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11 Optical Properties: Controlled Refractive Index Polymers and Polymeric Dyes

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74



Figure 3.5



Chapter 3



Dye molecules linked to a polyphosphazene chain, with co-substituents

such as trifluoroethoxy used to control polymer morphology and

transparency.



They are organic backbone polymers with cyclic trimeric phosphazene sidegroups that bear the functional units needed for specialized electrochemical

properties. The principles involved are as follows.

Considerable effort has been directed towards the use of alkyl ethersubstituted polyphosphazenes as solid or gel polymer electrolytes.51,52

Such polymers have potential uses in rechargeable lithium metal batteries.

The first example was poly[bis(methoxyethoxyethoxy)phosphazene],

[NP(OCH2CH2OCH2CH2OCH3)2]n, known as ‘‘MEEP.’’51 Later, its analogs

with longer or branched side-chains were also shown to have the ability to

serve as solvents for salts such as lithium triflate.52 Experimental rechargeable lithium metal batteries have been constructed and tested. Conductivities in the region of 10À4 S cmÀ1 were achieved, but the introduction of

small amounts (B10%) of solvents such as propylene oxide raised the conductivity to the 10À3 S cmÀ1 region, which is appropriate for rechargeable

lithium battery applications. MEEP-type polyphosphazenes resist combustion, which is an important requirement for large battery assemblies for

aircraft or automobiles.

This work was the starting point for the design of polymers for application

as electrolytes for primary lithium seawater batteries.53–55 This type of

battery is illustrated in Figure 3.6.

While protected from water, the battery is inert. However, following

immersion in seawater, lithium ions leave the metal electrode, pass



Structural Diversity in Fluorinated Polyphosphazenes



Figure 3.6



75



A lithium seawater battery requires an amphiphilic polymer electrolyte

membrane that allows lithium ions to pass through, but prevents water

from penetrating to the metallic lithium electrode. Trifluoroethoxy and

methoxyethoxyethoxy side-groups on cyclophosphazene pendant groups

allow tuning of the membrane.



though an ion-conductive membrane and escape to the external anode,

which is immersed in the surrounding water. Thus, electric current is

generated. However, it is essential to prevent water from penetrating the

membrane to reach the metallic lithium electrode. Therefore, the

membrane must be amphiphilic, i.e. it must be permeable to lithium ions

but impermeable to water, which is a difficult challenge to overcome.

Several closely related polymers have been synthesized in our program with

the objective of producing experimental model membranes with these

properties.49–51

Thus, ring-opening metathesis chemistry was used to produce a polyoctenamer chain with phosphazene cyclic trimer side-groups, some of

which bear hydrophilic methoxyethoxyethoxy groups together with others

carrying hydrophobic fluoroalkoxy side-groups or hybrids of the two

(structures 20 and 21).53 The ratio of the two cyclophosphazene side-groups

was varied to change the relative hydrophilic or hydrophobic character.

This system prevented water ingress but suffered from low lithium ion

conductivity.



76



Chapter 3



A second series54 utilized polynorbornenes with cyclotriphosphazene sidegroups. In this case, all the side-groups were decorated with both trifluoroethoxy and methoxyethoxyethoxy groups, with the ratio being varied

according to loadings of the two groups on each phosphazene ring and

variations in the ratio of two types of phosphazene rings each with different

ratios of the two groups. Polymer blends were also examined between

polymers with all hydrophobic and all hydrophilic side-groups. Several of

these systems showed lithium ionic conductivities in the marginal region of

10À5 S cmÀ1 coupled with high hydrophobicity and are thus candidates for

specialized energy storage applications.

A third model replaced the methoxyethoxyethoxy ion conduction groups

with lithium p-carboxyphenoxy units, together with trifluoroethoxy units, to

generate single ion conductors.55 This design generated low ionic conductivities (B10À6 S cmÀ1) and moderate hydrophobicities (B801 water contact

angles).

The main conclusion from these studies is that membranes can indeed be

designed to have these seemingly contradictory properties, although more

research is clearly needed to optimize the properties for practical applications. Amphiphilic membranes have other prospective uses apart from

specialized batteries. For example, gas, liquid and ion separations can utilize

these characteristics, as can stationary phases for chromatography or

specialized metal element purifications. Phosphazenes with fluorinated

side-groups could play an important role in these applications.



3.13 Prospects for the Future

A considerable amount of research has been devoted to the synthesis and

study of phosphazene polymers in attempts to explore the scope of this field



Structural Diversity in Fluorinated Polyphosphazenes



77



and solve some pressing materials challenges. Yet some serious questions

still remain. For example, the nature of the backbone bonding in

polyphosphazenes is not fully understood. In many cases, the mechanism of

chlorine replacement in the macromolecular substitution process remains

to be elucidated, especially when two or more different organic nucleophiles

are employed. The further development of an understanding of structure–

property relationships in these polymers is crucial for the development of

the engineering and medical aspects of the field.

Hence although a broad range of polymers and properties are now accessible by the use of organofluorine and other side-groups in the phosphazene system, this is not sufficient to guarantee the further development

of this field. Unique property combinations are certainly the driving force for

advanced applications, but the utilization of these polymers in technology

depends on their availability on a larger scale than is normal under

academic laboratory conditions. Hence the chemical engineering aspects

and the development of cooperative programs with industry, national

laboratories and specialist development initiatives are crucial for the wider

development of polyorganophosphazenes.



References

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78



Chapter 3



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Structural Diversity in Fluorinated Polyphosphazenes



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CHAPTER 4



Fluoroplastics and

Fluoroelastomers –

Basic Chemistry and

High-performance Applications

MASAHIRO OHKURA* AND YOSHITOMI MORIZAWA

Asahi Glass Co., Ltd, Research Center, 1150 Hazawa-cho, Kanagawa-ku,

Yokohama-shi, Kanagawa 221-8755, Japan

*Email: masahiro-ohkura@agc.com



4.1 Properties of Fluorine and Brief History of

Fluoropolymers

Fluorine is located at the top right of the Periodic Table and has the highest

electronegativity among all elements (Table 4.1). This attribute of elemental

fluorine makes fluorination reactions highly exothermic; however, the

resulting bonds are extremely strong, with the shared pair of electrons

pulled towards the fluorine atom. This electron withdrawal by fluorine also

tends to strengthen the C–C bonds in the fluoropolymer backbone. Since the

size of the fluorine atom is slightly larger than that of the hydrogen atom and

the C–F bond length is the shortest after the C–H bond, the replacement of

hydrogen with fluorine in an organic material causes a small change in

total bulk. Thus, unlike with the other halogens, fully fluorine-replaced,

i.e. perfluorinated, polymers can be synthesized. The C–C bonds in the

backbone of perfluoropolymers are almost completely covered by the

RSC Polymer Chemistry Series No. 24

Fluorinated Polymers: Volume 2: Applications

Edited by Bruno Ameduri and Hideo Sawada

r The Royal Society of Chemistry 2017

Published by the Royal Society of Chemistry, www.rsc.org



80



Fluoroplastics and Fluoroelastomers

Table 4.1



81



Some properties of elements.



Property



H



O (OH)



F



Cl



Br



Van der Waals radius/Å (Bondi)

Electronegativity (Pauling)

Bond length of CH3–X/Å

Bond energy of CH3–X/kJ molÀ1



1.20

2.20

1.09

432



1.52

3.44

1.43

378



1.47

3.98

1.39

472



1.75

3.16

1.77

342



1.85

2.96

1.93

290



surrounding sheath of fluorine. This replacement is reflected in the unique

characteristics of fluoropolymers.1–14

Owing to the strong C–F chemical bond, fluoropolymers show outstanding

resistance to chemicals, oxidation, heat and ultraviolet light. Fluoropolymers

are long-lasting in an outdoor environment, because fluorine is more

electronegative than oxygen, although hydrocarbon compounds undergo

photooxidation reactions with oxygen in sunlight. Perfluoropolymers do not

easily combust owing to their numerous fluorine bonds.

The low atomic polarizability of fluorine, which is closely related to its

high electronegativity,15 and the strong C–F bond lead to little distortion in

the presence of an external electric field. Therefore, perfluoropolymers have

the lowest refractive index, dielectric constant and dielectric loss tangent of

all polymeric materials. The low refractive index permits optical coating

with fluoropolymer to reduce reflection on the coated surface of a substrate.

The low dielectric constant and loss tangent allow fluoropolymers to be used

as an electrical insulator.

In addition, the low atomic polarizability of fluorine brings low surface

free energy, resulting in liquid repellency and a non-adhesive surface of

fluoropolymers. Conversely, the low polarizability is also a drawback of

fluoropolymers. Because of weak intermolecular interactions, fluoropolymers are inferior to other engineering plastics as structural materials in

terms of mechanical properties, especially creep resistance and stress-crack

resistance. To improve this drawback, typical commercialized fluoropolymers have much higher molecular weight than that of conventional

polymers obtained by radical polymerization.

Various fluoroplastics and fluoroelastomers with an exceptional

combination of characteristics have been developed to provide superior

performance in the food, medical, chemical, architectural, aerospace,

automotive, electrical and electronics industries. Most of the commercially

available fluoropolymers are formed from four fluoroethylenes, i.e. tetrafluoroethylene (TFE; CF2¼CF2), chlorotrifluoroethylene (CTFE; CF2¼CClF),

vinylidene fluoride (VdF; CH2¼CF2) and vinyl fluoride (VF; CH2¼CHF).

Homopolymers of fluoroethylenes, which started to be manufactured in

the early decades of fluoropolymer history (Table 4.2), still dominate in

production output. Polytetrafluoroethylene (PTFE) occupies more than half

of fluoropolymer production, followed by poly(vinylidene fluoride) (PVdF).

In the early 1930s, researchers at IG-Farbenindustrie in Frankfurt

(Germany) studied the first polymerization of fluoroethylenes, yielding PTFE

and polychlorotrifluoroethylene (PCTFE).16 They filed the first patent



82



Chapter 4



Table 4.2



Main events of commercialization and direction of developments in each

two decades in the fluoropolymer industry.a



1940–1960



1960–1980



1980–2000



Polymerization of Processing of

Diversification of

fluoroethylenes

fluoropolymers applications

1947

1953

1961

1961



a



PTFE

PCTFE

PVF

PVdF



2000–

Environmental and

economic

sustainability



Polymerization in

1958 VdF–HFP 1985 Amorphous

scCO2

perfluoroplastics

elastomer

Aromatic fluoropolymers Direct perfluorination

1960 FEP

Fluoropolymer alloys

1969 ePTFE

PFOA-free products

1972 PFA

1973 ETFE



PTFE, polytetrafluoroethylene; PCTFE, polychlorotrifluoroethylene; PVF, poly(vinyl fluoride);

PVdF, poly(vinylidene fluoride); HFP, hexafluoropropylene; FEP, tetrafluoroethylene–

hexafluoropropylene copolymer; ePTFE, expanded PTFE; PFA, perfluoroalkoxy copolymer;

ETFE, ethylene–tetrafluoroethylene copolymer; scCO2, supercritical carbon dioxide; PFOA,

perfluorooctanoate.



application for the manufacture of fluoropolymers in 1934.17 PTFE was also

discovered independently by Plunkett at DuPont in the USA in 1938.18 His

discovery has frequently been cited as scientific serendipity. PTFE was

recognized as a material withstanding highly corrosive environments and

contributed to the process of separating isotopes of uranium hexafluoride

(UF6) for the first atomic bomb during World War II.19,20 Following the war,

the high chemical stability of PTFE enabled commercial applications to be

broadened. PVdF and other fluorinated polyethylenes were developed and

commercialized by around 1960.1,6,21–25 Unlike other melt processable

homopolymers, PTFE has a high melt viscosity and lacks the moldability

required to produce complex shapes.

To improve the moldability of PTFE while keeping its other excellent

properties, researchers copolymerized TFE with hexafluoropropylene (HFP),

perfluoro(alkyl vinyl ether) (PAVE) and ethylene to yield melt processable

fluoroplastics called FEP, PFA and ETFE, respectively. During many struggles

to process PTFE, expanded PTFE (ePTFE) was discovered in 1969.26,27

Extension of PTFE at a temperature lower than the melting-point yielded a

uniformly porous membrane with hole diameters ranging from sub-microns

to microns, applicable to water-repellent clothes and chemical-proof filters.

In this period, the main fluoroelastomers having the excellent properties of

fluoroplastics were commercialized.2,7,10

Since the 1980s, special fluoropolymers containing a ring structure

have been developed. Researchers followed other engineering plastics and

started to study fluorine-containing aromatic polymers.28,29 Meanwhile,

perfluoroplastics having an aliphatic ring structure were commercialized

and applied as optical materials, taking advantage of their amorphousness

and transparency.30,31

In recent years, demands for various applications in almost all industrial

fields have led to the diversification of fluoropolymers, e.g. copolymers based



Fluoroplastics and Fluoroelastomers



83



on conventional fluoropolymers but modified with additional comonomers,

block copolymers and graft copolymers.11,32–34 In addition, the technology

of alloys, blends and composites has been developed to combine the

properties of fluoropolymers and other materials. To tackle environmental

issues and reduce costs, new process technologies, e.g. the use of a supercritical fluid at the time of polymerization or plastic processing,35–37

monomer synthesis via direct perfluorination of organic compounds,14,38,39 and a process for products free from bioaccumulative perfluorooctanoates (PFOA),40 have also been developed. The fluoropolymer

materials described above are commercially available as various molded

articles, films, sheets, filaments, dispersions of particles, dry powders,

elastomers, etc.



4.2 Perfluoroplastics

To produce a variety of commercially available perfluoroplastics, tetrafluoroethylene (TFE) is used not only as a monomer but also as a starting

material for other monomers, e.g. hexafluoropropylene (HFP)41 and perfluoro(propyl vinyl ether) (PPVE).1,38 Perfluoroplastics are prepared by a

free-radical polymerization reaction in water. TFE must be stored and

handled with maximum caution to prevent an explosion. TFE reacts with

oxygen to form a peroxide, homopolymerizes exothermically and sometimes

undergoes a violent disproportionation reaction to afford tetrafluoromethane

(CF4) and carbon.42 HFP and PPVE copolymerize with TFE, but homopolymerize only under forcing conditions.43,44

In addition to the careful handling of TFE, pureness of the monomers and

a clean environment are required when preparing fluoropolymers, since

organic contaminants tend to cause a chain-transfer reaction during the

free-radical polymerization of fluoromonomers. A polymer chain radical

abstracts a hydrogen atom from the weak C–H bonds in organic compounds,

resulting in a low molecular weight polymer. The strong C–F bonds in the

fluoropolymer chain do not allow fluorine atoms to be abstracted during

polymerization, which prevents long-chain branching in fluoropolymers, in

contrast to many polymers such as polyethylene prepared by free-radical

polymerization. The highly pure fluoropolymers obtained do not require any

stabilizers or plasticizers to exhibit their excellent properties, so they are

widely used in the field of semiconductor manufacture.45

Typical properties of perfluoroplastics are given in Table 4.3.46

Perfluoroplastics except for PTFE are melt processable. When PFA and FEP

are processed above 350 1C, generation of corrosive gas derived from thermal

degradation is observed. Hence a corrosion-resistant material is required for

molding machines and caution regarding ventilation is also necessary.

Crystalline perfluoroplastics, e.g. PTFE, FEP and PFA, have high thermal

stability and high chemical resistance. The same as for these plastics,

amorphous perfluoroplastics have high resistance to common solvents,

acids and bases but dissolve in fluorinated solvents. This solubility allows



84



Table 4.3 Typical properties of perfluorinated and non-fluorinated plastics.a

Property



PTFE



FEP



PFA



PBVE



PE



PVC



Monomer unit



–CF2CF2–



–CF2–cyclo

(–CFOCF2CF2CF–)–CF2–



o108 (Tg)



–CH2CHCl–



327

260



–CF2CF2– and

–CF2CF(ORF)–

310

260



–CH2CH2–



Melting-point/1C

Continuous-use

temperature/1C

Tensile strength/MPa

Refractive index

Dielectric constant

Solubility



–CF2CF2– and

–CF2CF(CF3)–

275

200



131

80





80



20–35

1.35

2.1

Insoluble



20–30

1.34

2.1

Insoluble



25–35

1.34

2.1

Insoluble



40–48

1.34

2.0

Soluble in fluorinated

solvents



18–33

1.54

2.3

Soluble in

hydrocarbons at

high temperature



41–52

1.54

3.0

Soluble in

ketones,

etc.



a



PBVE, perfluoro(3-butenyl vinyl ether) cyclopolymer (see Scheme 4.6); PE, polyethylene; PVC, poly(vinyl chloride); RF, perfluoroalkyl group; Tg, glass

transition temperature; PBVE is an amorphous perfluoroplastic.



Chapter 4



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