Fluoropolymeric Membranes E-Book

Zhaoliang Cui

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 519

Veröffentlichungsjahr: 2025

Beschreibung

Enables readers to gain a comprehensive understanding of fluoropolymeric membrane science and technology from a single resource

Fluoropolymeric Membranes: Fundamentals, Fabrication, and Applications comprehensively and systematically covers the basic science and technology of fluoropolymeric membranes, which have high mechanical strength and excellent chemical stability and thus have been employed for the last several decades as materials for membrane separation processes in a variety of applications.

Written by four highly qualified authors, Fluoropolymeric Membranes includes information on:

• Typical fluoropolymers like poly(vinylidene fluoride), poly(tetrafluoroethylene), and poly(ethylene chlorotrifluoroethylene)
• Structure and properties of fluoropolymeric membranes, and fabrication strategies of advanced fluoropolymeric membranes
• Fluoropolymeric membranes novel applications in chemical- and bio-separations, hybrid systems, energy, fuel cells, batteries, gas and air treatments, and more
• How fluoropolymeric membranes support common water treatment processes, such as ultrafiltration, microfiltration, MBR membrane processes, and more
• New membrane operations such as membrane distillation, membrane crystallization, membrane emulsification, and membrane contactors

Providing complete coverage on the subject, Fluoropolymeric Membranes is an essential resource for polymer chemists, membrane scientists, process engineers, materials scientists, water chemists, environmental chemists, and chemists in industry.

Leseprobe

Fluoropolymeric Membranes

Fundamentals, Fabrication, and Applications

Zhaoliang Cui, Enrico Drioli, Francesca Macedonio, and Young Moo Lee

Authors

Prof. Zhaoliang CuiNanjing Tech UniversityNo. 30 South Puzhu RoadPukou DistrictNanjing 211816China

Prof. Enrico DrioliUniversity of CalabriaInstitute on Membrane TechnologyNational Research Council of Italy (CNR‐ITM)Via P. Bucci, Cubo 17/C87036 Rende (CS)Italy

Dr. Francesca MacedonioInstitute on Membrane TechnologyNational Research Council of ItalyVia P. Bucci, Cubo 17/C87036 Rende (CS)Italy

Prof. Young Moo LeeHanyang UniversitySeoul 04763South Korea

Cover Image: © Татьяна Креминская/Adobe Stock Photos

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Print ISBN: 978‐3‐527‐34752‐0ePDF ISBN: 978‐3‐527‐82655‐1ePub ISBN: 978‐3‐527‐82656‐8oBook ISBN: 978‐3‐527‐82657‐5

This book is dedicated to Professor Enrico Drioli, a pioneer in membrane science and engineering, who sadly passed away on October 31, 2024, shortly before the publication of this book. He had trained generations of researchers around the world and had continuously envisioned new frontiers for membrane applications—from water, biomaterials, and energy to even space.

Preface

Membrane technology has witnessed remarkable growth in recent decades, with applications spanning water purification, gas separation, energy storage, and biomedical engineering. Despite the extensive literature on membrane materials and processes, relatively few publications have focused specifically on fluoropolymer membranes—a class of materials uniquely suited to meet the challenges of harsh chemical environments, elevated temperatures, and emerging clean energy systems.

Fluoropolymers such as polyvinylidene fluoride, polytetrafluoroethylene, ethylene‐chlorotrifluoroethylene, and other advanced copolymers exhibit exceptional chemical resistance, thermal stability, and tunable surface properties. These attributes render them indispensable in high‐performance membrane applications, including water treatment, battery separators, proton exchange membranes for fuel cells, and separation processes in aggressive industrial settings. As the global emphasis on sustainability and resource circularity intensifies, fluoropolymer membranes are increasingly recognized not only for their durability but also for their potential to enable the next generation of separation technologies.

This book offers a comprehensive and application‐oriented overview of fluoropolymer membranes. It opens with a foundational introduction to the chemistry and physicochemical properties of fluoropolymers (Chapter 1), followed by an in‐depth discussion of fabrication and modification techniques (Chapter 2). The subsequent chapters explore specific fluoropolymer systems in detail, including PVDF (Chapter 3), PTFE (Chapter 4), and ECTFE (Chapter 5), with particular focus on their structure–performance relationships and manufacturing strategies. Emerging fluoropolymers and hybrid materials are discussed in Chapter 6, highlighting recent advances in material innovation.

Chapters 7 through 10 shift the focus to application domains. These include water treatment (Chapter 7), membrane contactors and pressure‐driven processes (Chapter 8), and clean energy technologies, specifically fuel cells (Chapter 9) and lithium‐ion battery separators (Chapter 10). Together, these chapters illustrate how fluoropolymer membranes not only endure but often outperform alternatives in demanding operational environments.

The aim of this book is to bridge the gap between material fundamentals and practical engineering applications. It is intended to serve as a comprehensive resource for researchers, engineers, and graduate students, supporting both theoretical insights and applied developments in membrane science. By addressing both established materials and cutting‐edge advancements, we hope to provide readers with the knowledge and perspective needed to contribute meaningfully to the evolving field of high‐performance membranes.

As the global need for clean water, sustainable energy, and robust infrastructure continues to grow, the role of fluoropolymer membranes will become ever more critical. We hope this book will inspire continued innovation and interdisciplinary collaboration to fully realize the potential of these exceptional materials.

June 4, 2025

Zhaoliang CuiNanjing, ChinaEnrico DrioliRende, ItalyFrancesca MacedonioRende, ItalyYoung Moo LeeSeoul, South Korea

Acknowledgments

We express our deepest gratitude to our research team and colleagues, whose dedication and rigorous scientific inquiry have laid the foundation for much of the content presented herein. Zhaoliang Cui is particularly thankful to colleagues and PhD students from the Membrane Science & Technology Research Centre of Nanjing Tech University, especially Qian Wang, Qiuyueming Zhou, and Gongpu Wen, for their dedication and support. Francesca Macedonio expresses her heartfelt gratitude to the Institute on Membrane Technology of the National Research Council of Italy (CNR‐ITM) for fostering a stimulating and vibrant research environment. Young Moo Lee expresses sincere thanks to colleagues and former PhD students from the Membrane Laboratory of Hanyang University, particularly, Drs. Jun Tae Jung, Chuan Hu, and Sun Ju Moon, for their invaluable assistance in coordinating activities during the editing process.

A special thank you goes to Professor Enrico Drioli, founder of CNR‐ITM, an irreplaceable guide and mentor to generations in the field of membrane science and technology.

We thank the publisher for their dedication and hard work for their supporting role in realizing this ambitious project.

1Fluoropolymers for Membranes

1.1 Membrane Technology

The birth of membrane science is one of the symbols of the development of modern science. With the development of membrane science, it has spread to various fields of social production and has been applied to many fields worldwide. It will cause qualitative changes in the separation and purification processes in some industrial sectors.

In terms of environmental protection, membrane technology has been used for seawater desalination [1, 2], brackish water desalination [3–5], ultrapure water preparation, and industrial wastewater treatment [6, 7]. For example, in China, Jiangsu Province Membrane Science and Technology Research Institute uses membrane technology for the treatment of nickel and chromium electroplating wastewater, papermaking wastewater concentration, phenol removal from gas and petroleum wastewater, and municipal and factory wastewater treatment.

In chemical industry, membrane technology has been successfully employed for the separation, purification, and concentration of organic and inorganic salts, as well as for the concentration and recovery of high molecular organic materials and the purification of precious metals [8–10]. Some research institutes have used ultrafiltration membranes to concentrate and purify lignin from pulp waste in the world [11]. The membrane technology is also used to extract NaCl and NaSO4 from natural salt mines, to purify NaCl, and to concentrate Na2CO3[12].

In medical and pharmaceutical industries, membrane technology can be used not only for the separation of bacteria and viruses but also for the concentration and separation of milk, juice, and herbal preparations [13–15]. For example, membrane separation technology is considered a promising cleaner approach, along with chemical extraction, to produce ephedrine from Ephedra sinica Stapf [16]. Chlortetracycline (96%) and nitrate (99%) are removed by membrane biofilm reactors (MBfRs) [17]. At present, membrane technology has been used abroad to make artificial kidneys and artificial lungs [18].

In the field of biotechnology, membrane technology has been used in developed countries to improve enzyme and cell recovery, the development of new cell culture devices and the development of enzyme‐engineered membrane reactors, as well as the concentration or isolation of proteases, saccharification enzymes, etc. [19, 20].

Membrane technology is widely used in the food industry. It has been applied to the extraction of edible protein in soybeans, beans, and rapeseed, the removal of soy sauce, vinegar, and amino acids, and the purification of edible oils. It is also widely used in purification, concentration, and decontamination of fruit juice, fruit wine, beer, and mead [21, 22]. For example, reverse osmosis technology and ultrafiltration technology can be used to concentrate and purify jam, juice, milk, and vegetable juice and maintain their original flavor [13, 23].

Water is considered to be the world's most valuable renewable resource and an important aspect of life. The world's population tripled in the twenty‐first century and will increase by another 40–50% in the next 50 years. Due to population growth, coupled with industrialization and urbanization, the demand for freshwater is increasing rapidly. In addition, some existing freshwater resources have gradually become polluted due to human or industrial activities. In the coming decades, the problem of water scarcity worldwide will become increasingly serious. As a result, many researchers have been looking for suitable ways to obtain freshwater by purifying and reusing it to support future generations. Water purification is an important process of removing chemicals, organic and biological pollutants, and suspended solids from water to obtain satisfactory water [24, 25].

Membrane technology has dominated water purification technologies due to its low cost and high efficiency [26]. Unlike other types of membranes, fluoropolymer membranes are leading the membrane separation industry and market due to their economic and practical benefits. However, there are some limitations in its application, including chemical, mechanical, and heat resistance. Improving flux and selectivity and reducing membrane contamination are the most important problems in membrane applications [27]. In order to remove barriers and reduce problems in membrane technology, a great deal of research has been carried out to develop new materials and methods to manufacture and modify fluoropolymer membranes.

Fluoropolymer membranes are widely used in water treatment applications such as desalination, water softening, purification of industrial and municipal wastewater, production of ultra‐pure water, and in the food, chemical, and pharmaceutical industries. The membrane process has the significant advantages of simple operation, flexibility, high effectiveness, high reliability, low energy consumption, good stability, good environmental compatibility, easy control, handling, and scale‐up, and is suitable for a variety of operating conditions including temperature, pressure, and pH. However, in more serious applications, there are still unresolved problems with the application of fluoropolymer membranes. Membrane fouling, inadequate separation and retention, treatment of concentrates, membrane life, and resistance to certain chemicals are among the most important and well‐known problems associated with fluoropolymer membranes. Table 1.1 lists the representative membrane processes and requirements for membrane materials.

Table 1.1 Representative membrane processes and requirements for membrane materials.

Source: Reproduced from Cui et al. [28]/with permission of Elsevier.

Membrane process

General mechanism

Main properties

Ref.

MF/UF

Pressure‐driven liquid passes through the membrane pores

Hydrophilic used in aqueous systems, while hydrophobic used in oil systems

[

28

,

29

]

MD

Thermally driven, water vapor passes through the membrane pores

Hydrophobic, high porosity

[30]

Membrane crystallization (MCr)

Thermally driven, vapor passes through the membrane pores

Hydrophobic used for hydrophilic (aqueous) crystallizing solutions, while hydrophilic used for oleophilic solutions

[

31

,

32

]

Membrane emulsification (ME)

Pressure‐driven, contentious phase passed through the membrane pores

Hydrophobic used for producing O/W emulsions, while hydrophilic used for producing W/O emulsions

[

33

,

34

]

Osmotic distillation

Vapor pressure‐driven, vapor diffuses through the membranes

Hydrophobic typically

[35]

PV

Concentration‐driven, vapor passes through the membranes

Hydrophilic for dehydration of organic solvents or organic mixtures; hydrophobic for removal of organic solvents or volatile organic compounds (VOCs) from water; organophilic for organic/organic separation

[

36

,

37

]

Proton‐exchange membrane (PEM)

Proton transports in membranes

High proton conductivity, mechanical, chemical and thermal stability, good barrier properties for gas and methanol

[

38

,

39

]

Membrane separator for Li‐ion battery

Transport ionic charge carriers and prevent electric contact between anode and cathode electrodes

High ionic conductivity and good barrier for electron

[40]

Gas separation membrane

Pressure‐driven

High diffusivity and/or high solubility to permeate gases

[41]

Membrane gas absorption (MGA)

Concentration gradient‐driven, gas passes through the membrane pores

Hydrophobic

[

42

,

43

]

1.2 Fluoropolymers for Membranes

In the past few decades, there has been an increase in interest in the quest for innovative materials that exhibit the required characteristics for a certain application. A material with low polarizable and electronegative fluorine atoms (van der Waals radius of 1.32 Å) will have a short C—F bond with a high bond energy dissociation of about 4.85 kJ mol−1[44]. Because of their exceptional qualities, including thermal stability, chemical inertia (against solvents, oils, water, acids, and bases), low refractive index, dielectric constant, dissipation factor, and water absorption, as well as superior weather resistance, durability, and oxidation resistance, fluoropolymers are therefore good niche candidates. They thus have a wide range of high‐tech uses.

Fluoropolymers represent a significant advancement in modern high‐tech industries due to their exceptional properties, which confer high added value across various applications. Their unique combination of chemical resistance, thermal stability, and low surface energy positions them as critical materials in advanced technologies, offering substantial improvements in performance and durability in demanding environments. These polymers are applied widely in the following advanced technologies: automotive industries [45] (c. 300 g of fluoropolymers per car, and in components of fuel cells and lithium‐ion batteries (LIBs)), aerospace and aeronautics [46] (use of elastomers as seals, gaskets, O‐rings for application at extreme temperatures for tanks of liquid hydrogen, or hydrazine in boosters of space shuttles), petrochemical [47] (pipes and coatings as liners), microelectronics, chemical engineering [28] (high‐performance membranes), textile treatment, building (paints and coatings resistant to UV and graffiti, and stone protection, especially coatings of old monuments for the cultural heritage), and optics [48] (core and cladding of optical fibers). A special class of fluoropolymers can be obtained by photopolymerization, which is particularly attractive because it can be done at room temperature, has fast kinetics, and can be finished without any solvents [49]. Most fluoropolymers for membrane processes are shown in Table 1.2, and their respective chemical structures are presented in Table 1.3.

Table 1.2 Fluoropolymers for membrane process.

Source: Reproduced from Cui et al. [28]/with permission of Elsevier.

Polymer

Membrane process

PVDF homopolymer

MF, UF, MD, MCr, ME, PV

PVDF copolymer

P(VDF‐

co

‐TFE)

MF/UF, MD

P(VDF‐

co

‐HFP)

MF/UF, MD, PV, fuel cell, lithium‐ion battery

P(VDF‐

co

‐CTFE)

MF/UF, NF, MD, PV, fuel cell

PVDF‐

g

‐PSSA

Fuel cell

P(VDF‐TrFE)

Lithium‐ion battery, tissue regeneration

PTFE homopolymer

MD, MC, PV, MGA

PTFE copolymer

Perfluorosulfonic acid (PFSA)

Fuel cell, lithium‐ion battery, chlor‐alkali industry

Poly(tetrafluoroethylene‐

co

‐perfluoropropyl vinyl ether) (PFA)

Fuel cell

Poly(tetrafluoroethylene‐

co

‐hexafluoropropylene) (FEP)

MD, fuel cell

Poly(ethylene‐

alt

‐tetrafluoroethylene) (ETFE)

Fuel cell

Other fluoropolymers

Poly(ethylene chlorotrifluoroethylene) (ECTFE)

PV, MD, MC, and MF/UF

PCTFE

—

PVF

Fuel cell

PFE

Fuel cell

Hyflon

®

AD, Teflon

®

AF, Cytop

®

Gas separation, have potential in MD, MC

Table 1.3 Chemical structures of main fluoropolymers for membranes.

Source: Reproduced from Cui et al. [28]/with permission of Elsevier.

Polymer

Chemical structure

PVDF homopolymer

PVDF

PVDF copolymer

P(VDF‐

co

‐TFE)

P(VDF‐

co

‐HFP)

P(VDF‐

co

‐CTFE)

PVDF‐

g

‐PSSA

P(VDF‐TrFE)

Poly(tetrafluoroethylene) homopolymer

PTFE

PTFE copolymer

Perfluorosulfonic acid (PFSA)

Poly(tetrafluoroethylene‐

co

‐perfluoropropyl vinyl ether) (PFA)

Poly(tetrafluoroethylene‐

co

‐hexafluoropropylene) (FEP)

Poly(ethylene‐

alt

‐tetrafluoroethylene) (ETFE)

ECTFE

Poly(ethylene chlorotrifluoroethylene) (ECTFE)

Other fluoropolymers

PCTFE

PVF

Cytop

Hyflon AD

1.3 PVDF and Its Copolymer

Polyvinylidene fluoride (PVDF) is widely used in the membrane industry because of its excellent properties such as high mechanical resistance, thermal resistance, chemical resistance, and relatively high hydrophobicity. PVDF membranes are used for ultrafiltration and microfiltration, membrane contactors such as membrane distillation (MD), membrane crystallization (MCr), and membrane condensation [50–57].

Table 1.4 lists typical polymer materials and their critical surface tensions. PVDF remains the best choice for membrane contactors because PP and PTFE membranes cannot easily be fabricated by traditional phase conversion process [52].

Table 1.4 Critical surface tension membranes of major polymeric membrane materials.

Polymer

Critical surface tension (dynes/cm)

Polyvinylidene fluoride (PVDF)

25–28.5

Polyfluoroethylene (PFE)

22

Polytetrafluoroethylene (PTFE)

18.5

Fluorinated ethylene propylene (FEP)

16

Ethylentetrafluoroethylene (ETFE)

17

Polyacrylonitrile (PAN)

44

Polysulfone (PS)

41

Polyphenylene oxide (PPO)

41

Polyethylene (PE)

31

Polypropylene (PP)

29

PVDF is superior to other membrane materials due to its high mechanical strength and excellent chemical resistance, making it suitable for water treatment. Due to its low extractability, PVDF is also suitable for biomedical and bioseparation processes. PVDF also exhibits thermodynamic compatibility with other polymers such as polymethyl methacrylate (PMMA), polyvinylpyrrolidone (PVP), and polyethylene glycol (PEG) in a wide range of blended compositions, which can be used to manufacture membranes with desired properties [58–60]. PVDF can be further chemically modified to obtain some specific properties [59, 60], and can be cross‐linked by electron beam radiation or gamma radiation [61, 62].

Beside homopolymeric PVDF, PVDF copolymers also can be used as membrane materials, such as poly(vinylidenefluoride‐co‐hexafluoropropylene) (P(VDF‐HFP)), poly(vinylidene difluoride‐co‐chlorotrifluoroethylene) (P(VDF‐CTFE)), poly(vinylidene fluoride‐co‐tetrafluoroethylene) (P(VDF‐co‐TFE)), poly(vinylidene fluoride‐trifluoroethylene) (P(VDF‐TrFE)), and poly(vinylidene fluoride)‐graft‐poly‐(styrene sulfonic acid) (PVDF‐g‐PSSA). The properties of PVDF and PVDF copolymers will be introduced in the following.

1.3.1 Homopolymeric PVDF

PVDF is a kind of semicrystalline macromolecule, which contains 59.4 wt% fluorine and 3 wt% hydrogen [62, 63] (chemical structure is shown in Table 1.3). Commercial PVDF is generally produced by polymerization in emulsion or suspension using free radical initiators, forming a repeating unit of –CH2–CF2–. PVDF presents a crystallinity of 35–70%, depending on their process condition and heat history. A higher crystallinity endows PVDF with higher mechanical properties, such as higher stiffness, toughness, and creep resistance. Like most of polymers, molecular weight, molecular weight distribution, extent of irregularities along the long polymer chain, crystallinity, and crystalline form are the major factors for PVDF properties.

PVDF exists at least five crystalline phases as reported. The crystal polymorphs are named α, β, γ, δ, and ε. α‐, β‐, and γ‐phases are the most frequent PVDF phases [24] (Figure 1.1). The α‐ and δ‐phases display the same TGTG′ conformation. The β‐phase displays a TTT structure. Additionally, the ζ‐ and γ‐phases exhibit the same TTTGTTTG′ conformation. The most thermodynamically stable form is the β‐form, although the α‐form is kinetically advantageous.

Figure 1.1 Schematic representation of the chain conformation for the α, β, and γ phases of PVDF. The gray, white, and yellow spheres represent carbon, hydrogen, and fluorine atoms, respectively.

Source: Reproduced from Wang et al. [64]/with permission of Elsevier.

Table 1.5 provides an overview of the PVDF's characteristics throughout its several polymorphs. The slightly greater van der Waals radius of the fluorine atom (1.35 Å) compared to the hydrogen atom (1.20 Å) is directly related to the polymorphism of PVDF. In fact, for synthetic homopolymers, PVDF hardly ever has these five types of crystalline phases [67].

Table 1.5 Properties of PVDF with different polymorphs [65, 66].

α‐phase

β‐phase

γ‐phase

δ‐phase

ε‐phase

Crystal system

Monoclinic

Orthorhombic

Monoclinic

—

—

Polarity

Nonpolar

Polar

Polar

Polar

—

Lattice constants

a

 = 4.96 Å

b

 = 9.64 Å

c

 = 4.62 Å

a

 = 8.58 Å

b

 = 4.91 Å

c

 = 2.56 Å

a

 = 4.97 Å

b

 = 9.66 Å

c

 = 9.18 Å

a

 = 4.96 Å

b

 = 9.64 Å

c

 = 4.62 Å

a

 = 4.97 Å

b

 = 9.66 Å

c

 = 9.18 Å

Number of chains per lattice

2

2

2

—

—

Molecular conformation

TGTG′

TTT

TTTGTTTG′

TGTG′

TTTGTTTG′

Density, Observed at 30 °C (g ml

−1

)

1.77

1.81

1.80

—

—

FTIR peak (cm

−1

)

408, 532, 612, 766, 795, 855, 976, 1182, 1400

445, 470, 511, 600, 840, 1270

431, 512, 776, 795, 812, 833, 840, 1233, 18.5, 19.2, 20.4

—

—

Peak of 2

θ

of X‐ray diffraction (°)

17.66, 18.30, 19.90, 26.56

20.26

18.50, 19.20, 20.04

—

—

The α‐phase PVDF is non‐polar. On both sides of the C main chain, hydrogen and fluorine atoms alternate regularly to form a helix‐like shape (Figure 1.1a). The conformation is zigzag in the β‐phase. The piezo‐, pyrro‐, and ferroelectric characteristics of PVDF are derived from the regular organization of the C–F strong dipole along the polymer chain [67]. The intermediate, polar conformation of the γ‐phase is TTTGTTTG′ [68]. These different crystalline phase structures have already been characterized [67, 69].

PVDF is resistant to a wide range of oils, solvents, and acids. The amorphous and crystalline PVDF areas have glass transition (Tg) and melting temperatures (Tm) between −40–−30 °C and 155–192 °C, respectively. The density of amorphous PVDF is 1.68 g cm−3. The densities of the α, γ, and β polymorphs are 1.92, 1.93, and 1.97 g cm−3, in that order. PVDF typically has a density of 1.75–1.78 g cm−3 and a crystallinity degree of about 40%. At 230 °C and 1.0 bar, the melt density of a PVDF homopolymer is approximately 1.45–1.48 g cm−3[70]. A list of some physical and mechanical properties is shown in Table 1.6.

Table 1.6 Physical and mechanical properties of PVDF.

Source: Adapted from Saxena and Shukla [70].

Property (standard)

PVDF

Criterion

Color

White

—

Density (g cm

−3

)

1.78

—

Melting point (°C)

155–192

ASTM D‐3418

Glass transition temperature (°C)

−40 to −30

—

Heat deflection temperature (0.5 MPa)

148

—

Relative density (g cm

−3

)

1.76–1.80

ASTM D‐792 (Solid)

Tensile stress at 23 °C (MPa)

35–55

ASTM D‐638

Elongation at 23 °C (%)

25–500

ASTM D‐638

Young's modulus at 23 °C (MPa)

1340–2000

ASTM D‐638

Izod impact strength at 23 °C (J m

−1

)

160–530

ASTM D‐256

Thermal expansion coefficient

∼10

−4

ASTM D‐696

Processing temp. range (°C)

200–300

–

Relative permittivity (1 kHz)

7.5–13.2

ASTM D‐150

Dielectric strength (kV mm

−1

)

260–950

ASTM D‐149

Dissipation factor (1 kHz)

0.0163–0.019

ASTM D‐150

LOI (%)

44

ASTM D‐2863

PVDF's solubility is governed by “like‐dissolve‐like” principle. PVDF can dissolve in very few polar aprotic solvents, such as dimethylsulfoxide (DMSO), dimethylacetamide (DMAc), dimethylformamide (DMF), and N‐methylpyrrolidone (NMP).

PVDF has been recognized as one of the most excellent chemical‐resistant materials [71]. Although PVDF has strong corrosion resistance to chemicals such as halogens, oxidants, and inorganic acids, as well as aliphatic, aromatic, and chlorinated solvents, it is still incompatible with strong bases and many ketones [60]. Many investigations have fundamentally confirmed that PVDF membrane materials were susceptible to alkaline conditions [72–76]. PVDF membrane will change from white to brown, and then to black after exposure in the NaOH solution [72, 73], thus becoming brittle [72, 73, 77, 78]. This is interpreted as a dehydro‐fluorination reaction, which removes hydrogen fluoride (HF) units from PVDF chain, thus forming C=C bond [71, 74, 78, 80]. Scheme 1.1 shows the alkaline degradation process of PVDF.

Scheme 1.1 Diagram of the alkaline degradation mechanism of PVDF.

Source: Reproduced from Zhang et al. [71]/with permission of Elsevier.

According to Hashim et al., the mechanical strength and crystallinity of PVDF decreased as a result of the interaction between PVDF and NaOH, which was activated even at low NaOH concentrations. The reaction also became more intense as the treatment duration was prolonged. The reaction was expedited and enhanced as the sodium hydroxide concentration and treatment temperature were increased. PVDF membranes are more vulnerable to the effects of NaOH solutions than KOH solutions, according to Rabuni et al. [79–82]. A list of stability of PVDF membrane materials in harsh caustic environments is shown in Tables 1.7 and 1.8.

Table 1.7 Performance of PVDF resistance to organic solvents.

Source: Adapted from Saxena and Shukla [70].

Chemical solvent

20 °C

50 °C

Dimethyl formamide (DMF)

Not resistant

Not resistant

Dimethyl sulfoxide (DMSO)

Not resistant

Not resistant

Dimethylacetamide (DMAc)

Not resistant

Not resistant

N

‐Methylpyrrolidone (NMP)

Not resistant

Not resistant

Acetone

Not resistant

Not resistant

Tetrahydrofuran (THF)

Limited resistant

Limited resistant

Benzene

Resistant

Limited resistant

Chlorobenzene

Resistant

Resistant

Chloroform

Resistant

Resistant

Ethyl acetate

Resistant

Limited resistant

Cyclohexane

Resistant

Resistant

Table 1.8 Performance of PVDF resistance acids and alkali.

Source: Adapted from Saxena and Shukla [70].

Chemical solvent

Mass/molar concentration

20 °C, 30 days

Acetic acid

10%

Resistant

Acetic acid

100%

Resistant

Formic acid

(10%)

Resistant

Hydrochloric acid

Resistant

Hydrogen peroxide

(90%)

Resistant

Nitric acid

(10%)

Resistant

Sulfuric acid

(10%)

Resistant

Sulfuric acid

(90%)

Resistant

Sulfuric acid

fuming/monohydrate

Not resistant

NaOH

0.01 M (pH = 12)

Not resistant

KOH

0.01 M (pH = 12)

Not resistant

CaOH

0.01 M (pH = 12)

Not resistant

MgOH

0.01 M (pH = 12)

Not resistant

LiOH

0.01 M (pH = 12)

Not resistant

NH

3

·H

2

O

0.01 M (pH = 12)

Not resistant

1.3.2 Poly(Vinylidene Fluoride‐co‐Tetrafluoroethylene) (P(VDF‐co‐TFE))

Poly(vinylidene fluoride‐co‐tetrafluoroethylene) (P(VDF‐co‐TFE)) is produced by free radical copolymerization of two monomers, VDF and TFE. Scheme 1.2 shows the reaction path of the copolymerization process. The reaction path of the copolymerization process is depicted in Scheme 1.2. VDF or TFE monomers can be attacked by the initiator. In the case of VDF monomer, there are two alternative beginning points: the CH2 end or the CF2 end. The attack on the CF2 carbon results in the formation of an HOCF2CH2– end, which is unstable and can undergo HF elimination and hydrolysis to yield HOOCCH2– chain ends. Attack on the CH2 carbon is favored during propagation, such that the free radical is concentrated on the CF2 carbon. This method of chain propagation, known as normal addition, is preferred for steric and electronic reasons. In comparison, inverse addition, which forms head‐to‐head structures but is not common, occurs in around 1–6% of monomer additions and is affected by reaction parameters such as temperature. The precise structure of the chain and branch end will be determined by these types of monomer addition.

Scheme 1.2 Reaction pathways for copolymerization of VDF and TFE via free radical polymerization.

Source: Reproduced from Li et al. [83]/with permission of American Chemical Society.

As indicated in Scheme 1.2, chain branches can be formed via intramolecular or intermolecular chain transfer reactions. Intramolecular chain transfer reactions, also known as backbiting reactions, can occur and result in the formation of short‐chain branches. Because of the lower bond strength of the C—H bond, the assault on CH2 carbons is preferred over the attack on CF2 carbons, as illustrated in Scheme 1.2.

P(VDF‐co‐TFE) has an excellent mechanical character and can be dissolved in common solvents, indicating the possibility of fabricating microporous membranes using the phase inversion method. The hydrophobicity results in membrane capacity, which can be exploited in the MD process. The phase inversion procedure was used to create flat‐sheet microporous for MD [84]. Figure 1.2 depicts the P(VDF‐co‐TFE) membrane morphologies. A cross‐section of the P(VDF‐co‐TFE) membrane from LiCl revealed a finger‐like structure with a length of several microns reaching up to the membrane surface. P(VDF‐co‐TFE) membranes outperform PVDF membranes in terms of mechanical performance and hydrophobicity. In the MD process, the membrane was successfully used to the MD process and retained higher hydrophobicity than PVDF membrane.

Figure 1.2 Microphotographs taken with the SEM method of a P(VDF‐co‐TFE) membrane prepared from 12 wt% polymer solutions with an additive of 4.6 wt%. (1) LiCl, (2) LiClO4·3H2O; T: top surface, B: bottom surface, C: cross‐section.

Source: Feng et al. [84]/with permission of Elsevier.

P(VDF‐co‐TFE) has recently been used to create a range of new polymer inclusion membranes (PIMs) for Cr(VI) transport. The membranes demonstrated a high permeability coefficient and improved selectivity for Cr(VI) transport when bifunctional ionic liquid extractants were used as carriers in conjunction with an ionic liquid plasticizer [85]. They could be utilized to eliminate metal ions and contribute to environmental conservation.

In a different context, the utilization of P(VDF‐TFE) and polyethylene terephthalate (PET) copolymers involves the combined effects of co‐extrusion and biaxially oriented forced assembly of nanolayers. This process is employed to fabricate polymer multilayer films, which exhibit high energy density and can function as polymer film capacitors with significant potential, as depicted in Figure 1.3. The aforementioned films have a maximum breakdown field of 1000 kV mm−1 when subjected to a divergent field employing a needle/planar electrode configuration. The energy density of materials subjected to homogeneous electric fields, as measured using planar/planar electrodes, can reach up to 16 J cm−3. The observed breakdown characteristics and the extent of damage are shown to be associated with the limited morphology of PET and P(VDF‐TFE). The dielectric constant of the effective P(VDF‐TFE) layer is increased by the solid biaxial stretching of the edge P(VDF‐TFE) crystals, resulting in an enhanced dielectric contrast between the PET and P(VDF‐TFE) layers. The aforementioned phenomenon results in the accrual of supplementary charges at the interface of the layers, consequently resulting in the expansion of tree routes and branches, finally augmenting breakdown and energy storage properties. Furthermore, an assessment was conducted on the hysteresis features of these materials, in addition to their energy storage and breakdown characteristics. The observed hysteresis behavior can be altered by modifying the morphology of the P(VDF‐TFE) layer, which in turn allows for control over the low‐field dielectric loss (or ion migration behavior) [86].

Figure 1.3 Structures obtained in the PET/P(VDF‐TFE) multilayer system through (i) multilayer coextrusion and (ii) biaxial stretching of thick PET/P(VDF‐TFE) multilayer films at high draw ratios. The wide‐angle X‐ray scattering images were taken through the extrusion direction of the multi‐layer films. The arrows in the atomic force microscopy images indicate the P(VDF‐TFE) layers.

Source: Carr et al. [86]./with permission of John Wiley & Sons.

1.3.3 Poly(Vinylidene Fluoride‐co‐Hexafluoropropene) (P(VDF‐co‐HFP))

P(VDF‐HFP) (chemical structure is shown in Table 1.3) is an inert fluoropolymer that has lower crystallinity and good mechanical strength than PVDF, can withstand high temperatures, and has strong hydrophobicity. P(VDF‐HFP) can be produced from vinylidene fluoride (VDF) and hexafluoropropylene (HFP) by emulsion polymerization as shown in Scheme 1.3.

Scheme 1.3 Synthetic schemes of P(VDF‐HFP) copolymer.

The molecular chain structure of P(VDF‐HFP) is similar to that of PVDF. Relatively speaking, the crystallinity of P(VDF‐HFP) is lower than that of PVDF, so its piezoelectric performance is worse than that of PVDF, but its flexibility is much better than that of PVDF. Due to the introduction of hexafluoroethylene, which increases the F content, it is also more hydrophobic than PVDF. In addition, the gelability of PVDF‐HFP is better than that of PVDF, and it is easier to cast into a membrane. In addition, we can regulate the crystallization behavior and crystallinity of polymers by adjusting the ratio of VDF and HFP monomers during copolymerization. When the molar percentage of HFP is 5–15%, a flexible fluoropolymer material with low crystallinity can be obtained. If the molar percentage of HFP is further increased to more than 19%, HFP fragments with larger steric hindrance will completely obstruct the regular arrangement of VDF fragments and obtain completely amorphous fluoroelastomers [87]. In general, increasing the proportion of HFP fragments decreases the crystallinity of the polymer.

In terms of piezoelectric materials, P(VDF‐HFP) is an excellent dielectric material with a high dielectric constant, good physical properties, stable chemical properties, strong solvent resistance, excellent aging resistance, and UV resistance as well as good processability. Meanwhile, P(VDF‐HFP) is a semi‐crystalline thermoplastic polymer, which contains various crystal phases, such as α, β, and γ crystal phases. The β crystal phase is considered to be the main contributor to the piezoelectric properties of P(VDF‐HFP). The film process promotes the transition from the α crystal phase to the β crystal phase, thereby improving the dielectric properties of the material. Therefore, P(VDF‐HFP) is a feasible choice for piezoelectric materials. The addition of nanoparticles and other methods can also significantly improve the content of β crystal phase inside, thereby improving piezoelectric performance.

For the lithium battery, the existing polypropylene, polyethylene (PE), and other lithium battery isolation film have the shortcomings of low porosity, low liquid absorption rate, poor thermal stability, and cannot meet the requirements of high rate charge and discharge. However, P(VDF‐HFP) has become a hot material for isolation films because of its high dielectric constant, good heat resistance, chemical resistance, and electrochemical properties. In terms of adhesive application, the fluorinated adhesive prepared by chemically modified PVDF‐HFP grafted with maleic anhydride can effectively bond PTFE to metal.

1.3.4 Poly(Vinylidene Fluoride‐co‐Chlorotrifluoroethylene) P(VDF‐co‐CTFE)

P(VDF‐CTFE) (chemical structure is shown in Table 1.3) is another common PVDF‐based copolymer, similar to P(VDF‐HFP), which has high mechanical strength and good chemical and thermal stability. Its polymerization scheme is shown in Scheme 1.4.

Scheme 1.4 Synthetic schemes of P(VDF‐CTFE) copolymer.

The polymerization of CTFE and VDF is random copolymerization. The crystallinity of P(VDF‐co‐CTFE) copolymer is significantly lower than that of CTFE homopolymer. Its glass transition temperature is −40 °C (Tg of PVDF) to 45 °C (Tg of PCTFE). And according to the proportion of VDF added in the polymerization process, the properties of the final copolymer are different. When the P(VDF‐co‐CTFE) copolymer containing a small amount of VDF is a semi‐crystalline copolymer with a hexagonal structure, when the molar fraction of VDF is 25–70%, the prepared copolymer is an amorphous polymer with elastomer properties as the main component [65]. A VDF percentage above 70% results in a thermoplastic with a monoclinic crystal structure. These copolymers are called flexible PVDF. The type of P(VDF‐co‐CTFE) copolymer resin can be divided according to the molar ratio of CTFE to VDF in the polymer structure.

Additionally, PVDF‐CTFE's CTFE segment makes it simple to graft using atom transfer radical polymerization (ATRP) without sacrificing its chemical, mechanical or thermal stability. A PVDF‐CTFE‐g‐PSSA composite NF membrane, for instance, was created [88]. It is possible to directly modify P(VDF‐co‐CTFE) membranes by using ATRP polymerization [89] (Scheme 1.5).

Scheme 1.5 Schematic illustration of the processes for the modification of P(VDF‐co‐CTFE) by PEGMA via ATRP.

Source: Liu et al. [89]/with permission of Elsevier.

1.3.5 Poly(Vinylidene Fluoride)‐g‐Poly(Styrene Sulfonic Acid) PVDF‐g‐PSSA

PVDF‐g‐PSSA is a graft copolymer of PVDF with high conductivity and good structural properties. It is widely used in the preparation of proton exchange membranes, and proton exchange membranes can be used in microbial fuel cells (MFCs) and direct methanol fuel cells (DMFCs). Proton exchange membranes are an important part of MFC and DMFC. In other words, the proton exchange membrane is an indispensable heart for MFC and DMFC.

Figure 1.4 is the schematic representation of the current method of single‐step reaction to prepare sulfonic acid membrane (PVDF‐g‐PSSA) and two‐step method [90]. There are various synthesis methods for PVDF‐g‐PSSA. The reported synthetic methods include radiation grafting, which can be divided into co‐irradiation and pre‐irradiation, BPO‐initiated grafting and ATRP grafting. The most common synthesis method is radiation grafting.

Figure 1.4 A schematic representation of the current method of single‐step reaction to prepare sulfonic acid membrane (PVDF‐g‐PSSA) and two‐step method.

Source: Reproduced from Nasef et al. [90]/with permission of Elsevier.

PVDF‐g‐PSSA is widely used in vanadium redox flow batteries (VRBs) (Schematic illustration of a VRB is shown in Figure 1.5) [91]. The PVDF‐g‐PSSA membrane prepared by solution grafting has a high conductivity of 3.22 × 10−2 S cm−1 at 30 °C. ICP studies show that compared with Nafion 117, the vanadium ion permeability of PVDF‐g‐PSSA membrane is greatly reduced. Of all these membranes, pentavalent vanadium ions have the lowest permeability and trivalent vanadium ions the highest. With a low‐cost PVDF‐g‐PSSA membrane, VRB outperforms Nafion 117 under the same operating conditions, and its energy efficiency reaches 75.8% at 30 mA cm−2. After more than 200 cycles, the VRB with PVDF‐g‐PSSA membrane can continue to function at a current density of 60 mA cm−2.

Figure 1.5 Schematic illustration of a vanadium redox flow battery.

Source: Reproduced from Xuanli Luo et al. [91]./with permission of American Chemical Society.

1.3.6 Poly(Vinylidene Fluoride‐Trifluoroethylene) (P(VDF‐TrFE))

VDF can be copolymerized with trifluoroethylene (TrFE) [92, 93] in various proportions to form random semi‐crystalline thermoplastic copolymers. In contrast to PVDF, which requires mechanical stretching or poling to create net dipoles (β‐phase) in the material, P(VDF‐TrFE) can form a crystal structure with dipoles that permanently polarize the polymer without the need for these treatments [94]. It could be a useful starting material for tissue engineering applications, modifying cell behavior, and cell proliferation in a three‐dimensional matrix [94].

P(VDF‐TrFE) microporous membrane separators for LIBs can be prepared using the solvent‐cast approach (chemical structure described in Table 1.3) [95], and Figure 1.6 shows porous P(VDF‐TrFE) structures obtained by solvent evaporation at room temperature [96]. Additionally, electrospinning can produce P(VDF‐TrFE) copolymer membranes with low dielectric constant and good flexibility [97].

Figure 1.6 SEM microphotographs of the surface and cross‐section (a, b) of 5/95 P(VDF‐TrFE)/DMF and (c, d) 20/80 P(VDF‐TrFE)/DMF samples obtained by solvent evaporation at room temperature.

Source: California et al. [96]/with permission of Elsevier.

In a tissue engineering application known as directed tissue regeneration, a membrane is crucial to isolating periodontal abnormalities of the gingival connective and epithelial tissues and achieving the regeneration of bone, periodontal ligament, and cementum from their own cells. An excellent material should have acceptable electromechanical capabilities as well as biocompatibility to promote periodontal tissue regeneration. In vitro biocompatibility of a composite membrane of poly(vinylidene‐trifluoroethylene)/barium titanate (P(VDF‐TrFE)/BT was superior to that of ordinary expanded PTFE (ePTFE) [98].

1.4 PTFE and Its Copolymer

Porous polytetrafluoroethylene (PTFE) membranes exhibit chemical inertness and have found extensive applications in many membrane separation processes, such as MD, oil–water separation, and gas–solid separation. The prevalent types of Teflon membranes are plates and hollow fibers. PTFE membranes are primarily manufactured using drawing, spinning, and pore‐forming techniques. To enhance the performance of PTFE membranes and achieve enhanced results in the target application, several modification techniques were employed. These techniques included wet chemistry, plasma treatment, radiation exposure, atomic layer deposition, and high‐temperature melting. Table 1.9 presents the chemical structures of homopolymeric and copolymer PTFE.

Table 1.9 Chemical structures of homopolymeric and copolymer PTFE.

Polymer

Chemical structure

Poly(tetrafluoroethylene) homopolymer

PTFE

PTFE copolymer

Perfluorosulfonic acid (PFSA)

Poly(tetrafluoroethylene‐

co

‐perfluoropropyl vinyl ether) (PFA)

Poly(tetrafluoroethylene‐

co

‐hexafluoropropylene) (FEP)

Poly(ethylene‐

alt

‐tetrafluoroethylene) (ETFE)

1.4.1 Homopolymeric PTFE

PTFE (chemical structure is shown in Table 1.9) is a perfluoropolymer material in which all hydrogen atoms in PE are replaced with fluorine atoms. The structural formula is: –[CF2–CF2]n–. PTFE is a white, hydrophobic solid whose properties depend strongly on its molecular weight. PTFE surface free energy is very low and almost does not adhere to any substance, due to its strong C—C and C—F bonds and carbon skeleton, has excellent high‐temperature resistance, chemical resistance, environmental resistance, electrical insulation, oxidation resistance, strong hydrophobicity, and high fracture toughness, which is protected by a uniform spiral sheath formed by the electron cloud of fluorine atoms [28]. Table 1.10 lists the salient properties of PTFE.

Table 1.10 Physical and chemical properties of PTFE.

Source: Reproduced from Puts et al. [99]/with permission of American Chemical Society.

Property (standard)

PTFE

Criterion

as‐polymerized PTFE

335

Melting point (°C)

D3418

Glass transition (°C)

−103

—

Decomposition point (°C)

590

—

Phase transition (°C)

19

—

Processed PTFE

Theoretical density/g cm

−3

(at 23 °C)

2.16

ASTM D4895

Tensile strength/MPa (at 23 °C)

31

ASTM D4894

Compressive strength/MPa (at 23 °C)

4.4

ASTM D695

Hardness/shore D

55

ASTM D2240

These characteristics make it suitable for a variety of applications, such as exhaust‐gas treatment [100], MD [101, 102], osmotic distillation (OD) [103, 104], and oil–water separation [105].

Specific membrane qualities are required depending on the application. Different membrane applications can benefit from a custom‐made PTFE membrane. The most essential properties of porous membranes in gas–solid separation are solid phase rejection and gas permeability. A high pore size of about 5 μm, a narrow pore size distribution, and a thickness of less than 50 μm are necessary for effective membrane separation performance. Furthermore, for gas–solid separation, PTFE membranes with high porosity and specific surface area are required [106]. PTFE nanofibers are usually prepared by biaxial stretching and electrospinning to achieve a higher specific surface area, allowing for a larger contact area between particles and fibers while maintaining adequate particle retention and breathability. This is done to meet the requirements of the gas–solid separation process. MD is a thermally driven technique for separating molecules across hydrophobic membranes by taking advantage of the temperature differential. The mass transfer coefficient and heat transfer coefficient are the two main factors that determine how effective the MD process is [107]. The structure and chemical characteristics of the membrane influence mass and heat transfer optimization in MD. Because of its low thermal conductivity and hydrophobicity, PTFE membrane is employed in the MD process to minimize heat loss. Furthermore, PTFE has a pore structure suitable for MD and a low inlet pressure. The pore diameters of PTFE membranes employed in MD are usually around 0.5 μm. Additionally, the membrane in the oil–water separation process needs to be either oleophobic (also oleophilic and hydrophobic) or hydrophobic (also oleophilic and hydrophobic) [108–110], depending on the water–oil supply (oil‐in‐water emulsion or water‐in‐oil emulsion).

The biaxial stretching method used to prepare PTFE membranes was originally developed by Stein [111] and has been used for several years in the preparation of porous PTFE membranes. In the following decades, the biaxial stretching method was adopted and modified by many researchers. Bukchon, etc. [112] prepared porous PTFE membrane by mechanical operation, and the formation mechanism of the porous structure in PTFE was proposed. The fibril is formed in the crack by tensile action and oriented in the direction of tensile action. The spatial unit size of the periodic structure depends on the amount of PTFE, the average molecular weight, and the stretching conditions. The schematic of the various stages of the PTFE stretching process and the preparation procedure for the porous PTFE membrane [113] are shown in Figure 1.7.

Figure 1.7 (a) Preparation process of porous PTFE membrane [113] and (b) schematic of PTFE stretching process in different stages: (i) raw PTFE sheet, (ii) early‐stage stretching or strip crack process, (iii) node‐forming stage or uniaxial tension process, and (iv) uniform node connection or biaxial tension process.

Source: Wikol et al. / W. L. Gore & Associates, Inc.

Due to its perfect fiber shape and great productivity, electrospinning has garnered a lot of attention as a process for creating nano‐/submicron fibers. After the polymer fluid is exposed to a high‐voltage electric field via a micro‐nozzle, it hardens into a fiber membrane. Since PTFE has a high viscoelasticity, spinning molten PTFE into fibrils is challenging [114]. Consequently, in order to facilitate the electrospinning process of creating PTFE membranes, additives are added to the PTFE emulsion. Xiong et al. blended various quantities of poly(vinylalcohol) (PVA) into a PTFE emulsion in order to electrospun a porous PTFE membrane [115]. The PVA mass ratio and emulsion concentration both have significant impacts on the membrane shape. Electrospun composite fibers with varying PVA to PTFE mass ratios are shown in Figure 1.8 as scanning electron microscopy (SEM) images: (i) 10 : 90, (ii) 20 : 80, (iii) 30 : 70, (iv) 40 : 60, and (v) 50 : 50. The surfactant tends to be more stable at high emulsion concentrations, which causes the fiber's diameter to increase. The fibers are comparatively uniform when the PVA mass ratio exceeds 3 : 7.

Figure 1.8