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6 Co- and Terpolymers of Tetrafluoroethylene with Cyclic Monomers and Tetrafluoroethylene-based Photoresist Materials

6 Co- and Terpolymers of Tetrafluoroethylene with Cyclic Monomers and Tetrafluoroethylene-based Photoresist Materials

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Chlorotrifluoroethylene Copolymers for Energy-applied Materials



267



slightly basic reaction medium to avoid cationic polymerization of vinyl

ethers (VEs)].

The radical copolymerization of CTFE with a wide range of hydrogenated

compounds (e.g. ethylene, propylene, isobutylene, allylic, diallyldimethylammonium chloride, acrylates and styrene derivatives) and halogenated

compounds [e.g. vinyl chloride, vinylidene chloride (VDC) and including

fluorinated comonomers] has recently been reviewed,10 including radical

conventional (with comprehensive kinetics of radical copolymerization) and

controlled copolymerizations; these are not discussed further in this

chapter, although the sequential iodine transfer terpolymerization of CTFE

with VDC and VDF that was recently reported can be mentioned.12

The determination of the true molecular weights of fluoropolymers is a

challenge. Recently, we suggested an original method with the use of a

perfluoro-3-ethyl-2,4-dimethyl-3-pentyl persistent radical that releases  CF3

radicals above 90 1C (Scheme 10.1), allowing initiation of the radical

copolymerization CTFE with isobutyl vinyl ether (iBuVE) in good yields

(Scheme 10.2).13 The resulting series of poly(CTFE-alt-iBuVE) alternating

copolymers bearing CF3 end-groups was characterized by 19F NMR spectroscopy to assess the molecular weights from the integrals of the characteristic signals assigned to CF3 (À66 ppm) and CF2 (from À107 to À117 ppm)

of CTFE units. It was demonstrated that (i)  CF3 radicals preferentially attack

the methylene site of the vinyl ether monomer and then cross-propagation

occurs and (ii) the lower the PPFR concentration, the higher is the molecular

weight of the alternating copolymers obtained.13

From a fundamental point of view, the alternating nature of this copolymerization arises from the acceptor character of CTFE monomer

(e ẳ ỵ1.481.84), whereas VEs are highly donating (eE1.64).14 Additional

clues are the Q values of the two comonomers (QCTFE ¼ 0.020–0.031 and



F



CF3 F



F 3C



CF3



F 3C



F



CF3 F



F



CF3



Δ

β-scission



CF3



+



F

F 3C



F

CF3



F



CF3



Scheme 10.1



Mechanism of b-scission elimination from perfluoro-3-ethyl-2,4dimethyl-3-pentyl persistent radical for the generation of  CF3.



Scheme 10.2



Radical copolymerization of CTFE and iBuVE initiated by  CF3 to

yield a CF3–poly(CTFE-alt-iBuVE)–CF3 alternating copolymer13



268



Chapter 10



QVE ¼ 0.038). Consequently, it could be expected that comonomers showing

low Q values and hence being poorly stabilized by resonance, for which the

double bonds bear electron-donating group(s) (i.e. negative values of e), will

lead to alternating copolymers containing 50 mol% CTFE and 50 mol% VE.10

Further basic studies on the mechanism were carried out by Boutevin and

co-workers,15,16 who reported the radical cotelomerization of CTFE and

VEs in the presence of the thiol C6F13C2H4SH16 as the telogen. As expected,

this reaction led to low molecular weight derivatives including both monoadducts and a cotelomer that contained the CTFE–VE dyad. Surprisingly, the

resulting structure contrasts with that expected from the propagation of

the charge-transfer complex that was also determined by the same authors,

who suggested that the alternating polymerization mechanism involves free

monomers.16

Although the 13C, 1H and 19F NMR spectra are complex since each

CTFE–VE dyad contains two asymmetric carbon atoms, Carnevale et al.17

proposed a structural interpretation of poly(CTFE-alt-VE) alternating

copolymers. This was supported by a density functional theory (DFT) computational study to explore the diastereomeric relationships between the

single building blocks and their mutual influences along the polymer chain,

which clearly indicated chiral center inversion and not spin–spin J-coupling

interactions as the main issue causing the complexity of the spectra.

Deeper NMR characterizations of CTFE copolymers have been reported on

poly(CTFE-co-VDC) random copolymers,18 and by a new pulse sequence to

obtain 19F-detected DOSY (diffusion ordered spectroscopy) spectra,19 taking

into account the large spectral dispersion, the number and magnitude of

large couplings and the experimental temperature, providing evidence for a

heterogeneous composition composed of VDC–VDC, VDC–CTFE and a few

CTFE–CTFE dyads.



10.2.2



Kinetics of Radical Copolymerization of CTFE



Many kinetic studies of the radical copolymerization of CTFE with various

comonomers (M) have been reported and were reviewed recently.10

Figure 10.1 presents the composition curves for various copolymerizations

of CTFE with different comonomers. Actually, apart from methallyl monomers that yield random copolymers rich in CTFE,20 almost all comonomers

are more reactive than CTFE (this is mainly linked to the low QCTFE value

compared with high QM values14), except for VEs that alternate.10



10.2.3



Fluorinated Alternating Copolymers



It is well known that a copolymerization reaction is the result of the competition of four different propagation equations involving two monomers.21

As mentioned above, as CTFE is the accepting (A) monomer and vinyl

ether the donating (D) monomer, the kinetics of acceptor–donor (A–D)



Chlorotrifluoroethylene Copolymers for Energy-applied Materials



Figure 10.1



Scheme 10.3



269



Composition curves for the radical copolymerization of CTFE with various

comonomers (E ¼ ethylene, IB ¼ isobutylene, MMA ¼ methyl methacrylate, NVP ¼ N-vinylpyrrolidone, P ¼ propylene, TFE ¼ tetrafluoroethylene,

VAc ¼ vinyl acetate, VC ¼ vinyl chloride, VDC ¼ vinylidene chloride,

VDF ¼ vinylidene fluoride, VE ¼ vinyl ether and VCA ¼ vinylene carbonate).

Reproduced with permission from F. Boschet and B. Ameduri, Chem. Rev.,

2014, 114, 927–980. Copyright (2014) American Chemical Society.10



Propagation and cross-propagation equations for the radical copolymerization between the acceptor (A) and the donor (D) comonomers.17



copolymerization should lead to the determination of both reactivity ratios,

rA and rD, and the four equations can be derived as depicted in Scheme 10.3.

Ideally, this A–D copolymerization proceeds well if both comonomers do

not (or only poorly) homopolymerize but, to induce the alternation,21 both

comonomers contain specific functional groups of opposite polarity

(Scheme 10.4). This is the case with VEs that do not homopolymerize under



270



Chapter 10



Scheme 10.4



Expected radical copolymerization involving acceptor and donor

monomers from a macroradical bearing the acceptor monomeric unit.



radical initiation. The presence of both donor (VE) and withdrawing (CTFE)

groups allows the macro radical to react more or less efficiently with the

other comonomer, as displayed in Scheme 10.4.

More basic details of the mechanism of alternating copolymerization have

been reported in various articles and alternating copolymers based on CTFE

have been reviewed.10,15



10.3 CTFE Copolymers for Energy Material

Applications

CTFE copolymers have been used in many applications10 and E–CTFE

(E ¼ ethylene) copolymers (Haler ECTFE)22 and poly(CTFE-alt-VE) copolymers

have led to industrial achievements,10 the former as front and back sheets for

photovoltaic (PV) items and the latter as more than 20 year guaranteed

crosslinked outdoor paints, under the trade name Lumiflon marketed by

Asahi Glass,23 and well reviewed by Takakura23 (see Chapter 5 of Volume 2).

This section deals with specific materials devoted to energy-related

applications: polymer electrolytes for lithium ion batteries, electroactive

devices (e.g. relaxor ferroelectrics) and fuel cell membranes.



10.3.1



Polymer Electrolytes for Lithium Ion Batteries



The development of new polymer electrolytes for lithium ion batteries (LIBs)

endowed with good thermal, electrochemical and fire stabilities is still a real

challenge.24 Electrolytes based on poly(ethylene oxide) (PEO) derivatives25

display suitable conducting properties for LIBs, especially with plasticizers.

PEO forms complexes with the lithium salts and facilitates the transport of

Li1 cations while maintaining excellent interfacial stability.

Hence it was worth synthesizing novel poly(CTFE-g-oligoEO) graft

copolymers (EO ¼ ethylene oxide) and a recent strategy was adopted from

the radical copolymerization of CTFE with VE bearing oligoEO chains

(Scheme 10.5).26 First, two original VE macromonomers containing various

oligoOE chain lengths (3 and 10 EO units) were prepared by transetherification of o-hydroxyoligoEO with ethyl vinyl ether catalyzed by a palladium

complex in 70–84% yields. Then, radical copolymerization of these comonomers led to alternating poly(CTFE-alt-VE) copolymers that bore oligoOE



Chlorotrifluoroethylene Copolymers for Energy-applied Materials



271



F2

C



OH



CF



O



Cl



O

O



O



n“Pd”



O



x



O

F 2C



O



CFCl



O

Li



n



OCH3



O

n



OCH 3



H 3 CO



Scheme 10.5



Radical copolymerization of (2-oxo-1,3-dioxolan-4-yl)methyl vinyl

ether with chlorotrifluoroethylene (CTFE), initiated by tert-butyl

peroxypivalate (TBPPI).20



side-chains in satisfactory yield (65%). These novel poly(CTFE-g-oligoEO)

graft copolymers exhibited molecular weights up to 20 000 g molÀ1 and their

thermal properties were studied: their decomposition temperature was

270 1C with 10% weight loss (Td,10%) and their glass transition temperatures

varied between À42 and À36 1C.

These copolymers are of interest as polymer electrolytes in LIBs and

display room temperature conductivities ranging between 4.49 Â 10À7 and

1.45 Â 10À6 S cmÀ1 for unplasticized material (Figure 10.2).24

Using a similar strategy, our group prepared a VE bearing a cyclocarbonate

side-group that was subsequently copolymerized with CTFE (Scheme 10.6).27

The resulting copolymers should also have a certain potential for lithium ion

conduction thanks to the coordination of the Li1 with the cyclic carbonate

and thus can be useful as solid polymer electrolytes in LIBs. These copolymers

exhibit good thermal properties and chemical inertness and demonstrate

encouraging stability at both low and high electrochemical potentials.27



10.3.2



Electroactive Devices



CTFE has also been used as a precursor of electroactive polymers (piezo-,

pyro- or relaxor ferroelectrics), especially when this monomer is terpolymerized with VDF and trifluoroethylene (TrFE). These specific properties

arise from the crystalline phase transition, in which dipole moments and

lattice parameters are greatly modified, leading to significant changes in

dielectric constants and interesting electromechanical responses. In addition, high coercive fields induce relaxor ferroelectricity. Various strategies

were utilized and more recent reviews by the groups of Zhu,28–30 Wang31,32

and Zhang and Chung33 have brought significant improvements in the

understanding of their piezoelectric and electroactive properties from the

structure–property relationships. Although piezoelectricity was discovered

first on PVDF in 1969,34 followed by the well-known poly(VDF-co-TrFE)



272



Figure 10.2



Scheme 10.6



Chapter 10



Arrhenius plots of the conductivity of poly(CTFE-alt-PEOVEn) copolymerbased electrolytes. Circles mark the electrolyte containing an equal

Li1/repeating unit (r.u.) ratio and triangles represent the electrolyte

containing an equal O/Li1 ratio.20

Reproduced from ref. 20 with permission from John Wiley & Sons.

Copyright r 2012 Wiley Periodicals, Inc.



Radical copolymerization of (2-oxo-1,3-dioxolan-4-yl)methyl vinyl

ether with chlorotrifluoroethylene (CTFE), hexafluoropropylene

(HFP) and perfluoromethyl vinyl ether (PMVE) initiated by tert-butyl

peroxypivalate (TBPPI).21



copolymers,35 a more recent generation of electroactive polymers has been

of growing interest for the last decade, such as poly(VDF-ter-TrFE-ter-M)

copolymers [where M is a fluorinated monomer, especially chlorofluoroethylene

(CFE), chlorodifluoroethylene, hexafluoropropylene (HFP) or CTFE].36 The

polymer chain can be pinned between neighboring bulky comonomers. This

monomer, when inserted into the poly(VDF-co-TrFE) copolymer chain, enables the interchain distance to be enlarged but hardly rotates under



Chlorotrifluoroethylene Copolymers for Energy-applied Materials



273



relatively low electric fields because of the physical pinning effect, whereas a

poly(VDF-co-TrFE) ‘‘block’’ between the neighboring bulky monomeric units

can rotate freely and can be pulled back by the pinning sites upon removal of

the electric field. In poly(CTFE-ter-TrFE-ter-VDF) copolymers, the CTFE unit

induces several features: (i) it favors the trans-gauche conformation of VDF

chaining that imparts efficient dipole moment; (ii) it eliminates the normal

ferroelectric phase, leading to a relaxor ferroelectric material endowed with

an electromechanical strain higher than 7% and an elastic energy density of

0.7 J cmÀ3 under electrical fields up to 150 MV mÀ1;37 and (iii) it strongly

affects the strain response and the polarization hysteresis by changing the

spontaneous polarization, and also the crystallinity, the Young’s modulus,

the dielectric properties and the structural conformations.38

Two main strategies for synthesizing poly(CTFE-ter-TrFE-ter-VDF)

terpolymers have been reported. The simplest approach involves the

selective reduction of chlorine atoms in CTFE units from poly(CTFE-co-VDF)

copolymers39 as reported by Chung’s33,38 and Zhang’s40 groups, using

tributyltin hydride, or direct free radical terpolymerization of the three

fluoroalkenes can be used.31,32,40,41

In these terpolymers, the low amount of bulky Cl atoms in the CTFE units

induced some kinks (Figure 10.3), which reduced the crystalline lattice

without significantly reducing the overall crystallinity,33 as an efficient clue

to the piezoelectricity (low coercive field, satisfactory remanent polarization,

high dielectric constant and diffuse phase transition at room temperature).

In addition, the introduction of CTFE enhances the dipole mobility under

high electric fields and makes the dipole switching transition broader.31,32

The intended field of application is in electric energy storage (tactile sensors,

supercapacitors, haptics, artificial muscles, electric generators, ultrasonic

transducers and electroacoustic devices).

These terpolymers exhibit dielectric relaxation (large frequency dependence) for potential high-pulsed capacitors with low energy loss and high

energy density. Xu et al. reported similar bulky CTFE effects in these

terpolymers with decreases in both the melting and Curie temperatures.42

Wang’s group31,32 noted that these ferroelectric fluoropolymers display

high dielectric constants. Tuning the structure of the copolymers, the

authors proposed a ‘‘polymer structure–thermal and dielectric properties’’

correlation that gives insight into the parameters governing the responses



Figure 10.3



Sketch of the dipole moments in poly(CTFE-co-VDF) copolymers.

Reprinted with permission from Z. C. Zhang and T. C. Chung,

Macromolecules, 2007, 40, 783. Copyright (2007) American Chemical

Society.27



274



Chapter 10



of these electroactive materials. The many synthesized poly(CTFE-ter-TrFEter-VDF) terpolymers with different microstructures led to a wide range of

materials that display various Curie temperatures (ranging between 22 and

106 1C,31,32 while that of PVDF is 195–197 1C43) and dielectric constants at

room temperature varying from 11 to almost 50 at 1 kHz. Indeed, the highest

room temperature dielectric constant of 50 and a low dielectric loss

(tan do0.05) were obtanied31,32 for the relaxor ferroelectric poly(CTFE-terVDF-ter-TrFE) terpolymer containing 14.0, 78.8 and 7.2 mol% of CTFE, VDF

and TrFE, respectively. These values are higher than those of the terpolymers

prepared by direct radical terpolymerization of VDF with CTFE and TrFE44

(for which the dielectric constant is 37.545).

In addition, to circumvent that reduction and use of tin catalyst, Bauer’s

and Zhang’s groups40 and then Lu and co-workers31,32 studied the direct

terpolymerization of VDF with both CTFE and TrFE, the compositions of the

terpolymers and the dielectric permittivity properties and thus structure–

property relationships. For a large energy storage capacity, both high

dielectric permittivity and high electrical breakdown strength are required.

Zhao et al.39 revisited these fluorinated terpolymers and studied the effects

of molecular weight, molecular weight distribution and uniaxial stretching

on the dielectric properties over a wide range of temperatures and frequencies. Differential scanning calorimetry (DSC) thermograms and X-ray

diffraction patterns highlighted the coexistence of multiple phases in such

materials. The dielectric spectra provided evidence on the local relaxation

processes and relaxor ferroelectric behavior on the basis of dielectric loss

tangent versus temperature.39 Recently, the same group reported in depth on

the suspension terpolymerization of these three monomers, leading to

terpolymers with molecular weights of 60 000 g molÀ1.41

A similar approach was adopted by Li et al. to obtain original telechelic

poly(CTFE-co-VDF) copolymers bearing phosphonic acid end-groups (functionality B95%).46 They initiated the radical copolymerization of CTFE and

VDF using dibenzoyl peroxide that bore diethylphosphate end-groups. These

end-groups were used to induce direct coupling with zirconium oxide filler

during the preparation of nanocomposites. The choice of phosphonic endgroups versus side-groups (via a comonomer) was dictated by (i) the desire to

maintain the crystallinity and hence the ferroelectric properties and (ii) to

improve the affinity with various oxides such as TiO2, BaTiO3 and SrTiO3.

The resulting nanocomposites were fairly stable (more than 1.5 years at

room temperature) and exhibited remarkable dielectric strength with a high

energy density.

Zhu’s group29 compared the electroactive properties (displacement

versus electric field hysteresis loop) of two terpolymers based on VDF, TrFE

(in a molar ratio of B60 : 35) containing either CTFE (7.6 mol%) or CFE

(7.2 mol%). They noted different behaviors of strong or weak physical pinning

in crystals of poly(VDF-ter-TrFE-ter-CFE) and poly(VDF-ter-TrFE-ter-CTFE) terpolymers29 (Scheme 10.7). The right figure (D–E loop) show the resulting

double hysteresis loop (DHL) and relaxor ferroelectric (RFE) behaviors.



Chlorotrifluoroethylene Copolymers for Energy-applied Materials



Scheme 10.7



Schematic representation of weak versus strong physical pinning in crystals of poly(CTFE-ter-VDF-ter-TrFE) and poly(VDF-terTrFE-ter-CFE) terpolymers. The poling frequency is 10 Hz with a triangular wavefunction.

Reproduced from L. Yang et al., Novel polymer ferroelectric behavior via crystal isomorphism and the nanoconfinement

effect, Polymer, 54, 1709–1728, Copyright (2015), with permission from Elsevier.29



275



276



Chapter 10



A recent study revealed that normal ferroelectric property, a single hysteresis loop behavior and a double hysteresis loop behavior were achieved

with poly(CTFE-ter-VDF-ter-TrFE) terpolymers containing 5.4, 78.1 and

16.5 mol% of CTFE, VDF and TrFE, respectively, from post processing.41 In

addition, further efforts have been made by the same group,47 who synthesized some poly(CTFE-co-VDF)-g-PS (PS ¼ polystyrene) graft copolymers

by atom transfer radical polymerization (ATRP) of styrene from poly(CTFEco-VDF) macroinitiators for coating applications with confined electroactive

(ferroelectric) properties for dielectric capacitors. The resulting devices

displayed high energy density (10 J cmÀ3 at 600 MV mÀ1), low losses

(tan d ¼ 0.006 at 1 kHz) and a rapid discharge speed so as to provide a

reliable electrical power system. Phase segregation between the PS zones

and the crystalline PVDF lattices led to a low polarizable interface in which

the ferroelectric PVDF crystal was confined.

In conclusion, CTFE copolymers also contribute to the production of

electroactive materials (relaxor ferroelectrics for energy storage, sensors,

electrical generators and actuators). Various strategies have been reported, including two main routes: (i) from the reduction of the chlorine atoms in CTFE

units of poly(CTFE-co-VDF) copolymers that requires heavy metals (hence

leading to environmental issues) or (ii) the conventional radical terpolymerization of CTFE with VDF and TrFE. Zhu, and Wang and co-workers’ recent

comprehensive review articles28–30 have brought insights into this growing topic.



10.3.3 Fuel Cell Membranes

10.3.3.1 Introduction

As detailed in Chapter 5 in Volume 1 and Chapters 7 and 8 of this book, fuel

cell is an energy converter (exchanger) that converts, in an electrochemical

process, the energy of an oxidation–reduction to electrical energy, heat and

water as the only waste product. A fuel cell consists of a stack of various

elementary cells (420 or even440) that are composed of a membrane located

between an anode and a cathode. Fuel cell technology offers an attractive

combination of high energy conversion efficiency and a potential for large

reductions in power source emissions, including CO2.48–53 When the fuel

used is hydrogen, the device is called a hydrogen fuel cell (HFC). Polymer

electrolyte membranes (PEMs) for polymer electrolyte membrane fuel cells

(PEMFCs) are ideally suited for transportation (for automotive applications),

combined heat and power and mobile auxiliary power applications. Among

the many attractive features, the high power density, rapid start-up and high

efficiency make PEMFCs the system of choice for transport manufacturers.

Fuel cells are one of the most attractive approaches to energy conversion,

owing to their high flexibility and easy handling.48–53

Fluorinated copolymers bearing functional groups such as perfluorosulfonic acid (PFSA) have already led to commercially available products such as Nafion, Aquivion, 3MIonomer, Flemion and Fumion marketed



Chlorotrifluoroethylene Copolymers for Energy-applied Materials

Table 10.1



277



Names, structures and characteristics of the main commercially available

perfluorosulfonic acid copolymers for proton-exchange membrane fuel

cells (PEMFCs).



Structural parameters

m ¼ 1; x ¼ 5–13.5;

n ¼ 2; y ¼ 1



m ¼ 0.1; n ¼ 1–5



m ¼ 0; n ¼ 2–5;

x ¼ 1.5–14



Trade name and type



Equivalent weight Thickness/mm



DuPont

Nafion 120

Nafion 117

Nafion 115

Nafion 112



1200

1100

1100

1100



250

175

125

50



Asahi Glass

Flemion-T

Flemion-S

Flemion-R



1000

1000

1000



120

80

50



Asahi Chemicals

Aciplex-S



1000–1200



25–100



Solvay Specialty Polymers

m ¼ 0; n ¼ 2; x ¼ 3.6–10 Aquivion

800

m ¼ 0; n ¼ 4; x ¼ 4–9



3M

3M Membrane



1000



125

80–100



by DuPont, Solvay Specialty Polymers, 3M, Asahi Glass and Fumatec, respectively (Table 10.1).54 This subsection considers three main topics: CTFEcontaining copolymers for solid alkaline anion-exchange membrane fuel

cells (AAEMFCs) and proton-conducting FCs working either at room temperature or above 120 1C, which is a real challenge.



10.3.3.2



Alkaline Anion-exchange Membrane Fuel Cells

(AAEMFCs)



Many studies have been reported on alkaline anion-exchange membrane

fuel cells (AAEMFCs) from (co)polymers that bear specific functions able to

transport hydroxide ions such as ammonium, guanidinium, imidazolium,

phosphonium, pyridinium and sulfonium.55–58

AAEMFCs display several advantages, e.g. (i) they do not require precious

metal catalysts (reducing the cost per kilowatt of power) as needed for

proton-conducting FCs, (ii) they increase the electrode kinetics for fuel

(small organics) oxidation in an alkaline medium that in turn allows the easy

storage and transportation of fuels (methanol, ethanol, ethylene glycol, etc.)

and (iii) they are not involved in any metallic corrosion. The requirements

for high-performance AAEMFCs are high ionic conductivity in OHÀ media

(410À2 S cmÀ1), high chemical and thermal stabilities (80–90 1C), a barrier to

electrons, low gas and/or fuel permeability to reduce crossover, to be as

thin as possible (30–60 mm), to exhibit a high mechanical strength and low

degree of swelling, to have an excellent capability of being used in ionomer

solutions (impregnated electrodes) and to be inexpensive.



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