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- Herausgeber: John Wiley & Sons
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- Sprache: Englisch
APPLICATIONS OF POLYMER NANOFIBERS Explore a comprehensive review of the practical experimental and technological details of polymer nanofibers with a leading new resource Applications of Polymer Nanofibers delivers a complete introduction to the basic science of polymer nanofibers as well as a review of their diverse applications. The book assesses their potential for commercialization and presents contributions from leading experts emphasizing their practical and technological details. New and up to date research findings are presented throughout the book in areas including filters, fabric, energy, fuel cells, batteries, sensors, biomedicine, drug delivery, tissue engineering, and wound dressings. The book also presents a fulsome analysis of the technology of electrospinning, the most convenient and scalable technique for nanofiber production. It also provides readers with practical information on relevant surface modification techniques. Applications of Polymer Nanofibers effectively balances theoretical background with practical applications of the technology, including insights into polymer nanofiber materials that will be useful for advanced students and researchers. Students, researchers, and industry professionals will also enjoy the inclusion of: * A thorough introduction to electrospinning parameters and resulting nanofiber characteristics, including theoretical and practical considerations * An exploration of textile applications of nanofibers, like protective clothing, filter fabrics, wearable devices, functional fabrics, and biomedical textiles * A review of nanofiber mats as high-efficiency filters, including filtration developments, filters made with nanofibers, and the future outlook for nanofiber filters * A treatment of nanofiber-based chemical sensors, including sensor materials, approaches to nanofiber sensor design, and gravimetric nanofiber sensors Perfect for researchers and graduate students studying polymer science and engineering, chemical engineering, materials science, and nanotechnology. Applications of Polymer Nanofibers will also earn a place in the libraries of industrial researchers concerned with electrospinning, air filtration, fabrics, drug delivery, catalysis, and biomedicine.
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Table of Contents
Cover
Title Page
Copyright Page
Dedication Page
List of Contributors
Prefacepreface
1 Electrospinning Parameters and Resulting Nanofiber Characteristics
1.1 Electrospinning Overview
1.2 Effect of Process Parameters
1.3 Effect of Setup Parameters
1.4 Effect of Solution Parameters
1.5 Electrospinnable Systems
1.6 Advanced Fiber Characteristics
1.7 Process Scalability
References
2 Textile Applications of Nanofibers
2.1 Introduction of Nanofibers in Textile Applications
2.2 Fabrication of Nanofiber Yarns
2.3 Structure and Properties of Nanofiber Yarns
2.4 Fabrication of Nanofiber Fabrics
2.5 Characteristics and Specialized Applications of Nanofiber Fabrics
2.6 Summary and Future Trends
References
3 Nanofiber Mats as High‐Efficiency Filters
3.1 Introduction
3.2 Filters Made with Nanofibers
3.3 Filtration Developments
3.4 Outlook
Acknowledgments
References
4 Nanofiber‐Based Chemical Sensors
4.1 Introduction
4.2 General Features of Sensors
4.3 Nanofibers as a Sensor Material
4.4 Approaches to Nanofiber Sensor Design
4.5 Gravimetric Nanofiber Sensors
4.6 Optical Sensors
4.7 Electrochemical Sensors
References
5 Nanofibers in Energy Applications
5.1 Overview
5.2 Energy Storage Applications
5.3 Energy Conversion Applications
5.4 Concluding Remarks
References
6 Electrospun Nanofibers for Drug Delivery Applications
6.1 Introduction
6.2 Methods for Encapsulation of Bioactive Molecules in Electrospun Nanofibers
6.3 Conclusion
References
7 Interfacing Electrospun Nanofibers with Microorganisms: Applications from Killing to Repelling to Delivering Living Microbes
7.1 Introduction
7.2 Brief Background on the Electrospinning Process
7.3 Electrospinning Process and Variables
7.4 Why It Is Important to Understand the Interactions Between Biomaterials and Microorganisms
7.5 Background on Antibacterial Surface Engineering
7.6 Background on Antifouling Surface Engineering
7.7 Polymer Selection for Nanofibrous Biomaterials
7.8 Electrospinning Techniques Tailor the Location of Active Agents
7.9 Blend Electrospinning Yields a Dispersed Active Agent
7.10 Coaxial and Emulsion Electrospinning Enables the Controlled Delivery of Active Agents
7.11 Coating Electrospun Mats Tailors Their Interactions with Cells
7.12 Antibacterial Nanofiber Mats
7.13 Multifaceted Delivery from Nanofibrous Mats
7.14 Antifouling Nanofiber Mats
7.15 Nanofibrous Mats Containing Living Cells
7.16 Conclusion
Acknowledgments
References
8 Advances in Functionalizing the Interior and Exterior of Polymer Nanofibers
8.1 Introduction
8.2 Nanofibers with Controlled Nanoparticle Distribution
8.3 As‐spun Nanofibers with Bioresponsive Properties
8.4 Polymer Nanofibers with Postfunctionalized Surfaces
8.5 Nanofibers Produced by Directed Self‐Assembly
8.6 Concluding Remarks
Acknowledgments
References
9 Nanofiber Aerogels
9.1 Aerogels
9.2 Nanofiber‐Based Aerogels
9.3 Future Perspectives
References
10 Micro and Nanofibers
10.1 Electrospinning
10.2 The Melt‐blowing Process
10.3 “Splittable” Bicomponent Fibers
10.4 Partially “Soluble” Bicomponent Fibers
10.5 Fibrillating Bicomponent Fibers
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Key solvent properties for common electrospinning solvents.
Table 1.2 Nonpolymer electrospinning systems.
Table 1.3 Comparison of fiber production methods.
Chapter 4
Table 4.1 Selected examples of nanofiber‐based colorimetric and fluorescent ...
Table 4.2 Selected composite n–p heterojunction metal oxide nanofiber sensor...
Chapter 5
Table 5.1 Summary of LMO cathode materials with the highest theoretical capa...
Table 5.2 Theoretical capacities of anode materials.
Table 5.3 Summary of fuel cell devices and their respective electrolyte, key...
Chapter 9
Table 9.1 Processing conditions and applications of NFA developed using var...
List of Illustrations
Chapter 1
Figure 1.1 Schematic of conventional electrospinning setup and overview of p...
Figure 1.2 Schematic of various electrodes used to control the electrospinni...
Figure 1.3 Specific viscosity as a function of polymer concentration to dete...
Figure 1.4 Overview of interesting electrospun structures: (A (a–d)) ribbons...
Figure 1.5 Overview of advanced electrospun nanofiber cross sections: (a) po...
Chapter 2
Figure 2.1 Schematic image of electrospinning setup.
Figure 2.2 Cross‐section types of bicomponent fibers.
Figure 2.3 Photographic images of (a) twisting machine and (b) PU nanofiber ...
Figure 2.4 Schematic of a dual conjugate electrospinning setup.
Figure 2.5 Yarn‐spinning setup with water bath collecting electrode.
Figure 2.6 Schematic images of (a) the fabrication process for nanofiber yar...
Figure 2.7 Electrospun fiber yarns of (a) PVAc, (b) PVDF, and (c) PAN.
Figure 2.8
Scanning electron microscope
(SEM) images of (a, b) untreated and...
Figure 2.9 Surface morphology of PAN nanofiber yarn at a twisting air pressu...
Figure 2.10 (a) Schematic of the experimental setup for preparing nanofibrou...
Figure 2.11 SEM image of a typical ruptured end of the PA‐6 hollow nanofiber...
Figure 2.12 Melt blowing process for producing nanofiber nonwovens.
Figure 2.13 Layered fabric structure containing electrospun PU nanofibers....
Figure 2.14 (a) Photograph of PLA/TSF nanofiber woven fabric; SEM images of ...
Figure 2.15 Illustration of the fabrication process for the 3D woven fabric ...
Figure 2.16 (a) The weaving of the electrospun nanofibers in succession (fro...
Figure 2.17 The structure of (a) three‐layered and (b) five‐layered PI nanof...
Figure 2.18 (a–f) Schematic illustration of assembling flexible PLEDs by usi...
Chapter 3
Figure 3.1 Number of patents and articles published worldwide per year. The ...
Figure 3.2 Illustration of the various aspects of processing polymer to nano...
Figure 3.3 Schematic illustrating (a) nonslip flow and (b) slip flow of air ...
Figure 3.4 Comparison of the expressions used to extrapolate filter pressure...
Figure 3.5 Conceptual illustration of the first three mechanisms involved in...
Figure 3.6 (a) Illustration of efficiency of an air filter vs. particle size...
Figure 3.7 A scanning electron micrograph comparing commercial fiberglass fl...
Figure 3.8 Example of a nanofiber layer on a spunbond substrate.
Figure 3.9 Example of an electrospun Polyurethane filter in a disposable fil...
Figure 3.10 Example of a filter media composed of a melt‐blown fiber layer b...
Chapter 4
Figure 4.1 General response curve for a chemical sensor.
Figure 4.2 Generalized diagram of a sensor.
Figure 4.3 (a) Fluorescence emission of the copolymer PNNR2 at different con...
Figure 4.4 Conversion of {4‐rhodamine hydrazonomethyl‐3‐hydroxy‐phenyl metha...
Figure 4.5 (a) Binding of formaldehyde by the amine groups in poly(ethylenei...
Figure 4.6 (a) The frequency response to humidity, of a SAW sensor based on ...
Figure 4.7 (a) Reflectance spectra of the PANI–
leucoemeraldine base nanofibe
...
Figure 4.8 Typical response curves of glucose oxidase coated
electrospun fib
...
Figure 4.9 Dynamic response of PA6/TiO
2
/PANI and PA6/PANI composite nanofibe...
Figure 4.10 (a) A schematic diagram of the flow‐over and flow through geomet...
Chapter 5
Figure 5.1 General operating mechanisms and electrode materials of lithium‐i...
Figure 5.2 SEM micrographs of caterpillar‐like LiMnNiO
x
structures fabricate...
Figure 5.3 Electrospun carbon‐coated V
2
O
5
nanofibers with a hollow, porous s...
Figure 5.4 (A) Illustrations and SEM cross‐sections of PI–PVDF–PI sandwich m...
Figure 5.5 Operating mechanisms of (a) EDLCs and (b) pseudocapacitors. The e...
Figure 5.6 Illustrations and SEM micrographs of core–shell nanofibers with a...
Figure 5.7 The chemical structure of poly(perfluorosulfonic acid), or Nafion...
Figure 5.8 General schematic of a photovoltaic device based on a p–n junctio...
Figure 5.9 Progress of each photovoltaic technology in terms of peak researc...
Figure 5.10 General operation of DSSC. An incident photon is absorbed by a d...
Figure 5.11 Effect of calcination of highly porous ZSO nanofiber scaffold to...
Figure 5.12 Diagram of an organic photovoltaic (OPV). The photoactive layer ...
Chapter 6
Figure 6.1 Therapeutic effect of the 5‐FU patch in an orthotopic tumor model...
Figure 6.2 A schematic illustrating the proposed mechanism responsible for t...
Figure 6.3 Chemical structure of (a) sulfisoxazole; (b)
hydroxypropyl‐beta‐c
...
Figure 6.4 Disintegration of PVA/caffeine and PVA/riboflavin nanofibrous mat...
Figure 6.5 (a) Schematic representation of CD/linalool‐IC and CD‐linalool‐IC...
Figure 6.6 (a) Synthesis of poly(VBA‐
co
‐VBTAC). Schematic representation of ...
Figure 6.7 Illustration of possible diffusion and dissolution mechanism of h...
Figure 6.8 (a) Schematic representation of core–shell electrospinning and (b...
Figure 6.9 TEM (bottom) images of PVP/polycaprolactone (PCL) core/shell nano...
Figure 6.10 Preparation of
silk fibroin
(
SF
)/poly(ethylene oxide) (PEO)/
gela
...
Figure 6.11 A schematic illustrating the strategy underlying the design of t...
Figure 6.12 A proposed mechanism underlying the thermoresponsive properties ...
Figure 6.13 Single and emulsion electrospinning. (a) During the process of s...
Figure 6.14 (a) The representative images of skin wounds after being covered...
Chapter 7
Figure 7.1 (a) An electrospinning apparatus consists of a spinneret, two ele...
Figure 7.2 Schematic representation of how antibacterial materials and surfa...
Figure 7.3 Antifouling surfaces delay bacterial adhesion through (a) steric ...
Figure 7.4 A sink‐like flow pulls the bacteria toward the tip of the Taylor ...
Chapter 8
Figure 8.1 Illustrations and signature characteristics of (a) UCST and (b) L...
Figure 8.2 (a) SEM image of PEO/P2VP nanofibers composed of 80 wt% PEO. A se...
Figure 8.3 XRD profiles acquired from PEO powder and electrospun PEO/P2VP na...
Figure 8.4 In (a), the modified electrospinning setup designed to incorporat...
Figure 8.5 In (a–d), TEM images of GNRs aligned in electrospun PEO nanofiber...
Figure 8.6 Absorbance spectra acquired from (a) randomly arranged GNRs in a ...
Figure 8.7 In (a), a PAN film containing a charged Zn‐containing porphyrin a...
Figure 8.8 Schematic diagram of field‐driven surface biofunctionalization du...
Figure 8.9 In (a–h), a series of SEM images of electrospun PEO/(SEE)
3
–PEO na...
Figure 8.10 In (a–c), SEM images of PMMA nanofibers electrospun at different...
Figure 8.11 The surface PMMA content as a function of the bulk PMMA content ...
Figure 8.12 In (a–c), calculated electric‐field maps from three different ti...
Figure 8.13 Surface modification strategy for synthesizing functional polyme...
Figure 8.14 XPS spectra, including high‐resolution scans at the C 1s edge, a...
Figure 8.15 In (a, b), SEM images of PET nanofibers modified with PDMAEMA an...
Figure 8.16 In (a), an illustration of the CoDSA method for fabricating hier...
Figure 8.17 In (a), a schematic representation of block comicelles that are ...
Chapter 9
Figure 9.1 Schematic displaying various steps involved in the processing of ...
Figure 9.2 Different steps of phase change of water and carbon dioxide (CO
2
)...
Figure 9.3 Schematic to display various stages involved in NFA fabrication s...
Figure 9.4 Structure and properties of hybrid aerogels produced from electro...
Figure 9.5 Effect of change in the composition of solvent on NFA morphology....
Figure 9.6 Homogenization of electrospun nanofibers. (a) Photograph showing ...
Figure 9.7 (a) CDA‐silica hybrid NFA supported on a dandelion. SEM images di...
Figure 9.8 (a) The gravity‐driven separation of oil‐in‐water emulsions using...
Figure 9.9 Schematic illustration of particle filtration mechanisms by (a) a...
Figure 9.10 (a) Photographs of various dye solutions (100 mg/l) before and a...
Figure 9.11 (a) A fresh flower protected by NFA with a thickness of 2 cm und...
Figure 9.12 Compression properties of PISGs. (a) Typical compressive curves ...
Chapter 10
Figure 10.1 Schematic of a typical melt‐blowing equipment.
Figure 10.2 Exxon die.
Figure 10.3 Melt‐blown fiber stream.
Figure 10.4 Schematic of the air knives in an Exxon die.
Figure 10.5 Cross section view of Biax die and air distribution design.
Figure 10.6 The multirow die.
Figure 10.7 Schematic of a Hills die.
Figure 10.8 Schematic representation of bicomponent fibers: (a) tipped trilo...
Figure 10.9 Typical bicomponent segmented pie fiber: (a) solid; (b) hollow....
Figure 10.10 Schematic diagram of open bicomponent spun‐bond process with be...
Figure 10.11 Spun‐bonded fiber diameter as function of the number of segment...
Figure 10.12 Cross sections of the segmented pie fibers made of: (a) 8 segme...
Figure 10.13 Typical bicomponent islands‐in‐the‐sea fiber.
Figure 10.14 The effect of the number of islands and polymer composition on ...
Figure 10.15 Effect of number of islands on fabric tensile strength.
Figure 10.16 Effect of number of islands on fabric burst strength.
Guide
Cover Page
Title Page
Copyright Page
Dedication Page
List of Contributors
Preface
Table of Contents
Begin Reading
Index
Wiley End User License Agreement
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Applications of Polymer Nanofibers
Edited by
Anthony L. Andrady
Department of Chemical and Biomolecular Engineering,
North Carolina State University, Raleigh, NC, USA
Saad A. Khan
Department of Chemical and Biomolecular Engineering,
North Carolina State University, Raleigh, NC, USA
This edition first published 2022© 2022 John Wiley & Sons, Inc.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.
The right of Anthony L. Andrady and Saad A. Khan to be identified as the authors of the editorial material in this work has been asserted in accordance with law.
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Library of Congress Cataloging‐in‐Publication Data applied for:ISBN: 9781119267683
Cover Design: WileyCover Images: Image of silver‐silica hybrid nanofibers courtesy of Tahira Pirzada; Images of Elecrospun fibers of polymers of intrinsic microposity courtesy of Siyao Wang; Wiley (Advanced Functional Materials, Cellulose Silica Hybrid Nanofiber Aerogels, 2020)Tahira Pirzada
This volume is dedicated to the first responders and medical personnel worldwide, working tirelessly to contain the Covid‐19 pandemic
List of Contributors
Anthony L. AndradyDepartment of Chemical and Biomolecular EngineeringNorth Carolina State UniversityRaleigh, NCUSA
Zeynep AytacDepartment of Environmental HealthHarvard T.H. Chan School of Public HealthCenter for Nanotechnology and NanotoxicologyHarvard UniversityBoston, MA, 02115USA
Jessica L. BarlowDepartment of Chemical and Life Science EngineeringVirginia Commonwealth UniversityRichmond, VAUSA
Emily DiepDepartment of Chemical EngineeringUniversity of Massachusetts AmherstAmherst, MAUSA
Caitlin DillardBoeing1 S Stewart Ave, Ridley ParkPhiladelphia, PA, 19078USA
Arzan C. DotivalaDepartment of Chemical and Life Science EngineeringVirginia Commonwealth UniversityRichmond, VAUSA
David S. EnsorRetiredISO Technical Committee 209 Cleanrooms and Associated Controlled EnvironmentsSpokane, WAUSA
Nataliya FedorovaThe Nonwovens InstituteNC State UniversityRaleigh, NCUSA
Yeqian GeWilson College of TextilesNorth Carolina State UniversityFiber and Science ProgramRaleigh, NCUSA
Vibha KalraDepartment of Chemical and Biological EngineeringDrexel UniversityPhiladelphia, PAUSA
Saad A. KhanDepartment of Chemical and Biomolecular EngineeringNorth Carolina State UniversityRaleigh, NCUSA
Irene S. KurtzDepartment of Chemical EngineeringUniversity of Massachusetts AmherstAmherst, MAUSA
Shani L. LevitDepartment of Chemical and Life Science EngineeringVirginia Commonwealth UniversityRichmond, VAUSA
Benoit MazeThe Nonwovens InstituteNC State UniversityRaleigh, NCUSA
Bharadwaja S.T. PeddintiDepartment of Chemical and Biomolecular EngineeringNorth Carolina State UniversityRaleigh, NCUSA
Tahira PirzadaDepartment of Chemical and Biomolecular EngineeringNorth Carolina State UniversityRaleigh, NCUSA
Behnam PourdeyhimiThe Nonwovens InstituteNC State UniversityRaleigh, NCUSA
Vahid RahmanianDepartment of Chemical and Biomolecular EngineeringNorth Carolina State UniversityRaleigh, NCUSA
Kristen E. RoskovDepartment of Chemical and Biomolecular EngineeringNorth Carolina State UniversityRaleigh, NCUSAandBASF Agricultural SolutionsBASF CorporationResearch Triangle Park, NCUSA
Jessica D. SchiffmanDepartment of Chemical EngineeringUniversity of Massachusetts AmherstAmherst, MAUSA
Richard J. SpontakDepartment of Chemical and Biomolecular EngineeringNorth Carolina State UniversityRaleigh, NCUSAandDepartment of Materials Science and EngineeringNorth Carolina State UniversityRaleigh, NCUSA
Xiaoyu SunDepartment of Chemical and Biomolecular EngineeringNorth Carolina State UniversityRaleigh, NCUSAandIntegrated Diagnostic SolutionsBecton Dickinson & CompanyFranklin Lakes, NJUSA
Kathleen F. SwanaU.S. Army Combat Capabilities Development Command Soldier CenterNatick, MAUSA
Christina TangDepartment of Chemical and Life Science EngineeringVirginia Commonwealth UniversityRichmond, VAUSA
Breland T. ThorntonDepartment of Chemical and Life Science EngineeringVirginia Commonwealth UniversityRichmond, VAUSA
Tamer UyarDepartment of Fiber Science & Apparel DesignCollege of Human EcologyCornell UniversityIthaca, NY, 14853USA
Howard J. WallsAerosol Control Group LeadTechnology Advancement & CommercializationRTI InternationalResearch Triangle Park, NC, 27709‐2194USA
Xiangwu ZhangWilson College of TextilesNorth Carolina State UniversityFiber and Science ProgramRaleigh, NCUSA
Jiadeng ZhuWilson College of TextilesNorth Carolina State UniversityFiber and Science ProgramRaleigh, NCUSA
Note
1
Silk threads from spider species
Nephila clavipes
Vehoff, T., Glisović, A., Schollmeyer, H., Zippelius, A. et al. (2007). Mechanical properties of spider dragline silk: humidity, hysteresis, and relaxation.
Biophysical Journal
93 (12): 4425–4432.
1Electrospinning Parameters and Resulting Nanofiber Characteristics : Theoretical to Practical Considerations
Christina Tang1, Shani L. Levit1, Kathleen F. Swana2, Breland T. Thornton1, Jessica L. Barlow1, and Arzan C. Dotivala1
1Department of Chemical and Life Science Engineering, Virginia Commonwealth University, Richmond, VA, USA
2U.S. Army Combat Capabilities Development Command Soldier Center, Natick, MA, USA
1.1 Electrospinning Overview
Electrospinning has been widely used to produce nonwoven nanofibers for applications in biomaterials, energy materials, composites, catalysis, and sensors (Agarwal et al. 2008, 2009; Ahmed et al. 2014; Cavaliere et al. 2011; Chigome and Torto 2011; Ma et al. 2014; Mao et al. 2013; Yoon et al. 2008; Thavasi et al. 2008). On a bench scale, it is a simple, inexpensive process. To generate nanofibers by electrospinning, an electric potential is applied between a capillary containing a polymer solution or melt and a grounded collector (Figure 1.1). The applied electric field leads to free charge accumulation at the liquid‐air interface and electrostatic stress. When the electrostatic stress overcomes surface tension, the free surface deforms into a “Taylor cone.” Balancing the applied flow rate and voltage results in a continuous fluid jet from the tip of the cone. As the jet travels to the collector, it typically undergoes nonaxisymmetric instabilities such as bending and branching leading to extreme stretching. As the fluid jet is stretched, the solvent rapidly evaporates to form the polymer fibers that are deposited onto a grounded target (Reneker and Chun 1996; Helgeson et al. 2008; Rutledge and Fridrikh 2007; Thompson et al. 2007; Teo and Ramakrishna 2006; Li and Xia 2004). As a complex electrohydrodynamic process, the final fiber and mat/membrane properties depend on process parameters by process parameters, setup parameters, and solution properties.
Figure 1.1 Schematic of conventional electrospinning setup and overview of process, setup, and solution parameters that affect fiber and mat properties.
Source: Photograph of mat reprinted from Dror et al. (2008). Copyright (2008). American Chemical Society.
1.2 Effect of Process Parameters
Electrospun fibers from 30 nm to 10 μm in diameter have been reported (Greiner and Wendorff 2007). Despite its widespread use, electrospinning of new materials is typically done ad hoc varying polymer concentration and process variables. Although the nanofiber properties, namely fiber diameter, could be ideally controlled by varying the process parameters, precise control over the fiber diameter remains a technical bottleneck. The effect of process variables on fiber characteristics has been widely examined theoretically and experimentally.
1.2.1 Theoretical Analysis
To avoid the cost and time of experimental trial and error, modeling and theoretical analysis have been applied to predict how process parameters affect fiber diameter. Reneker and coworkers have developed a theoretical model based on simulating jet flow as bead‐springs. Their model describes the entire electrospinning process and accounts for solution viscoelasticity, electric forces, solvent evaporation and solidification, surface tension, and jet–jet interactions. Performing sensitivity analysis of 13 model input parameters, they determined that initial jet radius, tip‐to‐collector distance, volumetric charge density, and solution rheology, i.e. relaxation time and elongational viscosity, had strong effect on final fiber size. Initial polymer concentration, perturbation frequency, solvent vapor pressure, solution density, and electric potential had a moderate effect, whereas vapor diffusivity, relative humidity, and surface tension had minor effects on fiber diameter (Thompson et al. 2007).
Using a simple analytical model focusing on the whipping of the jet treats the jet as a slender viscous object. Rutledge and coworkers assume that the final fiber diameter is dictated by an equilibrium between Coulombic charge repulsion on the surface of the jet and the surface tension of the liquid jet. The model predicts that terminal jet radius (rj)
where Q is the volumetric flow rate, I is the electric current, γ is the surface tension, is the dielectric constant of the outside medium (typically air), and χ is the dimensionless wavelength of the instability of the normal displacement. The fiber diameter (d) is related to the terminal jet radius and the polymer concentration, c. When compared to experimental results, the model accurately predicted the diameter of polyethylene oxide (PEO) fibers (within 10%) and polyacrylonitrile fibers (within 20%). The theory overpredicted stretching for polycaprolactone fibers, which had relatively low conductivity and high solvent volatility (Fridrikh et al. 2003).
Recently, Stepanyan and coworkers developed an electrohydrodynamic model of the jet elongation in which kinetics of elongation and evaporation govern the nanofiber diameter. Using the timescale of elongation to nondimensionalize the force balance, the timescale of solvent evaporation, and concentration‐dependent material functions (e.g. relaxation time), they predict the scaling relationship for the final fiber radius (rf) is
where k is the solvent evaporation rate, ρs is the solution density, ηo is the solution viscosity, Q is the volumetric flow rate, and I is the electric current. The result for Eq. (1.2) reduces to Eq. (1.1) in the limit of very slow evaporation. The viscosity dependence, ηo1/3, and (Q/I)2/3 agree with experimental results. Using polyvinylpyrrolidone (PVP) in various alcohol solutions as well as polyamide/polyacrylonitrile in dimethylformamide (DMF) solutions, the fiber diameter was experimentally observed to scale with the evaporation rate, k, to the 1/3 power (Stepanyan et al. 2014, 2016).
The current in electrospinning (I) and the volumetric charge density (Q/I) is commonly used in modeling and theoretical approaches. Experimental investigations by Yarin and coworkers determined that
where V is applied voltage, Q is flow rate, C polymer concentration, M molecular weight, and H tip to collector distance. Using PEO/water/ethanol mixtures, the solvent properties also slightly affect the current and volumetric charge density. From this work, it is evident that the fiber size is affected by applied electric field strength (applied voltage and time to collector distance), flow rate, and polymer solution (Thompson et al. 2007). Thus, these predictions for fiber size rely on several model parameters that cannot be easily related to measurable variables.
In another work by Rutledge and coworkers, they experimentally determined that the total current in electrospinning, given by
varies electric field (E), volumetric flow rate (Q), and solution conductivity (K). In electrospray, the measured current scales linearly with Q. The authors attributed the observed Q0.5 dependence on combined jet and spray arising from secondary jetting. Secondary jetting was considered a mechanism for dynamic removal of charge from the surface of the jet which affects final fiber diameter and reducing jet stretching due to surface charge repulsion. The secondary jetting can be minimized by reducing the volumetric flow rate or solution conductivity (Bhattacharjee et al. 2010).
Alternative scaling analyses based on the spinning solution properties and electrospinning operating conditions alone have been developed. Based on electrohydrodynamic theory, the Taylor–Melcher slender body theory relates jet kinematics to measurable fluid properties and process variables. Helgeson et al. validated the electrohydrodynamic model with measurements of the jet radius and velocity via in situ high‐speed photography and velocimetry in the straight portion of viscoelastic electrospinning jet and PEO/NaCl as a model system. Dimensional analysis of the validated model involves two important quantities the electroviscous number characterizing the electromechanical stress relative to shear stress and the Ohnesorge number (Oh). The relationship for the final fiber diameter was determined to be
where wp is the mass fraction of polymer in solution, ρ is solution density, γ is surface tension, η0 is the zero shear viscosity, Q is the volumetric flow rate, is the extensional viscosity, ε is the dielectric constant of the fluid, is the dielectric constant of the surrounding medium (typically atmosphere), and E0 is the strength of the applied electric field. This analysis explicitly contains the extensional viscosity of the fluid known to control fiber formation. The extensional viscosity can be estimated as 3η0 for a Newtonian fluid. This approach assumes the final fiber is directly proportional to jet radius in the straight portion of the jet and the actual relationship may be system‐dependent, i.e. influenced by solution conductivity and mechanics of the bending instability (Helgeson et al. 2008). The predictions agree with experimental observations over a broad range of polymer concentrations and voltages. Upon addition of a significant amount of NaCl, the trends are consistent, but the scaling factor changes and may be attributed to the differences in the bending instability.
Recently, using a force balance on a bent jet considering electric field and surface charge for a Newtonian fluid, Gadkari predicted the final fiber diameter (df) scales as
where ηo is viscosity, K is conductivity, Q is flow rate, L is tip to collector distance, and V is applied voltage. When compared to experimental results, the scaling prediction qualitatively matches the observed trend for viscosity, flow rate, and applied voltage. The model overpredicts the dependence on volumetric flow rate (0.5 dependence predicted, whereas a 0.3 dependence has been observed experimentally). Experimental values for the scaling relationships for ΔV, L, and K were not available (Gadkari 2014).
1.2.2 Experimental Results
Collectively, theoretical considerations indicate that fiber size is affected by applied electric field strength (applied voltage and time to collector distance), volumetric flow rate, and polymer solution (viscosity and conductivity). However, experimental results have been system‐dependent. For example, experiments increasing the voltage has been observed to decrease fiber diameter for many systems such as polyacrylonitrile/DMF and aqueous polyvinyl alcohol due to greater stretching and a stronger electric field. (Andrady 2008). For polyacrylonitrile in DMF, fiber diameter was reduced from ~95 to 50 nm by increasing the voltage from 5 to 25 kV. Conversely, fiber size has increased with increasing voltage. For example, the polystyrene (PS) fibers increased in diameter from 0.31 to 1.72 μm when the applied voltage increased from 5 to 25 kV. The discrepancy in experimental observations indicates that the effect of voltage on fiber size needs to be considered with other process parameters, especially the feed rate and tip‐to‐collector distance. Notably, at higher applied voltages, there is a greater tendency for bead formation. The bead density increased with increasing voltage and the shape of the beads transitioned from spindle‐like to spherical‐like indicating instability of the jet (Ramakrishna 2005).
Tip‐to‐collector distance influences the time of travel, amount of drying, and electric field strength (depending on the applied voltage) and thus the resulting fiber diameter and morphology. Practically, the distance must be large enough to prevent corona discharge. Generally, increasing the tip‐to‐collector distance with other parameters kept constant reduces fiber diameter. For example, electrospinning polystyrene in chloroform, the fiber diameter decreased from 1 to 0.66 μm by increasing the distance from 5 to 25 cm due to increased time of travel and stretching. Conversely, increasing the distance has also been observed to increase fiber diameter due to the reduced electric field strength. Decreasing the tip‐to‐collector distance and resulting time of travel and amount of drying can lead to deposition of “wet” fibers that fuse on the collector. While higher electric field strengths can be achieved at shorter distances, it can often result in the formation of beads or an unstable Taylor cone if the distance is not sufficient for development of the whipping instability (Andrady 2008; Ramakrishna 2005).
Continuous nanofibers of uniform diameter are achieved when the feed rate matches the rate of at which solution is removed from the tip. At lower feed rates, fibers may form intermittently. Higher feed rates increase the tendency to form beads. Given sufficient applied voltage, the average fiber diameter increases with feed rate. Increasing the feed rate can also result in fused fibers. With larger volume of solution drawn from the needle top, the solvent may not completely evaporate. The residual solvent may cause the fibers to fuse together when deposited (Andrady 2008; Ramakrishna 2005).
Overall, the experimental results generally agree with the scaling analysis, i.e. the final fiber diameter is directly proportional to volumetric flow rate as well as polymer concentration/viscosity. The electric field strength, dictated by the applied voltage and tip to collector distance, also affects fiber diameter. However, various effects have been observed (e.g. increasing the applied voltage may increase, decrease, or have no effect on the fiber diameter depending on the system) due to the complexity of the process. Some authors have performed systematic experiments varying process parameters and used regression analysis (Cui et al. 2007) or neural network models (Sarkar et al. 2009) to establish quantitative relationships. However, these analyses are system‐dependent. There are no methods to date to predict the fiber size based on solution properties and process parameters (Helgeson et al. 2008; Thompson et al. 2007).
1.3 Effect of Setup Parameters
The effect of process parameters (e.g. flow rate, tip‐to‐collector distance, applied voltage) has been widely studied with conflicting experimental results. An alternative approach to tuning nanofiber and membrane properties has been adjusting parameters of the electrospinning setup.
Ambient conditions (temperature and humidity) affect the electrospinning process and fiber characteristics. The temperature of the spinning solution affects the evaporation rate and the viscosity. At higher temperatures, lower viscosities lead to increased stretching force and result in smaller fibers. Humidity affects solvent evaporation which can affect the resulting fiber characteristics. Using PEO in water as a model system, a twofold monotonic decrease in fiber size was observed with increasing relative humidity. At low humidity, solvent evaporation may be faster than removal of the solution away from the tip of the needle and can lead to needle clogging, especially with volatile solvents. Leveraging humidity to create porous fibers is further discussed in Section 1.6.2. Although, the relative humidity cannot always be readily controlled (Cai and Gevelber 2013), monitoring the ambient temperature and humidity during electrospinning is of practical importance.
Generally, electrospinning is performed in air. Controlling the gaseous environment can be advantageous for affecting fiber diameter. To slow the rate of drying, a gas‐jacketed capillary tip can be used to surround the jet with nitrogen saturated with spinning solvent. With slower solvent evaporation, stable Taylor cones could be achieved by electrospinning poly‐L‐lactic acid in dichloromethane (high volatility). Notably, the flow rate of gas affected the rate of electrospinning. Accelerating the rate of evaporation using an external heat source has also been reported to improve the mat quality of hyaluronic acid fibers spun from water. The improved fiber quality was attributed to increased stretching, enhanced solvent evaporation, and a threefold reduction in viscosity due to the flow of hot air (~60 °C). The composition of the gaseous environment is also an important consideration; it affects leakage of the surface charge on the jet to the surrounding environment and ultimately the fiber size. For example, when using Freon‐12 as the electrospinning environment, the fiber diameter was twofold larger than air at the same conditions. This result was attributed to the higher breakdown voltage of Freon‐12 compared to air. With a higher breakdown voltage, the fiber retained its electric charge for a longer period of time which would increase the jet velocity and ultimately result in a larger fibers (Ramakrishna 2005; Baumgarten 1971).
Polarity of the applied electric field also affects fiber quality and size. For nylon‐6 in formic acid, the average fiber diameter was approximately twofold smaller when the capillary was charged with a negative polarity compared to when a positive polarity at the same conditions. Further, the area over which the fibers deposited was smaller in the case of a positive polarity. The difference in fiber quality and size was attributed to increased charge density in the case of negative polarity (Andrady 2008).
Generally, DC voltage is used in electrospinning. The use of alternating current (AC) potential has also been reported. Since the charging of the solution is very rapid, jet initiation occurs before the voltage alternates. The jet contains positive and negative charged which reduces the repulsive forces and bending instability in the jet. Therefore, using AC the fibers are larger when compared to DC at the same voltage. In AC, there is reduced accumulation of like charges on the deposited fiber. Therefore, thicker layers of electrospun fibers can be achieved, especially when using an insulating collector (Ramakrishna 2005).
Notably, using sharp, pointed needles, i.e. capillary tips results in more efficient charging of the solution. The tip diameter is also an important consideration. Practically, the tip diameter selection is important in avoiding needle clogging due to solvent evaporation. Smaller internal diameters have been observed to reduce beading and reduce the diameter of the fibers (in some cases). As the internal diameter decreases, the surface tension increases and a greater electrostatic force is required for jet initiation leading to smaller fibers. Therefore, the smallest tip that facilitates extrusion of the solution is generally selected. Generally, electrospinning is performed with 16G–27G needles (Andrady 2008; Ramakrishna 2005).
In more complex setups, additional electrodes can be added to tune fiber deposition (Teo and Ramakrishna 2006; Teo et al. 2011). These auxiliary electrodes can be base electrodes, steering electrodes, focusing electrodes, and guiding electrodes (Figure 1.2). The base electrode is usually a conductive plate placed parallel to the collector at the needle tip to improve the uniformity of the electric field and minimize the effect of surrounding objects on the electric field. Since the base electrode increases the stability of the jet, fibers with smaller diameters have been observed. The base electrode should be level with the needle top. Notably, using a base electrode, a higher applied voltage is required to initiate spinning (Teo and Ramakrishna 2006; Teo et al. 2011). Focusing electrodes are used to damp the whipping of the electrostatic jet to achieve more localized deposition. The electrodes are ring‐shaped, cylinder, or conical and placed close to the needle tip. Multiple focusing electrodes can be used to reduce the spread of the fiber deposition. Electrodes with 400 μm diameter holes resulted in ~200 μm diameter nanofiber patches of randomly oriented nanofibers (Teo and Ramakrishna 2006; Teo et al. 2011). Steering electrodes are used to align the electric field in the vicinity of the collector. For example, a pair of parallel electrodes placed near the collector can be used to achieve uniaxially aligned nanofibers. Multiple pairs of steering electrodes are necessary to achieve more complex patterns (Teo and Ramakrishna 2006; Teo et al. 2011).
Figure 1.2 Schematic of various electrodes used to control the electrospinning process.
Source: Adapted from Teo et al. (2011).
The collector influences the electric field and is also an important factor in the electrospinning process (Teo and Ramakrishna 2006; Andrady 2008; Ramakrishna 2005; Teo et al. 2011). The simplest collector is a stationary metal plate placed at a fixed distance from the tip. The fibers generally collect as a symmetric circular batch of nanofibers on the plate. Since the plate is grounded, the residual charges on the deposited fibers are dissipated and the mat has high areal density. Moving the collector surface during processing provides some control in the areal density (Andrady 2008). Collectors with grids or charged needles can be used to create patterned nanofiber membranes which consist of regions of high‐ and low‐fiber density. Low‐fiber density occurs in regions where the collector is insulated. Another common collector is a rotating metal drum/mandrel. The rotating surface leads to an even deposition of fibers and a uniform nanofiber mat. The rotating drum can further stretch the fiber leading to reduced diameters as well as introduce alignment of nanofibers. When using high boiling point solvents, e.g. DMF, a rotating collector can provide a longer time for the solvent to evaporate to prevent fiber fusing. Combining electrospinning and mechanical drawing by collecting on a rotating mandrel can affect fiber size. For example, the diameter of PEO fibers spun from chloroform could be reduced from ~1600 to 600 nm by increasing the velocity of the rotating drum (Ogata et al. 2007). Rotating mandrels are also often used to make tubular constructs for potential application as vascular grafts. For tubular constructs, the wall thickness could be controlled linearly with electrospinning time (Teo and Ramakrishna 2006; Andrady 2008; Ramakrishna 2005; Teo et al. 2011).
The material of the collector is also an important consideration that affects the packing density. When nonconductive materials are used as the collector, charge accumulates, and fewer fibers are deposited resulting in lower‐packing densities when compared to fibers collected on conductive surfaces. Even when using conductive collectors, nonconductive behavior can be observed as the fibers (insulating) collect. Sensitivity analysis indicates that the dielectric properties and surface area of the collector are dominant variables that influence fiber diameter and fiber spacing (porosity). Using an auxiliary electrode supplied with AC voltage minimizes the effect of the material on the collector because it reduces the residual change on the deposited fibers. Collecting in a liquid has also been reported and significantly affects the fiber morphology. The choice of liquid can affect the surface characteristics of the fiber (Ramakrishna 2005).
The porosity of the collector also effects fiber deposition. Fibers collected on metal meshes had lower‐packing densities than smooth surfaces. This effect has been attributed to increased evaporation rate when using a porous collector. As the fibers dry faster, the residual charges persist and repel subsequent fibers. Notably, the topography of the deposited fiber mat will follow the texture of the collector (Ramakrishna 2005). Deposition of two‐dimensional patterned structures or three‐dimensional structures has also been observed. Honeycomb and dimpled structures have been observed using insulating collectors. Two‐dimensional and three‐dimensional patterning is attributed to charge repulsion of deposited fibers. The fibers of the three‐dimensional structures are loosely packed and easily compressed. The conditions to form such three‐dimensional structures are not well understood (Teo et al. 2011).
Controlling fiber deposition to achieve fiber patterning can be achieved using gap electrodes or open frame collectors. The two parallel electrodes cause the electrostatic field lines in their vicinity to align perpendicular to the edges of the electrodes. The jet aligns with the field lines and deposits uniaxially aligned nanofibers. Charge repulsion of the deposited fibers limits collection of aligned fibers to ~minutes so that samples of thick aligned fibers are difficult to achieve. Arrays of multiple electrodes have been used to achieve more complex patterns, e.g. orthogonal fibers (Teo and Ramakrishna 2006; Andrady 2008; Ramakrishna 2005; Teo et al. 2011).
Alternatively, fibers can be aligned by collecting on a rotating mandrel. The fibers align along the circumference of the mandrel. Typically, high rates of rotation ~1000 rpm are used. To achieve alignment, the rotation of the mandrel must be faster than fiber deposition so that the fibers are taken up on the surface of the mandrel and wound rather than randomly deposited. By replacing a solid mandrel with a wire drum, alignment can be achieved at much lower rates of rotation ~1 rpm. In the case of a wire drum, the fibers are thought to align due to the electric field profile created by the parallel wires. Use of a thin disk with a sharp edge as a collector provides more control of the electrostatic field to align fibers. The electrostatic field lines concentrate toward the knife‐edge and the jet tends to follow the direction of the electric field. As the disk rotates (~1000 rpm), the fibers wind continuously along the knife‐edge with a pitch of 1–2 μm (Ramakrishna 2005). To improve the alignment, the fibers must be collected before the onset of the whipping instability. Auxiliary electrodes can be used to suppress the whipping instability (Carnell et al. 2008). Alternatively, using solvents with low dielectric constants and high purity can suppress the whipping instability (Ogata et al. 2007). Practically, the highly aligned fibers are achieved for a short period of time ~ minutes after which alignment decreases, which may be attributed to fiber repulsion due to charge accumulation. Therefore, for complex patterns, mechanical drawing techniques that avoid the electric field and whipping stability are preferable (Nain and Wang 2013).
Nanofiber yarns have also been of interest. To produce yarns, electrospun nanofibers can be deposited on water. As the nanofibers are lifted off the water, the surface of tension bundles the fibers into a yarn. Collecting on water flowing in the form of a vortex is a means to achieving continuous yarn production. The disadvantage of this approach is that the yarn must then be dried. Self‐bundling nanofibers have been achieved using AC power. The jet splits and contains both negative and positive segments which bundle together midflight. Twisting the fibers can improve yarn strength. Such twisting can be achieved by collecting on two parallel ring electrodes and rotating one of the rings. Although simple, the length of the yarn is limited using this approach (Abbasipour and Khajavi 2013).
1.4 Effect of Solution Parameters
1.4.1 Polymer Solution Properties (Molecular Weight, Concentration, Viscosity, and Elasticity)
The effect of polymer solution properties is generally more significant than process and setup parameters on electrospinning and the resultant fibers. The solution properties, namely viscosity and viscoelasticity, surface tension, and conductivity are affected by the polymer, solvent(s), and additives (e.g. salts, surfactants).
Although the electrospinning process is relatively easy to implement on a lab scale, many polymer solutions do not form uniform fibers. Issues electrospinning uniform fibers arise when the polymer solution is too dilute and is limited by polymer solubility or when the polymer chains are short or rigid. Electrospinning new materials is typically done ad hoc varying solution properties and process variables; there are no generalizable approaches to predict if a polymer/solvent system will form nanofibers when electrospun. Significant efforts have yielded useful semiempirical approaches for predicting electrospinnability, i.e. production uniform fibers.
It is commonly observed that viscosity influences electrospinning and resulting fiber properties. The viscosity is affected by the molecular weight of the polymer, polymer concentration, and solvent quality. Generally, the higher the molecular weight, the higher the viscosity of the polymer solution. Increasing the polymer concentration also increases the viscosity of the solution. A frequent observation has been that at low viscosities/polymer concentrations, the electrospinning jet breaks up into droplets, rather than stretching to form a fiber. With increasing concentration, there is a transition to beaded fibers and a second transition to uniform fibers (Figure 1.3). The ability to form uniform fibers has been frequently attributed to polymer entanglement (Andrady 2008; Ramakrishna 2005; Li and Wang 2013).
Figure 1.3 Specific viscosity as a function of polymer concentration to determine entanglement concentration for PEO of various molecular weights. Scanning electron microscopy (SEM) (PEO 600 kDa) showing the transition from beaded fibers to uniforms as the polymer concentration increases above the entanglement concentration. For neutral polymers in a good solvent, e.g. aqueous PEO, concentrations above ~2.5× the entanglement concentration form uniform fibers.
Source: Image of beaded fibers is reprinted from Fong et al. (1999). Copyright (1999), with permission from Elsevier.
To quantify the degree of entanglement required to achieve uniform fibers, semiempirical relationships have been used (Shenoy et al. 2005; McKee et al. 2004, 2006). The entanglement concentration can be determined by measuring the viscosity (zero‐shear) as a function of polymer concentration and examining the scaling relationship between the specific viscosity and concentration. Note that the specific viscosity (ηsp) accounts for the viscosity of the solvent
where η0 is the zero‐shear viscosity of the polymer solution and ηs
