Nanocomposites: Materials, Manufacturing and Engineering 9783110267426, 9783110266443

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Advanced Composites Davim, Charitidis ∙ Nanocomposites

Also of Interest Nanocarbon-Inorganic Hybrids Next Generation Composites for Sustainable Energy Applications Eder, Schlögl (Eds.), 2013 ISBN 978-3-11-026971-0, e-ISBN 978-3-11-026986-4

Nanocellulose From Nature to High Performance Tailored Materials Dufresne, 2012 ISBN 978-3-11-025456-3, e-ISBN 978-3-11-025460-0

Nanoparticles Optical and Ultrasound Characterization Brodsky, 2011 ISBN 978-3-11-026591-0, e-ISBN 978-3-11-026734-1

Biomimetics A Molecular Perspective Jelinek, 2013 ISBN 978-3-11-028117-0, e-ISBN 978-3-11-028119-4

Nanotechnology Reviews Kumar, Challa (Editor-in-Chief) ISSN 2191-9089, e-ISSN 2191-9097

Science and Engineering of Composite Materials Hoa, Suong V. (Editor-in-Chief) ISSN 0792-1233, e-ISSN 2191-0359

Nanocomposites

Materials, Manufacturing and Engineering

Edited by J. Paulo Davim, Constantinos A. Charitidis

Editors Professor J. Paulo Davim University of Aveiro 3810-193 Aveiro Portugal Email: [email protected]

Professor Constantinos A. Charitidis National Technical University of Athens 15780 Athens Greece Email: [email protected]

This book has 122 Figures and 17 Tables.

ISSN 2192-8983 ISBN 978-3-11- 026644-3 e-ISBN 978-3-11- 026742-6 Library of Congress Cataloging-in-Publication Data A CIP catalog record for this book has been applied for at the Library of Congress. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available in the Internet at http://dnb.dnb.de. © 2013 Walter de Gruyter GmbH, Berlin/Boston The publisher, together with the authors and editors, has taken great pains to ensure that all information presented in this work (programs, applications, amounts, dosages, etc.) reflects the standard of knowledge at the time of publication. Despite careful manuscript preparation and proof correction, errors can nevertheless occur. Authors, editors and publisher disclaim all responsibility and for any errors or omissions or liability for the results obtained from use of the information, or parts thereof, contained in this work. The citation of registered names, trade names, trade marks, etc. in this work does not imply, even in the absence of a specific statement, that such names are exempt from laws and regulations protecting trade marks etc. and therefore free for general use. Typesetting: PTP-Berlin Protago-TEX-Production GmbH, Berlin Printing and binding: Hubert & Co., Göttingen Cover image: gettyimages/thinkstockphotos, Abalone Shell ♾ Printed on acid-free paper Printed in Germany www.degruyter.com

Preface Nowadays, it is usual to define nanocomposite “as a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm) or structures having nanoscale repeat distances between the different phases that make up the material”. Today, the use of nanocomposites has increased in various areas of engineering and technology due to their specific properties. It has been recognized that organic-inorganic nanocomposites are very important materials for photonic crystals, coatings, adhesives, pharmaceutical, biomedical and cosmetic formulations. Furthermore, the nanoscale dimensions of nanocomposites already suggest a variety of possible industrial applications: automotive (gas tanks, bumpers, interior and exterior panels), construction (building sections and structural panels), aerospace (flame retardant panels and high performance components), food packaging, textiles, etc. Their controlled production is a goal to synthesize and design devices at the nanoscale. Moreover, hollow nanocomposites have been a subject of great scientific and industrial interest ranging from molecular biology and electronic materials to medical imaging and photonic crystals since the intrinsic properties of spherical materials can be finely tuned by changing parameters such as sphere diameter, chemical composition, and crystallinity-structure. Furthermore, they have been of interest as fillers, coatings, capsule agents, etc., because of their lower density and optical properties. The present volume aims to provide recent information on nanocomposites (materials manufacturing and engineering) in six chapters. Chapter 1 of the book provides information on synthesis and characterization of ceramic hollow nanocomposites and nanotraps. Chapter 2 is dedicated to recent advances on preparation, properties and applications of polyurathane nanocomposites. Chapter 3 describes preparation, characterization and properties of organoclays, carbon nanofibers and carbon nanotubes based polymer nanocomposites. Chapter 4 discusses mechanical and wear properties of multi-scale phase reinforced composites. Chapter 5 describes modeling mechanical properties of nanocomposites. Finally, Chapter 6 is dedicated to polyaniline derivates and carbon nanotubes and their characterization. The present volume can be used as a research book for final year undergraduate engineering courses or as a topic on composites at the postgraduate level. Also, this book can serve as a useful reference for academics, researchers, materials, physics and mechanical engineers, professionals in composites and related industries. The scientific interest of this book is evident for many important centers of research, laboratories and universities as well as industry. Therefore, it is hoped that this book will inspire and enthuse others to undertake research in this field of nanocomposites.

VI   

   Preface

The Editors acknowledge De Gruyter for this opportunity and for their enthusiastic and professional support. Finally, we would like to thank all the chapter authors for their availability for this work. May, 2013

J. Paulo Davim Constantinos A. Charitidis

Contents Preface   V List of contributing authors 

1 1.1 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.2.6 1.2.7 1.2.8 1.3 1.4 1.5 1.6 1.7

2 2.1 2.2 2.2.1 2.2.1.1 2.2.1.2 2.2.1.3 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.3 2.2.3.1 2.2.3.2

 XI

I.A. Kartsonakis, C.A.Charitidis, G.C. Kordas Synthesis and characterization of ceramic hollow nanocomposites and nanotraps   1 Introduction   1 Hollow nanocomposites   4 Cerium oxide hollow nanocomposites   4 Titanium oxide hollow nanocomposites   6 Cerium molybdate hollow nanocomposites   8 Cerium titanium oxide hollow nanocomposites   9 Magnetic hollow nanocomposites   10 SiO2–CaO hollow nanocomposites   12 Water trapping nanocomposites   13 Chloride trap nanocomposites   14 Nanocomposites loaded with corrosion inhibitors   15 Antibacterial action of hollow nanocomposites   18 Nanocomposites incorporated into coatings   23 Properties   24 Summary and Conclusion   25 Acknowledgments   26 References   26 S.K. Srivastava, M. Kotal Recent advances on preparation, properties and applications of polyurethane nanocomposites   33 Introduction   33 Fillers used in PU nanocomposites   34 Sheet/platelets type inorganic nanofillers   35 Natural layered silicates   35 Layered double hydroxides   36 Graphene   37 Nanofillers with spherical and cubical shapes   38 Metal nanoparticles   38 Nanosilica   38 Polyhedral oligomeric silsesquioxane (POSS)   38 Rod/fiber type nanofillers   38 Carbon nanotubes   38 Carbon nanofibers   39

VIII   

   Contents

2.2.4 2.3 2.4 2.4.1 2.4.2 2.4.3 2.4.4 2.5 2.5.1 2.5.1.1 2.5.1.2 2.5.1.3 2.5.1.4 2.5.2 2.5.2.1 2.5.2.2

Other nanofillers   39 Preparation of PU nanocomposites   39 Nanostructure establishment in PU nanocomposites   39 Clay/PU nanocomposites   40 PU/LDH nanocomposites   42 PU nanocomposites of CNT and CNF   44 Nanocomposites of PU with POSS, SiO2 and Ag   45 Properties of PU nanocomposites   46 Mechanical properties   46 Clay/PU nanocomposites   46 LDH/PU nanocomposites   49 PU nanocomposites of CNT and CNF   51 Nanocomposites of PU with SiC, ZnO, SiO2 and Ag   53 Thermal properties   55 Thermogravimetric analysis   55 Differential scanning calorimetry and dynamic mechanical thermal analysis   61 Gas barrier properties   65 Adhesive properties   67 Flame retardant properties   68 Electrical conductivity   71 Thermal conductivity   76 Dielectric properties   77 Biological properties   78 Conclusions   81 References   81

2.5.3 2.5.4 2.5.5 2.5.6 2.5.7 2.5.8 2.5.9 2.6

3

3.1 3.2 3.2.1 3.2.2 3.2.3 3.3 3.3.1 3.4 3.4.1 3.4.2

A.K. Barick, D.K. Tripathy, B.P. Sahoo Preparation, characterization, and properties of organoclay, carbon nanofiber, and carbon nanotube based thermoplastic polyurethane nanocomposites   93 Introduction   93 Nanofillers   94 Layered silicates   94 Carbon nanofibers   96 Carbon nanotubes   97 Polyurethanes   99 Thermoplastic polyurethanes   100 Polymer nanocomposites   101 Polymer/organoclay nanocomposites   101 Preparation of polymer nanocomposites   102

Contents   

3.5 3.6 3.7 3.8

4 4.1 4.2 4.2.1 4.2.2 4.3 4.3.1 4.3.2 4.4 4.5

5 5.1 5.1.1 5.1.2 5.2 5.2.1 5.2.2 5.2.2.1 5.2.3 5.2.3.1 5.2.3.2 5.2.3.3 5.2.3.4 5.2.3.5 5.2.3.6 5.2.4 5.2.5 5.2.6

 103 TPU/organoclay nanocomposites  TPU/carbon nanofiber nanocomposites   104 TPU/carbon nanotube nanocomposites   105 Summary and future scope   106 References   106 Z. Jiang Mechanical and wear properties of multi-scale phase reinforced composites   111 Introduction   111 Preparation of multi-scale phase reinforced composites   112 MPRCs with nanofiller-modified matrix   112 MPRCs with nanotube-modified fibers   115 Properties of MPRCs based on nano-modified polymer matrix   118 Mechanical properties   118 Wear performance   127 Mechanical properties of MPRCs based on nano-engineered reinforcing fibers   132 Concluding remarks   136 References   138 J. Schjødt-Thomsen, L.R. Jensen, J.C. Rauhe Modeling mechanical properties of nanocomposites   145 Molecular modeling   146 Theory of molecular dynamics method   147 Applications of molecular dynamics simulations   151 Nano-, micro- and continuum mechanical modeling   155 Continuum mechanics   155 Micromechanics   156 Shape of reinforcing phase   157 Determination of tensors Aijkl and Bijkl   158 Voigt and Reuss   158 Eshelby’s equivalent inclusion method   159 The self-consistent model   161 The Mori–Tanaka Model   161 The Dvorak–Srinivas Model   162 The effective medium field approximation   163 Orientation effects   163 Effects of dispersion   166 Scale effects   169

   IX

X   

5.3 5.3.1 5.3.2

6 6.1 6.2 6.3 6.3.1 6.3.2 6.3.3 6.3.4 6.3.5

Index 

   Contents

Multiscale modeling   170 Sequential coupled methods  Concurrent coupled methods  References   176

 171  173

V. Bavastrello, C. Nicolini Polyaniline derivates and carbon nanotubes and their characterization   183 Introduction   183 Synthesis of nanocomposite materials   188 Characterization of nanocomposite materials and characterization of their physical chemistry properties   191 Deposition of thin films by Langmuir–Schaefer technique and study of the pressure-area isotherms   191 UV-vis spectroscopy and band gap calculations   194 Cyclic voltammetry   198 Determination of specific conductivity   201 Nanocomposite materials and their possible applications   203 Acknowledgements   204 References   204  209

List of contributing authors Aruna Kumar Barick Rubber Technology Centre Indian Institute of Technology Kharagpur Kharagpur, India [email protected] Chapter 3 Valter Bavastrello Laboratories of Biophysics and Nanobiotechnology Department of Medical Science University of Genova Genova, Italy [email protected] Chapter 6 Constantinos A. Charitidis School of Chemical Engineering National Technical University of Athens Zographos, Greece [email protected] Chapter 1 Lars R. Jensen Department of Mechanical and Manufacturing Engineering Aalborg University Aalborg East, Denmark [email protected] Chapter 5 Zhenyu Jiang Department of Engineering Mechanics South China University of Technology Guangzhou, China [email protected] Chapter 4

Ioannis A. Kartsonakis Sol-Gel Laboratory Institute for Advanced Materials, Physicochemical Processes, Nanotechnology & Microsystems NCSR Demokritos Agia Paraskevi, Greece [email protected] Chapter 1 George C. Kordas Sol-Gel Laboratory Institute for Advanced Materials, Physicochemical Processes, Nanotechnology & Microsystems NCSR Demokritos Agia Paraskevi, Greece [email protected] Chapter 1 Moumita Kotal Inorganic Materials and Nanocomposites Laboratory Indian Institute of Technology Kharagpur, India [email protected] Chapter 2 Claudio Nicolini Nanoworld Institute-CIRSDNNOB and Biophysics Division University of Genova Genova, Italy [email protected] Chapter 6

XII   

   List of contributing authors

Jens C. Rauhe Department of Mechanical and Manufacturing Engineering Aalborg University Aalborg East, Denmark [email protected] Chapter 5

Jan Schjødt-Thomsen Department of Mechanical and Manufacturing Engineering Aalborg University Aalborg East, Denmark [email protected] Chapter 5

Bibhu Prasad Sahoo Rubber Technology Centre Indian Institute of Technology Kharagpur Kharagpur, India [email protected] Chapter 3

Deba Kumar Tripathy Rubber Technology Centre Indian Institute of Technology Kharagpur Kharagpur, India [email protected] Chapter 3

Suneel Kumar Srivastava Inorganic Materials and Nanocomposites Laboratory Indian Institute of Technology Kharagpur, India [email protected] Chapter 2

I.A. Kartsonakis, C.A.Charitidis, G.C. Kordas

1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps 1.1 Introduction Nanocomposite materials encompass a large variety of systems such as one-dimensional, two-dimensional, three-dimensional and amorphous materials, made of distinctly dissimilar components and mixed at the nanometer scale. Organic nanoparticles and nanospheres have been a subject of great scientific and industrial interest, ranging from molecular biology and electronic materials to medical imaging and photonic crystals. These organic materials have been prepared by heterogeneous polymerization methods [1, 2]. Past studies have demonstrated the importance of composite particles consisting of organic cores with active surfaces covered with inorganic shells  [3–7] due to the customized properties of these composite dispersions (magnetic, optical, electric, adsorptive, etc.) which may be adjusted to meet specific requirements for a given application. Nanocomposites such as nanoparticles, nanospheres and micelles can be used as drug delivery and drug controlled release systems. Hollow nanocomposites are of great interest because of their ability to encapsulate substances in their hollow inner cavities and release them at a later stage. They have been of interest as fillers, coatings, capsule agents, etc., because of their lower density and optical properties [8–11]. Such “shells” are created either by hydrolysis, in situ, of the corresponding metallic salt in the presence of core materials [12–20] or calcinating polymer particles coated with uniform inorganic shells [21–32]. A very interesting topic is the production of “shells” that are constructed of corrosion inhibitor materials such as compositions of cerium oxides, cerium together with molybdenum oxides and cerium together with titanium oxides. Some of the most effective and environmentally friendly corrosion inhibitors for aluminum alloys are derived from cerium salts. Nanostructured sol-gel coatings doped with cerium ions were investigated as pretreatments for AA2024-T3 [33]. Studies on the corrosion inhibition of cerium oxide have been made too. A process developed for the spontaneous deposition of cerium oxide conversion coatings for corrosion protection of aluminum alloy 7075-T6 showed inhibited corrosion for up to two weeks (336 h) in salt fog testing [34]. Cerium molybdate has been used as a corrosion inhibitive component to a nonchromate protective solution useful for coating iron and iron alloys, particularly steel  [35]. The structure and catalytic properties of ultrafine Ce2(MoO4)3 particles have also been studied with good results due to the high mobility of lattice oxygen ions in the oxide and the high BET surface area [36].

2   

   1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps

Another interesting topic is the loading of hollow nanocomposites with corrosion inhibiting compounds such as 8-hydroxyquinoline (8-HQ), 2-mercaptobenzothiazole (MBT), p-toluenesulfonic acid (p-TSA) and 1-H-benzotriazole-4-sulfonic acid (1-BSA). The corrosion behavior of AA2024-T3 was studied in 3.5 % NaCl solution with 8-HQ. The results revealed that 8-HQ is a mixed type inhibitor by blocking the active sites of the metal surface [37]. The effect of 8-HQ on the corrosion inhibition of copper has been investigated in neutral aqueous NaCl solutions. It was stated that a protective film is formed on the surface by polymerization of Cu (II)–hydroxyquinoline complexes, films that play an essential role in the inhibition of Cu corrosion [38]. Moreover, 8-HQ exhibits antiseptic, disinfectant, and pesticidal properties. It functions as a bacteriostat and fungistat agent to prevent adverse growth of micro-organisms on red blood cells [39]. Furthermore, 1-BSA is a derivative of benzotriazole which has been used as a corrosion inhibitor for the protection of magnesium and aluminum alloys [40, 41]. 8-HQ and MBT compounds were studied as corrosion inhibitors by S.V. Lamaka and coworkers for AA2024-T3 [42]. They found that these inhibitors provide anticorrosion protection for AA2024-T3 forming a thin organic layer of insoluble complexes on the surface of the alloy. Inhibiting action is the consequence of suppression of dissolution of Mg, Al and Cu from the corrosion active intermetallic zones [43, 44]. K.A. Yasakau et al. examined the addition of 8-HQ at different stages of the synthesis process to understand the role of possible interaction of the inhibitor with the components of the sol-gel system [45]. MBT was evaluated by Zheludkevich et al. as a corrosion inhibitor for protection of AA2024-T3 in neutral chloride solutions [46]. A lot of attention has been focused on hollow spheres of titania. The reason is their photocatalytic activity and low density, which make them suitable for a number of applications such as catalysts, white pigments and filters. Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. The photocatalytic process includes chemical steps that produce reactive species that in principle can cause fatal damage to micro-organisms [47–49]. A heterogeneous photocatalytic system consists of semiconductor particles (photocatalyst) which are in close contact with a liquid or gaseous reaction medium. Titanium dioxide nanocontainers have been synthesized using PS cores as templates [50], using carbon spheres as templates [51], or by using Ti(SO4)2 instead of organometallic titanium as precursor [26]. Moreover, hollow microspheres of mesoporous titania with a thin shell of anatase structure have been prepared by a procedure of surfactant poly(ethylene oxide) assisted nanoparticle assembly [52]. Other methods for producing hollow titania spheres are the Kirkendall effect [53], the Ostwald ripening [54] and the gas phase synthesis [55]. Magnetic containers have received a lot of interest during the last years. Hematite and iron hollow spheres have been synthesized using PS templates [30, 56]. Moreover, iron oxide spheres were produced using hollow latex cages as templates [24]. Magnetic hollow spheres with a surface layer enriched in silica were fabricated using

1.1 Introduction   

   3

aerosol-assisted methods [25]. Magnetic biocompatible hybrid hollow spheres were prepared by a core-template-free route [57]. Hollow magnetic microspheres were fabricated by plasma treatment from precursor core-shell particles [58]. Encapsulation of various molecules with bioactivity is also a property that raises both scientific and industrial interest. In the last decade several works have focused on the preparation of nanospheres. Silica nanobottles [31], silica nanospheres with a magnetic core and a charged surface [59], fluorescent silica nanospheres  [60], polypyrrole-magnetite-silica particles [61], CdS–SiO2 core-shell particles [62] and silica nanospheres encapsulating enzyme [63] are some remarkable results in this area. Significant progress has also been observed in sol-gel bioglasses, where complex systems have been developed so as to improve bioglass properties [64]. In the last years there has been an attempt to decrease patients’ convalescence time and enforce bioglass functionality by the encapsulation of bio-molecules in the sol-gel silica networks [65]. These characteristics promote bioglasses for clinical use. Materials with the ability to absorb water have been known since the mid-1960s. Since then, many studies about these materials followed with the aim to develop super absorbent polymers (SAPs), which absorb water several hundred to a thousand times their own dry weight. SAPs are three-dimensional polymer networks, partially crosslinked, having the capability to expand during the absorption process and used in many fields, such as baby diapers, agricultural applications and other advanced technologies [66]. Among the disadvantages of these materials are the loss of their ability to absorb water after repeated absorption-drying cycles, the remarkable increase in their size during swelling and their scraggly shape both in dry and in expanded form. This review is focused on the preparation and characterization of cerium molybdate, cerium titanium oxide, ceria with/without conductive polymer coatings, magnetic and photocatalytic hollow nanocomposites. Furthermore, the nanocomposites are loaded with corrosion inhibitors to produce an inhibitor delivery system. Encapsulations of the corrosion inhibitors are proven by heat treatments and FT-IR spectroscopy. TGA measurements reveal the amount of inhibitors incorporated in the nanocontainers. The performance of the complex system (nanocontainers and inhibitors) in a corrosive environment is tested via potentiodynamic, electrochemical impedance spectroscopy, UV-vis spectroscopy measurements showing good results. Furthermore, experiments of the hollow nanocomposite antibacterial action on pure culture of E.  coli are reported. E.  coli are the most encountered bacterium in the clinical laboratory due to its clear structure. Additionally, the laboratory strains produce well-defined individual colonies when culturing the organisms on ager plates. A decrease of E. coli concentration is observed for the TiO2 and ceria nanocomposites with/without conductive polymer coatings. Finally, the production of nanocomposites that have the ability to absorb water without extensive swelling as well as to trap corrosive agents is reported. The water trapping nanocomposites can be regenerated after water removal by drying or washing with an excess of organic solvent, like ethanol. Such a system with nearly

4   

   1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps

constant dimension before and after water absorption can be incorporated into coatings to prevent interaction of water with the metal without causing cracks. The incorporation of the aforementioned complex inhibitors and trap nanocomposites into coatings on metals via sol-gel or electro-polymerization of conductive polymers can demonstrate efficient multiscale anticorrosion protection in systems used for airplanes, ships, and automobiles (Fig. 1.1).

Scratch

Trap nanocomposites Hollow nanocomposite loaded with corrosion inhibitor

Coating

Self-healed area

Metal substrate

Fig. 1.1: Multiscale anticorrosion protection system.

1.2 Hollow nanocomposites 1.2.1 Cerium oxide hollow nanocomposites The synthesis of cerium oxide hollow nanocomposites is based on an organic templates process [67]. The method of emulsion polymerization with potassium persulfate (KPS) as the initiator is used to produce anionic PS latex. The reaction is carried out in a 1,000 cm3 container under the conditions of Table 1.1. To eliminate the effect of oxygen, the solution is purged with nitrogen before the process is initiated. The

Table 1.1: The conditions used in the preparation of PS latex at 80 °C Material

Quantity (g)

Styrene KPS Sodium dodecyl sulfate Water

4.53 0.65 0.21 460

1.2 Hollow nanocomposites   

   5

polymerization process lasts for 12 hours. The resulting dispersions are centrifuged at 14,000  rpm for 30  min, the supernatant solutions are discarded, and then the particles are resuspended in doubly distilled water using a sonicator. This process is repeated three times. The PS lattices are coated via the sol-gel method to form a ceria oxide layer. The sol-gel coatings are prepared by controlled hydrolysis of cerium (III) acetylacetonate (Ce(acac)3) aqueous solution in the presence of PS latex, urea and polyvinylpyrrolidone (PVP). These dispersions are aged in an oven preheated to 100 °C for 24 hours (Table  1.2). The resulting dispersions are centrifuged at 14,000  rpm for 30  min, the supernatant solutions are discarded, and then the particles are resuspended in doubly distilled water with a sonicator. This process is repeated three times, and the purified powders are dried in a desiccator. Table 1.2: Conditions of preparation of coated nanospheres Material

Quantity (g)

PS PVP Ce(acac)3 Water

1.130 1.6 2.4 160

The production of hollow cerium dioxide nanocomposites encompasses the removal of the PS cores by calcination. The composite is calcinated for 4 h in air in a furnace at 600 °C with a heating rate of 10 °C min−1. Furthermore, the hollow ceria nanocomposites (approximately 90  nm in diameter) are coated with a conducting polymer layer via electrodeposition using cyclic voltammetry (CPCeO2). For this purpose, 0.1 g of the nanocomposites are added in 100 ml aqueous solution which contains 0.1 M pyrrole, 0.1 M aniline and 0.3 M oxalic acid. An aluminum panel is used as the working electrode, a platinum sheet as the counter while a saturated calomel electrode (SCE) serves as reference. Under these conditions, electropolymerization preferably occurs on the working electrode forming a conducting polymer layer containing ceria spheres. However, simultaneously the spheres remaining in the solution act as polymerization sites and are thus covered by a mixed conducting polymer layer. The potential is scanned between –1 and 3 V vs. SCE at a rate of 30 mVs−1. After the polymerization process, the coated spheres are collected by filtration, washed thoroughly with distilled water in order to eliminate solution residues (monomers, oxalic acid) and left in air to dry for 24h. The synthetic process leads to the formation of uniform hollow nanocomposites with a diameter 90 ± 10 nm (Fig. 1.2). The X-Ray Diffraction (XRD) analysis confirms the formation of crystalline cerianite CeO2 (4-0593 cerianite).

6   

   1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps

50 nm

Fig. 1.2: Transmission electron micrographs of CeO2 hollow nanocomposites [67].

1.2.2 Titanium oxide hollow nanocomposites The synthesis of titanium dioxide hollow nanocomposites consists of three experimental steps [50]. The first step involves the preparation of organic templates. Positive charged PS latex is synthesized via polymerization in suspension using 2,2’Azobis (2-methylpropionamidine) dihydrochloride (AMPA) as the initiator. For this purpose, the reaction is carried out in a 500 cm3 container under the conditions listed in Table 1.3. Table 1.3: The conditions used in the preparation of PS latex at 70 °C Material

Quantity (g)

Styrene AMPA Water

4.53 0.6521 400

To eliminate the effects of oxygen, the solution is purged with nitrogen before the process is initiated. The polymerization duration is 12 hours. The resulting dispersions are filtered, centrifuged at 14,000 rpm for 30 min, the supernatant solutions are discarded, and then the particles are resuspended in absolute ethanol using a sonicator. This process is repeated three times. The PS particles prepared by this procedure (Table 1.3) are homogeneous.

1.2 Hollow nanocomposites   

   7

The coating procedure consisted of controlled hydrolysis of ethanolic solution of titanium tetraisopropoxide (TTIP) in the presence of PS latex. PVP and NaCl 5mM solution are added to the mixture reaction to prevent aggregation of the core particles [84]. The resulting dispersions are centrifuged at 14,000  rpm for 30  min, the supernatant solutions are discarded, and then the particles are resuspended in absolute ethanol with a sonicator. This process is repeated three times, and the purified powders are dried in a desiccator. The conditions of preparation of coated spheres are listed in Table 1.4. Table 1.4: Conditions of preparation of coated spheres Material

Quantity

Ethanol (ml) PVP (g) NaCl 5mM (ml) PS (g) TTIP (ml)

400 4.0 10 4.5 4.5

The hollow titanium dioxide nanocomposites are produced after the removal of PS cores by calcination. Initially, the coated spheres are placed on a glass slide and dried, first at room temperature and then for 1 h at 60 °C. After that, the composite is further calcinated for 3 h in air in a furnace at 600 °C, at a heating rate of 10 °C min−1. After calcination the hollow titania dioxide nanocomposites have a diameter of approximately 240 ± 10 nm. The EDX analysis shows that the basic elements that constitute the spheres are titanium and oxygen (Fig. 1.3). The XRD analysis indicates that

24 1.3 nm

(s)

Ti

(s)

(s)

246 .9 nm

23 7.3 nm

O

Au Au

mag 160000 x

Ti

Au

2.00

Au

4.00

6.00

8.00

500 nm

Au

10.00

AuAu Au

12.00

Au

14.00

Fig. 1.3: Scanning electron micrographs of hollow TiO2 nanocomposites after calcination at 600 °C.

8   

   1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps

the TiO2 nanocomposites formed at temperature 600 °C are crystalline and consist of anatase (00-021-1272 Anatase, syn) and rutile (00-021-1276 Rutile, syn) type.

1.2.3 Cerium molybdate hollow nanocomposites Cerium molybdate hollow nanocomposites are synthesized using a two-step process [32]. First, PS nanospheres are produced using emulsion polymerization. Second, the PS spheres are coated via the sol-gel method to form a cerium molybdate layer. Finally, the nanocontainers are made by calcination of cerium molybdate coated PS nanospheres. The method of emulsion polymerization is employed to produce anionic PS latex, used as core particles. The experimental conditions for the PS preparation are similar to those in section 1.2.1 on cerium oxide nanocomposites. The PS lattices are coated via the sol-gel method to form a cerium molybdate layer. The sol-gel coatings are prepared by controlled hydrolysis of Ce(acac)3 and sodium molybdate aqueous solution in the presence of PS latex and PVP. These dispersions are aged for different time intervals in test tubes and placed in an oven preheated to 96 °C (Table 1.5). The resulting dispersions are centrifuged at 14,000 rpm for 30 min, the supernatant solutions are discarded, and then the particles are resuspended in doubly distilled water with a sonicator. This process is repeated three times, and the purified powders are dried in a desiccator.

Table 1.5: Conditions of preparation of coated spheres Material

Quantity (g)

Polystyrene PVP Ce(acac)3 sodium molybdate Water

10.0 10.0 5.0 0.5 1000

Hollow cerium molybdate nanocomposites are produced after the PS burn-off by calcination. Initially, the coated nanospheres are placed on a glass slide and dried, first at room temperature and then for 1 h at 60 °C. Then, the composite is calcinated for 4 h in air in a furnace at 550 °C with a heating rate of 10 °C min−1. After calcination of the coated nanospheres, the resulting hollow nanocomposites have an external diameter of 145 ± 10 nm (Fig. 1.4). The EDX analysis illustrates that the nanocomposites consist of cerium, molybdenum and oxygen. The XRD analysis depicts that the nanocomposites’ structures formed at temperature 550 °C are crystalline and consist of cerium molybdenum oxide (33-0330) type.

1.2 Hollow nanocomposites   

   9

142.4 nm

(s)

Mo

Ce

149.6 nm (s)

144.4 nm (s)

O Ce Na Ce C Ce Mo Cl F Mo

2.00

Ce Ce

4.00

mag 150000 x

Ce

500 nm

Mo

6.00

8.00

10.00 12.00 14.00 16.00 18.00

Fig. 1.4: Scanning electron micrographs of cerium molybdate hollow nanocomposites.

1.2.4 Cerium titanium oxide hollow nanocomposites The synthesis of cerium titanium oxide hollow nanocomposites consists of three experimental steps [69]. The first step involves the preparation of positive charged organic templates based on PS. Styrene is polymerized by polymerization in suspension. The conditions for the polymerization process of PS are similar to those of in section 1.2.2 above. During the second step the PS nanospheres are coated via sol-gel method. Sol-gel coating is prepared with controlled hydrolysis of the alcoholic solution of TTIP and Ce(acac)3 in presence of PS nanospheres, NaCl and PVP (Table 1.6). The positive charged polystyrene reacts with the negative charged product of the hydrolysis of TTIP and Ce(acac)3. Monomers or oligomers of hydrolyzed TTIP and Ce(acac)3 are condensed on the surface of the polystyrene. Aging of the solutions at 60 °C, centrifugation and washing of the coated nanospheres is followed. The formation of hollow nanospheres is achieved after heat treatments of the composites at 600 °C with heating rate 10 °C min−1, where the polystyrene cores are burned off.

Table 1.6: Conditions of preparation of coated spheres Material

Quantity (g)

Ethanol (ml) PVP (g) NaCl 5mM (ml) PS (g) TTIP (ml) Ce(acac)3

800 8.0 20.0 9.0 9.0 1.0

10   

   1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps

The size of the nanocomposites is 180 ± 10 nm as determined by Scanning Electron Microscopy (SEM) (Fig.  1.5). The EDX analysis shows that titanium, cerium and oxygen constitute the spectrum of the nanocomposites. XRD illustrates that the hollow nanoparticles consist of anatase and cerianite crystalline phases.

Ti 8 nm 179. (s)

183.3 nm

(s)

17 1.1 nm

(s)

176.1 nm (s)

mag 150000 x

Ce Ti

500 nm

O

Au Au

C

1.00

2.00

Au

Ce

3.00

4.00

Ce Ce

5.00

Ce

6.00

Fig. 1.5: SEM image and EDX analysis of cerium titanium oxide hollow nanocomposites.

1.2.5 Magnetic hollow nanocomposites Magnetic hollow submicrocomposites are synthesized through a two-step process [21]. First, PS spheres are produced in order to be used as core particles. Second, the PS spheres are coated via the sol-gel method to form an iron oxide layer. The composite is treated in air to burn off the PS latex. The method of emulsion polymerization is employed to produce anionic PS latex, used as core particles. The process is described in section 1.2.1 above. The size of the PS spheres is controlled by the concentration of the monomer (styrene), the initiator (KPS) and the emulsifier (sodium dodecylsulfate). The coating procedure involves the controlled hydrolysis of aqueous solution of iron (III) chloride in the presence of PS latex. PVP and urea are added to the mixture reaction to prevent aggregation of the core particles. For this purpose, dispersions containing polystyrene latex, iron (III) chloride, PVP and urea are aged for 3 days in test tubes, placed in an oven preheated to 95 °C. The resulting dispersions are centrifuged at 7,000 rpm for 15 min. The supernatant solutions are discarded and then the particles are resuspended in doubly distilled water with a sonicator. This process is repeated three times. The purified powders are dried in a desiccator (Table 1.7). Hollow hematite composites are synthesized via heat treatments where the PS cores are burnt off. Initially, the coated spheres are placed on a glass slide and dried,

1.2 Hollow nanocomposites   

   11

Table 1.7: Conditions of preparation of coated spheres Material

Quantity (g)

PS PVP Urea HCl 0.75×10−2 M Iron (III) Chloride Water

0.25 2.25 3.30 2.80 0.40 250

first at room temperature and then for 2 h at 80 °C. Then, the composite is further calcinated for 3 h in air in a furnace at 500 °C at a heating rate of 5 °C min−1. Moreover, the hollow hematite (Fe2O3) composites are heated for 1, 4, 12 and 24 h at 350 °C in a hydrogen oven and this results in hollow magnetite (heated for 1 h) and hollow composites containing both maghemite and iron phases (heated for 4, 12, 24 h). Calcination of the coated composites gives hollow spheres with a diameter of 400 ± 10 nm (Fig. 1.6). These spheres consist of hematite phase (XRD-Analysis). The thickness of the container’s wall is approximately 45 nm.

Pa 2 Pa 2 = 314.8 nm Pb 2 = 333.0 o

Pa R2 Da 1 = 402.3 nm Db 1 = 127097.nm2

Da 1 Pa 1 = 249.7 nm Pb 1 = 355.1 o

Pa 1 Pa R1

Mag = 21.09 KX 1 μm

Fig. 1.6: SEM of hollow hematite spheres after calcination [21].

12   

   1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps

1.2.6 SiO2–CaO hollow nanocomposites A binary system of SiO2–CaO nanocomposites is successfully prepared by the sol-gel method in W/O emulsion [64]. Tetraethyl orthosilicate (TEOS) and calcium nitrate tetra hydrate are used as silica and calcia source respectively, and an ammonia solution is used as catalyst. The choice of the components of the oil and water phase, which stabilizes the emulsion, is the key factor contributing to the formation of spherical particles. The surfactants Span 80 and Tween 20 are used to control the shell thickness, the surface form, the size and form of the pores. The resulting hollow nanocomposites with diameter from 110 to 180 nm are obtained after calcination at 600 °C (Fig. 1.7.).

Si 111.4 nm (s)

105.8 nm (s)

O

111.7 nm (s)

Au 110.9 nm (s)

C

Ca

1.00

2.00

3.00

mag 100000 x

500 nm

4.00

Fig. 1.7: SEM and EDX analysis of SiO2–CaO hollow nanocomposites [64].

Propanol, hydroxylpropyl cellulose (HPC) and Span 80 are used to prepare the oil phase. The water phase contained polyethylene glycol (PEG) to stabilize the water droplets, Tween 20 as a high-HLB surfactant and distilled water. An ammonia solution (ΝΗ3 33 %) is used as catalyst and ethanol (95 %) as a washing reagent. The exact compositions are shown in Table 1.8. Table 1.8: Conditions of preparation of SiO2–CaO nanocomposites Material

Quantity

Propanol HPC PEG Span 80 Tween 20 TEOS (Ca(NO3)2⋅4H2O) NH3

55.44 ml 0.48 g 0.23 g 1.45 ml 0.24 ml 11.58 ml 2.15 g 0.12 ml

1.2 Hollow nanocomposites   

   13

The water-in-oil emulsion is prepared in two stages. First, an external oil phase is prepared by dissolving HPC and Span 80 in propanol. Second, PEG and Tween 20 are added in distilled water. The emulsion is produced by mixing the water phase to the external oil phase and the weight ratio of water phase to oil phase emulsion is kept at 1 : 9. After the W/O emulsion has formed, TEOS and (Ca(NO3)2⋅4H2O) are added to the mixture. The nanocomposites obtained after the previous procedure are aged in a clave at 80 °C for 3 h and are finally heat treated in air at 600 °C for 6 h with a heating rate 5 °C/min.

1.2.7 Water trapping nanocomposites Spherical water traps are prepared via a two step process, which comprises the preparation of cross-linked poly(methacrylic acid) spheres by distillation-precipitation polymerization and the subsequent conversion of carboxylic groups to the corresponding sodium salts by treatment with aqueous sodium hydroxide solution. The influence of the cross-linker amount is investigated in order to achieve greater water absorption and a reversibility of the initial spheres after drying the formed hydrogel or washing with an excess of organic solvent [3]. Preparation of monodisperse PMAA spheres. PMAA nanospheres are prepared by distillation-precipitation polymerization, according to the procedure previously reported in the literature [82, 83]. Such a typical experiment is conducted as follows: Methacrylic acid (MAA) (46.9 mmol) and Ethylene dimethacrylate (EGDMA) (2.6 mmol) are dissolved in 500 ml of acetonitrile in a dried 1 L three-necked round bottom flask equipped with a thermometer, a condenser and a pad for nitrogen supply. The reaction solution is stirred under nitrogen flow at 80 °C in an oil bath for about 1 hour. After this time, 2,2’-Azobis(2-methylpropionitrile) (AIBN) (0.49 mmol) dissolved in 10 ml of acetonitrile is added and stirring is continued at the same temperature for another 1 hour. Then, the temperature is raised in order to achieve the boiling point of solvent. After distillation of 80 ml of acetonitrile (within 30 min), the reaction is ended and the milky solution is left to reach room temperature. The resultant product is collected and purified by repeated centrifugation (10,000 rpm, 4 min), decanting and resuspension in acetonitrile two times. The material is dried under vacuum. All others experiments are performed in the same manner, except the amount of EGDMA. Preparation of water traps. The ability of PMAA nanospheres to absorb water is succeeded by conversion of carboxylic groups to the corresponding sodium salts. The following procedure is used: 2 g of dry PMAA microspheres are suspended in 100  ml deionized water with ultrasonic bathing. After a homogenous suspension is achieved, sodium hydroxide (10.0 mmol, final concentration 0.1  M) is added and the milky solution is rapidly turned into a more transparent one (it depends on the molar ratio of monomers. Reducing the amount of EGDMA leads to more trans-

14   

   1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps

parent solutions), which is stirred for 30 min at room temperature. The solution is then centrifuged (10,000 rpm, 4 min) to give the swollen form of the product as a hydrogel. Resuspending to ethanol and centrifugation (6,000 rpm, 4 min) two times and drying subsequently under vacuum and at 90 °C gives the completely dry form of the spherical water traps. The result is that the water traps produced this way achieve highest water absorbency of about 70 times their weight occurring rapidly and reversible unlimited times after going through repeatable cycles of absorption and drying. Fig. 1.8 illustrates the SEM image of water trap composite. It can be seen that the water traps are spherical. EDX analysis certifies existence of sodium in the final product.

C

91.3 nm

91.1 nm

Na

92.2 n m

92.5 nm 92.4 nm

O

mag 200000 x

400 nm

Au

0.60 1.20 1.80 2.40 3.00 3.60 4.20 keV

Fig. 1.8: SEM image and EDX analysis of water trap composites.

1.2.8 Chloride trap nanocomposites Chloride nanotraps are synthesized by modification of inorganic templates. The synthetic route encompasses the production of a template and then the modification of the template in order to enable the final nanocomposite to trap chloride ions. The synthesis of a template is accomplished via the preparation of silicon dioxide templates (nanospheres). The reaction is carried out in a container under the conditions listed in Table 1.9. The process lasts for 12 hours. The resulting dispersions are centrifuged at 10,000 rpm for 10 min, the supernatant solutions are discarded. In order for the fabricated nanospheres (templates) to be modified, the nanospheres are resuspended in EtOH with a stirrer. Glycidoxypropyltrimethoxysilane and diethylenetriamine are added and the reactant mixture is left under stirring for 10  hours. Finally, the dispersions are centrifuged and washed with ethanol. After centrifugation the purified chloride nanotraps are dried in air.

1.3 Nanocomposites loaded with corrosion inhibitors   

   15

Table 1.9: Reagent Concentration in the preparation of inorganic templates Reagent

% w/w

Water TEOS NH3 EtOH

8.87 0.87 1.77 88.49

The synthesized chloride trap nanocomposites are presented in Fig.  1.9. Their size is 125 ± 20 nm. Their ability to trap Cl− was tested with solutions of 0.5 M NaCl and 4.33 M HCl. It was found that after exposure of the nanotraps in the above solutions, they consist of 2.5 % w/w Cl−.

124.2nm (s) 103.1nm (s) 116.2nm (s) 129.4nm (s) 116.2nm (s) 121.5nm (s) 105.9nm (s)

145.6nm (s)

500 nm

Fig. 1.9: SEM image of chloride trap nanocomposites.

1.3 Nanocomposites loaded with corrosion inhibitors The synthesized hollow nanocomposites are loaded with various corrosion inhibitors in order to be used as inhibitor storages in corrosion protective coatings. The cerium molybdate hollow nanocomposites that are obtained are loaded with the corrosion inhibitors MBT, 8-HQ and 1-BSA [32]. The process for the loading is the following. Firstly, a saturated solution of MBT in acetone is prepared. An amount of cerium molybdate hollow nanocomposites is placed in a sealed container. The air of the inner side of the nanocontainers is eliminated with a vacuum system. Then, the saturated

16   

   1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps

TGA-weight loss (mg)

solution of MBT in acetone is inserted into the sealed container and the whole mixture is stirred at room temperature for 2 hours. Finally, the cerium molybdate hollow nanocomposites loaded with MBT are collected through centrifugation and drying under vacuum at 60 °C overnight. The same process is repeated for the encapsulation of 8-HQ and 1-BSA using saturated solutions in acetone and water, respectively. The aforementioned loading process is used in order for cerium oxide hollow nanocomposites to be loaded with 8-HQ, cerium titanium hollow nanocomposites to be loaded with 8-HQ and MBT, and titania hollow nanocomposites to be loaded with 8-HQ and p-TSA. The loading of the inhibitors into the hollow nanocomposites is estimated by TGA. Fig. 1.10. illustrates the TGA diagrams of pure MBT and cerium molybdate hollow nanocomposites loaded with MBT. Pure MBT began to degrade at 150 °C until 350 °C where no residue left, corresponding to oxidative degradation of the inhibitor. On the other hand, the spectra of hollow nanocomposites loaded with MBT exhibit a mass loss percentage of 58.08 % between 220 °C and 500 °C corresponding to oxidative degradation of MBT. Comparing the TGA diagrams, it is observed that pure MBT is degraded at lower temperatures compare to the samples of containers loaded with MBT. This shift of the burn-off (approximately 150 °C higher than pure MBT) is due to the protection of the shell of the hollow nanocomposites. This result clearly denotes that MBT is encapsulated in the hollow nanocomposites. Similarly, according to TGA measurements, cerium molybdate hollow nanocomposites were 5.22 % w/w loaded with 8-HQ and 16.48 %

9000 8000 7000 6000 5000 4000 3000 2000 1000 0

pure MBT 0

100

200

300

400

500

600

400

500

600

TGA-weight loss (mg)

Temperature (°C) 9000 8000 7000 6000 5000 4000 3000 2000 1000 0

Cerium molybdate hollow nanocomposites loaded with MBT 0

100

200

300

Temperature (°C)

Fig. 1.10: TGA curve of pure MBT and cerium molybdate hollow nanocomposites loaded with MBT [68].

1.3 Nanocomposites loaded with corrosion inhibitors   

   17

w/w loaded with 1-BSA. Moreover, cerium oxide hollow nanocomposites were 4.28 % w/w loaded with 8-HQ, cerium titanium hollow nanocomposites were 4.37 % w/w loaded with 8-HQ and 25.36 % w/w loaded with MBT, and titania hollow nanocomposites were 3.56 % w/w loaded with 8-HQ and 6.13 % w/w loaded with p-TSA. The release study of the corrosion inhibitors from the loaded nanocomposites is conducted with UV spectroscopy [75]. The release spectrum of 8-HQ from titania nanocomposites is presented in Fig. 1.11. The 8-HQ concentration was determined in the release medium by measuring the % T at 305 nm. Calculations were based on a standard curve of 8-HQ obtained in double distilled H2O (ε = 2150 L mol−1 cm−1). Actually, 0.05  g of nanocontainers loaded with 8-HQ were placed in a beaker that contained 50 ml of distilled water. The mixture was placed under vigorous stirring and samples were withdrawn every 3 minutes and filtered prior to measurement. The presented diagram expresses the percentage (%) of released 8-HQ versus the immersion time. It can be seen that within the first 15 min a plateau is reached where most of the loaded 8-HQ is released (~87 %).

100

Release (%)

80 60 40 20 0

0

10

20

30

40

50

Time (min)

Fig. 1.11: (%) 8-HQ release from the titania nanocomposites [75].

Figs. 1.12 and 1.13 depict the release of 8-HQ and 1-BSA from cerium molybdate nanocomposites, respectively. These diagrams present % transmittance versus the immersion time. The spectrum of 8-HQ release was taken at a wavenumber of 326 nm and the spectrum of 1-BSA release was measured at a wavenumber of 365 nm. In the starting time the transmittance is 100 % because no inhibitor has been released. As time elapses, the inhibitor is released from the nanocontainers and starts to absorb at the specific wavenumber that is used. This is confirmed by the depression of % transmittance. 1-BSA is released faster than 8-HQ because it is more soluble in water than 8-HQ. After 1,500 minutes, the transmittance in the mixture of water with 8-HQ reaches 76.1 %. The mixture of water with 1-BSA shows transmittance 83.6 % after 350 minutes.

18   

   1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps

102 100

B ExpDec3 fit of Data1_B

98 96 Transmittance (%)

94 92 90 88 86 84 82 80 78 0

200

400

600

800

1000

1200

1400

Time (min)

Transmittance (%)

Fig. 1.12: The 8-HQ release from the cerium molybdate nanocomposites [32].

102 100 98 96 94 92 90 88 86 84 50

[T%] ExpDecay3 fit of [T%]

0

50 100 150 200 250 300 350 Time (min)

Fig. 1.13: The 1-BSA release from the cerium molybdate nanocomposites [32].

1.4 Antibacterial action of hollow nanocomposites An interesting application is the use of hollow nanocomposites in photocatalytic actions. Cerium molybdate (CeMo), cerium oxide (CeO2), titanium oxide (TiO2) and iron-titanium oxide (TiFe) hollow nanocomposites reveal improved photocatalytic action when they are treated with on pure culture of E. coli. The E. coli (DH size 5 Å) bacterial culture is grown aerobically in 15 ml conical glass flasks containing 2 ml of Liquid Broth (LB) at 37 °C on a rotary environment control shaker for 16 h. The speed of the shaker is set at 230 rpm. The constituents of 1 L of LB are listed in Table 1.10.

1.4 Antibacterial action of hollow nanocomposites   

   19

Continually, the cell culture is diluted 4.5 orders of magnitude in sterilized deionized water. The final cell concentration is determined by a viable count procedure [colonyforming units (CFU) counting] on LB agar plates. Table 1.10: Ingredients for the preparation of 1 L of LB Material

Quantity

Yeast extract (g) Bacto tryptone (g) NaCl (g) Sodium hydroxide 1N (ml) Bacto agar (g) Deionized water (ml)

5 10 10 3.5 17 1000

The photocatalytic experiments include the illumination of E. coli cells in the presence of nanocomposites according to the following process. Stock aqueous suspensions of nanocomposites are always prepared immediately prior to photocatalytic reaction and kept in the dark. Aliquots of 1 ml stock (10 mg⋅ml−1) aqueous nanocomposites are added to 50 ml glass beakers containing 8 ml of sterilized deionized water and 1 ml of cell solution. The beakers containing the spheres-cell slurry are placed on a magnetic stir plate with continuous stirring, and illuminated with two 8 W black light blue fluorescent lamps (SYLVANIA BLACKLIGHT-BLUE F 8 W / BLB, Japan), one lamp above the glass beaker and the other lamp below it. The wavelength range of the light is 350–400 nm. The viable count procedure is used for the estimation of the viability loss. An E. coli suspension without hollow spheres is illuminated as a control, and the reaction of the hollow spheres-cell slurry in the dark is also carried out. During the experiments, samples are taken at 10  min intervals for 60  min in triplicates. The viable count is performed on LB agar plates. All plates are incubated at 30 °C for 16 h. Fig. 1.14 illustrates the survival curve when E. coli cells (initial cell concentration is 1112 CFU / 0.5 ml) undergo constant illumination in the presence of 10 mg⋅ml−1 TiO2 [50]. After 20 min of illumination only 5 % of the cells retain their viability. After 40  min of treatment, almost all of the cells lose their viability. Controls run in the absence of light do not show significant bactericidal activity for the first 60 minutes and after 120  minutes 53 % of the cells retain their viability. Controls run in the absence of TiO2 depict that after 40 minutes 55 % of the cells retain their viability, demonstrating the photocatalytic effect. The survival curve in Fig. 1.15 presents the results when E. coli cells (initial cell concentration was 10096 CFU / 0.5 ml) undergo constant illumination in the presence of 12 mg⋅ml−1 TiO2. After 10 min of illumination only 3 % of the cells retain their viability. After 20 min of treatment, all of the cells lose their viability. Controls run in the absence of light do not show significant bactericidal activity for the first 20 minutes and after 70 minutes 81 % of the cells

Bacterial survival [as CFUs] (%)

20   

   1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps

100 80 60 40 20 0

0

20

40

60

80

100

120

Time (min)

Bacterial survival [as CFUs] (%)

Fig. 1.14: The effect of TiO2 photocatalytic reaction on cell viability. (▲) E. coli + TiO2, (●) E. coli + hv, (■) E. coli + hv + TiO2 [50].

100 80 60 40 20 0

0

10

20

30

40

50

60

70

80

Time (min)

Fig. 1.15: The effect of TiO2 photocatalytic reaction on cell viability. (▲) E. coli + TiO2 + 30 min hv, (●) E. coli + hv, (■) E. coli + hv + TiO2, (▼) E. coli + TiO2 [50].

retain their viability. Controls run in the absence of TiO2 do not show significant bactericidal activity for the first 20 minutes and after 70 minutes 27 % of the cells retain their viability. Furthermore, it can be clearly seen that if the light is turned off after 30 min followed by an additional 40 minutes incubation in darkness, the viable cell count obtained at 70 min is similar to the sample that undergoes illumination continuously for 70  min. These results are in common with Z. Huang and C. Wel who used TiO2 nanoparticles on E. coli and illumination with light in the same wavelength range as with this study [85]. The viability of CeO2-treated cells and CPCeO2-treated cells is also determined by CFU counting after 16 hours of incubation [67]. The initial cell concentration is 524 CFU / 0.5  ml. For the experiment with CeO2-treated cells, the survival curve in Fig. 1.16 reveals that when E. coli cells (524 CFU / 0.5 ml) undergo constant illumination for 10, 20 minutes in the presence of 10 mg⋅ml−1 CeO2, only 18 %, 10.3 % of the

1.4 Antibacterial action of hollow nanocomposites   

   21

Bacterial survival [as CFUs] (%)

cells respectively are alive. After 30 minutes of treatment, almost all of the cells lose their viability. Controls run in the absence of light depict that after 10 and 20 minutes, 92.7 % and 78.1 % of the cells retain their viability; finally after 60 minutes 50.3 % of the cells are alive. Controls run in the absence of CPCeO2 illustrate that after 10, 20 and 30 minutes, 66.9 %, 54.0 % and 18.1 % of the cells retain their viability.

100 80 60 40 20 0

0

10

20

30

40

50

60

Time (min)

Fig. 1.16: The effect of CeO2 photocatalytic reaction on cell viability. (▼) E. coli + CeO2, (●) E. coli + hv, (■) E. coli + CeO2 + hv [67].

Bacterial survival [CFU/0.5 ml] (%)

For experiment with CPCeO2-treated cells, the survival curve in Fig. 1.17 shows that when E. coli cells are illuminated for 10 min in the presence of 10 mg⋅ml−1 CPCeO2, only 6.8 % of the cells retain their viability [67]. After 20 min of treatment, almost all of the cells lose their viability. Controls run in the absence of light show that after 10 and 20  minutes, 81.6 % and 71.5 % of the cells respectively retain their viability; finally after 60 minutes 52.9 % of the cells are alive. Controls run in the absence of

100 80 60 40 20 0

0

10

20

30

40

50

60

Time (min)

Fig. 1.17: The effect of CPCeO2 photocatalytic reaction on cell viability. (▼) E. coli + CPCeO2, (●) E. coli + hv, (■) E. coli + CPCeO2 + hv [67].

22   

   1 Synthesis and characterization of ceramic hollow nanocomposites and nanotraps

Bacterial survival [as CFUs] (%)

CPCeO2 show that after 10, 20 and 30 minutes, 66.9 %, 54.0 % and 18.1 % of the cells respectively, retain their viability. Both CPCeO2 and CeO2 hollow nanocomposites provoke antibacterial action. The polymer coating of the CPCeO2 improves this action. The reason is the semiconductor properties of both PAni and PPy polymers. These results are in common with Thill et al.’s study that used CeO2 nanoparticles on E. coli [86]. The initial concentration is 704 CFU/0.5  ml for both TiFe-treated and CeMotreated cells. For the TiFe-treated cells, the survival curve in Fig. 1.18 reveals that the constant illumination of E. coli cells for 10 min in the presence of 10 mg⋅ml−1 TiFe, results only in 9.66 % bacterial survival. The continuous constant illumination for 20  min discloses that almost all the cells lose their viability. Controls run in the

100 90 80 70 60 50 40 30 20 10 0

E-colihv E-coliTiO2Fe2O3 E-colihvTiO2Fe2O3 0

10

20

30

40

50

60

Time (min)

Bacterial survival [as CFUs] (%)

Fig. 1.18: The effect of TiFe hollow spheres photocatalytic reaction on cell viability. (■) E. coli + hv, (●) E. coli + TiFe, (▲) E. coli + TiFe + hv.

100 90 80 70 60 50 40 30 20 10 0

E-colihv E-colihvCe2(MoO4)3 E-coliCe2(MoO4)3 0

10

20

30

40

50

60

Time (min)

Fig. 1.19: The effect of CeMo hollow spheres photocatalytic reaction on cell viability. (■) E. coli + hv, (►) E. coli + CeMo, (○) E. coli + CeMo + hv.

1.5 Nanocomposites incorporated into coatings   

   23

absence of light do not illustrate significant bactericidal activity for the first 20 min and after 60 min 49.71 % of the cells retained their viability. Fig. 1.19 depicts the survival curve of E. coli in the presence of 10 mg⋅ml−1 CeMo. It can be seen that in the absence of light or under constant illumination all of the cells lost their viability after 10 min. Controls run in the absence of composites (TiFe or CeMo) demonstrate that after 60 min 44.03 % of the cells retained their viability. These results clearly denote that the presence of nanocomposites provokes a photocatalytic effect under constant illumination. Furthermore, the CeMo composites provoke 100 % loss of viability to E. coli bacteria in the absence of light.

1.5 Nanocomposites incorporated into coatings The aforementioned synthesized cerium molybdate, cerium oxide and titanium oxide hollow nanocomposites loaded with corrosion inhibitors are incorporated into hybrid organic-inorganic, sol-gel and conductive polymer coatings in order to produce improved corrosion protective coatings for various metal alloys such as aluminum alloy 2024-T3 [68, 71, 74, 75, 78], hot dip galvanized steel [73], magnesium alloy ZK10 and ZK30 [76, 79]. The hollow nanocomposites work as corrosion inhibitor storages. The inhibitor has to be encapsulated and stored, otherwise, in a free form in the coating, it would degrade the coherence of the coating. The inhibitor should be released from the container (via diffusion or after the break down of the container) only after corrosive attack to the coating. Coatings consist of cross-linked polymers based on bisphenol A diglycidyl ether as well as organic modified silicates including cerium molybdate hollow nanocomposites loaded with MBT were synthesized and applied to magnesium ZK10. These coatings displayed improved corrosion protection properties after exposure to 0.5 M NaCl solution for 4 months. Studies on artificial defect coatings immersed in 1 mM NaCl solution for 73 hours presented partial self-recovery of the films. The incorporation into the films of containers loaded with corrosion inhibitor MBT enhanced the self-healing effect due to the increase of the charge transfer resistance [79]. HDG steel was coated via a dip-coating process with a hybrid organic-inorganic coating that contains cerium molybdate hollow nanocomposites loaded with MBT. The results reveal improved corrosion protection after immersion in 0.5 M NaCl solution for 744 hours (31 days). The addition of nanocomposites loaded with corrosion inhibitor improved the anticorrosive properties of the coatings compared to the coatings that have empty nanocomposites or the coatings that have only the inhibitor. The concentration of the loaded nanocomposites in the coating is an important factor for further improvement of the anticorrosive properties of the coating. Furthermore, the coatings present partial recovery action [73]. Improved corrosion protective coatings with additionally self-healing properties were synthesized for aluminum alloys by the incorporation of inorganic salts or

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organic corrosion inhibitors or nanoparticles into hybrid organic-inorganic sol-gel coatings [70, 87–90]. On the other hand, the incorporation of these compounds depressed the coherence of the coating matrix. This disadvantage was faced with the use of containers. The inorganic and organic corrosion inhibitors were encapsulated into containers and then added into the hybrid organic-inorganic sol-gel coatings [69, 71, 72, 74, 75, 78, 91–93].

1.6 Properties In previous studies, the incorporation of containers (with or without inhibitor) was proven to affect the mechanical integrity of the coatings, revealing a clear mechanical degradation of epoxy coating [79]. The decrease of hardness implies deterioration of the coating (further penetration into the coating does not significantly reveal H deviation, yet all coatings exhibited similar hardness (~0.3 GPa). Concerning modulus, a clear decrease of E was revealed. The scatter of H and E values was further investigated; deviation of nanomechanical properties is reported at the surface region, probably attributed to roundness of the tip and Indentation Size Effect (ISE), tending to reach constant values at greater displacements. The reasons for the wide range in H and E values obtained from these nanoindentation measurements were attributed to a combination of factors, e.g. graded surface structure due to containers concentration, adhesive forces between the tip and the sample or containers bundling. The empirical equation for describing the ISE in Meyer’s law [94] was used, which uses a correlation technique between the applied indentation test load and the resultant indentation size using a simple power law, Pmax = Chn, where C and n are constants derived directly from curve fitting of the experimental data. In particular, the exponent n, sometimes referred to as the Meyer index, is usually considered a measure of ISE. Compared to the definition of the apparent hardness, no ISE would be observed for n = 2 [94]. In nanocomposites case, n ranged from 1.96 to 1.99 implying no existence of ISE. The ratio of hardness to elastic modulus is of significant interest in tribology. Higher stresses are expected in high H/E, hard materials, and high stress concentrations develop towards the indenter tip, whereas in the case of low H/E and soft materials, the stresses are lower and are distributed more evenly across the cross-section of the material [95] and [96]. The high ratio of hardness to elastic modulus (H/E) is indicative of good wear resistance in a disparate range of materials [96]: ceramic, metallic and polymeric (e.g. c-BN, tool steel, nylon), which are equally effective in resisting attrition for their particular intended application. The change of H/E slope revealed that the addition of container and inhibitor amount strengthened (increase of wear resistance) the epoxy coating after ~600 nm of displacement, having no significant impact on surface region (0–600 nm), where all coatings exhibited similar (increased) H/E ratio.

1.7 Summary and Conclusion   

   25

The % plasticity values of the materials at different displacements were calculated by integrated areas under the loading curve and the unloading curve. At low displacements, the samples revealed elastoplastic behavior, while for higher displacements the samples exhibited the typical plastic behavior (~90 %). Taking into account the %plasticity, it was found that the change from elastic to plastic deformation is observed at almost identical displacement (~100 nm) for all coatings. Incorporation of nanocontainers increases the coefficients of friction, for the whole scratch path; however, when the tip further penetrates the sample, the behavior is almost identical for all samples. When an abrupt change in the coefficient of friction occurs, almost at the end of scratch path, it implies possible failure of the coating [73]. The contact area is influenced by the formation of pile-ups and sink-ins during the indentation process. To accurately measure the indentation contact area, pileups/sinks-ins should be appropriately accounted for. The presence of creep during nanoindentation has an effect on pile-up, which results in incorrect measurement of the material properties. Fischer-Cripps observed this behavior, in a case where the measured elastic modulus was much less than expected [97]. Rar et al. observed that the same material when allowed to creep for a long duration produced a higher value of pile-up/sink-in indicating a switch from an initial elastic sink-into a plastic pile-up [98]. Higher stresses are expected in hard materials and high stress concentrations develop towards the indenter tip, whereas in case of soft materials the stresses are lower and are distributed more evenly across the cross-section of the material [97]. Rate sensitive materials experience less pile-up compared to rate insensitive materials due to strain hardening. When hc/hm approaches 1, deformation is intimately dominated by pile-up [99] and [100]. On the other hand, when hc/hm approaches 0, it corresponds to purely elastic deformation and is apparently dominated by sink-in in a manner prescribed by Hertzian contact mechanics [101]. For the case of coatings and thin films, a switch from pile-up to sink-in deformation is often reported [74, 102].

1.7 Summary and Conclusion In this work, the synthesis, characterization and application of various nanocomposites are demonstrated. Hollow cerium oxide, cerium molybdate, titanium oxide, magnetic, cerium titanium oxide nanocomposites are produced based on the organic template method. An organic core is fabricated via radical polymerization, followed by an inorganic coating via the sol-gel method. The final nanocomposite is synthesized after heat treatment. Moreover, calcium silicon oxide hollow nanocomposites are produced via the emulsion method. The aforementioned nanocomposites are loaded with various corrosion inhibitors in order to act as corrosion inhibitor storage systems. Studies on the loading and release of the inhibitor from the nanocomposites

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are demonstrated. Furthermore, the photocatalytic action of the nanocomposites is presented based on E. coli cultures. Apart from hollow nanocomposites, the present work reports on the production of nanocomposites that act as water and chloride traps. All the aforementioned nanocomposites are used in hybrid organic-inorganic, sol-gel and conductive coatings in order to improve their anticorrosion protective properties as well as their partial recovery effect.

Acknowledgments Part of this work is encompassed in Dr. I.A. Kartsonakis post-doctoral thesis in the National Technical University of Athens, School of Chemical Engineering, Section III: Department of Materials Science and Engineering.

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Raps D., Hack T., Wehr J., Zheludkevich M.L., Bastos A.C., Ferreira M.G.S., Nuyken O. Electrochemical study of inhibitor-containing organic-inorganic hybrid coatings on AA2024, Corros. Sci., 2009, 51, 1012–1021. Rosero-Navarro N.C, Pellice S.A., Duran A., Aparicio M. Effects of Ce-containing sol-gel coatings reinforced with SiO2 nanoparticles on the protection of AA2024. Corros. Sci., 2008, 50, 1283–1291. Wang H., Akid R. A room temperature cured sol-gel anticorrosion pre-treatment for Al 2024-T3 alloys. Corros. Sci., 2007, 49, 4491–4503. Zheludkevich M.L., Poznyak S.K., Rodrigues L.M., Raps D., Hack T., Dick L.F., Nunes T., Ferreira M.G.S. Active protection coatings with layered double hydroxide nanocontainers of corrosion inhibitor. Corros. Sci., 2010, 52, 602–611. Raps D., Hack T., Kolb M., Zheludkevich M.L., Nuyken O. Development of corrosion protection coatings for AA2024-T3 using micro-encapsulated inhibitors, smart coatings III. In: Baghdachi J., Provder T. (eds.) ACS Symposium Series, Volume 1050, Chapter 12, 2010, 165–189. Skorb E.V., Fix D., Andreeva D.V., Möhwald H., Shchukin G. Surface-modified mesoporous SiO2 containers for corrosion protection. Adv. Funct. Mater., 2009, 19, 2373–2379. Sahin O., Uzun O., Kolemen U., Ucar N. Vickers microindentation hardness studies of β-Sn single crystals. Mater. Charact., 2007, 58, 197–204. Cheng Y.T., Cheng C.M. What is indentation hardness. Surf. Coat. Technol., 2000, 133–134, 417–424. Leyland A., Matthews A., Design criteria for wear-resistant nanostructured and glassy-metal coatings. Surf. Coat. Technol., 2004, 177–178, 317–324. Fischer-Cripps A.C. A simple phenomenological approach to nanoindentation creep. Mater. Sci. Eng. A., 2004, 385, 74–82. Rar A., Sohn S., Oliver W.C., Goldsby D.L., Tullis T.E., Pharr G.M. On the measurement of creep by nanoindentation with continuous stiffness techniques. In: MRS Fall Meeting 2005, Boston, MA, United States, Materials Research Society, Warrendale, PA, 2005, 119 –124. Hill R., Storakers B., Zdunek A.B. A theoretical study of the Brinell hardness test. Math. Phys. Sci., 1989, 423, 301–330. Biwa S., Storakers B., An analysis of fully plastic Brinell indentation. Mech. Phys. Sol., 1995, 43, 1303–1334. Hertz H. Experiments to determine an upper limit for the kinetic energy of an electric current. Miscellaneous Papers by H. Hertz, Macmillan, 1896. Koumoulos E.P., Charitidis C.A., Papageorgiou D.P., Papathanasiou A.G., Boudouvis A.G. Nanomechanical and nanotribological properties of hydrophobic fluorocarbon dielectric coating on tetraethoxysilane for electrowetting applications. Surf. Coat. Technol., 2012, 206, 19–20, 25.

S.K. Srivastava, M. Kotal

2 Recent advances on preparation, properties and applications of polyurethane nanocomposites 2.1 Introduction The discovery of nylon-6/clay nanocomposites [1, 2] in 1990 by the Toyota group of researchers provided a new beginning in the development of polymer nanocomposites with their remarkable mechanical, thermal, adhesive, catalytic, electronic, magnetic and optical properties, increased heat resistance, biodegradability, antibacterial effectiveness, decreased flammability and gas permeability at very low filler loading and which are not exhibited by the individual phases or by their macroor microcomposite counterparts [3–5]. The extent of such property enhancement depends on many factors including the geometrical shape and aspect ratio of the filler, its degree of dispersion, orientation in the polymer matrix and the degree of adhesion/interaction between the matrix and the filler at the filler-matrix interface [6–9]. However, “quantum size” effects, and sometimes coulombic-charging effects originating from the ultrafine sizes and morphology are also responsible for the unique improvements in the properties of polymer nanocomposites and allow them to live with the rising demands of their numerous potential applications in various fields [10–16]. Polyurethane (PU) is one of the most important classes of functional polymers with its multifaceted commercial applications in the form of foams, adhesives, coatings, sealants, synthetic leathers, membranes, rubbers and thermoplastic elastomers. It also finds extensive use in biomedical fields, e.g. vascular prostheses, endotracheal tubes, catheters and artificial hearts, drug release carriers, and as a suitable carrier for enzyme immobilization [17–19]. The properties of PU are determined by the presence of the molecular chain structure of soft and hard segments for its various applications. The hard segment, corresponding to isocyanates and triol or diol chain extenders, contributes to the stiffness (modulus and hardness) and strength (tensile stress at break), whereas the soft segment, consisting of long flexible polyester or polyether units, contributes to the elastic nature (strain at failure, elasticity, flexibility and damping ability). The repeating units in PUs are the urethane linkage produced from the polycondensation reaction of an isocyanate (–N=C=O) group of diisocyanate with one hydroxyl group (–OH) of diol (see Scheme 2.1) [20]. PU can be synthesized by bulk, solution and mini-emulsion polymerization methods [17, 18, 21, 22]. Although the bulk polymerization is generally preferred in industry, because the process is cost-effective and environmental-friendly, solution polymerization has invariably been used in laboratory synthesis. On the other

34   

   2 Preparation, properties and applications of polyurethane nanocomposites

Urethane group O HO

R Diol

OH

쎵 OCN

R’

NCO

HN OCN R’

O O R

Diisocyanate (excess)

HN R’ NCO O n

Scheme 2.1: Basic reaction scheme for urethane formation

hand, the mini-emulsion polymerization method has also been used for coating and adhesive purposes [22]. In all these methods, the key components required for the synthesis of PU are diisocyanate (aliphatic or aromatic), polyol (polyester or polyether), chain extender (low molecular weight diol or diamine) and catalyst [21, 22]. In addition, some additives like cross-linking agents (glycerol, trimethylol propane, sulfur) are also often used [21, 22]. PU can be classified into various types depending on its properties and applications, e.g. thermoplastic polyurethane (TPU), polyurethane foams, hyperbranched polyurethane (HBPU), castable polyurethane, millable polyurethane, adhesives, coatings, fibers, elastomers etc. In addition, the PU microsphere constitutes another form and can be obtained by cryogenic grinding/extrusion granulation of thermoplastic polyurethanes [23], and suspension polymerization of isocyanate terminated prepolymers in aqueous or non-aqueous medium [24, 25]. However, controlling the size of PU microspheres through these methods remained a major challenge. This problem can be overcome when PU microspheres are prepared from diol and diisocyanate by dispersion polymerization in the presence of organic solvent, steric stabilizer and catalyst at a controlled temperature [26]. PU has good tear strength, high elasticity, transparency, biocompatibility, excellent abrasion resistance and shock absorption, good resistance to oxygen, ozone, sunlight, oil, solvent and fat [21, 27]. Despite several advantages, it has poor thermal stability and barrier properties, and high combustibility. These limitations of PU could be overcome by changing the building blocks of either polyether/polyester polyol or chain extender or diisocyanates. The other alternatives involve the preparation of the PU nanocomposites at very low filler loadings [28–37] or PU blends with suitable polymers [38–42].

2.2 Fillers used in PU nanocomposites In the last few years, various types of nanofillers, e.g. layered silicates [28–31, 43–46], nanosilica [47–49], carbon nanotubes (CNT) [50–54], carbon nanofibers (CNF) [55–57], polyhedral oligomeric silsesquioxanes (POSS) [58–61], Au [36, 62], Ag [37, 63] and graphene [34, 35, 64] have been used in the development of PU nano-

2.2 Fillers used in PU nanocomposites    

   35

composites. But in recent years, layered double hydroxides (LDHs) have also received more attention as a new generation of two-dimensional nanofiller in the preparation of polymer nanocomposites with a lot of promise [12–15, 65–73]. Therefore, LDHs have been used in the present chapter as reinforcing nanofiller in the preparation of PU nanocomposites. The details about the different type of nanofillers used in the preparation of PU nanocomposites are described in the following.

2.2.1 Sheet/platelets type inorganic nanofillers 2.2.1.1 Natural layered silicates Natural layered silicates can be divided into three types depending on the relative ratio of two unit crystal sheets, 1:1 (mica, kaolin), 2:1 (montmorillonite, hectorite, vermiculite, saponite, bentonite, talc) and 2:2 (sepiolite) [74]. Among these, 2:1 and 2:2 types together belong to the smectite family of clays with a general formula: (Ca, Na, H) (Al, Mg, Fe, Zn)2(Si, Al)4O10(OH)2.xH2O, where x represents the variable amount of water. The structure of this group is composed of silicate layers sandwiching a gibbsite (or brucite) layer in between, in a silicate-gibbsite-silicate stacking sequence [75]. Among the smectite clay, montmorillonite (MMT) is one of the most commonly used nanofillers in preparation of polymer nanocomposites. Its structure consists of the two tetrahadrally coordinated silica (SiO44−) fused to an edge-shared octahedral sheets of alumina (AlO69−), as shown in Fig. 2.1 [76]. The isomorphic substitution of Si4+ in the tetrahedral lattice by Al3+and Al3+ in the octahedral sheet by Mg2+ or Fe2+ generate negative charges that are counterbalanced by alkali and alkaline earth cations, like Na+, Li+, Ca2+ situated between the layers and are hydrated [3, Oxygen Hydroxyl Silicon Aluminium

Exchangeable cations, nH2O

Fig. 2.1: Structure of montmorillonite.

36   

   2 Preparation, properties and applications of polyurethane nanocomposites

5, 77, 78]. The respective aspect ratio and cation exchange capacity of montmorillonite are generally greater than 1,000 and 65–150 meq/100 g respectively [75]. It is easily dispersible in water and the interlayer cations can be exchanged by organic cations, like alkyl ammonium, aryl ammonium etc. to make it compatible with organic polymers. The variations in the functionality, packing density and the length of the organic modifiers could also be tailored to optimize the miscibility of layered silicates with polymers [29, 30, 44–46, 79].

2.2.1.2 Layered double hydroxides LDHs are widely known as hydrotalcite-like materials and closely related to brucite, Mg(OH)2, which has a CdI2 type structure, typically associated with the small polarizing cations and anions [80, 81]. It consists of magnesium ions surrounded octahedrally by six OH− ions. These octahedral units form infinite layers by edge-sharing, with the hydroxide ions sitting perpendicular to the plane of the layers. The general formula of LDH can be represented as [MII1–x MIIIx(OH)2]x+(An–)x/n⋅yH2O, where MII is a divalent metal ion (Mg2+, Ca2+, Zn2+, Cu2+, Ni2+ etc.) MIII is a trivalent metal ion ( Al3+, Cr3+, Fe3+, Co3+, V3+ etc.); and An– is an anion (Cl–, CO32–, NO3–, etc.). The value of x lies in the range of 0.2≤x≤0.33, i.e., MII/ MIII ratios are in the range of 2–4 [82]. However, there are many exceptional examples of LDHs with x values outside the range, e.g., 0.67 for CoII/ AlIII LDH [83], 0.5 for FeII/FeIII LDH [84], 0.07 for MgII/GaIII-CO3 LDH [85], 0.41–0.48 for MII/MIII-CO3 LDHs (MII  =  Mg, Ni, Co, Cu; MIII  =  Al, V) [86–88]. The isomorphous substitution of some divalent cations by trivalent cations gives the brucite-like layers with the positive charge, which is counterbalanced by the anions, such as CO32−, NO3−, Cl−, SO42− in the interlayer region (gallery) resulting a hydrotalcite-like structure [89]. The interlayer anions in LDH are exchangeable with their order of preference [90, 91] as: NO3− > Q provides an orientation with all inhomogeneities aligned along the X3 axis, see Fig. 5.1. As is clear from Eq. (5.2), only the part involving the sum is orientation dependent, since the (1) stiffness Cijkl is isotropic, so only the part containing the sum is used in the integration. Furthermore, the stiffness to be integrated, Cpqrs , in Eq. (5.4) must be for a composite with unidirectionally aligned reinforcement, which is then averaged over all possible (1) directions via the integration and the final effective stiffness is obtained by adding Cijkl. This approach along with the Mori–Tanaka model was used in [94] to model the effective stiffness of montmorillonite (MMT) reinforced PVC. Experimental results for Young’s modulus, EEXP, and Mori–Tanaka modeling results, EMT, are shown along with the Hashin–Shtrikman lower, ELB, and upper bounds, EUB (see e.g. [95]) in Table 5.2. In the modeling it is assumed that the MMT particles are randomly oriented in the PVC matrix.

Table 5.2: Young’s moduli for the MMT-PVC composites (phr=Parts per hundred of resin). Vol. frac. (phr)

1

2

5

10

ELB (MPa) EEXP (MPa) EMT (MPa) EUB (MPa)

949 1186 1158 3161

968 1555 1387 5361

1028 1628 2077 11831

1137 1483 3245 22204

As can be seen, the agreement between experimental results and theory is quite good as long as the volume fraction of MMT is low, whereas for larger volume fractions the results deviate from each other. The reason for this deviation was found to be due to agglomeration of the MMT particles, which the theoretical model is not capable of handling in its present formulation. Another important observation is that for the volume fraction equal to 2 phr the experimental values are somewhat larger than the value predicted by the Mori–Tanaka model. This is believed to be due to size effects which tend to increase the overall stiffness. A similar approach was used by Zhang et al. [96] for modeling the stiffness of polyurethane (PU) foam reinforced with multiwalled carbon nanotubes (MWNT). The use of MWNT calls for additional considerations in relation to the load transfer between the matrix and the reinforcement. The two extremes are that all the tubes carry load or only the outer tube carries load. Thostenson and Chou [97] suggested an approach on how to deal with this situation and their approach was applied in the work of [96]. The Mori–Tanaka model was applied but with the modifications as described in Schjødt-Thomsen and Pyrz [98], in order to model cellular materials and the MWNTs were assumed to be randomly

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   5 Modeling mechanical properties of nanocomposites

oriented. Basically, the Mori–Tanaka model is applied in two steps. First, the reinforcement is taken to be spherical “particles” with zero stiffness, which resembles the stiffness of a cellular material which is used as matrix material for embedding the MWNT. Due to the two assumptions related to the load carrying mechanism and the fact that the tubes have the dimensions 3–5 nm (inner diameter), 10–30 nm (outer diameter) and lengths in the range 10–30 μm some intervals will be provided by the theoretical model. However, for brevity only the smallest and largest of the predicted values will be given in order to bound the experimental values as shown in Table 5.3. Also for brevity only some of the results from [96] will be shown. Table 5.3: Experimental and theoretical values for Young’s modulus of PU-MWNT reinforced composites. (The * indicates that the MWNT was functionalized).

MWNT (% wt.)

Mixing time (h) Cell. Vol. frac.

EMIN, (MPa)

EEXP, (MPa)

EMAX, (MPa)

0.2(*) 0.5 0.5(*) 1(*)

2 3 3 2

24.7 32.8 34 20.1

22.2 ± 1.9 35.5 ± 1.6 37.5 ± 1.3 15 ± 0.8

31.3 54.1 56.1 45.4

0.924 0.902 0.899 0.941

As can be seen from the data, the mixing time is very important in order for the MWNT to provide reinforcing effects. The increase from 0.2 % wt MWNTs to 1 % wt does not increase the stiffness. Contrarily, the stiffness decreases and it is believed to be due to agglomeration. Also the particular functionalization used does not seem to have much effect on the stiffness. The model is again in good agreement with the experimental values, and it can be concluded that it is likely that only the outer layer of the MWNT contribute to the stiffness, since this corresponds to the values of EMIN. The observations above are also in agreement with the findings of Wang and Pyrz [95, 99] that used various approaches, including the Mori–Tanaka model.

5.2.5 Effects of dispersion The actual dispersion of the reinforcement also influences the overall stiffness. The results from this analysis are very important when analyzing composites in which the reinforcement phase tends to agglomerate, as is the case for nano-sized reinforcements, because it is necessary to take clustering/agglomeration effects into account in the modeling. In the traditional Mori–Tanaka model spherical particles results in an isotropic stiffness tensor. If, however, these spherical particles are dispersed randomly in a very thin film, there may only be one plane of symmetry in the dispersion and the stiffness tensor must possess monoclinic symmetry. These effects on the overall prop-

5.2 Nano-, micro- and continuum mechanical modeling   

   167

erties were considered by several authors taking into account the statistics of the microstructure e.g. [100–103]. In general, in order to take the dispersion of the particles into account, statistical measures are used, i.e. n-point probability functions, see n e.g. [100, 103]. The n-point probability functions, s(i) , give the probability of finding n points at positions x1, x2, …, xN, in phase i. In fact the one point probability function corresponds to the volume fraction. The more information regarding the morphology that is needed, the more complex the statistical functions become and the exact microstructure may not even be possible to reproduce using a finite number of statistical descriptors. Another approach based on Eshelby’s equivalent inclusion method is given in [104]. The composite contains N inhomogeneities and Eshelby’s equivalency condition for inhomogeneity r is given as (I )

r ∞ r r∗ Cijkl ("∞ kl + "kl ) = Cijkl ("kl + "kl − "kl )

with Cijkl being the stiffness of the matrix material. The elastic strain disturbance due to the presence of the inhomogeneity, εijr , is given in terms of the contribution from inhomogeneity r itself plus the sum of contributions from all other inhomogeneities, i.e. "rij (x ) =

 Vr

Kijkl (x , x )"rkl∗ (x )d x +

N   Q =r

VQ

Kijkl (x , x )"Qkl ∗ (x )d x

The kernel in the integrals, Kijkl (x , x ), is a function of second order derivatives of Green’s function, Gij (x , x ). The interested reader may consult [104] for the relevant details. The equivalency condition, along with the expressions for "rij (x ), Kijkl (x , x ) and Gij (x , x ) form a system of 6N × 6N singular integral equations in the unknown equivalent eigenstrains "rkl∗ . This system of equations can be solved numerically, and having obtained the equivalent eigenstrains it is possible to calculate both the volume averaged stresses and volume averaged strains and subsequently obtain the effective stiffness. In Figs. 5.6, 5.7 and 5.8, dispersions of spherical particles are shown. The glass particles are assumed to be embedded in a polypropylene matrix and all the particles are dispersed in the same plane. Since there is only one plane of symmetry in dispersion #1 and #2, the effective stiffness must possess monoclinic symmetry. It turns out that this is exactly what the model returns, i.e. ⎡

C #1

⎢ ⎢ ⎢ ⎢ ⎢ =⎢ ⎢ ⎢ ⎢ ⎣

2627.9 1154 1138.7 0 0 10.4

1154 2612.6 1141.3 0 0 2.5

1138.7 1141.3 2592.7 0 0 −4.5

0 0 0 790.1 3 0

0 0 0 3 786.9 0

10.4 2. 5 −4.5 0 0 981.6

⎤ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎦

   5 Modeling mechanical properties of nanocomposites

0.2

0.2

0.1

0.1

y (mm)

y (mm)

168   

0 0

0.1

0 0

0.2

0.1

x (mm)

x (mm)

Fig.5.6: Dispersion #1.

Fig. 5.7: Dispersion #2.

0.2

y (mm)

0.2

0.1

0 0

0.1

0.2

x (mm)

Fig. 5.8: Dispersion #3.

and for dispersion #2 ⎡

C #2

2682.7 ⎢ ⎢ 1173.7 ⎢ ⎢ 1148.6 ⎢ =⎢ ⎢ 0 ⎢ ⎢ 0 ⎣ 10.1

1173.7 2650.1 1147.7 0 0 −17.6

1148.6 1147.7 2638 0 0 22.3

0 0 0 784.2 −35.9 0

0 0 0 −35.9 799.2 0

10.1 −17.6 22.3 0 0 1002.1

⎤ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎦

Finally, in the stiffness matrix for dispersion #3, the monoclinic symmetry should be absent since there is more than one plane of symmetry, thus ⎡

C #3

⎢ ⎢ ⎢ ⎢ ⎢ =⎢ ⎢ ⎢ ⎢ ⎣

2708.9 1178 1156.4 0 0 0

1178 2673.3 1152.9 0 0 0

1156.4 1152.9 2646.6 0 0 0

0 0 0 815.3 0 0

0 0 0 0 813.5 0

0 0 0 0 0 1039.4

⎤ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎦

5.2 Nano-, micro- and continuum mechanical modeling   

   169

As can be seen from these three examples the model captures the effects of the irregular dispersion. Also the more irregular the dispersion becomes (#2 is more irregular than #1), the greater the shear-normal coupling terms in the stiffness matrix increase.

5.2.6 Scale effects Yet another phenomenon that the theoretical models should account for is scale/size effects. The classical micromechanical approaches cannot predict such phenomena. In order to consider such effects, there are some alternatives such as considering the materials as higher order continua, to introduce interfacial effects through stress discontinuities at the interface or to introduce an interphase with differing material properties. The classical continuum theory only considers one kinematic variable, i.e. the displacements, ui, and first order derivatives of these, i.e. the strain. The higher order continuum theories consider additional kinematic variables and higher order gradients of the kinematic variables, see e.g. Toupin [105, 106], Mindlin [107, 108], Mindlin and Tiersten [109], Green and Rivlin [110] and Eringen [111]. In relation to nanocomposites, the approach involving interface/interphase considerations was used by e.g. [112–118]. Finally, Li et al. [119] and Chen et al. [120] used a combination of these approaches. A strain gradient based approach to Eshelby’s inclusion problem was used by Gao and Park [121] and Gao and Ma [122, 123]. The basic idea is that the strain energy is also a function of first order gradients of strain as opposed to the classic approach where only strains enter the strain energy. Only a simplified version is considered and involves one additional stiffness parameter. The final result of this is the strain gradient based Eshelby tensor, which can be expressed as the sum of the classical Eshelby tensor and a gradient dependent part which involves the scale parameter, L, which is an intrinsic material scale length and is related to a constitutive parameter c as L2 = c

where c enters the constitutive law as the strain gradient parameter flij = "ll ij + 2"ij − c ("ll ij + 2"ij ),kk

For obtaining the effective stiffness based on the strain gradient based Eshelby tensor, the volume average of the tensor is used. The expression for the strain gradient based Eshelby tensor is quite complex and for simplicity only the tensor for spherical particles is shown here. Thus, the strain gradient based volume averaged Eshelby tensor, SG Sijkl  , for spherical particles is

170   

   5 Modeling mechanical properties of nanocomposites

SG Sijkl =





1 2 L 1+ 15(1 − ) 3 R

3 

1−





R L

2

   R 2 − 2R − 1+ e L

L

 · (5 − 1)ij kl + (4 − 5)(ik jl + il jk )

where R is the radius of the spherical particle. For illustrating the predictions of this modeling approach, a material reinforced with spherical particles of volume fraction 5% is considered. The ratio of reinforcing E 160 particle stiffness to matrix stiffness is E21 = 3 . SG  is used along The strain gradient based volume averaged Eshelby tensor, Sijkl R with the Mori–Tanaka model. The ratio L is varied from 0.05 to 15 and the composite Young’s modulus Eeff normalized with respect to the classical Young’s modulus Eclassic is shown in Fig. 5.9 below.

3.5 3

Eeff/ Eclassic

2.5 2 1.5 1 0.5 0

0

3

6

9

12

15

R/ L

Fig. 5.9: Normalized Young’s modulus as a function of size parameter, L.

As can be seen, the size of the particles has a pronounced effect on the effective R stiffness of the composite. For L less than ≈ 3 the effective stiffness is larger than for “classic” composites, whereas for larger ratios (i.e. the particle size increases) the size effect diminishes. The same trend, i.e. a decrease of composite modulus as a function of particle size, was also observed by Cho and Sun [124] using molecular dynamics simulations.

5.3 Multiscale modeling A main goal within computational materials science is a fast and accurate prediction of the effective material properties. When developing new nanocomposite materials it is crucial to have a modeling method that can include information from different length and time scales. However, in the design of nanocomposite materials various

5.3 Multiscale modeling   

   171

length and time scales are involved which makes it difficult to predict the effective material behavior using only modeling methods from one length or time scale. Performing the modeling based solely on continuum based models results in some inaccuracies in describing both the dispersion of the particles and also the interface between the particle and matrix material. The reason is that both the dispersion and also the interface may be highly dependent on atomistic properties and behavior of the constituents. On the other hand, using purely atomistic based models to model the macroscopic behavior, as needed in engineering applications, is not realistic since the required amount of computational power and time needed to solve such a problem is not currently available. The largest systems that can be handled are of the order of 109 atoms corresponding to a volume of approximately 1 μm3 in size [125–129]. Therefore, in order to determine the general macroscopic behavior of a nanocomposite a multiscale approach has to be used where modeling techniques for different length and time scales are bridged together. In these modeling techniques it is utilized that the material behavior may be considered at different scales existing in a material system. In that way it becomes possible to model the macroscopic behavior of the nanocomposite including information all the way from atomistic to macroscopic scale. When bridging modeling methods appropriate for different length and time scales the information has to be transferred between the models in an efficient way which may be difficult [130]. Multiscale models are normally divided into two categories; sequential and concurrent coupled methods.

5.3.1 Sequential coupled methods In the sequential coupled methods, sometimes referred to as hierarchical methods, it is assumed that the different length scales involved in the material system can be separated into individual scales with their own characteristic behavior. Using this approach the different length and time scales in the model are completely separated from one another and modeling methods for each of the considered scales are used independently of each other. The results obtained at a smaller length/time scale, e.g. forces or displacements, are then used as boundary conditions or input for the modeling at a larger length/time scale. In that way information from a lower length scale is included in the modeling of a larger length scale. However, when using this approach it is not considered how the lower length scales are affected by phenomena taking place at a larger scale and in order to ensure that the multiscale simulations converges it may be necessary to feedback information to a lower scale [131]. This way of doing multiscale modeling is useful only if the coupling between the different length scales is weak because it may lead to inaccurate or nonrealistic results if there is a strong coupling between the modeled scales [132]. The sequential approach has been used in a number of applications, one of these is the work done by Doi [133] who developed an integrated simulation tool

172   

   5 Modeling mechanical properties of nanocomposites

for modeling polymeric materials which was used to predict the mechanical properties of block polymers. With this tool the polymer system can be modeled including information all the way from molecular scale to continuum scale. Each scale is modeled separately but the output from one model can be used directly as input for the next model without doing any further treatment of the output data. A similar approach has been used by [134] to include information from the atomistic to the macroscopic scales in the prediction of thermal, mechanical and rheological properties of polymer systems. Namilae and Chandra [135] used the sequential approach to couple molecular dynamics with the finite element method to determine the effective mechanical properties of a CNT-based nanocomposite. The effective properties are determined using the finite element method where the interface is modeled using cohesive zone elements. In the cohesive zone model a traction-displacement relation is needed and this relation is determined from the stress transferred from the carbon nanotube to the matrix material for a given applied displacement. This relation is determined from pullout simulations performed in molecular dynamics where it is assumed that the matrix material and the carbon nanotube are covalently bonded to one another and the load is transferred through these bonds. The averaged transferred stress is determined by calculating the reaction forces on the bonds and dividing by the surface area of the carbon nanotube. Thereby, they overcome the problem of defining stresses at the molecular level and they also have the possibility of varying the interfacial strength by varying the number of covalent bonds between the matrix and the nanotube. The elastic modulus is determined using a representative volume element in the finite element analysis and the influence of interface properties is determined. From the analysis they find that high interfacial strengths can be obtained through covalent bonding and that the interfacial strength significantly influences the properties of the nanocomposite. A similar approach is used by [136] to determine the elastic properties of a polymer reinforced with carbon fibers where carbon nanotubes have been grown on the surface of the carbon fibers. The numerically obtained properties are compared to experimentally determined values and generally the numerical method overestimates the properties. This large difference in numerically and experimentally determined values is attributed to insufficient bonding between the carbon nanotubes, the fiber and the matrix in the produced samples compared to the values used in the numerical model. Sheng et al. [137] combined molecular dynamics with Mori–Tanaka, Halpin– Tsai and the finite element method to determine the effective properties of a clayreinforced polymer. The properties of the silicate layers are determined from molecular dynamic simulations and structural parameters such as aspect ratio and volume fraction needed in the continuum models are determined from X-ray diffraction and transmission electron microscopy analysis of clay-reinforced polymers. Based on these parameters an effective clay particle is defined, made as a lamina from silicate sheets and a gallery of some polymeric material. In the continuum models the

5.3 Multiscale modeling   

   173

effective particles are considered as either a discrete stack, an anisotropic or isotropic homogenized particle. From the analysis it is found that in nanocomposites with aligned clay particles the determined axial properties are almost independent of whether the effective particle is modeled as a discrete stack, an anisotropic or isotropic homogenized particle. However, when the particles are misaligned or the loading is non-axial the anisotropic homogenized particle accounts better for the lower properties of the interlayer galleries. Furthermore, good agreement is obtained between experimentally and numerically determined properties of clay reinforced amorphous and semi-crystalline nylon.

5.3.2 Concurrent coupled methods In the concurrent methods, modeling of the different scales is performed simultaneously and information is continuously transferred back and forth between the different length scales in a seamless way. This continuous transfer of information makes the concurrent methods more suitable for modeling systems where the different scales are strongly coupled to one another. In the concurrent coupled methods it is common that the modeling of the atomic scale is done using molecular dynamics and the continuum scale is modeled using the finite element method [126]. The continuous transfer of information between the two modeling methods or length scales is done through a transition or “handshake” zone which is a critical point in the concurrent multiscale models. In the transition region the traditional finite element mesh used to model the continuum is reduced to atomic scale and the scale is thereby gradually changed from continuum scale to atomic scale. This reduction may lead to a number of difficulties such as mesh incompatibility resulting in numerical difficulties and a nonphysical behavior of the material in the transition zone. Due to the difficulties involved in modeling the transition zone a number of different approaches for establishing the concurrent coupling is found in the literature, see e.g. [131, 138, 139] Liu et al. [125] used the atomic potentials to derive the finite element stiffness matrices. This means that this approach is formally similar to the traditional finite element method. Furthermore, this makes the approach very fast in terms of computational time. Drawbacks are, however, that the element stiffness matrices must be derived for every material under consideration due to different atomic conformation and for nonlinear systems the computational speed may not be as fast due to the way the energy minimization is carried out. Wernik and Meguid [126] used a combination of appropriate atomistic potentials and continuum theory to model the behavior of carbon nanotube reinforced polymers. The nanotubes were modeled as beams connected by rotational springs. The polymer was modeled as a homogeneous solid. The interface between the nanotube and the polymer was modeled as truss rods with constitutive behavior described through

174   

   5 Modeling mechanical properties of nanocomposites

a Lennard-Jones potential providing the van der Waal interaction. The interatomic potentials were used to provide the constitutive behavior of the beams making up the nanotubes, whereas tensile tests of the polymer provided the constitutive behavior of the polymer. The uniaxial stress in the nanotube was determined as in traditional continuum mechanics as force divided by cross sectional area. When modeling mechanical behavior of materials the basic purpose is to find the equilibrium configuration of a given model geometry subjected to external forces or displacements. In the multiscale approach this also involves the determination of the equilibrium atomic configuration. Miller and Tadmor [139] proposed the quasi continuum method, which combines atomic potentials and the finite element approach. Initially, atom, “i”, is situated at position Xi. In the deformed configuration the atom is at position xi = Xi + ui. Obviously, ui has only physical meaning at atomic sites but is treated as a continuous field. The total potential energy of the atomic system, containing N atoms, is now taken to be Etot and is the sum of the energy from each individual atom i.e. E tot =

N 

Ei (u )

i =1

where Ei (u) is the energy from atom i. At present the specific expression for this energy is not interesting since merely the principle is described. The total potential energy of the system including applied loads can now be described as ˚(u ) = E tot (u ) −

N 

fi ui

i =1

and comparing this with the traditional finite element equation for the total potential energy of an elastic body 

⁄=

V

1 flij "ij dV − 2



 V

fi ui dV +

S



'i ui dS

reveals that Etot (u) is equivalent to the first integral in the expression for π and the terms in paranthesis, the applied loads, are equivalent to the sum in the expression for ϕ (u). The purpose is now to find the atomic displacements that minimizes ϕ (u), which is similar to minimizing π in the traditional finite element method. The total potential energy is approximated such that: 1. The degrees of freedom (DOF) are substantially reduced from 3N, but fully atomistic details are retained in critical/interesting regions. 2. The total energy computation is accurately approximated without having to explicitly compute the energy of each and every atom. 3. The fully atomistic regions can evolve with deformation during the simulation.

5.3 Multiscale modeling   

   175

In relation to the first point, a key measure of a displacement field is the deformation gradient, Fij Fij =

∂x i ∂u i = Iij + ∂X j ∂X j

If the deformation gradient changes gradually on the atomic scale it is not necessary to track displacements of every atom in the region and only representative atoms (repatoms) are treated explicitly – the others are dealt with through interpolation. Thus E tot ≈ E tot, h =

N 

Ei (u h )

i =1

where uh =

Nrep



˛=1

S˛ u˛

where S˛ are the interpolation functions (similar to what is known from finite element theory) and u˛ are the displacements of the representative atoms. Since Nrep < N, the DOF are reduced. Since atomistic behavior is non-local in nature, the model must also be non-local. This is handled through a weight function i.e. the approximation of the energy now takes the form E tot, h ≈

Nrep



˛=1

n˛ E ˛ (u h )

where Nrep



˛=1

n˛ = N

This means that the sum over all the atoms is replaced by a sum over only the repatoms. The weight function nα is a weight function for repatom α which will be large for repatoms in a region in which the density of repatoms is low and vice versa. The physical interpretation of the weight functions can be viewed as the number of atoms represented by each repatom. The energy is now computed from the deformed neighboring environment through the interpolated displacements. In order to minimize the energy, i.e. to obtain the solution of the problem, derivatives of the energy are needed. These are equivalent to the forces acting on the atoms and can be calculated as

176   

   5 Modeling mechanical properties of nanocomposites

N

f˛ =

N

 ∂ Ei ∂ u h  ∂ Ei ∂ E tot, h = = S˛ h ∂ u˛ ∂ u ∂ u˛ ∂u h i =1 i =1

Through suitable approximation by only considering clusters around the repatoms the complexity is reduced ([139]). The cluster size may be determined through the nearest neighbors, and depends on a trade-off between approximation error and computational efficiency. Finally, the evolution of the atomic and microstructure is handled by using adaptive mesh generation which is also known from traditional finite element theory. The principle is to apply an error estimate which in the quasi-continuum method is basically to minimize the “error” between the “average” deformation gradient, F¯ij and the deformation gradient of each element, Fije as 

1 "e = ˝e

 ˝e

(F¯ij − Fije )(F¯ij − Fije )d ˝

 21

Now an acceptable numerical value for "e is defined and F¯ij and Fije are calculated. In elements where the displacement gradient is differing too much compared to the average value, the mesh is refined, so in principle this is an iterative procedure.

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[120] Chen H., Hu G., Huang Z. Effective moduli for micropolar composite with interface effect. Int. J. Solids Struct., 2007, 44, 8106–8118. [121] Gao X.L., Park S.K. Variational formulation of a simplified strain gradient elasticity theory and its application to a pressurized thick-walled cylinder problem. Int. J. Solids Struct., 2007, 44, 7486–7499. [122] Gao X.L., Ma H.M. Green’s function and Eshelby’s tensor based on a simplified strain gradient elasticity theory. Acta. Mech., 2009, 207, 163–181. [123] Gao X.L., Ma H. M. Strain gradient solution for Eshelby’s ellipsoidal inclusion problem. P. Roy. Soc. A-Math. Phy., 2010, 466, 2425–2446. [124] Cho J., Sun C.T. A molecular dynamics simulation study of inclusion size effect on polymeric nanocomposites. Comp. Mater. Sci., 2007, 41, 54–62. [125] Liu B., Jiang H., Huang Y., Qu S., Yu M.F., Hwang K.C. Atomic-scale finite element method in multiscale computation with applications to carbon nanotubes. Phys. Rev. B, 2005, 72, 035435.1–8. [126] Wernik J.M., Meguid S.A. Multiscale modeling of the nonlinear response of nano-reinforced polymers. Acta. Mech., 2011, 217, 1–16. [127] Karakasidis T.E., Charitidis C.A. Multiscale modeling in nanomaterials science. Mat. Sci. Eng., 2007, 27, 1082–1089. [128] Abraham F.F., Walkup R., Gao H., Duchaineau M., Diaz De La Rubia T., Seager M. Simulating materials failure by using up to one billion atoms and the world’s fastest computer: Work-hardening. Proc. Nat. Acad. Sci., 2002, 99, 5783–5787. [129] Nakano A., Bachlechner M.E., Kalia R.K., Lidorikis E., Vashishta P., Voyiadjis G.Z., Campbell T.J., Ogata S., Shimojo F. Multiscale simulation of nanosystems. Comput. Sci. Eng., 2001, 3, 56–66. [130] Baeurle S. Multiscale modeling of polymer materials using field-theoretic methodologies: a survey about recent developments. J. Math. Chem., 2009, 46, 363–426. [131] Wernik J., Meguid S. Coupling atomistics and continuum in solids: status, prospects, and challenges. Int. J. Mech. Mater. Des., 2009, 5, 79–110. [132] Rudd R.E., Broughton J.Q. Concurrent coupling of length scales in solid state systems. Phys. Status Solidi B, 2000, 217, 251–291. [133] Doi M. OCTA (Open Computational Tool for Advanced material technology). Macromol. Symp., 2003, 195, 101–108. [134] Theodorou D.N. Hierarchical modeling of amorphous polymers. Comput. Phys. Commun., 2005, 169, 82–88. [135] Namilae S., Chandra N. Multiscale model to study the effect of interfaces in carbon nanotube-based composites. J. Eng. Mater., 2005, 127, 222–232. [136] Kulkarni M., Carnahan D., Kulkarni K., Qian D., Abot J.L. Elastic response of a carbon nanotube fiber reinforced polymeric composite: A numerical and experimental study. Compos Part B-Eng., 2010, 41, 414–421. [137] Sheng N., Boyce M.C., Parks D.M., Rutledge G.C., Abes J.I., Cohen R.E. Multiscale micromechanical modeling of polymer/clay nanocomposites and the effective clay particle. Polymer, 2004, 45, 487–506. [138] Curtin W.A., Miller R.E. Atomistic/continuum coupling in computational materials science. Model. Simul. Mater. Sc., 2003, 11, R33-R68. [139] Miller R.E., Tadmor E.B. The quasicontinuum method: Overview, applications and current directions. J. Comput-Aided Mater., 2002, 9, 203–239.

V. Bavastrello, C. Nicolini

6 Polyaniline derivates and carbon nanotubes and their characterization 6.1 Introduction In the last decades, two classes of organic materials such as carbon nanotubes (CNTs), an allotrope form of carbon, and conducting polymers have been the goal of full research because of their peculiar physical chemistry properties [1–3]. The firstly observed CNTs were obtained using an arc discharge process by Iijima in 1991 and consisted in needle-like systems constituted by graphite coaxial tubes, ranging between 4 and 30 nm in diameter and with a length of a few μm [4]. The structure of these materials can be imagined as a side-to-side rolled up sheet of graphite, thus obtaining a tube of very small diameter formed by sp2-hybridized carbon that can be closed at both ends by means of two fullerenes half-units as clearly visible in Fig. 6.1, which takes into account three possible “cappings” based on fullerenes C60, C70, and C80, respectively.

C60

C70

C80

Fig. 6.1: Representation of CNTs taking into account different cappings in function of different halfunit fullerenes.

The very peculiar physical chemistry properties of CNTs that make them unique in nature are identified in high length/diameter ratio, small radius of curvature at the ends, extraordinary electric and thermal conductivity as well as mechanical properties, and high stability. Because of these characteristics CNTs have been thought of as materials with possible applications in the field of molecular electronics, sensor devices, fuel storage, and medicine. For the fabrication of CNTs, obtained as MWNTs and SWNTs, different techniques are currently taken into account and the character-

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istics of the final product can differ in relation to the experimental conditions of the process. For example, in 1993 Iijima and Bethune completed the synthesis of SWNTs having a length/diameter ratio higher than 1000 and diameters ranging between 1–2  nm, using a catalyzed carbon DC arc discharge process [5, 6]; another process carried out in 1996 by Smalley of the Rice University allowed the synthesis in bulk of 100 g per day of highly symmetrical SWNTs with a very small diameter, and in this case the team of researchers used a pulsed laser vaporization technique on a carbon target containing 1–2 % of Ni/Co as catalyst at a temperature of about 1,200 °C, and was able to produce SWNTs with a performance between 70–90 % with a symmetrical growing process, and issuing materials having an average distance of about 0.30 nm [7]. The emission threshold and the electrical properties of CNTs depend on their main hexagonal structures rearrangement, formed by sp2-hybridized carbon, with respect to the axis passing through the center of the nanotube. This dependence is related to two chiral vectors (n,m), with n and m integers describing the vector equation R = na1 + ma2. The values of n and m therefore affect the chirality (twist level) of the nanotube and consequently the conductivity and other properties. Taking into account what is described above, a nanotube is considered metallic when the value n – m is divisible by three, otherwise it is semiconducting. Consequently, when CNTs are fabricated with random values of n and m, generally 1/3 behave as metal while the remaining 2/3 behave as semiconductors with a band gap energy of about 0.6 eV and 0.7 eV. The structures of CNTs related to both metallic and semiconducting forms as well as the application of vector equation R for a nanotube are illustrated in Fig. 6.2. In the current market, CNTs are synthesized by means of different methods such as cathode arc discharge, laser vaporization or laser ablation, chemical vapor deposition, and plasma enhanced chemical vapor deposition [8–12], respectively. All these techniques need a catalyst for the synthesis of CNTs, mostly nickel, cobalt, and iron oxides [13–17]. On the basis of the different kind of methods of synthesis and catalyst it is mainly possible to obtain SWNTs rather than MWNTs. A drawback associated with the use of CNTs is their poor solubility in common organic solvents making them not processable materials. This problem can be worked out by functionalizing the nanotube with functional groups soluble in determined solvents and capable of transferring their solubility to the whole nanotube. The process of functionalization can be carried out by forming either stable covalent bonds or weak interactions such as van der Waals forces, leading to the formation of a new material called nanocomposite. It has been demonstrated that the fluorination and hydrogenation processes carried out on CNTs issued new nanocomposite materials, used as intermediate for subsequently grafting hydrocarbon side chains by means of chemical bonds [18–21]. The possibility of chemically grafting hydrocarbon side chains to the sidewall of CNTs can represent a way of “tuning” the solubility of these materials in organic solvents.

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   185

(a)

Tube axis

(b)

A na1 R

ma2 (c)

B

Ar mc ha ir

na1  ma2  R

Fig. 6.2: Representation of carbon nanotubes: (a) metallic, (b) semiconducting, and application of the vector equation R (c).

Another way of increasing the solubility of CNTs in solvents can take advantage of processes carried out without the formation of covalent bonds, obtained by embedding CNTs inside matrices soluble in organic solvents [22–25]. Nanocomposite materials obtained with this method are synthesized by means of simpler and less expensive processes than ones undergoing severe experimental conditions to form more stable covalent bonds. The possibility of synthesizing nanocomposite materials by embedding processes can lead to the use of conducting polymers as organic matrices; this method has therefore the double advantage of combining the physical chemistry properties of both CNTs and conducting polymers, obtaining at the same time solvent processable materials. Among conducting polymers it is possible to find macromolecules such as polyaniline, polythiophene, polypyrrole, polyphenylenvinylene, polyvinylcarbazole, and polyacetylene. These materials can be considered as mono-dimensional systems formed by long polymer chains containing continuous conjugated double bonds [26–29]. These macromolecules can also contain aromatic rings and are insulating materials in their natural state. The molecular structures along with the related physi-

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cal chemistry properties can be changed by performing a process known as doping [30–36]. In particular, the electric conductivity can be increased by several orders of magnitude. Further evidence of the molecular rearrangement occurring during the doping process is given by the change in color. For the synthesis of the nanocomposite materials, as hereafter discussed, polyaniline derivatives can represent very good matrices for embedding CNTs. These conducting polymers usually show good physical chemistry properties that make them very interesting for the synthesis of nanocomposite materials [37–39]. Polyaniline was one of the earliest studied and characterized conducting polymers [40–42], and along with its derivatives can be synthesized by means of either electrochemical process or oxidative polymerization [43–45]. Like all conducting polymers, polyaniline and its derivatives are good insulators in their natural state and the related physical chemistry properties can be changed by performing the doping process. This change is even visually observable because of the different color related to the undoped (blue to violet) and doped (emerald green) forms. In order to explain the increment in conductivity during the doping process of polyaniline and its derivatives, it is important to highlight the polymer chains consist of reduced and oxidized units, whose representation can be visualized as [–B–NH–B–NH] and [–B–N=Q=N], respectively, where B stands for the benzenic ring, Q the quinonic ring, N is a nitrogen atom, and H is a hydrogen atom. The concentration of different units along the polymer chains issues three different forms changing in the segment oxidation ratio of the macromolecules, known as leucoemeraldine (fully reduced), pernigraniline (fully oxidized), and emeraldine base (different percentages of both reduced and oxidized forms), respectively, as illustrated in the structure of Fig. 6.3.

R

R H N

R’

R H N

R’

R N

y

R’

N R’

1– y n

Fig. 6.3: Chemical structure of a polyaniline derivative repeat unit. According to the concentration of reduced and oxidized segments we can have three different forms: y = 1 leucoemeraldine (fully reduced), y = 0 pernigraniline (fully oxidized), and 0 < y < 1 emeraldine base (different percentages of both reduced and oxidized forms), respectively.

The most important is the emeraldine base form and its protonation by means of H+ ions, obtained by addition of protic acids, gives the emeraldine salt form, responsible for the strong increment of the conducting properties [46]. The doping process carried out with protic acids leads to the imine nitrogen [–N=] protonation and issues local distortions of the polymer chain structure respon-

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   187

sible for the increment in conductivity of the doped form. This phenomenon can be explained by the polaronic state theory that describes the formation of two different forms of protonation known as polaron and dipolaron, strictly related to the amount of doping agent and the length of macromolecules, respectively [47–50]. The chemical structures of polaron and dipolaron are illustrated in Fig. 6.4.

H (1)

N

N

H

H

H

N

N

N

H

H

H

N

N

N

(2)

N

H

H

H

H

H

(3)

N

N

N

N

N

N

Fig. 6.4: Schematic of the structure related to the undoped polymer (1), polaron state (2) and dipolaron state (3).

The polaronic state is generated by the protonation of single imine nitrogen atom while the dipolaronic state takes into account the protonation of both imine nitrogen atoms grafted on the same aromatic rings, and the movement of the distortions generated by the protonation is likely to be supported by the macromolecule’s skeleton vibrations [51]. The doping process even affects the electronic structure around the distortions, so generating electronic states in the conducting polymers band gap. The use of the Magnetic Spin Resonance (MSR) technique highlighted the electric conductivity associated with the polaronic state, which becomes effective because of unpaired electrons (conductivity with spin), while the conductivity associated with the dipolaronic state becomes effective without unpaired electrons (spinless conductivity) [52–54]. It is important to highlight that the conduction mechanism of polyaniline and its derivatives involves the polaronic state, since it was demonstrated that the absorption spectra associated with polarons and bipolarons are consistent with the co-existence of both defects within short oligomers and to the predominance of polarons in larger oligomers, and consequently in polymers [47]. The stability of the doped form is attributed to the nature of the polymer itself, which contains an extended structure of conjugated double bonds and is therefore able to stabilize the molecule due the resonance phenomenon. The presence of counter-ions coming from the doping agent provides a further grade of stability by electrically neutralizing the positive charge generated by the protonation [55–57].

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The distortions occurring along the polymer backbone during the doping process are able to affect the morphology of the deposited films by varying their organization and play an important role in the electrical properties of the conducting polymer [58]. The number and kind of substituents along the aromatic rings of the polymer backbone seems to affect the molecular rearrangement occurring during the doping process and in some cases the sterical hindrance generated by “too close” substituents to the aromatic ring is responsible for the spontaneous undoping process [59]. The use of a determined conducting polymer rather than another one is connected to the ease of synthesis, conducting properties, stability toward environmental agents, its solubility in determined organic solvents, and these characteristics are very important if these materials are taken into account for industrial applications. In the last decade of research on conducting polymers, polyaniline and its derivatives have been shown to possess appropriate characteristics to be employed for the fabrication of nanocomposites.

6.2 Synthesis of nanocomposite materials In the previous section we took into consideration oxidative polymerization for the synthesis of conducting polymers and the possibility of fabricating nanocomposite materials containing CNTs without the formation of strong covalent bonds. These kinds of materials can be synthesized by embedding CNTs inside a polymer matrix, and a very easy method to achieve this result is the polymerization of monomers in the presence of dispersions of CNTs. In Fig. 6.5 the structural formulas of some monomers used for the synthesis of nanocomposite materials are shown.

NH2

NH2 CH3

OCH3

(a)

(b)

NH2

NH2 CH3

H3C

OCH3 H3CO

(c)

(d)

Fig. 6.5: Structural formulas of monomers used for the synthesis of nanocomposite materials: (a) o-methylaniline (o-toluidine), (b) o-methoxyaniline (o-anisidine), (c) 2,5-dimethylaniline (2,5-xylidine), (d) 2,5-dimethoxyaniline.

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   189

The synthesis of polyaniline and its derivatives by means of oxidative polymerization usually takes place in acid solutions, i.e. for hydrochloric acid, at temperature between 0 °C and 4 °C, and the polymerization process begins when an appropriate oxidizing agent is added to the medium of reaction. Some drawbacks may arise when CNTs are added into the medium of reaction because of their insolubility in all common organic solvents. In this case it is possible to setup a standard method of synthesis where the only changing experimental variable is given by the pH of the medium of reaction. Therefore, the methods of synthesis of nanocomposite materials based on polyaniline derivatives and CNTs embedded into the polymer matrix can be setup in a few steps for each fabricated material, and this can be an important characteristic if we take into account possible industrial applications, where low costs and easy methods of synthesis are important in large-scale production. In detail, the steps of synthesis can be summarized as follows. The first is the preparation of the medium of reaction containing the monomers and a dispersion of CNTs. Usually CNTs are dispersed into the medium of reaction itself where the addition of the monomers subsequently follows and the dispersion process is carried out by means of ultrasonic equipment. The ultrasonication working conditions have to be setup properly in order to prevent and avoid any possible breaking process of CNTs. Once obtained, a medium of reaction containing both a solution of the monomers, since they behave as bases and are completely soluble in acid solutions, and a dispersion of CNTs, previously cooled and maintained at an appropriate temperature between 0 °C and 4 °C by means of an ice bath, the polymerization begins with the addition of the oxidizing agent. The oxidizing agent for this kind of synthesis is usually ammonium persulfate [(NH4)2S2O8] dissolved in a solution having the same composition as the medium of reaction, previously cooled in an ice bath in order to avoid temperature oscillations during the addition. The amounts of CNTs and oxidizing agent added into the medium of reaction for the synthesis are calculated vs. the amount of monomers. Specifically, the amount of CNTs takes into account a monomer/MWNTs weight ratio of 100/1, while the amount of oxidizing agent takes into account a monomer/oxidant molar ratio of 4/1 [58,59]. In Table 6.1 an example of the amounts of reagents used for the synthesis of different nanocomposite materials, starting from different monomers, is shown. After the addition of the oxidizing agent the polymerization proceeds under continuous stirring, following the scheme of reaction shown in Fig. 6.6. The reaction is considered complete after 12 hours and the product of synthesis, which is found in the doped form due to the acid environment of the medium of reaction, undergoes a few treatments before obtaining the final pure nanocomposite material. These treatments are possible when the crude material from the synthesis is turned into the undoped form, due to the complete insolubility of the doped form in all solvents. For this reason, the product of synthesis is treated with a solution of ammonium hydroxide (NH4OH), which is capable of removing the H+ ions from the imine group basic sites and therefore turns the doped form into the undoped one. Further steps

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Table 6.1: Amounts of reagents used for the synthesis of different nanocomposite materials, starting from different monomers, calculated as monomer/MWNTs weight ratio of 100/1 and a monomer/oxidant molar ratio of 4/1, respectively.

Nanocomposite Material

MWNTs (mg)

SWNTs (mg)

Monomer (g)

Oxidant (g)

POTO-MWNTs POTO-SWNTs POAS-MWNTs POAS-SWNTs PDMA-MWNTs PDMA-SWNTs PDOA-MWNTs PDOA-SWNTs

100.8 ~ 99.2 ~ 99.5 ~ 100.2 ~

~ 100.5 ~ 100.7 ~ 100.9 ~ 99.7

10.08 10.05 9.92 10.07 9.95 10.09 10.02 9.97

5.32 5.30 4.56 4.63 4,65 4,71 3,71 3,69

R 4n

NH2



5 n (NH4)2S2O8



5 n HCl

R’

R

R NH

R’

R NH

R’



R’

3 n HCl



R NH

NH

Cl

Cl

n

R’

5 n (NH4)2SO4

Fig. 6.6: Scheme of reaction related to oxidative polymerization of polyaniline derivatives by means of ammonium persulfate in hydrochloric acid aqueous solution.

for the removal of the oligomers, shorter polymer chains formed during the synthesis, take into account a first treatment with methyl alcohol (CH3OH) and subsequently with diethyl ether (CH3OCH3). It is important to underline that among the various steps there is always a filtration process. After removing the oligomers, the final treatment is the elimination of residue solvents under vacuum. In Fig. 6.7 the schematic of the apparatus necessary to carry out the synthesis of nanocomposite materials is shown. The final product consists of thin grain powder completely soluble in chloroform, and the processability in this solvent is very important for the characterization of these materials by means of several techniques where the preparation of solutions and thin films plays an important role for the determination of the related physical chemistry properties.

6.3 Characterization of nanocomposite materials   

   191

Oxidant feeding

Ice

Monomers + CNTs

Magnetic stirrer

Fig. 6.7: Schematic of the apparatus employed for the synthesis of nanocomposite materials by oxidative polymerization.

6.3 Characterization of nanocomposite materials and characterization of their physical chemistry properties 6.3.1 Deposition of thin films by Langmuir–Schaefer technique and study of the pressure-area isotherms The Langmuir–Schaefer (LS) technique plays an important role in the deposition of thin films of nanocomposite materials and the preparation of samples for further characterization. For many applications it is desirable to have these materials in thin films structures, preferably with known thicknesses and molecular packing [60]. The Langmuir–Blodgett and LS techniques offer a unique control over architecture, thickness, and molecular orientation and have been proven to be powerful tools for the fabrication of thin polymeric films with controlled structures [61, 62]. Generally, when a monolayer is fabricated at the gas–liquid or liquid–liquid interface, the film is named Langmuir film. A Langmuir film can be deposited on a solid surface and is thereafter called either Langmuir–Blodgett film, in the case of vertical deposition, or LS film, in the case of horizontal deposition. In other words, LS is seen just as a variant of the Langmuir– Blodgett technique. LS assemblies are particularly attractive as they allow a very high control of the layer thickness, and require a very small amount of polymer material in contrast to the solution casting or spin coating techniques [63], respectively. The LS technique basically consists of a container, called trough, which contains a liquid subphase on which the monolayer is spread, a couple of barriers performing the film compression, and other apparatus of measurement for the characterization of the spread monolayer, such as surface pressure sensors and position detectors attached to the barriers to measure the surface area of the spread films. The container

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must be made of inert material, such as Teflon, which does not react with the subphase, and the complete setup of trough and barriers provides a means to constrain and compress the monolayer. The material is dissolved in an appropriate organic volatile solvent, e.g. chloroform in the case of polyaniline derivatives and related nanocomposite materials, and subsequently spread at the air–liquid interface. The solvent evaporates in a short time and the macromolecules monolayer remains spread over the whole surface of the subphase until it is compressed and subsequently aligned in a regular arrangement, as illustrated in the schematic of Fig. 6.8.

Balance Barrier

Barrier Subphase Trough

(a)

Balance Barrier Compression (b)

Barrier Compression Subphase Trough

Fig. 6.8: Schematic of the monolayer compression process leading to regular arrangement of molecules spread at the air–liquid interface.

Repeated dipping of the substrate results in the deposition of a multilayer structure, which in some cases can be up to hundreds of monolayers. The monolayer compression process is also very important for the calculation of the area per molecule value. Surface pressure–area isotherms, π–A isotherms or simply isotherms can be defined as a measurement, at constant temperature, of the surface pressure as a function of the available area per each molecule in a floating monolayer (Langmuir film), as shown in Figs. 6.9 and 6.10 where pressure–area isotherms related to POAN-MWNTs and POTO-MWNTs nanocomposite materials are obtained by spreading on water and 0.1 hydrochloric acid aqueous solution, respectively. It is therefore possible to use the pressure–area isotherms to calculate the related area/repeat unit ratio, where for the repeat unit the structure shown in Fig. 6.11 is intended.

6.3 Characterization of nanocomposite materials   

   193

60

Surface pressure (mN/m)

50

40

30

20

10

0

0

10

20

30

40

50

60

70

80

90

Area per molecule (A2/repeat unit)

Fig. 6.9: Pressure–area isotherms and related calculation of the area per molecule obtained for POAN-MWNTs nanocomposite material in the undoped (blue solid line) and doped (green solid line) forms, respectively.

80 70

Surface pressure (mN/m)

60 50 40 30 20 10 0

0

10

20

30

40

50

60

70

80

Area per molecule (A2/repeat unit)

Fig. 6.10: Pressure–area isotherms and related calculation of the area per molecule obtained for POTO-MWNTs nanocomposite material in the undoped (blue solid line) and doped (green solid line) forms, respectively.

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H

H

N

N

N

N

Fig. 6.11: Repeat unit of polyaniline derivatives taking into account the calculation of the area per molecule values expressed as area/repeat unit.

Taking advantage of the pressure–area isotherms study it was possible to study the sterical hindrance effect of different substituents (CH3– and CH3O– groups) along the aromatic rings on the conformational change occurring to the macromolecules on doping with protic acids, obtained by spreading LS films in a Langmuir–Blodgett trough. In this case, chloroform solutions of nanocomposite materials based on poly(omethoxyaniline) (POAN) and poly(o-methylaniline) (POTO) embedding MWNTs were spread on either distilled water or 0.1 M HCl aqueous solution (pH = 1) as subphases in order to obtain films of nanocomposites in both undoped and doped forms, respectively [64]. The experimental results highlighted that different substituents are able to affect in different grades the polymer chains conformational changes during the doping process and consequently the macromolecules’ packing grade. Furthermore, the presence of MWNTs inside the polymer matrix promotes more stretched conformations of macromolecules since the nanocomposite materials showed higher area/ repeat unit values with respect to the corresponding pure conducting polymers when spreading at the same air–liquid interface [65]. The study of pressure–area isotherms and the related information on the macromolecules’ packing grade provide an important support for the explanation of some properties of these nanocomposite materials such as the electrochemical behavior and the conductivity, where rearrangements of the polymer chains’ spatial conformation usually take place.

6.3.2 UV-vis spectroscopy and band gap calculations UV-vis spectroscopy is a widely applied technique of investigation for the characterization of polyaniline derivatives and therefore for nanocomposite materials; the wavelength range of spectra acquisition is usually included between 250  nm and 1,000 nm. Optimized sample can be prepared by depositing thin films of material in the undoped form on quartz substrates by means of LS technique. The doped form can be subsequently obtained after dipping the substrates into acid solutions, i.e. hydrochloric acid, for determined periods of time in order to perform a complete doping process. It is interesting to highlight that UV-vis spectra can be used to assess the thin film growing process after deposition of several monolayers, as illustrated in Fig. 6.12.

6.3 Characterization of nanocomposite materials   

   195

0.7

0.6

Absorbance (AU)

0.5

0.4

0.3

0.2

0.1

0 250

500

750

1000

Wavelength (nm)

Fig. 6.12: UV-vis spectrum of POAN-nanocomposite material obtained by recording after each five depositions; this method is useful to assess the growing process of the thin film.

The study of the UV-vis spectra recorded for nanocomposite materials in the undoped form shows that the oxidative polymerization usually leads to the formation of polymer chains in the emeraldine base form, evidenced by the presence of two characteristic bands as shown in the spectrum in Fig. 6.13. The first is assigned to the π–π* interband transition in the benzoid/quinoid ring structure, and the second to the n–π* transition from the nonbonding nitrogen lone pair to the conduction band π*, respectively [66]. A full study focused on the analysis of UV-vis spectra, carried out on different nanocomposite materials synthesized from four different monomers, highlighted, by the comparison between spectra of pure conducting polymers and related nanocomposite materials, that the presence of either MWNTs or SWNTs inside the polymer matrix produces no change in the π–π* transition, and this result suggests that the polymer chains simply wrap up around CNTs with no strong interaction [67, 68]. There is evidence that CNTs work as nucleation points where the polymerization begins, and the results obtained from the analysis of UV-vis spectra prove the presence of growing polymer chains associated with a wrapping process around CNTs [69]. The final result is CNTs embedded in the polymer matrix without the formation of covalent bonds. Contrarily, the n–π* transitions can show a perceptible red shift with respect to the corresponding pure conducting polymers, depending on the kind and number

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0.8

0.6

Absorbance (AU)

pp* 0.4 np*

0.2

0 250

500

750

1000

Wavelength (nm)

Fig. 6.13: UV-vis spectrum of POAN-nanocomposite material; it is possible to observe the presence of two bands due to the π–π* interband transition in the benzoid/quinoid ring structure at 310 nm, and the second at the n–π* transition from the nonbonding nitrogen lone pair to the conduction band π* at 620 nm, respectively.

of substituents along the aromatic ring [68]. Specifically, it was demonstrated the presence of electron donor substituents corresponds to lower energy in the n–π* transitions, and subsequently to major availability of the imine group basic sites lone pairs, responsible for the proton doping process illustrated in the schematic of Fig. 6.14. The same effect is not shown when the substituents are not electron donors. These results therefore give proof that the presence of CNTs inside the polymer matrix is able to affect the properties of the CH3O– group by enhancing the electron donor effect and thus the availability of the lone pairs of imine groups. This change in the optical properties is observed, i.e for nanocomposite materials based on POAN and poly(2,5-dimethoxyaniline) (PDOA) with CNTs embedded in the polymer matrix, while for nanocomposite materials based on POTO and 2,5-dimethylaniline (PDMA), where the aromatic rings of the polymer chains have CH3– groups as substituents, there is no evident change [68]. The analysis of the UV-vis spectra obtained after doping with protic acid is very important to assess the formation of the polaronic state responsible for the enhancement of conducting properties, evidenced by the presence of the relative bands due to the polaron–π* and π–polaron transitions, as shown in the spectrum of Fig. 6.15.

6.3 Characterization of nanocomposite materials   

R

R

R

H N

H N

   197

R N

N n

R’

R’

R’ 2H

2H



2Cl

2Cl R

R H N • Cl

R’

R’

R H N

R’

R H N • Cl

R’

H N n R’

Fig. 6.14: Doping process of an emeraldine base form repeat unit by means of hydrochloric acid; the protonation takes place on the imine groups basic sites.

0.8

p-polaron

0.6

Absorbance (AU)

polaron-p*

0.4

0.2

0 250

500

750

1000

Wavelength (nm)

Fig. 6.15: UV-vis spectrum of material; it is possible to observe the presence of two bands due to the polaron–π* at 390 nm and π–polaron transitions at 880 nm, respectively.

198   

   6 Polyaniline derivates and carbon nanotubes and their characterization

It was shown that the polanonic state is affected by the number and kind of substituents along the aromatic rings of the polymer backbone. In fact, the presence of the CH3– group seems to have a stronger effect than the CH3O– one on the doped macromolecules. The explanation takes into consideration the fact that the doping process generates a change in macromolecule configuration from planar (sp2 hybridization) to tetrahedral (sp3 hybridization) when protonation by H+ ions takes place on the imine groups basic sites’ nitrogen atoms, and this change in the atoms’ spatial disposition is affected in different levels by the presence of different substituents, i.e. CH3– and CH3O– ones. Besides the sterical hindrance, the electron donor effect of the CH3O– group favors the availability of lone pairs on imine group basic sites and consequently their protonation [68]. In the light of these results, there is the possibility of “tuning” the physical chemistry properties of polyaniline derivatives-CNTs nanocomposite materials in relation to the kind and number of substituents along the aromatic rings of the monomers used in the synthesis for the fabrication of materials with specific characteristics. UV-vis spectra can be successfully used also for the calculation of the ban gap by applying the Tauc equation (Equation (6.1)): ˛=B

(hv − Eg )n

hv

(6.1)

where α is the absorption coefficient, B is a fitting parameter, h is the Planck constant, ν is the photon frequency, Eg is the band gap, and n takes into account different possible electronic transitions responsible for the light absorption. On considering polyaniline and its derivatives, n = 1/2 [70], and the calculations of Eg are obtained from UV-vis spectra by plotting (αhν)2 vs. hν; the energy gap is obtained by the intercept on the abscissa of the best fitting of Equation (6.1), as shown in the example of Fig. 6.16. The calculations are carried out by taking into account the π–π* interband transition that is considered a good estimation of the material band gap [69]. Applications of the Tauc equation for the calculation of the undoped material band gaps showed no differences among mono- and disubstituted aromatic rings, either considering the same or different groups. This result thus supports the “wrapped up configuration with no covalent bonds” of conducting polymer macromolecules when CNTs embed in the polymer matrix [68].

6.3.3 Cyclic voltammetry Cyclic Voltammetry (CV) is a very effective and versatile electroanalytical technique, based on electrochemical measurements employing microelectrodes and an unstirred solution, so that the measured current is limited by diffusion of analytes at the electrode surface.

6.3 Characterization of nanocomposite materials   

   199

0.6

0.5

(ahu)2 (cm–2 eV2)

0.4

0.3

0.2

0.1

0 0

2

4

6

hu (eV)

Fig. 6.16: Band gap calculation from a UV-vis spectrum of POAN-MWNTs nanocomposite material; the value is obtained by plotting (αhν)2 vs. hν and the intercept on the abscissa of the best fitting of the Tauc equation.

The goal is the study of redox systems obtained by rapidly scanning a working electrode potential in search of redox couples. In a CV experiment, the working electrode potential is therefore ramped linearly versus time and this ramping is known as the scan rate (V/s) of the experiment, as shown in the cyclic voltammogram of Fig. 6.17. The potential is applied between the reference electrode and the working electrode while the current is measured between the working electrode and the counter electrode, and the experimental data obtained by applying this setup are subsequently plotted as current (I) vs. potential (E) (see Fig. 6.17). The scan process produces a current peak any time the analytes are reduced or oxidized, depending on the initial scan direction through the range of the scanned potential. During the redox processes the current thus increases as the potential reaches the reduction potential of the analyte, and then follows a decrement as the concentration of the analyte is depleted close to the electrode surface. If the redox couple is reversible when the potential sweeps back, the product formed in the first oxidation undergoes a reduction process and produces a current of reverse polarity from the forward scan. In most cases, the oxidation and reduction peaks are likely to have similar shapes, and the final result is to obtain information related to the redox potentials and electrochemical reaction rates of the materials.

200   

   6 Polyaniline derivates and carbon nanotubes and their characterization

5m V/s 10m V/s 20m V/s 50m V/s 100m V/s

1

200 uA

2 3

6

5 4

500

0

500

1000

1500

E/V (mV) vs Ag/AgCl

Fig. 6.17: Poly(o-methoxyaniline)-MWNTs nanocomposite material cyclic voltammogram obtained at different scan rates in a three electrodes configuration cell containing 0.1 M HCl aqueous solution. Arrows show the direction of the scan. Peaks (1/6) and (3/4) are related to leucoemeraldine/emeraldine and emeraldine/pernigraniline transitions, while the intermediate one (2/5) is due to polymer destruction and cross-linking.

The peak current, ip expressed in amperes, is described by the Randles–Sevcik equation: ip = (2.69 × 105) n3/2 A C D1/2 ν1/2

(6.2)

where n is the number of electrons transferred in the reaction, A is the area of the electrode (cm2), C is the analyte concentration (mol/cm3), D is the diffusion coefficient expressed in (cm2/s), and ν is the scan rate of the applied potential (V/s). The potential difference between the reduction and oxidation peaks is theoretically 59 mV for reversible reactions even though, in practice, the difference is typically 70–100 mV. Larger differences, or nonsymmetric reduction and oxidation peaks are an indication of a nonreversible reaction. The diffusion coefficient of electroactive species can be therefore determined by applying the relationships defined in Equation (6.2). Linear plots of ip vs. ν 1/2 provide evidence for chemically reversible redox processes in relation to the cases where they cause major structural changes in the analyte. For species where the diffusion coefficient either is known or can be estimated, the slope of the plot of ip vs. ν 1/2 provides information related to the stoichiometry of the redox process. Interesting applications of this technique for the characterization of nanocomposite materials were obtained by means of a three electrodes cell configuration where a silver chloride (AgCl) electrode acted as reference electrode, a platinum (Pt) one acted as counter electrode, and nanocomposite materials deposited as thin film of 60 LS monolayers on Indium Tin Oxide (ITO) glass plates acted as working electrode.

6.3 Characterization of nanocomposite materials   

   201

In this work [64], CV was performed in a cell containing 0.1 M HCl aqueous solution to investigate the redox transitions and the influence on this process of the substituents along the aromatic ring in association to MWNTs inside the polyaniline derivative matrix. The study of the voltammograms obtained at different sweep rates highlighted that the polyaniline derivatives containing less hindering substituents, such as CH3O– groups, along the aromatic rings undergo all the oxidation transitions with limited formation of degradation products; on the contrary, the presence of substituents with major sterical hindrance, such as CH3– groups, shows an extra production of degradation products. The support given by the CV technique by the analysis of the related voltammograms thus led to the conclusion that an increment of the substituents’ sterical hindrance generates slower electrochemical systems because of its capability to affect the conformational changes occurring in the redox transitions, which deeply strain the conducting polymer macromolecules. The analysis of redox peaks related to pure conducting polymers and related nanocomposite materials also highlighted that the presence of MWNTs inside the polymer matrix only issued a shift to higher potentials in the presence of electron donor substituents along the polymer chains, such as CH3O– group, which are able to promote the electron exchange during the redox transitions. Therefore, this kind of substituent not only diminishes the sterical hindrance responsible for the decrement in macromolecules’ degrees of freedom but, in association with MWNTs in the polymer matrix, even favors the electron exchange during the redox processes, while CH3– non-electron donor group gives no support to the electron exchange.

6.3.4 Determination of specific conductivity The study of nanocomposite materials’ conducting properties related to the doping process plays an important role in evaluating their possible applications. The specific conductivity can be calculated from specific resistance associated with the material based on current measurements, as a function of applied potential, by means of the first and second Ohm Equations: V=R·I

(6.3)

R=ρ·l/s

(6.4)

where R is the resistance (Ω), V is the potential (V), I is the current (A), ρ is the specific resistance (Ω × cm), l (cm) and S (cm2) are the length and the section of the conducting material. If we take into account a thin film deposited on a substrate, we can express Equation (6.4) as:

202   

   6 Polyaniline derivates and carbon nanotubes and their characterization

R=ρ·l/w·h

(6.5)

where l is the distance between contacts (cm), w is the width (cm), and h is the thickness (cm), respectively, according to the schematic of Fig. 6.18.

W

t ac nt o C

Material Substrate l

t ac nt o C h

Fig. 6.18: Schematic of dimensional coordinates of a thin film deposited on a substrate.

The specific resistance can be subsequently calculated from Equation (6.5): ρ=R·w·h/l

(6.6)

σ=1/ρ

(6.7)

For the current vs. resistance acquisitions an electrometer is contacted to the substrate by means of an experimental setup obtained on depositing thin films of nanocomposite materials by LS technique on a glass substrate. In Fig. 6.19 two setups for different methods of contacting the electrometer to the substrate are illustrated. One setup can be obtained by evaporating a layer of gold at both ends of a glass slide and the nanocomposite material is subsequently deposited to make contact. Another method can be performed by firstly depositing the nanocomposite material on a glass slide and then making contact by means of silver paste. Contacting obtained by means of silver paint was successfully used to determine and compare the specific conductivity of a nanocomposite materials and the corresponding pure conducting polymer as well as the specific conductivity of two nanocomposite materials synthesized by using different monomers. In the first case, it was possible to study the effective contribution given by the presence of MWNTs inside the polymer matrix to the conducting properties with respect to the pure polymer [58], while in the second it the influence of different substituents along the aromatic rings on the conformational changes occurring during the doping process and consequently to the related conductivity was demonstrated [64].

6.3 Characterization of nanocomposite materials   

Gold contact

   203

Gold contact

Nanocomposite material thin film

(a) Glass substrate

Silver wire Silver paint

Silver wire Nanocomposite material thin film

Silver paint

(b) Glass substrate

Fig. 6.19: Different setups used for contacting the electrometer to the substrate: (a) the contacting is obtained by means of gold; (b) the contacting is obtained by means of silver paint.

6.3.5 Nanocomposite materials and their possible applications The methods of investigation so far described are important for the characterization of nanocomposite materials, and interesting results were obtained from the study of PDMA-MWNTs nanocomposite material, synthesized by oxidative polymerization. UV-vis spectroscopy, cyclic voltammetry, and specific conductivity measurements were carried out in order to study the associated spontaneous undoping process and the possible applications of this material [59]. The experimental data highlighted that the presence of two methyl groups along the aromatic rings (see the structure formula of monomer illustrated in Fig. 6.5c) and MWNTs in the polymeric matrix are responsible for the instability of the nanocomposite material doped form. This behavior is caused by the sterical hindrance generated by two substituents and the simultaneous presence of MWNTs in the polymeric matrix, and the synergetic effect of both CH3– groups and MWNTs partially impede the macromolecules’ conformational changes that take place on doping and during the redox transitions. This instability was verified by the analysis of UV-vis spectra associated with the doped form because of the absence of the typical bands due to the polaron state [59], and the CV confirmed the experimental results because the related voltammograms showed only two redox peaks, because of the absence of emeraldine/pernigraniline

204   

   6 Polyaniline derivates and carbon nanotubes and their characterization

transition in the range of potential where usually polyaniline derivatives show all redox processes [59]. The final result is the spontaneous reversible undoping process. The calculation of the specific conductivity highlighted that this material possesses poor conducting properties, thus supporting the experimental data obtained from previous characterizations. Interestingly, even though this nanocomposite material cannot find application in molecular electronics, it can find application in the field of sensor devices for the determination of acid vapors.

Acknowledgements This project was supported by grants to Fondazione EL.B.A., by MIUR (Italian Ministry of Education, University and Resaerch) for “Funzionamento” and by a FIRB ItalNanoNet (RBPR05JH2P) from MIUR to CIRSDNNOB of the University of Genoa.

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Index ab initio method 147 abrasion resistance 106 additives 94 adhesive strength 67 adsorptive property 94 agglomeration 94 aggregation 94 allotropic modification 97 antistatic coatings 99 armchair 97 aspect ratio 93 asymmetry axis 97 barrier resistance 65 biocompatibility 100 biomedical applications 78 Brabender 104 carbon – nanofiber 93 – nanotube 93 cation exchange capacity 94 cerium – molybdate hollow nanocomposites 8 – oxide hollow nanocomposites 4, 16, 17 – titanium oxide hollow nanocomposites 9 chain extenders 100 chaotic mixer 105 chemical vapor deposition 96 chiral 97 chloride nanotraps 14 coatings 23, 31 coefficient of friction 128 compatibility 94 compatibilizer 95 complex viscosity 104 compliance 156 compressive strength 119 concentration tensors 156 concurrent coupled methods 173 conducting plastics 98 conductive – heating 105 – network 106 continuous phase 101 continuum mechanical modeling 155

continuum mechanics 155 corrosion resistant 99 covalent modification 98 crystallinity 62, 105 deformation gradient 175 delamination 101 dielectric – behavior 104 – properties 77 – relaxation spectroscopy 77 dispersed phase 96 dispersion 93 dispersion effects 166 distribution 96 Dvorak – Srinivas Model 162 effective Medium Field Approximation 163 elasticity 106 electrical – conductivity 71, 106 – percolation 105 – property 104 – volume resistivity 105 electromagnetic interference shielding 104 elongation at break 105 end-tethered 101 environmentally friendly processing 106 equation of motion 147 equivalent eigenstrains 158 equivalent inclusion method 159 Eshelby’s thought experiment 157 exfoliation 101 flame – retardancy 103 – retardant 68 flexibility 97 Flexural strength 119 flocculation 101 force fields 148 functionalization 94 functional materials 104 gallery spacing 94 gas barrier property 103 glass transition temperature 104

210   

   Index

graphene 37, 96 graphitic sheets 97 hardness 103 hard segment 100 Hashin-Shtrikman 162 heat and oil aging 104 heat distortion temperature 103 hierarchical methods 171 Hooke’s generalized law 156 hydrophilic 95 hydrophobic 95 hydroxides 36 inclusion 158 inhomogeneity 158 in situ intercalative polymerization 102 intercalation 101 interfacial interaction 93 interfacial shear strength 133 interlaminar fracture toughness 119 interlaminar shear strength 119 layered silicate 93 Lennard-Jones potential 149 loss modulus 104 low cost 97 magnetic hollow submicrocomposites 10 maleic anhydride grafted polypropylene 104 mass density 97 material properties 93 matrix phase 96 mechanical properties 46, 104, 145 melt – blending 105 – extrusion 105 – intercalation 93 – mixing 105 – processing 102 – viscosity 102 micromechanical modeling 155 micromechanics 156 microphase separation 106 microscale 97 miscibility 94 misfit strains 158 modeling 145 modification 94

modified organoclays 103 molecular – diffusion 102 – dynamics 147 – modeling 146 montmorillonite 94 Mori – Tanaka Model 161 morphology 39, 101 multi-functional 99 multi-scale modeling 170 multi-scale phase reinforced composites 111 multi-walled CNT 97 nanocomposites 145 nano-engineered fibers 115 nanofiller 93 nanofiller-modified matrix 114 nanofillers 34, 38 nanomechanical 31 nanomechanical modeling 155 nanoparticles 101 nanoscale 93 nanostructured materials 93 nanostructure morphology 104 nanotechnology 101 non-polar 102 organically modified LS 103 organically modified MMT 95 organoclay 103 organosilicates 103 orientation effects 163 percolation threshold 106 phase separation 100 photocatalytic action 18, 26 phyllosilicate 94 plasma treatment 98 platelet 102 polar 102 polyaddition polymerization 100 polyester 100 polyether 100 polyisocyanate 99 polymer – matrix 101 – melt 106 – nanocomposites 93 polyol 99

Index   

   211

polyurethane 33 polyurethane nanocomposites 103 potential energy 148, 156 potentials 148 processing 105 processing conditions 106 proton nuclear magnetic resonance 105

structure–properties relationships 93 surface – area 93 – energy 95 – reactivity 94 symmetry axis 97 synergistic effects 97

quasi continuum method 174

tensile modulus 105 tensile strength 105 thermal – conductivity 76, 106 – expansion 104 – properties 55 – stability 55, 103 thermodynamic incompatibility 100 thermo-oxidative stability 105 thermoplastic polyurethane 100 titanium oxide hollow nanocomposites 6, 9, 23 transformation strains 158 turbostratic graphite 97 twin screw extruder 104

radiation induced graft polymerization 98 reactivity 96 rectorite 104 reinforcement 103 reinforcing 94 representative volume element 146 resistive heating 105 Reuss compliance 158 scale effects 169 segmented block copolymers 100 self-Consistent Model 161 sensors 98 sequential coupled methods 171 shape memory behavior 103 shape recovery ratio 105 shear stress 102 silicates 35 single-walled CNT 97 SiO2–CaO nanocomposites 12 soft segment 100 solution intercalation 102 sp2 type 97 sp3 type 97 specific wear rate 127 stiffness 156 storage modulus 104 strain gradient parameter 169 structure 106

urethane 99 van der Waals force 97 vapor grown carbon nanofiber 96 viability 19, 20, 21, 22, 23 virial theorem 153 viscosity 104 Voigt stiffness 158 water absorption 103 water traps 13, 14 Young’s modulus 105, 152 zigzag 97