Electrospun Nanofibrous Technology for Clean Water Production (Nanostructure Science and Technology) [1st ed. 2023] 9819954827, 9789819954827

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Nanostructure Science and Technology Series Editor: David J. Lockwood

Rasel Das   Editor

Electrospun Nanofibrous Technology for Clean Water Production

Nanostructure Science and Technology Series Editor David J. Lockwood, FRSC National Research Council of Canada Ottawa, ON, Canada

Nanostructure science and technology now forms a common thread that runs through all physical and materials sciences and is emerging in industrial applications as nanotechnology. The breadth of the subject material is demonstrated by the fact that it covers and intertwines many of the traditional areas of physics, chemistry, biology, and medicine. Within each main topic in this field there can be many subfields. For example, the electrical properties of nanostructured materials is a topic that can cover electron transport in semiconductor quantum dots, self-assembled molecular nanostructures, carbon nanotubes, chemically tailored hybrid magnetic-semiconductor nanostructures, colloidal quantum dots, nanostructured superconductors, nanocrystalline electronic junctions, etc. Obviously, no one book can cope with such a diversity of subject matter. The nanostructured material system is, however, of increasing significance in our technology-dominated economy and this suggests the need for a series of books to cover recent developments. The scope of the series is designed to cover as much of the subject matter as possible – from physics and chemistry to biology and medicine, and from basic science to applications. At present, the most significant subject areas are concentrated in basic science and mainly within physics and chemistry, but as time goes by more importance will inevitably be given to subjects in applied science and will also include biology and medicine. The series will naturally accommodate this flow of developments in the sciences and technology of nanostructures and maintain its topicality by virtue of its broad emphasis. It is important that emerging areas in the biological and medical sciences, for example, not be ignored as, despite their diversity, developments in this field are often interlinked. The series will maintain the required cohesiveness from a judicious mix of edited volumes and monographs that while covering subfields in depth will also contain more general and interdisciplinary texts. Thus the series is planned to cover in a coherent fashion the developments in basic research from the distinct viewpoints of physics, chemistry, biology, and materials science and also the engineering technologies emerging from this research. Each volume will also reflect this flow from science to technology. As time goes by, the earlier series volumes will then serve as reference texts to subsequent volumes.

Rasel Das Editor

Electrospun Nanofibrous Technology for Clean Water Production

Editor Rasel Das Department of Chemistry Stony Brook University Stony Brook, NY, USA

ISSN 1571-5744 ISSN 2197-7976 (electronic) Nanostructure Science and Technology ISBN 978-981-99-5482-7 ISBN 978-981-99-5483-4 (eBook) https://doi.org/10.1007/978-981-99-5483-4 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

It has been an enormous challenge that exists worldwide is the unavailability of pure and safe water facilities. Even though developed countries have relatively good access to clean and safe water, it has remained a serious issue in many poor and underdeveloped nations. The situation is even getting worse as new water contaminants are emerging day by day, contributing to the spread of diseases transmitted through the water and bringing related adverse health issues. To solve this problem, electrospinning-based materials research has shown promising results in getting novel water purification technology or overhauling several existing water purification strategies like adsorption, catalysis, filtration, and membrane-based procedures, etc. Due to several advantages over traditional methods, electrospun nanofibers have found as a promising water purification material. Electrospinning offers controlled parameters for tuning the material’s properties, is relatively low-cost and less energyintensive, and does not require specialized personnel for operation and maintenance. Also, the method is simple and scalable that can produce fibers at macro-, micro-, and nano-scales with high surface area and porosity, which typically endorse the efficient removal of contaminants from water. Moreover, the nanofibers produced by electrospun techniques could be easily functionalized with other molecules or compounds and different nanoparticles that make composites highly selective toward specific pollutants (a significant requirement to treat industrial effluent). We attempt to give a comprehensive overview of the state-of-the-art in this book which first begin with a chapter(s) highlighting the principles of electrospinning cum different methods and engineering of electrospinning followed by various characterizations of electrospun nanofibers. Following these two chapters, we broadly introduced five more chapters, mainly answering how someone could effectively use electrospun nanofibers for water decontamination through adsorption, catalysis, membrane filtration, and phase (oil/water) separation and membrane-based distillation and pervaporation. The authors of these chapters have critically examined the material’s properties which will ensure a clear idea about the use of electrospun

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nanofibers in diverse water treatment procedures. The book also covers the challenges and restrictions currently associated with electrospinning and potential future lines of inquiry. Different experts worldwide write this book based upon our target readers, like researchers and professionals in the water treatment industry and graduate and advanced undergraduate students majoring in materials science, chemical engineering, and related disciplines. We hope that anyone interested in learning more about the possibilities of electrospinning for water purification will find this book helpful. Last, I would like to thank the contributing authors for the given chapters and the Springer team, who have greatly supported the publishing of this book. Stony Brook, NY, USA

Rasel Das

Contents

1 Introduction on Electrospun Nanofibers for Water Decontamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rasel Das

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2 Methods and Engineering of Electrospinning . . . . . . . . . . . . . . . . . . . . . Tomasz Blachowicz and Andrea Ehrmann

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3 Characterization of Electrospun Nanofibers . . . . . . . . . . . . . . . . . . . . . . Archana Samanta, Pratick Samanta, and Bhanu Nandan

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4 Electrospun Nanofibers Adsorbent for Water Purification . . . . . . . . . . Elham Tahmasebi and Roghayeh Ebadollahi

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5 Electrospun Nanofibers for Water Purification as Catalyst . . . . . . . . . 123 Pratick Samanta, Archana Samanta, and Bhanu Nandan 6 Electrospun Nanofibers for Membrane-Based Water Filtration . . . . . 153 Ragib Shakil, Yeasin Arafat Tarek, Md. Rabiul Hasan, Mahamudul Hasan Rumon, Rasel Das, and Al-Nakib Chowdhury 7 Electrospun Nanofibers for Oil–Water Separation . . . . . . . . . . . . . . . . . 181 Lin Zhang, Jing Wang, Saisai Lin, and Jing Dou 8 Electrospun Nanofibers for Water Distillation and Pervaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 Lin Zhang, Saisai Lin, and Zhikan Yao Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

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Chapter 1

Introduction on Electrospun Nanofibers for Water Decontamination Rasel Das

1.1 Background Developing fiber-based technology in continuous or noncontinuous ways for various applications has been introduced previously. Before developing synthetic fibers, nature decided to use bio-fibers or filaments to protect the plant and human body. For example, cellulose fibrils and various protein filaments provide sufficient strength in plants and human tissues, respectively. Also, a spider could successfully use web fibers from proteinaceous spider silk to catch its prey. Similarly, silkworm typically generates continuous filaments, a composite biopolymer that works with different environmental threats. This evidence suggests fibers have suitable physicochemical properties for tackling various challenges, which drive scientists to fabricate bio-inspired materials with comparable features. Nowadays, researchers commonly use synthetic polymers to fabricate fibers, having different shapes, diameters, length, and so on based on numerous traditional spinning routes like gel, wet, dry, and melt techniques [1]. The major challenge for these methods is to produce the fibers at nanoscale. On the other hand, viscoelastic liquid was successfully used to draw fibers in 1887 using an external electric field, according to Charles V. Boys report [2]. His innovation introduces the concept of electrospinning which has been extensively optimized by many authors over time through different studies which is comprehensively reviewed by Frenot and Chronakis in 2003 [3]. Electrospinning is considered to be efficient for generating the fibers at nanoscale dimension (fiber diameter < 100 nm). This is a versatile technique where viscoelastic fluid is commonly prepared by various polymers and solvents. The fluid’s

R. Das (B) Department of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794, USA e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Das (ed.), Electrospun Nanofibrous Technology for Clean Water Production, Nanostructure Science and Technology, https://doi.org/10.1007/978-981-99-5483-4_1

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Fig. 1.1 Simple illustration of a typical electrospinning. Reprinted with permission from ref. [1] (Copyright 2019 American Chemical Society)

droplet is electrified to produce a charged jet which needs to stretch due to electrostatic repulsion between surface charges and elongate to some extent that led to the formation of fibers, with controllable shapes, sizes, and thicknesses. Because of its simplicity, electrospinning setups, typically consist of a high-voltage power supply (alternating current/direct current), syringe pump, a spinneret, and a conductive collector (e.g., a grid or metal sheet). Because of its instrumental simplicity (as shown in Fig. 1.1), many labs be accessible to this electrospinning setup. The fibers either at microscale or nanoscale could be generated from a range of materials or their composites through the electrospinning process. Polymeric solution or melted polymers of polyacrylonitrile, poly(vinylidene fluoride), polyaniline, poly(lactic acid), polystyrene, and polypyrrole, to name just a few have been extensively used to electrospun into nanofibers. Also, some small molecules like amphiphiles and cyclodextrin derivatives could be successfully electrospun into the fibers, but one must ensure a stable electric jet by using the molecules with proper chain entanglements. Similarly, some colloidal particles (suspended medium and suspended particles) obeying this have been adopted for electrospinnng. Crosslinking of colloidal particles, having suitable sizes, could ensure a proper viscosity of the solution necessary for a stable electrospinning process. Therefore, scientists often prepare a composites where polymers and other metal or metal oxide nanoparticles are used to prepare a perfect solution which could generate the fibers with desired characteristics [1]. The fibers generated through electrospinning process have excellent physicochemical properties which are not only needed for clean water production but also

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suitable for smart surface coating, materials engineering, drugs delivery, sensor technologies, and so on. The polymers used for electrospinning often have functional moieties which could help to adsorb the water pollutants. Moreover, the ultra-fine solid fibers could entangle with each other that results in interconnected pore formation, where solutes diffusion is feasible. Tuning the fluid properties by using various polymers and nanoparticles and changing the electrospinning setup parameters will ensure to produce the fibers, having microporous and nanopores structures. It leads to an increase in surface area for maximizing the water pollutants’ adsorption. All these properties and advanced characterization followed by electrospun nanofibers application for water purification are extensively corroborated by different research groups in this book entitled ‘Electrospun Nanofibrous Technology for Clean Water Production.’ For example, in Chapter 2, Blachowicz and Ehrmann comprehensively discussed the different methods and engineering aspect of electrospinning. They first draw a basic discussion on needle-based electrospinning (comparative knowledge from single-jet to multiple-jet electrospinning has been given), wire-based electrospinning, needleless electrospinning techniques, free-surface electrospinning, etc. Then, they highlighted the importance of diverse static counter electrodes and rotating drums and other special counter electrodes. After discussing the spinnerets and collectors’ contribution in electrospinning at the far-field regime, the authors also gave recent findings on the electrospinning that can be done in the near-field regime at the end of the chapter. When readers are already introduced to the basics of these electrospinning methods in this chapter and Chapter 2, a clear picture of characterizing the electrospun fibers is given in Chapter 3. These include surface morphology analysis that includes the scanning electron microscopy and transmission electron microscopy; wettability assessment using contact angle analysis; porosity, pore volume fraction, and surface area measurement using the Brunauer–Emmett–Teller (BET); mechanical properties characterization through instron to detect tensile strength and Young’s modulus of fibers; crystal structure analysis by wide angle X-ray diffraction; thermal characterization using differential scanning calorimetry and thermogravimetric analysis, etc. Next, the authors summarized information on both the electrical conductivity and magnetic properties measurements of the fibers. Following that, a big section on how to determine the chemical properties of electrospun fibers was devoted. Over there, the authors gave an in-detail discussion on molecular weight determination of electrospun fibers made from polymers, and calculate polydispersity index (PDI) using Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry and gel permeation chromatography techniques. Readers would get much information on functionalities of electrospun fibers using Fourier-transform infrared spectroscopy, Raman spectroscopy, nuclear magnetic resonance as well as, X-ray photoelectron spectroscopy. Chapter 4 depicted a variety of electrospun fibers modification strategies especially needed for water pollutants adsorption. The authors first demonstrated the preparation and modification of polymer-based electrospun fibers which include both natural and synthetic polymers. The modification was achieved through chemically bonding functional groups/molecules which include cellulose, chitosan, polyacrylonitrile, thiol, cyclodextrins, calixarenes, ionic liquids, sodium alginate, etc., on the fibers. The effects of the functionalities on the adsorptive

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performances of the fibers were beautifully compiled in the table. Besides chemical bonding, the coating of different nanomaterials (polymeric and non-polymeric) on polymeric fibers was scrutinized. In addition, a range of organic/inorganic composites of polymeric electrospun fibers which include metal and metal oxide/zeolites/metal organic fibers/carbon-based nanoparticles, etc., was comprehensively discussed. The effects of these composites on the performance of the adsorption, i.e., achieving the maximum adsorption capacity of heavy metals, dyes, and other organic contaminants in the environment was compiled in a range of given tables. At the end of this chapter, a section was devoted to covering the carbonization of electrospinning of polymeric nanofibers to produce porous carbon materials with a high surface area with hydrophobic properties, providing efficient adsorbents for the removal of pollutants. Chapter 5 demonstrated the pathways to engineer electrospun nanofibers for fabricating an effective catalyst support. At first, the authors summarized the basic principles, materials, and methods for preparing the fibers through the electrospinning process. However, these pristine fibers often lack essential physicochemical properties which results in inefficient pollutants mineralization through the photocatalysis process. Therefore, a separate section demonstrating the various modification approaches on the fibers was added to the chapter. Then the authors comprehensively discussed different routes for modifying the fibers with a lot of semi-conducting materials and discussed the engineering of their bandgaps which is important for the removal of organic, inorganic, and inactivating the pathogens in wastewater. While Chapter 5 focused on demonstrating the photocatalytic degradation of water pollutants, the authors of Chapter 6 prioritized to highlight the filtration properties of electrospun fibers. Firstly, they introduced three types of common membrane types that include symmetric membrane, asymmetric membrane, and liquid membrane. Then a given discussion on how to fabricate these membranes using electrospinning fibers which include the layer-by-layer, functionalization, solution blending, wetchemical method, and chemical deposition and coating was dedicated. After that they highlighted the effects of different factors like voltage, flowrate of polymer, distance effects, polymer properties, etc. The authors prepared several tables mainly highlighting the performances of microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membranes, their mechanism for pollutants retention was also critically analyzed. In Chapter 7, Zhang and colleagues discussed the suitable properties of electrospun nanofibrous membranes for oil–water separation. To secure this, they first discussed the hydrophilic and oleophobic behavior of the fibers which can be tuned by controlling the desired physical morphology and chemical composition, and surface modification of the pristine electrospun materials. Next, a large section was devoted to discussing the performance of the fibers and drew several strategies to control the pollutants rejection and permeation behaviors of the electrospun nanofibers membrane. The authors also gave a good effort to discuss the mechanism of oil/water emulsions retention which could give possible clues for future experimental designs. Since the two-representative thermal-driven water treatment process like water distillation and pervaporation based on classical polymeric materials have been facing a lot of challenges, the electrospinning technique has brought

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new hopes for nanofiber-based composite membrane fabrications which were extensively discussed in Chapter 8. At first, the authors answered how to optimize the membrane distillation system by using different strategies like the facile approaches including spraying and dip-coating to fabricate advanced nanofibrous materials with super- and intrinsic-hydrophobicity. Such facile spraying approach could ensure a homogeneous distribution of a variety of nanoparticles on the membrane surface. Then, different materials for electrospinning nanofibrous membrane-based performances in continuous vacuum membrane distillation were tabulated. For different nanomaterials, the pore-size distribution and wetting property of the membrane would be different, and the authors extensively discussed these properties at the end of the membrane distillation section. On the other hand, pervaporation is a different process which is typically driven by the solution-diffusion model. At the end of the chapter, the authors showed how to use electrospun nanofibers as a suitable substrate for fabricating thin-film composites membrane for different pervaporation applications.

References 1. Xue, J., Wu, T., Dai, Y., Xia, Y.: Electrospinning and electrospun nanofibers: methods, materials, and applications. Chem. Rev. 119(8), 5298–5415 (2019) 2. Boys, C.V.: On the production, properties, and some suggested uses of the finest threads. Proc. Phys. Soc. London (1874–1925) 9(1), 8 (1887) 3. Frenot, A., Chronakis, I.S.: Polymer nanofibers assembled by electrospinning. Curr. Opin. Colloid Interface Sci. 8(1), 64–75 (2003)

Chapter 2

Methods and Engineering of Electrospinning Tomasz Blachowicz and Andrea Ehrmann

2.1 Introduction Electrospinning is based on the idea to spin supported by an electric field. Very first patents dealing with electrospraying or even electrospinning were already filed at the beginning of the twentieth century [1–4], with the well-known patent by Anton Formhals, describing electrospinning of a polymer, was filed in 1934 [5]. Nevertheless, it took more than 50 years until first researchers started working on electrospinning [6]. Nowadays, electrospinning is widely used in research and industry due to the possibility to create nanofibers with diameters down to few nanometers. The corresponding large surface-to-volume ratio enables using electrospun nanofiber mats in a broad range of applications, such as wound dressing [7–9], tissue engineering [10– 12], drug release [13–15] and other applications in the biomedical sector, but also for batteries [16–18], hydrogen production [19–21], solar cells [22–24], supercapacitors [25–27] and, of course, filters which are in the focus of this book. This chapter will concentrate on describing the different electrospinning techniques from the basics to the most recent developments, from the simple needlebased electrospinning to recent research on near-field electrospinning, to enable beginners fully understand the techniques, while specialists will be informed about latest development in this field of research and development.

T. Blachowicz Institute of Physics, Center for Science and Education, Silesian University of Technology, 44-100, Gliwice, Poland A. Ehrmann (B) Faculty of Engineering and Mathematics, Bielefeld University of Applied Sciences and Arts, 33619 Bielefeld, Germany e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Das (ed.), Electrospun Nanofibrous Technology for Clean Water Production, Nanostructure Science and Technology, https://doi.org/10.1007/978-981-99-5483-4_2

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2.2 Needle-Based Electrospinning—Basics Generally, needle-based electrospinning is a relatively easy technique, as visible from the large amount of home-built equipment reported in the literature. A syringe is filled with a polymer solution (or melt) and connected with a syringe pump which defines the flow rate of the polymer solution through the nozzle (Fig. 2.1a) [28]. Between nozzle (needle) and collector, a high voltage is applied, usually as DC (direct current) voltage. In the simplest form, the nozzle is set to a high positive voltage, while the collector is grounded (Fig. 2.1a). Typical values often found in the literature are in the range of 1 kV/cm with distances around 10–20 cm between needle and collector. While without electric field, the polymer solution would leave the nozzle drop by drop, the addition of a strong enough electric potential changes this situation. As calculated by Taylor [29], the impact of the electric field, superposing the surface tension of the drop, results in formation of a so-called Taylor cone (Fig. 2.1b) from which a jet of polymer solution is formed. The first part of this jet is normally assumed to be linear, which was modeled by diverse researchers based on Newtonian or viscoelastic fluids [30–34]. After this initial straight jet is ejected, it starts bending and curling, forming a spiral shape (Fig. 2.2, left panel), in this way reducing the fiber diameter and increasing its length [28, 35]. This phase mostly defines the final fiber properties and is thus important to understand in detail. Reneker et al. describe a cone as 3D envelope of all possible paths taken by a jet [36], i.e., the spiral shape depicted in Fig. 2.2 (left panel) is only the simplest

Fig. 2.1 a Principle of needle-based electrospinning; b Taylor cone formation. From [28], originally published under a CC-BY license

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Fig. 2.2 (left) Needle-based electrospinning principle, (upper right) isotropic and (lower right) oriented fiber growth. Reprinted with permission from [35]. Copyright (2019) American Chemical Society

case. They took stereographic images of the jet moving along this cone, allowing preparing 3D images from them, and modeled the observed bending instability by a system of beads connected with viscoelastic elements. They showed that different perturbations could develop further into a bending instability and thus form similar structures of the charged jet as depicted in Fig. 2.2 (left). Strongly correlated with the bending instability is the typical random (isotropic) arrangement of the nanofibers on the collector (Fig. 2.2, upper right). In general, by modifying the electric field intensity or using special collectors it is possible to receive an oriented (anisotropic) arrangement of fibers (Fig. 2.2, lower right). This will be discussed later. Besides the aforementioned bending instability, which is necessary to form thin fibers, Shin et al. used the calculation of a long, thin Newtonian fluid jet with perturbations of radius, velocity, surface charge distribution and local field strength to find three instabilities, the axisymmetric Rayleigh instability (dominated by surface tension) and conducting instability (resulting in a modulation of the jet radius) as well as the non-axisymmetric whipping instability in which the jet radius stayed constant, while the centerline was modulated [37, 38]. Figure 2.3 depicts axisymmetric and non-axisymmetric instabilities [37]. These theoretical explanations of the initial phase and the following phase of bending instability already give an idea of which spinning and solution parameters will influence the final fiber morphology. Shin et al. prepared a phase diagram of the electric field versus flow rate in which they identified regions of a stable jet, of Rayleigh and of whipping instabilities. Generally, the stable jet region was found for

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Fig. 2.3 a Axisymmetric and b Non-axisymmetric fluid jets induced by the surface electric charge density fluctuations σ0 ± . Reprinted from [37], Copyright (2001), with permission from Elsevier

relatively small electric field intensities, mostly below the aforementioned value of approx. 1 kV/cm, and for flow rates higher than approx. 5 mL/min [37]. Zuo et al. pointed out that axisymmetric beads, which occur often along the fibers and are mostly undesired, should logically result from axisymmetric instabilities. They used photographs of fibers caught on a glass slide at different points between nozzle and collector to show how the jet became thinner and thinner and at the same time formed beads, as visible in Fig. 2.3a [39]. Interestingly, they found more and thicker beads for higher flow rates. It must be mentioned that in contrast to Shin et al. who worked on electrospinning poly (ethylene oxide) (PEO), a common watersoluble polymer, Zuo et al. prepared fibers from poly(hydroxybutyrate-co-valerate) (PHBV) from chloroform and other solvents. Other factors found in their study which influenced fiber morphology were the applied voltage and the solvent which modified conductivity, surface tension and charge density of the spinning solution along with an increased voltage leading to smoother fibers, as well as low surface tension and high conductivity [39]. Fong et al. investigated the influence of solution viscosity, surface tension and resistivity by adding ethanol or NaCl to a solution of PEO in water [40]. They found increasing net charge density due to addition of NaCl favoring formation of fibers with small diameters without beads, while smaller surface tension coefficients resulted in larger fiber diameters, and higher viscosity suppresses bead formation [40]. As an example, Fig. 2.4 depicts morphologies from beaded to straight fibers with increasing solution viscosity [40].

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Fig. 2.4 Morphology of electrospun nanofibers for different solution viscosities. Reprinted from [40], Copyright (1999), with permission from Elsevier

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Finally, it must be mentioned that the polymer concentration is also an important factor for the formation—or avoidance—of beads since it significantly influences the aforementioned solution parameters [41]. During electrospinning, stretching ratios in the range of 105 can occur, correlated with stretching rates up to 105 /s [41], which is several orders of magnitude higher than common stretching ratios in other spinning techniques. Traveling durations of single molecules between nozzle and substrate are in the order of magnitude of 0.1 s, making final structure formation a very fast process [42]. This leads to the finding that electrospun nanofibers usually have very good mechanical properties since they are nearly defect-free. To understand this statement, it is necessary to distinguish between amorphous and partially crystalline polymers. For the first, it is important to know that the speed of glass formation defines the properties of the frozen state which is generally not the thermodynamic equilibrium [43]. This difference between frozen state and equilibrium state means that even in the solidified state, the molecules in amorphous materials are still not completely fixed, but tend to approaching equilibrium slowly. This means that during aging, amorphous materials will become stiffer, while their dielectric constants, creep- and stress-relaxation rates, etc. will decrease [42]. Measurements by differential scanning calorimetry (DSC) may show an additional peak in the first heating curve of electrospun fibers, as compared to bulk materials [44]. Semi-crystalline polymers, on the other hand, are expected to show the parts in which crystallization occurred. Here, the short durations of fiber formation may be expected to reduce the degree of crystallization. However, the amount of crystallization seems to be more related to other parameters. Chen et al., e.g., showed that adding multiwall carbon nanotubes (MWCNTs) to poly(ethylene terephthalate) (PET) significantly increased the rigid amorphous fraction and decreased the crystallinity, as compared to pure PET [45]. This behavior is depicted in Fig. 2.5 for different weight percentages of MWCNTs [45]. Su et al., on the other hand, found a preferential crystal growth of PEO along the c-axis direction due to an orientation of the crystallites along the fiber axis, while the crystalline order in the nanofibers was indeed lower than in cast films, which was attributed to short crystallization time and imperfect crystals due to the whipping instability [46]. A method which could be used to directly detect the frozen chain orientations is optical birefringence, i.e., measuring the difference of the refractive indices for polarizations parallel and perpendicular to the anisotropy axis of a fiber, which can be used to estimate the angle between fiber axis and polymer chain axis. Kolbuk et al. used a special instrument to measure birefringence by refraction patterns [47]. A simpler optical method to measure birefringence is given by polarized optical microscopy, detecting the absorbance for the electric field vector of the polarizer being oriented parallel and perpendicular to the fiber draw direction, respectively. In addition to optical birefringence, Fennessey and Farris used this method to investigate the difference of polyacrylonitrile (PAN) nanofibers, electrospun from N,Ndimethylformamide (DMF) solution on a stationary and a high-speed rotating target

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Fig. 2.5 Crystalline fraction as a function of temperature and weight percentage of MWCNTs in PET/MWCNT composite nanofibers. Reprinted from [45], Copyright (2009), with permission from Elsevier

[48]. They found the PAN nanofibers on the static target being not birefringent which they explained by the incomplete evaporation of DMF during electrospinning, allowing relaxation of the fibers after impinging on the collector so that the molecular orientation due to the stretching during electrospinning is lost. By changing to a fast rotating collection wheel, the fibers were not only oriented parallel to each other, but also became birefringent, as visible in Fig. 2.6 [48]. Another property directly influenced by spinning and solution parameters, besides the aforementioned fiber diameter and the amount of beads created, is the fiber surface. For diverse applications it is useful to increase the surface-to-volume ratio further, i.e., to introduce pores into the nanofibers. Different possibilities exist to introduce pores into the nanofiber surface. Mostly, either one of two components is removed, or polymer–solvent phase separation is induced by rapidly cooling the nanofibers before they are fully polymerized [49]. Li et al. prepared porous poly(lactic-acid) (PLA) nanofibers from solvents with different ratios of dichloromethane (DCM) and DMF [50], as shown in Fig. 2.7a–f [50]. McCann et al. used a bath of liquid nitrogen as collector to induce phase separation between different polymers (poly(styrene), PAN, poly(vinylidene fluoride) and poly(ε-caprolactone) (PCL)) and the solvent which resulted in a highly porous fiber surface, as visible in Fig. 2.7g [51]. Zhang et al. also used phase separation of gelatin/ PCL composite fibers by selectively removing gelatin in a warm aqueous solution of phosphate buffered saline, in this way more than doubling the surface area of the

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Fig. 2.6 a Aligned PAN nanofibers, gained by electrospinning on a fast rotating target, in the (subtraction) and b Addition position of the crossed polarizers. Reprinted from [48], Copyright (2004), with permission from Elsevier

fibers [52]. Lee et al. increased the fiber porosity by ultrasonication in an aqueous solution [53]. Besides these morphological modifications of nanofiber mats electrospun with a common needle, more sophisticated morphologies are enabled using special needles. Besides, using multiple-needle geometries, it is possible to increase the electrospinning productivity. The next section of this chapter will describe these possibilities more in detail.

2.3 Needle-Based Electrospinning—Multiple Needles Needle-based electrospinning has productivity far below other textile technologies. The simplest way to increase it is by adding an auxiliary electrode to modify the electric field. Liu et al. found a transition from single-jet to multiple-jet electrospinning

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Fig. 2.7 a–f Nanofibers of PLA with high and low crystallization ability (H-PLA and L-PLA, respectively), prepared with different DCM:DMF ratios. Reprinted from [50], Copyright (2015), with permission from Elsevier; g Highly porous nanofiber prepared by electrospinning into a liquid nitrogen bath. Reprinted with permission from [51]. Copyright (2006) American Chemical Society

by the addition of an auxiliary electrode perpendicular to the collector which led to an increase of the electric field strength at the needle tip when it was placed near to the spinneret (the spinning nozzle) [54]. Wu et al. tested a setup with an auxiliary electrode for different solutions of polystyrene (PS) and polyvinyl alcohol (PVA) in chloroform, DMF, water, tetrahydrofuran (THF) and N-methyl-2-pyrrolidinone (NMP) and concluded that polymer solutions with higher dielectric constant and surface tension produced more jets, while solutions with low dielectric constant and surface tension produced only one jet, but with a long, stable yet length clearly beyond the jet lengths of the first class of solutions [55]. Such auxiliary electrodes can also be used to control the orientation of the fibers on the collector. Teo et al. placed auxiliary electrodes in the form of aluminum strips or knife-edged bars behind a non-conductive rotating tube collector and found that in this way, it was possible to prepare a nanofiber coating on the tube with diagonal fiber orientation which may be supportive for blood vessel scaffolds [56]. Similarly, Carnell et al. used an additional electrode behind the conductive cylinder collector to reduce the bending and whipping instability, in this way preparing well-aligned nanofibers on the collector [57]. An even more sophisticated process was suggested by Gu et al. who applied a continuously rotating electric field along four auxiliary electrodes surrounding the direct line from nozzle to flat collector as shown in Fig. 2.8. In this way, they prepared twisted nanofibers, with the twist length being correlated with the rotation time of the electric field around the auxiliary electrode [58]. The second way to enable needle-based electrospinning with higher productivity is based on using more than one needle or spinneret hole. Zheng et al. compared a multi-hole spinneret with 7 holes with an arrangement with 7 needles. They found that the multi-hole spinneret produced a more uniform and in most positions also stronger electric field than the multi-needle arrangement, which resulted in a more concentrated nanofiber mat with smaller fiber diameters [59]. Kim et al. showed that

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Fig. 2.8 Needle-based electrospinning with an auxiliary electrode near the spinneret. From [55], originally published under a CC-BY license

adding a cylindrical auxiliary electrode around the multi-needle arrangement could be used to concentrate the electric field and thus also to narrow the substrate area on which electrospun fibers landed, as depicted in Fig. 2.9. They showed a linearly increasing deposition rate by using multiple needles together with the cylindrical auxiliary electrode, making this technique promising for further upscaling [60]. As this example already showed, it is not possible to simply increase the number of needles without taking into account other modifications of the spinning geometry. Tian et al. investigated the influence of different numbers of spinning needles in detail and found that the alignment of the fibers—which was more than 96% in case of 1 needle, as measured optically by estimating the angular differences in fiber orientation—was significantly reduced with increasing number of up to 5 needles. They also calculated and measured the offset of the electrostatic field distribution map center from the circular center, which was interpreted as electrostatic field interference, and found a strong correlation between this offset and reduced fiber alignment. Corresponding to the reduced alignment, they also found a reduced breaking stress with increasing number of needles, showing the importance to carefully model and measure the influence of a multi-needle arrangement on the electric field [61]. Another interesting effect was found by Varabhas et al. who used a porous hollow tube with embedded wire electrode for multi-hole electrospinning. In their experiment, no overlap between neighboring jets was found, resulting in a pattern of

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Fig. 2.9 a Electrospinning with 5 needles; b Detailed sketch of the 5 needles; c 5 needles with cylindrical auxiliary electrode; d Comparison of electrospinning results without and with auxiliary electrode. Reprinted from [60], Copyright (2006), with permission from Elsevier

alternating white and dark areas on the slowly moving collector belt, corresponding to areas with and without nanofibers [62]. A larger number of needles, up to 37, were positioned in a hexagon arrangement with cylindrical auxiliary electrode. By simulations and experiments, Yang et al. found that with the cylindrical shield, the electric field near the needle tips became more uniform and the electric field between needle tips and collector became stronger, resulting in creation of thinner fibers. On the other hand, they found that a longer distance for jet stretching was necessary for a higher amount of needles used [63]. Zhu et al. suggested another solution to increase the regularity of the electric field for a multi-needle technology. Based on the simulated tip charge distribution for three needles and measured jet flow displacement (Fig. 2.10), they used a COMSOL simulation and experimental investigations to increase the field uniformity for a linear arrangement of 16 needles. They found smaller undesired variations of the electric field for smaller needle diameters, but especially suggested surrounding the needles with a dielectric material. By this way, it was not only possible to significantly

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Fig. 2.10 Tip charge distribution and jet morphology for a three-needle arrangement. From [64], originally published under a CC-BY license

enhance the electric field along the needle tips, but also to reduce electric field variations between the needles, while the dielectric constant of the material interestingly was nearly without influence. Finally, they suggested the use of dielectric material around all but the edge needles, in this way further reducing field irregularities [64].

2.4 Needle-Based Electrospinning—Special Needles Besides such multiple-needle arrangements, there are also special needles which can be used to produce core–shell or Janus fibers. For the first, coaxial electrospinning is used, allowing for combining two different polymers or other solutions (Fig. 2.11) [65]. In this way, it is possible, e.g., to reduce the initial burst release of drug release fibers by combining a drug-containing core with a polymeric shell through which the drug has to diffuse [65]. Jiang et al. used this technique to combine protein-containing PEO core with a PCL shell, allowing for adjusting the release profile by controlling core and shell thicknesses [66]. PCL-starch core–shell nanofibers, prepared by coaxial electrospun, were found to support growth of NIH-3T3 mouse fibroblast cells, as compared to a common tissue culture plate surface [67]. Chen et al. found higher cell proliferation on coaxial thermoplastic polyurethane (TPU)/collagen nanofibers than on pure collagen or pure TPU fibers due to the combination of good mechanical strength and retained shape of TPU, combined with the high biocompatibility of collagen [68]. Generally, it is also possible to use an electrospinnable sheath fluid to enclose a non-spinnable core fluid [69]. Zhou et al. [70] as well as Wang et al. [71] used coaxial electrospinning in a reversed manner, combining a core from a spinnable polymer with a sheath or a nanocoating from a non-spinnable fluid [68, 69], as depicted in Fig. 2.12 [70]. In this way, ketoprofen (KET) loaded hydroxypropyl methylcellulose (HPMC) nanofibers with different diameters and correspondingly different KET release from the HPMC nanofibers could be prepared [70], as well as zein nanoribbons with a sheath from the electrolyte LiCl which was only used to control the zein ribbon width [69].

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Fig. 2.11 Scheme of a coaxial electrospinning setup. Reprinted from [65], Copyright (2018), with permission from Elsevier

Fig. 2.12 Scheme of the modified coaxial electrospinning process. From [70], originally published under a CC-BY license

It should be mentioned that coaxial electrospinning cannot only be used to prepare core–shell fibers, but also to create hollow fibers. Lavalle et al. used this technique to prepare solid and hollow lignin nanofibers as precursors for solid and hollow carbon nanofibers, the latter showing increased specific surface areas similar to those of activated carbon [72].

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Fig. 2.13 Scheme of a Janus nanofiber electrospinning setup. Reprinted from [73], Copyright (2020), with permission from Elsevier

Besides these possibilities given by coaxial electrospinning, another interesting sort of nanofibers is given by the so-called Janus fibers. Using a side-by-side spinneret (Fig. 2.13), Yang et al. combined hydrophilic polyvinylpyrrolidone (PVP) and hydrophobic ethyl cellulose with embedded Ag and ciprofloxacin nanoparticles, in this way increasing the antibacterial performance of the nanofibers [73]. Using different spinneret geometries, i.e., parallel, acentric and structured spinnerets, Yu et al. prepared even Janus fibers from three fluids with the structured spinneret, i.e., fibers with the inner structure visible in Fig. 2.13 and an additional shell [74]. Chen et al. showed that with varying port angles between the partial spinnerets for the two fluids, the morphology of the Janus fiber could be modified [75]. Wang et al. used zein/PVP Janus nanofibers for the controlled release of poorly water-soluble drugs [76]. As these few examples of special needle geometries show, the apparently simple needle-based electrospinning enables creating highly sophisticated nanofiber structures. The next sections will present other techniques which have been developed, based on further geometries of fiber emitter and/or collector.

2.5 Wire-Based Electrospinning The wire-based electrospinning technology was developed by Jirsak et al. [77]. The wire electrode is coated by the polymer solution and at the same time acts as the positive electrode of a strong electric field [78]. Wire-based electrospinning, as well as other free-surface electrospinning techniques, have the advantage of high

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Fig. 2.14 Wire-based electrospinning with the carriage including the polymer solution along the lower (positive) wire, the (blue) substrate and the upper wire acting as a counter electrode. From [79], originally published under a CC-BY license

productivity [78]. The fiber and mat morphologies can in a relatively broad range be controlled by modifying the solution and spinning parameters, such as the voltage, the distance between both wires which form the electrodes (Fig. 2.14, [79]), the distance between substrate and ground electrode, the carriage speed, etc. [41, 80]. Electrospinning polyvinylidene fluoride (PVDF) from a dimethylacetamide (DMac) solution by a wire-based equipment, Zhu et al. investigated the influence of solution and spinning parameters on the fiber diameter distribution. They found a strongly increasing viscosity and a slightly reduced conductivity with increasing solid content in the solution, resulting in larger fiber diameters for larger solid contents, while the spinning parameters under investigation had less influence on the average fiber diameters [81]. Comparing the possibilities to tailor fiber diameters by needlebased and wire-based systems, Wang et al. found that the latter opened a broader range of possible diameters which may be due to significantly different electric field intensity profiles [82]. Prahasti et al. prepared PVP fibers with a wire-based electrospinning process and also found the morphology and the diameters of the resulting nanofibers depend on the solid content in the solution and the process parameters, respectively [83], as depicted in Fig. 2.15. A different wire-based system was developed by Holopainen et al. [84]. They used a twisted wire as spinneret which was positioned vertically, opposite to the horizontal orientation shown in Fig. 2.14, and coated from the upper end by a downward flowing spinning solution, in this way creating multiple Taylor cones and corresponding jets at the same time. The whole spinneret was surrounded by cylindrical mesh collectors with different radii. In this way, PVP nanofibers with different ceramics were electrospun, resulting in a large-area nanofiber mat produced with a high throughput.

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Fig. 2.15 SEM images and fiber diameter distributions for different PVP concentrations in the spinning solution: a 2 wt%, b 4 wt%, c 6 wt%, d 8 wt%, e 10 wt%; f Numbers of jets and PVP fiber diameters as function of the PVP concentration. From [83], originally published under a CC-BY license

The authors suggested a cleaning system to avoid undesired drying of the polymer solution on the twisted wire [84]. Besides this relatively simple wire-based technique, diverse other needleless—or free-surface—electrospinning techniques exist which will be described in the next section.

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2.6 Other Free-Surface Electrospinning Techniques The needleless electrospinning techniques, also called free-surface electrospinning, can roughly be split into technologies with stationary (or static) and with rotating spinnerets. Among the static spinnerets, typical techniques are conical wire spinneret, plate spinneret, bowl spinneret, cylinder spinneret or bubble spinning, while rotating cylinder, rotating cone, rotating beaded chain spinneret, rotating ball spinneret, rotating disc spinneret or spiral coil spinneret [85]. Due to this large amount of possible technologies, here we report only on some of them to give the reader an idea of the possibilities given by needleless electrospinning. Yarin and Zussman reported on a special two-layer system, consisting of a ferromagnetic suspension below a polymer solution in an open box under which a strong magnet was placed, while an additional electric field was applied vertically. In this way, the vertical spike-like perturbations of the magnetic suspension through the fluid interface and the free surface of the polymer solution became origins of jets directed upward toward the counter electrode. An estimate of the production efficiency in comparison to a multi-needle electrospinning system with the same surface showed that this magnet-supported free-surface electrospinning technique should be able to produce approx. one order of magnitude more nanofibers per emitter area [86]. A combination of free-surface electrospinning with the aforementioned coaxial electrospinning was presented by Jiang and Qin (Fig. 2.16). They used a stepped pyramidal spinneret in which both polymer solutions were introduced, with the inner tube being slightly higher than the outer one to ensure that the upper layer solution stems from the inner tube. In comparison to conventional coaxial electrospinning, they report on avoiding clogging of the nozzle in combination with a productivity increase by two orders of magnitude [87]. Based on a similar stepped pyramidal spinneret, Jiang et al. prepared a microbubble solution system which produced nanofiber diameters which could be adjusted by the micro-bubble size and in addition a slightly increased productivity as compared to conventional free-surface electrospinning with a similar spinneret [88]. Using a cylinder rotating through a polymer solution reservoir, Kostakova et al. produced composite nanofibers from PVP with single-wall and (MW)CNTs after sonication of the solutions to create highly homogeneous solutions without agglomerations. They found that mass production of electrospun composite nanofibers was possible, but necessitated surface modifications of the CNT [89]. Different reservoir geometries for bubble-electrospinning (with air pump) and conventional free-surface electrospinning (without air pump) were developed by Fang and Xu. Spinning PAN from a DMF solution from these different reservoirs, they found different yet initiation and stabilizing processes, partly depicted in Fig. 2.17 [90]. Combining simulations and experimental findings, they found the most non-uniform electric field distribution for the Modified Bubble-Electrospinning (MBE) device, while the Modified Free-Surface Electrospinning (MFSE) device showed the smallest electric field intensities. The highest electric field intensity and the most uniform distribution of the electric field in the device were found for the

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Fig. 2.16 a Scheme of free-surface coaxial electrospinning by a stepped pyramidal spinneret; b Coaxial jets during electrospinning. Reprinted from [87], Copyright (2014), with permission from Elsevier

Spherical Section Free-Surface Electrospinning (SSFSE) setup. Interestingly, the MFSE method resulted in the largest nanofiber diameters, which fits to the normal finding in wire-based electrospinning that smaller electric fields result in thicker and more even fibers. Corresponding to the non-uniform field distribution, the MBE setup also gave the least uniform diameter distribution, while MFSE and SSFSE showed more uniform distributions. Besides, the SSFSE setup resulted in the highest nanofiber yield of approximately 20 g/h for an average voltage of 40 kV [90]. Tang et al. modified the rotating cylinder spinneret which in their setup was not coated by embedding it in the polymer solution, but a solution distributor above the rotating cylinder was used to place solution droplets on it. Due to the additional gravitational force from falling nearly half a meter onto the metal spinneret, the polymer solution droplets were pre-tensioned, changing their shapes from round to oval which resulted in a significant reduction of the necessary voltage to start electrospinning. In this way, a productivity enhancement compared to single-needle electrospinning of a factor of 24–45 was reached [91].

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Fig. 2.17 Photographs of jets ejecting from a Modified bubble-electrospinning (MBE), b Modified free-surface electrospinning (MFSE), c Oblique section free-surface electrospinning (OSFSE), d Spherical section free-surface electrospinning (SSFSE). From [90], originally published under a CC-BY license

Besides this short overview of interesting recent developments, there are several other strategies and electrode geometries used for free-surface electrospinning. Compared with the large amount of polymers, polymer blends and polymers filled with nanoparticles, a broad range of possibilities can still be discovered, besides the ideas described in recent literature. Another important factor defining the electric field and thus the properties of the electrospun nanofibers is the collector and/or the counter electrode. In the next section, we will present some rotating and other innovative counter electrodes.

2.7 Rotating Drum and Other Special Counter Electrodes An overview of diverse static counter electrodes is given in Fig. 2.18 [28]. Parallel and neighboring conductive plates (Fig. 2.18a–d, f), e.g., can be used to increase the electric field along their edges and by this to align fibers perpendicular to the collector edges, as shown by Li et al. [92] for needle-based electrospinning and recently by Storck et al. for a wire-based system [93]. More sophisticated is a parallel plate arrangement with alternating collectors (Fig. 2.18e), as described by Ishii et al. [94]. Finally, some authors suggested adding an external magnetic field to gain aligned nanofibers with decreased diameters (Fig. 2.18g) [95].

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Fig. 2.18 Needle-based electrospinning with different static counter electrodes. From [28], originally published under a CC-BY license

Besides such static counter electrodes or collectors, several groups investigated moving collectors, especially rotating ones. Xu et al. used electrospinning of cellulose on a rotating drum of parallel copper wires as counter electrode (Fig. 2.19) to avoid the contraction of the wet fibers. For cellulose, they found that spinning on a static aluminum foil was not possible; the rotating drum enabled fiber formation, while fiber orientation perpendicular to the wires was disturbed [96]. Besides such rotating drums, diverse other rotating collectors can be found in the literature, some of which were already mentioned briefly in the previous sections. Figure 2.20 gives an overview of some of them [28]. While the combination of an external electrode with a rotating collector (Fig. 2.20a) can be used for self-bundling of fibers [97], spinning into a liquid container is performed to neutralize the usual static charges on the nanofibers before the fiber yarn can be collected on a rotating drum (Fig. 2.20b) [98]. The combination of auxiliary electrodes with rotating field, combined with a static collector (Fig. 2.20c), was already described before as one possibility to produce twisted nanofiber yarn [58]. Another possibility is given by a couple of ring electrodes of which one is rotating (Fig. 2.20d), resulting in the fibers being suspended between the collector rings [99]. Finally, spinning from two needles

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Fig. 2.19 Needle-based electrospinning with a rotating drum as counter electrode. Reprinted from [96], Copyright (2008), with permission from Elsevier

with opposite polarities supports adhesion of the oppositely charged fibers to each other, while the rotating collector was used to align the neutral yarn [100]. All the preceding discussion about spinnerets and collectors has one thing in common—they work in the far-field regime, with a certain distance between emitter and substrate allowing the fibers to form. Nevertheless, electrospinning can also be done in the near-field regime, which has been investigated recently in detail. The next chapter will give a short overview of this geometry, its advantages and challenges.

2.8 Near-Field Electrospinning While common far-field electrospinning works on typical distances from ~5 to 25 cm, near-field electrospinning (NFES) describes spinning with smaller distances between emitter and collector, typically in the range of 0.5–50 mm [101]. To be more exact, it occurs when the distance between emitter and collector is smaller than the critical length of the straight first segment of the jet, before the aforementioned instabilities occur [102]. This means, on the other hand, that flow rate and voltage are significantly smaller than in common far-field electrospinning. Near-field electrospinning enables in principle producing fibers at exactly defined positions, i.e., “writing” with single fibers. Figure 2.21 shows an example of polystyrene patterns on glass slides [103]. Nevertheless, it must be mentioned that spinning parameters have to be tailored carefully to reach the desired patterns. Xin and Reneker, e.g., showed not only the nice

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Fig. 2.20 Needle-based electrospinning with different moving counter electrodes. From [28], originally published under a CC-BY license

patterns reprinted in Fig. 2.21, but also investigated in detail the influence of different spinning parameters on the printed patterns. Generally, they showed that uniform buckling patterns could be produced on a fast moving substrate, with the substrate speed, the tip-collector distance and the voltage defining the patterns. Straight fibers were found to be collected for high lateral collector speeds. They also mentioned the possibility to prepare fluffy fiber balls with sub-coiling structures from thicker buckled coiling structures [103]. Zheng et al. used near-field electrospinning to prepare different linear of arcshape micro-patterns. They managed connecting micro-pillars on the substrate by nanofibers placed in this way, especially by combining NFES with extruded structures which had concentrated the electric fields and thus attracted the applied nanofibers [104]. It should also be mentioned that near-field electrospinning can not only be used to place single fibers, but also to stack fibers sequentially, as shown in Fig. 2.22 [105]. This most recent electrospinning technique thus opens completely new opportunities to prepare defined nanofiber structures from 1 to 3D, in this way enabling new functionalities as sensors, nano-generators [105] and in diverse other fields of research.

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Fig. 2.21 Polystyrene patterns on glass slides, prepared with a tip-collector distance of 2 cm and a voltage of 2.2 kV. Reprinted from [103], Copyright (2012), with permission from Elsevier

2.9 Conclusions During the last few decades, electrospinning has developed from first experiments to a highly sophisticated technology, enabling producing fibers from diverse polymers and polymer blends as well as non-polymeric materials. Combining experiments and simulations resulted in understanding the electrospinning process better, and following from this, in developing new spinning equipment, either needle-based or needleless spinnerets and diverse geometries of static and dynamic collectors, partly even fluid ones. With near-field electrospinning, even more degrees of freedom of nanofiber positioning became available. This results in a broad and still emerging range of possible applications of electrospun nanofiber mats, from biomedicine to catalysis, from energy harvesting to filters and environmental protection. We hope that this chapter gives the readers a stable fundament to find the optimum electrospinning equipment for their own applications.

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Fig. 2.22 a Scheme of the NFES process used to produce stacked 3D structures from fibers placed at defined positions; b Aligned fiber structures on a paper substrate; c 3D square produced with sequential NFES; d SEM image of approx. 600 fiber layers building the “walls” of the square shown in (c). Reprinted from [105], Copyright (2016), with permission from Elsevier

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Chapter 3

Characterization of Electrospun Nanofibers Archana Samanta, Pratick Samanta, and Bhanu Nandan

3.1 Introduction Electrospun nanofibers have attracted attention owing to their intricate porous structures and high surface area to volume ratios [1]. Being a straight forward, costeffective, and continuous fabrication technique which has made them popular over other processes for developing nanofibers matrices, like self-assembly, 3D printing, or others [2, 3]. Versatility in selection of polymers along with ease in the incorporation of functional additives has made this process popular across various fields of science and research [4]. With the feasibility to control fiber diameters, nano- and micro-dimensional fibers produced from electrospinning technology have immensely been researched for potential applications in the field of fluid adsorption [5], catalysis [6], filtration [7], drug delivery [8], sensing membranes [9], and others. Solvent free approach using electrospinning where a polymer is heated above its melting point has also been explored to form fibers in a sustainable approach [10]. Whereas, this technique is limited to low melting polymers, solution electrospinning where a polymer is dissolvent in a suitable solvent with high dielectric constant has mostly been used for the fabrication of porous functional membranes. Over 100 synthetic and natural have been used for forming electrospun fibers, details of

A. Samanta Department of Applied Physics, KTH Royal Institute of Technology, 114 19 Stockholm, Sweden P. Samanta Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden B. Nandan (B) Department of Textile and Fibre Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Das (ed.), Electrospun Nanofibrous Technology for Clean Water Production, Nanostructure Science and Technology, https://doi.org/10.1007/978-981-99-5483-4_3

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polymers system along with electrospinning conditions for generation fibrous mats have been reviewed by Ramakrishna et al. [11]. Properties of the electrospun fibers are dependent on the electrospinning set up such as flow rate, potential difference, needle to collector distance, etc., polymer properties like molecular weight, chemical composition, etc., solvent properties such as viscosity and solution conductivity, and on spinning conditions like humidity and temperature [12]. Variation in any of these parameters can influence their properties and affect their behavior at the intendant application. Recently, the use of complex fluids like gels, emulsions, and co-extrusion techniques involving two different fluid systems has made it necessary to understand the influence of constituent system on the fiber properties [13–16]. It is therefore important to detail the various characterization techniques to understand their behavior and establish their structure–property relationship for the desired application. The characterization techniques can be classified into two types namely physical and chemical. In brief physical characteristics involve the observable properties of the fibers without altering the chemical composition. It encompasses morphology, surface wettability, electrical conductivity, mechanical, absorption, surface area, and others which are detailed in the continuing section. Chemical properties are dependent upon the basic constituent chemistry of the polymer from which the nanofiber is made of and involves properties such as reactivity or chemical stability, flammability, stability against oxidation, and others. The details against each of these techniques are derailed in the following section.

3.2 Physical Characterization Physical properties of fibers such as morphology, surface wettability, mechanical properties, thermal stability, etc., play a detrimental role in deciding the application effectiveness. These properties can be controlled by the electrospinning parameters and the fiber constituency. Therefore, a greater understanding of the physical properties can help achieve fiber fabrication with enhanced durability and desired properties. The various physical properties affecting fiber characteristics are discussed in the following section.

3.2.1 Surface Morphology Surface morphology including the diameter of nanofibers, textures like porosity or unevenness, embedment of nanoparticles for functionality, etc., are characterized using microscopy techniques. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are majorly used for carrying out the surface topographic analysis of electrospun nanofibers. In this technique, a focused beam of electrons (called primary

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beam) generated from an electron gun (for example a tungsten filament) strikes the surface of the sample. Thereafter, the primary electrons interact with the atoms of the sample, and certain amount of electronic signals are released from the sample which gives information about the sample [17]. For example, secondary electrons with a mean free path within the solid usually carrying a low energy of about 50 eV are emitted from the top few nanometers of the sample. A secondary electron detector collects the released secondary electrons from the sample by scanning an area on point basis, the intensity difference of which results in the formation of an image on a computer screen. When the primary electron beam strikes the sample surface, some electrons are reflected back in the form of electric scattering, usually emerging from atoms below the surface layers of secondary electrons, these electrons are called backscattered electrons (BSE). A back-scattered electron detector collects these electrons to give information about the element distribution (not identification). Interaction of a primary electron with the inner shell atoms of a sample results in the emission of high energy X-rays from the sample. These emissions are characteristics of particular element in the periodic table and help in identification and detection of elements in the form of energy-dispersive X-ray spectroscopy (EDS). Electrospinning of polymer solution can result in the formation of various morphologies depending on the polymer chemistry, viscosity, and electrospinning conditions of humidity, flow rate, etc. Morphological aspects, such as diameter, inherent porosity, core–sheath characteristics are crucial toward intendant applications. The morphology analysis is therefore an initial step toward optimization of spinning conditions to achieve uniform fibers with desired surface characteristics. In certain cases, the degradation of bio-polymeric poly (ε-caprolactone) (PCL)/ poly (L-lactic acid) (PLLA) electrospun fibers in phosphate buffer of esterase and water was studied using SEM to generate porous fibers [18]. Influence of viscosity of polymer solution on the morphology of cellulose acetate fibers was studied. It was observed that the morphology of the fibers improved from beaded to bead-free uniform diameters with an increase in polymer viscosity owing to attain optimum entanglement density to counteract the electrostatic and columbic repulsion forces arising from the stretching process during the electrospinning process [19]. In a study it was observed that an increase in relative humidity from 30 to 60% resulted in the formation of surface pores on poly(styrene) (PS) fibers caused by condensation of moisture at a higher relative humidity on the surface of the fibers [20]. Electrospinning of PLLA fibers was carried out to study the influence of flow rate on fiber diameter. It was observed from SEM that the diameters of the fibers increased with an increase in flow rate at a constant applied voltage [21]. Influence of working temperature on the morphology of electrospun fibers was studied in poly (acrylonitrile) (PAN), with N,N-dimethylacetamide as the solvent and poly (vinylpyrrolidone) (PVP), with ethanol as the solvent in which it was observed that an increase in working temperature reduces the fiber diameter, possibly arising from reduction in polymer viscosity. However, a very high temperature may result in quick evaporation of solvent and result in premature termination of fiber drawing [22]. Electrospinning can be used to characterize several surface structures such as perforated fibers, beaded fibers or

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Fig. 3.1 SEM images of various electrospun fibers. Electrospinning can be used to generate a perforated fibers; b beaded fibers, c straight fibers (a–c) [15] d porous fibers, e ribbons, f micro cups (e, f) [24] g–j represent multi-channel electrospun fibers [25]. Figures are taken with permission from a–c Elsevier (2016) Journal of Colloid and Interface Science Volume 471, 1, 29, e, f Elsevier (2015) European Polymer Journal Volume 69, 284, g–j Intechopen (2010) Core–Shell Nanofibers: Nano Channel and Capsule by Coaxial Electrospinning

straight fibers [15], porous fibers [23], ribbons, micro cups [24], or multi-channel electrospun fibers [25] for various applications as shown in Fig. 3.1. TEM is another technique to characterize the morphology of electrospun fibers. In TEM, a beam of electrons is passed through a thin (less than 100 nm) section of sample and a detector collects and quantifies the transmitted electrons passed through the sample. Image is created by the contrast obtained from the collection of transmitted electrons. The transmission property is dependent upon the elemental constituency and the material structure, for example, porous or dense, accordingly the image of the sample is created [26]. A major advantage of TEM over SEM is higher resolution capability owing to the smaller de Broglie wavelength of electrons. Often, TEM and SEM have attachments to connect with EDS systems to get information about sample morphology along with elemental composition if required. TEM is used to characterize core–shell or presence of functional particles on electrospun fibers. For example, electrospun fibers with poly (ethylene oxide) (PEO)/ gelatin as the shell layer and poly (vinyl alcohol) (PVA)/chitosan/glucantime as the core were assessed for the core and sheath thickness using TEM [27]. The

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core–shell thickness of antimicrobial fibers from PVA-(ethyl lauroyl arginate)/poly (lactic acid) (PLA) core/shell fibers for potential applications in food packaging, were studied using TEM [28]. Similarly core–sheath thickness was assessed using TEM for PVA/PCL pH-responsive core/sheath fibers for potential applications in biomedical research [29]. Fibers with graphene sheets were fabricated for the detection of environmental humidity. The graphene content was varied and TEM was used to characterize the location and distribution of graphene on PVA fibers [30]. Zinc oxide (ZnO)-Titanium dioxide (TiO2 ) doped PAN fibers were developed for the elimination of Cr (VI) from polluted water, in which the distribution of ZnOTiO2 was characterized using TEM [31]. TEM has been used for characterization of magnetic particles on electrospun fibers [32], assessment of thickness of core and sheath from PEO/gelatin-PVA/chitosan [27], TiO2 nanoparticles in/on the poly (3-hydroxybutyrate) fibers [33], nylon-6/chitosan core/shell fibers [34], electrospun LiFePO4 composites heat treated at 700 °C [35], SnO2 nanofibers decorated with N-doped ZnO nanonodules [36], and others, as shown in Fig. 3.2. Fluorescence microscopy is used to detect the presence of fluorescent compounds (dyes, quantum dots, etc.) in the electrospun fibers. Laser scanning confocal microscope is used for detecting fluorescent compounds with much higher resolution with

Fig. 3.2 TEM images of electrospun fibers showing different surface features of a magnetic particles on electrospun fibers [32], b assessment of thickness of core and sheath from PEO/gelatinPVA/chitosan, [27] c TiO2 nanoparticles in/on the poly (3-hydroxybutyrate) fibers [33], d nylon-6/ chitosan core/shell fibers [34], e electrospun LiFePO4 composites heat treated at 700 °C [35], f SnO2 nanofibers decorated with N-doped ZnO nanonodules [36]. Figures are taken with permission from a Nature (2020) Scientific Reports 10, 367, b Elsevier (2020) International Journal of Biological Macromolecules, 163, 288, c MDPI (2020) Polymers, 12 (6), 1384, d Springer Nature (2020) Journal of Nanobiotechnol, 18 (1), 51, e Elsevier (2019) Composite Fibers for Use as Cathodes in Li-Ion Batteries. Data in Brief, 26, 104,364 f RSC (2012) Journal of Material Chemistry, 22 (29), 14,565

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3D imaging technique. In these techniques a monochromatic light beam is used to excite the sample which is absorbed by the fluorophores, the result of which leads to the emission of fluorescent light (at longer wavelength) at selective wavelengths characteristics of the fluorescent compound. Detection of protein/antibodies by labeling the antigen with an antibody-conjugated fluorescent dye helps in sensing the fluorescent signal. Use of fluorescent and confocal microscopy are mostly used to evaluate the cell growth tendencies and cell viability of electrospun matrices for several tissue engineering applications [37–39]. Coaxial electrospinning was carried out to fabricate core–shell fiber of silk fibroin/poly (PLLA-co-PCL)-PEO) for tissue engineering applications. Core–sheath structure was determined using red fluorescence Rhodamine dye in core and green fluorescence FITC in shell, shown in Fig. 3.3a [40]. Fluorescence microscope was used to characterize the distribution of tangeretin flavonoid in PVA/poly (acrylic acid) (PAA) electrospun fibers for waterinsoluble drug delivery approach [41]. Distribution of curcumine in gelatin electrospun fibers developed for therapeutic effects was studied using fluoresce microscopy [42]. Core–shell fibers of PEO/PCL encapsulating luminescent chromophores or proteins were developed for controlled molecular release in which the luminescent chromophores were characterized for their distribution across the fiber length using confocal microscopy [43]. Similarly, the core–shell morphology of Poly (γ-glutamic acid) (γ-PGA) as the core and PLA as the shell materials were characterized using confocal microscopy using rhodamine and coumarin-6 [44]. Poly (d,l)-lactide-coglycolide (PLGA) containing antibiotics (vancomycin and ceftazidime) were electrospun in which confocal microscopy was used to characterize the homogeneous distribution of antibiotics in the fiber [45]. Atomic force microscopy (AFM) is also used to characterize the surface topography and the surface roughness of electrospun fibers. This technique is quite useful

Fig. 3.3 a Fluorescence image of core–shell fiber of Silk Fibroin/poly (PLLA-co-PCL)-PEO electrospun fibers [40] and b AFM image of bi-component electrospun fibers of PCL and gelatin [54]. Figures are taken with permission from a Nature (2017) Scientific Reports, 7 (1), 8509 and b MDPI (2018) Journal of Functional Biomaterial, 9 (2), 27

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for assessing wet samples under vacuum, ambient, or selective fluid environment. AFM has a cantilever with a sharp tip which works on a raster scanning mechanism. AFM consists of a cantilever and a sharp tip attached to one end of the cantilever. This is called as AFM probe. Usually, the cantilever tip is brought in contact with the sample surface and the X–Y grid raster scanning allows the cantilever tip to move across the sample surface while maintaining a constant force. The displacement of the tip in z direction in relation to the original position is monitored by a laser beam. The reflection of the laser beam from the cantilever is tracked by a position sensitive photo-detector (PSPD) which records the vertical and lateral displacement of the probe. In contact mode, the AFM probe is in contact with the sample surface during scanning which causes the cantilever to deflect caused by repulsive forces originating from the topographical features of sample. In non-contact mode, the cantilever oscillates above the surface of the sample as it scans. A high speed feedback loop prevents the cantilever tip from crashing into the sample surface. As the tip approaches the sample’s surface the oscillation amplitude reduces due to long-range force such as Van der Waals forces. The feedback loop helps monitor these amplitude deviations and provides information about the sample topography. In tapping mode, the cantilever oscillates at much higher amplitudes of oscillations compared to non-contact mode. The bigger oscillations make the deflection signal (arising from dipole–dipole interactions, electrostatic forces, etc.) large enough for the control circuit, and hence an easier control for the topography feedback results in the production of most AFM results. In this mode the tip contacts the sample surface intermittently, and therefore, the chances of tip damage are less compared to the contact mode. Further details on the working principle of AFM are described elsewhere [46]. Some examples of morphology assessment of electrospun fibers using AFM include coaxial electrospun Poly (methyl methacrylate) (PMMA)–PAN nanofibers [47], electrospun conjugated polymer/fullerene hybrid fibers [48], molecular orientations in poly (vinylidene fluoride) (PVDF) and PAN nanofibers [49], morphology of electrospun silk fibers [50], and others. Non-contact AFM mode was used for assessing the surface roughness of PVDF/PAN blend fibers in which it was observed that the surface roughness reduces with an increase in fiber diameters [51]. Electrospun fibers from glycerophosphate-polylactic copolymer (GP-PLA) with intrinsic surface pores were developed for tissue engineering applications. The smoothness of these fibers was compared with native PLA electrospun fibers using AFM in which it was observed that the surface of the GP-PLA film is smoother than PLA fibers and the prominent crystallizing points observed in PLA fibers were absent in GP-PLA. The authors concluded that the GP-PLA is more hydrophilic encompassing lesser crystallinity and dominant surface pores compared to PLA [52]. Electrospun fibers from blends of poly (thiophene phenylene) (PThP) and PLGA were produced for tissue engineering applications and the morphology of the resultant fibers indicating the formation of uniform bead free fibers were studied using AFM [53]. Bi-component electrospun fibers were produced from PCL and gelatin for tissue engineering applications. The morphology and diameter of these fibers were determined using AFM as shown in Fig. 3.3b [54].

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3.2.2 Wettability The wettability of electrospun fibers is influenced by the surface free energy of the constituent polymers and is measured by determining the contact angle between solid–liquid interfaces. In general, if the adhesive force between solid and liquid is higher than the cohesive force between the liquid molecules, this force can overcome the surface tension of the liquid and results in the spread of the liquid drop over the solid surface [55]. This measurement is usually carried out in a goniometer where a measured volume of water drop is allowed to drop on a flat electrospun film (over a glass slide). Contact angle is the angle formed by the liquid between the liquid–solid and liquid–air interface. It is derived from Young’s equations, as given in Eq. (3.1). γSA = γSL + γLA cos θ

(3.1)

where, γ SA , γ SL , and γ LA are the surface tension values between solid–air, solid– liquid, and liquid–air, respectively, shown in Fig. 3.4. If the θ value is less than 90°, the liquid spread over the solid (electrospun fiber matrices) and the sample is considered hydrophilic. Likewise, if the value of θ value is over 90°, the liquid tends to roll over the surface without wetting it and the surface is considered hydrophobic. Samples with contact angle over 150° are classified as super hydrophobic [55]. Utmost importance should be given to the sample preparation technique to ensure a flat surface as electrospun fibers often have uneven surfaces. There are several literatures on assessing the wettability of electrospun matrices using contact angle studies [56–59]. For example, the hydrophobicity of electrospun PS fibers containing additives with 1–3 fluoroalkyl groups end groups was assessed using a goniometer in which a contact angle of 158° was observed [60]. Contact angle was used to assess the distribution of PVA and lignin on the surface of the composite electrospun fibers containing PVA, lignin, and cellulose nanocrystals (CNC) by using the surface hydrophilicity [61]. Contact angle measurements were carried out in another study to determine the asymmetric wettability of electrospun fibrous membranes for smart moisture-wicking fabrics [62]. Contact angle studies were carried out on electrospun fibers from PMMA, PLGA, poly (carbonate) (PC), PCL, PS, PVDF, and most hydrophilic Nylon 6. It was studied that surface

Fig. 3.4 Schematic representing extent of wettability: a interfacial tensions of all three phases that co-exist and static contact angle; b hydrophilic surface contact angle (θ ) < 90°; c hydrophobic surface contact angle (θ) > 90°; d super hydrophobic surface contact angle (θ) > 150° [55]. Figures are taken with permission from MDPI (2016) Materials, 9 (11), 892

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Fig. 3.5 Surface wettability assessment via contact angle assessments a PVDF and THV fibers [66] b c-PBNPs/PVA fibers [67]. Figures are taken with permission from a RSC (2017) RSC Advances, 7 (89), 56,183 and b RSC (2014) RSC Advances, 4 (103), 59,571 The white bar indicates a scale of 1 μm in the SEM images in Fig. 3.5b

roughness and fiber diameter affected the wettability of these hydrophobic matrices, making it feasible to alter the surface energy of the system without the use of any chemical treatment [63]. Yadav et al. studied that Ni coating on PAN fibers can enhance the static contact angle from 8 to 40°. Further, the PAN fibers were metallized with deposition of gold particles resulting in achieving a metastable contact angle of 91.3° [64]. Hydrophilicity of PVA-chitosan electrospun fibers was found to be enhanced compared to neat PVA fibers as studied from surface contact angle measurements, the authors considered the presence of additional OH- groups from chitosan resulted in enhanced hydrogen bonding [65]. In another study it was reported that increasing the surface roughness enhanced the hydrophobicity of the electrospun mats produced from poly (TFE-co-HFP-co-VDF) (THV), a terpolymer of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride. It was studied that increase in surface roughness increases the trapment of air inside the network structure which in turn increases the space between the solid masses and results in enhanced hydrophobicity, shown in Fig. 3.5a [66]. In another study it was observed that c-PBNPs/PVA fibers having a rough surface than pure PVA fibers showed low contact angle implying higher surface hydrophilicity due to the presence of unreacted (uncross-liked) –OH and –CHO groups, shown in Fig. 3.5b [67].

3.2.3 Porosity, Pore Volume Fraction, and Surface Area Electrospun fibers are considered to be porous non-woven mesh where the individual fibers are interconnected with several cross-over points. Sometimes, the individual fiber can bear intrinsic porosity, desirable for specific applications like tissue engineering, filtration, or absorption. Quantification of porosity and pore volume fraction can be crucial for this application. A simple gravimetric mechanism to determine pore volume fraction involves the use of the following Eq. (3.2),

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 Pv = 1 −

Pb Pc

 (3.2)

where, Pv , Pb , and Pc depict the pore volume fraction, the bulk density of the constituent polymer, and the density of the electrospun composite matrices, respectively. In case of polymer blends, Pc is determined from the ratio of their individual mass and corresponding densities [68, 69]. Porosity meter is used to measure pore sizes. Usually a known weight of material is filled inside the sample chamber which is then flushed with non-wetting mercury. The pressure of the force required to fill a certain volume is inversely proportional to the size of the pores and results in the determination of pore sizes in this approach. Washburn’s equation is used to calculate the average pore diameter and the equation is as given in Eq. (3.3). Dp =

−4Y Cos θ P

(3.3)

where, Dp is the pore diameter, Y is the surface tension of mercury, θ the contact angle between mercury and the sample, and P is the pressure through which mercury is forced inside to fill the pores of the sample inside the sample chamber [69]. This technique is used for the determination of micron level porosity; however, the fibers may deform under pressure to fill nano level pores under this technique. Brunauer–Emmett–Teller (BET) is used to determine nano level porosity in samples. Under this technique gases like nitrogen gas are allowed to flow through the sample. The amount of gas physisorbed or the sample surface is proportional to the surface area corresponding to a single layer of absorbate on the sample surface. BET in combination with Barrett-Joyner-Halenda (BJH) is studied for analyzing pore size distribution of meso level pores [70]. Specific surface area (SA) is determined from Eq. (3.4), SA = N A σ Q

(3.4)

where, N A is the Avogadro constant, σ is the area of the sample covered by one molecule of the adsorbing gas, and Q is the amount of gas adsorbed in 1 g of sample in a monolayer (mol/g) [71]. Porosity content and comparison of pore size distribution were studied for electrospun fibers from polylactide, PCL, gelatin, and polyamide using porosimetry [71]. The use of porosity meter for determination of pore characteristics influenced by web density and fiber diameter was studied in electrospun fibers from PVA [72]. In another study, porosity meter was used to determine an increase in average pore size of PCL electrospun fibers with an increase in fiber diameters [69]. Porosity meter was used to study porosity characteristics of nanofibrous electrospun composite fibers from PAN and PVDF where an increase in the total specific surface area could be achieved due to an increase in the temperature of oxidative warming up of mats from 250 to 330 °C followed by vacuum pyrolysis at 900–1000 °C [73]. BET of carbonized electrospun

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Fig. 3.6 a Surface area analysis using BET [76] and b pore size distributions of the 6FDA–TrMPD polymer powder, the nanofibrous mat, and the film [77]. Figures are taken with permission from a PLoS ONE (2019), 14 (2), e0211731 and b RSC (2020) Environmental Science Nano, 7, 1365

fibers from PAN exhibited an increase in specific SA as a function of nanofiber diameter, with values of 684 m2 /g at 407 nm, for potential applications as electrodes [74]. PVP-TiO2 fiber electrospun at higher humidity showed higher surface porosity and BET revealed the highest BET-specific SA of ∼127.5 m2 /g compared to fibers produced at lower humidity conditions [75]. It has been reported that the surface area of pitted electrospun fibers is higher than smooth electrospun fibers from PLLA as assessed using BET. The authors claim this enhancement of surface area can enable the extension of long neurites, shown in Fig. 3.6a [76]. A comparison between film, powder, and electrospun mat from 6FDA–TrMPD showed a noteworthy development of pore size distribution in the fibers mesoporous range (2–50 nm), shown in Fig. 3.6b [77]. Use of BET for measuring the pore characteristics in electrospun has been studied in several other literatures [78–81].

3.2.4 Mechanical Properties Mechanical properties of polymeric fibers are influenced by the relative humidity of the environment and therefore samples are usually preconditioned at desired humidity and temperature conditions for certain duration before measuring their mechanical properties as per ASTM standards. Mechanical properties of individual electrospun fiber can be determined using AFM. In this technique, a load is applied perpendicular to the longitudinal axis of fiber (usually deposited on a quartz slide) with the help of a cantilever and the deformations induced by the external force on the fiber are then measured [82]. Generally, one end of the fiber is attached to the cantilever to act as the force-sensing element and the other end of the individual fiber is attached to a movable pin which can pull the sample. Starting with a fixed gauge length between the cantilever and the movable pin, the pin gradually moves apart at a constant strain rate and the deflection of cantilever induced by the fiber pull is measured. Taking

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into account the spring constant of the cantilever, the deflection induced by each single strain step is converted to load values, and a stress–strain graph is obtained by taking into consideration the strain rate at the movable end of the pin and the crosssectional area of the fiber [83]. For example, Young’s modulus of poly [2-methoxy5-(2-ethylhexyl-oxy)-1,4-phenylene-vinylene] (MEH-PPV) electrospun fibers was determined using AFM. The crystallinity affected the indentation and the mechanical properties across the axis of the fibers [82]. In another study mechanical properties of poly (urethane) (PU) fibers were determined using AFM technique. The authors noticed an increase in Young’s modulus with a decrease in fiber diameter due to the surface tension effect [84]. 2 or 3-point bending modes have also been carried out using the AFM cantilever deflection technique to determine the elastic bending modulus and the shear modulus of individual electrospun fibers. Details of the measurement process are described elsewhere [85, 86]. In one study, the impact of temperature on the mechanical properties was determined using 3-point bending technique. A decrease in mechanical properties with an increase in temperature was observed, possibly arising from increased polymer chain mobility with temperature [84]. For example, the elastic modulus of individual electrospun PVA fibers was measured using AFM-based bending tests where an increase in elastic modulus with decrease in fiber diameter was observed [87]. Micromechanical bending tests in a scanning mode in AFM were used for analyzing the mechanical properties of single electrospun collagen fibers in which it was observed that bending moduli of the cross-linked electrospun collagen fibers at ambient conditions range from 1.3 to 7.8 GPa with decrease ∼20 times when the fibers are immersed in PBS buffer. Fibers were developed for potential application in tissue engineering [88]. In another study the total stretch modulus of dried electrospun fibrinogen fibers was determined to be 4.2 GPa while the relaxed elastic modulus was 3.7 GPa [89]. The bulk mechanical properties of electrospun matrices are measured using an instron mechanical tester. Usually the sample with specific dimensions according to ASTM standards is fixed between two clamps and one end of the clamp is subjected to pre-defined strain rates. The force experienced at the other end of the clamp is constantly recorded. Taking into consideration, the specimen dimensions and the elongation rate of the jaw, stress–strain curves of the sample are obtained. Information about braking modulus, ultimate tensile strength, work of rupture, etc., can be obtained from this analysis. The use of instron for mechanical property analysis and comparison of the composite properties has been detailed in several literatures [16, 90, 91]. For example, Young’s modulus and tensile strength of PCL and PLGA fibers at various blend ratios in the presence and absence of hydrophilic drug, tenofovir (TFV) was assessed using instron [92]. In another study instron was used to detect an increase in tensile strength and Young’s modulus of hordein/zein fibers from 4.36 ± 0.29 to 7.79 ± 0.36 MPa and from 195.80 ± 13.02 to 396.64 ± 18.33 MPa, respectively, with the inclusion of 3 wt.% of surface-modified cellulose nanowhiskers [93]. In another study it was observed that an increase in diameter of PVA and cellulose acetate bi-component electrospun fibers resulted in a reduction in elastic modulus, caused

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Fig. 3.7 a Modulus versus fiber diameter in PVA and cellulose acetate bi-component electrospun fibers [94] and b Relation between Young’s modulus to the degree of crystallinity of PVA composite fibers [95]. Figures are taken with permission from a Elsevier (2019) Alexandria Engineering Journal 58 (3), 885 and b RSC (2017) RSC Advances, 7 (69), 43,994

by reduction in number of fibers per cross-sectional area of the fibrous fat at the jamming region, shown in Fig. 3.7a [94]. The tensile properties of electrospun PVA fibers were reported to be enhanced by subjecting them to a secondary crystallization through freezing/thawing process which was further confirmed by XRD. The tensile strength was increased up to ∼165% by increasing the crystallinity from 23.5 to 43.6%, shown in Fig. 3.7b [95]. Use of instron for characterization of mechanical properties of electrospun fibers has also been reported in several studies [96–98].

3.2.5 Crystal Structure XRD The crystal structure of the constituent components in electrospun matrices can be determined using Wide Angle X-ray diffraction (WAXD), a non-destructive technique. The interaction of X-ray depends upon the crystal structure (periodic array of order) of a material. In brief, when X-rays strike a material, electrons/photons can emerge from the sample surface. The emitted radiation can undergo constructive or destructive interference [99]. Constructive interference follows Bragg’s law which helps to determine the crystal structure. Bragg’s equation is given as Eq. (3.5), nλ = 2d Sin θ

(3.5)

where, n is an integer, λ is the wavelength of the radiation, d is the spacing between atomic planes, and θ is the angle between the radiation and the atomic planes (Bragg angle) [100]. Crystalline materials show sharp peaks at characteristic diffraction angles, whereas amorphous materials show broad hump in the absence of sharp peaks. The crystallographic planes in a crystal are characterized by Miller indices (hkl), integral numbers that are related to the reciprocal values of the intersection of a given plane with the crystallographic unit-cell axes. Semi-crystalline materials possessing crystalline and amorphous regions are quantified in terms of degree of crystallinity,

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indicating the percent of crystalline components. The degree of crystallinity (D) for a semi-crystalline material is given by Eq. (3.6), D=

Ac At

(3.6)

where, Ac and At stand for the crystalline area fraction and total area in a WAXD graph, respectively [101]. Crystal sizes are determined using Scherrer’s equation, given as represented in Eq. (3.7), L=

Ks λ (Cos θ )τ

(3.7)

where, L is the mean size of the ordered (crystalline) domains, K s is a shape factor constant in the range 0.8–1.2 (typically equal to 0.9), λ is the X-ray wavelength, τ is peak width at full width at half maxima, and θ is the Bragg angle [101]. Long order arrangement/periodicity is determined using the small angle X-ray scattering (SAXS) technique. Some examples involving the study of crystal structures of electrospun fibers using WAXD include; crystal structure of poly (oxymethylene) nanofibers was studied using WAXD where the authors have related and modified the crystal structure with the electrospinning conditions [102]. In-situ determination of crystal structures of PVDF electrospun fibers was determined using WAXD where α to β crystal phase transformation was determined with alteration in strain rate [103]. Crystalline properties of polyamide-6/organic-modified Fe-montmorillonite composite nanofibers were also determined using this technique [104]. Crystalline phase of nano-hydroxyapatite (nHAP) was characterized by 2θ peaks at 25.6°, 31.7°, 32.2°, 33°, 34.1°, and 39.8° in Gelatin-PCL-nHAP composite electrospun fibers developed for effective osteoblasts cells adhesion and proliferation [105]. Gelatin electrospun mat with ZnO/graphene oxide (GO) was developed for antimicrobial wound dressing applications. The development of ZnO/GO in the electrospun fibers was characterized using WAXD by analyzing the intensity of characteristics peaks at 31.8°, 34.5°, 36.3°, 47,6°, 56,6°, 62.9°, 68.0°, and 69.1°, respectively, shown in Fig. 3.8a [106]. Presence of Si crystals in carbon electrospun fibers was characterized by 2θ peaks at 28.5°, 47.4°, 56.3°, 69.2°, 76.8°, and 88.3°, respectively, developed for electrode materials for lithium-ion batteries, shown in Fig. 3.8b [107].

3.2.6 Thermal Characterization Differential scanning calorimetry (DSC) is used for determining the glass transition, phase transition temperatures (melting and crystallization), enthalpy of fusion and crystallization, and crystallinity percentage of thermoplastic polymers [108]. This

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Fig. 3.8 XRD patterns of a ZnO/GO [106] and b Si–C electrospun fibers [107]. Figures are taken with permission from a MDPI (2020) Molecules, 25 (5), 1043 and b Frontiers in Chemistry (2020) 7, 867

instrument is equipped with a sample and reference pan both connected to a thermal sensor which is designed to maintain the same temperature in both pans. A sample can undergo exothermic or endothermic reactions at particular temperatures, and the differential heat provided to maintain both pans at the same temperature is calculated and plotted against temperature regimes. In electrospun composites, the impact of external moiety like nanoparticles of the crystallization and fusion behavior of the neat polymer is often compared and studied. This technique is also used for evaluating the extent of curing and studying the crystallization kinetics of polymer systems. In a study the Tg of PLGA and drugincorporated PLGA was observed at 53 °C and 50 °C, respectively, analyzed by using DSC. The authors report that the decrease in Tg in the inclusion of drugs can arise from the interference of drug molecules on the ordering of PLGA polymer chains. This also resulted in lowering of melting temperature (T m ) of the mats from 349 °C to 329 °C for PLGA and PLGA-drug incorporated fibers, respectively. These fibers were developed for potential antimicrobial applications, shown in Fig. 3.9a, b [109]. In another study, it was observed that the presence of activated carbon lowered the intensity of the crystallinity peaks of the Poly (butylene adipate-co-terephthalate) (PBAT)/PCL blend, possibly due to the plasticization effect by the inclusion of activated carbon [110]. Also, it was observed that the inclusion of nano clay and NiFe2 O4 nanoparticles in polymer poly (vinylidene fluoride-co-hexafluoropropylene) electrospun fibers β phase in PVDF and shifted the melting temperature to higher values as β has a higher melting point than α phase [111]. Use of DSC for quantification and comparison of enthalpy of crystallization and fusion along with melting and crystallization temperatures of a polymer in presence of a functional moiety for example TiO2 , silver, antimicrobial particles, etc., have been reported in several literatures [112–115]. Thermogravimetric analysis (TGA) gives information about the thermal stability, onset of degradation, oxidation/combustion, residual chars %, and degradation

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Fig. 3.9 Thermal properties of electrospun fibers. a and b DSC, c TGA thermograms of the PLGA and PLGA-TSC-10 fiber mats [109]. Figures a–c are taken with permission from Wiley (2020) Polymer for Advanced Technology, 31 (12), 3182

steps involved (if any). In this analysis the mass loss of the sample is owing to thermal degradation is continuously monitored with change in temperature under air/nitrogen/argon environment. TGA instruments are coupled with spectroscopy to determine the oxidation by-products by alteration of temperature [116]. For example, thermal degradation analysis of poly (aniline) (PANI) electrospun fibers for supercapacitor applications was studied using TGA [117]. Thermal decomposition temperature of PVA was improved by 40 °C by the inclusion of 6% of graphene, as analyzed using TGA in one of the studies [30]. In another study it was observed that the inclusion of Aloe Vera into PVA electrospun fibers resulted in the enhancement of thermal stability to 80 °C [118]. ZnO rods were grown onto the surface of poly ethersulfone electrospun fiber for antimicrobial applications. The presence of ZnO was found to advance the thermal resistance of the polymer by retarding its degradation process [119]. The degradation of PLGA fibers was insignificantly affected by incorporation of drugs, as observed from TGA, shown in Fig. 3.9c [109]. TGA of hydroxyapatite-glass fibers for removal of cadmium (Cd+2 ) and lead (Pb+2 ) from aqueous media reveal an initial moisture loss of 4% at 90 °C, loss of 29% from organic contents took place at 350 °C and 62% weight loss was observed at 444–636 °C corresponding to the sintering of the ceramic materials [120]. The use of TGA to study and compare the thermal stability of a polymer in presence of functional substances has been extensively studied and reported [121–125]. Degradation studies of several electrospun fibers are also studied using TGA elsewhere [126–128].

3.2.7 Electrical Conductivity Electrical conductivity determination is crucial for applications involving supercapacitors, electromagnetic shielding, sensors, photovoltaics, and others. Electrical conductivity (σ) is given as represented in Eq. (3.8).

3 Characterization of Electrospun Nanofibers

σ =

53

L AR

(3.8)

where, L, A, and R are the distance between 2 electrodes, the cross-sectional area of electrospun fiber mat, and the electrical resistance of the fiber mat, respectively [129]. Conductive polymers like olyacetylene (PA), PANI, poly (pyrrole) (PPY), poly (thiophene) (PT), and its derivatives of poly (3,4-ethylene dioxythiophene) (PEDOT) and poly (3-hexylthiophene) (P3HT), etc., containing sp2 hybridization for charge carriers have been used for fabricating conductive electrospun mats [3]. Inclusion of fillers like graphene [130], carbon nanotubes [131], etc. [132], have been studied extensively for the fabrication of conductive electrospun composites. Conductive particles and fillers like carbon, and graphene are generally incorporated into the polymer phase to generate conductive electrospun fibers [132]. Ceramic nanoparticles included inside the matrix of the polymer in electrospun composites are sintered to generate conductive matrices of Cu [133], Pt [134], and other metal nanoparticles [135].

3.2.8 Magnetic Properties One-dimensional (1D) magnetic nanoparticles have been incorporated recently into the polymer matrix of the electrospun fibers for their applications in biomedical engineering, for example targeted drug delivery [136], magnetic resonance imaging (MRI) [137], magnetic cell separation, and others [137]. Magnetic properties can be measured using a superconducting quantum interference device (SQUID) [138]. The details about its working principles are detailed elsewhere [139–141]. Vibrating sample magnetometer [142] and alternating gradient magnetometer [143, 144] have also been used for detecting the magnetic properties of electrospun fibers. Magnetic properties of electrospun fibers are described in detail in a recent review article by Blachowicz et al. [145].

3.3 Chemical Characteristics of Electrospun Fibers Chemical properties of electrospun fibers are dependent upon their elemental composition, molecular weight, and functional groups. And these properties dictate their reactivity or resistance to various solvents and chemicals [146]. Some of the important characterization techniques for determining the chemical composition of electrospun fibers are detailed below.

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3.3.1 Molecular Weight Determination of Polymers in Electrospun Fibers Electrospun fibers are made from polymers with selective molecular weight, but in some cases, electrospinning is performed from monomers which are polymerized during or after the electrospinning process [126, 147]. In such cases the determination of molecular weight which directly impacts the mechanical properties of the generated mat plays a crucial role. Absolute, equivalent, or relative ways of molecular weight determination can be followed for this. Absolute methods count the number of molecules on the basis of colligative methods (membrane osmometry, ebulliometry, cryoscopy, and vapor-phase osmometry), light scattering, and others, and often help in determination of number average molecular weight of the samples [148]. Equivalent methods help in determination of molecular weight based on the chemistry for example end group analysis with titration. Relative methods help in determining the molecular weight of polymers in relation to associative physical or chemical property for example viscosity which varies with change in molecular weight of the polymers or gel permeation chromatography. Details of each of these methods along are described elsewhere [149, 150]. In brief, M n , M w , M z , and M v give indication about the number average, weight average, size average, and viscosity average molecular weight of a polymer. These values are given ∞ by, Mn = x=1 nx M x, where, nx and Mx are the mole fraction and molecular weight, respectively, of the xth repeating unit.  Mw = ∞ wx M x, wx and Mx are the weight fraction and molecular weight, x=1 respectively, of the xth repeating unit. M w and M v can be represented as depicted in Eqs. 3.9 and 3.10, respectively. Mz =

∞ 

nx M x 3

  ∞ 

x=1

Mv =

 ∞  x=1

nx M x

 nx M x 2

x=1

1+α

  ∞ 

(3.9) α

nx M x

2

(3.10)

x=1

where α is a constant that is intrinsic viscosity to molar mass. Polydispersity index (PDI) is obtained by dividing M w with M n and is used to indicate the molecular weight variations in a polymer system. M n can be determined using osmometry techniques or end group analysis. M w is determined using light scattering technique, and M v is determined using viscometry. Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF) is another absolute technique to determine the M n and M w molecular weights of polymers [151]. Working principles of MALDI-TOF are detailed elsewhere [152]. Gel permeation chromatography, a relative technique working on the principle of size exclusion can be used to determine the M n , M w , and M z of a

3 Characterization of Electrospun Nanofibers

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polymer [153]. In brief, a dilute solution of polymer is allowed to pass through a column consisting of small cross-linked beads which do not react with the polymer system. Molecules of different sizes elude at different rates from the column. The larger ones are exuded before the smaller ones with a higher retention time as they get trapped within the domains of the beads. The detailed working mechanism for GPC is described elsewhere [154, 155]. The influence of M w of PEG determined using the release profile of drug pDNA or pDNA polyplexes loaded in the core with a sheath of PLLA poly (ethylene glycol) (PEG) was studied. Inclusion of PEG resulted in sustained drug release, the authors also observed a faster pDNA polyplexes release with increase in molecular weight. For example, the release time was reduced from 6 to 20 days with the use of 10% of PEG with a molecular weight of 6 and 2 kDa, respectively. The authors presume larger molecular chains could lead to the formation of larger channels for drug diffusion on exposure to aqueous conditions [156]. PLA of different molecular weights was synthesized and their M n was determined from SEC. Electrospinning of PLA fibers with different molecular weight was carried out, and the melting point of the fiber was found to increase from 171 to 179 °C with an increase in M n from 22 × 103 to 132 × 103 , respectively. The crystallinity was observed to increase with the increase in M w which resulted in the formation of fibers with enhanced tensile strength with higher M w [157]. Most studies use commercial polymers of specified M w which are determined from the above techniques at the suppliers end.

3.3.2 Functional Group Identification—FTIR, Raman, NMR Identification of functional groups and their possible interactions can be studied using FTIR, Raman, or NMR spectroscopy. FTIR is a non-destructive analytical tool in which a material is exposed to infrared radiations and the absorbance of the sample across various wavelengths is measured to define its molecular composition and structure. Change in dipole moment results in absorption of infrared radiation in this technique, therefore symmetric bonds which do not undergo a change in dipole moment like diatomic molecules (such as H2 , O2 , N2 , Cl2 , etc.) are infrared inactive. FTIR works by detecting the stretching, bending, and vibrational or rotational modes in a molecule after absorbing energy [158]. The number of possible vibrational or rotational modes with ‘n’ number of atoms in a molecule can be calculated using the formula of 3n-5 and 3n-6 for linear and non-linear molecules, respectively [159]. Among these theoretically calculated modes, infrared active modes (with dipoles) can only be noticed in the spectroscopy. Samples are analyzed in pellet modes formed by mixing a measured content of sample with KBr (as KBr has a 100% transmission window in the range of wave number 4000–400 cm− 1 ). Attenuated total reflectance (ATR) can be used in conjugation with FTIR for measuring the IR spectroscopy of solid/liquid samples directly instead of forming a sample pellet [160]. The conventional FTIR mode of sample analysis with penetration depth below 5 microns, whereas in the ATR mode it is below 2 microns enabling the possibility

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of determining the surface characteristics. In this mode, a small quantity/section of sample is placed above an optically dense crystal with a high refractive index than the sample to be measured [161]. Crystals of Ge, ZnSe, or diamond are generally used for this. ATR adopts the property of total internal reflection which results in the formation of an evanescent wave to analyze the functional groups. An infrared beam is allowed to strike the crystal placed directly beneath the sample to be analyzed. The evanescent wave formed as a result of this extends into the sample and a detector records the attenuated reflected IR beam exiting from the crystal and generates an IR spectrum [162]. Examples of studies using FTIR-ATR for the characterization of electrospun fibers are as follows. Study of molecular orientation using FTIR in electrospun nylon6 nanofibers with variation in collector drum velocity from 0 to 300 m/min by monitoring characteristics of –NH stretching band at 3303 cm−1 and the amide I and II vibrations at 1647 and 1543 cm−1 , respectively [163]. FTIR has been used immensely to characterize functional groups and analyze interactions between functional substances incorporated into neat polymer electrospun matrices. For example, the interaction between constituent materials in electrospun fiber from cellulose acetate (CA), PEG, and silver particles was studied using FTIR. For example, the intensity of the peak at 3310 cm−1 , representing O–H stretch in CA appeared more prominent in CA/PEG/AgPs. The authors suggested the intermolecular hydrogen bonding of CA with PEG would have resulted in this. The shifting of C–O bond from 1239 cm−1 in CA to a slightly higher frequency of 1242 cm−1 was considered to be formed from the binding of C=O and C–O with Ag particles [164]. In another study, the IR spectrum of gelatin-PCL-nano hydroxyapatite composite scaffold, the shifting of these bands toward the smaller wave numbers, and the presence of characteristic bands of nHAp indicated the deposition of nHAp on Gelatin-PCL scaffold, as reported by the authors [105]. Starch nanofiber mat containing antimicrobial peptide ε-poly-lysine was produced for potential wound dressing applications. The composite fibers showed a decrease in absorption peak at 1100 cm−1 , and presence of new absorption peaks at 2930, 2890, 1670, and 1540 cm−1 compared to control starch fibers, indicating that peptide reacted with the aldehyde groups in oxidized starch, and had been successfully incorporated into the nanofibers [165]. Calcium peroxide was incorporated into silk fibroin electrospun fibers for tissue engineering applications. Methanol treatment of the fibers resulted in a significant shift of amide I, II, and III functional group signals to lower wavelengths owing to the formation of more β-sheets, as characterized by FTIR [166]. Gluten nanofibrous films incorporated with glycerol monolaurate (GML) with antimicrobial and water stability properties were developed for food-packaging applications. In the gluten film, the broad characteristic absorption bands of –OH and –NH stretching vibrations observed at around 3425 cm−1 , shifted to lower wavelengths of 3418, 3343, 3323, 3314 cm−1 with the increasing concentration of GML, indicating the formation of intermolecular hydrogen bonds between them [167]. In another study, PVA-zinc acetate fibers were formed and calcinated to develop ZnO fibers. IR bands at 1063 cm−1 and between 500 and 700 cm−1 , formed from the deformation and

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frequency modes of the CH3 and the stretching modes of the Zn–O bond, respectively, confirmed the formation of composite PVA-zinc acetate nanofibers. FTIR spectra of PVA-zinc acetate fibers in mass ratio 1:2 calcinated at 500 °C showed the absence of any organic matter [168]. FTIR was used to quantify residual solvents of HFP and chloroform from PLLA electrospun mats for ensuring the complete removal of organic solvents aimed toward developing matrices for tissue engineering applications [169]. ATR-FTIR was used for determining surface functionalization of (PCL) electrospun fibers with fibronectin by monitoring the characteristics peaks of each in the composite electrospun fibers [170]. Electrospun fibers from PMMA/PANI were developed as a support for laccase immobilization and for use in dye de-colorization. PMMA was characterized by 1770 cm−1 and 1150 cm−1 , assigned to stretching vibrations of C=O bonds in ester groups and to stretching vibrations of C–O–C bonds, respectively. PANI peaks in the wavenumber range 1550–1450 cm−1 , characteristic for stretching vibrations of the CAr–CAr bonds were also observed. In the enzyme modified electrospun fibers, an additional peak appeared, indicating effective surface functionalization, associated with stretching vibrations of amide III bonds, and has its maximum at 1255 cm−1 . After enzyme immobilization, irrespective of the technique used, additional signals with maxima at 3410, 1645, 1550, 1250, and 650 cm−1 can be observed in the FTIR spectra, and these peaks can be assigned, respectively, to stretching vibrations of –OH groups and amide I, amide II, and amide III bonds and bending vibrations of C–C bonds, and thus indicate effective deposition of the enzyme via covalent bonds and adsorption interactions, shown in Fig. 3.10a [171]. Raman spectroscopy is another non-destructive technique used to characterize the functional group and chemical composition of samples. Unlike dipole interaction as in FTIR, Raman detects changes in molecular bond polarizability (deformation of its electron cloud). Interaction of light with matter can result in scattering of photons with same energy as that of incident photons this is called as Rayleigh scattering. Some photons, (approximately 1 photon in 10 million) will scatter at a different

Fig. 3.10 Functional group analysis of electrospun fibers. a FTIR spectra of PMMA/PANI electrospun fibers containing laccase; [171] b Raman spectra of PEP (a), PCL (b), PCLPEP1.5 (c), PCLPEP3 (d), and PCLPEP6 (e) electrospun fiber mats; [179] c 1H NMR spectra of pure HPCD: (a) and Chitosan:HPCD 2:20 fiber (b) prepared in 1% (w/w) THF/DMSO-d6 solution for HPCD content determination [192]. Figures are taken with permission from a Elsevier (2020) Environmental Research, 184, 109,332, b Frontier in Bioengineering and Biotechnology (2019) 7, 346 c MDPI (2019) Fibers, 7 (5), 48

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frequency than the incident photon, this process is called inelastic scattering, or the Raman effect. The difference in energy of the incident photon to the scattered photon is called as Raman shift and is very useful for detecting bonds (such as S–H, C=S, N=N, C=C, etc., that are difficult to observe in FTIR). Raman spectroscopy was used for detecting real-time solvent/polymer ratio and polymer orientation of PS electrospun fibers [172]. Distribution of PEO and PCL in the electrospun fibers spun from a 50:50 blend of both polymers was studied using Raman spectroscopy [173]. Similar study to determine the localized composition in electrospun fibers made from combinations of PCL or PEO and hyaluronic acid was also studied using Raman [174]. ZnO fibers were produced by electrospinning aqueous PVA solution containing zinc acetate and then calcination of the resultant fibers. Characterization of ZnO nanofibers was done by identification of ZnO peaks at 436.17 cm−1 which ascertains the hexagonal quartzite structure of the ZnO nanofibers and from the peak at 580.86 cm−1 corresponding to the structural defects such as oxygen vacancy [168]. Formation of anatase form of TiO2 fibers obtained by calcinating PVP-titanium (IV) isopropoxide electrospun fibers was characterized by Raman peak at 143 cm−1 and at 396 cm−1 corresponding to bending vibrational modes of O–Ti–O. The Raman bands at 515 cm−1 and 638 cm−1 were assigned to stretching vibrational modes of Ti–O. The purity of the anatase phase was analyzed by the presence of strong peak at 143 cm−1 [175]. PCL/GO/Fe3 O4 electrospun fibers were produced for enhanced cell proliferation in tissue engineering applications. Raman analysis was used to identify GO by observing its characteristics D band at 1343 cm−1 , G band at 1592 cm−1 , and a 2D peak around 2680 cm−1 . The authors indicate that graphite was oxidized strongly during the hummer’s method and the graphite layers were affected by oxygen atoms forming various bonds with graphene [176]. PAN fibers with iron acetylacetonate were electrospun and calcinated to generate Fe-carbon fibers. Raman was carried out to characterize iron peaks at 216 cm−1 and 279 cm−1 and D peak and G peaks at 1354 cm−1 and 1585 cm−1 , respectively, indicating carbon in the material, thus demonstrating the formation of FeS-CNF composite material. The intensity ratio of D band to G indicating the degree of structural graphitization in the carbon materials was 0.97, signifying a higher degree of graphitization [177]. Sol–gel electrospinning of PVP and zirconium butoxide as precursor was carried out followed by calcination of the fibers to obtain zirconia fibers for potential uses in adsorption and catalytic processes. Raman analysis was carried out to identify the crystal phases of zirconia and was observed that the phase transition from tetragonal to monoclinic zirconium oxide occurred at 1000 °C [178]. PCL electrospun fibers were loaded with peppermint essential oil (PEP) for antibacterial applications. Raman peaks at 2870 cm−1 were assigned to terpenoid vibrations of the PEP. Peaks at 2916 and 2864 cm−1 were assigned to the asymmetric and symmetric vibrations of the CH2 groups of PCL, thus indicating the successful incorporation of PEP in PCL fibers, shown in Fig. 3.10b [179]. Nuclear Magnetic Resonance (NMR) is another useful technique to detect functional groups, degree of polymerization, residual moieties in electrospun fibers, and polymeric substrates [180]. Atoms have nuclei which carry charges [181]. Nuclei can have a magnetic spin and on exposure to an external magnetic field energy transfer can

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occur between the base energy to a higher energy level. When the nuclei return to its base spin, it releases energy at the same frequency. This energy transfer can occur at wavelengths corresponding to radio frequencies. The intramolecular magnetic field around an atom in a molecule changes the resonance frequency, and the resonance frequency is proportional to the magnetic field [182]. Therefore, each active nucleus should give one characteristic signal, thus giving access to details of the electronic structure of a molecule and its individual functional groups [183]. Signal that matches these changes in frequency is processed and plotted across wavelengths to generate NMR spectra. Some studies involving use of NMR for characterizing properties of electrospun fibers are as follows; Interaction of oil with PLA electrospun fibers indicating surface relaxation and restricted diffusion was studied using NMR [184]. Copolymer of styrene and 4-vinylbenzyl 2-bromopropionate was synthesized for electrospinning and the composition ratios of the corresponding blocks were determined using NMR [185]. NMR was also used to determine the chemical stability of curcumin and vanillin encapsulated within phospholipid electrospun fibers [186]. β sheet conformation electrospun silk fibroin treated with methanol to generate surface grooves conducive toward cell attachment was confirmed using NMR in one of the studies [187]. γ -Cyclodextrin (γ-CD) and Ferulic acid (FA) were encapsulated into PVA to form electrospun nanofibers for food-packaging applications. 1 H NMR spectra of PVA/γ-CD/FA showed the presence of characteristics peaks of FA and γ-CD, confirming the presence of FA and γ-CD in PVA nanofibers. The molar ratio of FA/γ–CD was determined from NMR obtained by integrations of γ-CD peak (H-2, H-3, H-4, and H-5) 1.02, and FA peak (H–d) 0.23 was found to be 0.92 [188]. Thiosemicarbazone (TSC) N4-(S)-(1-phenylethyl)-2-(pyridin-2ylmethylene)hydrazine-1-carbothioamide (HfpyTSCmB) were incorporated to poly (lactic-co-glycolic acid) (PLGA) to produce antibacterial electrospun fibers. The chemical integrity of HfpyTSCmB in the fiber mats was studied via 1 H NMR by observing the characteristics peaks of HfpyTSCmB and PLGA, respectively [109]. Electrospinning of flax lignin/PEO followed by carbonization was carried out to obtain carbon nanofibers. Lignin with higher proportions of p-hydroxyphenyl termed as H-lignin was determined to be 0.38 mmol/g from [31] P NMR, representing 20% of the total number of free phenolics in the lignin [189]. PEO/PS matrix and poly (ferrocenylphosphinoboranes) were electrospun for potential fabrication of generation of miniaturized devices. 1 H NMR indicated the characteristics peaks from signals of the ferrocenyl protons at 4 ppm and showed that the poly (phosphinoboranes) are present in very along with the organic polymers [190]. Thermo- and pH-sensitive poly (N-isopropylacrylamide-N-methylolacrylamide-acrylic acid) electrospun fibers were containing drugs developed for potential application as drug carriers. Characteristic peaks in 1 H NMR of the constituent components revealed the successful fabrication of the composite fibers [191]. Chitosan/hydroxypropyl-β-cyclodextrin (HPCD) electrospun fibers were developed for potential coatings for functional applications. HPCD content in the resultant fibers was determined from 1 H NMR on basis of integral peak intensities of a –CH3 group of HPCD (δ = 0.99 ppm) relative to the

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internal standard THF (δ = 1.60 ppm) and was found to be ~40 and 75% in fibers with Chitosan: HPCD of 2:20 and 2:50, respectively, shown in Fig. 3.10c [192].

3.3.3 X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) is a surface characterization technique for detecting elements along with their specific chemical bonding with a surface penetration of depth of 2–5 nm. It is based on the principle that when X-ray with energy hυ (where h is the plank’s constant and υ is the frequency of photon) strikes the surface of a material, information from the kinetic energy and the number of the ejected electrons are used for depicting elemental properties. It is based on the energy conservation equation given as represented in Eq. (3.11), hυ = E(Fermibinding) + φspectrometer + E(kinetic) + V (charge) + V (bias) (3.11) where, E(kinetic) is the kinetic energy from the emitted electron as measured in the spectrometer, E(Fermibinding) is the binding energy relative to the Fermi level or electron chemical potential; φ spectrometer is the work function of the spectrometer used to measure kinetic energy, V (charge) is a possible charging potential on the sample that may build up if the emitted photoelectron and secondary electron current is not fully compensated by flow from the sample ground, and V (bias) is a timedependent bias potential that between the sample and the spectrometer [193]. From this, is it possible to calculate the binding energies of various core or valence electrons involved in chemical bonding to characterize a given material. Details about the working principle are described elsewhere [194]. Studies involving XPS characterization include determination of atomic compositions of poly (methyl methacrylate)-co-poly (2-(2-bromoisobutyryloxy)ethyl methacrylate) electrospun fiber mats before and after surface-initiated radical polymerization [195], detecting the presence of NCO-terminated star-shaped PEG on PCL electrospun surface indicating enrichment of hydrophilic polymer using XPS, [196] showing presence of the grafted molecules on electrospun PLLA fibers [197], indicating encapsulation of fluorescein isothiocyanate-conjugated bovine serum albumin (fitcBSA) within PCL electrospun fibers [198] and others. PVA and sodium alginate (SA) nanofibers were developed for resistant cell scaffolds. XPS demonstrates that the chemical composition of the nanofibers varies depending on the depth profile. The carbon content decreased along the depth profile while the oxygen and the nitrogen content remained constant at various composition profiles while the sodium content increased from 0.72% up to 11.27%. This signified that the external layer of the nanofibers did not contain sodium, whereas the internal core was mainly made up of sodium demonstrating the fabrication of natural coaxial-type nanofibers [199]. Thermally reduced graphene oxide (TrGO) was incorporated into PVA to produce electrospun fibers for potential hyperthermia applications in photo-thermal therapy.

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XPS was used to characterize several functional groups in the reduced (TrGO) and it was concluded that the presence of oxygenated functional groups such as ether, alcohol, or ketone, suggests a partial affinity of TrGO with polar PVA polymer [200]. Several studies have used XPS to characterize surface properties in electrospun fibers [201–203].

3.4 Conclusions Electrospinning is a versatile system for the fabrication of nanofibers from various polymers and functional moieties. The properties of these fibers can be altered by varying the polymer properties and spinning conditions. In recent years, electrospinning of complex fluids like gels, emulsions, coaxial spinning, and usage of polymers containing nano sols and monomers are been explored for functional and advanced applications. Highly porous structures with high surface area to volume ratios make them a material of choice for applications in energy storage, healthcare, biotechnology, environmental engineering, and defense and security [204]. Owing to these significances in research, understanding their physical and chemical properties and correlating their structure–property relations are of paramount importance for designing nanofiber mats. Physical and chemical characterization techniques when used in combinations can be useful for controlling surface and bulk properties and obtaining desired properties. Acknowledgements This work was supported by the Science and Engineering Research Board (SERB), India (Project No. EMR_2017_001675 and Project No. CRG/2021/003082).

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Chapter 4

Electrospun Nanofibers Adsorbent for Water Purification Elham Tahmasebi

and Roghayeh Ebadollahi

4.1 Introduction The discharge of industrial effluents, such as heavy metal ions, dyes, persistent organic pollutants, and biological and pharmaceutical pollutants, to name a few into the environment, is a serious environmental problem. These pollutants can enter the human body through food chain and accumulate in living tissues. Even at low concentrations, they are toxic and cause disturbances in physiological activities and lead to various diseases and even death. Therefore, the presence of these pollutants in the environmental water can cause serious problems to human health due to their continuous accumulation over time [1, 2]. Accordingly, removing these pollutants from wastewaters has become a particular choice and various methods have been extensively studied for this purpose. These methods include ion exchange, membrane separation, chemical precipitation, electrolysis, adsorption, flotation, reverse osmosis, solvent extraction, etc. [3–8]. Among these, removal through the adsorption process can be considered a simple, convenient, cheap, and effective method with the ability to recover the adsorbent and reuse [9]. In the adsorption method choice of adsorbent is a very important subject and a good adsorbent should have some requirements including high adsorption capacity, fast adsorption rate, chemical and mechanical stability, reusability, low cost, and easy separation and recovery [10]. So far, several inorganic and organic adsorbents such as carbon-based adsorbents like activated carbon, graphene, carbon nanotubes, etc., and inorganic adsorbents such as zeolites and metal oxides, as well as biological adsorbents have been introduced for this aim [9]. Some of these adsorbents have low adsorption capacity and efficiency as well as no or less selectivity. Due to their unique characteristics such as high surface E. Tahmasebi (B) · R. Ebadollahi Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), 444 Prof. Yousef Sobouti Blvd., 45137-66731 Zanjan, Iran e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Das (ed.), Electrospun Nanofibrous Technology for Clean Water Production, Nanostructure Science and Technology, https://doi.org/10.1007/978-981-99-5483-4_4

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area, high adsorption efficiency, and fast adsorption rates, nanomaterials have been considered interesting adsorbents [9]. Despite the high surface area and good adsorption properties, powder nanosorbents have the problem of difficult separation and recovery from the solution and may lead to secondary pollution in the solution. So their application has been with some challenges. Fibrous nanomaterials, in addition to having high porosity and surface area, providing large adsorption capacities, allow for simple collection and separation from solution, which makes the adsorbent recovery process easier and faster. So, their application in the adsorption process due to the fast and easy separation recovery can be a promising issue [11]. Various methods and techniques have been used for the synthesis of polymeric, inorganic, and composite nanofibrous membranes. Electrospinning is an easy and cheap method, needs simple equipment, and can provide good quality nanofibers with high porosity and surface area comprised of different polymers and composite materials [12]. Therefore, in recent years, electrospun nanofibrous membranes (EFMs) with a variety of functional groups such as amine, carboxyl, hydroxyl, etc. prepared from natural or synthetic polymers, as well as composites of polymers and inorganic materials have attracted much attention in utilization as efficient adsorbents to remove environmental pollutants such as heavy metal ions, toxic dyes, etc. from environmental waters, resulting in satisfactory results [13]. Until now, different natural and synthetic polymers have been electrospun for various aspects. Some of the polymers have low electrospinning ability or produce nanofibers with poor mechanical strength. To overcome these drawbacks, some strategies have been introduced. On the other hand, surface characteristics and functional groups of electrospun nanofibers have an influential role in their adsorption performance for metal ions and organic pollutants [14]. Since a large number of polymers solely have no proper functional groups for this aim, their pure electrospun nanofibers are not suitable for such applications. So, it is necessary to be modified before or after the electrospinning process and/or by preparing a composite with other polymers or inorganic materials, enhancing nanofibers’ adsorption performance towards various pollutants as well as improving their mechanical strength and electrospinnability [15]. One of the important features of electrospun nanofibers is that their surface chemistry can be modified through which different functional groups are attached to their surfaces. This is attributed to their porosity and high surface area as well as their stability. Recently, many studies have been conducted on the modification of electrospun nanofibers [16]. Based on the fabrication and modification strategy, EFMs could be classified into the following categories: a. Electrospinning of a blended solution of two (or more) polymers so that one polymer has good electrospinnabilty while the other one has proper functional groups [17]. b. Functionalization of a polymer with desired functional groups before or after the electrospinning process [18]. c. Fabrication of composite electrospun nanofibers with organic and/or inorganic materials. For this purpose, a blend of nanomaterials and polymer solution is

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electrospun [19], or after the formation of polymeric EFMs, the desired nanomaterials are coated on the surface of the as-prepared EFMs. It is even possible to synthesize two-layered nanofibers using coaxial electrospinning. The resulting composite nanofibers will have better adsorption performance compared to pure polymeric nanofibers. d. Preparation of non-polymeric EFMs. In this method, the polymeric EFM is subsequently carbonized to prepare carbon-based FEMs. In another strategy, after preparing composite EFMs based on inorganic materials, the polymer part of the nanofibers is removed through dissolving in a solvent or heat treatment and finally, inorganic nanofibers are obtained [20]. This chapter efforts to give an outline of the classification of EFMs based on their composition and synthesis strategy. Moreover, recently published researches on the synthesis and application of EFMs for the removal of heavy metals and organic pollutants in water with emphasis on synthesis strategy, adsorption mechanisms as well as adsorption efficiencies are discussed and reviewed. Figure 4.1 depicts different strategies of modification of EFMs that have been used for the preparation of adsorbent for water treatment application that is discussed here. The majority of papers employing the electrospinning technique to fabricate nanofibrous membranes for water treatments have been devoted to heavy metal

Fig. 4.1 Schematic diagram of different modification strategies of EFMs used for water treatment application

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ions removal. However, some reports have focused on the field of organic pollutants, especially toxic dyes, removal from aqueous solutions. Nowadays, a great number of EFMs modification techniques have been utilized to construct nanofibrous membranes, working through adsorption principles, with the required functionality and suitable chemical and physical properties as described here.

4.2 Application of EFMs in Water Treatment as Adsorbent and Their Modification Processes 4.2.1 Polymeric EFMs In order to electrospin a polymer into nanofibers, it should have good electrospinnablity. For this purpose, it needs sufficient molecular weight and good solubility in a suitable solvent. Besides, the polymer solution must have sufficient viscoelasticity and surface tension for electrospinning [12]. However, many functional polymers have poor electrospinnability and/or produce nanofibers with low mechanical stability. Electrospinning of blending two polymers can effectively improve their spinnability and stability and along with the resulting nanofibers with suitable functional groups can adsorb metal ions or organic compounds. In fact, in the electrospinning of a dual-component system, one component has a good electrospinnability, improving the electrospinning properties of the polymer solution, and the other one is selected based on the desired functional groups required in the adsorption process and improves the adsorptive performance of the hybrid nanofibers. Until now, various natural and synthetic polymers have been electrospun solely or in a mixture. Table 4.1 depicts the use of polymeric EFMs adsorbents and their maximum adsorption capacities for the removal of pollutants. Natural polymers have advantages such as abundance in nature, non-toxicity, availability, low cost, and biocompatibility, while, synthetic polymers have better mechanical properties, suitable viscoelasticity, tunable properties, and more stability than natural polymers [39].

4.2.1.1

Natural Polymers EFMs

A variety of nanofibrous membranes based on natural polymers such as chitosan, cellulose, alginate, keratin, silk fibroin, and collagen have been electrospun (via direct electrospinning or blending with synthetic polymers and co-electrospinning) and applied to adsorption of environmental pollutants [39]. Chitosan (CS) is an abundant biopolymer with the advantages of being cheap and accessible, biocompatible, and biodegradable. Moreover, it has a polycationic nature and contains a large number of functional groups such as amino and hydroxyl groups, providing to form chelates with many metal ions [39]. Among various functional

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Table 4.1 Polymeric EFMs and their application as adsorbents for pollutants removal Polymer

Modification (via blending)

Pollutant

Maximum adsorption capacity (mg g–1 )

Ref.

PVA and PEI

Cd(II), Cu(II), Ni(II)

112.13, 86.09, 75.5

[17]

Natural polymers CS CS

PCL

Cr(VI)

114.7

[21]

CS

–

Acid blue-113 dye

1377

[22]

CS

PMMA

Cr(VI)

67.0

[23]

CS

PEO

Cu(II), Zn(II), Pb(II),

120, 117, 108

[24]

CS

PEO

Ibuprofen

141.21

[25]

CA

PAT

Hg(II)

177

[26]

CA

P(DMDAAC-AM)

Acid black-172 dye

231

[27]

Alkali lignin

PVA

Fluoxetine

29

[28]

Keratin

–

MB

170

[29]

Keratin

PA6

Cu(II)

103.5

[30]

WK

SF

Cu(II)

2.88

[31]

α-Chitin and CS NWs

PVA

MB

–

[32]

PEN

Activated with NaOH

Cu(II)

52.77

[33]

PES

AA-MMA copolymer

MB

2257.88

[34]

Synthetic polymers

PVA

PAN

Cr(VI)

133

[35]

M-PEI

PVDF

MO

633.3

[36]

Branched PEI

Amidated PAN

Cu(II)

209.53

[37]

PANI

PEO

MO

81.96

[38]

CS chitosan, PVA polyvinyl alcohol, PEI polyethyleneimine, PCL polycaprolactam, PMMA polymethylmethacrylate, PEO poly (ethylene oxide), CA cellulose acetate, PAT poly (2-aminothiazole), P(DMDAAC-AM) poly(dimethyldiallylammonium chloride-acrylamide), MB methylene blue, PA6 polyamide 6, WK wool keratose, SF silk fibroin, NWs nanowhiskerson, PEN polyarylene ether nitrile, PES polyethersulfone, AA-MMA acrylic acid and methyl methacrylate, PAN polyacrylonitrile, M-PEI methacrylate-modified polyethyleneimine, PVDF polyvinylidene fluoride, MO methyl orange, PANI polyaniline

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polymers, CS is the most applied polymer in the preparation of EFMs. However, due to the high viscosity, electrospinning of pure CS aqueous solution is difficult. Thus, electrospinning of its mixture with other suitable polymers with good electrospinnability is a proper solution to get the desired electrospinning conditions [21]. In 2017, a pure CS nanofibrous membrane was synthesized using the electrospinning technique and used as an adsorbent for acid blue-113 dye. The nanofibers were prepared in different diameters of 86 ± 18, 114 ± 17, and 164 ± 28 nm, and their adsorption capacities for acid blue-113 were investigated. The highest amount of adsorption (1377 mg g–1 ) was obtained with the fibers with a diameter of 86 nm [22]. In another work, a nanofibrous membrane was prepared via electrospinning of an aqueous acetic acid solution of CS and polymethylmethacrylate (PMMA) to remove Cr(VI). The different characteristics of the membrane were investigated with the Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS), and it was observed that amino groups of CS play an influential role in the adsorption of Cr(VI). A maximum adsorption capacity of 67 mg g–1 was observed with a CS: PMMA ratio of 0.3:1.0, which is almost three times larger than that of CS powder (22.9 mg g–1 ) [23]. Li et al. fabricated a filter paper based on CS/polycaprolactam EFM to remove Cr(VI) via filtration [21]. The maximum adsorption capacity obtained by filtration (114.7 mg g–1 ) was much higher than static and shaking adsorption processes (81.7 mg g–1 and 80.7 mg g–1 , respectively). Further, XPS analysis demonstrated that amino groups of CS, in addition to adsorbing Cr(VI) anions via electrostatic attraction, have a role in reducing Cr(VI) to the less toxic form of Cr(III). In this method, polycaprolactam was blended into CS to improve the strength and stability of the resulting composite membrane. CS was also co-electrospun with poly(ethylene oxide) (PEO) to improve the physical properties of the resulting nanofibers. Shariful et al. investigated the fabrication of CS/PEO nanofibrous membranes with different weight ratios of CS: PEO through the electrospinning method [24]. It was observed that the ratio of 60:40 of CS: PEO was the best ratio to produce beadless fibers with high surface area as well as to obtain high adsorption capacities of 120, 117, and 108 mg g–1 for divalent heavy metal ions of Cu(II), Zn(II), and Pb(II), respectively. Also, PEO blended with CS was electrospun into nanofibrous membranes to remove ibuprofen from urban wastewater. Blending of PEO with CS was done to improve the viscosity and electrospinnability of the polymer solution [25]. In 2019, Sahebjamee et al. presented a new EFM by adding a third polymer (Polyethylenimine, PEI), with a large number of amine functional groups, to the CS/ polyvinyl alcohol (CS/PVA) blend to increase the active sites for adsorbing Ni2+ , Cu2+ , and Cd2+ ions [17]. The adsorption capacity of the modified membrane was compared with activated carbon and the membrane prepared without PEI. It was observed that the modified membrane has a higher adsorption capacity than the other two adsorbents due to an increase in adsorption sites. However, it was demonstrated that adding too much PEI results in a decline in the adsorption capacity due to the decrease in the porosity of the membrane.

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Cellulose is another naturally existing polymer that has attracted great attention for a wide range of applications [39]. However, due to the abundant hydroxyl groups in cellulose structure and less stability, electrospinning of pure cellulose is a difficult process. So it is necessary to be functionalized and usually, its derivative, i.e., cellulose acetate (CA) is introduced to the electrospinning process. Zou et al. prepared an EFM based on poly(2-aminothiazole) (PAT) and CA by modified coaxial electrospinning method using a blended solution of PAT and CA, as the core fluid, and acetone as the sheath solvent. PAT with a large number of S and N heteroatoms has a high affinity for metal ions, however, due to its low electrospinnability, fabrication of its electrospun nanofibers is not feasible. So in this study, CA as a derivative of cellulose, with good electrospinnability, was chosen as the base material. The PAT/CA EFM was evaluated for the removal of Hg(II) ions from water. The results revealed that a maximum adsorption capacity of 177 mg g–1 can be obtained at pH 6.5 for Hg(II) [26]. Likewise, the fabrication of CA/poly (dimethyl diallyl ammonium chlorideacrylamide) (P(DMDAAC-AM)) nanofibrous membrane was demonstrated using the electrospinning method [27]. The synthesized membrane was tested as an adsorbent for the removal of an anionic dye of acid black-172 (AB-172) at a pH range of 4–10, and the maximum adsorption capacity was achieved 231 mg g–1 . The results also indicated that the adsorption mechanism is based on electrostatic interaction between the cationic copolymer of P(DMDAAC-AM) and the anionic dye AB-172. The combination of CA, as an easy electrospinning polymer, and P(DMDAAC-AM), with cationic functional groups, for preparing a nanofibrous membrane could take their own advantage and resulted in excellent adsorption performance. In another work, nanofibrous membranes were prepared by combining alkali lignin, as a renewable and abundant biopolymer, with PVA using the electrospinning method to apply for adsorption of a pharmaceutical contaminant (fluoxetine) in aqueous solution. The results indicated that the optimum ratio of 50:50 (lignin: PVA) gives a maximum adsorption capacity of 29 mg g–1 [28]. Moreover, some protein-based polymers, including keratin, collagen, and silk fibroin (SF), have been utilized as EFMs for adsorption purposes due to the presence of a large number of polar groups such as amino and carboxyl in their structures, providing good affinities towards heavy metal ions. However, because of the poor electrospinnability of these polymers as well as the low mechanical strength of the protein-based electrospun nanofibers, their co-electrospinning with other polymers has been performed to improve the property of the hybrid fibers. Recently, in some reports composite EFMs have been fabricated based on proteins and other materials to improve their adsorption efficiencies. Keratin, a natural polymer with abundant amino acids, has a high affinity to ionic species. In 2014, Aluigi et al. fabricated electrospun keratin nanofibrous membranes using formic acid as a solvent and examined them for methylene blue (MB) adsorption [29]. The prepared nanofibers had an average diameter of 220 nm with a specific surface area of 13.59 m2 g–1 . The parameters of temperature, pH, and adsorbent dosage were optimized and the maximum adsorption capacity of 170 mg g–1 was obtained at pH = 6 for MB. In another study, polyamide 6(PA6)/keratin nanofibrous mat was prepared using the electrospinning technique and investigated for the

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adsorption of copper (II) ions [30]. The results showed that due to the increase in the surface area of the membranes, the adsorption efficiency of Cu2+ ions by keratin based-membranes increases. Also, the maximum adsorption capacity (103.5 mg g–1 ) of the nanofibrous mats increased via increasing keratin content. Moreover, the FTIR analysis revealed that the complex formation between Cu2+ ions and carboxyl groups of keratin plays a role in the adsorption process. SF and wool keratin (WK, oxidized wool protein) consist of different amino acids with high affinity for ionic species such as metal ions. In a study in 2007, a blended solution of WK and SF (50:50) was electrospun into the nanofibers using 98% formic acid as solvent [31]. The WK/SF EFM exhibited excellent performance for Cu2+ adsorption. Both SF and WK have numerous functional groups that can bind metal ions; however, WK has much more amino acids than SF and is more proper for metal ions adsorption. Besides, due to the low electrospinnability of WK as well as its structural instability in water, EFMs using pure WK cannot be easily prepared. In this work, by blending the WK with SF, these problems were resolved and a higher adsorption capacity for Cu2+ ions was obtained than using SF nanofibrous membrane. Kim et al. proposed the synthesis of PVA nanofiber membranes containing different contents of α-chitin nanowhiskers (CtNWs) or CS nanowhiskers (CsNWs) with electrospinning method to remove the cationic dye of MB from contaminated water [32]. Results demonstrated that the introducing of highly crystalline CtNWs or CsNWs into the fibers causes to increase in thermal stability and tensile strength of the resulting EFMs. Also, it was observed that by increasing the content of CtNWs or CsNWs, the adsorption capacity decreases due to the electrostatic repulsion between the positive charge of the nanofibrous membrane and the cationic dye. So it was concluded that the charge of the membrane has an important role in the adsorption efficiency of organic dyes from water.

4.2.1.2

Synthetic Polymers EFMs

In addition, some reports have focused on the electrospinning of synthetic polymers, mostly in a mixture with other polymers in order to have a spinnable polymeric solution, resulting in a functional nanofibrous membrane, for application in the removal of environmental pollutants. Polyarylene ether nitrile (PEN), with carboxylic acid functional groups, was electrospun into nanofibers mats and then activated with NaOH solution to introduce chemical functionality for chelating metal ions [33]. The adsorption capability of the PEN membrane towards metal ions was then evaluated. According to the obtained results, it was found that the synthesized adsorbent has a maximum adsorption capacity of 52.77 mg g–1 to adsorb Cu2+ ions from the aqueous solution. It was also observed that the synthesized EFM has the ability to be recycled and reused after several adsorption–desorption cycles. Xu et al. produced a nanofibrous membrane to remove MB by electrospinning a mixture of a copolymer of acrylic acid and methyl methacrylate and polyethersulfone

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(PES) [34]. Due to the porosity and large surface area as well as having abundant carboxyl groups, the prepared EFM had a high adsorption capacity (2257.88 mg g–1 ) for selective adsorbing MB among the previously reported membranes for removal of MB. In addition, it was also observed that the proposed membrane has high recyclability for up to 5 cycles. Polyacrylonitrile (PAN) with good characteristics such as excellent solvent resistance, high mechanical strength, and good spinning ability has been widely used in the fabrication of EFMs. Moreover, not only does direct chelating of metal ions with the nitrile (–C≡N) groups of PAN allow the removal of heavy metal ions but also the introduction of different functional groups via simple chemical reactions with its nitrile groups provides further functionality for adsorption purposes. In a study, PVA and PAN nanofibrous membranes were synthesized through two-nozzle electrospinning techniques. The membrane included a thicker skeleton scaffold of PVA nanofibers and a thinner PAN functional scaffold [35]. The composite membrane showed high permeability to the water and more mechanical strength compared to the single-component membrane. In addition, PAN nanofibers of the membrane were modified through surface-grafting to create positively charged functional groups and then used for the adsorption of Cr(VI) ions with an adsorption capacity of 133 mg per gram of PAN nanofibers. Polyethylenimine (PEI) is a positively charged polymer with a large number of amino and imino functional groups which has a high affinity towards the metal ions as well as negatively charged compounds, so it can be used for the removal of such pollutants from water. However, pure PEI is difficult to electrospun due to having a linear cationic structure, excellent water solubility, and poor mechanical properties. Hence, recently in some works electrospinning of its mixture with other polymers was accomplished to overcome these drawbacks. In a work, PEI was modified by introducing the methacrylate groups, and then the blending of the modified PEI with polyvinylidene fluoride (PVDF) in N,N-dimethyl formamide (DMF) provided a feed solution for electrospinning [36]. The final cationic EFM, with good water resistance and mechanical strength, was used to remove anionic dyes from the aqueous solution. It was observed that this EFM is an effective adsorbent for the removal of the anionic dye of methyl orange (MO) with a maximum adsorption capacity of 633.3 mg g–1 . In another work, in 2021, Shao et al. reported a strategy based on electrospinning and subsequent hydrothermal treatment for the fabrication of an amidated PAN/ branched polyethyleneimine nanofibrous membrane (aPAN/BPEI NMs) [37]. Since the electrospinning of pure BPEI is difficult, its blending with low-cost PAN with good electrospinnability provided a suitable solution for the fabrication of nanofibrous membranes based on BPEI to apply in heavy metal adsorption. Accordingly, a blended solution of PAN and BPEI was electrospun into the nanofibers and then a hydrothermal treatment was applied to the resultant fibers to allow grafting of more amine groups of BPEI on the surface of PAN/BPEI nanofibers, providing more amino groups to capture Cu2+ ions. Due to the high binding capacity of BPEI towards Cu2+ ions, aPAN/BPEI NMs exhibited a considerable adsorption capacity of 209.53 mg g–1 . In addition, the color change of a PAN/BPEI NMs from yellow to blue caused by

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their reaction with Cu2+ ions provided simultaneous removal and detection of Cu2+ ions via colorimetric detection. Polyaniline (PANI) as a conductive polymer contains protonated amino functional groups that have an affinity towards anionic species. However, low solubility and poor mechanical properties, as well as low molecular weight, limit the electrospinning of pure PANI into the nanofibrous membrane. Hence, the electrospinning of PANI with other polymers has been reported by some researchers to resolve these problems [40–42]. In a study, to remove the anionic dye of MO, a composite membrane of polyethylene oxide/polyaniline (PEO/PANI) was prepared by electrospinning of their blended solution. PEO was used as a hosting polymer to improve the poor mechanical property of PANI via creating hydrogen bonding and making a flexible membrane, increasing the membrane’s durability by up to 8 cycles. It was demonstrated that the adsorption mechanism is based on electrostatic attraction between the positively charged PANI at acidic pHs and the anionic dye of MO [38].

4.2.2 Surface-Modified EFMs Modification of the nanofibers can be achieved by chemically bonding functional groups on EFMs via reactions with different reagents as well as by coating other functional polymers or inorganic materials onto EFMs [14]. To improve the adsorption performance of EFMs, suitable functional groups or ligands can be attached to their surfaces, increasing their affinity for metal ions or organic compounds. Electrospun nanofibers can provide a suitable scaffold for the introduction of functional groups due to their high surface area and porosity. Besides, various functional groups such as carboxyl, hydroxyl, amino, etc. can be bound to the surface of the nanofibers. Various methods have been used to functionalize polymeric nanofibers, among which functionalization of the polymer skeleton and subsequent electrospinning, as well as introducing functional groups to the surfaces of electrospun nanofibers, have attracted more attention [43]. In the post-functionalization method, nanofibers must have sufficient mechanical strength to be introduced to the chemical reaction, and usually, harsh reaction conditions can cause the destruction of nanofibers. Also, in some works, co-electrospinning of the ligand and polymer has been accomplished. In this case, no chemical reaction takes place between the ligand and the polymer, and the process only involves their physical blending. In this method, because the ligand is not chemically bonded to the polymer, there is a possibility of leaching the ligand from the nanofibers during the adsorption and recovery processes, so the modification method is not very effective. Coating the surfaces of EFMs with polymers or other materials is an alternative strategy to modify them, introducing suitable functionality as well as stability to the resultant nanofibrous membrane [44].

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85

Surface Modification Through Functionalization

Owing to large specific surface area and high porosity as well as good chemical and mechanical stabilities, EFMs can be functionalized by the introduction of various groups such as amino, carboxyl, thiol, amidoxime, etc., providing an enhancement in their adsorption performance for heavy metals and organic compounds in water [11]. Nowadays, a great number of EFMs modification techniques have been utilized to construct nanofibrous membranes with the required functionality and suitable chemical and physical properties as described in this section. Recent examples of the removal application of different EFMs modified through functionalization are summarized in Table 4.2. a. Modified-cellulose EFMs In some studies, modified cellulose EFMs have been synthesized and applied for the removal of pollutants from water. Direct electrospinning of cellulose solution into nanofibers requires higher temperatures (80 to 130 °C) to perform, and relatively less uniform cellulose nanofibers are obtained. To solve these problems, the cellulose derivative (i.e., cellulose acetate) is electrospun and then the deacetylation process is carried out using an alkaline solution. In a study, amine-functionalized cellulose nanofibers membrane was prepared via deacetylation of electrospun CA nanofibers followed by treatment with ethylenediamine to incorporate the amine functional groups onto the obtained cellulose nanofibers [45]. The efficiency of the synthesized nanofibrous membrane was evaluated to remove fluoride ions dissolved in groundwater. The results showed a high adsorption performance for fluoride ions even at very low concentrations ( 420 nm) [170]. Clerck and coworker systematically studied polymer nanofibers as support for titania nanoparticles for catalytic application. The nanoparticles were immobilized polyamide 6 and silica nanofibrous membranes [172]. The electrospun nanofibers were modified with metal-containing polyhedral oligomeric silsesquioxanes (MPOSS). The catalytic activity of poly(styrene-co-maleic anhydride) (PSMA)/amino hexaisobutyl titanium (Ti-POSS-NH2 ) was studied via the degradation of sulforhodamine B under UV light [173]. Later, electrospun nanofibers were prepared from bismuth vanadate (BiVO4 ). It showed higher photocatalytic activity on degradation of Rhodamine B in the presence of visible light. The improved photocatalytic action is accredited to the development of monoclinic sheelite (s-m) phase and phase junction of tetragonal sheelite (s-t) in the electrospun nanofibers [174]. An et al. prepared nylon-6 nanofibers containing titania nanoparticles for potential application in water purification and control of toxicity of chlorophenols. Methylene blue solution was used as a pollutant here. The nanofibers membrane degraded 100% methylene blue solution after 90 min of treatment by a relatively weak UV irradiation (0.6 mW/cm2 ) [175]. Prior studies on photocatalysis behavior of nanofibers showed that the presence of heterostructure in nanofibers improved the catalytic efficiency of the materials [137, 178]. The TiO2 nanofibers were prepared with CuO plate like structure [179]. Such heterostructure was used for degradation of water pollutants through photocatalysis (Fig. 5.8). The nanofibers containing 1.25 and 0.5 wt% CuO showed the highest photocatalytic efficiency in the presence of UV. The photocatalytic activity of the heterojunction containing nanofibers showed 3.3 times greater activity than pure TiO2 fibers. Such higher catalytic activity was noticed because of increase in light harvesting potential, particle-fiber structure, better charge separation, and p-n junctions in the heterostructure. The heterostructure containing nanofibers is shown in Fig. 5.8. In another study, ZnO/ZnFe2 O4 /Ag hollow nanofibers containing heterojunctions were prepared through electrospinning method. Such heterojunctions showed better catalytic activity as compared to pure Zn/ZnFe2 O4 nanofibers [176]. The multicomponent heterojunctions promoted the separation of photogenerated carriers, better abilities absorption of optic, reduced the recombination of electron–hole pairs. Therefore, continuous and efficient formation of holes and electrons took place aiding reduction and oxidation reactions. This led to effective target

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Fig. 5.6 a The schematic illustration of preparation of PAN@TiO2 /Ag membrane, b PAN@TiO2 / Ag nanofibers, c recyclability performance of the membrane PAN@TiO2 /Ag [162] surface morphology of ZnO nanofibers prepared d 450 °C and e 650 °C calcination temperature [171]. Figures are taken with permission from a–c Elsevier (2017) Appl. Surf. Sci., 426, 622, d, e Elsevier (2013) J. Colloid Interface Sci., 394, 208

reaction for all types of pollutants. The hollow nature helped to reduce the path for charge transfer to the solution at the time of photocatalytic process [176]. Lin et al. prepared a dimer-type heterostructure (Ag−ZnO) through electrospinning process. The average diameter of the prepared nanofibers was varied from 80 to 150 nm. And, the size of the Ag nanoparticles was in the range of 15 nm. The photocatalytic activity of such nanofibers was studied via degradation of Rhodamine B (RhB) in the presence of UV. The nanofibers showed 25 times more higher efficiency than pure ZnO nanofibers [180]. Later, similar heterostructure nanofibers of TiO2 -SiO2 were prepared through electrospinning. Such porous nanofibers showed potential application for catalytic application. These nanofibers were used for hydrolysis of

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Fig. 5.7 a The schematic illustration of possible degradation mechanism [168] b the photocatalysis reaction mechanism of ZnO/ZnFe2 O4 /Ag heterojunction multicomponent material in presence of visible light [176] c the surface modification technique of PAN nanofibers d the photodegradation mechanism of methylene blue (MB) dye [177]. Figures are taken with permission from a Elsevier (2020) Chemosphere, 239, 124764, b Elsevier (2019) Ceram. Int., 45, 23522, c, d Elsevier (2017) J. Ind. Eng. Chem., 45, 277

polluted water which contained inorganic alkoxides. Such mesoporous fibers (TiO2 – SiO2 ) showed outstanding photocatalytic activity for degradation of Rhodamine B as compared to Degussa P25. The higher catalytic activity was found because of mesoporosity, high specific surface area, anatase–rutile heterojunction, and high adsorption ability [181]. The electrospun nanofibers were prepared from foam-assisted technique to prepare mesoporous structures. The mesoporosity in the nanofibers enhanced the surface area of the nanofibers. However, the photocatalytic activity and stability of these TiO2 nanofibers was much higher than normal solid nanofibers and commercially available sample P25. The incorporation of the foaming agent was responsible for formation of pore on the precursor fibers and this led to formation of mesopores inside the structure [182]. Prahsarn et al. prepared polyacrylonitrile (PAN)/TiO2 nanofibers through electrospinning method. During preparation of nanofibers, water was used and it helped to generate pores in the structure through phase separation. The diameter of the composite nanofibers was found 170–430 nm and its surface was rough. The photocatalytic activity of these nanofibers was 80% for methylene blue after 24 h of light treatment [184]. Similar mesoporous TiO2 /SiO2 nanofibers were prepared through co-electrospinning by sol–gel path. The composite nanofibers had a diameter of 100–200 nm and the thickness of silica shell was varied from 5 to 50 nm. These nanofibers showed selective photocatalytic activity on the disintegration of

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Fig. 5.8 a The schematic illustration of preparation of Pdopa modified PU nanofibers and preparation of ZnO decorated nanofibers, b surface morphology of Pdopa-ZnONPs/PU nanofibers [160], c surface morphology of TiO2 /ZIF-8-12 nanofibers [163], d surface morphology of Pdopa-ZNRs/ PU nanofibers [160] e TiO2 /MoS2 composite nanofibers [183], f surface morphology of mesoporous TiO2 nanofibers [182]. Figures are taken with permission from a, b, d Elsevier (2018) J. Colloid Interface Sci., 513, 566, c Elsevier (2019) J. Alloys Compd., 777, 982, e Elsevier (2020) Appl. Surf. Sci., 504, 144361, f ACS Publications (2014) J. Am. Chem. Soc., 136, 16716

methylene blue, disperse red and active yellow [185–187]. A number of studies have been carried out on the preparation of composite nanofibers via electrospinning method followed by calcination. The schematic illustration of the preparation of catalytic nanofibers through electrospinning and calcination technique is shown in Fig. 5.9. These nanofibers contained photocatalytic materials such as ZnO, SnO2 , In2 O3 , TiO2 , CO3 O4 , Fe2 O3 , CeO2 , and WO3 and were used for waste water treatment [145, 166, 188–191]. For example, porous polytetrafluoroethylene (PTFE) nanofibers containing Fe2 O3 (Fe2 O3 /PTFE) were prepared by three step methods (electrospinning, immersion, and calcination). These nanofibers used to degrade the acid red in the presence of hydrogen peroxide under UV irradiation. The Fe2 O3 /PTFE nanofibers have high photocatalytic strength and it could be recycled with simple filtration mechanism [192]. By combining carbonizing and electrospinning methods, porous carbon

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nanofibers (CNFs) decorated by silver (Ag) nanoparticles (NPs) were prepared using a one-pot/self-template synthesis strategy. The prepared membrane showed enhanced photocatalytic activity in the presence of visible-light irradiation. This membrane also exhibited outstanding dye degradation in batch experiments. The catalytic activity was significantly enhanced because of good light absorption ability, highly reachable surface area, and high separation competence of photogenerated electron–hole pairs. The membranes could be recycled because of their 1D structure [193]. The template free highly porous carbon nanofibers surrounded with WO3 (WO3 –CNF) were developed with the combination of electrospinning and carbonization. Semiconductor precursor and porogen purpose, ammonium tungstate hydrate was used. The formed pores in CNF formed pathways between the surrounded WO3 and the surface of the CNFs. This facilitated photogenerated electron transfer [194]. Later, Doh et al. also prepared TiO2 nanofibers through similar electrospinning process and annealing at 550 °C for 30 min. The average diameter of the nanofibers was 236 nm and it contained anatase phase of crystals. Further, these nanofibers were coated with TiO2 particles through sol–gel method to enhance the photocatalytic activity. The degradation rate (k  = 85.4 × 10−4 min−1 ) of TiO2 coated nanofibers was higher than TiO2 nanofibers (15.7 × 10−4 min−1 ) and TiO2 particles (14.3 × 10−4 min−1 ) [195]. Ofori et al. prepared tungsten trioxide (WO3 ) nanofibers via electrospinning method with a precursor solution of polyvinyl pyrrolidone (PVP)/ citric acid/tungstic acid. The properties of WO3 nanofibers were decided by the PVP concentration. The average diameter of nanofibers was also controlled by PVP concentration. The photocatalytic activity of the prepared nanofibers was studied through the degradation of methylene blue color. It showed two times higher efficiency than commercial WO3 microparticles [196]. Electrospun fibers prepared from polystyrene-block-poly(ethylene oxide)-containing titanium-tetraisopropoxide and tungsten hexaphenoxide. In these nanofibers, WO3 filled the gaps and TiO2 acted as frames which enhanced the photocatalytic performance against degradation of acetaldehyde compared to neat WO3 and neat TiO2 [197]. Sharma and coworker used the precursor solution of zinc acetate and polyacrylonitrile (PAN) in N,N-dimethylformamide (DMF) to fabricate free-standing mesoporous ZnO nanofibers through the electrospinning. After removal of PAN from nanofibers by calcination introduced porosity in the nanofibers. The porous ZnO nanofibers diameters changed from 50 to 150 nm. The ZnO nanofibers with an average fiber diameter of 60 nm showed higher photocatalytic degradation of the polycyclic aromatic hydrocarbon (PAH) dyes—naphthalene and anthracene [171]. Watthanaarun et al. prepared electrospun nanofibers through sol–gel method with TiO2 and silicon-doped titanium (IV) oxide. The calcination temperature and dopant significantly influenced chemical and physical properties of TiO2 fibers. The increase in calcination temperature decreased the anatase fraction but the crystal dimensions were increased. The presence of silicon dopant, increased the anatase phase but reduced the crystal dimensions. The photocatalytic activity was strongly controlled by the percentage of anatase phase in the material. The photocatalytic activity of both, silicon-doped titania and neat was investigated via photooxidative breakdown of methylene blue. Both fibers revealed

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better activity compared to titania powder. The silicon dopant boosted the photocatalytic activity of the titania fibers [198]. Liu et al. prepared polyaniline (PANi) coated TiO2 /SiO2 composite nanofibers through electrospinning process followed by calcination and in-situ polymerization technique. The nanofibers were used to decompose organic pollutants like methyl orange under visible light. [199, 200] Liu et al. prepared TiO2 /ZnO composite nanofibers with a diameter range from 200 to 800 nm through electrospinning by titanium tetraisopropoxide and zinc acetate as precursors and cellulose acetate as a template. Later, it was treated with 0.1 mol/L NaOH solution and TiO2 /zinc acetate/cellulose acetate nanofibers transformed into TiO2 /Zn(OH)2 /cellulose nanofibers. The TiO2 /ZnO composite fibers were prepared after calcination at 500 and 700 °C for 5 h. The blending of TiO2 and ZnO developed new crystallites which showed better UV absorption potential, hence, it had better photocatalytic activity for Rhodamine B and phenol. It was found that almost 100% Rhodamine B and 85% phenol were degraded by the TiO2 /ZnO nanofibers [201, 202].

Fig. 5.9 a The schematic presentation of development of polyaniline (PANi) coated TiO2 /SiO2 nanofiber [199], b the schematic diagram of the photocatalytic mechanism for the phase junction of s-m/s-t BiVO4 and c describes the absorption spectrum of RhB solution in presence of nanofibers. The inset photos are the solutions of RhB solutions after photocatalytic degradation for different times with SEM image of reclaimed samples after photocatalytic degradation [174]. Figures are taken with permission from a, b, d Elsevier (2014) J. Colloid Interface Sci., 424, 49, b ACS Publications (2015) ACS Appl. Mater. Interfaces, 7, 9638

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5.3.2 Inorganic Pollutants Removal The inorganic pollutant like metal ions (Pd, Cd, Cr, Ni, As, and selenium), radioactive ions, and anions (sulfide, fluoride ions, and nitrate) could be found in waste water. Such ions come from industrial waste, agricultural waste, or nuclear power plant. Such inorganic pollutants should be removed from water to reduce water pollution [203, 204]. Mahapatra et al. prepared iron oxide–alumina mixed composite nanofibers via electrospinning technique followed by sintering at 1000 °C. These fibers were used to remove heavy metals like Cu2+ , Pb2+ , Ni2+ , and Hg2+ [205]. Zhang et al. prepared TiO2 nanofibers via electrospinning technique and In2 S3 nanosheets were assembled on it by the hydrothermal technique (Fig. 5.8). These one-dimensional hierarchical heterostructure of nanofibers showed better catalytic activity for the reduction of Cr(VI) under visible light [206]. The composite carbon nanofiber/tin (IV) sulfide (CNF@SnS2 ) core-sheath fibers were prepared through electrospinning technique. These nanofibers were used to remove heavy metal Cr(VI) from waste water (250 mg/L) through photocatalytic degradation under visible light. It was found that the nanofiber membrane needed 90 min to decompose the Cr(VI) [207]. Mohamed et al. prepared PAN-CNT/TiO2 -NH2 composite nanofibers thorough electrospinning technique followed by chemical cross linking. These nanofibers showed excellent photoreduction of Cr(VI) under visible light [208]. Li et al. also prepared PA6@Fex Oy composite nanofibers membranes through electrospinning method for adsorption of Cr(VI). The capacity of adsorption reached upto 150 mg Cr/g by the nanofibrous membrane [144]. The schematic illustration of preparation of membrane for the removal of heavy metal ions by the electrospun nanofibers is shown in Fig. 5.10. A new approach was developed to adsorb pollutants from water through the grafting mechanism on nanofibers. For example, the surface of the γ-alumina (γAl2 O3 ) nanofibers was grafted by thiol and octyl groups. The thiol functional groups containing nanofibers adsorbed heavy metal like Pb+2 and Cd+2 from waste water. The prepared membranes from these functional nanofibers showed higher adsorption ability to remove the heavy metal ions selectively at high flux. The modified nanofibers with octyl groups showed hydrophobicity with contact angle 145 ± 2° and it has the ability to remove hydrophobic 4-nonylphenol from water [209]. The composite titanate nanofibers (Na2 Ti3 O7 and Na1.5 H0.5 Ti3 O7 ) were able to remove heavy metal and radioactive ions from waste water. Such nanofibers were fabricated easily with the reaction of titania and caustic soda. The TiO6 octahedra formed layers with negative charges and the cations of sodium present inside between inter layers. These structures adsorbed bivalent radioactive ions and heavy metal ions from water by the ion exchange process [204]. α-Fe2 O3 nanofibers were fabricated to remove heavy metal like Cr(VI) from waste water. The nanofibers were prepared through simple hydrothermal process, followed by calcination process. The nanofibers showed excellent adsorption of Cr(VI) 16.17 mg g–1 from waste water. It was also found that the adsorption capacity remained unchanged after recycling and reuse [210].

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Fig. 5.10 a Schematic of synthesis of CNF@SnS2 membranes and its catalytic reduction process of Cr(VI) to Cr(III) [207]. b Photo reduction of Cr(VI) with different catalyst using visible light and c photoreduction mechanism of Cr(VI) under visible light with 1D In2 S3 /TiO2 H-HSs [206]. Figures are taken with permission from a ACS Publications (2016) ACS Appl. Mater. Interfaces, 8, 28671, b, c Elsevier (2013) J. Hazard. Mater., 260, 892

5.3.3 Pathogenic Microorganisms Bacteria and virus are serious pollutants in water. These are responsible for various diseases of human body. Therefore, all these pathogens are desired to be removed from water before consumption [211–214]. For this, highly porous titania nanoparticles decorated nanofibers were fabricated by electrospinning [215]. Further, soy protein-containing nanofibers were prepared by solution blowing method. Later it was further decorated with silver nanoparticles. All these nanofibers showed strong antibacterial activity against E. coli colonies without UV light [215]. Figure 5.11 shows the application electrospun nanofibers for the removal of pathogens from waste water. Similarly, silver nanoparticles containing TiO2 nanofibers were fabricated for waste water treatment. Such membrane showed higher permeate flux as compared to commercial P25 deposited film. The bacteria inactivation achieved up to 99.9%. These membranes also showed excellent antibacterial activity without solar radiations [216]. In another study, nanofibers were prepared from the colloidal solution of titanium isopropoxide, copper nanoparticles and poly(vinylpyrrolidone) (PVP) and calcinated at 700 °C in the presence of air for 1 h to prepare pure TiO2 / CuO nanofibers. The Klebsiella pneumoniae was used as model to check the antibacterial activity. The cell wall and cell wall membrane were degraded after exposed on the nanofibers in the presence of visible light. These nanofibers could be reused for

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Fig. 5.11 a Schematic illustration of preparation of PAN/AgBr/Ag membrane [161], b The surface morphology of Ag/TiO2 nanofibers, c Ag/TiO2 under nanofibers transmission electron microscopy, d schematic of antibacterial test [161], e the inhibition of E. coli on the nanofiber membrane decorated with titania in presence of UV light for 1 h, f Schematic of proposition of Ag/TiO2 membrane [216] and g absorbance at different time of irradiation of UV [215]. Figures are taken with permission from a–d Elsevier (2019) Chem. Eng. J., 361, 1255, e, f Elsevier (2012) Water Res., 46, 1101, g Elsevier (2013) Catal. Commun., 34, 35

further waste water treatment. The synergistic effect of TiO2 and CuO enhanced the photocatalyst and antibacterial activity [217].

5.4 Summary and Future Outlook In recent decades, electrospinning method has been found to be the easiest way to prepare nanofibers from various polymeric materials in single step. The processing parameters of this spinning process could be controlled to achieve desired electrospun nanofibers-based catalytic membranes (ENCMs). These membranes show better performance due to high specific surface area, interconnected pores, small pore, the narrow pore size distribution, and ease of functional moiety incorporation into the nanofibers. Also, these nanofibers are chemically and thermally stable. The flexibility and versatility of the process enable it to be a robust method for fabrication of ENCMs for waste water treatment. Here, electrospinning method and the

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critical factors which influence the nanofibers properties have been described such as thickness, porosity, and pore size distribution. This directly control the selectivity and flux behaviors of the membrane. The surface of membranes could be modified by adding various nanoparticles into it. However, there are several issues which need to be addressed in this area before industrial scale-up of ENCMs for waste water treatment. The average diameter of the electrospun nanofibers is few hundred nanometers. The diameter of such nanofibers could be reduced further to reduce the pore size and better rejection rate of pollutants from water. Mechanical property of nanofibers also limits its applications in various fields. Furthermore, electrospinning process is highly sensitive to processing conditions and polymer sources. Hence, the membrane’s behaviors are strongly controlled via fiber dimeter, inter-fibers spacing, surface roughness, hydrophobicity nature of surface, and selectivity of pollutants. This could be studied further in detail. The large-scale development is not economical and it is difficult to process the materials. Also, the material cost and sources are not well arranged. Therefore, suitable materials and better machines are required for the industrial production of ENCMs. The functional nanofibers are produced through post treatment process like physical or chemical or thermal treatments. Therefore, the surface modification of the materials could be studied in more detail to develop more robust materials with high efficacy for rejection of pollutants. Most membranes lose their efficiency due to membrane structure deterioration with time. Therefore, durable reusable membrane or self-cleaning membranes could be developed in future. Significant progress has done so far on preparation of composite membranes for waste water treatment through catalytic activity. However, above issues need to be addressed to prepare ENCMs for waste water treatment at industrial scale.

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Chapter 6

Electrospun Nanofibers for Membrane-Based Water Filtration Ragib Shakil , Yeasin Arafat Tarek , Md. Rabiul Hasan, Mahamudul Hasan Rumon , Rasel Das , and Al-Nakib Chowdhury

6.1 Introduction In the twenty-first century, due to fast industrialization, exponential expansion in the global population, and urbanization, water is becoming alarmingly scarce and is being invaded with hazardous contaminants. It is worth mentioning that 75% of the surface of the planet is covered with water and roughly 97% of that surface water is excessively salty. The remaining 3.0% are entrapped in ice caps, and glaciers or underground, leaving less than 1.0% of the entire water supply available for human consumption [1]. However, these water sources have been continually polluted by urban, industrial, and agricultural contaminants. Even small amounts of contaminants present in freshwater supplies may have adverse impacts on human health. Hence, several studies showed that almost 50% of this population is suffering from diverse health issues due to drinking the contaminated water [2]. In response to the lack of adequate freshwater supplies, the development of effective and sustainable water purifying technologies is a matter of massive concern with unprecedented challenges. To ensure safe and potable water, the development of cost-effective and efficient technologies, such as ultraviolet (UV)-radiation [3], membrane technology [4], ionexchange technology [5], biological filtration [6], etc. are hot topics in the research field. Among these technologies, the membrane-based filtration methods have significant contributions in the purification and desalination compared to the conventional water treatment methods and feed quality variations which have a considerably lower footprint. Moreover, membrane-based desalination methods are more R. Shakil · Y. A. Tarek · Md. R. Hasan · M. H. Rumon · A.-N. Chowdhury Department of Chemistry, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh R. Das (B) Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Das (ed.), Electrospun Nanofibrous Technology for Clean Water Production, Nanostructure Science and Technology, https://doi.org/10.1007/978-981-99-5483-4_6

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energy-efficient than thermal processes [4]. Usually, membrane materials and manufacturing processes are mostly based on empirical approaches and are not designed in a molecular-level approach. The improper architecting of the membrane layer, therefore, limits membrane performance and increases the cost of water purification and desalination. The inherent material restrictions such as permeability–selectivity could reduce the solute selectivity, and high fouling propensity has marginalized the recent advancements in water purifying membranes. Nowadays, researchers have been more interested in nanomaterials due to their unique characteristics and enormous advantages [7]. The developments in nanotechnology (nanoparticles, or nanofiber) have broadened the potential area in the field of membrane-based water filtration. However, nanofibers are a special type of nanomaterial as they have shown some fascinating characteristics in membrane fabrication. Other than nanoparticles, nanofiber membranes are a relatively recent approach to enhance the purification efficiency by modifying the functionalities of the membrane surface. For manufacturing nanofibers, with great flexibility and upscaling the production potential, electrospinning is widely regarded as the best and simplest method [8]. It is highly likely that nanofiber membranes fabricated via electrospinning are a suitable alternative for either replacing the traditional water-filtration membrane technologies or overhauling the drawbacks of classical membrane technologies. Electrospinning is the process of generating nanofibers from a polymer mix solution or melt by applying high electric fields and depositing them on a metal collector. Basically, the process introduces a high voltage electricity supply on a liquid polymer solution contained in a reservoir and that is extruded from the reservoir nozzle and collected on a collector as shown in Fig. 6.1. Basically, there are three fundamental constituents required for fabricating electrospun nanofibers membranes (ENMs); (a) a high voltage power supply (either direct current or alternating current) (b) a syringe pump with needles, and (c) a conductive collector [9]. During electrospinning, the polymer solution ejects from spinneret formed electrically charged droplets after applying high voltage. Meanwhile, the charged droplet formed a conical shape also known as tailor cone as a result of electrostatic repulsion among surface charges. Subsequently, the solution undergoes evaporation and solidification and is finally deposited on the collector plate as electrospun nanofibers [10]. ENMs are superior in water purification for their tunable morphology and controlled surface functionalization. The surface of the ENMs possesses a highly interconnected porous network while maintaining controlled thickness. The micro or nano porous network usually facilitates excellent high surface area which is essential for inducing the active sites toward the pollutants of wastewater. Due to these characteristics, highly selective and specific membranes such as microfiltration (MF), nanofiltration (NF), ultrafiltration (UF), etc. can easily be fabricated via ENMs techniques [11, 12]. However, membrane filtration processes have major drawbacks such as concentration polarization (CP) and fouling [13]. The present chapter will cover a comprehensive outline of electrospinning including methods of fabrication, factors, and modification of ENMs for wastewater filtration.

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Fig. 6.1 Schematic illustration of typical electrospun tools, setup, and nanofiber mesh formation

6.2 Types of Membranes In the most general sense, membranes are semipermeable barriers that separate two or more species by allowing a certain species to pass through and restricting the transport of other chemicals depending on their physical or chemical properties. Usually, membranes can be classified into two groups, either solid or liquid. Solid membranes consist of stiff and highly vacuous interlinked porous or nonporous structures, whereas liquid membranes contain a solution of an organic liquid membrane in the pores of polymer support. The solid membranes are categorized as symmetrical or asymmetrical on the basis of morphology.

6.2.1 Symmetric Membrane Symmetric membranes also known as isotropic membranes may be porous, nonporous, and electrically charged where the pores are almost equal in diameter across the membrane depth. The flow of unwanted particles faces selective barriers through the whole membrane. In addition to keeping material on the filter surface, isotropic membranes tend to retain components around the same size resembling the pores in the membrane. The membrane serves in this way as a profound filter and as a screen or surface filter. Filtration using an isotropic membrane can be increasingly less effective as the removal of particles trapped inside the membrane is difficult [14]. Porous membranes with randomly distributed interconnected pores have a stiff and high void structure. The separation process heavily relies on the particle size, and the membrane pores, such porous membranes can successfully isolate only molecules that differ significantly in size [15]. UF and MF membranes are excellent examples of porous membranes. The pore diameter of a microporous membrane is relatively

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lower (0.1–5 µm) than conventional filters where the pore size ranges from 5 to 10 µm [16]. In the case of nonporous membranes made up of a thick film, the separation is carried out via diffusion through these membranes, and the diffusion is driven by pressure, electrical potential gradient, or concentration [17]. For example, in RO, the separation of a material depends on its relative transportation rate through the membrane [18]. Again, the transportation rate is influenced by the particle’s diffusion rate as well as solubility. However, the electrically charged membranes can be porous, nonporous thick films or microporous structures with positively or negatively charged ions, enriched on the membrane. The concentration of analyte ions and charge have a major effect for separating solutes where solute with the different charge adheres to the membrane walls and solute with the same charge is rejected via Coulombic repulsion [14]. Ion exchange and electrodialysis are good examples of electrically charged membranes [19]. However, there is a great need for filtration membranes with strong anti-fouling performance and solvents resistance (e.g., organic solvents or very acidic/alkaline/ saline solvents) that can efficiently filter complicated polluted water systems. Here, using the composite membrane comprising stabilized polyacrylonitrile (PAN) nanofibers and ferric oxide (FeOOH) nanorods as a case study, a straightforward approach is shown to overcome the challenges [20]. In this study, significant solventresistance against organic solvents and strong inorganic acidic/alkaline/saline solutions may be achieved by simply stabilizing PAN nanofibers in air. This electrospun membrane, which has hydrophilic -FeOOH nanorods tethered onto SPAN nanofibers, is superhydrophilic (0°) and underwater superoleophobic (>155°) for a wide range of oils. Not only does the SPAN/-FeOOH nanofibrous membrane have a Young modulus of 274 MPa, but it is also chemically stable, has high separation fluxes (2532–10,146 L m−2 h−1 ), and achieves satisfactory removal ratios (>98.2%) when confronted with both insoluble oils and soluble cationic dyes.

6.2.2 Asymmetric Membranes In membrane-based filtration technology, a major breakthrough came in 1960 owing to the development of anisotropic membrane also known as asymmetric membrane. Like isotropic membranes, anisotropic membranes are not uniform in cross-sections but rather formed from differently structured layers and different materials [21]. Usually, anisotropic membranes are classified as phase-separation membranes and composite membranes [22]. The chemical composition of phaseseparation membranes is often similar to isotropic membranes but separated because of their random/diverse pores and porosity like Loeb-Sourirajan membranes. Typically, composite membranes involve a layer-by-layer arrangement and are composed of a dense and thin top layer supported by a porous substrate layer in the same polymer matrix [23]. The thin layer termed as a surface layer or active layer directly affects the separation process and provides a significant advantage in retaining the solutes, whereas the thicker, porous membrane is called a sub-layer or bottom

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layer that ensures mechanical support to the top layer. The top layer fabrication processes and their optimization play a governing role in water-filtration technology. Usually, composite membranes including polysulfone (PSf), polypropylene, polyamide, polyethersulfone, polyimide, cellulose acetate, etc. [20, 24, 25] membranes are noteworthy for their utilization in industrial water-filtration application. However, while small-pore membranes have their advantages, the drastic drop in volumetric flow that comes with them is a major drawback. An electrospun polycaprolactone fibers membrane was developed by Weerapha Panatdasirisuk et al., which has a very high porosity (about 88%) [20]. Anisotropic tunnels with an aspect ratio (pore length/pore width) of up to 5.3/3.0 were created when the membranes were stretched uniaxially at varying strain levels. Tween 80, a hydrophilic surfactant, was added to PCL solutions before electrospinning to increase their wettability. With a tensile strength of 6.59 ± 1.67 MPa and an elongation at break of up to 130 ± 21%, the modified PCL membranes demonstrated outstanding mechanical qualities befitting their use as free-standing separators. By extending the membranes, they were able to reduce the pore size while keeping the porosity the same.

6.2.3 Liquid Membranes In 1968, Li et al. created liquid membranes and suggested remedies to a number of waste disposal difficulties [26]. Liquid membranes are emulsions that are incapable of blending with water generally, composed of two phases i.e. (i) oil phase contains surfactants, and (ii) a hydrocarbon solvent with various additives, which encapsulates microscopic droplets of an aqueous solution of appropriate reagents and removing wastewater contaminants. For example, the removal of ammonia from wastewater such as that from a sewage plant [27]. In the effluent from the sewage plant, ammonia exists in mobile equilibrium with the ammonium ion: NH3 + H+ = NH+ 4 The molecular form of ammonia has appreciable oil solubility and readily permeates from the outside aqueous phase through the oil membrane into the encapsulated aqueous acid where it is trapped in the form of the oil-insoluble ion. The example shown depends upon the pollutant having appreciable oil solubility. This is not a requirement since additives can be incorporated into the membranes to enhance solubility and permeation rates. For example, heavy metal ions that cannot normally permeate oil membranes can be removed from aqueous solutions by incorporating a suitable ion carrier in the oil phase [28]. However, liquid membranes can be divided into supported liquid membranes and unsupported liquid membranes. The structure of supported liquid membranes is microporous, featuring mechanical strength and the liquid-filled pores operate as a selective separation barrier, whereas unsupported

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fluid membranes are composed of thin fluid films which are stabilized in an emulsion composition by the surfactant on each side.

6.3 Methods of Electrospun Membrane Fabrications There are several electrospinning processes including single-spinneret electrospinning, multi-spinneret electrospinning, coaxial electrospinning, etc. [29–31] for surface modified or doped nanofiber fabrication. For laboratory uses, single-spinneret electrospinning is the easiest one to produce single or mixed nanofiber. Singlespinneret electrospinning uses one nozzle for plunging just one polymeric solution [32], whereas, the multi-spinneret electrospinning consists of multiple nozzles loaded with different polymeric solutions, and thus able to manufacture hybrid nanofiber membranes by electrospinning the polymeric solutions on a single metal collector [33]. Usually, two or more nozzles are present in multi-spinneret which enables quicker and large-scale nanofiber fabrication. In coaxial electrospinning shown in Fig. 6.2, unlike single spinneret, a concentric tube is employed in the place of the single spinneret. Coaxial electrospinning is a useful technique if solutions to be implemented are hardly soluble or difficult to electrospun as well as hollow fiber membrane production [34]. The electrospun membranes might be modified by various surface treatment techniques and or by including chemical groups [35]. In the following section, various types of modification methods shown in Fig. 6.2 will be discussed which might help future research in water treatment technologies.

Fig. 6.2 Schematic illustration of the modification of nanofiber in ENMs

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6.3.1 Layer-By-Layer (LbL) Technique Recently, the chemical mixing or coating of a functional group moiety has been developed with several new techniques, including self-assembly and the layer-bylayer approach [36, 37]. Among those, the LbL technique is a convenient method to introduce some charged functional molecules on the polymeric surface. This method is primarily based on the binding of macromolecules having positive and negative charge properties on the polymer surface [38]. This approach is special because it is possible to modify the membrane surface with desired surface functional groups so that either a single property can be improved, or several new properties can be introduced. By introducing different functional moieties in the membrane surface, a highly rough surface with an abundant surface area can be attained which is generally favorable for the membrane-based water purification efficiency. Although LbL coating mainly occurs through oppositely charged organic molecules, it may also be possible to coat positively charged inorganic nanomaterials on oppositely charged organic molecules. According to recent reports, cellulose acetate, polystyrene, poly(llactic acid) are used as parent polymeric materials in the LbL technique [39, 40]. Among them, cellulose acetate having negatively charged functional moiety on the surface is the most used material for the LbL approach [41]. Usually, cellulose acetate may not require any surface treatment but other than that, various pre-treatments may require for surface activation of the polymers to be electrospun. Some prior works on LbL-treated ENMs are summarized in Table 6.1.

Table 6.1 Some ENMs fabricated by LbL technique Polymer

Pre-treatment reagent

Alternate layers sequence

Refs

Polystyrene

sulfuric acid for sulfonation

Fluorescein isothiocyanate-labeled poly (allylamine hydrochloride) Poly (styrene sulfonate, sodium salt)

[42]

Cellulose acetate (CA)

None

Polyethylenimine Polyoxometalate

[43]

Poly (L-lactic acid)

None

Poly(ethyleneimine)

[44]

CA

None

TiO2 Poly (acrylic acid)

[45]

CA

NaOH

Chitosan Alginate

[46]

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6.3.2 Functionalization of Electrospun Membrane Functionalization of nanofiber adds some fascinating features to a given electrospun nanofiber. In the functionalization approach, mechanical properties and chemical stability can be combined in a material with other essential properties like catalytic, biocompatibility, and antibacterial effects [47, 48]. Introducing functionality to the nanofibers improves their performance and flexibility, allowing them to be compatible with desired applications. The electrospun fibers are dipping onto the conductive polymer surface to enhance their surface area with different functional groups as well as surface adhesive properties. Recent studies have shown that nanoparticles, including TiO2 , MgO, Al2 O3, etc., can be introduced into the surface of nanofibrous by using the electrospinning technique [49–51]. Therefore, the efficacy and adaptability of the nanofiber are enhanced by imparting additional functional characteristics that increase its potential for the specific application.

6.3.3 Solution Blending Approach Solution blending is a popular method for introducing a new functional moiety or property to a membrane surface [52]. According to the solution blending approach principle, two components are blended to form a solution, followed by the electrospinning process. In several studies, the blending strategy is used when the desired materials are unable to produce nanofibers individually. As a result, an electrospinnable polymer is blended with the desired material to overcome this issue. In the electrospinning process, the desired materials are suspended in a polymer solution where the suspended materials can migrate to the nearest polymeric surface to form modified ENMs. The hydrophilic or hydrophobic properties of the desired material determine the membrane characteristics. For instance, hydrophobic polymers like PSf, polyvinylidene fluoride (PVDF), polyethylene, polyester polymers interact very well with hydrophobic particles like hydroxyapatite, silica particles, carbon nanotubes, etc. [53–55]. However, blending methods are often categorized into three techniques: materials that are dissolvable in a specific solvent, the second technique uses different solvents for the two ingredients, and the third technique uses materials that are insoluble in a common solvent [56]. But the solution blending technique is inefficient for producing core–shell structures which can be overcome by using coaxial electrospinning.

6.3.4 Wet Chemical Treatment Covalent bonding of functional molecules on the material is likely the best way to ensure long-term structural functionality. In the wet chemical treatment process, the

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Fig. 6.3 Wet chemical treatment of ENMs for wastewater treatment

relatively inert polymer nanofiber is treated with a chemical to introduce various reactive functional groups such as hydroxyl (−OH), amine (−RNH2 ), or carboxylic (−OH), depending on whether the polymer molecule’s sites are sensitive to nucleophilic or electrophilic reaction [57, 58]. The polymeric material is immersed in a basic solution to generate a hydroxyl or carboxylic group on the nanofiber surface. Additionally, a cross-linking or bonding agent may be employed to attach molecules to the nanofiber surface. Wet chemical treatment increases multiple applications through selective treatment of the material. Wet chemical etching contributes to surface energy enhancement by localizing oxidized functional groups on the surface. The technique is successful because the solvent penetrates deeply into the polymer matrix pores, allowing for the treatment of multiple characteristics at substantially lower prices. The oxidizers like potassium permanganate or dichromate, as well as wet solution swellers like 2-(2-butoxyetoxy)ethanol enhances the surface activity of the polymer [59]. Although both reagents improved the surface roughness of the parent material, the oxidizers resulted in more etching of the upper layers. Nylon has been chemically treated with an iodine-potassium iodide solution to increase microstructural crystallinity and molecular weight and improve surface adherence [60]. Treatment of polypropylene mesh with a highly alkaline solution (sodium hydroxide) improved cell adhesion owing to increased adhesion without sacrificing mechanical characteristics, whereas acid treatment boosted its adherence to epoxies [61]. A typical diagrammatic representation of wet chemical treatment, which can be used for the functionalization of the ENMs is shown in Fig. 6.3.

6.3.5 Chemical Depositions and Coating Chemical vapor deposition (CVD) is a coating technique that employs a temperaturedependent chemical approach at the interface of heated material, with chemicals supplied in the vapor phase [62]. These reactions may or may not include the substrate material itself. This approach actually performs to increase the mechanical properties of nanofiber material. In this approach, the exterior surface of a suitable fiber material undergoes a chemical reaction, and therefore the result self-assembles and deposits to make a thin layer over the substrate. It is common practice to employ semi-crystalline

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polymers as well as crystalline inorganic components in the self-assembled film fabrication process. Guo et al. demonstrated that by initiating CVD on a hydrophilic membrane, the liquid entry pressure increased substantially to 373 kPa from 15 kPa, resulting in an increase in membrane distillation (MD) efficiency [63]. The coating process might be achieved by immersing the membranes in a coating solution. Dopamine has recently gained popularity as an interface because of its ease of use and efficacy for adherence. In addition, Liao et al. increased the contact angle of the MD membranes to 158° from 138° by using dopamine as an interface in the coating process [64].

6.4 Key Factors That Affecting the Electrospinning Process The outlook of the electrospinning setup looks very simple at a glance, but in reality, the nanofiber production process by electrospinning requires a lot of optimizations in the parameters. Due to the high operational process, electrospinning renders the diversity in morphology, structure, and functionalization in the fabricated nanofibers. The parameters which mainly influence the fabrication process are the applied voltage on the polymeric material, type of polymeric materials, and their concentration, viscosity, and molecular weight. Even the distance between the collector and the needle tip, tip size, the speed of the collector, and the total ambient process (temperature, humidity) can bring a lot of varieties in the fabricated membrane [65]. One can specifically design a desired membrane by altering the conditions during the fabrication process.

6.4.1 Applied Voltage The exact function of voltage in the electrospinning process is overcoming the surface tension of the polymer solution contained in the syringe. The high voltage between the electrodes is dependent on the applied electric field. The higher the applied electric field, the higher the applied voltage would be imposed on the polymeric solution. However, an optimized applied voltage which is different for different polymers forces the polymeric solution to form a Taylor cone and upon a further increase in the voltage, it ultimately forms an emitting jet of nanofiber. The regular scale of the applied voltage is about 5–40 kV but a variety of higher voltages have also been reported in many works [66]. It is rational that the high voltage can induce a high repulsion force on the stretched nanofiber ejecting from the needle tip during the fabrication process, hence producing the small diameter-based fiber and vice versa. But exceptions are also being seen in many prior works where the direct correlation between the applied voltage and the diameter of the fabricated membrane couldn’t be established. Even some studies have reported that the applied voltage merely

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contributes to the total process [67]. Nonetheless, it is worth mentioning that besides the effect of applied voltage varies on the polymeric properties such as its viscosity and concentration.

6.4.2 Flow Rate of the Polymeric Solution The industrial production of high-performing membrane fabrication with welldefined morphology demands a uniform nanofiber formation throughout the whole membrane. The uniformity of ENMs is highly dependent on the diameter of the nanofibers in the fabricated membrane by the electrospinning method [68]. The jet formed on the needle tip of the syringe is determined by the overall polymer properties. However, the continuous production of the uniform nanofiber in the fabricated membranes requires a controlled flow of polymer solution through the syringe needle, because an appropriate flow rate can form a stable Taylor cone upon optimized voltage. Usually, the standard flow rate of most of the polymers is below 1.0 ml/h [64]. This amount of flow provides the required time to evaporate the solvents from the polymer solution while depositing them on the collector surface. This time ensures the smoothness and uniformity of the membrane. Below or higher of this flow rate, the process shows difficulties in the continuous production of the stable and uniform jet. Thus, the lower feed amount produces receded and higher flow rate produces beaded nanofiber in the ultimate membrane. Receding means the ejected fibers tend to move backward and form a bent and unstable jet due to insufficient flow. Again, beaded means the agglomerated form of polymeric solution is formed instead of a Taylor cone when the flow rate is too high [69]. Besides, the high flow rate doesn’t give spacious time for solvent evaporation and thus several diameters of unsmooth nanofibers are formed during the extrusion process. Sometimes the excessiveness of flow can result in a film-like portion in the membrane having ribbon and garland-like defects [70, 71]. The non-uniformity in the membrane is not considered as favorable for the high performance in the separation process.

6.4.3 Distance Between Tip and Metal Collector The distance between the needle tip and metal collector can play an interesting role as the voltage intensity of the applied electric field fluctuates with the change in the fluctuation of the distance between the tip and metal collector [72]. Usually, the shorter distance induces a higher electric field and hence generates a higher voltage which is ultimately favorable for the narrow diameter of the fabricated membrane. On the contrary, the longer distance will generate low voltage and result in a wider diameter due to the reduced repulsion on the needle tip [73, 74]. Subsequently, the distance has to be ample for solvent evaporation because the quality of the nanofibers from the polymeric solution will be determined by smoothness and uniformity. As

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a result, the optimization of the distance between the tip and metal collector is required based on the polymer’s intrinsic properties (viscosity, concentration, etc.) [75]. The variation of metal collectors can also bring up some changes in the size of the fabricated nanofiber. Usually, the plate-type collectors have a narrow area to collect the nanofibers by electrospinning compared to the drum-based collector. The drum-based collectors are mostly popular for larger size membrane production and the high-speed drum collectors can help to produce a more oriented nanofiber structure in the fabricated membrane [76].

6.4.4 Properties of Polymer In the whole electrospinning process, the quality and pattern of the fabricated nanofiber to construct a high-performance membrane is mainly dependent on the polymeric intrinsic properties. The intrinsic properties are including polymer concentration, viscosity, molecular weight, conductivity, and surface tension. The electrospun capability of a certain polymer is very crucial and does vary with the variation of the concentration (40–50% weight ratio) based on different polymers. The access low concentration may result in uneven nanofiber formation, or beads formation and thus results in nano sputtering instead of the electrospinning process [77]. Most importantly, the diameter of the nanofibers can be controlled by optimizing the concentration, and usually, moderately higher concentrations can help to form a wider diameter. But access to polymer concentration in the polymer blend may create a jam in the needle tip. However, the high molecular weight-based polymers are usually preferable for the formation of the well-defined polymer chain entanglement in the fabricated membrane [78]. The entanglement affects the viscosity and surface tension as well as the electrical properties of the nanofibers. Usually, low molecular weight-based polymers possess low entanglement and viscosity, hence produce beads instead of nanofibers [12]. The solution conductivity of polymers depends on polymers and the solvents used with some other additives during the preparation of the polymeric blends for the electrospinning process. The conductivity issue is important because the electrospinning process produces nanofiber upon applying external voltage on the solution of the polymers. Usually, some inorganic salts (NaCl, KCl, CaCO3 ) and nanomaterials including carbon nanotubes ((CNT) or graphene) are suggested to enhance the conductivity of the solution [79]. The highly conductive polymers are responsible for bearing extra charge, and hence extra repulsion is induced on the polymeric solution to generate thinner nanofibers. On the contrary, the lesser conductivity can slow up the jet formation process and lead to the formation of the beads. So, upon tailoring those polymeric properties, the desired nanofiber formation is possible in the electrospinning process.

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6.4.5 Ambient Conditions The ambient conditions, mainly temperature and humidity can massively affect the electrospinning process. The relatively smaller relative humidity is usually favorable for solvent evaporation during the electrospinning and ensures smooth nanofibers formation [80]. But the exceptions are also available and sometimes the relative humidity can favor the formation of bigger fiber in the case of different polymers. This is due to the presence of water which induces a faster precipitation process. Similarly, the temperature has also dominated effects on the quality of the nanofibers. This is also optimizable for a different range of polymers. Some polymers with lower viscosity require high temperature and some others having higher viscosity require a lower temperature to get expected diameters [81]. However, the drying process is also important after forming the nanofibers to get the electrospun membrane. Usually, the drying temperature is set below the boiling point of the solvent used and to evaporate the residual solvent from the readily formed nanofibers. Both the temperature and humidity should be controlled to avoid the pore formation generated from the moisture content present in the formation site. Some recent work on the ENMs experimental setup and their performance for water filtration is summarized in Table 6.2.

6.5 Mechanisms of Membrane-Based Water-Filtration Process In the removal of pollutants from wastewater, electrospun nanofibers are prominent due to their intrinsic electrostatic properties and active sites originating from different functional groups on their surface. Sometimes several pollutants are retained by chelation and complexation with the surface of ENMs during the water treatment process. Thus, the total active surface area and functional sites are highly required for the best efficacy of the fabricated electrospun nanofibers. Nowadays, surface modification is not only limited to introduce the functional groups but also the presence of micro or nanopores on the fiber membrane surface are considered as the promising structural demand toward the efficient filtration by the electrospun membrane. The entanglement area along with the pores and functional sites facilitates different filtration mechanisms like—NF, UF, MF, RO, etc. [87]. A brief summary of recent work on the membrane characteristics and separation efficacy of nanofiber membranes for MF, UF, NF, FO is shown in Table 6.3.

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Table 6.2 Summary of recent work on ENMs experimental setup and their performance for water filtration Types of polymers

Electrospinning operational condition

PAN

Target separation

Performance

Ref

High voltage applied Suspended particle at 18 kV, Qd: 0.5 mL/ MF h, needle ID-0.4 mm, membrane uniform thickness: 60 µm

Membrane eliminated 0.1 µm particle from water without permeate flux reduction

[82]

CA and PSf

Voltage is applied 17 UF and 25 kV, neddle-18 and 22 gauges, TCD of about 10 cm and needle: 18 and 22 gauges, Relative humidity: 55 and 23%

fouling performance enhance about 90%

[83]

Polyetherimide

Applied voltage 30 kV, Qd: 15 µL/ min, TCD is about 12 cm, Humidity-50% at room temperature

Forward osmosis (FO)

Exhibited higher water flux which is about 42 L/m2 h

[84]

PSf/PAN

Solvent-DMF/NMP, drum rotating speed-150 rpm, Applied voltage-20 kV, flow rate-1 ml/h

FO

Excellent permeability with salt rejection 97%

[85]

PAN

Voltage applied 28.5 kV, flowrate 1.0 mL/h, TCD is 16 cm, Relative humidity-50% ambient temperature

FO, reverse osmosis (RO), pressure retarded osmosis

Membrane showed sevenfold higher permeability than commercial membrane

[86]

6.5.1 Nanofiber Membrane for Microfiltration MF is a commercially used separation technique by which suspended water contaminants are removed by means of a membrane having pore size ranges from 0.1 to 1.0 µm [88]. Coagulation, flocculation, and final sedimentation are three sequential phases included in the present water pre-treatment [89]. Instead of the three-stage pre-treatment procedure, the appropriate design of the water purification module enables the use of ENMs. Though conventional membranes now can achieve smaller porosity (0.2 um), ENMs have shown their advantages like easy fabrication, controllable pore size, and high porosity over conventional membranes [65]. However, in determining the usage of the nanofiber membrane in separation processes and particularly in the separation performance, two key aspects viz. flux and selectivity affect

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Table 6.3 Summary of recent work on the membrane characteristics and separation efficacy of nanofiber membranes for MF, UF, NF, FO Types of membrane Fabrication process

Membrane flux (kg/m2 . h)

Solute rejection (%)

Method

References

PSf ENMs

Electrospinning/ heat post-treatment

232

Humic acid -63.9

MF

[103]

NF membrane based on polyethylyne terephthalate and polyamide

Electrospinning/ reverse interfacial

34 ± 2.3

Salt-78

NF

[104]

Polyimide supported microporous NF membrane

Electrospinning/ interfacial polymerization

11.6 ± 2

Salt- 49.8

FO

[105]

Polyvinyl alcohol (PVA)-MWCNT/ PAN

Electrospinning/ 270.1 solution treatment

Salt -99.5

UF

[106]

LbL assembly high Electrospinning/ flux NF LbL assembly/ membranes phase inversion

75

Salt-80

NF

[107]

Hydrolyzed PAN supported NF based membrane

55.05

Salt-97

FO

[108]

Electrospinning/ interfacial polymerization

the practicality of a nanofiber membrane. Selectivity is governed by the surface characteristics of the nanofiber membrane, which differentiate the class of species that may be transported, and flux represents the species penetration rate through the membrane. The two previously mentioned variables depend on both the morphological and structural characteristics, such as weight resistance, thickness, pore size, distribution, etc., ENMs including polyurethane, PVA, PSf, PVDF, PAN, etc. can achieve higher flux than conventional MF membranes. [90] These membranes can also be used for removing bacteria (>0.3 µm) and as pre-filters before NF or UF [47]. However, a relationship between pore size and fiber diameter was established by Hsiao and coworkers indicating that the mean and maximum membrane pore size is about 3 and 10 times the mean fiber diameter [91]. With this relationship, one can have a better design of the nanofiber structure and morphology.

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6.5.2 Ultrafiltration and Nanofiltration Membranes ENMs have introduced an extra dimension in UF and NF technology. Both UF and NF processes are pressure-driven [92]. In UF membranes, typically the pore size is approximately 0.002 to 0.1 microns and the operating pressures are about 200 to 700 kPa whereas NF membranes have a pore size of approximately 0.001 microns and operating pressures are usually near 600 kPa [93]. In terms of UF membrane fabrication, the conventional phase inversion technique is commonly used. Usually, those membranes have a higher fouling rate, low permeate flux, and passing contamination phenomena which limits their application in water filtration. In that case, researchers have shown ENMs would be the best way to overcome these problems. For example, the separation efficiency of hydrophobic PSf and hydrophilic cellulose membrane can be enhanced by incorporating electrospun nanofiber [94]. Though the nanofibers have no effects on the parental membrane structure as well as selectivity, the enormous porosity of the nanofiber contributes to the enhancement in the permeability even at low pressure. For effective heavy metal adsorption from wastewater, the PAN nanofibers-based membrane is developed where the (-CN) group coordinates with the metal ions. Additionally, researchers are also focusing on polyaniline (PANI), PVA, PVDF nanofibers for developing the UF membrane [95]. Similar to the UF membrane fabrication, NF membranes are mostly fabricated via interfacial polymerization technique. The performance of NF membranes mainly depends on the structure of the substrate layer and its porosity [96]. This criterion opens the exciting promising route for the fabrication of NF membranes via ENMs due to their remarkable porosity. Through the ENMs technique, the interfacial polymerization can easily be fabricated, and the supporting layer can be optimized properly. Recently, Yoon and coworkers fabricated a thin-film NF membrane with nanofibrous composite structures via the ENMs technique [97]. The membranes showed remarkable separation performance with higher permeate flux (over 2.4 times) compared with conventional NF membranes.

6.5.3 Reverse Osmosis and Forward Osmosis Membranes Water desalination by employing RO membrane has expanded globally and became a major substitute for the conventional method of distillation. Size exclusion is the most common removal method in RO membrane filtration (pore diameter ≥ 0.01 um), allowing the process to potentially attain 100% efficiency regardless of factors like solution pressure and concentration [98]. There are three predominant layers (an ultrathin layer, a microporous supporting layer, and a mechanically resistant nonwoven fabric) in the RO membrane which requires hydraulic pressure for clean water flow. Though industrially, RO membranes with very thin selective layers were manufactured, a major issue in RO membrane research was to increase water flow rate than the conventional thin-film composite membranes by decreasing the thickness of

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their selective layers [99]. The utilization of the electrospun nanofiber as a supporting layer for next-generation RO membranes may result in increasing the water flow rate while requiring lower hydraulic pressure. However, nanofiber membranes are found to be inappropriate for high-pressure systems, since the nanofiber structures have lower mechanical strength, resulting in active layer breakdown during operation. In the electrospun technique, parallel plate electrodes or rotating collectors have allowed the creation of well-aligned fiber systems with enhanced mechanical strength [100]. However, FO uses natural osmotic pressure to facilitate the flow of the water molecule, whereas the flow of any foreign particles or ions is inhibited through the FO membrane. FO simply maintains that only freshwater is retrieved from the feed solution and which makes the FO membrane’ suitability for various types of industrial applications, for instance, water recycling, wastewater treatment, etc. [101]. Since FO membrane-based separation technologies depend on natural osmotic pressure, it also requires less energy than any other technology using pump hydraulic pressure for clean water flow like RO. Industrial wastewater (feed solution) flows in a FO system through one side of the membrane, and a solution with high solute concentration flows on the other side of the membrane. The difference in solute concentration in both sides of the membrane generates osmotic pressure and causes the water to pass through the membrane while retaining all pollutants in the feed solution. Nevertheless, FO is in underdevelopment and less progressive because of the unavailability of the proper FO membranes, unlike RO and NF. The thin-film composite membranes are very commonly used in FO with a thick active layer and porous support where the design of the supporting layers should receive more attention in order to minimize the effects of CP while maintaining a high-water flow rate. Electrospun nanofiber is one of the best techniques to produce a highly interconnected porous structure for the superior osmotic flux of thin-film composite membrane with improved mechanical strength. Various types of polymers support including PSf, PVA, PVDF, PANI, etc. can be utilized for the synthesis of electrospun nanofibers. Park et al. developed a PVDF membrane supported by electrospun PVA nanofiber where glutaraldehyde is used as a crosslinker [102]. The hydrophilic PVA nanofiber remarkably upgrades the mechanical properties (in presence of water) of the hydrophobic PVDF membrane without reducing the membrane porosity. Moreover, this led to a significant reduction in minimal CP effects as a result, improvements in FO performance concerning water flow as well as the ratio of solute concentration to water.

6.6 Solute and Solvent Transport Mechanism The ENMs act as a nanocage where the contaminants are selectively separated from water by their physical structure. Those semipermeable nanocages are efficient enough for the permeability of water molecules while the contaminant solute particles are selectively capped in those nanocages’ surfaces. The capture of the solute particles usually follows the sieving and diffusion mechanism. Both of those mechanisms are pressure-driven water separation techniques including MF, NF, UF, RO,

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Fig. 6.4 Schematic illustration of MF, UF, NF, RO process showing the pollutants that are captured by the membranes along with the pores

etc. [92]. The separation efficacy of these membranes depends on the membrane’s selectivity, permeability, and pore size. The component transfer rate is generally defined as the product of the driving force and the mass transfer coefficient through the pores. The main driving forces can be the chemical potential difference and the pressure, temperature, and electrochemical potential differences. On the other hand, mass transport depends strongly on the operating conditions and the interaction between the permeate molecules and the surface of the membrane material. Mass transfer across a membrane layer is an irreversible process. The process is shown in Fig. 6.4.

6.7 Concentration Polarization and Fouling There is a good relationship between membrane performance and transmembrane flux where transmembrane flux decreases as a function of time resulting in a decreasing trend in the membrane performance. Both CP and fouling depend on each other but are different phenomena. CP or electrolyte concentration is responsible for fouling and may lead to reductions in transmembrane flux over time [109]. Furthermore, processes involving a thermal pressure, including MD, may again undergo thermal polarization, which is known as CP. During the separation process, CP possesses a damaging effect on transmembrane permeability which limits the permeation ability of the membrane. The convective flow of contaminant particles to the surface is greater before steady state than the diffusion backflow to the bulk solution. As a result, rejected particles are accumulated near the membrane surface which leads to CP phenomena [110]. Some factors related to CP usually occurring in the separation process are mentioned below:

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(a) When the surface chemical potential increases, the driving force for the filtration reduces. (b) If the solute concentration of the membrane wall reaches the saturation conditions, the gel formation, or precipitation, happens, for which hydrostatic resistance increases. (c) Membrane composition may change by a chemical attack when the solute concentration reaches a high concentration. (d) The depositions of solute particles at the membrane surface can change the separation properties of the membrane. However, CP has a significant effect on membrane fouling although it can also be caused by other factors. Fouling can be observed as a result of an increase in CP, when the particles accumulate on the membrane surface and block the pores or cover the surface, ultimately leading to a reduction of the permeate flux. In this case, permeation flux is weakened, productivity is reduced, and the service life of filters is limited [111]. Electrospun nanofibers have some unique structural orientation which is prospective for avoiding the possibility of fouling in water treatment. Especially the neutral zwitterionic polymers for the fabrication of ENMs have shown effective antifouling features [112]. Those polymers form an excellent hydration layer with water molecules which provides a barrier to the adsorption of contaminants on the surface. Besides, there are some hydrophilic ENMs like cellulose, chitin, etc. which are able to reduce the CP as well as the fouling rate by decreasing the water contact angle to zero degrees. [47] To reduce the biofouling, the ENMs can also be modified with antimicrobial metal nanoparticles viz. Cu, Ag, Au, etc. [113, 114]. Thus, the incoming pathogens of the wastewater trapped by the ENMs nanofibers, and the denaturation of the protein cell inhibit the growth of microbial fouling. The inhibition process is illustrated in Fig. 6.5.

6.8 Critical Assessment and Future Vision Recently, the electrospinning process has received much interest for the fabrication of nanofiber-based membranes owing to their economical feasibility and environmental friendliness. Moreover, electrospinning is a flexible process and a new type of nanotechnology that allows researchers to access a wide range of nanomaterials with unique characteristics [116]. For instance, to produce biocompatible and biodegradable polymeric nanofiber membrane with PSf, PVDF, PVA, PA that have been extensively used in water filtration and treatment, the ENMs are highly applicable [117]. Electrospinning facilitates the development of a three-dimensional porous nanofiber with functionalized surfaces, resulting in a large surface area, high aspect ratio, consistent porosities, and customizable interconnected pores. Such features of ENMs can be used to enhance the functionality of the membrane, and

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Fig. 6.5 Schematic illustration of CP and fouling. The concept for this illustration is acquired from [115]

selectively eliminate various water pollutants like salt, metal particles, and biological pollutants [118]. Furthermore, the performance of ENMs for wastewater treatment and filtration can be improved by including inorganic nanofillers (discussed in Sect. 6.3) into the polymeric matrix with uniform dispersion. There have been several advancements in the field of ENMs during the last few decades. But there are certain factors to be considered, such as the mechanical support for fiber deposition and the limitations of large-scale manufacturing [119]. Next generation of fiber filtration media has established promising features and they have been shown to be capable candidates for good opportunities of advanced filtrations in the future. Based upon the current view of development and application, nanofibrous membranes are extremely promising for water and wastewater treatment. The process parameters that affect the properties of the electrospun nanofibers and their functionality, to enhance their performance in water and wastewater purification, are summarized in this review. Electrospun nanofibers exhibit great potential as antibacterial coating and reduction of bacteria in fibrous media provides efficient separation of pollutants. Significant progress has been achieved on the fundamental understanding of the water-filtration mechanism and there is currently a big paradigm shift toward the industrialization and commercialization of the electrospun nanofiber formation. In view of their application in filtration, electrospun nanofibrous

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Table 6.4 Possible features of electrospun nanofiber membrane process for water treatment industries Process

Upcoming decades

Microfiltration and Ultrafiltration NFMs

– Possibility of isoporous and ceramic membranes – Standardized membrane modules – Introduce responsive membranes and vibratory systems in wide range – Cost-effective MF/UF membrane developed

Nanofiltration and Reverse osmosis NFMs – Smart hollow module with ultra-permeable – Development of cost-effective chemically robust NF/RO membrane with negligible fouling – Availability of hybrid NF/RO, RO/FO with sensors – Feasibility of using vibration in hollow fibers module

membranes have exhibited high efficiency for filtration because solute and water permeability play important roles in the membrane performance, which are mainly ascribed to their high aspect ratio and vast interconnected porosity. The future of MF, UF, NF, and RO has been tabulated in Table 6.4.

6.9 Conclusions Electrospun PAN, CNT, PVDF, and PS nanofibers have been acclaimed to be the most efficient composite nanofibers for filtration applications. It is also notable that the surface functionalization of nanofibers with controlled porous structures determines performance to enhance the permeability and cost efficiency of nanofibrous membranes. This study would certainly provide a platform for novel study toward understanding properties of these composite nanofibers in terms of long-term stability, large-scale, low-cost, separation, catalytic, degradation, anti-fouling, and antibacterial applications. In spite of their benefits, the use of electrospun nanofibrous membranes for filtration techniques also faces a few challenges such as drawbacks of wettability, insufficient nanoscale selectivity, mechanical weakness and the adhesion between substrate and nanofibers that needs to be addressed well. Many challenges have to be overcome by the collaborative effort of research institutions and industrial companies in view of low-cost, industrial-scale modules, and efficient electrospun nanofibrous membranes for desalination and water treatment. Acknowledgements Supports from the University Grants Commission (UGC) of Bangladesh and Bangladesh University of Engineering and Technology (BUET) are highly appreciated.

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Chapter 7

Electrospun Nanofibers for Oil–Water Separation Lin Zhang , Jing Wang, Saisai Lin , and Jing Dou

7.1 Introduction Growing demands for oily industrial wastewater and oil spill treatment have increased the need for efficient oil–water separation methods [1, 2]. Different from the common filtration process that removes solid particles, achieving effective separation of oil/ water mixtures is quite arduous, especially for emulsions with complex compositions, and small droplet sizes in the dispersed phase that are prone to secondary emulsification. At present, membrane separation technology has the advantages of high separation efficiency and simple operation and is considered to be one of the most promising approaches for oily wastewater treatment and oil purification. Membrane separation techniques, such as microfiltration (MF) and ultrafiltration (UF) processes, have been used for the separation of the mixture of oil and water [3–6]. Nevertheless, traditional polymer-based separation membranes are inherently oleophilic and therefore prone to quick fouling [4, 7, 8]. To effectively separate oil/water emulsions, membranes need to meet three criteria: selective wetting capacity for the oil or water; proper selection of pore size according to the droplet size of emulsion and high porosity to guarantee an acceptable permeation under a certain operating pressure. Electrospun nanofibrous membranes have attracted interest due to their thinner fiber diameter, high porosity, large surface area and ease of use. ENMs with opencell pores, high porosity and easily tunable structures possess high permeability and have emerged as potential candidates for the treatment of oil/water emulsions [9– 11]. In general, the flux of ENMS membranes is approximately 2–3 times higher fluxes than that of commercial microfiltration membrane (GSWP, Millipore) with the same pore size and membrane thickness [12]. Furthermore, by means of surface L. Zhang (B) · J. Wang · S. Lin · J. Dou Engineering Research Center of Membrane and Water Treatment of MOE, College of Chemical & Biological Engineering, Zhejiang University, Hangzhou 310027, China e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Das (ed.), Electrospun Nanofibrous Technology for Clean Water Production, Nanostructure Science and Technology, https://doi.org/10.1007/978-981-99-5483-4_7

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physical- and chemical-modification, oil/water droplets with micron size can be effectively separated by hydrophilic-modified ENMS membranes [13, 14]. The next part of the sections in this chapter will introduce the advanced ENMs in the application of oil-water separation.

7.2 Hydrophilic-Oleophobic Electrospun Nanofiber Membranes Enlightened by the oleophobic capacity of natural organisms such as fish scales, a hydrophilic and underwater oil-repellent could be created by combining hydrophilic surface with proper roughness. A series of super-wetting ENMs for oil-water separation have been fabricated based on the synergistic effect between multi-scale roughness and low surface energy.

7.2.1 Hierarchically Structured Electrospun Nanofiber Membranes According to Cassie–Baxter theory, the interfacial contact area between liquid and solid play a crucial part in designing super-wettable materials [15]. The intrinsic wetting threshold theory furtherly ensured that the enhanced surface hydrophilicity can be obtained by creating a hierarchical structure with multi-scale [16, 17]. During the oil-water separation, water can be trapped in the hierarchical structures, and then a repelling liquid phase which can greatly decrease the chance of oil droplets touching the membrane surface was formed. Therefore, building micro or nano-scale hierarchical structures on the membrane surface is an effective way to enable the membrane with superhydrophilicity and underwater superoleophobicity. With the combination of super-wettable hierarchical structure, super-hydrophilic surface property and high pore tortuosity, ENMs membranes are capable of efficiently separating immiscible oil/water emulsions even under gravity alone. To obtain the desired physical morphology and chemical composition, surface modification of the as prepared ENMs has been well developed, including the in-situ polymerization of functional polymer and nanoparticles. Generally, most hydrophilic ENMs with large pores and small emulsion droplets are not effective for the treating of emulsified oil/water solutions. In addition, the functional polymer or nanoparticles only partially covered the mesh fibers, while the uncoated part maintained oleophilic. The author’s group deposited the commonly used hydrophobic polymer, polydopamine (PDA), onto a crosslinked polyacrylonitrile (PAN)/hyperbranched polyethyleneimine (HPEI) ENMs and obtained a super-hydrophilic and underwater super-oleophobic polyacrylonitrile/

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hyperbranched polyethyleneimine/polydopamine (PAN/HPEI/PDA) ENMs hierarchical surface structures with nanoscale [18]. Take this as an example, the characteristics of surface morphology, wettability and oil–water separation performance of hierarchically structured ENMs were comprehensively described.

7.2.1.1

Characteristics of Surface Morphology and Wettability

The hierarchically structured PAN/HPEI/PDA ENMs were fabricated by the formation of PDA nanoclusters on the cross-linked PAN/HPEI ENMs, as shown in Scheme 7.1. PAN/HPEI electrospun precursor solution was prepared by dissolving PAN powders and HPEI in N, N-dimethylformamide. The PAN/HPEI ENMs were fabricated by lab-made electrospun equipment in a stable environment. Then, the collected PAN/HPEI membrane was cross-linked at 160 °C for 1 h. The polydopamine nanoparticular was formed and attached to the membrane surface by mixing dopamine into prepared PAN/HEPEI membranes under rotation. PDA nanoclusters composed of larger sparse and smaller dense nanoparticles were formed via adjusting the self-polymerization time of dopamine. The surface morphologies of the hierarchically structured ENMs were shown in Fig. 7.1. Compared to pristine PAN ENMs, much smoother PAN/HPEI ENMs surface is obtained by the incorporation of hydrophilic-modified HPEI (Fig. 7.1a, b). Figure 7.1c shows that after DA treatment, smaller but dense and larger but sparse nanoparticles are found on the surface of the membrane, forming the nanoscale

Scheme 7.1 Schematic illustration of the preparation of hierarchically structured PAN/HPEI/PDA ENMs [18]. This scheme is adapted with permission from ref. [18]

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hierarchical structure due to the spontaneous self-polymerization of DA-generating PDA nanoparticles [19]. Subsequently, hierarchical structures on the membrane are generated through Michael addition or a Schiff base reaction between catechol of PDA nanoparticles and amine of PAN/HPEI ENMS [20–22]. The uniformly interpenetrated networks enable ENMs with open-cell pores and high porosities due to the stacking of nanofibers. As shown in Table 7.1, large pore diameters (>1 µm) and high porosity (>70%) are characteristics shared by all three ENMs membranes. PAN/HPEI and PAN/HPEI/PDA ENMs are prepared under the

Fig. 7.1 SEM images of a Pristine PAN ENMS membrane, b PAN/HPEI ENMS membrane and c PAN/HPEI/PDA ENMS membrane [18]. Figure (a–c) is adapted with permission from ref. [18]

7 Electrospun Nanofibers for Oil–Water Separation Table 7.1 Porosity and pore diameter of ENMS membranes [18]

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Sample

Porosity (%) Average pore diameter (µm)

PAN

70.4

1.33

PAN/HPEI

72.2

1.48

PAN/HPEI/PDA 73

1.45

This table is adapted with permission from ref. [18]

same electrospinning conditions; therefore, the impact of the porosity and fiber diameter on the fabricated nano-cluster structure can be neglected. Consequently, the porosity and fiber diameter of ENMs exhibit no significant difference. Nevertheless, PAN/HPEI and PAN/HPEI/PDA ENMs possess larger pore diameter, higher porosity and smaller fiber diameter compared to PAN ENMs, which can be attributed to the lower viscosity of PAN/HPEI electrospun solutions than that of PAN [23]. Ascribed to its high viscosity (5.9 and 48 Pa.s for PAN/HPEI and PAN electrospun solutions, respectively), PAN electrospun fibers become easier to dry and solidify. Compared with PAN electrospun fibers, PAN/HPEI electrospun fibers require longer drying time, thus the duration time of electric field drag force lasts longer and the fibers are thinner [24]. Water contact angle (WCA) and underwater oil contact angle (OCA) were utilized to characterize the surface wettability of the ENMS membranes. According to the optical snapshots of dynamic contact processes shown in Fig. 7.2, both the WCA and ultrafast water permeability of PAN/HPEI and PAN/HPEI/PDA ENMS membranes are lower than those of PAN ENMS membranes. This can be ascribed to the presence of super-hydrophilic component of the ENMS membranes, which facilitate the spreading and permeating process of water droplets. The oleophobicity of ENMS membranes was tracked by the introduction of oil (1,2-dichloroethane) droplets on the membrane surface underwater. Variations in the oil-adhesion properties with different DA treatment times are shown in Fig. 7.3. When the DA treatment time was prolonged, the OCA increased from 139° to 163° (Fig. 7.3a), and the affinity for oil droplets was significantly decreased (Fig. 7.3b), indicating that the formation of the underwater superoleophobic property was acquired. The OCA did not change greatly as the DA treatment time increased from 1 to 2 h, so the shortest DA treatment time was set to 1 h to provide sufficiently high oleophobicity. These nanoscale hierarchical structures are key to obtain super-wetting characteristics of ENMs. The nanocluster-modified PAN/HPEI/ PDA ENMs adsorbed water to its equilibrium state, which can trap water in rough nanoscale hierarchical structures and then form an oil/water/solid composite interface. This oil-adhesion force is greatly decreased as these trapped water molecules greatly reduce the contact area between the oil droplet and the membrane surface. Figure 7.4 further confirmed the function of nanoscale hierarchical structures. With the increase of DA treatment time, more obvious nanoscale hierarchical structures of ENMs were observed, and the underwater oleophobicity was significantly improved. In conclusion, it is shown that the underwater superoleophobicity of the membrane

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Fig. 7.2 Snapshots of dynamic contact processes of water droplets on a PAN, b PAN/HPEI and c PAN/HPEI/PDA [18]. Figure (a–c) is adapted with permission from ref. [18]

was successfully enhanced by the PDA nanoclusters-modified hierarchical structure. Furthermore, the oil droplets could easily roll off the surface of oleophobic PAN/ HPEI/PDA ENMs (Fig. 7.3c).

7.2.1.2

Separation Performance for Oil/Water Emulsions

PAN/HPEI/PDA ENMs possess excellent oil-water separation ability due to its superhydrophilic and underwater super-oleophobic properties. In this work, surfactant, sodium dodecyl sulfate (SDS) and stabilized oil/water emulsions with a micronsized droplet were constructed and utilized to test the emulsified oil-water separation performance of ENMs membrane. The flux and rejection of various emulsions were tested under gravity (~1 kPa) via maintaining the height of the feed constant at 10 cm. The flux was obtained by collecting a certain volume of permeate as shown in Fig. 7.5a. The permeation flux of PAN/HPEI and PAN ENMs decreased rapidly. As the volume of permeate reached 60 mL, the flux dropped to a lower level (1600 L·m−2 ·h−1 ), significantly higher than commercial UF membranes (typically exhibiting a flux less than 300 L·m−2 ·h−1 ·bar−1 ) [25] The PAN/HPEI/PDA ENMs can also be operated under higher transmembrane pressure. Initially, the dyed pure oil could not pass through PAN/HPEI/PDA ENMs at a transmembrane pressure of 0.02 MPa; however, the toluene-in-water emulsions could also be separated under the same transmembrane pressure. It can be seen in Fig. 7.7 that the ultra-high permeate flux with stable oil rejection under a more

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Fig. 7.5 Separation performance of PAN/HPEI/PDA, PAN/HPEI and PAN ENMs for SDSstabilized toluene-in-water emulsions, a Permeation flux and b Rejection ratio [18]. Figure (a, b) is adapted with permission from ref. [18]

Fig. 7.6 a Photographs of the feed and the filtrate of the SDS-stabilized toluene-in-water emulsions, scale bar: 20 µm, b Separation performance of a series of SDS-stabilized oil-in-water emulsions [18]. Figure (a, b) is adapted with permission from ref. [18]

realistic transmembrane pressure was observed in the membranes. At the same time, the permeation flux dropped greatly in the initial stage and then reached a steady state. This can be ascribed to the fact that the enrichment of oil/water emulsions on the membrane surface resulted in the reduction of water permeation channels and then water permeated through the channels between oil/water emulsions and nanofibers due to the presence of nano-scale hierarchical surface structures. A deduction concerning the plausible mechanism for the separation of oil/water emulsions is obtained. As shown in Fig. 7.8a, b, the micro-scale oil/water emulsions can be rejected due to the underwater superoleophobicity capacity and pore size exclusion. The oil/water emulsions separation process can be seen in Fig. 7.9. Oil/water feeds were poured into the membrane fixed between the glass funnel and conical flask. Water quickly penetrated through the membrane and was collected in the conical flask. Meanwhile, oil was maintained above the membrane attributed to its underwater superoleophobicity ability. The whole separation process was completed

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Fig. 7.7 Separation performance of PAN/HPEI/PDA ENMs for SDS-stabilized toluene-in-water emulsions at 0.02 MPa Fig. 7.8 Schematic illustration of the interaction between micro-scale emulsions and ENMS membranes, a PAN/HPEI ENMS membrane and b PAN/HPEI/PDA ENMS membrane [18]. Figure (a, b) is adapted with permission from ref. [18]

quickly (less than 150 s) without any external force engaged. The permeate flux can be affected by the thickness of ENMS membranes, as thinner membranes possess higher permeation flux ascribed to their lower transmembrane pressure. The endurance of the PAN/HPEI/PDA ENMS membrane can be reflected by the antifouling performance. And this antifouling ability was tested by repeating filtration

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Fig. 7.9 The solely gravity-driven separation for oil/water mixtures using PAN/HPEI/PDA membrane [18]. This Figure is adapted with permission from ref. [18]

Fig. 7.10 Performance cycles of PAN/HPEI/PDA ENMs when separating SDS-stabilized toluene [18]. This figure is adapted with permission from ref. [18]

experiments and the results were shown in Fig. 7.10. After 10 cycles of separation, the membrane still retained a steady separation performance of SDS-stabilized toluene. A slight reduction of permeation flux from 1700 to 1600 L·m−2 ·h−1 was observed. The flux was kept at a high level (>1500 L·m−2 ·h−1 ) and the oil content of the filtrate was still below 20 ppm. These phenomena indicated that the PAN/HPEI/ PDA ENMS membrane has excellent antifouling ability because of its low affinity to oil. This makes membranes a bright prospect for long-term use in the treatment of industrial oil-water effluent.

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7.3 Conclusions As a simple and effective method for the preparation of nanofiber materials, electrospinning technology has aroused intense interest in the field of oil–water separation attributed to its advantages of single-fiber structure, controllable pore structure and adjustable wettability. On the basis of these advantages, researchers have developed a variety of electrospun nanofiber materials tailor-designed for oil–water separation applications, including nanofiber membranes or aerogels with high oil absorption property and separation performance, which are significantly important for the practical applications in oily effluent treatment and oil purification. This chapter mainly discussed the application of electrospun nanofiber membranes in oil–water separation. Although electrospinning fiber membranes have made remarkable progress in the application of oil–water separation, there still existed many challenges. Firstly, in order to achieve the super-wettability of nanofiber materials, it is necessary to construct multiscale roughness on the surface of nanofibers. However, under the action of external factors, the as-structure is easily damaged, which will fundamentally abridge the service life of oil–water separation materials. Secondly, in contrast with commercial oil–water separation materials, currently prepared electrospun nanofiber membranes still have poor mechanical properties in the long-term operation process, which greatly limits their practical application. Finally, the current research mainly focused on the design of various separation materials with diverse wettability. While the design of nanofiber materials for oil–water separation is a multiphase flow process, which involves microfluid mechanics, interfacial chemistry, engineering science, etc. The revelation and construction of the universal principle for such multiphase flow process needs more theoretical and basic research. The further application of ENMs in oil–water separation is still under exploration, and it is expected that electrospinning technology will have bright prospects in the design and generation of the next-generation oil-water separation materials in the coming decades. Declaration Some portions of this chapter have been published in Ref. [18].

References 1. Shannon, M.A., Bohn, P.W., Elimelech, M., Georgiadis, J.G., Marinas, B.J., Mayes, A.M.: Science and technology for water purification in the coming decades. Nature 452, 301–310 (2008) 2. Kota, A.K., Kwon, G., Choi, W., Mabry, J.M., Tuteja, A.: Hygro-responsive membranes for effective oil-water separation. Nat. Commun. 3, 8 (2012) 3. Pan, Y.Q., Wang, T.T., Sun, H.M., Wang, W.: Preparation and application of titanium dioxide dynamic membranes in microfiltration of oil-in-water emulsions. Sep. Purif. Technol. 89, 78–83 (2012)

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4. Yi, X.S., Yu, S.L., Shi, W.X., Wang, S., Sun, N., Jin, L.M., Ma, C.: Estimation of fouling stages in separation of oil/water emulsion using nano-particles Al2 O3 /TiO2 modified PVDF UF membranes. Desalination 319, 38–46 (2013) 5. Lipp, P., Lee, C.H., Fane, A.G., Fell, C.J.D.: A fundamental study of the ultrafiltration of oil-water emulsions. J. Membr. Sci. 36, 161–177 (1988) 6. Chakrabarty, B., Ghoshal, A.K., Purkait, M.K.: Ultrafiltration of stable oil-in-water emulsion by polysulfone membrane. J. Membr. Sci. 325, 427–437 (2008) 7. Zhang, S., Lu, F., Tao, L., Liu, N., Gao, C., Feng, L., Wei, Y.: Bio-inspired anti-oil-fouling chitosan-coated mesh for oil/water separation suitable for broad pH range and hyper-saline environments. ACS Appl. Mater. Interfaces 5, 11971–11976 (2013) 8. Faibish, R.S., Cohen, Y.: Fouling-resistant ceramic-supported polymer membranes for ultrafiltration of oil-in-water microemulsions. J. Membr. Sci. 185, 129–143 (2001) 9. Peng, X., Jin, J., Nakamura, Y., Ohno, T., Ichinose, I.: Ultrafast permeation of water through protein-based membranes. Nat. Nanotechnol. 4, 353–357 (2009) 10. Wu, J., Wang, N., Zhao, Y., Jiang, L.: Electrospinning of multilevel structured functional micro-/ nanofibers and their applications. Journal of Materials Chemistry A 1, 7290–7305 (2013) 11. Zhang, J., Xue, Q., Pan, X., Jin, Y., Lu, W., Ding, D., Guo, Q.: Graphene oxide/polyacrylonitrile fiber hierarchical-structured membrane for ultra-fast microfiltration of oil-water emulsion. Chem. Eng. J. 307, 643–649 (2017) 12. Wang, R., Liu, Y., Li, B., Hsiao, B.S., Chu, B.: Electrospun nanofibrous membranes for high flux microfiltration. J. Membr. Sci. 392, 167–174 (2012) 13. Raza, A., Ding, B., Zainab, G., El-Newehy, M., Al-Deyab, S.S., Yu, J.: In situ cross-linked superwetting nanofibrous membranes for ultrafast oil-water separation. J. Mater. Chem. A 2, 10137–10145 (2014) 14. Ge, J., Zhang, J., Wang, F., Li, Z., Yu, J., Ding, B.: Superhydrophilic and underwater superoleophobic nanofibrous membrane with hierarchical-structured skin for effective oil-in-water emulsion separation. J. Mater. Chem. A 5, 497–502 (2017) 15. Cassie, A., Baxter, S.: Wettability of porous surfaces. Trans. Faraday Soc. 40, 546–551 (1944) 16. Tian, Y., Jiang, L.: Wetting: Intrinsically robust hydrophobicity. Nat. Mater. 12, 291–292 (2013) 17. Zhang, J., Liu, H., Jiang, L.: Membrane-based strategy for efficient ionic liquids/water separation assisted by superwettability. Adv. Funct. Mater. 27, 1606544 (2017) 18. Wang, J., Hou, L., Yan, K.K., Zhang, L., Yu, Q.J.: Polydopamine nanocluster decorated electrospun nanofibrous membrane for separation of oil/water emulsions. J. Memb. Sci. 547, 156–162 (2018) 19. Yuan, T., Meng, J., Hao, T., Wang, Z., Zhang, Y.: A scalable method toward superhydrophilic and underwater superoleophobic PVDF membranes for effective oil/water emulsion separation. ACS Appl. Mater. Interfaces 7, 14896–14904 (2015) 20. Jiang, J., Zhu, L., Zhu, L., Zhu, B., Xu, Y.: Surface characteristics of a self-polymerized dopamine coating deposited on hydrophobic polymer films. Langmuir 27, 14180–14187 (2011) 21. Yang, H.-C., Liao, K.-J., Huang, H., Wu, Q.-Y., Wan, L.-S., Xu, Z.-K.: Mussel-inspired modification of a polymer membrane for ultra-high water permeability and oil-in-water emulsion separation. J. Mater. Chem. A 2, 10225–10230 (2014) 22. LaVoie, M.J., Ostaszewski, B.L., Weihofen, A., Schlossmacher, M.G., Selkoe, D.J.: Dopamine covalently modifies and functionally inactivates parkin. Nat. Med. 11, 1214–1221 (2005) 23. Liao, Y., Wang, R., Tian, M., Qiu, C.Q., Fane, A.G.: Fabrication of polyvinylidene fluoride (PVDF) nanofiber membranes by electro-spinning for direct contact membrane distillation. J. Membr. Sci. 425, 30–39 (2013) 24. Zong, X.H., Kim, K., Fang, D.F., Ran, S.F., Hsiao, B.S., Chu, B.: Structure and process relationship of electrospun bioabsorbable nanofiber membranes. Polymer 43, 4403–4412 (2002) 25. Zhang, W.B., Shi, Z., Zhang, F., Liu, X., Jin, J., Jiang, L.: Superhydrophobic and superoleophilic PVDF membranes for effective separation of water-in-oil emulsions with high flux. Adv. Mater. 25, 2071–2076 (2013)

Chapter 8

Electrospun Nanofibers for Water Distillation and Pervaporation Lin Zhang , Saisai Lin , and Zhikan Yao

8.1 Introduction Electrospinning is a facile and versatile technology that can fabricate continuous ultrafine polymeric fibers with a vast variety of structure and morphology ranging from nano- to micro-scale [1]. Generally, in the typical electrospinning process, a polymer solution held by its surface tension at the end of a capillary tube is subjected to an electric field that generates a charge on the liquid surface. When the applied electric field reaches a critical value, the repulsive electrical forces overcome the surface tension. Eventually, a charged jet of the solution is ejected from the tip of the Taylor cone, and an unstable and fast jet occurs in the space between the capillary tip and the collector which leads to evaporation of the solvent, leaving a polymer deposit randomly on the collector in the form of a nonwoven textile [2]. Electrospinning has begun to be extensively applied in various fields as a convenient, practical, versatile, and cost-effective technique. Due to the distinctive properties of nanofibers, the deposited electrospun nanofiber membranes (ENMs) possess many unique characteristics such as interconnected porous structure, high porosity, large surface area, and light weigh. Moreover, electrospun nanofiber membranes can be easily functionalized to satisfy some certain desirable applications, such as mixing functional nanoparticles or additives into the electrospinning solution [3], surface modification [4], or interfacial polymerization (IP) [5]. Such nanocomposite or composite ENMs have proved the enormous potential in a wide spectrum of emerging areas including desalination, biomedicines, and energy storage, etc. The shortage of water resources has been one of the global challenges that will restrict the sustainable development of human beings. Recent estimates suggest that L. Zhang (B) · S. Lin · Z. Yao Engineering Research Center of Membrane and Water Treatment of MOE, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Das (ed.), Electrospun Nanofibrous Technology for Clean Water Production, Nanostructure Science and Technology, https://doi.org/10.1007/978-981-99-5483-4_8

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about 4 billion people experience water scarcity for at least one month each year, and about 500 million are exposed to severe water scarcity throughout the year. Many emerging water problems are exacerbated by the pollution of existing ground or surface water resources, what has aroused the attention of many countries. For example, China has invested thousands of billions to enhance wastewater treatment in the recent decade. To cope with the increasing water shortage and pollution, new, better and more efficient separation techniques need to be developed to remove harmful pollutants. Membrane-based separation techniques have been a mainstay to solve the contemporary issues in energy, resource, and environment in view of their cost-effective and energy-efficient. Two thermal-driven membrane process, including membrane distillation (MD) and pervaporation (PV), are considered as effective ways to remove harmful species from waste or contaminated water, and have attracted arousing interest because of their two unique characteristics. Firstly, their driving force (i.e., temperature) is not strongly influenced by the feed concentration or composition. Thus, they are applicable in desalinating high-salinity brines and dehydration of high-purity organic solvents. Secondly, both MD and PV occur at mild operation conditions, without high temperatures or pressures. Thus, they can use low-grade thermal energy. Despite these positive characteristics, their widely industrial applications in water treatment are still greatly hindered by the membrane materials currently used with low productivity, poor long-term stability, and low energy efficiency. Compared with the conventional membrane, electrospun nanofiber membranes are good alternatives to break through the intrinsic limitations in expense and efficiency, benefiting from the interconnected pore structure, high porosity, large surface area, flexible functionality, and safety in use. The interconnected pore structure, large surface area, and high porosity of ENMs endow them possess excellent permeable properties, while the controllable fiber diameter and pore size also could ensure the filter precision. Meanwhile, by virtue of the chemical modification or the incorporation of functional components, electrospun nanofiber membranes could obtain some unique pore structure and physico-chemical properties, and further express much more effective in a wide range of water treatment application. In this chapter, we will focus on advances in nanofiber-based composite membranes for several representative water purification systems, including MD and PV. Undoubtedly, electrospinning has been a remarkably facile and powerful means for offering effective membrane materials in water treatment.

8.2 Electrospun Nanofibers for Membrane Distillation Water scarcity is one of the global problems and fresh water supply has become a top priority, thus desalination is considered as one of the most important ways to alleviate the water shortage issue. MD is a promising desalination process among

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various membrane separation technologies [6–8], which separates volatile solvents from solutions thermally driven by the vapor pressure gradient across a hydrophobic microporous membrane. Since the MD process is usually operated under the boiling point of the solvent, it can be carried out at relatively low temperatures [9, 10]. Therefore, MD process can be operated under low-grade heat source conditions and is expected to be an energy-saving desalination technology. In MD system, the membranes act as a liquid/vapor interface through which water vapor diffuses. The hydrophobicity of the membrane preserves the water solution supply, while the transmembrane microporous structure allows vapor penetration [11]. To improve the transfer efficiency of the mass and energy, the optimal properties of MD membranes should include medium pore size, high porosity, and hydrophobic surface. At the same time, the hydrophobic surface plays a more critical role. If the hydrophobicity is not strong enough, the membrane will be wetted by the feed solution and cannot run stably. Revolved around the above properties of membranes, there are still two critical deficiencies in MD need to be further improved through: (i) the relatively low permeation flux (compared to other desalination technologies such as reverse osmosis); and (ii) general permeation flux attenuation due to membrane pores wetting. To overcome these disadvantages and exploit the advantages of MD, more efforts have been put into improving the permeation flux and water resistance of MD membranes. Compared with the conventionally hydrophobic MD membranes (flatsheet membrane and hollow-fiber membrane) prepared by phase inversion, which have poor porosity and disconnected inner pore structure, the ENMs have excellent properties such as high porosity, interconnected open pores, and hydrophobic surface, which can significantly improve the disadvantages of low permeation flux and membrane pore wetting during long-term MD operation. So far, the mostly utilized polymers in ENMs for MD application are polyvinylidene fluoride (PVDF) and PVDF-co-hexafluoropropylene (PVDF-HFP), as they can be easily dissolved in common solvents compared with commercial MD membrane materials such as polypropylene and polytetrafluoroethylene (PTFE). However, compared with the above commercially PTFE-based MD membrane, the availably electrospinnable polymeric materials with instinctive hydrophobicity used for MD are extremely limited [12–15], and is still not strong enough to avoid the wetting phenomenon especially for long-term operation. Thus, significant efforts should be paid to developing novel electrospun membranes with more hydrophobic surface. Herein, the advanced ENMs materials with super- and intrinsic-hydrophobicity will be demonstrated, and the characteristic of structure, morphology, hydrophobicity, and their potential in MD application will be comprehensively reviewed.

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8.2.1 Super-Hydrophobic Nanocomposite Electrospun Nanofiber Membranes The hydrophobicity of ENMs depends on the polymers used. Due to the easy doping and a wide range of choice, several inorganic nanoparticles, such as SiO2 , carbon nanotubes (CNTs), and graphene, have been widely incorporated to obtain nanocomposite ENMs [16–18]. When nanoparticles are added to ENMs, the roughness of the nanofiber surface is increased, and micro-nano structure is formed on the surface of ENMs, and the water contact angles of the membrane is also increased. However, in the preparation process of nanocomposite ENMs, due to the relatively long fabrication time, it is difficult for nanoparticle to be completely distributed in the aqueous environment, or it is easier to reaggregate electrospun polymer solution, so the problem of nanoparticles agglomeration is still faced. As a result, the prepared nanocomposite ENMs are easy to form microbeads, causing the inner pores blockage of the membranes. In addition, nanoparticles embedded in nanofibers cannot fully exhibit their hydrophobicity in nature. Carbon nanotubes (CNTs) are considered as the ideal hydrophobic material and are usually incorporated in ENMs to fabricate nanocomposite membranes. The CNTs-hybrid ENMs showed an enhanced water contact angle than that of commercial PVDF membrane [19, 20]. Aimed to solve the problem of microbeads formation and nanoparticles embedding, the author’s group took the commonly used CNTs as an example and developed a superhydrophobic nanocomposite ENMs via the surface spraying method [21]. Such facile spraying method provides an effective way to achieve the homogeneous distribution of nanoparticles on the ENMs surface. The principle of nanoparticle distribution control, the characteristics of structure, morphology, and performance of the superhydrophobic nanocomposite ENMs will be comprehensively described as below.

8.2.1.1

Principle of Nanoparticle Homogeneous Distribution

In the surface spraying technique, nanoparticles are dispersed in a proper solvent and then sprayed on the ENMs surface by a spray gun. To control the uniform distribution of nanoparticles on ENMs surface without obvious aggregation, the key parameters are the proper solvent with low surface tension and the low content of nanoparticle dispersion. For example, herein ethanol was chosen as the dispersion solvent, whose contact angle on PVDF was about 0°. Thus, ethanol cannot form isolate droplet on PVDF ENMs surface after sprayed, and the CNTs agglomeration was prevented during the rapid ethanol evaporation process. On the other hand, the low content of CNTs dispersion also contributed to prevent aggregation. CNTs dispersion with the concentration of 1 mg/mL can be stably suspended for more than 20 h, far exceeding the whole spraying duration. The sprayed density of CNTs on ENMs surface depends on the spray time. Table 8.1 listed the dope solution and CNTs spray density of the fabricated nanocomposite ENMs.

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Table 8.1 Dope solutions and CNTs spray density of fabricated membranes [21]. This table is adapted with permission from ref. [21] Membrane

PVDF (wt.%)

Acetone (wt.%)

DMF (wt.%)

LiCl (wt.%)

CNTs spraying time (min)

CNTs sprayed density (g/ m2 )

C0

5

76

19

0.004

0

0

C4

5

76

19

0.004

2

4

C8

5

76

19

0.004

4

8

C12

5

76

19

0.004

6

12

C20

5

76

19

0.004

10

20

C30

5

76

19

0.004

15

30

8.2.1.2

Influence on Surface Morphology and Hydrophobicity

The spray density of nanoparticle directly influences the surface structure and morphology of ENMs. As shown in Fig. 8.1a, the surface of the pristine ENMs appeared as interconnected open pores formed by nanofibers, and no microbeads were found on the nanofibers. The nanofiber diameter distribution of the pristine membrane was narrow, with an average diameter of 294 nm. While as shown in Fig. 8.1b–f, the CNTs sprayed ENMs had a totally different morphology due to the coverage of CNTs. CNTs were uniformly distributed on membrane surface without large aggregation, which was attributed to the volatile ethanol as the dispersion solvent discussed above and the low content of CNTs in the dispersion solvent. Under different spraying density in Figs. 8.1b and d, the CNTs on membrane surface all had a homogeneous distribution. Even when sprayed at relatively high densities, CNTs not only crossed among nanofibers, but also interlaced with each other, thus forming a thin layer of CNTs network on the surface of PVDF ENM. Because of the one-dimensional materials sprayed on the membrane surface, the possibility of blocking the pores inside the ENMS was very little. As can be seen in Table 8.2, the average pore size of the membranes slightly decreased from 0.26 to 0.20 μm, and the maximum pore size decreased from 0.32 to 0.23 μm. This decrease in pore size is because the surface pores of ENMs were encapsulated by CNTs. The decrease of pore size was proportional to the density of CNTs coated layer. However, the average pore size did not change much. The diameter of CNTs (20–40 nm) was much thinner than that of PVDF nanofibers (~290 nm), and as shown in the cross-sectional view of C20 membrane in Fig. 8.2a, CNTs were only sprayed on membrane surface, and the thickness of CNTs layer was quite limited. In this case, the CNTs network did not significantly affect the pore size of PVDF ENMs. The pore diameter distribution did not change much as shown in Fig. 8.2b, which was also attributed to the thin layer of CNTs. Water contact angle (WCA) was used to describe the hydrophobicity of the pristine and CNTs-coated membranes, and the results were present in Fig. 8.3a. The

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Fig. 8.1 SEM images of ENMs with different CNTs spraying density: a C0, b C4, c C8, d C12, e C20, and f C30 membrane [21] (The figures [a–f] are adapted with permission from ref. [21]) Table 8.2 Properties of fabricated nanocomposite ENMs. [21]. This table is adapted with permission from ref. [21] Membrane code

Thickness (μm)

Mean pore size (μm)

Maximum pore size (μm)

C0

101 ± 3.0

0.26

0.32

C4

99 ± 2.5

0.27

0.34

C8

101 ± 3.0

0.23

0.29

C12

98 ± 3.5

0.22

0.27

C20

101 ± 3.0

0.19

0.24

C30

100 ± 2.5

0.20

0.23

Fig. 8.2 a SEM images of cross-sectional view of C20 membrane, b Pore volume at different pore diameter of C0 and C20 membrane [21] (The figures [a, b] are adapted with permission from ref. [21])

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Fig. 8.3 a Images of water droplets on the membranes surface and the corresponding WCAs and b LEP of C0-C20 membranes [21] (The figures [a, b] are adapted with permission from ref. [21])

image of water droplet on C0 membrane exhibited a WCA of 130.6°, indicating that it had instinctive hydrophobic property. The WCAs of CNTs coated ENMs increased from 135.4° to 159.3° with the density of coated CNTs increased from 4 to 30 g/m2 , implying that the C0 membrane was modified to be superhydrophobic. Compared with C20 membrane, the C30 membrane showed less increase in WCA. This can be contributed to the saturated CNTs on membrane surface, the increased of coating density did not change the surface properties of the membrane, and the hydrophobicity of the membrane did not change. The increased contact angle is introduced by the Cassie-Baxter model, as shown in Eq. 8.1 [22]:   cos θ c = f 1 + cos θ c − 1

(8.1)

where, θ c is the apparent contact angle, θ is the intrinsic contact angle, and f is the solid faction, which is related to surface roughness. The superhydrophobic surface given by CNTs increases the intrinsic contact angle (θ ), compared with the fact that the micro/nano structure formed by mixed CNTs reduces the solid part ( f ). It was clear that the CNTs network layer on the membrane surface changed the property of PVDF ENM due to the hydrophobic nature of CNTs. The wetting resistance of fabricated membranes was evaluated by measuring the liquid entry pressure (LEP). According to Laplace’s equation, LEP is proportional to contact angle, liquid surface tension, and inversely proportional to the maximum pore size, as shown in Eq. 8.2: LEP =

−2Bγ cosθ rmax

(8.2)

where B is the geometric factor determined by pore structure, γ is the surface tension of liquid, θ is the contact angle between water and membrane, and r max is the maximum pore size of the membrane. Figure 8.3b showed that the LEP value significantly increased from 138 to 188 kPa when the CNTs coating density increased

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Scheme 8.1 Schematic illustration of a pristine PVDF ENM, b CNTs bridges coated ENM, and c CNTs network coated ENM surface in feed solution [21] (The scheme is adapted with permission from ref. [21])

from 4 to 30 g/m2 . According to Eq. (8.2), LEP is related to θ and r max . The contact angles (θ ) increased proportionally with the increase of the CNTs coating density due to the change in membrane surface properties, which was demonstrated in the WCA test. At the same time, as shown in Table 8.2, the maximum pore size decreased after CNTs coating. As expected, the decrease in r max resulted in an increase in LEP. To further investigate the changes in membrane surface properties, we studied the essential difference between the ENMs coated with CNTs network and the pristine PVDF ENMs in the wetted state, as described in Scheme 8.1. On the surface of C0 membrane, PVDF nanofibers formed pores (Scheme 8.1a). After coating with a certain amount of CNTs, PVDF nanofibers were crossed with one-dimensional CNTs (Scheme 8.1b). Because of the superhydrophobic nature of CNTs, CNTs bridges can prevent the permeation of the feed solution through the pores. When CNTs were coated at a higher density, a CNTs network was formed (Scheme 8.1c), which significantly reduced the pore wetting tendency.

8.2.1.3

Continuous Vacuum Membrane Distillation

The water flux and wetting time of the prepared membranes were shown in Fig. 8.4. All CNTs coated membranes exhibited higher permeation flux (Fig. 8.4a), with the highest flux in the C20 membrane, which was significantly increased by 58.5% compared with the C0 membrane. Although the thin layer of CNTs resulted in a decrease in the average pore size, the matrix structure of ENMs substrate did not change. It possessed the similar water vapor transport resistance to C0 membrane. Thus, the corresponding water flux was not affected by the smaller pore size measured, but is significantly increased by the presence of surperhydrophobic surface. In the MD process shown in Scheme 8.1a, the feed solution was directly in contact with the PVDF membrane and partially retained in the membrane pores. The trapped feed solution can reduce the effective evaporation area of the liquid.

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Fig. 8.4 a The initial flux and b wetting time of all membranes (feed concentration = 3.5 wt.% NaCl, feed temperature = 80 °C, flow rate = 130 mL/min, permeated pressure = 3 kPa) [21] (The figures [a, b] are adapted with permission from ref. [21])

However, according to Scheme 8.1b and c, the CNTs not only reduced the pore wetting tendency but also increased the effective liquid evaporation area (Sc > Sb > Sa ) in ENMs sprayed by CNTs. Increasing the effective liquid evaporation area can increase the mass transfer coefficient and thus improve the water flux. Similar results had been reported in other works [19, 23, 24]. However, the flux of C30 membrane did not increase continually. The CNTs were already saturated on membrane surface. The excess CNTs on C30 membrane surface added additional mass transfer resistance in MD. Similarly, as shown in Fig. 8.4b, the wetting time of CNTs sprayed EMNs increased proportionately with the increase of CNTs coating density. The wetting time of C20 mebrane was extended to 26 h, which was 78.6% higher than that of C0 membrane. The results showed that the wetting time increased with the increase of hydrophobicity. On the other hand, when coated with CNTs, the increase in hydrophobicity also contributed to the increase in LEP values, thereby enhancing the anti-wetting property. The continuous VMD tests of the fabricated membranes were shown in Fig. 8.5. Initially, the flux of C0 membrane was 18.8 kg/m2 h, and the salt rejection was >99.99%. The permeated conductivity began to rise after 14 h. However, after coated with CNTs, the initial flux of the C20 membrane was 28.5 kg/m2 h, while the conductivity of the permeate remained at a low level. The VMD results suggested that the CNTs coated membranes had higher flux and longer wetting time compared with the pristine PVDF ENM. These results indicated that the CNTs network, as a superhydrophobic layer, had excellent anti-wetting property during the long-term VMD operation. The stability of CNTs sprayed membrane was conducted via bending tests. According to test results, the membrane was not cracked after bending. The stability of CNTs network was also investigated by the observation of the surface morphology of membranes after VMD tests. The surface morphology of C20 membrane had not

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Fig. 8.5 Flux and permeated conductivity in continuous VMD test (feed concentration: 3.5 wt.% NaCl, feed temperature: 80 °C, flow rate: 130 mL/ min, permeated pressure: 3 kPa) [21] (The figure is adapted with permission from ref. [21])

Fig. 8.6 Photographs of a the pristine C20 membrane, b and c the bended C20 membrane, and d the C20 membrane after bended [21] (The figures [a–d] are adapted with permission from ref. [21])

significantly alter after VMD, indicating that the CNTs had not been washed away (Fig. 8.6). To understand the potential application of CNTs network-coated nanofiber membranes, the VMD performance comparison among other nanofiber membranes in the literatures was presented in Table 8.3. For a better comparison, we also assembled the VMD performance of PVDF membrane prepared by vapor-induced phase separation and the DCMD performance of CNTs nanofillers ENMs. From the results in Table 8.3, the MD performance of the membrane sprayed with CNTs was considerably enhanced. The significant enhancement was attributed to the thin CNTs network on membrane surface.

179

379

99

120

185

158

139

158.5

130.6

159.0

PVA [30]

PVDF [31]

CNT-PcHd [19]

C0 (this study)

C20 (this study)

137

152.2

PVDF-PTFE [29]

195

165

160.5

150

PVDF-SiO2 [27]

PVAc -PTFE [28]

119

–

135

145

PVDFb [26]

Thickness (μm)

0.28

0.19

0.26

0.29

2.1

0.46

0.35–0.49

–

0.26–0.38

0.49

48

70.2

70.1

84

–

82

69–72

81.5

77–83

78

111

101

101

81

61

100

81–98

156

98–102

82

60

80

80

60

27

60

70

80

60

73

0.097

0.097

–

0.094

0.091

0.097

0.030

0.091

0.685

–

Vacuum (MPa)

b PVDF

contact membrane distillation (DCMD) membrane was prepared by vapor-induced phase separation; others were prepared by electrospinning c PVA: polyvinyl alcohol d PcH: polyvinylidene fluoride-co-hexafluoropropylene; direct contact membrane distillation

a Direct

Porosity (%)

Tf (°C)

Pore size (μm)

WCA (°)

LEP (kPa)

Operation parameter

Membrane properties

Commercial membranea [25]

Membrane

28.4

18.8

29.5

6.5

25.2

18.5

15.8

31.5

22.4

10.6

Flux (kg/m2 h)

>99.9

>99.9

99.99

>99.98

>99.9

>99.9

99.11

>99.9

99.9

>99.9

Rejection (%)

Separating property

Table 8.3 Properties and VMD performances of different nanofiber membranes for desalination (35 g/L NaCl solution as feed)

26

14

5

6

>16

>15

>10

15

>6

>8

Stable time (h)

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8.2.2 Instinctive Hydrophobic Composite Electrospun Nanofiber Membranes Inspired by lotus leaves, constructing hierarchical structures on ENMs is an alternative route to enhance the wetting resistance [24, 32, 33]. The hierarchical structure can provide more air pockets in the nano-hairy region, resulting in a large number of dispersed gas–liquid-solid interface. Based on the Cassie-Baxter model [22], the gas–liquid-solid interface had a higher apparent contact angle on the hierarchical surface. However, the water-repellency does not completely depend on the apparent contact angle. For example, in some MD configurations (such as vacuum membrane distillation, VMD), the air pocket in the nano-hairy region is discharged by the vacuum pump [34, 35], thus the gas–liquid-solid interface would disappear, and water will wet the gas–solid interface. On this basis, we can estimate that if the membrane material is not intrinsically hydrophobic, the hierarchically superhydrophobic surface would not help to improve the water-repellency or wetting resistance of the membranes throughout the VMD process. Cheng et al. [36] verified that when water condensed from vapor phase to the hierarchical surface, the superhydrophobic surface would disappear. In the MD process, the feed aqueous solution is in contact with the membrane for a long time, and the water vapor will condense in hierarchical membranes during the water phase transition process, and the surface of the membrane pores will become wet. Therefore, the increase in instinctive hydrophobicity of the membrane materials is the key to the MD process, rather than the formation of hierarchical structure. As mentioned above, the intrinsic hydrophobicity of ENMs mainly depends on the membrane material. PVDF is the most used material in the manufacture of MD ENM. However, although the water contact angle of PVDF ENM can reach 130°, compared to some more promising compounds such as PTFE, the instinctive hydrophobicity of PVDF is not strong enough due to its high surface energy (30.3 mN/m) [37], resulting in the limited wetting resistance of MD. Liao et al. [25] also found that the original PVDF ENMs did not show a high-performance stability in MD. Moreover, the contact angle of ENMs partially depends on the hierarchical morphology of nanofibers. The author’s group carried out a series of research on the fabrication of the instinctive hydrophobic composite ENMs. Herein, taken the mostly used PVDF as the matrix of ENMs, polydimethylsiloxane (PDMS) with very low surface energy (~21.0 mN/m) was selected to cover on PVDF nanofibers surface through a facile dip-coating [38], and then crosslinked and solidified by a traditional reported method [39], as shown in Scheme 8.2. In this way, the PDMS coating on PVDF nanofibers obscures the wrinkled hierarchical structure on nanofibers while simultaneously reducing surface energy of nanofibers. The surface structure and instinctive hydrophobicity of the PDMS-PVDF composite ENMs is comprehensively reviewed.

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Scheme 8.2 Schematic representation of composite ENM fabrication procedure [38] (The scheme is adapted with permission from ref. [38])

8.2.2.1

Characteristics of Surface Morphology and Hydrophobicity

The changes of membrane morphology before (Fig. 8.7a) and after (Fig. 8.7c) PDMS coating were observed by SEM. Compared with the original ENMs, the composite ENMs maintained original nanofiber morphology. The details in the differences of membrane morphologies were observed by high magnification SEM. The original PVDF nanofibers showed a wrinkled hierarchical structure (Fig. 8.7b), due to the rapid solvent evaporation during electrospinning. However, after coated by PDMS, the pleats on the nanofiber almost disappeared (Fig. 8.7d). Since the pleats of PVDF nanofibers were indeed wetted and filled with PDMS/hexane solution during the dip-coating process, all PVDF nanofibers were completely coated after crosslinking and solidify, and the nanofibers became smoother. As shown in Fig. 8.8, the two membranes exhibited similar internal morphologies. The adhesion between nanofiber and nanofiber, and the aggregation of PDMS in membrane pores were not observed. Such procedure successfully prevented the

Fig. 8.7 SEM images of a, b PVDF ENMs; c, d PDMS-PVDF composite ENMs [38] (The figures [a–d] are adapted with permission from ref. [38])

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curing of residual PDMS in ENMs pores, because the residual PDMS solution in the membrane pores was drained before the curing reaction during the coating process. The results showed that the ENMs kept high porosity after PDMS coating, thus maintaining high mass transfer coefficient of composite ENMs in MD. Nanofiber is the core components of ENMs, whose diameter has a great influence on the pore size of the membrane. The diameters of nanofiber coated with PDMS were statistically analyzed and compared with that of the original ENMs (Fig. 8.9). The results showed that the two ENMs had similar diameter distribution curves, and the nanofiber diameter of composite ENMs only increased slightly. With an additional layer coated, the ENM nanofibers of the composite were slightly thicker (~0.01 μm) than the original PVDF nanofibers. The results showed that the effect of PDMS coating on the nanofiber size was negligible. By measuring the pore size distribution, the pore diameters of the two ENMS was quantitatively characterized. As shown in Fig. 8.10, pore diameter distribution

Fig. 8.8 SEM images of a PVDF ENMs and b PDMS-PVDF composite ENM [38] (The figures [a, b] are adapted with permission from ref. [38]) Fig. 8.9 Fiber diameter distribution of pristine and composite membranes [38] (This figure is adapted with permission from ref. [38])

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of two membranes was similar. When the diameter was larger than 120 nm, the volume difference between these two ENMS was not distinct. However, when pore size is less than 120 nm, the volume of composite ENMs is significantly smaller than that of the original ENM. After coating with PDMS, some pores even disappeared at the pore diameter of 32–40 nm. Compared with larger pores, smaller pores are more susceptible to changes in fiber diameter, resulting in significant changes in pore volume. However, smaller pores are not the effective mass transfer zones in MD [40, 41]. Moreover, these pores made up only a small fraction of the total membrane pores. Therefore, MD flux would not be affected by pore volume changes in smaller pores. The data of membrane porosity and pore size before and after PDMS coating were shown in Table 8.4. As the nanofibers in ENMs were coated with a thin layer of PDMS, the space between nanofibers was slightly narrowed, resulting in a slight decrease in membrane porosity. However, according to the cross-section images in Fig. 8.8, the PDMS coating did not change the internal structure of membranes Therefore, the porosity did not decrease significantly. Interestingly, as shown in Table 8.4, the mean and median pore diameters increased after coating PDMS. WCA and LEP were used to characterize the hydrophobicity of the membranes. According to Cassie-Baxter theory [22], the hierarchical surfaces with micro-nano structures are conducive to the formation of surfaces with higher WCA. Composite ENMs were smooth and had no obvious hierarchical structure as shown in Fig. 8.7 which was not conducive to improving WCA [42, 43]. However, the composite ENMs had greater WCA than the original ENM as shown in Table 8.5, and we can conclude that the enhancement of WCA was mainly due to the strongly instinctive hydrophobicity of PDMS. LEP is proportional to the surface tension and inversely proportional to the maximum pore diameter [44]. In this work, LEP was mainly related to the surface tension, thus the LEP of composite ENM was higher than that of original ENM for the same reason. Fig. 8.10 Pore diameter distribution of original and composite membranes [38] (This figure is adapted with permission from ref. [38])

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Table 8.4 Porosity and pore diameter of membrane before and after PDMS coating [38]. This table is adapted with permission from ref. [38] Membrane

Porosity (%)

Mean pore diameter (nm)

Median pore diameter (nm)

Original

68.1

508

210

Composite

65.6

612

238

Table 8.5 WCAs and LEP of membrane before and after PDMS coating [38]. This table is adapted with permission from ref. [38]

Membrane

WCA (°)

Original

130.8 ± 0.8

98 ± 3

Composite

148.7 ± 1.3

138 ± 3

LEP (kPa)

The instinctive hydrophobicity of the fabricated membranes was tested by the water vapor condensation experiment. Cheng et al. suggested that if the superhydrophobic surface of the material was not sufficiently hydrophobic, the superhydrophobic surface would disappear when water condensed from vapor phase to the surface. As shown in Fig. 8.11a, after the condensing process, parts of the original ENMs were covered with condensed water. In addition, the highlights showed that the water condensed on the inside of the membrane, and that these dots have become completely wet. In contrast, for the composite ENM (Fig. 8.11b), the condensed water attached to the membrane surface in the form of isolated droplets did not spread out, or even wet the membrane. When a large droplet rolled off the membrane, the captured droplet images exhibited a high contact angle (~130°). The results revealed the composite ENM had intrinsic hydrophobicity. The condensation state of water vapor on different membrane surfaces was illustrated in Scheme 8.3. For the hierarchical micro-nano structure surface (Scheme 8.3a), once the air pockets were replaced by water vapor, the water vapor condensed, and the water droplets were trapped in the nanosized regions of the

Fig. 8.11 Images of original (a) and composite ENM (b) after water condensed test [38] (The figures [a, b] are adapted with permission from ref. [38])

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Scheme 8.3 Illustrations of water vapor condensed on the membrane surface a with the hierarchical structure and, b without hierarchical structure [38] (The figures [a, b] are adapted with permission from ref. [38])

micro-nano surface [36]. Among them, although the water droplets in contact with the micro-nano surface would disperse, when the water vapor condensed in the membrane, the surface would quickly wet, damaging the stability of the hydrophobic micro-nano surface. Obviously shown in Fig. 8.7b, PVDF nanofibers had some nanohairs and were partially wetted. However, PDMS supplies instinctive hydrophobicity on the nanofiber surface, despite the vanishment of nano regions on nanofibers (Scheme 8.3b), the membrane surface would not intercept the condensed water, and the membrane would not be wet under the condition of condensed water vapor.

8.2.2.2

Continuous Vacuum Membrane Distillation

ENM is woven by nanofibers and its wetting resistance in MD also relies on the thickness of the membranes. Generally, thicker membranes have stronger wetting resistance [45]. However, the membrane is also a mass transfer barrier during the MD process, so reducing the thickness of the membrane can obtain a higher MD flux. Avoid being wet by water, only thinner membranes with excellent hydrophobicity can be applied to MD. On the basis of this principle, the VMD fluxes of original and composite membranes with different membrane thickness were studied. As shown in Fig. 8.12, the thinner membane brought the higher flux for both original and composite ENM, due to the smaller mass transfer resistance in the membrane. At the same thickness, the flux of the original ENMs was slightly higher than that of the composite ENMs, due to its relatively higher porosity. When the membrane thickness was reduced to 70 μm, the original ENM was almost infiltrated and there was no real flux, but the composite ENM still had a higher flux up to 35 kg/ m2 ·h.

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Fig. 8.12 The original and composite membrane VMD permeation flux at different membrane thickness (feed concentration = 3.5 wt.% NaCl, feed temperature = 80 °C, flow rate = 130 mL/ min, permeated pressure = 3 kPa) [38] (This figure adapted with permission from ref. [38])

To better understand the differences in VMD operation, Fig. 8.13 showed the flux and conductivity of the original and composite membranes with a thickness of 80 μm as a function of time. For the original PVDF membrane, the flux increased after 1 h, accompanied by a sharp increase in conductivity. The reason for the erratic VMD performance can be described as its lower thickness. The LEP of the original PVDF membrane (Table 8.6) was slightly higher than the transmembrane pressure difference during VMD process. This suggested that it was possible for the small amount of water vapor condensed in some regions of the membrane at the same time causing the membrane to be permeated by the feed solution. Under the same conditions, the PDMS-PVDF composite membrane could run stably for 22 h. The difference of VMD further supported that coating PDMS with strong hydrophobicity is an effective way to enhance the water-repellency. Fig. 8.13 Comparison of P80 and S80 membranes in continuous VMD test (feed concentration: 3.5 wt.% NaCl, feed temperature: 80 °C, flow rate: 130 mL/ min, permeated pressure: 3 kPa) [38] (This figure adapted with permission from ref. [38])

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Table 8.6 LEP values (kPa) of pristine and composite membrane at different membrane thickness [38]. This table is adapted with permission from ref. [38] Membrane thickness (μm)

100

90

80

70

Pristine

125 ± 4

110 ± 2

105 ± 5

98 ± 3

Composite

170 ± 5

155 ± 3

149 ± 4

138 ± 3

8.2.3 Micro-Mechanism Underlying Membrane Wetting Behavior Revolving around the membrane wetting issue widely existed in MD, the superhydrophobic nanocomposite ENMs and the instinctive hydrophobic ENMs is typically demonstrated in the former sections, respectively. Although a variety of anti-wetting strategies have been developed, including the structure design of the nanofibers as we demonstrated above, the membrane wetting mechanism, especially the micromechanism in pore-level, is still far from clear. To understand the wetting behavior of membranes, many methods have been utilized to detect the wetting process, such as in-situ optical measurement [46], AC impedance method [47], and Detection of Dissolved Tracer Intrusion (DDTI) [48, 49]. However, due to the limitations of the size and structure of membrane pores, it is difficult to find direct evidence inside membrane pores at the microscopic scale and establishing a link between membrane wetting and microscopic changes in membrane pores remains a challenge. To further understand the pore-level behavior of membrane wetting in desalination process via VMD, the author’s group utilized the above-developed PVDFPDMS composite ENMs in Sect. 8.2.2 to thoroughly analyze the wetting process of sodium chloride (NaCl) solution [50]. The open pores scattering on the PVDF-PDMS composite ENMs surface were greatly helpful to overcome the visualizing limit of conventional porous membranes. On this basis, the micro-mechanism underlying the wetting behavior of vacuum membrane distillation in desalination was systematically investigated, and a regeneration process was also proposed as following.

8.2.3.1

Membrane Wetting Behavior in Continuous VMD

The VMD desalination performances of the PVDF-PDMS ENMs were shown in Fig. 8.14. The conductivity of the permeate remained constant, and the flux gradually decreased during the first 10 h. After 11 h operation, the conductivity of the permeate increased sharply, while the flux increased slightly, indicating that the membrane wetting phenomenon was serious. After regeneration, the flux and conductivity of the permeate returned to the initial values, indicating that the wettability of the membrane disappeared. According to the mass transfer model of membrane distillation, the membrane flux is related to the vapor pressure difference between the two sides of the membrane during the MD process, and the relation is given by Eq. 8.3 [51, 52]:

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Fig. 8.14 Continuous vacuum MD test of electrospun membrane (feed side: 3.5 wt.% NaCl, 80 °C, flow rate: 130 mL/min; permeate side pressure: 3 kPa) [50] (This figure is adapted with permission from ref. [50])

  J = Dm P = Dm P f − Pp

(8.3)

In Eq. 8.3, Dm is the mass transfer coefficient through the membrane, Pf is the vapor pressure at the feed side of water, Pp is the vapor pressure at the permeate side, ΔP = Pf − Pp . During the process of membrane distillation, the temperature and vapor pressure differences across the membrane remain constant. Therefore, membrane flux is mainly affected by Dm , which depends on membrane structure parameters and can be expressed as Eq. 8.4 [51, 53]: Dm ∝

εda ιδ

(8.4)

In Eq. 8.4, ε is the porosity of membrane, d is the average pore diameter, ι is the tortuosity of membrane, δ is the thickness of membrane, and a is a constant. The ENMs used in this experiment were prepared in the same batch, so the ι values of membrane tortuosity were fixed, and membrane thickness δ was the actual distance of water vapor transmission. According to Fig. 3.14 and the above analysis, the decrease of flux dropping should be caused by the comprehensive effect of the decreasing values of ε、d、δ, and it is speculated that the pores structures would change continuously before the membrane was completely wetted. Of course, the feed passed through the membrane as a fluid after being wetted, thus rapidly increasing the flux. Figure 8.15 showed the change of the membrane cross-section morphology by SEM images during VMD process. After 6 h of VMD test (Fig. 8.15b), a small number of crystals appeared on the membrane surface, and the flux decreased slightly (from 36.4 ± 0.6 kg/m2 h to 33.9 ± 0.3 kg/m2 h in Fig. 8.14), while the conductivity remained basically unchanged. After approximately 12 h (Fig. 8.15c), crystals were observed in the pores on the permeation side, resulting in a significant increase in flux and conductivity. In contrast, the deposited crystals dissolved after regeneration (Fig. 8.15d). Figure 8.16 showed the chemical composition changes during VMD process via EDS spectra. It was confirmed that the crystals were composed of NaCl. Therefore, it

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Fig. 8.15 Cross-section images of different stages: a before distillation, b after 6 h, feed side, c after 12 h, permeate side, d regenerated membrane (The white arrows point out NaCl. The red scalebar is 2 μm) [50] (The figures [a–d] are adapted with permission from ref. [50])

could be inferred that the wetting of membranes was due to the gradual precipitation and crystallization of NaCl from the surface pore to the interior of membranes.

8.2.3.2

Correspondence of Pore-Size Distribution and Wetting Property

To further analyze the effect of NaCl precipitated on the membrane structure and properties, the variation in the pore volume and porosity of membrane was measured by the mercury intrusion method. Compared with the original membrane, the porosity ε of the wetting membrane decreased from 65.6 to 58.8% (see Table 8.7). Correspondingly, the flux decreased from 36.4 ± 0.6 kg/m2 h to 32.8 ± 0.5 kg/m2 h (see Fig. 8.14), with a decrease of about 10%. These results were consistent with the quantitative relationship described in Eqs. (8.3) and (8.4), and both porosity and flux were returned to their initial values after regeneration. Therefore, it can be concluded that the decrease in porosity (ε) caused by the NaCl crystallization leads to the declination in flux. As for the average pore size (d), it can be seen from Fig. 8.17 that the pore volume of smaller pores with diameter less than 256 nm decreased significantly, while the

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Fig. 8.16 Energy-dispersive X-ray spectroscopy (EDS) of different states. (Peak position: C: 0.27, F:0.68, Na:1.04, Si:2.06, Cl:2.63) [50] (This figure is adapted with permission from ref. [50])

Table 8.7 The membrane porosity and pore size results [50]. This table is adapted with permission from ref. [50] Membrane status

Porosity/%

Average pore diameter/nm

Median pore diameter/nm

Virgin (0 h)

65.6

612

238

Wetted (12 h)

58.8

631

296

Regenerated

65.2

610

242

pores of larger diameter did not change much. This indicated that the smaller the pores, the easier it was to be filled with NaCl. In this case, mercury can be injected into the larger pores, but not into smaller pores that are occupied, although the pore distribution was narrowed. Therefore, statistically, the peak pore volume of the wet membrane will shift to the right as the average and median pore diameter became larger. Nevertheless, for every single pore, compared with its origin status, the pore diameter d decreased. To confirm the correspondence between pore size variation and wetting property, WCA and LEP of virgin, wetted and regenerated membranes were measured. In Fig. 8.18a, the WCA of the wetted membrane decreases from 148.7 ± 1.3° to 116.2 ± 2.2°, which indicated the precipitation of NaCl crystals on the nanofiber surface. The structure and composition changes of the solid–liquid interface resulted in the WCA reduction and the gradual decrease of membrane hydrophilicity. The LEP of the wetted membrane reduced from 138 ± 3 kPa to 90 ± 5 kPa (see Fig. 8.18b). Thus, with a steady pressure (ΔP = 98.3 kPa) applied, the feed solution could penetrate

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Fig. 8.17 Pore size distribution of virgin, wetted and regenerated membrane (Peak position: Virgin: 252.0 nm, Wetted: 284.3 nm, Recovered: 252.0 nm) [50] (This figure is adapted with permission from ref. [50])

the membrane and entered the permeate side. After regeneration, the WCA and LEP of ENMs were recovered. Some literatures [47, 54] believed that pore deformation was one of the main factors leading to the membrane wetting during MD process. The precipitated particles may lead to the pore expansion and deformation, increasing r max , and decreasing LEP. In this study, however, the mercury injection test confirmed that the maximum allowable membrane pore size r max was almost constant, with larger pores remaining the same as in the original membrane (see Fig. 8.17). At the same time, the average pore diameter of the membrane did not increase significantly. However, the LEP of the wetted membrane decreased significantly after immersion (from 138 ± 3 kPa to 90 ± 5 kPa, as shown in Table 8.8), indicating that the pore size r max was not the main factor of membrane wetting during VMD process desalting of NaCl aqueous solution.

Fig. 8.18 The a WCA and b LEP results of membranes in different states [50] (The figures [a, b] are adapted with permission from ref. [50])

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Table 8.8 The WCA and LEP results of membranes in different states [50]. This table is adapted with permission from ref. [50]

Membrane

WCA (°)

LEP (kPa)

Virgin (0 h)

148.7 ± 1.3

138 ± 3

Wetted (12 h)

116.2 ± 2.2

90 ± 5

Regenerated

149.1 ± 2.5

137 ± 4

Therefore, the change of cos θ caused by the NaCl crystallization is the main factor affecting both WCA and LEP. Moreover, according to the LEP results, this transformation occurred not only on the surface of ENMs but also inside the pores, which was consistent with the SEM results (see Fig. 8.15c).

8.2.3.3

Membrane Wetting Formation Mechanism

Based on the above analysis of the morphology, porosity, and wettability of ENMs, we divided the whole MD process into four stages to elaborate the wetting mechanism. Initially, the membrane pores are cleared and interpenetrated, the LEP >ΔP, and the feed can be maintained on the membrane surface (Stage 1, Scheme 8.4a). With continuous evaporation at the interface of the feed-membrane, the concentration polarization near the pore entrance will increase. Subsequently, NaCl will crystallize and precipitate, and the vapor–liquid interface will go deep into the membrane, and surface wetting occurred (Stage 2, Scheme 8.4b). At this point, the LEP is still greater than ΔP, and neither wetting nor significant changes in flow rate and conductivity occur throughout the membrane. Thus, the MD process is still in effect. However, the deeper evaporation interface may accelerate the precipitation of NaCl due to the greatly increased concentration polarization of the feed in the pores. At the same time, the invasive feed reduces the distance between the “hot (feed)” and “cold (permeate)” sides in the pore (Stage 3, Scheme 8.4c). Soon, the LEP drops below ΔP. Driven by the pressure difference, the evaporation interface moves forward, and the wetting speed of the membrane is faster. As a result, the feed solution can rapidly pass through the membrane as a liquid along the surface of the NaCl crystal, leading to a sharp increase in flux and conductivity at the permeate side (Stage 4, Scheme 8.4d). The above four stages are consistent with the previous research results [46, 55]. At the micro-scale, many studies have mentioned wetting caused by water bridges [55, 56]. In fact, the evidence from this study suggests that the NaCl crystals do not fill the entire membrane pores when membrane wetting occurs, but rather preferentially precipitate in the small pores. This provides evidence that water bridges must be present when wetting occurs. Of the above four stages, stage 2 and 3 are considered to be critical for membrane wetting. The crystalline-wetting closed-loop process dominates the wetting transitions between the two stages (see Scheme 8.4e). Because of the concentration polarization, once the feed is supersaturated, NaCl particles will precipitate out and fill the membrane pores, leading to a decrease in porosity and membrane flux, which changes the interface between ENM fiber and feed from hydrophobic to hydrophilic. These transformed interfaces then expand

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Scheme 8.4 Schematic diagrams of four stages of membrane wetting kinetics a Stage 1: initial operation, b Stage 2: surface wetting, c Stage 3: rapid wetting, d Stage 4: total wetting and e crystalline-wetting closed-loop process mechanism [50] (The figures [a–e] are adapted with permission from ref. [50])

the scope of local wetting, creating channels for the feed incursion. Driven by pressure, the feed enters deep into the membrane and the evaporation boundary moves toward the permeate side. The operation of the evaporation interface aggravates the concentration polarization, leading to the further crystallization of NaCl. This crystallization and wetting process complement each other, which will accelerate the wetting process of membrane and deteriorate the separation performance rapidly.

8.2.3.4

Regenerability of VMD Performance

It can be seen that Fig. 8.15 that the membrane was seriously wetting at about 10 h. At the same time, based on the above mechanism, if the crystalline-wetting closedloop process cannot be cut off before the feed directly passes through the membrane, then a large amount of membrane wetting is inevitable. Therefore, the membrane regeneration operation was performed every 10 h to dissolve the precipitated NaCl and break the crystalline-wetting closed-loop process. As shown in Fig. 8.19, after each recycles by regeneration, the membrane flux returned to the initial value and the permeate conductivity remained stable (RNaCl = 99.9%). The SEM (Fig. 8.15d) and EDS (Fig. 8.16) indicated that NaCl particles were eluted, and the surface of hydrophobic fiber was exposed after regeneration. In this way, the LEP of the membrane was maintained, and the wetting of the membrane was prevented, so that the conductivity of the permeate was kept at a low level. After the dissolution of NaCl particles in the pores, the porosity of the regenerated membrane rose to the level of the original membrane (see Table 8.7), and the flux was restored. This evidence suggested that the membrane wetting was mainly caused by the NaCl crystallization in the pores.

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35 80

30 25

60

Flush

Flush

20

Flush 40

Flux

15

Conductivity

10

20

5

Conductivity (µS/cm)

100

40

Flux (kg/m2h)

Fig. 8.19 Flux and permeation conductivity for continuous VMD test under periodic flushing [50] (This figure is adapted with permission from ref. [50])

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0

0 0

5

10

15

20

25

30

35

40

Time (h)

8.3 Electrospun Nanofibrous for Pervaporation Pervaporation (PV) is the process driven by differences in chemical activity across a membrane, and mass transfer across membrane is controlled selectively by membrane affinity for specific component. It is another thermally driven membrane-based separation technique that has gained increasing interest as a potential alternative to conventional distillation due to its high efficiency in separating azeotropic and nearboiling mixtures. Different from MD requiring porous and hydrophobic membranes, PV uses a dense membrane or a membrane with a dense molecular sieving active layer. The PV membrane needs to have suitable surface properties to achieve high selectivity and affiliation with the target permeating components. The current top challenge that restricts PV from industrial applications lies in membranes with the relatively low permeability comparing with that in other water treatment. To date, several approaches have been employed in the development of novel pervaporation membranes with the aims of increasing flux while maintaining selectivity, obtaining higher selectivity at constant flux, or increasing both. Electrospinning is a simple and versatile technique to produce ultrafine fibers with diameters ranging from nano- to micro-scale using various polymer materials. Electrospun nanofiber membranes have been good choice for water treatment applications because of their unique and interesting features including high surface areato-volume ratio, high porosity, high flexibility, good modifiability, and good water permeabilities, as discussed in the above section. In pervaporation, electrospinning technique has been integrated in the development of composite membranes that consist of a ENMs substrate with highly porous structure and a top layer with a compact structure. Such membranes typically exist as thin-film composites (TFC) ENMs. The support provides for the permeability, and the layer provides for the selectivity. Currently, the formation of a thin, dense, and selective layer on a highly porous ENMS substrate has been the Achilles heel in TFC ENMs preparation for PV application, as it is a great challenge to prevent the selective layer from penetrating into the ENF support with three-dimensional pore structure. It is not easy to directly

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deposit a selective thin layer (