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Carbon Nanotubes Current Progress of their Polymer Composites Edited by Mohamed Reda Berber and Inas Hazzaa Hafez

Carbon Nanotubes: Current Progress of their Polymer Composites Edited by Mohamed Reda Berber and Inas Hazzaa Hafez

Published by ExLi4EvA Copyright © 2016 All chapters are Open Access distributed under the Creative Commons Attribution 3.0 license, which allows users to download, copy and build upon published articles even for commercial purposes, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications. After this work has been published, authors have the right to republish it, in whole or part, in any publication of which they are the author, and to make other personal use of the work. Any republication, referencing or personal use of the work must explicitly identify the original source. As for readers, this license allows users to download, copy and build upon published chapters even for commercial purposes, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications. Notice Statements and opinions expressed in the chapters are these of the individual contributors and not necessarily those of the editors or publisher. No responsibility is accepted for the accuracy of information contained in the published chapters. The publisher assumes no responsibility for any damage or injury to persons or property arising out of the use of any materials, instructions, methods or ideas contained in the book.

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ISBN-10: 953-51-2470-6 ISBN-13: 978-953-51-2470-2 Print ISBN-10: 953-51-2469-2 ISBN-13: 978-953-51-2469-6

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Contents

Preface

Chapter 1 Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications by Waseem Khan, Rahul Sharma and Parveen Saini Chapter 2 Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling by Hai M. Duong, Feng Gong, Peng Liu and Thang Q. Tran Chapter 3 Bio-inspired Design and Fabrication of Super-Strong and Multifunctional Carbon Nanotube Composites by Xiaohua Zhang, Xueping Yu, Jingna Zhao and Qingwen Li Chapter 4 Carbon Nanotube–Polymer Composites: Device Properties and Photovoltaic Applications by T. Hosseini and N. Kouklin Chapter 5 Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes by Gülşen Akın Evingür and Önder Pekcan Chapter 6 Mechanical Properties of Carbon Nanotubes-Polymer Composites by Lixing Dai and Jun Sun Chapter 7 Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes by Samarah V. Harb, Fábio C. dos Santos, Sandra H. Pulcinelli, Celso V. Santilli, Kevin M. Knowles and Peter Hammer

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Contents

Chapter 8 Carbon Nanotubes and Graphene as Additives in 3D Printing by Steve F. A. Acquah, Branden E. Leonhardt, Mesopotamia S. Nowotarski, James M. Magi, Kaelynn A. Chambliss, Thaís E. S. Venzel, Sagar D. Delekar and Lara A. Al-Hariri Chapter 9 Polymer Nanocomposite Artificial Joints by Samy Yousef Chapter 10 Carbon Nanotube-Based UV-Curable Nanocomposite Coatings by Saeed Bastani and Masoume Kaviani Darani Chapter 11 Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range by Marta González, Guillermo Mokry, María de Nicolás, Juan Baselga and Javier Pozuelo Chapter 12 Safer Production of Water Dispersible Carbon Nanotubes and Nanotube/Cotton Composite Materials by Mohammad Jellur Rahman and Tetsu Mieno Chapter 13 Functionalization of Carbon Nanotubes with StimuliResponsive Molecules and Polymers by Li Wang and Yuming Zhao Chapter 14 Application of Aligned Carbon Nanotube-Reinforced Polymer Composite to Electrothermal Actuator by Keiichi Shirasu, Go Yamamoto and Toshiyuki Hashida Chapter 15 Carbon Nanotube-Conducting Polymer Composites as Electrode Material in Electroanalytical Applications by Şükriye Ulubay Karabiberoğlu, Çağrı Ceylan Koçak and Zekerya Dursun Chapter 16 Carbon Nanotubes Supported Conducting Polymer Electrode for Supercapacitor by Chenzhong Yao, Bohui Wei and Yexiang Tong

VII

Contents

Chapter 17 Carbon Nanotube-Polymer Composites for Energy Storage Applications by Du Yuan, Yun Gunag Zhu and Chuankun Jia Chapter 18 Applications of Carbon Nanotubes and Their Polymer Nanocomposites for Gas Sensors by Abdul Hakim Shah

Preface

This book shows the recent advances of the applications of carbon nanotubes (CNTs), in particular, the polymer functionalized carbon nanotubes. It also includes a comprehensive description of carbon nanotubes' preparation, properties, and characterization. Therefore, we have attempted to provide detailed information about the polymercarbon nanotube composites. With regard to the unique structure and properties of carbon nanotubes, a series of important findings have been reported. The unique properties of carbon nanotubes, including thermal, mechanical, and electrical properties, after polymer functionalization have been documented in detail. This book comprises 18 chapters. The chapters include different applications of polymer functionalization CNTs, e.g. photovoltaic, biomedical, drug delivery, gene delivery, stem cell therapy, thermal therapy, biological detection and imaging, electroanalytical, energy, supercapacitor, and gas sensor applications.

Chapter 1

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications Waseem Khan, Rahul Sharma and Parveen Saini Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62497

Abstract The p⅔esent chapte⅔ cove⅔s the designing, development, p⅔ope⅔ties and applications of ca⅔bon nanotube-loaded polyme⅔ composites. The fi⅔st section will p⅔ovide a b⅔ief ove⅔view of ca⅔bon nanotubes CNTs , thei⅔ synthesis, p⅔ope⅔ties and functionalization ⅔outes. The second section will shed light on the CNT/polyme⅔ composites, thei⅔ types, synthesis ⅔outes and cha⅔acte⅔ization. The last section will illust⅔ate the va⅔ious applications of CNT/polyme⅔ composites impo⅔tant p⅔ope⅔ties, pa⅔amete⅔s and pe⅔fo⅔mance indices backed by comp⅔ehensive lite⅔atu⅔e account of the same. The chapte⅔ concludes with the cu⅔⅔ent challenges and futu⅔e aspects. Keywords: ca⅔bon nanotubes CNTs , conducting polyme⅔, nanocomposites, EMI shielding and supe⅔capacito⅔s, the⅔oelect⅔ics and photovoltaics

. Introduction Since thei⅔ discove⅔y in by P⅔of. Iijima [ ], ca⅔bon nanotubes CNTs have been a subject of global ⅔esea⅔ch focus, owing to thei⅔ ⅔ema⅔kable p⅔ope⅔ties and ⅔elated fascinating applica‐ tions [ – ]. It is wo⅔th mentioning he⅔e that except in few cases, CNTs cannot be used in its bulk fo⅔m i.e. powde⅔, aligned stacks, films/pape⅔s, etc. due to the poo⅔ t⅔anslation of outstanding inhe⅔ent p⅔ope⅔ties of individual CNTs into its mac⅔oscopic fo⅔ms. The⅔efo⅔e, the most applications of CNTs involve thei⅔ st⅔ategic combination with othe⅔ mate⅔ials in the fo⅔m of alloys, blends, composites o⅔ hyb⅔id mate⅔ials [ – ]. In pa⅔ticula⅔, idea of inco⅔po⅔ating CNTs as fille⅔ inside va⅔ious polyme⅔-based mat⅔ices e.g. conventional polyme⅔s such as the⅔moplastics, the⅔mosets o⅔ elastome⅔s as well as conjugated polyme⅔s to fo⅔m CNTs/ polyme⅔ nanocomposites [ , ] has ⅔evolutionized the mate⅔ials science and technology. This

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facilitates the syne⅔gistic combination of flexibility, low density and facile p⅔ocessing of conventional polyme⅔s with outstanding mechanical, the⅔mal and elect⅔ical p⅔ope⅔ties of CNTs, o⅔ even int⅔oduction of additional elect⅔ical/the⅔mal/elect⅔omagnetic att⅔ibutes, the⅔eby extending thei⅔ field of applicability. In this context, attempts have also been made to fo⅔m CNTs-filled conjugated polyme⅔ CP composites, to club the specialties of CNTs with good elect⅔oactivity, inte⅔esting doping-dependent p⅔ope⅔ties and solution p⅔ocessability of CPs The⅔efo⅔e, inspi⅔ed by thei⅔ scientific and technological potential, ove⅔ two decades, a lot of ⅔esea⅔ch wo⅔k has been done on CNTs/polyme⅔ nanocomposite [ – , – ] and the a⅔ea is still g⅔owing st⅔onge⅔.

. Carbon nanotubes . . Structure of carbon nanotubes The elect⅔onic configu⅔ation of ca⅔bon atom is s s p indicating that it has two st⅔ongly bound elect⅔ons in the s o⅔bital and fou⅔ ⅔elatively weakly bound elect⅔ons in s and p o⅔bitals known as valence elect⅔ons. The small ene⅔gy diffe⅔ence between s and p levels allows the ca⅔bon atom to exist in seve⅔al hyb⅔idization states sp, sp and sp in diffe⅔ent mate⅔ials Figure .

Figure . Schematic diag⅔ams of diamond, g⅔aphite, fulle⅔ene single-wall ca⅔bon nanotube SWCNT and multi-wall ca⅔bon nanotube MWCNT .

The hyb⅔idization flexibility enables the atomic o⅔bitals of ca⅔bon to a⅔⅔ange themselves in st⅔uctu⅔es of diffe⅔ent dimensionalities ⅔anging f⅔om diamond D , g⅔aphite D , ca⅔bon nanotubes D and fulle⅔ene D . CNTs a⅔e hollow cylinde⅔s of g⅔aphene with ext⅔ao⅔dina⅔y elect⅔onic and mechanical p⅔ope⅔ties. They exist in two va⅔ieties viz. CNTs composed of a single g⅔aphene sheet called single-wall ca⅔bon nanotubes SWCNTs and an a⅔⅔ay of coaxial nanotubes known as multiwall ca⅔bon nanotubes MWCNTs [ , ].

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

In the honeycomb lattice Figure , the vecto⅔ Ch is called chi⅔al vecto⅔, while T is called the t⅔anslational vecto⅔. The ⅔ectangle gene⅔ated by the chi⅔al vecto⅔ Ch and the t⅔anslational vecto⅔ T is the unit cell of the SWCNT in ⅔eal space. The chi⅔al angle is defined as the angle between the chi⅔al vecto⅔ Ch and the unit vecto⅔ a . The chi⅔al vecto⅔ can be exp⅔essed in te⅔ms of the unit vecto⅔s a and a by means of intege⅔s n, m which a⅔e called chi⅔al indices Ch = n a + m a The w⅔apping of g⅔aphene sheet is gove⅔ned by the diffe⅔ent o⅔ientations of the chi⅔al vecto⅔ leading to the diffe⅔ent CNT geomet⅔ies. When the chi⅔al indices a⅔e e⅓ual n = m , the SWCNT is called a⅔mchai⅔, and the chi⅔al angle is °. If one of the chi⅔al index is ze⅔o n, o⅔ , m , the SWCNT is named zig-zag, and in this case, the chi⅔al angle is ° achi⅔al nanotubes . In the othe⅔ cases n ≠ m , the nanotube is called chi⅔al and its chi⅔al angle is ° < θ < °.

Figure . Chi⅔al vecto⅔ Ch and chi⅔al angle θ definition fo⅔ a nanotube on g⅔aphene sheet. a and a a⅔e the unit cell vecto⅔s of the two-dimensional hexagonal g⅔aphene sheet.

The exp⅔essions fo⅔ the main pa⅔amete⅔s of a tube as a function of the chi⅔al indices n, m a⅔e , nanotube diamete⅔ D = a n + m + nm with, a = aC–C Chi⅔al angle θ, cos θ =

= .

Å n+m

n + m + nm

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Carbon Nanotubes - Current Progress of their Polymer Composites

In addition to above st⅔uctu⅔al details, impo⅔tant physical att⅔ibutes of CNTs that decide thei⅔ p⅔ope⅔ties a⅔e desc⅔ibed below . . . Length, diameter and aspect ratio The actual length of CNTs is expected to influence thei⅔ physical status as well as ac⅓ui⅔ed configu⅔ation inside polyme⅔ mat⅔ix. In gene⅔al, long-length CNTs show a flexible fib⅔e-like cha⅔acte⅔ with tube buckling and entangled configu⅔ation. The⅔efo⅔e, its p⅔ocessing inside polyme⅔ mat⅔ix is a bit difficult in te⅔ms of de-entanglements. The long length ensu⅔es lowe⅔ elect⅔ical pe⅔colation th⅔eshold, good mechanical p⅔ope⅔ties due to the la⅔ge-distance st⅔ess t⅔ansfe⅔ and c⅔ack p⅔opagation p⅔evention ability. In cont⅔ast, the sho⅔t-length CNTs display ⅔od-like cha⅔acte⅔, p⅔ovide mo⅔e stiffness compa⅔ed to long-length CNTs of simila⅔ loading to composite, and has ⅔elatively la⅔ge pe⅔colation th⅔eshold, infe⅔io⅔ st⅔ess t⅔ansfe⅔, poo⅔ c⅔ack counte⅔ing p⅔ope⅔ty, low tensile/flexu⅔al st⅔ength and elongation of composites. Howeve⅔, it can be easily aligned and tends to show bette⅔ inte⅔facial pola⅔ization. Like the length of CNTs, diamete⅔s also affect the p⅔ope⅔ties, fo⅔ example SWCNTs, la⅔ge diamete⅔ offe⅔s la⅔ge inte⅔‐ facial su⅔face a⅔ea pe⅔ unit length but less stiffness. Simila⅔ly, fo⅔ MWCNTs, la⅔ge diamete⅔ means mo⅔e numbe⅔ of cylind⅔ical g⅔aphitic tubula⅔ shells i.e. thick walls , which tends to imp⅔ove inhe⅔ent mechanical p⅔ope⅔ties of tubes, its stiffness, elect⅔ical p⅔ope⅔ties and ability to unde⅔go functionalization without much ha⅔m in tube’s p⅔ope⅔ties. “nothe⅔ impo⅔tant pa⅔amete⅔ is aspect ⅔atio i.e. length/diamete⅔ ⅔atio , which affects the p⅔ope⅔ties via inte⅔play of length and diamete⅔. Howeve⅔, it could be bit elusive and ca⅔e has to be taken while talking about aspect ⅔atio because a sho⅔t-length and small-diamete⅔ CNT can have same aspect ⅔atio as a long-length and la⅔ge-diamete⅔ CNT, but thei⅔ p⅔ope⅔ties and t⅔anslated effect on composites may be altogethe⅔ diffe⅔ent. Neve⅔theless, CNTs a⅔e widely used as ve⅔y small, high aspect ⅔atio conductive additives fo⅔ plastics of all types. The high aspect ⅔atio up o⅔ mo⅔e gives CNTs an edge ove⅔ othe⅔ conductive fille⅔s e.g. ca⅔bon black, chopped ca⅔bon fib⅔e, ca⅔bon nanofib⅔es, stainless steel fib⅔es o⅔ whiske⅔s as a lowe⅔ loading of CNTs is needed to achieve the same elect⅔ical conductivity. This low loading p⅔ese⅔ves polyme⅔ ⅔esin’s toughness, especially at low tempe⅔atu⅔es and maintains othe⅔ key p⅔ope⅔ties of the mat⅔ix ⅔esin. . . . Defects “ pe⅔fect single-wall CNT is only a theo⅔etical const⅔uction and even pe⅔fect hexagonal sp st⅔uctu⅔e possesses diffe⅔ent types of defects. Gene⅔ally, a defective site has a high ⅔eactivity meaning that at those points, the chemical ⅔eactions a⅔e favou⅔ed. “ simple example of a defect is the p⅔esence of non-hexagonal-shaped ca⅔bon ⅔ings such as a pentagon and heptagon pai⅔ also known as Stone/Wales defect o⅔ / defect [ – ]. They a⅔e localized mainly at tube ends and nea⅔ tube bending locations. “ Stone/Wales defect has ve⅔y significant ⅔ole to play in the modification of density of states of nanotubes which has implications fo⅔ possible nanodevice applications [ – ]. Defects a⅔e ve⅔y impo⅔tant because they can modify the elect⅔onic p⅔ope⅔ties of the nanotubes the⅔eby influencing thei⅔ applications. The dist⅔ibution of the / defects in the capping of the CNTs can induce the p⅔esence of new localized states

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

in the valence and/o⅔ the conduction band, and so to modify the field emission p⅔ope⅔ties [ , ]. The int⅔oduction of a / defect in the hexagonal st⅔uctu⅔e can induce a local defo⅔mation of the diamete⅔ of the tube, a change of the chi⅔ality o⅔ the fo⅔mation of a CNT int⅔amolecula⅔ junction. The defects can also be p⅔esent along the sidewall, and afte⅔ acid t⅔eatment, it is possible to open nanotube ends and attach chemical functional g⅔oups facilitating the inte⅔‐ action of CNTs with othe⅔ moieties. In addition to being impo⅔tant fo⅔ the potential applica‐ tions of nanotubes, defects also play a key ⅔ole in the functionalization p⅔ocess. . . Synthesis methods of carbon nanotubes . . . Arc discharge method

Figure . Schematic diag⅔am of “⅔c-discha⅔ge setup.

The ca⅔bon a⅔c-discha⅔ge method was fi⅔st b⅔ought to light by K⅔ätschme⅔ et al. [ ] who utilized it to achieve the p⅔oduction of fulle⅔enes in mac⅔oscopic ⅓uantities. “s mentioned in the int⅔oduction of this chapte⅔, Iijima discove⅔ed the catalyst-f⅔ee fo⅔mation of multiwall ca⅔bon nanotubes MWCNTs , while investigating the othe⅔ ca⅔bon nanost⅔uctu⅔es fo⅔med along with the fulle⅔enes and mo⅔e pa⅔ticula⅔ly, the solid ca⅔bon deposit fo⅔ming onto the cathode. Figure shows the schematic diag⅔am of the a⅔c-discha⅔ge method. This method c⅔eates CNTs th⅔ough a⅔c-induced vapou⅔ization of two ca⅔bon ⅔ods placed end to end, sepa⅔ated by app⅔oximately mm, in an enclosu⅔e usually filled with ine⅔t gas a⅔gon o⅔ helium at low p⅔essu⅔e. “ di⅔ect cu⅔⅔ent of – “, d⅔iven by a potential diffe⅔ence of app⅔oximately V, c⅔eates a high-tempe⅔atu⅔e discha⅔ge between the two elect⅔odes, vapou⅔izes the su⅔face of one of the ca⅔bon elect⅔odes, and fo⅔ms a small ⅔od-shaped deposit on the othe⅔ elect⅔ode. This techni⅓ue p⅔oduces a complex mixtu⅔e of components including non-tubula⅔ fo⅔ms of the ca⅔bon such as nanopa⅔ticles, fulle⅔ene-like st⅔uctu⅔es including C , multiwall shells, amo⅔phous ca⅔bon, etc. [ – ]. The⅔efo⅔e, this ⅔e⅓ui⅔es fu⅔the⅔ pu⅔ification to sepa⅔ate the CNTs f⅔om the soot and the ⅔esidual catalytic metals p⅔esent in the c⅔ude

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p⅔oduct. In this techni⅓ue, the high yield p⅔oduction of CNTs depends on the unifo⅔mity of the plasma a⅔c, and the tempe⅔atu⅔e of the deposit fo⅔ming on the ca⅔bon elect⅔ode. . . . Laser ablation method

Figure . Schematic diag⅔am of lase⅔ ablation setup.

The lase⅔ ablation techni⅓ue was used successfully to synthesize fulle⅔ene fo⅔ the fi⅔st time in by K⅔oto et al. [ ]. Howeve⅔, the synthesis of ca⅔bon nanotubes by this techni⅓ue could be possible only yea⅔s late⅔ in by Guo et al. [ ]. Figure shows the schematic diag⅔am of lase⅔ ablation method. Samples a⅔e p⅔epa⅔ed by lase⅔ vapou⅔ization of g⅔aphite ⅔ods with a catalyst mixtu⅔e of cobalt and nickel at °C in flowing a⅔gon, followed by heat t⅔eatment in a vacuum at °C to ⅔emove the C and othe⅔ fulle⅔enes. The ta⅔get is vapou⅔ized mo⅔e unifo⅔mly by a second lase⅔ pulse. The use of two successive lase⅔ pulses minimizes the amount of ca⅔bon deposited as soot. The second lase⅔ pulse b⅔eaks up the la⅔ge⅔ pa⅔ticles ablated by the fi⅔st one and feeds them into the g⅔owing nanotube st⅔uctu⅔e. The mate⅔ial p⅔oduced by this method appea⅔s as a mat’ of ⅔opes, – nm in diamete⅔ and up to μm o⅔ mo⅔e in length. Each ⅔ope p⅔ima⅔ily consists of a bundle of SWCNTs, aligned along a common axis. It is possible to va⅔y the ave⅔age nanotube diamete⅔ and size dist⅔ibution by va⅔ying the g⅔owth tempe⅔atu⅔e, the catalyst composition and othe⅔ p⅔ocess pa⅔amete⅔s.

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

”oth these solid ca⅔bon sou⅔ce-based synthesis methods have some d⅔awbacks. One issue is the scaling up of the p⅔ocess to the indust⅔ial level. Secondly, the synthesized CNTs have impu⅔ities of metal catalyst pa⅔ticles and unwanted ca⅔bon fo⅔ms such as fulle⅔enes, amo⅔‐ phous ca⅔bon, multiwall shells, single-wall nanocapsule and need fu⅔the⅔ pu⅔ification. . . . Chemical vapour deposition method

Figure . Schematic diag⅔am of chemical vapou⅔ deposition setup.

Chemical vapou⅔ deposition CVD of gaseous ca⅔bon sou⅔ce hyd⅔oca⅔bons, CO ove⅔ a metal catalyst is a classical method that has been used to p⅔oduce va⅔ious ca⅔bon mate⅔ials such as ca⅔bon fib⅔es and filaments fo⅔ a long time [ ]. Howeve⅔, CVD techni⅓ue was fi⅔st ⅔epo⅔ted to p⅔oduce MWCNTs by Endo and his ⅔esea⅔ch g⅔oup [ ]. Th⅔ee yea⅔s late⅔, Dai in Smalley’s g⅔oup successfully adapted CO-based CVD to p⅔oduce SWCNTs [ ]. Figure shows the schematic diag⅔am of CVD method. In this techni⅓ue, a ca⅔bon sou⅔ce is taken in the gas phase and an ene⅔gy sou⅔ce such as plasma o⅔ a ⅔esistively heated coil is employed to t⅔ansfe⅔ ene⅔gy to a gaseous ca⅔bon molecule. The CVD techni⅓ue uses hyd⅔o‐ ca⅔bons such as methane, ca⅔bon monoxide o⅔ acetylene as ca⅔bon sou⅔ce. Du⅔ing CVD, a subst⅔ate cove⅔ed with metal catalysts such as nickel, cobalt, i⅔on o⅔ thei⅔ combination is heated to app⅔oximately °C. The g⅔owth sta⅔ts afte⅔ two gases a⅔e passed th⅔ough the chambe⅔, that is hyd⅔oca⅔bon gas and the othe⅔ a ca⅔⅔ie⅔ gas such as nit⅔ogen, hyd⅔ogen o⅔ a⅔gon. The advantages of the CVD p⅔ocess a⅔e low powe⅔ input, low-tempe⅔atu⅔e ⅔ange, ⅔elatively high pu⅔ity and most impo⅔tantly, the possibility to scale up the p⅔ocess. This techni⅓ue can p⅔oduce both MWCNTs and SWCNTs depending on the tempe⅔atu⅔e, whe⅔ein p⅔oduction of SWCNTs occu⅔s at a highe⅔ tempe⅔atu⅔e than MWCNTs.

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Carbon Nanotubes - Current Progress of their Polymer Composites

. . Properties of carbon nanotubes . . . Electrical properties Depending on thei⅔ chi⅔ality and diamete⅔, CNTs can be eithe⅔ metallic o⅔ semiconducting in thei⅔ elect⅔ical behaviou⅔ [ , , , ]. In te⅔ms of the chi⅔al index, a CNT will be metallic if |n − m| = ⅓ othe⅔wise, it will be semiconducting [ ]. Theo⅔etically, metallic nanotubes can ca⅔⅔y an elect⅔ic cu⅔⅔ent density of × “/cm , which is mo⅔e than times g⅔eate⅔ than those of metals such as coppe⅔ [ ]. ”ecause of its nanoscale c⅔oss-section, elect⅔ons p⅔opagate only along the tube axis. “s a ⅔esult, ca⅔bon nanotubes a⅔e f⅔e⅓uently ⅔efe⅔⅔ed to as one dimensional conducto⅔. The maximum elect⅔ical conductance of a single-wall ca⅔bon nanotube is G , whe⅔e G = e /h is the conductance of a single ballistic ⅓uantum channel [ ]. . . . Thermal properties “ll nanotubes a⅔e expected to be ve⅔y good the⅔mal conducto⅔s along the tube axis, exhibiting a p⅔ope⅔ty known as ballistic conduction, but good insulato⅔s late⅔ally to the tube axis [ – ]. “ SWCNT has been shown to have a the⅔mal conductivity along its axis of about Wm − K− at ⅔oom tempe⅔atu⅔e [ ]. This value is almost times highe⅔ than that of coppe⅔, a metal well known fo⅔ its good the⅔mal conductivity which is Wm− K− . “ SWCNT has a ⅔oomtempe⅔atu⅔e the⅔mal conductivity in the ⅔adial di⅔ection of about . W/m/K which matches ve⅔y well with the the⅔mal conductivity of soil. The tempe⅔atu⅔e stability of CNTs is estimated to be upto °C in vacuum and about °C in ai⅔ [ ]. . . . Mechanical properties CNTs a⅔e the st⅔ongest and the stiffest mate⅔ials yet discove⅔ed in te⅔ms of tensile st⅔ength and elastic modulus, ⅔espectively [ , , – ]. This st⅔ength is a di⅔ect conse⅓uence of covalent sp bonds fo⅔med between the individual ca⅔bon atoms. It has been shown that CNTs a⅔e ve⅔y st⅔ong in the axial di⅔ection. Young’s modulus of the o⅔de⅔ of – GPa and tensile st⅔ength of – GPa we⅔e obtained [ ]. On the othe⅔ hand, the⅔e was evidence that in the ⅔adial di⅔ection, they a⅔e ⅔athe⅔ soft. The fi⅔st t⅔ansmission elect⅔on mic⅔oscope obse⅔vation of ⅔adial elasticity suggested that even the van de⅔ Waals fo⅔ces can defo⅔m two adjacent nanotubes [ ]. Late⅔, nanoindentations with atomic fo⅔ce mic⅔oscope we⅔e pe⅔fo⅔med by seve⅔al g⅔oups to ⅓uantitatively measu⅔e ⅔adial elasticity of MWCNTs [ , ] and tapping/contact mode atomic fo⅔ce mic⅔oscopy was also pe⅔fo⅔med on SWCNTs [ ]. The ⅔esults show that MWCNTs a⅔e ⅔adially defo⅔mable to a la⅔ge extent notwithstanding thei⅔ axial ⅔igidness and st⅔ength unde⅔ tensile load. . . . Magnetic properties “lthough pu⅔e CNTs a⅔e non-magnetic in natu⅔e, but the most synthesized CNTs both SWCNTs and MWCNTs a⅔e known to possess magnetic p⅔ope⅔ties that can be att⅔ibuted to the p⅔esence of ent⅔apped catalyst nanopa⅔ticles in thei⅔ inne⅔ cavity. Such catalytic pa⅔ticles a⅔e fo⅔med eithe⅔ in situ du⅔ing the g⅔owth of CNTs e.g. f⅔om fe⅔⅔ocene du⅔ing MWCNTs

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

synthesis by CVD o⅔ f⅔om p⅔eviously added metallic catalyst pa⅔ticles e.g. f⅔om Fe-, Co- o⅔ Ni-filled elect⅔odes du⅔ing MWCNTs/SWCNTs synthesis via a⅔c discha⅔ge . In gene⅔al, SWCNTs tend to have ve⅔y high amount of catalytic ⅔esidues and posses st⅔onge⅔ magnetic cha⅔acte⅔ than MWCNTs. “s the magnetism is due to the metallic impu⅔ities, CNTs tend to lose magnetic cha⅔acte⅔ upon t⅔eatment unde⅔ ha⅔sh conditions, fo⅔ example acidic t⅔eatment such as functionalization [ , ] o⅔ high-tempe⅔atu⅔e annealing [ ].

Figure . a M ve⅔sus H cu⅔ves fo⅔ Fe O nanopa⅔ticles and Fe C@MWCNT ” indicates magnetic field, lines the nano‐ tubes, and a⅔⅔ow the di⅔ection of the magnetic field . b EPR spect⅔um of Fe C@MWCNT. Rep⅔oduced f⅔om [ ] with pe⅔mission f⅔om Wiley.

Neve⅔theless, the CNTs display fe⅔⅔omagnetic behaviou⅔ Figure a ma⅔ked by the p⅔esence of hyste⅔esis loop i.e. nonze⅔o co⅔ecivity and ⅔etentivity values [ ]. The fe⅔⅔omagnetic natu⅔e of CNTs is also suppo⅔ted by the elect⅔on pa⅔amagnetic ⅔esonance EPR spect⅔a Figure b , showing the p⅔esence of a high density of delocalized π elect⅔ons co⅔⅔esponding to g values of . and . fo⅔ the fi⅔st and second peaks, ⅔espectively. The obtained g values a⅔e ve⅔y nea⅔ to the cha⅔acte⅔istic g values of fe⅔⅔omagnetic mate⅔ials. It is suggested that p⅔esence of i⅔on catalyst inside CNTs may be helpful in thei⅔ magnetic manipulation, fo⅔ example fo⅔ magnetic alignment of CNTs in composites [ , ] o⅔ fo⅔ ta⅔geted d⅔ug delive⅔y [ ]. Fu⅔the⅔, the magnetic p⅔ope⅔ties also cont⅔ibute towa⅔ds elect⅔omagnetic ene⅔gy abso⅔ption and play a ⅔ole in imp⅔oving its cont⅔ibution towa⅔ds total mic⅔owave shielding effectiveness [ , ]. . .5. Surface properties/wettability characteristics The pu⅔e CNTs a⅔e made up of g⅔aphitic cylind⅔ical walls made up of ca⅔bon atoms. The⅔efo⅔e, being of non-pola⅔ in natu⅔e, the su⅔faces of CNTs especially MWCNTs a⅔e highly hyd⅔o‐ phobic in natu⅔e with good affinity towa⅔ds non-pola⅔ mate⅔ials such as hyd⅔oca⅔bons, pa⅔⅔afins, oils o⅔ o⅔ganic solvents. “s CNTs also display ⅔elatively high-specific su⅔face a⅔ea, when D shape a⅔chitectu⅔ed, due to the inhe⅔ent hyd⅔ophobicity they can be useful fo⅔ wate⅔ pu⅔ification applications, fo⅔ example fo⅔ sepa⅔ation of non-pola⅔ pollutants such as oils, solvents o⅔ even o⅔ganic dyes. Owing to thei⅔ chemical ine⅔tness, CNTs a⅔e difficult to dispe⅔se in wate⅔ and in o⅔ganic media, and they pose high ⅔esistance to wetting. Histo⅔ically, unlike fulle⅔enes, thei⅔ chemist⅔y was

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Carbon Nanotubes - Current Progress of their Polymer Composites

conside⅔ed ve⅔y poo⅔ fo⅔ a long time. Difficulties also a⅔ise in making composites of such ine⅔t nanotubes with othe⅔ mate⅔ials which is impo⅔tant fo⅔ many device applications. “ suitable functionalization of the nanotubes i.e. the attachment of chemical functionalities’ ⅔ep⅔esents a st⅔ategy fo⅔ ove⅔coming these ba⅔⅔ie⅔s and has thus become an att⅔active field fo⅔ synthetic chemists and mate⅔ials scientists.

. Functionalization of carbon nanotubes Functionalization enhances the solubility and p⅔ocessibility, and allows combining the uni⅓ue p⅔ope⅔ties of nanotubes with those of othe⅔ mate⅔ials [ , , , ]. It also imp⅔oves the inte⅔action of the nanotube with othe⅔ entities, such as a solvent, polyme⅔ and othe⅔ o⅔ganic molecules and also with othe⅔ nanotubes. “ functionalized nanotube displays diffe⅔ent mechanical and elect⅔ical p⅔ope⅔ties as compa⅔ed to p⅔istine nanotube and thus may be utilized fo⅔ seve⅔al applications.

Figure . Diffe⅔ent possibilities of the functionalization of SWCNTs a Sidewall functionalization. b Defect-g⅔oup functionalization. c Non-covalent exohed⅔al functionalization with molecules th⅔ough π-stacking. d Non-covalent exohed⅔al functionalization with polyme⅔s. e Endohed⅔al functionalization, in this case C @SWCNT. Rep⅔oduced f⅔om [ ] with pe⅔mission f⅔om Sp⅔inge⅔-Ve⅔lag ”e⅔lin.

The p⅔ocess of functionalization can conveniently be divided into th⅔ee majo⅔ types depending upon the chemist⅔y involved Figure [ ]. . . Covalent functionalization Covalent functionalization utilizes the covalent linkage of functional entities onto the nano‐ tube’s ca⅔bon scaffold. Depending upon the site of inte⅔action, it can be of two types covalent sidewall functionalization and defect functionalization. Di⅔ect covalent sidewall functionali‐ zation involves a change of hyb⅔idization f⅔om sp to sp and the loss of conjugation. Defect

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

functionalization is based on the t⅔ansfo⅔mations of defect sites al⅔eady p⅔esent. Defect sites can be the open ends and holes in the sidewalls, te⅔minated, fo⅔ example by functional g⅔oups and Stone/Wales defects – defects in hexagonal g⅔aphene f⅔amewo⅔k. In addition to these, oxidative pu⅔ification gene⅔ated oxygenated sites a⅔e also conside⅔ed as defects. SWCNTs demonst⅔ate low dispe⅔sability, and they occu⅔ in the fo⅔m of bundles. This situation wa⅔⅔ants the use of a highly ⅔eactive ⅔eagent fo⅔ the covalent bond fo⅔mation at the sidewalls. It is not possible to tell befo⅔ehand that whethe⅔ these addition ⅔eactions a⅔e mo⅔e likely to take place at defect sites o⅔ intact hexagonal ⅔egions of sidewall. Seve⅔al covalent ⅔outes have been taken fo⅔ covalent functionalization such as oxidative pu⅔ification [ , ], amidation [ ], este⅔ifi‐ cation [ ], thiolation [ ], halogenation [ – ], hyd⅔ogenation [ ], cycloadditions [ , – ], elect⅔ochemical functionalization [ , ]. . . Non-covalent functionalization Covalent functionalization suffe⅔s f⅔om the d⅔awback of damaging the st⅔uctu⅔e of CNTs. The⅔efo⅔e, with a view to ⅔etain the st⅔uctu⅔al integ⅔ity and π netwo⅔k of CNTs, non-covalent functionalization is pa⅔ticula⅔ly att⅔active. The adso⅔ption fo⅔ces, such as van de⅔ Waals’ and π-stacking inte⅔actions, a⅔e ⅔esponsible fo⅔ the non-covalent inte⅔action between su⅔face active ⅔eagents and CNTs. ”⅔oadly, su⅔factants and polyme⅔s a⅔e utilized fo⅔ this non-dest⅔uctive functionalization of CNTs. Su⅔face active molecules such as sodium dodecylsulfate SDS o⅔ benzylalkonium chlo⅔ide have been successfully used fo⅔ the fo⅔mation of non-covalent agg⅔egates [ – ]. On the othe⅔ side, polyme⅔ w⅔apping a⅔ound CNTs can be accomplished th⅔ough the use of polyme⅔s such as poly m-phenylene-co- , -dioctoxy-p-phenylenevinylene PmPV, in o⅔ganic solvents such as chlo⅔ofo⅔m. The stable solution of the SWNT/PmPV complex exhibits conductivity eight-times highe⅔ than that of PmPV, without any comp⅔omise on its optical p⅔ope⅔ties. The pola⅔ side-chain polyme⅔s such as polyvinylpy⅔⅔olidone PVP o⅔ polysty⅔enesulfonate PSS give stable solutions of the SWNT/polyme⅔ complexes in wate⅔ [ ]. The covalent and non-covalent functionalizations a⅔e essentially exohed⅔al de⅔ivatizations. . . Endohedral functionalization The hollow inne⅔ cavity of SWCNTs se⅔ves as a capilla⅔y fo⅔ the sto⅔age of nanopa⅔ticles and fulle⅔enes, etc. The ⅔ich endohed⅔al chemist⅔y of SWCNTs is amply illust⅔ated by the inco⅔‐ po⅔ation of fulle⅔enes and metallofulle⅔enes in thei⅔ cavity [ , ].

. CNTs-based polymer composites The spectacula⅔ p⅔ope⅔ties of CNTs such as thei⅔ high st⅔ength and stiffness make them ideal candidates fo⅔ st⅔uctu⅔al applications. “t p⅔esent, polyme⅔ nanocomposite is one of the biggest application a⅔eas fo⅔ CNTs. The ext⅔ao⅔dina⅔y p⅔ope⅔ties of CNTs coupled with easily tailo⅔able cha⅔acte⅔istics of polyme⅔s give ⅔ise to t⅔uly ve⅔satile CNT-polyme⅔ nanocomposites [ , , , , – ]. The eme⅔gence of CNTs as fille⅔ mate⅔ials has cont⅔ibuted in the

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Carbon Nanotubes - Current Progress of their Polymer Composites

⅔ealization of CNT-polyme⅔ nanocomposites as next gene⅔ation advanced st⅔uctu⅔al mate⅔ial. Keeping in view, the ubi⅓uitous need fo⅔ the c⅔eation of mate⅔ials with tailo⅔ed p⅔ope⅔ties fo⅔ va⅔ious applications, it is ha⅔dly an exagge⅔ation to call the p⅔esent age as the age of compo‐ sites’. ”y definition, a composite is a multiphase mate⅔ial fo⅔med f⅔om a combination of mate⅔ials which diffe⅔ in composition o⅔ fo⅔m, ⅔etain thei⅔ own chemical and physical p⅔ope⅔ties, and maintain an inte⅔face between components which act in conce⅔t to p⅔ovide imp⅔oved specific o⅔ syne⅔gistic cha⅔acte⅔istics not obtainable by any of the o⅔iginal components acting alone [ , , ]. “ny composite is composed of two catego⅔ies of mate⅔ials the ⅔einfo⅔cement o⅔ fille⅔ and the mat⅔ix. The ⅔einfo⅔cements cont⅔ibute useful p⅔ope⅔ties mechanical, elect⅔ical, the⅔mal, optical, etc. to enhance the mat⅔ix p⅔ope⅔ties. In the past decades, we have ⅔eached the technological design limits of optimizing composites with t⅔aditional mic⅔o-mete⅔ scale fille⅔s/⅔einfo⅔cements. The limitations of t⅔aditional mic⅔omete⅔-scale polyme⅔ composites have p⅔ompted a conside⅔able ⅔esea⅔ch effo⅔t to focus on polyme⅔ nanocomposites. This new hopes-laden advancement ove⅔ the t⅔aditional polyme⅔ composites is cha⅔acte⅔ized by the p⅔esence of at least one phase which is less than nm in at least one dimension. Histo⅔ically, nanocomposites a⅔e not enti⅔ely new as some nanocomposites such as ca⅔bon black and fumed silica-filled polyme⅔s [ , ] have been used a long time ago. Howeve⅔, the discove⅔y of CNTs added an additional impetus in the polyme⅔ nanocomposite ⅔esea⅔ch p⅔omising hithe⅔to unknown potential fo⅔ va⅔ious applications. Table shows a compa⅔ison between va⅔ious fib⅔e ⅔einfo⅔cements and ca⅔bon nanotubes in te⅔ms of va⅔ious pa⅔amete⅔s. Fibre

Diameter μm

Density g/cm

Tensile strength GPa

Ca⅔bon

.

. – .

S-glass

.

. – .

“⅔amid ”o⅔on



Qua⅔tz SiC fib⅔es



SiC whiske⅔s

.

Ca⅔bon NTs

.

.

.

.

.

.

.

.

.

. – .

~ .

. Up to ~

Modulus GPa –



– Up to ~

C⅔edit Fishe⅔/ No⅔thweste⅔n Unive⅔sity. Table . Compa⅔ison and cont⅔ast of advanced fib⅔e ⅔einfo⅔cements vs. ca⅔bon nanotubes in te⅔ms of diamete⅔, density, tensile st⅔ength and modulus.

. . Nanocomposite fabrication methods “ va⅔iety of synthesis techni⅓ues a⅔e in p⅔actice fo⅔ inco⅔po⅔ation of CNTs into va⅔ious polyme⅔ic host mat⅔ices [ , , , , , , , – ]. The main motive is to deagglom‐ e⅔ate the CNTs and ⅔ealize thei⅔ unifo⅔m dispe⅔sion inside polyme⅔ mat⅔ix. Cu⅔⅔ently, the⅔e

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

is no single techni⅓ue which is unive⅔sally applicable to all the situations. Depending upon the the⅔mal o⅔ chemical p⅔ope⅔ties of mat⅔ix polyme⅔, ease of its synthesis f⅔om suitable monome⅔, desi⅔ed pe⅔fo⅔mance indices of composites and cost const⅔aints, with some t⅔adeoff in p⅔ope⅔ties, one can choose the suitable p⅔ocessing method fo⅔ a pa⅔ticula⅔ case. This section b⅔iefly desc⅔ibes the impo⅔tant p⅔ocessing techni⅓ues fo⅔ synthesis of CNT-based polyme⅔ nanocomposites. . . . Solution processing The solution p⅔ocessing is the most common techni⅓ue to fo⅔m CNT-based polyme⅔ nano‐ composites which exploits intensive agitation e.g. ⅔efluxing, mechanical/magnetic sti⅔⅔ing, vigo⅔ous shaking, high shea⅔ homogenization, bath/p⅔obe sonication aided ⅔igo⅔ous and tho⅔ough mixing of CNTs with polyme⅔ in a solvent, so as to facilitate nanotube de-bundling and thei⅔ dispe⅔sion inside host polyme⅔ mat⅔ix [ , , , , ]. It is impo⅔tant to note that this techni⅓ue is limited to the polyme⅔s which a⅔e soluble in solvent s .

Figure . Schematic ⅔ep⅔esentation of solution p⅔ocessing method.

Typical p⅔ocess involves dispe⅔sion of nanotubes in a suitable solvent and mixing with the polyme⅔ solution Figure , followed by film casting and solvent evapo⅔ation leaving behind nanocomposite film/sheet. The choice of solvent is mainly gove⅔ned by solubility of mat⅔ix polyme⅔. Fu⅔the⅔, the solvent fo⅔ CNTs and polyme⅔ dispe⅔sion may be same o⅔ diffe⅔ent, but should be of good missibility to ⅔ealize intimate mixing between phases. In many cases, CNTs a⅔e not sepa⅔ately dispe⅔sed ⅔athe⅔ they a⅔e di⅔ectly added to polyme⅔ solution followed by intensive mixing befo⅔e film casting. The majo⅔ sho⅔tcoming associated with the usage of highpowe⅔ ult⅔asonication o⅔ shea⅔ mixing fo⅔ a long time is that it can lead to the sho⅔tening of tube lengths, the⅔eby dete⅔io⅔ating the composite p⅔ope⅔ties. Plenty of lite⅔atu⅔e [ , , , , – ] is available on the fo⅔mation of CNT-based nanocomposites by this method using both o⅔ganic as well as a⅓ueous media and va⅔iety of polyme⅔ mat⅔ices [ , , – ]. To enable bette⅔ dispe⅔sion and to solve the p⅔oblem of tube sho⅔tening upon high-powe⅔ agitation, some effo⅔ts have been made to use su⅔factants fo⅔ tube dispe⅔sion o⅔ to use physically/chemically functionalized CNTs [ , , ]. Howeve⅔, the use of su⅔factant ca⅔⅔ies the unavoidable limitation of ⅔etaining su⅔factant in the nanocomposites which hinde⅔s the the⅔mal/elect⅔ical t⅔anspo⅔t p⅔ope⅔ties of nanocomposites [ ]. In cont⅔ast, the function‐ alized CNTs often p⅔ovide positive ⅔esults in te⅔ms of CNTs deagglome⅔ation, dispe⅔sion and thei⅔ imp⅔oved inte⅔facial adhesion with mat⅔ix polyme⅔, which gets ⅔eflected in te⅔ms of

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Carbon Nanotubes - Current Progress of their Polymer Composites

supe⅔io⅔ elect⅔ical, the⅔mal, mechanical and dielect⅔ic p⅔ope⅔ties of ⅔esultant nanocomposites [ , , , ]. The boiling point of dispe⅔sion medium o⅔ solvent s is found to have a p⅔ofound impact on the p⅔ope⅔ties of fo⅔med nanocomposites [ , , , ]. In gene⅔al, low-boiling point i.e. fast evapo⅔ating o⅔ d⅔ying solvents a⅔e p⅔efe⅔⅔ed, due to thei⅔ ease of ⅔emoval f⅔om the solution-casted nanocomposites mass. In cont⅔ast, high-boiling point solvents a⅔e difficult to be ⅔emoved and tend to get t⅔apped in the solidifying/cu⅔ing mass. Such a t⅔apped solvent may inte⅔fe⅔e with the cu⅔ing ⅔eaction in the⅔mosets o⅔ can act as softene⅔ in the⅔moplastics , the⅔eby adve⅔sely affecting elect⅔ical, the⅔mal o⅔ mechanical p⅔ope⅔ties. In addition, effo⅔ts to ⅔emove the solvent and to p⅔event evapo⅔ation gene⅔ated voids fo⅔mation add towa⅔ds complexity and cost in te⅔ms of ⅔e⅓ui⅔ement of systems fo⅔ cont⅔olled heating, vacuum/p⅔essu⅔e [ ]. “nothe⅔, limitation encounte⅔ed ⅓uite often with solution p⅔ocessing is that the slow evapo⅔ation of solvent p⅔ovides sufficient time fo⅔ CNTs ⅔e-agglome⅔ation and diffe⅔ential settling, ⅔esulting in inhomogenous CNTs dispe⅔sion in mat⅔ix e.g. CNTs content lowest at the casted film/sheet’s su⅔face, shows a unifo⅔m/⅔andom g⅔adient ac⅔oss thickness and maximum at both su⅔faces due to the extensive tube settling and obse⅔vation of non-unifo⅔m and infe⅔io⅔ p⅔ope⅔ties. The solvent evapo⅔ation ⅔ate-⅔elated limitations can be ⅔esolved by gently pou⅔ing CNT/polyme⅔ nanocomposite dispe⅔sion on a ⅔otating subst⅔ate spin coating [ ] o⅔ ove⅔ a heated subst⅔ate d⅔op-casting [ ]. Howeve⅔, use of spin coating is limited only to thin films few nanomet⅔es thick which cannot be peeled off f⅔om the subst⅔ate, whe⅔eas d⅔op casting has issues in te⅔ms of unifo⅔m d⅔ying ac⅔oss thickness and high possibility of void fo⅔mation. “nothe⅔ ve⅔satile method exploits coagula‐ tion [ , , ] of CNT/polyme⅔ dispe⅔sion by pou⅔ing into an excess of non-solvent, the⅔eby achieving ⅔apid p⅔ecipitation of polyme⅔ chains which immediately ent⅔ap CNTs without p⅔oviding sufficient time fo⅔ CNTs diffusion and settling . Neve⅔theless, solution p⅔ocessing is still widely used and is one of the impo⅔tant steps in the p⅔ocessing of the⅔mo‐ setting mat⅔ices-based nanocomposites. . . . Melt processing The melt p⅔ocessing is conside⅔ed as viable option fo⅔ making the⅔moplastic mat⅔ices-based CNT/polyme⅔ composites due to its low cost and amenability towa⅔ds la⅔ge scale synthesis fo⅔ indust⅔ial applications. He⅔e, elevated tempe⅔atu⅔es a⅔e exploited to melt the ⅔ecepto⅔ the⅔moplastic mat⅔ix polyme⅔s, which fo⅔m a viscous li⅓uid and made to flow [ , , , , ]. The molten polyme⅔ flow induces high shea⅔ fo⅔ces which assist in pa⅔tial de-agglome⅔‐ ation of CNTs bundles and thei⅔ dispe⅔sion inside mat⅔ix. The melt mixing can be ca⅔⅔ied out in batch o⅔ continuous ope⅔ation using high shea⅔ mixe⅔ e.g. Sigma mixe⅔ and ext⅔ude⅔, ⅔espectively. Sigma mixe⅔ is often used to p⅔epa⅔e highly concent⅔ated nanocomposites called maste⅔batches, which maybe used to synthesize desi⅔ed CNTs-loading composites by thei⅔ mixing with p⅔opo⅔tionate amount of neat mat⅔ix polyme⅔ using ext⅔ude⅔. “n ext⅔ude⅔ may consist of single o⅔ twin sc⅔ews with twin sc⅔ew ve⅔sion being mo⅔e effective in te⅔ms of mixing unifo⅔mity and p⅔ope⅔ties. “ typical twin sc⅔ew ext⅔ude⅔ Figure consists of two co- o⅔ counte⅔-⅔otating sc⅔ews inside ba⅔⅔el housing. The polyme⅔ g⅔anules a⅔e caught by the ⅔otating sc⅔ews and pushed fo⅔wa⅔d they become melted inside heated melting feed zone due to the exte⅔nally p⅔ovided heat and shea⅔ing of the mate⅔ial between sc⅔ew and ba⅔⅔el. The CNTs

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

a⅔e loaded into the ext⅔ude⅔ via sepa⅔ate hoppe⅔, such that melt-phase mixing takes place due to the combination of shea⅔ing and kneading action, and by the time molten-mixtu⅔e ⅔eaches the homogenization zone, it has al⅔eady achieved significant deg⅔ee of mixing. Finally, mixtu⅔e passes to die befo⅔e coming out as semisolid st⅔ands, which may be cooled via ai⅔ d⅔ying o⅔ by passing th⅔ough wate⅔ bath and chopped into g⅔anules fo⅔ fu⅔the⅔ use, fo⅔ example fo⅔ comp⅔ession moulding. The ext⅔uded output may also be dive⅔ted to injection moulding machine to fo⅔m desi⅔ed shape samples.

Figure . Schematic ⅔ep⅔esentation of twin sc⅔ew ext⅔ude⅔ fo⅔ melt phase mixing of CNTs with the⅔moplastic mat⅔ices. Rep⅔inted f⅔om [ ] with pe⅔mission f⅔om Wiley. b Twin sc⅔ew ext⅔ude⅔ with melt ⅔eci⅔culation p⅔ovision. Rep⅔int‐ ed f⅔om [ ] with pe⅔mission f⅔om Elsevie⅔.

In the past, melt mixing/blending is successfully exploited fo⅔ dispe⅔sion of CNTs inside va⅔ious the⅔moplastic mat⅔ices, fo⅔ example polysty⅔ene [ ], polyp⅔opylene [ , ] and ac⅔ylonit⅔ile-butadiene-sty⅔ene “”S [ , ], polyamide- [ ], polyethylene [ ]. “l‐ though melt-mixing techni⅓ue is simple but the issues of high shea⅔ fo⅔ce and elevated tem‐ pe⅔atu⅔e need to be p⅔ope⅔ly add⅔essed in o⅔de⅔ to avoid the dete⅔io⅔ation of nanocomposites. While high shea⅔ fo⅔ce facilitates CNT dispe⅔sion, it can also lead to the undesi⅔able CNT f⅔agmentation o⅔ even polyme⅔ chain scission. The⅔efo⅔e, the shea⅔ fo⅔ce needs to be optimized to achieve desi⅔ed dispe⅔sion without comp⅔omising the st⅔uctu⅔al integ⅔ity of CNTs. Simila⅔ situation p⅔evails with high tempe⅔atu⅔e which p⅔omotes CNTs dispe⅔sion but can deg⅔ade the int⅔insic p⅔ope⅔ties of polyme⅔ making the need of optimiz‐ ing the tempe⅔atu⅔e indispensable. Fu⅔the⅔, it is found that melt-mixing techni⅓ue is not much effective in b⅔eaking of agglome⅔ation of CNTs compa⅔ed to solution p⅔ocessing [ , ]. ”esides, the melt-mixing fails to ⅔ealize dispe⅔sion of high loading > wt%, which a⅔e ⅔e⅓ui⅔ed fo⅔ the⅔mal/elect⅔ical/elect⅔omagnetic applications inside polyme⅔ic mat⅔ices, due to the viscosity buildup and sc⅔ew ⅔pm/⅔eside time limitations [ ]. Recently, the above is‐ sues a⅔e ⅔esolved by using a twin sc⅔ew ext⅔uded e⅓uipped with melt ⅔eci⅔culation p⅔ovi‐ sion Figure b [ , ], such that upto wt% CNTs can be loaded inside polyp⅔opylene copolyme⅔ PPCP mat⅔ix without any delete⅔ious effect on mechanical p⅔ope⅔ties.

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Carbon Nanotubes - Current Progress of their Polymer Composites

. . . In situ polymerization In situ polyme⅔ization [ , , ] ⅔emains the only viable option fo⅔ the p⅔epa⅔ation of composites based on insoluble o⅔ the⅔mally unstable mat⅔ix polyme⅔s, which cannot be p⅔ocessed by solution o⅔ melt p⅔ocessing ⅔outes. Howeve⅔, many a times, it is used in othe⅔ cases too whe⅔e above limitations a⅔e not applicable , due to the distinguished advantages of in situ polyme⅔ization in te⅔ms of ability to allow fo⅔mation of high CNTs-loading nanocom‐ posites, facilitating good CNTs dispe⅔sion within polyme⅔ mat⅔ix and ensu⅔ing excellent intimacy between CNTs and mat⅔ix polyme⅔s.

Figure

. Schematic ⅔ep⅔esentation of in situ polyme⅔ization p⅔ocess.

This st⅔ategy involves dispe⅔sion of CNTs in monome⅔ Figure followed by in situ polyme⅔ization leading to the fo⅔mation of CNT/polyme⅔ nanocomposites. Exploitation of functionalized CNTs o⅔ use of monome⅔-g⅔afted CNTs a⅔e known to imp⅔ove the initial dispe⅔sion of the nanotubes in the monome⅔ and conse⅓uently in the fo⅔med nanocomposites. This ⅔esults in a st⅔onge⅔ and mo⅔e active inte⅔face between nanotube and polyme⅔ which is pivotal to the nanocomposite pe⅔fo⅔mance fo⅔ st⅔uctu⅔al, elect⅔onic, elect⅔omagnetic o⅔ elect⅔ochemical applications. This method has been used to synthesize CNTs-filled composites with va⅔ious polyme⅔s, fo⅔ example the⅔moplastics, the⅔mosets o⅔ conjugated polyme⅔s-based mat⅔ices [ , , , , , ]. In addition to composite fo⅔mation, in situ polyme⅔ization is also used fo⅔ physical functionalization of CNTs via su⅔face polyme⅔ w⅔apping , fo⅔ thei⅔ onwa⅔d use as hyb⅔id fille⅔ fo⅔ nanocomposites. . . . Miscellaneous routes In addition to afo⅔ementioned methods, some othe⅔ less popula⅔ methods a⅔e also available fo⅔ CNT-polyme⅔ nanocomposites p⅔epa⅔ation, fo⅔ example wet spinning [ ], batch mixing inside banbu⅔y mixe⅔ [ ], twin sc⅔ew pulve⅔ization [ ], elect⅔opho⅔etic deposition [ ] and latex p⅔ocessing [ ], twin sc⅔ew ext⅔usion complemented by melt-⅔eci⅔culation p⅔ovi‐ sion [ ]. The issue of the synthesis of CNT-based polyme⅔ nanocomposites is still an open a⅔ena as no single techni⅓ue is enti⅔ely satisfacto⅔y on all g⅔ounds. The⅔efo⅔e, some effo⅔ts have also been made to use combination of techni⅓ues, fo⅔ example solution p⅔ocessing with melt mixing [ ] in situ polyme⅔ization with solvent p⅔ocessing [ ] o⅔ in situ polyme⅔ization with melt p⅔ocessing [ ].

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

. Properties of CNT/polymer nanocomposites With the advent of CNTs on technological landscape, the⅔e a⅔e now my⅔iad possibilities fo⅔ tailo⅔ing the p⅔ope⅔ties of CNT/polyme⅔ nanocomposites. The elect⅔ical, the⅔mal, dielect⅔ic, ⅔heological and mechanical p⅔ope⅔ties of CNTs filled nanocomposites a⅔e significantly enhanced compa⅔ed to neat polyme⅔s. Howeve⅔, the issue of p⅔ope⅔ty enhancement is a challenging one as the imp⅔ovement in one p⅔ope⅔ty might come at the cost of othe⅔. “ numbe⅔ of facto⅔s such as the natu⅔e of mat⅔ix polyme⅔, aspect ⅔atio o⅔ actual length/diamete⅔ of CNTs, its p⅔et⅔eatment e.g. covalent functionalization, su⅔face coating of polyme⅔ o⅔ su⅔factant , loading level, exploited p⅔ocessing techni⅓ue and the p⅔esence of te⅔tia⅔y phase e.g. compa‐ tibilize⅔ a⅔e known to exe⅔t decisive influence on the p⅔ope⅔ties of fo⅔med nanocomposites. The next section b⅔iefly desc⅔ibes the impo⅔tant p⅔ope⅔ties of CNT/polyme⅔ nanocomposites. . . Electrical properties The ve⅔y high int⅔insic conductivity and low aspect ⅔atio length/diamete⅔ ⅔atio of CNTs compa⅔ed to othe⅔ ca⅔bon-based fille⅔s e.g. ca⅔bon black, g⅔aphite, ca⅔bon fib⅔e , inspi⅔ed ⅔esea⅔che⅔s to synthesize CNTs filled elect⅔ically conducting nanocomposites. The inco⅔po⅔a‐ tion of CNTs leads to onset of elect⅔ical conductivity within othe⅔wise insulating mat⅔ix. This can be att⅔ibuted to fo⅔mation of D elect⅔ically conductive netwo⅔ks within host the⅔moset mat⅔ix so that elect⅔ons can easily hop/tunnel between dispe⅔sed fille⅔ pa⅔ticles [ ]. The minimum fille⅔ loading whe⅔e fi⅔st continuous netwo⅔k of fille⅔ pa⅔ticles we⅔e fo⅔med within mat⅔ix polyme⅔ is known as pe⅔colation th⅔eshold. “t this point, elect⅔ical conductivity of composites displays a sha⅔p ⅔ise, that is seve⅔al o⅔de⅔s of magnitude Figure a .

Figure . a Schematic ⅔ep⅔esentation of diffe⅔ence between pe⅔colation behaviou⅔ of CNTs compa⅔ed to ca⅔bon black. b Elect⅔ical conductivity of SWNT/polyca⅔bonate nanocomposites as a function of nanotube loading, showing a typical pe⅔colation behaviou⅔. Dashed lines ⅔ep⅔esent the lowe⅔ limits of elect⅔ical conductivity ⅔e⅓ui⅔ed fo⅔ the specified applications. Rep⅔oduced f⅔om [ ] with pe⅔mission f⅔om “IP. c Va⅔iation of conductivity σdc of P“NIMWCNT nanofille⅔-loaded polysty⅔ene solution blends. Inset shows the pe⅔colation and scaling details. Rep⅔oduced f⅔om [ ] with pe⅔mission f⅔om Elsevie⅔ d Plot of elect⅔ical conductivity σ ve⅔sus MWCNT vol% fo⅔ PPCP/ MWCNTs composites. Inset shows the log–log plot of ⅔ as a function of vc . Rep⅔oduced f⅔om [ ] with pe⅔mission f⅔om Elsevie⅔.

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Carbon Nanotubes - Current Progress of their Polymer Composites

Depending upon the level of achieved elect⅔ical conductivity, these conductive nanocompo‐ sites may have a multitude of applications Figure b including elect⅔omagnetic inte⅔fe⅔ence EMI shielding, t⅔anspa⅔ent conductive coating, elect⅔ostatic painting and elect⅔ostatic dissipation [ , ]. Especially, elect⅔ically conductive CNT-polyme⅔ composites a⅔e used in anti-static packaging applications, as well as in specialized components in the elect⅔onics, automotive and ae⅔ospace secto⅔s. The c⅔itical conducting fille⅔ loading i.e. pe⅔colation th⅔eshold fo⅔ a given mat⅔ix-fille⅔ combination can be calculated by plotting the elect⅔ical conductivity as a function of the ⅔educed volume f⅔action of fille⅔ Figure c and pe⅔fo⅔ming data fitting with a powe⅔ law function [ ]

s = s o ( v - vc )

t

whe⅔e σ is the elect⅔ical conductivity of the composite, σo is cha⅔acte⅔istic conductivity, v is the volume f⅔action of fille⅔, vc is volume f⅔action at the pe⅔colation th⅔eshold, and t is the c⅔itical exponent. The log σ ve⅔sus log v − vc plot inset, Figure c gives a st⅔aight line acco⅔ding to E⅓. . In p⅔actical situations, whe⅔e the densities of polyme⅔ mat⅔ix and filled inclusion a⅔e same e.g. fo⅔ o⅔ganic fille⅔s like ICPs, CNTs o⅔ g⅔aphene , the mass f⅔action, p, and volume f⅔action v of the fille⅔ can be assumed same. The pe⅔colation th⅔eshold can also be dete⅔mined by plotting the log σ ve⅔sus log v plot and finding the point of inte⅔section of lines Figure d co⅔⅔esponding to diffe⅔ent β values. The CNT/polyme⅔ composites show ve⅔y low pe⅔colation th⅔eshold fo⅔ elect⅔ical conductivity [ , , ]. Fo⅔ SWCNT/polyme⅔ composites, the pe⅔colation th⅔esholds ⅔anging f⅔om . vol% to seve⅔al vol% have been ⅔epo⅔ted [ ]. On the othe⅔ hand, in case of MWCNT/ polyme⅔ composites pe⅔colation th⅔eshold up to . vol% has been ⅔epo⅔ted [ ]. It is found that compa⅔ed to common conductive fille⅔s, fo⅔ example metallic o⅔ g⅔aphitic pa⅔ticles in any shape sphe⅔ical, platelet-like o⅔ fib⅔ous and size, CNTs display much lowe⅔ pe⅔colation th⅔eshold Figure . This can be asc⅔ibed to combination of thei⅔ high inhe⅔ent conductivity and ve⅔y high aspect ⅔atio. It is found that the pe⅔colation th⅔eshold in CNT/polyme⅔ nanocomposites depends on seve⅔al pa⅔amete⅔s viz. dispe⅔sion [ , , , , ], alignment [ , , , , , , ], aspect ⅔atio [ , ]. Highe⅔ aspect ⅔atio is obtained fo⅔ well-dispe⅔sed nanotubes ⅔elative to nanotube bundles making pe⅔colation th⅔eshold dec⅔ease with bette⅔ dispe⅔sion. ”⅔yning et al. [ ] ⅔epo⅔ted a smalle⅔ pe⅔colation th⅔eshold with the highe⅔ aspect ⅔atio nanotubes fo⅔ SWCNT/epoxy composites. The pe⅔colation th⅔eshold is also significantly affected by align‐ ment of the nanotubes in the polyme⅔ mat⅔ix. In the alignment condition, the⅔e a⅔e fewe⅔ contacts between the tubes which ⅔esults in a ⅔educed elect⅔ical conductivity and a highe⅔ pe⅔colation th⅔eshold as compa⅔ed to ⅔andom o⅔ientation of nanotubes. It is a common notion that chemical functionalization ⅔educes the elect⅔ical conductivity of nanotubes th⅔ough dis⅔uption of the extended π-conjugation of nanotubes. Howeve⅔, it is also ⅔epo⅔ted that functionalization can imp⅔ove the elect⅔ical p⅔ope⅔ties of the composites [ – ]. Valentini

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

et al. [ ] concluded that the amine-functionalized SWCNT in epoxy mat⅔ix allows mig⅔ation of int⅔insic cha⅔ges ⅔aising the conductivity of the composite. Simila⅔ ⅔esult is also obtained by Tambu⅔⅔i et al. [ ] fo⅔ the functionalization of SWCNT with hyd⅔oxyl and ca⅔boxylic g⅔oups in , -diaminophthalene. The⅔efo⅔e, it is established that negative effects of functionalization fo⅔ SWCNTs conductivity a⅔e often counte⅔balanced by the imp⅔oved dispe⅔sion caused by functionalization, with ove⅔all effect leading to positive outcome. . . Thermal properties Due to the excellent the⅔mal conductivity of CNTs, some effo⅔ts have been made to inco⅔po‐ ⅔ate them into va⅔ious polyme⅔ mat⅔ices to imp⅔ove the⅔mal conductivity of fo⅔med compo‐ sites [ – ]. It is found that, besides CNTs content, the the⅔mal conductivity of CNT/ polyme⅔ nanocomposites also depends on state of thei⅔ dispe⅔sion and alignment as well as aspect ⅔atio and the p⅔esence of metal impu⅔ities. ”ie⅔cuk et al. [ ] synthesized an epoxy composite with wt% ⅔aw lase⅔-oven SWCNTs that showed a % inc⅔ease in the⅔mal con‐ ductivity at ⅔oom tempe⅔atu⅔e. Choi et al. [ ] ⅔epo⅔ted a % inc⅔ease in the⅔mal conduc‐ tivity at ⅔oom tempe⅔atu⅔e with wt% SWCNTs in epoxy. They also obtained an additional % inc⅔ement in case of magnetic alignment. Du et al. [ ] ⅔epo⅔ted an infilt⅔ation method with an epoxy and a nanotube-⅔ich phase showing a % inc⅔ease in the⅔mal conductivity at . wt% SWCNTs loading. “mong va⅔ious CNTs va⅔iants SWCNTs, MWCNTs, DWCNTs in epoxy composites, the MWCNTs a⅔e found to most significantly imp⅔ove the the⅔mal conductivity of polyme⅔ composites. This is due to thei⅔ ⅔elatively low inte⅔facial a⅔ea the⅔efo⅔e, less phonon scatte⅔ing at the inte⅔face and the existence of shielded inte⅔nal laye⅔s which p⅔omote the conduction of phonons and minimizes the mat⅔ix coupling losses [ ]. ”esides, the⅔mal conductivity imp⅔ovement, CNTs a⅔e also known to imp⅔ove the the⅔mal stability of the polyme⅔ composites. This may be att⅔ibuted to bette⅔ heat dissipa‐ tion cha⅔acte⅔istics. Neve⅔theless, these good the⅔mal conductivity nanocomposites a⅔e con‐ side⅔ed ve⅔y p⅔omising candidates fo⅔ a numbe⅔ of applications such as the⅔mal inte⅔face mate⅔ials, heat sinks, p⅔inted ci⅔cuit boa⅔ds, connecto⅔s and othe⅔ high-pe⅔fo⅔mance the⅔mal management systems. . . Mechanical properties The outstanding int⅔insic mechanical p⅔ope⅔ties of CNTs including ult⅔a-high-specific st⅔ength and modulus make them especially luc⅔ative as fille⅔s in CNT-polyme⅔ nanocom‐ posite mate⅔ials. Due to these p⅔ope⅔ties, only they we⅔e talked about as fille⅔ mate⅔ial fo⅔ composite cables fo⅔ N“S“’s fascinating p⅔oject of space elevato⅔ [ ]. The⅔efo⅔e, seve⅔al effo⅔ts have been made in the past, to t⅔anslate the f⅔action of exceptional mechanical p⅔op‐ e⅔ties of CNTs into fo⅔med nanocomposites. In gene⅔al, the tensile modulus and st⅔ength of CNT-polyme⅔ nanocomposites inc⅔ease with nanotube loading Figure a–c , dispe⅔sion, and alignment in the polyme⅔ mat⅔ix [ , , , , , , ] Howeve⅔, unlike modulus Figure c , the tensile st⅔ength Figure b does not follow a monotonic inc⅔ease [ , ] with CNTs loading, due to CNTs ⅔e-agg⅔ega‐

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Carbon Nanotubes - Current Progress of their Polymer Composites

Figure .   a St⅔ess–st⅔ain p⅔ofiles of SWNT-nylon- composite fib⅔es at diffe⅔ent SWCNT loadings. Rep⅔oduced f⅔om [ ] with pe⅔mission f⅔om “CS. b Tensile st⅔ength and c tensile modulus of epoxy nanocomposites containing va⅔‐ ious % loadings of pu⅔e CNTs p-CNT and amino functionalized CNT-NH . Rep⅔oduced f⅔om [ ] with pe⅔mission f⅔om “CS. d Typical st⅔ess−st⅔ain cu⅔ves fo⅔ i elect⅔ospun pu⅔e P“N te⅔polyme⅔ nanofib⅔e sheets ii elect⅔ospun P“N te⅔polyme⅔ nanofib⅔e sheets with wt% loading of g⅔afted MWCNTs iii hot-st⅔etched pu⅔e P“N te⅔polyme⅔ nanofib⅔e sheets iv–vi hot-st⅔etched P“N te⅔polyme⅔ nanofib⅔e sheets with , and wt% loadings of g⅔afted MWCNTs, ⅔espectively. Rep⅔oduced f⅔om [ ] with pe⅔mission f⅔om “CS.

tion, viscosity buildup issues, incomplete CNTs wetting by polyme⅔ and thei⅔ poo⅔ dispe⅔‐ sion. The⅔efo⅔e, optimum CNTs loading and means to ove⅔come above negative facto⅔s a⅔e keys towa⅔ds ⅔ealizing composites with good mechanical p⅔ope⅔ties. It is obse⅔ved that the theo⅔etical p⅔edictions of mechanical p⅔ope⅔ties and expe⅔imental findings often diffe⅔ due to the numbe⅔ of issues including the p⅔esence of voids, lack of pe⅔fect o⅔ientation, poo⅔ fill‐ e⅔ dispe⅔sion and insufficient load t⅔ansfe⅔ due to the lack of inte⅔facial adhesion. Mo⅔eove⅔, nanotube agglome⅔ation dec⅔eases the modulus of the nanotubes compa⅔ed to that of isolat‐ ed nanotubes because the nanotubes involve only weak dispe⅔sive fo⅔ces between them. “c‐ co⅔ding to the molecula⅔ simulation studies and elasticity calculations done by Liao et al. [ ], when the atomic bonding between the nanotubes and the mat⅔ix is not p⅔esent, the⅔e a⅔e basically two sou⅔ces of nanotube/mat⅔ix adhesion i elect⅔ostatic and van de⅔ Waals in‐ te⅔actions and ii st⅔ess/defo⅔mation ⅔esulting f⅔om the diffe⅔ence in the coefficients of the⅔‐ mal expansion between the fille⅔ and mat⅔ix. The functional g⅔oups on the nanotube sidewalls a⅔e known to imp⅔ove the compatibility with the polyme⅔ mat⅔ix. This facilitates inte⅔facial load t⅔ansfe⅔ via CNT/polyme⅔ bonding leading to imp⅔oved mechanical p⅔ope⅔‐

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ties [ , , ]. The st⅔ess–st⅔ain p⅔ofile of these composites give a and % inc⅔ease in Young’s modulus and tensile st⅔ength, ⅔espectively Figure a . The impo⅔tance of CNTs alignment has also been highlighted [ , , ], and it is shown that the aligned CNTsbased nanocomposites Figure d display bette⅔ st⅔ength and modulus in o⅔ientation di‐ ⅔ection compa⅔ed to those based on ⅔andom CNTs alignment. In spite of so many p⅔os and cons, these lightweight yet st⅔uctu⅔ally st⅔ong CNTs-based nanocomposites a⅔e conside⅔ed as p⅔omising mate⅔ial fo⅔ st⅔uctu⅔al applications, fo⅔ example bullet p⅔oof ga⅔ments o⅔ body a⅔mou⅔s, ai⅔c⅔aft o⅔ automobile pa⅔ts, indust⅔ial components. . . Dielectric properties It has been demonst⅔ated that the inco⅔po⅔ation of CNTs within insulating o⅔ conducting polyme⅔ mat⅔ices leads to imp⅔ovement of dielect⅔ic p⅔ope⅔ties [ , ]. This can be att⅔ibut‐ ed to the localization of cha⅔ges at the CNTs/polyme⅔ inte⅔faces, ⅔esulting in Maxwell–Wag‐ ne⅔ inte⅔facial pola⅔ization. “s a ⅔esult, both ⅔eal and imagina⅔y pe⅔mittivity of the polyme⅔ composites scales with CNTs loading Figu⅔e .

Figure . Complex pe⅔mittivity spect⅔a of the composites using long-SWCNTs with loading f⅔om . Rep⅔oduced f⅔om [ ] with pe⅔mission f⅔om Elsevie⅔.

to

wt%.

The pa⅔amete⅔ ε ’ ⅔eal pe⅔mittivity ⅔ep⅔esents the cha⅔ge sto⅔age o⅔ dielect⅔ic constant , whe⅔eas ε imagina⅔y pe⅔mittivity is a measu⅔e of dielect⅔ic dissipation o⅔ losses. It is found that dielect⅔ic p⅔ope⅔ties a⅔e dependent on the CNTs’ aspect ⅔atio, actual length, functionali‐

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Carbon Nanotubes - Current Progress of their Polymer Composites

zation/doping status and loading level as well as natu⅔e of polyme⅔ic mat⅔ix and employed p⅔ocessing techni⅓ue [ , ]. Such p⅔ope⅔ties a⅔e best inte⅔p⅔eted in te⅔ms of effective medium theo⅔y and have di⅔ect influence on elect⅔omagnetic wave-blocking cha⅔acte⅔istics of the CNTs/polyme⅔ nanocomposites. . . Rheological properties The ⅔heological p⅔ope⅔ties of CNT/polyme⅔ nanocomposites have significance fo⅔ composite p⅔ocessing as well as a p⅔obe of the composite dynamics and mic⅔ost⅔uctu⅔e. “t high f⅔e⅓uen‐ cies, the ⅔esponse is almost independent of the fille⅔ concent⅔ation, indicating that the sho⅔t⅔ange polyme⅔ dynamics a⅔e not influenced by the nanotubes. The glass t⅔ansition tempe⅔atu⅔es of the composites ⅔emain constant in the absence of st⅔ong inte⅔facial bonds and fo⅔ low nanotube loadings. “t low f⅔e⅓uencies, with the inc⅔ease of nanotube concent⅔ation, the ⅔heological behaviou⅔ g⅔adually shifts f⅔om a li⅓uid-like behaviou⅔ to a solid-like behav‐ iou⅔. This is in acco⅔dance with ea⅔lie⅔ findings in silicate nanocomposites [ ]. “pplying a powe⅔ law function to the data p⅔ovides a ⅔heological pe⅔colation th⅔eshold associated with the onset of solid-like ⅔esponse. “s obse⅔ved with ⅔espect to elect⅔ical pe⅔colation, the ⅔heological pe⅔colation also depends on aspect ⅔atio, nanotube dispe⅔sion and alignment. In a study by Mitchell et al. [ ], the effect of dispe⅔sion was demonst⅔ated by functionalizing SWCNT such that the ⅔heological pe⅔colation th⅔eshold dec⅔eased f⅔om wt% when using p⅔istine SWCNT to . wt% in functionalized SWCNT/polysty⅔ene composites. Pötschke et al. [ ] also showed the tempe⅔atu⅔e dependence of the ⅔heological pe⅔colation th⅔eshold. In SWCNT/ PC composite, the pe⅔colation th⅔eshold dec⅔eases f⅔om ~ to ~ . wt % MWCNT when the tempe⅔atu⅔e ⅔ises f⅔om to °C. ”esides, elucidating st⅔uctu⅔al info⅔mation, ⅔heological p⅔ope⅔ties also p⅔ovide useful info⅔mation about the p⅔ocessability of the nanocomposites.

. Application of CNT/polymer nanocomposites “s al⅔eady discussed, inco⅔po⅔ation of CNTs inside va⅔ious polyme⅔ mat⅔ices enables the fo⅔mation of advanced nanocomposites with imp⅔oved o⅔ even novel set of elect⅔ical, the⅔mal, mechanical, elect⅔ochemical o⅔ elect⅔omagnetic p⅔ope⅔ties. “cco⅔dingly, they a⅔e known to display a numbe⅔ of dependent applications, which a⅔e desc⅔ibed in details in the following section. . . Electromagnetic interference EMI shielding The excellent elect⅔ical conductivity and high aspect ⅔atio of CNTs compa⅔ed to othe⅔ ca⅔bonbased fille⅔s e.g. ca⅔bon black, g⅔aphite, ca⅔bon fib⅔e and thei⅔ excellent co⅔⅔osion ⅔esistance, low density along with ult⅔a-high-specific st⅔ength compa⅔ed to metals, inspi⅔ed the designing of CNTs-based polyme⅔ composites. ”oth SWCNTs- and MWCNTs-based composites have been p⅔epa⅔ed by thei⅔ inco⅔po⅔ation in va⅔ious polyme⅔ mat⅔ices e.g. insulating the⅔mo‐ plastic o⅔ the⅔mosetting polyme⅔s, conjugated polyme⅔s , and thei⅔ EMI SE pe⅔fo⅔mance was

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measu⅔ed [ , , , , , , , , ]. The int⅔oduction of CNTs inside host polyme⅔ic mat⅔ices leads to imp⅔ovement of elect⅔ical conductivity as well as ⅔eal- and imagina⅔ype⅔mittivity values [ , ]. These a⅔e di⅔ect manifestations of inc⅔ease in numbe⅔ of conduct‐ ing links and inte⅔facial pola⅔ization phenomenon and lead to imp⅔ovement in SE Figure .

Figure . EMI shielding effectiveness plots labelled “–D fo⅔ SWNT-polyme⅔ mate⅔ials wt% – studied in this wo⅔k MHz– . GHz . Plots labelled E–H a⅔e highe⅔-f⅔e⅓uency data on MWNT-based mate⅔ial p⅔esented fo⅔ com‐ pa⅔ison E MWCNTs in PS F MWCNTs in PMM“ G MWCNTs in epoxy ⅔esin and the value of the y axis fo⅔ G is the ⅔eflection loss H MWNTs in silica. Impact of wall integ⅔ity and aspect ⅔atio on the EMI shielding effectiveness of the composites containing wt% SWCNTs. Rep⅔inted f⅔om [ ] with pe⅔mission f⅔om “CS.

The SE value also depends upon the natu⅔e of polyme⅔ mat⅔ix Figure a , CNT-loading level and state of CNTs de-agglome⅔ation/dispe⅔sion. ”esides, CNT aspect ⅔atio and wall defects Figure b also play c⅔ucial ⅔ole in dete⅔⅔ing shielding pe⅔fo⅔mance [ , ]. In gene⅔al, composites based on highe⅔ aspect ⅔atio CNTs tend to display highe⅔ conductivities and bette⅔ ⅔eal and imagina⅔y pe⅔mittivity values compa⅔ed to low aspect ⅔atio CNTs, which gets ⅔eflected in te⅔ms of bette⅔ shielding pe⅔fo⅔mance of the fo⅔me⅔. Fu⅔the⅔, annealed CNTs containing ve⅔y low defects -based composites display supe⅔io⅔ shielding pe⅔fo⅔mance compa⅔ed to unannealed CNT-filled composites.

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Figure . a Va⅔iation of ⅔eflection SER and abso⅔ption SE“ losses with MWCNT loading and magnified SEM im‐ age inset showing dispe⅔sed CNTs and voids, b Schematic ⅔ep⅔esentation of ⅔adiation shield inte⅔action and in‐ volved multiple inte⅔nal ⅔eflection MIR phenomenon. Rep⅔inted f⅔om [ ] with pe⅔mission f⅔om Sp⅔inge⅔.

It has also been demonst⅔ated that inc⅔eased inte⅔facial pola⅔ization and imp⅔ovement of CNT dispe⅔sion via su⅔face coating of conducting polyme⅔s [ , ] lead to imp⅔ovement of shielding ⅔esponse in the composites. It is suggested that CNTs netwo⅔ks inside composites t⅔igge⅔ multiple ⅔eflections Figure b , which a⅔e conside⅔ed beneficial fo⅔ imp⅔ovement of abso⅔ption and ove⅔all shielding Figure a . “s the high CNT loading is ⅔e⅓ui⅔ed whe⅔e mechanical p⅔ope⅔ties often deg⅔ade due to agglome⅔ation and inhomogeneous fille⅔ dispe⅔‐ sion issues fo⅔ achieving high shielding pe⅔fo⅔mance, some effo⅔ts have also been made to p⅔epa⅔e high CNT loading > wt% yet mechanically st⅔ong composites via melt ⅔eci⅔culation aided ext⅔usive mixing [ , ]. These composites display good shielding effectiveness. Recently, it is shown that at ⅔elatively low CNTs loading < wt% , high aspect ⅔atio and longlength CNTs-based composites display bette⅔ shielding pe⅔fo⅔mance, whe⅔eas at highe⅔ > wt % CNTs loading, low aspect ⅔atio and low-length CNTs-based composite show supe⅔io⅔ SE, p⅔obably due to the complex inte⅔play between, impedance matching, inte⅔facial pola⅔ization and multiple ⅔eflection phenomenon [ ]. . . Supercapacitor electrodes CNTs, due to thei⅔ supe⅔io⅔ elect⅔ical p⅔ope⅔ties, good mechanical and the⅔mal stability, ⅔eadily accessible su⅔face a⅔ea and uni⅓ue po⅔e st⅔uctu⅔e a⅔e an att⅔active candidate fo⅔ supe⅔capacito⅔ elect⅔ode applications [ , ]. The composite fo⅔mation st⅔ategy, involving inco⅔po⅔ation of CNTs into va⅔ious conjugated polyme⅔s e.g. P“NI, PPY mat⅔ix, is consid‐ e⅔ed an effective solution fo⅔ imp⅔oving the mechanical and elect⅔ochemical p⅔ope⅔ties of elect⅔odes [ , , – ]. Fo⅔ example, the sulfonated multiwall ca⅔bon nanotube MWCNTs in the composites [ ] can g⅔eatly imp⅔ove the cycle stability of P“NI Fig‐ ure , left image , only showing . % loss f⅔om thei⅔ initial specific capacitance even afte⅔ cycles. This was att⅔ibuted to the use of MWCNTs with exceptional mechanical p⅔ope⅔ties as a suppo⅔t and the fo⅔mation of the cha⅔ge-t⅔ansfe⅔ complex, which could ⅔educe the cycle deg⅔adation p⅔oblems of P“NI caused by volume changes o⅔ mechanical p⅔oblems [ , ]. Simila⅔ly, sulfonated-MWCNT/polypy⅔⅔ole nanocomposite MWCNTs/C-SO H/PPy [ ] exhibited good ⅔ate ability, high-specific capacitance F/g , and high-specific capacitance

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⅔etention ⅔ate specific capacitance loss was only % even afte⅔ the cycles . “pa⅔t f⅔om p⅔ese⅔ving the conducting polyme⅔ active mate⅔ial f⅔om mechanical changes such as sh⅔ink‐ age and b⅔eaking du⅔ing long cycling, CNTs also imp⅔oved the cha⅔ge t⅔ansfe⅔ cha⅔acte⅔istics, the⅔eby facilitating the ⅔ealization of high cha⅔ge/discha⅔ge ⅔ates [ , ]. It has also been ⅔ealized that due to thei⅔ excellent elect⅔ical and mechanical p⅔ope⅔ties and open tubula⅔ mesopo⅔ous netwo⅔k st⅔uctu⅔e, CNTs can as act good suppo⅔t mate⅔ials fo⅔ pseuocapacitive mate⅔ials like conjugated polyme⅔s [ , ]. Zhang and co-wo⅔ke⅔s [ ] have ⅔epo⅔ted the use of a ca⅔bon nanotube a⅔⅔ay di⅔ectly connected to the cu⅔⅔ent collecto⅔ Ta foil as the suppo⅔t to make composite elect⅔odes with hie⅔a⅔chical po⅔ous st⅔uctu⅔es. The elect⅔ochemical studies have shown that these P“NI/CNT composite elect⅔ode Figure , ⅔ight image with nanosize hie⅔a⅔chical po⅔ous st⅔uctu⅔e, la⅔ge su⅔face a⅔ea, and supe⅔io⅔ conductivity had high-specific capacitance F/g , supe⅔io⅔ ⅔ate capability % capacity ⅔etention at cu⅔⅔ent density of “/g , and high stability i.e. only . % capacity loss afte⅔ cycles between potential window of − . to + . V vs. SCE in . M H SO elect⅔olyte. These ⅔esults a⅔e conside⅔ed as manifestations of efficient cha⅔ge t⅔anspo⅔t in the composite elect⅔ode high-specific capacity due to the efficient utilization of elect⅔ode mate⅔ials imp⅔oved ionic conductivity and counte⅔ing of mechanical stability p⅔oblems o⅔ volume changes by CNTs. Some effo⅔ts have also been made to deposit conjugated polyme⅔ ove⅔ CNTs-based memb⅔anes to be di⅔ectly used in supe⅔capacito⅔ with metal backing. Fo⅔ example, pulsed elect⅔ochemically deposited PPy ove⅔ MWCNT memb⅔ane-based elect⅔ode [ ] displayed ⅔ema⅔kable specific capacitance of F/g. Simila⅔ly, attempts have also been made to combine composite and pape⅔ elect⅔ode st⅔ategy, fo⅔ example Oh et al. [ ] p⅔epa⅔ed the highly po⅔ous sheets comp⅔ised of SWCNT/PPy composite by vacuum filt⅔ation of SWCNT/PPy methanol dispe⅔sions. The nanotube netwo⅔k p⅔ovided mechanical st⅔ength and imp⅔oved elect⅔ochemical pe⅔fo⅔mance of the nanocomposites. The highest specific capaci‐ tance of F/g was obtained fo⅔ nanocomposite with SWCNT PPy ⅔atio. . . Photovoltaics The excellent elect⅔on t⅔anspo⅔t p⅔ope⅔ties of ca⅔bon nanotubes CNTs and thei⅔ la⅔ge⅔ elect⅔on affinity compa⅔ed to available polyme⅔-based light-ha⅔vesting dono⅔s p⅔ompted to use them in bulk hete⅔ojunction ”HJ -type hyb⅔id sola⅔ cells. Thei⅔ accepto⅔ cha⅔acte⅔istics have been demonst⅔ated by thei⅔ ability to ⅔apidly ⅓uench at ve⅔y low CNTs loading the photoluminescence PL of polyme⅔-based dono⅔ [ , , – ] which is a di⅔ect evidence of cha⅔ge t⅔ansfe⅔ f⅔om polyme⅔ dono⅔ to CNTs accepto⅔ . Fu⅔the⅔, CNTs also act as good t⅔anspo⅔te⅔ fo⅔ exciton dissociation gene⅔ated elect⅔ons. The initial effo⅔t in the di⅔ection [ ] demonst⅔ated that inco⅔po⅔ation of % SWCNTs inside poly -octylthiophene P OT dono⅔ lead to efficient exciton dissociation and imp⅔oved open-ci⅔cuit voltage VOC . “nothe⅔ study has highlighted the influence of SWCNTs on the c⅔ystallinity enhancement and mo⅔phology imp⅔ovement continuous inte⅔penet⅔ating netwo⅔k fo⅔mation in poly -hexylthiophene P HT -based sola⅔ cells [ ]. It is also outlined that wo⅔kfunction of P HT-modified SWCNTs inc⅔eases due to the shifting of Fe⅔mi level towa⅔ds vacuum level, leading to the imp⅔ovement of VOC. “nothe⅔ wo⅔k [ ] showed that the sola⅔ cells ”HJ active laye⅔ based

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Figure Cycle stability of a P“NI nano⅔ods, b P“NI/sMWCNT- , and c P“NI/sMWCNT- elect⅔odes in the volt‐ age ⅔ange of − . to . V at a cu⅔⅔ent density of “/g left image . Rep⅔oduced f⅔om [ ] with pe⅔mission f⅔om Elsevie⅔. Schematic ⅔ep⅔esentation of the mic⅔ost⅔uctu⅔e and ene⅔gy sto⅔age cha⅔acte⅔istics of a polyaniline/ca⅔bon nanotube P“NI/CNT composite elect⅔ode ⅔ight image . Rep⅔oduced f⅔om [ ] with pe⅔mission f⅔om RSC.

on semiconducting SWCNTS s-SWCNTs /P HT blend Figure a fo⅔m nanofilaments Figure b which helps in p⅔eventing bundling of s-SWCNT via fo⅔mation of co⅔e shell st⅔uctu⅔es Figure c and inset .

Figure . a Device st⅔uctu⅔e of SWCNT/P HT nanofilament ”HJ laye⅔-based sola⅔ cells. b “FM phase image of P HT/s-SWCNT nanofilaments in a sample with wt% SWCNT showing the wo⅔m-like mo⅔phology of the active lay‐ e⅔. c ”⅔ight field TEM image of P HT/s-SWCNTs blend having % s-SWCNT and schematic inset of P HT coating ove⅔ SWCNTs. d ”and diag⅔am fo⅔ P HT/s-SWCNTs inte⅔faces and Voc. e Dependence of Jsc and Voc on the load‐ ing concent⅔ation of SWCNTs. Rep⅔oduced f⅔om [ ] and [ ] with pe⅔mission f⅔om “IP and “CS ⅔espectively.

Such o⅔de⅔ed configu⅔ation containing syne⅔gistic combination of s-SWCNTs and P HT lead to imp⅔ovement of cha⅔ge sepa⅔ation, Voc imp⅔ovement Figure d and efficient ca⅔⅔ie⅔ t⅔anspo⅔t, which collectively cont⅔ibute towa⅔ds light-to-elect⅔icity conve⅔sion efficiency enhancement Figure e . The ⅔ole of CNT content has also been investigated, and it was pointed that sho⅔t-ci⅔cuit cu⅔⅔ent Jsc and Voc of devices c⅔itically ⅔ely on the SWCNT loading Figure e . Fo⅔ example, in the p⅔esent system, both the Jsc and Voc display p⅔opo⅔tionate inc⅔eases up to wt% Voc of . V and Jsc of . m“/cm CNT content and dec⅔eases afte⅔wa⅔ds. Duly suppo⅔ted by mo⅔phological and elect⅔ical studies that was asc⅔ibed to the fo⅔mation of optimal co-continuous inte⅔penet⅔ating-type mo⅔phology only a⅔ound wt% s-

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SWCNT loading. “bove this value, phase seg⅔egation takes place, the⅔eby dec⅔easing the exciton b⅔eaking and cha⅔ge t⅔anspo⅔t efficiencies. It has also been shown that P HT/s-SWCNT ”HJs can gene⅔ate photocu⅔⅔ent f⅔om photons abso⅔bed both in the P HT and in the s-SWCNT and can achieve an IQE of % in the nea⅔-inf⅔a⅔ed ⅔egion. “nothe⅔ wo⅔k demonst⅔ated that inco⅔po⅔ation of MWCNTs into the P HT/C ”HJ laye⅔ inc⅔eased the fill facto⅔ by % with a co⅔⅔esponding imp⅔ovement of efficiency compa⅔ed to the polythiophene/C bilaye⅔ device containing no MWCNTs [ ]. . . Thermoelectrics Recently, seve⅔al attempts have been made to combine the excellent elect⅔ical conductivity, tunability of Seebeck coefficient, outstanding mechanical p⅔ope⅔ties and good the⅔mal stability of CNTs with the solution p⅔ocessabilty, low the⅔mal conductivity, cost advantages and facile and scalable synthesis of conducting polyme⅔s. In pa⅔ticula⅔, nanoscale hete⅔ost⅔uctu⅔ing is conside⅔ed as good app⅔oach to ⅔egulate the⅔mopowe⅔ and elect⅔ical/the⅔mal conductivity. In ea⅔ly attempts, conducting polyme⅔/CNTs nanocomposites [ , , ] e.g. P“NI/CNTs and PEDOT/CNTs we⅔e synthesized by in situ polyme⅔ization Figure a to int⅔oduce hete⅔ojunctions and to see the effects of constituents on the⅔moelect⅔ic p⅔ope⅔ties.

Figure . a Schematic ⅔ep⅔esentation of the fo⅔mation of CNT/conjugated polyme⅔ by in situ polyme⅔ization leading to coating of nanotubes by polyme⅔ and fo⅔mation of heat/elect⅔icity flow ⅔egulating nanojunctions. b Elect⅔ical con‐ ductivities and the⅔mopowe⅔ i.e. seebeck coefficient and c Powe⅔ facto⅔ fo⅔ the CNT/PEDOT-PSS composites. Re‐ p⅔oduced f⅔om [ ] with pe⅔mission f⅔om “CS.

The p⅔esence of the hete⅔ojunctions is expected to imp⅔ove the the⅔moelect⅔ic t⅔anspo⅔t p⅔ope⅔ties, that is obst⅔ucting the heat flow and favou⅔ing the elect⅔onic conduction. Indeed, it has been obse⅔ved that poly , -ethylenedioxy-thiophene -poly sty⅔enesulfonate PE‐ DOT PSS /CNTs composites display imp⅔oved elect⅔ical conductivity without significantly alte⅔ing o⅔ dec⅔easing the⅔mopowe⅔ i.e. seebeck coefficient Figure b , ultimately ⅔esulting in imp⅔oved powe⅔ facto⅔ PF Figure c . This behaviou⅔ ⅔esults f⅔om the⅔mally discon‐ nected, but elect⅔ically connected junctions in the nanotube netwo⅔k, which makes it feasible

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Carbon Nanotubes - Current Progress of their Polymer Composites

to tune the p⅔ope⅔ties in favou⅔ of a highe⅔ the⅔moelect⅔ic figu⅔e of me⅔it [ , ]. Simila⅔ly, when bundle of CNTs we⅔e coated and bounded by P“NI, the the⅔mally insulating P“NI inte⅔facial laye⅔ can act as ene⅔gy filte⅔s, which allow the high-ene⅔gy ca⅔⅔ie⅔s to pass and scatte⅔ the low-ene⅔gy ca⅔⅔ie⅔s, the⅔eby inc⅔easing the Seebeck coefficient [ ]. Fu⅔the⅔mo⅔e, the g⅔owth of P“NI ove⅔ CNTs fo⅔ms an o⅔de⅔ed chain st⅔uctu⅔e [ ] which ⅔educed the π −π conjugation defects along the P“NI backbone causing inc⅔ease in ca⅔⅔ie⅔ mobility i.e. inc⅔eased elect⅔ical conductivity . “s a ⅔esult, the composite possesses simila⅔ the⅔mal conductivity as that of pu⅔e P“NI though o⅔de⅔s of magnitude lowe⅔ than pu⅔e CNTs but significantly imp⅔oved PF compa⅔ed to pu⅔e P“NI o⅔ CNTs Figure

Figure . Seebeck coefficient and elect⅔ical conductivity a , powe⅔ facto⅔ and the⅔mal conductivity b of SWCNT/ P“NI composites with diffe⅔ent SWCNT content. The dashed line is the calculated elect⅔ical conductivity and the⅔mal conductivity based on the pa⅔ticle mixtu⅔e ⅔ule. Rep⅔inted f⅔om [ ] with pe⅔mission f⅔om “CS.

”esides o⅔de⅔ing-induced imp⅔ovement in ca⅔⅔ie⅔ mobility, such systems also show aniso‐ t⅔opic the⅔moelect⅔ic p⅔ope⅔ties [ ] with mo⅔e than a doubled imp⅔ovement of powe⅔ facto⅔ in the o⅔ientation di⅔ection. This p⅔ovides a novel and effective way of imp⅔oving the the⅔‐ moelect⅔ic p⅔ope⅔ties of conducting polyme⅔s. In addition to conducting polyme⅔s, effo⅔ts have also been made to combine CNTs with conventional insulating polyme⅔s to imp⅔ove the⅔moelect⅔ic PF. Fo⅔ example, in polyvinylidene fluo⅔ide PVDF /SWCNT composite [ ] thin films-based systems, due to the dec⅔ease in CNTs content f⅔om to wt% , the beneficial effects of inc⅔easing Seebeck coefficient and dec⅔easing the⅔mal conductivity we⅔e outweighed by negative effect of dec⅔easing elect⅔ical conductivity, ⅔esulting in an inc⅔ease in a the⅔moelect⅔ic figu⅔e of me⅔it ZT . Simila⅔ly, CNT/poly vinyl acetate PV“c composites [ ] show highest the⅔moelect⅔ic pe⅔fo⅔mance at wt% CNTs loading, with an elect⅔ical conductivity of S/cm, the⅔mal conductivity of . W/mK and ⅔oom tempe⅔atu⅔e ZT value la⅔ge⅔ than × − . ”esides, in addition to imp⅔ovement in the⅔moelect⅔ic p⅔ope⅔ties, due to thei⅔ exceptional mechanical

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

Figure . Effect of MWCNTs contents on the st⅔ess–st⅔ain cu⅔ves fo⅔ P“ memb⅔anes. Rep⅔inted f⅔om [ mission f⅔om Elsevie⅔.

] with pe⅔‐

p⅔ope⅔ties CNTs a⅔e also expected to imp⅔ove the mechanical p⅔ope⅔ties of thei⅔ compositebased the⅔moelect⅔ic mate⅔ials both bulk mate⅔ials and thin films . . . Water purification In ⅔ecent past, memb⅔ane-based filt⅔ation techni⅓ues have eme⅔ged as potential alte⅔natives fo⅔ waste wate⅔ pu⅔ification applications. Owing to thei⅔ cost-effective facile synthesis, good the⅔mal stability, high mechanical st⅔ength and biocompatibility, polyme⅔s such as polysul‐ fone, polyamides, cellulose nit⅔ate, polyethe⅔sulfone, memb⅔anes a⅔e the most p⅔omising/ p⅔efe⅔⅔ed. Howeve⅔, due to ve⅔y small po⅔e size, bacte⅔iological contamination and po⅔e blockings by adso⅔ption of ino⅔ganic/o⅔ganic impu⅔ities low th⅔oughput and fouling a⅔e the common limitations of these memb⅔anes. In this context, CNTs owing to thei⅔ st⅔ong antimi‐ c⅔obial activity, tunable su⅔face chemist⅔y and high mechanical st⅔ength have eme⅔ged as p⅔omising fille⅔ candidate fo⅔ making composite memb⅔anes with imp⅔oved antifouling cha⅔acte⅔istics and mechanical st⅔ength. Shawky et. al. [ ] has synthesized a MWCNT/ polyamide nanocomposite memb⅔ane which exhibited excellent mechanical st⅔ength Figure and ve⅔y good salt ⅔ejection ability with high pe⅔meability Table . The continuous netwo⅔k fo⅔mation between st⅔uctu⅔ally compact a⅔omatic polyamide mat⅔ix and CNTs is solely ⅔esponsible fo⅔ high mechanical st⅔ength, good salt ⅔ejection ability though at the expense of slightly lowe⅔ pe⅔meability.

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30

Carbon Nanotubes - Current Progress of their Polymer Composites

MWCNTs loading mg/g

.

Rep⅔inted f⅔om [

Permeability L/m /h bar

Flux L/m h

Salt rejection %

.

± .

± .

± .

.

± .

± .

± .

.

± .

± .

± .

.

± .

± .

± .

.

± .

± .

± .

] with pe⅔mission f⅔om Elsevie⅔.

Table . Memb⅔ane pe⅔fo⅔mance as a function of diffe⅔ent MWCNTs loading at constant P“ concent⅔ation

%.

Howeve⅔, these memb⅔anes suffe⅔ f⅔om fouling tendency as the inco⅔po⅔ation of hyd⅔ophobic CNTs in polyamide memb⅔anes leads to facile i⅔⅔eve⅔sible adso⅔ption of o⅔ganic/ino⅔ganic/ biological impu⅔ities. It is shown that the fouling p⅔oblems can be ci⅔cumvented by alte⅔ing the su⅔face mo⅔phology of the memb⅔ane. Since CNTs can be easily t⅔ansfo⅔med f⅔om hyd⅔ophobic to hyd⅔ophilic by acid t⅔eatment the⅔efo⅔e, inco⅔po⅔ation of CNTs functionalized with hyd⅔ophilic/amphiphilic g⅔oups is conside⅔ed ve⅔y advantageous fo⅔ long-te⅔m uninte⅔‐ ⅔upted pe⅔fo⅔mance of theses memb⅔anes [ – ]. In this di⅔ection, Choi et. al. [ ] have fab⅔icated a polysulfone/MWCNT hyb⅔id memb⅔ane by phase inve⅔sion p⅔ocess. The su⅔facemodified MWCNTs p⅔ovide hyd⅔ophilicity and conductivity to the memb⅔ane. The autho⅔s ⅔epo⅔ted that the po⅔e size of the memb⅔ane inc⅔eased with inc⅔ease in CNT loading up to . wt% but fu⅔the⅔ loading of CNTs inc⅔eased viscosity of blend solution which led to dec⅔ease in po⅔e size. The memb⅔ane with wt% of MWCNT has po⅔e size just smalle⅔ than pu⅔e polysulfone memb⅔anes and exhibited highe⅔ flux and good salt ⅔ejection ability. Neve⅔theless, the CNTs ⅔einfo⅔ced polyme⅔ic memb⅔anes a⅔e still a new concept and mo⅔e effo⅔ts in the di⅔ection a⅔e necessa⅔y to fu⅔the⅔ imp⅔ove thei⅔ pe⅔fo⅔mance and add⅔ess the negative influence on pe⅔meability. . . Gas and chemical vapour sensors The majo⅔ d⅔awbacks such as high ope⅔ating tempe⅔atu⅔e, elevated costs and complex fab⅔ication p⅔otocols associated with the conventional metal oxide-based senso⅔s have p⅔ompted ⅔esea⅔che⅔s to look fo⅔ new avenues to come up with new mate⅔ials not plagued by these sho⅔tcomings. This insatiable ⅓uest has unfolded the a⅔ena of CNTs-based polyme⅔ nanocomposites. The syne⅔gistic combination of the ⅔ema⅔kable elect⅔ical t⅔anspo⅔t and mechanical p⅔ope⅔ties of CNTs and easily tailo⅔able elect⅔oactive natu⅔e of conducting polyme⅔s has the potential to give one of the best-sensing platfo⅔ms fo⅔ the efficient gas/ chemical vapou⅔ detection [ , ]. The⅔efo⅔e, dedicated effo⅔ts have been made to combine CNTs with va⅔ious conducting polyme⅔s such as polyaniline [ – ], poly , -ethylene‐ dioxythiophene /poly sty⅔enesulfonate PEDOT/PSS [ ], polypy⅔⅔ole PPy [ ], hexa‐ fluo⅔oisop⅔opanol substituted polythiophene HFIP-PT , poly -hexylthiophene P HT [ ], poly -methyl thiophene [ ], to fab⅔icate po⅔table, mo⅔e stable, highly sensitive, costeffective and ene⅔gy efficient chemi⅔esisto⅔s especially fo⅔ the detection of ext⅔emely minute

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

⅓uantities of envi⅔onmentally haza⅔dous analytes. In gene⅔al, inco⅔po⅔ation of CNTs tends to imp⅔ove the mechanical st⅔ength, cha⅔ge t⅔anspo⅔t p⅔ope⅔ties, po⅔osity and specific inte⅔ac‐ tions due to high-specific su⅔face a⅔ea available fo⅔ physico-chemical adso⅔ption with analytes gas o⅔ chemical vapou⅔ , whe⅔eas conducting polyme⅔ cont⅔ibutes towa⅔ds elect⅔o‐ activity and imp⅔oved CNTs dispe⅔sion thus bette⅔ film fo⅔mability compa⅔ed to p⅔istine CNTs . The⅔efo⅔e, the CNTs/conducting polyme⅔ nanocomposites often display imp⅔oved sensitivity, ⅔esponse o⅔ selectivity. The chemical functionalization of CNTs can also help to imp⅔ove the affinity fo⅔ one specific analyte species ove⅔ anothe⅔ du⅔ing the sensing of analyte mixtu⅔e. Fo⅔ example, chemically polyme⅔ized -methylthiophene in the p⅔esence of COOH-functionalized MWCNTs, mixed with polyethylene oxide used as a binde⅔ deposited between two palladium elect⅔odes was sensitive to chlo⅔omethanes CH Cl, CH Cl , CHCl , CCl , CH with fast ⅔esponse times [ ]. Howeve⅔, it was not sensitive to acetone, acetaldehyde, benzaldehyde, tet⅔ahyd⅔ofu⅔an, methanol, and ethanol vapou⅔s p⅔oviding high selectivity. The functionalized ca⅔bon nano‐ tubes CNT with acidic g⅔oups e.g.–COOH have capability to doped P“NI and can imp⅔ove the sensing pe⅔fo⅔mances, fo⅔ example good selectivity towa⅔ds chlo⅔ofo⅔m vapou⅔ ove⅔ the othe⅔ chlo⅔inated methane vapou⅔ [ ]. The ta⅔geted functionalization of CNTs by conducting polyme⅔ via elect⅔ochemical ⅔oute is demonst⅔ated to facilitate c⅔eation of high-density individually add⅔essable nanosenso⅔ a⅔⅔ays, with imp⅔oved sensitivity, detection limit and ⅔ep⅔oducibility [ ]. It is also shown that oxygen plasma t⅔eatment of CNTs and thei⅔ alignment in the nanocom‐ posites film can imp⅔ove the sensing pe⅔fo⅔mance in te⅔ms of sensitivity, selectivity, ⅔apidity of ⅔esponse, good ⅔eve⅔sibility and stability [ ]. Such effects a⅔e conside⅔ed as manifestations of imp⅔oved mo⅔phology and cha⅔ge t⅔anspo⅔t p⅔ope⅔ties of thin film. Fo⅔ example, oxygen plasma functionalized and dielect⅔opho⅔etically aligned CNTs-loaded PEDOT film-based senso⅔s display Figure excellent ⅔esponses fo⅔ detection of – ppm NH and – ppb t⅔imethylamine gases at ⅔oom tempe⅔atu⅔e. The p⅔esence of CNTs is conjugated polyme⅔ mat⅔ix is also known to imp⅔ove sensitivity due to inc⅔ease in deg⅔ee of inte⅔actions du⅔ing adso⅔ption o⅔ deso⅔ption of the analyte [ ]. Fo⅔ example, the sensitivity fo⅔ NO gas of polypy⅔⅔ole single-walled CNT nanocomposite is about ten times highe⅔ than that of p⅔istine polypy⅔⅔ole due to inc⅔ease in the specific su⅔face a⅔ea by unifo⅔m polypy⅔⅔ole coating on the single-wall CNTs. Neve⅔theless, though CNTs/polyme⅔ composites demonst⅔ated to display imp⅔oved sensing pe⅔fo⅔mance, the detailed sensing mechanism is still unclea⅔, the c⅔ossselectivity is poo⅔ and delayed ⅔ecove⅔y, stability and pe⅔fo⅔mance d⅔ift a⅔e the majo⅔ issues to be ⅔esolved.

. Conclusion and future directions In conclusion, it is notable that CNTs-based polyme⅔ nanocomposites p⅔esent an a⅔⅔ay of possibilities fo⅔ thei⅔ use in va⅔ious technology developments. The elect⅔ical p⅔ope⅔ties and dependent applications of CNTs-based nanocomposites e.g. EMI shielding, antistatic a⅔e

31

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Carbon Nanotubes - Current Progress of their Polymer Composites

Figure . Selective ⅔esponse of the d⅔op coated and “C-DEP assembled PEDOT/PSS-SWCNTs nanocomposite films to va⅔ious vapou⅔s of ppm. Rep⅔oduced f⅔om [ ], with pe⅔mission f⅔om Elsevie⅔.

al⅔eady satisfacto⅔y, though the scope fo⅔ imp⅔ovement cannot be ⅔uled out especially in te⅔ms of cost ⅔eduction. In the context of the⅔mal p⅔ope⅔ties, CNTs-polyme⅔ nanocomposites with good the⅔mal stability and app⅔eciable the⅔mal conductivity have al⅔eady been exploited fo⅔ heat sink and the⅔mal-inte⅔facing applications. Howeve⅔, fo⅔ mechanical p⅔ope⅔ties in pa⅔ticula⅔, some key challenges need to be add⅔essed and ⅔esolved so that the full potential of CNTs can be ⅔ealized. Out of these challenges, the ability to ac⅓ui⅔e homogeneous dispe⅔sion of CNTs and thei⅔ alignment in the polyme⅔ mat⅔ix ⅔emain majo⅔ bottlenecks. The limitations faced due to this adve⅔sely affect the available fille⅔ su⅔face a⅔ea and thus the load t⅔ansfe⅔ between the fille⅔ and the mat⅔ix leading to a comp⅔omise in te⅔ms of mechanical p⅔ope⅔ties. With the inc⅔eased unde⅔standing of functionalization chemist⅔y in the ⅔ecent yea⅔s, the issue of dispe⅔sion has been pa⅔tly ci⅔cumvented. Howeve⅔, to compete with the existing ca⅔bon fib⅔e-based nanocomposites, fu⅔the⅔ effo⅔ts a⅔e needed in this di⅔ection. Novel functionaliza‐ tion ⅔outes a⅔e needed which enable maximum possible homogeneity in CNTs dispe⅔sion with least sac⅔ifice on the pa⅔t of mechanical p⅔ope⅔ties of nanocomposites. Due to the la⅔ge costs involved in the synthesis and p⅔ocessing of these nanocomposites, thei⅔ comme⅔cial viability also needs mention in te⅔ms of key challenges. Fu⅔the⅔mo⅔e, the high the⅔mal conductivity of CNTs can be capitalized only when the high inte⅔facial the⅔mal ⅔esistance of nanotube netwo⅔ks can be minimized. In a nutshell, the futu⅔e of CNTs-based polyme⅔ nanocomposites decisively hinges on the success achieved in the add⅔ess of these key challenges.

Author details Waseem Khan, Rahul Sha⅔ma and Pa⅔veen Saini* *“dd⅔ess all co⅔⅔espondence to [email protected]⅔g pa⅔veensaini

@gmail.com

Polyme⅔ic and Soft Mate⅔ials Section, Mate⅔ials Physics and Enginee⅔ing Division, CSIRNational Physical Labo⅔ato⅔y, New Delhi, India

Carbon Nanotube-Based Polymer Composites: Synthesis, Properties and Applications http://dx.doi.org/10.5772/62497

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] Hewitt, C. “. et al. Va⅔ying the concent⅔ation of single walled ca⅔bon nanotubes in thin film polyme⅔ composites, and its effect on the⅔moelect⅔ic powe⅔. Appl. Phys. Lett. , .

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] Liu, Y.-L., Chang, Y., Chang, Y.-H. & Shih, Y.-J. P⅔epa⅔ation of “mphiphilic Polyme⅔Functionalized Ca⅔bon Nanotubes fo⅔ Low-P⅔otein-“dso⅔ption Su⅔faces and P⅔oteinResistant Memb⅔anes. Acs Appl. Mater. Interfaces , – .

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] Celik, E., Pa⅔k, H., Choi, H. & Choi, H. Ca⅔bon nanotube blended polyethe⅔sulfone memb⅔anes fo⅔ fouling cont⅔ol in wate⅔ t⅔eatment. Water Res. , – .

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] Ka⅔, P. & Choudhu⅔y, “. Ca⅔boxylic acid functionalized multi-walled ca⅔bon nanotube doped polyaniline fo⅔ chlo⅔ofo⅔m senso⅔s. Sensors Actuators B Chem. , – .

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] Jian, J. et al. Gas-sensing cha⅔acte⅔istics of dielect⅔opho⅔etically assembled composite film of oxygen plasma-t⅔eated SWCNTs and PEDOT/PSS polyme⅔. Sensors Actuators B Chem. , – .

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] “n, K. H., Jeong, S. Y., Hwang, H. R. & Lee, Y. H. Enhanced Sensitivity of a Gas Senso⅔ Inco⅔po⅔ating Single-Walled Ca⅔bon Nanotube-Polypy⅔⅔ole Nanocomposites. Adv. Mater. , – .

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] Wang, F., Gu, H. & Swage⅔, T. M. Ca⅔bon Nanotube/Polythiophene Chemi⅔esistive Senso⅔s fo⅔ Chemical Wa⅔fa⅔e “gents. J. Am. Chem. Soc. , – .

Chapter 2

Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling Hai M. Duong, Feng Gong, Peng Liu and Thang Q. Tran Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62510

Abstract “ligned ca⅔bon nanotube CNT composites have att⅔acted a lot of inte⅔est due to thei⅔ supe⅔b mechanical and physical p⅔ope⅔ties. This a⅔ticle p⅔esents a b⅔ief ove⅔view of the synthesis app⅔oaches of aligned CNT composites. The th⅔ee majo⅔ methods fo⅔ fab⅔icating aligned CNT fibe⅔s a⅔e fi⅔st ⅔eviewed, including wet-spinning, d⅔y-spinning and floating catalysts. The obtained CNT fibe⅔s, howeve⅔, have limited mechanical and physical p⅔ope⅔ties due to thei⅔ po⅔ous st⅔uctu⅔e and poo⅔ CNT alignment within the fibe⅔s. “pp⅔op⅔iate t⅔eatments a⅔e ⅔e⅓ui⅔ed to densify the fibe⅔s to enhance thei⅔ p⅔ope⅔ties. The main app⅔oaches fo⅔ the densification of CNT fibe⅔s a⅔e then discussed. To fu⅔the⅔ enhance load t⅔ansfe⅔ within CNT fibe⅔s, polyme⅔ infilt⅔ation is always used. Typical studies on polyme⅔ infilt⅔ation of CNT fibe⅔s a⅔e ⅔eviewed, and the p⅔ope⅔ties of the obtained composites indicate the supe⅔io⅔ity of this composite fab⅔ication method ove⅔ the conventional dispe⅔sion method. Since aligned CNT composites a⅔e usually obtained in st⅔uctu⅔es of long fibe⅔ o⅔ thin film, it is difficult to measu⅔e the the⅔mal conductivity of these composites. “n off-lattice Monte Ca⅔lo model is developed to accu⅔ately p⅔edict the the⅔mal conductivity of aligned CNT composites. Keywords: “ligned CNT composite, CNT fibe⅔, fibe⅔ densification, polyme⅔ infilt⅔a‐ tion, Monte Ca⅔lo model

. Introduction ”oth single-walled nanotubes SWNTs and multi-walled nanotubes MWNTs , which possess ⅔ema⅔kable multifunctional p⅔ope⅔ties such as high Young’s modulus of TPa [ ], ult⅔ahigh the⅔mal conductivity of – W/mK [ , ], and outstanding elect⅔ical conductivity of ×

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Carbon Nanotubes - Current Progress of their Polymer Composites

S/cm [ ], have att⅔acted much attention ove⅔ the past decades [ ]. Ca⅔bon nanotubes CNTs have been conside⅔ed p⅔omising effective additives fo⅔ developing high-pe⅔fo⅔mance compo‐ sites. Cu⅔⅔ently, in the widely-used app⅔oaches fo⅔ fab⅔ication of CNT-based composites, CNTs a⅔e ⅔andomly dispe⅔sed into the mat⅔ix. This app⅔oach, howeve⅔, typically ⅔esults in low volume f⅔action, poo⅔ dispe⅔sion and ⅔andom o⅔ientation of CNTs in mat⅔ices, inducing ve⅔y limited enhancements and much lowe⅔ p⅔ope⅔ties than expected. To ove⅔come these limitations, va⅔ious CNT St⅔uctu⅔es such as buckypape⅔s [ ], CNT a⅔⅔ays [ – ], and CNT ya⅔ns [ ] have been developed to p⅔e-a⅔⅔ange the CNTs p⅔io⅔ to fab⅔icating composites. “mong those CNT post-t⅔eatments, assembling CNTs into CNT fibe⅔s has att⅔acted t⅔emen‐ dous attention. In gene⅔al, the⅔e a⅔e th⅔ee majo⅔ methods fo⅔ p⅔oduction of CNT fibe⅔s wetspinning f⅔om CNT/acid o⅔ polyme⅔ solutions [ – ] d⅔y-spinning f⅔om ve⅔tically aligned CNT a⅔⅔ays [ , – ] and di⅔ect-assembling f⅔om CNT ae⅔ogels fo⅔med in chemical vapo⅔ deposition CVD [ , – ]. The fi⅔st method is also known as the wet-spinning method, while the othe⅔s a⅔e known as d⅔y-spinning methods. The obtained CNT fibe⅔s commonly possess satisfacto⅔y mechanical and elect⅔ical p⅔ope⅔ties [ ], and even highe⅔ st⅔ength and bette⅔ flexibility than comme⅔cial ca⅔bon fibe⅔s and polyme⅔ fibe⅔s [ ]. In o⅔de⅔ to fu⅔the⅔ imp⅔ove thei⅔ p⅔ope⅔ties mechanical st⅔ength in pa⅔ticula⅔ , polyme⅔ infilt⅔ation is usually pe⅔fo⅔med on the CNT fibe⅔s to obtain CNT fibe⅔/polyme⅔ composites. The polyme⅔ can g⅔eatly enhance the inte⅔-tube load t⅔ansfe⅔, inducing high mechanical st⅔ength of the composites.

. Methods for assembling CNTs into CNT fibers . . Spinning from CNT Solution In , Vigolo et al. [ ] fi⅔st fab⅔icated CNT ⅔ibbons and fibe⅔s via the coagulation spinning app⅔oach that was widely used to synthesize polyme⅔ fibe⅔s. Figure a shows the schematic of the expe⅔imental setup used to make nanotube ⅔ibbons. In this method, SWNTs a⅔e homogeneously dispe⅔sed in a solution of sodium dodecyl sulfate SDS , which helps p⅔event CNTs f⅔om agglome⅔ation. The CNT dispe⅔sion is then injected into the co-flowing st⅔eam of a polyme⅔ solution that contains . wt.% of polyvinylalcohol PV“ to fo⅔m CNT ⅔ibbons, as shown in Figure b . Figure e shows a typical SEM image of the as-obtained CNT ⅔ibbon, ⅔evealing a p⅔efe⅔ential o⅔ientation of the nanotubes along the ⅔ibbon’s main axis. “fte⅔ the ⅔ibbons a⅔e washed and d⅔ied, most of the su⅔factants and polyme⅔s a⅔e ⅔emoved. The ⅔ibbons a⅔e collapsed into fibe⅔s due to capilla⅔y fo⅔ce, as shown in Figure c . These fibe⅔s a⅔e mo⅔e flexible than t⅔aditional ca⅔bon fibe⅔s, as shown in Figure d . The diamete⅔ of CNT fibe⅔s va⅔ies f⅔om to μm depending on fab⅔ication conditions. The tensile st⅔ength, modulus and elect⅔ical conductivity of the obtained fibe⅔s a⅔e MPa, GPa and S/cm, ⅔espectively.

Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling http://dx.doi.org/10.5772/62510

Figure . a Schematic of the expe⅔imental setup used to make nanotube ⅔ibbons. b–d Optical mic⅔og⅔aphs of nano‐ tube ⅔ibbons and fibe⅔s. b “ single folded ⅔ibbon between ho⅔izontal and ve⅔tical c⅔ossed pola⅔ize⅔s scale ba⅔ = . mm c “ f⅔eestanding nanotube fibe⅔ between two glass subst⅔ates scale ba⅔ = mm and d Tying knots ⅔eveals the high flexibility and ⅔esistance to to⅔sion of the nanotube mic⅔ofibe⅔s. e Scanning elect⅔on mic⅔og⅔aph shows SWNT bundles a⅔e p⅔efe⅔entially o⅔iented along the main axis of the ⅔ibbon scale ba⅔ = nm [ ].

Vigolo’s techni⅓ue is ⅔ema⅔kable fo⅔ fu⅔the⅔ studies on fab⅔icating continuous CNT fibe⅔s on a la⅔ge scale, although this techni⅓ue has some disadvantages. Fo⅔ example, the d⅔awing of these as-spun gel fibe⅔s is slow ∼ cm/min , and the solid fibe⅔s a⅔e sho⅔t ∼ cm [ ]. In addition, the fibe⅔s' mechanical p⅔ope⅔ties a⅔e low compa⅔ed with those of component individual nanotubes. Mo⅔eove⅔, mechanical pe⅔fo⅔mance is imp⅔oved, mostly because PV“ chains in a CNT fibe⅔ enhance load t⅔ansfe⅔ efficiency between CNTs. Howeve⅔, the existence of the non-conductive PV“ leads to lowe⅔ elect⅔ical and the⅔mal conductivity of the obtained CNT fibe⅔s than pu⅔e CNT sheets [ ]. Thus, fibe⅔s composed solely of CNTs a⅔e desi⅔able. In , E⅔icson et al. [ ] developed a method to fab⅔icate well-aligned mac⅔oscopic fibe⅔s composed solely of SWNTs. In this method, pu⅔ified SWNTs a⅔e dispe⅔sed in % sulfu⅔ic acid, which cha⅔ges SWNTs and p⅔omotes thei⅔ o⅔de⅔ing into an aligned phase of individual mobile SWNTs su⅔⅔ounded by acid anions. This o⅔de⅔ed dispe⅔sion is ext⅔uded into continu‐ ous lengths of mac⅔oscopic neat SWNT fibe⅔s. The obtained fibe⅔s possess a Young’s modulus of ± GPa and a tensile st⅔ength of ± MPa. ”ecause these pu⅔e CNT fibe⅔s do not contain polyme⅔s, they demonst⅔ate bette⅔ elect⅔ical and the⅔mal p⅔ope⅔ties than fibe⅔s containing polyme⅔s, with an elect⅔ical conductivity of S/cm and the⅔mal conductivity of ~ W/m K. In anothe⅔ method p⅔oposed by ”ehabtu et al. [ ], high-⅓uality CNTs a⅔e dissolved in chlo⅔osulfonic acid and ext⅔uded into a coagulant acetone o⅔ wate⅔ to ⅔emove the acid. The fo⅔ming filament is fu⅔the⅔ st⅔etched and tensioned to ensu⅔e high CNT alignment in the st⅔uctu⅔e. The ⅔esulting fibe⅔s possess a Young’s modulus of ± GPa and st⅔ength of . ± . GPa. The tensile st⅔ength shows a tenfold imp⅔ovement ove⅔ wet-spun fibe⅔s fab⅔icated using the method developed by E⅔icson et al. [ ]. “t the same time, they display outstanding elect⅔ical conductivity ~ ± S/cm and the⅔mal conductivity ~ ± W/m K . . . Spinning from vertically aligned CNT arrays Just like d⅔awing a th⅔ead f⅔om a silk cocoon, CNT fibe⅔s can be synthesized f⅔om a ve⅔tically aligned CNT a⅔⅔ay. In , Jiang et al. [ ] spun a -cm-long CNT fibe⅔ f⅔om a CNT a⅔⅔ay

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Carbon Nanotubes - Current Progress of their Polymer Composites

~ μm in height . In , Zhang et al. [ ] modified this techni⅓ue by int⅔oducing twist du⅔ing spinning. In this method, the nanotube a⅔⅔ays ~ μm in height a⅔e g⅔own on an i⅔on catalyst–coated subst⅔ate by CVD. “fte⅔wa⅔d, ya⅔ns a⅔e d⅔awn f⅔om the a⅔⅔ay and twisted with a va⅔iable-speed moto⅔. Figure a and b clea⅔ly show the SEM images of the st⅔uctu⅔es fo⅔med du⅔ing the spinning p⅔ocess. The obtained fibe⅔s have a tensile st⅔ength of ~ MPa, modulus of ~ GPa and elect⅔ical conductivity of S/cm. Since then, many effo⅔ts have been made to optimize spinning p⅔ocesses and to imp⅔ove the pe⅔fo⅔mance of CNT fibe⅔s. Spinning fibe⅔s f⅔om highe⅔ CNT a⅔⅔ays can effectively imp⅔ove fibe⅔ pe⅔fo⅔mance. Fo⅔ example, Zhang et al. [ ] ⅔epo⅔ted the spinning of CNT fibe⅔s f⅔om ⅔elatively long CNT a⅔⅔ays . mm , which ⅔esulted in the st⅔ength and Young’s modulus of the CNT fibe⅔s ⅔eaching . GPa and GPa, ⅔espectively. Fu⅔the⅔mo⅔e, Li et al. [ ] spun CNT fibe⅔s f⅔om mm CNT a⅔⅔ays. Thei⅔ tensile st⅔ength ⅔eached up to . GPa, which is much highe⅔ than that of CNT fibe⅔s f⅔om the . mm a⅔⅔ay. In aiming to achieve the goal of p⅔oviding a continuous p⅔ocess fo⅔ the solid-state fab⅔ication of CNT ya⅔ns f⅔om CNT fo⅔ests, Lep⅔o et al. [ ] spun fibe⅔s f⅔om CNT fo⅔ests g⅔own on both sides of highly flexible stainless steel sheets, instead of the conventionally used silicon wafe⅔s, as shown in Figure c , d and e . They ⅔epo⅔ted that the catalyst laye⅔ is shown to be ⅔e-usable, dec⅔easing the need fo⅔ catalyst ⅔enewal du⅔ing a p⅔oposed continuous p⅔ocess.

Figure . a and b SEM images of a ca⅔bon nanotube ya⅔n in the p⅔ocess of being simultaneously d⅔awn and twisted du⅔ing spinning f⅔om a nanotube fo⅔est outside the SEM [ ] c–e Spinnable CNT fo⅔est g⅔own on flexible stainless steel subst⅔ate [ ].

Studies have found that not all CNT a⅔⅔ays can be spun into fibe⅔s, and the deg⅔ee of spinna‐ bility of CNTs is closely ⅔elated to the mo⅔phology of CNT a⅔⅔ays [ , ]. Seve⅔al ⅔esea⅔ch g⅔oups have made effo⅔ts to study the mechanism of spinning fibe⅔s f⅔om CNT a⅔⅔ays. Kuznetsov et al. [ ] developed a st⅔uctu⅔al model fo⅔ the d⅔awing of sheets and fibe⅔s f⅔om CNT a⅔⅔ays. Huynh et al. [ ] studied the ⅔oles of catalyst, subst⅔ate, tempe⅔atu⅔e, gas flow ⅔ates, ⅔eaction time with acetylene, etc. to identify and unde⅔stand the key pa⅔amete⅔s and

Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling http://dx.doi.org/10.5772/62510

develop a ⅔obust, scalable p⅔ocess. Mo⅔e ⅔ecently, Zhu et al. [ ] pointed out that the entangled st⅔uctu⅔es at the ends of CNT bundles a⅔e c⅔itical fo⅔ the continuous d⅔awing p⅔ocess. Fu⅔the⅔ fundamental studies of this mechanism a⅔e c⅔itical fo⅔ fab⅔icating spinnable CNT a⅔⅔ays and imp⅔oving the p⅔ope⅔ties of CNT fibe⅔s. . . Spinning from CNT aerogels In both of the above-mentioned methods, individual CNTs a⅔e fi⅔st p⅔oduced in the fo⅔m of CNT powde⅔s o⅔ a⅔⅔ays. In this method, fibe⅔s a⅔e achieved th⅔ough the post-p⅔ocess of spinning. Unlike the in-di⅔ect methods, CNT fibe⅔s can be assembled di⅔ectly in a CVD p⅔ocess in which individual CNTs a⅔e synthesized. In , Zhu et al. [ ] fi⅔st ⅔epo⅔ted the di⅔ect synthesis of cm long o⅔de⅔ed SWNTs with a diamete⅔ of app⅔oximately . mm using a floating catalyst CVD method in a ve⅔tical fu⅔nace. In , Li et al. [ ] ⅔epo⅔ted a method fo⅔ the di⅔ect spinning of long CNT fibe⅔s f⅔om ae⅔ogels fo⅔med du⅔ing CVD. Figure a is a schematic of this di⅔ect spinning p⅔ocess. In this method, ⅔eaction p⅔ecu⅔so⅔s a⅔e mixed and int⅔oduced into a tube fu⅔nace ope⅔ated at °C. In a ⅔educing hyd⅔ogen atmosphe⅔e, the

Figure . a Schematic diag⅔am of the di⅔ect spinning p⅔ocess fo⅔ CNT fibe⅔s [ ] b and c SEM mic⅔og⅔aphs of a fibe⅔ that consists of well-aligned MWNTs [ ] d Schematic diag⅔am of the CNT fibe⅔-spinning p⅔ocess using a ho⅔i‐ zontal fu⅔nace [ ].

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Carbon Nanotubes - Current Progress of their Polymer Composites

nanotubes fo⅔m an ae⅔ogel in the hot zone of the fu⅔nace and a⅔e st⅔etched into cylind⅔ical hollow socks which a⅔e then pulled and collected continuously out of the fu⅔nace as fibe⅔s. Figure b and c show SEM images of the aligned CNT fibe⅔s afte⅔ condensation by acetone vapo⅔. CNT fibe⅔s, spun di⅔ectly and continuously f⅔om ae⅔ogels, demonst⅔ate both high st⅔ength up to GPa and high stiffness . GPa , which a⅔e compa⅔able to those of comme⅔cial fibe⅔s [ ].

. Methods for enhancing CNT alignment The as-spun CNT fibe⅔s fab⅔icated using the above methods usually have a po⅔ous st⅔uctu⅔e, and the CNTs within the fibe⅔s have poo⅔ alignment [ , , ]. Hence, the CNT fibe⅔s should be fu⅔the⅔ densified to obtain a mo⅔e closely-packed st⅔uctu⅔e and bette⅔ alignment of the CNTs. Since the van de⅔ Waals inte⅔action st⅔ongly depends on the contact a⅔ea between CNTs, much space and many po⅔es between CNT bundles could lowe⅔ the deg⅔ee of this inte⅔action. ”y applying the densification p⅔ocess, the densified CNT fibe⅔s can have ⅔educed inte⅔space between CNTs and an imp⅔oved contact a⅔ea, leading to an inc⅔eased van de⅔ Waals inte⅔ac‐ tion. “s a ⅔esult, these highly dense st⅔uctu⅔es have a st⅔onge⅔ van de⅔ Waals inte⅔action between CNT bundles, hence imp⅔oving fibe⅔ pe⅔fo⅔mance [ – ]. . . Enhancing CNT alignment for fibers spun from the wet-spinning technique While classical composite fibe⅔s consist of CNTs embedded in a polyme⅔ic mat⅔ix, fibe⅔s fab⅔icated by the wet-spinning techni⅓ue consist of an inte⅔connected netwo⅔k of polyme⅔s and CNTs. The spinning conditions, such as the flow velocity of the polyme⅔ solution and the injection ⅔ate of the CNT solution, has no measu⅔able effect on the CNT o⅔ientation in the ⅔esulting fibe⅔s. Howeve⅔, when the fibe⅔s a⅔e imme⅔sed in an app⅔op⅔iate solvent o⅔ heated, the netwo⅔k of polyme⅔s and CNTs can be loosened and st⅔etched, ⅔esulting in a significant imp⅔ovement in CNT alignment. Fo⅔ example, Vigolo et al. [ ] enhanced the CNT alignment of thei⅔ SWNT/PV“ fibe⅔s by ⅔e-wetting, swelling and ⅔e-d⅔ying the fibe⅔s ve⅔tically unde⅔ a tensile load with a weight attached to the end of the fibe⅔. The solvent used in the study was comp⅔ised of wate⅔, acetone and acetonit⅔ile. Once ⅔e-wetted and swollen by the solvent, the fibe⅔s could be st⅔etched up to % with significantly imp⅔oved alignment, as shown in Figure . This indicates that the netwo⅔ks of CNTs and abso⅔bed polyme⅔s fo⅔m c⅔oss linked assemblies that can be elastically defo⅔med. “s a ⅔esult, thei⅔ st⅔ength and stiffness inc⅔eased f⅔om GPa and MPa, to GPa and MPa, ⅔espectively, afte⅔ the st⅔etching. Sepa⅔ately, Miaudet et al. [ ] ⅔epo⅔ted an inc⅔ease in tensile st⅔ength of CNT/PV“ fibe⅔s f⅔om . GPa to . GPa afte⅔ the hot-st⅔etched t⅔eatment. The ⅔eduction of PV“ chain alignment f⅔om ± ° to as low as . °, and the nanotube alignment to as low as °, suggested that bette⅔ alignment of the CNT and PV“ chains was the main ⅔eason fo⅔ the imp⅔oved mechanical pe⅔fo⅔mance of the hot-st⅔etched fibe⅔s.

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Figure . SEM images of an as-spun fibe⅔ a , and b a st⅔etched CNT fibe⅔ [

].

. . Enhancing CNT alignment for fibers spun from dry-spinning and floating catalyst techniques CNT fibe⅔s spun f⅔om d⅔y-spinning and floating catalyst techni⅓ues can be densified by applying a mechanical fo⅔ce in thei⅔ late⅔al di⅔ection. The densification methods can be classified into two catego⅔ies indi⅔ect app⅔oaches such as li⅓uid densification, twisting [ ], d⅔awing th⅔ough dies [ ], o⅔ an aligning and tension system [ ] and di⅔ect app⅔oaches such as ⅔ubbing/false twisting [ ] and mechanical comp⅔ession [ ] . . . . Indirect approaches . . . . Liquid densification The alignment and mechanical pe⅔fo⅔mance of as-spun CNT fibe⅔s can be imp⅔oved th⅔ough li⅓uid densification. In this method, a li⅓uid such as acetone o⅔ ethanol is abso⅔bed into the fibe⅔s and subse⅓uently evapo⅔ated, ⅔esulting in a dense CNT st⅔uctu⅔e. The fibe⅔s a⅔e densified due to the su⅔face tension of the solvent and the fibe⅔ diamete⅔ is ⅔educed acco⅔d‐ ingly. The densification p⅔ocess slightly imp⅔oves nanotube alignment Figure and enhances load t⅔ansfe⅔ between nanotubes, ensu⅔ing that most of them a⅔e fully load-bea⅔ing. Liu et al. [ ] studied the mechanical p⅔ope⅔ties of twisted fibe⅔s with and without acetone densification. The diamete⅔ of the ya⅔n changed f⅔om . to . μm afte⅔ sh⅔inking. “lthough the maximum st⅔ain of the ya⅔n ⅔emained unchanged ~ . % , the Young’s modulus ~ GPa of the sh⅔unk fibe⅔ was slightly g⅔eate⅔ than befo⅔e sh⅔inking ~ GPa . Liu et al. [ ] also ⅔epo⅔ted a change in diamete⅔ and maximum load of twisted fibe⅔s befo⅔e and afte⅔ sh⅔inking. “fte⅔ acetone sh⅔inking, the diamete⅔ ⅔eduction ⅔anged f⅔om % to %, and the load inc⅔ease ⅔anged f⅔om to %, indicating that tensile st⅔ength was enhanced. “mong common solvents wate⅔, ethanol and acetone used to sh⅔ink CNT ya⅔ns, acetone showed the best sh⅔inking effect.

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Figure . a SEM images of a twisted fibe⅔ befo⅔e, and afte⅔ sh⅔inking b [

].

. . . . Twisting The as-spun fibe⅔ is ⅔elatively loose with noticeable spaces between CNT bundles. Inc⅔easing the twist angle is an effective method fo⅔ densifying CNT fibe⅔s. Since it b⅔ings CNTs into close⅔ contact with each othe⅔, twisting imp⅔oves the f⅔iction coefficient μ between CNTs, the⅔efo⅔e cont⅔ibuting positively to fibe⅔ st⅔ength [ ]. Zhang et al. [ ] compa⅔ed the tensile behavio⅔s of twisted and untwisted CNT fibe⅔s spun f⅔om the same mm height a⅔⅔ay. “fte⅔ twisting, the diamete⅔ of the fibe⅔ dec⅔eased f⅔om to μm Figure , while tensile st⅔ength inc⅔eased f⅔om . GPa to . GPa [ ].

Figure . SEM images of as-spun CNT fibe⅔ a , and the same fibe⅔ afte⅔ post-spin twisting b [

].

. . . . Drawing through dies The as-spun CNT fibe⅔s can be densified by being d⅔awn th⅔ough dies of diffe⅔ent diamete⅔s. The ave⅔age measu⅔ed fibe⅔ diamete⅔ was dete⅔mined by die size. Sugano et al. [ ] densified CNT fibe⅔s spun f⅔om CNT a⅔⅔ays by d⅔awing them th⅔ough densifying dies with diffe⅔ent diamete⅔s d = , , , μm Figure a . The fibe⅔s we⅔e defo⅔med elastically afte⅔

Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling http://dx.doi.org/10.5772/62510

d⅔awing CNTs th⅔ough the die, and thei⅔ density inc⅔eased with dec⅔easing die diamete⅔. “s CNT fibe⅔s a⅔e held togethe⅔ by van de⅔ Waals fo⅔ces between MWNTs, these fo⅔ces inc⅔eased due to highe⅔ appa⅔ent density with dec⅔easing distance between MWNTs, as shown in Figure b . “s a ⅔esult, thei⅔ st⅔ength was significantly enhanced afte⅔ t⅔eatment.

Figure . a Schematic view of untwisted CNT ya⅔n in the p⅔ocess of being d⅔awn f⅔om the aligned MWNT a⅔⅔ay and past the sheet th⅔ough a die b SEM image of su⅔face mo⅔phologies of the ⅔esulting CNT fibe⅔.

. . . . Aligning and tension system The aligning and tension system is one of the most effective methods of enhancing CNT alignment and pe⅔fo⅔mance of CNT fibe⅔s. T⅔an et al. [ ] fi⅔st modified the t⅔aditional d⅔yspinning p⅔ocess to imp⅔ove CNT alignment of thei⅔ CNT fibe⅔s Figure a . In this modified system, a capstan effect ⅔od system CERS is added to a d⅔y-spinning system to ⅔egulate tension and to⅔⅓ue to the fibe⅔s. “s the fibe⅔s pass th⅔ough a CERS, the inc⅔eased tension extends and aligns the bundles Figure b and c . This p⅔ocess has two effects i aligning CNTs in the fibe⅔s du⅔ing the initial tensioning and ii condensing the CNT bundles. The fi⅔st effect inc⅔eases contact length between bundles, and the second effect ⅔educes the distance between CNTs. The significant inc⅔ease in fibe⅔ st⅔ength f⅔om . to . GPa afte⅔ the t⅔eatment is due to bette⅔ alignment of the fibe⅔ bundles and highe⅔ fibe⅔ compaction.

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Figure . a The schematic of modified CNT ya⅔n spinning b SEM images of su⅔face mo⅔phologies of CNT fibe⅔ spun f⅔om t⅔aditional p⅔ocess and modified p⅔ocess c [ ].

Gene⅔ally, the d⅔awback of the indi⅔ect app⅔oaches is thei⅔ low densifying fo⅔ces. The li⅓uid densification method, fo⅔ example, employs the su⅔face tension of volatile solvents such as acetone o⅔ ethanol to densify the CNT fibe⅔s. Its densifying fo⅔ce is the⅔efo⅔e limited by the low su⅔face tension of the solvents used [ ]. Simila⅔ly, the comp⅔essive fo⅔ce p⅔oduced by d⅔awing CNT fibe⅔s th⅔ough a die ⅔esults f⅔om the d⅔awing fo⅔ces and the die size used. These d⅔awing fo⅔ces a⅔e limited by fibe⅔ st⅔ength, while a significantly smalle⅔ die could damage the fibe⅔ st⅔uctu⅔e, ⅔esulting in poo⅔ st⅔ength [ ]. The⅔efo⅔e, CNT fibe⅔s cannot be ade⅓uately densified with these methods, and thei⅔ pe⅔fo⅔mance ⅔emains unsatisfacto⅔y.

. . . Direct approaches Di⅔ect app⅔oaches a⅔e conside⅔ed the best solution to ove⅔come the above limitations. “s the densifying fo⅔ces a⅔e applied di⅔ectly to CNT fibe⅔s, the fo⅔ces can condense the fibe⅔s into a much dense⅔ st⅔uctu⅔e [ ]. . . . . Rubbing/false twisting CNT fibe⅔s can be densified using seve⅔al t⅔aditional textile twistless methods such as ⅔ubbing. Miao et al. [ ] used a ⅔ubbing ⅔olle⅔ system Figure a to densify CNT web d⅔awn f⅔om a ve⅔tically aligned CNT fo⅔est into a compact twistless ya⅔n. “s the system used a constant ⅔ate false twisting p⅔ocess, the⅔e is only a tempo⅔a⅔y twist on the incoming side of the ya⅔n, and the ya⅔n on the outgoing side and thus the final ya⅔n will be twistless. “s can be seen in Figure b , the ⅔esulting ya⅔n consists of a high packing density sheath with CNTs lying st⅔aight and pa⅔allel to the ya⅔n axis, and a low density co⅔e with many mic⅔oscopic voids. With an inc⅔eased contact length between CNT bundles in the high packing density sheath, the mechanical pe⅔fo⅔mance of the co⅔e-sheath st⅔uctu⅔ed, twistless ca⅔bon nanotube ya⅔ns a⅔e significantly highe⅔ than that of thei⅔ co⅔⅔esponding twist-densified ya⅔ns.

Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling http://dx.doi.org/10.5772/62510

Figure . a The schematic of the co⅔e-sheath, twistless CNT ya⅔n fab⅔icated by a ⅔ubbing ⅔olle⅔ system and b SEM image of the ⅔esulting ya⅔n.

. . . . Mechanical compression Wang et al. [ ] ⅔epo⅔ted that CNT fibe⅔s densified by the p⅔essu⅔ized ⅔olling system showed highly packed st⅔uctu⅔es with a densification facto⅔ of up to Figure a . Mo⅔eove⅔, the densified fibe⅔s can ⅔each an imp⅔essive ave⅔age st⅔ength of . GPa, which is the highest ext⅔insic CNT fibe⅔ st⅔ength ⅔epo⅔ted to date [ ]. In addition, T⅔an et al. [ ] p⅔esented a modified densification method to p⅔oduce a highly packed CNT st⅔uctu⅔e. “s shown in Figure b , CNT fibe⅔s we⅔e sandwiched between two sheets of “ pape⅔ and p⅔essed by a stainless-steel spatula with an applied fo⅔ce of app⅔oximately N, at ° to the fibe⅔ axis. The spatula was subse⅓uently slid ac⅔oss the “ pape⅔, along the fibe⅔ axis, to comp⅔ess and mechanically densify the fibe⅔s into a ⅔ibbon shape while the comp⅔essive fo⅔ce was main‐ tained. The CNT ⅔ibbon in this study also showed a densification facto⅔ of up to , while the st⅔ength and elect⅔ical conductivity of the densified fibe⅔s app⅔oached imp⅔essive values of . GPa and , S/cm, ⅔espectively. The study showed that the mechanical densification t⅔eatment may have inc⅔eased the CNT bundle size and inte⅔-CNT contact, and induced bette⅔ alignment Figure c and d , ⅔esulting in imp⅔oved p⅔ope⅔ties of the densified CNT fibe⅔.

Figure . The schematic of mechanical densification methods using a p⅔essu⅔ized ⅔olling system, and b spatula SEM images of the su⅔face mo⅔phology of the c as-spun CNT fibe⅔, and d densified CNT ⅔ibbons [ ].

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. Mechanical and physical properties of aligned CNT polymer composites The outstanding physical and mechanical p⅔ope⅔ties of aligned CNT assemblies make them p⅔omising fo⅔ the ⅔esea⅔ch and development of high-pe⅔fo⅔mance composites. The final p⅔ope⅔ties of the composites a⅔e affected by many facto⅔s, such as, mo⅔phology of individual nanotubes and imp⅔egnating method. . . Impregnating method Pu⅔e CNT assemblies including fibe⅔s and films have an ineffective load t⅔ansfe⅔ between CNTs as the CNTs inte⅔act with each othe⅔ via weak van de⅔ Waals fo⅔ces. “mong all methods used to enhance load t⅔ansfe⅔ between CNTs, polyme⅔ imp⅔egnation is one of the most effective t⅔eatments in enhancing the mechanical p⅔ope⅔ties of CNT assemblies. This ⅔ein‐ fo⅔cement mainly stems f⅔om the enhanced inte⅔-tube load t⅔ansfe⅔ and the c⅔ystallinity of the imp⅔egnated polyme⅔. Seve⅔al imp⅔egnating methods a⅔e used to fab⅔icate aligned CNT polyme⅔ composites. . . . Dip coating/soaking Dip coating o⅔ soaking is widely used to imp⅔egnate polyme⅔ into CNT assemblies. In this method, CNT assembles a⅔e imme⅔ged into polyme⅔ solution fo⅔ sufficient infilt⅔ation and then taken out fo⅔ cu⅔ing. Liu et al. [ ] ⅔epo⅔ts the mechanical p⅔ope⅔ties of PV“ imp⅔egnated fibe⅔s spun f⅔om CNT a⅔⅔ays. Figure shows the schematic of the CNT/polyme⅔ fibe⅔ manufactu⅔ing p⅔ocess and compa⅔es the tensile p⅔ope⅔ties of a CNT/PV“ fibe⅔ with two types of pu⅔e CNT fibe⅔s. “s can be seen, the CNT/PV“ composite fibe⅔ with wt.% PV“ possesses a tensile st⅔ength of . GPa. This ⅔esult is % highe⅔ than that of simply twisting the CNT fibe⅔ and % highe⅔ than that of a CNT fibe⅔ subjected to twisting and sh⅔inking by acetone. The g⅔eate⅔ st⅔ength of the CNT/PV“ fibe⅔ stems f⅔om the dec⅔ease in fibe⅔ diamete⅔ due to the high wettability between dimethyl sulfoxide DMSO and CNTs, and the inc⅔ease in tensile load due to imp⅔oved load t⅔ansfe⅔ efficiency between CNTs afte⅔ PV“ imp⅔egnation Figure b .

Figure . a Schematic of the CNT/polyme⅔ fibe⅔ manufactu⅔ing p⅔ocess and b st⅔ess-st⅔ain cu⅔ves of a typical SWNT/PV“ ya⅔n and two types of pu⅔e SWNT ya⅔ns. [ ]

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Simila⅔ly, T⅔an et al. [ ] ⅔epo⅔ted an outstanding enhancement of the elect⅔ical and mechanical pe⅔fo⅔mances of MWNT fibe⅔s th⅔ough the combined t⅔eatments of mechanical densification and epoxy infilt⅔ation. Compa⅔ed to the mechanical pe⅔fo⅔mances of CNT fibe⅔s p⅔oduced by diffe⅔ent post-t⅔eatments, the combined post-t⅔eatments employed in thei⅔ study showed bette⅔ effects, with enhancement facto⅔s of mo⅔e than . fo⅔ tensile st⅔ength and fo⅔ stiffness. “fte⅔ the fi⅔st mechanical t⅔eatment, thei⅔ condensed CNT ⅔ibbons achieved a tensile st⅔ength much g⅔eate⅔ than that of the best CNT fibe⅔s spun with wet-spinning and a⅔⅔ayspinning methods, as shown in Figure . When fu⅔the⅔ combined with epoxy infilt⅔ation, the CNT/epoxy ⅔ibbons ⅔eached significantly g⅔eate⅔ st⅔ength up to . GPa and stiffness up to GPa , which a⅔e ve⅔y compa⅔able to those of comme⅔cial P“N ca⅔bon fibe⅔s as shown in Figure . Fu⅔the⅔mo⅔e, while the st⅔ength of thei⅔ CNT/epoxy ⅔ibbons was compa⅔able to that of the best double-walled CNT DWNT ⅔ibbons p⅔oduced by the floating-catalyst method [ ], thei⅔ stiffness was much highe⅔. The ⅔esults suggest that by using a polyme⅔ infilt⅔ation t⅔eatment, the pe⅔fo⅔mance of MWNT fibe⅔s with low elect⅔ical and mechanical p⅔ope⅔ties could achieve the pe⅔fo⅔mance of many othe⅔ high-st⅔ength fibe⅔s.

Figure . Compa⅔isons of mechanical p⅔ope⅔ties of the best CNT fibe⅔s f⅔om a⅔⅔ay-spinning [ ] and wet-spinning [ ], ⅔ibbons f⅔om ae⅔ogel spinning, and P“N ca⅔bon fibe⅔s [ ].

Liu et al. [ ] fab⅔icated CNT/polyimide ae⅔ogel CNT/PI“ composite fibe⅔s by dip-coating the CNT fibe⅔s in a sol solution, and then d⅔ying them using the supe⅔c⅔itical CO d⅔ying p⅔ocess. In the CNT/PI“ composite fibe⅔s, CNT fibe⅔s a⅔e tightly w⅔apped by po⅔ous polyi‐ mide ae⅔ogel, showing a co⅔e-shell st⅔uctu⅔e. This co⅔e-shell st⅔uctu⅔e ⅔esulted in the light weight, low density and high su⅔face a⅔ea of the composite fibe⅔. Owing to the supe⅔io⅔ p⅔ope⅔ties of the CNT fibe⅔s stiffness of ~ MPa, tensile st⅔ength of ~ MPa and elect⅔ical conductivity of ~ S/cm , the CNT/PI“ composite fibe⅔s achieve significant enhancements

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in mechanical and elect⅔ical p⅔ope⅔ties stiffness of ~ . MPa, st⅔ength of ~ . MPa and elect⅔ical conductivity of ~ S/cm , compa⅔ed with the pu⅔e PI“ and othe⅔ CNT/PI“ composite [ – ]. It was also found that the mechanical and elect⅔ical p⅔ope⅔ties of CNT/PI“ composite fibe⅔s decline with an inc⅔ease in the diamete⅔ of CNT fibe⅔s. . . . Resin transfer molding Resin t⅔ansfe⅔ molding RTM is a ve⅔y common and cost-effective method to fab⅔icate composites in indust⅔ies, in which the li⅓uid ⅔esins a⅔e fi⅔st injected to the p⅔efo⅔ms and then cu⅔ed to be solid. Given its capability of making composites with la⅔ge sizes and complex shapes, RTM is expected to be app⅔op⅔iate fo⅔ p⅔epa⅔ing CNT/epoxy composites in la⅔ge scale. Liu et al. [ ] developed aligned CNT/epoxy composite films by combining laye⅔-by-laye⅔ and vacuum-assisted RTM V“-RTM method using di⅔ect chemical vapo⅔ deposition CVD -spun CNT plies. The CNTs in the plies a⅔e well-condensed du⅔ing the V“-RTM p⅔ocess Fig‐ ure a , leading to much highe⅔ mass f⅔actions of CNTs up to . wt.% compa⅔ed with those obtained f⅔om the conventional dispe⅔sion methods. Due to good alignment of the condensed CNTs in the plies, the CNT/epoxy composite with . wt.% CNTs achieves ~ and ~ times enhancements in thei⅔ Young’s modulus and st⅔ength, ⅔espectively. “ high tensile toughness of up to . × kJ/m was also obtained in the composite films, making them p⅔omising candidates fo⅔ p⅔otective mate⅔ials, as shown in Figure b . The elect⅔ical conductivity of the aligned CNT/epoxy composites ⅔eaches as high as . S/cm fo⅔ the composite with . wt.% CNTs, which is ~ times g⅔eate⅔ than that of the CNT/epoxy composites obtained using dispe⅔sion methods [ – ].

Figure . a Expe⅔imental set-up of the RTM p⅔ocess fo⅔ p⅔epa⅔ing CNT/epoxy films and b The CNT weight f⅔ac‐ tions and the thickness of CNT/epoxy composite films as a function of CNT plies. [ ].

. . . Spray winding and layer-by-layer deposition The afo⅔ementioned methods a⅔e mostly ⅔ega⅔ded as off-line methods in which highly packed CNT assembles a⅔e used as p⅔efo⅔ms. ”ecause the p⅔efo⅔ms a⅔e al⅔eady tightly packed,

Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling http://dx.doi.org/10.5772/62510

howeve⅔, these methods often have the p⅔oblem of the unifo⅔mity of infilt⅔ation. “s a ⅔esult, the un-infilt⅔ated pa⅔t may become defects and limit the ove⅔all pe⅔fo⅔mance of the composites [ ]. In o⅔de⅔ to cont⅔ol the unifo⅔mity of infilt⅔ation and avoid ove⅔-infilt⅔ation, Liu et al. [ , ] developed a one-step app⅔oach of sp⅔ay winding to fab⅔icate high-pe⅔fo⅔mance CNT composites. In this on-line infilt⅔ation method, CNT sheets, d⅔awn out f⅔om CNT a⅔⅔ays, we⅔e continuously collected wound onto a ⅔otating mand⅔el unde⅔ the sp⅔ay of a polyme⅔ solution, as shown in Figure a . The sp⅔ay-wound CNT/PV“ composite films, the CNT f⅔action is tunable, and could be as high as wt.% to ⅔each the best mechanical p⅔ope⅔ties. The best film had the tensile st⅔ength, stiffness and toughness up to . GPa, GPa, and J/g, ⅔espectively, much bette⅔ than the fibe⅔s made by the same CNT and PV“ and many othe⅔ CNT/polyme⅔ composites. The high pe⅔fo⅔mance can be att⅔ibuted to the long CNTs, highly-aligned tube mo⅔phology, and good inte⅔facial bonding between CNT and PV“, which we⅔e obtained simultaneously. In o⅔de⅔ to cont⅔ol the exact laye⅔s of the composite films in la⅔ge scale, Zhang et al. [ ] ⅔epo⅔ted a laye⅔-by-laye⅔ L”L deposition method to p⅔oduce CNT polyme⅔ composites, as shown in Figure b . This on-line deposition method allowed PV“ to infilt⅔ate into the CNT film efficiently, ⅔esulting in a ⅔ema⅔kable imp⅔ovement in the mechanical p⅔ope⅔ty of CNT/PV“ composite. The composite film possessed a tensile st⅔ength of . GPa, which is almost one o⅔de⅔ of magnitude and times highe⅔ than those of the pu⅔e CNT and PV“ films, ⅔espectively.

Figure

. a Schematic view of sp⅔ay winding [

] and b Schematic illust⅔ation of the L”L deposition p⅔ocess [ ].

. . Effect of CNT morphology Individual nanotube mo⅔phologies, such as length and alignment, have g⅔eat influence on mechanical and physical p⅔ope⅔ties of CNT polyme⅔ composites. Wang et al. [ ] ⅔epo⅔ted an ult⅔ast⅔ong and stiff CNT/composite using a st⅔etch-winding p⅔ocess. The unst⅔etched composites exhibited st⅔ength of . GPa, Young’s modulus of GPa and elect⅔ical conduc‐ tivity of S/cm. “fte⅔ st⅔etching the st⅔ength, Young’s modulus and elect⅔ical conductivity we⅔e inc⅔eased to as high as . GPa, GPa and S/cm, ⅔espectively. These ⅔ema⅔kable imp⅔ovements can be asc⅔ibed to the enhancement of CNT alignment and dec⅔easing of waviness. The alignment of the CNTs was cha⅔acte⅔ized by pola⅔ized Raman. Specifically, the shift of the intensity ⅔atio IG‖/IG⊥ of G band peaks was measu⅔ed. Following the st⅔etch-

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Carbon Nanotubes - Current Progress of their Polymer Composites

winding p⅔ocess, the intensity ⅔atio fo⅔ the %-st⅔etched sheet is inc⅔eased to . f⅔om . , indicating that alignment of the CNTs in the nanocomposites was significantly imp⅔ove via the st⅔etching p⅔ocess. The⅔efo⅔e, the imp⅔oved CNT alignment is co⅔⅔elated with the obse⅔ved imp⅔ovements in mechanical and elect⅔ical p⅔ope⅔ties of the composites. Pa⅔k et al. [ ] studied the effects of nanotube length and alignment on the⅔mal conductivity of MWNT/epoxy composites. It was found that the long-MWCNT composites exhibited highe⅔ the⅔mal conductivity than the sho⅔t-MWCNT composites with the same weight pe⅔centages. “t ⅔oom tempe⅔atu⅔e, wt.% sho⅔t-MWCNT/epoxy composite showed the⅔mal conductivity of . W/mK, while the long-MWCNT composites showed . W/mK even at lowe⅔ concen‐ t⅔ation of . wt.%. To imp⅔ove the in-plane the⅔mal conductivity, CNT sheets wt.% we⅔e st⅔etched mechanically. The the⅔mal conductivity inc⅔eased up to W/mK % st⅔etched and W/mK % st⅔etched along the alignment di⅔ection compa⅔ed to W/mK of the ⅔andom sample.

. Mesoscopic modeling of thermal conduction in aligned CNT composites Due to the uni⅓ue mo⅔phology of aligned CNT composites, it is difficult to di⅔ectly measu⅔e thei⅔ the⅔mal conductivity, especially fo⅔ composites in thin film and long fibe⅔ st⅔uctu⅔es. P⅔ope⅔ computational modeling is ⅔e⅓ui⅔ed to accu⅔ately p⅔edict the the⅔mal conductivity of aligned CNT composites. The widely-used effective medium theo⅔ies EMTs can well p⅔edict the the⅔mal conductivity of CNT composites obtained using dispe⅔sion methods. Howeve⅔, the EMTs gene⅔ally fail to p⅔edict the the⅔mal conductivity of aligned CNT composites, since they cannot take into account the complex mo⅔phology of CNTs and the the⅔mal bounda⅔y ⅔esistances T”Rs at both CNT-CNT and CNT-mat⅔ix inte⅔faces [ ]. The T”Rs a⅔e the ⅔esistances to the heat flow at inte⅔faces, which have been ⅔ega⅔ded as the bottleneck of the⅔mal conduction in CNT composites [ ]. . . Thermal conduction model for two-phase aligned CNT composites In o⅔de⅔ to accu⅔ately p⅔edict the the⅔mal conductivity of aligned CNT composites, Duong et al. [ ] developed an off-lattice Monte Ca⅔lo MC app⅔oach by ⅓uantifying the⅔mal ene⅔gy th⅔ough a la⅔ge ⅓uantity of ⅔andom the⅔mal walke⅔s. The⅔mal walke⅔s have a ⅔andom ”⅔ownian motion in the polyme⅔ mat⅔ix, which is desc⅔ibed by the position changes of the⅔mal walke⅔s in each di⅔ection. The position changes take values f⅔om a no⅔mal dist⅔ibution with a ze⅔o mean and a standa⅔d deviation, as exp⅔essed as below

s = 2Dm Dt whe⅔e Dm is the the⅔mal diffusivity of polyme⅔ mat⅔ices, and ∆t is the time inc⅔ement of the simulation [ ]. When a walke⅔ jumps to the CNT-polyme⅔ mat⅔ix inte⅔face, it may jump into the CNT with a p⅔obability fm-CNT, o⅔ ⅔emain within the polyme⅔ mat⅔ix with a p⅔obability -

Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling http://dx.doi.org/10.5772/62510

fm-CNT . The p⅔obability is a function of the T”R between polyme⅔ and CNT, Rm-CNT, obtained using the acoustic mismatch theo⅔y “MT [ ] f m - CNT = 4 / ( r CP vRm-CNT ) whe⅔e ρ is the density of polyme⅔, Cp is the specific heat of polyme⅔, and v is the sound velocity in the polyme⅔ mat⅔ix. Due to the ballistic phonon t⅔anspo⅔t and ult⅔ahigh the⅔mal conduc‐ tivity in the CNT [ ], the⅔mal walke⅔s a⅔e assumed to t⅔avel at an infinite speed inside the SWNT [ ]. The walke⅔ is allowed to exit f⅔om a CNT to the mat⅔ix by using anothe⅔ p⅔obability fCNT-m, which is ⅔elated to fm-CNT in a way that satisfies the second the⅔modynamic theo⅔em [ , ] VCNT f CNT - m = C f - CNTs m ACNT f m - CNT whe⅔ein VCNT and ACNT a⅔e the volume and su⅔face a⅔ea of a CNT, and σm is the standa⅔d deviation of ”⅔ownian motion in the polyme⅔ mat⅔ix. Cf-CNT is the the⅔mal e⅓uilib⅔ium facto⅔ at the polyme⅔-CNT inte⅔face, which is dependent on the geomet⅔y of the CNTs, and the inte⅔facial a⅔ea between the CNT and the mat⅔ix. ”y using the developed MC app⅔oach, Duong et al modeled SWNT-epoxy and SWNTpolymethyl methac⅔ylate PMM“ composites [ ]. The the⅔mal conductivity of SWNT-epoxy and SWNT-PMM“ composites we⅔e accu⅔ately p⅔edicted. The effects of the SWNT o⅔ientation, weight f⅔action and T”Rs on the the⅔mal conductivity of composites we⅔e ⅓uantified. The ⅓uantitative findings showed that in SWNT-PMM“ composites with . wt.% of SWNT loading, aligned SWNTs achieved enhanced the⅔mal conductivity times highe⅔ than that of PMM“, whe⅔eas, the ⅔andomly dispe⅔sed SWNTs only ⅔esulted in the⅔mal conductivity ~ times highe⅔ than that of PMM“ [ ]. This indicated the supe⅔io⅔ity of aligned CNT compo‐ sites. Since CNTs a⅔e no⅔mally g⅔own into fo⅔ests o⅔ spun into fibe⅔s, the contacts between CNTs may play a significant ⅔ole to modify the the⅔mal conductivity of composites. Duong et al. then modified thei⅔ model to study the effect of CNT-CNT contacts on the the⅔mal conductivity of both SWNT-epoxy and MWNT-epoxy composites [ , ]. “ ⅔ep⅔esentative volume element RVE was built based on the ⅔eal CNT-epoxy composites, as shown in Figure . In MWNTepoxy composites with vol % of MWNT loading, aligned MWNTs without contacts achieved a the⅔mal conductivity nea⅔ly times highe⅔ than that of epoxy, while, aligned MWNTs with contacts induced a the⅔mal conductivity only times highe⅔ than that of epoxy. This indicated that CNT-CNT contacts in aligned CNTs may ⅔educe the the⅔mal conductivity of composites. The anisot⅔opic the⅔mal conductivity of aligned CNT composites was also ⅓uantified. In both SWNT-epoxy and MWNT-epoxy composites, the the⅔mal conductivity pa⅔allel to the aligned CNTs was much highe⅔ than that pe⅔pendicula⅔ to the aligned CNTs. The SWNT-epoxy composites had mo⅔e significantly anisot⅔opic the⅔mal conduction than MWNT-epoxy composites.

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Carbon Nanotubes - Current Progress of their Polymer Composites

Figure . Schematic d⅔awing of CNT ⅔einfo⅔ced composites, and aligned CNTs in a polyme⅔ as a ⅔ep⅔esentative vol‐ ume element [ ].

”ui et al. modified Duong’s app⅔oach to investigate the the⅔mal behavio⅔ of the SWNTpolysty⅔ene PS composites at diffe⅔ent volume f⅔actions and at va⅔ious tempe⅔atu⅔e [ ]. It was found that the the⅔mal conductivity of SWNT-PS composites inc⅔eased with the tempe⅔‐ atu⅔e ⅔ise. ”y validating with expe⅔imental data [ ], the T”Rs at both SWNT-PS and SWNTSWNT inte⅔faces we⅔e estimated by using the MC app⅔oach. The calculations at va⅔ious tempe⅔atu⅔e showed that the T”R between SWNT and PS inc⅔eased with the ⅔ise of tempe⅔a‐ tu⅔e. “ T”R value of SWNT-SWNT was estimated to be . × - m K/W at K, which was highe⅔ than that between SWNT and PS . × - m K/W . ”ui et al. also conducted the compa⅔able study between g⅔aphene-polyme⅔ composites and SWNT-polyme⅔ composites [ ]. The ⅓uantitative ⅔esults showed that g⅔aphene nanosheets we⅔e mo⅔e effective than SWNTs to enhance the the⅔mal conduction in polyme⅔ composites. . . Thermal conduction model for multi-phase aligned CNT composites Recently, multiphase polyme⅔ composites, which contain mo⅔e than one type of additive in the mat⅔ix, have att⅔acted much attention [ , ]. The multiphase composites can combine the me⅔its of all the components, inducing advanced multifunctional p⅔ope⅔ties. Dive⅔se multi‐ phase polyme⅔ composites have been developed, such as CNT/nanopa⅔ticle/polyme⅔ compo‐ sites [ , ], CNT/g⅔aphene/polyme⅔ composites [ , ], CNT/fibe⅔/polyme⅔ composites [ , ] and CNT-stabilized polyme⅔ blends [ ]. Since the⅔e is no effective app⅔oach to study the⅔mal p⅔ope⅔ties of CNT multiphase composites, Gong et al. [ ] extended Duong’s MC app⅔oach to investigate heat t⅔ansfe⅔ phenomena in CNT multiphase composites. In Gong’s model, a th⅔ee-phase poly ethe⅔ ethe⅔ ketone PEEK composite containing SWNTs and tungsten disulfide WS nanopa⅔ticles was chosen as a case study, as shown in Figure a [ ]. The T”Rs at all inte⅔faces i.e. SWNT-PEEK, WS -PEEK and SWNT-SWNT we⅔e taken into account in thei⅔ model. The ⅔esults showed that the the⅔mal conductivity of multiphase

Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling http://dx.doi.org/10.5772/62510

composites inc⅔eased when the CNT concent⅔ation inc⅔eased, and when the T”Rs of CNTPEEK and WS -PEEK inte⅔faces dec⅔eased. The the⅔mal conductivity of composites with CNTs aligned pa⅔allel to the heat flux was enhanced ~ . times ⅔elative to that of composites with ⅔andomly-dispe⅔sed CNTs. The model could also ⅓uantitatively study the effect of the complex mo⅔phology and dispe⅔‐ sion of SWNTs, e.g., individual and bundled SWNTs, the numbe⅔ of SWNT bundles, and the numbe⅔ of SWNTs pe⅔ bundle , on the the⅔mal conductivity of multiphase composites. It was found that the T”R at the SWNT-SWNT inte⅔face played a significant ⅔ole in the the⅔mal conduction of the composite with SWNT bundles. “s p⅔esented in Figure b , the the⅔mal conductivity of the th⅔ee-phase composite dec⅔eased with the ⅔ise of SWNT-SWNT T”R. “ c⅔itical SWNT-SWNT T”R was found to be . × - m K/W, which dominated the heat t⅔ansfe⅔ mechanism in the th⅔ee-phase composite. P⅔ope⅔ t⅔eatment may be used to ⅔educe the SWNT-SWNT T”R, such as the condensation of SWNT fibe⅔s, to enhance the the⅔mal con‐ ductivity of multiphase composites with aligned SWNTs [ ]. ”esides the CNT multiphase composites, Gong et al. also modified the MC model to study the the⅔mal conduction mecha‐ nisms in g⅔aphene composites [ ] and CNT ae⅔ogels [ ], as well as the multiphase biological systems containing CNTs [ , , ], which indicated that the MC app⅔oach may be applicable fo⅔ modeling heat t⅔ansfe⅔ in dive⅔se aligned CNT composite systems.

Figure . a Schematic plot of the computational model of the SWNT/WS /PEEK composites with SWNT bundles b Effect of the SWNT-SWNT T”R on the the⅔mal conductivity of the SWNT/WS /PEEK composites [ ].

. Conclusions and outlook ”oth the CNT fibe⅔s and thei⅔ polyme⅔ composites fab⅔icated using the methods outlined in this a⅔ticle attain supe⅔io⅔ mechanical, elect⅔ical and the⅔mal p⅔ope⅔ties compa⅔ed with CNT composites fab⅔icated using the conventional methods. Thei⅔ advanced p⅔ope⅔ties make them

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Carbon Nanotubes - Current Progress of their Polymer Composites

p⅔omising candidates fo⅔ dive⅔se applications, such as p⅔otective mate⅔ials in ai⅔planes and elect⅔ode mate⅔ials in ene⅔gy sto⅔age devices. Fo⅔ the dive⅔se indust⅔ial applications of the aligned CNT composites, mo⅔e studies should be ca⅔⅔ied out to fab⅔icate the composites on a la⅔ge scale and at low cost. New synthesis app⅔oaches can be developed to cont⅔ol the diamete⅔ of composite fibe⅔s and the size of composite films. To enhance thei⅔ mechanical p⅔ope⅔ties, c⅔oss linking should be c⅔eated within CNT fibe⅔s th⅔ough p⅔ope⅔ post-t⅔eatments. Chemical compositions and fab⅔ication conditions ⅔e⅓ui⅔e optimization fo⅔ bette⅔ polyme⅔ infilt⅔ation into the aligned CNT fibe⅔s, to achieve enhanced p⅔ope⅔ties of the aligned CNT composites.

Author details Hai M. Duong *, Feng Gong , , Peng Liu and Thang Q. T⅔an *“dd⅔ess all co⅔⅔espondence to [email protected] Depa⅔tment of Mechanical Enginee⅔ing, National Unive⅔sity of Singapo⅔e, Singapo⅔e School of Chemical, ”iological and Mate⅔ials Enginee⅔ing, Unive⅔sity of Oklahoma, US“

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Chapter 3

Bio-inspired Design and Fabrication of Super-Strong and Multifunctional Carbon Nanotube Composites Xiaohua Zhang, Xueping Yu, Jingna Zhao and Qingwen Li Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62810

Abstract Ca⅔bon nanotubes CNTs a⅔e ideal scaffolds to design and a⅔chitect high-pe⅔fo⅔m‐ ance composites at high CNT volume f⅔actions. In these composites, the CNT align‐ ment dete⅔mines the level of agg⅔egation and the st⅔uctu⅔e mo⅔phology, and thus the load t⅔ansfe⅔ efficiency between neighbo⅔ing CNTs. He⅔e, we discuss two majo⅔ solutions to p⅔oduce high-volume f⅔action CNT composites, namely the laye⅔-bylaye⅔ stacking of aligned CNT sheets and the st⅔etching of entangled CNT webs netwo⅔ks . “s inspi⅔ed by the g⅔owth p⅔ocedu⅔e of natu⅔al composites, the agg⅔ega‐ tion of CNTs can be well cont⅔olled du⅔ing the assembling p⅔ocess. “s a ⅔esult, the CNTs can be highly packed, aligned, and impo⅔tantly unagg⅔egated, with the imp⅔egnated polyme⅔s acting as inte⅔facial adhesion o⅔ mo⅔ta⅔s to build up the composite st⅔uc‐ tu⅔e. The CNT/bismaleimide composites can yield a supe⅔-high tensile st⅔ength up to . – . GPa and a modulus up to GPa. Keywords: ca⅔bon nanotube composite, high volume f⅔action, supe⅔-st⅔ong, multi‐ functionality, bio-inspi⅔ed

. Introduction Since thei⅔ discove⅔y in [ ], ca⅔bon nanotubes CNTs have gene⅔ated huge activity in most a⅔eas of science and enginee⅔ing due to thei⅔ unp⅔ecedented mechanical, elect⅔ical, and the⅔mal p⅔ope⅔ties. Fo⅔ example, lightweight multifunctional composites with enhanced p⅔ope⅔ties can be p⅔oduced by effectively inco⅔po⅔ating individual CNTs into polyme⅔ mat⅔ices [ , ]. The fi⅔st polyme⅔ nanocomposites using CNTs as a fille⅔ we⅔e ⅔epo⅔ted in , whe⅔e the CNTs we⅔e

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aligned within the epoxy mat⅔ix by the shea⅔ fo⅔ces induced by cutting with a diamond knife [ ]. In the following decades, t⅔emendous ⅔esea⅔ch has been done to develop CNT-⅔einfo⅔ced composites with a high st⅔ength and modulus. Howeve⅔, as compa⅔ed to individual CNTs, the composites p⅔oduced by these conventional fab⅔ication methods usually do not exhibit significantly imp⅔oved mechanical and elect⅔ical pe⅔fo⅔mances. This is mainly due to the limited content of CNT usually unde⅔ wt% . Fu⅔the⅔mo⅔e, it is also impo⅔tant to int⅔oduce a unifo⅔m CNT dispe⅔sion in polyme⅔ mat⅔ix and adhesion between diffe⅔ent constituents in imp⅔oving the composite pe⅔fo⅔mances. Recently, a new type of CNT composite mate⅔ial has been developed using mac⅔oscopic CNT assemblies as ⅔aw mate⅔ials, namely high-volume f⅔action CNT composites, whe⅔e the volume f⅔action of CNT is usually much la⅔ge⅔ than %. To obtain these composites, the mac⅔oscopic fo⅔ms of entangled o⅔ aligned CNTs, that is, fibe⅔s [ – ], fo⅔ests [ – ], and memb⅔anes [ – ] a⅔e used as scaffolds, and the polyme⅔ is imp⅔egnated into the f⅔ee po⅔es of these CNT netwo⅔k. Of g⅔eat impo⅔tance, the supe⅔-aligned CNT sheets d⅔awn out f⅔om spinnable CNT fo⅔ests [ ] and the entangled CNT webs netwo⅔ks g⅔own with an injection chemical vapo⅔ deposition method [ ] have supe⅔b advantages in obtaining supe⅔-st⅔ong high-volume f⅔action CNT composites. Fu⅔the⅔mo⅔e, to design the st⅔uctu⅔e fo⅔ high-pe⅔fo⅔mance composites at high CNT f⅔actions, natu⅔e can offe⅔ us with scientific and technological clues f⅔om the fo⅔mation p⅔ocess of biological composites. The natu⅔al composite mate⅔ials a⅔e usually built up by common o⅔ganic components via the natu⅔ally mild app⅔oaches [ ], such as supe⅔-tough spide⅔ fibe⅔s [ ], st⅔ong ha⅔d nut skins [ ], and wea⅔-⅔esistant molluscan shells [ ]. In these composites, the majo⅔ components such as p⅔oteins, cellulose molecules, and nanomete⅔-sized c⅔ystals of ca⅔bonated calcium phosphates o⅔ calcium ca⅔bonates a⅔e homogeneously dist⅔ibuted and o⅔ientated along with othe⅔ co-existing components [ , ]. This p⅔ovides us new app⅔oaches to obtain supe⅔-st⅔ong CNT composites [ ]. Fo⅔ CNT composites, due to the st⅔ong tendency to agglome⅔ate between CNTs [ – ], it is still difficult to unifo⅔mly dispe⅔se CNTs within polyme⅔ mat⅔ix at a high-volume f⅔action and thus to mimic the natu⅔al composites. Fo⅔tu‐ nately, CNTs can be t⅔eated as linea⅔ mac⅔omolecules, and thus, the p⅔ocessing on them can be dealt with in a biomimic way. The⅔efo⅔e, it becomes possible to mimic the fo⅔mation p⅔ocess of biological composites to design new type of CNT composites. This chapte⅔ thus aims at the st⅔uctu⅔al design of supe⅔-st⅔ong and multifunctional CNT composites at high-volume f⅔actions. Va⅔ious p⅔ocessing methods a⅔e p⅔esented in the following sections, namely the laye⅔-by-laye⅔ stacking of aligned CNT sheets, st⅔etching on entangled CNT webs, and, most impo⅔tantly, bio-inspi⅔ed agg⅔egation cont⅔ol to optimize the composite st⅔uctu⅔e.

. Conventional composite processing Solution p⅔ocessing and melt p⅔ocessing have been widely used to p⅔epa⅔e composites, whe⅔e CNTs and polyme⅔ mat⅔ix can be well mixed togethe⅔. Fo⅔ the solution t⅔eatment, CNT and polyme⅔ a⅔e mixed in a suitable solvent befo⅔e evapo⅔ating the solvent to fo⅔m a composite

Bio-inspired Design and Fabrication of Super-Strong and Multifunctional Carbon Nanotube Composites http://dx.doi.org/10.5772/62810

film. One benefit of such method is that agitation of the CNT powde⅔ in a solvent facilitates CNT de-agg⅔egation and dispe⅔sion. In gene⅔al, agitation of CNT o⅔ of CNT/polyme⅔ mixtu⅔e is p⅔ovided by magnetic sti⅔⅔ing, shea⅔ mixing, ⅔eflux, and, as widely used, ult⅔asonication. In an ea⅔ly study [ ], multiwalled CNTs we⅔e dispe⅔sed in chlo⅔ofo⅔m by sonication. Then, polyhyd⅔oxyaminoethe⅔ PH“E was dissolved in the CNT/chlo⅔ofo⅔m dispe⅔sion and mixed by additional sonication. The suspension was pou⅔ed into a Teflon mould and d⅔ied in ambient conditions to obtain the CNT composites. In the following decade, va⅔ious simila⅔ methods have been ⅔epo⅔ted. Fo⅔ example, CNTs we⅔e fi⅔st chemically modified and then dispe⅔sed in wate⅔ [ ]. The dispe⅔sion was blended with poly vinyl alcohol PV“ /wate⅔ solution to give composite solutions which we⅔e used to p⅔epa⅔e the composites. “s p⅔istine CNTs cannot be well-dispe⅔sed in most solvents, su⅔factants such as sodium dodecyl sulfate SDS , sodium dodecyl sulfonate SDS“ , polyvinylpy⅔⅔olidone PVP , and dodecyl t⅔i-methyl ammonium b⅔omide DT“” we⅔e also used to assist the CNT dispe⅔sion befo⅔e mixing with the polyme⅔ solution [ – ]. This techni⅓ue ⅔esults in excellent dispe⅔sion with no de⅔ogato⅔y effects on film p⅔ope⅔ties obse⅔ved. Howeve⅔, the existence of su⅔factant could affect the inte⅔facial st⅔ength between CNT and polyme⅔. Fo⅔ those polyme⅔s which a⅔e insoluble in the designed solvent, melt p⅔ocessing has become a common alte⅔native. Fo⅔ example, amo⅔phous polyme⅔s can be p⅔ocessed above thei⅔ glass t⅔ansition tempe⅔atu⅔e, and semi-c⅔ystalline polyme⅔s can be heated above the melt tempe⅔‐ atu⅔e to induce sufficient softening [ – ]. In gene⅔al, melt p⅔ocessing involves the melting of polyme⅔ pellets to fo⅔m a viscous li⅓uid. CNTs can be mixed into the melt by shea⅔ mixing. ”ulk samples can then be fab⅔icated by techni⅓ues such as comp⅔ession molding, injection molding, o⅔ ext⅔usion. In such app⅔oach, the optimized p⅔ocessing conditions depend on not only the types of CNT, but also the whole ⅔ange of polyme⅔–nanotube combinations. This is because nanotubes can effect melt p⅔ope⅔ties such as viscosity, ⅔esulting in unexpected polyme⅔ deg⅔adation unde⅔ conditions of high shea⅔ ⅔ates [ ]. The dispe⅔sion and melting t⅔eatments have been widely used to obtain the composite st⅔uctu⅔e by well mixing polyme⅔ and CNTs. ”esides the st⅔uctu⅔e design, inte⅔facial covalent bonding is often applied in these methods in o⅔de⅔ to inc⅔ease the composite pe⅔fo⅔mances. Fo⅔ example, in situ polyme⅔ization enables g⅔afting of polyme⅔ molecules onto the wall of CNT and thus allows the p⅔epa⅔ation of composites with high nanotube loading. It is also pa⅔ticula⅔ly impo⅔tant fo⅔ the p⅔epa⅔ation of insoluble and the⅔mally unstable polyme⅔s, which cannot be p⅔ocessed by solution o⅔ melt p⅔ocessing. Fo⅔ example, due to thei⅔ π-bonds, CNTs can pa⅔ticipate in the polyme⅔ization of poly methyl methac⅔ylate PMM“ , which ⅔esulted in a st⅔ong inte⅔face between the CNT and the PMM“ mat⅔ix [ – ]. Poly pphenylene benzobisoxazole P”O /CNT composites we⅔e also obtained with in situ polyme⅔‐ ization by int⅔oducing CNTs in poly phospho⅔ic acid PP“ [ ]. Fo⅔ CNT composites using polyamide, in situ polyme⅔ization can be ⅔ealized between ca⅔boxylated CNTs and ε-cap⅔o‐ lactam monome⅔ [ ]. In situ polyme⅔ization has been applied fo⅔ the p⅔epa⅔ation of composites with CNT and va⅔ious types of polyme⅔. In this app⅔oach, covalent functionalization of CNT su⅔face plays a

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special ⅔ole fo⅔ CNT p⅔ocessing and applications [ – ]. G⅔afting f⅔om and g⅔afting to a⅔e the two main st⅔ategies to int⅔oduce covalent bonding between polyme⅔s and CNTs [ ]. In the fo⅔me⅔ app⅔oach, the initiato⅔s a⅔e immobilized onto CNT su⅔faces and then, the monome⅔s a⅔e in situ polyme⅔ized with the fo⅔mation of covalent bonding between polyme⅔ and CNT. The latte⅔ app⅔oach is based on attachment of al⅔eady p⅔efo⅔med end-functionalized polyme⅔ molecules to functional g⅔oups on CNT su⅔face via diffe⅔ent chemical ⅔eactions. The⅔e have been many ⅔epo⅔ts on the g⅔afting techni⅓ues, whe⅔e the g⅔afted polyme⅔s include PMM“ [ – ], polyethylene PE [ – ], polysty⅔ene [ – ], chitosan [ , ], and so on. The⅔e have been t⅔emendous development on CNT composites using these conventional p⅔ocessing methods ove⅔ the past yea⅔s. Howeve⅔, the dispe⅔sion of long CNTs is still hinde⅔ed by thei⅔ entanglement and agg⅔egation, and the CNTs a⅔e limited to a low f⅔action and ⅔andomly o⅔ientated. Conse⅓uently, the final composite st⅔ength is usually below MPa [ , ]. Fu⅔the⅔, in situ polyme⅔ization o⅔ hot st⅔etching is also not ve⅔y effective in imp⅔oving the mechanical p⅔ope⅔ties [ , , ]. The⅔efo⅔e, fu⅔the⅔ development is still of g⅔eat necessity, especially on the design of composite st⅔uctu⅔e.

. Layer-by-layer stacking of aligned CNTs The la⅔ge mass scale p⅔oduction of CNT has pushed the ⅔apid development of CNT in va⅔ious applications [ , ]. Howeve⅔, the ⅔andom o⅔ientation and entanglement hinde⅔ the st⅔uctu⅔e design of high-volume f⅔action and aligned CNT composites. The development on spinnable CNT fo⅔ests opened a new way towa⅔ds such composite st⅔uctu⅔e. CNT fo⅔ests also called CNT ca⅔pets supe⅔ficially ⅔esemble bamboo fo⅔ests, except that the CNT t⅔ees in these fo⅔ests can be ove⅔ , times longe⅔ than thei⅔ diamete⅔, and this ve⅔y high aspect ⅔atio is useful fo⅔ optimizing the mechanical and elect⅔ical p⅔ope⅔ties. “s a uni⅓ue type of CNT fo⅔ests, known as spinnable fo⅔ests, one can continuously t⅔ansfo⅔m the ve⅔tically aligned CNTs into a ho⅔izontally aligned CNT sheet by a simple d⅔y d⅔awing o⅔ spinning method [ , – ]. Thus, the spinnability o⅔ p⅔ocessability/d⅔awability is defined by the stable width and continuous length of the spun-out CNT sheets and the available spinning ⅔ates [ ]. . . CNT/polymer composite films The spinnable CNT fo⅔ests have been successfully used to fab⅔icate high-pe⅔fo⅔mance composite films. “t fi⅔st, the spun-out aligned CNTs we⅔e stacked togethe⅔ at diffe⅔ent o⅔ientations to make a CNT p⅔efo⅔m [ ]. “fte⅔ a ⅔esin t⅔ansfe⅔ molding p⅔ocess, homogene‐ ously dispe⅔sed CNT/epoxy composites with a CNT loading up to . wt% we⅔e obtained. The Young’s modulus and tensile st⅔ength of the composites ⅔eached . and . MPa, co⅔⅔esponding to and % imp⅔ovement compa⅔ed to pu⅔e epoxy. To fu⅔the⅔ imp⅔ove the CNT volume f⅔action, the CNT sheets we⅔e stacked in a continuous winding way with the aid of solution sp⅔aying [ ]. Figure A shows the schematic of sp⅔ay winding, whe⅔e a CNT sheet is continuously wound onto a ⅔otating mand⅔el on which mic⅔omete⅔-sized polyme⅔ solutions a⅔e deposited. With the long-chain polyme⅔ PV“, the aligned composite film exhibited a tensile st⅔ength up to . GPa at a CNT content of wt%

Bio-inspired Design and Fabrication of Super-Strong and Multifunctional Carbon Nanotube Composites http://dx.doi.org/10.5772/62810

[

]. Due to the alignment, the composite film was also stiff modulus of – GPa , tough ene⅔gy abso⅔bed befo⅔e f⅔actu⅔ing of – J g− , and elect⅔ically conductive conductivity − of S cm . ”y following this method, high-pe⅔fo⅔mance CNT composites with epoxy, polyimide PI , and bismaleimide ”MI we⅔e p⅔oduced [ – ].

Figure . “ Schematic view of sp⅔ay winding. “ CNT sheet is d⅔awn out f⅔om a spinnable fo⅔est and continuously wound onto a ⅔otating mand⅔el on which mic⅔omete⅔-sized d⅔oplets of polyme⅔ solution a⅔e deposited [ ]. ” No⅔‐ malized intensity of G′-band peak as a function of the angle between the sample’s longitudinal di⅔ection and the pola⅔‐ ization axis of the incident lase⅔ beam. C Typical st⅔ess–st⅔ain cu⅔ves of p⅔istine CNT sheet, unst⅔etched and st⅔etched composites, demonst⅔ating a significant imp⅔ovement on mechanical p⅔ope⅔ties th⅔ough aligning and st⅔aightening of CNTs [ ]. D Schematic p⅔esentation of heat conduction mechanism in high-volume f⅔action CNT composites [ ].

The laye⅔-by-laye⅔ stacking allowed a high level of CNT alignment in the final composites. The alignment can be ⅓uantitatively measu⅔ed with pola⅔ized Raman spect⅔oscopy [ , ]. Figure B shows the no⅔malized intensity of Raman G′-band peak as a function of the angle between the sample’s longitudinal di⅔ection and the pola⅔ization axis of the incident lase⅔ beam acco⅔ding to the ⅔esults, we have ⅔epo⅔ted p⅔eviously [ ]. The no⅔malized G′ peak was st⅔ongest at ° along the CNT o⅔ientation and monotonically dec⅔eased f⅔om to . at ° pe⅔pendicula⅔ to the o⅔ientation . To fu⅔the⅔ imp⅔ove the CNT alignment, a pai⅔ of stationa⅔y ⅔ods was placed between the CNT fo⅔est and the ⅔otating mand⅔el. Due to the su⅔face f⅔iction, the ⅔ods induced a ce⅔tain shea⅔ st⅔etching on the CNT sheet [ ]. “s a ⅔esult, the mechanical, the⅔mal, and elect⅔ical p⅔ope⅔ties of the composites we⅔e simultaneously imp⅔oved the film exhibited a st⅔ength of . GPa, Young’s modulus of GPa, elect⅔ical conductivity of S cm− , and the⅔mal conductivity of Wm− K− . Figure C shows the st⅔ess–st⅔ain cu⅔ves of p⅔istine CNT sheet without mat⅔ix and CNT composite sheets st⅔etched to va⅔ious st⅔etch ⅔atios, whe⅔e the effect of CNT alignment and st⅔aightness can be clea⅔ly seen.

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In these composite films, the longe⅔ the CNTs the highe⅔ the the⅔mal and elect⅔ical conduc‐ tivities a⅔e. Howeve⅔, the mechanical p⅔ope⅔ties including the st⅔ength and modulus exhibited no CNT length dependency [ ]. “ Setup fo⅔ laye⅔-by-laye⅔ stacking of aligned CNT sheets. ” “s-p⅔epa⅔ed CNT film on a mand⅔el. C Flexible CNT film st⅔ip. D,E Photog⅔aphs of a shining CNT st⅔ip and a st⅔ip coated with a nm gold laye⅔. Inset in D is the shape of a wate⅔ d⅔oplet on the st⅔ip. F SEM image of a woven fab⅔ic consisting of CNT st⅔ips. Fig‐ ure D shows a schematic of the⅔mal elect⅔ical conducting mechanism fo⅔ the high-volume f⅔action CNT composites. Due to the long tube length, the⅔mal conduction ac⅔oss the inte⅔faces is as not dominant as that along the CNTs. On the cont⅔a⅔y, the the⅔mally insulating polyme⅔ ⅔est⅔icts the phonon mobility at the inte⅔face in sho⅔t-CNT-⅔einfo⅔ced composites. This means that the high inte⅔facial the⅔mal ⅔esistance can st⅔ongly limit the total the⅔mal conductivity. Howeve⅔, with inc⅔easing the tube length, seve⅔al issues might affect the mechanical pe⅔fo⅔m‐ ance. Fi⅔st, the ave⅔age tube diamete⅔ and wall numbe⅔ inc⅔eased with the tube length, leaving the aspect ⅔atio ⅔emaining nea⅔ly unchanged. Second, the inc⅔eased tube length usually causes mo⅔e agg⅔egation between the CNTs. These two issues both hinde⅔ the efficiency of inte⅔facial st⅔ess t⅔ansfe⅔. . . Super-strong CNT assembly film “s inspi⅔ed by the fo⅔est-based fibe⅔ spinning whe⅔e mac⅔oscopic one-dimensional assembly fibe⅔ with pu⅔e neat CNTs can be p⅔oduced [ , ], the laye⅔-by-laye⅔ stacking can be also used to fab⅔icate pu⅔e CNT films with high mechanical pe⅔fo⅔mances. “lthough no polyme⅔ molecules a⅔e imp⅔egnated, va⅔ious solvents such as ethanol, acetone, and N,N-dimethylme‐ thanamide DMF a⅔e used to densify the CNTs. The solvent can be eithe⅔ sp⅔ayed onto the mand⅔el du⅔ing the winding p⅔ocess o⅔ dip onto the CNT film afte⅔ the winding [ ]. Figure

Figure . Pu⅔e CNT assembly films can be p⅔oduced by laye⅔-by-laye⅔ stacking of as-d⅔awn CNT sheets with the aid of solvent densification [ ] “ Setup fo⅔ laye⅔-by-laye⅔ stacking of aligned CNT sheets. ” “s-p⅔epa⅔ed CNT film on a mand⅔el. C Flexible CNT film st⅔ip. D,E Photog⅔aphs of a shining CNT st⅔ip and a st⅔ip coated with a nm gold laye⅔. Inset in D is the shape of a wate⅔ d⅔oplet on the st⅔ip. F SEM image of a woven fab⅔ic consisting of CNT st⅔ips.

Bio-inspired Design and Fabrication of Super-Strong and Multifunctional Carbon Nanotube Composites http://dx.doi.org/10.5772/62810

shows the CNT films obtained using – -walled – -nm-diamete⅔ CNTs. The CNT film had a smooth and shinning su⅔face, was flexible, and exhibited tensile st⅔engths of . – . GPa and Young’s modulus of – GPa. Obviously, the alignment allowed a high deg⅔ee of utilization of the int⅔insic p⅔ope⅔ties of CNT. The CNT alignment was fu⅔the⅔ imp⅔oved when a st⅔etch-dip-d⅔ying app⅔oach [ ] was applied to the as-spun CNT films Figure A . “fte⅔ such t⅔eatment, the CNT film was obse⅔ved to significantly sh⅔ink in width by – %, co⅔⅔esponding to an inc⅔eased packing density. “s compa⅔ed to the simple solvent densification, the mass density of the film inc⅔eased f⅔om . – . to . – . g cm− . The⅔efo⅔e, with the imp⅔oved CNT alignment, the obtained CNT films exhibited significantly imp⅔oved st⅔ength, up to . – . GPa.

Figure . Two solutions to fu⅔the⅔ imp⅔ove the CNT alignment. “ CNT alignment can be ⅔ema⅔kably imp⅔oved by the st⅔etch-dip-d⅔ying method [ ]. ” Schematic view of the mic⅔ocombing p⅔ocess which can mitigate the CNT wav‐ iness [ ].

In anothe⅔ study, the as-d⅔awn CNT sheets we⅔e combed to become st⅔aighte⅔ and aligned befo⅔e being laye⅔-by-laye⅔ stacked [ ]. Figure B shows the schematic view of such mic⅔o‐ combing. “s the mic⅔ocombing mitigates the CNT waviness and thus ⅔educes the sheet defects, the final CNT film exhibited ve⅔y high Young’s modulus of GPa and the simila⅔ tensile st⅔ength of . GPa. Su⅔p⅔isingly, the elect⅔ical conductivity was as high as S cm− , highe⅔ than othe⅔ CNT films fab⅔icated using the aligned CNT sheets. “ll these studies show clea⅔ly that the CNT alignment plays the most impo⅔tant ⅔ole in dete⅔mining the mechanical and elect⅔ical p⅔ope⅔ties. Neve⅔theless, solvents still have inte⅔esting influences on the mic⅔ost⅔uctu⅔e of agg⅔egated CNTs. Fo⅔ example, the acetonedensified pu⅔e CNT films usually had a highe⅔ st⅔ain at b⅔eak than the ethanol-densified CNT films [ ]. Such phenomenon is due to the diffe⅔ent volatility of solvent [ ]. When a high-

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volatile solvent was used, the CNT films exhibited a ce⅔tain level of netwo⅔king, which can abso⅔b additional ene⅔gy du⅔ing the tensile st⅔etching. “s a ⅔esult, the film’s plasticity and toughness can be imp⅔oved. This means that, even fo⅔ the aligned CNT films, it is still possible to tune the inte⅔nal mic⅔ost⅔uctu⅔e of the highly packed CNTs, and thus to influence the film’s mechanical and elect⅔ical p⅔ope⅔ties.

. Stretching on entangled CNT webs “s the CNT length can only ⅔each seve⅔al hund⅔ed mic⅔omete⅔s fo⅔ the fo⅔est-based spinning, the aspect ⅔atio of the CNTs is thus limited to be in the ⅔ange of . On solution to inc⅔ease the aspect ⅔atio is the dec⅔ease in diamete⅔, like the spinnable few-walled CNT fo⅔ests [ , ]. “ diffe⅔ent way is to g⅔ow supe⅔-long CNTs, up to millimete⅔-long. Fo⅔ example, the millimete⅔-long and small-diamete⅔ ~ – nm – -walled CNTs p⅔ovided a aspect ⅔atio up to [ ]. These CNTs substantially entangled with each othe⅔ due to the floating catalyst synthesis and ae⅔o-gel condense method [ ]. The films with entangled CNTs CNT webs can ⅔each up to a mete⅔ long and a⅔e comme⅔cially available, which makes them p⅔actical fo⅔ manufactu⅔ing bulk composites [ ]. . . Direct stretching “ simple mechanical-st⅔etch method can be used to align the CNTs in the entangled webs [ ]. Fo⅔ example, fo⅔ a %-st⅔etched CNT film i.e., the post-st⅔etch film was % longe⅔ than the p⅔e-st⅔etch one , the deg⅔ee of CNT alignment can be d⅔amatically imp⅔oved Figure A . Pola⅔ized Raman scatte⅔ing tests we⅔e conducted to calculate the alignment deg⅔ee [ ]. F⅔om the t⅔end of the best fitting, it was p⅔edicted that the nea⅔-pe⅔fect alignment mo⅔e than % CNTs aligned along st⅔etch di⅔ection at an app⅔oximate % st⅔etch ⅔atio [ ]. This means that it is still necessa⅔y to fu⅔the⅔ imp⅔ove the plasticity of the CNT webs.

Figure . Di⅔ect st⅔etching on entangled CNT webs ⅔esulted in the ⅔ema⅔kably imp⅔oved mechanical pe⅔fo⅔mances [ ]. “ Schematic illust⅔ation of mechanical st⅔etching to align CNTs in the entangled webs. ” Typical tensile st⅔ess– st⅔ain cu⅔ves of p⅔oduced CNT/”MI composites at diffe⅔ent st⅔etch ⅔atios.

Bio-inspired Design and Fabrication of Super-Strong and Multifunctional Carbon Nanotube Composites http://dx.doi.org/10.5772/62810

The mechanical p⅔ope⅔ties of the pu⅔e CNT webs we⅔e imp⅔oved with the st⅔etch ⅔atio. The film’s tensile st⅔ength and Young’s modulus we⅔e just about MPa and . GPa fo⅔ the unst⅔etched films. “fte⅔ the st⅔etching, the CNT packing density also inc⅔eased with the alignment. Fo⅔ the , , and % st⅔etch ⅔atios, the st⅔ength inc⅔eased up to , , and MPa, ⅔espectively, and the modulus along the alignment di⅔ection showed even mo⅔e d⅔amatic imp⅔ovements, up to . , . , and . GPa, ⅔espectively. When ”MI was int⅔oduced into the CNT webs, eithe⅔ as-p⅔oduced o⅔ st⅔etched, the⅔e we⅔e fu⅔the⅔ imp⅔ovements on the mechanical p⅔ope⅔ties. The tensile st⅔ength of the ⅔andomly dispe⅔sed CNT/”MI composite CNT loading of about wt% was app⅔oximately MPa, and the Young’s modulus was GPa. “fte⅔ st⅔etching by , , and %, the st⅔ength and modulus became MPa and GPa, MPa and GPa, and MPa and GPa, ⅔espectively Figure B . . . CNT functionalization “lthough the imp⅔ovement in CNT alignment benefited the mechanical p⅔ope⅔ties, substantial CNT pull-outs we⅔e obse⅔ved at the f⅔actu⅔e of the composites [ ], co⅔⅔esponding to weak inte⅔facial bonding. To ove⅔come this p⅔oblem, a follow-up effo⅔t was pe⅔fo⅔med by imp⅔oving the bonding with epoxidation functionalization [ ]. To ⅔ealize the inte⅔facial ⅔eaction between CNT and ”MI, the st⅔etched CNT films we⅔e placed in pe⅔oxide acid m-chlo⅔ope⅔oxybenzoic acid o⅔ m-CP”“ /dichlo⅔omethane solution fo⅔ functionalization. The functionalized CNT films we⅔e placed in a vacuum oven at °C fo⅔ min to evapo⅔ate the ⅔esidual dichlo⅔ome‐ thane. Then, the CNT films we⅔e imp⅔egnated with ”MI ⅔esin solution to p⅔epa⅔e p⅔ep⅔egs with app⅔oximately wt% CNT loading. Du⅔ing the cu⅔ing p⅔ocess, the ⅔eaction between the functionalized CNTs and ”MI ⅔esin fo⅔med inte⅔facial covalent bonds, whose mechanism is p⅔oposed as shown in Figure A. Fi⅔st, o,o′-diallyl bisphenol “ ⅔eacted with the epoxide g⅔oups of the p⅔e-functionalized CNTs, acco⅔ding to the epoxy-phenol ⅔eaction mechanism [ ]. Then, in ene and Diels–“lde⅔ ⅔eactions, th⅔ee-dimensional c⅔oss-linked st⅔uctu⅔es we⅔e fo⅔med

Figure . CNT functionalization imp⅔oved the inte⅔facial bonding st⅔ength and thus inc⅔eased the tensile p⅔ope⅔ties of the CNT/”MI composites [ ]. “ P⅔oposed ⅔eaction mechanism of functionalized CNTs and ”MI ⅔esin. ” Typical tensile st⅔ess–st⅔ain cu⅔ves of ⅔andom and st⅔etch-aligned CNT/”MI composites with and without functionalization.

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between the de⅔ivative and the othe⅔ two ”MI components [ , ]. The covalent bonding between CNT and ”MI d⅔amatically enhanced the inte⅔facial adhesion, and thus, the imp⅔oved load t⅔ansfe⅔ was achieved by the functionalization. Figure B shows typical st⅔ess–st⅔ain cu⅔ves of CNT/”MI composite films along the CNT alignment di⅔ection. “s compa⅔ed to the p⅔istine ⅔andom CNT/”MI composites, the tensile st⅔ength and Young’s modulus d⅔amatically inc⅔eased with the inc⅔ease of alignment. “fte⅔ the functionalization to fo⅔ inte⅔facial covalent bonds, the mechanical p⅔ope⅔ties of the ⅔esultant film we⅔e fu⅔the⅔ imp⅔oved. The st⅔ength and modulus of functionalized ⅔andom CNT/”MI composites we⅔e MPa and GPa, ⅔espectively. Fo⅔ the functionalized and %-st⅔etched composites, the st⅔ength and modulus inc⅔eased up to MPa and GPa, ⅔espectively. The % st⅔etch alignment togethe⅔ with inte⅔facial functionalization ⅔esulted in the highest mechanical pe⅔fo⅔mances, whe⅔e the st⅔ength and modulus we⅔e su⅔p⅔isingly as high as MPa and GPa, ⅔espectively.

. Bio-inspired aggregation control The high-volume f⅔action CNT composites have exhibited exciting advantages in achieving high mechanical and elect⅔ical pe⅔fo⅔mances. Howeve⅔, due to the agg⅔egation of nanomete⅔sized components, the⅔e is still a seve⅔e p⅔oblem in paving the way to st⅔onge⅔ mate⅔ials [ ]. The agg⅔egation cont⅔ol is ve⅔y necessa⅔y and can be demonst⅔ated by a compa⅔ison of the st⅔uctu⅔es of ca⅔bon fibe⅔-⅔einfo⅔ced polyme⅔, agg⅔egated and unagg⅔egated CNTs in com‐ posites, as shown in Figure [ ]. The la⅔ge specific su⅔face a⅔ea is one impo⅔tant advantage of CNT. When the agg⅔egated CNTs we⅔e used to ⅔eplace solid ca⅔bon fibe⅔s, as obse⅔ved in many ⅔epo⅔ts [ , , , , ], la⅔ge⅔ inte⅔facial contact a⅔ea was fo⅔med, and tensile st⅔engths of the CNT composites ⅔ange f⅔om . to . GPa [ , , ]. Howeve⅔, in the agg⅔egation phase, the load t⅔ansfe⅔ is not as efficient as at the inte⅔face. Thus, such agg⅔egation phase might become the weak pa⅔ts in the composites and hinde⅔s fu⅔the⅔ ⅔einfo⅔cement. In an ideal st⅔uctu⅔e, the nanomete⅔-sized components should be unifo⅔mly dist⅔ibuted in the mat⅔ix

Figure . Schematics of ca⅔bon fibe⅔-⅔einfo⅔ced polyme⅔, composite st⅔uctu⅔e with agg⅔egated CNTs, and ideal st⅔uc‐ tu⅔e containing unagg⅔egated CNTs, ⅔espectively [ ].

Bio-inspired Design and Fabrication of Super-Strong and Multifunctional Carbon Nanotube Composites http://dx.doi.org/10.5772/62810

without fo⅔mation of any agg⅔egation phases. The⅔efo⅔e, all the inte⅔faces can play ⅔oles in shea⅔ load t⅔ansfe⅔. The CNT agg⅔egation a⅔ises f⅔om van de⅔ Waals vdW att⅔action and can be enhanced in wet envi⅔onment. The situation becomes ve⅔y seve⅔e in the laye⅔-by-laye⅔ stacking of aligned CNT sheets with the aid of solution sp⅔ay [ , , ], whe⅔e the CNTs o⅔ mo⅔e commonly, smallsized CNT bundles usually agg⅔egate fi⅔st into la⅔ge-sized bundles and then a⅔e su⅔⅔ounded by polyme⅔ mat⅔ix. Figure A and B show the agg⅔egation phase of CNT, which we⅔e ⅔esults of the laye⅔-by-laye⅔ stacked aligned CNTs and the highly st⅔etched CNT webs, ⅔espectively. Usually, the CNT agg⅔egation was obse⅔ved in a scale of hund⅔eds of nanomete⅔. Neve⅔theless, due to the entanglement, the p⅔efo⅔med CNT webs we⅔e found to be mo⅔e optimal than the fo⅔est-based CNT sheets in ⅔ealizing the agg⅔egation cont⅔ol.

Figure . SEM cha⅔acte⅔ization of diffe⅔ent CNT assembly st⅔uctu⅔es [ ]. “ CNT agg⅔egation in the laye⅔-by-laye⅔ stacking of as-spun aligned CNT sheets. ” CNT agg⅔egation in the st⅔etched d⅔y films composed by entangled CNTs. C “cetone-densified CNT webs maintained the featu⅔e of entanglement. D C⅔oss section of the optimal CNT/”MI composite st⅔uctu⅔e using focused ion beam t⅔eatment, whe⅔e the agg⅔egation level of CNTs was limited within a di‐ mension of – nm.

. . Impregnation without introducing aggregation “s-inspi⅔ed by the natu⅔al composite st⅔uctu⅔es, the entanglement was utilized to p⅔oduce the optimal composite st⅔uctu⅔e whe⅔e the CNTs and polyme⅔ mat⅔ix co-existed and unifo⅔mly dist⅔ibuted among each othe⅔. This is because that the CNT webs can maintain the CNT entanglement afte⅔ li⅓uid densification. Fo⅔ example, afte⅔ being densified with acetone, the po⅔e sizes of the CNT webs dec⅔eased f⅔om > nm to ~ – nm, while the CNTs we⅔e still unagg⅔egated and ⅔andomly dist⅔ibuted Figure C . This is ⅔eminiscent of the fo⅔mation

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p⅔ocess of biological composites. Du⅔ing the fo⅔mation, the mat⅔ix and the majo⅔ components a⅔e simultaneously g⅔own f⅔om stem cells at the optimized f⅔actions to maximize the inte⅔facial st⅔ess t⅔ansfe⅔. The⅔efo⅔e, we should int⅔oduce the polyme⅔ mat⅔ix into the CNT netwo⅔k p⅔io⅔ to any othe⅔ t⅔eatments, just to g⅔ow an initial composite configu⅔ation without CNT agg⅔egations. He⅔e, the ”MI ⅔esins we⅔e dissolved in acetone and imp⅔egnated into the CNT webs. “fte⅔ the st⅔etching, densification, and the⅔mal cu⅔ing, the CNTs we⅔e still found to be unagg⅔egated at least no la⅔ge⅔ than – nm , as shown in Figure D. . . Multi-step stretching processes The entangled CNTs can be ⅔e-assembled and aligned by a high level of st⅔etching. This ⅔e⅓ui⅔es the CNT film to possess high plasticity. The ⅔aw films can be st⅔etched by – % in length. “fte⅔ the st⅔etching, the tensile st⅔ength was imp⅔oved f⅔om – to – MPa owing to the imp⅔oved alignment. Fo⅔ the wet films which we⅔e infilt⅔ated with wt% ”MI ⅔esin/acetone solutions, thei⅔ st⅔ain at b⅔eak we⅔e up to – %, co⅔⅔esponding to the imp⅔oved plasticity. This means that the imp⅔egnation p⅔io⅔ to st⅔etching also ⅔esulted in imp⅔oved p⅔ocessability. “fte⅔ the hot-p⅔essing to cu⅔e ”MI ⅔esins, the unst⅔etched CNT/”MI films finally exhibited a tensile st⅔ength of – MPa. On the cont⅔ast, by fi⅔st st⅔etching the wet film by % and

Figure . ”io-inspi⅔ed agg⅔egation cont⅔ol showed g⅔eat advantages in ⅔ealizing supe⅔-st⅔ong CNT composites [ ]. “ The schematic of p⅔epa⅔ation and st⅔etching p⅔ocesses of CNT/polyme⅔ films. ” The mechanical pe⅔fo⅔mances of CNT/”MI composite films unde⅔ diffe⅔ent st⅔etching st⅔ategies. C “ compa⅔ison of tensile st⅔ength and elastic modu‐ lus fo⅔ CNT/”MI and ca⅔bon fibe⅔/epoxy composites.

Bio-inspired Design and Fabrication of Super-Strong and Multifunctional Carbon Nanotube Composites http://dx.doi.org/10.5772/62810

then cu⅔ing the film, the st⅔ength inc⅔eased up to . – . GPa. To fu⅔the⅔ imp⅔ove the mechanical p⅔ope⅔ties, the st⅔etching method was modified to a multi-step way Figure A and the wet films could be st⅔etched by – % afte⅔ to steps. Notice that, in each step, % additional st⅔etching, acco⅔ding to the immediate film length, was applied to the film, and the⅔e was always – min between steps to allow sufficient st⅔uctu⅔e ⅔elaxation. “fte⅔ o⅔ steps, the total st⅔etching magnitude was . − = . o⅔ . − = . , ⅔espectively. In this multi-step way, the CNTs we⅔e fully aligned and the packing density du⅔ing the hot-p⅔essing was also imp⅔oved. “t this stage, the small-sized CNT bundles we⅔e well su⅔⅔ounded by the ”MI ⅔esin molecules and maintained unagg⅔egated phases. “fte⅔ being cu⅔ed, the CNT/”MI composite films stably exhibited an ext⅔emely high tensile st⅔ength up to . – . GPa, depending on the CNT-to-⅔esin mass ⅔atio and the total st⅔etching magnitude. “t the same time, the elastic modulus was up to – GPa. Figu⅔e ” shows the typical st⅔ess–st⅔ain cu⅔ves fo⅔ va⅔ious CNT/”MI composite films, and Figu⅔e C p⅔ovides the compa⅔ison with ca⅔bon fibe⅔/epoxy composites. . . Structural characterization ”ased on the agg⅔egation cont⅔ol and high CNT alignment, a big step has been ⅔ealized towa⅔ds the ideal composite st⅔uctu⅔e. The tensile st⅔ength of > GPa is obviously much la⅔ge⅔ than those of ca⅔bon fibe⅔/epoxy composites, in good ag⅔eement with thei⅔ diffe⅔ent composite st⅔uctu⅔es Figu⅔e . Figu⅔e D shows the c⅔oss-sectional pictu⅔e of the CNT/”MI composite st⅔uctu⅔e obtained by focused ion beam. “lthough individual CNTs we⅔e difficult to distin‐ guish, CNT bundles with small size we⅔e found pe⅔pendicula⅔ to the c⅔oss section and thei⅔ su⅔faces we⅔e all su⅔⅔ounded by ”MI ⅔esins. Obviously, the maximized inte⅔face contacts can allow the most efficient inte⅔facial st⅔ess t⅔ansfe⅔. Notice that the level of CNT agg⅔egation was within – nm. This means that the⅔e is still a big challenge to obtain the ideal composite st⅔uctu⅔e whe⅔e individual CNTs a⅔e aligned, highly packed, and unagg⅔egated. . . Electrical properties The ability to conduct elect⅔icity of a thin film is usually cha⅔acte⅔ized by sheet ⅔esistance o⅔ s⅓ua⅔e ⅔esistance, in units of ohms pe⅔ s⅓ua⅔e . The s⅓ua⅔e ⅔esistance of the as-p⅔oduced CNT web was . Ω s⅓− . “fte⅔ being imp⅔egnated with ”MI ⅔esins, the ⅔esistance dec⅔eased to . Ω s⅓− , as a ⅔esult of the enhanced densification. “fte⅔ being st⅔etched, the CNTs became aligned and the connection between CNT bundles we⅔e sepa⅔ated by the mat⅔ix. “s a ⅔esult, the ⅔esistance inc⅔eased to . and . Ω s⅓− befo⅔e and safe⅔ the cu⅔ing p⅔ocess. The elect⅔ical conductivity of the final film was ~ S cm− , about . o⅔ % of coppe⅔’s o⅔ stainless steel’s elect⅔ical conductivity.

. Conclusion High-volume f⅔action CNT composites have exhibited exciting mechanical and elect⅔ical pe⅔fo⅔mances due to the high utilization of the CNT’s int⅔insic p⅔ope⅔ties. The laye⅔-by-laye⅔ stacking of aligned CNT sheets and the st⅔etching of entangled CNT webs have p⅔ovided two diffe⅔ent solutions to ⅔ealize the high-st⅔ength CNT composite films. Conside⅔ing the easy

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agg⅔egation between CNTs, the bio-inspi⅔ed agg⅔egation cont⅔ol makes a big step to app⅔oach the ideal composite st⅔uctu⅔e whe⅔e the CNTs a⅔e highly packed, aligned, and unagg⅔egated. The highest tensile st⅔ength of the CNT/”MI composite film can be up to . GPa, much highe⅔ than the st⅔ength of ca⅔bon fibe⅔-⅔einfo⅔ced polyme⅔s. We anticipate that the p⅔esent fab⅔ica‐ tion method can be gene⅔alized fo⅔ developing multifunctional and sma⅔t nanocomposites.

Acknowledgements The autho⅔s thank financial suppo⅔ts f⅔om the National Natu⅔al Science Foundation of China , , , , , and the Youth Innovation P⅔omotion “ssociation of the Chinese “cademy of Sciences , G⅔ant to X.Z. .

Author details Xiaohua Zhang*, Xueping Yu, Jingna Zhao and Qingwen Li *“dd⅔ess all co⅔⅔espondence to xhzhang

@sinano.ac.cn

Key Labo⅔ato⅔y of Nano-Devices “pplications, Suzhou Institute of Nano-Tech Nano”ionics, Chinese “cademy of Sciences, Suzhou, China

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Chapter 4

Carbon Nanotube–Polymer Composites: Device Properties and Photovoltaic Applications T. Hosseini and N. Kouklin Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62692

Abstract This chapte⅔ p⅔ovides an in-depth cove⅔age of ⅔ecent advances in the a⅔eas of the development and cha⅔acte⅔ization of elect⅔o-optically active, device-g⅔ade ca⅔bon nanotube CNT –polyme⅔ blends. These new o⅔ganic–ino⅔ganic multifunctional nanocomposites sha⅔e many advanced cha⅔acte⅔istics which make them ideally suited fo⅔ indust⅔ial scale, high-th⅔oughput manufactu⅔ing of lightweight, flexible elect⅔onic, light switching and emitting as well as ene⅔gy ha⅔vesting devices of ext⅔emely low cost. The fundamental aspects and the physical mechanisms cont⅔olling light–matte⅔ inte⅔action, photo-conve⅔sion, and photo-gene⅔ated cha⅔ge-ca⅔⅔ie⅔ t⅔anspo⅔t in these nanotube–polyme⅔ composites as well as the influence of the p⅔ocessing conditions on the elect⅔onic p⅔ope⅔ties and device-⅔elated pe⅔fo⅔mances a⅔e fu⅔the⅔ ⅔eviewed and discussed. Keywords: Ca⅔bon nanotube, Polyme⅔, Composite, Exciton, Photovoltaic, Sola⅔ cell, Diode, Detecto⅔

. Introduction ”lends of conjugated polyme⅔s and high pe⅔fo⅔mance ca⅔bon-based nanosemiconducto⅔s a⅔e an eme⅔ging class of easy-to-fab⅔icate o⅔ganic–ino⅔ganic nanocomposite mate⅔ials with the potential to p⅔ofoundly influence many elect⅔onic device ma⅔ket segments, including optoe‐ lect⅔onics. Ext⅔ao⅔dina⅔y cha⅔acte⅔istics of ca⅔bon nanotubes CNTs and p⅔evalence of inte⅔facial ⅔egions and the nanoscopic phase a⅔e a sou⅔ce of d⅔astic change and gain in the optoelect⅔ical ⅔esponse of the polyme⅔ mat⅔ix that typically falls outside of classical scaling behav‐ io⅔ of conventional polyme⅔ composites. These novel nanocomposites and thei⅔ based devices

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can be fab⅔icated using ⅔oll-to-⅔oll techni⅓ues that makes them ideally suited to indust⅔ial scale, high-th⅔oughput manufactu⅔ing of lightweight, flexible elect⅔onic, light switching and emitting as well as ene⅔gy ha⅔vesting devices at ext⅔emely low cost [ – ]. Conjugated polyme⅔s exhibit elect⅔onic and light emission p⅔ope⅔ties that a⅔e simila⅔ to those of c⅔ystalline semiconducto⅔s and have been al⅔eady implemented in o⅔ganic optoelect⅔onic devices such as o⅔ganic light-emitting diodes OLEDs , switches, and o⅔ganic photovoltaic OPV cells [ ]. Inco⅔po⅔ating n-type dopants in the fo⅔m of metallic CNTs into p-type polyme⅔ mat⅔ix has been shown to g⅔eatly enhance pe⅔fo⅔mance of such OPV cells by inc⅔easing the ⅔ates of non-⅔adiative dissociation of excitons as well as cha⅔ge-ca⅔⅔ie⅔ collection efficiency. The fo⅔mation of optimally loaded netwo⅔ks of elect⅔ically conductive nanotube netwo⅔k in tu⅔n entails detailed the conside⅔ation of the influence of the p⅔ocess pa⅔amete⅔s on the physical cha⅔acte⅔istics and inte⅔action of the polyme⅔ with the nanotubes in a li⅓uid phase. “s the abso⅔ption coefficient of photosensitive polyme⅔s ⅔emains la⅔ge, light is typically abso⅔bed within a ve⅔y thin laye⅔, which d⅔astically benefits the efficiency-to-cost ⅔atio fo⅔ these cells [ , ]. The π-conjugation in polyme⅔s ⅔esults in an ene⅔gy sepa⅔ation of ~ – eV between the lowest unoccupied molecula⅔ o⅔bital LUMO and the highest occupied molecula⅔ o⅔bital HOMO . “s a ⅔esult, the light abso⅔ption–emission spect⅔um falls in the visible nea⅔inf⅔a⅔ed NIR spect⅔al ⅔ange that complements that of single-walled ca⅔bon nanotubes SWNTs , that is, the nea⅔ IR–UV [ , – ]. “n ab⅔upt, type-II band alignment between polyme⅔ mat⅔ix and ca⅔bon phase is ⅔e⅓ui⅔ed and can be ⅔ealized fo⅔ many nanotube–polyme⅔ composites to achieve sufficiently fast inte⅔facial cha⅔ge sepa⅔ation and p⅔onounced photo‐ voltaic effect [ , ]. “mong diffe⅔ent classes of nanomate⅔ials including semiconducto⅔ ⅓uantum dots and fulle⅔enes, SWNTs have been p⅔oven pa⅔ticula⅔ly suitable fo⅔ uses in OPV, photodetecto⅔, and light-emitting diode applications based on conjugated polyme⅔s because of thei⅔ la⅔ge aspect ⅔atio and ⅔ema⅔kable optoelect⅔onic p⅔ope⅔ties including bandgap tunability, st⅔ong optical abso⅔ptivity, ballistic t⅔anspo⅔t, solution-p⅔ocessability, and excellent chemical stability [ , ]. Owing to thei⅔ ⅓uasi-one-dimensional st⅔uctu⅔e and imp⅔oved t⅔anspo⅔t cha⅔acte⅔istics, a class of SWNTs has been confi⅔med to exhibit many favo⅔able device functionalities which make them att⅔active fo⅔ application in a va⅔iety nano-elect⅔onic and mechanical devices and systems, among which a⅔e inte⅔connects, ⅔ectifies, field-effect t⅔ansisto⅔s, analyte, and light senso⅔s. Compa⅔ed with othe⅔ nanost⅔uctu⅔es, SWNTs a⅔e also known to exhibit st⅔ong multi⅔ange abso⅔ption in pa⅔t associated with ⅔esonance-type inte⅔band elect⅔onic t⅔ansitions e.g., S , M , S as well as f⅔ee ca⅔⅔ie⅔ and plasmonic excitations. Recent expe⅔iments fu⅔the⅔ confi⅔med on the p⅔esence of a st⅔ong photoconduction ⅔esponse in the inf⅔a⅔ed IR which can in tu⅔n affo⅔d many new oppo⅔tunities in enginee⅔ing nanophotovoltaic and optoelec‐ t⅔onic o⅔ganic polyme⅔–SWNT-based devices ope⅔ating ove⅔ multiple spect⅔al ⅔anges, including IR [ – ]. High conve⅔sion efficiencies of ~ and % we⅔e ext⅔acted in case of polyme⅔-based OPV cells featu⅔ing C molecules and CNTs, ⅔espectively. Yet, unlike to C , polyme⅔s inco⅔po⅔ating aligned CNTs demonst⅔ate much la⅔ge⅔ int⅔insic cha⅔ge mobility at lowe⅔ pe⅔colation th⅔esh‐ old/limit. “t the same time, the inc⅔eased photo-gene⅔ated cha⅔ge t⅔anspo⅔t and in tu⅔n

Carbon Nanotube–Polymer Composites: Device Properties and Photovoltaic Applications http://dx.doi.org/10.5772/62692

collection efficiency facilitate the development of OPV cells featu⅔ing la⅔ge⅔ light abso⅔ption thicke⅔ active device laye⅔ and elect⅔ical powe⅔ output, which t⅔anslates into ove⅔all highe⅔ efficiency-to-cost ⅔atio fo⅔ these cells. Combining SWNTs with elect⅔ooptically active polyme⅔s thus p⅔ovides an att⅔active ⅔oute to c⅔eating a new gene⅔ation of multifunctional device-g⅔ade o⅔ganic–ino⅔ganic elect⅔onic mate⅔ials fo⅔ uses as senso⅔s, OLEDs, PV cells, elect⅔omagnetic abso⅔be⅔s, and othe⅔ elect⅔onic devices [ – ]. In this chapte⅔, we ⅔eview the p⅔og⅔ess while focusing on the fundamental aspects behind the light–matte⅔ inte⅔action, photo-conve⅔sion, and photo-ca⅔⅔ie⅔ gene⅔ation as well cha⅔geca⅔⅔ie⅔ t⅔anspo⅔t in SWNT–polyme⅔ composites. The fab⅔ication, st⅔uctu⅔al–mechanical, and t⅔anspo⅔t cha⅔acte⅔istics of va⅔ious nanotubes–polyme⅔-based composites a⅔e ⅔eviewed in Section . Key photo-physical p⅔ocesses that take place at the inte⅔face between SWNT and polyme⅔ molecules including ene⅔gy t⅔ansfe⅔, exciton dissociation, cha⅔ge t⅔ansfe⅔, and ⅔elated effects a⅔e ⅔eviewed in Section . Section discusses the elect⅔onic and optoelect⅔onic devices built based on SWNT/polyme⅔ composites including OPV cells, light-emitting diodes, and IR senso⅔s.

. Carbon nanotubes/polymer composites: synthesis and properties Recent studies involving fab⅔ication and cha⅔acte⅔ization of st⅔uctu⅔al and unde⅔lying device cha⅔acte⅔izations have identified seve⅔al p⅔ocessing-⅔elated challenges pe⅔taining to p⅔oduc‐ ing polyme⅔/nanotubes composites of high pu⅔ity, st⅔uctu⅔al anisot⅔opy/alignment, and unifo⅔m dispe⅔sion [ , ]. ”ecause of the π-o⅔bitals of the sp -hyb⅔idized C atoms, CNTs show a tendency fo⅔ st⅔ong inte⅔molecula⅔ inte⅔action and spontaneous agg⅔egation van de⅔ Waals inte⅔action into la⅔ge diamete⅔ bundles that a⅔e not ⅔eadily dispe⅔sible in o⅔ganic solvents o⅔ polyme⅔ mat⅔ix. To add⅔ess the dispe⅔sion-⅔elated and mixing challenges, the use of su⅔factants [ – ], pe⅔fo⅔ming shea⅔ mixing [ – ], sidewall chemical modification [ , ], and in situ polyme⅔ization [ – ] we⅔e p⅔oposed. “mong all these st⅔ategies, covalent chemical functionalization and int⅔oduction of defects into SWNT su⅔faces have been p⅔oven highly effective in achieving stable SWNT suspensions in pola⅔ solvents as discussed below. . . Defect functionalization In defect functionalization, nanotubes a⅔e t⅔eated by oxidative methods that also help ⅔emove metal pa⅔ticles and amo⅔phous ca⅔bon deposits, that is, ⅔aise pu⅔ity. The ⅔esultant SWNTs oftentimes gain in localized su⅔face defect density most of which a⅔e in the fo⅔m as ca⅔boxyl, that is, –COOH attachments. Mawhinney et al. [ ] studied su⅔face defect site density of oxidatively t⅔eated SWNTs by p⅔obing the amounts of CO g and CO g ⅔eleased du⅔ing heating to up to K. The ⅔esults indicated that as much as ~ % of the ca⅔bon atoms in such SWNTs can be defect-associated. “cid–base tit⅔ation method [ ] yielded simila⅔ ⅔esults, that is, – % of acidic sites in pu⅔ified SWNTs. The density of defective sites c⅔eated at the su⅔faces by this method is viewed gene⅔ally insufficient fo⅔ good nanotubes dispe⅔sion in the polyme⅔ mat⅔ix. Howeve⅔, the st⅔ategy can be used fo⅔ covalent attachment of o⅔ganic g⅔oups by fi⅔st

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conve⅔ting them into acid chlo⅔ides that can be next linked to amines to fo⅔m amides. Such modified CNTs show significantly highe⅔ solubility in o⅔ganic solvents as compa⅔ed with unp⅔ocessed nanotubes [ ]. . . Non-covalent functionalization Non-covalent functionalization ⅔outes a⅔e of fu⅔the⅔ inte⅔est because they do not comp⅔omise the integ⅔ity of the nanotube backbone while helping imp⅔ove thei⅔ solubility and p⅔ocessa‐ bility. This type of functionalization is p⅔ima⅔ily done with the aid of su⅔factants, bio-mac⅔o‐ molecules, o⅔ non-covalent su⅔face attachments, such as w⅔apping sidewalls with polyme⅔s. Successful implementation of both cationic and anionic su⅔factants as well as nonionic su⅔factants has been demonst⅔ated in seve⅔al studies involving non-covalent functionalization of nanotubes via mic⅔oemulsion [ – ], ult⅔asonication [ ], sonication [ ], and emulsion polyme⅔ization [ – ]. ”iological mac⅔omolecules such as p⅔otein/DN“ and glucose [ , ] have been also linked to CNTs via dialysis [ ], elect⅔o-active inte⅔action [ ], and ult⅔asoni‐ cation [ ]. Poly -vinyl py⅔idine , poly phenyl acetylene , poly sty⅔ene –poly methac⅔ylic acid [ ], poly m-phenylenevinylene-co- , -dioctoxy-p-phenylenevinylene [ ], and poly[ methoxy- - -ethylhexyloxy , -phenylene vinylene] [ ], have been ⅔epo⅔ted to noncovalently w⅔ap nanotubes using sol–gel chemist⅔y, solution mixing, and immobilization methods, Figure .

Figure . a Scheme fo⅔ the fo⅔mation of MEH-PPV/ca⅔bon nanotubes composites, b top-view SEM image of the MEH-PPV encased nanotubes films fab⅔icated by immobilization method. The lowe⅔ inset shows a digital photo of the suspensions of the SWNTs and MEH-PPV–SWNTs.

Carbon Nanotube–Polymer Composites: Device Properties and Photovoltaic Applications http://dx.doi.org/10.5772/62692

. . Covalent functionalization Despite the fact that sp -hyb⅔idized C atoms fo⅔m a chemically stable backbone, a numbe⅔ of st⅔ategies we⅔e developed to covalently link chemical g⅔oups to CNTs [ – ]. In the case of covalent functionalization, the t⅔anslational symmet⅔y of nanotubes is dis⅔upted by changing sp ca⅔bon atoms to sp ca⅔bon atoms that we⅔e ⅔epo⅔ted to affect the elect⅔onic and t⅔anspo⅔t p⅔ope⅔ties of nanotubes [ , ]. This ⅔oute is highly effective in inc⅔easing solubility as well as dispe⅔sion of nanotubes in many o⅔ganic solvents as well as polyme⅔s. Covalent function‐ alization can be accomplished eithe⅔ by the modification of su⅔face-bound ca⅔boxyl g⅔oups on the nanotubes o⅔ by the di⅔ect elemental ⅔eaction with ca⅔bon atoms such as in the case of CHx-modified nanotubes. Poly ε-cap⅔olactone [ , ], poly L-lactide [ , ], poly methyl methac⅔ylate [ – ], Polysty⅔ene [ – ], poly N-isop⅔opyl ac⅔ylamide [ – ], polyimide [ , ], polyvinyl acetate-co-vinyl alcohol [ ], have been used to covalently attach to CNTs. F⅔om the standpoint of device application, non-covalent functionalization ⅔emains p⅔efe⅔⅔ed ove⅔ the covalent app⅔oach, as the latte⅔ has the p⅔opensity to induce st⅔ong st⅔uctu⅔al damage [ , ]. The dispe⅔sion of nanotubes in polyme⅔ mat⅔ices is one of the most c⅔itical bottlenecks in the p⅔epa⅔ation of CNTs/polyme⅔ composites. “dditional st⅔ategies to enhance dispe⅔sion of nanotubes included melt mixing and in situ polyme⅔ization, whe⅔eas Ni et al. confi⅔med conside⅔able imp⅔ovement in the dispe⅔sion of multi-walled CNTs in poly vinyl alcohol PV“ mat⅔ix th⅔ough gum “⅔abic t⅔eatment [ ].

. Photo-physical properties of carbon nanotube–polymer composites . . Energy transfer in carbon nanotube–polymer composites “bso⅔ption of a photon by a⅔omatic polyme⅔s leads to a fo⅔mation of bound elect⅔on–hole pai⅔ known as exciton, which can dissociate ⅔adiatively by emitting a lowe⅔ ene⅔gy photon. The p⅔esence of semiconducting SWNTs has been shown to st⅔ongly affect the ⅔ate of ⅔adiative ⅔ecombination by inducing the t⅔ansfe⅔ of eithe⅔ holes o⅔ elect⅔ons to the nanotubes which depends on the elect⅔onic band alignment between SWNTs and polyme⅔ [ ]. “lte⅔natively, ⅔esonant ene⅔gy t⅔ansfe⅔ f⅔om polyme⅔s to SWNTs has been confi⅔med expe⅔imentally [ , ]. In Umeyama et al. [ ] study, a conjugated polyme⅔, poly [ p-phenylene- , -vinylene -cop-phenylene- , -vinylidene ] coPPV , was synthesized and used to study the influence of SWNTs on the light emission cha⅔acte⅔istics of the fo⅔me⅔. UV–vis–NIR abso⅔ption and “FM measu⅔ements ⅔evealed that SWNTs we⅔e dispe⅔sed well in o⅔ganic solvents likely via π–π inte⅔action. The composite solution of coPPV–SWNTs exhibited a st⅔ong NIR emission o⅔iginating f⅔om SWNT when the polyme⅔ was subject to a di⅔ect optical excitation with the light sou⅔ce ope⅔ating at ~ – nm. The efficiency and ⅔ate of the ene⅔gy t⅔ansfe⅔ f⅔om polyme⅔s to SWNTs have been shown to be st⅔ongly dependent on the polyme⅔ concent⅔ation/agg⅔egation on SWNTs [ , ]. Fu⅔the⅔ studies point to the polyme⅔ π-conjugation chain that gove⅔ns the ene⅔gy t⅔ansfe⅔ in the polyme⅔–SWNT system to ⅔emain mo⅔e extended compa⅔ed with that of the pu⅔e polyme⅔

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system [ ]. Massuyeau et al. [ ] studied ene⅔gy t⅔ansfe⅔ between the polyme⅔ and nanotubes by examining steady state PL spect⅔a of a se⅔ies of composite films containing both metallic and semiconducting nanotubes. The ⅔esults of these studies show that the⅔e is a substantial spect⅔al ove⅔lap of PL and optical abso⅔ption of SWNTs, which favo⅔s the Fö⅔ste⅔ ene⅔gy t⅔ansfe⅔ between polyme⅔ chains and CNTs. . . Charge transfer in carbon nanotube–polymer composites Combining CNTs with polyme⅔s offe⅔s an att⅔active ⅔oute not only to mechanically ⅔einfo⅔cing polyme⅔ films but also to enhancing polyme⅔s’ cha⅔ge t⅔anspo⅔t p⅔ope⅔ties and modifying elect⅔onic p⅔ope⅔ties th⅔ough a mo⅔phological modification o⅔ elect⅔onic coupling between the two [ ]. The effect of nanotube doping has been systematically investigated by embedding nanotube powde⅔s in the emission, elect⅔on t⅔anspo⅔t, and hole t⅔anspo⅔t laye⅔s of OLEDs [ ]. Such polyme⅔/nanotube composites have been successfully exploited fo⅔ va⅔ious applications including OPV [ – ], OLEDs [ ], and o⅔ganic field-effect t⅔ansisto⅔s [ , ]. “mong diffe⅔ent t⅔anspo⅔t models [ – ], pe⅔colation of the nanotube netwo⅔k within the polyme⅔ mat⅔ix has been suggested to play a p⅔ima⅔y ⅔ole behind imp⅔oved cha⅔ge mobility of up to two o⅔de⅔s of magnitude compa⅔ed with that in the p⅔istine polyme⅔. This p⅔ovides a techno‐ logically simple pathway to imp⅔oving the pe⅔fo⅔mance of o⅔ganic elect⅔onic and optoelec‐ t⅔onic devices, while keeping thei⅔ fab⅔ication costs as low as possible [ ]. The low dielect⅔ic constant of conjugated polyme⅔s ⅔esults in la⅔ge Coulomb inte⅔actions between cha⅔ge ca⅔⅔ie⅔s, inc⅔easing exciton binding ene⅔gy and photo-⅔esponse cha⅔acte⅔istics. The majo⅔ity of OPV devices ope⅔ate based on exciton dissociation at the inte⅔face fo⅔med by two dissimila⅔ mate⅔ials with a type-II band alignment that favo⅔s inte⅔facial cha⅔ge sepa⅔ation and fo⅔mation of f⅔ee pola⅔ons. If the ⅔ate of bound elect⅔on–hole pai⅔ sepa⅔ation is low, othe⅔, that is, ⅔adiative and non-⅔adiative ⅔ecombinations will p⅔evail, which is a p⅔ima⅔y ⅔eason behind efficiency loss. Inte⅔nal elect⅔ic fields at the polyme⅔–metal inte⅔faces inte⅔face dipoles o⅔ dissociation cente⅔s, fo⅔ instance, oxygen impu⅔ities that can act as elect⅔on t⅔aps monop‐ oles p⅔omote fast exciton dissociation. “s the elect⅔on affinity ⅔emains smalle⅔ fo⅔ conjugated polyme⅔s [ ], pe⅔colated CNTs act as high mobility elect⅔on ext⅔action paths o⅔ excitonic antennas. Even at low doping levels, highly conductive pathways can be still established due to a la⅔ge aspect ⅔atio and p⅔opensity of SWNT to bundling. While photo-gene⅔ated elect⅔ons will tend to t⅔ansfe⅔ to SWNT, the photo-gene⅔ated holes a⅔e to ⅔emain in the polyme⅔ mat⅔ix that helps to lowe⅔ the ⅔ate of inte⅔nal ⅔ecombinations and to mitigate cha⅔ge-ca⅔⅔ie⅔ losses [ , ]. The fi⅔st solid evidence of the cha⅔ge t⅔ansfe⅔ between SWCNTs and conjugated polyme⅔s MEH-PPV was p⅔ovided by Yang et al. [ ] by pe⅔fo⅔ming photoinduced abso⅔ption spect⅔oscopy. In thei⅔ study, photoinduced cha⅔ge t⅔ansfe⅔ was deduced by obse⅔ving a ⅔eduction of the emission f⅔om the polyme⅔ accompanied by an inc⅔ease of the pola⅔on peak in the MEH-PPV-SWCNT hyb⅔ids. ”indl et al. [ ] examined exciton dissociation and cha⅔ge t⅔ansfe⅔ at s-SWCNT hete⅔ojunction fo⅔med with a⅔chetypical polyme⅔ic photovoltaic mate⅔ials including fulle⅔enes, poly thiophene , and poly phenylenevinylene using an exciton dissociation-sensitive photo-capacito⅔ measu⅔ement techni⅓ue that is advantageously

Carbon Nanotube–Polymer Composites: Device Properties and Photovoltaic Applications http://dx.doi.org/10.5772/62692

insensitive to optically induced the⅔mal photoconductive effects. It was found that fulle⅔ene and polythiophene de⅔ivatives induce exciton dissociation ⅔esulting in elect⅔on and hole t⅔ansfe⅔ away f⅔om optically excited s-SWCNTs. Significantly weake⅔ and almost no cha⅔ge t⅔ansfe⅔ was obse⅔ved fo⅔ la⅔ge bandgap polyme⅔s la⅔gely due to insufficient ene⅔gy band offsets. In anothe⅔ study, Ham et al. [ ] fab⅔icated a plana⅔ nano-hete⅔ojunction comp⅔ising wellisolated millimete⅔-long SNWTs placed unde⅔neath a poly -hexylthiophene P HT laye⅔. The ⅔esulting junctions displayed photovoltaic efficiencies pe⅔ nanotube in the ⅔ange of – %, which exceeded those of polyme⅔/nanotube bulk hete⅔ojunctions by almost two o⅔de⅔s of magnitude. The inc⅔ease was att⅔ibuted to an absence of agg⅔egates in case of the plana⅔ device geomet⅔y. It was shown that the polyme⅔/nanotube inte⅔face itself can be ⅔esponsible fo⅔ the exciton dissociation with the best efficiency ⅔ealized fo⅔ ~ nm thick P HT laye⅔. “mong diffe⅔ent classes of nanomate⅔ials, semiconducting CNTs ⅔emain the p⅔ima⅔y candi‐ dates to enhance the cha⅔ge sepa⅔ation when inte⅔faced with conjugated polyme⅔s. The diffe⅔ence in the behavio⅔ of semiconducting and metallic CNTs in polyme⅔ was studied theo⅔etically by Kanai et al. [ ] who employed a density functional theo⅔y. Case studies involving poly- -hexylthiophene P HT inte⅔faced with semiconducting and metallic CNTs we⅔e ca⅔⅔ied out. In case of semiconducting nanotubes, the theo⅔y p⅔edicts a fo⅔mation of typeII hete⅔ojunction, c⅔itical to photovoltaic applications. In cont⅔ast, in case of the metallic nanotubes, substantial cha⅔ge ⅔edist⅔ibution occu⅔⅔ed and the built-in-potential was ⅓uite small, whe⅔eas P HT became elect⅔ostatically mo⅔e att⅔active fo⅔ elect⅔ons. These obse⅔vations confi⅔m that in case of mixed single-walled nanotubes, a majo⅔ity of inte⅔faces would be made by metallic components to comp⅔omise the device pe⅔fo⅔mance. Simila⅔ conclusions we⅔e d⅔awn by Holt et al. [ ] in his study of P HT-polyme⅔/SWNT blends containing va⅔ying ⅔atios of metallic to semiconducting SWNTs.

. Electronic and optoelectronic applications of carbon nanotube/polymer composites . . Organic photovoltaic devices OPV devices based on π-conjugated polyme⅔s have been suggested as low-cost alte⅔natives to silicon-based sola⅔ cells [ , ]. Unlike to ene⅔gy conve⅔sion devices based on semicon‐ ducto⅔s, in o⅔ganic sola⅔ cell devices, a dono⅔/accepto⅔ D/“ inte⅔face is ⅔e⅓ui⅔ed to b⅔eak f⅔ee photo-gene⅔ated excitons into f⅔ee cha⅔ges ca⅔⅔ies befo⅔e they can be collected by the elect⅔odes [ , ]. The list of the ⅔e⅓ui⅔ements fo⅔ the mate⅔ials fo⅔ application in bulk PV devices includes the following st⅔ong light abso⅔ption ove⅔ the whole sola⅔ emission spect⅔um sufficient sepa⅔ation between HOMO and LUMO la⅔ge elect⅔on and hole mobilities within the device active laye⅔ and low device fab⅔ication cost [ , ]. In addition to a detailed conside⅔ation of int⅔insic elect⅔onic aspects of the constituent components, geomet⅔ic aspects and chemical stability play e⅓ually impo⅔tant ⅔ole. Fo⅔ example, the dimensions of active laye⅔ should not exceed the exciton diffusion length, ⅔epo⅔tedly on the o⅔de⅔ of ~ nm [ , ].

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In CNTs/polyme⅔ photovoltaic devices, the dissociation of excitons can be accomplished th⅔ough the fo⅔mation of a stagge⅔ed gap dono⅔/accepto⅔, type-II hete⅔ojunction fo⅔med between the s-SWCNTs and the polyme⅔ in which the ene⅔gy offsets at the hete⅔o-inte⅔face exceed the exciton binding ene⅔gy, E”. Recent expe⅔imental and theo⅔etical studies by Schuett‐ fo⅔t [ ] and Kanai [ ], ⅔espectively, demonst⅔ate that a type-II band alignment only exists fo⅔ ce⅔tain inte⅔faces, such as between small diamete⅔ semiconducting SWNTs and P HT. Even fo⅔ such blends, ene⅔gy t⅔ansfe⅔ f⅔om the polyme⅔ to SWNTs ⅔emains one of the fastest deexcitation channels that compete with the cha⅔ge t⅔ansfe⅔ p⅔ocesses, with the fo⅔me⅔ facilitated by la⅔ge⅔ su⅔face a⅔ea and elect⅔on affinity of the nanotubes vs. polyme⅔s [ , ]. Kymakis et al. [ ] examined both da⅔k and photocu⅔⅔ent–voltage J–V cha⅔acte⅔istics of poly -octylthiophene P OT /SWNT composite photovoltaic cells as a function of SWNT concent⅔ation. “n open-ci⅔cuit voltage VOC as high as . V was obtained fo⅔ % doped SWNTs/ P OT composite which se⅔ved as a device active laye⅔. “n almost -fold inc⅔ease in the photo-⅔esponse was pa⅔tly att⅔ibuted to a -fold inc⅔ease in the hole mobility due to a ⅔eduction in the density of the localized states in P OT mat⅔ix, and in pa⅔t due to enhanced exciton ext⅔action at the polyme⅔/nanotube junctions. Despite the imp⅔ovement in the ⅔ate of the cha⅔ge sepa⅔ation, the powe⅔ conve⅔sion efficiency was only . % unde⅔ mW/cm illumination conditions. “ poo⅔ dispe⅔sion of SWNT and the p⅔esence of a mixtu⅔e of metallic and semiconducting tubes we⅔e believed the p⅔ima⅔y facto⅔s behind the low efficiency numbe⅔s. In , the same g⅔oup investigated the use of spin-coated SWNTs as a hole t⅔anspo⅔t laye⅔ HTL in o⅔ganic bulk hete⅔ojunction photovoltaic devices shown schematically in Figure to ⅔aise the conve⅔sion efficiency [ ]. Va⅔ying thickness SWNT films we⅔e ⅔epetitively spin coated with dichlo⅔oethane and next evaluated as the HTL in P HT and - -methoxy-ca⅔bonyl -p⅔opyl- -phenyl- , C PC”M photovoltaic devices. It was shown that inse⅔tion of ~ -nm-thick SWNT laye⅔ led to powe⅔ conve⅔sion efficiencies as high as . %, compa⅔ed with . and . % fo⅔ the devices without and with the t⅔aditional PEDOT PSS acting as the HTL. The imp⅔oved efficiency was att⅔ibuted to imp⅔oved hole t⅔anspo⅔t in the polyme⅔ mat⅔ix due to a highe⅔ deg⅔ee of c⅔ystallinity p⅔ovided with SWNT.

Figure . a Schematic d⅔awings of the P HT PC”M photovoltaic cell with the SWNTs acting as the HTL. b Ene⅔gy level diag⅔ams of photovoltaic device components ⅔efe⅔enced to the vacuum level.

In anothe⅔ study, June et al. implemented homogeneously dispe⅔sed CNTs using alkyl-amide g⅔oups to chemically modify nanotubes to imp⅔ove thei⅔ dispe⅔sion in o⅔ganic medium [ ].

Carbon Nanotube–Polymer Composites: Device Properties and Photovoltaic Applications http://dx.doi.org/10.5772/62692

The ⅔esultant composites and thei⅔ based OPV cells exhibited gain in thei⅔ optical and elect⅔ical p⅔ope⅔ties with the device efficiency app⅔oaching ~ . %. The schematic of the fab⅔icated sola⅔ cell is shown in Figure .

Figure . Schematics of the functionalization of nanotubes with the alkyl-amide g⅔oup fo⅔ a homogeneous dispe⅔sion in o⅔ganic solvent and the PV devices fab⅔icated in [ ].

In most OPV cells that host nanotubes, the open-ci⅔cuit voltage Voc gene⅔ally stayed below V, anothe⅔ pe⅔fo⅔mance limiting facto⅔. Rodolfo et al. [ ] was able to ⅔aise Voc by ~ % by inse⅔ting continuous polyme⅔ laye⅔ between the elect⅔ode and SWNTs, which helped add⅔ess p⅔oblems with elect⅔ical sho⅔ting and shunts by the metallic tubes. Some p⅔io⅔ studies pointed out that uncont⅔olled inte⅔actions at the CNT–polyme⅔ inte⅔face can not only ⅔educe the ability of the tubes to t⅔anspo⅔t cha⅔ge but also inte⅔fe⅔e with the photophysical p⅔ocesses, which act as a sou⅔ce of ⅔ecombination cente⅔s fo⅔ excitons metallic tubes and ene⅔gy ⅓uenche⅔s polyme⅔–s-SWCNT , o⅔ by elect⅔ically sho⅔ting the ci⅔cuit long tubes . F⅔om the standpoint of device enginee⅔ing p⅔actices, a mo⅔e ⅔ational design of the CNTs– polyme⅔ inte⅔face ac⅔oss diffe⅔ent length scales, that is, nano to meso and ca⅔eful conside⅔ation/ cont⅔ol of inte⅔molecula⅔ level inte⅔actions via dispe⅔sion will be ⅔e⅓ui⅔ed [ , , ].

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On this f⅔ont, “⅔⅔anz-“nd⅔es and ”lau [ ] investigated the influence of the nanotube dimensions length and diamete⅔ and concent⅔ation on the pe⅔fo⅔mance of a CNT–polyme⅔ device. They found that adding % of nanotubes by weight inc⅔eased the powe⅔ conve⅔sion efficiency PCE by th⅔ee o⅔de⅔s of magnitude compa⅔ed with that of the native polyme⅔. The inco⅔po⅔ation of nanotubes into the P HT mat⅔ix favo⅔ably affected the ene⅔gy levels of the P HT and the mo⅔phology of the active laye⅔. They also found that the nanotubes can act as nucleation sites fo⅔ P HT chains, imp⅔oving cha⅔ge sepa⅔ation and elect⅔on t⅔anspo⅔t.

Figure . J–V cha⅔acte⅔istics of se⅔ies-connected inve⅔ted tandem sola⅔ cell. Tandem cell cu⅔ves ⅔ed , back cell blue , f⅔ont cell black .

The th⅔ee-component a⅔chitectu⅔es based on nanotubes-fulle⅔ene–conjugated polyme⅔ composites we⅔e p⅔oposed to achieve bette⅔ photovoltaic efficiencies. Li et al. [ ] suggested using C as an elect⅔on accepto⅔ and nanotubes fo⅔ the photo-gene⅔ated cha⅔ge t⅔anspo⅔t. Two types of chemically functionalized nanotubes we⅔e tested ca⅔boxylated and octadecylamine functionalized multi-walled nanotubes, in sho⅔t c-MWNT and o-MWNT. “ll th⅔ee photovol‐ taic pa⅔amete⅔s, namely sho⅔t-ci⅔cuit cu⅔⅔ent density, open-ci⅔cuit voltage, and fill facto⅔ of the P HT c-MWNT/C -based cells showed imp⅔ovements ove⅔ those of the P HT o-MWNT/ C cell as a ⅔esult of a faste⅔ elect⅔on t⅔ansfe⅔ f⅔om C to the nanotube backbone. De⅔balHabak et al. [ ] ⅔epo⅔ted o⅔ganic PV cells with powe⅔ conve⅔sion efficiency of . % by inco⅔po⅔ating functionalized SWNTs within P HT PC”M laye⅔ that helped imp⅔ove both the cu⅔⅔ent density Jsc and open-ci⅔cuit voltage, Voc att⅔ibuted to a pa⅔tial c⅔ystallization of the

Carbon Nanotube–Polymer Composites: Device Properties and Photovoltaic Applications http://dx.doi.org/10.5772/62692

RR-P HT as ⅔evealed by XRD studies. Nismy [ ] p⅔obed the optical and elect⅔onic ⅔esponse of the composite devices comp⅔ising dono⅔ polyme⅔ and localized MWNTs also featu⅔ing t⅔iple hete⅔ojunction a⅔chitectu⅔e/scheme. “ significant imp⅔ovement in photoluminescence ⅓uenching was obse⅔ved fo⅔ the devices with nanotubes embedded into the polyme⅔ mat⅔ix, with the fo⅔me⅔ facilitating the fo⅔mation of the t⅔ap states. The t⅔iple scheme is gene⅔ally confi⅔med to yield a lowe⅔ da⅔k cu⅔⅔ent and hence a significantly imp⅔oved photovoltaic pe⅔fo⅔mance with the PCE app⅔oaching ~ . %. Relatively high PCEs of ~ . % we⅔e demonst⅔ated by int⅔oducing coppe⅔-phthalocyanine de⅔ivative TSCuPc /SWNT laye⅔ into the se⅔ies-connected inve⅔ted tandem devices featu⅔ing f⅔ont P HT–IC”“ and back PC”M–PCDT”T active laye⅔s, Figure . “s summa⅔ized in Table , ⅔epeated ⅔esults f⅔om studies on CNTs/polyme⅔ OPV devices ⅔eveal that the pe⅔fo⅔mance of nanotubes inco⅔po⅔ated OPV cells is dependent on seve⅔al facto⅔s such as the device a⅔chitectu⅔e, t⅔eatment o⅔ functionalization method of nanotubes, type of CNTs, concent⅔ation of nanotubes as well as thickness of the nanotube-inco⅔po⅔ated active laye⅔. Type of CNT

Type of polymer

Preparation method

SWNT

P HT PC”M

Spin coating of su⅔factant-f⅔ee

PCE %

FF % References

.

.

[

]

. – .



[

.

[

.

[

.

[

]

[

]

[

]

[

]

CNTs as the hole t⅔anspo⅔t laye⅔ SWNT

P HT PC”M

Homogenous dispe⅔sion by

]

alkyl-amide functionalization of CNT SWNT

wt%

SWNT

P HT

Dispe⅔sion in chlo⅔ofo⅔m

RR-P HT

Solution mixing

. .

] ]

PC”M DWNT

P HT

Dispe⅔sion in chlo⅔ofo⅔m

.

double-walled CNT c-MWNT/C

P HT

Dispe⅔sion of ca⅔boxylated

.

nanotubes in o-dichlo⅔obenzene ODC” solutions MWCNT

P HT

Dispe⅔sion in chlo⅔ofo⅔m

O-MWCNTs

P HT, PC”M

Solution mixing

. .

Table . Summa⅔y of o⅔ganic photovoltaic devices ⅔eviewed in this chapte⅔

To ove⅔come p⅔oblems with poo⅔ pe⅔fo⅔mance of bi-laye⅔ devices that stem f⅔om sho⅔t exciton diffusion length in polyme⅔s, poo⅔ exciton dissociation and absence of a pe⅔colated netwo⅔k ⅔e⅓ui⅔ed fo⅔ imp⅔oved photo-gene⅔ated cha⅔ge t⅔anspo⅔t, the devices inco⅔po⅔ating polyme⅔fulle⅔ene-based dono⅔–accepto⅔ D–“ mate⅔ial have been ⅔econside⅔ed. Compa⅔ative studies

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on bulk hete⅔ojunction devices vs. those with a nanotube-inco⅔po⅔ated active laye⅔ fo⅔med by se⅓uential deposition show that the latte⅔ a⅔chitectu⅔e is p⅔one to a highe⅔ ⅔ecombination of ca⅔⅔ie⅔s due to the int⅔oduction of t⅔ap states associated with the nanotubes. Photo-gene⅔ated excitons a⅔e also ⅓uenched at the D/“ mate⅔ial inte⅔face due to these additional ene⅔gy levels and ⅔ende⅔ lowe⅔ Jsc values. On the othe⅔ hand, the hete⅔ojunction scheme yields lowe⅔ da⅔k cu⅔⅔ents and bette⅔ photovoltaic pe⅔fo⅔mance confi⅔ming a ve⅔y c⅔itical ⅔ole of the hete⅔ojunc‐ tion in devices with o⅔ganic/hyb⅔id a⅔chitectu⅔es. Fo⅔ the nanotube/polyme⅔-based OPV cells, the nanotube type is also to influence the pe⅔fo⅔mance of such devices. While the⅔e is no clea⅔ link between the numbe⅔ of walls o⅔ the diamete⅔ of the nanotubes and the pe⅔fo⅔mance of the OPV device, the semiconducting nanotubes we⅔e concluded to fo⅔m a needed, type-II hete⅔ojunction. In cont⅔ast, in case of metallic nanotubes, a substantial cha⅔ge ⅔edist⅔ibution is to take place at the inte⅔face. “s a ⅔esult, the built-in-potential is ⅓uite small and unlikely to cont⅔ibute significantly to the subse⅓uent cha⅔ge sepa⅔ation at this inte⅔face, leading to an inefficient PV device. The photovoltaic cha⅔acte⅔istics of the PV cells a⅔e also to depend on the concent⅔ation of nanotubes. In pa⅔ticula⅔, the inco⅔po⅔ation of low concent⅔ations of nanotubes in the photoactive laye⅔ leads to an inc⅔ease of the cu⅔⅔ent density Jsc. The functional g⅔oups as well as the p⅔epa⅔ation methodology a⅔e among the othe⅔ facto⅔s that we⅔e found to influence the pe⅔fo⅔mance of OPV cells. . . Organic light-emitting diodes OLED OLEDs a⅔e indispensible to flexible light displays because of thei⅔ excellent p⅔ope⅔ties They a⅔e lightweight and featu⅔e low powe⅔ consumption, wide angle of view, fast ⅔esponse, low ope⅔ational voltage, and excellent mechanical flexibility [ , ]. Light-emitting polyme⅔s demonst⅔ate excellent ⅓uantum efficiencies and can be solution p⅔ocessed to build elect⅔oluminescent devices of ve⅔y low cost. OLEDs a⅔e gene⅔ally conside⅔ed as dual-injected devices as holes and elect⅔ons a⅔e injected f⅔om the anode and cathode, ⅔espectively, into active molecula⅔/mac⅔omolecula⅔ medium, whe⅔e they fo⅔m excitons that ⅔ecombine ⅔adiatively [ , ]. Recent p⅔og⅔ess in OLEDs stems not only f⅔om the advancement of the polyme⅔ science but also f⅔om achieving bette⅔ cont⅔ol ove⅔ the cha⅔ge t⅔anspo⅔t in the elect⅔oluminescent laye⅔s and doping of the emissive mate⅔ials [ ]. “ p⅔ope⅔ laye⅔ se⅓uence in OLEDs ensu⅔es that the injected cha⅔ges a⅔e p⅔ope⅔ly balanced within the emissive laye⅔ to achieve high exte⅔nal efficiency. SWNTs int⅔oduced into conducting polyme⅔s lowe⅔ the cha⅔ge injection ba⅔⅔ie⅔ fo⅔med at the elect⅔ode–o⅔ganic inte⅔face and hence favo⅔ably affect the device pe⅔fo⅔mance [ ]. One of the fi⅔st studies to combine SWNT with conjugated polyme⅔-based OLEDs was attempted by Cu⅔⅔an et al. [ ]. The obse⅔ved inc⅔ease in the ⅓uantum yield was att⅔ibuted to inte⅔molecula⅔ π–π stacking inte⅔actions that take place between the polyme⅔ and nano‐ tubes. “ polyme⅔ stiffening is anothe⅔ facto⅔ that can lead to an inc⅔ease in the luminescence output. Mo⅔eove⅔, when SWNTs a⅔e added the st⅔ength of the polyme⅔–polyme⅔ inte⅔action becomes weake⅔, which is a sou⅔ce of self-⅓uenching effects. The concent⅔ation of SWNTs of % by weight is conside⅔ed optimal/sufficient fo⅔ the polyme⅔ st⅔ands to expe⅔ience inte⅔‐

Carbon Nanotube–Polymer Composites: Device Properties and Photovoltaic Applications http://dx.doi.org/10.5772/62692

action with the nanotubes. Excess concent⅔ations of SWNTs lead to a d⅔op in the luminescence. Woo et al. [ ] p⅔epa⅔ed double-emitting OLEDs DE-OLEDs based on SWNTs-PmPV. “ low bias I-Vs obtained on the devices made f⅔om the composites we⅔e ⅓uad⅔atic, while in the devices with pu⅔e PmPV, the dependence was significantly mo⅔e nonlinea⅔ I ~ V the ⅔esult was explained by the p⅔esence of st⅔uctu⅔al and chemical defects in the PmPV composite that favo⅔s continuous t⅔ap-limited cha⅔ge t⅔anspo⅔t. In a ⅔ecent study, Gwinne⅔ et al. [ ] investigated the influence of small amounts of semi‐ conducting SWNTs on cha⅔acte⅔istics of ambipola⅔ light-emitting field-effect t⅔ansisto⅔s LEFETs comp⅔ising polyfluo⅔enes such as poly , -di-n-octylfluo⅔ene-alt-benzothiadiazole F ”T and poly , -dioctylfluo⅔ene F -conjugated polyme⅔s, Figure . Inco⅔po⅔ating SWNTs within a semiconducting laye⅔ at the concent⅔ations below the pe⅔colation limit significantly augments both hole and elect⅔on injections, even fo⅔ a la⅔ge bandgap semicon‐ ducto⅔ such as F , without invoking a significant luminescence ⅓uenching. In gene⅔al, owning to lowe⅔ contact ⅔esistance and th⅔eshold voltage, la⅔ge⅔ ambipola⅔ cu⅔⅔ents and in tu⅔n highe⅔ output/light emissions can be ⅔ealized.

Figure . “ Schematic illust⅔ation of bottom contact/top gate polyme⅔ field-effect t⅔ansisto⅔ with ca⅔bon nanotubes dispe⅔sed in the semiconducting polyme⅔s F ”T and F . ” Ene⅔gy level diag⅔am of a semiconducting SWNT with , chi⅔ality, gold injecting elect⅔ode , and HOMO/LUMO levels of both F and F ”T. Rep⅔inted f⅔om [ ] with pe⅔‐ mission. Copy⅔ight © “me⅔ican Chemical Society.

Divya et al. [ ] investigated the use of a diketone ligand, , , , , -pentafluo⅔o- -hyd⅔oxy- phenanth⅔en- -yl pent- -en- -one Hpfppd , containing a polyfluo⅔inated alkyl g⅔oup, by covalently immobilizing it onto the multi-walled CNT host via ca⅔boxylic acid functionaliza‐ tion pathway. The ⅔esultant nanocomposite displayed intense ⅔ed emissions with an ove⅔all ⅓uantum yield of % unde⅔ a wide excitation ⅔ange f⅔om UV to visible ~ – nm , making it p⅔ime candidate fo⅔ application in OLEDs. Indium tin oxide ITO featu⅔es a high t⅔ansmittance at a low sheet ⅔esistance [ ] and is ubi⅓uitously employed as an OLED anode but not without d⅔awbacks. ITO is b⅔ittle and can suffe⅔ f⅔om c⅔acks that lead to elect⅔ical sho⅔ting it can se⅔ve as a sou⅔ce of oxygen that diffuses into emissive laye⅔s, while it has insufficiently high wo⅔k function of ~ . eV [ , ].

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On this f⅔ont, SWNT sheets have been conside⅔ed as viable alte⅔native and we⅔e studied fo⅔ possible use as anodes in OLEDs, Figure [ ]. Some ⅔ecent p⅔ototypes exhibited b⅔ightness of ~ cd m− that was compa⅔able to that of OLED featu⅔ing ITO anodes.

Figure . a Schematic of the SWNT OLED device and b co⅔⅔esponding c⅔oss-sectional scanning elect⅔on mic⅔oscopy image at a b⅔oken edge taken at a ° angle f⅔om the su⅔face no⅔mal. Rep⅔inted f⅔om [ ] with pe⅔mission. Copy⅔ight © “me⅔ican Institute of Physics.

Zhang et al. [ ] showed a⅔c-discha⅔ge nanotubes we⅔e ove⅔whelmingly bette⅔ elect⅔odes than HiPCO-nanotube-based films in all of the c⅔itical aspects, including su⅔face ⅔oughness, sheet ⅔esistance, and t⅔anspa⅔ency. “⅔c-discha⅔ge nanotube films that we⅔e PEDOT passivated showed high su⅔face smoothness and featu⅔ed sheet ⅔esistance of ~ Ω/s⅓ at % t⅔anspa⅔‐ ency. Pa⅔ekh et al. [ ] was able to imp⅔ove the conductivity of t⅔anspa⅔ent SWNT thin films by t⅔eating the samples with nit⅔ic acid and thionyl chlo⅔ide. Geng et al. [ ] was able to achieve a fou⅔fold sheet conductance imp⅔ovement by exposing SWNT films p⅔oduced by sp⅔ay techni⅓ue to a nit⅔ic acid with the t⅔eated samples demonst⅔ating sheet ⅔esistance of ~ and Ω/s⅓, at and % t⅔ansmittance, ⅔espectively. To b⅔eak inte⅔dependence of the sheet conductance and the t⅔anspa⅔ency, a magnetic field was applied du⅔ing d⅔op-casting of SWNT–polyme⅔ films onto ITO-coated glass and ITO-coated PET subst⅔ates [ ]. This led to sample de-wetting and enhancement in the elect⅔ical conductivity of the films. Fo⅔ a function‐ alized SWNT–PEDOT PSS film fo⅔med on an ITO-coated PET subst⅔ate, a sheet ⅔esistance of Ω/s⅓ at % t⅔ansmittance was obtained. SWNT–PEDOT PSS composite devices fo⅔med on PET subst⅔ate we⅔e p⅔oposed as a way to combat the p⅔oblem, with the films featu⅔ing a sheet ⅔esistance of Ω/s⅓, and having a t⅔ansmittance of % at ~ nm. The ⅔atio of DC to optical

Carbon Nanotube–Polymer Composites: Device Properties and Photovoltaic Applications http://dx.doi.org/10.5772/62692

conductivity was highe⅔ fo⅔ composites with mass f⅔actions of wt% than fo⅔ nanotubes only films. Fo⅔ ~ -nm-thick composite filled with wt% a⅔c discha⅔ge nanotubes, this conductivity ⅔atio was maximized at σDC/σ P = , with the t⅔ansmittance at nm and sheet ⅔esistance of and Ω/s⅓, ⅔espectively. These composites also have excellent elect⅔ome‐ chanical stability, with < % ⅔esistance change ove⅔ bend cycles. “s outlined above, CNTs/polyme⅔ composites could be inco⅔po⅔ated into conducting poly‐ me⅔s as the buffe⅔ laye⅔, o⅔ in the fo⅔m of plain sheets as flexible anode elect⅔ode in OLEDs. The cha⅔acte⅔istics exhibited by the CNTs/polyme⅔ composite as the t⅔anspo⅔t laye⅔ in OLEDs have been obse⅔ved to change with the polyme⅔ system as influenced by the natu⅔e of the polyme⅔–nanotube inte⅔actions. “dditionally, nanotube sheets can se⅔ve as t⅔anspa⅔ent elect⅔odes in OLEDs which make them a viable alte⅔native to the conventional ITO elect⅔odes. . . Infrared sensors Infra f⅔om Latin means below thus, IR ⅔efe⅔s to a spect⅔al ⅔ange beyond the ⅔ed bounda⅔y of the visible elect⅔omagnetic spect⅔um, which co⅔⅔esponds app⅔oximately to ~ . μm. Since all objects emit IR ⅔adiation, the effect is known as a black body ⅔adiation, seeing in the da⅔k o⅔ th⅔ough obscu⅔ed conditions, by detecting the IR ene⅔gy emitted by objects is possible. IR imaging has the⅔efo⅔e become a co⅔ne⅔stone technology fo⅔ many milita⅔y and civilian applications including night vision, ta⅔get ac⅓uisition, su⅔veillance, and the⅔mal photovoltaic devices. ”iomedical imaging and light-activated the⅔apeutics ⅔ep⅔esent anothe⅔ c⅔itical a⅔ea that pa⅔ticula⅔ly benefits f⅔om high tissue t⅔anspa⅔ency to IR light. Despite a ⅔ecent p⅔og⅔ess in the field of IR sensing and imaging, high cost, ⅔e⅓ui⅔ement fo⅔ c⅔yogenic cooling, and spect⅔ally limited sensitivity still ⅔emain the main disadvantages of this technology today. Two p⅔ima⅔y methods of IR detection exist ene⅔gy and photon detection. Ene⅔gy detecto⅔s ⅔espond to tempe⅔atu⅔e changes gene⅔ated f⅔om incident IR ⅔adiation th⅔ough changes in mate⅔ial p⅔ope⅔ties. Ene⅔gy detecto⅔s, the well-known examples of which a⅔e bolomete⅔s, py⅔oelect⅔ic, and the⅔mopile detecto⅔s, a⅔e no⅔mally low cost and p⅔ima⅔ily used in singledetecto⅔ applications such applications include fi⅔e and motion detection systems as well as automatic light switches and ⅔emote the⅔momete⅔s. In cont⅔ast to ene⅔gy detecto⅔s, light inte⅔acts di⅔ectly with the semiconducto⅔s in photon detecto⅔s to gene⅔ate elect⅔ical ca⅔⅔ie⅔s. Mo⅔e specifically, incident light with ene⅔gy g⅔eate⅔ than o⅔ e⅓ual to the ene⅔gy gap of the semiconducto⅔ d⅔ives the semiconducto⅔ out of e⅓uilib⅔ium by gene⅔ating excess majo⅔ity elect⅔ical ca⅔⅔ie⅔s. This t⅔anslates into a change in the net ⅔esistance of the detecto⅔. The wellestablished examples of photon detecto⅔ mate⅔ials a⅔e lead sulfide PbS , lead selenide PbSe . Since these detecto⅔s do not function by changing tempe⅔atu⅔e, they ⅔espond much faste⅔ than ene⅔gy detecto⅔s and in p⅔inciple can be sensitive to a single photon, if used, fo⅔ instance, in conjunction with the eme⅔ging class of single elect⅔on devices. ”oth, inc⅔eased sensitivity and ⅔educed ⅔esponse time p⅔ovided with the use of small bandgap semiconducto⅔ mate⅔ials, have ⅔ecently led to the development of advanced and ve⅔y sophisticated IR detection systems, which a⅔e of high technological ⅔elevance today. The highe⅔ the tempe⅔atu⅔e of an object, the la⅔ge⅔ the amount of the⅔mal ⅔adiation it emits, while its peak intensity also shifts to a sho⅔te⅔ wavelength. The demonst⅔ated st⅔ong spect⅔al

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dependence of the⅔mal ⅔adiation on the tempe⅔atu⅔e, also known as a Wien’s law, necessitates the use of mate⅔ials with optimized sensitivity at multiple wavelengths fo⅔ two p⅔ima⅔y ⅔easons to inc⅔ease sensitivity and to enable highly selective milita⅔y/civilian ta⅔get identification and ac⅓uisition. Until ⅔ecently, the p⅔oblem was add⅔essed th⅔ough simultane‐ ous use of seve⅔al mate⅔ials with peak sensitivity co⅔⅔esponding to diffe⅔ent wavelengths. “s fab⅔ication and p⅔ocessing change d⅔amatically f⅔om one mate⅔ial system to anothe⅔, engi‐ nee⅔ing of wavelength-specific and ult⅔a-sensitive IR detecto⅔s cu⅔⅔ently ⅔emains uneconom‐ ical. “ ⅔ecent p⅔og⅔ess in the field of nanotechnologies, and in pa⅔ticula⅔, in the a⅔ea of nonlithog⅔aphic fab⅔ication of multi-functional nanomate⅔ials such as ⅓uantum wells, wi⅔es, dots and CNTs opens new oppo⅔tunities fo⅔ advancing IR sensing technology beyond today’s confines. Unlike semiconducto⅔ alloys, the effective ene⅔gy bandgap of nanomate⅔ials and pa⅔ticula⅔ly CNTs can be easily tailo⅔ed by simply changing thei⅔ size which enables engi‐ nee⅔ing of futu⅔e IR-devices with expected spect⅔al ⅔ange of ope⅔ation f⅔om ~ to ~ . μm i.e., f⅔om ~ . to eV . Fu⅔the⅔mo⅔e, as elect⅔on scatte⅔ing is supp⅔essed in mate⅔ials featu⅔ing one-dimensional elect⅔onic configu⅔ations, nanotube-based IR photo-detecto⅔s a⅔e expected to demonst⅔ate o⅔de⅔s of magnitude imp⅔oved sensitivity at ⅔oom tempe⅔atu⅔e as compa⅔ed with the detecto⅔s ope⅔ating on thin films o⅔ ⅓uantum wells. This p⅔ope⅔ty could potentially mitigate the ⅔e⅓ui⅔ement fo⅔ c⅔yogenic cooling cu⅔⅔ently implemented in most IR photon-type sensing devices. Fo⅔ IR-detection application, aligning of many nanotubes would be highly c⅔itical f⅔om two points of view to inc⅔ease packing density of nanotubes and thus device sensitivity and to ⅔ealize pola⅔ization sensitive IR optical devices. In cont⅔ast to conventional semiconducto⅔s, conjugated polyme⅔s p⅔ovide d⅔amatic benefits fo⅔ enginee⅔ing active optical nano-elect⅔onic and photonic devices this includes ⅔educed p⅔ocessing cost, excellent physical flexibility, and la⅔ge a⅔ea cove⅔age. Until now, application of polyme⅔s in elect⅔onic devices was p⅔ima⅔ily limited to a visible ⅔ange of elect⅔omagnetic spect⅔um [ , ]. While stability of most polyme⅔s ⅔ep⅔esents a ba⅔⅔ie⅔ to thei⅔ use as UV senso⅔s, extending thei⅔ use in the IR ⅔ange becomes possible by implementing CNTs fo⅔ both light abso⅔ption and f⅔ee ca⅔⅔ie⅔ gene⅔ation. The exciton dissociation ⅔ate can be inc⅔eased by int⅔oducing hete⅔ojunctions o⅔ applying exte⅔nal elect⅔ic fields. The fo⅔me⅔ can be ⅔ealized by inco⅔po⅔ating p-type nanotubes into ntype polyme⅔ mat⅔ix, such as PPy py⅔idine- , -diyl conjugated polyme⅔, which is also known to exhibit ⅔elatively high ⅔esistance to oxidation. Composites of CNTs/polyme⅔ featu⅔e ⅔elatively high abso⅔ption in a wide spect⅔al ⅔ange of ~ . – μm and an emissivity coefficient close to unity while. Mo⅔eove⅔, such composites a⅔e ⅔esistive to ha⅔d ⅔adiation damages and can wo⅔k in high magnetic fields [ ]. Unlike MWNTs and g⅔aphene which possesses featu⅔eless visible/NIR abso⅔ption, semiconducting SWNTs in pa⅔ticula⅔ exhibit st⅔ong and disc⅔ete abso⅔ption in the visible/NIR ⅔egion owing to fi⅔st optically active inte⅔band t⅔ansition S with its ene⅔gy scaling inve⅔sely p⅔opo⅔tional to the nanotube diamete⅔. Lu et al. [ ] ⅔epo⅔ted a ve⅔y la⅔ge photocu⅔⅔ent in the device comp⅔ising semiconducting single-walled ca⅔bon nanotube s-SWCNT /polyme⅔ with type-II inte⅔face,

Carbon Nanotube–Polymer Composites: Device Properties and Photovoltaic Applications http://dx.doi.org/10.5772/62692

Figure . The detecto⅔ featu⅔ed significantly enhanced NIR detectivity of ~

cm·Hz / /W,

which is compa⅔able to that of the many conventional uncooled IR senso⅔s, Figure .

Figure . a Diag⅔am of s-SWCNT/P HT nanohyb⅔id. b ”and st⅔uctu⅔e of the s-SWCNT/P HT type-II hete⅔ojunction. c “FM image of s-SWCNT/P HT. d Optical abso⅔bance spect⅔a of sSWCNT and s-SWCNT/P HT. Rep⅔inted f⅔om [ ] with pe⅔mission. Copy⅔ight © “me⅔ican Chemical Society.

Figure . a ”⅔ief diag⅔am of the elect⅔ical setup fo⅔ IR detections. b Rep⅔esentative V−I cu⅔ves of s-SWCNT/P HT in da⅔k and unde⅔ NIR illumination of mW/mm . The inset shows the voltage-biased V−I cu⅔ves of s-SWCNT cont⅔ol sample. c Diffe⅔ential conductance dI/dV of s-SWCNT/P HT The data we⅔e calculated f⅔om b . Rep⅔inted f⅔om [ ] with pe⅔mission. Copy⅔ight © “me⅔ican Chemical Society.

“mong othe⅔ composites, polyaniline–CNTs composite thin film senso⅔s showed an IR photosensitivity enhancement of mo⅔e than two o⅔de⅔s of magnitude unde⅔ ambient condi‐ tions [

]. The attained enhancement in the sensitivity bolomet⅔ic effect is att⅔ibuted to a

highe⅔ heat gene⅔ation by CNTs and la⅔ge tempe⅔atu⅔e dependence of the ⅔esistance of

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polyaniline. In anothe⅔ study, “liev [ ] built an uncooled bolomet⅔ic senso⅔ based on SWNTs/polyme⅔ composite with voltage ⅔esponsivity of ~ V/W. “nothe⅔, all-p⅔inted NIR senso⅔ was enginee⅔ed by Gohie⅔ et al. [ ] by depositing multi-walled CNTs on a flexible polyimide subst⅔ate the senso⅔ showed ult⅔a-high ⅔esponsivity of ~ . kV/W. “ st⅔ong dependence of the device ⅔esponse on the su⅔⅔ounding atmosphe⅔e was though noted and att⅔ibuted to deso⅔ption of wate⅔ molecules that negatively affected the photosensitivity. Glamazda et al. [ ] ⅔epo⅔ted on a st⅔ong bolomet⅔ic ⅔esponse in SWNT–polyme⅔ composite featu⅔ing highe⅔ deg⅔ee of inte⅔nal alignment. “ bette⅔ alignment d⅔amatically inc⅔eased the tempe⅔atu⅔e sensitivity of the ⅔esistance explained within the f⅔amewo⅔k of fluctuationinduced tunneling theo⅔y. “ spect⅔ally flat mid-IR ⅔esponsivity of V W− was obse⅔ved and is among the highest ⅔epo⅔ted fo⅔ nanotube-based bolomete⅔s.

Author details T. Hosseini and N. Kouklin* *“dd⅔ess all co⅔⅔espondence to [email protected] Depa⅔tments of Elect⅔ical Enginee⅔ing Compute⅔ Science, Unive⅔sity of WisconsinMilwaukee, Milwaukee, US“

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Chapter 5

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes Gülşen Akın Evingür and Önder Pekcan Additional information is available at the end of the chapter http://dx.doi.org/10.5772/63054

Abstract Th⅔ee-dimensional netwo⅔ks can be hyd⅔ophilic and/o⅔ hyg⅔oscopic. Optical, mechani‐ cal, and elect⅔ical p⅔ope⅔ties of these mate⅔ials encompass many fields of technology. Composites of ca⅔bon nanotubes CNTs in polyme⅔ic mate⅔ials have att⅔acted consid‐ e⅔able attention in the ⅔esea⅔ch and indust⅔ial communities due to thei⅔ uni⅓ue optical, mechanical, and elect⅔ical p⅔ope⅔ties. CNT polyme⅔ nanocomposites possess high stiffness, high st⅔ength, and good elect⅔ical conductivity at ⅔elatively low concent⅔a‐ tions of CNT fille⅔. He⅔e, in this chapte⅔, we su⅔vey the optical, mechanical, and elect⅔ical mechanisms fo⅔ va⅔ious polyme⅔ic systems. Composite gels and films we⅔e p⅔epa⅔ed with va⅔ious mola⅔ pe⅔centages of multiwalled ca⅔bon nanotubes MWCNTs . The optical, mechanical, and elect⅔ical behavio⅔s of va⅔ious composite gels doped by MWCNT a⅔e also discussed in the each sections of the chapte⅔. The optical behavio⅔s of the compo‐ sites we⅔e pe⅔fo⅔med by the UV-Vis spect⅔oscopy and fluo⅔escence spect⅔oscopy in the fi⅔st pa⅔t of the chapte⅔. On the othe⅔ hand, comp⅔essive testing techni⅓ue and ⅔heologi‐ cal measu⅔ements we⅔e employed to dete⅔mine the va⅔iations of mechanical p⅔ope⅔ties of the composites in the second pa⅔t of the chapte⅔. Lastly, we ⅔eview the elect⅔ical p⅔ope⅔ties of the composites imp⅔oved significantly by addition of MWCNTs ⅔esea⅔ches. Keywords: multiwalled ca⅔bon nanotubes MWCNTs , d⅔ying, swelling, elasticity, conductivity, polyac⅔ylamide P““m , polysty⅔ene PS , poly vinyl acetate-co-butyl ac⅔ylate P V“c-co-”u“ , latex, PET poly ethylene te⅔ephthalate

. Introduction Polyme⅔-ca⅔bon nanotube CNT , discove⅔ed by Iijima [ ], composites we⅔e studied by “jayan fi⅔stly. Ca⅔bon nanotubes and thei⅔ composites have many application a⅔eas such as batte⅔ies,

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flat panel sc⅔eens, senso⅔s, and nanop⅔obes [ ]. When compa⅔ing with diffe⅔ent mate⅔ials, nanotubes a⅔e st⅔onge⅔ and mo⅔e conductive than othe⅔s. The⅔efo⅔e, the⅔e a⅔e many studies on polyme⅔-CNT composites at last decade [ ]. Nanotubes can be desc⅔ibed as long and slende⅔ fulle⅔enes, in which the walls of the tubes a⅔e hexagonal ca⅔bon g⅔aphite st⅔uctu⅔e . These tubes can eithe⅔ be single-walled ca⅔bon nanotube SWCNT o⅔ multiwalled ca⅔bon nanotube MWCNT . N-vinylca⅔bazole NVC was polyme⅔ized in bulk o⅔ in toluene in the p⅔esence of MWCNT without any ext⅔aneous catalyst [ ]. “ composite of PNVC with MWCNT isolated f⅔om the polyme⅔ization system showed high dc conductivity va⅔ying f⅔om . to S cm− depending upon the extent of MWCNT loading in the composite. In situ polyme⅔ization and photophysical p⅔ope⅔ties of poly p-phenylene benzobisoxazole P”O /MWCNT composites we⅔e pe⅔fo⅔med by optical techni⅓ues such as UV-Vis abso⅔ption, photoluminescence [ ]. The investigation of UV-Vis abso⅔ption and fluo⅔escence emission spect⅔a exhibited that in situ P”O-MWCNT composite had a st⅔onge⅔ abso⅔bance and obvious t⅔end of ⅔ed-shift compa⅔ed with blend P”O/MWCNT composites fo⅔ all compositions. The cha⅔acte⅔ization of poly octylthiophene P OT /polysty⅔ene PS -MWCNT polyme⅔ hyb⅔id system pe⅔fo⅔med by Fou⅔ie⅔ t⅔ansfo⅔m inf⅔a⅔ed spect⅔oscopy, ult⅔aviolet UV -visible abso⅔ption, and elect⅔ical measu⅔ements shows significant effects [ ]. This study p⅔esents that the inco⅔po⅔ation of wt % functionalized MWCNT onto P OT/PS polyme⅔ hyb⅔id blend will conve⅔t this mate⅔ial f⅔om insulato⅔ to conducto⅔. Modulation of single-walled ca⅔bon nanotubes photoluminescence by hyd⅔ogel swelling demonst⅔ated that the shift of nanotubes photoluminescence occu⅔⅔ed in a hyd⅔ogel mat⅔ix. “s the hyd⅔ogel c⅔oss-linking density and hyd⅔ation state is changed, the nanotubes expe⅔ience lattice defo⅔mations and a shift in photoluminescence emission maxima [ ]. CNTs we⅔e added into poly vinyl alcohol PV“ hyd⅔ogels to modify thei⅔ mechanical p⅔ope⅔ties. F⅔eezing/thawing method was used fo⅔ p⅔epa⅔ing the hyd⅔ogels. The tensile modulus, tensile st⅔ength, and st⅔ain at b⅔eak of CNTP- . specimen with . % w/w CNTs a⅔e inc⅔eased by . , . , and . %, ⅔espectively, [ ]. Dual stimuli ⅔esponsive poly N,Ndiethylac⅔ylamide-co-ac⅔ylic acid composite hyd⅔ogels functionalized with MWCNTs we⅔e obtained. Swelling, deswelling, comp⅔ession p⅔ope⅔ties of the composites we⅔e g⅔eatly imp⅔oved [ ]. The ⅔elaxation p⅔ocess of the composites was modeled by Fickian diffusion mechanism. Synthesis, elect⅔ical, and mechanical p⅔ope⅔ties of polyethylene oxide PEO -MWCNT composites we⅔e investigated [ ]. The conductivity measu⅔ements on the PEO-MWCNT composite films with the highest concent⅔ation of MWCNT wt% showed an inc⅔ease of eight o⅔de⅔s . S cm− of magnitude in conductivity f⅔om ba⅔e PEO film. The elastic modulus and tensile st⅔ength of a PEO-MWCNT film we⅔e inc⅔eased by about fivefold and tenfold, ⅔espectively, as compa⅔ed to the co⅔⅔esponding values fo⅔ a PEO film. Polyvinyl alcohol PV“ -vapo⅔ g⅔owth ca⅔bon fibe⅔ VGCF and PV“-MWCNT we⅔e p⅔epa⅔ed by gelation/c⅔ystallization [ ]. The pe⅔colation th⅔eshold of elect⅔ical conductivity fo⅔ the PV“/MWCNT was < wt% MWCNT loading which was much lowe⅔ than that of PV“/VGCFs composites. The mechanical p⅔ope⅔ties of the PV“ composite films we⅔e significantly by adding VGCFs and MWCNTs. Poly ethylene te⅔ephthalate PET -MWCNT nanocomposites

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

we⅔e p⅔epa⅔ed by coagulation method [ ]. The pe⅔colation th⅔eshold . wt% fo⅔ ⅔heological p⅔ope⅔ty and . wt% fo⅔ elect⅔ical conductivity has been found. The less ⅔heological pe⅔co‐ lation th⅔eshold than elect⅔ical pe⅔colation th⅔eshold is mainly att⅔ibuted to the fact that a dense⅔ MWCNT netwo⅔k is ⅔e⅓ui⅔ed fo⅔ elect⅔ical conductivity, while a less dense MWCNT netwo⅔k sufficiently impedes PET chain mobility ⅔elated to the ⅔heological pe⅔colation th⅔eshold. The elect⅔ical and ⅔heological cha⅔acte⅔istics of poly vinyl acetate PV“c /multiwall ca⅔bon nanotube nanocomposites we⅔e investigated [ ]. Small amount of MWCNT was obse⅔ved to ⅔ema⅔kably dec⅔ease ⅔esistivity of the nanocomposites. The G′ and G inc⅔ease with the addition of MWCNT loading compa⅔ed with that of the PV“c mat⅔ix. The ⅔heological and conductivity th⅔eshold of semic⅔ystalline syndiotactic polysty⅔ene sPS composites filled with CNTs and ca⅔bon nanocapcules CNCs we⅔e dete⅔mined dynamic ⅔heological tests of samples in the melt state and f⅔om elect⅔ical tests in the solid state, ⅔espectively [ ]. The sPS composites filled with CNT with a highe⅔ aspect ⅔atio exhibited a lowe⅔ th⅔eshold than the CNC filled ones in both ⅔heological and conductivity pe⅔colation. The elect⅔ical and ⅔heological p⅔ope⅔ties of polyp⅔opylene PP -CNT composites we⅔e studied [ ]. Thus, the conductivity of the composites was inc⅔eased with CNT content and the content pe⅔colation th⅔eshold is between and wt% MWCNTs. Poly N-isop⅔oplac⅔ylamide PNIP““m containing single-walled ca⅔bons and single-walled nanoho⅔ns showed phase t⅔ansitions [ ]. P⅔epa⅔ation and cha⅔acte⅔ization of P““m/ MWCNTs monohyb⅔id hyd⅔ogels with mic⅔opo⅔ous st⅔uctu⅔es we⅔e p⅔esented by mechanical, pH, and tempe⅔atu⅔e sensitive ⅔esponse and swelling kinetics [ ]. The addition of nanotubes p⅔oduced inte⅔esting p⅔ope⅔ties, including tailo⅔ ability of tempe⅔atu⅔e ⅔esponsive swelling and mechanical st⅔ength of the PNIP““m-MWCNT composites. Polysty⅔ene PS -MWCNT composites have been widely studied and published in the lite⅔atu⅔e. The ⅔esults of Yu et al. [ ] yield an inc⅔ement in conductivities of such composites which we⅔e p⅔oduced by latex technology. Poly vinyl acetate PV“c is a non-c⅔ystalline, amo⅔phous the⅔moplastic poly‐ me⅔. The PV“c is gene⅔ally fab⅔icated via mixing p⅔ocess with ce⅔tain polyme⅔ic mate⅔ials in o⅔de⅔ to ⅔einfo⅔ce the st⅔uctu⅔al p⅔ope⅔ties [ , ]. PV“c-based composites a⅔e widely used in adhesive, pape⅔, emulsifie⅔, paint, and textile indust⅔ies due to its high-bond ⅔einfo⅔ced, film-like, nonflammable, and odo⅔less cha⅔acte⅔istics [ ]. In this chapte⅔, afte⅔ the int⅔oduction section, optical p⅔ope⅔ties of va⅔ious polyme⅔-CNTs composite will be given and then mechanical p⅔ope⅔ties of them which doped by MWCNTs will be p⅔ovided. In the late⅔ section, elect⅔ical p⅔ope⅔ties of them will be discussed.

. Optical properties of different polymer-CNTs composites Polyac⅔ylamide P““m -MWCNT composite gels we⅔e p⅔epa⅔ed by f⅔ee ⅔adical c⅔oss-linking copolyme⅔ization [ ]. P““m was doped with va⅔ious . – wt% of MWCNTs. ”efo⅔e d⅔ying was sta⅔ted [ ], composites we⅔e cut into discs with mm in diamete⅔ and mm in

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thickness f⅔om the injecto⅔. Disc-shaped gel samples we⅔e placed on the wall of a cm path length, s⅓ua⅔e ⅓ua⅔tz cell filled with ai⅔ and wate⅔ fo⅔ d⅔ying and swelling expe⅔iments, ⅔espectively. D⅔ying [ ] and swelling [ , ] p⅔ocess we⅔e pe⅔fo⅔med by a Model LS- spect⅔omete⅔ of Pe⅔kinElme⅔, e⅓uipped with tempe⅔atu⅔e cont⅔olle⅔. “ll measu⅔ements we⅔e made at ° position, and spect⅔al bandwidths we⅔e kept at nm. Py⅔anine as a fluo⅔escence p⅔obe in the composite gels was excited at nm du⅔ing in situ fluo⅔escence expe⅔iments, and emission intensities of the py⅔anine we⅔e monito⅔ed at nm as a function of d⅔ying, and swelling time, ⅔espectively. It was obse⅔ved that the fluo⅔escence intensity of py⅔anine inc⅔eased as d⅔ying time was inc⅔eased du⅔ing the d⅔ying p⅔ocess. ”y combining the Ste⅔n-Volme⅔ e⅓uation with the moving bounda⅔y model, wate⅔ deso⅔ption coefficients, D, we⅔e dete⅔mined fo⅔ the d⅔ying gels p⅔epa⅔ed with va⅔ious MWCNT content at diffe⅔ent tempe⅔atu⅔es.

Figure . Emission spect⅔a of py⅔anine f⅔om composites du⅔ing d⅔ying in ai⅔, fo⅔ . and at °C and in min [ ].

wt% MWCNT content gels

Figure shows the emission spect⅔a of py⅔anine f⅔om P““m-MWCNT composite gel with . and wt% MWCNT contents du⅔ing d⅔ying in ai⅔ at °C and in min. It can be seen that as the MWCNT content is inc⅔eased, fluo⅔escence intensity, Iem, dec⅔eases ⅔elative to the scatte⅔ed light intensity, Isc. Since the dec⅔ease in Isc co⅔⅔esponds to the dec⅔ease in tu⅔bidity of the d⅔ying gel [ ], the co⅔⅔ected fluo⅔escence intensity, I, was int⅔oduced as Iem/Isc to eliminate the tu⅔bidity effect. “s fa⅔ as the co⅔⅔ection of fluo⅔escence emission is conce⅔n, totally empi⅔ical fo⅔mula was int⅔oduced to p⅔oduce the meaningful ⅔esults fo⅔ the fluo⅔escence ⅓uenching mechanisms. In o⅔de⅔ to ⅓uantify these ⅔esults, a collisional type of ⅓uenching mechanism may be p⅔oposed fo⅔ the fluo⅔escence intensity, I, f⅔om the gel samples du⅔ing the d⅔ying p⅔ocess using E⅓. [ ]. æ I ö u W = ç1 - ÷ è I 0 ø k qt 0

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

= ns is known fo⅔ py⅔anine [ ] so W can be calculated using E⅓. values, in each d⅔ying step.

and the measu⅔ed I

Figure . The plots of the wate⅔ ⅔elease, W ve⅔sus d⅔ying time, t fo⅔ P““m-MWCNT composite gel d⅔ied in ai⅔ meas‐ u⅔ed by fluo⅔escence techni⅓ue fo⅔ . and wt% MWCNT content samples at °C [ ].

Figure p⅔esents the WI ve⅔sus d⅔ying time. The plots of W ve⅔sus t / fo⅔ . and wt% MWCNT content sample at °C a⅔e p⅔esented in Figure , whe⅔e the fit of the data to E⅓. p⅔oduced the deso⅔ption coefficients, DdI which a⅔e plotted in Figure . WI éD ù = 2 ê dI2 ú WIf ëp a û

1/ 2

t1/ 2

Figure . Deso⅔ption diffusion coefficients, DdI ve⅔sus wt% MWCNT content measu⅔ed by fluo⅔escence techni⅓ue at and °C [ ].

Fluo⅔escence techni⅓ue measu⅔es the DdI values at a molecula⅔ level. The deso⅔ption coeffi‐ cients, DdI, we⅔e obtained f⅔om E⅓. and measu⅔ed by fluo⅔escence techni⅓ue fo⅔ va⅔ious

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MWCNT content samples, whe⅔e it was obse⅔ved that the deso⅔ption coefficient dec⅔eased as the MWCNT content is inc⅔eased up to wt% MWCNT and ⅔eached to a plateau by p⅔esenting the diffe⅔ent behavio⅔s below and above the c⅔itical MWCNT wt% content at which the conducting pe⅔colation cluste⅔ sta⅔ts to appea⅔ [ ]. Monito⅔ing of swelling expe⅔iments shows that emission light intensity, Iem, was dec⅔eased which is modeled by the Ste⅔n-Volme⅔ e⅓uation. The Li-Tanaka e⅓uation was used to dete⅔‐ mine the swelling time constants, S, and coope⅔ative diffusion coefficients, DSI, f⅔om fluo⅔es‐ cence intensity Table . Figure p⅔esents the fluo⅔escence spect⅔a of py⅔anine f⅔om the P““m-MWCNT composite du⅔ing the swelling p⅔ocess in pu⅔e wate⅔ at °C fo⅔ and wt% MWCNT at min, ⅔espectively. When the wate⅔ uptake is inc⅔eased, the emission light intensity, Iem, was dec⅔eased and the scatte⅔ed light intensity, Isc, was inc⅔eased, ⅔espectively, because of the ⅓uenching of excited py⅔anines and tu⅔bidity.

Figure . Fluo⅔escence spect⅔a of py⅔anine f⅔om the composite du⅔ing the swelling p⅔ocess at MWCNT content samples at min [ ].

°C fo⅔

and

wt%

To p⅔oduce the meaningful ⅔esults fo⅔ the fluo⅔escence ⅓uenching mechanisms, empi⅔ical e⅓uation was int⅔oduced [ – ]. The co⅔⅔ected fluo⅔escence intensity, I, is p⅔oduced by dividing emission light intensity, Iem, to scatte⅔ing intensity, Isc, to exclude the effect of tu⅔bidity. On the othe⅔ hand, when ⅓uenching of excited py⅔anines inc⅔eased, the swelling time, t, inc⅔eased. This behavio⅔ of the composites was modeled by Ste⅔n-Volme⅔ Model have been p⅔oposed using E⅓. [ ].

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

DdI ×



m /s

τS min

.

DSI ×



m /s

. . .

.

. .

.

. .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

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.

.

.

.

.

.

.

.

.

.

.

.

.

.

. .

.

.

. .

.

.

.

.

.

.

.

. .

.

.

. .

.

.

.

.

.

.

.

.

.

Table . Expe⅔imentally measu⅔ed pa⅔amete⅔s of P““m-MWCNT composites fo⅔ va⅔ious tempe⅔atu⅔e and wt% MWCNT contents du⅔ing d⅔ying and swelling p⅔ocesses, ⅔espectively.

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Figure . The plots of fluo⅔escence data using E⅓. ve⅔sus swelling time, t, fo⅔ P““m-MWCNT composite gels swol‐ len in wate⅔ measu⅔ed by fluo⅔escence techni⅓ue fo⅔ . and wt% MWCNT content sample at °C, ⅔espectively.

Plots of wate⅔ uptake, W, ve⅔sus swelling time a⅔e p⅔esented in Figure fo⅔ . and wt% MWCNT content samples at °C, ⅔espectively. These a⅔e typical solvent uptake cu⅔ves, obeying the Li-Tanaka e⅓uation is E⅓. [ – ]. W = 1 - B1 exp(-t / t S ) Wf The loga⅔ithmic fo⅔m of the data was fitted to the following ⅔elation p⅔oduced f⅔om E⅓. ln(1 -

W t ) = ln B1 ts Wf

whe⅔e S is the time constant which was calculated f⅔om linea⅔ ⅔eg⅔ession of the cu⅔ves in Figure and E⅓. desc⅔ibed by Li and Tanaka [ ]. DSI =

3a 2f

t Sa I2

Using E⅓. , coope⅔ative diffusion coefficients DSI we⅔e dete⅔mined fo⅔ these disc-shaped composites and found to be a⅔ound − m /s. “ll DSI values fi⅔st inc⅔eased up to wt% MWCNT content and ⅔eached its highest value at this c⅔itical point, whe⅔e the pe⅔colation cluste⅔ f⅔om MWCNT sta⅔ts to fo⅔m as given in Figure and Table .

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

Figure . Coope⅔ative diffusion coefficient, DSI, ve⅔sus MWCNT content measu⅔ed by fluo⅔escence methods at °C, ⅔espectively [ ].

and

The pe⅔colation cluste⅔ fo⅔med f⅔om CNTs helps wate⅔ molecules flow faste⅔ in thei⅔ channels and causes the composite gel swell faste⅔ p⅔esenting la⅔ge DSI values fo⅔ all samples unde⅔ conside⅔ation. Howeve⅔, above the c⅔itical point wt% MWCNT , composite gel is ⅓uite stiff due to the fo⅔mation of infinite netwo⅔k f⅔om MWCNT. The fo⅔mation of inelastic composite gel above the c⅔itical point then lowe⅔s the DSI values to the smalle⅔ numbe⅔s. The ⅔esults we⅔e inte⅔p⅔eted in te⅔ms of the swelling time constants s, dec⅔eased and the coope⅔ative diffusion coefficient, DSI, inc⅔eased ve⅔sus wt% MWCNT content. It was obse⅔ved that high MWCNT content composites swell much slowe⅔ p⅔oducing smalle⅔ DSI coefficients fo⅔ all measu⅔ements at a given tempe⅔atu⅔e. The effect of MWCNT addition in insulating PS mat⅔ix was investigated [ ]. Composite films we⅔e p⅔epa⅔ed using polysty⅔ene . % w/v with va⅔ious . – wt% of MWCNTs stock content on . × . cm glass plates at ⅔oom tempe⅔atu⅔e. Va⅔iations in optical p⅔ope⅔ties of ve⅔y-thin PS-MWCNT composite films we⅔e measu⅔ed using photon t⅔ansmission techni⅓ues. Elect⅔ical and optical pe⅔colation th⅔esholds we⅔e dete⅔mined. Classical and site pe⅔colation theo⅔ies we⅔e used to calculate the c⅔itical exponents fo⅔ optical t⅔ansmission data. UV spect⅔ophotomete⅔ Lambda S of Pe⅔kinElme⅔, US“ was used to monito⅔ the va⅔iation of optical t⅔anspa⅔ency of the composite films. The t⅔ansmittances of the composite films we⅔e detected at nm wavelength. Measu⅔ements we⅔e pe⅔fo⅔med at six diffe⅔ent positions on the film su⅔face in o⅔de⅔ to lowe⅔ the e⅔⅔o⅔. “ll the photon t⅔ansmission measu⅔ements we⅔e ca⅔⅔ied out at ⅔oom tempe⅔atu⅔e. Figure shows that the behavio⅔ of t⅔ansmitted It⅔ and scatte⅔ed Isc light intensities ve⅔sus the ⅔atio of MWCNT contents M in composite films. When t⅔ansmitted light intensity, It⅔, sha⅔ply dec⅔eases, and scatte⅔ing cente⅔s in the film a⅔e inc⅔eased, ⅔espectively, due to ⅔ef⅔active indices between two medium in the composite system [ ]. In Figure , the scatte⅔ed light intensity inc⅔eases ⅔apidly, even though the MWCNT content was . wt% at the beginning which shows the pe⅔colation th⅔eshold value is in between and . wt%. Since M − Mop → M fo⅔ the ext⅔emely low Mop values, the pe⅔colation p⅔obability fo⅔ the optical data can be w⅔itten as follows

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I sc = I 0 M b op The c⅔itical exponent βop was calculated and found to be as . ve⅔sus log M plot.

f⅔om the slope of log Isc/I

The effect of MWCNT addition in insulating PV“c mat⅔ix was investigated [ ]. Mass f⅔actions of the composites we⅔e obtained between . and wt%. Va⅔iations in optical p⅔ope⅔ties of P V“c-co-”u“ /MWCNT composite films we⅔e measu⅔ed using photon t⅔ansmission and fluo⅔escence emission measu⅔ement techni⅓ues. The optical and fluo⅔escence pe⅔colation th⅔esholds we⅔e dete⅔mined. The classical and site pe⅔colation theo⅔ies we⅔e used to calculate the c⅔itical exponents fo⅔ two data sets f⅔om the techni⅓ues, ⅔espectively.

Figure . Va⅔iation in t⅔ansmitted It⅔ and scatte⅔ed Isc light intensities on PS-MWCNT composite films ve⅔sus mass f⅔actions [ ].

UV spect⅔ophotomete⅔ Lambda S of Pe⅔kinElme⅔, US“ was used to monito⅔ the va⅔iation of optical t⅔anspa⅔ency of the composite films. Since the abso⅔bance ⅔egion of the py⅔anine molecule was – nm, the t⅔ansmittances of the composite films we⅔e detected at nm wavelength. T⅔ansmission measu⅔ements we⅔e pe⅔fo⅔med at six diffe⅔ent positions on the film su⅔face in o⅔de⅔ to lowe⅔ the e⅔⅔o⅔. Thus, the ave⅔age value of t⅔ansmitted light intensity It⅔

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

was obtained. T⅔anspa⅔ency va⅔iations of the composite films ve⅔sus mass f⅔actions M of MWCNT we⅔e monito⅔ed by t⅔ansmitted light intensity, It⅔, f⅔om the films. It was obse⅔ved that It⅔ sha⅔ply dec⅔eases as MWCNT content inc⅔eases in the composite system, and the⅔e is almost no light t⅔ansmission f⅔om the films above the mass f⅔action of . wt% M . wt% .

Figure . The log-log plot of Isc ve⅔sus M. The slope of the st⅔aight line p⅔oduces the optical c⅔itical exponent, βop as . [ ].

The behavio⅔ of It⅔ ve⅔sus M p⅔edicts that the composite system owns a pe⅔colative st⅔uctu⅔e having a pe⅔colation th⅔eshold at . wt% MWCNT content. “s M is inc⅔eased, the scatte⅔ed light intensity, Isc = Io − It⅔, inc⅔eases due to the concent⅔ation fluctuations. The va⅔iation of log Isc/Io ve⅔sus log M is given in Figure . The c⅔itical exponent, βop, was calculated as . f⅔om the slope of Figure using E⅓. . The obtained value of βop = . is not fa⅔ f⅔om the theo⅔etical site pe⅔colation value of . [ ]. The va⅔iation of fluo⅔escence emission of the composite films was measu⅔ed using Va⅔ian Ca⅔y Eclipse fluo⅔escence spect⅔ophotomete⅔. Excitation and emission wavelengths we⅔e used as and nm, ⅔espectively. The emission wavelength at maximum intensity of py⅔anine is nm, which va⅔ies up to – nm depending on the st⅔uctu⅔e of the polyme⅔ molecules. The fluo⅔escence emission measu⅔ements we⅔e pe⅔fo⅔med at six diffe⅔ent positions on the film su⅔face in o⅔de⅔ to lowe⅔ the e⅔⅔o⅔ level, and the ave⅔age value of the fluo⅔escence emission intensity Ifl at the maximum was obtained. The maxima of the emission intensity Ifl ve⅔sus mass f⅔actions M of the samples a⅔e shown in Figure , whe⅔e it can be seen that the emission intensities of py⅔anine show a ⅔apid dec⅔ease fo⅔ fu⅔the⅔ addition of MWCNTs in the composite films. “s the MWCNT concent⅔ation is inc⅔eased, the numbe⅔ of the scatte⅔ing cente⅔s in the film also inc⅔eases.

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Figure . The fluo⅔escence emission intensity, Ifl, ve⅔sus MWCNT mass f⅔action, M [ ].

The fluo⅔escence emission data in Figure can be t⅔eated by the pe⅔colation theo⅔y. The pe⅔colation p⅔obability fo⅔ the data of fluo⅔escence emission could be a⅔⅔anged as follows 1 = (M - M fl )βfl Ifl whe⅔e M is identical to lattice occupation p⅔obability, p, then the pe⅔colation th⅔eshold value, pc, is e⅓ual to Mfl. Since M − Mfl → M fo⅔ low Mfl then E⅓.

becomes as follows

1 = M βfl In ”y assuming, the pe⅔colation p⅔obability P∞ p is inve⅔sely p⅔opo⅔tional to the fluo⅔escence emission intensity. The va⅔iation of log /Ifl − log M is p⅔esented in Figure , whe⅔e two diffe⅔ent pe⅔colation ⅔egions can be detected.

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Figure . The log-log plot of /Ifl ve⅔sus M. The slopes of the st⅔aight lines p⅔oduce the fluo⅔escence c⅔itical exponents, βfl, as . and . fo⅔ the low and high MWCNT concent⅔ation ⅔egions, ⅔espectively [ ].

The c⅔itical exponents, βfl, was calculated and found to be as . the st⅔aight lines in Figure , acco⅔ding to E⅓. .

and . f⅔om the slopes of

“t low MWCNT content, py⅔anine simply p⅔obes the scatte⅔ing sites in the composite film howeve⅔, at high MWCNT concent⅔ation, py⅔anine now t⅔aces the conducting netwo⅔k [ ]. Polysty⅔ene-MWCNT composite was p⅔epa⅔ed by Ugu⅔ et al. showed that healing and inte⅔ diffusion p⅔ocess using photon t⅔ansmission techni⅓ues. “fte⅔ annealing step, the t⅔ansmitted light intensity, It⅔, was monito⅔ed to obse⅔ve the film fo⅔mation p⅔ocess as shown in Figure . The inc⅔ease in It⅔ up to healing tempe⅔atu⅔e, Th, and above Th du⅔ing annealing was explained by void closu⅔e and inte⅔s diffusion p⅔ocesses, ⅔espectively, [ ].

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Figure . T⅔ansmitted photon intensities, It⅔, ve⅔sus annealing tempe⅔atu⅔es depending on MWCNTs content in the films. Numbe⅔s on each figu⅔e shows the MWCNTs content [ ].

Tempe⅔atu⅔e dependence of oxygen diffusion into polyme⅔-MWCNT composite films was examined by fluo⅔escence spect⅔oscopy [ ]. The diffusivity of the composite films was dete⅔mined by pe⅔fo⅔ming oxygen O diffusion measu⅔ements within a tempe⅔atu⅔e ⅔ange of – °C fo⅔ each film, and py⅔ene P was used as a fluo⅔escence p⅔obe Figure .

Figure [ ].

. Plot of the diffusion coefficients, D, ve⅔sus tempe⅔atu⅔es, T, fo⅔ the ,

, and

wt% MWCNT content films

The diffusion coefficients inc⅔eased d⅔astically with both inc⅔eases of MWCNT content and also of the tempe⅔atu⅔e, and this inc⅔ease was explained via the existence of la⅔ge amounts of

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

po⅔es in composite films which facilitate oxygen penet⅔ation into the st⅔uctu⅔e. The⅔efo⅔e, PS/ MWCNT nanocomposites have useful p⅔ope⅔ties as fluo⅔escent oxygen senso⅔s, and a simple SSF techni⅓ue can be used to measu⅔e the diffusion coefficient of oxygen molecules into these films ⅓uite accu⅔ately [ ]. PS/MWCNT films we⅔e p⅔epa⅔ed by va⅔ious contents of MWCNT at ⅔oom tempe⅔atu⅔e. “fte⅔ annealing at °C which is above glass t⅔ansition Tg tempe⅔atu⅔e of PS, fluo⅔escence ⅓uenching p⅔ocesses we⅔e ⅔ealized on oxygen diffusion [ ]. Figure illust⅔ates the loga⅔ithmic plots of the fluo⅔escence intensity of py⅔ene behavio⅔ within time du⅔ing oxygen diffusion into the composite films fo⅔ wt %MWCNT content.

Figure . Loga⅔ithmic plots of the fluo⅔escence intensity of py⅔ene behavio⅔ within time du⅔ing oxygen diffusion into the composite films fo⅔ wt %MWCNT content [ ].

The diffusion coefficients inc⅔eased d⅔astically with the inc⅔ease of MWCNT content, and this inc⅔ease was explained via the existence of la⅔ge amounts of po⅔es in composite films which facilitate oxygen penet⅔ation into the st⅔uctu⅔e. Oxygen pe⅔meability of nanocomposite films consisting of MWCNT and PS we⅔e dete⅔mined to investigate the oxygen diffusion depending on MWCNT and tempe⅔atu⅔e [ ].

Figure . The time behavio⅔ of py⅔ene, fluo⅔escence intensity, I, du⅔ing oxygen diffusion into the composite films with diffe⅔ent MWCNT content. Numbe⅔s on each cu⅔ve indicates the MWCNT content % in the film [ ].

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In Figure no⅔malized py⅔ene intensity, Ip cu⅔ves a⅔e p⅔esented against diffusion time fo⅔ films having diffe⅔ent MWCNT content exposed to oxygen. It is seen that as oxygen diffused th⅔ough the plana⅔ film, the emission intensity, Iem, of the py⅔ene dec⅔eased fo⅔ each MWCNT content film. “fte⅔ completing oxygen diffusion, Iem was satu⅔ated. “s shown in Figure , the ⅓uenching ⅔ate depends on the MWCNT contents in the film. Rapid ⅓uenching of excited py⅔enes by O molecules is possible fo⅔ the high MWCNT content composite films [ ].

Figure . The time behavio⅔ of the py⅔ene, P, fluo⅔escence intensity, I, du⅔ing oxygen diffusion into the MWCNT content film at va⅔ious tempe⅔atu⅔es. Numbe⅔s on each cu⅔ve indicate the tempe⅔atu⅔e [ ].

wt%

The ⅔ate of dec⅔ease in intensity is highe⅔ at highe⅔ tempe⅔atu⅔es p⅔edicting the mo⅔e ⅔apid ⅓uenching of excited py⅔ene molecules by O molecules diffused into the films. It is wo⅔thy to note that in Figure as expected the D inc⅔eases with inc⅔ease in tempe⅔atu⅔e fo⅔ all composite films. Inc⅔ease in tempe⅔atu⅔e natu⅔ally inc⅔eases the ”⅔ownian motion of oxygen molecules given them mo⅔e chance to meet the P molecules in the composite film [ ]. The ⅔esults of Figures and showed that the diffusion of oxygen was accele⅔ated by both inc⅔ease in MWCNT f⅔action and tempe⅔atu⅔e. PS-MWCNT latex composite films we⅔e p⅔epa⅔ed by mixing of py⅔ene P -labeled PS latex with diffe⅔ent amounts of MWCNTs va⅔ying in the ⅔ange between and wt% [ ]. To monito⅔ the stages of film fo⅔mation of PS/MWCNT latex composite films, scatte⅔ed light Is and fluo⅔escence intensities IP f⅔om P we⅔e pe⅔fo⅔med afte⅔ each annealing step. Some tempe⅔atu⅔es such as minimum film fo⅔mation To , void closu⅔e Tv , and healing, Th tempe⅔atu⅔es we⅔e decided as given in Figure .

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

Figure . Plot of co⅔⅔ected fluo⅔escence intensity =fluo⅔escence intensity/scatte⅔ed fluo⅔escence intensity of compo‐ site films with diffe⅔ent MWCNT content ve⅔sus annealing tempe⅔atu⅔es. a , b . , c % MWCNT content, ⅔espec‐ tively [ ]. T , minimum film fo⅔mation tempe⅔atu⅔e Th, healing tempe⅔atu⅔e .

The existence of MWCNT delays the latex film fo⅔mation p⅔ocess because of the behavio⅔ of T . Howeve⅔, healing p⅔ocesses a⅔e not affected by the p⅔esence of MWCNT [ ].

. The mechanical properties of polymer-CNTs composites The elastic modulus of the swollen P““m-MWCNT composites was measu⅔ed to dete⅔mine the effect of MWCNTs content. Elasticity measu⅔ement was pe⅔fo⅔med by comp⅔essive testing techni⅓ue and modeled by the theo⅔y of ⅔ubbe⅔ elasticity [ , ].

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Figure

. St⅔ess ve⅔sus st⅔ain cu⅔ves a lowe⅔ and b highe⅔ MWCNT contents at

°C, ⅔espectively [

].

St⅔ess Pa -st⅔ain plots of low and high MWCNTs content gels p⅔oduced using the data obtained f⅔om the linea⅔ ⅔egion, in the plots of F N ve⅔sus comp⅔ession cu⅔ves fo⅔ P““mMWCNT composites at °C, a⅔e p⅔esented in Figure , ⅔espectively. The st⅔ess ve⅔sus st⅔ain displays a good linea⅔ ⅔elationship at °C, which ag⅔ees with E⅓. .

t = Gl The elastic moduli we⅔e obtained by a least s⅓ua⅔e fit to the linea⅔ ⅔egion of Figure . The addition of MWCNT into P““m caused an inc⅔ease in elastic modulus of the composite as expected. In P““m– wt% MWCNT composite, the measu⅔ed elastic modulus is found to be . MPa, two times la⅔ge⅔ than pu⅔e P““m sample. It is seen in Figure a that P““m– .

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

wt% MWCNT composite has smalle⅔ initial slope than wt% MWCNT content composite. In this case, it appea⅔s that the alignment effect of MWCNT plays an impo⅔tant ⅔ole fo⅔ getting the diffe⅔ent onset behavio⅔ [ ]. The st⅔ess of the P““m-MWCNT – wt% MWCNT composites inc⅔eases d⅔amatically when the st⅔ain exceeds . %, whe⅔e the alignment is taking place in the composite. On the othe⅔ hand, at high MWCNT ⅔egion, the ⅔andom motion of MWCNT impedes alignment as p⅔edicted in Figure b. The⅔efo⅔e, in a gel with low MWCNT content, alignment of MWCNTs with each othe⅔ is much easie⅔ than in high content MWCNT composite.

Figure

. Dependence of elastic modulus on content of wt% MWCNT in the composite [ ].

Figure shows the plot of elastic modulus, G ve⅔sus MWCNTs content in the composite gel. Elastic modulus inc⅔eases d⅔amatically up to wt% MWCNT with inc⅔easing nanotube content and dec⅔eases p⅔esenting a c⅔itical MWCNT value indicating that the⅔e is a sudden change in the mate⅔ial elasticity. The sudden change in G p⅔edicts that the composites have ⅔eached a supe⅔-elastic pe⅔colation netwo⅔k [ ]. “t contents above wt% MWCNTs, the elastic modulus is dec⅔eased ma⅔ginally with inc⅔easing MWCNTs content. On the othe⅔ hand, at low MWCNT content ≤ wt% , the elastic modulus inc⅔eases up to . MPa, only exceeding it when the MWCNT content is above wt%, and then dec⅔eases fu⅔the⅔ as the MWCNTs content is ⅔aised. “t the pe⅔colation th⅔eshold, wt% the nanotubes fo⅔m an inte⅔connecting st⅔uctu⅔e, call pe⅔colation cluste⅔ exhibiting a high deg⅔ee of nanotube inte⅔actions and/o⅔ entanglement [ ]. The inc⅔easing MWCNT content p⅔oduces infinite netwo⅔k in ⅔educing the swelling and dec⅔easing elastic modulus as was expected fo⅔ the composites at high MWCNT content. The dec⅔ease in elastic modulus, G, can be explained by the fo⅔mation of a ca⅔bon nanotube netwo⅔k which significantly imp⅔oves the stiffness of

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Carbon Nanotubes - Current Progress of their Polymer Composites

composite gel. That is due to the high stiffness the mechanical p⅔ope⅔ties should be influenced substantially [ ]. On the othe⅔ hand, when the MWCNT content is below wt%, the elastic modulus is p⅔esenting lowe⅔ values and inc⅔eases as MWCNT is added. ”ecause of molecula⅔ tube-tube and tube-polyme⅔ inte⅔actions, the mechanical p⅔ope⅔ties of MWCNT-polyme⅔ composites we⅔e affected. On the othe⅔ hand, to dete⅔mine mechanical p⅔ope⅔ties of MWCNTpolyme⅔ composites, load t⅔ansfe⅔ and inte⅔facial bonding should be known. The⅔efo⅔e, nanotube dispe⅔sion in the polyme⅔ plays a c⅔itical ⅔ole fo⅔ this phenomenon [ ].

Figure . Loga⅔ithmic plot of the elastic modulus ve⅔sus MWCNTs contents cu⅔ves fo⅔ p < pc. The exponent, y, was dete⅔mined f⅔om the slope of the st⅔aight line [ ].

In Figure , it is unde⅔stood that wt% is the c⅔itical pe⅔colation th⅔eshold, pc, at which gel system owns a pe⅔colation cluste⅔ fo⅔med f⅔om MWCNTs. He⅔e, the composite gel passes the highest elasticity p⅔esenting the highest G value. G ( p ) » ( pc - p ) - y E⅓uation now can be used to fit the G ve⅔sus wt% MWCNT cu⅔ve below the c⅔itical point below wt% . The value of the fitting exponent y in E⅓. was estimated f⅔om the slope of the linea⅔ ⅔elation between log G and log Ip_pcI, as shown in Figure . Elastic pe⅔colation occu⅔s below wt% MWCNTs Figure with a c⅔itical exponent a⅔ound y = . , which is close to the theo⅔etical p⅔ediction of this value in the D pe⅔colated system known as a SEPN. The c⅔itical exponent, y, ag⅔ees with the lite⅔atu⅔e values [ ]. He⅔e, x is the c⅔itical exponent which is taken f⅔om lite⅔atu⅔e by the uppe⅔ limit x = . . Mo⅔eove⅔, Δ was found to be . , ve⅔y close to the uppe⅔ limit of Δ, as given in ⅔ange between . and . . The mechanical p⅔ope⅔ties of PV“c-MWCNT nanocomposites [ ] we⅔e investigated as shown in Figure .

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

Figure

. Sto⅔age modulus as a function of angula⅔ f⅔e⅓uency at

°C fo⅔ PV“c-MWCNTs nanocomposites [ ].

“s shown in Figure , the sto⅔age modulus G′ inc⅔eases with an addition of MWCNT loading compa⅔ed with that of the PV“c mat⅔ix. The ⅔heological p⅔ope⅔ties of PET-MWCNT nanocomposites we⅔e dete⅔mined using low st⅔ain values with f⅔e⅓uency . – ⅔ad/s [ ]. The sto⅔age modulus ve⅔sus f⅔e⅓uency is given in Figure .

Figure

. Sto⅔age modulus G′ of PET-MWCNT nanocomposites ve⅔sus f⅔e⅓uency w at

°C [

].

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The ⅔heological behavio⅔ of the nanocomposites depends on the MWCNT content with ⅔espect to the f⅔e⅓uencies because of the ⅔est⅔iction of PET chain ⅔elaxation and the sho⅔t-⅔ange dynamics o⅔ local motion of the PET chains in the nanocomposites [ ].

. The electrical properties of polymer-CNTs composites The elect⅔ical ⅔esistivity of the PV“c/MWCNT composite films was pe⅔fo⅔med by a Keithley Model “ elect⅔omete⅔ and Keithley Model ⅔esistivity test fixtu⅔e. – V DC potential fo⅔ eve⅔y s pe⅔iods was applied to measu⅔e thei⅔ su⅔face ⅔esistivity, Rs, Ohm/ s⅓ua⅔e o⅔ Ohm which was fou⅔ diffe⅔ent o⅔ientations and ⅔epeated measu⅔ements we⅔e ⅔epeated. The su⅔face conductivity values we⅔e calculated f⅔om the following e⅓uation.

s = 1 / Rs Then, the obtained ⅔esults f⅔om E⅓. a⅔e p⅔esented in Figure . It can be seen f⅔om Figure that the conductivity, σ, d⅔amatically inc⅔eases above Mσ = . wt%. This behavio⅔ can be explained by the existence of ve⅔tical conductive paths of MWCNTs in the composite film. “fte⅔ Mσ = . wt%, the insulating system sta⅔ts to t⅔ansfo⅔m to the conductive system. The⅔efo⅔e, Mσ = . wt% is the pe⅔colation th⅔eshold of conductivity. Fo⅔ a dilute composite st⅔uctu⅔e, classical pe⅔colation theo⅔y was given in E⅓. can be used [ ].

s = s 0 ( M - Ms )

b

s

He⅔e, σ is the conductivity Siemens , σo is the conductivity of pu⅔e MWCNT film, M is the volume o⅔ mass f⅔action of MWCNT, Mσ is the pe⅔colation th⅔eshold value, and βσ is the c⅔itical exponent fo⅔ the conductivity, ⅔espectively, which was calculated f⅔om E⅓. and dete⅔‐ mined f⅔om the slope of the log σ and log M − Mσ plot. The βσ = . value is well ag⅔eed with the theo⅔etical and the expe⅔imental data in the lite⅔atu⅔e [ ]. The ave⅔age values of the su⅔face ⅔esistivity, Rs, of PV“c/MWCNT composite films ve⅔sus mass f⅔actions of MWCNT, M, we⅔e measu⅔ed and obse⅔ved that the su⅔face ⅔esistivity Rs of the composite films do not change much below . wt% M ≤ . wt% [ ]. Howeve⅔, Rs values of the composite films d⅔amatically dec⅔ease f⅔om to Ohm/s⅓ua⅔e in the bandgap of M = . – . wt%. This behavio⅔ indicates that the elect⅔ical pe⅔colation occu⅔s at low levels of M. The su⅔face conductivity values we⅔e calculated f⅔om E⅓. . The pe⅔colation th⅔eshold of the su⅔face conductivity Mσ is . wt%. βσ was calculated f⅔om the slope of the cu⅔ve in Figure , which was d⅔awn f⅔om the loga⅔ithms of the su⅔face conductivity data t⅔eated with E⅓. , and found to be as . which is well ag⅔eed with the theo⅔etical and the expe⅔imental ⅔esults in the lite⅔atu⅔e [ ].

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

Figure

. Va⅔iations in conductivities, σ, which we⅔e calculated by E⅓.

ve⅔sus mass f⅔actions [ ].

The su⅔face conductivity p⅔ope⅔ties of PS-MWCNT composite films we⅔e measu⅔ed at ⅔oom tempe⅔atu⅔e using a two p⅔obe techni⅓ue [ ]. Figure shows the elect⅔ical conductivity σ of PS-MWCNT composite films as a function of MWCNT ⅔atio, R. While low MWCNT content composites R < . show simila⅔ conductivity between − and − , the conductivity of high MWCNT content films R > . inc⅔eases d⅔amatically to − – − S.

Figure . The log-log plot of σ ve⅔sus M → Mσ. The slope of the st⅔aight line p⅔oduces the elect⅔ical c⅔itical exponent, βσ as . [ ].

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Figure

. Conductivity ve⅔sus MWCNT content, R w/w [

].

Figure . Elect⅔ical conductivity σ of the PET-MWCNT nanocomposites as a function of MWCNT loading. Inset a log-log plot of elect⅔ical conductivity ve⅔sus ⅔educed MWCNT loading. The solid lines a⅔e fits to a powe⅔ law depend‐ ence of elect⅔ical conductivity on the ⅔educed MWCNT loading [ ].

Figure shows the elect⅔ical conductivity σ of the PET-MWCNT [ ]. The elect⅔ical conductivity of pu⅔e PET is . × − S cm− . ”y loading MWCNT f⅔om . to wt%, the conductivity of PET-MWCNT nanocomposites inc⅔eases in o⅔de⅔s of magnitude, because pe⅔colative path was fo⅔med in the nanocomposite [ ].

Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

. Conclusion In this chapte⅔, we ⅔eview the optical, elect⅔ical, and mechanical behavio⅔s of polyme⅔s doped by multiwalled ca⅔bon nanotubes MWCNTs . This chapte⅔ cove⅔ed the wo⅔ks p⅔oduced f⅔om diffe⅔ent types of polyme⅔s. We t⅔y to give the expe⅔imental ⅔esults based on the mentioned theo⅔etical models. In the fi⅔st section of the chapte⅔, some examples we⅔e given about optical behavio⅔ of polyme⅔-MWCNT composites which we⅔e pe⅔fo⅔med by fluo⅔escence and UV-Vis spect⅔oscopy. The⅔efo⅔e, the behavio⅔ of them at a molecula⅔ level we⅔e discussed, and measu⅔ed some physical ⅓uantities we⅔e ⅔epo⅔ted. The second section of the chapte⅔ has demonst⅔ated that the mechanical and ⅔heological measu⅔ements can be used to dete⅔mine the va⅔iations of the elastic and sto⅔age modulus of the composites. The thi⅔d pa⅔t of the chapte⅔ has shown that the insulato⅔-conducto⅔ t⅔ansition takes place by the addition of a small amount of MWCNT in the polyme⅔ composite system. The insulato⅔ system sta⅔ts to t⅔ansfo⅔m to a mo⅔e conductive state by consisting of conductive paths of MWCNTs between the elect⅔odes. The size of MWCNTs and elect⅔on hopping and/o⅔ tunneling effects play impo⅔tant ⅔oles in the ea⅔ly pe⅔colation behavio⅔ of the films. In conclusion, we t⅔y to give the expe⅔imental evidences based on the mentioned theo⅔etical models. “s fa⅔ as the enginee⅔ing applications a⅔e conce⅔ned, optical, mechanical, and elect⅔ical p⅔ope⅔ties of polyme⅔ composites p⅔epa⅔ed by MWCNT contents a⅔e ve⅔y impo⅔tant in coating, food, elect⅔onic, and pha⅔maceutical indust⅔ies. This chapte⅔ int⅔oduces basic pa⅔amete⅔s fo⅔ the given p⅔ocesses in the polyme⅔ composites which can find impo⅔tant applications in the mentioned fields.

Author details G(lşen “kın Eving(⅔ * and 5nde⅔ Pekcan *“dd⅔ess all co⅔⅔espondence to gulsen.evingu⅔@pi⅔i⅔eis.edu.t⅔ Pi⅔i Reis Unive⅔sity, Tuzla, İstanbul, Tu⅔key Kadi⅔ Has Unive⅔sity, Cibali, İstanbul, Tu⅔key

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Optical, Mechanical, and Electrical Properties of Polymer Composites Doped by Multiwalled Carbon Nanotubes http://dx.doi.org/10.5772/63054

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Chapter 6

Mechanical Properties of Carbon Nanotubes-Polymer Composites Lixing Dai and Jun Sun Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62635

Abstract Ca⅔bon nanotubes CNTs , as a one-dimensional mate⅔ial, have outstand‐ ing mechanical p⅔ope⅔ties such as ext⅔eme tensile st⅔ength and Young’s modulus. “t p⅔esent, to p⅔epa⅔e pu⅔e CNTs mate⅔ials is ⅓uite difficult and the mechanical p⅔ope⅔ties of the mate⅔ials a⅔e limited in a low level. ”e‐ cause of thei⅔ ext⅔ao⅔dina⅔y mechanical p⅔ope⅔ties and high aspect ⅔atio, CNTs a⅔e conside⅔ed to be ideal candidates fo⅔ polyme⅔ ⅔einfo⅔cement. In addition, CNTs/polyme⅔ composite mate⅔ials a⅔e much easie⅔ to p⅔epa⅔e than pu⅔e CNTs mate⅔ials, so they have been paid much attention by ⅔e‐ sea⅔che⅔s ⅔ecently. Howeve⅔, challenges must be faced to p⅔epa⅔e the CNTs/polyme⅔ composite with ultimate mechanical p⅔ope⅔ties. So in this chapte⅔, the main conce⅔ns a⅔e how to dispe⅔se CNTs in polyme⅔ mat⅔ix to p⅔epa⅔e homogenous composite dispe⅔sions, how to p⅔epa⅔e homoge‐ nous CNTs/polyme⅔ composite using possible fab⅔icating p⅔ocesses based on the homogenous dispe⅔sions, how to inc⅔ease the fibe⅔ mechanical p⅔ope⅔ties especially th⅔ough the enhancing inte⅔action between polyme⅔ and CNTs, cont⅔olling the amount of CNTs and enhancing thei⅔ o⅔ienta‐ tion in the mat⅔ix. Keywords: mechanical p⅔ope⅔ty, ca⅔bon nanotube, polyme⅔, composite, dispe⅔sibility

. Introduction In ⅔ecent yea⅔s, because st⅔onge⅔, lighte⅔, o⅔ less expensive compa⅔ed to t⅔aditional mate⅔ials, high-pe⅔fo⅔mance composite mate⅔ials have inc⅔easingly become an essential in a wide ⅔ange

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of st⅔uctu⅔al applications [ , ]. “s ea⅔ly as in ancient times, people have ⅔ealized that the combination of two o⅔ mo⅔e mate⅔ials could fo⅔m new mate⅔ials which had bette⅔ p⅔ope⅔ties than eithe⅔ of the mate⅔ials compositions. Fo⅔ example, ancient people in China began to use st⅔aw to ⅔einfo⅔ce mud to fo⅔m b⅔icks as building mate⅔ials, and to use hemp o⅔ silk to ⅔ein‐ fo⅔ce paint and so on [ , ]. Nowadays, composite mate⅔ials a⅔e used extensively in va⅔ious a⅔eas, not only in the ae⅔ospace indust⅔y but also in a la⅔ge and inc⅔easing numbe⅔ of comme⅔‐ cial mechanical enginee⅔ing applications. Composites a⅔e made up of ⅔einfo⅔cements and mat⅔ix mate⅔ials, in which the latte⅔ su⅔⅔ounds and suppo⅔ts the fo⅔me⅔ by maintaining thei⅔ ⅔elative positions, and conve⅔sely, the ⅔einfo⅔ce‐ ments impa⅔t thei⅔ special mechanical and physical p⅔ope⅔ties to enhance the mat⅔ix p⅔ope⅔ties [ ]. With thei⅔ ⅔apid development and because of thei⅔ advantages in good p⅔ocessibility and high mechanical p⅔ope⅔ties, polyme⅔s have g⅔adually ⅔eplaced many of the conventional mate⅔ials as mat⅔ix of composites in va⅔ious applications. In o⅔de⅔ to obtain high-st⅔ength, high-modulus, and light-weight composites used in ha⅔sh loading conditions such as ae⅔o‐ space components, anti-bullet, high tempe⅔atu⅔e ⅔esistance, etc., st⅔ong and light ⅔einfo⅔cing fibe⅔s such as Kevla⅔, ca⅔bon fibe⅔, ult⅔ahigh-molecula⅔-weight polyethylene fibe⅔ and glass fibe⅔ a⅔e always selected to suit the ⅔e⅓ui⅔ements [ ]. P⅔ovided the fibe⅔s a⅔e mechanically well attached to the mat⅔ix, they can g⅔eatly imp⅔ove the ove⅔all p⅔ope⅔ties of composites. Recently, fibe⅔-⅔einfo⅔ced composite mate⅔ials have inc⅔easingly gained popula⅔ity in high-pe⅔fo⅔mance mate⅔ials. Ca⅔bon nanotubes CNTs as one of the st⅔ongest and stiffest mate⅔ials in te⅔ms of tensile st⅔ength and elastic modulus in natu⅔e have ⅔eceived much attention since thei⅔ discove⅔y by Iijima in [ ]. Thei⅔ excellent mechanical p⅔ope⅔ties ⅔esult f⅔om the covalent sp bonds fo⅔med between the individual ca⅔bon atoms. It has been shown that CNTs a⅔e ve⅔y st⅔ong in the axial di⅔ection, Young’s modulus on the o⅔de⅔ of – GPa and tensile st⅔ength – GPa [ ]. Fu⅔the⅔mo⅔e, CNTs as one-dimensional mate⅔ials possess some basic cha⅔acte⅔istic and advantages of fibe⅔, which have inspi⅔ed inte⅔est in using CNTs as a fille⅔ in polyme⅔based composites to obtain ult⅔a–high pe⅔fo⅔mance st⅔uctu⅔al mate⅔ials with enhanced mechanical p⅔ope⅔ties [ , ]. “ lot of effo⅔ts have been made to explo⅔e CNTs/polyme⅔ composite mate⅔ials in both academy and indust⅔y. The st⅔uctu⅔e, mo⅔phology, mechanical p⅔ope⅔ties, and possible applications of these composite mate⅔ials have been extensively investigated with some ve⅔y p⅔omising ⅔esults. They have been p⅔oven to be ve⅔y effective fille⅔s in polyme⅔s. Howeve⅔, CNTs have la⅔ge specific su⅔face a⅔ea, bending fibe⅔-like shape, and st⅔ong van de⅔ Waals inte⅔actions which a⅔e easy to make the CNTs agglome⅔ate and entangled, and difficult to be dissolved in wate⅔ o⅔ o⅔ganic solvents, o⅔ dispe⅔se in polyme⅔ [ ]. The agglome⅔ation of CNTs will lead to difficulties of st⅔ess t⅔ansfe⅔ f⅔om mat⅔ix to the ⅔einfo⅔ced mate⅔ials CNTs efficiently, and adve⅔sely lowe⅔ the mechanical p⅔ope⅔ties of the CNTs/polyme⅔ composite. So, how to enhance the dispe⅔sibility of CNTs in polyme⅔ mat⅔ix has been one of the most conce⅔ns in the field of CNTs ⅔einfo⅔ced polyme⅔-based composite mate⅔ials. The⅔efo⅔e, functionalization of CNTs is ext⅔emely impo⅔tant fo⅔ thei⅔ dispe⅔sion, st⅔ess-t⅔ansfe⅔, and potential applications in polyme⅔ composites [ ]. In this chapte⅔, we will focus on the

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dispe⅔sibility of CNTs in polyme⅔ mat⅔ix th⅔ough enhancing the inte⅔action between CNTs and polyme⅔, the p⅔ocessing of the composites, and the mechanical p⅔ope⅔ties of the compo‐ sites.

. Dispersion of CNTs in polymers Dispe⅔sity of CNTs in polyme⅔ and the st⅔ess t⅔ansfe⅔ f⅔om the mat⅔ix to CNTs have significant effects on ⅔eaching optimum mechanical p⅔ope⅔ties fo⅔ CNTs/polyme⅔ composites. Only if CNTs dispe⅔se homogeneously in polyme⅔ mat⅔ix, effective st⅔ess t⅔ansfe⅔ in the whole composite mate⅔ials will be gua⅔anteed conve⅔sely, poo⅔ dispe⅔sity of CNTs in polyme⅔ mat⅔ix will lead to poo⅔ st⅔ess t⅔ansfe⅔, which not only cannot play thei⅔ ⅔ole in inc⅔easing mechanical p⅔ope⅔ties but also can possibly weaken the o⅔iginal p⅔ope⅔ties of the mat⅔ix. The⅔efo⅔e, the ⅔einfo⅔cement of CNTs to polyme⅔ mat⅔ix is to g⅔eat extent dependant on the dispe⅔sity of CNTs in the mat⅔ix.

To imp⅔ove the dispe⅔sity of CNTs has two app⅔oaches mac⅔oscopical mechanical mixing and functionalizing CNTs to build inte⅔action between the inte⅔faces of CNTs and polyme⅔ mat⅔ix. The inte⅔action th⅔ough bonding has anothe⅔ significant effect, that is, st⅔engthening st⅔ess t⅔ansfe⅔ effect. Obtaining a good dispe⅔sion of CNTs in the polyme⅔ mat⅔ix is ve⅔y challenging. Ult⅔asonication, physical o⅔ chemical functionalization a⅔e common to achieve a good dispe⅔sity of CNTs in polyme⅔ mat⅔ix with enhanced mechanical p⅔ope⅔ties. . . Ultrasonic dispersion Ult⅔asonication gene⅔ates alte⅔nating low-p⅔essu⅔e and high-p⅔essu⅔e waves in li⅓uids, causing high-speed impinging li⅓uid jets and st⅔ong hyd⅔odynamic shea⅔-fo⅔ces. So, ult⅔asonic device can be used as a high-speed mixe⅔ and agitato⅔ [ ]. Ult⅔asonication is a common tool used to b⅔eak up CNTs agglome⅔ates in solution, but pu⅔e ult⅔asonication t⅔eatment of CNTs is not common, fo⅔ the dispe⅔sion effect is not so satisfacto⅔y some of CNTs ⅔e-agg⅔egate o⅔ some of them deposit in a sho⅔t pe⅔iod of time once sonication stops. Figure shows the photos of MWCNTs a⅓ueous solutions in diffe⅔ent standing time afte⅔ sonication. The solutions sonicated min, h⅔ and h⅔ all deposited afte⅔ standing min. Gene⅔ally, ult⅔asonication of CNTs always combines with application of su⅔factants to obtain bette⅔ dispe⅔sion effect ult⅔asonication is used to dispe⅔se CNTs, and su⅔factants a⅔e used as stabilize⅔s. Sodium dodecyl sulfate SDS [ , ], dodecyl benzene sulfonic acid D”S“ [ , ], and sodium dodecyl benzene sulfonate SD”S [ ] a⅔e commonly used as su⅔factants. The p⅔ocess is simply that CNTs mix with su⅔factants and the mixtu⅔e a⅔e then sonicated. Du⅔ing sonication, CNTs a⅔e g⅔adually exfoliated and disentangled f⅔om agg⅔egates and bundles and

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Figure . Digital images of MWNTs a⅓ueous suspensions afte⅔ ult⅔asonication and standing fo⅔ diffe⅔ent time. Ult⅔aso‐ nication time “. min ”. min C. h⅔ D. h⅔.

stabilized by su⅔factants. Figure shows the photos of MWCNTs a⅓ueous solutions added SD”S in diffe⅔ent standing time afte⅔ sonication. Clea⅔ly, the addition of SD”S distinctly imp⅔oves the dispe⅔sity and stability of MWCNTs in wate⅔. CNTs dispe⅔sions often foam heavily du⅔ing ult⅔asonication, and these foams conse⅓uently ⅔educe the dispe⅔sion efficiency. This bubble p⅔oblem can often be solved by adding antifoam agents such as oligome⅔s of polyethe⅔ and polysiloxane to eliminate ai⅔ laye⅔s o⅔ to ⅔educe foams [ ]. The dest⅔uction of CNTs in suspensions by use of ult⅔asonication was desc⅔ibed as ea⅔ly as . Ult⅔asonic t⅔eatment of CNTs causes a conside⅔able amount of defects including buckling, bending, and dislocations in the ca⅔bon st⅔uctu⅔es [ ]. So suitable ult⅔asonication time and ene⅔gy output have to be conside⅔ed befo⅔e CNTs a⅔e t⅔eated. . . Covalent functionalization Covalent functionalization of CNTs can g⅔eatly imp⅔ove the st⅔ess t⅔ansfe⅔ f⅔om the mat⅔ix to CNTs besides dispe⅔sity, but it usually int⅔oduces st⅔uctu⅔al defects to the su⅔face of CNTs. The functionalization of CNTs can be achieved by eithe⅔ di⅔ect int⅔oduction of oxygen

Mechanical Properties of Carbon Nanotubes-Polymer Composites http://dx.doi.org/10.5772/62635

Figure . Digital images of MWNTs a⅓ueous suspensions changed with the standing time afte⅔ ult⅔asonication fo⅔ min in the p⅔esence of SD”S.

containing g⅔oups to the sidewalls of CNTs by acid t⅔eatment, o⅔ on this basis fu⅔the⅔ int⅔o‐ duction of polyme⅔ chains th⅔ough its chain ends di⅔ectly ⅔eacting with the oxygen containing g⅔oups on CNTs ex situ , o⅔ commonly th⅔ough the monome⅔s ⅔eacting with the g⅔oups on CNTs to int⅔oduce polyme⅔ chains, that is, in situ polyme⅔ization. . . . Acid treatment The most common p⅔ocedu⅔e used fo⅔ covalent attachment of oxygen containing ⅔eactive g⅔oups to CNTs is the t⅔eatment with ino⅔ganic acids. Usually, the nanotubes a⅔e t⅔eated th⅔ough being ⅔efluxed with a mixtu⅔e of concent⅔ated nit⅔ic and sulfu⅔ic acid with the ⅔atio of volume to although thei⅔ concent⅔ations may be slightly diffe⅔ent [ – ]. Howeve⅔, accidently, the ⅔atio of the acids changes, fo⅔ example, CNTs we⅔e oxidized with the mixtu⅔e of mL of sulfu⅔ic acid and mL of nit⅔ic acid % [ ]. Concent⅔ated nit⅔ic acid [ ] and the mixtu⅔e of hyd⅔ogen pe⅔oxide and sulfu⅔ic acid vol = [ ] a⅔e also selections fo⅔ oxidizing CNTs. These oxidative t⅔eatments usually ⅔esult in sho⅔tening of the CNTs’ length and fo⅔mation of su⅔face ⅔eactive g⅔oups, such as hyd⅔oxyl, ca⅔bonyl, and mainly Ca⅔boxyl. Fu⅔the⅔mo⅔e, the ino⅔ganic impu⅔ities of the “l-Fe catalyst used in the p⅔epa⅔ation of CNTs a⅔e solubilized by the acids and the concent⅔ation of the impu⅔ities is g⅔adually dec⅔eased with t⅔eatment time [ , , , ]. It has been shown that acid functionalized CNTs has good dispe⅔sibility in wate⅔ and o⅔ganic solvents, and the ca⅔boxylic functional g⅔oups give a st⅔onge⅔ nanotube-polyme⅔ inte⅔action, leading to enhanced values in mechanical p⅔ope⅔ties of the CNTs/polyme⅔ composite [ , ]. “cid-t⅔eated MWCNTs show homogeneous dispe⅔‐ sion and sca⅔cely any p⅔ecipitates a⅔e obse⅔ved. ”ecause the sidewalls of the MWCNTs ca⅔⅔y mo⅔e dissociated ca⅔boxyl g⅔oups afte⅔ oxidization, the nanotubes can stabilize via an elect⅔ostatic stabilization mechanism [ ]. Saito et al. conside⅔ that this sho⅔tened MWCNTs afte⅔ acid t⅔eatment could be dispe⅔sed in the pola⅔ solvents such as ethanol, DMSO, and DMF mo⅔e easily than the c⅔ude MWCNTs. Mo⅔eove⅔, these dispe⅔sion li⅓uids of the MWCNTs a⅔e stable without agg⅔egation fo⅔ mo⅔e than one month. This phenomenon can be asc⅔ibed to the

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⅔eduction of thei⅔ length and the effect of the solvation b⅔ought by the int⅔oduced hyd⅔ophilic g⅔oup, i.e., the ca⅔boxyl g⅔oup [ ]. “t mode⅔ate tempe⅔atu⅔e, only little sidewall damage happens with the inc⅔ease of the acid t⅔eatment time, whe⅔eas at high tempe⅔atu⅔e, much sho⅔te⅔ nanotubes a⅔e ⅔esulted [ ]. ”eing diffe⅔ent f⅔om ⅔eaction of CNTs with concent⅔ated acids, ca⅔boxylic functional g⅔oups can be int⅔oduced on the CNTs th⅔ough a photosensitized d⅔y oxidation induced by ult⅔aviolet ⅔adiation [ , ]. O⅔ganic acid could also be used as an oxidative, fo⅔ example, t⅔ifluo⅔oacetic anhyd⅔ide/H O , % H O we⅔e used to functionalize single-walled ca⅔bon nanotubes SWCNTs , in addition to oxygen-based functional g⅔oups, t⅔ifluo⅔oacetic g⅔oups we⅔e covalently attached to the SWCNTs. Mo⅔eove⅔, these modified SWCNTs we⅔e sho⅔tened into ca. nm in length du⅔ing functionalization, ⅔esulting in exfoliation of nanotube ⅔opes to yield small bundles and individual nanotubes. The ⅔esultant SWCNTs we⅔e easily dispe⅔sed in pola⅔ solvents such as dimethylfo⅔mamide, wate⅔, and ethanol [ ]. . . . Grafting macromolecular chains Polyme⅔-functionalized CNTs, compa⅔ed to the unmodified ones, usually show ⅔elatively good dispe⅔sibility in o⅔ganic solvents and high compatibility with polyme⅔ mat⅔ix in CNTs/ polyme⅔ composites. The functionalized CNTs containing active g⅔oups p⅔edominantly ca⅔boxyl can fu⅔the⅔ co⅔po⅔ate with polyme⅔s th⅔ough este⅔ification o⅔ amidation ⅔eactions between the g⅔oups on the CNTs and those on the polyme⅔ chains, [ , , ] and conve⅔se‐ ly, t⅔eated o⅔ functionalized mac⅔omolecula⅔ chains can also di⅔ectly attach on p⅔istine CNTs [ , , ]. Most of the app⅔oaches ⅔e⅓ui⅔e the p⅔e-modification of CNTs to int⅔oduce functional g⅔oups to CNTs su⅔faces, which can ⅔eact to the g⅔oups on polyme⅔ chains, leading to the chains g⅔afted onto CNTs. Fo⅔ example, afte⅔ CNTs a⅔e acidic functionalized, polyvinyl alcohol PV“ g⅔afts to the tubes th⅔ough este⅔ification as shown in Figure . PV“-functionalized CNTs a⅔e soluble in the same solvent of neat PV“, thus allowing the intimate mixing of the nanotubes with the mat⅔ix polyme⅔, so the functionalization of CNTs by polyme⅔ is an effective way fo⅔ the homogeneous nanotube dispe⅔sion to obtain high-⅓uality polyme⅔ic ca⅔bon nanocompo‐ site mate⅔ials [ ]. G⅔afting diffe⅔ent kind of polyme⅔ onto CNTs f⅔om the mat⅔ix polyme⅔ which is howeve⅔ compatible with the fo⅔me⅔ polyme⅔ is anothe⅔ example. Massoumi et al fi⅔st p⅔epa⅔ed ca⅔boxylated MWCNTs by acid t⅔eatment, then poly ethylene glycol -g⅔afted MWCNTs PEG-MWCNTs , whe⅔eas finally obtained PV“/MWCNTs-PEG nanocomposite and PV“/sta⅔ch/MWCNTs-PEG nanocomposite [ ]. Sometimes, polyme⅔ chains a⅔e not necessa⅔y to g⅔aft onto CNTs, but only some g⅔oups which a⅔e well compatible with polyme⅔ a⅔e needed to connect with CNTs. Fo⅔ instance, amino g⅔oups g⅔afted MWCNTs possess good compatibility with the mat⅔ix PEEK, so simply by mixing the MWCNTs with PEEK, the composite with significantly inc⅔eased sto⅔age modulus, glass t⅔ansition tempe⅔atu⅔e, and f⅔iction coefficient was p⅔epa⅔ed [ ].

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Figure . Schematic of functionalized CNTs with PV“ in ca⅔bodiimide-activated este⅔ification ⅔eactions [ ].

On the othe⅔ hand, the use of p⅔istine o⅔ unmodified CNTs in the p⅔epa⅔ation of polyme⅔ functionalized CNTs has shown seve⅔al advantages such as p⅔eventing the CNTs f⅔om damage. It is ⅔epo⅔ted that polyme⅔ g⅔afted CNTs have bette⅔ ⅔einfo⅔cement than p⅔istine CNTs. This kind of polyme⅔ functionalized CNTs can be ⅔ealized th⅔ough f⅔ee-⅔adical polyme⅔ization, ozonization of polyme⅔, F⅔iedel-C⅔afts ⅔eaction and etc. [ , , , ]. Th⅔ough a ⅔adical coupling ⅔eaction involving polyme⅔-cente⅔ed ⅔adicals, polysty⅔ene and poly[ te⅔t-butyl ac⅔ylate -b-sty⅔ene] a⅔e used to functionalize sho⅔tened SWCNTs [ ]. Ozonization of polyme⅔s p⅔oduces alkylpe⅔oxide and hyd⅔ope⅔oxide g⅔oups in polyme⅔ chains, which can decompose into ⅔adicals by heating. “s the ⅔adicals a⅔e ⅔eactive towa⅔d the sp hyb⅔id ca⅔bons of CNTs, the ozonized polyme⅔ chains can be g⅔afted onto the CNTs th⅔ough the ⅔eaction between CNTs sidewalls and the ⅔adicals of polyme⅔ chains as shown in Figure [ ].

Figure . Reaction mechanism of ozonized PV“ g⅔afting onto MWCNT [ ].

“fte⅔ F⅔iedel–C⅔afts ⅔eaction has been successfully applied fo⅔ the chemical functionalization of CNTs th⅔ough g⅔afting small molecules onto CNTs [ , ], ⅔esea⅔che⅔s a⅔e inc⅔easingly inte⅔ested in application of F⅔iedel-C⅔afts fo⅔ functionalization of CNTs using polyme⅔. The app⅔oach is mo⅔e feasible than the t⅔aditional ca⅔boxylation because the su⅔face t⅔eatment of

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CNTs befo⅔e the g⅔afting of polyme⅔ chains to the CNTs is not needed. Wu et al. [ ] success‐ fully functionalized MWCNTs with high pe⅔centage of g⅔afting via F⅔iedel-C⅔afts alkylation. The PVC-g⅔afted MWCNTs could be dispe⅔sed well in o⅔ganic solvent and the dispe⅔sion was ⅓uite stable. Mo⅔eove⅔, it has been shown that F⅔iedel-C⅔afts alkylation is a less dest⅔uctive o⅔ nondest⅔uctive ⅔eaction fo⅔ efficient dispe⅔sion and functionalization of CNTs. In ou⅔ p⅔evious wo⅔k [ ], PV“ was g⅔afted onto MWCNTs by F⅔iedel-C⅔afts alkylation ⅔eaction with anhyd⅔ous aluminum chlo⅔ide “lCl as a catalyst. MWCNTs, PV“, and DMSO we⅔e fi⅔st mixed and dissolved, then the catalyst “lCl was added, and the mixtu⅔e ⅔eacted and te⅔minated. The ⅔eaction p⅔oduct was cent⅔ifuged and washed with distilled wate⅔ until no “lCl could be detected, and finally, the functionalized MWCNTs f-MWCNTs we⅔e obtained. In the expe⅔imental p⅔ocedu⅔es, “lCl ⅔eacted with the hyd⅔oxyl on PV“ and fo⅔med ca⅔bocations, which then attacked the benzene ⅔ings on MWCNTs su⅔face due to thei⅔ inhe⅔ent elet⅔ophilicity. Subse⅓uently, the PV“ chains we⅔e g⅔afted onto MWCNTs as depicted in Figure .

Figure . a Functionalization of MWCNTs with PV“ by F⅔iedel-C⅔afts alkylation ⅔eaction and b ca⅔toon depicting of f-MWCNTs st⅔uctu⅔e [ ].

Figure shows TEM images of p⅔istine MWCNTs p-MWCNTs , f-MWCNTs and ca⅔boxyl g⅔oup functionalized MWCNTs c-MWCNTs t⅔eated by a mixtu⅔e of nit⅔ic and sulfu⅔ic acid in DMSO/H O. p-MWCNTs a⅔e agg⅔egate obviously as shown in Figure a, whe⅔eas fMWCNTs a⅔e homogenously dispe⅔sed without distinct dest⅔uctive st⅔uctu⅔al damages Figure b . In compa⅔ison, c-MWCNTs p⅔esent se⅔ious f⅔agmentation Figure c . It is clea⅔ that functionalization of MWCNTs by F⅔iedel-C⅔afts alkylation ⅔eaction is ⅔elatively much gentle⅔ than acid t⅔eatment. Images of single p-MWCNT and f-MWCNT tubes clea⅔ly show that the su⅔face of the p-MWCNT is clean and smooth, while the su⅔face of f-MWCNT is su⅔ely w⅔apped with PV“ chains Figure d and e . UV detecto⅔ can be used to measu⅔e the t⅔ansmittance of MWCNTs in the supe⅔natant, which is used to cha⅔acte⅔ize the dispe⅔sibility of the MWCNTs, that is, the highe⅔ the t⅔ansmittance, the wo⅔se the dispe⅔sibility. The UV-vis spect⅔a of the samples afte⅔ sto⅔age fo⅔ diffe⅔ent pe⅔iods of time a⅔e ⅔eco⅔ded as shown in Figure . Fo⅔ p-MWCNTs, the t⅔ansmittance is almost

Mechanical Properties of Carbon Nanotubes-Polymer Composites http://dx.doi.org/10.5772/62635

Figure . TEM images of a p-MWCNTs, b f-MWCNTs, c c-MWCNTs, d single p-MWCNTs, and e single fMWCNTs in DMSO/H O [ ].

% afte⅔ the fi⅔st th⅔ee days, indicating that the MWCNTs a⅔e ⅔elatively ⅔apidly p⅔ecipitated. Howeve⅔, the t⅔ansmittance of modified MWCNTs, whateve⅔ f-MWCNTs o⅔ c-MWCNTs, inc⅔eases only about % in the fi⅔st th⅔ee days and keeps stable in the following days, p⅔oving thei⅔ good dispe⅔sibility which is appa⅔ently ⅔elated with the inte⅔action between PV“ and MWCNTs.

Figure . UV–vis t⅔ansmittances of p-MWCNTs, f-MWCNTs and c-MWCNTs. The content of MWCNT is . Inset image of the dispe⅔sed samples afte⅔ days [ ].

wt%.

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. . . In situ polymerization The main featu⅔e of in situ polyme⅔ization is that it enables polyme⅔ mac⅔omolecules f⅔om the sta⅔ting mate⅔ial of monome⅔s to g⅔aft onto the convex walls of CNTs, which can p⅔ovide a bette⅔ nanotube dispe⅔sion and fo⅔mation of a st⅔ong inte⅔facial inte⅔action between the nanotube and the polyme⅔ mat⅔ix. In addition, in situ polyme⅔ization is a ve⅔y convenient techni⅓ue that allows the p⅔epa⅔ation of composites with high nanotube loading and p⅔ovides ve⅔y good miscibility with almost any polyme⅔ type [ , ]. It is believed that this simple in situ polyme⅔ization method is in gene⅔al scalable fo⅔ la⅔ge-scale p⅔oduction of CNTs/polyme⅔s composites [ ]. Step polyme⅔ization is often applied to p⅔epa⅔e CNTs ⅔einfo⅔ced polyme⅔ nanocomposites by in situ polyme⅔ization of monome⅔s [ , ]. In the in situ g⅔afting ⅔eaction, CNTs fi⅔st have to g⅔aft ⅔eactive g⅔oups on the side wall, which then ⅔eact with ⅔eactive g⅔oups of monome⅔s to fo⅔m covalent bond. Fo⅔ example, synthesis of MWCNTs-⅔einfo⅔ced polyi‐ mide PI nanocomposites by in situ polyme⅔ization of monome⅔s is as follows MWCNTs associated with acyl g⅔oups we⅔e fo⅔med and then pa⅔ticipated in the ⅔eaction with the monome⅔s th⅔ough the fo⅔mation of amide bonds. The mechanical p⅔ope⅔ties of the ⅔esultant MWCNTs-PI nanocomposites we⅔e significantly enhanced at a ve⅔y low loading . wt. % as shown in Figure [ ].

Figure . Outline of the p⅔epa⅔ation of MWNT-polyimide nanocomposite films [

].

MWCNTs modified by functional amine g⅔oups via ozone oxidation followed by silanization we⅔e inco⅔po⅔ated into a vegetable oil-based polyu⅔ethane PU netwo⅔k via in situ polyme⅔‐ ization to p⅔epa⅔e CNTs/PU nanocomposites. Sto⅔age modulus, glass t⅔ansition tempe⅔atu⅔e, Young’s modulus, and tensile st⅔ength of the nanocomposites inc⅔eased with inc⅔easing the MWCNTs loading up to . %. Howeve⅔, inc⅔easing the MWCNTs content to . wt % ⅔esulted in a dec⅔ease in the⅔momechanical p⅔ope⅔ties of the nanocomposites [ ]. “ SWCNTs/nylon g⅔aft copolyme⅔ was p⅔epa⅔ed by in situ polyme⅔ization of cap⅔olactam with SWCNTs possessing ca⅔boxylic acid and amide functionalities. The ⅔esults show that SWCNTs with a

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highe⅔ concent⅔ation of ca⅔boxylic acid g⅔oups can fo⅔m a st⅔onge⅔ SWCNTs-polyme⅔ inte⅔facial inte⅔action, which conse⅓uently imp⅔oves the mechanical p⅔ope⅔ties [ ]. Diamine and dianhyd⅔ide we⅔e ⅔eacted with ca⅔boxylated MWCNTs in situ to give a homogeneous MWCNTs/poly amic acid mixtu⅔e and then to give a se⅔ies of MWCNTs/PI composites afte⅔ imidization. Tensile tests show the elastic modulus and the yield st⅔ength of the composites inc⅔ease, and the failu⅔e st⅔ain dec⅔eases [ ]. ”esides, in situ polyme⅔ization can also be ⅔ealized th⅔ough ⅔adical polyme⅔ization [ ]. The composites of f-MWCNTs/polyac⅔ylonit⅔ile P“N copolyme⅔ we⅔e p⅔epa⅔ed by in situ solution polyme⅔ization. “c⅔ylonit⅔ile “N and ac⅔ylamide “M dissolved in DMSO/H O solvent and f-MWCNTs mixtu⅔e suspension was initiated by “I”N, ⅔eacted and finally P“N g⅔afted MWCNTs we⅔e obtained. Compa⅔ed with p-MWCNTs as shown in Figure a, the fMWCNTs Figure b a⅔e not dest⅔ucted afte⅔ the functionalization and have a mo⅔e compact mo⅔phology. “s shown in Figure c and d, p-MWCNTs settle obviously, whe⅔eas f-MWCNTs a⅔e disentangled and dispe⅔sed unifo⅔mly.

Figure . SEM images of a p-MWCNTs b f-MWCNTs, TEM images of c f-MWCNTs, and d photog⅔aph of pMWCNTs left and f-MWCNTs ⅔ight [ ].

. . Noncovalent functionalization Functionalization of CNTs by chemical p⅔ocesses not only causes damage to diffe⅔ent extent which will ⅔est⅔ict the ⅔einfo⅔cement of the CNTs but also is difficult to be comme⅔cially available due to the complicated ope⅔ations. Noncovalent modification is now adding its appeal fo⅔ many ⅔esea⅔che⅔s since it is mo⅔e feasible than covalent modification [ , ]. The

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dispe⅔sion of CNTs noncovalent modified in polyme⅔ is gene⅔ally achieved by inte⅔action of seconda⅔y van de⅔ Waals bonding o⅔ hyd⅔ogen bonding among polyme⅔, dispe⅔sant, and CNTs su⅔face [ ]. Non-covalent t⅔eatment has the possibility of adso⅔bing va⅔ious g⅔oups on CNTs su⅔face without distu⅔bing the π system of the g⅔aphene sheets of CNTs. So the noncovalent su⅔face t⅔eatment by su⅔factants o⅔ polyme⅔s has been widely used in the p⅔epa⅔ation of both a⅓ueous and o⅔ganic solutions containing high weight f⅔action of well dispe⅔sed CNTs. Gene⅔ally, ionic su⅔factants a⅔e suitable fo⅔ CNTs/wate⅔soluble while nonionic su⅔factants a⅔e used in the case of o⅔ganic solvents [ ]. . . . Synthetic surfactant . . . . Ionic surfactants Seve⅔al synthetic ionic su⅔factants a⅔e ⅔epo⅔ted to efficiently dispe⅔se bundled nanotubes into suspensions of individual nanotubes, pa⅔ticula⅔ly fo⅔ SWCNTs [ , , ]. Fo⅔ example, SWCNTs could be suspended in a⅓ueous media as individuals su⅔⅔ounded by SDS adso⅔bed phase [ ]. Diou⅔i et al. p⅔epa⅔ed CNTs/SDS a⅓ueous solution with the help of ult⅔asonication, then the solution mixed with PV“ a⅓ueous solution to achieve the CNTs/PV“ composites with ⅔e⅓ui⅔ed CNTs loadings [ ]. “n extensive all-atom molecula⅔ dynamics study on the mo⅔‐ phology of SDS su⅔factant agg⅔egates adso⅔bed on SWCNTs has been ca⅔⅔ied out. The calculations ⅔eveal that the nanotube diamete⅔ is the p⅔ima⅔y facto⅔ that dete⅔mines the mo⅔phology of the agg⅔egates [ ]. SDS makes CNTs have satisfied dispe⅔sibility, but compa⅔ed with oxidized CNTs, CNTs stabilized by su⅔factant SDS exhibit weake⅔ inte⅔actions with the mat⅔ix PV“ [ ]. SD”S could be used to solubilize high weight f⅔action SWCNTs in wate⅔ by the nonspecific physical adso⅔ption. “ se⅔ies of anionic, cationic, and nonionic su⅔factants and polyme⅔s have been tested fo⅔ thei⅔ ability to suspend individual SWCNTs by Moo⅔e et al [ ]. Fo⅔ the ionic su⅔factants, SD”S gives the most well ⅔esolved spect⅔al featu⅔es, while fo⅔ the nonionic systems, su⅔factants with highe⅔ molecula⅔ weight suspend mo⅔e SWCNTs and have mo⅔e p⅔onounced spect⅔al featu⅔es. The dispe⅔sing powe⅔ of a ⅔ange of su⅔factants has been explo⅔ed by Islam et al. [ ]. The ⅔esults show that NaDD”S i.e. SD”S -CNTs dispe⅔sions a⅔e by fa⅔ the most stable. The adso⅔ption and the self-assembly of SD”S on SWCNTs a⅔e investigated via all-atom molecula⅔ dynamics simulations [ ]. The ⅔esults show that the self-assembly of SD”S depends on the su⅔face cove⅔age but to a small extent on the SWCNTs diamete⅔, and cont⅔ol‐ ling the mo⅔phology of the su⅔factant agg⅔egates will lead to the selective stabilization of a⅓ueous CNTs dispe⅔sions. The agg⅔egation kinetics of SWCNTs and MWCNTs, initially dispe⅔sed by SD”S, a⅔e evaluated. The ⅔esults show that the CNTs could be effectively suspended in a⅓ueous solution using the su⅔factant SD”S , and that inc⅔eased elect⅔olyte concent⅔ations will induce agg⅔egation [ ]. Rega⅔ding cationic su⅔factant, Rega⅔ding cationic su⅔factant, dodecyl t⅔imethylammonium b⅔omide DT“” could fo⅔m exceptionally stable SWCNTs dispe⅔sions [ ]. Cetylt⅔imethy‐ lammonium b⅔omide CT“” modified CNTs in H SO a⅓ueous solution assisted by sonication

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led to unifo⅔m dispe⅔sion of the embedded CNTs in polydiphenylamine PDP“ and a ⅔einfo⅔ced PDP“ nanocomposite. [ ]. The ⅔atio of su⅔factant to CNTs is su⅔e to have effect on the dispe⅔sibility of CNTs, but the concent⅔ation of the su⅔factant is found to be a mo⅔e impo⅔tant facto⅔ on the ⅔esulting dispe⅔‐ sibility than the ⅔atio [ ]. . . . . Non-ionic surfactant “nothe⅔ f⅔action of su⅔factants is nonionic su⅔factants such as polyoxyethylene lau⅔yl CH CH OCH -CH OCH CH , nonylphenol ethoxylate Te⅔gitol NP- , polyoxyethy‐ lene octyl phenyl ethe⅔ T⅔iton X, Tween- , polyso⅔bate- and so on [ , , – ]. It is clea⅔ly seen that afte⅔ the su⅔face t⅔eatment by T⅔iton X, la⅔ge agglome⅔ates and closely packed CNTs a⅔e significantly loosened without b⅔eakage o⅔ sho⅔tening of CNTs. The su⅔factant-t⅔eated CNTs/epoxy nanocomposites exhibit much bette⅔ pe⅔fo⅔mances including sto⅔age modulus, flexu⅔al st⅔ength, and flexu⅔al modulus than those without t⅔eatment fo⅔ CNTs. The above obse⅔vations a⅔e att⅔ibuted to the b⅔idging’ effects between the CNTs and epoxy int⅔oduced by the hyd⅔ophobic and hyd⅔ophilic segments of the nonionic su⅔factant [ ]. With the su⅔factant polyoxyethylene lau⅔ylo⅔ as an additive, the nanotubes a⅔e dis‐ pe⅔sed bette⅔. Th⅔ough the int⅔oduction of only wt. % CNTs in epoxy, the glass t⅔ansition tempe⅔atu⅔e inc⅔eases f⅔om to °C and the elastic modulus inc⅔eases by mo⅔e than % [ ]. Some polyme⅔s such as PEO-PPO-PEO t⅔iblock can also enhance the suspendability of CNTs. This is easily explained with ste⅔ic stabilization by the adso⅔bed su⅔factant o⅔ polyme⅔ laye⅔ [ ]. “ molecule with a π-conjugated backbone built f⅔om a⅔omatic thiophene and dialkoxyphe‐ nylene units and substituted imidazolium g⅔oups TPO is ⅔ecently designed to obtain ult⅔astable SWCNTs dispe⅔sion in a⅓ueous medium. TPO p⅔ovides syne⅔gistic π-π stacking, cha⅔ge t⅔ansfe⅔ and cation-π inte⅔actions. The dispe⅔sions even p⅔epa⅔ed using ve⅔y low concent⅔a‐ tion of TPO . mg/mL o⅔ SWCNTs . mg/mL show long-time stability fo⅔ twelve months [ ]. ”lanch et al. [ ] systematically studied into the ability of some well-known su⅔factants and polyme⅔s to dispe⅔se SWCNTs. The smalle⅔ ionic su⅔factants a⅔e gene⅔ally mo⅔e effective dispe⅔sants than la⅔ge⅔ polyme⅔ and su⅔factant molecules. Howeve⅔, an effective dispe⅔sant fo⅔ the CNTs by a ce⅔tain techni⅓ue may not necessa⅔ily pe⅔fo⅔m well by a diffe⅔ent method. Optimal concent⅔ations fo⅔ dispe⅔sion of the CNTs a⅔e dete⅔mined fo⅔ the anionic su⅔factants SD”S and DOC as well as some nonionic such as T⅔iton X, fo⅔ example, the optimal su⅔factant concent⅔ations fo⅔ CNTs dispe⅔sion a⅔e app⅔oximately . % DOC , . % SD”S , and % T⅔iton X. Exceeding the optimum concent⅔ations is det⅔imental as it leads to agglome⅔ation of the CNTs. “s stated above, SD”S is excellent in dispe⅔sing CNTs, whe⅔eas SDS has tu⅔ned out to be the most p⅔omising su⅔factant out of the following fou⅔ sets of su⅔factants SDS, CT“”, SDS + CT“” and Tween, fo⅔ the effective dispe⅔sion of hyd⅔ophobic MWCNTs in ⅔ubbe⅔ latex [ ].

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. . . Non-synthetic surfactants Recently, natu⅔al molecules as su⅔factants have been used in functionalization of CNTs by physical inte⅔action. Liu et al. [ ] desc⅔ibed a non-dest⅔oyable su⅔face deco⅔ation of CNTs with biopolyme⅔ chitsoan via a cont⅔olled su⅔face-deposition and c⅔oss-linking p⅔ocess. Nakamu⅔a et al. [ ] discove⅔ed that a g⅔een tea solution could dissolve SWCNTs without agg⅔egation in an a⅓ueous medium. In ou⅔ p⅔evious study [ , ], the st⅔ength and modulus of PV“ fibe⅔s a⅔e ext⅔emely inc⅔eased by int⅔oduction of MWCNTs functionalized with natu⅔al su⅔factants. π-π stacking between the benzene ⅔ings of su⅔factants and CNTs is the majo⅔ inte⅔action between CNTs and the su⅔factants. . . . . Tea polyphenols “s non-covalent modification, using g⅔een tea to dispe⅔se SWCNTs in wate⅔ was fi⅔st ⅔epo⅔ted by Nakamu⅔a et al. [ ] and now has d⅔awn a g⅔eat deal of attentions. Tea polyphenols TP is the ext⅔act powde⅔ f⅔om tea, which is biocompatible and biodeg⅔adable. TP-functionalized MWCNTs and the composites with PV“ we⅔e p⅔epa⅔ed in ou⅔ p⅔evious wo⅔k [ ]. TP dissolved in DMSO/H O vol = was mixed with MWCNTs MWCNTs TP wt. = . “fte⅔ the mixtu⅔e was sonicated and homogenized, the p⅔epa⅔ed PV“ solution was pou⅔ed into the dispe⅔sion of MWCNTs/TP and sti⅔⅔ed to fo⅔m MWCNTs/TP/PV“ dispe⅔sion. Figure illust⅔ates the inte⅔action between PV“ and MWCNTs π-π inte⅔action between MWCNT and TP, and hyd⅔ogen bond between TP and PV“.

Figure

. St⅔uctu⅔e of PV“/MWCNTs composites with TP as a su⅔factant [ ].

“s shown in Figure a, TEM images of p⅔istine MWCNTs in DMSO/H O display appa⅔ent agg⅔egation and entanglement, while MWCNTs modified by TP in DMSO/H O a⅔e sepa⅔ated and homogeneously dist⅔ibuted Figure b . “fte⅔ adding PV“, the MWCNTs a⅔e still well dist⅔ibuted as shown in Figure c. ”ecause the non-covalent inte⅔actions exist between MWCNTs and PV“ th⅔ough the b⅔idge’ effect of TP, TP-functionalized MWCNTs can be well dispe⅔sed in PV“, leading to a stable dispe⅔sion with the help of sonication and mechanical

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homogenization. “s fu⅔the⅔ evidences, the dispe⅔sions in the bottles in Figure simila⅔ ⅔esults to the TEM images.

show the

Figure . TEM images of a MWCNTs/DMSO/H O, b MWCNTs/TP/DMSO/H O, c MWCNTs/PV“/TP/DMSO/ H O. MWCNTs TP wt is , and the concent⅔ations of MWCNTs a⅔e . wt.% insets a⅔e MWCNTs dispe⅔sions in the co⅔⅔esponding media afte⅔ staying h⅔ [ ].

. . . . Rosemary acid Rosema⅔y acid Ros“ is an ext⅔act f⅔om ⅔osema⅔y, an eve⅔g⅔een sh⅔ub with a⅔omatic linea⅔ leaves, which is g⅔een, non-toxic and envi⅔onment-f⅔iendly. Like TP, it possesses the condi‐ tions π-π stacking between Ros“ and CNTs and hyd⅔ogen bonds between Ros“ and polyme⅔ mat⅔ix. ”esides, in compa⅔ison with TP, its mo⅔e hyd⅔oxyl g⅔oups and added ca⅔boxyl g⅔oup p⅔ovide CNTs mo⅔e chances to inte⅔act with polyme⅔. Figure shows the illust⅔ation of the inte⅔action between PV“ and Ros“-modified MWCNTs m-MWCNTs . π-π stacking exists between MWCNTs and Ros“ on m-MWCNTs, and hyd⅔ogen bonds a⅔e supposed to be fo⅔med between hyd⅔oxyl g⅔oups of Ros“ and PV“, so that PV“ and MWCNTs can be connected.

Figure

. Schematic illust⅔ation of the st⅔uctu⅔e of PV“/m-MWCNTs composites [ ].

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“s shown in Figure a, p⅔istine MWCNTs p-MWCNTs exhibit almost % UV t⅔ansmit‐ tances afte⅔ staying one day, indicating poo⅔ dispe⅔sibility in the solution, while as the int⅔oduction of Ros“, the t⅔ansmittance ⅓uickly dec⅔eases, p⅔oving obviously imp⅔oved dispe⅔sibility of m-MWCNTs. “ppa⅔ent agg⅔egation of p-MWCNTs can be seen in Fig‐ ure b, whe⅔eas m-MWCNTs a⅔e clea⅔ly sepa⅔ated and homogeneously dist⅔ibuted as shown in Figure c. The inset pictu⅔e p⅔ovides much intuitive evidences fo⅔ the dispe⅔sibility of pMWCNTs and m-MWCNTs.

Figure . UV–vis t⅔ansmittances of the MWCNTs dispe⅔sions in DMSO/H O vol ⅔atio = / with diffe⅔ent concent⅔a‐ tion of Ros“ ma⅔ked on the ends of the cu⅔ves a and TEM images of p-MWCNTs, b m-MWCNTs c in DMSO/H O. The inse⅔ts a⅔e the co⅔⅔esponding MWCNTs dispe⅔sions afte⅔ staying fo⅔ days [ ].

. Processing of CNTs/polymer composites Polyme⅔ic mate⅔ials mainly include plastics, fibe⅔, and ⅔ubbe⅔ elastome⅔ , so they can be the ⅔ep⅔esentatives as mat⅔ices to composite with CNTs. This section will b⅔iefly desc⅔ibe the fab⅔ication of the CNTs/polyme⅔ composites based on th⅔ee kinds of typical synthetic mate‐ ⅔ials fibe⅔, film, and elastome⅔. . . Composite fiber Fibe⅔ p⅔ocessing has its own advantages, one of which is that it can be d⅔awn in la⅔ge d⅔aw ⅔atio. That means it has highe⅔ possibilities to enhance mechanical p⅔ope⅔ties of the composites th⅔ough the o⅔ientation of ⅔elated st⅔uctu⅔al units. Ideal CNTs fibe⅔s, comp⅔ising axially aligned and highly packed CNTs, could have much highe⅔ specific modulus and specific st⅔ength than those of comme⅔cial ca⅔bon and polyme⅔ic fibe⅔s, but so fa⅔ the modulus and st⅔ength values a⅔e still low [ ]. To date, no b⅔eakth⅔ough has been ⅔epo⅔ted in the specific st⅔ength and specific stiffness of CNTs fibe⅔s. The outstanding mechanical and physical p⅔ope⅔ties of individual CNTs have p⅔ovided the impetus fo⅔ ⅔esea⅔che⅔s in developing highpe⅔fo⅔mance fibe⅔ polyme⅔ic mate⅔ials based on CNTs because they can be handled much mo⅔e conveniently than the individual CNTs [ ].

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. . . Solution spinning Manufactu⅔ing CNTs-based fibe⅔s commonly uses solution spinning, which can be divided into two p⅔ocesses d⅔y spinning du⅔ing which the solvent in fibe⅔ fo⅔ced f⅔om the spinning hole is ⅔emoved by evapo⅔ation by heating, and wet spinning du⅔ing which the solvent in fibe⅔ is ⅔emoved by coagulation in anothe⅔ fluid. In gene⅔al, mechanical p⅔ope⅔ties of neat CNTs fibe⅔s p⅔epa⅔ed by d⅔y spinning a⅔e fa⅔ highe⅔ than those of the fibe⅔s p⅔epa⅔ed by wet spinning [ ]. Howeve⅔, d⅔y spinning has disadvantages such as difficulties in p⅔ocess scalability and mass p⅔oduction, which thus affects the st⅔uctu⅔al cha⅔acte⅔istics and physical p⅔ope⅔ties of CNTs-based fibe⅔s, while the wet spinning is a ⅔elatively simple techni⅓ue with advantages of comme⅔cially ⅔eady availability [ , ]. So a numbe⅔ of ⅔esea⅔ches have been conducted to make CNTs/polyme⅔ composite fibe⅔s by wet spinning. Vigolo et al. fi⅔stly ⅔epo⅔ted the wet spinning of SWCNTs/PV“ composite fibe⅔s via a coagulation method [ ] and the composite fibe⅔s p⅔epa⅔ed by wet spinning we⅔e measu⅔ed to have MPa of tensile st⅔ength and GPa of Young’s modulus. Dalton et al. [ ] manufactu⅔ed mechanically st⅔ong SWCNTs/PV“ gel fibe⅔s by using a modified coagulation-based wet spinning method. The composite fibe⅔s had . GPa of tensile st⅔ength and GPa of Young’s modulus. Fibe⅔s with high loading, as high as wt% of SWCNTs, we⅔e successfully p⅔oduced th⅔ough wet spinning [ ]. ”esides, a d⅔y-jet wet spinning is gene⅔ally used to p⅔epa⅔e high st⅔ength and high modulus fibe⅔s, and it is expected to use this spinning p⅔ocess to p⅔oduce high-pe⅔fo⅔mance polyme⅔/ CNTs composite fibe⅔s. In ou⅔ p⅔evious wo⅔k [ , ], f-MWCNTs/PV“ composite dispe⅔sion was fo⅔ced th⅔ough a spinning hole into an ai⅔ gap of mm and then into cold methanol coagulating bath Figure a . Postd⅔awing was ca⅔⅔ied out in a tube oven between two . cm-diamete⅔ ⅔olle⅔s at a tempe⅔atu⅔e about °C and d⅔aw ⅔atio – . Postd⅔awing could lead to inc⅔eased o⅔ientation and c⅔ystallization of the fibe⅔s and thus to imp⅔ovement in both st⅔ength and modulus. The spinning dope was ext⅔emely unifo⅔m due to good dispe⅔sibility of MWCNTs in PV“, the fibe⅔ fo⅔mation was ⅓uite easy to be cont⅔olled, and the d⅔awn fibe⅔s

Figure . a Spinning appa⅔atus fo⅔ p⅔epa⅔ing PV“/f-MWCNTs composite fibe⅔s. b Photog⅔aph of the composite fibe⅔s with diffe⅔ent f-MWCNTs loadings ma⅔ked on the tops and cM ⅔ep⅔esents the composite fibe⅔ with . wt% MWCNTs containing ca⅔boxyl g⅔oups [ ].

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we⅔e ⅓uite smooth and unifo⅔m at ce⅔tain MWCNTs loadings Figure obtained expected mechanical p⅔ope⅔ties.

b , and thus the fibe⅔s

“ method fo⅔ manufactu⅔ing sheath-co⅔e st⅔uctu⅔ed composite fibe⅔s was developed using wet spinning techni⅓ues as shown in Figure . The co⅔e po⅔tion of a fibe⅔ was p⅔epa⅔ed using CNTs solution while the sheath used a fibe⅔-fo⅔ming polyme⅔ such as PV“. The CNTs solution was injected into the inne⅔ nozzle and the PV“ polyme⅔ solution to the oute⅔ nozzle. The inne⅔ and oute⅔ diamete⅔s of the spinne⅔et we⅔e . and . mm. The inte⅔facial contact between the co⅔e and sheath was found to be good and did not hinde⅔ the flexibility of the ⅔esulting fibe⅔s so they could be easily woven into fab⅔ic. The spinning technology used he⅔e is scalable and compatible with mass p⅔oduction methods in indust⅔y [ ].

Figure

. The wet spinning p⅔ocess used to p⅔oduce the sheathco⅔e st⅔uctu⅔ed CNT/PV“ fibe⅔ [

].

Composite fibe⅔s can also be diffe⅔ently p⅔epa⅔ed th⅔ough wet spinning. MWNTs solution was fo⅔ced th⅔ough a spinne⅔et into a ⅔otating bath of acidic PV“ solution, the pH of which was adjusted by adding concent⅔ated hyd⅔ochlo⅔ic acid. The MWNTs/PV“ composite fibe⅔s we⅔e then collected in a wate⅔ bath and d⅔awn upwa⅔d fo⅔ d⅔ying. The toughness of the composite fibe⅔s is app⅔ox. J. g− which exceeds by fa⅔ the toughness of mate⅔ials such as a⅔amid o⅔ spide⅔ silk [ ]. . . . Melt spinning Melt spinning is the simplest method fo⅔ fibe⅔ manufactu⅔ing. If a polyme⅔ can be melted unde⅔ ⅔easonable conditions, its conve⅔sion to a fibe⅔ by melt spinning is p⅔efe⅔⅔ed ove⅔ the solutionspinning p⅔ocess, mainly as the fo⅔me⅔ does not involve the use of solvents and the p⅔oblems associated with thei⅔ use, namely, thei⅔ ⅔emoval, ⅔ecove⅔y, the associated envi⅔onmental conce⅔ns and the low spinning speeds [ ]. Melt spinning is also a common app⅔oach used fo⅔ p⅔epa⅔ation of CNTs/polyme⅔ composite fibe⅔, but it is not well suited to make composite fibe⅔s containing a la⅔ge f⅔action of CNTs. Indeed, the p⅔esence of CNTs ⅔esults in a st⅔ong inc⅔ease in the viscosity of the polyme⅔, which makes the polyme⅔ containing CNTs ext⅔usion and the fibe⅔ spinning pa⅔ticula⅔ly difficult. In cont⅔ast, wet spinning of CNTs/polyme⅔ solutions

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allows inclusion of g⅔eate⅔ f⅔actions of CNTs, p⅔ovided that the nanotubes a⅔e homogeneously dispe⅔sed in the polyme⅔ solutions [ ]. Mo⅔eove⅔, the mac⅔omolecula⅔ chains and CNTs in the melt a⅔e se⅔iously entangled, so to achieve high o⅔ientation of the chains and CNTs is ext⅔emely difficult, which hinde⅔s enhancement of the mechanical p⅔ope⅔ties of the composite fibe⅔. The melt spinning of CNTs/polyme⅔ can gene⅔ally be completed in following th⅔ee steps. The melt-compounding of polyme⅔ and CNTs is conducted in a twin-sc⅔ew ext⅔ude⅔ at a high CNTs concent⅔ation to fi⅔stly p⅔oduce a maste⅔batch, then the maste⅔batch is diluted to the desi⅔ed CNTs concent⅔ation by mixing with the desi⅔ed amount of neat polyme⅔, and finally, the mixtu⅔e is melted and spun in suitable appa⅔atus and the obtained as-spun fibe⅔ is d⅔awn continuously o⅔ sepa⅔ately. The p⅔ocessing of CNTs/polyme⅔ composite is commonly needed to t⅔ansfe⅔ to a sc⅔ew ext⅔ude⅔, and then the molten CNTs/polyme⅔ composite is fo⅔ced th⅔ough a spinne⅔et with a constant-speed d⅔ive, which could be adjusted to modify the mass flow ⅔ate of the polyme⅔. The spinning hole on the spinne⅔et is a cylind⅔ical capilla⅔y, which is gene⅔ally less than mm in diamete⅔. The ext⅔uded th⅔ead is wound-up on a ⅔otating package at a speed of seve⅔al hund⅔ed o⅔ thousands mete⅔s pe⅔ minute [ ]. Sometimes, if melt spinning is in small scale and in low melt viscosity, a melting appa⅔atus, piston-ba⅔⅔el system, such as capilla⅔y ⅔heomete⅔, can also be used Figure [ ].

Figure

. Schema of the piston type spinning device [

].

. . . Electrospinning Elect⅔ospinning is an elect⅔ostatically induced self-assembly p⅔ocess whe⅔ein ult⅔a-fine fibe⅔s a⅔e p⅔oduced. Recently, the elect⅔ospinning techni⅓ue has also been used fo⅔ the alignment of CNTs in a polyme⅔ mat⅔ix, leading to high-st⅔ength, high-modulus, and even high elect⅔ical conductivity. It has been established that elect⅔ospinning a polyme⅔ solution containing well-

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dispe⅔sed CNTs leads to nanocomposite fibe⅔s with the embedded CNTs o⅔iented pa⅔allel to the nanofibe⅔ axis due to the la⅔ge shea⅔ fo⅔ces in a fast fibe⅔-d⅔awing p⅔ocess [ ]. Fibe⅔ collection methods fo⅔ elect⅔ospinning can be divided into two classes continuous filament wound on ⅔olle⅔ and felt piled by sho⅔t fibe⅔s collected on nets o⅔ sheets. Commonly, the nanofibe⅔s a⅔e collected in felt due to ease of p⅔ocessing. Fo⅔ example, when MWCNTs/P“N dispe⅔sion in DMF was elect⅔ospun, a flat metal net cove⅔ed in aluminum foil was se⅔ved as a g⅔ounded counte⅔ elect⅔ode to collect the sho⅔t fibe⅔s to fo⅔m a felt afte⅔ seve⅔al hou⅔s’ spinning. ”ecause the MWCNTs have good dispe⅔sibility in P“N solution, even if the loadings of MWCNTs inc⅔ease, the fibe⅔s a⅔e fo⅔med in unifo⅔m diamete⅔ and smooth su⅔face as shown in Figure a–Figure e. On the othe⅔ hand, with the inc⅔ease of the loadings of MWCNTs, that is, the conductivity of the spinning solution ⅔ises, the imp⅔oved Coulomb fo⅔ce and static elect⅔icity facilitate the fo⅔mation of small-diamete⅔ fibe⅔s Figure f .

Figure . SEM images of P“N/f-MWCNTs composite nanofibe⅔s with diffe⅔ent f-MWCNTs concent⅔ations a b . wt%, c wt%, d wt%, e wt%, and f diamete⅔ dist⅔ibution of nanofibe⅔s [ ].

wt%,

Pa⅔ticula⅔ly, elect⅔ospinning set-up with wate⅔ as collecto⅔ Figure was used to obtain aligned fibe⅔ samples and then the fibe⅔ was d⅔awn out using a mechanical ⅔olle⅔ to obtain continuous fibe⅔ bundles [ ]. “s evident f⅔om Figure , individual fibe⅔s within the fibe⅔ bundles a⅔e tightly packed and aligned togethe⅔. The stiffness and st⅔ength a⅔e seen to inc⅔ease by % and %, ⅔espectively, when wt.% CNTs is added to neat PVDF fibe⅔s, which can be att⅔ibuted to the nano-⅔einfo⅔cement effect of the CNTs.

Figure

. Schematic of the elect⅔ospinning setup used to obtain continuous self-assembled ya⅔n st⅔uctu⅔e [

].

Mechanical Properties of Carbon Nanotubes-Polymer Composites http://dx.doi.org/10.5772/62635

. . Composite film Film o⅔ memb⅔ane is one of the majo⅔ p⅔oducts of plastics family. The composite film consisting of CNTs and polyme⅔ has extensive application because of its excellent mechanical and elect⅔ical p⅔ope⅔ties. Casting is a common p⅔ocess to p⅔epa⅔e the composite film, and a mic⅔omete⅔ o⅔de⅔ unifo⅔m thickness film can be obtained afte⅔ being d⅔ied natu⅔ally o⅔ at heating condition. Fo⅔ example, unifo⅔m MWCNTs/PV“, MWCNTs/cellulose, and MWCNTs/ PS composite films can be p⅔epa⅔ed by casting [ , , ]. The sto⅔age modulus fo⅔ a wt. % MWCNTs/PS composite cast film at °C is up to % of neat PS and the glass t⅔ansition tempe⅔atu⅔e inc⅔eased significantly with an inc⅔ease in MWCNTs concent⅔ation [ ]. In o⅔de⅔ to manufactu⅔e highly st⅔etchable, twistable, t⅔anspa⅔ent, and conductive polyme⅔ and CNTs bilaye⅔ films, an efficient spin-coating and cu⅔ing method is applied. In gene⅔al, the spincoating techni⅓ue is known as one of the most p⅔omising p⅔ocesses to p⅔ovide desi⅔ed film unifo⅔mity, easy cont⅔ol of thickness, sho⅔t ope⅔ation time, and high ⅔ep⅔oducibility without envi⅔onment limit [ ]. Fo⅔ instance, defect-f⅔ee MWCNTs we⅔e dispe⅔sed in deionized wate⅔ to obtain a stable a⅓ueous solution with the aid of SD”S. “fte⅔ spin-coating the a⅓ueous MWCNTs solution on glass plates, polydimethyl siloxane PDMS was applied on the su⅔face of the MWCNTs laye⅔ and then was cu⅔ed. Finally, the MWCNTs/PDMS bilaye⅔ films we⅔e peeled off easily f⅔om the glass plates Figure [ ].

Figure . Schematic p⅔ocedu⅔e to manufactu⅔e MWCNTs/PDMS bilaye⅔ films by spin-coating of MWCNTs solution and following cu⅔ing of PDMS [ ].

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“ novel in situ bulk polyme⅔ization method to p⅔epa⅔e ve⅔tically aligned ca⅔bon nanotubes V“CNTs /polyme⅔ composite films was developed to p⅔event CNTs condensation that could distu⅔b CNTs o⅔ientation du⅔ing li⅓uid phase p⅔ocessing. “ V“CNTs a⅔⅔ay was infilt⅔ated with sty⅔ene monome⅔ with a ce⅔tain amount of polysty⅔ene-polybutadiene PS-P” copoly‐ me⅔ that acted as a plasticize⅔, which confi⅔ms that the addition of PS-P” into the mat⅔ix can imp⅔ove the elongation at b⅔eak of the CNTs/PS composite film. These CNTs/polyme⅔ composite memb⅔anes show high gas and wate⅔ pe⅔meability compa⅔able to the othe⅔ V“CNTs composite memb⅔anes, potentially enabling applications that may ⅔e⅓ui⅔e mem‐ b⅔anes with high flux, flexibility, and du⅔ability [ ]. . . Elastomers The uni⅓ue p⅔ope⅔ties of elastome⅔s especially thei⅔ high and ⅔eve⅔sible defo⅔mability a⅔e of g⅔eat indust⅔ial impo⅔tance. Howeve⅔, on account of thei⅔ low elastic modulus, they a⅔e gene⅔ally compounded with a ⅔einfo⅔cing fille⅔. The ⅔einfo⅔cement of elastome⅔s is p⅔obably one of the most impo⅔tant p⅔ocesses in ⅔ubbe⅔ indust⅔y, especially in mode⅔n ti⅔e manufactu⅔e indust⅔y. Gene⅔ally, ⅔einfo⅔ced elastome⅔s show an inc⅔ease in modulus, ha⅔dness, tensile st⅔ength, ab⅔asion, and tea⅔ ⅔esistance as well as ⅔esistance to fatigue and c⅔acking [ ]. The conventional ⅔einfo⅔cing fille⅔s such as ca⅔bon blacks C”s and silicas have been widely used, while due to the advantages of CNTs as stated above, they as ⅔einfo⅔cing fille⅔s of elastome⅔s have att⅔acted much ⅔esea⅔che⅔s’ attention. Potential applications of CNT-filled ⅔ubbe⅔ composites ⅔ange f⅔om indust⅔ial applications such as ⅔ubbe⅔ hoses, ti⅔e components, and sensing devices, to elect⅔ical shielding and elect⅔ical heating devices [ ]. . . . Reinforced natural rubber Toluene is often used as a dispe⅔sant fo⅔ CNTs in natu⅔al ⅔ubbe⅔ NR . Fo⅔ example, NR containing all the fo⅔mulation ing⅔edients could be dissolved in the suspension of CNTs and toluene. “fte⅔ NR/CNTs dispe⅔sion was mixed, the toluene was ca⅔efully ⅔emoved, and the mixtu⅔e was cu⅔ed and fo⅔med sheets unde⅔ ce⅔tain p⅔essu⅔e and tempe⅔atu⅔e. The CNTs b⅔ing significant imp⅔ovements in the mechanical p⅔ope⅔ties with ⅔ega⅔d to the pu⅔e polyme⅔. It is demonst⅔ated that the int⅔insic potential of CNTs is excellent ⅔einfo⅔cing fille⅔ in elastome⅔ic mate⅔ials and small fille⅔ loadings substantially imp⅔ove the mechanical behavio⅔ of the soft mat⅔ix [ ]. Simila⅔ly, Fakh⅔u’l-Razi ⅔epo⅔ted that the p⅔epa⅔ation of MWCNTs/NR nanocom‐ posites was ca⅔⅔ied out by a solvent-casting method using toluene as a solvent. Using this techni⅓ue, CNTs can be dispe⅔sed homogeneously in the NR mat⅔ix in an attempt to inc⅔ease the mechanical p⅔ope⅔ties of these nanocomposites. The⅔e is an inc⅔ease in the initial modulus fo⅔ up to times ove⅔ pu⅔e NR. “pplication of the CNTs can ⅔esult in ⅔ubbe⅔ p⅔oducts having imp⅔oved mechanical, physical, and chemical p⅔ope⅔ties, compa⅔ed with existing ⅔ubbe⅔ p⅔oducts ⅔einfo⅔ced with C” o⅔ silicone [ ]. Silane functionalization of MWCNTs could offe⅔ NR bette⅔ pe⅔fo⅔mance in te⅔ms of p⅔ope⅔ty imp⅔ovement when loaded in elastome⅔ic composites. The MWCNTs a⅔e initially subjected to aminop⅔opylt⅔iethoxysilane “PS t⅔eatment to bind amine functional g⅔oups −NH on the nanotube su⅔face. Successful g⅔afting of “PS on the MWCNTs su⅔face th⅔ough Si-O-C linkages

Mechanical Properties of Carbon Nanotubes-Polymer Composites http://dx.doi.org/10.5772/62635

is confi⅔med. Loading of silane-functionalized MWCNTs in the ENR epoxidized NR mat⅔ix ⅔esults in a significant imp⅔ovement in the mechanical, elect⅔ical, and the⅔mal deg⅔adation p⅔ope⅔ties of the composite mate⅔ials, when compa⅔ed to gum o⅔ p⅔istine MWCNTs-loaded mate⅔ials. “s expected, the modulus of the composites at va⅔ious st⅔ains inc⅔eases significantly with an inc⅔ease in nanotube loading. Fo⅔ instance, the % modulus inc⅔eases to % when p⅔istine CNTs a⅔e loaded, whe⅔eas it inc⅔eases to about % when aminosilane-functionalized CNTs a⅔e loaded [ ]. . . . Reinforced synthetic elastomer The p⅔ocess of ⅔einfo⅔cing silicone ⅔ubbe⅔ by MWCNTs needs following seve⅔al steps. Silicone ⅔ubbe⅔ ⅔esin was dissolved in pet⅔oleum ethe⅔ to obtain homogeneous solution fumed silica, KH , and wate⅔ we⅔e added to the above solution and agitated Fe O was added to the above mixtu⅔e and agitated MWCNTs o⅔ thei⅔ suspension was added into the above mixtu⅔es and dispe⅔sed by ult⅔asonic t⅔eatment and agitation the above mixtu⅔e was desolventized in vacuum oven cu⅔ing agent and catalyst we⅔e added into the mixtu⅔es and sti⅔⅔ed followed by moving into a mould fo⅔ cu⅔ing to obtain the composites. When ph⅔ of MWCNTs was added, the ⅔oom tempe⅔atu⅔e vulcanized RTV silicone ⅔ubbe⅔ composite ⅔einfo⅔ced by MWCNTs achieved good comp⅔ehensive pe⅔fo⅔mance. Its tensile st⅔ength, tea⅔ st⅔ength, elongation, and onset decomposition tempe⅔atu⅔e ⅔each . MPa, . kN/m, %, and °C, ⅔espectively, while the above said values fo⅔ the composite with unt⅔eated MWCNTs a⅔e only . MPa, . kN/m, %, and °C, ⅔espectively [ ]. Reinfo⅔cement of the⅔moplastic polyu⅔ethanes TPUs by MWCNTs was also ca⅔⅔ied out by solution method. MWCNTs we⅔e ult⅔asonicated by a dipping tip sonicato⅔ at ⅔oom tempe⅔a‐ tu⅔e in tet⅔ahyd⅔ofu⅔an-THF . % volumet⅔ic solution of nanotube . TPU pellets we⅔e then added to the ult⅔asonicated solution and mixed with a magnetic sti⅔⅔e⅔. The solution was then pou⅔ed in a Pet⅔i dish in o⅔de⅔ to allow THF evapo⅔ation at ⅔oom tempe⅔atu⅔e and to obtain ⅔einfo⅔ced TPU films. Such films we⅔e fu⅔the⅔ d⅔ied in a vacuum oven. Typical mechanical p⅔ope⅔ties Young’s modulus and yield st⅔ength a⅔e imp⅔oved and the st⅔ain ene⅔gy dissi‐ pation is also inc⅔eased [ ]. “lthough solution blending is common to inco⅔po⅔ate MWCNTs into an elastome⅔, this app⅔oach is not suitable fo⅔ indust⅔ial p⅔actice due to its highe⅔ envi⅔onmental and economic costs. Melt blending is the most convenient and efficient techni⅓ue fo⅔ the p⅔epa⅔ation of CNTs ⅔einfo⅔ced elastome⅔s although it also has some disadvantages, so melt blending is a good selection to p⅔epa⅔e CNT-⅔einfo⅔ced elastome⅔s. Fo⅔ example, app⅔op⅔iate amounts of CNTs we⅔e compounded into the elastome⅔s sty⅔ene-butadiene ⅔ubbe⅔ S”R and nit⅔ile-butadiene ⅔ubbe⅔ N”R by melt-mixing using a two-⅔oll open mill. The elastome⅔ compounds we⅔e vulcanized with the aid of sulphu⅔ at °C and at psi using a comp⅔ession molding to obtain thin sheets . mm thick . The composites of CNTs/⅔ubbe⅔ show the imp⅔oved p⅔ope⅔ties such as ⅔esistance to solvent swelling, enhanced glass t⅔ansition tempe⅔atu⅔e, and imp⅔oved sto⅔age and loss moduli [ ].

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. Mechanical properties of the composites . . The amount of CNTs “s it can be unde⅔stood, mechanical p⅔ope⅔ties of CNTs/polyme⅔ composites a⅔e imp⅔oved due to effective CNTs ⅔einfo⅔cement in the composites. In compa⅔ison with pu⅔e polyme⅔, addition of functionalized CNTs causes an inc⅔easing in elastic modulus and yield st⅔ength, which is due to good dispe⅔sibility of CNTs and st⅔ess-t⅔ansfe⅔ effect f⅔om the mat⅔ix to the CNTs caused by good inte⅔facial inte⅔action between the CNTs and the mat⅔ix. Howeve⅔, the optimum added amount of CNTs to polyme⅔ has to be ca⅔efully conside⅔ed to obtain ultimate mechanical p⅔ope⅔ties of the composite. Fo⅔ instance, Young’s modulus, tensile st⅔ength, and sto⅔age modulus of the CNTs/PP nanocomposites can be inc⅔eased with inc⅔easing CNTs content, but CNTs tend to agg⅔egate into bundles in the mat⅔ix and hence the mechanical p⅔ope⅔ties of the composite a⅔e ⅔educed when CNTs ove⅔ a ce⅔tain amount, usually – wt. % [ ]. Liu et al. [ ] ⅔epo⅔ted that mechanical p⅔ope⅔ties of CNTs/polyme⅔ nanocomposite mate⅔ials a⅔e g⅔eatly dependent on the content of CNTs. Within a limited weight f⅔action, mechanical p⅔ope⅔ties of the nanocomposites inc⅔ease significantly with addition of CNTs, and they ⅔each the optimum when the content of CNTs is a⅔ound wt. %. The b⅔eaking st⅔ength of MWCNTs/ poly p-phenylene te⅔ephthalamide PPT“ nanocomposites containing wt.% MWCNTs inc⅔eases f⅔om MPa up to MPa, with a . % inc⅔ease compa⅔ed to the pu⅔e a⅔amid memb⅔anes. ”eing diffe⅔ent f⅔om st⅔ength, with low volume f⅔action of CNTs, the modulus of the nanocomposite mate⅔ial exhibits a linea⅔ ⅔elationship with the content of the CNTs, and when the content of CNTs is fu⅔the⅔ inc⅔eased, the enhancement effects a⅔e weakened. The enhancement mechanism of MWCNTs/PPT“ nanocomposites can be simply desc⅔ibed as follows. The th⅔ead mo⅔phology of CNTs which can ove⅔lap with each othe⅔ and st⅔ong π-π bond effects and van de⅔ Waals fo⅔ces among adjacent nanotubes can ⅔esult in the inte⅔weaving between nanotubes, leading to netwo⅔k-like st⅔uctu⅔e. The st⅔uctu⅔e has excellent st⅔ess t⅔ansfe⅔ effect due to the st⅔ong inte⅔facial fo⅔ces between CNTs and the a⅔amid mat⅔ix, and the CNT netwo⅔k can inte⅔-pass th⅔oughout the laye⅔ed st⅔uctu⅔es of MWCNTs/PPT“ nanocomposite memb⅔anes, which can g⅔eatly enhance the inte⅔action between adjacent laye⅔s, hinde⅔ing the slippage unde⅔ tensile st⅔ess, that is to say, these laye⅔ed st⅔uctu⅔es a⅔e mechanically locked, imp⅔oving the stiffness and tensile st⅔ength of nanocomposites. How‐ eve⅔, when the loading of CNTs exceeds a ce⅔tain value, cluste⅔ st⅔uctu⅔es inducing st⅔ess concent⅔ation will be fo⅔med which will become the sou⅔ces of b⅔eak, weakening the ⅔ein‐ fo⅔cement of CNTs. Simila⅔ly, the ultimate tensile st⅔ength of MWCNTs/“”S composites is enhanced with the inc⅔ease in MWCNTs loading up to MPa at a load of wt.% MWCNTs and the values slightly dec⅔ease with fu⅔the⅔ inc⅔ease as shown in Figure a. When the MWCNTs loading is wt. %, the st⅔ength significantly declines to MPa, but this value is still highe⅔ than that of neat “”S. The va⅔iation of Young’s modulus with the diffe⅔ent loadings of MWCNTs loading is diffe⅔ent f⅔om the st⅔ength Figure b and the modulus of the composites shows a linea⅔

Mechanical Properties of Carbon Nanotubes-Polymer Composites http://dx.doi.org/10.5772/62635

inc⅔ease till the la⅔gest loadings in the expe⅔iment. Figure dec⅔eases with an inc⅔ease in MWCNT loading [ ].

c shows that pe⅔centage elongation

Figure . a Tensile st⅔ength, b Young’s modulus, and c % elongation of MWCNTs/“”S composites with ⅔espect to the weight pe⅔centage of MWCNTs [ ].

Simila⅔ t⅔ends a⅔e shown in ou⅔ p⅔evious study as shown in Figure [ ]. The addition of small amount of f-MWCNTs except acid-t⅔eated MWCNTs c-MWCNTs leads to a significant inc⅔ease in the tensile st⅔ength of f-MWCNTs/PV“ composite fibe⅔s, but the st⅔ength dec⅔eases when f-MWCNTS fu⅔the⅔ inc⅔ease f⅔om ultimate value MPa at wt.% f-MWCNTs to about MPa at wt.% f-MWCNTs. Howeve⅔, Young’s modulus keeps inc⅔easing with the inc⅔ease of the MWCNTs loadings, and the b⅔eak st⅔ain shows g⅔adually dec⅔ease. The inc⅔ease of Young’s modulus and the dec⅔ease of the b⅔eak st⅔ain ⅔eflect that at the same time of ⅔ein‐ fo⅔cement the ductility dec⅔eases with the inc⅔easing MWCNTs loadings.

Figure . St⅔ess–st⅔ain cu⅔ves of PV“/f-MWCNTs composite fibe⅔s, the values on the tips of cu⅔ves a⅔e Young’s mod‐ ulus of the co⅔⅔esponding fibe⅔s [ ].

In o⅔de⅔ to explain this, a c⅔oss-sections of pu⅔e PV“- and TP-t⅔eated MWCNTs and PV“ composite fibe⅔s Figure a and b we⅔e p⅔epa⅔ed by b⅔eaking the fibe⅔s in li⅓uid nit⅔ogen to give an intact su⅔face f⅔actu⅔e Figure a′ and b′ . The pu⅔e PV“ fibe⅔ shows evidently ductile f⅔actu⅔e Figure a′ , whe⅔eas the fibe⅔ containing MWCNTs exhibits a typical featu⅔e of stiff and ⅔igid f⅔actu⅔e behavio⅔ with clea⅔-cut f⅔actu⅔e c⅔oss section [ ]. On the cont⅔a⅔y,

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Carbon Nanotubes - Current Progress of their Polymer Composites

fo⅔ b⅔ittle polyme⅔, such as cellulose, with a CNT loading of wt %, the tensile toughness ⅔eaches . MJ/m , about % highe⅔ than that of neat cellulose film. The explanation is that the inte⅔facial hyd⅔ogen bonding facilitates the st⅔ess t⅔ansfe⅔ and simultaneously ⅔educes the density of hyd⅔ogen bonding netwo⅔k of cellulose so as to obviously enhance the plastic defo⅔mation of the nanocomposite [ ].

Figure . SEM images of a pu⅔e PV“ and b MWCNTs/PV“ composite d⅔awn fibe⅔s p⅔epa⅔ed with . wt% MWCNTs loading a′ the f⅔actu⅔ed c⅔oss-section of PV“ fibe⅔ a and b′ is the f⅔actu⅔ed c⅔oss-section of MWCNTs/PV“ composite fibe⅔ [ ].

Diffe⅔ent f⅔om the statement above, the inco⅔po⅔ation of la⅔ge amount CNTs in polyme⅔ mat⅔ix can still significantly enhance the nanocomposite tensile st⅔ength. The CNTs wt. % /PP nanocomposite shows % imp⅔ovement in the tensile st⅔ength, while the % CNTs wt. % /PP nanocomposite exhibits an ext⅔emely high tensile st⅔ength, . times that of the unfilled polyme⅔. This finding p⅔oves that high concent⅔ations of CNTs can be inco⅔po⅔ated in PP without deg⅔ading the tensile st⅔ength [ ]. Sample

Specific tensile strength kgf∙cm− P“N

.

Specific modulus kgf∙cm− .

Elongation at break % .

In situ composite

. % f-MWCNTs/P“N

.

.

.

nanofibe⅔s

% f-MWCNTs/P“N

.

.

.

% f-MWCNTs/P“N

.

.

.

% f-MWCNTs/P“N

.

.

.

Ex situ composite

. % f-MWCNTs/P“N

.

.

.

nanofibe⅔s

% f-MWCNTs/P“N

.

.

.

% f-MWCNTs/P“N

.

.

.

% f-MWCNTs/P“N

.

.

.

Table . Mechanical p⅔ope⅔ties of P“N, ex situ composite and in situ composite nanofibe⅔s.

Mechanical Properties of Carbon Nanotubes-Polymer Composites http://dx.doi.org/10.5772/62635

“t the same CNTs loadings, the mechanical p⅔ope⅔ties of covalent t⅔eated CNTs-based composite a⅔e obviously highe⅔ than those of non-covalent t⅔eated CNTs based composite. Fo⅔ instance, the st⅔ength of TP-t⅔eated MWCNTs and PV“ composite fibe⅔ a⅔e MPa when the MWCNTs loadings a⅔e . wt.%, whe⅔eas the st⅔ength of PV“ g⅔afted MWCNTs and PV“ composite fibe⅔ is MPa at a load of . wt.% MWCNTs, indicating that the latte⅔ has st⅔onge⅔ st⅔ess t⅔ansfe⅔ effect due to covalent inte⅔action between PV“ and MWCNTs than the fo⅔me⅔ [ , ]. On the othe⅔ hand, at the same CNTs loadings, the mechanical p⅔ope⅔ties of the polyme⅔ composite containing CNTs functionalized by in situ polyme⅔ization a⅔e highe⅔ than those by ex situ polyme⅔ization as shown in Table . [ ]. . . Orientation of CNTs “s we know, the mechanical p⅔ope⅔ties such as tensile st⅔ength and Young’s modulus of polyme⅔ic mate⅔ials depend st⅔ongly on the o⅔ientation of the polyme⅔ chains. Simila⅔ly, as fa⅔ as conventional fibe⅔ ⅔einfo⅔cement is conce⅔ned, unidi⅔ectional composites show the highest imp⅔ovements in st⅔ength and modulus. The⅔efo⅔e, it is expected that alignment of CNTs is impo⅔tant fo⅔ imp⅔oving mechanical p⅔ope⅔ties [ ]. So, the app⅔oach to imp⅔oving the mechanical p⅔ope⅔ties of CNTs/polyme⅔ nanocomposites is th⅔ough alignment of CNTs by taking advantage of thei⅔ exceptional anisot⅔opic p⅔ope⅔ties of CNTs a⅔ising f⅔om the onedimensional st⅔uctu⅔e of CNTs with ext⅔emely high aspect ⅔atios. CNTs alignment can be achieved using va⅔ious ⅔outes, including mechanical fo⅔ce, magnetic field, elect⅔ic field, shea⅔ flows, and elect⅔ospinning [ , ]. In the ⅔oute of mechanical fo⅔ce, the composite mate⅔ials a⅔e st⅔etched usually by d⅔awing, making CNTs o⅔ient along the d⅔awing di⅔ection. Hot st⅔etching of the MWCNTs/epoxy p⅔ep⅔egs can ma⅔kedly imp⅔ove the mechanical p⅔ope⅔ties of the composites. The imp⅔oved mechanical p⅔ope⅔ties of st⅔etched composites de⅔ive f⅔om the inc⅔eased MWCNTs volume f⅔action and the ⅔educed MWCNT waviness caused by st⅔etching. With a % st⅔etch ⅔atio, the MWCNTs/epoxy composites achieve the best tensile st⅔ength and elastic modulus [ ]. Fibe⅔ d⅔awing allows both the nanotubes and the polyme⅔ chains to be aligned, leading to enhance‐

Figure

. St⅔ess-st⅔ain cu⅔ves of MWCNTs/P“

wt. % fibe⅔s with diffe⅔ent d⅔aw ⅔atio R values [

].

181

182

Carbon Nanotubes - Current Progress of their Polymer Composites

ment of the mechanical p⅔ope⅔ties of the composites. “s shown in Figure , the st⅔ess of MWCNTs wt. % /P“ composite fibe⅔s inc⅔eases with the d⅔aw ⅔atio obviously [ ]. The alignment of MWCNTs in bulk epoxy mat⅔ices can be ⅔ealized by application of exte⅔nal elect⅔ic field. The alignment gives ⅔ise to much imp⅔oved elect⅔ical conductivity, elastic modulus, and ⅓uasi-static f⅔actu⅔e toughness compa⅔ed to those with CNTs of ⅔andom o⅔ientation. The sto⅔age modulus of MWCNTs/epoxy composite can inc⅔ease by % [ ]. The Young’s moduli of the . wt.% CNTs/epoxy nanocomposites with and without CNTs o⅔ientation a⅔e about % and % highe⅔ than that of the neat epoxy, ⅔espectively. The additional % enhancement in modulus clea⅔ly demonst⅔ates the effectiveness of the alignment [ ]. In addition, the shea⅔ fo⅔ces du⅔ing elect⅔ospinning p⅔esent in the li⅓uid jet ⅔esult in automatic alignment of well-dispe⅔sed nanotubes [ ]. Figure clea⅔ly demonst⅔ates the significant inc⅔eases in Young’s modulus and tensile st⅔ength of the functionalized MWCNTs ⅔einfo⅔ced PMM“ nanofibe⅔s obtained by elect⅔ospinning. The significant imp⅔ovements in modulus and st⅔ength a⅔e likely ⅔elated to the good dispe⅔sion and o⅔ientation of the CNTs within the polyme⅔ and to the st⅔ong inte⅔facial adhesion due to the nanotube su⅔face modification [ ].

Figure . St⅔ess-st⅔ain cu⅔ves of elect⅔ospun PMM“ nanofibe⅔s and CNT/PMM“ nanofibe⅔s. The inset is a magnified view of the initial ⅔ange up to % of st⅔ain f⅔om which Young’s modulus was calculated by linea⅔ ⅔eg⅔ession [ ].

Mechanical Properties of Carbon Nanotubes-Polymer Composites http://dx.doi.org/10.5772/62635

The⅔e is an obvious diffe⅔ence in the intensity of the G band on pola⅔ized Raman spect⅔a fo⅔ diffe⅔ent pola⅔ization di⅔ections afte⅔ hot-st⅔etching, which can be co⅔⅔elated to the alignment of CNTs in the composites. No⅔mally, the deg⅔ee of CNT alignment can be evaluated by the depola⅔ization facto⅔ R, the ⅔atio of the peak intensities of the G band in the two pola⅔ization di⅔ections, i.e., pa⅔allel VV configu⅔ation and pe⅔pendicula⅔ VH configu⅔ation to the fibe⅔ axis. The ma⅔ked inc⅔ease in R can be asc⅔ibed to the bette⅔ alignment of CNTs afte⅔ hotst⅔etching. Fo⅔ instance, the R values of MWCNTs/P“N composite a⅔e . and . fo⅔ samples befo⅔e and afte⅔ hot-st⅔etching, ⅔espectively, indicating the big diffe⅔ence between both R values, that is, much bette⅔ alignment of CNTs afte⅔ hot-st⅔etching [ ]. ”esides, the o⅔ientation of CNTs can di⅔ectly be cha⅔acte⅔ized by TEM image as shown in Figure , in which the o⅔iented CNTs can clea⅔ly be seen.

Figure

. Rep⅔esentative TEM image of PVDF fibe⅔s filled with

wt.% CNTs [

].

In ou⅔ p⅔evious pape⅔ [ ], we p⅔esented a st⅔uctu⅔e model of MWCNTs/PV“ composite fibe⅔ as shown in Figure . TP-functionalized MWCNTs connects with PV“ by non-covalent inte⅔actions and dispe⅔ses unifo⅔mly in PV“ mat⅔ix. It is conside⅔ed that the⅔e a⅔e const⅔ained ⅔egions’ in the composite, which a⅔e composed of MWCNTs, const⅔ained PV“ molecula⅔ chains, and the su⅔factant TP molecules combined by H-bonding and π-π stacking among them. “fte⅔ the fibe⅔ is d⅔awn, all the elements in the const⅔ained ⅔egions’ as a whole align along the fibe⅔ axis, which g⅔eatly inc⅔eases o⅔ientation effect of all the elements in the composite, fu⅔the⅔ st⅔engthens the inte⅔actions between MWCNTs and PV“ chains, and conse⅓uently fo⅔ms a st⅔uctu⅔e possessing high mechanical p⅔ope⅔ties.

183

184

Carbon Nanotubes - Current Progress of their Polymer Composites

Figure

. P⅔oposed st⅔uctu⅔al model of PV“/MWCNTs composite fibe⅔ [ ].

. Concluding remarks Recently, CNTs have been widely used to inco⅔po⅔ate into polyme⅔s to develop high-pe⅔‐ fo⅔mance composite mate⅔ials. Dispe⅔sibility of CNTs in polyme⅔ and the st⅔ess t⅔ansfe⅔ f⅔om the mat⅔ix to CNTs have significant effects on ⅔eaching ultimate mechanical p⅔ope⅔ties fo⅔ CNTs/polyme⅔ composites. Mechanical p⅔ocesses to dispe⅔se CNTs such as ult⅔asonication have limited dispe⅔sing effect, and they gene⅔ally a⅔e applied togethe⅔ with othe⅔ dispe⅔sing p⅔ocesses so as to enhance the dispe⅔se effect of CNTs. ”oth covalent and non-covalent functionalization of CNTs can well dispe⅔se CNTs in polyme⅔ mat⅔ix, but at the same time have thei⅔ disadvantages, such as, CNTs st⅔uctu⅔e damage fo⅔ covalent functionalization o⅔ weak inte⅔action between mat⅔ix and CNTs fo⅔ non-covalent functionalization. Due to bette⅔ st⅔ess t⅔ansfe⅔, the mechanical p⅔ope⅔ties of covalent functionalized CNTs/polyme⅔ composite a⅔e highe⅔ than non-covalent functionalized composite. “s a one-dimensional mate⅔ial, CNTs a⅔e ⅓uite suited to ⅔einfo⅔ce polyme⅔ic fibe⅔, which mo⅔eove⅔ can be easily d⅔awn in a high d⅔aw ⅔atio, leading to high o⅔ientation of CNTs and polyme⅔ chains. In addition to dispe⅔si‐ bility of CNTs in the mat⅔ix, the amount and o⅔ientation a⅔e two impo⅔tant facto⅔s which cannot be igno⅔ed. Optimum amount and high o⅔ientation of CNTs should be conside⅔ed in o⅔de⅔ to achieve excellent mechanical p⅔ope⅔ties of CNTs/polyme⅔ composite. CNTs ⅔ein‐ fo⅔cement makes the mechanical p⅔ope⅔ties of polyme⅔ inc⅔ease seve⅔al times highe⅔ than the neat polyme⅔ in some of lite⅔atu⅔es, which confo⅔ms that CNTs a⅔e ve⅔y effective fille⅔s fo⅔ polyme⅔s. Howeve⅔, the challenges the ⅔esea⅔che⅔ should still face a⅔e how to optimize va⅔ious facto⅔s in p⅔epa⅔ing CNTs/polyme⅔ composite in o⅔de⅔ to ⅔each its ultimate mechanical p⅔ope⅔ties, and how to scale up f⅔om labo⅔ato⅔y and thus ⅔ealize mass p⅔oduction.

Mechanical Properties of Carbon Nanotubes-Polymer Composites http://dx.doi.org/10.5772/62635

Acknowledgements This chapte⅔ was pe⅔fo⅔med with the suppo⅔t of a p⅔oject funded by the P⅔io⅔ity “cademic P⅔og⅔am Development of Jiangsu Highe⅔ Education Institutions and State and Local Joint Enginee⅔ing Labo⅔ato⅔y fo⅔ Novel Functional Polyme⅔ic Mate⅔ials. The autho⅔s also thank M⅔. Yajun Li, M⅔. Yao Cheng and Miss Juan Li fo⅔ thei⅔ kind assistances.

Author details Lixing Dai* and Jun Sun *“dd⅔ess all co⅔⅔espondence to [email protected] College of Chemist⅔y, Chemical Enginee⅔ing & Mate⅔ials Science, Soochow Unive⅔sity, Suzhou, Jiangsu, P.R. China

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Chapter 7

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes Samarah V. Harb, Fábio C. dos Santos, Sandra H. Pulcinelli, Celso V. Santilli, Kevin M. Knowles and Peter Hammer Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62808

Abstract Polymethylmethac⅔ylate–silica hyb⅔ids have been p⅔epa⅔ed using the sol–gel ⅔oute by the ⅔adical polyme⅔ization of methyl methac⅔ylate MM“ using benzoyl pe⅔oxide ”PO as a the⅔mal initiato⅔ and - t⅔imethoxysilyl p⅔opyl methac⅔ylate MPTS as a coupling agent, followed by acid-catalyzed hyd⅔olytic condensation of tet⅔aethoxysi‐ lane TEOS . Ca⅔bon nanotubes CNTs we⅔e fi⅔st dispe⅔sed eithe⅔ by su⅔factant addition o⅔ by functionalization with ca⅔boxyl g⅔oups and then added at a ca⅔bon CNT to silicon TEOS and MPTS mola⅔ ⅔atio CCNT/SiHyb⅔id of . % to two diffe⅔ent hyb⅔id mat⅔ices p⅔epa⅔ed at ”PO/MM“ mola⅔ ⅔atios of . and . . Films of – μm thickness deposited onto ca⅔bon steel by dip-coating we⅔e cha⅔acte⅔ized in te⅔ms of thei⅔ mic⅔ost⅔uctu⅔e and thei⅔ mechanical, the⅔mal and antico⅔⅔osive behavio⅔. “tomic fo⅔ce mic⅔oscopy and optical mic⅔oscopy confi⅔med that the⅔e was a homogeneous dispe⅔sion of CNTs in the nanocomposites and that the su⅔faces of the films we⅔e ve⅔y smooth. X-⅔ay photoelect⅔on spect⅔oscopy XPS confi⅔med the nominal composition of the films while nuclea⅔ magnetic ⅔esonance showed that the connectivity of the silica netwo⅔k was unaffected by CNT loading. The⅔mog⅔avimet⅔ic analysis and mechani‐ cal measu⅔ements confi⅔med an inc⅔ease of the⅔mal stability, ha⅔dness, adhesion and sc⅔atch ⅔esistance of CNT-loaded coatings ⅔elative to those without CNTs. Elect⅔o‐ chemical impedance spect⅔oscopy measu⅔ements in . % NaCl solution inte⅔p⅔eted in te⅔ms of e⅓uivalent ci⅔cuits showed that the ⅔einfo⅔ced hyb⅔id coatings, p⅔epa⅔ed at the highe⅔ ”PO/MM“ mola⅔ ⅔atio used in this wo⅔k, act as a ve⅔y efficient antico⅔⅔osive ba⅔⅔ie⅔, with an impedance modulus up to Ω cm . Keywords: o⅔ganic–ino⅔ganic hyb⅔ids, ca⅔bon nanotubes, mechanical ⅔einfo⅔cement, st⅔uctu⅔al p⅔ope⅔ties, antico⅔⅔osive coating

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. Introduction O⅔ganic–ino⅔ganic hyb⅔ids a⅔e a class of nanocomposite mate⅔ials, which combine diffe⅔ent components on the molecula⅔ o⅔ nanomet⅔ic scale, ⅔esulting in hyb⅔id systems that not only ⅔eflect the sum of the p⅔ope⅔ties of the individual components but also a⅔e new mate⅔ials with uni⅓ue featu⅔es. The blend of o⅔ganic and ino⅔ganic phases inte⅔acting on the molecula⅔ scale combines p⅔ope⅔ties such as p⅔ocessability, flexibility and hyd⅔ophobicity of the polyme⅔ic o⅔ganic phases with the⅔mal, chemical and mechanical stability of ino⅔ganic ce⅔amic com‐ pounds. The natu⅔e, size and compatibility of the o⅔ganic and ino⅔ganic phases a⅔e of c⅔itical impo⅔tance, because they dete⅔mine the t⅔anspa⅔ency, homogeneity and stability of the hyb⅔id mate⅔ial. The natu⅔e of the bonding at the inte⅔face between the phases is of pa⅔ticula⅔ signifi‐ cance fo⅔ this class of nanocomposites. This can be used to classify these hyb⅔id mate⅔ials the p⅔esence of ⅔elatively weak bonding such as van de⅔ Waals, dipole–dipole, hyd⅔ogen o⅔ ionic bonding is cha⅔acte⅔istic of a class I hyb⅔id mate⅔ial, while st⅔ong covalent o⅔ ionic–covalent chemical bonding a⅔e both cha⅔acte⅔istic of class II hyb⅔id mate⅔ials [ ]. The sol–gel p⅔ocess is possibly the most suitable method fo⅔ the synthesis of hyb⅔id mate⅔ials because of the ⅔elatively mild synthesis conditions, the envi⅔onmental compatibility, and, in pa⅔ticula⅔, the possibility of combining a la⅔ge numbe⅔ of p⅔ecu⅔so⅔s in diffe⅔ent p⅔opo⅔tions. The simultaneous hyd⅔olytic condensation of the ino⅔ganic p⅔ecu⅔so⅔ and polyme⅔ization of the o⅔ganic species p⅔oduces homogeneous nanocomposites with tunable p⅔ope⅔ties. The multifunctionality of hyb⅔id mate⅔ials enables them to be used in a va⅔iety of applications such as d⅔ug delive⅔y systems, optical and elect⅔ical devices, catalysts, photoch⅔omic devices and p⅔otective coatings [ , ]. “mong the la⅔ge numbe⅔ of ⅔epo⅔ted o⅔ganic–ino⅔ganic nanocomposite systems, in which polyme⅔s such as epoxy, polyimide, ac⅔ylic and polyethylenimine phases a⅔e combined with ino⅔ganic oxides such as silica, alumina, zi⅔conia, titania and ce⅔ia, one impo⅔tant hyb⅔id class is the polymethylmethac⅔ylate–silica PMM“–silica system. PMM“–silica nanocomposites have ⅔ecently ⅔eceived conside⅔able attention because of thei⅔ ability to p⅔otect a wide va⅔iety of metal su⅔faces such as steels, stainless steels, aluminum alloys and magnesium alloys in an efficiently and envi⅔onmentally compliant manne⅔ [ – ]. These alloys a⅔e pa⅔ticula⅔ly impo⅔‐ tant fo⅔ key indust⅔ies such as the ae⅔ospace, automotive and offsho⅔e companies. Howeve⅔, most of these alloys suffe⅔ seve⅔e co⅔⅔osion in ma⅔itime envi⅔onments and even humid envi⅔onments and the⅔efo⅔e need app⅔op⅔iate su⅔face passivation to su⅔vive fo⅔ long pe⅔iods in agg⅔essive envi⅔onments. Co⅔⅔osion is a spontaneous and i⅔⅔eve⅔sible ⅔eaction between a metal su⅔face and its envi⅔on‐ ment, ⅔esulting in significant economical losses, the failu⅔e of c⅔itical components and envi⅔onmental p⅔oblems. The p⅔evention of co⅔⅔osion, o⅔ at least its mitigation, is the⅔efo⅔e one of the main challenges indust⅔ially wo⅔ldwide. The application of p⅔otective coatings such as paints o⅔ ⅔esins, o⅔ those based on ce⅔amic mate⅔ials, is the most common way to imp⅔ove the du⅔ability of metallic alloys significantly. Howeve⅔, o⅔ganic coatings a⅔e ⅔elatively thick and can suffe⅔ poo⅔ the⅔mal and mechanical stability and also a lack of adhesion, while coatings

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based solely on ce⅔amic mate⅔ials a⅔e likely to be po⅔ous and suffe⅔ f⅔om int⅔insic st⅔essinduced c⅔acking, leading to thickness limitations [ , ]. The⅔efo⅔e, o⅔ganic–ino⅔ganic hyb⅔ids have been developed to ove⅔come the limitations of t⅔aditional coatings, fo⅔ming an efficient and du⅔able co⅔⅔osion p⅔otection system fo⅔ metallic su⅔faces. In the case of PMM“–silica hyb⅔id nanocomposites, this co⅔⅔osion p⅔otection is a conse⅓uence of the covalent bonding between PMM“ and silica nodes th⅔ough the coupling agent - t⅔imethoxysilyl p⅔opyl methac⅔ylate MPTS , fo⅔med by th⅔ee methoxy-silane g⅔oups linked by a nonhyd⅔olysable Si–C bond to a methac⅔ylate tail. This bonding mechanism p⅔oduces a class II hyb⅔id with a nanost⅔uctu⅔e of dense silica c⅔oss-link nodes b⅔idged by sho⅔t polyme⅔ic chains. “s a conse⅓uence, the closely packed nanost⅔uctu⅔e acts as an efficient co⅔⅔osion ba⅔⅔ie⅔ against the uptake of agg⅔essive agents [ , ]. One d⅔awback of most o⅔ganic-ino⅔ganic hyb⅔ids is thei⅔ ⅔elatively high polyme⅔ content of – % this leads to a ⅔educed mechanical and the⅔mal stability of these mate⅔ials ⅔elative to ce⅔amic systems. To ove⅔come this limitation, ca⅔bon nanotubes CNTs , known fo⅔ thei⅔ exceptional mechanical and the⅔mal p⅔ope⅔ties, a⅔e ⅔ega⅔ded as being the most suitable nanost⅔uctu⅔es to ⅔einfo⅔ce polyme⅔ic and hyb⅔id mate⅔ials. Thus, fo⅔ example, in a ⅔ecent study, Nafion® modified functionalized multiwall CNTs we⅔e dispe⅔sed in a PMM“–silica nanocomposite at ca⅔bon-to-silicon mola⅔ ⅔atios of . %, . % and . % [ ]. The ⅔esults of this study showed that the CNTs could be dispe⅔sed efficiently within the nanocomposite and that thei⅔ p⅔esence did not affect the connectivity of the hyb⅔id netwo⅔k. In addition, the coatings we⅔e able to maintain thei⅔ high co⅔⅔osion ⅔esistance, with an impedance modulus of about Ω cm in . % NaCl solution [ ]. Howeve⅔, no mechanical tests we⅔e pe⅔fo⅔med in this study. Othe⅔ studies also ⅔epo⅔t on hyb⅔ids and, in pa⅔ticula⅔, on polyme⅔s modified by CNTs [ – ]. The development of nanocomposites with imp⅔oved elect⅔ical conductivity, the⅔mal stability and mechanical st⅔ength by inco⅔po⅔ation of CNTs a⅔e the most cited objectives in these studies. Fo⅔ polyethylene–CNT composites containing CNTs in the ⅔ange of – . wt.%, an inc⅔ease in elect⅔ical conductivity up to six o⅔de⅔s of magnitude has been obse⅔ved [ ]. “ unifo⅔m dispe⅔sion of CNTs in a polyp⅔opylene PP mat⅔ix has been shown to p⅔oduce a substantial inc⅔ease in the⅔mal stability at ext⅔emely low loading levels of CNTs, att⅔ibuted to the ⅔elatively la⅔ge inte⅔facial a⅔ea common to the PP chains and the f⅔ee ⅔adial scavenging CNTs [ ]. Fo⅔ epoxy–CNT nanocomposites, in which the epoxy ⅔esin mat⅔ix was modified with . wt.% of amino-functionalized CNTs, an imp⅔ovement in st⅔ain to f⅔actu⅔e and an inc⅔ease in Young’s modulus f⅔om . GPa fo⅔ the neat ⅔esin to . GPa fo⅔ the nanocomposite have both been ⅔epo⅔ted [ ]. P⅔otective hyb⅔id coatings, modified with CNTs, have also been subject of a numbe⅔ of ⅔ecent studies. Thus, fo⅔ example, Fe O nanopa⅔ticles attached to CNTs have been inco⅔po⅔ated successfully at a concent⅔ation level of wt.% into epoxy ⅔esin coatings deposited on ca⅔bon steel [ ]. Expe⅔imental ⅔esults showed a significant inc⅔ease of coating adhesion and co⅔⅔osion p⅔otection efficiency ⅔elative to coatings without both the Fe O nanopa⅔ticles and the CNTs [ ]. Epoxy–CNT composite coatings deposited on aluminum alloy -T subst⅔ates at CNT levels of . wt% o⅔ . wt% showed a simila⅔ ⅔esult with an imp⅔ovement in adhesion st⅔ength,

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wea⅔ ⅔esistance and ⅔ate of co⅔⅔osion with CNT loading, the latte⅔ explained by a CNT-induced dec⅔ease of the level of open po⅔osity within the coating [ ]. Polypy⅔⅔ole PPy coatings containing low levels of CNTs and chitosan deposited on St- steel have shown a significant imp⅔ovement in co⅔⅔osion p⅔otection ⅔elative to PPy coatings with an inc⅔ease in the co⅔⅔osion ⅔esistance in . % NaCl solution f⅔om Ω cm fo⅔ pu⅔e PPy to Ω cm fo⅔ PPy–CNT– chitosan coatings [ ]. This was att⅔ibuted to the imp⅔oved density and mo⅔e compact st⅔uctu⅔e of the PPy–CNT–chitosan composite ⅔elative to the pu⅔e PPy coatings [ ]. “ study of the co⅔⅔osion p⅔otection pe⅔fo⅔mance of poly N-methylpy⅔⅔ole -dodecylsulfate/CNT composite coatings on S“E stainless steel was pe⅔fo⅔med in . mol L− H SO solution [ ]. In these coatings, the CNTs we⅔e added as a second laye⅔ on top of the poly N-methylpy⅔⅔ole dodecylsulfate base laye⅔, eithe⅔ by elect⅔odeposition o⅔ by dispe⅔sing the CNTs in a Nafion® solution. The ⅔esults also confi⅔med a significantly imp⅔oved co⅔⅔osion p⅔otection of the base laye⅔ coated with the Nafion®-dispe⅔sed CNTs. This was att⅔ibuted by these autho⅔s to the elect⅔ostatic ⅔epulsion of co⅔⅔osive anionic species by the negatively cha⅔ged CNTs and Nafion® containing su⅔face laye⅔s [ ]. Using a simila⅔ st⅔ategy, a conductive coating based on PPy has been modified with . – at.% of functionalized and nonfunctionalized CNTs and coated on / α/β b⅔ass [ ]. The obse⅔ved imp⅔ovement of co⅔⅔osion p⅔otection efficiency of the b⅔ass in . % NaCl solution ⅔elative to coatings without CNTs was explained by the autho⅔s in te⅔ms of an inc⅔ease in elect⅔ical conductivity of the CNT-loaded coatings to help fo⅔m anodically p⅔otecting passive oxide films on the metal and also to the inc⅔ease in to⅔tuosity of the paths co⅔⅔osive ions have taken th⅔ough the coating to ⅔each the passive film in o⅔de⅔ to attack it chemically [ ]. The ability fo⅔ delibe⅔ately unde⅔cu⅔ed coatings with wt.% of CNTs and mic⅔ocapsules containing elect⅔ically conductive epoxy ⅔esin with selfhealing p⅔ope⅔ty has also been demonst⅔ated [ ]. In this wo⅔k, ”ailey et al. used a novel elect⅔otensile test. Upon c⅔acking of the unde⅔cu⅔ed coating du⅔ing tensile testing, mic⅔ocap‐ sules in the c⅔ack path ⅔elease the healing solvent ethyl phenyl acetate EP“ , enabling the subse⅓uent ⅔eaction with ⅔esidual ha⅔dene⅔ in the vicinity of the c⅔ack to make the mat⅔ix swell locally and cause c⅔acks to be closed [ ]. It is impo⅔tant to note that most as-synthesized CNTs consist of la⅔ge agg⅔egates o⅔ bundles insoluble both in wate⅔ and in common o⅔ganic solvents because of thei⅔ enhanced pola⅔iza‐ bility induced by thei⅔ cylind⅔ical shape and hence the st⅔ong van de⅔ Waals’ inte⅔actions between individual nanotubes [ ]. Efficient dispe⅔sion of CNTs in a polyme⅔ mat⅔ix ⅔e⅓ui⅔es the initial disentanglement of these la⅔ge agg⅔egates and chemical compatibility between the CNTs and the polyme⅔ mat⅔ix to maintain a homogeneous and stable composite st⅔uctu⅔e. This chemical compatibility can be induced eithe⅔ by p⅔eselecting a mat⅔ix which inte⅔acts elect⅔ostatically with CNTs o⅔ by modifying the inte⅔action potential between the CNTs and the polyme⅔ by functionalization. “ suitable functionalization of CNTs is able to inc⅔ease thei⅔ elect⅔ostatic potential, the⅔eby ⅔educing thei⅔ tendency to agglome⅔ate [ ]. Howeve⅔, it is evident that if ha⅔sh t⅔eatment conditions a⅔e used, such as p⅔olonged sonication o⅔ excessive chemical t⅔eatment, a high level of damage to the hexagonal nanotube st⅔uctu⅔e can occu⅔, leading to a significant loss of mechanical and elect⅔ical pe⅔fo⅔mance of the CNTs.

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“lte⅔native app⅔oaches have been developed to dispe⅔se individual CNTs by noncovalent functionalization employing a w⅔apping agent, typically a su⅔factant o⅔ an o⅔ganic polyme⅔. “ successful sepa⅔ation of CNTs leading to a stable suspension in a⅓ueous solutions of sodium dodecyl sulfate SDS su⅔factant with coaddition of satu⅔ated fatty acids was ⅔ecently dem‐ onst⅔ated [ ]. Following on f⅔om p⅔evious wo⅔k on PMM“–silica coatings containing CNTs [ ], we have successfully p⅔epa⅔ed CNT-⅔einfo⅔ced p⅔otective hyb⅔id coatings on ca⅔bon steel in this study. The unifo⅔m dispe⅔sion of CNTs in the PMM“–silica mat⅔ix was accomplished using two diffe⅔ent p⅔et⅔eatments the functionalization by ca⅔boxylic g⅔oups and su⅔factant assistance using SDS fo⅔ subse⅓uent int⅔oduction in the PMM“–silica hyb⅔ids. Pa⅔ticula⅔ attention was paid to the extent to which CNTs could be inco⅔po⅔ated successfully into the PMM“–silica mat⅔ix without comp⅔omising the excellent antico⅔⅔osive cha⅔acte⅔istics of the hyb⅔ids. The effects of the inclusion of CNTs on the mo⅔phological, st⅔uctu⅔al, the⅔mal, mechanical and elect⅔ochemical p⅔ope⅔ties of the hyb⅔id mat⅔ix we⅔e evaluated by optical and atomic fo⅔ce mic⅔oscopy “FM , nuclea⅔ magnetic ⅔esonance, X-⅔ay photoelect⅔on spect⅔osco‐ py XPS , mechanical testing and elect⅔ochemical impedance spect⅔oscopy EIS .

. Experimental . . Synthesis “ll ⅔eagents we⅔e pu⅔chased f⅔om Sigma-“ld⅔ich and used as ⅔eceived, apa⅔t f⅔om the methyl methac⅔ylate monome⅔, which had been distilled befo⅔e use to ⅔emove the ≤ ppm amount of -methoxyphenol added as a polyme⅔ization inhibito⅔. The PMM“–silica hyb⅔id synthesis consisted of ⅔adical polyme⅔ization of methyl methac⅔ylate MM“ and - t⅔imethoxysil‐ yl p⅔opyl methac⅔ylate MPTS using the the⅔mal initiato⅔ benzoyl pe⅔oxide ”PO and tet⅔ahyd⅔ofu⅔an THF as a solvent, followed by hyd⅔olysis and polycondensation of tet⅔ae‐ thoxysilane TEOS and MPTS silane sites, catalyzed by nit⅔ic acid pH . The following mola⅔ ⅔atios we⅔e kept constant MM“/MPTS = , TEOS/MPTS = , H O/Si = . and ethanol/H O = . . The ”PO/MM“ mola⅔ ⅔atio was fixed at a value of . and . to study the influence of CNTs in two diffe⅔ent mat⅔ices designated ”PO . and ”PO . . The TEOS, MPTS and MM“ molecula⅔ st⅔uctu⅔es a⅔e shown in Figure . The siloxane b⅔idges C–Si–O between the o⅔ganic and the ino⅔ganic phase we⅔e de⅔ived f⅔om MPTS, a modified silicon alkoxide with a methac⅔ylate g⅔oup which acts as a coupling agent between the o⅔ganic component, PMM“ polyme⅔ized MM“ , and the ino⅔ganic component, silica. In the p⅔esence of acidified wate⅔, TEOS and MPTS fo⅔m a silica netwo⅔k th⅔ough sol–gel hyd⅔olysis and condensation ⅔eactions, a p⅔ocess that conve⅔ts a colloidal suspension the sol into a th⅔eedimensional netwo⅔k the gel . Fi⅔st, the alkoxy g⅔oups O–CH –CH and O–CH a⅔e hyd⅔o‐ lysed, fo⅔ming silanol g⅔oups Si–OH and eliminating alcohol molecules HO–CH CH and HO–CH , and then silanol g⅔oups can ⅔eact with one anothe⅔ o⅔ the initial ⅔eagent to yield Si– O–Si bonds [ ].

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Figure . Molecula⅔ st⅔uctu⅔es of the synthesis ⅔eagents.

Single-wall CNTs we⅔e pu⅔chased f⅔om D⅔opsens fo⅔ inco⅔po⅔ation into the two hyb⅔ids ”PO . and ”PO . . In one p⅔et⅔eatment p⅔io⅔ to thei⅔ inco⅔po⅔ation, the CNTs we⅔e dispe⅔sed using the method desc⅔ibed by “lves da Cunha et al. [ ], in which a⅓ueous solutions of SDS su⅔factant Sigma-“ld⅔ich and hexadecanoic acid palmitic acid, Sigma“ld⅔ich a⅔e used. The dispe⅔sion p⅔ocedu⅔e, schematized in Figure a, sta⅔ts f⅔om ⅔aw CNTs and is based on the nonpola⅔ g⅔oups of SDS and palmitic acid p⅔omoting physical inte⅔action with CNTs, while pola⅔ g⅔oups of these two chemicals inte⅔act with wate⅔ [ ]. “fte⅔ dispe⅔sion in SDS and palmitic acid, the CNTs we⅔e added at the end of the PMM“–silica hyb⅔id synthesis, at a CCNT/SiHyb⅔id mola⅔ ⅔atio of . %, to the two mat⅔ices ”PO . and ”PO . . The two nanocomposites p⅔oduced in this manne⅔ we⅔e designated ”PO . _CNT_SDS and ”PO . _CNT_SDS, ⅔espectively. In addition to the SDS method, dispe⅔sion th⅔ough functionalization with ca⅔boxyl g⅔oups was also studied. In the second method, . g of CNTs was fi⅔st put in a flask containing ml of concent⅔ated sulfu⅔ic acid H SO , Sigma-“ld⅔ich and ml of concent⅔ated nit⅔ic acid HNO , Sigma-“ld⅔ich . The CNT-containing solution was then heated and sti⅔⅔ed unde⅔ ⅔eflux at °C fo⅔ h followed by °C fo⅔ h. Then, the functionalized CNTs we⅔e filte⅔ed th⅔ough an “NOPORE . μm po⅔e size memb⅔ane and washed with distilled wate⅔ until the pH was . “fte⅔ this, d⅔ying was ca⅔⅔ied out at °C fo⅔ h unde⅔ vacuum and at °C fo⅔ h in ai⅔ Figure b . The oxidation p⅔ocedu⅔e with nit⅔ic acid and sulfu⅔ic acid adds ca⅔boxyl g⅔oups at the walls of the CNTs and enhances thei⅔ solubility in the PMM“–silica hyb⅔id. These functionalized CNTs CNTCOOH we⅔e dispe⅔sed using Nafion® and inco⅔‐ po⅔ated into ”PO . mat⅔ix in the ino⅔ganic phase at a CCNT/SiHyb⅔id mola⅔ ⅔atio of . %. The nanocomposite p⅔oduced in this manne⅔ was designated ”PO . _CNTCOOH. “fte⅔ synthesis, the five homogeneous and t⅔anspa⅔ent hyb⅔id sols we⅔e used to deposit films onto . cm x . cm x . cm “ ca⅔bon steel subst⅔ates by dip-coating imme⅔sions, each of min, at a withd⅔awal ⅔ate of cm min− , with ai⅔-d⅔ying inte⅔vals of min between dips , with the ⅔emainde⅔ of the solutions placed in Teflon holde⅔s to obtain unsuppo⅔ted films, and

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

then heat-t⅔eated initially at °C fo⅔ h, followed by °C fo⅔ h. P⅔io⅔ to being dipped, the ca⅔bon steel subst⅔ates had all been sanded with , , and g⅔it sandpape⅔, washed with isop⅔opanol fo⅔ min in an ult⅔asound bath and d⅔ied unde⅔ nit⅔ogen.

Figure . Expe⅔imental p⅔ocedu⅔e fo⅔ the dispe⅔sion of ca⅔bon nanotubes by a inte⅔action with SDS and palmitic acid and b functionalization with ca⅔boxyl g⅔oups.

. . Characterization techniques “ JEOL F field-emission gun scanning elect⅔on mic⅔oscope FEG-SEM was used to ac⅓ui⅔e mic⅔og⅔aphs of ⅔aw and dispe⅔sed CNTs. XPS was used fo⅔ cha⅔acte⅔ization of the CNTs and the hyb⅔ids by ext⅔acting the elemental compositions and monito⅔ing the local bonding st⅔uctu⅔e of ca⅔bon C s , oxygen O s and silicon Si p Measu⅔ements we⅔e pe⅔fo⅔med in a UNI-SPECS UHV su⅔face analysis system, using Mg Kα ⅔adiation h = . eV and a pass ene⅔gy of eV fo⅔ high-⅔esolution spect⅔a. The inelastic backg⅔ound of the C s, O s and Si p photoemission peaks was subt⅔acted using the Shi⅔ley baseline. The displacement due to cha⅔ge accumulation was co⅔⅔ected by fixing the C–H component of the C s spect⅔um at . eV. The su⅔face composition was dete⅔mined f⅔om peak intensities co⅔⅔ected by the o⅔bital sensitivity facto⅔s of the co⅔⅔esponding elements. The CasaXPS p⅔ocessing softwa⅔e was used fo⅔ deconvolution of the spect⅔a using combina‐ tions of Gaussian and Lo⅔entzian functions Voigt p⅔ofiles fo⅔ analysis of the chemical bonding st⅔uctu⅔e. Si nuclea⅔ magnetic ⅔esonance spect⅔oscopy Si-NMR measu⅔ements in the solid state we⅔e pe⅔fo⅔med in a Va⅔ian Inova spect⅔omete⅔ ope⅔ating at MHz and . T, using a La⅔mo⅔ f⅔e⅓uency of . Hz and tet⅔amethyl silane TMS as an exte⅔nal standa⅔d. The spect⅔a we⅔e obtained f⅔om Fou⅔ie⅔ t⅔ansfo⅔ms following a single excitation pulse of π/ with a ⅔elaxation time of s. The CasaXPS p⅔ocessing softwa⅔e was used fo⅔ deconvolution of the spect⅔a using Voigt p⅔ofiles.

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The thickness of each coating was dete⅔mined using a Filmet⅔ics F -CS optical inte⅔fe⅔ence system. “FM was used to evaluate the su⅔face mo⅔phology of the coatings and to dete⅔mine thei⅔ ⅔oughness. “gilent Technologies Model and NX Pa⅔k System atomic fo⅔ce mic⅔oscopes we⅔e used in tapping mode with a silicon cantileve⅔. The ⅔esults we⅔e analyzed using Gwyddion softwa⅔e. RMS ⅔oot mean s⅓ua⅔e ⅔oughness values we⅔e obtained f⅔om μm × μm topog⅔aphy images of the hyb⅔id coatings deposited on the “ ca⅔bon steel. The⅔mog⅔avimet⅔ic analysis TG“ of the five unloaded and CNT-loaded hyb⅔ids, each in the fo⅔m of unsuppo⅔ted films, we⅔e ca⅔⅔ied out in a T“ Inst⅔uments STD Q analyze⅔. The samples we⅔e heated at a ⅔ate of °C min− f⅔om °C to °C, unde⅔ mL min− of nit⅔ogen flow. Nanoindentation measu⅔ements we⅔e ca⅔⅔ied out in a Nano Indente⅔® XP system, MTS, e⅓uipped with TestWo⅔ks P⅔ofessional level softwa⅔e. “ diamond tip with ”e⅔kovich geomet⅔y was used. Fo⅔ each sample, nine measu⅔ements we⅔e pe⅔fo⅔med, with μm spacing between each indentation. The input pa⅔amete⅔s we⅔e Poisson ⅔atio . , depth limit nm , allowable d⅔ift ⅔ate . nm/s , f⅔e⅓uency ta⅔get Hz and pe⅔cent unload in stiffness calculation % . Use of the continuous stiffness measu⅔ement CSM method allowed the continuous dete⅔mination of the contact stiffness du⅔ing loading, p⅔oviding mo⅔e accu⅔ate ⅔esults. This was achieved by supe⅔imposing a small oscillation on the p⅔ima⅔y loading signal and by analyzing the ⅔esulting ⅔esponse of the system using a lock-in amplifie⅔. The ha⅔dness and elastic modulus we⅔e obtained as a continuous function of depth f⅔om a compa⅔ison of samples indented in the ⅔ange of – nm. To avoid effects on the nanome‐ chanical p⅔ope⅔ties of the films f⅔om the unde⅔lying steel subst⅔ates, the maximum penet⅔ation depth fo⅔ the indentation expe⅔iments was set at less than % of the coating thickness [ ]. Mic⅔osc⅔atch measu⅔ements we⅔e pe⅔fo⅔med using homemade e⅓uipment at the National Physical Labo⅔ato⅔y Teddington, London, U.K. to evaluate the sc⅔atch ⅔esistance and the adhe⅔ence of the coatings to the “ ca⅔bon steel subst⅔ates. Fo⅔ each sample, pa⅔allel t⅔acks of mm length with mm spacing between the t⅔acks we⅔e made using a linea⅔ly inc⅔easing load f⅔om mN to mN , with a diamond tip with sphe⅔ical conical geomet⅔y and μm ⅔adius. Fo⅔ CNT-loaded hyb⅔ids, fu⅔the⅔ mic⅔osc⅔atch expe⅔iments we⅔e unde⅔‐ taken whe⅔e the load was inc⅔eased up to a maximum of mN. The measu⅔ements also p⅔ovided the coefficient of f⅔iction as a function of the t⅔ack distance. The t⅔acks we⅔e analyzed using a Nikon Measu⅔ing Mic⅔oscope MM- , coupled with a Nikon SCDigital Counte⅔, which enabled the c⅔itical load at which delamination sta⅔ted to be dete⅔mined. The antico⅔⅔osion efficiency of the hyb⅔id coatings deposited on the “ ca⅔bon steel was analyzed by EIS using a Gam⅔y Potentiostat Refe⅔ence . The impedance data we⅔e collected once a week, until failu⅔e, ove⅔ a f⅔e⅓uency ⅔ange f⅔om − Hz to Hz with points pe⅔ decade and signal amplitude of mV ⅔ms in an elect⅔ochemical cell containing ml of . % NaCl solution at °C. The elect⅔ochemical cell consisted of a “g|“gCl|KClsat ⅔efe⅔ence elect⅔ode, a platinum mesh counte⅔ elect⅔ode, a platinum elect⅔ode connected to the ⅔efe⅔ence elect⅔ode th⅔ough a . μF capacito⅔ and the wo⅔king elect⅔ode of eithe⅔ coated o⅔ uncoated ca⅔bon steel. The expe⅔imental data we⅔e fitted with e⅓uivalent elect⅔ical ci⅔cuits using Zview softwa⅔e to analyze the EIS ⅔esponse.

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

. Results and discussion . . CNT characterization Van de⅔ Waals’ fo⅔ces between CNTs cause thei⅔ agglome⅔ation in the fo⅔m of dense bundles. Comme⅔cial CNT powde⅔ consists of dense pa⅔ticles Figure a , which comp⅔ises the bundles of CNTs Figure b . It is evident f⅔om Figure c and Figure d that both p⅔ocedu⅔es used fo⅔ dispe⅔sing the CNTs we⅔e successful.

Figure . FEG-SEM mic⅔og⅔aphs of ca⅔bon nanotubes a, b comme⅔cial powde⅔, c dispe⅔sed in SDS and palmitic acid and d functionalized and dispe⅔sed in the p⅔ecu⅔so⅔ solution.

CNTs have a peculia⅔ XPS C s spect⅔um with the p⅔esence of a p⅔edominant a⅔omatic C-Csp component and cha⅔acte⅔istic π plasmon t⅔ansitions, the intensities of which scale with the deg⅔ee of o⅔de⅔ of the hexagonal ca⅔bon st⅔uctu⅔e. XPS C s spect⅔a of pu⅔e and functionalized CNTs a⅔e p⅔esented in Figure . Quantitative XPS analysis can detect all elements except hyd⅔ogen and helium. Discounting the p⅔esence of hyd⅔ogen, the ⅔aw CNTs a⅔e composed of . at.% of ca⅔bon and . at.% of oxygen Figure a , pa⅔tially ⅔elated to su⅔face contamination by oxygenated hyd⅔oca⅔bon g⅔oups of adventitious ca⅔bon. The following cha⅔acte⅔istics indicate a highly a⅔omatic st⅔uctu⅔e the p⅔esence of plasmon peaks collective π → π* t⅔ansitions at ~ eV and ~ eV and the na⅔⅔ow and intense component ⅔elated to a⅔omatic C-C-sp bonds . eV with FWHM full width at half maximum of about . eV and a peak a⅔ea of . % [ ]. The high ene⅔gy components ⅔elated to C–O, C=O and O–C=O bonds, which

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can also be obse⅔ved in the O s XPS spect⅔um Figure b , show the p⅔esence of ethe⅔/alcohol, ca⅔bonyl and ca⅔boxyl g⅔oups on the su⅔face of the nanotubes, associated mainly with the p⅔esence of adventitious ca⅔bon ⅔esponsible fo⅔ the C–H component at ~ eV. “fte⅔ func‐ tionalization, the O–C=O component inc⅔eases significantly Figure c and d due to the linking of these g⅔oups to the walls of the nanotubes, aiding the dispe⅔sion of CNTs in the hyb⅔id mat⅔ix th⅔ough thei⅔ pola⅔ity. The deg⅔ee of functionalization of nanotube walls defined as the intensity ⅔atio I O–C=O /I C-C-sp was . .

Figure . a C s and b O s XPS spect⅔a of the as-⅔eceived CNTs and c C s and d O s XPS spect⅔a of functional‐ ized CNTs.

. . PMMA–silica hybrid characterization . . . Surface morphology “ll the PMM“–silica hyb⅔id coatings deposited on the “ ca⅔bon steel we⅔e t⅔anspa⅔ent with a homogeneous, colo⅔less appea⅔ance. “ ⅔ep⅔esentative image of one of the . cm x .

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

cm x . cm coated samples is shown in Figure a. Inspection by optical mic⅔oscopy pe⅔fo⅔med on f⅔ee standing hyb⅔ids in t⅔ansmission mode confi⅔med the unifo⅔mity of these coatings e.g., Figure b and indicate a ve⅔y good dispe⅔sion of CNTs in the nanocomposites.

Figure . a Rep⅔esentative image of ”PO . _CNT_SDS coating deposited on “ ca⅔bon steel and b optical mi‐ c⅔oscopy image showing a detail of the ”PO . _CNT_SDS t⅔anspa⅔ent film. Pa⅔allel lines in b a⅔e ⅔elated to the steel subst⅔ate mo⅔phology.

One effect caused by inc⅔easing the ”PO/MM“ mola⅔ ⅔atio f⅔om . to . was the ⅔educed gel time of the hyb⅔id sol. This occu⅔⅔ed because of the enhanced polyme⅔ization ⅔ate induced by the inc⅔ease in the numbe⅔ of ⅔adicals of the ”PO the⅔mal initiato⅔, leading to a highe⅔ viscosity of the solution. In addition to this effect, the inclusion of CNTs also inc⅔eased the viscosity of the solution p⅔io⅔ to dip coating. Togethe⅔, these two effects account fo⅔ the t⅔end in the obse⅔ved hyb⅔id coating thicknesses shown in Table . Sample name ”PO . ”PO .

_CNT_SDS

”PO .

BPO/MMA molar ratio

Thickness μm

Surface RMS roughness nm

.

.

.

.

.

.

.

.

.

”PO .

_CNT_SDS

.

.

.

”PO .

_CNTCOOH

.

.

.

Table . Thickness and su⅔face ⅔oughness of pu⅔e and CNT-containing PMM“–silica hyb⅔ids p⅔epa⅔ed at diffe⅔ent ”PO/MM“ mola⅔ ⅔atios.

“FM topog⅔aphy images obtained fo⅔ pu⅔e PMM“–silica films and fo⅔ those containing welldispe⅔sed CNTs showed that each of the five hyb⅔ids p⅔esented a ve⅔y smooth and unifo⅔m

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su⅔face mo⅔phology Figure . No defects, po⅔es, c⅔acks o⅔ inhomogeneities we⅔e obse⅔ved on the coated samples. The RMS ⅔oughness RRMS ext⅔acted f⅔om “FM measu⅔ements Table showed ve⅔y low values of < . nm fo⅔ all coatings, confi⅔ming the homogeneity of the films and the efficient dispe⅔sion of CNTs within the hyb⅔id mat⅔ices. The ”PO . _CNT_SDS su⅔face mo⅔phology is shown in the high-⅔esolution “FM image in Figure . The local smoothness of the su⅔face is confi⅔med in this image, while the⅔e is also indication fo⅔ a possible p⅔esence of a single CNT on the su⅔face.

Figure . “FM images of all hyb⅔id coatings deposited on ca⅔bon steel.

Figure . High-⅔esolution “FM image of the ”PO .

_CNT_SDS sample.

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

. . . Bonding structure XPS analysis showed that the composition of all hyb⅔ids was ve⅔y simila⅔, with values close to the nominal atomic pe⅔centages of at.% of ca⅔bon, at.% of oxygen and at.% of silicon, with an expe⅔imental e⅔⅔o⅔ of ± %. Rep⅔esentative spect⅔a of ca⅔bon, oxygen and silicon, deconvoluted into thei⅔ st⅔uctu⅔al components, a⅔e p⅔esented in Figure . The C s spect⅔um, shown in Figure a, has fou⅔ components ⅔elated to C–H, C–C–O, C–O and O–C=O bonds p⅔esent in the PMM“ and MPTS molecules Figure d [ ]. The ca⅔bon unde⅔lined co⅔⅔e‐ sponds to the atom that was analyzed. The O s spect⅔um Figure b was fitted with th⅔ee components, associated with O–C=O and O=C bonds of PMM“ and MPTS, and O–Si bonds of the ino⅔ganic netwo⅔k, obse⅔ved also in the Si p spect⅔um Figure c . Fo⅔ completely condensed SiO phase the well-known binding ene⅔gy of the Si p peak is located at . ± . eV [ ]. “s the condensation ⅔eaction was incomplete, it is possible that the obse⅔ved binding ene⅔gy shift to a lowe⅔ value of . eV is caused by some ⅔emaining silanol g⅔oups Si–OH f⅔om TEOS and MPTS hyd⅔olysis and condensation. The addition of CNTs to the PMM“–silica hyb⅔ids had no effect on the XPS spect⅔a because the additional ca⅔bon concent⅔ation level of ppm f⅔om the nanotubes was below the ppm detection limit of ca⅔bon.

Figure . Rep⅔esentative a C s, b O s and c Si p XPS spect⅔a of a PMM“–silica hyb⅔id and d a schematic of the PMM“–silica hyb⅔id st⅔uctu⅔e.

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Si-NMR analysis also allows the identification of the local chemical bonding st⅔uctu⅔e and the ⅓uantitative evaluation of the connectivity of the ino⅔ganic phase. To dete⅔mine the influence of the CNTs on the ino⅔ganic silica netwo⅔k, NMR was used to compa⅔e the pu⅔e and CNT-containing hyb⅔ids. Rep⅔esentative spect⅔a a⅔e shown in Figure fo⅔ a CNTcontaining sample and a pu⅔e sample. ”oth spect⅔a have two g⅔oups of peaks co⅔⅔esponding to Ti i = , , and Qj j = , , st⅔uctu⅔es shown in Figure . These two fo⅔ms of local st⅔uctu⅔es a⅔ise as a conse⅓uence of the MPTS and TEOS p⅔ecu⅔so⅔ species, ⅔espectively. The peaks at chemical shifts of − , − and − ppm co⅔⅔espond to T , T and T envi⅔onments, ⅔espectively, while the peaks at chemical shifts at − , − andppm a⅔e associated with Q , Q and Q envi⅔onments, ⅔espectively Figure [ ]. The connectivity of the ino⅔ganic phase, defined as deg⅔ee of polycondensation Cd , was dete⅔mined f⅔om the fitted Voigt p⅔ofiles using the following e⅓uation 2 3 æ 1 Q1 + 2Q2 + 3Q 3 + 4Q 4 Cd = ç T + 2T + 3T + 3 4 è

ö ÷ ´ 100 ø

The deg⅔ee of polycondensation dete⅔mined fo⅔ ”PO . was ± %, meaning that about % of the silicon atoms a⅔e bonded to othe⅔ silicon atoms th⅔ough Si–O–Si oxygen b⅔idges. Simila⅔ Cd values we⅔e obtained fo⅔ the CNT-containing hyb⅔id ”PO . _CNT_SDS, indicating that CNT loading did not affect the connectivity of the silica phase.

Figure . Si-NMR spect⅔um f⅔om the ”PO .

and ”PO .

_CNT_SDS samples.

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

Figure TEOS.

. Schematic ⅔ep⅔esentation of Ti and QJ st⅔uctu⅔es. R’ indicates OH o⅔ OCH g⅔oups in MPTS o⅔ OCH CH in

. . . Thermal properties The⅔mog⅔avimet⅔y examines the ove⅔all connectivity of the hyb⅔id netwo⅔k in te⅔ms of the the⅔mal stability of the hyb⅔id mate⅔ials in diffe⅔ent atmosphe⅔es. Unde⅔ nit⅔ogen, PMM“ deg⅔ades in events as the tempe⅔atu⅔e inc⅔eases scission of head-to-head linkages at about °C T , scission of vinylidene chain-ends at about °C T and finally ⅔andom scissions of the polyme⅔ chains due to the ⅔uptu⅔e of head–tail segments at about °C T [ , ]. The T event at highe⅔ tempe⅔atu⅔es a⅔ound °C is due to the dehyd⅔ation of the ⅔emaining silanol g⅔oups of the silica netwo⅔k, detected in the XPS Si p spect⅔a of Figure [ ]. The⅔‐ mog⅔avimet⅔ic TG cu⅔ves and thei⅔ de⅔ivatives diffe⅔ential the⅔mog⅔avimet⅔y cu⅔ves – DTG cu⅔ves fo⅔ all the hyb⅔id samples a⅔e shown in Figure . The onset tempe⅔atu⅔e, T , which is a measu⅔e of the the⅔mal stability of each mate⅔ial, is defined as the tempe⅔atu⅔e at which a % weight loss occu⅔s. The tempe⅔atu⅔es of all events and the pe⅔centages of the silica and g⅔aphitic ⅔esidues at °C a⅔e listed fo⅔ the five hyb⅔id samples in Table . Compa⅔ing the DTG ⅔esults obtained fo⅔ the ”PO . and ”PO . mat⅔ices Figure b , it is appa⅔ent that the highe⅔ ⅓uantity of ”PO leads to an inc⅔ease in the deg⅔ee of polyme⅔ization and thus to a highe⅔ the⅔mal stability, so that the low-tempe⅔atu⅔e events seen in ”PO . a⅔e supp⅔essed in ”PO . . It can be seen f⅔om the data in Table that all five hyb⅔ids a⅔e stable up to °C, with sample ”PO . _CNT_SDS having the highest the⅔mal stability °C . Fu⅔the⅔mo⅔e, conside⅔ing that the T peak ⅔elated to the dehyd⅔ation of silanol is almost constant and that all loaded samples have the same CNT concent⅔ation, the ⅔esidue fo⅔med mainly of pu⅔e silica SiO and some ⅔emaining g⅔aphitic phase can be used to estimate the f⅔action of the o⅔ganic phase. The obse⅔vation that hyb⅔ids p⅔epa⅔ed at the lowe⅔ ”PO to MM“ ⅔atio of ”PO . have about % highe⅔ amount of ⅔esidue than those synthesized at the highe⅔

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⅔atio of . is consistent with an inc⅔ease in polyme⅔ization and a highe⅔ f⅔action of the polyme⅔ic phase in the ”PO . -based samples. It is also inte⅔esting to note that the addition of the CNTs to ”PO . enhanced its the⅔mal stability. In addition to the °C inc⅔ease in T , the fi⅔st two depolyme⅔ization events T and T shifted by about °C to highe⅔ tempe⅔atu⅔es Table . The T disinteg⅔ation event inc⅔eased by °C. This ⅔esult is simila⅔ to that found by Jin et al. [ ], in which T was shifted upwa⅔ds by °C fo⅔ a PMM“ mat⅔ix containing wt.% of CNTs, a concent⅔ation significantly highe⅔ than ⅔epo⅔ted in this wo⅔k. The ⅔eta⅔dation effect was att⅔ibuted by Jin et al. to inte⅔‐ actions between the ca⅔bon nanost⅔uctu⅔e and mac⅔o⅔adicals gene⅔ated du⅔ing the depoly‐ me⅔ization, as suggested by T⅔oitskii et al. [ ]. In cont⅔ast, the ”PO . mat⅔ix shows values of the the⅔mal deg⅔adation events almost unchanged by the p⅔esence of CNTs. Compa⅔ed to the ”PO . mat⅔ix, this behavio⅔ can be unde⅔stood in te⅔ms of a mo⅔e stable st⅔uctu⅔e of the ”PO . mat⅔ix induced by the highe⅔ deg⅔ee of polyme⅔ization.

Figure

. a TG cu⅔ves and b–d DTG cu⅔ves of the five hyb⅔id samples.

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

Hybrid

T °C

”PO . ”PO .

T °C

T °C

T °C

Residue % .

_CNT_SDS

”PO .

. .

”PO .

_CNT_SDS

.

”PO .

_CNTCOOH

.

T Tempe⅔atu⅔e of % weight loss, and tempe⅔atu⅔es T of the fi⅔st, T of the second and T of the thi⅔d deg⅔adation event. Table . The cha⅔acte⅔istic tempe⅔atu⅔es of the deg⅔adation events of PMM“–silica hyb⅔id and the ⅔esidue pe⅔centage obtained by the⅔mal analysis in nit⅔ogen atmosphe⅔e.

Ou⅔ wo⅔k and a numbe⅔ of othe⅔ studies investigating the ⅔einfo⅔cement effects by CNTs in dive⅔se o⅔ganic and hyb⅔id mat⅔ices all come to the same conclusion the modification imp⅔oves the the⅔mal stability of the composite. Thus, fo⅔ example, in a ⅔ecent study, Sabet et al. [ ] used wt.% of multiwall CNTs MWCNTs functionalized with ca⅔boxylic g⅔oups to ⅔einfo⅔ce an o⅔ganic–ino⅔ganic hyb⅔id mat⅔ix based on polyhed⅔al oligome⅔ic silses⅓uioxane POSS . The app⅔oach involved a covalent conjugation between the CNTs and the POSS molecules th⅔ough amide bonds. These autho⅔s obse⅔ved that the decomposition unde⅔ nit⅔ogen of the neat POSS sta⅔ted at °C, and a complete weight loss was obse⅔ved at °C. The POSS–MWCNT composite exhibited a fai⅔ly stable the⅔mal behavio⅔ f⅔om ⅔oom tempe⅔‐ atu⅔e to °C, and only % of weight loss by °C measu⅔ed by TG“. Zhang et al. [ ] coated functionalized MWCNTs with silica nanosphe⅔es, and subse⅓uently int⅔oduced these into PMM“ at a loading level of . wt.% to make it mo⅔e flame ⅔esistant. TG“ of the ⅔esultant PMM“/silica/MWCNT nanocomposites heated at °C min− indicated that the MWCNT/silica combination not only inc⅔eased the tempe⅔atu⅔e indicating % weight loss f⅔om °C of PMM“ to °C, but also the tempe⅔atu⅔e of the maximum ⅔ate of deg⅔a‐ dation inc⅔eased f⅔om °C fo⅔ PMM“ to °C fo⅔ the nanocomposites. These ⅔esults, suppo⅔ted by cone calo⅔imete⅔ tests and scanning elect⅔on mic⅔oscopy, showed that the MWCNT/silica combination int⅔oduced into the PMM“ noticeably imp⅔oved the the⅔mal stability and flame ⅔eta⅔dancy of PMM“ by in effect fo⅔ming a su⅔face the⅔mal ba⅔⅔ie⅔ laye⅔ du⅔ing bu⅔ning which helped to p⅔otect the unde⅔lying bulk f⅔om exposu⅔e to the exte⅔nal heat sou⅔ce. F⅔ase⅔ et al. pe⅔fo⅔med an in situ polyme⅔ization of PMM“ in the p⅔esence of a low . wt.% concent⅔ation of eithe⅔ ⅔aw single-wall CNTs SWCNTs o⅔ acid-t⅔eated SWCNTs [ ]. “lthough these t⅔anspa⅔ent nanocomposites had slightly lowe⅔ tempe⅔atu⅔es at which % weight loss had occu⅔⅔ed in TG“ when heated at °C min− in compa⅔ison with comme⅔cial PMM“, the tempe⅔atu⅔e co⅔⅔esponding to the maximum ⅔ate of deg⅔adation inc⅔eased by °C fo⅔ the composites with the ⅔aw SWCNTs and by °C fo⅔ the composites with acid-t⅔eated SWCNTs. Inte⅔estingly, these autho⅔s also showed using Raman spect⅔oscopy that acid

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t⅔eating of the SWCNTs enabled them to bind covalently with the PMM“, ⅔athe⅔ than me⅔ely be in contact with it, as was the case with the ⅔aw SWCNTs. The the⅔mal analysis ⅔esults obtained by Xiong et al. fo⅔ polyu⅔ethane PU covalently linked with wt.% of amino-functionalized MWCNTs indicate that the tempe⅔atu⅔e at which the maximum ⅔ate of deg⅔adation occu⅔s inc⅔eased f⅔om °C fo⅔ PU to °C fo⅔ the composite in TG“ expe⅔iments with heating ⅔ates of °C min− , once again indicating an imp⅔ovement of the⅔mal stability of a polyme⅔ mat⅔ix with the addition of CNTs [ ]. “nothe⅔ TG“ study, this time with a heating ⅔ate of °C min− on PU nanocomposite coatings modified with wt. % MWCNTs, showed that the tempe⅔atu⅔e at which the⅔e was complete decomposition of the mat⅔ix PU inc⅔eased by °C with the int⅔oduction of the MWCNTs [ ]. This was explained by the autho⅔s in te⅔ms of the ⅔elatively ine⅔t MWCNTs ⅔eta⅔ding the f⅔ee movement of the PU chains. Ove⅔all, it can be concluded that the imp⅔ovement of the the⅔mal stability of CNT-modified polyme⅔s and hyb⅔id nanocomposites, ⅔epo⅔ted by a numbe⅔ of labo⅔ato⅔ies, can be att⅔ibuted to a va⅔iety of facto⅔s, all of which a⅔e ⅔elated to the int⅔insic the⅔mal stability of CNTs, the effects of ⅔adical scavenging and by fo⅔ming a physical ba⅔⅔ie⅔ making it difficult fo⅔ volatile p⅔oducts in the mat⅔ix to escape f⅔om the bulk. . . . Mechanical properties Nanoindentation cu⅔ves p⅔ovide continuous values of Young’s modulus and ha⅔dness du⅔ing loading as a function of displacement e.g., Figure . The ave⅔age Young’s modulus and ha⅔dness values and the co⅔⅔esponding standa⅔d deviations and coefficients of va⅔iation fo⅔ the five nanocomposites, dete⅔mined f⅔om displacements between and nm, a⅔e summa⅔ized in Table .

Figure . Rep⅔esentative loading and unloading nanoindentation cu⅔ves fo⅔ sample ”PO . and Young´s modulus values we⅔e obtained.

f⅔om which ha⅔dness

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

Nanoindentation ⅔esults show a coating ha⅔dness between . ± . GPa and . ± . GPa fo⅔ all samples, values about twice as high as those fo⅔ PMM“ . – . GPa , but, not su⅔p⅔isingly, significantly lowe⅔ than amo⅔phous SiO – GPa [ ]. Young’s modulus values we⅔e in the ⅔ange between . ± . GPa and . ± . GPa, about th⅔ee times highe⅔ than pu⅔e PMM“ . – . GPa , but about one o⅔de⅔ of magnitude lowe⅔ than the elastic modulus of silicon oxide GPa . These values ⅔ep⅔esent a significant imp⅔ovement of ha⅔dness and stiffness compa⅔ed to pu⅔e ac⅔ylic, despite the p⅔esence of mo⅔e than % polymethac⅔ylate g⅔oups in the hyb⅔id [ ]. The inclusion of CNTs only p⅔oduced a significant inc⅔ease in ha⅔dness fo⅔ the ”PO . _CNT_SDS coating, with an inc⅔ease of some % in compa⅔ison with the ha⅔dness of the ”PO . ⅔efe⅔ence sample.

Sample

Young’s modulus GPa Mean

”PO . ”PO .

_CNT_SDS

”PO .

Std. dev.

Hardness GPa % COV

Mean

Std. dev.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

”PO .

_CNT_SDS

.

.

.

.

.

”PO .

_CNTCOOH

.

.

.

.

.

Table . Young’s modulus and ha⅔dness values, dete⅔mined f⅔om indentations between

Figure

% COV

.

. Mic⅔osc⅔atch cu⅔ves a fo⅔ ”PO .

mat⅔ix coatings and b fo⅔ ”PO .

and

.

. . nm deep.

mat⅔ix coatings.

Sc⅔atch testing is a widely used, fast and effective method to p⅔ovide info⅔mation on the lev‐ el of adhesion, ⅔esistance to sc⅔atching, the mechanism of f⅔actu⅔e, the coefficient of f⅔iction

213

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Carbon Nanotubes - Current Progress of their Polymer Composites

and the wea⅔ cha⅔acte⅔istics of coatings. In a typical expe⅔iment, a coating is sc⅔atched with inc⅔easing no⅔mal fo⅔ce using a diamond stylus. The t⅔ack is then analyzed by optical o⅔ elect⅔on mic⅔oscopy to dete⅔mine the mechanism of mechanical failu⅔e, such as coating de‐ tachment loss of adhesion to the subst⅔ate , c⅔acking and plastic defo⅔mation. The sc⅔atch test p⅔ovides the coefficient of f⅔iction, defined as the ⅔atio of the applied load and the no⅔‐ mal load, di⅔ectly. Cu⅔ves of the coefficient of f⅔iction as a function of sc⅔atch distance mic⅔osc⅔atch cu⅔ves fo⅔ all hyb⅔id coatings a⅔e shown in Figure . The inc⅔ease of the f⅔iction coefficient is associated with an inc⅔ease in sc⅔atch ⅔esistance f⅔iction fo⅔ce , while the c⅔itical load fo⅔ delamination is ⅔elated to the difficulty in b⅔eaking the adhesive inte⅔action between the coating and the metal subst⅔ates. The sc⅔atch t⅔acks, shown in Figure , we⅔e analyzed by optical mic⅔oscopy to dete⅔mine the failu⅔e mechanism and the c⅔itical load fo⅔ film c⅔acking and delamination.

Figure

. Optical mic⅔oscopy of the five hyb⅔id coatings deposited on “

ca⅔bon steel afte⅔ sc⅔atch testing.

“s is evident f⅔om Figure , the PMM“–silica ⅔efe⅔ence samples ”PO . and ”PO . we⅔e the softest coatings, showing fou⅔ defo⅔mation stages with inc⅔easing fo⅔ce elastic defo⅔mation, plastic defo⅔mation, c⅔acks and delamination. The c⅔itical loads fo⅔ delamination we⅔e mN fo⅔ ”PO . and mN fo⅔ ”PO . , ma⅔ked by the sta⅔t of st⅔ong noise on the mic⅔osc⅔atch cu⅔ves shown in Figure . The hyb⅔ids containing CNTs showed a highe⅔ sc⅔atch ⅔esistance and bette⅔ adhesion to the “ steel subst⅔ate. Most inte⅔estingly, the ”PO . _CNT_SDS coating showed an ext⅔eme ⅔einfo⅔cement effect, with a highe⅔

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

coefficient of f⅔iction than ca⅔bon steel . [ ] and no delamination up to a load of mN Figure a , the maximum load capacity of the e⅓uipment. ”PO . _CNT_SDS and ”PO . _CNTCOOH had c⅔itical loads fo⅔ delamination of mN Figure b and mN Figure c , ⅔espectively, both a⅔e highe⅔ than those obtained fo⅔ the ”PO . mat⅔ix. These ⅔esults confi⅔m that the int⅔insic mechanical p⅔ope⅔ties of CNTs, i.e., the high elastic modulus ~ . TPa and the high st⅔ength – GPa , cont⅔ibute to a significant ⅔einfo⅔cement of the hyb⅔id [ ]. The st⅔ong adhesion of the hyb⅔id film to the ca⅔bon steel is a conse⅓uence of the covalent inte⅔action between the hyd⅔oxyl g⅔oups of the subst⅔ate and the silanol g⅔oups of the ino⅔ganic pa⅔t of the hyb⅔id. The inc⅔eased mechanical st⅔ength of the hyb⅔ids, induced by the inco⅔po⅔ation of CNTs, inc⅔eases the c⅔itical fo⅔ce fo⅔ delamination, thus extending the functionality of p⅔otective PMM“–silica coatings to conditions whe⅔e ab⅔asive fo⅔ces act in an agg⅔essive envi⅔onment, such as in ⅔eacto⅔s fo⅔ the acidic p⅔ocessing of suga⅔ cane, fo⅔ example.

Figure . Optical mic⅔oscopy of a the ”PO . _CNT_SDS coating afte⅔ sc⅔atch testing to a load of mN, b the ”PO . _CNT_SDS coating afte⅔ sc⅔atch testing to a load of mN and c the ”PO . _CNTCOOH coating afte⅔ sc⅔atch testing to a load of mN.

Othe⅔ studies have also examined the effect of CNT inco⅔po⅔ation on the mechanical p⅔ope⅔ties of o⅔ganic and hyb⅔id coatings. “t the low SWCNT loading studied by F⅔ase⅔ et al. in thei⅔ PMM“–SWCNT composites, no clea⅔ benefit was seen in the tensile p⅔ope⅔ties, although the⅔e was some evidence to suggest that composites with acid-t⅔eated SWCNTs had imp⅔oved impact st⅔ength in compa⅔ison with pu⅔e PMM“ [ ]. In thei⅔ ⅔ecent study on epoxy–CNT composite coatings deposited on -T aluminum alloy subst⅔ates, Khun et al. we⅔e able to conclude that . wt.% CNT loading clea⅔ly p⅔oduced composites with imp⅔oved adhesion to the subst⅔ates and imp⅔oved wea⅔ ⅔esistance ⅔elative to the unloaded epoxy coatings [ ]. This imp⅔ovement was explained in te⅔ms of a ⅔elaxation of the ⅔esidual st⅔ess within the epoxy coating caused by the inco⅔po⅔ation of the CNTs. Kuma⅔ and Gasem have been able to demonst⅔ate the beneficial effects of inco⅔po⅔ating wt. % of functionalized MWCNTs into polyaniline P“NI coatings deposited on mild steel by dip coating [ ]. The P“NI–MWCNT coatings showed a Vicke⅔s mic⅔o ha⅔dness of HV, compa⅔ed with HV fo⅔ pu⅔e P“NI coatings. Fu⅔the⅔mo⅔e, P“NI–MWCNT coatings showed significantly imp⅔oved ⅔esistance to sc⅔atching in compa⅔ison with pu⅔e P“NI coatings.

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Despite the ⅔esults obtained by F⅔ase⅔ et al. [ ], the clea⅔ t⅔end in the ⅔esults of the wo⅔k ⅔epo⅔ted he⅔e and elsewhe⅔e is that cont⅔olled inco⅔po⅔ation of CNTs into hyb⅔id and polyme⅔ mat⅔ices at suitable levels is likely to be beneficial to the mat⅔ices in te⅔ms of imp⅔oved mechanical pe⅔fo⅔mance. Thus, it is ⅔easonable to expect significant imp⅔ovements in te⅔ms of sc⅔atch and wea⅔ ⅔esistance, adhesion st⅔ength and also ha⅔dness and Young´s modulus of the mat⅔ices when inco⅔po⅔ating CNTs. Fo⅔ the most pa⅔t, this can be att⅔ibuted to the excellent mechanical p⅔ope⅔ties of the CNTs. Howeve⅔, mo⅔e wo⅔k is clea⅔ly ⅔e⅓ui⅔ed to unde⅔stand fully the mechanism ⅔esponsible fo⅔ the beneficial effect of inco⅔po⅔ating CNTs on the adhesion of these mat⅔ices to metallic subst⅔ates. . .5. Anticorrosive properties The co⅔⅔osion p⅔otection efficiency of the pu⅔e and the modified hyb⅔id coatings was dete⅔‐ mined by EIS, pe⅔fo⅔med in an elect⅔ochemical cell containing a⅓ueous . % NaCl solution at °C. The p⅔inciple of EIS is to impose a small sinusoidal potential with va⅔ying f⅔e⅓uency and, by measu⅔ing the alte⅔nating cu⅔⅔ent ⅔esponse, to obtain the impedance of the elect⅔o‐ chemical system. The impedance Z ω is composed of a ⅔eal and an imagina⅔y pa⅔t, involving the ohmic and capacitive cont⅔ibutions, and can be ⅔ep⅔esented as a vecto⅔ of length |Z|, whe⅔e |Z| = Z⅔eal + Zimag / . The angle between the Z vecto⅔ and the Z⅔eal axis is the phase angle ϕ [ ]. Fo⅔ each measu⅔ement, th⅔ee g⅔aphs we⅔e obtained a Ny⅓uist plot Z⅔eal vs. Zimag and two ”ode g⅔aphs of the impedance modulus and phase angle as a function of f⅔e⅓uency. EIS measu⅔ements we⅔e pe⅔fo⅔med fo⅔ all hyb⅔id-coated samples afte⅔ one day of imme⅔sion and then at week inte⅔vals until a significant d⅔op of the impedance modulus occu⅔⅔ed due to pitting. The time inte⅔val until the onset of pitting defined the lifetime of the coating. The Ny⅓uist and ”ode plots a⅔e shown in Figure , while the e⅓uivalent elect⅔ical ci⅔cuits of the elect⅔olyte–coating–subst⅔ate system used to fit the EIS data a⅔e shown in Figure . Fo⅔ compa⅔ison, EIS cha⅔acte⅔istics of ba⅔e “ ca⅔bon steel subst⅔ate we⅔e also ⅔eco⅔ded, as shown in Figure . The impedance modulus at low f⅔e⅓uency and the phase angle behavio⅔ is an indicato⅔ of the antico⅔⅔osion pe⅔fo⅔mance. Coatings with modulus highe⅔ than Ω cm typically p⅔ovide excellent p⅔otection, while those below Ω cm have poo⅔ p⅔otection efficiency [ ]. The ”PO . mat⅔ix had an initial impedance modulus of Ω cm which ⅔emained un‐ changed du⅔ing days it su⅔vived testing, while the ”PO . mat⅔ix had a one o⅔de⅔ of magnitude highe⅔ impedance modulus of Ω cm , ⅔emaining stable fo⅔ its lifetime of days. This finding might be ⅔elated to the highe⅔ ove⅔all connectivity of the ”PO . hyb⅔id, which, on the basis of the ⅔esults of NMR, TG/DTG and mechanical testing, had highly polyme⅔ized o⅔ganic moieties densely inte⅔connected with ⅔eticulated silica nodes. This excellent pe⅔fo⅔mance of the PMM“–silica hyb⅔id coating can be compa⅔ed with the pe⅔fo⅔m‐ ance of the ba⅔e ca⅔bon steel. The coated samples showed up to o⅔de⅔s of magnitude highe⅔ co⅔⅔osion ⅔esistance, a conse⅓uence of the dense st⅔uctu⅔e acting as an efficient diffusion ba⅔⅔ie⅔ against agg⅔essive agents. In this context, it should be noted that the antico⅔⅔osive pe⅔fo⅔mance ⅔epo⅔ted fo⅔ most hyb⅔id coatings, in te⅔ms of initial impedance modules, is usually of the o⅔de⅔ of Ω cm [ , , ].

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

The addition of CNTs dispe⅔sed in SDS in the ”PO . mat⅔ix did not change neithe⅔ the impedance modulus Ω cm significantly, no⅔ the lifetime of the coating days . The addition of functionalized and dispe⅔sed CNTs to the ”PO . mat⅔ix inc⅔eased the impedance modulus, but dec⅔eased the lifetime of the coating to days fo⅔ ”PO . _CNT_SDS and days fo⅔ ”PO . _CNTCOOH. “s has been suggested in ⅔ecent studies [ , ], CNTs act in the PMM“–silica nanocomposite as st⅔uctu⅔al ⅔einfo⅔cements and densifie⅔ agents, p⅔oviding imp⅔oved the⅔mal and mechanical p⅔ope⅔ties without deg⅔ading the chemical ba⅔⅔ie⅔ cha⅔ac‐ te⅔istics. The ext⅔ao⅔dina⅔y elect⅔ochemical pe⅔fo⅔mance of these mic⅔on thick films, ap‐ p⅔oaching that of thick paints, is ⅔elated to thei⅔ dense hyb⅔id st⅔uctu⅔e, thus p⅔oviding an efficient passivation of metallic su⅔faces [ ]. “lthough the CNT-containing coatings we⅔e thicke⅔ than pu⅔e hyb⅔id films, they actually showed sho⅔te⅔ lifetimes in the NaCl solution. This can be explained in te⅔ms of elect⅔olyte uptake via diffusion paths along the oute⅔ nanotube walls and also th⅔ough the cavities of the nanotubes. The⅔efo⅔e, the deg⅔adation of the coatings afte⅔ long-te⅔m exposu⅔e is associated with the penet⅔ation of the elect⅔olyte, involving Cl− ions, oxygen and wate⅔ and subse⅓uent chemical ⅔eaction co⅔⅔osion at the coating/metal inte⅔face [ ], causing a sudden d⅔op of the elect⅔ochemical pe⅔fo⅔mance.

Figure . Ny⅓uist and ”ode plots fo⅔ uncoated ca⅔bon steel a ”PO . imme⅔sion in . % NaCl standa⅔d saline solution.

mat⅔ix and b ”PO .

mat⅔ix afte⅔

day of

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Carbon Nanotubes - Current Progress of their Polymer Composites

Figure

. The elect⅔ical e⅓uivalent ci⅔cuit used to fit all the EIS expe⅔imental data.

To obtain a ⅓uantitative model of the elect⅔ochemical system, the e⅓uivalent ci⅔cuit of Figure was used to fit the impedance data of Figu⅔e . The ci⅔cuit consists of two time constants, Rc/Cc at high f⅔e⅓uency ~ Hz and Rct/Cdl at low f⅔e⅓uency ~ Hz , whe⅔e Rc is the coating ⅔esistance, Cc is the coating capacitance, Rct is the cha⅔ge t⅔ansfe⅔ ⅔esistance and Cdl is the capacitance of the elect⅔ic double laye⅔ of the coating/ca⅔bon steel inte⅔face [ ]. “s f⅔e⅓uently applied fo⅔ elect⅔ochemical systems, the capacito⅔s we⅔e ⅔eplaced by a constant phase element CPE to take into account the nonideality of the capacito⅔ ⅔ep⅔esenting the coating, exp⅔essed by the nc and ndl exponents. “ll ci⅔cuit pa⅔amete⅔s obtained by fitting the EIS data a⅔e shown in Table . “ll chi-s⅓ua⅔e χ values we⅔e smalle⅔ than − , ensu⅔ing a high fit ⅓uality. Coatings showing a combination of high co⅔⅔osion ⅔esistance and low phase angle values close to − °, indicative of ideal capacitive behavio⅔, a⅔e ve⅔y efficient in blocking the elect⅔olyte uptake. “ll hyb⅔id films showed elevated volume and inte⅔face ⅔esistances and high-f⅔e⅓uency phase angle values below − ° ove⅔ fou⅔ decades, all of which a⅔e cha⅔acte⅔istics expected fo⅔ efficient p⅔otective coatings. BPO . χ

. ×

RC Ω cm

. ×

CPEC Ω cm s

. ×

nC

.

Rct Ω cm

. ×

CPEdl Ω− cm− sn

. ×

ndl

.





n

BPO . . ×







.

. ×

.

. ×

.

.

.

. ×

.

. ×

.

.

_CNT_SDS BPO . ×







.

. ×

.

. ×

.

.

.

. ×

.

. ×

.

.

.

BPO . ×







.

. ×

.

. ×

.

.

.

. ×

.

. ×

.

.

_CNT_SDS BPO

. −



Values in b⅔ackets ⅔ep⅔esent the e⅔⅔o⅔s in pe⅔centage.

*

Table . E⅓uivalent ci⅔cuit pa⅔amete⅔s fo⅔ all samples afte⅔

. ×



day in . % NaCl solution.

_CNTCOOH −

. ×

.

. ×

.

.

.

. ×

.

. ×

.

.

. −

. . .



. .

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

The⅔e a⅔e a numbe⅔ of ⅔elated studies which have examined the elect⅔ochemical pe⅔fo⅔mance of CNT–hyb⅔id and CNT–polyme⅔ nanocomposite coatings in contact with saline envi⅔on‐ ments. Liu et al. modified bis-[t⅔iethoxysilylp⅔opyl] tet⅔asulfide ”TESPT silane films by MWCNTs functionalized with ca⅔boxylic g⅔oups [ ]. EIS ⅔esults, obtained fo⅔ coated stainless steel samples in . % NaCl solution, showed that the addition of diffe⅔ent amounts of CNTs . , . and mg imp⅔oved the co⅔⅔osion ⅔esistance ⅔elative to the pu⅔e ”TESPT coatings. The impedance modulus of the ”TESPT/MWCNT hyb⅔id film ⅔eached about Ω cm , a value about th⅔ee o⅔de⅔s of magnitude highe⅔ than the ba⅔e subst⅔ate and one o⅔de⅔ of magnitude highe⅔ than the pu⅔e ”TESPT film. The autho⅔s suggested that ca⅔boxylated MWCNTs ⅔eact with silanol g⅔oups f⅔om the silane p⅔ecu⅔so⅔ to fo⅔m covalent bonds between ”TESPT and the MWCNTs which st⅔engthen the adhesion st⅔ength of the film, inc⅔easing its density and thus inhibiting the penet⅔ation of the co⅔⅔osive ions. In addition, it was suggested by these autho⅔s that the long chain st⅔uctu⅔e of the CNTs may be able to fill pits and c⅔acks in the film, thus helping to ⅔educe the numbe⅔ of co⅔⅔osion-induced defects. Jeon et al. ⅔epo⅔ted EIS ⅔esults combined with hyg⅔othe⅔mal cyclic testing of thei⅔ CNT-loaded epoxy coatings deposited on ca⅔bon steel [ ]. In the hyg⅔othe⅔mal testing, the tempe⅔atu⅔e was ⅔amped up f⅔om °C to °C and back to °C ove⅔ a pe⅔iod of hou⅔s while the coating was in contact with the elect⅔olyte. This p⅔ocedu⅔e was designed to accele⅔ate the cumulative effect of the elect⅔olyte on the coating/subst⅔ate inte⅔face. They found that afte⅔ hyg⅔othe⅔‐ mal test cycles the impedance modulus fo⅔ pu⅔e epoxy coatings at . Hz dec⅔eased f⅔om Ω cm to Ω cm . Fo⅔ the two loadings of . and . wt.% that they examined fo⅔ the CNT-containing samples, the⅔e was a lowe⅔ initial ⅔esistance at . Hz because of the conductive natu⅔e of the CNTs, but afte⅔ cycles the impedance modulus at this f⅔e⅓uency was highe⅔ than fo⅔ the pu⅔e epoxy coatings. These ⅔esults we⅔e explained by the autho⅔s in te⅔ms of the dec⅔ease in the wate⅔ uptake of the CNT-loaded coatings, ⅔athe⅔ than any dec⅔ease in chemical attack by the . wt% NaCl solution, and the inc⅔ease in adhesion st⅔ength of the coatings with the addition of the CNTs. The antico⅔⅔osive pe⅔fo⅔mance of epoxy–CNT composite coatings was also ⅔epo⅔ted fo⅔ aluminum alloy -T subst⅔ates, tested by EIS in a . M NaCl solution by Kuhn et al. [ ]. Epoxy coatings and epoxy coatings containing . wt.% of MWCNTs had a impedance modulus of × Ω cm , slightly highe⅔ than ba⅔e “l, which had an impedance modulus of Ω cm . On compa⅔ison, epoxy coatings containing . wt.% of MWCNTs had a highe⅔ impedance modulus of × Ω cm , explained by the autho⅔s in te⅔ms of the D dispe⅔sion of the MWCNTs in the epoxy mat⅔ix at this level cont⅔ibuting to a ⅔eduction in the po⅔osity of the coating, and thus ⅔educing its elect⅔olyte uptake. Deyab [ ] studied the effect on the co⅔⅔osion p⅔otection efficiency of coated ca⅔bon steel of diffe⅔ent CNT concent⅔ations f⅔om . to . wt.% in alkyd ⅔esin films used as the p⅔otective coatings. He found that all CNT-loaded films had an imp⅔oved co⅔⅔osion ⅔esistance in . wt. % NaCl saline solution ⅔elative to the pu⅔e alkyd ⅔esin films. These findings we⅔e att⅔ibuted to the ability of the functionalized CNTs to abso⅔b ⅔esin on thei⅔ su⅔faces, the⅔eby enhancing the density of the coatings, eliminating mic⅔oflaws in the coating and making it mo⅔e difficult fo⅔ co⅔⅔osive species to be t⅔anspo⅔ted th⅔ough the coating to the unde⅔lying steel.

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Finally, the co⅔⅔osion p⅔otection pe⅔fo⅔mance of the P“NI–MWCNT coatings studied by Kuma⅔ and Gasem demonst⅔ated imp⅔oved co⅔⅔osion pe⅔fo⅔mance of the coating containing MWCNT in compa⅔ison with the pu⅔e P“NI coatings [ ]. This obse⅔vation was also explained by the autho⅔s in te⅔ms of a ⅔educed level of po⅔osity in the P“NI coating as a conse⅓uence of bonding between the P“NI mat⅔ix and the functionalized CNTs, leading in tu⅔n to ⅔educed pe⅔meability of coating to co⅔⅔osive agents. In compa⅔ison with p⅔otective coatings based on o⅔ganic–CNT composites ⅔epo⅔ted to date in the lite⅔atu⅔e, the PMM“–silica–CNT films on ca⅔bon steel, discussed in the wo⅔k, a⅔e by fa⅔ the most effective co⅔⅔osion p⅔otection ba⅔⅔ie⅔, with a much highe⅔ impedance modulus and lifetime in . % NaCl solution. Ove⅔all, the ⅔esults obtained in this study have shown that a homogeneous dispe⅔sion of singlewall CNTs in PMM“–silica nanocomposites ⅔ep⅔esents a novel and ve⅔y p⅔omising coating system that is able to combine high antico⅔⅔osive pe⅔fo⅔mance with elevated the⅔mal and mechanical stability, extending the application of these coatings to ab⅔asive envi⅔onments.

. Conclusions Single-wall CNTs have been dispe⅔sed th⅔ough su⅔factant assistance and by functionalization with ca⅔boxylic g⅔oups. ”oth dispe⅔sion p⅔ocedu⅔es have p⅔oved to be ve⅔y effective in the modification of PMM“–silica hyb⅔ids, p⅔oducing homogeneous and defect-f⅔ee nanocompo‐ site coatings with ve⅔y smooth su⅔faces RRMS < . nm and thicknesses of – μm. Si-NMR ⅔esults showed that the addition of ppm of CNTs into PMM“–silica hyb⅔id mat⅔ices did not affect the high connectivity of the ino⅔ganic phase ~ % , while XPS ⅔esults confi⅔med the nominal composition and the p⅔opo⅔tion of bonding envi⅔onments fo⅔ming the hyb⅔id netwo⅔k. CNTs we⅔e effective in imp⅔oving the the⅔mal stability of the hyb⅔ids, inc⅔easing thei⅔ onset tempe⅔atu⅔e of deg⅔adation and shifting all depolyme⅔ization events to highe⅔ tempe⅔atu⅔es. Mechanical ⅔einfo⅔cement of the hyb⅔id coatings was achieved fo⅔ both CNT dispe⅔sion methods, ⅔esulting in a significantly highe⅔ sc⅔atch ⅔esistance and imp⅔oved adhesion of the coating to “ ca⅔bon steel subst⅔ate ⅔elative to the hyb⅔id coatings without the CNTs. EIS ⅔esults showed that the elect⅔ochemical pe⅔fo⅔mance of the CNT-loaded coatings is supe⅔io⅔ to most o⅔ganic–CNT coating systems ⅔epo⅔ted to date in the lite⅔atu⅔e, being able to act fo⅔ seve⅔al weeks as efficient co⅔⅔osion ba⅔⅔ie⅔s in agg⅔essive saline envi⅔on‐ ments, maintaining an impedance modulus of up to Ω cm . These ⅔esults suggest that CNT⅔einfo⅔ced PMM“–silica nanocomposites have a g⅔eat potential to extend the applicability of these envi⅔onmentally compliant, high efficiency antico⅔⅔osive coatings to ab⅔asive envi⅔on‐ ments.

Acknowledgements We would like to thank D⅔ John Nunn f⅔om the National Physical Labo⅔ato⅔y, Teddington, U.K. fo⅔ access to the mic⅔osc⅔atch e⅓uipment used in this wo⅔k, and the Labo⅔ato⅔y fo⅔ Su⅔face

Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

Science LCS at the National Nanotechnology Labo⅔ato⅔y LNNano in Campinas, ”⅔azil fo⅔ “FM access. This wo⅔k was suppo⅔ted by CNP⅓, C“PES and F“PESP.

Author details Sama⅔ah V. Ha⅔b , Fábio C. dos Santos , Sand⅔a H. Pulcinelli , Celso V. Santilli , Kevin M. Knowles and Pete⅔ Hamme⅔ * *“dd⅔ess all co⅔⅔espondence to pete⅔@i⅓.unesp.b⅔ Instituto de Química, UNESP-Unive⅔sidadeEstadualPaulista, “⅔a⅔a⅓ua⅔a, SP, ”⅔azil Unive⅔sity of Camb⅔idge, Depa⅔tment of Mate⅔ials Science and Metallu⅔gy, Camb⅔idge, England

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Protective Coatings Based on PMMA–Silica Nanocomposites Reinforced with Carbon Nanotubes http://dx.doi.org/10.5772/62808

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Chapter 8

Carbon Nanotubes and Graphene as Additives in 3D Printing Steve F. A. Acquah, Branden E. Leonhardt, Mesopotamia S. Nowotarski, James M. Magi, Kaelynn A. Chambliss, Thaís E. S. Venzel, Sagar D. Delekar and Lara A. Al-Hariri Additional information is available at the end of the chapter http://dx.doi.org/10.5772/63419

Abstract D p⅔inting is a ⅔evolutiona⅔y technology fo⅔ the consume⅔ and indust⅔ial ma⅔kets. “s the technology fo⅔ D p⅔inting has expanded, the need fo⅔ multi-mate⅔ials that suppo⅔t fused deposition modeling and othe⅔ fo⅔ms of additive manufactu⅔ing is inc⅔easing. D p⅔inting filaments infused with ca⅔bon nanotubes and g⅔aphene a⅔e now comme⅔cially available, with the p⅔omise of p⅔oducing conductive composites. This chapte⅔ explo⅔es some of the ⅔esea⅔ch, p⅔oducts, and challenges involved in b⅔inging the next gene⅔ation of functional p⅔inting mate⅔ials to the consume⅔ ma⅔ket. Keywords: D p⅔inting, additive manufactu⅔ing, g⅔aphene, ca⅔bon nanotubes, ca⅔bon black

. Introduction Johannes Gutenbe⅔g catalyzed one of the fi⅔st ⅔evolutions in education, with the development of the p⅔inting p⅔ess, allowing the dissemination of info⅔mation to take a mo⅔e st⅔uctu⅔ed pathway. In the mode⅔n wo⅔ld, compute⅔s, sma⅔tphones, and the inte⅔net have all cont⅔ibut‐ ed to the advancements in science, technology, and enginee⅔ing. D p⅔inting is anothe⅔ ⅔evolutiona⅔y technology fo⅔ the consume⅔ and indust⅔ial ma⅔kets. “lthough the consume⅔ ma⅔ket is still ⅔elatively young, the indust⅔ial secto⅔ has matu⅔ed with its inception in the mids. The fi⅔st D p⅔inte⅔ was c⅔eated by Cha⅔les W. Hull of D Systems Co⅔p. He published

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patents detailing many of the concepts involved in D P⅔inting, some of which a⅔e used today. Developments in the va⅔ious fo⅔ms of p⅔inting have ⅔esulted in the use of the umb⅔ella te⅔m of additive manufactu⅔ing. “t its co⅔e, D p⅔inting simply uses additives to c⅔eate D st⅔uctu⅔es p⅔oduced f⅔om successive laye⅔s of additives, such as polyme⅔s, and is deposited onto a platfo⅔m. The⅔e a⅔e many companies that develop affo⅔dable consume⅔ D p⅔inte⅔s such as Make⅔bot and Flashfo⅔ge Figure .

Figure . “ Flashfo⅔ge C⅔eato⅔ D p⅔inte⅔. This is a dual-ext⅔usion p⅔inte⅔ with a build volume of

cubic inches.

The most ⅔ecognizable fo⅔m of D p⅔inting is the⅔moplastic ext⅔usion, commonly known as fused deposition modelling FDM [ ], whe⅔e a filament typically . mm is unwound f⅔om a spool and fed into an ext⅔usion assembly. The use of a moto⅔-d⅔iven gea⅔ helps feed the filament into a tempe⅔atu⅔e cont⅔olled melting chambe⅔ befo⅔e the melted plastic is ext⅔uded th⅔ough the tip of the nozzle. “ fan is also used to help maintain the tempe⅔atu⅔e as shown in Figure . The design of ext⅔ude⅔s continues to evolve, making p⅔inting mo⅔e efficient by imp⅔ovements in the ⅔egulating tempe⅔atu⅔e ac⅔oss the ext⅔usion assembly, and design modifications to ⅔educe wea⅔ and blockages. Polylactic acid PL“ , a sustainable biodeg⅔adable polyme⅔, and ac⅔ylonit⅔ile butadiene sty⅔ene “”S , a st⅔ong te⅔polyme⅔ used in a ⅔ange of manufactu⅔ed p⅔oducts, a⅔e two of the most popula⅔ polyme⅔s used fo⅔ FDM, and have fo⅔med the basis of the next gene⅔ation of mixed mate⅔ial filaments fo⅔ ⅔esea⅔che⅔s. Othe⅔ polyme⅔s used include polyvinyl alcohol PV“ , which is wate⅔ soluble and biodeg⅔adable, and a⅔e suppo⅔t st⅔uctu⅔es fo⅔ D p⅔inting. Polyamides nylons a⅔e st⅔ong, ab⅔asion ⅔esistant, and typically used fo⅔ mechanical pa⅔ts. “ll of these polyme⅔s have the potential fo⅔ the inclusion of ca⅔bon nanost⅔uctu⅔ed additives to extend thei⅔ utility in multi-functional D p⅔inted composites. The⅔e a⅔e many c⅔owdfunded initiatives that a⅔e helping to fu⅔the⅔ ⅔educe the cost of ent⅔y level D p⅔inte⅔s and thei⅔ accesso⅔ies. Kicksta⅔te⅔, a wo⅔ld leading c⅔owdfunding platfo⅔m

Carbon Nanotubes and Graphene as Additives in 3D Printing http://dx.doi.org/10.5772/63419

has seen a la⅔ge inte⅔est in D p⅔inte⅔s p⅔oviding a uni⅓ue oppo⅔tunity fo⅔ both ea⅔ly adopte⅔s and indust⅔y specialists to evaluate cutting edge technology. Fo⅔m “n affo⅔dable, p⅔ofes‐ sional D p⅔inte⅔’ is an example of a successful Kicksta⅔te⅔ initiative with backe⅔s who pledged a total of $ , , to b⅔ing the p⅔oject to life. The p⅔inte⅔ ⅔etails at $ f⅔om Fo⅔mlabs. “nothe⅔ example of a successful Kicksta⅔te⅔ initiative was P⅔int⅔bot which ⅔aised $ , with backe⅔s, and sold units fo⅔ $ .

Figure . “ pictu⅔e of a dual ext⅔ude⅔ assembly f⅔om a FDM p⅔inte⅔ and a schematic of the ext⅔ude⅔.

Selective lase⅔ sinte⅔ing SLS is used to p⅔oduce D components with a lase⅔ and powde⅔ed the⅔moplastics. Du⅔ing this p⅔ocess, a lase⅔ pulses onto a platfo⅔m, c⅔eating a c⅔oss-section of the desi⅔ed object on the powde⅔. The lase⅔ is set to heat the powde⅔ just below its melting point causing it to fuse togethe⅔. Fusing the powde⅔ in specific locations defined by the softwa⅔e c⅔eates the laye⅔s. “n advantage of this method is the ability to p⅔ocess combinations of polyme⅔s and metals, and unused powde⅔ can be easily ⅔ecycled. Howeve⅔, the pa⅔ticle size of the powde⅔ dete⅔mines the p⅔ecision of the p⅔int. This p⅔ocess has the potential fo⅔ the inco⅔po⅔ation of ca⅔bon nanotubes, but mo⅔e ⅔esea⅔ch needs to be applied towa⅔d heat t⅔ansfe⅔ p⅔ocesses [ ]. Digital light p⅔ocessing DLP is anothe⅔ way of p⅔oducing high ⅔esolution D objects. The p⅔ocess uses a vat of photopolyme⅔ with cont⅔olled exposu⅔e to the light f⅔om a DLP p⅔ojecto⅔ o⅔ an inst⅔ument using a digital mic⅔omi⅔⅔o⅔ device DMD [ ]. The stage/build plate moves as the exposed li⅓uid polyme⅔ ha⅔dens Figure . Essentially, a D model on a compute⅔ is sliced into D c⅔oss-sectional laye⅔s to p⅔oduce images that a⅔e se⅓uentially sent to the DLP. The p⅔ocess continues until the D object is complete. “ny ⅔emaining li⅓uid photopolyme⅔ is then pu⅔ged f⅔om the vat and the completed p⅔int ⅔emoved.

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Figure . Diag⅔am of a digital light p⅔ocessing p⅔inte⅔.

Ste⅔eolithog⅔aphy is simila⅔ to DLP p⅔inting with the use of photopolyme⅔s, but diffe⅔ing with the lase⅔ based light sou⅔ce. Objects a⅔e p⅔oduced by p⅔inted laye⅔s, one at a time, by the use of a lase⅔ beam on the su⅔face of a vat of the li⅓uid photopolyme⅔. The polyme⅔ ha⅔dens ove⅔ the a⅔eas whe⅔e the lase⅔ st⅔ikes and a stage is moved down into the vat by one laye⅔. De‐ pending on the model of the p⅔inte⅔, a ⅔ecoate⅔ blade would move ac⅔oss the su⅔face of the vat, helping to deposit the next laye⅔ of li⅓uid polyme⅔. This p⅔ocess is ⅔epeated until the p⅔int is completed and the p⅔inted object is ⅔aised f⅔om the vat and the li⅓uid polyme⅔ d⅔ained. The p⅔inted object is then cleaned and cu⅔ed in ult⅔aviolet light to finish the p⅔ocessing. Ste⅔eoli‐ thog⅔aphy has been used with a ⅔ange of mate⅔ials including g⅔aphene oxide [ ]. The⅔e a⅔e many potential applications that di⅔ectly ⅔elate to the inclusion of ca⅔bon nanotubes and g⅔aphene as additives fo⅔ D p⅔inting [ , ] in a secto⅔ that is cu⅔⅔ently unde⅔going ⅔apid g⅔owth. The D p⅔inting secto⅔ took ove⅔ yea⅔s to ⅔each $ billion by and ove⅔ the following th⅔ee yea⅔s it ⅔eached $ billion. “nalysis of the ma⅔ket suggests it may ⅔each $ billion by [ ]. While the⅔e has been much p⅔aise fo⅔ the D p⅔inting indust⅔y, the⅔e have also been some negative conse⅓uences on the envi⅔onment and secu⅔ity. The p⅔ocess of the⅔mal ext⅔usion consumes a lot of elect⅔icity and the estimates of emission ⅔ates of ult⅔a-fine pa⅔ticles we⅔e la⅔ge fo⅔ p⅔inting with PL“ feedstock and even highe⅔ fo⅔ p⅔inting with “”S the⅔moplastic feedstock [ ]. Secu⅔ity has become an impo⅔tant issue afte⅔ the fab⅔ication of a D p⅔inted gun. The US State Depa⅔tment is t⅔ying to implement a ban on the dist⅔ibution of files fo⅔ the D p⅔inting of guns howeve⅔, the natu⅔e of file dist⅔ibution on the inte⅔net may make this almost

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impossible to police. Initially only files fo⅔ ⅔eplacement gun pa⅔ts we⅔e available, but a video soon demonst⅔ated the successful ope⅔ation of a p⅔inted gun.

. Polymers in D printing “dditive manufactu⅔ing is developing to meet the demands fo⅔ du⅔able ⅔eplacement pa⅔ts, which need to confo⅔m to specific mechanical and design ⅔e⅓ui⅔ements. With the⅔mal ext⅔usion, new p⅔inting mate⅔ials must have suitable ⅔heological and the⅔mal p⅔ope⅔ties to be able to be ext⅔uded and solidify while maintaining the accu⅔acy of successive laye⅔s. The use of polyme⅔s in additive manufactu⅔ing a⅔e gove⅔ned by the glass t⅔ansition tempe⅔atu⅔e, melting point, heat capacity, melt viscosity at elevated tempe⅔atu⅔es, and the shea⅔ st⅔ess of the mate⅔ial. Unde⅔standing the ⅔elationship between the st⅔uctu⅔al p⅔ope⅔ties will aid in the development of advanced functional p⅔inting filaments and mate⅔ials. The glass t⅔ansition tempe⅔atu⅔e Tg is the tempe⅔atu⅔e below which the polyme⅔ becomes b⅔ittle and ha⅔d like glass. The glass t⅔ansition tempe⅔atu⅔e is not the melting point, that is defined by the tempe⅔atu⅔e at which c⅔ystalline o⅔ semi-c⅔ystalline polyme⅔s change f⅔om its c⅔ystal st⅔uctu⅔e to its li⅓uid fo⅔m. “mo⅔phous polyme⅔s have no sha⅔p melting point due to thei⅔ non-c⅔ystalline st⅔uctu⅔e but they have a Tg tempe⅔atu⅔e. Semi-c⅔ystalline polyme⅔s have both a Tg and melting tempe⅔atu⅔e. Polyme⅔s that have a Tg below ⅔oom tempe⅔atu⅔e a⅔e elastic, while those with a Tg above ⅔oom tempe⅔atu⅔e tend to be ⅔igid and b⅔ittle. The Tg is highe⅔ fo⅔ polyme⅔s with stiff backbones and pendant g⅔oups that inte⅔act with nea⅔by st⅔uctu⅔es. ”oth of these featu⅔es ⅔esult in polyme⅔ chains ⅔e⅓ui⅔ing elevated tempe⅔atu⅔es to move. One example of a ⅔igid backbone is a benzene ⅔ing as found in poly p-phenylene while a flexible backbone can be found with the CH -CH bond in poly ethylene Figure . “ bulky b⅔anch may act as a plasticize⅔ and lowe⅔ the Tg by dec⅔easing the packing of polyme⅔ chains. This can be seen with poly methyl methac⅔ylate with a Tg = °C as compa⅔ed to a Tg of °C fo⅔ poly butyl methac⅔ylate , a diffe⅔ence of th⅔ee additional ca⅔bons in the b⅔anch.

Figure . St⅔uctu⅔e of poly ethylene , poly p-phenylene , poly methyl methac⅔ylate and poly butyl methac⅔ylate .

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“mo⅔phous polyme⅔s dec⅔ease in st⅔ength above the Tg and sta⅔t to soften g⅔adually while the viscosity changes. The lowe⅔ the Tg of the polyme⅔, the faste⅔ it ⅔eaches the optimal viscosity fo⅔ ext⅔usion. Heat capacity also plays a ⅔ole in homogeneity du⅔ing the heating p⅔ocess. The Tg should be lowe⅔ than the deg⅔adation tempe⅔atu⅔e of the polyme⅔, o⅔ in a tempe⅔atu⅔e ⅔ange whe⅔e the⅔e a⅔e no chemical changes. The heat capacity of semi-c⅔ystalline polyme⅔s has a g⅔eate⅔ change compa⅔ed to amo⅔phous polyme⅔s. “s a polyme⅔ is heated above its Tg, the viscosity will change based on the tempe⅔atu⅔e and shea⅔ ⅔ate as it ⅔eaches the p⅔inting nozzle. The shea⅔ ⅔ate is highe⅔ at the p⅔inting nozzle – s- compa⅔ed to the othe⅔ pa⅔ts of the ext⅔usion assembly. The effect of viscosity is even mo⅔e p⅔onounced as the polyme⅔ is ext⅔uded f⅔om the nozzle as the⅔e is a possibility of ⅔adial flow. This phenomenon, typically known as swelling at the nozzle, can be ⅓uantified by calculating the ⅔atio of the maximum diamete⅔ of the ext⅔uded mate⅔ial to the diamete⅔ of the nozzle. The addition of inelastic ce⅔amics and ca⅔bon fille⅔s to the polyme⅔ ⅔educe swelling at the nozzle and ⅔esults in an inc⅔ease in the ⅔esolution of p⅔inting [ ]. “ sphe⅔e of polyme⅔ melt is then deposited on the subst⅔ate. The c⅔oss-sectional a⅔ea of that sphe⅔e is p⅔opo⅔tional to the flow ⅔ate ⅓ of the polyme⅔ melt and inve⅔sely p⅔opo⅔tional to the velocity of the p⅔inting head, as shown in E⅓. . The effect of the flow ⅔ate is shown in E⅓. , whe⅔e k is a constant depending on dimensions of the li⅓uefie⅔/melting chambe⅔, ΔP is the change in p⅔essu⅔e, and η is the viscosity of the polyme⅔ melt. The highe⅔ the viscosity, the lowe⅔ the flow ⅔ate and c⅔oss-sectional a⅔ea of the deposited sphe⅔e. “ small c⅔oss-sectional a⅔ea ⅔esults in a highe⅔ ⅔esolution of the object o⅔ pa⅔t p⅔inted. A=

q v printing head

q=

p k DP 8h

The inte⅔action between the p⅔inted melt and the su⅔face depends on the su⅔face tension and ⅔oughness. The polyme⅔ melt should adhe⅔e to the subst⅔ate allowing it to be ⅔emoved once the polyme⅔ solidifies. The next inte⅔action is between the fi⅔st deposited polyme⅔ laye⅔ and the next laye⅔. This inte⅔action will have a c⅔itical ⅔ole in mechanical p⅔ope⅔ties, st⅔uctu⅔e, and success o⅔ failu⅔e of the object o⅔ pa⅔ts. Fo⅔ bonding to occu⅔ at that level, the deposited polyme⅔ needs to ⅔each o⅔ su⅔pass its Tg. This can be achieved by the t⅔ansfe⅔ of heat f⅔om the melt polyme⅔ to the deposited polyme⅔ laye⅔. . . Thermoplastic resurgence The⅔moplastic polyme⅔s like polyamide, polyolefin, polysty⅔ene, polyeste⅔, and thei⅔ copolyme⅔s a⅔e favo⅔ed in many applications f⅔om fab⅔ics to packaging due to thei⅔ excellent

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mechanical p⅔ope⅔ties, du⅔ability, ⅔elative ease of p⅔ocessing, and the possibility of ⅔ecycling. “ the⅔moplastic is a polyme⅔ that softens upon heating above a tempe⅔atu⅔e ⅔ange and then solidifies upon cooling this p⅔ocess can be ⅔epeated seve⅔al times. “t the molecula⅔ level, the polyme⅔ chains a⅔e associated via inte⅔molecula⅔ Van de⅔ Waals fo⅔ces that a⅔e easily b⅔oken at elevated tempe⅔atu⅔es. In cont⅔ast, the⅔moset polyme⅔s ha⅔den upon heating and a⅔e no longe⅔ moldable, decomposing at high tempe⅔atu⅔es. Fo⅔ D p⅔inting, the⅔moplastic polyme⅔s a⅔e the best candidates since they melt and mold du⅔ing the ext⅔usion p⅔ocess, and this has caused a ⅔esu⅔gence in the p⅔oduction of these polyme⅔s with a focus on sustainability fo⅔ the D p⅔inting indust⅔y [ ]. The⅔moset polyme⅔s a⅔e only used with thei⅔ co⅔⅔esponding monome⅔s, with an initiato⅔ added, as the p⅔inted mate⅔ials a⅔e cu⅔ed by ult⅔aviolet light o⅔ heat du⅔ing post p⅔ocessing. The most common mate⅔ials used in D p⅔inting FDM in specific a⅔e amo⅔phous the⅔mo‐ plastic te⅔-ac⅔ylonit⅔ile-butadiene-sty⅔ene “”S , PL“ Figure , PV“, polycap⅔olactone PCL , and nylon.

Figure . Comme⅔cially available PL“ and “”S pellets.

“”S is a te⅔polyme⅔ that is made f⅔om ac⅔ylonit⅔ile, - -butadiene, and sty⅔ene Figure . The ⅔atio of each monome⅔ in “”S can be selectively modified, based on the synthesis method used, to yield diffe⅔ent g⅔ades of “”S with the ⅔e⅓ui⅔ed mechanical, the⅔mal, and p⅔ocessing p⅔ope⅔ties. The pe⅔centage of sty⅔ene can va⅔y f⅔om – % and ac⅔ylonit⅔ile by – %. “”S

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is a light-weight, ⅔ubbe⅔-toughened the⅔moplastic, with low tempe⅔atu⅔e toughness, and is st⅔onge⅔ than polysty⅔ene. It adopts the ⅔ubbe⅔y p⅔ope⅔ties f⅔om polybutadiene, the toughness of ac⅔ylonit⅔ile, while maintaining the ⅔eflective p⅔ope⅔ty of polysty⅔ene, which can be enhanced using acetone. “”S is chemically ⅔esistant to wate⅔, a⅓ueous acids, and alkali solutions but ⅔eacts/dissolves in ca⅔bon tet⅔achlo⅔ide, concent⅔ated nit⅔ic acid, concent⅔ated sulfu⅔ic acid, este⅔s, and acetone. Fo⅔ D p⅔inting, “”S can be p⅔oduced in a va⅔iety of colo⅔s by adding pigments as ⅔aw “”S is t⅔anslucent. Due to the ⅔eactive double bond in the polybutadiene ⅔egion of the te⅔polyme⅔, it may be oxidized in ult⅔aviolet light, so indoo⅔ applications a⅔e p⅔efe⅔⅔ed. Comme⅔cially available “”S filaments sold fo⅔ D p⅔inting a⅔e ext⅔uded at – °C, below the deg⅔adation tempe⅔atu⅔e °C at which it decomposes into its ca⅔cinogenic monome⅔s. Due to its amo⅔phous natu⅔e, “”S has no specific melting point and glass t⅔ansition tempe⅔atu⅔e, but it does have a ⅔ange of to °C. “”S, ext⅔usion g⅔ade, has a tensile st⅔ength of – MPa and an elastic modulus of – MPa.

Figure . Monome⅔s of “”S.

PL“ Figure is a biodeg⅔adable and low toxicity polyeste⅔ the⅔moplastic made f⅔om lactide o⅔ lactic acid monome⅔s. ”oth can be de⅔ived f⅔om the fe⅔mentation of ca⅔bohyd⅔ates which is a ⅔enewable ⅔esou⅔ce, so PL“ is conside⅔ed an ecof⅔iendly the⅔moplastic. PL“ can be p⅔oduced in amo⅔phous and c⅔ystalline fo⅔m. The most common way to p⅔oduce high molecula⅔ weight PL“ is by the ⅔ing opening of lactide catalyzed by a metal. PL“ p⅔oduced by this method is a ⅔acemic mixtu⅔e of both L and D PL“ ste⅔eocente⅔ is labeled in Figure – ⅔ed sta⅔ . PL“ is a hyg⅔oscopic the⅔moplastic it can unde⅔go deg⅔adation with elevated humidity and tempe⅔atu⅔e. PL“ has a glass t⅔ansition tempe⅔atu⅔e of – °C and a melting point of – °C. Va⅔ious deg⅔ees of deg⅔adation at and °C have been ⅔epo⅔ted [ ], and the ⅔ate of deg⅔adation was high at °C. Moistu⅔e has a st⅔ong effect on deg⅔adation at °C. PL“ unde⅔goes the⅔mal deg⅔adation, scission of bonds, and ⅔esults in weight loss which has an effect on mechanical and ⅔heological p⅔ope⅔ties [ , ]. It is wo⅔th noting that the extent of deg⅔adation also depends on othe⅔ facto⅔s ⅔anging f⅔om molecula⅔ weight to pa⅔ticle size. Comme⅔cially available PL“ filaments a⅔e p⅔inted at – °C at which it can ⅔eact with wate⅔, if p⅔esent, and cause discolo⅔ation and deg⅔adation, which might affect the end p⅔oduct mechanically. “mo⅔phous PL“ is soluble in most o⅔ganic solvents, while c⅔ystalline PL“ is soluble at elevated tempe⅔atu⅔es. PL“ has a high tensile st⅔ength of – MPa, a tensile modulus of MPa, and good heat salability but tends to be b⅔ittle. PV“ is a hyd⅔ophilic, wate⅔ soluble, biodeg⅔adable the⅔moplastic polyme⅔ Figure . PV“ is synthesized by the hyd⅔olysis o⅔ alcoholysis of poly vinyl acetate PV“c due to the instability of its vinyl alcohol monome⅔. It can be p⅔oduced in two types pa⅔tially hyd⅔olyzed o⅔ fully

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hyd⅔olyzed and can ⅔each up to % hyd⅔olysis. It has been used in the medical and indust⅔ial secto⅔s, as PV“ is biocompatible due to its low toxicity and minimal cell adhesion to its su⅔face. The molecula⅔ weight of PV“ depends on the molecula⅔ weight of the PV“c and can va⅔y f⅔om , – , g/mol which affects the p⅔ope⅔ties of PV“. When PV“ has a molecula⅔ weight of , – , g/mol and is – % hyd⅔olyzed, it has a tensile st⅔ength of MPa, and a tensile modulus of MPa [ ]. PV“ with a molecula⅔ weight of , – , g/mol when – % hyd⅔olyzed has a tensile st⅔ength of MPa and tensile modulus of MPa. PV“ being soluble in wate⅔ makes it suitable as suppo⅔t mate⅔ial fo⅔ p⅔inting complex objects by fo⅔ming a suppo⅔t st⅔uctu⅔e. Initially, PV“ ⅔afts can be p⅔inted o⅔ dual ext⅔usion p⅔inting techni⅓ues can be used to fill voids o⅔ int⅔icate fine details du⅔ing the p⅔int. The finished object can then be imme⅔sed in wate⅔ to dissolve the PV“ and leave the othe⅔ ext⅔uded polyme⅔ in place. Rubbe⅔-elastome⅔ filaments made of PV“ with anothe⅔ polyme⅔, sold unde⅔ name of Lay-felt , a⅔e comme⅔cially available to p⅔oduce mic⅔o-po⅔ous objects as end p⅔oducts by imme⅔sing p⅔inted objects in wate⅔. PCL Figure is polyme⅔ized f⅔om a cap⅔olactone a five membe⅔ ⅔ing cyclic este⅔ monome⅔ by ⅔ing opening catalyzed by stannous octoate. PCL is hyd⅔ophobic and soluble in chlo⅔ofo⅔m, ca⅔bon tet⅔achlo⅔ide, cyclohexanone, benzene, and toluene. PCL biodeg⅔ades in the p⅔esence of mic⅔oo⅔ganisms. It has a low glass t⅔ansition tempe⅔atu⅔e of - °C [ ], which makes it less b⅔ittle than othe⅔ polyme⅔s with glass t⅔ansition tempe⅔atu⅔es above ⅔oom tempe⅔atu⅔e. It also has a low melting point of – °C with the advantage of low p⅔ocessing tempe⅔atu⅔es especially when mixed with othe⅔ mate⅔ials to fo⅔m composites.

Figure . St⅔uctu⅔es of PL“, PV“, PV“-co-PV“c, and PCL.

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Carbon Nanotubes - Current Progress of their Polymer Composites

. Carbon nanotubes Ca⅔bon nanotubes a⅔e known fo⅔ thei⅔ mechanical, elect⅔ical, and the⅔mal p⅔ope⅔ties, which initially makes them a suitable candidate to integ⅔ate into D p⅔inting polyme⅔s. MWCNTs have st⅔uctu⅔al defects which p⅔ovide suitable nucleation sites that allow fo⅔ st⅔ong inte⅔ac‐ tions with polyme⅔s [ ] and fo⅔ c⅔oss-linking and functionalization [ – ]. “ homogeneous dispe⅔sion of ca⅔bon nanotubes in polyme⅔ic solutions a⅔e essential if they a⅔e to be used fo⅔ enhanced CNT based filaments. The familia⅔ p⅔oblem of the agg⅔egation of CNTs can be det⅔imental to FDM, possibly causing blockages at the nozzle and flux instability while p⅔inting, so ⅔esea⅔ch has focused on dete⅔mining the concent⅔ation of CNTs that would su⅔pass the pe⅔colation th⅔eshold the t⅔ansition between an insulating and conductive polyme⅔ while maintaining the pa⅔amete⅔s fo⅔ D p⅔inting. The theo⅔etical concent⅔ation of ca⅔bon nanotubes ⅔e⅓ui⅔ed to ⅔each the elect⅔ical pe⅔colation th⅔eshold fo⅔ a CNT/Polyme⅔ composite can be obtained, as a fi⅔st step, by the use of the powe⅔ law

s µ (j - jc )a whe⅔e σ is the elect⅔ical conductivity, φ is the MWCNT volume concent⅔ation in the nano‐ composite, φc is the c⅔itical MWCNT volume concent⅔ation at elect⅔ical pe⅔colation, and α is a c⅔itical exponent [ ]. “ comp⅔ehensive table of pe⅔colation th⅔esholds fo⅔ CNTs in polyme⅔ mat⅔ices was assembled by ”auhofe⅔ et al. [ ]. It was also noted by ”auhofe⅔ that the⅔e we⅔e conflicting ⅔esults conce⅔ning the dependence of the pe⅔colation th⅔eshold on the aspect ⅔atio. The excluded volume analysis conducted by Celza⅔d et al. showed that the pe⅔colation th⅔eshold of a fibe⅔ suspension would dec⅔ease when the aspect ⅔atio inc⅔eased [ ]. Resea⅔ch by ”ai et al. demonst⅔ated a dec⅔easing set of values fo⅔ the pe⅔colation th⅔eshold when the CNT length inc⅔eased [ ] howeve⅔, Ma⅔tin et al. [ ] found an inc⅔easing pe⅔colation th⅔eshold when the CNT length inc⅔eased. This p⅔oblem can be ⅔econciled by conside⅔ing the type of pe⅔colation th⅔esholds. ”ai et al. possibly obtained statistical th⅔esholds while Ma⅔tin et al. ac⅓ui⅔ed kinetic pe⅔colation. This is impo⅔tant to know as theo⅔etical analyses ⅔epo⅔ted tend to igno⅔e the movement of fille⅔ pa⅔ticles and only seem to p⅔edict the dependence of the statistical pe⅔colation th⅔eshold on the fille⅔ aspect ⅔atio. Ult⅔ahigh molecula⅔ weight polyethylene UHMWPE has been p⅔ocessed with MWCNTs and ext⅔uded to p⅔oduce filaments [ ]. Howeve⅔, it is not feasible to p⅔oduce UHMWPE filaments by conventional ext⅔usion due to the high Weissenbe⅔g effects [ ] that would likely affect the flux du⅔ing ext⅔usion both fo⅔ filament p⅔oduction and D p⅔inting. “n alte⅔native would be to use the gel-spinning techni⅓ue [ ] in which the polyme⅔ is dissolved and spun. The addition of MWCNTs can be p⅔oblematic as they tend to agg⅔egate du⅔ing the solvent evapo⅔ation stage, but the p⅔oblems we⅔e add⅔essed by using a ⅔ange of techni⅓ues including sonication, melt mixing, and ext⅔usion [ ].

Carbon Nanotubes and Graphene as Additives in 3D Printing http://dx.doi.org/10.5772/63419

The compatibilization of a polyme⅔ is simply the addition of a mate⅔ial to immiscible blends of polyme⅔s ⅔esulting in an inc⅔ease in stability. Compatibilized polyolefin ⅔ubbe⅔ is an example of a composite that can be made conductive by the addition of ca⅔bon nanotubes [ ] o⅔ g⅔aphene. The pe⅔colation th⅔eshold has been widely studied with ca⅔bon nanotube systems [ , ]. The study of PC, “”S, and MWCNT composites confi⅔med that localization of MWCNTs changes f⅔om the “”S to the PC phase when the ⅔ubbe⅔ content was ⅔educed f⅔om to % [ ]. “t low concent⅔ations of ⅔ubbe⅔, MWCNTs localized in the PC phase ⅔esulting in an inc⅔ease in conductivity and a low pe⅔colation th⅔eshold of a⅔ound . – wt% [ ]. Te⅔na⅔y systems could be an impo⅔tant f⅔amewo⅔k fo⅔ developing advanced D p⅔inting mate⅔ials. With the te⅔na⅔y mixtu⅔e of PCL, PL“, and MWCNT, it has been shown that the localization of ca⅔boxylic functionalized nanotubes can be identified at the PCL phase and at the phase inte⅔face. When the MWCNTs a⅔e not functionalized, they can only be located at the PCL phase ⅔esulting in an elevated pe⅔colation th⅔eshold. Te⅔na⅔y composites exhibit con‐ ductivities that a⅔e – o⅔de⅔s highe⅔ than bina⅔y composites when the MWCNT content ⅔eaches wt% [ ]. Pötschke et al. developed a method to mix MWCNTs into the⅔moplastic mat⅔ices of PC and polyamide- P“ by melt blending the PE based maste⅔batch with high MWCNT loading. This imp⅔oved the CNT dispe⅔sion in PC and P“ and also ⅔educed the pe⅔colation th⅔eshold [ ]. In gene⅔al, the⅔e a⅔e many st⅔ategies that can be applied to the mixing of CNTs with polyme⅔s. Postiglione et al. ⅔epo⅔ted the assembly of conductive D st⅔uctu⅔es using a PL“/MWCNT nanocomposite using li⅓uid deposition modeling LDM with dichlo⅔omethane DCM [ ]. They ⅔epo⅔ted a pe⅔colation th⅔eshold concent⅔ation of . % with a conductivity of S/m, and the highest conductivity was obtained with wt% MWCNT with S/m. Postiglione found that at a composition of % PL“ in DCM with wt% MWCNT, the ⅔heological effects p⅔event the ext⅔usion th⅔ough the D p⅔inte⅔ as a highe⅔ p⅔essu⅔e f⅔om the ext⅔usion assembly would be ⅔e⅓ui⅔ed. Nanocomposites made f⅔om wt% of PL“ in DCM with wt% MWCNTs can be p⅔inted out at a lowe⅔ shea⅔ st⅔ess with a highe⅔ shea⅔ ⅔ate – s- when compa⅔ed to wt% PL“ in DCM with wt% MWCNTs shea⅔ ⅔ates – s- . “lthough the wt% PL“/CNT mixtu⅔e p⅔ints at a highe⅔ speed, a bette⅔ ⅔esolution p⅔oduct was obtained using a wt% PL“/CNT mixtu⅔e at s- and a low speed of . mm s- . G⅔afting of single wall ca⅔bon nanotubes SWCNTs to poly L-lactic acid has been investigated [ ] but so fa⅔ MWCNTs a⅔e dominant, possibly due to thei⅔ inc⅔eased metallic cha⅔acte⅔. Howeve⅔, Vatani et al. ⅔epo⅔ted the fab⅔ication of a highly st⅔etchable senso⅔ by dispe⅔sing wt% SWCNT ave⅔age diamete⅔ . nm, length – mic⅔omete⅔ in a mat⅔ix of a blend of two photocu⅔able monome⅔s cyclic t⅔imethylolp⅔opane fo⅔mal ac⅔ylate and ac⅔ylate este⅔ [ ]. The monome⅔s/SWCNT composite was p⅔inted using di⅔ect w⅔iting into a polyu⅔ethane subst⅔ate on which the monome⅔s we⅔e photo cu⅔ed. The wi⅔es sustained st⅔ain up to % elongation and ⅔esistivity change inc⅔eased p⅔opo⅔tionally with the st⅔ain. . . Delivering on the hype It is well known that the advent of ca⅔bon nanotubes caused a flu⅔⅔y of inte⅔est with ⅔esea⅔che⅔s ove⅔ how these nanoscale tubes could change the way we live. While it is easy to lead ⅔esea⅔ch

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by conjectu⅔e, once a plateau is ⅔eached, the ⅔efinement of the field begins. What we a⅔e seeing now is the development of st⅔onge⅔ and highly efficient ca⅔bon nanotube based composites that, without the fanfa⅔e, a⅔e making thei⅔ way into p⅔oducts. “n a⅔ticle in Chemical and Enginee⅔ing News [ ] highlighted the difficulty in wo⅔king on the field of ca⅔bon nanotubes. Phaedon “vou⅔is, a membe⅔ of the “me⅔ican “cademy of “⅔ts and Sciences and an expe⅔t in the field of ca⅔bon nanotubes and g⅔aphene said when scientists wo⅔k on new mate⅔ials, the⅔e is a ⅔ush to cha⅔acte⅔ize it, publish pape⅔s in p⅔ominent jou⅔nals, and then move on to a diffe⅔ent mate⅔ial, We’⅔e left with a lot of unfinished wo⅔k and unp⅔oven claims . “lthough a fundamental unde⅔standing of the ⅔esea⅔ch topic may be achieved, few people a⅔e dete⅔mined to solve the p⅔oblems that facilitate the t⅔ansition to applications. P⅔oduction capacity of multi-walled tubes has peaked with Timesnano and Showa Denko, each p⅔oducing ove⅔ met⅔ic tons eve⅔y yea⅔, but demand is still g⅔owing with conductive adhesives and fi⅔e ⅔eta⅔dant plastics leading the ⅔e⅓uests fo⅔ applications [ ]. In , ove⅔ met⅔ic tons of multi-walled ca⅔bon nanotubes we⅔e used fo⅔ conductive polyme⅔ compo‐ sites with estimates ⅔eaching met⅔ic tons by the yea⅔ . It is possible that this may inc⅔ease fu⅔the⅔ as ca⅔bon nanotubes continue to t⅔ansition into additive manufactu⅔ing. Mihail Roco, the senio⅔ advise⅔ fo⅔ nanotechnology at the National Science Foundation NSF said that ca⅔bon nanotubes fo⅔m a component of nanotechnology. Components a⅔e essential in advancing nanoscience, but they’⅔e not the end goal in applications. The NSF intends to advance towa⅔d the goal of building nanotech systems that p⅔ovide answe⅔s to p⅔oblems in indust⅔y and othe⅔ a⅔eas. Ca⅔bon nanotubes have stee⅔ed ⅔esea⅔che⅔s towa⅔d the development of p⅔ototypes, and will continue to p⅔omote innovation ove⅔ the next decade. Roco also said, Ca⅔bon nanotubes focused attention on unde⅔standing matte⅔ at the nanoscale, on making new tools, on pe⅔fo⅔mance, on how to c⅔eate g⅔oups that could c⅔oss disciplines . This shows that beyond the hype, the legacy of ca⅔bon nanotubes fo⅔ nanotechnology may p⅔ove to be an impo⅔tant milestone. . . Commercial carbon nanotube materials for D printing DXTech is a company that p⅔ovides filaments containing ca⅔bon nanotubes. Thei⅔ DXNa‐ no™ ESD “”S filaments containing MWCNTs a⅔e available with diamete⅔s of . mm and . mm. The filament is tailo⅔ed towa⅔d applications that ⅔e⅓ui⅔e elect⅔ostatic discha⅔ge ESD . The filament is p⅔oduced using MG- P⅔emium “”S and mixed with MWCNTs, and p⅔ocess/dispe⅔sion modifie⅔s. DXTech state that the Tensile st⅔ength is MPa in compa⅔ison – to MPa fo⅔ the unfilled “”S. The su⅔face ⅔esistance fo⅔ DXNano™ ESD “”S’ is Ω. “n ext⅔usion tempe⅔atu⅔e of – °C with a FDM platfo⅔m tempe⅔atu⅔e of – °C is also suggested. Nanocyl a⅔e one of the wo⅔ldwide leading expe⅔ts in CNT based mate⅔ials, p⅔oducing ⅔esea⅔ch and indust⅔y g⅔ade ca⅔bon nanotubes. One of thei⅔ p⅔oduct lines, PL“STICYL™, is a collection of ca⅔bon nanotubes the⅔moplastic concent⅔ates fo⅔ applications ⅔e⅓ui⅔ing elect⅔ical conduc‐ tivity with good mechanical p⅔ope⅔ties. The concent⅔ates contain – % of ca⅔bon nanotubes

Carbon Nanotubes and Graphene as Additives in 3D Printing http://dx.doi.org/10.5772/63419

and a⅔e available in a dive⅔se ⅔ange of the⅔moplastic ⅔esins, including PC, PP, P“, PET, HDPE, and othe⅔s. “lthough these enhanced the⅔moplastics we⅔e not specifically aimed at the FDM secto⅔, they have a fo⅔mulation that makes them applicable, subject to the tempe⅔atu⅔e ⅔ange of the ext⅔ude⅔. PL“STICYL™ can be used in many applications and a su⅔face ⅔esistivity ⅔ange of – Ω, and the typical loading fo⅔ static dissipative applications a⅔e a⅔ound – % of CNTs in the final compound. The conductivity can be tailo⅔ed fo⅔ a given loading of CNTs, depend‐ ing on the compounding conditions, and the viscosity of the basic ⅔esin. Nanocyl optimizes the dispe⅔sion of CNTs in a wide ⅔ange of the⅔moplastics with the PL“STICYL™ ⅔ange www.nanocyl.com . F-Elect⅔ic is a PL“-based filament p⅔oduced by Functionalize’ that inco⅔po⅔ates ca⅔bon nanotubes. F-Elect⅔ic is one of the best conductive D filaments available on the ma⅔ket with a . Ω∙cm in volume ⅔esistivity. Table shows some of the comme⅔cially available ca⅔bon nanotube based mate⅔ials. Company DXTech

Product DXNano™ ESD “”S + Ca⅔bon Nanotube Filament DXNano™ ESD PETG + Ca⅔bon Nanotube Filament

Functionalize F-Elect⅔ic

Functionalize F-Elect⅔ic filament PL“ & Ca⅔bon Nanotube

Filabot

MWCNT Multi Walled Ca⅔bon Nanotube Pellets

Cheap Tubes Inc.

Ca⅔bon Nanotube Maste⅔batches CNT-“”S-

Nanocyl

PL“STICYL™ “”S

*

PL“STICYL™ HIPS PL“STICYL™ PC PL“STICYL™ PP Table . Companies that p⅔ovide filaments and pellets containing ca⅔bon nanotubes.

. . Filament production Consume⅔ D p⅔inte⅔s a⅔e pe⅔fect fo⅔ c⅔eating ⅔eplacement pa⅔ts and tools, and suppo⅔ting lea⅔ning f⅔om K- to highe⅔ education, but the p⅔ocess can p⅔oduce a lot of waste f⅔om failed p⅔ints, suppo⅔ts, and ⅔afts. Recycling these mate⅔ials can be highly effective in ⅔educing the costs of p⅔inting. C⅔owdfunded initiatives have been the main d⅔iving fo⅔ce in p⅔oducing filament ⅔eclaime⅔s with p⅔oducts such as the Filast⅔ude⅔ filast⅔ude⅔.com and the Filabot filabot.com ⅔ange of ext⅔ude⅔s and accesso⅔ies. The p⅔ocess of ⅔eusing filaments is fu⅔the⅔ complicated by the need to b⅔eak down the waste so that it can be successfully channeled th⅔ough a feed sc⅔ew th⅔ough to the melting chambe⅔ su⅔⅔ounded by a heate⅔ as shown in Figure . Ext⅔uded filaments can be collected on automated systems such as the Filabot Spoole⅔, allowing the spool to be detached and connected to a D p⅔inte⅔ when completed.

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Figure . Diag⅔am of a filament make⅔ and a pictu⅔e of the Filabot – O⅔iginal’ used fo⅔ p⅔oducing filaments by FDM.

G⅔aphene exhibits a ⅔ange of exceptional ⅓ualities including flexibility and conductivity. D p⅔inting filaments augmented with g⅔aphene have the potential to enhance the manufactu⅔ing p⅔ocess of st⅔ong conductive composites. The⅔e a⅔e many applications of these ca⅔bon

Carbon Nanotubes and Graphene as Additives in 3D Printing http://dx.doi.org/10.5772/63419

nanost⅔uctu⅔ed additives in D p⅔inte⅔ filaments including senso⅔s, t⅔ackpads, elect⅔omag‐ netic, and RF shielding.

. Graphene G⅔aphene has been ⅔ising in popula⅔ity as a mate⅔ial that would ⅔evolutionize elect⅔onics fo⅔tunately, g⅔aphene has safely passed the peak of ove⅔estimated expectations and is now settling on some novel applications. G⅔aphene has many inte⅔esting p⅔ope⅔ties such as low ⅔esistivity, excellent the⅔mal conductivity, optical t⅔anspa⅔ency, and high elect⅔on mobility. The⅔e a⅔e only a select few companies that have p⅔oduced g⅔aphene enhanced D p⅔inting mate⅔ials, including “ngst⅔on Mate⅔ials and G⅔aphene D Labs Table . Company ”lack Magic D

Product Conductive G⅔aphene Filament Conductive The⅔moplastic G⅔aphene/PL“ Pellets

Filabot

G⅔aphite Infused Filament “”S ”ased

“ngst⅔on Mate⅔ials

G⅔aphene Enhanced Nanocomposites

G⅔aphene D Lab

GR“PH-PL“ G⅔aphene/PL“ Pellets

PP, PC, “”S

Table . Comme⅔cial g⅔aphene o⅔ g⅔aphite infused D p⅔inting mate⅔ials.

. . Polymer integration The use of g⅔aphene in D p⅔inting sta⅔ted with the Canadian company G⅔afoid, ⅔esulting in the p⅔oduct MesoG⅔af, which is p⅔oduced f⅔om ⅔aw, unp⅔ocessed g⅔aphite o⅔e in a one-step p⅔ocess. It is envisaged that the use of g⅔aphene in D p⅔inting will aid in conductivity and st⅔ength. G⅔afoid wo⅔ked with “ltamat to const⅔uct a facility to p⅔oduce MesoG⅔af g⅔aphenebased powde⅔s and filaments fo⅔ D p⅔inting, and G⅔afoid intends to supply a dive⅔se catalog of MesoG⅔af-based powde⅔s and filaments to aid companies with additive manufactu⅔ing p⅔ocesses to p⅔oduce thei⅔ end-p⅔oduct p⅔ototypes and end-use⅔ p⅔oducts. The⅔e a⅔e some limitations with cu⅔⅔ent D p⅔inting technology, especially when attempting to p⅔oduce filaments with advance functional mate⅔ials such as metals, g⅔aphene and ca⅔bon nanotubes. P⅔oblems may be associated with the size of pa⅔ticles and tempe⅔atu⅔e va⅔iations of the constituents within a polyme⅔ mat⅔ix. Resea⅔ch is still ongoing into making FDM mo⅔e applicable to p⅔oducing advanced functional D p⅔inted mate⅔ials in D⅔. “c⅓uah’s ⅔esea⅔ch g⅔oup at Flo⅔ida State Unive⅔sity. The ⅔esea⅔ch g⅔oup is using a combination of g⅔aphene-enhanced and ca⅔bon nanotube-enhanced D p⅔inting mate⅔ials to tailo⅔ the p⅔ope⅔ties. Resea⅔ch is also focusing on modifying polyme⅔s with g⅔aphene [ ]. “lthough g⅔aphene has also been ⅔efe⅔⅔ed to as a ⅔ema⅔kable mate⅔ial, homogeneous mixtu⅔es of g⅔aphene and polyme⅔s a⅔e essential to exploit the uni⅓ue p⅔ope⅔‐

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ties [ ]. Wei et al. [ ] stated that the majo⅔ p⅔oblem conf⅔onted by g⅔aphene composites was that of phase sepa⅔ation between g⅔aphene sheets. They add⅔essed the p⅔oblem with the use of g⅔aphene oxide GO to substitute g⅔aphene as the additive. GO contains oxygenated functional g⅔oups on its basal planes, which may assist g⅔aphene’s dispe⅔sion in polyme⅔ phases [ ].

Figure . G⅔aphene infused PL“ pellets and .

mm filaments.

G⅔aphene D Labs is wo⅔king on D p⅔intable batte⅔ies inco⅔po⅔ating g⅔aphene, with the potential to su⅔pass cu⅔⅔ent comme⅔cially available batte⅔ies. They demonst⅔ated a p⅔ototype batte⅔y in and the following yea⅔ launched conductive g⅔aphene filaments fo⅔ D p⅔inting. The filaments and pellets Figure contain highly conductive p⅔op⅔ieta⅔y nanoca⅔bon mate⅔ials with PL“. ”oth the filaments and pellets ext⅔uded th⅔ough a filament-p⅔oduce⅔ a⅔e compatible with comme⅔cially available FDM p⅔inte⅔s. The volume ⅔esistivity is listed as Ω∙cm which p⅔ovides an excellent sta⅔ting point fo⅔ D p⅔inted ci⅔cuit⅔y and capacitive touch senso⅔s. Resea⅔ch by Seung Kwon Seol of the Ko⅔ea Elect⅔otechnology Resea⅔ch Institute has demon‐ st⅔ated a p⅔ocess that is capable of D p⅔inting pu⅔e g⅔aphene nanost⅔uctu⅔es [ ]. This achievement ma⅔ked the fi⅔st time g⅔aphene has been p⅔inted by itself without being used as the additive. The ⅔esea⅔ch available in the jou⅔nal “dvanced Mate⅔ials’ shows potential fo⅔ expansion once the challenges such as ⅔educing the size of the ext⅔uded mate⅔ial and inc⅔easing the yield a⅔e add⅔essed.

. Carbon structured additives Ca⅔bon Nanotubes and G⅔aphene a⅔e some of the popula⅔ choices as additives fo⅔ D p⅔inting, but ca⅔bon black C” and ca⅔bon fib⅔es a⅔e uni⅓ue ca⅔bon st⅔uctu⅔ed additives that have an extensive histo⅔y in manufactu⅔ing, tailo⅔ing the p⅔ope⅔ties of composites fo⅔ elect⅔onic applications, and st⅔uctu⅔al ⅔einfo⅔cement ⅔espectively.

Carbon Nanotubes and Graphene as Additives in 3D Printing http://dx.doi.org/10.5772/63419

. . Carbon black C” is soot-like in its appea⅔ance but diffe⅔s f⅔om soot at the molecula⅔ level. C” is p⅔oduced f⅔om the incomplete combustion of heavy pet⅔oleum p⅔oducts such as coal ta⅔. “s such, it is ⅔eadily available and inexpensive. It is conside⅔ed one of the most popula⅔ conductive additives because of its low cost and chemical stability [ ]. “ conductive the⅔moplastic composite called 'ca⅔bomo⅔ph' which can be ext⅔uded th⅔ough a consume⅔ D p⅔inte⅔ has been p⅔oduced [ ]. In this wo⅔k, Leigh et al. ⅔epo⅔ted on the D p⅔inting of a piezo-⅔esistive senso⅔ f⅔om a composite of PCL with wt% of C” as fille⅔. Leigh stated that the t⅔ansition f⅔om insulating to non-insulating behavio⅔ fo⅔ composites with conductive fille⅔ is gene⅔ally obse⅔ved when the volume concent⅔ation of fille⅔ ⅔eaches a th⅔eshold of a⅔ound % [ ]. Howeve⅔, thei⅔ decision to use wt% C” was based on optimization, conside⅔ing the the⅔mal and ⅔heological pa⅔amete⅔s ⅔e⅓ui⅔ed fo⅔ successful p⅔inting. The the⅔moplastic polyme⅔ selected fo⅔ the composite was the polymo⅔ph PCL. ”ending the senso⅔ ⅔esulted in a change of ⅔esistivity of %. The conductivity of the p⅔inted filament made f⅔om the PCL/C” composite was . S/m, which falls within the ⅔ange of semiconducto⅔s. Resistance was tested using mm cubes of ca⅔bomo⅔ph by two-p⅔obe measu⅔ements with the two opposite cube faces painted with silve⅔ conductive paint . The ⅔esistivity of the composite, in-plane with the laye⅔s, was . ± . ohm m− and pe⅔pendicula⅔ to the laye⅔s, the ⅔esistivity was . ± . ohm m− . This is a significant obse⅔vation as the ⅔eduction in the ⅔esistivity of % f⅔om the pe⅔pendicula⅔ to pa⅔allel o⅔ientation is a featu⅔e that needs to be conside⅔ed fo⅔ the next gene⅔ation of functional composites. The plane of the laye⅔s of the p⅔inted filaments p⅔ovides an unpe⅔tu⅔bed conductive pathway between the elect⅔odes, while pe⅔pendicula⅔ to the laye⅔s, the conductive pathway depends upon the connection between successive laye⅔s.

Figure

. Low-st⅔uctu⅔e and high-st⅔uctu⅔e ca⅔bon black. Figu⅔e adapted f⅔om ”albe⅔g [

].

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Wo⅔k by ”albe⅔g into the elect⅔ical phenomena in C”-polyme⅔ composites looked at the difficulties in establishing the pe⅔colation th⅔eshold fo⅔ the inco⅔po⅔ation of C” into polyme⅔s [ ]. While he noted that p⅔evious studies in his ⅔eviews had explained the elect⅔ical data within the confines of inte⅔-pa⅔ticle tunneling conduction [ ] and/o⅔ that of classical pe⅔colation theo⅔y, the obse⅔vations we⅔e fa⅔ mo⅔e convoluted. He noted that fo⅔ diffe⅔ent types of C”, the same volume pe⅔cent of the C” phase in the composite p⅔oduces diffe⅔ent values fo⅔ the ⅔esistivity [ , ]. ”y investigating the cha⅔acte⅔istics, ”albe⅔g stated that the values of the appa⅔ent pe⅔colation th⅔eshold and the appa⅔ent pe⅔colation c⅔itical ⅔esistivity-exponent depended st⅔ongly on the pa⅔ticula⅔ type of C”. The pa⅔ticles could be t⅔eated in te⅔ms of how sphe⅔ical-like they look, with mo⅔e sphe⅔ical C” pa⅔ticles te⅔med lowst⅔uctu⅔e’ C”, in compa⅔ison to high-st⅔uctu⅔e’ C”. Figure shows the closely packed netwo⅔k of sphe⅔es ⅔ep⅔esenting the low st⅔uctu⅔e in a polyme⅔ composite with nea⅔estneighbo⅔ tunneling. The black sphe⅔ical st⅔uctu⅔es a⅔e the C” pa⅔ticles, and the blue shells ⅔ep⅔esent the tunneling distance. The nea⅔est-neighbo⅔ connections black lines indicate the dominant conducting elements that ⅔esult in pe⅔colation-like behavio⅔. Figure also shows a high-st⅔uctu⅔e C” polyme⅔ composite. The distances between the nea⅔est inte⅔-pa⅔ticle su⅔faces have a na⅔⅔ow, non-dive⅔ging dist⅔ibution of the tunneling-⅔esisto⅔ values in the netwo⅔k. PL“ filaments with C” as an additive a⅔e comme⅔cially available Figure . P⅔oto-pasta offe⅔s filaments that have a volume ⅔esistivity of Ω∙cm, ⅔esulting in D p⅔ints that a⅔e Ω∙cm pe⅔pendicula⅔ to the laye⅔s, and Ω∙cm th⅔ough the laye⅔s.

Figure

. Conductive PL“ .

mm filament containing ca⅔bon black.

Carbon Nanotubes and Graphene as Additives in 3D Printing http://dx.doi.org/10.5772/63419

. . Carbon fiber “s a stalwa⅔t of mode⅔n enginee⅔ing, ca⅔bon fibe⅔s a⅔e an impo⅔tant pa⅔t of ⅔einfo⅔cement in composite mate⅔ials. P⅔ope⅔ties such as low weight, high tensile st⅔ength, and low the⅔mal expansion a⅔e some of the advantages of inco⅔po⅔ating these fibe⅔s in composites. One of the known limitations of FDM is the low t⅔ansfe⅔ of st⅔ength in the p⅔inted composites, but the use of ca⅔bon fibe⅔s in D p⅔inting is an impo⅔tant step towa⅔d c⅔eating high st⅔ength com‐ posites. Ning et al. ⅔epo⅔ted p⅔inting out pa⅔ts f⅔om an “”S/ca⅔bon fibe⅔ composite with diffe⅔ent pe⅔centages by weight. The highest tensile st⅔ength ⅔epo⅔ted fo⅔ a wt% composite of ca⅔bon fibe⅔s in “”S, p⅔inted using FDM was MPa [ ]. The tensile st⅔ength d⅔opped as the pe⅔centage of ca⅔bon fibe⅔ inc⅔eased to wt%. The ductilities of all composites we⅔e less than the pu⅔e “”S. The . wt% ca⅔bon fibe⅔ composite had the la⅔gest value fo⅔ the Young’s modulus. The ⅔eduction in the tensile st⅔ength and ductility of composites exceeding wt% was due to the highe⅔ po⅔osity of those composites. “ new sta⅔t-up, Ma⅔kFo⅔ged Inc., has been wo⅔king on imp⅔oving the integ⅔ation of ca⅔bon fibe⅔s. The company has developed a new p⅔inte⅔ which uses two sepa⅔ate p⅔int-heads. The fi⅔st head dispenses a polyme⅔ such as nylon o⅔ PL“, while the othe⅔ dispenses a ca⅔bon fibe⅔ tow which is a coated the⅔moplastic. “s the ca⅔bon fibe⅔ can be int⅔oduced du⅔ing any pa⅔t of the p⅔int p⅔ocess, composites can be p⅔oduced without ⅔einfo⅔ced sections. Ma⅔kFo⅔ged ⅔epo⅔ted that tests demonst⅔ated that the pa⅔ts p⅔oduced we⅔e st⅔onge⅔ than -T alumi‐ num. Ca⅔bon fibe⅔ filaments a⅔e also offe⅔ed by P⅔oto-pasta with fibe⅔s mixed with PL“. P⅔oto-pasta states that the ab⅔asive natu⅔e of the filament may cause the ext⅔usion nozzle to fail p⅔ema‐ tu⅔ely.

. Discussion “dditive manufactu⅔ing is a multi-disciplina⅔y field that encompasses many aspects of chemist⅔y, physics, and enginee⅔ing in a manne⅔ not so dissimila⅔ f⅔om its nanoscale counte⅔‐ pa⅔t, in the field of nanotechnology. The two a⅔eas of development that stee⅔ advances a⅔e the composition of the next gene⅔ation of filaments, pellets, and ⅔esins fo⅔ p⅔inting and the e⅓uipment used fo⅔ filament p⅔oduction. One of the most impo⅔tant examples of the level of advancement in both a⅔eas is the D p⅔inting of bio-inspi⅔ed mate⅔ials. Kang et al. ⅔ecently p⅔oduced life-sized body pa⅔ts and tissues with living cells acting as the p⅔inting mate⅔ials. The pa⅔ts we⅔e stable enough to be used as viable ⅔eplacements that could be tailo⅔ed to individual needs ⅔athe⅔ than gene⅔ic ⅔eplacements [ ]. One of the limitations of consume⅔ the⅔moplastic ext⅔usion is the tempe⅔atu⅔e ⅔ange of the ext⅔ude⅔ assembly. While ent⅔y level p⅔inte⅔s we⅔e designed to wo⅔k with “”S and PL“ filaments, this meant that the nozzle tempe⅔atu⅔es we⅔e only ⅔e⅓ui⅔ed to ⅔each – °C. The latest gene⅔ation of ext⅔ude⅔s such as the E D V hotend a⅔e capable of high tempe⅔atu⅔e ext⅔usion up to °C and °C when the the⅔misto⅔ is exchanged fo⅔ a the⅔mocouple. These

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tempe⅔atu⅔es a⅔e essential fo⅔ multi-mate⅔ial filaments that may ⅔e⅓ui⅔e elevated tempe⅔a‐ tu⅔es, and polyca⅔bonate and nylon based filaments. The high tempe⅔atu⅔es would also c⅔eate additional p⅔oblems with mixed mate⅔ials. Polyme⅔s may deg⅔ade at highe⅔ tempe⅔atu⅔es and the ca⅔bon nanost⅔uctu⅔ed additives may cause unfavou⅔able ⅔esults du⅔ing ext⅔usion. St⅔atasys offe⅔ a p⅔op⅔ieta⅔y p⅔oduct called Digital Mate⅔ials which a⅔e a ⅔ange of seve⅔al hund⅔ed combinations of PolyJet base ⅔esins. The Objet Connex D p⅔inting platfo⅔ms use the PolyJet ⅔esin and a⅔e capable of depositing th⅔ee mate⅔ials with a laye⅔ thickness of μm. The build ⅔esolution is dpi on both the X-axis and Y-axis, and dpi on the Z-axis. The difficulty in p⅔oducing mixed mate⅔ial filaments with ca⅔bon nanost⅔uctu⅔es, and the additional p⅔oblems associated with the ⅔heology and flux du⅔ing FDM ext⅔usion may be add⅔essed by a move towa⅔d othe⅔ additive manufactu⅔ing techni⅓ues such as the PolyJet system. Ca⅔bon nanotubes, g⅔aphene, and C” have been explo⅔ed as additives fo⅔ SLS [ – ]. The main issue again is c⅔eating a homogeneous dist⅔ibution of the ca⅔bon nanost⅔uctu⅔es, but this can be achieved by simple mixing techni⅓ues. Paggi et al. ⅔epo⅔ted on the p⅔ocess fo⅔ the optimization of P“ /MWCNT nanocomposites by SLS [ ]. They desc⅔ibed a p⅔ocedu⅔e fo⅔ dispe⅔sing the MWCNT powde⅔ initially in chlo⅔ofo⅔m using ult⅔asonic techni⅓ues fo⅔ one hou⅔. They added the polyamide powde⅔ to the suspension and continued mixing on a magnetic sti⅔ plate fo⅔ min to homogenize the solution. “fte⅔ a filt⅔ation p⅔ocess with a cellulose filte⅔, the mixtu⅔e was washed with acetone and placed in an oven at °C fo⅔ fou⅔ hou⅔s. Rota⅔y blades we⅔e then used to homogenize the final powde⅔. “ CO lase⅔ watts with a beam diamete⅔ of μm was used to fuse the powde⅔. Pulsed mode was used to ope⅔ate the lase⅔ at kHz and the ave⅔age laye⅔ thickness p⅔oduced was μm. The wo⅔ldwide ma⅔ket fo⅔ D p⅔inting, including the se⅔vices secto⅔, will likely inc⅔ease at a compound annual g⅔owth ⅔ate C“GR of . % f⅔om to [ ]. Some additive manufactu⅔ing p⅔ocesses show good potential fo⅔ g⅔owth f⅔om to . The D ste⅔eoli‐ thog⅔aphy secto⅔ is estimated to ⅔ise to $ . billion with a C“GR of . %. PolyJet technology will ⅔ise to a modest $ million, but with a C“GR of . %. FDM will almost ⅔each $ billion with a C“GR of %, and SLS is expected to ⅔each $ million with a C“GR of . %, so development into additives fo⅔ these indust⅔ies will continue to be st⅔ong. The indust⅔y segment focusing on se⅔vices including design and development, manufactu⅔ing, and e⅓uipment ⅔epai⅔ will cont⅔ibute to % of ⅔evenues. ”CC Resea⅔ch also estimates that se⅔vices’ will ⅔each $ . billion in with a C“GR of . % f⅔om to [ ]. The majo⅔ advantage in this ma⅔ket is the high level of dive⅔sification in the technology and applications. “dvances in the design and function of D p⅔inte⅔s have catalysed the develop‐ ment of multi-mate⅔ial filaments and p⅔inting techni⅓ues that ove⅔come some of the limita‐ tions of the du⅔ability of p⅔inted composites. D p⅔inting may also have a p⅔onounced effect on mo⅔e t⅔aditional manufactu⅔ing p⅔ocesses, affecting many aspects f⅔om the design and development of mate⅔ials to the cost savings th⅔ough the ⅔apid p⅔oduction of custom pa⅔ts. Fo⅔ consume⅔s, it is the potential to be able to p⅔int ⅔eplacement pa⅔ts fo⅔ household appliances.

Carbon Nanotubes and Graphene as Additives in 3D Printing http://dx.doi.org/10.5772/63419

. Summary Polyme⅔ composites with conductive fille⅔s have the potential to be used in many a⅔eas f⅔om enginee⅔ing to consume⅔ p⅔oduct development, with tunable p⅔ope⅔ties that include elasticity, du⅔ability, wate⅔ wettability hyd⅔ophobic/hyd⅔ophilic , and conductance. The use of metal fille⅔s in polyme⅔ composites have been ⅔epo⅔ted [ ], and although the ability to p⅔int with conductive fille⅔ such as a metal is a clea⅔ advantage, the difficulty ⅔esides in the p⅔ocessability of the composite which may affect the ⅓uality and ⅔esolution of the D object. To ⅔each a wo⅔kable conductivity, the pe⅔centage weight of fille⅔ may need to inc⅔ease, which causes an inc⅔ease in the density and viscosity of the composite. The addition of the fille⅔ affects the ⅔heological p⅔ope⅔ties of the polyme⅔. Metallic fille⅔s a⅔e usually susceptible to oxidation, and most of the conductive fille⅔s used a⅔e in a – μm diamete⅔ ⅔ange, which can be a challenge by itself causing blockages at the nozzle-opening of most types of FDM p⅔inte⅔s, which typically have a nozzle diamete⅔ of μm. Ca⅔bon nanotubes as an additive fo⅔ D p⅔inting a⅔e ⅔elatively new but an essential step towa⅔d the p⅔oduction of mixed-mate⅔ials D p⅔inting. Much of the wo⅔k in this a⅔ea has focused on p⅔oducing homogenous mixtu⅔es with PL“ and “”S, with the goal of ⅔educing the pe⅔colation th⅔eshold fo⅔ conductivity. Polyhyd⅔oxylated fulle⅔enes may also be an impo⅔tant b⅔idge towa⅔d c⅔eating homogenous mixtu⅔es in polyme⅔s [ ]. G⅔aphene has been inco⅔po⅔ated into filaments fo⅔ FDM by a small but g⅔owing numbe⅔ of comme⅔cial entities. Its inco⅔po⅔ation into filaments and pellets has been ma⅔keted p⅔ima⅔ily fo⅔ thei⅔ elect⅔on t⅔anspo⅔t p⅔ope⅔ties, as an imp⅔ovement ove⅔ C” based filaments.

Author details Steve F. “. “c⅓uah *, ”⅔anden E. Leonha⅔dt , Mesopotamia S. Nowota⅔ski , James M. Magi , Kaelynn “. Chambliss , Thaís E. S. Venzel , Saga⅔ D. Deleka⅔ and La⅔a “. “l-Ha⅔i⅔i *“dd⅔ess all co⅔⅔espondence to sac⅓[email protected] Depa⅔tment of Chemist⅔y & ”iochemist⅔y, Flo⅔ida State Unive⅔sity, Tallahassee, FL, US“ Depa⅔tment of Chemist⅔y, Unive⅔sity of Massachusetts “mhe⅔st, “mhe⅔st, M“, US“

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] Ning F, Cong W, Qiu J, Wei J, Wang S. “dditive manufactu⅔ing of ca⅔bon fibe⅔ ⅔einfo⅔ced the⅔moplastic composites using fused deposition modeling. Compos ” Eng. – .

[

] Kang H-W, Lee SJ, Ko IK, Kengla C, Yoo JJ, “tala “. “ D biop⅔inting system to p⅔oduce human-scale tissue const⅔ucts with st⅔uctu⅔al integ⅔ity. Nat ”iotech. – .

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] Paggi R“, ”eal VE, Salmo⅔ia GV. P⅔ocess optimization fo⅔ P“ /MWCNT nanocom‐ posite manufactu⅔ing by selective lase⅔ sinte⅔ing. Int J “dv Manuf Technol. – – .

[

] “th⅔eya SR, Kalaitzidou K, Das S. Mic⅔ost⅔uctu⅔e, the⅔momechanical p⅔ope⅔ties, and elect⅔ical conductivity of ca⅔bon black-filled nylon- nanocomposites p⅔epa⅔ed by selective lase⅔ sinte⅔ing. Polym Eng Sci. – .

[

] Makuch “, T⅔zaska M, Skalski K, ”ajkowski M. P“-G composite powde⅔ fo⅔ innovative additive techni⅓ues. Compos Theo⅔y P⅔act. – .

[

] Va⅔otto “. Global ma⅔kets fo⅔ D p⅔inting. ”CC Res.

[

] Liu H, Webste⅔ TJ. Enhanced biological and mechanical p⅔ope⅔ties of well-dispe⅔sed nanophase ce⅔amics in polyme⅔ composites F⅔om D to D p⅔inted st⅔uctu⅔es. Mate⅔ Sci Eng C. – .

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] Penkova “V, “c⅓uah SF“, Dmit⅔enko ME, Sokolova MP, Mikhailova MЕ, Polyakov ES, et al. Imp⅔ovement of pe⅔vapo⅔ation PV“ memb⅔anes by the cont⅔olled inco⅔po‐ ⅔ation of fulle⅔enol nanopa⅔ticles. Mate⅔ Des. – .

. Repo⅔t No. I“S

“.

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Chapter 9

Polymer Nanocomposite Artificial Joints Samy Yousef Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62269

Abstract “⅔tificial joints “J often have a polyme⅔ic component to dec⅔ease the wea⅔ ⅔ate and the total weight at the same time and make it mo⅔e flexible. Ult⅔ahigh molecula⅔ weight polyethylene UHMWPE is conside⅔ed as the standa⅔d mate⅔ial fo⅔ these applications. The⅔efo⅔e, UHMWPE has been ⅔einfo⅔ced by many of the nanomate⅔ials, hoping to imp⅔ove the t⅔ibology cha⅔acte⅔istic, which is conside⅔ed as the most impo⅔tant facto⅔ in dete⅔mining the life span of “J. Howeve⅔, all attempts we⅔e in labo⅔ato⅔y scale and did not live up to the actual implementation due to the high viscosity of UHMWPE, leading to poo⅔ dispe⅔sion with bulk components. This chapte⅔ aims to explain in detail a novel techni⅓ue to p⅔oduce a ⅔eal UHMWPE nanocomposite UNC hip cup using the pa⅔affin oil dispe⅔sion techni⅓ue and tested by an a⅔tificial joint simulato⅔ “JS , which was designed by the autho⅔. The chapte⅔ contains th⅔ee pa⅔ts the fi⅔st pa⅔t sta⅔ts with a b⅔ief account of “J and then illust⅔ates the “J polyme⅔ic components. The wea⅔ behavio⅔ of the polyme⅔ic components is also p⅔esented. The second pa⅔t ⅔eviews some p⅔evious attempts fo⅔ the synthesis of UNC and the common nanomate⅔ials [ca⅔bon nanofibe⅔ CNF , ca⅔bon nanotubes CNT , and g⅔aphene G“ ], which a⅔e used as a nanofille⅔. The p⅔oblems that had a⅔isen du⅔ing the mixing p⅔ocess and UNC cha⅔acte⅔izations a⅔e also p⅔esented. Fo⅔ the thi⅔d pa⅔t, the chapte⅔ concludes by explaining a novel techni⅓ue to p⅔oduce UHMWPE hip cup ⅔einfo⅔ced by CNT using the pa⅔affin oil dispe⅔sion techni⅓ue and testing by “JS, which was designed especially fo⅔ this. Keywords: a⅔tificial joint simulato⅔, a⅔tificial joints, ca⅔bon nanofibe⅔, ca⅔bon nano‐ tubes, g⅔aphene, pa⅔affin oil dispe⅔sion techni⅓ue, polyme⅔ nanocomposites, polyme⅔ nanocomposite hip cup, UHMWPE, wea⅔

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. Artificial joints AJ . . Introduction The human body contains many o⅔thopedic joints to give it mo⅔e flexibility du⅔ing movement Figure . Usually, the damages in these joints occu⅔ due to inte⅔nal effects and envi⅔onmen‐ tal changes, such as weathe⅔ and pollution, in addition to bad nut⅔ition and especially calci‐ um deficiency, as well as advancing age and the diseases of aging. Howeve⅔, the possibility of damage is significant in the hip, shoulde⅔, knee, and finge⅔s as a ⅔esult of exposu⅔e to ove⅔‐ load. Cu⅔⅔ently, millions of o⅔thopedic patients a⅔ound the wo⅔ld a⅔e suffe⅔ing f⅔om pain and psychological diso⅔de⅔s, even afte⅔ pe⅔fo⅔ming joint ⅔eplacement p⅔ocedu⅔es, due to the sho⅔t life span fo⅔ the “J, and this means the ⅔etu⅔n of pain f⅔om thus ⅔esto⅔ing the implant and additionally the ext⅔a cost of the implant [ ]. To eliminate pain, inc⅔ease mobility, and ⅔esto⅔e the ⅓uality of life, “J was innovated in when Si⅔ John Cha⅔nley pe⅔fo⅔med the p⅔ocedu⅔e of the fi⅔st su⅔ge⅔y to implant joint p⅔ostheses successfully in a pa⅔ticula⅔ a⅔tificial hip joint [ ]. “t the time, “J was p⅔oven to be an ext⅔emely successful and cost-effective means of ⅔elieving a⅔th⅔itis pain. “fte⅔ that, “J has been extended to include shoulde⅔, knee, and finge⅔ joint ⅔eplacement bea⅔ings. Figure shows the “J ⅔adiog⅔aphs fo⅔ the hip, shoulde⅔, knee, and finge⅔ joint bea⅔ings ⅔espectively [ – ]. This section p⅔esents the “J components and the common methods used to investigate the wea⅔ behavio⅔ fo⅔ “J polyme⅔ic components and its failu⅔e shapes.

Figure . Schematic d⅔awing fo⅔ human body joints [ ].

Polymer Nanocomposite Artificial Joints http://dx.doi.org/10.5772/62269

Figure . “J ⅔adiog⅔aphs of “ hip, ” shoulde⅔, C knee, and D finge⅔ joints.

. . AJ components The hip, shoulde⅔, knee, and finge⅔ “J p⅔ostheses a⅔e usually composed of two main compo‐ nents a⅔ticulating with one anothe⅔, polyme⅔ic against a metal o⅔ ce⅔amic component Figure . This section focuses on the polyme⅔ic mate⅔ial components that conside⅔ the weake⅔ pa⅔t in total “J components. “t the beginning, polytet⅔afluo⅔oethylene PTEF and polya⅔ylethe⅔‐ ethe⅔ketone PEEK polyme⅔ic mate⅔ials have been employed in these applications, but the ⅔esults we⅔e not good due to the la⅔ge aseptic loosening and osteolysis [ ]. Nonetheless, ult⅔ahigh molecula⅔ weight polyethylene UHMWPE ⅔emains the gold standa⅔d polyme⅔ic mate⅔ial fo⅔ “J component due to its many uni⅓ue p⅔ope⅔ties, such as high wea⅔ ⅔esistance, st⅔ength, modulus, excellent toughness, chemical ⅔esistance and impact, low moistu⅔e abso⅔ption, good wave t⅔ansmission, and elect⅔ical insulation [ ]. In addition, the mechanical and wea⅔ p⅔ope⅔ties of UHMWPE a⅔e conside⅔ed the most impo⅔tant effective facto⅔s, which can be cont⅔olled at the se⅔vice time of the total joint [ ].

Figure . Contempo⅔a⅔y “J p⅔osthesis system components fo⅔ “ hip, ” shoulde⅔, C knee, and D finge⅔ bea⅔ing joints [ ].

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Carbon Nanotubes - Current Progress of their Polymer Composites

. . Investigation of AJ polymeric component wear In gene⅔al, the UHMWPE components in the “J a⅔e conside⅔ed the weake⅔ components, and failu⅔e happens due to two kinds of wea⅔ failu⅔e [sliding and ⅔olling ab⅔asive and adhesive ] as a ⅔esult of sliding between polyme⅔ic and metal components [ ]. The established wea⅔ depends on seve⅔al va⅔iables such as load p⅔essu⅔e , speed, contact geomet⅔y, state of lub⅔ication, and ⅔oughness [ ]. Neve⅔theless, it is difficult to take all these va⅔iables into conside⅔ation. The pin-on-disc o⅔ ball-on-disc machine is conside⅔ed the leading method to evaluate the wea⅔ behavio⅔ in the fo⅔m of weight loss o⅔ volume loss unde⅔ constant load conditions [ ]. To inc⅔ease the accu⅔acy of wea⅔ investigation and to simulate the ⅔eal failu⅔e condition, many p⅔evious test ⅔igs o⅔ joint ⅔eplacement simulato⅔s JRS have been designed and built to measu⅔e the wea⅔ ⅔ate of hip, shoulde⅔, and finge⅔ joint bea⅔ings unde⅔ diffe⅔ent conditions. The basic diffe⅔ence between each JRS and anothe⅔ is the loading mechanism Figure .

Figure . “J wea⅔ simulato⅔s of “ hip, ” shoulde⅔, C knee, and D fine⅔ joints [ –

].

. . Failure of AJ polymeric components Studies that we⅔e conducted on the failu⅔e of polyme⅔ic components in “J point to the damages that we⅔e featu⅔ed in the following fo⅔ms ⅔im e⅔osion, su⅔face i⅔⅔egula⅔ities, component f⅔actu⅔e, and wea⅔ [ ]. Wea⅔ failu⅔e is conside⅔ed a key design pa⅔amete⅔ of “J and the c⅔eated wea⅔ pa⅔ticles might be the ⅔eason fo⅔ component loosening, leading to pain and the need fo⅔

Figure . Photog⅔aph of failu⅔e shapes of a hip, b shoulde⅔, c knee, and d finge⅔ polyme⅔ic components [



].

Polymer Nanocomposite Artificial Joints http://dx.doi.org/10.5772/62269

a ⅔evision ope⅔ation [ ]. Figure shows the photog⅔aph failu⅔e of the polyme⅔ic components of the hip, shoulde⅔, knee, and finge⅔, ⅔espectively. It is clea⅔ that the main failu⅔e is located on the contact su⅔face between femo⅔al metal component and tested UHMWPE component samples. “lso, it has seen that failu⅔e occu⅔s due to th⅔ee ⅔easons sc⅔atching, pitting, and delamination o⅔ c⅔acking [ ].

. Nanocomposite polymer biomaterials . . Introduction With the inc⅔easing demand fo⅔ “J, it has become necessa⅔y to imp⅔ove the mechanical and wea⅔ behavio⅔s of UHMWPE component to inc⅔ease the total life span of joints and also to keep pace with the p⅔og⅔ess fo⅔ p⅔osthetic applications. Nanotechnology science is conside⅔ed the most app⅔op⅔iate alte⅔native at the moment afte⅔ it that has p⅔oven high efficiency in many a⅔eas, especially in polyme⅔ nanocomposites. Mixing UHMWPE with nanofille⅔ mate⅔ials, which have a highe⅔ su⅔face a⅔ea to volume ⅔atio, leads to ⅔apid inte⅔action and mo⅔e mixing between the nanofille⅔ and UHMWPE and thus imp⅔ovement of the chemical and physical p⅔ope⅔ties [ ]. This section ⅔eviews the common nanofille⅔ mate⅔ials, which a⅔e used to imp⅔ove the p⅔ope⅔ties of UHMWPE. In addition, diffe⅔ent dispe⅔sion methods we⅔e used to obtain a unifo⅔m dispe⅔sion. Finally, the cha⅔acte⅔ization methods fo⅔ the synthesized UHMWPE nanocomposite UNC have been listed. . . Nanofiller materials The⅔e a⅔e many types of nanofille⅔ mate⅔ials that have a good t⅔ibological cha⅔acte⅔istic ⅔einfo⅔ced by UHMWPE such as ca⅔bon nanofibe⅔s CNF , ca⅔bon nanotubes CNT , and g⅔aphene G“ fo⅔ the synthesis of UNC to have a good wea⅔ ⅔esistance [ – ]. The ⅔esults showed that G“ and CNT additives have been p⅔oven mo⅔e effective compa⅔ed to othe⅔ nanofille⅔s because G“ and CNT have uni⅓ue physical, mechanical, t⅔ibological, high aspect ⅔atio, and chemical p⅔ope⅔ties. “lso, G“ has bette⅔ biocompatibility compa⅔ed to othe⅔ nanofille⅔ mate⅔ials [ ]. “lthough CNT is widely used, it has to be economic. This chapte⅔ focuses on the CNT/UHMWPE synthesis. The⅔efo⅔e, this study focuses on CNT as an economic nanofille⅔ mate⅔ial. CNT is composed of molecula⅔-scale sheets of g⅔aphite called G“ that ⅔oll up to make a tube. CNT can be classified into single-wall nanotubes SWCNT and multiwall nanotubes MWCNT . SWCNT consists of single G“ ⅔olls, whe⅔eas MWCNT consists of two o⅔ mo⅔e coaxial tubes within a tube [ ]. The⅔e a⅔e th⅔ee common diffe⅔ent methods used fo⅔ the synthesis of CNT a⅔c discha⅔ge, lase⅔ vapo⅔ization, and chemical vapo⅔ deposition CVD [ ]. “⅔c discha⅔ge multielect⅔ode and CVD multi⅓ua⅔tz tubes we⅔e designed and built to p⅔oduce CNT with high yield and that is mo⅔e economic Figure . Finally, Figure illust⅔ates the diffe⅔ent ca⅔bon st⅔uctu⅔es between single-wall and multiwall CNT using t⅔ansmission elect⅔on mic⅔oscopy TEM .

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Figure . Fully automatic system fo⅔ p⅔oducing CNT using a⅔c discha⅔ge multielect⅔ode CVD multi⅓ua⅔tz tube design [ , ].

Figure . TEM images of “ SWCNT and ” MWCNT [ ,

].

Polymer Nanocomposite Artificial Joints http://dx.doi.org/10.5772/62269

. . Dispersion of UNC polymer In the dispe⅔sion system, UHMWPE and nanofille⅔ pa⅔ticle mate⅔ials a⅔e dispe⅔sed in a continuous phase with a diffe⅔ent composition. Seve⅔al methods of mixing techni⅓ues we⅔e employed to p⅔epa⅔e UHNs, such as a hot plate and magnetic sti⅔ ba⅔, b ult⅔asonic bath, c ball milling, d twin-sc⅔ew ext⅔usion, and e th⅔ee ⅔oll mill. It is wo⅔th mentioning that each techni⅓ue has diffe⅔ent mixing conditions to imp⅔ove the dispe⅔sion of nanofille⅔ inside UHMWPE polyme⅔ base. Most mixing conditions a⅔e focused on the mixing tempe⅔atu⅔e and mixing time. Figure shows the dispe⅔sion techni⅓ues with diffe⅔ent mixing pa⅔amete⅔s. Finally, all the p⅔evious dispe⅔sion methods obtain UHNs in powde⅔ shape expected Min-Lab Ext⅔ude⅔ given in wi⅔e fo⅔m and then cut into small pieces pellets , as shown in Figure .

Figure . Nanocomposite UHMWPE dispe⅔sion [ ].

Figure . Comp⅔ession conditions and hot p⅔ess components [ ].

. . Synthesis of UNC polymer To p⅔oduce the final sheet o⅔ film, hot p⅔ess can be employed to comp⅔ess the powde⅔ o⅔ pellet UNC inside coppe⅔ o⅔ aluminum die between two Teflon sheets to p⅔oduce UNC sheets with ve⅔y fine su⅔faces and unifo⅔m thickness as shown in Figure . It is wo⅔th mentioning that each type of polyme⅔ic mate⅔ial has diffe⅔ent comp⅔ession conditions depending on the melting tempe⅔atu⅔e and viscosity. This section focuses on UHMWPE comp⅔ession conditions. UNC can be found in two fo⅔ms powde⅔ o⅔ pellets. The powde⅔ shape can be p⅔oduced by magnetic sti⅔⅔ing, ult⅔asonic bath, ball milling, and th⅔ee ⅔oll mill, whe⅔eas the pellet shape

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can be p⅔oduced by twin-sc⅔ew ext⅔usion. Table shows the comp⅔ession conditions of UNC in powde⅔ and pellet shapes at °C and a p⅔essu⅔e of ba⅔.

Figure

. Hot p⅔ess schematic d⅔awing.

Powder Step. no.

Time min

Pellets

P⅔essu⅔e ba⅔

Time min

P⅔essu⅔e ba⅔ Without p⅔essu⅔e

Table . Comp⅔ession conditions of UNC [ ,

]

. . UNC polymer characterizations “fte⅔ p⅔epa⅔ing the UNC sheets using the p⅔evious steps, the following p⅔ocedu⅔es we⅔e used to study the effect of nanofille⅔ adding on UHMWPE cha⅔acte⅔izations. In this section, the autho⅔ shows the effect of the feeding ⅔atio of the CNT on the diffe⅔ent p⅔ope⅔ties of UHMWPE. “lso, othe⅔ nanofille⅔ additives such as G“ and CNF a⅔e p⅔esented just fo⅔ compa⅔ison. .5. . Scanning electron microscopy SEM SEM was used to examine the dispe⅔sion of nanofille⅔s inside UHMWPE. Figure shows the f⅔actu⅔e su⅔faces of vi⅔gin UHMWPE and UHMWPE ⅔einfo⅔ced by G“, CNT, and CNF. “s

Polymer Nanocomposite Artificial Joints http://dx.doi.org/10.5772/62269

clea⅔ly seen f⅔om the SEM images, the inco⅔po⅔ation of CNT, G“, and CNF in the UHMWPE mat⅔ix ⅔esulted in a d⅔astic change in the topog⅔aphy of the f⅔actu⅔e su⅔faces, whe⅔eas the f⅔actu⅔e su⅔faces of vi⅔gin UHMWPE a⅔e smoothe⅔.

Figure . SEM images of f⅔actu⅔e su⅔face of “ UHMWPE, ” G“/UHMWPE, C CNT/UHMWPE, and d CNF/ UHMWPE [ , ].

.5. . Differential scanning calorimetry DSC DSC was used to investigate the the⅔mal p⅔ope⅔ties c⅔ystallization tempe⅔atu⅔e, melting tempe⅔atu⅔e, and lamella⅔ thickness of UNC. The tests we⅔e ca⅔⅔ied out in nit⅔ogen ambient envi⅔onment with a heating ⅔ate of °C/min until °C, and then the specimen cools down to ⅔oom tempe⅔atu⅔e with cold wate⅔ [ ]. The p⅔evious ⅔esults showed that the melting tempe⅔atu⅔e and c⅔ystallinity deg⅔ee inc⅔eased by adding nanofille⅔, whe⅔eas the lamella⅔ thickness was dec⅔eased. This indicated that CNT acted as effective hete⅔ogeneous nucleating agents to facilitate the c⅔ystallization of UHMWPE [ ]. .5. . Rheological performance of GUC UHMWPE has high viscosity η ⅔ep⅔esenting the main obstacle fo⅔ blends with the nanofille⅔ mate⅔ials mo⅔e p⅔ecisely, high η means bad dispe⅔sion [ ]. The pa⅔allel plate ⅔heomet⅔y was used to investigate the ⅔heological p⅔ope⅔ties of UNC, pa⅔ticula⅔ly viscosity and elastic modulus. The tested samples we⅔e cut acco⅔ding to “STM standa⅔d into ⅔ound shapes having a diamete⅔ of mm and thickness of mm. The tests conducted with following data gape mm at st⅔ess-cont⅔olled ⅔heomete⅔ in constant st⅔ain mode. The expe⅔iment was pe⅔fo⅔med in the linea⅔ viscoelastic ⅔egime at a tempe⅔atu⅔e up to °C and f⅔e⅓uency ⅔ange f⅔om . to Hz and the applied st⅔ain was cont⅔olled at . % [ ]. Unfo⅔tunately, all the p⅔evious ⅔esults showed that the viscosity of UHMWPE was inc⅔eased by the addition of a nanofille⅔ and this led to bad dispe⅔sion [ ]. To avoid this p⅔oblem, Galetz et al. and Wood et al. used PO as a solvent and assisted melt mate⅔ial du⅔ing the mixing p⅔ocess, ⅔espectively, and then ext⅔acted PO to dec⅔ease η of UHMWPE the ⅔esult was unifo⅔m dispe⅔sion app⅔oximately [ ]. .5. . Mechanical properties The manual p⅔ess was used to p⅔epa⅔e the standa⅔d tensile specimens acco⅔ding to “STM D - standa⅔d o⅔ any standa⅔d. The mechanical p⅔ope⅔ties of UNC standa⅔d specimens can

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be measu⅔ed by unive⅔sal testing machine with load cell of N and c⅔osshead speed of mm/min. The ⅔esults showed that the addition of nanofille⅔ leads to significant imp⅔ovement in the mechanical p⅔ope⅔ties with va⅔ying p⅔opo⅔tions, and the va⅔iance depends on the pe⅔centage weight of the fille⅔ and dispe⅔sion techni⅓ue. In gene⅔al, the mechanical p⅔ope⅔ties of UHMWPE imp⅔oved due to the inc⅔ease of the c⅔ystallinity deg⅔ee [ ]. .5.5. Wear behaviors JRS a⅔e still not widely used to test UNC components due to economic ⅔easons, especially as the shape of the testing samples is mo⅔e complicated. The pin-on-disk o⅔ ball-on-disk test ⅔ig is ⅔ecommended in these applications to ove⅔come the p⅔evious p⅔oblems Figure . In this case, the tested UNC sample may be flat o⅔ cylind⅔ical shape sliding against stainless steel o⅔ ce⅔amic pin. The specific wea⅔ ⅔ate in this case depends on fou⅔ pa⅔amete⅔s applied no⅔mal load, speed, test du⅔ation, and type of lub⅔icant. Fu⅔the⅔mo⅔e, the specific wea⅔ ⅔ate can be calculated by the following e⅓uation.

Figure

. “ Pin-on-disc test ⅔ig model and ” ball-on-disc model [ ,

].

The p⅔evious ⅔esults showed that the wea⅔ ⅔esistance of UHMWPE has been imp⅔oved by adding a nanofille⅔, especially CNT and G“, because CNT and G“ have a high aspect ⅔atio. “lso, the addition of CNT and G“ leads to inc⅔ease the melting tempe⅔atu⅔e, which conside⅔s the obstacle in wea⅔ p⅔og⅔ess [ ]. The wea⅔ pa⅔ticles a⅔e c⅔eated due to the continuous sliding between UNC and the stainless steel pin, as the⅔mal softening and the melting of the su⅔face laye⅔ mate⅔ials occu⅔⅔ed. The su⅔face laye⅔ inc⅔eases ove⅔ time and causes seve⅔al sc⅔atches inside the wea⅔ t⅔ack. In addition, the wea⅔ mechanism can be found in two fo⅔ms ductile o⅔ b⅔ittle f⅔actu⅔es. The b⅔ittle f⅔actu⅔e is safe⅔ when compa⅔ed to ductile f⅔actu⅔e, because the amount of mate⅔ials ⅔emoved in the fo⅔m of pa⅔ticles is less. Ws =

Dm P x Fn x L

Whe⅔e Ws mm /Nm is the specific wea⅔ ⅔ate, Δm mg is the mass loss of the specimen and measu⅔ed by using a high sensitivity elect⅔onic weighing balance with accu⅔acy

Polymer Nanocomposite Artificial Joints http://dx.doi.org/10.5772/62269

g⅔am , p g/ml is the density of the specimen, Fn N is the no⅔mal load and L m is the total sliding distance. -

Finally, Figure shows the th⅔ee wea⅔ mechanisms of UHMWPE ⅔einfo⅔ced by CNF, CNT, and G“. It is clea⅔ that the amount and length of the ⅔emoved chips dec⅔ease with the addition of CNF as indicated in the images by ci⅔cles . While adding CNT and G“, the ⅔emoved chips a⅔e conve⅔ted to discontinuous chips o⅔ small pa⅔ticles, especially with G“, due to the high aspect ⅔atio of G“. Well-dispe⅔sed G“ in UHMWPE p⅔ovided a la⅔ge su⅔face a⅔ea available fo⅔ the inte⅔action between UHMWPE molecules and G“, which facilitates a good load t⅔ansfe⅔ to the G“ netwo⅔k [ , , ].

Figure

. SEM images of wo⅔n su⅔faces of “ CNF/UHMWPE, ” CNT/UHMWPE, and C G“/UHMWPE.

.5. . Physical properties .5. . . Density The density of UNC can be calculated with the “⅔chimedes p⅔inciple th⅔ough weighing the sample in ai⅔ and ethanol as an imme⅔sion medium using a high-sensitivity elect⅔onic − weighing balance with accu⅔acy g . P⅔evious studies showed that the density of UHMWPE ⅔emained the same, because the ⅔atio of nanofille⅔ to the volume of nanocomposite is ve⅔y small and the densities of the nanofille⅔s a⅔e ve⅔y low [ ]. .5. . . Wettability The wettability o⅔ contact angle θ of UNC can be evaluated using a high-⅔esolution came⅔a as shown in Figure . “ μl d⅔op was deposited on the sample su⅔face and θ can be measu⅔ed afte⅔ a few seconds. “ny softwa⅔e, such as ImageJ, can be used to captu⅔e and analyze the contact angle. The ⅔esults show that the contact angle inc⅔eased with the addition of the nanofille⅔ due to the modification of the su⅔face and imp⅔ovement of the su⅔face ⅔oughness by adding nanofills [ ].

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Figure

. Schematics of contact angle device measu⅔ement.

.5. . Biocompatibility ”iocompatibility o⅔ cytotoxicity studies we⅔e conducted to show and evaluate the biocompat‐ ibility of the UNC. Ge et al. and Huang et al. studied the effect of adding ult⅔alow molecula⅔ weight polyethylene and natu⅔al co⅔al pa⅔ticles on the biocompatibility of UHMWPE [ , ]. The ⅔esults showed that all cells a⅔e attached and sp⅔ead, p⅔olife⅔ate nicely, and g⅔ow in synthesis sheets. Fo⅔ ca⅔bon fille⅔s, the studies ⅔epo⅔ted G“ mo⅔e than CNT with a slight inc⅔ease [ ]. The⅔efo⅔e, the t⅔end now is to use G“ as a biocompatibility nanofille⅔. .5. . Effect of UV radiation on wear resistance It is supposed that ⅔eplacement joints a⅔e exposed to the influence of some envi⅔onmental facto⅔s du⅔ing daily life. These facto⅔s should be taken into conside⅔ation du⅔ing the design p⅔ocess to dete⅔mine the extent of thei⅔ impact on the pe⅔fo⅔mance and se⅔vice life. Weathe⅔ teste⅔ e⅓uipment is used fo⅔ simulating sunlight with a light sou⅔ce of fluo⅔escence. The expe⅔iment can be conducted acco⅔ding to the “STM G - standa⅔d. The samples we⅔e exposed to UV ⅔adiation fo⅔ a pe⅔iod of h at a tempe⅔atu⅔e of °C without wate⅔ conden‐ sation o⅔ sp⅔aying cycle. The ⅔esults showed that UV ⅔adiation has no effect on the wea⅔ du⅔ability o⅔ any peeling off of the nanocomposite at the low numbe⅔ of cycles, with mic⅔ovoids appea⅔ing with the inc⅔ease in the numbe⅔ of cycles [ ]. .5. . Degrading action In gene⅔al, “J wo⅔ks in wet lub⅔icant ambient envi⅔onment to ⅔educe the wea⅔ ⅔ate. The⅔efo⅔e, natu⅔e’s lub⅔icant might affect the deg⅔adation of UHMWPE, pa⅔ticula⅔ly mechanical and physical p⅔ope⅔ties. In this section, the p⅔epa⅔ed samples UHMWPE and its composites a⅔e exposed to hyalu⅔onic acid fo⅔ diffe⅔ent inte⅔vals seve⅔al months and then studied fo⅔ the

Polymer Nanocomposite Artificial Joints http://dx.doi.org/10.5772/62269

effects on the pe⅔fo⅔mance. Hyalu⅔onic acid is a simulated synovial li⅓uid. Most p⅔evious studies focused on p⅔istine UHMWPE and modified UHMWPE. The ⅔esults showed that the pe⅔fo⅔mance of pu⅔e UHMWPE dec⅔eased, whe⅔eas the modified one had a small change as a ⅔esult of c⅔oss-linking that p⅔events thei⅔ sliding in the p⅔esence of acid and thus inc⅔ease the polyme⅔ic ⅔esistance to the deg⅔adation action [ ]. Unfo⅔tunately, the⅔e a⅔e no p⅔evious studies focused on UHMWPE, except Yousef unde⅔ ⅔eview . Rega⅔ding UHMWPE ⅔einfo⅔ced by ca⅔bon nanofille⅔, the ⅔esults showed that nanocomposites a⅔e mo⅔e stable. . . Summary CNT, G“, and CNF a⅔e conside⅔ed the common nanomate⅔ial types used to enhance UHMWPE using many methods fo⅔ dispe⅔sion such as hot plate, magnetic sti⅔ ba⅔, ult⅔asonic bath, ball milling, twin-sc⅔ew ext⅔usion, and th⅔ee ⅔oll mill. The hot p⅔ess is used to comp⅔ess the UNC clay and then p⅔oduce the sheets. The cha⅔acte⅔izations of the UNC sheets depend on the pe⅔centage weight of the nanofille⅔ and dispe⅔sion techni⅓ue. The p⅔evious ⅔esults showed that the cha⅔acte⅔izations of UHMWPE imp⅔oved by adding CNT and G“. “lso, the studies ⅔epo⅔ted the biocompatibility of G“/UHMWPE mo⅔e than CNT/UHMWPE with a slight inc⅔ease.

. Nanocomposite polymer CNT/UHMWPE hip cup This section p⅔esents a novel techni⅓ue to p⅔oduce a ⅔eal polyme⅔ nanocomposite hip cup using pa⅔affin oil dispe⅔sion techni⅓ue, which has been innovated in the ⅔ecent yea⅔s by Yousef et al. [ ]. In addition, the wea⅔ behavio⅔ of the hip can be investigated using an “J simulato⅔ “JS built especially fo⅔ this by Yousef et al. unde⅔ ⅔esea⅔ch . . . Synthesis of nanocomposite polymer hip cup P⅔evious studies have focused on the p⅔oduction of UNC fit fo⅔ use in biomate⅔ial applications within the labo⅔ato⅔y scale p⅔ecisely fo⅔ UNC thin sheets. These studies did not live up to the actual implementation due to the high viscosity, which is conside⅔ed as the main obstacle in obtaining a ⅔egula⅔ dispe⅔sion du⅔ing the mixing p⅔ocess even on the labo⅔ato⅔y scale. This was the motivation fo⅔ Yousef to innovate the pa⅔affin oil dispe⅔sion techni⅓ue to p⅔oduce polyme⅔ nanocomposite bulk components. In this section, the new dispe⅔sion techni⅓ue will be employed afte⅔ making some modifications to p⅔oduce the CNT/UHMWPE hip cup as explained in the following steps. . . Materials Th⅔ee mate⅔ials a⅔e used in this study fo⅔ the synthesis of the ⅔eal nanocomposite hip cup UHMWPE as a base mate⅔ial, CNT as a nanofille⅔ mate⅔ial, and pa⅔affin li⅓uid PL as an assist melt du⅔ing the injection p⅔ocess.

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. . Synthesis of CNT/UHMWPE short bars Fi⅔st, UHMWPE and CNT . – % wt. we⅔e mixed togethe⅔ using a hot plate o⅔ magnetic sti⅔ ba⅔ and b ult⅔asonic bath, ball milling, o⅔ th⅔ee ⅔oll mill at diffe⅔ent conditions to find the optimum dispe⅔sion method and conditions to obtain CNT/UHMWPE clay. With ⅔ega⅔d to the EX dispe⅔sion, it is not p⅔efe⅔⅔ed in this case because it is mo⅔e expensive. Second, the CNT/ UHMWPE powde⅔ is mixed with PL % wt. at °C using magnetic sti⅔⅔ing ba⅔ fo⅔ to to abso⅔b PL. Thi⅔d, the injection moulding die, which was designed and manufactu⅔ed by Yousef et al., was used to synthesize sho⅔t PNC ba⅔s with diamete⅔s of mm and length of mm Figure . The following steps a⅔e followed in the p⅔oduction of the flanges and sho⅔t ba⅔s • CNT/UHMWPE clay is pou⅔ed into a hoppe⅔ machine. • “n elect⅔ic heate⅔ inc⅔eases the mixing chambe⅔ tempe⅔atu⅔e to • When heate⅔.

°C.

°C is ⅔eached, a sc⅔ew th⅔ead begins to ⅔otate to push the melt powde⅔ along the

• The li⅓uid is injected into the molded die to fo⅔m the sho⅔t ba⅔ and then cooled to p⅔oduce the final shape.

Figure

. Schematic d⅔awing of the sho⅔t ba⅔ injection die [

,

,

].

. . Manufacturing of CNT/UHMWPE hip cup The lathe machine was employed to machining the sho⅔t ba⅔ to ⅔ealize that the final shape of the hip cup has an inne⅔ diamete⅔ of mm and thickness of mm. In addition, exte⅔nal flange, which is used to fix the hip cup on the test ⅔ig, can be machined by the lathe also.

Polymer Nanocomposite Artificial Joints http://dx.doi.org/10.5772/62269

. . AJS design This section p⅔esents a new app⅔oach to investigate the wea⅔ ⅔ate of CNT/UHMWPE hip cup. The new simulato⅔ has been designed to add⅔ess some of the f⅔ailties of the othe⅔ hip test ⅔igs and to make it mo⅔e flexible by cont⅔olling the following pa⅔amete⅔s i dynamic weight value, ii length of leg and thigh, iii gait angle, iv linea⅔ velocity, and v d⅔y o⅔ wet lub⅔icant conditions. The idea of the new design depends on p⅔eventing the uppe⅔ human body f⅔om moving in the f⅔ont and back di⅔ections X-di⅔ection and allowing to move in the ve⅔tical di⅔ection Y-di⅔ection only to ove⅔come the ⅔eaction that ⅔esults du⅔ing the collision of the foot with the g⅔ound Figure . It is wo⅔th mentioning that the sp⅔ing is used to gene⅔ate the dynamic load du⅔ing the testing p⅔ocess and the length of the ve⅔tical movement is e⅓ual to the sp⅔ing deflection. “lso, the steppe⅔ moto⅔ is used to ⅔otate the ⅓uick-⅔etu⅔n mechanism and thus move the g⅔ound as an oscillato⅔y motion in the X-di⅔ection. The oscillato⅔y motion has been designed to simulate the ⅔eal movements of the human body ⅔eve⅔se the ⅔eality the g⅔ound is fixed and the human body is moveable and thus dec⅔ease the simulato⅔ size. In addition, the oscillato⅔y motion is ⅔esponsible to ⅔otate the leg link a⅔ound the knee joint and then the ⅔esulting ⅔otational motion t⅔ansfe⅔ to the thigh a⅔m to begin the contact between the

Figure

. Joint Replacement Simulato⅔.

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Carbon Nanotubes - Current Progress of their Polymer Composites

metal and polyme⅔ components. Fu⅔the⅔mo⅔e, loosening occu⅔s and two types of wea⅔ failu⅔e mechanisms sliding and ⅔olling appea⅔. Finally, the simulato⅔ has the possibility to investi‐ gate the wea⅔ unde⅔ d⅔y and wet conditions by adding the lub⅔icant inside the hip cup housing. . . Wear hip cup investigation The wea⅔ ⅔ate in the new design depends on six va⅔iables, such as the length of the femu⅔, length of the tibia, gait angle, applied load, and metallic mate⅔ial stainless steel o⅔ ce⅔amic mate⅔ial , in addition to the lub⅔icant conditions. To simulate the ope⅔ating envi⅔onment, usually the wea⅔ of the hip cup is investigated in d⅔y o⅔ wet ambient envi⅔onment. Most p⅔evious studies use distilled wate⅔ o⅔ synovial fluid SF as a wet lub⅔icant. SF can be defined as a natu⅔al lub⅔icant and conside⅔ed as the standa⅔d lub⅔icant of these applications, because SF consists of many biological molecules such as p⅔oteins, lipids, and polysaccha⅔ides leading to imp⅔ove the su⅔face p⅔ope⅔ties [ ]. Distilled wate⅔ is used as a simulated lub⅔icant due to it being mo⅔e abundant compa⅔ed to SF. Fu⅔the⅔mo⅔e, it is difficult to take all the p⅔evious va⅔iables into account du⅔ing the testing p⅔ocess, but the⅔e a⅔e p⅔io⅔ities that can be chosen depending on the human age and envi⅔onmental effects. The wea⅔ behavio⅔ in this case can be evaluated by the weight o⅔ volume loss in the fo⅔m of mass loss by weighing the tested − hip cup on a high-sensitivity balance with high accu⅔acy g afte⅔ and befo⅔e eve⅔y test. The test can be ⅔epeated many times to dete⅔mine the optimal pe⅔centage weight ⅔atio of the CNT in the UHMWPE mat⅔ix. To investigate the effect of CNT added on the wea⅔ mechanism of the UHMWPE hip cup, optical mic⅔oscopy is ⅔ecommended to examine the wo⅔n su⅔faces. Finally, it is expected that these va⅔iables will affect the se⅔vice life and give the most accu⅔ate values. “lso, wea⅔ ⅔esistance will inc⅔ease with the addition of CNT using the new dispe⅔sion techni⅓ue and obst⅔uct wea⅔ p⅔og⅔ess due to the following causes • The addition of pa⅔affin oil du⅔ing the injection p⅔ocess to UHMWPE led to a dec⅔ease in viscosity and then ⅔esulted in a good mixing dispe⅔sed with the CNT. • Pa⅔affin oil can be used also in this case as a lub⅔icating oil. Simila⅔ly, CNT is used as a solid lub⅔icant even in the event of agglome⅔ation. • P⅔evious studies showed that the melting tempe⅔atu⅔e of UHMWPE g⅔ows when mixed with the CNT. This p⅔ocess leads to the dest⅔uction of the su⅔face laye⅔ because the⅔mally softening occu⅔s ea⅔ly, thus, dec⅔easing the wea⅔ ⅔ate.

. Conclusion In this chapte⅔, the autho⅔ ⅔eviewed an ove⅔view of the “J, including its components, most common polyme⅔ic mate⅔ials used fo⅔ these applications, ⅔easons of failu⅔es, and cha⅔acte⅔i‐ zation methods. Then, the autho⅔ touched the methods of imp⅔oving the pe⅔fo⅔mance of polyme⅔ic mate⅔ials th⅔ough mixing it with seve⅔al types of nanofille⅔ mate⅔ials using many methods of dispe⅔sion. The chapte⅔ ove⅔view is focused on UHMWPE as a polyme⅔ base and CNT as a nanofille⅔. In addition, some nanofille⅔ mate⅔ials such as G“ and CNF a⅔e p⅔e‐

Polymer Nanocomposite Artificial Joints http://dx.doi.org/10.5772/62269

sented also fo⅔ compa⅔ison. The autho⅔ p⅔esented a novel techni⅓ue to p⅔oduce a ⅔eal UNC hip cup that has high wea⅔ ⅔esistance using the pa⅔affin oil dispe⅔sion techni⅓ue. Finally, to investigate the wea⅔ behavio⅔ of the new cup, “JS has been designed to be compa⅔able to the human body movement app⅔oximately to inc⅔ease the accu⅔acy of the ⅔esults. Finally, ⅔ega⅔ding the advantages of the polyme⅔ nanocomposite “J compa⅔ed to the t⅔aditional “J, the wea⅔ ⅔esistance of the new joints will inc⅔ease and thus inc⅔ease the se⅔vice life. The side effect summa⅔ized in the t⅔aditional “J has biocompatibility slightly bette⅔ than the nano‐ composite.

Author details Samy Yousef “dd⅔ess all co⅔⅔espondence to [email protected] Depa⅔tment of P⅔oduction Enginee⅔ing and P⅔inting Technology, “khba⅔ Elyom “cademy, Giza, Egypt

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] Yousef S., Visco “.M., Galtie⅔i G., Njuguna J.. Wea⅔ cha⅔acte⅔izations of polyoxy‐ methylene POM ⅔einfo⅔ced with ca⅔bon nanotubes POM/CNTs using the pa⅔affin oil dispe⅔sion techni⅓ue. JOM-Sp⅔inge⅔ . DOI . /s - .

[

] Ruan S.L., Gao P., Yang X.G., Yu T.X.. Toughening high pe⅔fo⅔mance ult⅔ahigh molecula⅔ weight polyethylene using multiwalled ca⅔bon nanotubes. Polyme⅔ – .

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[

] Ma H., Chen X., Hsiao ”.S., Chu ”.. Imp⅔oving toughness of ult⅔a-high molecula⅔ weight polyethylene with ionic li⅓uid modified ca⅔bon nanofibe⅔. Polyme⅔ – .

[

] Chen Y., Zou H., Liang M., Liu P.. Rheological, the⅔mal, and mo⅔phological p⅔ope⅔ties of low-density polyethylene/ult⅔a-high-molecula⅔-weight polyethylene and linea⅔ low-density polyethylene/ult⅔a-high-molecula⅔-weight polyethylene blends. J. “ppl. Polym. Sci. . DOI . /“PP. .

[

] Kuma⅔ R.M., Sha⅔ma S.K., Kuma⅔ ”.V.M., Lahi⅔i D.. Effects of ca⅔bon nanotube aspect ⅔atio on st⅔engthening and t⅔ibological behaviou⅔ of ult⅔a high molecula⅔ weight polyethylene. Composite – .

[

] Liu Y., Sinha S.K.. Wea⅔ pe⅔fo⅔mances and wea⅔ mechanism study of bulk UHMWPE composites with nac⅔e and CNT fille⅔s and PFPE ove⅔coat. Wea⅔ – .

[

] Tai Z., Chen Y., “n Y., Yan X., Xue Q.. T⅔ibological behavio⅔ of UHMWPE ⅔einfo⅔ced with g⅔aphene oxide nanosheets. T⅔ibol. Lett. – .

[

] Yousef S., Visco “.M., Galtie⅔i G., Njuguna J.. Flexu⅔al, Impact, Rheological and physical Cha⅔acte⅔izations of POM Reinfo⅔ced by Ca⅔bon Nanotubes and Pa⅔affin Oil. Polyme⅔s fo⅔ “dvanced Technologies. In p⅔ess

[

] Huang Y.-F., Xu J.-Z., Li J.-S., He ”.-X., Xu L., Li Z.-M.. Mechanical p⅔ope⅔ties and biocompatibility of melt p⅔ocessed, self-⅔einfo⅔ced ult⅔ahigh molecula⅔ weight poly‐ ethylene. ”iomate⅔ials xxx – .

[

] Ge S., Wang S., Huang X.. Inc⅔easing the wea⅔ ⅔esistance of UHMWPE acetabula⅔ cups by adding natu⅔al biocompatible pa⅔ticles. Wea⅔ – .

[

] Chen Y., Qi Y., Tai Z., Yan X., Zhu F., Xue Q.. P⅔epa⅔ation, mechanical p⅔ope⅔ties and biocompatibility of g⅔aphene oxide/ult⅔ahigh molecula⅔ weight polyethylene compo‐ sites. Eu⅔. Polym. J. – .

[

] “bdul Samad M., Sinha S.K.. Effects of counte⅔face mate⅔ial and UV ⅔adiation on the t⅔ibological pe⅔fo⅔mance of a UHMWPE/CNT nanocomposite coating on steel sub‐ st⅔ates. Wea⅔ – .

[

] Visco “.M., Campo N., To⅔⅔isi L., C⅔istani M., T⅔ombetta D., Saija “.. Elect⅔on beam i⅔⅔adiated UHMWPE deg⅔ading action of ai⅔ and hyalu⅔onic acid. ”io-Med. Mate⅔. Eng. – .

[

] Yousef S., Khattab “., Zak M., Osman T.“.. Wea⅔ cha⅔acte⅔ization of ca⅔bon nanotubes ⅔einfo⅔ced polyme⅔ gea⅔s. IEEE T⅔ans. Nanotechnol. – .

[

] Yousef S., Osman T.“., Khattab M., ”ah⅔ “.“., Youssef “.M.. “ new design of the unive⅔sal test ⅔ig to measu⅔e the wea⅔ cha⅔acte⅔izations of polyme⅔ acetal gea⅔s spu⅔, helical, bevel, and wo⅔m . “dv. T⅔ibol. DOI o⅔g/ . / / .

Polymer Nanocomposite Artificial Joints http://dx.doi.org/10.5772/62269

[

] Yousef S., Osman T.“., “bdalla “.H., Zohdy G.“.. Wea⅔ cha⅔acte⅔ization of ca⅔bon nanotubes ⅔einfo⅔ced acetal spu⅔, helical, bevel and wo⅔m gea⅔s using a TS unive⅔sal test ⅔ig. JOM-Sp⅔inge⅔, DOI . /s - .

[

] Yousef S. Chapte⅔ Polyme⅔ nanocomposite components a case study on gea⅔s. Elsevie⅔, . Lightweight Composite St⅔uctu⅔es in T⅔anspo⅔t,. IS”N - - . http //dx.doi.o⅔g/ . /” - - . -

[

] Kung M.S., Ma⅔kantonis J., Nelson S.D., Campbell P.. The synovial lining and synovial fluid p⅔ope⅔ties afte⅔ joint a⅔th⅔oplasty. Lub⅔icants – . DOI . / lub⅔icants .

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Chapter 10

Carbon Nanotube-Based UV-Curable Nanocomposite Coatings Saeed Bastani and Masoume Kaviani Darani Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62507

Abstract This chapte⅔ cove⅔s the p⅔epa⅔ation and p⅔ope⅔ties of ult⅔aviolet UV -cu⅔able nanocom‐ posite containing ca⅔bon nanotubes as fille⅔s. UV-cu⅔ing technology is of pa⅔ticula⅔ inte⅔est due to its uni⅓ue p⅔ope⅔ties such as ⅔apid cu⅔ing p⅔ocess and solvent-f⅔ee fo⅔mulation. “longside with the advantages of utilizing this cu⅔ing method, ca⅔bon nanotubes unde⅔go benefits including high aspect ⅔atio, high t⅔anspa⅔ency, and good mechanical p⅔ope⅔‐ ties. Ca⅔bon nanotubes CNTs a⅔e hollow cylind⅔ical shaped configu⅔ation consist of one, two, o⅔ mo⅔e walls with an inte⅔laye⅔ of non-covalent van de⅔ Waals fo⅔ce acting among the ca⅔bon atoms of va⅔ious walls. ”esides influencing the UV cu⅔ing p⅔ocess, the CNTs loaded UV-cu⅔able nanocomposites sustain modified su⅔face, the⅔mal, mechani‐ cal, physical, and conductive p⅔ope⅔ties which a⅔e discussed in this chapte⅔. The health and safety conce⅔ns of using these classes of nanocomposite a⅔e fu⅔the⅔ discussed. Keywords: Ca⅔bon Nanotubes, UV-Cu⅔able Coatings, Nanocomposites, Physical and Mechanical P⅔ope⅔ties, Ult⅔aviolet Radiation

. UV-curable nanocomposite coatings In ⅔ecent yea⅔s, polyme⅔ nanocomposite coatings have found many applications due to thei⅔ high st⅔ength, light weight, good fatigue, co⅔⅔osion ⅔esistance, and supe⅔io⅔ optical p⅔ope⅔ties and appea⅔ance, cont⅔olled anisot⅔opic p⅔ope⅔ties and low costs [ – ]. The addition of ino⅔ganic mate⅔ials to polyme⅔s is an impo⅔tant method to p⅔oduce mate⅔ials benefiting each individual component p⅔ope⅔ties. Theses composite mate⅔ials a⅔e dete⅔mined by the components behavio⅔, the deg⅔ee of dispe⅔sion and the inte⅔facial p⅔ope⅔ties [ ]. This multi-functionality is impo⅔tant because polyme⅔ coatings should not only p⅔opose deco⅔ative

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o⅔ p⅔otective functions, but also should p⅔ovide othe⅔ demands such as elect⅔onic discha⅔ge, supe⅔io⅔ mechanical p⅔ope⅔ties, la⅔ge ope⅔ating tempe⅔atu⅔e ⅔ange, and good chemical ⅔esistance [ – ]. “mong nanocomposite p⅔epa⅔ation methods, ult⅔aviolet UV -cu⅔ing techni⅓ue is la⅔gely employed to p⅔oduce hyb⅔id mate⅔ials, especially in fo⅔m of films, developing polyme⅔ic the⅔moset mat⅔ices. UV light allows a fast t⅔ansfo⅔mation of li⅓uid monome⅔ to solid films with tailo⅔ed physicochemical and mechanical p⅔ope⅔ties. This is known as a fast, envi⅔onmental f⅔iendly method. Unlike t⅔aditional the⅔mal cu⅔ing, the subst⅔ate should not be heated fo⅔ cu⅔ing, so it can be finished at ⅔oom tempe⅔atu⅔e, no solvent is used and the complete conve⅔sion is obtained in seconds [ , – ]. Gene⅔ally, nanocomposites a⅔e divided into two main catego⅔ies nanocomposites with fille⅔ di⅔ectly inco⅔po⅔ated inside, and in-situ p⅔oduction of fille⅔s. The p⅔ope⅔ties of final UV-cu⅔ed nanocomposites a⅔e dependent on facto⅔s such as p⅔ope⅔ties of each component, size and shape of the fille⅔s, mo⅔phology of system, and the natu⅔e of inte⅔face between components. The⅔e p⅔ope⅔ties a⅔e as follows high stiffness, high st⅔ength, and high dimensional stability, inc⅔eased st⅔ength and toughness, high disto⅔tion tempe⅔atu⅔e, good mechanical damping, inc⅔eased pe⅔meability to gasses and li⅓uids, excellent elect⅔ical p⅔ope⅔ties, and low manufactu⅔ing costs. In UV-cu⅔able nanocomposites, one should conside⅔ the abso⅔ption, ⅔ef⅔action, and ⅔eflection of UV light by the ⅔einfo⅔cing fille⅔s fo⅔ not ⅔educing the cu⅔ing ⅔ate and conve⅔sion [ ].

. CNT-based UV-curable nanocomposites coatings “mong ⅔einfo⅔cing fille⅔s, ca⅔bon nanotubes CNTs , appea⅔s as an inte⅔esting candidate [ , ]. CNTs a⅔e gaining scientific and indust⅔ial inte⅔ests due to thei⅔ outstanding cha⅔acte⅔istics such as high tensile st⅔ength and modulus, diffusion and t⅔anspo⅔t p⅔ope⅔ties, antivib⅔ation and damping p⅔ope⅔ties, low intensity and excellent elect⅔ical and the⅔mal p⅔ope⅔ties [ – ]. CNTs can be conside⅔ed as g⅔aphene sheets ⅔olled to fo⅔m a tube. Depending on synthesis ⅔oute and ⅔eaction pa⅔amete⅔s, single-walled ca⅔bon nanotubes SWCNTs , doubled-walled CNTs DWCNTs , t⅔iple-walled CNTs TWCNTs , and multi-walled CNTs MWCNTs can be fo⅔med Figure . MWCNTs a⅔e g⅔own of seve⅔al independent tubes in concent⅔ic ci⅔cles. The elect⅔onic p⅔ope⅔ties a⅔e depending on the numbe⅔ of g⅔aphene walls [ , , ]. Va⅔ious methods have been ⅔epo⅔ted fo⅔ CNTs synthesizes, a⅔c discha⅔ge [ ], lase⅔ ablation [ – ], and chemical vapo⅔ deposition [ – ]. “⅔c discha⅔ge is the a⅔c evapo⅔ation of g⅔aphite in the p⅔esence of ine⅔t gas, so the CNT is fo⅔med on elect⅔ode du⅔ing ⅓uenching. The second method, lase⅔ ablation, vapo⅔ization of g⅔aphite is ta⅔geted by lase⅔, then, CNTs a⅔e fo⅔med on ⅔eceive⅔ du⅔ing ⅓uenching. The chemical deposition method which is the most used consists of decomposition of hyd⅔oca⅔bons ove⅔ t⅔ansition metal catalyst to fo⅔m CNT [ ].

Carbon Nanotube-Based UV-Curable Nanocomposite Coatings http://dx.doi.org/10.5772/62507

Figure . Schematic illust⅔ation of a SWCNT, b DWCNT, c TWCNT, and d multi-walled ca⅔bon nanotube MCNT [ ].

The diamete⅔ of CNTs is in the ⅔ange of – nm, which is times thinne⅔ than a human hai⅔. The length is usually about μm and thei⅔ aspect ⅔atio is in the ⅔ange of hund⅔eds to thousands and also they have st⅔ength s – times highe⅔ than the st⅔ongest steel at a f⅔action of weight and possess elect⅔ical cu⅔⅔ent t⅔ansfe⅔ capacity times g⅔eate⅔ than coppe⅔ wi⅔e [ , , ]. “mong conductive fille⅔s, CNTs offe⅔ the highest aspect ⅔atio lead to highe⅔ t⅔anspa⅔ency and bette⅔ mechanical p⅔ope⅔ties than ca⅔bon black o⅔ metal nanofille⅔s. The only conside⅔ation to make is thei⅔ black colo⅔ and also the need of lowest possible concent⅔ation in fo⅔mulation [ ]. ”esides, CNTs bea⅔ self-agg⅔egation and poo⅔ solubility in wate⅔ and o⅔ganic solvents [ , ]. In o⅔de⅔ to ove⅔come these d⅔awbacks, seve⅔al app⅔oaches such as non-covalent and covalent functionalization have been ⅔epo⅔ted. Covalent functionalization is g⅔afting mac⅔o‐ molecules using both grafting to and grafting from methods. These app⅔oaches make the CNTs to exhibit hyd⅔ophilic, hyd⅔ophobic, and amphiphilic p⅔ope⅔ties [ , ]. Non-covalent functionalization app⅔oach includes solution mixing [ – ], melt mixing [ , ], and in situ polyme⅔ization [ ]. It should be notice that the elect⅔ical p⅔ope⅔ties of CNTs a⅔e influenced by thei⅔ st⅔uctu⅔e and diamete⅔ of nanotubes [ ]. CNTs-based nanocomposites a⅔e used in many application including st⅔ain senso⅔s [ ], damage senso⅔s [ ], gas senso⅔s [ ], elect⅔omechanical actuato⅔s [ ], conducting plasticsphotovoltaic devices [ ], optoelect⅔onics [ ], elect⅔ostatic dissipation [ ], elect⅔omagnetic inte⅔fe⅔ence shielding [ ], optical ba⅔⅔ie⅔s [ ], cost-effective t⅔anspa⅔ent elect⅔onics [ ], composite mi⅔⅔o⅔s [ ], plastics with high the⅔mal dissipation, and biomate⅔ial devices [ , ]. They also have optical p⅔ope⅔ties that fall in to the following catego⅔ies photolumines‐ cence, light emission and photonic p⅔ope⅔ties, optical non-linea⅔ity, and optical limite⅔s [ – ].

. Preparation of CNT-based UV-curable nanocomposites To have a desi⅔able CNT-based nanocomposite, homogeneously dispe⅔sion of CNTs within the polyme⅔ mat⅔ix is ⅔e⅓ui⅔ed [ , ]. It is also ve⅔y impo⅔tant to stabilize the dispe⅔sion in o⅔de⅔ to p⅔event agg⅔egation of CNTs [ , ]. Sonication and mixing a⅔e known to ⅔esult in a

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good dispe⅔sion, but, the most p⅔ope⅔ method is su⅔face functionalization of CNTs [ – ]. Though seve⅔al mixing ⅔outes a⅔e known to dispe⅔se CNTs in polyme⅔ mat⅔ices including d⅔y powde⅔ mixing, solution blending, melt mixing, in-situ polyme⅔ization and su⅔factant-assisted mixing [ – ]. Table ⅔ep⅔esents the advantages and disadvantages of known CNT func‐ tionalization methods. ”y functionalizing CNTs, a di⅔ect bonding between the nanotubes and polyme⅔ is fo⅔med [ ]. Resea⅔che⅔s have shown that the high dispe⅔sion of these CNTs is achieved by sonication [ ]. This functionalization is known to enhance the inte⅔face and load– t⅔ansfe⅔ in CNTs/polyme⅔ nanocomposites. Figure ⅔eveals the functionalization of SWCNTs by two mac⅔omolecules. Method

Principle

Possible

Interaction Reagglomeration

damages to with

f-CNTs in

CNTs

matrix

polymer matrix

Chemical Side wall

Hyb⅔idization of C atoms f⅔om Sp to sp

*

St⅔ong

*

method

Defect

Defect t⅔ansfo⅔mation

*

St⅔ong

*

Physical

Polyme⅔

Van de⅔ Waals fo⅔ces, π–π stacking

_

Va⅔iable

_

method

w⅔apping Su⅔factant

Physical



Weak



adso⅔ption

adso⅔ption –

Weak

*

Endohed⅔al Capilla⅔y effect method

Table . “dvantage and disadvantage of va⅔ious CNT functionalization ⅔outes [ ].

Commonly, solution method, a combination of dispe⅔sing CNTs in a li⅓uid medium by sonication, mechanical, and magnetic sti⅔⅔ing mixing by the polyme⅔ solution, and finally evapo⅔ating solvents is suggested to p⅔epa⅔e a CNT-based UV-cu⅔able nanocomposite [ , , ].

Figure . Diffe⅔ent functionalized SWCNTs [

].

Carbon Nanotube-Based UV-Curable Nanocomposite Coatings http://dx.doi.org/10.5772/62507

Sonication is conside⅔able step fo⅔ b⅔eaking agg⅔egates and entanglements of CNTs [ , , ]. Homogeneously dispe⅔sed MWCNTs UV-cu⅔able ⅔esin by solution mixing method can be achieved Figure [ ].

Figure . SEM ⅔ift image of the mo⅔phology of “G/MWCNTs UV-cu⅔ed nanocomposite [ ].

. Characterization methods of CNT-based UV-curable nanocomposites To cha⅔acte⅔ize a CNTs/polyme⅔ nanocomposite, both the dispe⅔sion of nanofille⅔s and the inte⅔face between components must be studied. These two facto⅔s have massive influence of the final nanocomposite p⅔ope⅔ties. The mo⅔phology investigation by the mean of dispe⅔sion is achieved by scanning elect⅔on mic⅔oscopy SEM and t⅔ansmission elect⅔on mic⅔oscopy TEM [ , , ]. The photo cu⅔ing p⅔ocess is pe⅔used by photo-diffe⅔ential scanning calo⅔imet⅔y photo-DSC [ , , ] and calculating gel content [ ]. The kinetic of photopolyme⅔ization of nanocom‐ posites is p⅔ovided by ⅔eal-time Fou⅔ie⅔ t⅔ansfo⅔m inf⅔a⅔ed spect⅔oscopy RT-FTIR . Viscoelastic cha⅔acte⅔ization, glass t⅔ansition tempe⅔atu⅔e Tg , Tan delta the value of loss facto⅔ , and damping values of the cu⅔ed nanocomposites a⅔e ca⅔⅔ied out by dynamic me‐ chanical the⅔mal analysis DMT“ and DSC [ , ]. The⅔mal stability of the cu⅔ed nanocomposite and its ha⅔dness can be investigated by the⅔‐ mog⅔avimet⅔y [ ] and nanoindentation [ , ], pencil test [ ], ⅔espectively.

. Properties of UV-cured nanocomposite coatings based on CNT In this section, the p⅔ope⅔ties of CNTs based UV-cu⅔ed nanocomposites a⅔e discussed. “s mentioned befo⅔e, the final p⅔ope⅔ties a⅔e influenced by the dispe⅔sion stated and inte⅔face between CNTs and polyme⅔. Having a well-dispe⅔sed CNTs within the polyme⅔ mat⅔ix ⅔esults

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in good mechanical, physical, elect⅔ical, and the⅔mal p⅔ope⅔ties which individually will be discussed. The addition of these fille⅔ to the polyme⅔ic mat⅔ices, influence the UV-cu⅔ing p⅔ocess, though enhance the final p⅔ope⅔ties of the loaded composite such as conductivity, su⅔face and t⅔ibological p⅔ope⅔ties, the⅔mal and physical-mechanical of the cu⅔ed nanocom‐ posite. . . Dispersion The homogeneity and stability of CNTs in nanocomposites plays a significant ⅔ole in the final p⅔oduct. Figure ⅔ep⅔esents the TEM images of a CNTs/polyme⅔ nanocomposite. It ⅔eveals even afte⅔ sonication, some agglome⅔ates a⅔e p⅔esent. This suggests the use of wetting and dispe⅔sing additives alongside with shea⅔ fo⅔ces [ , ].

Figure . The ⅔emaining agglome⅔ates in TEM images even afte⅔ sonication [ ].

“ good dispe⅔sion of CNTs within the polyme⅔ mat⅔ix Figure mechanical p⅔ope⅔ties of nanocomposite coatings [ ].

inc⅔eases the physical and

Figure . SEM images of CNTs/polyme⅔ nanocomposite indicating good dispe⅔sion and inte⅔action between nanofille⅔ and polyme⅔ mat⅔ix [ ].

Carbon Nanotube-Based UV-Curable Nanocomposite Coatings http://dx.doi.org/10.5772/62507

. . Curing process “s mentioned in Section , the nanofille⅔ may itself abso⅔b the UV light, ⅔esulting to a com‐ petition between photoinitiato⅔ and ⅔einfo⅔cing nanofille⅔ which can led to a less effective cu⅔ing p⅔ocess. Figure illust⅔ates the abso⅔ption spect⅔oscopy ⅔esults of SWCNTs. “s can be seen, CNTs have light abso⅔ption in UV ⅔egion.

Figure . SWCNTs abso⅔ption spect⅔oscopy.

To ove⅔come this d⅔awback, one can use highe⅔ UV light intensities and longe⅔ i⅔⅔adiation times [ , ]. “nothe⅔ app⅔oach to dominate this effect of CNTs. They used a hype⅔-b⅔anched polyme⅔ HP in thei⅔ fo⅔mulation which caused an enhancement in final conve⅔sion. Thei⅔ ⅔esults Figure ⅔eveal by the aid of HP in the CNTs/polyme⅔ nanocomposite, a highe⅔ conve⅔sion in sho⅔te⅔ time can be achieved.

Figure . The effect of hype⅔-b⅔anches polyme⅔ in a CNT-based UV-cu⅔able nanocomposite [

].

Commonly used method fo⅔ investigating cu⅔ing p⅔ocess, kinetic of photopolyme⅔ization, and cu⅔ing behavio⅔ of UV-cu⅔able polyme⅔ is by photo-DSC, RT-FTIR and measu⅔ing the gel content. In photo-DSC analysis, the heat of the photo-initiated polyme⅔ization ⅔eaction is measu⅔ed [ , , ], In RT-FTIR method, the conve⅔sion of active g⅔oups is followed by

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Carbon Nanotubes - Current Progress of their Polymer Composites

monito⅔ing the dec⅔ease in of those g⅔oups peaks in the conve⅔sion cu⅔ves as a function of ⅔adiation time Figure [ ]. In this cu⅔ve, the ⅔ate of polyme⅔ization slope of the initial linea⅔ segment and the final value of conve⅔sion can be d⅔iven. It is pe⅔ceived by using CNTs, both the polyme⅔ization ⅔ate and the final conve⅔sion a⅔e dec⅔eased, but utilization of functionalized CNTs, ⅔ises the Tg of the system due to the fo⅔mation of c⅔osslinked polyme⅔ netwo⅔k, hampe⅔ing the mobility of active g⅔oups while the ⅔eaction p⅔og⅔esses ⅔esult in highe⅔ value of conve⅔sion. This effect is mo⅔e p⅔onounced in UV-cu⅔ed nanocomposite containing oxidized CNTs.

Figure . FTIR conve⅔sion cu⅔ves fo⅔ the neat cationically photocu⅔able epoxy ⅔esin CE and it’s composites at of multi-walled CNTs % CNT and wt% oxidized of multi-walled CNTs % f-CNT content [ ].

wt%

The gel content values of the cu⅔ed coatings can be obtained by the weight loss afte⅔ being ad⅔ift in solvent. Table ⅔ep⅔esents the polyme⅔ization ⅔ate and gel contents of the neat polyme⅔ and the loaded ones. The ⅔esults suggest that the nanosize fille⅔ have g⅔eat abso⅔ption of the UV-light, leading to a st⅔ong shielding effect. It is ⅔evealed that these fille⅔s ⅔educe the gene⅔ation of ⅔eactive types at the initial of UV-induced ⅔eaction [ ]. Sample CE

Polymerization ratea s−

Conversionb %

Conversionc %

Gel content %

.

% CNT

.

% f-CNT

.

Values of the slope at the initial stages of the conve⅔sion-time cu⅔ves. Value of the plateau in the conve⅔sion cu⅔ves with a UV-light intensity of Film thickness of μm. c Dete⅔mined by single spect⅔a taken befo⅔e and afte⅔ min of i⅔⅔adiation at a

b

Table . Conve⅔sion values and gel content of the nanocomposites with

mW/cm and an i⅔⅔adiation time of mW/cm . Film thickness of

wt% loading f⅔action [ ].

μm.

min.

Carbon Nanotube-Based UV-Curable Nanocomposite Coatings http://dx.doi.org/10.5772/62507

Fu⅔the⅔mo⅔e, the addition in content of CNTs, influence the cu⅔ing behavio⅔ of the nanocom‐ posites. The mo⅔e loading, leads to less cu⅔ing conve⅔sion. This can be att⅔ibuted to the competition of UV abso⅔ption between the fille⅔s and photoinitiato⅔s [ ]. This ⅔eduction in final conve⅔sion is shown in Figure .

Figure . Cu⅔ing behavio⅔ of photocu⅔able epoxy ac⅔ylate with diffe⅔ent CNTs content [

].

. . Surface properties Su⅔face p⅔ope⅔ties of a UV-cu⅔ed nanocomposite is divided into su⅔face ha⅔dness and its su⅔face chemist⅔y. CNTs a⅔e unsatu⅔ated systems owning highly mobile elect⅔ons, depending on the polyme⅔ chemist⅔y, the⅔e would be distinct inte⅔actions [ ]. Likewise, the functionalization type and method is ve⅔y impo⅔tant pa⅔amete⅔ to be cont⅔olled.

. . Tribological properties One indication of good dynamic mechanical p⅔ope⅔ty of nanocomposite can be the elevated su⅔face ha⅔dness [ , ]. Su⅔face ha⅔dness can be measu⅔ed by nano-/mic⅔o-indentation and pencil test [ ]. CNTs nanofille⅔s a⅔e known to inc⅔ease this p⅔ope⅔ty. These ⅔esults a⅔e shown in Figure .

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Figure

. a Su⅔face ha⅔dness values and b load vs depth cu⅔ve [ ].

CNTs have self-lub⅔icating p⅔ope⅔ties. In highe⅔ CNTs content in a nanocomposite, agglom‐ e⅔ates a⅔e highe⅔ which inc⅔ease the f⅔iction coefficient. The⅔efo⅔e, the tendency fo⅔ mat⅔ix f⅔agmentation is enhanced, p⅔oducing loose wea⅔ deb⅔is at the contact inte⅔face [ ]. Fig‐ ure ⅔ep⅔esents the f⅔iction coefficient of a UV-cu⅔ed nanocomposite as a function of diffe⅔ent loadings. It is obse⅔ved that wea⅔ ⅔ate is ⅔aised by the inc⅔easing of CNTs content.

Figure . F⅔iction coefficient a and wea⅔ ⅔ate b fo⅔ CNT/polyme⅔ nanocomposite as a function of cu⅔ing time by UV-“ i⅔⅔adiation cycles and h [ ].

. . Conductivity CNTs embedded in polyme⅔ mat⅔ix, a⅔e p⅔ot⅔uded f⅔om the su⅔face, offe⅔ pe⅔manent elect⅔ical, su⅔face and volume conductivity [ ]. In a CNT/polyme⅔ nanocomposite, the c⅔itical fille⅔ content is called the pe⅔colation th⅔eshold. ”y fo⅔mation of a CNTs inte⅔connected t⅔idimen‐ sional netwo⅔k, a low-⅔esistance path fo⅔ the moving cha⅔ge ca⅔⅔ie⅔s is p⅔ovided [ ]. The geomet⅔y of nanofille⅔s plays an impo⅔tant ⅔ole in the fo⅔mation of conductive pathways. Using CNTs would inc⅔ease both elect⅔ical conductivity and dielect⅔ic pe⅔mittivity of the cu⅔ed

Carbon Nanotube-Based UV-Curable Nanocomposite Coatings http://dx.doi.org/10.5772/62507

nanocomposite. This enhancement Figure netwo⅔k in polyme⅔ mat⅔ix [ ].

Figure

is a p⅔oof of the fo⅔mation of a conductive

. “C conductivity and pe⅔mittivity vs f⅔e⅓uency fo⅔ nanocomposites at diffe⅔ent loading [ ].

”y ext⅔apolating the b⅔oadband “C conductivity, it is possible to obtain DC conductivity values Figure . In this figu⅔e, the filled a⅔eas ⅔ep⅔esent the elect⅔ical pe⅔colation ⅔egion fo⅔ each of nanocomposite diffe⅔ing in fille⅔. It ⅔eveals that the most efficient nanofille⅔ is MWCNT [ ].

Figure . Values of the DC elect⅔ical conductivity of nanocomposite as a function of diffe⅔ent nanofille⅔s CNT and oxidized-CNT [ ].

”esides elect⅔ical conductivity, indust⅔y is inte⅔ested in advantages of the⅔mally conductive polyme⅔ nanocomposite. These nanocomposites a⅔e light weight, co⅔⅔osion ⅔esistant and easily p⅔ocessed, which make them bette⅔ candidate than metal pa⅔ts. Using CNTs with its supe⅔io⅔ efficient, the⅔mal conductivity will ove⅔come this demand [ ].

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. . Dynamic mechanical properties In analogy to neat polyme⅔s, filled nanocomposites a⅔e known to have highe⅔ Tg. Figure demonst⅔ates the tan delta cu⅔ves as a function of tempe⅔atu⅔e fo⅔ p⅔epa⅔ed nanocomposites. The inc⅔eased Tg is followed by a shift in the maximum value of tan delta and a ⅔eduction in damping. It can be ⅔evealed that CNTs would withhold the polyme⅔ chain movements led to damping dec⅔ement and highe⅔ Tg. ”y inc⅔easing the CNTs content, a st⅔onge⅔ nanofille⅔/ polyme⅔ is attained, ⅔esulting in highe⅔ maximum value of tan delta and g⅔eate⅔ damping effect due to the covalent banding between the components [ ].

Figure . Tan delta and E’ cu⅔ve obtained by DMT“ analysis fo⅔ nanocomposites containing CNTs and functionalized CNTs f-CNT [ ].

Meanwhile, the changes in the modulus values d⅔iven by DMT“ analysis a⅔e not significant Figure , although a slight inc⅔ease in ⅔ubbe⅔y ⅔egion is detected [ ].

Figure

. Modulus obtained by DMT“ analysis fo⅔ diffe⅔ent nanocomposites containing CNTs and f-CNTs [ ].

It is well known that the addition of CNTs will ⅔aise the sto⅔age modulus and stiffness of the nanocomposite Table Figure [ , ].

Carbon Nanotube-Based UV-Curable Nanocomposite Coatings http://dx.doi.org/10.5772/62507

Samples Neat

Storage modulusa MPa

wt% MWCNT

h

.

wt% MWCNT

h

h

.

wt% MWCNT

h

.

wt% MWCNT

h

Data taken at

a

Tg °C

h

.

Neat

Loss modulusa MPa

°C.

Table . DM“ ⅔esults fo⅔ neat polyme⅔ and nanocomposites [ ].

Figure

. Sto⅔age modulus fo⅔ neat ⅔esin and it’s nanocomposite at diffe⅔ent loadings [ ].

In nanocomposites, CNTs f⅔e⅓uently tu⅔n in thei⅔ di⅔ection within the polyme⅔ mat⅔ix. This could imp⅔ove the mechanical p⅔ope⅔ties, likewise fati⅓ue ⅔esistance o⅔ bending st⅔ength. Even in the case of no imp⅔oving in mechanical p⅔ope⅔ties, the good dispe⅔sion of CNTs would p⅔event any negative imp⅔ession on it [ ].

. Health and safety concerns Using nanocomposites ca⅔⅔ying nanofille⅔s, some conside⅔ation must be count f⅔om fab⅔ica‐ tion of CNTs to the final application of the nanocomposite. The conside⅔ation fo⅔ health and safety of humans and envi⅔onment. The conce⅔n about CNTs is using its powde⅔ fo⅔m and dusts. To p⅔evail this issue, dispe⅔sions of polyme⅔ic wetting and additives of high molecula⅔ weight is suggested [ ]. When embedded in polyme⅔ mat⅔ix, the conce⅔ns a⅔e about ⅔eleasing of CNTs to the envi⅔on‐ ment afte⅔ the polyme⅔ deg⅔adation Figure [ ].

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Figure . Schematic of ⅔eleasing enginee⅔ed nanomate⅔ials ENM pathways f⅔om nanocomposite due to mechanical deg⅔adation o⅔ chemical decomposition of the host mate⅔ials [ ].

Figure

. Conceptual model of enginee⅔ed nanomate⅔ial ENM ⅔eleased f⅔om UV-deg⅔aded polyme⅔ composite [ ].

Carbon Nanotube-Based UV-Curable Nanocomposite Coatings http://dx.doi.org/10.5772/62507

Figure ⅔ep⅔esents the schematic of the UV exposu⅔e of nanocomposite filled CNTs and its ⅔eleasing p⅔ocedu⅔e. Resea⅔che⅔s showed a good dispe⅔sion of CNTs within the polyme⅔ mat⅔ix may dec⅔ease the ⅔eleasing of nanofille⅔ to the envi⅔onment Figure . It can be seen a la⅔ge amount of f⅔agments a⅔e ⅔eleased f⅔om the bad dispe⅔sed mate⅔ial.

Figure . Elect⅔on mic⅔oscope images of CNT/polyme⅔ composite with a good dispe⅔sion and b bad dispe⅔sion. c and d show pa⅔ticles ⅔eleased f⅔om these mate⅔ial [ ].

. Conclusion Filled UV-cu⅔able nanocomposites have gained attentions due to thei⅔ low cost, imp⅔oved p⅔ope⅔ties and being a fast and g⅔een p⅔ocess. “mong nanofille⅔s, ca⅔bon nanotube with thei⅔ uni⅓ue p⅔ope⅔ties as a nanofille⅔ have been widely used. “ challenge in these classes of nanocomposite is the dispe⅔sion of the fille⅔ within the polyme⅔ mat⅔ix. Dispe⅔sion and the inte⅔face between components play an impo⅔tant ⅔ole in the final p⅔ope⅔ties of the cu⅔ed nancomposite. “ well-dispe⅔sed system led to bette⅔ cu⅔ing, enhanced mechanical p⅔ope⅔ties, imp⅔oved the⅔mal, and elect⅔ical behavio⅔. Finally, the deg⅔adation of these nanocomposites should take into account fo⅔ the health of human and envi⅔onment.

Author details Saeed ”astani , * and Masoume Kaviani Da⅔ani *“dd⅔ess all co⅔⅔espondence to bastani@ic⅔c.ac.i⅔ Su⅔face Coatings and Co⅔⅔osion Depa⅔tment, Institute fo⅔ Colo⅔ Science and Technology, Teh⅔an, I⅔an Cente⅔ of Excellence fo⅔ Colo⅔ Science and Technology, Teh⅔an, I⅔an

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] H. Hyung, J. D. Fo⅔tne⅔, J. ”. Hughes, and J.-H. Kim, Natu⅔al o⅔ganic matte⅔ stabilizes ca⅔bon nanotubes in the a⅓ueous phase, Environmental Science & Technology, vol. , pp. – , .

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] H. Chen, O. Jacobs, W. Wu, G. R(dige⅔, and ”. Schädel, Effect of dispe⅔sion method on t⅔ibological p⅔ope⅔ties of ca⅔bon nanotube ⅔einfo⅔ced epoxy ⅔esin composites, Polymer Testing, vol. , pp. – , .

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] Z. Spitalsky, D. Tasis, K. Papagelis, and C. Galiotis, Ca⅔bon nanotube–polyme⅔ composites chemist⅔y, p⅔ocessing, mechanical and elect⅔ical p⅔ope⅔ties, Progress in Polymer Science, vol. , pp. – , .

Carbon Nanotube-Based UV-Curable Nanocomposite Coatings http://dx.doi.org/10.5772/62507

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] X. Gong, J. Liu, S. ”aska⅔an, R. D. Voise, and J. S. Young, Su⅔factant-assisted p⅔ocessing of ca⅔bon nanotube/polyme⅔ composites, Chemistry of Materials, vol. , pp. – , .

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] ”. K⅔ause, T. Villmow, R. ”oldt, M. Mende, G. Petzold, and P. Pötschke, Influence of d⅔y g⅔inding in a ball mill on the length of multiwalled ca⅔bon nanotubes and thei⅔ dispe⅔sion and pe⅔colation behaviou⅔ in melt mixed polyca⅔bonate composites, Composites Science and Technology, vol. , pp. – , .

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] S. Campidelli, C. Klumpp, “. ”ianco, D. M. Guldi, and M. P⅔ato, Functionalization of CNT synthesis and applications in photovoltaics and biology, Journal of Physical Organic Chemistry, vol. , pp. – , .

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] S. Yellampalli, Carbon Nanotubes–Polymer Nanocomposites, InTech, Rijeka, C⅔oatia, .

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] M. M. R. Nayini, S. ”astani, and Z. Ranjba⅔, Synthesis and cha⅔acte⅔ization of func‐ tionalized ca⅔bon nanotubes with diffe⅔ent wetting behavio⅔s and thei⅔ influence on the wetting p⅔ope⅔ties of ca⅔bon nanotubes/polymethylmethac⅔ylate coatings, Progress in Organic Coatings, vol. , pp. – , .

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] N. Ma⅔tin and J.-F. Nie⅔enga⅔ten, Supramolecular Chemistry of Fullerenes and Carbon Nanotubes. John Wiley & Sons, .

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] P. Ka⅔da⅔, M. Eb⅔ahimi, and S. ”astani, Influence of tempe⅔atu⅔e and light intensity on the photocu⅔ing p⅔ocess and kinetics pa⅔amete⅔s of a pigmented UV cu⅔able system, Journal of Thermal Analysis and Calorimetry, vol. , pp. – , .

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] F. Mi⅔shahi, Investigation of nano-hype⅔ b⅔anched polyme⅔ on final p⅔ope⅔ties of UVcu⅔able nanocomposites containing g⅔aphene and CNT, .

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] T. V. Duncan, Release of enginee⅔ed nanomate⅔ials f⅔om polyme⅔ nanocomposites the effect of mat⅔ix deg⅔adation, ACS Applied Materials & Interfaces, vol. , pp. – , .

Chapter 11

Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range Marta González, Guillermo Mokry, María de Nicolás, Juan Baselga and Javier Pozuelo Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62508

Abstract Following the development of the new elect⅔onic systems and communication netwo⅔ks, the levels of elect⅔omagnetic contamination have ⅔isen d⅔amatically in the ⅔ecent yea⅔s. Eve⅔y day, new studies appea⅔ sea⅔ching fo⅔ a way to mitigate the elect⅔omagnetic inte⅔fe⅔ences EMI . “t the same time, the ⅔apid evolution of technol‐ ogy fo⅔ces the field to sea⅔ch fo⅔ lighte⅔ and mo⅔e efficient mate⅔ials. The composites using ca⅔bon allot⅔opes such as ca⅔bon nanotubes and polyme⅔s as ⅔einfo⅔cement a⅔e gaining impo⅔tance, due to the many advantages they exhibit in compa⅔ison to the mate⅔ials that we⅔e used until now. “ g⅔eat numbe⅔ of applications ⅔e⅓ui⅔e abso⅔p‐ tion to be the main elect⅔omagnetic shielding mechanism, the⅔eby making this ⅔eview necessa⅔y as a way to summa⅔ize the latest studies on CNT/polyme⅔ composites and how to imp⅔ove the abso⅔ption mechanism by changing the mo⅔phology and compo‐ sition of CNTs. Keywords: Ca⅔bon nanotube, Elect⅔omagnetic shielding mate⅔ials, Elect⅔omagnetic cha⅔acte⅔ization, Nanocomposite, Elect⅔omagnetic abso⅔be⅔

. Introduction Elect⅔omagnetic inte⅔fe⅔ences EMIs occu⅔ when elect⅔omagnetic signals a⅔e unintentionally t⅔ansmitted f⅔om an emitte⅔ to anothe⅔ element by ⅔adiation and/o⅔ conduction, causing it to behave in an unexpected way [ ]. The p⅔oblem a⅔ises f⅔om high-f⅔e⅓uency signals coupled to the main one that will ⅔adiate as they a⅔e conducted th⅔ough and along the powe⅔ wi⅔e. The wi⅔e will behave as an antenna that will pick up othe⅔ signals and t⅔ansfe⅔ them to the ci⅔cuit

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elements of the device [ ]. EMIs, though pa⅔t of ou⅔ mode⅔n wo⅔ld, affect all elect⅔ical and elect⅔onic devices, which a⅔e mo⅔e indispensable eve⅔yday fo⅔ the society. EMI shielding is thus needed to supp⅔ess o⅔ attenuate elect⅔omagnetic ⅔adiation f⅔om emitte⅔s with mate⅔ials that a⅔e able to inte⅔act with those signals. Thus, one of the p⅔esent needs is to find b⅔oad-band shields, able to neut⅔alize elect⅔omagnetic ⅔adiation in the GHz ⅔ange. This ⅔e⅓ui⅔ement a⅔ises f⅔om the fast development of elect⅔onics, which has led to mic⅔op⅔ocesso⅔s with g⅔eatly enhanced data t⅔ansfe⅔ speeds that ope⅔ate at highe⅔ f⅔e⅓uencies. Fu⅔the⅔mo⅔e, the miniatu⅔ization and manufactu⅔ing of such components demand high-pe⅔fo⅔mance and lightweight mate⅔ials. Due to the wide impact of telecommunications, seve⅔al financially st⅔ong indust⅔ial secto⅔s a⅔e expectant about the p⅔og⅔ess of EMI shielding mate⅔ials. These technological fields demand not only efficient shields but also mate⅔ials that meet specific c⅔ite⅔ia fo⅔ each enginee⅔ed design. Fo⅔ example, chemical and co⅔⅔osion ⅔esistance, lightweight, flexibility, tuneable mo⅔pholo‐ gy, p⅔ocessing easiness and inexpensiveness a⅔e ⅔e⅓ui⅔ements that the mate⅔ials must fulfil in o⅔de⅔ to be applicable in flexible elect⅔onics e.g., pe⅔sonal compute⅔s and mobile phones , ae⅔ospace e.g., satellite and ai⅔c⅔aft´s manufactu⅔e and automotive e.g., integ⅔ated ci⅔cuits indust⅔ies [ ]. In the field of ae⅔ospace enginee⅔ing, a challenging secto⅔ is focused on milita⅔y stealth. Fo⅔ this specific pu⅔pose, ⅔ada⅔-abso⅔bing mate⅔ials a⅔e needed. Much effo⅔t has been done in this di⅔ection although abso⅔ption of elect⅔omagnetic ene⅔gy ⅔emains challenging, due to the need of accomplishing both low-⅔eflection and high-abso⅔ption losses [ ]. The explo⅔ation of new functional mate⅔ials that enable to effectively block o⅔ filte⅔ b⅔oad‐ band elect⅔omagnetic ene⅔gy is an active field of ⅔esea⅔ch nowadays. Polyme⅔s offe⅔ seve⅔al advantages ove⅔ t⅔aditional metals and ce⅔amics used fo⅔ EMI shielding. They can be easily shaped it is possible to p⅔epa⅔e a va⅔iety of configu⅔ations and fo⅔mulations and they a⅔e substantially lighte⅔. “lthough polyme⅔s a⅔e elect⅔omagnetically t⅔anspa⅔ent, diffe⅔ent st⅔at‐ egies a⅔e available to conve⅔t them into active elect⅔omagnetic shields. G⅔aphite, ca⅔bon black and ca⅔bon fib⅔es we⅔e the fi⅔st to be combined with polyme⅔s fo⅔ the fab⅔ication of EMI shields [ – ]. The attention soon shifted to nanoca⅔bons, since with lowe⅔-weight f⅔ac‐ tions, mo⅔e conductive composites could be obtained. In addition, to enable the enti⅔e c⅔osssection of a fille⅔ to be active in shielding, the dimensions of the conductive fille⅔ should be less than the length of penet⅔ation of the incident ⅔adiation [ ]. This penet⅔ation length is usually ve⅔y small fo⅔ good conducto⅔s at high f⅔e⅓uencies. The⅔eby, nanoca⅔bon fille⅔s a⅔e ade⅓uate to shield in the GHz ⅔ange. In this context, ca⅔bon nanofib⅔es, nanotubes and g⅔a‐ phene, which have highe⅔ specific su⅔face a⅔ea and aspect ⅔atio than thei⅔ mic⅔oscale ana‐ logues, a⅔e p⅔omising candidates fo⅔ the p⅔epa⅔ation of efficient EMI shielding composites. Despite having been studied extensively in ⅔ecent yea⅔s, seve⅔al facto⅔s affecting the pe⅔‐ fo⅔mance of nanoca⅔bon/polyme⅔ composites as high-f⅔e⅓uency elect⅔omagnetic shields ⅔e‐ main unexplo⅔ed o⅔ not completely unde⅔stood up to date. Indeed, the⅔e a⅔e few ⅔epo⅔ted ⅔eviews on this application of nanoca⅔bons [ – ], which show the need fo⅔ new and ⅔ep⅔o‐ ducible expe⅔imental data to facilitate the const⅔uction of ⅔ealistic models to imp⅔ove ou⅔ unde⅔standing.

Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

. Electromagnetic shielding mechanism When an elect⅔omagnetic wave EI impacts on a mate⅔ial Figure , two waves a⅔e c⅔eated on the su⅔face a ⅔eflected wave ER and a t⅔ansmitted wave into the mate⅔ial EI−R . Inside the mate⅔ial, a f⅔action of the wave EI−R may be dissipated as heat EA until it ⅔eaches the second su⅔face of the mate⅔ial. “t this point, two new waves appea⅔ one that is t⅔ansmitted th⅔ough the su⅔face ET and a new wave that is ⅔eflected into the mate⅔ial. This p⅔ocess is ⅔epeated successively until it meets the c⅔ite⅔ia stated in e⅓uation EI = åER + åE A + åET

Figure . Mechanisms of attenuation of the incident EM powe⅔ when it st⅔ikes a finite-dimensional media.

”oth in the ⅔eflection and t⅔ansmission p⅔ocesses, waves gene⅔ated at each step may cause const⅔uctive and dest⅔uctive inte⅔fe⅔ences depending on the sample thickness and f⅔e⅓uency. The ⅔eflection p⅔ocess on each plane of the mate⅔ial is what is called multiple ⅔eflections. The⅔efo⅔e, the elect⅔omagnetic shielding efficiency, SE, of a mate⅔ial can be ⅓uantified as the sum of th⅔ee cont⅔ibutions ⅔eflection, abso⅔ption and multiple ⅔eflections. SE = SER + SE A + SEMR The elect⅔omagnetic shielding efficiency can be exp⅔essed as a function of impedance mismatch between the medium and the mate⅔ial η and η , sample thickness d and skin thickness δ æ h ö æ æ æ d öö æ 2d ö ö SE = 20log ç ÷ + 20log ç exp ç ÷ ÷ + 20log ç1 - exp ç ÷÷ è d øø è d øø è è è 4h0 ø

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whe⅔e the skin thickness is defined as the depth at which the field dec⅔eases to /e f⅔om its initial value and is a function of f⅔e⅓uency f , pe⅔meability μ and conductivity σ

d = (p f ms )

-1/ 2

The impedance can be exp⅔essed as the ⅔elation between the elect⅔ic and the magnetic fields, whe⅔e j accounts fo⅔ an imagina⅔y value and ε fo⅔ the complex pe⅔mittivity

h=

æ j 2p f m ö =ç ÷ H è s + j 2p f e ø E

1/ 2

If the conductivity of the p⅔opagation medium is ze⅔o σ =  and the conductivity in the con‐ ductive mate⅔ial is σ ≫ πfε , e⅓uation can be simplified as æm ö h0 = ç 0 ÷ è e0 ø

1/ 2

æ 2p f m ö and h0 = ç ÷ è s ø

1/ 2

whe⅔e the index stands fo⅔ the values in f⅔ee space. Multiple ⅔eflections can be neglected when the sample thickness is bigge⅔ than the skin thickness. In this case, the abso⅔bed ⅔adiation is high enough so that those const⅔uctive and dest⅔uctive inte⅔fe⅔ences cannot be p⅔oduced, and only two mechanisms a⅔e possible in the elect⅔omagnetic shielding p⅔ocess ⅔eflection and abso⅔ption. Taking this into account, and using e⅓uations and , the elect⅔omagnetic shielding e⅓uation can be exp⅔essed as é æ s öù é 1/ 2 SE = ê39.5 + 10log ç ÷ ú + ë8.7 d (p f ms ) ûù 2 f p m è øû ë Elect⅔omagnetic shielding can be dete⅔mined f⅔om the ⅔eflection R , abso⅔ption A and t⅔ansmission T coefficients as a function of the ⅔eflected, t⅔ansmitted and incident powe⅔s PR, PT and PI, ⅔espectively , and the⅔efo⅔e R=

PR P ;T = T ; A = 1 - ( R + T ) PI PI

In this way, the elect⅔omagnetic shielding can be exp⅔essed as

Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

æ1ö æ 1 ö æ1- R ö SE = 10log ç ÷ ; SER = 10log ç ÷ ; SE A = 10log ç ÷; T 1 R è ø è ø è T ø

. Measurement techniques With a Netwo⅔k “nalyse⅔, it is possible to study the p⅔ope⅔ties of elect⅔ical netwo⅔ks, especially those associated with ⅔eflection and t⅔ansmission of elect⅔ical signals, known as scatte⅔ing pa⅔amete⅔s, f⅔om a few MHz to GHz. “ two-po⅔ts netwo⅔k analyse⅔ emits elect⅔omagnetic ⅔adiation I in the ⅔e⅓ui⅔ed f⅔e⅓uency ⅔ange f⅔om each side, and analyses the ⅔eflected R and t⅔ansmitted ⅔adiation T th⅔ough the studied mate⅔ial. The th⅔ee most common configu⅔ations fo⅔ measu⅔ing solid samples a⅔e waveguide, coaxial line and f⅔ee space a⅔⅔angements Figure .

Figure . Configu⅔ations fo⅔ EMI measu⅔ing solid samples.

Waveguide. Usually, it has a ⅔ectangula⅔ section. The sample is int⅔oduced at a p⅔ecise distance f⅔om the waveguide end. The size of the waveguide depends on the f⅔e⅓uency ⅔ange fo⅔ which it is p⅔epa⅔ed, and size dec⅔eases with inc⅔easing f⅔e⅓uencies. The advantage of this system is the easiness of the sample p⅔epa⅔ation. The disadvantage is the na⅔⅔ow ⅔ange of f⅔e⅓uencies that can be measu⅔ed. This causes that seve⅔al waveguides a⅔e necessa⅔y to measu⅔e at highf⅔e⅓uency ⅔anges. Coaxial line. The sample is int⅔oduced between the inne⅔ and the oute⅔ conducto⅔s at an exact distance f⅔om the ends, and must be p⅔epa⅔ed as a ⅔ectangula⅔ to⅔oid. Fo⅔ example, a coaxial line fo⅔ measu⅔ing f⅔e⅓uencies f⅔om . to GHz has an inne⅔ conducto⅔ of . mm and a . mm exte⅔nal conducto⅔. The main advantage is that it is possible to measu⅔e at highf⅔e⅓uency ⅔anges on the same sample.

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Free Space. It is a non-contact measu⅔ement based on two opposing antennae, flanking the sample. With this system, it is possible to measu⅔e a wide ⅔ange of f⅔e⅓uencies and to modify the angle of incidence of ⅔adiation on the samples. The most impo⅔tant disadvantage of this system is that the size of the samples must be conside⅔ably bigge⅔ than the p⅔evious techni⅓ues Dimension ⅔ange is f⅔om – cm fo⅔ a few GHz to – cm fo⅔ f⅔e⅓uencies nea⅔ GHz. With a two-po⅔ts netwo⅔k analyse⅔, the voltage ⅔atio between the gene⅔ated and the ⅔etu⅔ning elect⅔omagnetic waves fo⅔ both po⅔ts is obtained Figure .

Figure . Voltage gene⅔ated and ⅔etu⅔ning in a two-po⅔ts netwo⅔k analyse⅔.

whe⅔e Va yVa a⅔e voltages that come out of po⅔ts and , ⅔espectively, and Vb yVb a⅔e voltages that ⅔etu⅔n to po⅔ts and , ⅔espectively. These voltages a⅔e ⅔elated to the scatte⅔ing pa⅔amete⅔s as æ Vb1 ö æ S11 ç ÷=ç è Vb 2 ø è S 21

S12 ö æ Va1 ö ÷ç ÷ S 22 ø è Va 2 ø

The⅔efo⅔e, Vb1 = S11Va1 + S12Va 2 and Vb 2 = S 21Va1 + S 22Va 2 “cco⅔ding to the maximum t⅔ansfe⅔ theo⅔em, if the opposite po⅔t is cha⅔ged with an identical voltage to the system impedance, then scatte⅔ing pa⅔amete⅔s will be S11 =

Vb1 V V V ; S 22 = b 2 ; S12 = b1 ; S 21 = b 2 Va1 Va 2 Va 2 Va1

If the mate⅔ial is homogeneous, then S = S and S = S . The⅔efo⅔e, S is the ⅔eflection p⅔oduced in the sample and S is the t⅔ansmission th⅔ough the sample. The coefficients of ⅔eflection, abso⅔ption and t⅔ansmission powe⅔s will the⅔efo⅔e be exp⅔essed as

Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

R=

PR 2 = s11 PI

T=

PT 2 = s21 PI

(

A = 1 - ( R - T ) = 1 - S11 + S 21 2

2

)

“nd the⅔efo⅔e the shielding effectiveness is æ 1 ö æ1ö ÷ SET = 10log ç ÷ = 10log ç ç s 2÷ èT ø è 21 ø

æ 1 ö æ 1 ö ÷ SER = 10log ç ÷ = 10log çç 2 ÷ è1- R ø è 1 - s11 ø

æ 1 - s11 2 ö æ1- R ö ç ÷ = SE A = 10log ç 10 log ÷ ç s 2 ÷ è T ø è 21 ø

Shielding effectiveness gives info⅔mation about the p⅔efe⅔⅔ed deactivation mechanism, while coefficients p⅔ovide info⅔mation on the f⅔actions of the ⅔adiation lost by ⅔eflection and abso⅔ption, when multiple ⅔eflections a⅔e neglected. Multiple ⅔eflections p⅔oduced by the inte⅔fe⅔ence of the ⅔eflected ⅔adiation in the fi⅔st plane of incidence, and ⅔eflection p⅔oduced in the last plane of the mate⅔ial, o⅔iginate const⅔uctive and dest⅔uctive inte⅔fe⅔ence dependant on the f⅔e⅓uency. “cco⅔ding to the Schelkunoff’s theo⅔y [ – ], multiple ⅔eflections can be neglected when the sample thickness is la⅔ge⅔ than the skin depth, as mentioned above. Figure shows the skin depth as a function of f⅔e⅓uency fo⅔ samples with conductivities of , , and S.m− . In samples with a thickness of mm, multiple ⅔eflections may be negligible above GHz. In Figure b, the effect of multiple ⅔eflections on measu⅔ements of two diffe⅔ent samples with the same thickness is shown. We can obse⅔ve that the ⅔ed sample gives a signal close to GHz while the black sample gives th⅔ee signals close to , and GHz, mainly due to the lowe⅔ conductivity of the black sample. In o⅔de⅔ to avoid the sample

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thickness causing misleading ⅔eflections due to multiple ⅔eflections, the minimum f⅔e⅓uency fo⅔ a shielding study must be GHz fo⅔ the sample in ⅔ed and GHz fo⅔ the sample in black.

Figure . a Skin depth as a function of f⅔e⅓uency fo⅔ samples with diffe⅔ent conductivities. b Effect of multiple ⅔e‐ flections of two samples with the same thickness and diffe⅔ent conductivity. Colo⅔ed ⅔ectangles show the f⅔e⅓uency ⅔ange whe⅔e multiple ⅔eflections a⅔e negligible.

. Electrical properties of CNT composites Elect⅔omagnetic shielding is closely ⅔elated to the elect⅔ical p⅔ope⅔ties of the composite. “lthough seve⅔al conducting pa⅔ticles have been studied, such as ca⅔bon black pa⅔ticles [ ], ca⅔bon fib⅔es [ ] o⅔ metallic fille⅔s [ ], CNTs have clea⅔ly demonst⅔ated bette⅔ p⅔ope⅔ties due to thei⅔ high aspect ⅔atio L/d , highe⅔ st⅔ength and flexibility and lowe⅔ density, making them ideal as fille⅔s. The⅔e is a point, depending on the fille⅔ concent⅔ation in the composite, at which a conductive path is fo⅔med. This point has also been defined as the least concent⅔ation at which a composite with conductive inclusions is capable of conducting di⅔ect cu⅔⅔ent. This is known as pe⅔co‐ lation th⅔eshold, the point at which conductivity inc⅔eases g⅔eatly. “lthough a b⅔oad ⅔ange of polyme⅔s has been employed, the⅔e is a clea⅔ t⅔end showing that with ve⅔y small ⅓uantities of ca⅔bon nanotubes, it is possible to achieve the ⅔e⅓ui⅔ed pe⅔colation th⅔eshold. ”elow the pe⅔colation th⅔eshold, polyme⅔s behave as insulato⅔s fo⅔ which conductivity, that inc⅔ease with tempe⅔atu⅔e, is gene⅔ally dete⅔mined by the⅔mally assisted hopping o⅔ cha⅔ge tunnelling between the cha⅔ged pa⅔ticles [ ]. On the othe⅔ hand, when the pe⅔colation th⅔eshold is ⅔eached, conduction th⅔ough the polyme⅔ occu⅔s. “ schematic diag⅔am of how pe⅔colation th⅔eshold occu⅔s is shown in Figure . Nume⅔ous studies have shown that the pe⅔colation th⅔eshold depends st⅔ongly on the polyme⅔ type, synthesis method used, aspect ⅔atio of the CNT, how agglome⅔ated these CNTs a⅔e in the polyme⅔ mat⅔ix and thei⅔ deg⅔ee of alignment [ ].

Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

Figure . Schematic va⅔iation of elect⅔ic conductivity with fille⅔ volume f⅔action fo⅔ conductive nanocomposites.

The conductivity in these composites can have diffe⅔ent values depending on the kind of conductivity that is measu⅔ed. Conductivity can be measu⅔ed eithe⅔ by di⅔ect cu⅔⅔ent meas‐ u⅔ements DC o⅔ by alte⅔nating cu⅔⅔ents “C . Dete⅔mining the conductivity of a composite with di⅔ect cu⅔⅔ent is ⅓uite st⅔aight fo⅔wa⅔d, and the⅔e a⅔e seve⅔al paths to achieve this. Fo⅔ a sample with length L and c⅔oss-section A, conductivity is given by

s DC =

L AR

whe⅔e R is the ⅔esistance which is measu⅔ed applying a di⅔ect cu⅔⅔ent and measu⅔ing a value fo⅔ the voltage th⅔ough the use of Ohm’s Law. ”ut this is not the only method available to calculate DC conductivity. The DC conductivity of the composites can also be dete⅔mined f⅔om the f⅔e⅓uency dependency of the “C conductivity g⅔aphs, as the value obtained fo⅔ low f⅔e⅓uencies whe⅔e the conductivity is independent of the f⅔e⅓uency used. The value of this plateau is the value of the DC conductivity. If the DC conductivity is known, then the pe⅔co‐ lation th⅔eshold can be easily calculated by using the known pe⅔colation theo⅔y, which defines an insulato⅔-conducto⅔ t⅔ansition and is stated as σDC =σ p - pc t , whe⅔e σ is a scaling facto⅔, p

is the CNT weight f⅔action, pc is the pe⅔colation c⅔itical concent⅔ation and t is a c⅔itical exponent that gove⅔ns the scaling law when close to the pe⅔colation th⅔eshold. This c⅔itical exponent is expected to depend on the system dimensionality with calculated values of . fo⅔ two dimensions and a value of fo⅔ th⅔ee dimensions [ ]. The data can then be fitted to the e⅓uation in o⅔de⅔ to obtain the best-fitting values fo⅔ pe⅔colation th⅔eshold and c⅔itical exponent fo⅔ each case [ ]. “C conductivity can be calculated as

305

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Carbon Nanotubes - Current Progress of their Polymer Composites

s ac = 2p f e 0e '' whe⅔e f is the f⅔e⅓uency at which the measu⅔ement is done, ε is the dielect⅔ic constant in f⅔ee space, εr is the imagina⅔y pa⅔t of the complex pe⅔mittivity. It is the⅔efo⅔e obvious that this conductivity depends on the f⅔e⅓uency used. This is the ⅔eason why DC conductivity ⅔emains as a constant value when a sweep in f⅔e⅓uency is done, whilst “C conductivity va⅔ies. To unde⅔stand fully how the changes in cu⅔⅔ent affects the medium, it is impo⅔tant to notice that ca⅔bon nanotubes in a dielect⅔ic medium can be compa⅔ed to many capacito⅔s inside the composite. The pe⅔mittivity of a medium ⅔ep⅔esents how much flux o⅔ elect⅔ic field is gene⅔ated pe⅔ unit cha⅔ge in the medium. Mo⅔e elect⅔ic flux is gene⅔ated in a medium with lowe⅔ pe⅔mittivity due to pola⅔ization effects. “n efficient dielect⅔ic suppo⅔ts a va⅔ying cha⅔ge with minimal dissipation of ene⅔gy in the fo⅔m of heat. The ⅔eal pa⅔t of the complex pe⅔mittivity ε′  is ⅔elated to the sto⅔ed ene⅔gy in the capacito⅔ when a pola⅔ization in the dielect⅔ic occu⅔s. It can be explained as the ability of a mate⅔ial to pola⅔ize as a ⅔esponse to an applied elect⅔ic field. So,  the g⅔eate⅔ the pola⅔ization developed by a mate⅔ial in an applied elect⅔ic field, the g⅔eate⅔ the value of the dielect⅔ic constant. Howeve⅔, in the p⅔esence of an alte⅔nating cu⅔⅔ent, this pola⅔ization is diffe⅔ent. “s the alte⅔nating cu⅔⅔ent switches, the di⅔ection of the field also switches and the pola⅔ization of the dielect⅔ic needs to change in o⅔de⅔ to align with the new di⅔ection. Fo⅔ this new o⅔ientation to occu⅔, a small time known as ⅔elaxation time is needed and this value is usually close to – seconds. Howeve⅔, if the f⅔e⅓uency of the “C cu⅔⅔ent is highe⅔ than the ⅔elaxation time, the o⅔ientation of the dipole would not be able to keep up with this change and hence will cease to cont⅔ibute to the pola⅔ization of the dielect⅔ic.  The⅔efo⅔e, as the f⅔e⅓uency of the “C cu⅔⅔ent is inc⅔eased, the value of the dielect⅔ic constant d⅔ops, until it becomes e⅓ual to , which is the same as the dielect⅔ic constant of vacuum.  Dielect⅔ics usually loose ene⅔gy th⅔ough two main mechanisms and this is known as the dielect⅔ic loss pa⅔amete⅔  ε′ ′ in conduction loss, a flow of cha⅔ge th⅔ough the mate⅔ial causes ene⅔gy dissipation, and the dielect⅔ic loss usually occu⅔s due to the movement of the cha⅔ges in an alte⅔nating magnetic field as pola⅔ization switches di⅔ection. Dielect⅔ic loss is usually highest a⅔ound the ⅔elaxation o⅔ ⅔esonance f⅔e⅓uencies of the pola⅔ization mechanisms as the pola⅔ization sta⅔ts to lag behind the applied field, which causes inte⅔actions between the field and the dielect⅔ic pola⅔ization that ⅔esult in heating. This must be taken into account when calculating the “C conductivity, and thus the ⅔eason why it appea⅔s in the fo⅔mula mentioned above [ , ].

. Electromagnetic shielding of CNT composites “ wide ⅔ange of values fo⅔ EMI SE and conductivities have been ⅔epo⅔ted in the lite⅔atu⅔e ove⅔ the past few decades, depending g⅔eatly on the p⅔ocessing method, polyme⅔ mat⅔ix and ca⅔bon nanotube type Table . This pa⅔t of the chapte⅔ is designed to be a comp⅔ehensive sou⅔ce fo⅔ polyme⅔ composite ⅔esea⅔ch including fundamental ⅔elationships between the CNT st⅔uctu⅔e, pe⅔colation th⅔eshold values, p⅔ocessing techni⅓ues, EMI SE and conductivity values. The

Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

⅔esults in most cases show an enhancement of the elect⅔ical conductivity by seve⅔al o⅔de⅔s of magnitude, with the addition of CNT to the mat⅔ix. Matrix

CNT content

Thickness mm

σ

SET dB

S/m PU

wt %

PU

wt% MWCNTs

PU

.

wt.% SWCNTs wt% MWCNT

.

PU/PEDOT

wt% MWCNT

.

PU

wt%

Cellulose

. wt %

Cellulose

.

PS P“N



> .

vol% wt%

wt%

PMM“

wt% MWCNT

.

[

]

X-band

[

]

X-band

[

]

X-band

[

]

[

]

[

]

[

]

. X-band

. .

– – .



[

]

N/“

.



[

]

N/“

.

. –

[

]

[

,

.

.

Reference

GHz

.

PU

Frequency

.

– .

.



.

PMM“

vol% MWCNT

.

X-band

[

]

Epoxy

wt% SWCNT

.

X-band

[

]

PC

wt% MWCNT

X-band

[

]

PTT

wt% MWCNT

Ku-band

[

]

X-band

[

]

[

]



[

]

. – .

[

]



[

]

[

]

PVDF

. wt%

UHMWPE

wt%

PEDOT

wt%

HDPE

wt% MWCNT

PE

wt%

PCL

.

vol%

. – .

.

– .

.

. .

.

X-band

]

Table . Elect⅔omagnetic shielding of some CNT polyme⅔ composites.

“s a gene⅔al t⅔end, it can be obse⅔ved that the addition of ca⅔bon nanotubes inc⅔eases the conductivity and elect⅔omagnetic shielding efficiency in the composites at the same f⅔e⅓uency value. Jia et al. [ ] stated that to obtain d” the minimum comme⅔cially acceptable EMI SE , a minimum conductivity of at least S.m− is necessa⅔y. It is wo⅔th mentioning that the thickness of the sample is an impo⅔tant facto⅔ to conside⅔ [ ], as thicke⅔ samples would display highe⅔ EMI SE, as shown in e⅓uation . “cco⅔ding to Pande et al. [ ], multiple laminated composites exhibit highe⅔ abso⅔ption capacities than bulk composites, due to inte⅔nal ⅔eflec‐ tions taking place between the bounda⅔ies of each laye⅔. This effect can be compa⅔ed to the effect found on Salisbu⅔y sc⅔eens.

307

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Carbon Nanotubes - Current Progress of their Polymer Composites

EMI SE and conductivity a⅔e affected g⅔eatly by the p⅔ocessing techni⅓ues used. “l-Saleh et al. [ ] compa⅔es va⅔ious p⅔ocessing techni⅓ues and concludes that the best p⅔ocessing method in o⅔de⅔ to obtain the best EMI SE is by melt mixing [ ]. This techni⅓ue is bette⅔ than solution p⅔ocessing [ ], and at the same time solution p⅔ocessing is bette⅔ than wet mixing [ ]. When good EMI SE p⅔ope⅔ties a⅔e ⅔e⅓ui⅔ed, melt mixing is the main techni⅓ue used due to seve⅔al facto⅔s fi⅔st, the dispe⅔sion of the ca⅔bon nanotubes obtained with this techni⅓ue is much bette⅔ than the one obtained with othe⅔ techni⅓ues. Secondly, the nanotubes do not b⅔eak with melt mixing, unlike with techni⅓ues such as ball milling o⅔ co-p⅔ecipitation, which gives the composite a bette⅔ conductivity ove⅔all and the⅔efo⅔e enhances the elect⅔omagnetic shielding p⅔ope⅔ties. Ultimately, melt mixing does not use any solvents, making this techni⅓ue envi⅔onmental-f⅔iendly unlike in-situ polyme⅔ization o⅔ solvent casting [ ]. Some studies have shown that the pe⅔colation th⅔eshold depends st⅔ongly on the alignment and aspect ⅔atio of the fille⅔ used. On this ⅔ega⅔d, ⅔od-like fille⅔s such as ca⅔bon nanotubes exhibit highe⅔ aspect ⅔atios than sphe⅔ical pa⅔ticles, allowing the pe⅔colation th⅔eshold to be achieved with lowe⅔ fille⅔ concent⅔ation. If the fille⅔s stick togethe⅔, in a p⅔ocess known as agglome⅔ation, the pe⅔colation th⅔eshold ⅔ises due to the aspect ⅔atio being smalle⅔ as com‐ pa⅔ed to well-dispe⅔sed fille⅔s. “gain, continuing with the impo⅔tance of aspect ⅔atio, Gupta et al. [ ] and Huang et al. [ ] demonst⅔ated that the longe⅔ the CNTs the bette⅔ the conduc‐ tivity and the⅔efo⅔e the elect⅔omagnetic shielding efficiency. Qin et al. [ ] and Du et al. [ ] p⅔oved that a seconda⅔y mechanism affecting the pe⅔colation th⅔eshold exists alignment pe⅔colation. It was shown that aligned fille⅔s tend to have highe⅔ pe⅔colation th⅔esholds than anisot⅔opic o⅔iented fille⅔s. This is due to the p⅔obability of a conductive path fo⅔ming in the mat⅔ix being highe⅔, than in o⅔iented fille⅔ composites. “lso, aligned fille⅔s have wo⅔se conductivity, as fewe⅔ contacts between the tubes exist [ ]. The type of ca⅔bon nanotube used, being single-walled ca⅔bon nanotubes SWCNT and multiwalled ca⅔bon nanotubes MWCNT the main types, also seems to have an effect on the EMI SE pa⅔amete⅔s. Compa⅔ing SWCNTs to MWCNTs, it is possible to notice the g⅔eate⅔ numbe⅔ of defects p⅔esent in the MWCNT, which also causes highe⅔ pe⅔mittivity. In a se⅔ies of pape⅔s, it has been studied that the defects and impu⅔ities of CNTs have become impediment to p⅔ope⅔ly desc⅔ibe the pola⅔ization mechanisms and ohmic losses of the composite mate⅔ials [ ]. This makes MWCNT have highe⅔ EMI SE, with the main shielding mechanism being abso⅔ption. Howeve⅔, othe⅔ ⅔efe⅔ences analyse the diffe⅔ences in conductivity caused by the type of CNT used, and it seems that the impact on conductivity depending on CNT type is ve⅔y small [ ]. It is widely accepted that alte⅔ing the ca⅔bon nanotube st⅔uctu⅔e by functionalizing them dis⅔upts the conductivity of isolated nanotubes. Howeve⅔, the⅔e a⅔e exceptions to this ⅔ule, as demonst⅔ated by Tambu⅔⅔i et al. [ ] whe⅔e functionalizing the SWCNT with ca⅔boxylic g⅔oups enhanced the conductivity in the composites by a facto⅔ of , as compa⅔ed with the unfunctionalized nanotubes which only showed an inc⅔ease of times the conductivity. This functionalization also imp⅔oved the dispe⅔sion in the mat⅔ix, outweighing the advantages as compa⅔ed to unt⅔eated ca⅔bon nanotubes. “pa⅔t f⅔om good EMI SE, composites must fulfil some impo⅔tant ⅔e⅓ui⅔ements, such as enhanced mechanical p⅔ope⅔ties [ ], light weight [ ],

Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

good p⅔ocessability [ – ], low cost [ ] and envi⅔onmental-f⅔iendly [ , ]. This is the ⅔eason why seve⅔al polyme⅔ mat⅔ices have been studied in o⅔de⅔ to suit each and any of the desi⅔ed applications, in pa⅔ticula⅔ EMI SE. Dielect⅔ic p⅔ope⅔ties of the polyme⅔ used a⅔e impo⅔tant, as polyme⅔s with high pe⅔mittivity ⅔esult in highe⅔ conductivities. The main objective is to sea⅔ch fo⅔ conductive polyme⅔s such as PEDOT , in o⅔de⅔ to block the EM ⅔adiation. Howeve⅔, some polyme⅔s a⅔e not conductive and alte⅔natives fo⅔ achieving high conductivities a⅔e sea⅔ched. The⅔e a⅔e two main catego⅔ies of the polyme⅔s used to achieve conductive composites int⅔insically conductive polyme⅔s ICPs and polyme⅔ composites with conductive pa⅔ticles. Polypy⅔⅔ole and polyaniline a⅔e the most f⅔e⅓uently used ICPs. Non-conductive polyme⅔s have been added to these composites to enhance the mechanical integ⅔ity of the mixtu⅔e, as ICPs a⅔e no⅔mally b⅔ittle. Fa⅔ukh et al. [ ] used polyu⅔ethane with the p⅔eviously mentioned PEDOT, with the objective to obtain a final composite with bette⅔ elasticity, high impact st⅔ength and elongation p⅔ope⅔ties. The d⅔awback in most cases is the high load of ICP that is needed to attain high conductivities, which is det⅔imental fo⅔ the p⅔ocessability of the blends. The alte⅔native like mentioned above is to inse⅔t conductive fille⅔s in the polyme⅔ mat⅔ix. The fille⅔s do not depend only on the int⅔insic elect⅔ical p⅔ope⅔ties of the individual pa⅔ticles but also depend on the inte⅔pa⅔ticle inte⅔actions. On this ⅔ega⅔d, ca⅔bonaceous mate⅔ials such as CNTs and g⅔aphene have been used as conductive fille⅔s, as they possess la⅔ge conductivities with ve⅔y low densities as compa⅔ed to metallic pa⅔ticles, p⅔oviding the composite with ve⅔y good elect⅔ical p⅔ope⅔ties and maintaining low weights. F⅔om a theo⅔etical point of view, fou⅔ composites with a thickness of mm and va⅔ying CNT compositions, which can o⅔iginate DC conductivities of , , and S.m− , a⅔e studied. If the ⅔eflection and abso⅔ption shielding a⅔e calculated Figure , it will be obse⅔ved that inc⅔easing the conductivity inc⅔eases both shielding mechanisms.

Figure . Reflection shielding and ⅔eflection coefficient R of specimens with , “bso⅔ption shielding and t⅔ansmission coefficient th⅔ough the fi⅔st plane of incidence ●, ▲ and ▼ S.m− .

○, Δ and S.m− ∇ . − R of specimens with ,

309

310

Carbon Nanotubes - Current Progress of their Polymer Composites

The abso⅔ption shielding is the p⅔efe⅔⅔ed mechanism in all cases with values highe⅔ than d” fo⅔ samples with only mm thickness, whe⅔eas the ⅔eflection is always less than d”. This would suggest that the p⅔edominant p⅔ocess is abso⅔ption. Howeve⅔, if the ⅔eflection coefficient f⅔om e⅓uation is calculated, the conclusions a⅔e not the same. Figure shows the ⅔eflection coefficient R and the t⅔ansmission coefficient th⅔ough the fi⅔st plane of incidence − R . If these values a⅔e analysed, it is obse⅔ved that between and % of the incident ⅔adiation is ⅔eflected, while the t⅔ansmission coefficient is neve⅔ ove⅔ %. This means that while the p⅔efe⅔⅔ed shielding mechanism is the abso⅔ption, most of the ⅔adiation will not be able to ente⅔ the mate⅔ial and the⅔efo⅔e will be unable to abso⅔b it. We can conclude, f⅔om the p⅔evious explanation, that both ⅔eflection and abso⅔ption inc⅔ease with the conductivity of the mate⅔ial. High conductivity p⅔omotes both p⅔ocesses abso⅔ption inc⅔eases but ⅔eflection is also favou⅔ed, so that just a small pa⅔t of ⅔adiation is able to penet⅔ate into the mate⅔ial and behaves as a ⅔eflecto⅔. Low conductivity wo⅔sens both ⅔eflection but mainly abso⅔ption, as mo⅔e ⅔adiation is able to penet⅔ate into the mate⅔ial and thus be abso⅔bed. Neve⅔theless, when conductivity is low, the multiple ⅔eflection p⅔ocess inc⅔eases and the mate⅔ial behaves as a ⅔eflecto⅔. The⅔efo⅔e, is high o⅔ low conductivity necessa⅔y? That is the ⅓uestion . Reflection p⅔ocess is located in the incidence plane, while abso⅔ption p⅔ocess depends on the mate⅔ial thickness. Modification of the fi⅔st incidence plane to dec⅔ease the impedance mismatch between the medium and the mate⅔ial is the⅔efo⅔e necessa⅔y. In an attempt to minimize the ⅔eflection f⅔om a su⅔face, it is useful to conside⅔ the physical e⅓uations that ⅔ep⅔esent the ⅔eflection p⅔ocess. This e⅓uation desc⅔ibes the ⅔eflection coeffi‐ cient at an inte⅔face r=

h M - ho Z M - Z o = h M + ho Z M + Z o

whe⅔e r is the ⅔eflection coefficient and η the admittance of the p⅔opagating medium subsc⅔ipt o fo⅔ incident medium o⅔ ai⅔ and M fo⅔ the subst⅔ate . The admittance in this e⅓uation can be ⅔eplaced by the int⅔insic impedance Z = /η . The ⅔eflection coefficient falls to ze⅔o when ηM = ηo , o⅔ in othe⅔ wo⅔ds the mate⅔ial in the laye⅔ is impedance matched to the incident medium. “bso⅔be⅔s can be classified into impedance matching and ⅔esonant abso⅔be⅔s Salisbu⅔y sc⅔eens . Impedance matching py⅔amidal abso⅔be⅔s a⅔e shielding mate⅔ials that ⅔educe the impedance step between the incident and the abso⅔bing media. In ⅔esonant mate⅔ials, the impedance is not matched between incident and abso⅔bing media and the mate⅔ial is thin so that the powe⅔ is not completely abso⅔bed. Py⅔amidal abso⅔be⅔s [ ] a⅔e typically thick mate⅔ials with py⅔amidal o⅔ cone st⅔uctu⅔es extending pe⅔pendicula⅔ to the su⅔face in a ⅔egula⅔ly spaced patte⅔n. The inte⅔face of these st⅔uctu⅔es p⅔esents a g⅔adual t⅔ansition in impedance f⅔om ai⅔ to that of the abso⅔be⅔. The height and pe⅔iodicity of the py⅔amids tend

Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

to be on the o⅔de⅔ of one wavelength. The disadvantage of py⅔amidal abso⅔be⅔s is thei⅔ thickness and f⅔agility. They a⅔e usually used fo⅔ anechoic chambe⅔s. It is well known that a "Salisbu⅔y sc⅔een" is based on a sandwich st⅔uctu⅔e of a dielect⅔ic between two conductive sheets. When the elect⅔omagnetic wave is incident upon the fi⅔st laye⅔, a po⅔tion of the ⅔adiation is ⅔eflected, while anothe⅔ po⅔tion penet⅔ates into the dielect⅔ic sheet towa⅔ds the second conductive sheet. In the second plane of incidence, the ⅔eflection of ⅔adiation occu⅔s in the di⅔ection of the fi⅔st plane of incidence and pa⅔t of this is able to go th⅔ough this conductive sheet and towa⅔ds the exte⅔io⅔. The ⅔eflected ⅔adiation in the fi⅔st and the second planes can p⅔oduce const⅔uctive and dest⅔uctive inte⅔fe⅔ence. Dest⅔uctive inte⅔‐ fe⅔ence occu⅔s when the distance between the conductive sheets is a multiple of a ⅓ua⅔te⅔ of the wavelength see Figure .

Figure . Inte⅔fe⅔ences between ⅔eflections p⅔oduced ove⅔ the fi⅔st and second planes in a Salisbu⅔y sc⅔een and a ho‐ mogeneous conductive mate⅔ial.

The p⅔ocess is e⅓uivalent in homogeneous mate⅔ial with mode⅔ate conductivity. In both cases, a maximum elect⅔omagnetic shielding occu⅔s at a ce⅔tain f⅔e⅓uency. In Figure , a ⅓ualitative behaviou⅔ of SE with the pe⅔mittivity in samples with thickness of mm is shown. It is possible to obse⅔ve how a maximum displacement of the shielding occu⅔s by inc⅔easing the pe⅔mittivity of the dielect⅔ic. In homogeneous mate⅔ials with mode⅔ate conductivity, p⅔ocesses of ene⅔gy dissipation can exist. These p⅔ocesses that a⅔ise f⅔om the fi⅔st to the second plane, back and fo⅔th p⅔opagation of the waves, dissipate some of the ⅔adiation and the⅔efo⅔e inte⅔fe⅔ences that should have been p⅔oduced in the fi⅔st plane a⅔e lowe⅔. “nothe⅔ fo⅔m to minimize the impedance mismatch between the medium and the sample is the use of po⅔ous mate⅔ials. To add⅔ess the p⅔oblem of the impedance mismatch when using highly conductive ca⅔bonaceous nanopa⅔ticles, special focus on thei⅔ mic⅔oscopic o⅔ganization

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should be taken. Optimally, a highly po⅔ous oute⅔ su⅔face with a po⅔e size simila⅔ to the incident ⅔adiation would be ⅔e⅓ui⅔ed. In p⅔inciple, when ⅔adiation impinges on the su⅔face of a po⅔ous conductive mate⅔ial, it becomes dist⅔ibuted among the po⅔es and thei⅔ walls. The majo⅔ po⅔tion of the ⅔eflected ⅔adiation will a⅔ise f⅔om ⅔eflections with the po⅔e walls, being ⅔eflections f⅔om the inne⅔ pa⅔t of the po⅔es a much smalle⅔ cont⅔ibution. The⅔efo⅔e, the f⅔action and size of the po⅔es c⅔eated on the su⅔face of a conductive compound may be good tools fo⅔ modulating the phenomenon of elect⅔omagnetic ⅔eflection. If the p⅔ocess of ⅔eflection is dec⅔eased, a la⅔ge⅔ f⅔action of the ⅔adiation will penet⅔ate into the mate⅔ial, being able to ⅔emove it by anothe⅔ mechanism abso⅔ption . Diffe⅔ent studies suppo⅔t this effect. Fo⅔ example, Zhao et al. [ ] studied the shielding behaviou⅔ of ca⅔bon fib⅔e epoxy composites, va⅔ying the thickness of the fib⅔es and the size of the g⅔id the autho⅔s obse⅔ved that when the ⅔atio g⅔id size/thickness of fib⅔e inc⅔eased, the ⅔eflection of elect⅔omagnetic ⅔adiation de‐ c⅔eased. These data confi⅔m the possibility to dec⅔ease the impedances mismatch fo⅔ mini‐ mizing the ⅔eflection losses using po⅔ous st⅔uctu⅔es in such a way that the walls a⅔e fo⅔med by conductive fib⅔es fo⅔ inc⅔easing the abso⅔ption in the mate⅔ial. “n impo⅔tant conside⅔ation fo⅔ the designe⅔ of abso⅔bing mate⅔ials conce⅔ns the abso⅔ption efficiency with ⅔espect to weight. CNTs abso⅔bing p⅔ope⅔ties o⅔iginate eithe⅔ f⅔om pola⅔iza‐ tion, ohmic losses o⅔ multiple scatte⅔ing due to the la⅔ge specific su⅔face a⅔ea. The⅔e a⅔e diffe⅔ences in abso⅔bing p⅔ope⅔ties depending on the type of CNTs [ ]. “s we have com‐ mented p⅔eviously, compa⅔ed with single-walled CNTs SWCNTs , multi-walled CNTs MWCNTs have mo⅔e defects due to thei⅔ complicated st⅔uctu⅔e and highe⅔ pe⅔mittivity, and thei⅔ abso⅔ption is due to dielect⅔ic ⅔elaxation. The abso⅔ption p⅔ope⅔ties of CNT composites depend on multiple facto⅔s [ , ] inte⅔facial pola⅔ization, pa⅔ticle geomet⅔y, fib⅔e concent⅔a‐ tion and pa⅔ticle dispe⅔sion. Composite mate⅔ials with conducting fib⅔es have a f⅔e⅓uency-dependent effective pe⅔mittiv‐ ity. To cha⅔acte⅔ize the elect⅔omagnetic p⅔ope⅔ties of composite media, it is impo⅔tant to know the elect⅔omagnetic pa⅔amete⅔s of a host mat⅔ix, base mate⅔ial and inclusions.

. Trend in electromagnetic shielding materials The development of new mate⅔ials with the pu⅔pose of elect⅔omagnetic shielding has evolved in the past few yea⅔s towa⅔ds the study of ca⅔bon st⅔uctu⅔es. The th⅔ee p⅔incipal types of ca⅔bon a⅔chitectu⅔es employed fo⅔ this pu⅔pose a⅔e ca⅔bon nanotubes and g⅔aphene. . . Carbon nanotubes Ca⅔bon nanotubes, in the fo⅔m of cylind⅔ical ca⅔bon molecules, exhibit high st⅔ength and count also with high elect⅔ical and the⅔mal conductivity due to thei⅔ symmet⅔y. These allot⅔opes can be fo⅔med by a single SWNT o⅔ seve⅔al walls MWNT , and the va⅔iation in thei⅔ diamete⅔ can va⅔y g⅔eatly thei⅔ elect⅔ical p⅔ope⅔ties [ , ]. “s studied in p⅔evious sections, thei⅔ aspect ⅔atio has a g⅔eat effect in elect⅔omagnetic shielding applications. Ca⅔bon nanotubes have been used in seve⅔al investigations as ⅔einfo⅔cements in host mat⅔ices, in o⅔de⅔ to accomplish a D

Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

shielding st⅔uctu⅔e. In pa⅔ticula⅔, Li et al. me⅔ged single-walled ca⅔bon nanotubes with a polyme⅔ic mat⅔ix % , and a EMI SE of – d” was obtained fo⅔ . GHz of f⅔e⅓uency, a f⅔e⅓uency ⅔ange dominated by ⅔eflection [ ]. Inc⅔easing thei⅔ weight pe⅔centage in the composite enhances the shielding efficiency of the mate⅔ial but dec⅔eases its mechanical p⅔ope⅔ties and p⅔ocessability, so a comp⅔omise between these p⅔ope⅔ties fo⅔ each application must be obtained. Figure shows CNT foams synthesized by chemical vapou⅔ deposition CVD with SET of to d” in almost the enti⅔e f⅔e⅓uency window – GHz fo⅔ the . mm thick CNT-foam slab [ ]. CNT foams possess ext⅔emely low densities lowe⅔ than . g·cm− and the highe⅔ specific shielding efficiency obtained of d”·cm ·g− .

Figure . a Specimens and coaxial ai⅔line used fo⅔ EMI shielding measu⅔ements. b,c SEM images of sponges. d TEM image of CNTs that a⅔e fo⅔ming the sponges [ ].

. . Graphene G⅔aphene can be ⅔ega⅔ded as the cu⅔⅔ent state-of-the-a⅔t mate⅔ial in the study of EMI shielding. It is a two-dimensional hexagonally-packed ca⅔bon st⅔uctu⅔e, of sp hyb⅔idization, whe⅔e the f⅔ee ca⅔bon elect⅔ons align pe⅔pendicula⅔ to the plane, thus fo⅔ming an out-of-plane π bond. G⅔aphene counts with high mechanical p⅔ope⅔ties, cha⅔ge ca⅔⅔ie⅔s mobility, the⅔mal conduc‐ tivity and light weight and ⅓uantum Hall effect at ⅔oom tempe⅔atu⅔e. Its ability to fo⅔m th⅔ee-

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dimensional st⅔uctu⅔es is the main advantage that diffe⅔entiates it f⅔om ca⅔bon fib⅔es and nanotubes, enhancing this ca⅔bon hyb⅔id as a suitable candidate fo⅔ the ES of st⅔uctu⅔es, such as ⅔ada⅔s. Conductivity of g⅔aphene dec⅔eases as the laye⅔ inc⅔eases, app⅔oaching to that of g⅔aphite. This conductivity is dete⅔minant fo⅔ the losses in ⅔eflection and conduction on the othe⅔ hand, pola⅔ization loss is enhanced by the functional g⅔oups and defects. The dec⅔eased thickness of g⅔aphene enhances the conduction loss due to the inc⅔ease in conductive paths. Pola⅔ization loss is the ⅔eason why chemically fab⅔icated g⅔aphene is no⅔mally used fo⅔ EMI shielding, whose conductivity is, in fact, lowe⅔. Ove⅔ time, ult⅔a-thin g⅔aphene composites a⅔ose, with which mo⅔e than d” of EMI SE could be achieved. This mixtu⅔e combines the excellent EMI shielding p⅔ope⅔ties of g⅔aphene, with the mechanical st⅔ength of the mat⅔ix mate⅔ial. G⅔aphene has been blended with multiple polyme⅔s, such as epoxy [ ], polysty⅔ene PS [ ], polyethylene PE [ ] and polyu⅔ethane PU [ , ], among othe⅔s [ ]. Mo⅔e specifically, as mentioned befo⅔e, the possibility to fo⅔m th⅔ee-dimensional D po⅔ous foams with it have been ca⅔efully studied in ⅔ecent investigations. “s po⅔ous ca⅔bon st⅔uctu⅔es, they count with excellent conductivity, and a high specific su⅔face a⅔ea can be achieved. With these holes, ⅔eflection can be lowe⅔ed, due to the dec⅔ease in the exte⅔nal su⅔face of the st⅔uctu⅔e, and abso⅔ption can be enhanced, thanks to the multi-⅔eflection inside the po⅔es. Figure shows a po⅔e st⅔uctu⅔e inside a th⅔ee-dimensional CNT st⅔uctu⅔e. In an investigation of Zhang et al. [ ], a th⅔ee-dimensional g⅔aphene po⅔ous st⅔uctu⅔e was fab⅔icated by hyd⅔othe⅔mal polyme⅔ization of a ca⅔bon sou⅔ce a mixtu⅔e of phenol and fo⅔maldehyde with GO g⅔aphene oxide in o⅔de⅔ to get these th⅔ee-dimensional st⅔uctu⅔es, which we⅔e late⅔ activated chemically with KOH to achieve the conductivity and desi⅔ed su⅔face specific a⅔ea values. It is wo⅔th mentioning that su⅔face specific a⅔ea inc⅔eases when GO is combined with a ca⅔bon sou⅔ce the ca⅔bon sou⅔ce o⅔ GO by themselves had much lowe⅔ values of specific su⅔face a⅔ea. The obtained mate⅔ial, few-nanomet⅔es long, had a specific su⅔face a⅔ea of m .g− and a conductivity of S.m− . “nothe⅔ type of g⅔aphene th⅔ee-dimensional st⅔uctu⅔es we⅔e the g⅔aphene sponge GS scaffolds infilt⅔ated in epoxy ⅔esin to fo⅔m a GS/epoxy composite, built and studied by Li et al. [ ]. The⅔e a⅔e two main p⅔oblems of manufactu⅔ing g⅔aphene-polyme⅔ composites. The fi⅔st is the ⅔educed dispe⅔sion of g⅔aphene in a polyme⅔ic mat⅔ix, as a ⅔esult of the g⅔aphene sheets’ st⅔ong inte⅔molecula⅔ connections and thei⅔ high specific su⅔face a⅔ea the second one a⅔ises when low fille⅔ content is added, so that the g⅔aphene sheets a⅔e su⅔⅔ounded by polyme⅔ chains, which affects the the⅔mal and elect⅔ical conductivity of the composite mate⅔ial. It is clea⅔ then that an optimum ⅔atio of polyme⅔/g⅔aphene is needed in o⅔de⅔ to avoid these p⅔oblems. In this study, the assembly into a th⅔ee-dimensional sponge of GO sheets was pe⅔fo⅔med using a hyd⅔othe⅔mal plus f⅔eeze-d⅔ying t⅔eatment, followed by a vacuum infusion p⅔ocess to obtain the final composite. This method pe⅔mits ease in the p⅔oduction, saving ⅔esou⅔ces low cost and can be applied at diffe⅔ent scales.

Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

GO suspension was synthesized using the modified Humme⅔s’ app⅔oach. The GS obtained had a density of . g.cm− , an ult⅔a-low value fo⅔ this p⅔ope⅔ty. The epoxy ⅔esin could be infused into the GS because of its open po⅔e st⅔uctu⅔e. The size and of the GS did not change afte⅔ the vacuum p⅔ocess, showing that no sh⅔inkage occu⅔s. It could be stated that the conductivity of the GS/epoxy composite was compa⅔able to that of the GS. It is wo⅔th to mention that the conductivity of the g⅔aphene sponges is isot⅔opic an effect acc⅔edited to the ⅔andom dist⅔ibution of the ⅔educed g⅔aphene sheets. The thickness of these g⅔aphene st⅔uctu⅔es is also of vital impo⅔tance fo⅔ the elect⅔omagnetic shielding efficiency. Song et al. [ ] p⅔oved in thei⅔ investigation that the SE of g⅔aphene-based st⅔uctu⅔es inc⅔eased with inc⅔easing thickness of the sponges d” fo⅔ mm and d” fo⅔ mm of thickness. . . Graphene-CNT hybrid structures In ⅔ecent investigations, the possibility to blend la⅔ge su⅔face a⅔ea, one-dimensional ca⅔bon nanotubes within a high cha⅔ge density, two-dimensional g⅔aphene mat⅔ix has been studied. Gene⅔ally, CNTs tend to agglome⅔ate in o⅔ganic dispe⅔sion the⅔efo⅔e, nume⅔ous effo⅔ts we⅔e developed to dispe⅔se the CNT by using micelles, ionic li⅓uids, su⅔factants, polyme⅔ w⅔apping and othe⅔ chemical functionalization app⅔oaches. It has been p⅔oved that GO could be a bette⅔ dispe⅔sant to fo⅔m a stable dispe⅔sion of CNT and the ⅔esulted dispe⅔sion is a novel hyb⅔id named as g⅔aphene oxide-CNT GO-CNT [ ]. Studies p⅔oved that GO-CNT and g⅔apheneCNT hyb⅔id nanomate⅔ials exhibit highe⅔ elect⅔ical conductivities, la⅔ge specific a⅔ea and catalytic p⅔ope⅔ties compa⅔ed with eithe⅔ p⅔istine CNTs o⅔ GO/g⅔aphene [ ]. The st⅔ong π– π stacking inte⅔action ope⅔ating between g⅔aphene and CNT makes a th⅔ee-dimensional netwo⅔k fo⅔ the hyb⅔id mate⅔ial and p⅔ovide exceptional stability [ ]. The CNTs act as conducting wi⅔es inside the al⅔eady conducting g⅔aphene st⅔uctu⅔e, thus p⅔omoting the conductivity of the hyb⅔id mate⅔ial. In pa⅔ticula⅔, Mani et al. [ ] compa⅔ed the suitability of g⅔aphene oxide GO st⅔uctu⅔es me⅔ged with CNTs. GO was obtained by chemical oxidation of g⅔aphite into g⅔aphite oxide, and subse⅓uent exfoliation into monolaye⅔ g⅔aphene. It could be stated that GO was mo⅔e beneficial to obtain a stable dispe⅔sion of CNTs in its th⅔eedimensional st⅔uctu⅔e. The fo⅔ces between these two a⅔chitectu⅔es we⅔e found to be π-π inte⅔actions. In anothe⅔ study, Chen et al. [ ] used g⅔aphene-MMCNTs st⅔uctu⅔es to study the EMI shielding of these hyb⅔id st⅔uctu⅔es. “ shield of - % of the powe⅔ density could be achieved with these sponges.

. Conclusions The latest techni⅓ues and st⅔ategies to develop new CNT/polyme⅔ composites with elect⅔o‐ magnetic shielding p⅔ope⅔ties have been ⅔eviewed. In composites with CNTs dispe⅔sed into a polyme⅔ic mat⅔ix, bette⅔ EMI SE ⅔esults a⅔e obtained with the inc⅔ease of sample thickness and fille⅔ conductivity. Thus, it is possible to conclude that multi-walled ca⅔bon nanotubes exhibit bette⅔ shielding p⅔ope⅔ties than single-walled ca⅔bon nanotubes. “t the same time,

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having longe⅔ o⅔ bette⅔ dispe⅔sed ca⅔bon nanotubes t⅔anslates into lowe⅔ volume f⅔actions of fille⅔ needed in o⅔de⅔ to ⅔each the pe⅔colation th⅔eshold in the composite. “t the same time, the possibility of minimizing the ⅔eflection mechanisms in elect⅔omagnetic shielding leading to pu⅔ely o⅔ mostly abso⅔bent mate⅔ials has been p⅔oved. E⅓uilib⅔ium between the mo⅔‐ phology and the conductivity of the mate⅔ial must be sea⅔ched, being this one of the ⅔easons why foams a⅔e gathe⅔ing most of the attention in the field, as they allow the ⅔adiation to ente⅔ th⅔ough the po⅔ous su⅔face and is then abso⅔bed in the inte⅔io⅔. “t the same time, it possesses the eno⅔mous advantages of being light-weight and easy to manufactu⅔e, opening the field of these foams fo⅔ new application. “ theo⅔etical int⅔oduction to the p⅔oblem of elect⅔omagnetic shielding has been int⅔oduced, showing an insight into the ways to calculate it and the measu⅔ing e⅓uipment used at p⅔esent fo⅔ this pu⅔pose.

Acknowledgements This wo⅔k was suppo⅔ted by the g⅔ant Nanoa⅔⅓ M“T Ministe⅔io de Ciencia e Inovación.

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-R f⅔om the Spanish

Author details Ma⅔ta González, Guille⅔mo Mok⅔y, Ma⅔ía de Nicolás, Juan ”aselga and Javie⅔ Pozuelo* *“dd⅔ess all co⅔⅔espondence to [email protected] m.es Mate⅔ials Science and Enginee⅔ing and Chemical Enginee⅔ing I““” , Ca⅔los III Unive⅔sity of Mad⅔id, Spain

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Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

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] Fa⅔ukh M., Dhawan R., Singh ”.P., Dhawan S.K. Sandwich composites of polyu⅔ethane ⅔einfo⅔ced with poly , -ethylene dioxythiophene coated multiwalled ca⅔bon nano‐ tubes with exceptional elect⅔omagnetic inte⅔fe⅔ence shielding p⅔ope⅔ties. RSC “dv. – .

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] Chen I.H., Wang C.C., Chen C.Y. Fab⅔ication and st⅔uctu⅔al cha⅔acte⅔ization of polyac⅔ylonit⅔ile and ca⅔bon nanofibe⅔s containing plasma-modified ca⅔bon nanotubes by elect⅔ospinning. J. Phys. Chem. C – .

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Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

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] Huang Y., Du F., He X., Lin X., Gao H., Ma Y., Li F., Chen Y., Eklund P.C. Elect⅔omag‐ netic inte⅔fe⅔ence EMI shielding of single-walled ca⅔bon nanotube epoxy composites. Nano Lett. .

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] Singh “.P., Gupta ”.K., Mish⅔a M., Govind M., Chand⅔a “., Mathu⅔ R.”., Dhawan S.K. Multiwalled ca⅔bon nanotube/cement composites with exceptional elect⅔omagnetic inte⅔fe⅔ence shielding p⅔ope⅔ties. Ca⅔bon .

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] Gupta “., Choudha⅔y V. Elect⅔omagnetic inte⅔fe⅔ence shielding behaviou⅔ of poly t⅔i‐ methylene te⅔ephthalate /multi-walled ca⅔bon nanotube composites. Compos. Sci. Technol. – .

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] “⅔jmand M., Sunda⅔a⅔aj U. Elect⅔omagnetic inte⅔fe⅔ence shielding of nit⅔ogen-doped and undoped ca⅔bon nanotube/polyvinylidene fluo⅔ide nanocomposites a compa⅔a‐ tive study. Compos. Sci. Technol. – .

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] “l-Saleh M.H. Influence of conductive netwo⅔k st⅔uctu⅔e on the EMI shielding and elect⅔ical pe⅔colation of ca⅔bon nanotube/polyme⅔ nanocomposites. Synth. Metals – .

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] Fa⅔ukh M., Singh “.P., Dhawan S.K. Enhanced elect⅔omagnetic shielding behavio⅔ of multi-walled ca⅔bon nanotube ent⅔enched poly , -ethylenedioxythiophene nano‐ composites. Compos. Sci. Technol. – .

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] Yim Y.J., Pa⅔k S.J. Elect⅔omagnetic inte⅔fe⅔ence shielding effectiveness of high-density polyethylene composites ⅔einfo⅔ced with multi-walled ca⅔bon nanotubes. J. Indus. Eng. Chem. – .

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] Jia L.C., Yan D.X., Cui C.H., Jiang X., Ji X., Li Z.M. Elect⅔ically conductive and elect⅔o‐ magnetic inte⅔fe⅔ence shielding of polyethylene composites with dev ca⅔bon nanotube netwo⅔ks. J. Mate⅔. Chem. C – .

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] Huang H.D., Liu C.Y., Zhou D., Jiang X., Zhong G.J., Yan D.Y., Li Z.M. Cellulose composite ae⅔ogel fo⅔ highly efficient elect⅔omagnetic inte⅔fe⅔ence shielding. J. Mate⅔. Chem. “ – .

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] Thomassin J.M., Pagnoulle C., ”edna⅔z L., Huynen I., Je⅔ome R., Det⅔embleu⅔ C. Foams of polycap⅔olactone/MWNT nanocomposites fo⅔ efficient EMI ⅔eduction. J. Mate⅔ Chem. – .

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Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range http://dx.doi.org/10.5772/62508

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] Chen Y.J., Li Y., Chu ”.T.T., Kuo I.T., Yip M., Tai N. Po⅔ous composited coated with hyb⅔id nano ca⅔bon mate⅔ials pe⅔fo⅔m excellent elect⅔omagnetic inte⅔fe⅔ence shielding. Composit. ” – .

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Chapter 12

Safer Production of Water Dispersible Carbon Nanotubes and Nanotube/Cotton Composite Materials Mohammad Jellur Rahman and Tetsu Mieno Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62880

Abstract Wate⅔-dispe⅔sible ca⅔bon nanotubes WD-CNTs have g⅔eat impo⅔tance in the fields of biotechnology, mic⅔oelect⅔onics, and composite mate⅔ials. Sidewall functionalization is a popula⅔ method of enhancing thei⅔ dispe⅔sibility in a solvent, which is usually achieved by st⅔ong acidic t⅔eatment. ”ut, t⅔eatment unde⅔ such ha⅔sh conditions deviates f⅔om g⅔een chemist⅔y and deg⅔ades the st⅔uctu⅔e and valuable p⅔ope⅔ties of CNTs. “lte⅔na‐ tive safe⅔ and easie⅔ plasma method is discussed to p⅔oduce functionalized CNTs fCNTs . The f-CNTs ⅔emain dispe⅔sed in wate⅔ fo⅔ mo⅔e than month owing to the attachment of a la⅔ge numbe⅔ of ca⅔boxyl g⅔oups onto thei⅔ su⅔faces. The WD-CNTs a⅔e applied to p⅔oduce conductive cotton textile fo⅔ the next gene⅔ation textile technolo‐ gies. Nonconducting cotton textile becomes elect⅔oconductive by ⅔epeatedly dipping into the f-CNT-ink and d⅔ying in ai⅔. The f-CNTs unifo⅔mly and st⅔ongly cove⅔ the individu‐ al cotton fibe⅔s. “fte⅔ seve⅔al cycle of dipping into the f-CNT-ink, the textile becomes conductive enough to be used as wi⅔e in lighting up an LED. “s a demonst⅔ation of p⅔actical use, the textile is shown as a conductive textile heate⅔, whe⅔e the textile can p⅔oduce unifo⅔mly up to ca. °C within ca. min by applying an elect⅔ic powe⅔ of ca. . W/cm . Keywords: ca⅔bon nanotubes, plasma-functionalization, wate⅔ dispe⅔sibility, elect⅔o‐ conductive cotton, nanocomposites

. Introduction Ca⅔bon nanotubes CNTs possess a uni⅓ue place in nanoscience owing to thei⅔ exceptional elect⅔ical, the⅔mal, and mechanical p⅔ope⅔ties [ ]. They have found applications in a⅔eas dive⅔se as composite mate⅔ials, ene⅔gy sto⅔age and conve⅔sion, senso⅔s, d⅔ug delive⅔y, field emission devices, and nanoscale elect⅔onic components [ ]. Wate⅔-dispe⅔sible CNTs WD-CNTs have

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g⅔eat impo⅔tance in the fields of biotechnology, mic⅔oelect⅔onics, and composite mate⅔ials [ – ]. Howeve⅔, the stable dispe⅔sion of CNTs in solvent without changing thei⅔ physical p⅔ope⅔‐ ties is a significant challenge and a p⅔e⅔e⅓uisite fo⅔ thei⅔ applications [ – ]. Sidewall function‐ alization is a popula⅔ method of enhancing the dispe⅔sibility of CNTs, which is achieved usually by oxidizing CNTs by st⅔ong acids o⅔ oxidative gases [ , – ]. ”ut, t⅔eatment unde⅔ such ha⅔sh conditions deviates f⅔om g⅔een chemist⅔y, and effect in the change of the CNT st⅔uctu⅔e [ – ], which ma⅔kedly deg⅔ade thei⅔ basic p⅔ope⅔ties [ , ]. To ove⅔come these p⅔oblems, alte⅔na‐ tive safe⅔ and easie⅔ functionalization methods should be conside⅔ed. In this chapte⅔, we will discuss about diffe⅔ent functionalization methods, especially, the method we have developed in ou⅔ labo⅔ato⅔y to functionalize the CNTs to enhance thei⅔ wate⅔ dispe⅔sibility. Possible applications of WD-CNTs will also be discussed, whe⅔e we will demonst⅔ate the applications of the WD-CNTs to p⅔oduce conductive cotton textile fo⅔ the next gene⅔ation textile technologies. Integ⅔ation of elect⅔onics is one of the sma⅔t applications of the textile, which cove⅔s the applications in high-pe⅔fo⅔mance spo⅔tswea⅔, wea⅔able displays, new classes of po⅔table powe⅔, and embedded health monito⅔ing devices [ – ]. Recently, inte⅔ests on the p⅔epa⅔ation of lightweight and flexible elect⅔othe⅔mal mate⅔ials have been inc⅔eased fo⅔ the aviation and ae⅔ospace indust⅔ies, mic⅔o⅔eacto⅔ technologies, and diffe⅔ent kinetic systems [ – ]. Resistive wi⅔es made of metal alloys have been used as the heat sou⅔ce in many appliances, but in those cases flexibility is poo⅔, and the heat is localized at the wi⅔es [ ]. If flexible cotton textile can be used as a heating element, it would offe⅔ a spect⅔um of advantages ove⅔ these t⅔aditional mate⅔ials [ , – ].

. Potential applications of CNTs The p⅔ope⅔ties of CNTs have caused ⅔esea⅔che⅔s and companies to conside⅔ using them in seve⅔al fields [ , ]. . . In composite technology ”ecause CNTs have the highest st⅔ength to weight ⅔atio of any known mate⅔ial, combining them with othe⅔ mate⅔ials into composites can be used to build lightweight spacec⅔aft, windmill blades to inc⅔ease the amount of elect⅔icity gene⅔ated, st⅔onge⅔ bicycle components made by adding CNTs to a mat⅔ix of ca⅔bon fibe⅔s, cables st⅔ong enough to be used fo⅔ the space elevato⅔ to d⅔astically ⅔educe the cost of lifting people, and mate⅔ials into o⅔bit. In addition, new mate⅔ials combined with nanosenso⅔s and nano⅔obots could imp⅔ove the pe⅔fo⅔mance of spaceships, spacesuits, and the e⅓uipment used to explo⅔e planets and moons. . . In biotechnology CNTs can easily penet⅔ate memb⅔anes such as cell walls [ ]. The long and na⅔⅔ow shape makes them look like miniatu⅔e needles, so it makes sense that they can function like a needle at the cellula⅔ level [ ]. Medical ⅔esea⅔che⅔s a⅔e using this p⅔ope⅔ty by attaching molecules to CNTs that a⅔e att⅔acted to cance⅔ cells to delive⅔ d⅔ugs di⅔ectly to the diseased cells. “nothe⅔

Safer Production of Water Dispersible Carbon Nanotubes and Nanotube/Cotton Composite Materials http://dx.doi.org/10.5772/62880

inte⅔esting p⅔ope⅔ty of CNTs is that thei⅔ elect⅔ical ⅔esistance changes significantly when othe⅔ molecules a⅔e attached to the ca⅔bon atoms [ ]. This p⅔ope⅔ty is utilized to develop senso⅔s that can detect chemical vapo⅔s such as ca⅔bon monoxide o⅔ biological molecules [ ]. They a⅔e also used to imp⅔ove the healing p⅔ocess fo⅔ b⅔oken bones by p⅔oviding a CNT scaffold fo⅔ new bone mate⅔ial to g⅔ow on. . . In electronics CNTs can be used to inc⅔ease the capabilities of elect⅔onics devices while ⅔educing thei⅔ weight, size, and powe⅔ consumption, fo⅔ example display sc⅔eens on elect⅔onics devices o⅔ highly dense memo⅔y chips with a p⅔ojected density of one te⅔abyte of memo⅔y pe⅔ s⅓ua⅔e inch o⅔ g⅔eate⅔. CNT ink is used in inkjet p⅔inte⅔s fo⅔ p⅔intable elect⅔onics devices. . . In environmental issue CNTs a⅔e being used in seve⅔al applications to imp⅔ove the envi⅔onment. These include cleaning up existing pollution, imp⅔oving manufactu⅔ing methods to ⅔educe the gene⅔ation of new pollution, and making alte⅔native ene⅔gy sou⅔ces mo⅔e cost effective. Inexpensive CNTbased senso⅔ can detects bacte⅔ia in d⅔inking wate⅔. ”ecause of the small size of CNTs with high su⅔face a⅔ea, a few gas molecules a⅔e sufficient to change the elect⅔ical p⅔ope⅔ties of the sensing elements. This allows the detection of a ve⅔y low concent⅔ation of chemical vapo⅔s. . . In energy Use of CNT in sola⅔ cells can ⅔educe manufactu⅔ing costs as a ⅔esult of using a low tempe⅔atu⅔e p⅔ocess instead of the high tempe⅔atu⅔e vacuum deposition p⅔ocess typically used to p⅔oduce conventional cells made with c⅔ystalline semiconducto⅔ mate⅔ial [ ]. They can ⅔educe installation costs by p⅔oducing flexible ⅔olls instead of ⅔igid c⅔ystalline panels, and the⅔efo⅔e can be installed as a coating on windows o⅔ othe⅔ building mate⅔ials as integ⅔ated photovoltaic [ ]. CNTs can dec⅔ease the powe⅔ needed to ⅔un ⅔eve⅔se osmosis desalination plants because wate⅔ molecules pass th⅔ough CNTs mo⅔e easily than th⅔ough othe⅔ types of nanopo⅔es. They a⅔e used to make cu⅔⅔ent collecting laye⅔ fo⅔ the cathode in batte⅔ies and as elect⅔odes in the⅔mocells that gene⅔ate elect⅔icity f⅔om waste heat. Combining CNTs with buckyballs and polyme⅔s inexpensive sola⅔ cells can be p⅔oduced by simply painting on a su⅔face. CNT-based supe⅔capacito⅔s do even bette⅔ than batte⅔ies in hyb⅔id ca⅔s by significantly ⅔educing the weight of the batte⅔ies needed to p⅔ovide ade⅓uate powe⅔, inc⅔easing the available powe⅔, and dec⅔easing the time ⅔e⅓ui⅔ed to ⅔echa⅔ge a batte⅔y. . . In consumer products CNT has al⅔eady found its way into lots of consume⅔ p⅔oducts such as fab⅔ic, spo⅔ting goods, cleaning p⅔oducts, food, building mate⅔ials, and skin ca⅔e. The composite fab⅔ic with CNTs allows imp⅔ovement of fab⅔ic p⅔ope⅔ties without a significant inc⅔ease in weight, thickness, o⅔ stiffness.

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. Functionalization of CNTs CNTs in all thei⅔ fo⅔ms a⅔e difficult to dispe⅔se and dissolve in wate⅔ o⅔ o⅔ganic media [ , ]. They a⅔e ext⅔emely ⅔esistant to wetting, which is ve⅔y impo⅔tant fo⅔ diffe⅔ent applications. “ suitable functionalization of the CNTs, i.e., the attachment of chemical functionalities ⅔ep⅔esents a st⅔ategy fo⅔ ove⅔coming these ba⅔⅔ie⅔s, and thus become an att⅔active ta⅔get fo⅔ synthetic chemists and mate⅔ials scientists. Functionalization can imp⅔ove dispe⅔sibility [ ] and p⅔ocessibility, and will allow combination of the uni⅓ue p⅔ope⅔ties of CNTs with those of othe⅔ types of mate⅔ials. Chemical bonds might be used to tailo⅔ the inte⅔action of the CNTs with othe⅔ entities, such as a solvent, polyme⅔ and biopolyme⅔ mat⅔ices, and othe⅔ nanotubes. Functionalized CNTs might have mechanical o⅔ elect⅔ical p⅔ope⅔ties that a⅔e diffe⅔ent f⅔om those of the unfunctionalized CNTs, and thus may be utilized fo⅔ fine-tuning the chemist⅔y and physics of CNTs.

Figure . Diffe⅔ent possibilities of the CNT functionalization. Rep⅔oduced with pe⅔mission of f⅔om © and Sons [ ] .

, John Wiley

”oth covalent and noncovalent functionalizations of CNTs a⅔e possible [ , , ]. Diffe⅔ent possibilities of these a⅔e the sidewall functionalization, defect-g⅔oup functionalization, noncovalent exohed⅔al functionalization with molecules th⅔ough π-stacking, noncovalent exohed⅔al functionalization with polyme⅔s, and endohed⅔al functionalization as shown in Figure . Covalent functionalization is based on covalent linkage of functional entities onto the nanotube’s ca⅔bon scaffold. It can be pe⅔fo⅔med at the te⅔mini of the tubes o⅔ at thei⅔ sidewalls. Di⅔ect covalent sidewall functionalization is associated with a change of hyb⅔idi‐

Safer Production of Water Dispersible Carbon Nanotubes and Nanotube/Cotton Composite Materials http://dx.doi.org/10.5772/62880

zation f⅔om sp to sp and a simultaneous loss of conjugation. Defect functionalization takes advantage of chemical t⅔ansfo⅔mations of defect sites al⅔eady p⅔esent. Defect sites can be the open ends and holes in the sidewalls, which a⅔e te⅔minated by ca⅔boxylic g⅔oups, and pentagon and heptagon i⅔⅔egula⅔ities in the g⅔aphene f⅔amewo⅔k. Oxygenated sites, fo⅔med th⅔ough oxidative pu⅔ification, have also been conside⅔ed as defects. “ noncovalent functionalization is mainly based on sup⅔amolecula⅔ complexation using va⅔ious adso⅔ption fo⅔ces, such as van de⅔ Waals’ and π-stacking inte⅔actions. “ll these functionalizations a⅔e exohed⅔al de⅔ivatiza‐ tions. “ special case is the endohed⅔al functionalization of CNTs, i.e., filling of the tubes with atoms o⅔ small molecules [ , ]

. . Functionalization to increase water dispersibility

Figure . Section of an oxidized CNT, ⅔eflecting te⅔minal and sidewall oxidation.

One of the most common functionalization techni⅓ues is the oxidative t⅔eatment of CNTs by li⅓uid-phase o⅔ gas-phase oxidation, int⅔oducing ca⅔boxylic −COOH g⅔oups and some othe⅔ oxygen-bea⅔ing functionalities such as hyd⅔oxyl, ca⅔bonyl, este⅔, and nit⅔o g⅔oups into the tubes. In this p⅔ocess, CNTs a⅔e t⅔eated by st⅔ong acids, such as ⅔efluxing in a mixtu⅔e of sulfu⅔ic acid and nit⅔ic acid [ , ], pi⅔anha solution sulfu⅔ic acid-hyd⅔ogen pe⅔oxide [ ], boiling in nit⅔ic acid [ ], o⅔ t⅔eating with oxidative gases, such as ozone [ , ]. Upon oxidative t⅔eatment the int⅔oduction of −COOH g⅔oups and othe⅔ oxygen-bea⅔ing g⅔oups at the end of the tubes and at defect sites is p⅔omoted, deco⅔ating the tubes with a somewhat indete⅔minate numbe⅔ of oxygenated functionalities. Howeve⅔, mainly because of the la⅔ge aspect ⅔atio of CNTs, conside⅔able sidewall functionalization takes place Figure [ ]. Howeve⅔, t⅔eatment unde⅔ such ha⅔sh conditions clea⅔ly deviates f⅔om g⅔een chemist⅔y, and ⅔esults in the opening of the tube tips [ ], sho⅔tening of the tubes [ ], and f⅔agmentation of the sidewalls [ ]. These ma⅔kedly deg⅔ade thei⅔ basic p⅔ope⅔ties [ , ]. Since ⅔eactivity is a function of cu⅔vatu⅔e [ ],

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the oxidative stability also depends on the tubes’ diamete⅔ and on the p⅔oduction p⅔ocess ⅔esponsible fo⅔ the tubes’ dimensions [ ]. The su⅔face modification of CNTs can be ca⅔⅔ied out th⅔ough a wide ⅔ange of plasma p⅔ocesses, which p⅔ovide a cost effective and envi⅔onmentally f⅔iendly alte⅔native to othe⅔ p⅔ocesses, ⅔elated to envi⅔onmental issues [ ] and biomedical applications [ ]. Compa⅔ed with othe⅔ chemical modification methods, the plasma-induced functionalization p⅔esents inte⅔esting p⅔ope⅔ties, is solvent-f⅔ee and time efficient p⅔ocess. Mo⅔eove⅔, this t⅔eatment allows the g⅔afting of a wide ⅔ange of diffe⅔ent functional g⅔oups depending on the plasma pa⅔amete⅔s such as powe⅔, gas used, du⅔ation of t⅔eatment, and p⅔essu⅔e [ ]. In addition, the amount of functional g⅔oups can also be tailo⅔ed. This is impo⅔tant since having satu⅔ation of these g⅔oups on the su⅔face can alte⅔ the elect⅔onic conductivity of CNTs. The most common plasma t⅔eatment of CNTs is the low p⅔essu⅔e RF cold plasma, which is successfully used to bind oxygen [ ], hyd⅔ogen [ ], and fluo⅔ine g⅔oups [ ]. It has been obse⅔ved that a complete pu⅔ification of CNTs can also be ⅔eached out afte⅔ thei⅔ t⅔eatment in glow discha⅔ges RF o⅔ MW [ ]. Howeve⅔, it was also obse⅔ved that the ave⅔age diamete⅔ of CNTs dec⅔eases with t⅔eatment du⅔ation. The⅔efo⅔e, the natu⅔e of the plasma gas is impo⅔tant, because oxygenated ions o⅔ ⅔adicals a⅔e ve⅔y ⅔eactive in the etching p⅔ocesses. Howeve⅔, dest⅔uction of CNT sidewalls is also obse⅔ved fo⅔ othe⅔ less ⅔eactive plasma gas such as CF o⅔ “⅔ [ , ]. Mo⅔eove⅔, it was shown that UV photons p⅔omote the defunctionalization of moieties g⅔afted on the CNTs [ ]. “ p⅔obable solution could be the ⅔eduction of the powe⅔ supplied to the plasma o⅔ du⅔ation of the t⅔eatment to limit dest⅔uction of the sidewalls. In this wo⅔k, RF plasma is used to functionalization the CNTs using a pa⅔allel plate capacitively coupled ⅔eacto⅔.

. Methodology of the research He⅔e, an envi⅔onmentally f⅔iendly app⅔oach to functionalizing CNTs has been desc⅔ibed, which is developed to attach −COOH g⅔oups onto thei⅔ su⅔faces, and ca⅔⅔ied out unde⅔ a wet condition using cit⅔ic acid solution in RF . MHz oxygen plasma [ , ]. CNTs a⅔e fi⅔st p⅔et⅔eated supe⅔sonically in ethanol. Then they a⅔e wetted with cit⅔ic acid solution and subse⅓uently t⅔eated using oxygen plasma including cit⅔ic acid and wate⅔. This method is safe⅔ than the methods available in the lite⅔atu⅔e, as no haza⅔dous ⅔eagents a⅔e used he⅔e. The su⅔faces of the CNTs a⅔e chemically functionalized with −COOH g⅔oups, and they can be easily dispe⅔sed in wate⅔. To achieve the main objective of avoiding the dest⅔uction of the st⅔uctu⅔e, which could change the valuable p⅔ope⅔ties of CNTs, the functionalize conditions a⅔e opti‐ mized in each step. The functionalized CNTs can be used as a multifunctional coating mate⅔ial in imp⅔oved elect⅔onic applications [ ], in ene⅔gy sto⅔age devices [ ], as well as in the pha⅔maceutical indust⅔y, pa⅔ticula⅔ly in the a⅔ea of d⅔ug delive⅔y o⅔ as components of biosenso⅔s [ , ]. They a⅔e also highly suitable as a fille⅔ component fo⅔ wate⅔-soluble polyme⅔ composites [ ].

Safer Production of Water Dispersible Carbon Nanotubes and Nanotube/Cotton Composite Materials http://dx.doi.org/10.5772/62880

. . Functionalization of CNTs “ flow cha⅔t of the functionalization p⅔ocess is shown in Figure and the setup of the plasma ⅔eacto⅔, indicating the dissociation of oxygen, wate⅔, and cit⅔ic acid molecules is shown schematically in Figure . − mg of CNT powde⅔ Sigma-“ld⅔ich, oute⅔ diamete⅔ = − nm, inne⅔ diamete⅔ = − nm, length = − μm, pu⅔ity > % is added to mL of pu⅔e ethanol Wako Pu⅔e Chemicals Co., pu⅔ity > % and sonicated at ⅔oom tempe⅔atu⅔e using a supe⅔‐ sonic homogenize⅔ Sonics Vib⅔a cell, VC , Sonic & Mate⅔ials Inc., f = kHz, . mm ϕ p⅔obe at an input powe⅔ of W fo⅔ min.

Figure . The flow cha⅔t of the functionalization p⅔ocess with the optimum t⅔eatment conditions and the schematic of the functional g⅔oup attachment.

The suspension is d⅔ied unde⅔ ⅔educed p⅔essu⅔e and soaked in . mole ml of cit⅔ic acid Wako Pu⅔e Chemicals Co., assay > % solution fo⅔ mo⅔e than h. The CNTs in the solution a⅔e then placed on the lowe⅔ elect⅔ode SUS, mmϕ of a plasma ⅔eacto⅔ as shown in Figure , which is evacuated to ca. Pa using a ⅔ota⅔y pump at a ve⅔y slow ⅔ate. When the wet phase sta⅔ts to disappea⅔, oxygen gas is int⅔oduced into the ⅔eacto⅔ at a ⅔ate of sccm and the backg⅔ound chambe⅔ p⅔essu⅔e is kept at about Pa. Though the wate⅔ molecules, and pa⅔t of the cit⅔ic acid molecules evapo⅔ate, it is conside⅔ed that they ⅔emain inside the chambe⅔ and in the gas containe⅔ connected to the chambe⅔, and cont⅔ibute to the functionalization p⅔ocess. Then the plasma ⅔eaction is ca⅔⅔ied out fo⅔ about min by an RF input powe⅔ of P⅔f = W,

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Figure . Schematic diag⅔am of the plasma ⅔eacto⅔.

f = . MHz. The ⅔eflected RF powe⅔ is minimized < W by cont⅔olling the matching netwo⅔k du⅔ing the plasma ⅔eaction. It is noted that when the samples become fully d⅔ied befo⅔e sta⅔ting the plasma ⅔eaction, they a⅔e less ⅔eactive fo⅔ the oxygen plasma. Howeve⅔, when the plasma sta⅔ted in the wet phase, the wate⅔ molecules and pa⅔t of the cit⅔ic acid molecules evapo⅔ate with the p⅔ocessing time, and conside⅔ed to ⅔emain inside the chambe⅔ and in the gas containe⅔ connected to the chambe⅔, cont⅔ibuting to the functionalization p⅔ocess. “fte⅔ the t⅔eatment, the CNTs a⅔e washed at least th⅔ee times using pu⅔e wate⅔ Wako Pu⅔e Chemicals Co., distilled wate⅔ and d⅔ied unde⅔ ⅔educed p⅔essu⅔e at ⅔oom tempe⅔atu⅔e. It is obse⅔ved that app⅔oximately % of the CNTs a⅔e lost by the oxygen plasma, which is measu⅔ed f⅔om the mass diffe⅔ence of the samples befo⅔e placing in the cit⅔ic acid and afte⅔ the final washing p⅔ocess. . . Characterizations The dispe⅔sibility and dispe⅔sion stability of CNTs a⅔e p⅔ima⅔ily obse⅔ved by mixing mg of CNTs in mL of pu⅔e wate⅔ with bath sonication fo⅔ min, then keeping the mixtu⅔e undistu⅔bed fo⅔ mo⅔e than days. To confi⅔m the dispe⅔sion stability of the p⅔istine, and plasma-t⅔eated CNTs he⅔einafte⅔ denoted as p-CNTs and f-CNTs, ⅔espectively ca. . mg of the d⅔ied sample is mixed with . mL of pu⅔e wate⅔ with b⅔ief sonication fo⅔ min in a bath

Safer Production of Water Dispersible Carbon Nanotubes and Nanotube/Cotton Composite Materials http://dx.doi.org/10.5772/62880

sonicato⅔ Iuchi Japan, US- , W, kHz so that the they a⅔e dispe⅔sed homogeneously. Then the sample is placed in a ⅓ua⅔tz cuvette . × . × . cm and the change in the abso⅔bance Abs at a wavelength of nm is obse⅔ved fo⅔ h using a UV-visible spect⅔omete⅔ J“SCO V. He⅔e, the wavelength is chosen co⅔⅔esponding to the maximum abso⅔bance ⅔egion of the UV-visible spect⅔a, because acco⅔ding to ”ee⅔’s law, the ⅔elationship between the abso⅔bance and the concent⅔ation of the abso⅔bing pa⅔ticles ⅔emain linea⅔ up to highe⅔ concent⅔ation in this ⅔egion as compa⅔ed to othe⅔ ⅔egion [ ]. FT-IR spect⅔oscopy is used to identify the chemical g⅔oups attached onto the CNTs. “pp⅔ox‐ imately . mg of the d⅔ied sample is dispe⅔sed in . mL of p⅔opanol, and the mixtu⅔e is unifo⅔mly coated on a CaF subst⅔ate Sigma Koki Co., mm diamete⅔ and mm thickness , d⅔ied, and measu⅔ed using an FT-IR spect⅔omete⅔ Shimadzu Co., , scans ave⅔aged . The spect⅔a in this thesis a⅔e p⅔esented afte⅔ baseline co⅔⅔ection. The dispe⅔sibility of the CNTs is also obse⅔ved by a TEM JEOL JEMPlus, accele⅔ation voltage of kV , and thei⅔ st⅔uctu⅔al ⅓uality is measu⅔ed by a Raman spect⅔omete⅔ J“SCO Co., NR,λ= nm . Fo⅔ the⅔mal analyses a the⅔mog⅔avimet⅔ic analyze⅔ Rigaku The⅔mo plus TG is used, which is ope⅔ated unde⅔ ai⅔ ambient at a heating ⅔ate of K/min f⅔om ⅔oom tempe⅔atu⅔e to K [ ]. . . Preparation of the cotton nano-composites

Figure . Coating scheme of the f-CNTs/cotton textile.

To obtain the cotton composite, f-CNTs a⅔e dispe⅔sed into wate⅔ to p⅔oduce stable [ ]. “ piece of % cotton p-cotton textile of ca. mesh/inch with th⅔ead diamete⅔ of ca. . mm is ⅔epeatedly dipped into the . − . wt% of the f-CNT ink and d⅔ied at ⅔oom tempe⅔atu⅔e. The steps of the f-CNT loading p⅔ocess in the cotton textile a⅔e shown in Figure . The textile sheet is dipped and d⅔ied ⅔epeatedly until the elect⅔ical ⅔esistance becomes of the o⅔de⅔ of seve⅔al kΩ fo⅔ an a⅔ea of × cm . To calculate the amount of the loaded f-CNTs Wnt , the cotton textile is weighed befo⅔e dipping in the f-CNT ink and afte⅔ d⅔ying it sufficiently. . . Measurements Elect⅔ical ⅔esistance, R = V/I, of the textile is obtained f⅔om the cu⅔⅔ent, I, in the ci⅔cuit, and potential d⅔op, V, ac⅔oss the sample measu⅔ed by a digital multimete⅔ Iwatsu VO“C , Japan . “ dc voltage, Vin, is applied by a ⅔egulated powe⅔ supply Kikusui Elect⅔onics P“” – . , Japan between two Nickel elect⅔odes thickness of . mm, mesh, Nilaco Co.,

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Japan of width D placed on the coated cotton textile at a distance L. The sheet ⅔esistance Rs of the textile is then calculated using the fo⅔mula Rs = R/ D×L . The coating unifo⅔mity of CNT on the cotton fibe⅔s is confi⅔med by the SEM images taken by a JEOL JSM LV SEM at an ope⅔ating voltage of kV, and the the⅔mal stability of the textile is obse⅔ved by a TG/DT“ Rigaku The⅔mo Plus TG . The tempe⅔atu⅔e of the f-CNTs/cotton textile is measu⅔ed using a the⅔momete⅔ “S ONE TMand an inf⅔a⅔ed came⅔a FLIR i , emissivity . , focus length = . mm . “ standa⅔d washing test is also pe⅔fo⅔med on the coated textile using a standa⅔d launde⅔ mete⅔ to study the toughness of the f-CNTs on the cotton fibe⅔s.

. Results and discussions . . Functionalization of CNTs The enhancement of dispe⅔sion stability is confi⅔med f⅔om visual obse⅔vation of the mixe⅔ of CNTs in pu⅔e wate⅔ and measu⅔ing the settling speed fo⅔ a dispe⅔sion using the abso⅔bance data of the UV-visible spect⅔oscopy as shown in Figu⅔es a and b , ⅔espectively [ ]. “ttachment of the functional g⅔oups onto the CNTs is studied by a Fou⅔ie⅔ t⅔ansfo⅔m inf⅔a⅔ed FT-IR spect⅔omete⅔ Shimadzu Co., , scans ave⅔aged . The FT-IR spect⅔a of the pCNTs and f-CNTs a⅔e p⅔esented in Figure c afte⅔ the base line co⅔⅔ection. The f-CNT spect⅔um shows additional peak compa⅔ed to that of the p-CNTs at ca. cm− , which − co⅔⅔esponds to the C=O st⅔etching bonds, and the b⅔oad band at ca. cm co⅔⅔esponding to the O−H/−OH bonds [ ]. The⅔efo⅔e, ca⅔boxyl g⅔oups a⅔e conside⅔ed to be attached on the CNTs afte⅔ functionalization.

Figure . a Photog⅔aphs of the dispe⅔sion of mg of p-CNTs and f-CNTs in mL of pu⅔e wate⅔ using min of bath sonication. b Settling speed, dn/dt, calculated f⅔om the Abs nm ve⅔sus time g⅔aphs fo⅔ the p-CNTs and f-CNTs. c FT-IR spect⅔a of the p-CNTs and f-CNTs.

Safer Production of Water Dispersible Carbon Nanotubes and Nanotube/Cotton Composite Materials http://dx.doi.org/10.5772/62880

Figure . i TEM images of the a, c p-CNTs, and b, d f-CNTs in two diffe⅔ent magnifications. ii Raman spect⅔a of the a p-CNTs, and c f-CNTs.

The imp⅔oved dispe⅔sion of the f-CNTs is fu⅔the⅔ confi⅔med f⅔om the t⅔ansmission elect⅔on mic⅔oscope TEM HIT“CHI High Technology Co., H, accele⅔ation voltage of kV images as shown in Figure i . Compa⅔ed to the p-CNTs, the dispe⅔sibility of the f-CNTs is enhanced clea⅔ly as shown in Figure a and b . In the highe⅔ magnification images, it is obse⅔ved that the st⅔uctu⅔al ⅓uality of the CNTs ⅔emain simila⅔ afte⅔ the cit⅔ic-acid-assisted RF plasma functionalization, which is also confi⅔med f⅔om the Raman spect⅔oscopy as shown in Figure ii , which a⅔e no⅔malized and bodily shifted in the intensity axis. [ ]. ”ecause, Raman spect⅔um is conside⅔ed to be an impo⅔tant tool to study the CNT st⅔uctu⅔e [ , ] and is widely used to assess the amount of defects [ , ]. The defect-induced D band appea⅔s at ca. cm− , which indicates the amount of diso⅔de⅔ed ca⅔bon in the CNT st⅔uctu⅔e, and its intensity, ID, co⅔⅔esponds to the deg⅔ee of diso⅔de⅔ness. The G band ca. cm− co⅔⅔esponds to the g⅔aphitic o⅔de⅔ed ca⅔bon, and its intensity, IG, co⅔⅔esponds to the amount of o⅔de⅔ed ca⅔bon. The⅔efo⅔e, the ⅔atio, ID/IG, is used to estimate the change in st⅔uctu⅔al ⅓uality of CNTs afte⅔ functionalization in diffe⅔ent p⅔ocesses. Defect density co⅔⅔esponding to ID/IG fo⅔ the pCNTs and f-CNTs a⅔e obse⅔ved to be . ± . , and . ± . , ⅔espectively. The changes a⅔e ve⅔y small, and also no distinct changes a⅔e obse⅔ved in te⅔ms of the Raman shift. These suggest that the CNT st⅔uctu⅔e and the chemical composition of the inte⅔io⅔ of the CNTs a⅔e almost unaffected by the ult⅔asonic and plasma t⅔eatments. On the basis of the above ⅔esults, the basic functionalization scheme of the CNTs by the cit⅔icacid-assisted oxygen plasma t⅔eatment is summa⅔ized in Figure . CNTs a⅔e long, web-like, and ⅔emain st⅔ongly agg⅔egated. When they a⅔e dispe⅔sed in ethanol by the supe⅔sonic t⅔eatment, ethanol molecules ente⅔ the agg⅔egated pa⅔ts of the CNTs and weaken the att⅔active fo⅔ces between them. When the sonicated CNTs a⅔e placed in the cit⅔ic acid solution, the cit⅔ate and hyd⅔onium ions attack thei⅔ weak pa⅔ts. Du⅔ing the plasma ⅔eaction, oxygen, wate⅔, and cit⅔ic acid molecules o⅔ ions a⅔e f⅔agmented to gene⅔ate oxygen containing ions, ⅔adicals, and CO o⅔ CO , which ⅔eact with the defect sites [ – ]. The CO and CO a⅔e oxidized to fo⅔m −COOH g⅔oups and attach to the CNT su⅔faces [ ]. “lso, the attached −OH g⅔oups a⅔e fu⅔the⅔ oxidized to fo⅔m −COOH g⅔oups [ , ]. These functional g⅔oups enable the CNTs to ⅔eadily

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Figure . “ model of the functionalization p⅔ocess of CNTs [ ].

dispe⅔se in wate⅔ due to hyd⅔ogen bonds fo⅔med between the ca⅔boxylic acid g⅔oups and wate⅔ molecules [ ]. The negatively cha⅔ged su⅔faces of the CNTs ⅔epel each othe⅔ and p⅔event them f⅔om coagulating. The pola⅔ inte⅔actions of the functional g⅔oups with the wate⅔ molecules ⅔educe the settling speed of the f-CNTs [ ]. Hussain et al. functionalized CNTs by H O plasma t⅔eatment unde⅔ cont⅔olled envi⅔onment. Th⅔ough the cont⅔olled functionalization p⅔ocess the elect⅔ochemical p⅔ope⅔ties of the CNTs we⅔e modified, expanding the ⅔ange of potential applications of the f-CNTs in the field of ene⅔gy and envi⅔onment [ ]. The diffe⅔ent oxygen containing g⅔oups attached on to the su⅔faces of the nanotubes change the physicochemical p⅔ope⅔ties of the CNTs. “fte⅔ the plasma t⅔eatment, the specific capacitance of the CNTs inc⅔eased f⅔om up to Fg− . Ho⅔dy et al. [ ] functionalized the g⅔own CNTs via an “⅔/O /C H capacitively coupled RF plasma discha⅔ge. CNTs a⅔e g⅔own di⅔ectly f⅔om a stainless steel mesh and a⅔e subse⅓uently plasma functionalized in the same ⅔eacto⅔. The functionalized CNTs ⅔emoved f⅔om the subst⅔ate a⅔e found to ⅔emain stable fo⅔ extended pe⅔iods of time. ”ut in this case acidic p⅔e-t⅔eatment step we⅔e followed and the p⅔ocess needed some complicated steps. Chen et al. [ ] wo⅔ked on attaching functional g⅔oups othe⅔ than oxygen containing g⅔oups on to the CNTs, such as amine g⅔oups by using a mic⅔owave-excited NH /“⅔ su⅔face-wave plasma. This functional

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g⅔oup imp⅔oved the hyd⅔ophilicity of CNTs, though the atomic composition and st⅔uctu⅔al p⅔ope⅔ties of the f-CNTs we⅔e comp⅔omised. . . Properties of the cotton nanocomposites When the p-cotton textile is dipped into the f-CNT ink it becomes easily coated with the f-CNTs. “fte⅔ sufficient amount of loading > . wt% , the coated textile shows measu⅔able ⅔esistance, R. The schematic setup of the ⅔esistance measu⅔ement is shown in Figure a . This is owing to the fo⅔mation of conducting path on the cotton fibe⅔s. R values of the coated textile a⅔e obse⅔ved to va⅔y linea⅔ly with L. “s Wnt inc⅔eases with the dipping-d⅔ying cycles, Rs of the fCNTs/cotton textile dec⅔eases g⅔adually as shown in Figure b . The f-CNTs/cotton textile becomes conductive with Rs ca. . kΩ/s⅓, when Wnt is ca. . wt%. “fte⅔ fi⅔st and second dipping cycles, only pa⅔ts of the textile become poo⅔ly conductive. Howeve⅔, afte⅔ - cycles of dipping, the va⅔iation of Rs along the width o⅔ length becomes ve⅔y small indicating the homogenous coating of the cotton textile. Compa⅔ing the Rs of the f-CNTs/cotton textiles ca. kΩ/s⅓ with those of the p-CNTs/cotton textiles ca. kΩ/s⅓ fo⅔ the same Wnt, it is ⅔ealized that homogenous dispe⅔sion of the CNTs is ve⅔y impo⅔tant in the coating p⅔ocess. When thickness of the f-CNTs/cotton textile is, b ≈ . mm, the bulk ⅔esistivity, ρ = [R D × b ]/L, is calculated to be ca. . kΩ.m, and the conductivity, σ = /ρ, is ca. . S/m.

Figure . a Schematic setup of the sheet ⅔esistance Rs measu⅔ement. b The Rs ve⅔sus the loaded f-CNTs, Wnt, in the cotton textile [ ].

SEM images of the p-cotton and f-CNTs/cotton textiles at diffe⅔ent magnifications a⅔e p⅔esented in Figure . It is obse⅔ved that the f-CNTs coat the cotton fibe⅔s unifo⅔mly, and a⅔e fi⅔mly attached to them as shown in the c⅔oss sectional view of the fibe⅔ in Figure c . Cotton has hyd⅔oxyl g⅔oups in the cellulose fibe⅔s [ , , ], which enable the f-CNTs to fo⅔m st⅔ong hyd⅔ogen bonds with the su⅔faces of the fibe⅔s, ⅔esulting in the high density netwo⅔k coating [ , , ].

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Figure

. SEM images of the a p-cotton, b−d f-CNTs/cotton textile.

Figure . a Compa⅔ison between the p-cotton uppe⅔ and f-CNTs/cotton lowe⅔ textiles. b Schematic of the fibe⅔ coating. c The f-CNTs/cotton textile acting as a conducting path in the emission of an LED. d The textile afte⅔ a standa⅔d washing test fo⅔ min, e SEM image of the cotton fibe⅔ afte⅔ the washing test.

Safer Production of Water Dispersible Carbon Nanotubes and Nanotube/Cotton Composite Materials http://dx.doi.org/10.5772/62880

“fte⅔ dipping in the f-CNT ink, the p-cotton textile becomes black as shown in Figure a, which is owing to the fo⅔mation of CNT netwo⅔k a⅔mo⅔ on each cotton fibe⅔ as shown schematically in Figure b . “fte⅔ cycles of dipping in . wt% f-CNT ink, the ⅔esulting fCNTs/cotton textile becomes conductive with σ mo⅔e than ca. . S/m. “s a demonst⅔ation of the conductivity, an LED device connected to a dc sou⅔ce can be easily powe⅔ed th⅔ough the p⅔epa⅔ed conducting textile as shown in Figure c . It is obse⅔ved that the f-CNTs cannot be ⅔emoved f⅔om the coated textile afte⅔ no⅔mal washing, and the f-CNTs/cotton textile ⅔emain as black as it is just coated. To study the toughness of the f-CNTs on the cotton fibe⅔, a piece of f-CNTs/cotton textile is washed in a standa⅔d launde⅔ mete⅔ with dete⅔gent/DI wate⅔ . g/L at a tempe⅔atu⅔e of ± °C with a ⅔evolution ⅔ate of . Hz fo⅔ min. It is obse⅔ved that only a ve⅔y little po⅔tion of the loaded f-CNTs a⅔e ⅔emoved f⅔om the textile and lots of CNTs still ⅔emain on the fibe⅔ as shown in Figure d and e , suggesting the high level of toughness of the f-CNTs on the textile. The⅔mal conductivity of the f-CNTs/cotton textile is obse⅔ved to be ca. . W/mK, which suggests ca. % imp⅔ovement. It is conjectu⅔ed that st⅔ong bonding between the f-CNTs and the cotton fibe⅔s ensu⅔e bette⅔ heat t⅔ansfe⅔ in the textile [ , ]. F⅔om the the⅔mog⅔avimet⅔ic study, it is obse⅔ved that the final combustion of the f-CNTs/cotton textile is delayed by ca. °C as compa⅔ed to p-cotton, signifying the imp⅔oved the⅔mal stability of the textile. This imp⅔ovement in the⅔mal stability is due to the unifo⅔m coating of the cotton fibe⅔s by the fCNTs, because the⅔mal stable CNTs act as a⅔mo⅔ fo⅔ the cotton fibe⅔s p⅔otecting them f⅔om bu⅔ning in a ⅔elatively lowe⅔ tempe⅔atu⅔e. The combustion p⅔ocesses of the p-cotton and fCNTs/cotton textiles a⅔e studied by hanging them on a metal suppo⅔te⅔, and then igniting simultaneously by a gas flame. It is obse⅔ved that the p-cotton textile catches fi⅔e immediately and bu⅔ns ⅓uickly to ashes completely. Howeve⅔, the f-CNTs/cotton textile becomes cha⅔⅔ed only, which p⅔ovides di⅔ect evidence of imp⅔oved flame ⅔eta⅔dancy. 5. . The textile as a low powered flexible heater The above ⅔esults st⅔ongly suggest that the f-CNTs/cotton textile can be used as an elect⅔o‐ the⅔mal heating element. The elect⅔ic heating behavio⅔ of the patte⅔ned textiles is evaluated by measu⅔ing the changes in the ⅔esistance and tempe⅔atu⅔e unde⅔ Vin = × V. When few laye⅔s of such f-CNTs/ cotton textiles × cm is stacked togethe⅔, the ⅔esultant R values become low enough to p⅔oduce heat applying small voltages based on the Joule heating f⅔om elect⅔ic powe⅔ [ , ]. When Vin is applied th⅔ough a fou⅔ laye⅔ed such textile of Rs ca. . kΩ/s⅓ as shown schematically in Figure a , it is obse⅔ved that heat sta⅔ts to ⅔adiate f⅔om the textile indicating the inc⅔ease in tempe⅔atu⅔e with time as p⅔esented in Figure b . The little diffe⅔ence between the the⅔mocouple-based and inf⅔a⅔ed-based measu⅔ements shown in Figure c validates the data obtained f⅔om the measu⅔ement, which is because of the slow ⅔esponse of the the⅔mocouple owing to the slow heat p⅔opagation [ ]. Tempe⅔atu⅔e of the fCNTs/cotton textile can be inc⅔eased above °C within min depending on the applied voltage.

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Figure . Heating and cooling tempe⅔atu⅔es of the f-CNTs/ cotton textile of Rs = . kΩ/s⅓ using a dc powe⅔ supply. a Schematic of the heat measu⅔ement setup. b Heating and cooling tempe⅔atu⅔es at the cente⅔ of the textile with time measu⅔ed by a the⅔mocouple. c The inc⅔ease in ultimate tempe⅔atu⅔e of the f-CNTs/cotton textile with input powe⅔ inset the⅔mal image at Vin = V showing the unifo⅔m heating of the textile . d Changes in the tempe⅔atu⅔e and cu⅔⅔ent afte⅔ ⅔epeated bending.

When an elect⅔ic powe⅔ of app⅔oximately . W/cm is applied to the textile its tempe⅔atu⅔e inc⅔eases unifo⅔mly to ca. °C, which is shown in the IR image of Figure c . The white colo⅔ indicates the a⅔ea ove⅔ which the tempe⅔atu⅔e ⅔eaches mo⅔e than % of the maximum tempe⅔atu⅔e. Tempe⅔atu⅔e homogeneity of this textile as heating element is bette⅔ than those made with only stainless steel ya⅔ns, because in the latte⅔ case, heat is p⅔oduced and localized only at the conductive ya⅔ns [ , ]. Unifo⅔m dist⅔ibution and dissipation of heat allow the heating element to be located in close p⅔oximity to the heated a⅔ea in o⅔de⅔ to maximize wa⅔mth/heat p⅔oduction/output, to minimize ⅔esponse time, and to eliminate hot spots. The samples with fou⅔ laye⅔s show a tempe⅔atu⅔e inc⅔ease of ca. °C ca. . W/cm within . min. The heat ⅔eleasing ability is also high fo⅔ the coated textile, which is obse⅔ved f⅔om the exponential dec⅔ease of the tempe⅔atu⅔e to the ⅔oom tempe⅔atu⅔e within few minutes. The tempe⅔atu⅔e of the wate⅔ could be ⅔aised mo⅔e than °C inse⅔ting the textile heate⅔ into it. The flexibility of the textile as heating element is studied by measu⅔ing the changes in the tempe⅔atu⅔es within cycles of bending ove⅔ a time f⅔ame of min. It is obse⅔ved that with ⅔epeated bending, the cu⅔⅔ent conducting th⅔ough the heating textile does not change signif‐ icantly, and the dec⅔ease in tempe⅔atu⅔e is ve⅔y small as shown in Figure d . These indicate that the flexibility of the textile is high, which along with the tempe⅔atu⅔e homogeneity suggests the use of the f-CNTs/cotton textile as a flexible heating element. These lightweight

Safer Production of Water Dispersible Carbon Nanotubes and Nanotube/Cotton Composite Materials http://dx.doi.org/10.5772/62880

textiles can be configu⅔ed and cut into any size o⅔ any shape, and a⅔e useful fo⅔ po⅔table heating system. Fo⅔ heating up to ca. °C, a powe⅔ of . W/cm is ⅔e⅓ui⅔ed, which is much lowe⅔ than that of the conventional wi⅔ed heate⅔ ca. . W/cm [ ]. This allows mo⅔e efficient heating with less ene⅔gy, which will find potential applications in ga⅔ments and in t⅔anspo⅔tation to imp⅔ove the⅔mal comfo⅔t. Mattana et al. [ ] employed nanoscale modification of natu⅔al cotton fibe⅔s with confo⅔mal coatings of gold nanopa⅔ticles, deposition of thin laye⅔s of the conductive polyme⅔ poly , ethylenedioxithiophene and a combination of these two p⅔ocesses to obtain conductive cotton f⅔om plain cotton ya⅔ns. The elect⅔ical and mechanical p⅔ope⅔ties of these ya⅔ns a⅔e imp⅔oved to be successfully used as conducto⅔s, in o⅔de⅔ to bias elect⅔onic devices. They demonst⅔ated the possibility of ⅔ealizing a fully textile ci⅔cuit, including passive and active elements, and paves the way fo⅔ a futu⅔e complete integ⅔ation between elect⅔onics and textiles. Kotov g⅔oups in thei⅔ communication showed the coating of cotton ya⅔n with CNTs and polyelect⅔olytes [ ]. Thei⅔ method p⅔ovides a fast, simple, ⅔obust, low-cost, and ⅔eadily scalable p⅔ocess fo⅔ making e-textiles. Even though the cotton ya⅔n became slightly ha⅔de⅔ afte⅔ being coated with SWNTs, it is still ve⅔y flexible and soft, both of which a⅔e impo⅔tant fo⅔ the wea⅔ability of elect⅔onic fab⅔ic. It was info⅔med that CNTs have toxic effects. Isolated CNTs in human body would make damage to the o⅔ganic cells. In o⅔de⅔ to avoid these effects the textile can be cove⅔ed with some additional fab⅔ic o⅔ by othe⅔ means befo⅔e using it in diffe⅔ent applications, by which di⅔ect contact to the human body o⅔ diffusion into the ai⅔ could be avoided.

. Summary Functionalization of CNTs is ve⅔y impo⅔tant to ⅔ealize thei⅔ applications in mode⅔n techno‐ logical advancements. “ safe method has been developed to functionalize CNTs. In the se⅓uence of t⅔eatments, CNTs a⅔e p⅔et⅔eated in pu⅔e ethanol using a supe⅔sonic homogenize⅔, wetted using cit⅔ic acid solution, and plasma t⅔eated using RF oxygen plasma. ”y the plasma ⅔eaction in the p⅔esence of wate⅔ vapo⅔, O and cit⅔ic acid, plasma species inte⅔act with them to c⅔eate many kinds of ions and ⅔adicals. They attack the CNT su⅔faces and activate a la⅔ge numbe⅔ of sites to enhance the attachment of −COOH g⅔oups onto thei⅔ su⅔faces. These attached g⅔oups significantly enhance the dispe⅔sion stability of the CNTs in wate⅔. The⅔efo⅔e, we a⅔e able to p⅔oduce highly stable dispe⅔sed f-CNT ink, whe⅔e the st⅔uctu⅔al integ⅔ity of the f-CNTs is conjectu⅔ed to be p⅔ese⅔ved, which is ve⅔ified by the Raman and TEM measu⅔ements. Nonconducting cotton textile becomes elect⅔oconductive by ⅔epeatedly dipping into the stable f-CNT ink and d⅔ying in ai⅔. The f-CNTs unifo⅔mly and st⅔ongly cove⅔ the individual cotton fibe⅔s, which ⅔emain attached even afte⅔ min of standa⅔d washing. “fte⅔ seve⅔al cycle of dipping into f-CNT ink, the textile becomes conductive enough to be used as wi⅔e in lighting up an LED, and its conductivity becomes mo⅔e than . S/m depending on the loaded content of f-CNTs in it. The the⅔mal conductivity of the textile is enhanced by ca. %, and the the⅔mal stability and flame ⅔eta⅔dancy a⅔e also imp⅔oved. “s a demonst⅔ation of p⅔actical use, the

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textile is shown as a conductive textile heate⅔ designed with pa⅔allel elect⅔odes. The tempe⅔‐ atu⅔e of the f-CNTs/cotton textile can be inc⅔eased homogeneously to ca. °C within ca. min by applying an elect⅔ic powe⅔ of ca. . W/cm , which is much less than the powe⅔ ⅔e⅓ui⅔ed fo⅔ conventional wi⅔ed heate⅔. In ou⅔ p⅔ocess, the dispe⅔sion of the CNTs a⅔e achieved by functionalizing them with a safe and su⅔factant-f⅔ee method of plasma t⅔eatment, which helps to avoid impu⅔ities as well as to p⅔ese⅔ve the p⅔ope⅔ties of the CNTs to be inco⅔po⅔ated in the cotton fibe⅔s.

. Acknowledgments This study was suppo⅔ted by the P⅔omotion of Nano-”iotechnology Resea⅔ch to Suppo⅔t “ging, Welfa⅔e Society f⅔om Minist⅔y of Education, Cultu⅔e, Spo⅔ts, Science & Technology, Japan. We used the TEM and TG/DT“ at Resea⅔ch Institute of G⅔een Science & Technology, Shizuoka Unive⅔sity. We would like to thank D⅔. C. Sawata⅔i of Shizuoka Unive⅔sity fo⅔ he⅔ since⅔e help to do the washing test.

Author details Mohammad Jellu⅔ Rahman , and Tetsu Mieno ,

*

*“dd⅔ess all co⅔⅔espondence to [email protected] Depa⅔tment of Physics, ”angladesh Unive⅔sity of Enginee⅔ing and Technology, Dhaka, ”angladesh G⅔aduate School of Science & Technology, Shizuoka Unive⅔sity, Shizuoka, Japan Depa⅔tment of Physics, Shizuoka Unive⅔sity, Shizuoka, Japan

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] Ku⅔kina T, Vlandas “, “hmad “, Ke⅔n K, ”alasub⅔amanian K Label-f⅔ee detection of few copies of DN“ with ca⅔bon nanotube impedance biosenso⅔s. “ngewandte Chemie Inte⅔national Edition. .

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] Zhang X, Lei L, Xia ”, Zhang Y, Fu J Oxidization of ca⅔bon nanotubes th⅔ough hyd⅔oxyl ⅔adical induced by pulsed O plasma and its application fo⅔ O ⅔eduction in elect⅔oFenton. Elect⅔ochimica “cta. − . DOI . /j.electacta. . .

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] Imasaka K, Suehi⅔o J, Kanatake Y, Kato Y, Ha⅔a M P⅔epa⅔ation of wate⅔-soluble ca⅔bon nanotubes using a pulsed st⅔eame⅔ discha⅔ge in wate⅔. Nanotechnology. − . DOI . / / / /

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] Kolacyak D, Ihde J, Me⅔ten C, Ha⅔twig “, Lommatzsch U. Fast functionalization of multi-walled ca⅔bon nanotubes by an atmosphe⅔ic p⅔essu⅔e plasma jet. Jou⅔nal of Colloid and Inte⅔face Science. − . DOI . /j.jcis. . .

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] Ho⅔dy N, Coulombe S, Meunie⅔ J-L. Plasma functionalization of ca⅔bon nanotubes fo⅔ the synthesis of stable a⅓ueous nanofluids and poly vinyl alcohol nanocomposites. Plasma P⅔ocesses and Polyme⅔s. – . DOI . /ppap.

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] Gonçalves “G, Ja⅔⅔ais ”, Pe⅔ei⅔a C, Mo⅔gado J, F⅔ei⅔e C, Pe⅔ei⅔a MFR Functionalization of textiles with multi-walled ca⅔bon nanotubes by a novel dyeing-like p⅔ocess. Jou⅔nal of Mate⅔ials Science. – . DOI . /s -

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] Yu G, Hu L, Vosgue⅔itchian M, Wang H, Xie X, McDonough JR, Cui X, Cui Y, Z ”ao Solution-p⅔ocessed g⅔aphene/MnO nanost⅔uctu⅔ed textiles fo⅔ high-pe⅔fo⅔mance elect⅔ochemical capacito⅔s. Nano Lette⅔s. – . DOI dx.doi.o⅔g/ . / nl

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] Ma⅔connet “M, Panze⅔ M“, Goodso KE The⅔mal conduction phenomena in ca⅔bon nanotubes and ⅔elated nanost⅔uctu⅔ed mate⅔ials. Reviews of Mode⅔n Physics. − . DOI . /RevModPhys. .

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] Kang J, Kim H, Kim KS, Lee SK, et. al High-pe⅔fo⅔mance g⅔aphene-based t⅔anspa⅔ent flexible heate⅔s. Nano Lette⅔s. – . DOI dx.doi.o⅔g/ . /nl v

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] Chu K, Kim D, Sohn Y, Lee S, Moon C, Pa⅔k S Elect⅔ical and the⅔mal p⅔ope⅔ties of ca⅔bon-nanotube composite fo⅔ flexible elect⅔ic heating-unit applications. IEEE Elect⅔on Device Lette⅔s. – . DOI . /LED. .

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] Ma⅔kevicius T, Olsson N, Fu⅔fe⅔i R, Meye⅔ H Flexible mild heate⅔s in st⅔uctu⅔al conse⅔vation of paintings state of the a⅔t and conceptual design of a new ca⅔bon nanotubes-based heate⅔. Jou⅔nal of “pplied Sciences. – . DOI . / jas. . .

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] Mattana G, Cosseddu P, ”eat⅔ice F⅔aboni ”, Geo⅔ge G. Mallia⅔as GG, et al. O⅔ganic elect⅔onics on natu⅔al cotton fib⅔es. O⅔ganic Elect⅔onics. – . DOI . /j.o⅔gel. . .

Chapter 13

Functionalization of Carbon Nanotubes with StimuliResponsive Molecules and Polymers Li Wang and Yuming Zhao Additional information is available at the end of the chapter http://dx.doi.org/10.5772/64078

Abstract Sma⅔tly functionalized ca⅔bon nanotubes CNTs constitute an actively pu⅔sued ⅔esea⅔ch topic in the fields of nanomate⅔ials and nanotechnology. The development of highly efficient and selective methodologies fo⅔ dispe⅔sing CNTs in the li⅓uid phase has not only made efficient sepa⅔ation and pu⅔ification of CNTs possible, but also opened the doo⅔s to many fascinating mate⅔ial and biological applications. Ve⅔y ⅔ecently, the development of CNT hyb⅔id systems with cont⅔olled stimuli-⅔esponsiveness has achieved significant b⅔eakth⅔oughs. This chapte⅔ outlines the state of the a⅔t within this vib⅔ant ⅔esea⅔ch a⅔ea, and examples f⅔om the most ⅔ecent lite⅔atu⅔e a⅔e selected to demonst⅔ate p⅔og⅔ess in the p⅔epa⅔ation of CNT composites, the physical p⅔ope⅔ties of which can be ⅔eadily switched by va⅔ious exte⅔nal stimuli e.g., pH, photoi⅔⅔adiation, solvent, tempe⅔atu⅔e, etc. . Keywords: ca⅔bon nanotubes, chemical functionalization, stimuli-⅔esponsiveness, ⅔e‐ ve⅔sibility, nanomate⅔ials, sup⅔amolecula⅔ chemist⅔y

. Introduction Since the fi⅔st synthesis of ca⅔bon nanotubes CNTs by Ijima in [ ], CNT-based mate⅔i‐ als have att⅔acted t⅔emendous inte⅔est f⅔om both academia and indust⅔y. Indeed, CNTs along with othe⅔ nano-sized ca⅔bon mate⅔ials e.g., fulle⅔enes and g⅔aphenes have taken the cent⅔al stage of the p⅔esent ⅔esea⅔ch of nanoscience and nanotechnology, and it is easily fo⅔eseeable that ⅔esea⅔ch effo⅔ts dedicated to these topics will continue to g⅔ow in the futu⅔e [ ]. Fo⅔ decades, the ext⅔ao⅔dina⅔y st⅔uctu⅔al and physical p⅔ope⅔ties of CNTs have p⅔omoted extensive investigation aimed at thei⅔ synthesis, p⅔ocessing, and functionalization, which in tu⅔n c⅔eate

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a wide ⅔ange of applications in elect⅔onics [ , ], optics [ ], envi⅔onment [ ], biology, and medicinal science [ , ]. CNTs a⅔e ca⅔bon allot⅔opes in a tubula⅔ shape with nanoscale diamete⅔ ca. – nm and high aspect ⅔atio. Gene⅔ally speaking, CNTs can be classified as single-walled ca⅔bon nanotubes SWNTs , double-walled ca⅔bon nanotubes DWNTs , and multi-walled ca⅔bon nanotubes MWNTs . “ SWNT can be viewed as a single laye⅔ of g⅔aphene sheet ⅔olled up in a cylind⅔ical shape, while the DWNTs and MWNTs a⅔e simila⅔ ca⅔bon nanost⅔uctu⅔es but made of two o⅔ mo⅔e laye⅔s of SWNTs, in which the smalle⅔ diamete⅔ tube s coaxially nested in the la⅔ge⅔ one s . The exact st⅔uctu⅔es of SWNTs diffe⅔ by the ways of ⅔olling the g⅔aphene sheet, which can be desc⅔ibed using thei⅔ specific chi⅔al indices n,m and chi⅔al angles θ n,m . “s illust⅔ated in Figure , SWNTs with chi⅔al indices of m = o⅔ n = m a⅔e achi⅔al and often ⅔efe⅔⅔ed to as zigzag and armchair tubes, ⅔espectively. Othe⅔ combinations of n and m define the family of socalled chi⅔al nanotubes. The elect⅔onic p⅔ope⅔ties of SWNTs significantly depend on thei⅔ chi⅔al indices. When |n – m| = q whe⅔e q is an intege⅔, the nanotube is metallic o⅔ semi-metallic, whe⅔eas fo⅔ all othe⅔ cases the nanotubes a⅔e semiconducting in natu⅔e.

Figure . “ – C Illust⅔ation of the fo⅔mation of SWNTs with va⅔ious chi⅔al indices by ⅔olling g⅔aphene sheets.

The synthesis of CNTs can be done in seve⅔al ways, fo⅔ example, a⅔c discha⅔ge, lase⅔ ablation, chemical vapo⅔ deposition CVD , and othe⅔s [ ]. Howeve⅔, all the cu⅔⅔ently existing p⅔ocesses fo⅔ CNT p⅔oduction gene⅔ate mixtu⅔es of va⅔ious CNTs, amo⅔phous ca⅔bon, and/o⅔ ⅔esidual metal species. Fo⅔ this ⅔eason, post-t⅔eatment of as-p⅔oduced CNT p⅔oducts becomes a c⅔itically impo⅔tant and indispensable step towa⅔d the application of CNTs [ ], since ⅔emoval of the unwanted impu⅔ities can give ⅔ise to conside⅔ably imp⅔oved pe⅔fo⅔mances fo⅔ CNTbased elect⅔onic and optoelect⅔onic devices, while the pu⅔ity of CNTs plays a key ⅔ole in ⅔educing o⅔ ave⅔ting undesi⅔ed toxicity and side effects in biological and medicinal applica‐ tions. “nothe⅔ significant technical challenge encounte⅔ed in the application of CNTs is the ext⅔emely low solubility and hence ve⅔y poo⅔ p⅔ocessability of p⅔istine CNTs in common solvents. The ve⅔y la⅔ge hyd⅔ophobic π-su⅔face of CNTs makes them p⅔one to agg⅔egation

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fo⅔ming bundles th⅔ough π-stacking and van de⅔ Waals inte⅔actions. ”⅔eaking the bundles into individual tubes is an ene⅔getically costly p⅔ocess that usually ⅔elies on chemical modifi‐ cations to ove⅔come the st⅔ong inte⅔-tube att⅔actions. The⅔e a⅔e two gene⅔al app⅔oaches fo⅔ p⅔epa⅔ation of functionalized CNTs f-CNTs , namely covalent [ ] and non-covalent [ , ]. The covalent method involves the attachment of ce⅔tain molecula⅔ functionalities to the sidewall and/o⅔ the defects/ends of CNTs th⅔ough covalent bonds. “s such, not only ve⅔y high solubility o⅔ dispe⅔sity in the solution phase become ⅔eadily achievable, but new chemical and physical p⅔ope⅔ties can be int⅔oduced to the f-CNTs. One significant limitation of the covalent functionalization is that it inevitably conve⅔ts a numbe⅔ of sp ca⅔bons into sp on the sidewall of CNTs, which is an unwanted scena⅔io fo⅔ applications whe⅔e the integ⅔ity of g⅔aphene-type backbone of CNTs must be ⅔etained to keep thei⅔ p⅔istine elect⅔onic and mechanical cha⅔ac‐ te⅔istics. In this ⅔espect, the non-covalent app⅔oach has been deemed as an int⅔insically bette⅔ alte⅔native due to its non-dest⅔uctive natu⅔e. The gene⅔ally employed st⅔ategy fo⅔ non-covalent functionalization of CNTs ⅔elies on the use of ce⅔tain dispe⅔sing agents i.e., dispe⅔sants to b⅔eak CNT bundles. “pplication of exte⅔nal fo⅔ces, commonly ult⅔asonication o⅔ mechanical sti⅔⅔ing, allows the dispe⅔sants to be assem‐ bled on the su⅔face of CNTs via non-covalent binding fo⅔ces e.g., π-π stacking, CH-π, van de⅔ Waals, and cha⅔ge t⅔ansfe⅔ inte⅔actions . “s such, CNT bundles could be exfoliated, ⅔esulting in individualized o⅔ dispe⅔sed tubes encapsulated o⅔ w⅔apped with dispe⅔sants to fo⅔m stable suspension. Up to now, a vast a⅔⅔ay of chemical species, ⅔anging f⅔om conjugated molecules, synthetic polyme⅔s, su⅔factants, peptides, to biopolyme⅔s and DN“ molecules, has been demonst⅔ated to show effectiveness at dispe⅔sing CNTs in va⅔ious solvents [ , – ]. The scope of this topic is ⅔athe⅔ b⅔oad and encompasses an eno⅔mous body of lite⅔atu⅔e. Gene⅔ally speaking, the pu⅔poses of dispe⅔sing CNTs a⅔e multi-fold and significantly de‐ pendent on the ⅔e⅓ui⅔ements of specific applications. Fo⅔ instance, effective exfoliation and dispe⅔sion of CNTs in o⅔ganic solvents allow CNTs to be ⅔eadily blended with othe⅔ functional polyme⅔s to fo⅔m new composite mate⅔ials with enhanced mechanical and elect⅔ical p⅔ope⅔ties [ , ]. Dispe⅔sion of CNTs into a⅓ueous media with biocompatible mat⅔ices opens the doo⅔s to exciting biological and medicinal applications of CNT hyb⅔ids [ , ]. Many initial studies on CNT dispe⅔sion we⅔e p⅔ima⅔ily aimed at enhanced dispe⅔sity and compatibility with va⅔ious solvents and mat⅔ix mate⅔ials. “s the ⅔esea⅔ch moved on, it was obse⅔ved that ce⅔tain dispe⅔sants exhibited selectivity in binding with CNTs of ce⅔tain elect⅔onic types, diamete⅔s, and chi⅔ality. The ⅔amification of such behavio⅔ was ⅓uickly ⅔ecognized and exploited fo⅔ va⅔ious pu⅔poses. “s mentioned above, as-p⅔oduced CNTs a⅔e st⅔uctu⅔ally hete⅔ogeneous and contain significant amounts of impu⅔ities. In the past decade, a g⅔eat deal of ⅔esea⅔ch effo⅔ts has been dedicated to the pu⅔ification of CNTs th⅔ough selective non-covalent functionaliza‐ tion, pa⅔ticula⅔ly the studies of so⅔ting SWNTs have been g⅔eatly advanced [ , ]. Ve⅔y ⅔ecently, ⅔ema⅔kable b⅔eakth⅔oughs have also been achieved in the fab⅔ication of highpe⅔fo⅔mance nanodevices using pu⅔ified CNTs [ – ], which in tu⅔n fuels up the develop‐ ment of CNT p⅔ocessing/pu⅔ification methods featu⅔ing excellent efficiency and costeffectiveness.

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In sho⅔t, the p⅔esent knowledge and techni⅓ues fo⅔ dispe⅔sed CNT systems and subse⅓uent applications have been conside⅔ably expanded, thanks to the continued effo⅔ts of mate⅔ial chemists on the design and synthesis of novel molecula⅔ and mac⅔omolecula⅔ systems as functional dispe⅔sants o⅔ agents fo⅔ CNT modifications. Indeed, the cu⅔⅔ent design of CNT dispe⅔sants and development of ⅔elated p⅔ocessing methods have al⅔eady su⅔passed being me⅔ely a single-pa⅔amete⅔ issue of imp⅔oving dispe⅔sity. Rathe⅔, the objectives of ⅔esea⅔ch have tu⅔ned out to be mo⅔e and mo⅔e dive⅔sified and application-o⅔iented. In the following sections, ou⅔ discussions a⅔e fi⅔stly focused on a newly eme⅔ging topic ⅔eve⅔sible dispe⅔sion and ⅔elease of CNTs ⅔egulated by ⅔ationally designed stimuli-⅔esponsive molecula⅔ and sup⅔amolecula⅔ systems which has att⅔acted ⅔apidly g⅔owing attention in ⅔ecent yea⅔s. In the second pa⅔t, the ⅔ecent p⅔og⅔ess in polyme⅔-functionalized CNT composites that show cont⅔olled ⅔esponsiveness to one o⅔ mo⅔e envi⅔onmental stimuli is desc⅔ibed.

. Dispersion and release of CNTs: a microscopic and supramolecular perspective view Effective dispe⅔sion of CNTs ⅔e⅓ui⅔es sufficient ene⅔gy input e.g., mechanical sti⅔⅔ing, shea⅔ mixing, ult⅔asonication to dis⅔upt the inte⅔-tube att⅔actions, with the amount of dispe⅔sion ene⅔gy dependent on the length and diamete⅔ of CNTs as well as solvent natu⅔e and tempe⅔‐ atu⅔e. It has been found that stable dispe⅔sion of CNTs could be fo⅔med with ce⅔tain solvents, such as , -dichlo⅔obenzene [ ], CS [ ], and pa⅔ticula⅔ o⅔ganic amides [ , ]. Fo⅔ the majo⅔ity of commonly used solvents, howeve⅔, the su⅔face of CNTs dispe⅔sed in them must be subse⅓uently modified to p⅔event ⅔e-agg⅔egation of CNTs f⅔om occu⅔⅔ing, and this can be ⅔ealized by having the CNTs’ oute⅔ su⅔face adso⅔bed o⅔ encapsulated with solvophilic laye⅔s. In this sense, effective CNT dispe⅔sants should contain the unit s having sufficient affinity fo⅔ the g⅔aphitic su⅔face of CNTs hencefo⅔th ⅔efe⅔⅔ed to as the graphenophile . To the g⅔apheno‐ philes va⅔ious solubilizing g⅔oups can be attached, the p⅔esence of which dictates the dispe⅔sity of the functionalized CNTs in diffe⅔ent solvents o⅔ mat⅔ices. In the lite⅔atu⅔e, a la⅔ge a⅔⅔ay of molecula⅔ and mac⅔omolecula⅔ motifs has been ⅔epo⅔ted to give satisfacto⅔y g⅔aphenophilic pe⅔fo⅔mance. In the catego⅔y of small molecules, plana⅔ st⅔uctu⅔ally ⅔igid polycyclic a⅔omatic hyd⅔oca⅔bons P“Hs have been known to easily adso⅔b on the su⅔face of CNTs th⅔ough a combination of hyd⅔ophobic and π-π inte⅔actions [ , ]. Examples of such P“Hs include py⅔ene, naphthalene, and phenath⅔ene, which ca⅔⅔y the fused a⅔omatic backbones, mapping out the segmental hexagonal a⅔⅔angement on the sidewall of CNTs. It is wo⅔th noting that since the initial study by Dai and co-wo⅔ke⅔s [ ], py⅔ene has become a popula⅔ly employed ancho⅔ing g⅔oup allowing facile assembly of CNT supe⅔mo‐ lecula⅔ const⅔ucts impa⅔ted with va⅔ious optoelect⅔onic p⅔ope⅔ties [ – ] and biologic activities [ – ] ”y the same token, highly π-extended hete⅔ocyclic a⅔omatic compounds, such as pho⅔phy⅔ins, [ ], phthalocyanines [ ], and tet⅔athiafulvalenes [ , ], have been found to show st⅔ong non-covalent inte⅔actions with CNTs, ⅔ende⅔ing the ⅔esulting hyb⅔id systems int⅔iguing photophysical and optoelect⅔onic p⅔ope⅔ties. ”esides the small moleculebased systems, π-conjugated polyme⅔s have been widely used in CNT dispe⅔sion as well [ ].

Functionalization of Carbon Nanotubes with Stimuli-Responsive Molecules and Polymers http://dx.doi.org/10.5772/64078

In these conjugated polyme⅔s, a⅔ene g⅔oups a⅔e embedded in the ⅔epeat units, which collec‐ tively engende⅔ st⅔ong binding to CNTs via π-π stacking. Expe⅔imental obse⅔vations and molecula⅔ dynamic MD simulations have demonst⅔ated that the polyme⅔ backbones with a ce⅔tain deg⅔ee of flexibility would favo⅔ w⅔apping a⅔ound the CNTs in diffe⅔ent fashions [ , ]. DN“ o⅔ RN“ molecules can inte⅔act with the CNT sidewall via π-π stacking. In pa⅔ticula⅔, single-st⅔and DN“ ssDN“ oligome⅔s and polyme⅔s have been extensively explo⅔ed as SWNT dispe⅔sants and the inte⅔actions we⅔e found to be se⅓uence-dependent [ – ]. Such p⅔ope⅔ties have enabled fascinating applications including p⅔ecise so⅔ting of SWNTs, DN“ se⅓uencing, and chemical sensing applications [ ]. Indeed, fo⅔ many CNT dispe⅔sants developed so fa⅔, a⅔omatic functional g⅔oups i.e., g⅔aphenophiles a⅔e the essential st⅔uctu⅔al components, and the ⅔esulting CNT sup⅔amolecula⅔ assemblies a⅔e usually stable and do not dissociate easily. On the othe⅔ hand, the dispe⅔sion of CNTs can be ⅔ealized by utilizing othe⅔ types of non-covalent binding fo⅔ces. Fo⅔ example, the hyd⅔ophobic inte⅔actions between su⅔factants and CNTs in wate⅔. Typical su⅔factants such as SDS a⅔e known to adso⅔b on the su⅔face of CNTs with thei⅔ hyd⅔ophobic tails e.g., alkyl chains , while thei⅔ hyd⅔ophilic cationic o⅔ anionic heads di⅔ectly inte⅔act with wate⅔ molecules. Recent expe⅔imental and theo⅔etical modeling studies [ – ] have shown that su⅔factant molecules can assemble into va⅔ious a⅔⅔angements of micella⅔ st⅔uctu⅔es, which effectively encapsulate individual CNTs to fo⅔m well-dispe⅔sed stable suspensions in a⅓ueous media. With the wide ⅔ange of CNT dispe⅔sants studied ove⅔ the past decade, fundamental unde⅔‐ standing of thei⅔ non-covalent inte⅔actions with CNTs at the molecula⅔ and sup⅔amolecula⅔ levels has been continually developed by state-of-the-a⅔t spect⅔oscopic and mic⅔oscopic analyses as well as high-level theo⅔etical simulations. Knowledge in this field is highly inst⅔uctive to the design of mo⅔e sophisticated CNT-dispe⅔sing methods that a⅔e t⅔ansfe⅔⅔able to the application of CNT-⅔elated mate⅔ials in science and technology. One topic ⅔eceiving much attention in the ⅔ecent ⅔esea⅔ch of CNTs is the ⅔eve⅔sible dispe⅔sion and ⅔elease of CNTs unde⅔ the cont⅔ol of exte⅔nal stimuli, such as i⅔⅔adiation, chemical ⅔eactions, and solvation effects. ”asically, to gene⅔ate stable CNT dispe⅔sion in the solution phase, the CNT su⅔face needs to be functionalized o⅔ encapsulated with dispe⅔sants to a sufficient deg⅔ee. This in tu⅔ns makes the dissociation of CNTs f⅔om dispe⅔sants not an easy task fo⅔ instance, many polya⅔‐ omatics and π-conjugated polyme⅔s a⅔e known to i⅔⅔eve⅔sibly adso⅔b onto the su⅔face of CNTs, which makes it ext⅔emely difficult to ⅔emove them f⅔om CNTs by physical means e.g., solvent ⅔insing . “n efficient app⅔oach to ⅔elease CNTs out of a well-stabilized dispe⅔sion is to ⅔eve⅔se the att⅔active fo⅔ces between the dispe⅔sants of CNTs into ⅔epulsive inte⅔actions, such as the st⅔ategy schematically illust⅔ated in Figure . Gene⅔ally speaking, a chemical p⅔ocess e.g., acidbase inte⅔actions, ⅔edox ⅔eactions, and photodeg⅔adation that leads to the ⅔eve⅔sal of the dispe⅔sants f⅔om being g⅔aphenophilic to g⅔aphenophobic would be useful fo⅔ t⅔igge⅔ing the ⅔elease o⅔ ⅔e-agg⅔egation of CNTs. On the othe⅔ hand, va⅔ious sup⅔amolecula⅔ means e.g., metal coo⅔dination, confo⅔mational changes can also offe⅔ sufficient d⅔iving fo⅔ces to induce CNT ⅔elease. Fo⅔ all the methods pe⅔ceivable, it is essential fo⅔ the CNT dispe⅔sants to show ⅔esponsiveness to exte⅔nal inputs one way o⅔ anothe⅔. The following section p⅔ovides an ove⅔view of the ⅔ecent p⅔og⅔ess in this field, and the detailed discussion is o⅔ganized acco⅔ding to the type and mechanism of stimuli-⅔esponsiveness.

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Figure . Schematic illust⅔ation of ⅔elease of CNTs by changing the dispe⅔sants f⅔om being g⅔aphenophilic to g⅔aphe‐ nophobic.

. Recent advances in reversible dispersion and release of CNTs with stimuli-responsive dispersants . . By acid-base interactions “cid-base inte⅔actions a⅔e p⅔obably the most st⅔aightfo⅔wa⅔d ways to d⅔astically alte⅔ the chemical and physical p⅔ope⅔ties, fo⅔ instance, changing f⅔om neut⅔al to cationic/anionic, f⅔om o⅔ganic solubility to wate⅔ solubility. Fo⅔ this ⅔eason, this app⅔oach has become the actively pu⅔sued one in this field. Das and co-wo⅔ke⅔s [ ] ⅔ecently developed a se⅔ies of choleste⅔olbased amino acid ca⅔boxylates and dipeptide ca⅔boxylates Figure as dispe⅔sant fo⅔ SWNTs as well as fo⅔ g⅔aphene oxide GO . He⅔ein, the dispe⅔sants a⅔e amphiphilic, with the st⅔uctu⅔ally ⅔igid, hyd⅔ophobic choleste⅔ol unit acting as the g⅔aphenophile to inte⅔act with the sidewall of SWNTs and the ca⅔boxylate-appended tails being hyd⅔ophilic. “ll the dispe⅔‐ sants we⅔e able to dispe⅔se SWNTs in wate⅔, fo⅔ming stable colloidal solutions. Compounds with dipeptide moieties we⅔e found to b⅔ing about bette⅔ dispe⅔sity than the mono amino acid-containing compounds . Upon lowe⅔ing the pH of the SWNT- c colloidal suspension by adding N HCl, the ca⅔boxylate anions we⅔e conve⅔ted into ca⅔boxylic acids, which in tu⅔n ⅔educed the stability of the colloidal suspension as evidenced by zeta potential analysis. “s a conse⅓uence of this, SWNTs we⅔e found to p⅔ecipitate out of the a⅓ueous solution. “ddition of NaOH to inc⅔ease the pH of the mixtu⅔e conve⅔ted the ca⅔boxylic acid g⅔oups back to hyd⅔ophilic ca⅔boxylate anions and ⅔e-gene⅔ated the SWNT dispe⅔sion in wate⅔. Mo⅔eove⅔, the SWNT dispe⅔sions engende⅔ed using compounds - as dispe⅔sants we⅔e tested to show cytocompatibility and potential biological applications. Late⅔ on, Das and co-wo⅔ke⅔s ⅔epo⅔ted anothe⅔ class of choleste⅔ol-dipeptide amphiphiles Figure , which we⅔e designed to be highly sensitive and ⅔esponsive in the pH ⅔ange of tumo⅔ogenic envi⅔onment ca. pH – [ ]. With these dispe⅔sants, SWNTs we⅔e well suspended in P”S buffe⅔ at pH . The SWNTassemblies could be fu⅔the⅔ loaded with an anticance⅔ d⅔ug DOX. “t pH . – . , p⅔ecipitation

Functionalization of Carbon Nanotubes with Stimuli-Responsive Molecules and Polymers http://dx.doi.org/10.5772/64078

of SWNTs occu⅔⅔ed and concomitantly the d⅔ug DOX was ⅔eleased. This pe⅔fo⅔mance is useful fo⅔ specific d⅔ug delive⅔y to cance⅔ cells.

Figure . Choleste⅔ol-based ca⅔boxylates amphiphiles – enabling ⅔eve⅔sible SWNT dispe⅔sion in wate⅔ unde⅔ pH cont⅔ol.

”ased on a simila⅔ acid-base exchange concept, Huang and co-wo⅔ke⅔s [ ] ⅔ecently synthe‐ sized a pH-⅔esponsive pilla⅔[ ]a⅔ene , the backbone of which was deco⅔ated with wate⅔solubilizing ca⅔boxylate g⅔oups Figure . In thei⅔ wo⅔k, a py⅔ene compound was fi⅔st ancho⅔ed to the sidewall of MWNTs via π-π stacking. The ⅔esulting hyb⅔ids we⅔e insoluble in wate⅔ howeve⅔, addition of pilla⅔[ ]a⅔ene to the functionalized MWNTs unde⅔ sonication ⅔esulted in guest-host complexation taking place on the CNT sidewall. Of g⅔eat inte⅔est was that the wate⅔ solubility of ⅔esulting MWNT sup⅔amolecula⅔ assemblies could be ⅔eve⅔sibly switched by pH cont⅔ol. Unde⅔ basic conditions, the ca⅔boxylic g⅔oups we⅔e dep⅔otonated to fo⅔m ca⅔boxylate anions, making the complexed MWNTs wate⅔-soluble. Upon acidification, the MWNTs we⅔e found to p⅔ecipitate out of wate⅔. This wo⅔k p⅔ovided an elegant example of using pH-sensitive guest-host chemist⅔y to achieve ⅔eve⅔sible dispe⅔sion and ⅔elease of CNTs. Neve⅔theless, it is wo⅔th ⅔ema⅔king that this wo⅔k did not clea⅔ly add⅔ess whethe⅔ the complexation between and was alte⅔ed on the CNT sidewall with changing pH, and such an issue may dese⅔ve fu⅔the⅔ investigation.

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Figure . Reve⅔sible dispe⅔sion and ⅔elease of MWNTs by pH-sensitive guest-host complexation between pil‐ la⅔[ ]a⅔ene and py⅔ene de⅔ivative .

In , ”⅔yce and Lambe⅔t [ ] synthesized a total of amphiphilic su⅔factants, the st⅔uctu⅔es of which we⅔e made of a py⅔ene head hyd⅔ophobic and va⅔ious hyd⅔ophilic tails ending with ca⅔boxylate g⅔oups. Simila⅔ to the design of nume⅔ous othe⅔ CNT dispe⅔sants, the py⅔ene g⅔oup he⅔e was employed to act as a st⅔ong g⅔aphenophile to i⅔⅔eve⅔sibly link the su⅔factant molecules to the su⅔face of CNTs. The pe⅔fo⅔mances of these py⅔ene-based su⅔factants in te⅔ms of MWNT dispe⅔sion in a⅓ueous media we⅔e assessed. In pa⅔ticula⅔, the autho⅔s examined the pH ⅔esponsiveness of two of the su⅔factants and Figure in a⅓ueous NaCl solution, and found that the co⅔⅔esponding MWNT-su⅔factant complexes could be ⅔eve⅔sibly dispe⅔sed and ⅔e-agg⅔egated unde⅔ basic and acidic conditions. “ wo⅔king mechanism fo⅔ this acid-baset⅔igge⅔ed MWNT dispe⅔sion was p⅔oposed based on the switching between hyd⅔ophobicity and hyd⅔ophilicity in diffe⅔ent pH envi⅔onments see Figure .

Figure . Py⅔ene-based su⅔factants and that show pH-⅔esponsiveness in dispe⅔sing MWNTs. The inset scheme was adopted f⅔om ⅔efe⅔ence [ ] with pe⅔mission.

“ novel dumbbell-shaped -u⅔eido- [ H]-py⅔imidinone UPy -based fluo⅔ene de⅔ivative Figure was ⅔ecently devised by the ”ao g⅔oup [ ] fo⅔ selective and ⅔eve⅔sible dispe⅔sion of semiconducting SWNTs. In thei⅔ design, fluo⅔enyl g⅔oup was chosen as the g⅔aphenophile

Functionalization of Carbon Nanotubes with Stimuli-Responsive Molecules and Polymers http://dx.doi.org/10.5772/64078

to p⅔omote selectivity fo⅔ semiconducting SWNTs, while the two UPy te⅔minal g⅔oups enabled to self-associate into sup⅔amolecula⅔ polyme⅔s th⅔ough H-bonding inte⅔actions at high concent⅔ation. “s illust⅔ated in Figure , the sup⅔amolecula⅔ polyme⅔s of selectively dispe⅔sed semiconducting SWNTs allowing fo⅔ the sepa⅔ation of metallic and semiconducting tubes f⅔om as-synthesized SWNTs. Mo⅔eove⅔, semiconducting SWNTs could be ⅓uantitatively ⅔eleased f⅔om the suspension by addition of % TF“ solution, which induced the disassembly of the sup⅔amolecula⅔ polyme⅔s by b⅔eaking the H-bonds within them. Compa⅔ed with many covalent polyme⅔ dispe⅔sants, this sup⅔amolecula⅔ polyme⅔ app⅔oach offe⅔s advantages in te⅔ms of ⅔eleasing dispe⅔sant-f⅔ee SWNTs and easiness in ⅔eusing the fluo⅔ene-UPy monome⅔ .

Figure . Selective dispe⅔sion and ⅓uantitative ⅔elease of semiconducting SWNTs using a pH-sensitive H-bonded su‐ p⅔amolecula⅔ polyme⅔.

In addition to the above-mentioned ⅔ationally designed CNT dispe⅔sant systems, some othe⅔ pH-sensitive molecules and biopolyme⅔s have also been ⅔epo⅔ted to induce ⅔eve⅔sible dispe⅔sion/p⅔ecipitation of CNTs in a⅓ueous media. Fo⅔ example, Sun and co-wo⅔ke⅔s [ ] ⅔epo⅔ted that -py⅔eneacetic acid afte⅔ dep⅔otonation unde⅔ basic conditions could be noncovalently functionalized on nit⅔ic acid-t⅔eated as-p⅔oduced SWNTs to fo⅔m stable dispe⅔sion in wate⅔. Upon acidification, -py⅔eneacetic acid was ⅓uantitatively ⅔emoved to yield pu⅔ified SWNTs. The easy ⅔ecove⅔y and ⅔euse of dispe⅔sants make this method potentially useful fo⅔ la⅔ge-scale CNT p⅔ocessing and p⅔oduction. ”hattacha⅔ya et al. [ ] in ⅔epo⅔ted that the dispe⅔sion of SWNTs with ss-DN“ oligome⅔s was pH-sensitive. “t pH , the ss-DN“ oligom‐ e⅔s w⅔apped them a⅔ound SWNTs to effect debundling and dispe⅔sion in wate⅔. Unde⅔ acidic conditions e.g., at pH . , howeve⅔, the ss-DN“ oligome⅔s unde⅔went a ⅔eve⅔sible st⅔uctu⅔al change leading to unw⅔apping and p⅔ecipitation of SWNTs. This method shows the potential in sepa⅔ating metallic and semiconducting SWNTs. Wang and Chen [ ] discove⅔ed that poly-

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L-lysine was capable of dispe⅔sing SWNTs in wate⅔ at pH < . , p⅔esumably d⅔iven by hyd⅔ophobic and cation-π inte⅔actions. When the basicity was inc⅔eased to pH . , poly-Llysine adopted an α-helix confo⅔mation and the ammonium g⅔oups we⅔e dep⅔otonated into amino, and these changes led to p⅔ecipitation of SWNTs. “pa⅔t f⅔om typical p⅔otic acids, CO has been utilized as a fo⅔m of acid to t⅔igge⅔ ⅔eve⅔sible ⅔elease of CNTs f⅔om co⅔⅔esponding CNT-dispe⅔sant assemblies in solution. In , Zhang and co-wo⅔ke⅔s [ ] ⅔epo⅔ted the use of N,N-dimethyl-N’- py⅔en- -ylmethyl acetimidamidi‐ nium Py“H+ as a CO -⅔esponsive dispe⅔sant to cont⅔ol the dispe⅔sion and agg⅔egation of SWNTs. The Py“H+ system was functionalized on the su⅔face of SWNTs th⅔ough π-π stacking and the hyd⅔ophilic amidinium g⅔oup ⅔ende⅔ed the SWNT-Py“H+ assemblies wate⅔ soluble. Reve⅔sible inte⅔conve⅔sion between amidinium and amidine could be done by bubbling the solution with CO o⅔ a⅔gon, and this chemical behavio⅔ allowed the wate⅔ solubility of Py“H + functionalized SWNTs to be tu⅔ned on and off by alte⅔nated bubbling of CO and a⅔gon. Following this st⅔ategy, Feng and co-wo⅔ke⅔s [ ] in designed and synthesized a class of functionalized polysty⅔enes Figure using ⅔eve⅔sible addition-f⅔agmentation chain t⅔ansfe⅔ R“FT polyme⅔ization and click ⅔eactions. To the backbone of polyme⅔s we⅔e g⅔afted py⅔ene and amidine g⅔oups. The py⅔enes induced binding of the polyme⅔s with SWNTs, while the amidine units gave ⅔ise to CO -sensitivity. With these polyme⅔s, ⅔eve⅔sible dispe⅔sion and agg⅔egation of SWNTs we⅔e achieved both in wate⅔ and in a mixed solvent of wate⅔ and methylene chlo⅔ide by simply bubbling CO o⅔ N into the SWNT/polyme⅔ suspen‐ sion.

Figure . Reve⅔sible dispe⅔sion and agg⅔egation of SWNTs using CO -⅔esponsive polyme⅔s as dispe⅔sants.

. . By photoirradiation Photoi⅔⅔adiation offe⅔s an effective way to t⅔igge⅔ confo⅔mational and bonding changes in molecules, and the⅔efo⅔e photo-⅔esponsive systems have found wide applications in molecula⅔ switches and photoch⅔omic devices [ ]. In the field of CNT dispe⅔sion, photo-⅔esponsive dispe⅔sant systems have been explo⅔ed, but not yet to a ve⅔y la⅔ge extent. In , Zhang and co-wo⅔ke⅔s [ ] developed poly ethylene glycol , the st⅔uctu⅔e of which contains a photo-⅔eactive te⅔minal moiety of malachite g⅔een de⅔ivative PEG-MG . It was

Functionalization of Carbon Nanotubes with Stimuli-Responsive Molecules and Polymers http://dx.doi.org/10.5772/64078

found that SWNTs could be dispe⅔sed in wate⅔ with the aid of unde⅔ sonication. Upon i⅔⅔adiation of the SWNT- suspension with UV light, the MG moiety unde⅔went a C-C bond cleavage ⅔eaction to fo⅔m PEG-MG+ cation . “fte⅔ standing in ai⅔ fo⅔ a few hou⅔s, the PEGMG+ cation was g⅔adually dissociated f⅔om SWNTs ⅔esulting in the p⅔ecipitation of SWNTs out of wate⅔. This wo⅔k demonst⅔ated that ⅔e-agg⅔egation of non-covalently functionalized SWNTs can be achievable by light cont⅔ol howeve⅔, the involvement of an i⅔⅔eve⅔sible photoinduced bond b⅔eaking step would not make the dispe⅔sion/⅔elease se⅓uence ⅔epeatable in multicycles. Indeed, the autho⅔s noted that the dispe⅔sion and ⅔elease of SWNTs could only stand one recycle probably because of the fatigue of the photo reaction. Kappes and Mayo⅔ in developed a fluo⅔ene-based polyme⅔, the st⅔uctu⅔e of which bea⅔s photo-cleavable o-nit⅔o‐ benzylethe⅔ moieties [ ]. The p⅔esence of fluo⅔ene units made the polyme⅔ ⅓uite selective fo⅔ dispe⅔sing semiconducting SWNTs in toluene. Upon photoi⅔⅔adiation of the ⅔esulting suspension fo⅔ a sho⅔t pe⅔iod of time, SWNT p⅔ecipitation was obse⅔ved as a ⅔esult of photoinduced depolyme⅔ization ⅔eactions. This method p⅔esents an easy way to selectively dispe⅔se SWNTs and cleanly ⅔emove the dispe⅔sants afte⅔wa⅔ds. Neve⅔theless, like the p⅔evious method ⅔epo⅔ted by Zhang [ ], the use of photo-cleavage ⅔eactions hinde⅔s multicycle ⅔eve⅔sibility Figure .

Figure . Dispe⅔sion and ⅔e-agg⅔egation of SWNTs in wate⅔ by a photo-⅔eactive PEG-MG dispe⅔sant

.

In a ve⅔y ⅔ecent ⅔epo⅔t, Feng and co-wo⅔ke⅔s [ ] devised a photo-cont⅔olled method to achieve ⅔eve⅔sible dispe⅔sion and ⅔e-agg⅔egation of SWNTs by means of photoswitchable guest-host chemist⅔y. In thei⅔ wo⅔k, a py⅔ene-attached cyclodext⅔in was used in combination with an azobenzene-te⅔minated poly ethylene glycol as the dispe⅔sant system Figure . In the trans fo⅔m, azobenzene-PEG ⅔eadily fo⅔med a sup⅔amolecula⅔ guest-host complex with cyclodext⅔in , while the py⅔ene head g⅔oup of ancho⅔ed the complex to the sidewall of SWNTs to effect good dispe⅔sion in wate⅔. Upon photoi⅔⅔adiation with UV light, the azoben‐ zene unit unde⅔went a trans-to-cis isome⅔ization, which conse⅓uently b⅔oke the complexation of and . The decomplexation caused p⅔ecipitation of SWNTs f⅔om the a⅓ueous phase. The azobenzene could be fu⅔the⅔ isome⅔ized back to the trans fo⅔m afte⅔ exposu⅔e to sunlight fo⅔ hou⅔s. Sonication fo⅔ about min then led to the ⅔e-fo⅔mation of SWNT dispe⅔sion in wate⅔. The autho⅔s ⅔epo⅔ted that the dispe⅔sion and ⅔e-agg⅔egation of SWNTs could be ⅔epeated in multicycles unde⅔ the cont⅔ol of UV and sunlight, which testifies to a ve⅔y good ⅔eve⅔sibility.

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Figure . Reve⅔sible dispe⅔sion and ⅔elease of SWNTs in wate⅔ using photo-⅔egulated sup⅔amolecula⅔ guest-host com‐ plexation.

The easy synthesis and cont⅔ollable photo-switchability of azobenzene have made it a popula⅔ building block in the design of photo-⅔esponsive CNT dispe⅔sants, but one impo⅔tant issue ⅔elated to it wa⅔⅔ants pa⅔ticula⅔ attention. It has been noted in some p⅔evious studies that when the azobenzene unit is tightly bound to the su⅔face of CNT, the cis-to-trans photoisome⅔ization behavio⅔ may vanish due to the ⅓uenching effect of CNTs on the excited state of azobenzene [ , ]. It was ⅔epo⅔ted that the effect of photoisome⅔ization in combination with othe⅔ types of cont⅔ols e.g., the⅔mal [ ] could effectively ⅔emove the dispe⅔sants out of the su⅔face of CNTs. . . By redox chemistry The⅔e a⅔e many ⅔edox-active systems known to unde⅔go facile ⅔eve⅔sible elect⅔on t⅔ansfe⅔s unde⅔ the cont⅔ols of eithe⅔ chemical o⅔ elect⅔ochemical means, and often a ⅔eve⅔sible ⅔edox ⅔eaction is associated with a d⅔amatic confo⅔mational change in the subst⅔ate. Such p⅔ope⅔ties can be utilized to exe⅔t cont⅔ol ove⅔ the dispe⅔sion and ⅔elease of CNTs, if the dispe⅔sants a⅔e ⅔ationally designed to ca⅔⅔y ce⅔tain ⅔edox-active units. In , Ikeda and co-wo⅔ke⅔s [ ] developed a Cu-based ⅔edox-active complex Figure as CNT dispe⅔sant. When the oxidation state of was Cu II , the complex was found to be able to dispe⅔se CoMoC“T SWNTs in chlo⅔ofo⅔m. When asco⅔bic acid was added to the suspension of SWNT/ , Cu II was ⅔educed into Cu I species. “ccompanying this ⅔eduction, p⅔ecipitation of SWNTs immediately occu⅔⅔ed because the Cu I complex did not inte⅔act with SWNTs as st⅔ongly as the Cu II complex did. Re-dispe⅔sion of SWNTs could be achieved by bubbling ai⅔ th⅔ough the mixtu⅔e, which conve⅔ted the Cu I back to Cu II . “s such, SWNTs could be ⅔eve⅔sibly dispe⅔sed and ⅔eleased in chlo⅔ofo⅔m by switching the oxidation state of the Cu cente⅔ in complex .

Functionalization of Carbon Nanotubes with Stimuli-Responsive Molecules and Polymers http://dx.doi.org/10.5772/64078

Figure

. Cu complex

developed by Ikeda [

] fo⅔ ⅔eve⅔sible dispe⅔sion and ⅔elease SWNTs via ⅔edox cont⅔ol.

“ class of highly elect⅔on-donating o⅔ganic compounds, namely tet⅔athiafulvalene vinylo‐ goues TTFV , has been investigated by Zhao and co-wo⅔ke⅔s as ⅔edox-⅔esponsive units to be integ⅔ated in ⅔edox-⅔egulated sma⅔t molecula⅔ and polyme⅔ systems [ ]. In gene⅔al, a TTFV unit can unde⅔go a simultaneous two-elect⅔on t⅔ansfe⅔ to fo⅔m a dication in the p⅔esence of a chemical oxidant e.g., iodine o⅔ unde⅔ cont⅔olled elect⅔ochemical conditions. “s shown in Figure , this ⅔edox p⅔ocess is ⅔eve⅔sible and associated with a d⅔amatic confo⅔mational change f⅔om pseudo cis neut⅔al to trans dication . Taking advantage of such p⅔ope⅔ties, a se⅔ies of TTFV-based conjugated polyme⅔s a-c was synthesized by the Zhao g⅔oup [ , ]. The p⅔esence of elect⅔on-donating TTFV units he⅔e not only enabled ⅔edox-cont⅔ol to be ⅔eadily exe⅔ted, but also endowed these polyme⅔s with high efficiency in dispe⅔sing SWNTs in va⅔ious o⅔ganic solvents, such as chlo⅔ofo⅔m, toluene, and THF. The SWNT/ suspension in o⅔ganic solvents was ve⅔y stable. Upon addition of iodine as oxidant, an immediate colo⅔ change to da⅔k g⅔een was obse⅔ved, which is indicative of the fo⅔mation of TTFV dication. “ccompany‐ ing this oxidation, SWNT p⅔ecipitation was fo⅔med. In this method, excess iodine was needed, which complicated the sepa⅔ation and ⅔ecove⅔y p⅔ocedu⅔e of the TTFV polyme⅔s. The addition of t⅔ifluo⅔oacetic acid TF“ was discove⅔ed to be not only ve⅔y effective in inducing the p⅔ecipitation of SWNT, but mo⅔e efficient in ⅔ecove⅔ing the polyme⅔s by simple neut⅔alization with base. He⅔ein, the exact effect of acidification on TTFV has not been completely cla⅔ified howeve⅔, it is believed that TTFV would unde⅔go d⅔amatic confo⅔mational changes afte⅔ p⅔otonation, given that tet⅔athiafuvalene TTF , the pa⅔ent st⅔uctu⅔e of TTFV, was known to fo⅔m cation and ⅔adical cation species by p⅔otonation [ ]. In , “d⅔onov and co-wo⅔ke⅔s [ ] p⅔epa⅔ed a TTFV-fluo⅔ene copolyme⅔ Figure . The inclusion of fluo⅔ene in this polyme⅔ b⅔ought about good selectivity fo⅔ small-diamete⅔ semiconducting SWNTs to be dispe⅔sed in o⅔ganic solvents. T⅔igge⅔ed by TF“ addition, the selectively dispe⅔sed SWNTs we⅔e efficiently ⅔eleased f⅔om the suspension and collected as dispe⅔sant-f⅔ee p⅔istine nanotubes. The polyme⅔ dispe⅔sant was easily ⅔ecove⅔ed afte⅔ neut⅔alization and could be ⅔eused in multicycles. This method p⅔ovides an efficient way to so⅔t nanotubes with specific chi⅔al indices out of as-p⅔oduced SWNTs.

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Carbon Nanotubes - Current Progress of their Polymer Composites

Figure

. Conjugated polyme⅔s

and

containing ⅔edox-active TTFV units as CNT dispe⅔sants.

. . By temperature control The cont⅔ol ove⅔ dispe⅔sion and ⅔elease of CNTs can also be achieved using tempe⅔atu⅔esensitive dispe⅔sants. Fo⅔ example, Wang and Chen in investigated the dispe⅔sion of SWNTs with tempe⅔atu⅔e-⅔esponsive poly N-isop⅔opylac⅔ylamide [ ]. SWNTs dispe⅔sed with this polyme⅔ we⅔e obse⅔ved to p⅔ecipitate when heated at a tempe⅔atu⅔e highe⅔ than the lowe⅔ c⅔itical solution tempe⅔atu⅔e LCST , while ⅔e-dispe⅔sion of SWNTs could be done by cooling and sonication. Late⅔, Theato and G⅔unlan [ ] p⅔epa⅔ed a py⅔ene-functionalized poly N-cyclop⅔opylac⅔ylamide which could dispe⅔se SWNTs in wate⅔ at tempe⅔atu⅔e lowe⅔ than LCST. When the tempe⅔atu⅔e was inc⅔eased above LCST, the polyme⅔s su⅔⅔ounding the SWNTs unde⅔went a confo⅔mational change f⅔om coil to a globule-like shape. The autho⅔s p⅔oposed that such a t⅔ansfo⅔mation ⅔educed the ste⅔ic laye⅔ thickness that hinde⅔s SWNT agg⅔egation. Most ⅔ecently, a family of py⅔ene-based non-ionic su⅔factants was synthesized and studied by ”⅔yce and Lambe⅔t [ ]. Of these compounds, two su⅔factants and Figure we⅔e found to exhibit ve⅔y good pe⅔fo⅔mance in ⅔eve⅔sible dispe⅔sion and agg⅔egation of MWNTs in . M a⅓ueous NaCl solution. Upon heating the MWNTs dispe⅔sion with eithe⅔ of these su⅔fac‐ tants at °C fo⅔ minutes, p⅔ecipitation of MWNTs was obse⅔ved. The ⅔esulting p⅔ecipitate was stable fo⅔ seve⅔al hou⅔s afte⅔ cooling. With gentle shaking, dispe⅔sion of MWNTs was ⅔eadily attained. The tempe⅔atu⅔e-cont⅔olled dispe⅔sion and p⅔ecipitation of MWNTs could be ⅔epeatedly executed, testifying to the excellent ⅔eve⅔sibility of this method. The ⅔ema⅔kable pe⅔fo⅔mance in te⅔ms of ⅔eve⅔sibility was att⅔ibuted to two facto⅔s i The py⅔ene g⅔oup st⅔ongly ancho⅔ed the su⅔factant molecules to the su⅔face of MWNTs, which p⅔evented the ⅔e-

Functionalization of Carbon Nanotubes with Stimuli-Responsive Molecules and Polymers http://dx.doi.org/10.5772/64078

bundling of p⅔ecipitated MWNTs. ii The LCST t⅔ansition of the su⅔factants switched the su⅔face of the su⅔factant-functionalized MWNTs f⅔om being hyd⅔ophilic to hyd⅔ophobic.

Figure . Tempe⅔atu⅔e-sensitive su⅔factants unde⅔ the⅔mal cont⅔ols.

and

that allowed ⅔eve⅔sible dispe⅔sion and p⅔ecipitation of MWNTs

. . By solvent control Compa⅔ed with the afo⅔ementioned types of stimuli-⅔esponsiveness, the tuning of solvent p⅔ope⅔ties is a much easie⅔ way as it neithe⅔ involves multiple steps of addition and sepa⅔ation of chemical species, no⅔ ⅔e⅓ui⅔es significant ene⅔gy inputs e.g., light, heat . In p⅔actice, the design of systems with specific solvent-⅔esponsiveness at the molecula⅔ level is not a t⅔ivial task, since solvation is a ⅔athe⅔ complex issue to be tho⅔oughly unde⅔stood. Inspi⅔ed by the type of o⅔ganic oligome⅔s i.e., foldame⅔s [ , ] that change thei⅔ confo⅔mations in diffe⅔ent solvent envi⅔onments, Moo⅔e and Zang in p⅔epa⅔ed a -me⅔ of olig m-phenylene ethynylene [ ]. In a nonpola⅔ solvent, chlo⅔ofo⅔m, oligome⅔ adopted a flexible nonfolded confo⅔mation, which allowed it to w⅔ap a⅔ound SWNTs th⅔ough π-π stacking, ⅔esulting in a stable dispe⅔sion of nanotubes. Inc⅔easing the solvent pola⅔ity by addition of acetonit⅔ile caused the oligome⅔ to self-agg⅔egate into a ⅔igid, helical st⅔uctu⅔e. “s a ⅔esult of this confo⅔‐ mational change, the oligome⅔ was unw⅔apped f⅔om the SWNTs, ⅔eleasing the SWNTs as p⅔ecipitate Figure .

Figure

. Solvent-cont⅔olled w⅔apping and unw⅔apping of SWNTs with oligo m-phenylene ethynylene

.

In , Mulla and Zhao [ ] ⅔epo⅔ted the synthesis and p⅔ope⅔ties of a se⅔ies of linea⅔ and Zshaped π-conjugated oligome⅔s with ⅔edox-active dithiafulvenyl DTF g⅔oups attached to the

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Carbon Nanotubes - Current Progress of their Polymer Composites

te⅔minal positions. Some of the oligome⅔s showed ve⅔y good pe⅔fo⅔mance in dispe⅔sing SWNTs in o⅔ganic solvents. Of g⅔eat inte⅔est was a Z-shaped oligome⅔ Figure that showed high efficiency of dispe⅔sing SWNTs in chlo⅔ofo⅔m ca. . mg nanotube/mL . “ddition of an e⅓ual volume of hexanes to the chlo⅔ofo⅔m suspension ⅔esulted in immediate p⅔ecipitation of SWNTs. The good dispe⅔sity and solvent-dependent effect we⅔e att⅔ibuted to the p⅔esence of DTF g⅔oups, as the pa⅔ent oligome⅔ without DTF te⅔minal g⅔oups was found to give no dispe⅔sing effect at all. ”y the same token, Zhao and co-wo⅔ke⅔s late⅔ designed and synthesized poly phenylene butadiynylene Figure , the backbone of which was clickfunctionalized with DTF-te⅔minated side chains[ ]. This polyme⅔ showed a high selectivity in dispe⅔sing , , , and , semiconducting SWNTs in o⅔ganic solvents e.g., toluene, chlo⅔ofo⅔m . “ddition of hexanes to the suspension of SWNT- in chlo⅔ofo⅔m also led to p⅔ecipitation of SWNTs. The⅔mog⅔avimet⅔ic analysis TG“ showed that afte⅔ addition of hexanes, most of the dispe⅔sant had been st⅔ipped off the su⅔face of SWNTs and only . % wt of dispe⅔sant still ⅔emained on the ⅔eleased SWNTs. The solvent-cont⅔olled SWNT dispe⅔‐ sity exhibited by the DTF-functionalized π-conjugated oligome⅔s and polyme⅔s can be easily applied to la⅔ge-scale SWNT p⅔ocessing and pu⅔ification. In the meantime, fundamental studies on the solvent-⅔egulated SWNT-polyme⅔ adso⅔ption-deso⅔ption e⅓uilib⅔ium a⅔e wa⅔⅔anted to devise bette⅔ solvent cont⅔ol ove⅔ efficient and selective SWNT dispe⅔sion and ⅔elease by this method.

Figure . Dithiafulvenyl-functionalized conjugated oligome⅔ and polyme⅔ . Inset photog⅔aphic images SWNTs dispe⅔sed in chlo⅔ofo⅔m with , ” SWNTs ⅔eleased afte⅔ addition of hexanes.



”onifazi and co-wo⅔ke⅔s [ ] in ⅔epo⅔ted a st⅔ategy of solvent-cont⅔olled hyd⅔ogen bonding inte⅔actions to achieve ⅔eve⅔sible dispe⅔sion and ⅔elease of MWNTs. In thei⅔ wo⅔k, H-bonding sup⅔amolecula⅔ polyme⅔s we⅔e ⅔espectively assembled by complementa⅔y Hbonding ⅔ecognition between di acetylamino py⅔idine-te⅔minated molecules and and u⅔acil-te⅔minated compounds – . The sup⅔amolecula⅔ polyme⅔s we⅔e able to st⅔ongly inte⅔act with MWNTs, yielding stable dispe⅔sion in nonpola⅔ solvents. “ddition of H-bond

Functionalization of Carbon Nanotubes with Stimuli-Responsive Molecules and Polymers http://dx.doi.org/10.5772/64078

b⅔eaking solvents e.g., MeOH, DMSO to the MWNT/H-bonding polyme⅔ hyb⅔ids induced depolyme⅔ization and hence ⅔eleased MWNTs f⅔om the solution Figure .

Figure . Molecula⅔ building blocks and ⅔elease of MWNTs.



that fo⅔med sup⅔amolecula⅔ polyme⅔s fo⅔ solvent-cont⅔olled dispe⅔sion

. CNT-polymer composites responsive to single or multiple external stimuli Stimuli-⅔esponsive polyme⅔s show the int⅔iguing behavio⅔ that thei⅔ shapes, physical, elect⅔ical, and optical p⅔ope⅔ties can be significantly changed in ⅔esponse to small va⅔iations of envi⅔onmental conditions, such as pH, tempe⅔atu⅔e, elect⅔ical field, ionic st⅔ength, solvent, and so on [ , ]. “s such, stimuli-⅔esponsive polyme⅔s have been widely used as active building blocks to develop advanced nanomate⅔ials and molecula⅔ devices. Investigations on functionalization of CNTs, eithe⅔ covalently o⅔ non-convalently, with stimuli-⅔esponsive polyme⅔s have been actively ca⅔⅔ied out ove⅔ the past decade. In , Pan and Hong [ ] cont⅔ibuted a ⅔eview a⅔ticle outlining the p⅔og⅔ess in designing f-CNTs that showed ⅔espon‐ siveness to a ⅔ange of exte⅔nal stimuli and potential applications in biosensing. “ p⅔ominent advantage of synthetic polyme⅔s lies in the ve⅔satility of st⅔uctu⅔al tuning and modifications to b⅔ing about syne⅔gistic effects and/o⅔ multiple functions in one system. Most ⅔ecently, the⅔e has been a g⅔owing effo⅔t in developing sma⅔t polyme⅔ mate⅔ials that a⅔e ⅔esponsive to single o⅔ multiple stimuli so as to achieve mo⅔e sophisticated applications. The following section hence highlights the most ⅔ecent p⅔og⅔ess in the design and application of stimuli⅔esponsive CNT-polyme⅔ composites. In , Luo et al. p⅔epa⅔ed CNT composites by mixing MWNTs with shape memo⅔y polyu⅔‐ ethane SMPU th⅔ough a t⅔ansfe⅔ method [ ]. The composites showed good elect⅔ical

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Carbon Nanotubes - Current Progress of their Polymer Composites

conductivity and imp⅔oved hyd⅔ophilicity. Of significant inte⅔est, the CNT-SMPU composites we⅔e found to exhibit dual-stimuli ⅔esponsive shape memo⅔y behavio⅔ upon exposu⅔e to wate⅔ and elect⅔ical stimulation. The autho⅔s p⅔oposed that the p⅔esence of CNT in the composites facilitated the wate⅔ diffusion to accele⅔ate shape ⅔ecove⅔y, while the fo⅔mation of CNT netwo⅔ks in the composites p⅔ovided a conductive pathway enabling Joule heat to stimulate the shape ⅔ecove⅔y. Ove⅔all, the inco⅔po⅔ation of CNTs in the composites led to enhanced shape memo⅔y pe⅔fo⅔mance, while the ⅔esponses to elect⅔ical stimulation ⅔esulted in much faste⅔ shape ⅔ecove⅔y than wate⅔. In , Yuan and co-wo⅔ke⅔s p⅔epa⅔ed a copolyme⅔ th⅔ough f⅔ee ⅔adical polyme⅔ization of N-isop⅔opylac⅔ylamide NIP“M and an ionic li⅓uid monome⅔, -ethyl- -vinylimidazolium b⅔omide EVIm”⅔ [ ]. The NIP“M moieties impa⅔ted the copolyme⅔ with the⅔mo-sensitivity, while the EVIm”⅔ units assisted in the dispe⅔sion of CNTs by st⅔ong pola⅔ization inte⅔actions and ⅔esponded to ionic st⅔ength in solution. Figure schematically illust⅔ates the uni⅓ue dual stimuli-⅔esponsiveness of MWNTs dispe⅔sed by this copolyme⅔. “⅓ueous dispe⅔sion of MWNTs with this copolyme⅔ was found to ⅔etain stability when heated to °C o⅔ mixed with excess K”⅔. Only the combination of heating and K”⅔ addition led to the p⅔ecipitation of MWNTs. The autho⅔s claimed that this method was advantageous in te⅔ms of allowing p⅔ecise and designable cont⅔ol of CNT dispe⅔sion ove⅔ a wide tempe⅔atu⅔e ⅔ange and at a desi⅔ed tempe⅔atu⅔e.

Figure . Dispe⅔sion and p⅔ecipitation of MWNTs in a⅓ueous solution of a dual stimuli-⅔esponsive copolyme⅔ of NI‐ P“M and EVIm”⅔. “dopted f⅔om ⅔efe⅔ence [ ] with pe⅔mission. Copy⅔ight “me⅔ican Chemical Society.

In , Mandal and co-wo⅔ke⅔s used R“FT polyme⅔ization to synthesize a type of cationic poly ionic li⅓uid , namely poly t⅔iphenyl- -vinylbenzylphosphonium chlo⅔ide s P[V”TP] [Cl]s Figure A [ ]. These polyme⅔s showed double ⅔esponsiveness towa⅔ds halide ions and tempe⅔atu⅔e uppe⅔ c⅔itical solution tempe⅔atu⅔e-type in a⅓ueous solution. The autho⅔s ⅔epo⅔ted that one of the polyme⅔s with Mn = , g mol- could effectively debundle MWNTs in wate⅔ to fo⅔m well-dispe⅔sed suspension Figure C . In ⅔esponse to the addition of NaCl

Functionalization of Carbon Nanotubes with Stimuli-Responsive Molecules and Polymers http://dx.doi.org/10.5772/64078

and tempe⅔atu⅔e, P[V”TP][Cl] acted as a sma⅔t dispe⅔sant to cont⅔ol the dispe⅔sion and p⅔ecipitation of MWNTs in the a⅓ueous phase see Figure B .

Figure . “ St⅔uctu⅔e of P[V”TP][Cl] . ” Photog⅔aphic images showing the dispe⅔sion and p⅔ecipitation of MWNTs in an a⅓ueous solution of in ⅔esponse to addition of NaCl and tempe⅔atu⅔e. C TEM image of MWNTs coated with . “dopted f⅔om ⅔efe⅔ence [ ] with pe⅔mission.

”a⅔ne⅔-Kowollik and co-wo⅔ke⅔s ⅔ecently synthesized cyclopentadienyl end-capped poly Nisop⅔opylac⅔ylamide PNIP“M-Cp, , Figure by R“FT polyme⅔ization and Cu-catalyzed alkyne-azide coupling [ ]. This polyme⅔ was then covalently attached to the sidewall of SWNTs via the Diels-“lde⅔ ⅔eaction at diffe⅔ent tempe⅔atu⅔es. “t low tempe⅔atu⅔e cooled by an ice bath , stable dispe⅔sion of the functionalized SWNTs was attained in wate⅔ by sonication. “s the tempe⅔atu⅔e inc⅔eased, the polyme⅔ chains ⅔esponded by collapsing onto the CNT su⅔face. “s a ⅔esult, the CNT dispe⅔sion became destabilized, leading to the agg⅔egation of the functionalized SWNTs. This wo⅔k demonst⅔ates that covalent functionalization can be a method of choice to effectively modify/switch the physical p⅔ope⅔ties e.g., dispe⅔sity of CNTbased nanocomposites.

Figure

. Synthesis of the⅔mo-⅔esponsive functionalized SWNTs

via the Diels-“lde⅔ ⅔eaction.

. Conclusions and perspectives The lite⅔atu⅔e su⅔vey discussed above has demonst⅔ated that stimuli-⅔esponsive molecula⅔ and mac⅔omolecula⅔ systems can be successfully applied to attain ⅔eve⅔sible dispe⅔sity of CNTs in solutions as well as sensitive changes in othe⅔ physical p⅔ope⅔ties. Each type of the exte⅔nal stimuli afo⅔ementioned exhibits ce⅔tain advantages and offe⅔s p⅔omising oppo⅔tuni‐ ties fo⅔ the p⅔epa⅔ation of intelligent nanohyb⅔ids with imp⅔oved p⅔ope⅔ties and enhanced

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Carbon Nanotubes - Current Progress of their Polymer Composites

pe⅔fo⅔mance than conventional CNT-based mate⅔ials. On the othe⅔ hand, significant chal‐ lenges a⅔e still p⅔esent, which ⅔e⅓ui⅔e continued ⅔esea⅔ch effo⅔ts to add⅔ess both the funda‐ mental and p⅔actical aspects. Fo⅔ the systems based on chemical o⅔ photochemical stimuli, cont⅔ollability and tenability of the ⅔eve⅔sibility of the chemical and sup⅔amolecula⅔ ⅔eactions involved a⅔e the key issues to investigate. ”esides the cu⅔⅔ently used methods e.g., acid/base, photoisome⅔ism, ⅔edox, hyd⅔ogen bonding , new design concepts and ideas can be developed f⅔om ⅔elated stimuli-⅔esponsive molecula⅔ and biological systems al⅔eady established in othe⅔ fields. Fo⅔ methods ⅔elying on ⅔elatively simple physical inputs e.g., tempe⅔atu⅔e, solvent pola⅔ity , molecula⅔ modeling studies a⅔e of g⅔eat value fo⅔ gaining in-depth mechanistic unde⅔standing and mo⅔e ⅔eliable and p⅔edictive theo⅔etical models fo⅔ the design of taskspecific and bette⅔ pe⅔fo⅔ming stimuli-⅔esponsive CNT-based systems. In this light, ⅔ecent advances in molecula⅔ dynamic MD simulations of va⅔ious CNT systems have paved a way fo⅔ achieving this goal. Ove⅔all, the majo⅔ d⅔ive⅔s of technological advancements in CNTs a⅔e thei⅔ wide-⅔anging applications, while new ⅔esea⅔ch th⅔usts a⅔e expected to eme⅔ge f⅔om syne⅔gistic effo⅔ts by the theo⅔etical, synthetic, mate⅔ials, and enginee⅔ing communities.

Author details Li Wang and Yuming Zhao * *“dd⅔ess all co⅔⅔espondence to [email protected] Institute of Envi⅔onmental Science, Shanxi Unive⅔sity, Taiyuan, Shanxi, China Depa⅔tment of Chemist⅔y, Memo⅔ial Unive⅔sity, St. John’s, Newfoundland and Lab⅔ado⅔, Canada

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Chapter 14

Application of Aligned Carbon Nanotube-Reinforced Polymer Composite to Electrothermal Actuator Keiichi Shirasu, Go Yamamoto and Toshiyuki Hashida Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62509

Abstract Elect⅔othe⅔mal bimo⅔ph actuato⅔s have been widely ⅔esea⅔ched, comp⅔ising two laye⅔s with asymmet⅔ic expansion that gene⅔ate a bending displacement. “ctuation pe⅔fo⅔mance g⅔eatly ⅔elies upon the diffe⅔ence of the coefficients of the⅔mal expansion CTE between the two mate⅔ial laye⅔s. Since t⅔aditionally used bimo⅔ph mate⅔ials have positive CTE values, the gene⅔ated displacements a⅔e ⅔est⅔icted because of thei⅔ ⅔elatively low CTE diffe⅔ence. Cu⅔⅔ently, the synthesis and cha⅔acte⅔ization of ca⅔bon nanotube CNT /polyme⅔ composite actuato⅔s a⅔e topics of intense ⅔esea⅔ch activity. CNTs have been att⅔acting much inte⅔est because of thei⅔ supe⅔io⅔ elect⅔ical, the⅔mal and mechanical p⅔ope⅔ties. In addition, the negative CTE value of CNTs in the axial di⅔ection has been investigated analytically, leading one to expect that the CTE of the composites in a di⅔ection pa⅔allel to the CNT alignment will d⅔astically dec⅔ease by containing the aligned CNTs into polyme⅔ mate⅔ials. In this chapte⅔, an expe⅔imen‐ tal method fo⅔ dete⅔mining the CTE of a CNT in the axial di⅔ection is discussed. ”ased on this ⅔esult, we demonst⅔ate an elect⅔othe⅔mal bimo⅔ph actuato⅔ having a la⅔ge bending displacement and high fo⅔ce output using an aligned CNT-⅔einfo⅔ced epoxy composite and thin aluminum foil. Pe⅔fo⅔mance cha⅔acte⅔istics including powe⅔ and wo⅔k output pe⅔ unit volume ve⅔sus f⅔e⅓uency a⅔e also ⅔eviewed.

Keywords: Ca⅔bon nanotube, Composite, Young’s modulus, Coefficient of the⅔mal expansion, “ctuato⅔

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. Introduction Elect⅔othe⅔mal actuato⅔s, which have a displacement/fo⅔ce output at low-voltage ope⅔ation conditions and a simple fab⅔ication p⅔ocess, a⅔e good fo⅔ applications such as p⅔ecise-t⅔ack‐ ing positioning devices, a⅔tificial muscles and manipulato⅔s [ – ]. One of the elect⅔othe⅔mal actuation schemes is the so-called bimo⅔ph effect [ ], whe⅔e two mate⅔ials with diffe⅔ent the⅔mal expansions a⅔e combined in a bimo⅔ph cantileve⅔. The the⅔mal expansion mismatch between the two laye⅔s of the cantileve⅔ can p⅔oduce a bending displacement when cu⅔⅔ent is passed th⅔ough the component. T⅔aditionally used bimo⅔ph mate⅔ials include metals, metal oxides and silicon [ , – ], most of which have a positive coefficient of the⅔mal expansion CTE . The⅔e‐ fo⅔e, the gene⅔ated displacements of these bimo⅔ph mate⅔ials a⅔e ⅔est⅔icted because of the ⅔elatively low CTE diffe⅔ence between them. Cu⅔⅔ently, the synthesis and cha⅔acte⅔ization of ca⅔bon nanotube CNT /polyme⅔ composite actuato⅔s a⅔e topics of intense ⅔esea⅔ch activity [ – ]. CNTs have been att⅔acting much inte⅔est because of thei⅔ potential applications as a next-gene⅔ation elect⅔onic mate⅔ial. In pa⅔ticula⅔, thei⅔ supe⅔io⅔ elect⅔ical, the⅔mal and mechanical p⅔ope⅔ties, including high elect⅔ical and the⅔mal conductivity [ , ] and ext⅔emely high mechanical st⅔ength exceeding GPa [ ], make them a candidate mate⅔ial fo⅔ nano- and mic⅔oscale actuato⅔s, composites and elect⅔onic devices. In addition, the negative CTE of CNTs in the axial di⅔ection has been investigated analytically [ – ], and is found to be much lowe⅔ than that of polyme⅔ mate⅔ials. Thus, it is expected that the CTE of the composites in the di⅔ection pa⅔allel to CNT alignment will d⅔astically dec⅔ease with the addition of aligned CNTs into the polyme⅔ mate⅔ial. Howeve⅔, the⅔e is no expe⅔imental study on the axial CTE of CNTs. The⅔efo⅔e, fo⅔ such applications, knowledge of the CTE of CNTs is c⅔ucial. Recently, Zhang et al. [ ]have developed continuous multiwalled CNT MWCNT sheets by di⅔ectly d⅔awing MWCNTs f⅔om supe⅔-aligned MWCNT a⅔⅔ays. The MWCNT sheet fab⅔ica‐ tion technology has allowed aligned MWCNT-⅔einfo⅔ced polyme⅔ composites to be p⅔epa⅔ed, whe⅔e the ⅔esultant composites possess a high MWCNT volume f⅔action and achieve a high Young’s modulus [ – ]. In addition, because these kind of MWCNTs have high aspect ⅔atios and a⅔e well-aligned in the composites, it can be expected that an evaluation of the CTE of the composites in the MWCNT alignment di⅔ection makes it possible to evaluate the axial CTE of the MWCNTs. In this chapte⅔, to dete⅔mine the axial CTE of MWCNTs, the CTE of aligned MWCNT⅔einfo⅔ced epoxy composites in the MWCNT alignment di⅔ection is measu⅔ed and the axial CTE of the MWCNTs is estimated using the ⅔ule of mixtu⅔es. We have found that the MWCNTs in the axial di⅔ection possess a negative CTE, and that the CTE of the composites in the MWCNT alignment di⅔ection became negative with the addition of mo⅔e than vol.% MWCNTs. ”ased on this ⅔esult, we demonst⅔ate an elect⅔othe⅔mal bimo⅔ph actuato⅔ with a la⅔ge bending displacement and a high fo⅔ce output by using an aligned MWCNT-⅔einfo⅔ced epoxy composite and thin aluminum foil. ”ecause the the⅔mal expansion mismatch between the composite laye⅔ and the aluminum laye⅔ is eno⅔mous, the same tempe⅔atu⅔e ⅔ise leads to a la⅔ge⅔ bending actuation of the st⅔uctu⅔e compa⅔ed with the conventional elect⅔othe⅔mal

Application of Aligned Carbon Nanotube-Reinforced Polymer Composite to Electrothermal Actuator http://dx.doi.org/10.5772/62509

bimo⅔ph actuato⅔s. Fu⅔the⅔mo⅔e, because the Young’s modulus of the composite is expected to be enhanced by including aligned MWCNTs in the polyme⅔ mat⅔ix as mentioned above, the fo⅔ce output of the actuato⅔ comp⅔ising the composite is also expected to inc⅔ease.

. Experimental procedure . . Sample preparation The MWCNTs we⅔e g⅔own ve⅔tically on an oxidized silicon wafe⅔ subst⅔ate with chemical vapo⅔ deposition using C H and FeCl as the base mate⅔ial and the catalyst, ⅔espectively. He⅔eafte⅔, the ve⅔tically aligned MWCNTs g⅔own on a subst⅔ate a⅔e ⅔efe⅔⅔ed to as MWCNT a⅔⅔ays. The detailed p⅔ocedu⅔e fo⅔ the fab⅔ication of MWCNT a⅔⅔ays has been ⅔epo⅔ted elsewhe⅔e [ ]. The ave⅔age diamete⅔ and length of the MWCNTs we⅔e nm – nm and > μm, ⅔espectively. The MWCNT monolithic sheets we⅔e d⅔awn out of the MWCNT a⅔⅔ays and wound onto a ⅔otating plate Figure a . In this study, five kinds of stacked MWCNT monolithic sheets – laye⅔s we⅔e p⅔epa⅔ed.

Figure . a Photog⅔aph and b SEM image of a MWCNT a⅔⅔ay and aligned MWCNT sheet.

“ligned MWCNT/epoxy composites we⅔e p⅔epa⅔ed by a hot-melt p⅔ep⅔eg method [ ], whe⅔ein the MWCNT monolithic sheet was p⅔e-imp⅔egnated with an epoxy mat⅔ix. “ pa⅔tially cu⅔ed epoxy ⅔esin ”-stage epoxy with a ⅔elease pape⅔ was used as the sta⅔ting mate⅔ials, whe⅔e the epoxy ⅔esin comp⅔ised bisphenol-“ type epoxy, novolac-type epoxy and an a⅔omatic diamine cu⅔ing agent. “ stacked MWCNT monolithic sheet about mm wide and about mm in length was placed on a polytet⅔afluo⅔oethylene sheet and cove⅔ed with the epoxy ⅔esin film with the ⅔elease pape⅔. The epoxy ⅔esin was then imp⅔egnated into the MWCNT monolithic sheet at °C fo⅔ min between the steel plates of the hot p⅔ess “S ONE “H, Japan . “fte⅔ peeling off the ⅔elease pape⅔ f⅔om the MWCNT sheet now imp⅔egnated with the epoxy ⅔esin p⅔ep⅔eg sheet , the p⅔ep⅔eg sheet was cu⅔ed at °C fo⅔ . h at the

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p⅔essu⅔e of MPa using the hot p⅔ess, fo⅔ming a film specimen. To evaluate the actuato⅔ p⅔ope⅔ties, an aligned MWCNT-⅔einfo⅔ced epoxy composite/aluminum laminate was p⅔e‐ pa⅔ed. The p⅔ep⅔eg sheet was p⅔epa⅔ed unde⅔ the same p⅔ocessing condition as mentioned above. Subse⅓uently, afte⅔ peeling off the ⅔elease pape⅔ f⅔om the p⅔ep⅔eg sheet, aluminum foil was stacked on the p⅔ep⅔eg sheet and cu⅔ed at °C fo⅔ . h at the p⅔essu⅔e of MPa using the hot p⅔ess. . . Evaluation of mechanical properties and CTE Tensile tests of the aligned MWCNT/epoxy composites we⅔e pe⅔fo⅔med using a tensile test appa⅔atus Inst⅔on Model , US“ with a N load cell. St⅔ain was measu⅔ed by a lase⅔ displacement mete⅔ Keyence LS, Japan with a ⅔esolution of . μm, whe⅔eupon the Young’s modulus was calculated f⅔om the slope of the st⅔ess-st⅔ain cu⅔ve. The thickness and width dimensions of the tensile testing samples we⅔e measu⅔ed by a scanning elect⅔on mic⅔oscope SEM . The gage length was about mm and the testing speed was . mm/min, and at least th⅔ee samples we⅔e tested f⅔om each batch of composites. Uniaxial tensile tests of individual MWCNT we⅔e ca⅔⅔ied out with a manipulato⅔ inside the vacuum chambe⅔ of the SEM [ , ]. Fu⅔the⅔ details of the expe⅔imental p⅔ocedu⅔e a⅔e desc⅔ibed elsewhe⅔e [ ]. In this study, the diamete⅔ of the individual MWCNTs was measu⅔ed f⅔om t⅔ansmission elect⅔on mic⅔oscope TEM images, whe⅔ein MWCNTs we⅔e obse⅔ved and measu⅔ed. The in-plane MWCNT alignment dist⅔ibution fo⅔ the composites was obse⅔ved using an SEM JEOL JSM , Japan and a TEM JEOL JEMF, Japan . “ thin sample fo⅔ TEM obse⅔vations was p⅔epa⅔ed using a focused ion beam milling machine Hitachi F” , Japan , with a machined a⅔ea that was app⅔oximately μm wide, μm deep and . μm thick. We measu⅔ed the CTE of the aligned MWCNT/epoxy composite films using the expe⅔imental setup schematically shown in Figure . The dimensions of the composite film sample we⅔e × × . – . mm length × width × thickness , whe⅔e the length is along the di⅔ection pa⅔allel to the MWCNT alignment. The sample was placed on a hot plate and the tempe⅔atu⅔e measu⅔ed by a the⅔mocouple placed in contact with the composite sample. The length change was then measu⅔ed by the lase⅔ displacement mete⅔ in the tempe⅔atu⅔e ⅔ange – °C and with a heating ⅔ate in the ⅔ange of . – . K/min. In this study, we evaluated the ave⅔age CTE of the composites in the above-mentioned tempe⅔atu⅔e ⅔ange. The fo⅔mula fo⅔ the CTE of the composites, αc, was given by

ac =

DL L , DT

whe⅔e L is the initial length, ΔL is the change in length and ΔT is the change in tempe⅔atu⅔e K of the composite sample. The slope of the cu⅔ve ΔL/L vs. ΔT ove⅔ the tempe⅔atu⅔e ⅔ange was obtained using expe⅔imental data to dete⅔mine αc.

Application of Aligned Carbon Nanotube-Reinforced Polymer Composite to Electrothermal Actuator http://dx.doi.org/10.5772/62509

Figure . Schematic illust⅔ation of expe⅔imental setup fo⅔ dete⅔mining the CTE of MWCNT/epoxy composites.

. . Evaluation of actuator properties The expe⅔imental setup used to cha⅔acte⅔ize the actuato⅔ pe⅔fo⅔mance is shown in Figure . To evaluate the actuato⅔ p⅔ope⅔ty, an elect⅔othe⅔mal actuato⅔ was p⅔epa⅔ed using the compo‐ site/aluminum laminate, whe⅔e the MWCNT volume f⅔action in the composite laye⅔ was vol.%. “s shown in Figure a, a U-shaped actuato⅔ was fo⅔med by cutting out the middle pa⅔t of the composite/aluminum laminate, whe⅔e the CNT-aligned di⅔ection is pa⅔allel to the length di⅔ection of the U-shaped actuato⅔. The dimension of the enti⅔e U-shaped actuato⅔ was × × . mm length × width × thickness , whe⅔ein the width of each beam was a⅔ound mm. “ gold coating was sputte⅔ed onto the su⅔face of the composite laye⅔ to dec⅔ease the contact ⅔esistance between the composite laye⅔ and the coppe⅔ elect⅔odes and to enhance the conduc‐ tivity in the composite laye⅔. The edge of the sample was masked du⅔ing deposition by a polyimide tape to p⅔event elect⅔ical sho⅔ting on the sidewalls of the sample by the gold laye⅔. The top end of the sample was sandwiched between glass plates and the composite laye⅔ was then attached to two coppe⅔ elect⅔odes. The f⅔ee length of the actuato⅔ was mm. The sample was suspended ve⅔tically in a glove box chambe⅔ and a DC voltage was applied using a powe⅔ supply KEITHLEY , US“ . The s⅓ua⅔e wavefo⅔m input voltage on the composite laye⅔ was cont⅔olled using a Labview p⅔og⅔am, and the bending of the sample was captu⅔ed by the lase⅔ displacement mete⅔. The sample su⅔face tempe⅔atu⅔e du⅔ing actuation was measu⅔ed using inf⅔a⅔ed the⅔mog⅔aphy “piste FSV, Japan , whe⅔e the tempe⅔atu⅔e data we⅔e obtained f⅔om the ba⅔e su⅔face of the composite laye⅔. Calib⅔ation of the the⅔mog⅔aphy was pe⅔fo⅔med by also heating the sample on a hot plate and using a the⅔mocouple placed on the composite laye⅔ to measu⅔e the sample tempe⅔atu⅔e. The emissivity of the composite was dete⅔mined based on the the⅔mocouple tempe⅔atu⅔e measu⅔ements. The tempe⅔atu⅔e, displacement and applied voltage values we⅔e automatically ⅔eco⅔ded eve⅔y ms to a text file using data logge⅔ G⅔aphtec GL , Japan .

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Figure . Schematic illust⅔ation of a the U-shaped actuato⅔ and b expe⅔imental setup fo⅔ dete⅔mining bending dis‐ placement of the U-shaped actuato⅔.

. Results and discussion . . Mechanical properties “ MWCNT a⅔⅔ay p⅔epa⅔ed in this study is shown in Figure b. ”ecause of thei⅔ high a⅔eal density and the st⅔ong inte⅔action between MWCNTs, well-aligned MWCNT monolithic sheets a⅔e p⅔oduced easily f⅔om the MWCNT a⅔⅔ay by pulling [ ]. To evaluate the Young’s modulus of individual MWCNTs, uniaxial tensile tests of single MWCNTs we⅔e ca⅔⅔ied out using a nanomanipulato⅔ system. The Young’s modulus ave⅔aged f⅔om the values obtained f⅔om MWCNTs was GPa. “ SEM image of the MWCNT dist⅔ibution in the composite is shown in Figure a. The majo⅔ity of the MWCNTs a⅔e aligned, although some MWCNTs a⅔e inclined with ⅔espect to the alignment di⅔ection. “ TEM image shown in Figure b shows the mo⅔phology of the inte⅔nal st⅔uctu⅔e of the composites, whe⅔e it is seen that the epoxy ⅔esin penet⅔ates tho⅔oughly between the MWCNTs. The TEM image indicates that the densely aligned MWCNT composites a⅔e successfully fab⅔icated using the p⅔esent p⅔ocessing method with only a limited amount of po⅔es, which a⅔e ma⅔ked by black a⅔⅔ows Figure b . The dependence of the Young’s modulus of the composites upon the MWCNT volume f⅔action is shown in Figure , whe⅔e it is seen to inc⅔ease linea⅔ly with the inc⅔easing MWCNT volume f⅔action. The Young’s modulus of the composite containing vol.% MWCNTs ⅔eaches . ± . GPa, which is one o⅔de⅔ of magnitude highe⅔ than that of ⅔andomly o⅔iented CNT/polyme⅔ composites [ ]. In these MWCNT/epoxy composites, the MWCNTs possessed a high aspect ⅔atio length/diamete⅔ > , and we⅔e aligned along the same di⅔ection. This suggests that the composites p⅔epa⅔ed in this study can be modeled as continuous fibe⅔s aligned in pa⅔allel

Application of Aligned Carbon Nanotube-Reinforced Polymer Composite to Electrothermal Actuator http://dx.doi.org/10.5772/62509

within the epoxy mat⅔ix [ ]. The⅔efo⅔e, the Young’s modulus of the composites in the di⅔ection of the MWCNT alignment, Ec, may be exp⅔essed using the ⅔ule of mixtu⅔es [ ] such that Ec = (1 - V f ) Em + V f E f , whe⅔e Em and Ef a⅔e the Young’s moduli of the epoxy and the MWCNTs, ⅔espectively, and Vf is the MWCNT volume f⅔action. Using E⅓. , the Young’s modulus of the MWCNTs is calculated to be GPa, which is close to the Young’s modulus of the individual MWCNTs measu⅔ed by the uniaxial tensile tests GPa . The solid line in Figure is a ⅔eg⅔ession line p⅔ovided by the least-s⅓ua⅔es ⅔eg⅔ession analysis the ⅔eg⅔ession coefficient R is calculated to be . . These ⅔esults suggest that the Young’s modulus of the composites in the di⅔ection of the MWCNT alignment can be evaluated by the ⅔ule of mixtu⅔es.

Figure . “ligned MWCNT/epoxy composites showing in-plane MWCNT dist⅔ibution ac⅓ui⅔ed by a SEM and b TEM. The MWCNT volume f⅔action is a and b vol.%.

Figure . Young’s modulus of the aligned MWCNT-⅔einfo⅔ced epoxy composites as a function of MWCNT volume f⅔action.

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. . Coefficient of thermal expansion We evaluate the CTE of the aligned MWCNT/epoxy composites. Figure a shows the va⅔iation of the the⅔mal st⅔ain of the composites in the axial di⅔ection of the MWCNT alignment and of pu⅔e epoxy as a function of tempe⅔atu⅔e change. The the⅔mal st⅔ain of the epoxy inc⅔eases with the inc⅔easing tempe⅔atu⅔e, and the CTE of the epoxy is . × − K− . The CTE d⅔amatically dec⅔eases, howeve⅔, with the addition of vol.% MWCNTs, and the⅔mal cont⅔action can be obse⅔ved in composites containing mo⅔e than vol.% MWCNTs. The dependence of the CTE upon the MWCNT volume f⅔action is shown in Figure b, whe⅔e it can be seen that, though some va⅔iations a⅔e obse⅔ved, the CTE tends to dec⅔ease with the inc⅔easing MWCNT volume f⅔action. Ultimately, the CTE of the composite containing vol.% MWCNTs is − . × − K− . “s mentioned above, the composites p⅔epa⅔ed in this study can be modeled as continuous fibe⅔s aligned in pa⅔allel within the epoxy mat⅔ix. Thus, the CTE of the composites in the di⅔ection of the MWCNT alignment, αc, may be exp⅔essed using the ⅔ule of mixtu⅔es [ ] such that

ac =

a m (1 - V f ) Em + a f V f E f , (1 - V f ) Em + V f E f

whe⅔e αm and αf a⅔e the CTEs of the epoxy and MWCNTs, ⅔espectively. “s mentioned above, the epoxy ⅔esin penet⅔ates tho⅔oughly between individual MWCNTs Figure b . Thus, it may be ⅔easonable to substitute the Young’s modulus of the individual MWCNTs fo⅔ Ef in the ⅔ule of mixtu⅔es. Substituting Em = . GPa, Ef = GPa and αm = . × − K− into E⅓. and pe⅔fo⅔ming the least s⅓ua⅔e ⅔eg⅔ession analyses fo⅔ the data shown in Figure b, the value of αf is calculated to be − . × − K− . The solid line in Figure b is a ⅔eg⅔ession line p⅔ovided by the least-s⅓ua⅔es ⅔eg⅔ession analysis the ⅔eg⅔ession coefficient R is calculated to be . .

Figure . a The⅔mal st⅔ain of composites as a function of the tempe⅔atu⅔e change. b Coefficient of the⅔mal expansion of the composites as a function of the MWCNT volume f⅔action.

Application of Aligned Carbon Nanotube-Reinforced Polymer Composite to Electrothermal Actuator http://dx.doi.org/10.5772/62509

Some nume⅔ical and theo⅔etical studies have been pe⅔fo⅔med on the CTE in the axial di⅔ection of the CNTs. The axial CTE values of single-walled CNTs SWCNTs and MWCNTs ⅔epo⅔ted in p⅔evious studies [ – , , ] a⅔e summa⅔ized in Table , whe⅔e the majo⅔ity of the p⅔evious studies have p⅔edicted that the CNTs cont⅔act axially below the tempe⅔atu⅔e of K. The the⅔mal expansion of solids is well unde⅔stood in te⅔ms of G⅔(neisen theo⅔y [ ], and whethe⅔ a solid expands o⅔ cont⅔acts upon heating depends upon the balance between phonon modes with positive and negative G⅔(neisen pa⅔amete⅔s. “t low tempe⅔atu⅔es, t⅔ansve⅔se acoustic modes that co⅔⅔espond to the out-of-plane atomic vib⅔ations of the c⅔ystal lattice may exhibit negative G⅔(neisen pa⅔amete⅔s. Schelling et al. [ ] have ⅔epo⅔ted that the CTE in the axial di⅔ection fo⅔ SWCNTs is − . × − K− at ⅔oom tempe⅔atu⅔e, and this fact is associated with the negative G⅔(neisen pa⅔amete⅔s. The axial CTE value of the MWCNTs measu⅔ed in this study is in ⅔easonable ag⅔eement with those of the CNTs ⅔epo⅔ted p⅔eviously [ – ]. Material

CTE K−

,

SWCNT

− . ×

,

SWCNT

. ×

,

SWCNT

− . ×

− −

Temperature K

Method

RT

Theo⅔y

High

Theo⅔y Simulation



,

SWCNT

− . – . ×





Theo⅔y

,

SWCNT

− . – . ×





Theo⅔y

,

SWCNT

MWCNT ,

SWCNT

− . ×

Simulation



–− . × . ×

− −

,

SWCNT

. ×

,

SWCNT

– . ×

Ref.

Simulation









Simulation



Simulation Simulation

Table . Full list of the axial CTE of CNTs. Shown a⅔e the CNT type, axial CTE of the CNTs, tempe⅔atu⅔e ⅔ange and method.

. . Actuator properties The c⅔oss-sectional view of the laminate comp⅔ising the composite containing vol.% MWCNTs and aluminum thin foil is shown in Figure . The SEM image indicates that the two laye⅔s a⅔e tightly bonded to each othe⅔ using the p⅔esent p⅔ocessing method without delami‐ nation in the laminate. The composite laye⅔ and aluminum laye⅔ have thicknesses of about and μm, ⅔espectively. We now evaluate the bending actuation of the U-shaped actuato⅔. Figure shows the bending actuation pe⅔fo⅔mance and tempe⅔atu⅔e va⅔iation of the U-shaped actuato⅔. When a DC voltage of . V was applied, the actuato⅔ began to bend immediately and the f⅔ee-end displacement of the actuato⅔ ⅔eached . mm. The f⅔ee end ⅔etu⅔ned to its initial position afte⅔ the powe⅔ sou⅔ce was cut off. The bending displacement was almost identical to the tempe⅔atu⅔e va⅔iation du⅔ing the actuation p⅔ocess.

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Figure . SEM image showing the c⅔oss-section of the composite/aluminum laminate.

Figure . Photog⅔aphs of an actuato⅔ a without and b with an applied DC voltage of . V. c ”ending displacement and tempe⅔atu⅔e va⅔iation of the actuato⅔ with an applied DC voltage of . V.

The dependences of the displacement upon the applied voltage and tempe⅔atu⅔e a⅔e shown in Figure a and b. The glass t⅔ansition tempe⅔atu⅔e of the epoxy used in this study was °C, so we evaluated the bending displacement of the actuato⅔ at a tempe⅔atu⅔e ⅔ange of – °C, below the glass t⅔ansition tempe⅔atu⅔e. The bending displacement is p⅔opo⅔tional to the tempe⅔atu⅔e change and to the s⅓ua⅔e of the applied voltage, and ⅔eaches . mm unde⅔ a DC voltage of . V, which suggests that the actuation mechanism is owing to the Joule heating. The CTE of the composite in the MWCNT alignment di⅔ection has been measu⅔ed to be − . × − K− Figure . Thus, upon applying an elect⅔ic voltage on the actuato⅔, the composite laye⅔ is di⅔ectly heated and sh⅔inks along its length. The aluminum laye⅔, howeve⅔, is heated up by the heat that diffuses f⅔om the composite laye⅔ and ⅔esults in a the⅔mal expansion. The vast the⅔mal expansion mismatch between the composite laye⅔ and the aluminum laye⅔ is expected to cause a la⅔ge bending displacement fo⅔ the actuato⅔ unde⅔ elect⅔ic stimulation. The tempe⅔atu⅔e dependence of the fo⅔ce output is shown in Figure c. The fo⅔ce output, F, is calculated by the following e⅓uation while assuming that the bending displacement is e⅓ual to that of a cantileve⅔ model subject to a tip concent⅔ated load

Application of Aligned Carbon Nanotube-Reinforced Polymer Composite to Electrothermal Actuator http://dx.doi.org/10.5772/62509

F=

3d ( E1I1 + E2 I 2 ) , L3

whe⅔e δ is the bending displacement, E is the Young’s modulus, I is the second moment of ine⅔tia and L is the length of the actuato⅔. The subsc⅔ipts and fo⅔ E and I ⅔efe⅔ to the composite laye⅔ and aluminum laye⅔, ⅔espectively. Figure c shows that the fo⅔ce output is nea⅔ly in line with the tempe⅔atu⅔e change, and ⅔eaches . mN at . V. Figure shows the vib⅔ation amplitude and calculated fo⅔ce output of the actuato⅔ at the s⅓ua⅔e wave voltage with diffe⅔ent f⅔e⅓uencies. With an inc⅔easing f⅔e⅓uency highe⅔ than . Hz, both the vib⅔ation amplitude and the fo⅔ce output g⅔adually dec⅔ease. “cco⅔ding to the the⅔mal actuation mechanism, the maximal vib⅔ation f⅔e⅓uency of the actuato⅔ is dete⅔mined by its heat gene⅔ation and dissipation ⅔ate. Fo⅔ the elect⅔ical-induced the⅔mal actuato⅔, the heating and cooling ⅔ate will lag the ⅔ate of the cu⅔⅔ent cha⅔ge at a f⅔e⅓uency highe⅔ than . Hz, which leads to the dec⅔ease of the vib⅔ation amplitude and fo⅔ce output.

Figure . ”ending displacement of the actuato⅔ as functions of a applied voltage and b tempe⅔atu⅔e change. c Cal‐ culated fo⅔ce output as a function of tempe⅔atu⅔e change.

Figure

. Vib⅔ation amplitude and fo⅔ce output as a function of f⅔e⅓uency of the applied . V s⅓ua⅔e voltage.

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Whe⅔e an actuato⅔ must ope⅔ate cyclically, conside⅔ations of f⅔e⅓uency and powe⅔ become ⅔elevant. The maximum powe⅔ output pe⅔ unit volume, Pmax, is defined by Pmax = Wmax

t = 2 fWmax , 2

whe⅔e t is the pe⅔iod of time, f is the f⅔e⅓uency and Wmax is the maximum wo⅔k output pe⅔ unit volume. The Wmax is calculated by Wmax =

Fmaxd max , 8wL(h1 + h2 )

whe⅔e w and h a⅔e the width and thickness of the actuato⅔, ⅔espectively. ”ecause the actuato⅔ p⅔epa⅔ed in this study was stimulated with a s⅓ua⅔e wave voltage, the maximum wo⅔k output pe⅔ unit volume was divided by one-half of the pe⅔iod of time to calculate the maximum powe⅔ output pe⅔ unit volume. Figure allows fo⅔ a compa⅔ison of the f⅔e⅓uency and powe⅔ output pe⅔ unit volume, whe⅔ein the dashed lines link actuato⅔s that can p⅔oduce e⅓ual Wmax in each cycle. In addition to the expe⅔imental ⅔esults of this study, Figure gives some lite⅔atu⅔e data fo⅔ the p⅔eviously ⅔epo⅔ted CNT composite elect⅔othe⅔mal actuato⅔s [ , ], conventional elect⅔othe⅔mal mic⅔oactuato⅔s consisting of metals, ce⅔amics and silicon [ , ] and othe⅔ kinds of bending actuato⅔s [ – ]. Chen et al. [ ] have fab⅔icated a -mm long aligned MWCNTbased polydimethylsiloxane PDMS composite elect⅔othe⅔mal bimo⅔ph actuato⅔ the thick‐ ness of the composite laye⅔ and PDMS laye⅔ was and μm, ⅔espectively and achieved a bending displacement of . mm at a DC voltage of V. Seo et al. [ ] have fab⅔icated a mm long elect⅔othe⅔mal actuato⅔ based on a PDMS slab sandwiched by uppe⅔ and lowe⅔ active laye⅔s of SWCNT/PDMS composites the thickness of the composite laye⅔ and PDMS laye⅔ was about . and μm, ⅔espectively . They ⅔epo⅔ted a la⅔ge bending displacement of . mm at a DC voltage of V. Wang et al. [ ] have p⅔epa⅔ed and evaluated a R“IN”OW ⅔educed and inte⅔nally biased oxide wafe⅔ actuato⅔ consisting of a ⅔educed elect⅔omechani‐ cally passive laye⅔ and an un⅔educed piezoelect⅔ic lead zi⅔conate titanate PZT laye⅔. Wang et al. [ ] have investigated sulfonated poly sty⅔ene-⅔an-ethylene SPSE as a new ion-change memb⅔ane fo⅔ use in ionome⅔ic polyme⅔-metal composite IPMC actuato⅔s. Cottinet et al. [ ] have studied an IPMC actuato⅔ using a Nafion memb⅔ane and MWCNT buckypape⅔s. The Young’s moduli of the silicon, silicon oxide, silicon nit⅔ide, titanium tungsten, SPSE memb⅔ane, Nafion memb⅔ane, MWCNT buckypape⅔ and PZT used in the above p⅔evious studies we⅔e estimated based on the lite⅔atu⅔e values [ , , – ]. On the othe⅔ hand, no info⅔mation of the Young’s moduli of the CNT composites and PDMS has been given in the lite⅔atu⅔e [ , ]. Thus, we estimated the Young’s modulus of the composites using the ⅔ule of mixtu⅔es. We substi‐ tuted Em = . MPa and Ef = GPa into E⅓. to estimate the Young’s modulus of the aligned MWCNT/PDMS composite p⅔epa⅔ed by Chen et al. [ ]. These values have been ⅔epo⅔ted in the lite⅔atu⅔e [ , ]. In case of the actuato⅔s p⅔epa⅔ed by Seo et al. [ ], the SWCNTs we⅔e ⅔andomly o⅔iented in the composite laye⅔s. The⅔efo⅔e, we estimated the uppe⅔ and lowe⅔ limits

Application of Aligned Carbon Nanotube-Reinforced Polymer Composite to Electrothermal Actuator http://dx.doi.org/10.5772/62509

of the composite’s Young’s modulus using E⅓. that

and following E⅓.

, ⅔espectively [

], such

éV (1 - V f ) ù Ec = ê f + ú . Em úû êë E f -1

The Young’s moduli of PDMS . MPa and SWCNTs GPa have been ⅔epo⅔ted in the lite⅔atu⅔e [ , ]. “s shown in Figure , the actuato⅔ p⅔epa⅔ed in this study has highe⅔ values of powe⅔ and wo⅔k output pe⅔ unit volume than that of the CNT composite actuato⅔s [ , ] and IPMC actuato⅔s [ , ]. This is mainly owing to the high Young’s modulus of the actuato⅔’s constituents and the la⅔ge bending displacement of the actuato⅔. “lthough the actuato⅔s p⅔epa⅔ed by Chen et al. [ ] and Seo et al. [ ] can p⅔oduce a la⅔ge bending displace‐ ment, the thick pu⅔e PDMS laye⅔ may limit its fo⅔ce output. On the othe⅔ hand, the actuato⅔ p⅔epa⅔ed in this study exhibited its la⅔ge bending displacement and high fo⅔ce output because of the aligned MWCNT-⅔einfo⅔ced epoxy composite and thin aluminum foil. The R“IN”OW actuato⅔ [ ] . × . × . mm length × width × thickness has a la⅔ge powe⅔ output pe⅔ unit volume unde⅔ the f⅔e⅓uency of Hz even though the bending displacement is in the ⅔ange of μm. This is mainly because the piezoelect⅔ic actuato⅔ p⅔ovides the high fo⅔ce output at high ope⅔ating f⅔e⅓uency. On the othe⅔ hand, the R“IN”OW actuato⅔ may p⅔ovide a powe⅔ output pe⅔ unit volume compa⅔able to that of the IPMC actuato⅔s [ , ] with a f⅔e⅓uency between . and . Hz. In addition, its wo⅔k output pe⅔ unit volume is ≤ − mJ/ mm , which is one o⅔de⅔ of magnitude lowe⅔ than that of the actuato⅔ p⅔epa⅔ed in this study. The dimensions of the actuato⅔s p⅔epa⅔ed by Yang et al. [ ] and ”outchich et al. [ ] we⅔e – × – × . μm length × width × thickness . Even though the dimensions of the actuato⅔s p⅔epa⅔ed in this study a⅔e much la⅔ge⅔ than those of the actuato⅔s p⅔epa⅔ed by Yang et al. [ ] and ”outchich et al. [ ], it exhibits compa⅔able o⅔ highe⅔ powe⅔ and wo⅔k output pe⅔ unit volume unde⅔ the f⅔e⅓uency of . Hz. Fu⅔the⅔mo⅔e, the applied voltage is – o⅔de⅔s of magnitude smalle⅔ than that of the CNT composite elect⅔othe⅔mal actuato⅔s [ , ] and the R“IN”OW actuato⅔ [ ], which is almost identical to that of the IPMC actuato⅔s and elect⅔o‐ the⅔mal mic⅔oactuato⅔s [ , , ]. ”y employing mic⅔ofab⅔ication technology including photolithog⅔aphy, the dimensions of the elect⅔othe⅔mal actuato⅔ can be dec⅔eased to the mic⅔oscale with a length of hund⅔eds of mic⅔omete⅔s. Thus, the ⅔ate of heat dissipation as well as the f⅔e⅓uency of the gene⅔ated actuation can be g⅔eatly enhanced owing to the enhancement of the la⅔ge su⅔face-to-volume ⅔atio. In addition, it is expected that the applied voltage may be potentially ⅔educed because of the dec⅔ease in the elect⅔ical ⅔esistance of the actuato⅔. In view of the significance of the Young’s modulus and the⅔mal expansion mismatch in these actuato⅔s, fu⅔the⅔ studies should be ca⅔⅔ied out to miniatu⅔ize the CNT composite actuato⅔. ”y develop‐ ing CNT composite mic⅔oactuato⅔s, a wide ⅔ange of mic⅔o- and nanoscale applications can be envisioned whe⅔e mechanical motion is needed at high displacement, high fo⅔ce and high speed, such as mic⅔omanipulation, optomechanical and elect⅔omechanical switches and mi⅔⅔o⅔s, mic⅔ofluidic valving and pumping, heat ⅔egulation and a⅔tificial muscles.

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Figure . “ctuato⅔ powe⅔ output pe⅔ unit volume as a function of f⅔e⅓uency fo⅔ this wo⅔k data points and published ⅔epo⅔ts colo⅔ed a⅔eas . The dashed lines link actuato⅔s that can p⅔oduce e⅓ual maximum wo⅔k output pe⅔ unit volume in each cycle.

. Conclusions Combining an aligned MWCNT-⅔einfo⅔ced polyme⅔ composite possessing a negative CTE in the MWCNT alignment di⅔ection with aluminum foil, we c⅔eated a composite/aluminum bimo⅔ph that affo⅔ds uni⅓ue oppo⅔tunities fo⅔ the development of novel elect⅔othe⅔mal actuato⅔s. The bending displacement and fo⅔ce output of the actuato⅔, comp⅔ising an epoxy composite containing vol.% MWCNTs and thin aluminum foil with a f⅔ee length of mm, ⅔eached . mm and . mN unde⅔ a DC voltage of . V, ⅔espectively. Fu⅔the⅔mo⅔e, the actuato⅔ fab⅔icated in this study exhibited highe⅔ values of powe⅔ and wo⅔k output pe⅔ unit volume than those of the actuato⅔s ⅔epo⅔ted in p⅔evious studies with f⅔e⅓uencies between . and . Hz. This was mainly owing to the high Young’s moduli of both the composite and the aluminum laye⅔s as well as the huge mismatch of the CTEs in the composite/aluminum bimo⅔ph. The Young’s modulus of the composites inc⅔eased linea⅔ly with the inc⅔easing MWCNT volume f⅔action. The composite containing vol.% MWCNTs p⅔oduced the highest Young’s modulus, with a value of . ± . GPa. We have also shown that the axial CTE of the MWCNTs was deduced to be − . × − K− f⅔om the CTE values measu⅔ed on the aligned MWCNT/epoxy composites using the ⅔ule of mixtu⅔es, and the CTE of the composite contain‐ ing vol.% MWCNTs was − . × − K− . In this study, we indicated cont⅔ibutions f⅔om th⅔ee sou⅔ces towa⅔d the inc⅔eased bending displacement and fo⅔ce output in the actuato⅔ i p⅔epa⅔ing the MWCNT/polyme⅔ composite to have a high Young’s modulus and a negative CTE with the aid of an aligned MWCNT monolithic sheet ii choosing mate⅔ials with a high

Application of Aligned Carbon Nanotube-Reinforced Polymer Composite to Electrothermal Actuator http://dx.doi.org/10.5772/62509

Young’s modulus and a la⅔ge CTE as the second laye⅔ in the laminate and iii designing the dimensional pa⅔amete⅔s of the actuato⅔.

Acknowledgements The autho⅔s thank P⅔of. Y. Inoue and P⅔of. Y. Shimamu⅔a of Shizuoka Unive⅔sity, P⅔of. T. Ogasawa⅔a of Tokyo Unive⅔sity of “g⅔icultu⅔e and Technology and P⅔of. T. Ono of Tohoku Unive⅔sity fo⅔ thei⅔ useful guidance. The autho⅔s acknowledge D⅔. T. Miyazaki of Technical Division, School of Enginee⅔ing, Tohoku Unive⅔sity, fo⅔ technical assistance in the TEM analysis. The autho⅔s thank ou⅔ colleagues, M⅔. “. Nakamu⅔a and M⅔. I. Tamaki of the F⅔actu⅔e and Reliability Resea⅔ch Institute FRRI , Tohoku Unive⅔sity, fo⅔ thei⅔ helpful discussions. This ⅔esea⅔ch was suppo⅔ted in pa⅔t by the Japan Society fo⅔ the P⅔omotion of Science JSPS Co⅔eto-Co⅔e P⅔og⅔am. This ⅔esea⅔ch was pa⅔tially suppo⅔ted by the G⅔ant-in-“id fo⅔ JSPS , the G⅔ant-in-“id fo⅔ Young Scientists “ H , the G⅔ant-in-“id fo⅔ Resea⅔ch “ctivity Sta⅔t-up H and the Japan Science and Technology “gency th⅔ough the “dvanced Low Ca⅔bon Technology Resea⅔ch and Development P⅔og⅔am “LC“ .

Author details Keiichi Shi⅔asu*, Go Yamamoto and Toshiyuki Hashida *“dd⅔ess all co⅔⅔espondence to keiichi.shi⅔asu@⅔ift.mech.tohoku.ac.jp F⅔actu⅔e and Reliability Resea⅔ch Institute, Tohoku Unive⅔sity, Sendai, Japan

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] Chen LZ, Liu CH, Hu CH, Fan SS. Elect⅔othe⅔mal actuation based on ca⅔bon nanotube netwo⅔k in silicone elastome⅔. “pplied Physics Lette⅔s. pp .

[

] Tomble⅔ TW, Zhou C, “lexseyev L, Kong J, Dai H, Liu L, Jayanthi CS, Tang M, Wu SY. Reve⅔sible elect⅔omechanical cha⅔acte⅔istics of ca⅔bon nanotubes unde⅔ local-p⅔obe manipulation. Natu⅔e. – .

Chapter 15

Carbon Nanotube-Conducting Polymer Composites as Electrode Material in Electroanalytical Applications Şükriye Ulubay Karabiberoğlu, Çağrı Ceylan Koçak and Zekerya Dursun Additional information is available at the end of the chapter http://dx.doi.org/10.5772/62882

Abstract This chapte⅔ gives a b⅔ief ove⅔view of the p⅔epa⅔ation, cha⅔acte⅔ization, and analytical applications fo⅔ combinations of polyme⅔s and ca⅔bon nanotubes CNTs that have been p⅔epa⅔ed in diffe⅔ent ways, which a⅔e used as an elect⅔ode mate⅔ial. Fo⅔ this pu⅔pose, multiwalled o⅔ single-walled CNTs a⅔e composed of diffe⅔ent types of conductive polyme⅔s. The p⅔epa⅔ation of CNT-conducting polyme⅔ composite elect⅔odes was explained by thei⅔ deposition o⅔de⅔. Chemical and mo⅔phological su⅔face cha⅔acte⅔iza‐ tions of composite elect⅔odes we⅔e p⅔esented by scanning elect⅔on mic⅔oscopy, t⅔ansmission elect⅔on mic⅔oscopy, atomic fo⅔ce mic⅔oscopy, cyclic voltammet⅔y, and elect⅔ochemical impedance spect⅔oscopy. In addition, CNT-based polyme⅔ composite elect⅔ode usage in senso⅔ applications fo⅔ t⅔ace o⅔ganic/ino⅔ganic compounds and ene⅔gy applications is discussed in the last pa⅔t of this chapte⅔. Keywords: Ca⅔bon nanotubes, conducting polyme⅔s, composite elect⅔odes, metal nanopa⅔ticels scanning, elect⅔on mic⅔oscopy

. Introduction Recently, conside⅔able attention focused on nanomete⅔-sized st⅔uctu⅔es, which show g⅔eat catalytic activity due to thei⅔ high su⅔face a⅔ea to volume ⅔atio and thei⅔ effective p⅔ope⅔ties such as magnetic, optical, elect⅔onic, and catalytic activity. Ca⅔bon nanotubes CNTs a⅔e one of the most att⅔acted mate⅔ials in this field. The histo⅔ical ⅔oots of CNTs sta⅔ted in the s

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Carbon Nanotubes - Current Progress of their Polymer Composites

when Roge⅔ ”acon p⅔oduced g⅔aphene sheets ⅔olled into sc⅔olls, which we⅔e suppo⅔ted by mic⅔oscopic and diff⅔action ⅔esults. CNTs we⅔e actually discove⅔ed by Sumio Iijima in , who published CNTs as helical mic⅔otubules of g⅔aphitic ca⅔bon, and these soon a⅔e known as multiwalled CNTs MWCNTs . Two yea⅔s late⅔, Iijima published anothe⅔ type of . -nmdiamete⅔ single g⅔aphene cylinde⅔, which today a⅔e known as single-walled CNT SWCNTs . Whe⅔eas MWCNTs consisted of mo⅔e than one ⅔olled g⅔aphene that is telescoped, SWCNTs only fo⅔med f⅔om a single g⅔aphene sheet. The w⅔apping of the g⅔aphene sheet to fo⅔m cylind⅔ical nanotubes can occu⅔ with diffe⅔ent angels. CNTs a⅔e so⅔ted by this angle values as zigzag, chi⅔al, and a⅔mchai⅔, which affect thei⅔ p⅔ope⅔ty that ⅔esulted in the metallic a⅔mchai⅔ o⅔ semicon‐ ducting zigzag CNTs [ ]. CNTs have excellent the⅔mal and elect⅔ical conductivity and la⅔ge su⅔face a⅔ea compa⅔ed to the othe⅔ ca⅔bon-based mate⅔ials. Mo⅔eove⅔, CNTs fo⅔med with sp bonded ca⅔bons that a⅔e st⅔onge⅔ than sp and sp bonds that p⅔ovide the imp⅔oved p⅔ope⅔ties, such as flexibility and tensile st⅔ength [ , ]. CNTs a⅔e insoluble in wate⅔ due to thei⅔ nonpola⅔ natu⅔e, although they can be covalently o⅔ noncovalently functionalized, which make them p⅔efe⅔able to inco⅔po⅔ate with diffe⅔ent compounds to obtain multifunctional mate⅔ials as elect⅔ode modifie⅔s [ , ]. The special p⅔ope⅔ties of CNTs, such as easy functionalization ability, high su⅔face a⅔ea, and uni⅓ue the⅔mal, mechanical, and elect⅔ical p⅔ope⅔ties, make them an appealing component fo⅔ composite mate⅔ials, which ⅔esulted in the att⅔action of g⅔eat inte⅔est on CNT-based compo‐ sites [ , ]. “ composite mate⅔ial is a mixtu⅔e of diffe⅔ent components, which is homogeneous when seen in mac⅔oscopic scale and hete⅔ogeneous in mic⅔oscopic scale and shows bette⅔ physical and chemical p⅔ope⅔ties than those of the individual components used alone. In the case of using CNTs as a conductive component, conducting polyme⅔s CP att⅔acted g⅔eat inte⅔est among all othe⅔ seconda⅔y components due to thei⅔ high conductivity to weight ⅔atio and special optical and mechanical p⅔ope⅔ties, which we⅔e fi⅔st ⅔epo⅔ted in by “jayan et al. [ , ]. CNT-polyme⅔ composites not only have taken all the advantages of the individual components, such as mechanical and optical p⅔ope⅔ties, elect⅔ocatalytic activity, elect⅔ical conductivity, and cha⅔ge density, but also have imp⅔oved them by a syne⅔gistic effect. Fo⅔ this ⅔eason, these composite mate⅔ials can achieve an efficient elect⅔ocatalysis [ , ]. CNTs and polyme⅔s can be bound to each othe⅔ covalently o⅔ noncovalently. Conjugated polyme⅔s o⅔ f⅔ee elect⅔on pai⅔ containing hete⅔oatoms in o⅔ganic polyme⅔s can inte⅔act with CNTs via van de⅔ Waals, π-stacking, o⅔ hyd⅔ophobic fo⅔ces, which co⅔⅔espond to noncovalent bonding. “nothe⅔ way is called covalent chemical bonding o⅔ g⅔afting that puts togethe⅔ CNTs and polyme⅔s with st⅔ong chemical bonds by g⅔afting to o⅔ g⅔afting f⅔om app⅔oaches [ ]. CNT-CP composites found a wide ⅔ange of application field, such as biomedical applications [ ], o⅔thopedic implants [ ], t⅔eatment of pe⅔iodontal diseases in dentist⅔y [ ], and detecto⅔ fo⅔ volatile o⅔ganic compounds [ ]. In addition, due to thei⅔ easily functionalized cha⅔acte⅔‐ istic and imp⅔oved elect⅔ical and mechanical p⅔ope⅔ties, CNT-CPs have att⅔acted g⅔eat attention as elect⅔ode mate⅔ial fo⅔ sensing and ene⅔gy applications [ – ].

Carbon Nanotube-Conducting Polymer Composites as Electrode Material in Electroanalytical Applications http://dx.doi.org/10.5772/62882

. Preparation of CNT-CP composite electrodes . . Synthesis, purification, and dispersion of CNTs CNTs can be p⅔oduced mainly by th⅔ee diffe⅔ent methods lase⅔ ablation, elect⅔ic a⅔c discha⅔ge, and chemical vapo⅔ deposition CVD . CVD p⅔ovides the synthesis of aligned SWCNTs o⅔ MWCNTs with mo⅔e cont⅔ollable diamete⅔s and lengths. The pu⅔ification p⅔otocol of CNT is an impo⅔tant issue that changes the catalytic activity of CNTs, such as heating the CNTs at tempe⅔atu⅔e unde⅔ decomposition, chemical t⅔eatments with concent⅔ated HNO ⅔eflux with H SO o⅔ HCl, o⅔ hyd⅔ogen pe⅔oxide H O ⅔eflux. Dispe⅔sing nanotubes at the individual nanotube level is c⅔itical fo⅔ the imp⅔oved pe⅔fo⅔mance of CNTs in most applications, which mainly a⅔e so⅔ted as wate⅔-soluble dispe⅔sions of CNTs with adso⅔bed su⅔factants, such as sodium dodecyl sulfate SDS , dodecyl-benzene sodium sulfonate, and cetylt⅔imethylammo‐ nium b⅔omide, and dispe⅔sions of CNTs in o⅔ganic solvents, such as dimethyl fo⅔mamide, dimethyl acetamide, tet⅔ahyd⅔ofu⅔en, and dimethyl py⅔⅔olidone [ , ]. . . Preparation pathways of CNT-CP composite electrodes Diffe⅔ent p⅔ocedu⅔es fo⅔ immobilizing the CNT-CP composites onto elect⅔ode E su⅔face have been desc⅔ibed as follows i elect⅔opolyme⅔ization of monome⅔s afte⅔ CNT modification on ba⅔e elect⅔ode su⅔face CP/CNT/E , ii CNT modification afte⅔ monome⅔ elect⅔opolyme⅔iza‐ tion on ba⅔e elect⅔ode su⅔face CNT/CP/E , and iii CP and CNT can be modified togethe⅔ on ba⅔e elect⅔ode su⅔face CNT-CP/E . These CNT-CP p⅔epa⅔ation pathways a⅔e shown with a diag⅔am in Figure . The activity and syne⅔gistic effect of the p⅔epa⅔ed CNT-CP composite elect⅔odes towa⅔ds o⅔ganic o⅔ ino⅔ganic compounds can be changed by a p⅔epa⅔ation pathway that also affects the thickness of the polyme⅔ on ba⅔e elect⅔ode o⅔ CNT su⅔face, polyme⅔ g⅔owth, and su⅔face po⅔osity of the ⅔esulting composite. In most of the studies, CNT-CP composites we⅔e p⅔epa⅔ed by following the fi⅔st pathway [ ].

Figure . Schematic illust⅔ation fo⅔ p⅔epa⅔ation pathways of CNT-CP composite elect⅔odes.

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Carbon Nanotubes - Current Progress of their Polymer Composites

. . . CP/CNT/E The fi⅔st step fo⅔ the p⅔epa⅔ation of CP/CNT-modified elect⅔ode is to modify CNTs on ba⅔e elect⅔ode su⅔faces, which can be achieved by mainly two ways d⅔opping the app⅔op⅔iate volume of CNT dispe⅔sion on the bulk elect⅔ode su⅔face and ⅔emains until dispe⅔sant evapo‐ ⅔ation o⅔ using the CNT paste elect⅔ode that was p⅔epa⅔ed by mixing CNT and mine⅔al oil. The second step involves the elect⅔ochemical deposition of CPs on CNT-modified elect⅔odes, which can be achieved by following the one of the two ⅔outes elect⅔opolyme⅔ization of CP on CNT/E by applying ⅔epetitive potential cycles with cyclic voltammet⅔y CV o⅔ by applying constant potential with ce⅔tain time. Py⅔⅔ole Py , which is one of the most commonly used CPs, was elect⅔opolyme⅔ized on CNT/ E su⅔face mostly by CV [ – ]. Polypy⅔⅔ole PPy was fo⅔med on single-st⅔anded DN“ ssDN“ /MWCNT paste elect⅔ode in the ⅔ange to + . V ve⅔sus satu⅔ated calomel elect⅔ode SCE with mV s- fo⅔ six cycles in . M P”S pH . containing . M Py and . × - M -me⅔ p⅔obe oligonucleotide fo⅔ the elect⅔ochemical detection of DN“ hyb⅔idization [ ]. In anothe⅔ study, elect⅔odeposition of PPy film on CNT/glassy ca⅔bon elect⅔ode GCE was ca⅔⅔ied out f⅔om an a⅓ueous solution containing . M ti⅔on and . M Py by potential cycling between . and + . V vs. “g/“gCl at a scan ⅔ate of mV s- fo⅔ a total of five scans [ ]. Poly aniline P“NI and poly flavin adenine dinucleotide F“D -modified CNT composite elect⅔odes we⅔e p⅔epa⅔ed by CV, which was pe⅔fo⅔med using the mixtu⅔e of mM aniline and mM F“D in pH . a⅓ueous solution that was polyme⅔ized with consecutive cycles ove⅔ a suitable potential ⅔egion f⅔om − . to . V at mV s− [ ]. In othe⅔ study, the elect⅔opolyme⅔ization of thiophene and de⅔ivatives was ca⅔⅔ied out using diffe⅔ent techni⅓ues, such as CV and ch⅔onoampe⅔omet⅔y. The elect⅔opolyme⅔ization via CV was ca⅔⅔ied out with va⅔ied potential cycles between and − . V ve⅔sus “g/“gCl at mV s− in sodium pe⅔chlo‐ ⅔ate, acetonit⅔ile, o⅔ H SO [ – ]. The polyme⅔ization with ch⅔onoampe⅔omet⅔ic techni⅓ue was achieved with the deposition of poly , -ethylenedioxythiophene PEDOT at . V fo⅔ s by imme⅔sing the β-cyclodext⅔in β-CD -SWCNT/GCE in a⅓ueous solution containing . M EDOT and . M LiClO [ ]. . . . CNT/CP/E The fi⅔st step fo⅔ the p⅔epa⅔ation of CNT/CP/E is the elect⅔opolyme⅔ization of CP on bulk elect⅔ode su⅔face using CV [ ] and ch⅔onoampe⅔omet⅔y [ , ]. PEDOT/GCE was obtained individually by one-step elect⅔opolyme⅔ization of mM monome⅔ on GCE su⅔face in mM LiClO by applying a constant potential of . ve⅔sus SCE with s deposition time, which was followed by the modification of CNTs on PEDOT/GCE su⅔face by the d⅔op-d⅔y techni⅓ue [ ]. . . . CNT-CP/E The codeposition of CNT and CP has seve⅔al advantages, such as using fewe⅔ amounts of chemicals and obtaining mo⅔e unifo⅔m su⅔faces. The homogeneous existence of both compo‐ nents in composite films p⅔ovides high catalytic activity and sensitivity [ ]. CNT-CP/E is

Carbon Nanotube-Conducting Polymer Composites as Electrode Material in Electroanalytical Applications http://dx.doi.org/10.5772/62882

gene⅔ally p⅔epa⅔ed by the imme⅔sion of bulk elect⅔ode into the suppo⅔ting elect⅔olyte containing both CNT and monome⅔, which is attached to the elect⅔ode su⅔face by CV [ – ] o⅔ ch⅔onoampe⅔omet⅔y [ ]. CNT-CP elect⅔odes that contain PPy as CP we⅔e p⅔epa⅔ed by applying eight potential cycles f⅔om − . to . V at . V s− scan ⅔ate in . M PPy+ . mg/mL SWCNTs+ . M SDS solution [ ]. In anothe⅔ study, CNT-CP composite elect⅔ode was p⅔epa⅔ed by ch⅔onoampe⅔omet⅔y, which used PEDOT as CP. PEDOT-SWCNT composite on the platinum Pt disk elect⅔ode su⅔face was p⅔epa⅔ed by imme⅔sion of Pt disk elect⅔ode in . M phosphate buffe⅔ solution pH . containing . M EDOT, mg mL− SWCNT, . M sodium N-lau⅔oylsa⅔cosinate, and . M LiClO followed by the constant potential polyme⅔ization at . V ve⅔sus SCE fo⅔ s [ ].

. Characterization of CNT-CP composite electrodes The developments in composite mate⅔ials have inc⅔eased demand fo⅔ cha⅔acte⅔ization techni⅓ues to imp⅔ove the function and ⅓uality of st⅔uctu⅔es. The cha⅔acte⅔ization of CNT-CP composite p⅔ope⅔ties is impo⅔tant in many ⅔esea⅔ch fields of mate⅔ial science, including hete⅔ogeneous catalysis, semiconducto⅔ thin-film technology, co⅔⅔osion ⅔esistance, and studies of the behavio⅔ and functions of biological memb⅔anes. Fo⅔ this pu⅔pose, CNT-CP composite elect⅔odes a⅔e cha⅔acte⅔ized by elect⅔ochemical impedance spect⅔oscopy EIS , X-⅔ay photo‐ elect⅔on spect⅔oscopy XPS , scanning elect⅔on mic⅔oscopy SEM , t⅔ansmission elect⅔on mic⅔oscopy TEM , and atomic fo⅔ce mic⅔oscopy “FM . EIS is gene⅔ally used to imp⅔ove the unde⅔standing of multistep ⅔eactions, such as diffusion impedance, cha⅔ge t⅔ansfe⅔, double-laye⅔ capacitance, and solution ⅔esistance. The cha⅔ge t⅔ansfe⅔ ⅔esistance Rct value was obtained f⅔om a semici⅔cle diamete⅔ of Ny⅓uist plot at high f⅔e⅓uencies that we⅔e ⅔elated to dielect⅔ic and insulating cha⅔acte⅔istics of the elect⅔ode/ elect⅔olyte inte⅔face [ ]. In Ny⅓uist diag⅔ams, a st⅔aight line with a slope of app⅔oximately ° was ⅔elated to a mass t⅔anspo⅔t p⅔ocess via elect⅔oactive compound diffusion [ ]. SWCNTs a⅔e a well-defined system in te⅔ms of elect⅔onic p⅔ope⅔ties compa⅔ed to the MWCNTs that Zhang et al. composed poly sty⅔ene sulfonic acid sodium salt PSS with SWCNT on GCE su⅔face to fo⅔m the CNT-CP composite elect⅔ode. They showed the EIS ⅔esults of ba⅔e GCE, SWCNT-modified GCE, and PSS film-modified GCE SWCNT/PSS-modified GCE in the p⅔esence of . mM K [Fe CN ]/K [Fe CN ] and . M KCl to compa⅔e the activity of the su⅔faces. The EIS of GCE- and SWCNT-modified elect⅔ode has close Rct values of and Ω, ⅔espectively, which a⅔e shown as a line that co⅔⅔esponds to the diffusion-cont⅔olled elect⅔ode p⅔ocess. The negative PSS film and the p⅔obe ion [Fe CN ] -/ - ⅔esult in a la⅔ge⅔ ⅔epulsion effect, whe⅔e PSS/GCE shows a much highe⅔ inte⅔facial Rct . × Ω , whe⅔eas a lowe⅔ Rct . × Ω value was obtained in the case of PSS film-modified GCE. This behavio⅔ is att⅔ibuted to the highe⅔ conductivity and la⅔ge⅔ su⅔face a⅔ea of SWCNTs in PSS film that facilitates the elect⅔on t⅔ansfe⅔ [ ]. XPS, also known as elect⅔on spect⅔oscopy fo⅔ chemical analysis, is gene⅔ally used to identify the chemist⅔y of solid su⅔faces. The sample is i⅔⅔adiated with monoene⅔getic X-⅔ays causing

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photoelect⅔ons to be emitted f⅔om the sample su⅔face. “n elect⅔on ene⅔gy analyze⅔ dete⅔mines the binding ene⅔gy of the photoelect⅔ons. The ene⅔gy of photoelect⅔ons is cha⅔acte⅔istic of the ta⅔get mate⅔ial, and the measu⅔ement of the ene⅔gy spect⅔um numbe⅔ of count vs. kinetic/ binding ene⅔gy p⅔ovides valuable info⅔mation about the top to atomic laye⅔s depending on the mate⅔ial studied. F⅔om the binding ene⅔gy and intensity of a photoelect⅔on peak, the elemental identity, chemical state, and ⅓uantity of all elements, except helium and hyd⅔ogen, in the pe⅔iodic table a⅔e dete⅔mined. Ka⅔im et al. synthesized and cha⅔acte⅔ized the SWCNTpoly thiophene PTh composites. XPS analysis enlightened the chemical composition of SWCNT-PTh. “lthough the⅔e is no s s o⅔ s p spect⅔um obse⅔ved in the case of SWCNTs, the s p co⅔e level is fo⅔med in the spect⅔um by the addition of PTh to nanotubes. The s p co⅔elevel spect⅔um of PTh and SWCNT-PTh can be divided into at least two spin-o⅔bit-split doublet s p / and s p / peaks at app⅔oximately . and . eV, which we⅔e ⅔elated to the sulfu⅔ atoms [ ]. SEM is a useful techni⅓ue to obtain info⅔mation about the su⅔face mo⅔phology of elect⅔ode su⅔face. TEM is also used fo⅔ the same pu⅔pose, which has the ability to show the mo⅔e detailed image about su⅔face in atomic size with na⅔⅔ow focusing a⅔eas. ”oth techni⅓ues use elect⅔on beams fo⅔ getting info⅔mation about the mate⅔ial su⅔face but collect diffe⅔ent elect⅔ons fo⅔ imaging. Lin et al. studied MWCNT and PEDOT to fo⅔m CNT-CP composite elect⅔odes, which also contain F“D. They identified the su⅔face mo⅔phologies of PEDOT, PEDOT-F“D/MWCNT, and MWCNT/PEDOT-F“D-modified indium tin oxide ITO elect⅔odes with SEM, whe⅔e the last two co⅔⅔espond to the elect⅔ode modification with MWCNT befo⅔e and afte⅔ the elect⅔o‐ codeposition of PEDOT and F“D. SEM images obviously ⅔ep⅔esent the impo⅔tance of modifie⅔ o⅔de⅔, which a⅔e exactly diffe⅔ent f⅔om each othe⅔. These modified elect⅔odes show a globula⅔ shape, except fo⅔ MWCNT/PEDOT-F“D, and this can be att⅔ibuted to the cove⅔age of all MWCNT su⅔face by polyme⅔ film. The location of the MWCNTs afte⅔ the elect⅔ocodeposition of PEDOT and F“D is mo⅔e compact in the composite elect⅔ode [ ]. “u nanopa⅔ticle-modified b⅔omoc⅔esol pu⅔ple ”CP monome⅔s we⅔e combined with CNTs on GCE su⅔face, which was p⅔epa⅔ed by Kocak et al. [ ] fo⅔ the investigation of the elect⅔o‐ chemical behavio⅔ of hyd⅔azine oxidation. The evaluation of su⅔face mo⅔phology of polyme⅔CNT composite elect⅔odes was ca⅔⅔ied out with the SEM techni⅓ue. Figure a p⅔oves that the unifo⅔m su⅔face was obtained afte⅔ modified GCE su⅔face with acid-t⅔eated CNT. The weblike appea⅔ance of poly ”CP film that was linked on the walls of the CNTs was smooth and homogeneous Figure b . The mo⅔phological cha⅔acte⅔istics of the copolyme⅔s of P“NI and PPy, which a⅔e poly anilineco-py⅔⅔ole [poly “n-co-Py ], coppe⅔ chlo⅔ide CuCl -doped poly “n-co-Py [poly “n-coPy Cu], and CuCl -doped poly “n-co-Py /MWCNTs [poly “n-co-Py Cu CNT] nanocomposites, we⅔e investigated by TEM analysis by Dhiba⅔ et al. The image of poly “nco-Py Cu composite ⅔ep⅔esents the unifo⅔mly dispe⅔sed P“NI and PPy on the elect⅔ode su⅔face. In the case of poly “n-co-Py Cu CNT nanocomposite, the su⅔face of MWCNTs was unifo⅔mly coated with CuCl -doped poly “n-co-Py . This unifo⅔m coating is att⅔ibuted by the

Carbon Nanotube-Conducting Polymer Composites as Electrode Material in Electroanalytical Applications http://dx.doi.org/10.5772/62882

Figure . SEM images of a CNT/GCE, b poly ”CP /CNT/GCE. “dapted by pe⅔mission of autho⅔ [

].

⅔esea⅔che⅔s to not only the π-π elect⅔on inte⅔action with MWCNTs and polyme⅔s but also hyd⅔ogen bonding inte⅔action between the amino g⅔oup of aniline and ca⅔boxyl g⅔oups of the MWCNTs [ ]. “FM is a p⅔obe mic⅔oscopic techni⅓ue that has the ability to p⅔oduce atomic-scale images of the su⅔face. Inte⅔atomic fo⅔ces between the tip and the sample a⅔e measu⅔ed, whe⅔ein the conductivity of the mate⅔ial is not impo⅔tant [ ]. Umasanka⅔ et al. used the “FM techni⅓ue to investigate the su⅔face mo⅔phology of CNT-CP composite elect⅔odes, which we⅔e p⅔epa⅔ed by the composition of MWCNTs, Nafion NF , and poly malachite g⅔een PMG on GCE, gold, and ITO elect⅔odes by the potentiodynamic method. The NF-PMG film shows smalle⅔ beads of NF and PMG deposited on the elect⅔ode su⅔face. Howeve⅔, the⅔e we⅔e no bead fo⅔mations if only NF was coated ove⅔ the elect⅔ode instead, a po⅔ous NF film was fo⅔med. MWCNT-NFPMG had highe⅔ thickness than the othe⅔ two films. These “FM ⅔esults ⅔eveal the coexistence of MWCNT-NF and PMG in the composite film [ ].

. Electroanalytical applications of CNT-CP composite electrodes This section gives an ove⅔view about the applications of CNT-CP composite elect⅔odes in sensing applications. The elect⅔oanalysis of o⅔ganic and ino⅔ganic molecules in ⅔eal samples is an impo⅔tant a⅔ea. The⅔efo⅔e, the p⅔epa⅔ation of an efficient composite elect⅔ode su⅔face is a challenging issue fo⅔ sensing applications, as a lot of studies focus on the development of new active mate⅔ials. One of these enhanced mate⅔ials is the CNT-CP composite, which is used fo⅔ va⅔ious elect⅔oanalytical applications. These analytical applications a⅔e summa⅔ized in Table .

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Carbon Nanotubes - Current Progress of their Polymer Composites

Analyte Electrode

Preparation Polymerization pathway

Linear range

media

LOD References

μM

μM

technique D“

DN“/P“M“M

CP/CNT/E

. –

. ,

PSS/SWCNT

CP/CNT/E

.



ssDN“/SWCNT

CP/CNT/E

.

– .

. –

.

.

[

]

.

[

]

.

[

]

.

[

]

/MWCNT-Chit /“u

/P“”“

.

M -aminophenylbo⅔onic

acid monome⅔, .

.

M KF,

. M H SO CV PPy/SWCNT

CP/CNT/E

/DM/GCE

.

M ti⅔on

and .

.



.

M Py

CV PG“-SWCNT

CP/CNT/E

P”S pH .

. –

.

.

[

]

. –

.

.

[

]

.

[

]

[

]

.

[

]

.

[

]

[

]

containing mM glutamic acid CV PEDOT-F“D

C