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11 Optical Properties: Controlled Refractive Index Polymers and Polymeric Dyes
Dye molecules linked to a polyphosphazene chain, with co-substituents
such as trifluoroethoxy used to control polymer morphology and
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
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
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
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
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
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.
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Structural Diversity in Fluorinated Polyphosphazenes
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Basic Chemistry and
MASAHIRO OHKURA* AND YOSHITOMI MORIZAWA
Asahi Glass Co., Ltd, Research Center, 1150 Hazawa-cho, Kanagawa-ku,
Yokohama-shi, Kanagawa 221-8755, Japan
4.1 Properties of Fluorine and Brief History of
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
Fluoroplastics and Fluoroelastomers
Some properties of elements.
Van der Waals radius/Å (Bondi)
Bond length of CH3–X/Å
Bond energy of CH3–X/kJ molÀ1
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
Main events of commercialization and direction of developments in each
two decades in the fluoropolymer industry.a
Polymerization of Processing of
1958 VdF–HFP 1985 Amorphous
Aromatic fluoropolymers Direct perfluorination
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,
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
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
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,
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
Table 4.3 Typical properties of perfluorinated and non-fluorinated plastics.a
Soluble in fluorinated
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.