Polyphosphazenes elastomers

Figure 12.30 Potential uses of polyphosphazenes (a) A thin film of a poly(aminophosphazene) sueh materials are of interest for biomedical applications, (b) Fibres of poly[bis(trifluoroethoxy)phosphazene] these fibres are water-repellant, resistant to hydrolysis or strong sunlight, and do not burn, (c) Cotton cloth treated with a poly(fluoroalkoxyphosphazene) showing the water repellaney eonferred by the phosphazene. (d) Polyphosphazene elastomers are now being manufaetured for use in fuel lines, gaskets, O-rings, shock absorbers, and carburettor eomponents they are impervious to oils and fuels, do not bum, and remain flexible at very low temperatures. Photographs by eourtesy of H. R. Allcock (Pennsylvania State University) and the Firestone Tire and Rubber Company.

Allcock s discovery that stable polyphosphazenes could be prepared under controlled conditions opened the door for the commercial development of this class of polymers, and In 1970, the first polyphosphazene elastomers were synthesized and the technology subsequently developed by Firestone Tire and Rubber Company (5). Today, polyphosphazenes are commercially available, and represent... 

Although questions still exist concerning the structure of polyphosphazene elastomers, several of these polymers are of technological interest and are undergoing commercial development (3-5). 

Figure 3.1 Polyphosphazene elastomers of general formula, [NP(OCH2CF3) (OCH2(CF2), CF2H)]b, fabricated into fuel lines, O-rings, gaskets, and other hydrocarbon-resistant devices. Reproduced by permission of the Firestone Tire and Rubber Company, and Ethyl Corporation.

As a coating offers increased anti-icing effectiveness and durability than fluorocarbon and silicone elastomers. These icephobic coats can reduce the accumulation of ice on products such as rooftops, aircraft, radomes, antennas, ships, and power-transmission lines. The weight of such accumulations of ice has led to aircraft crashes, fallen power lines, etc. The icephobic coats reduce the adhesive force between ice and a surface. Polyphosphazene elastomers possess these desired properties, in addition have low glass transition temperature (Tg), good environmental stability, curability, and moderate cost. 

Polyphosphazene elastomers undergo crosslinking when 7-irradiation is performed in a vacuum. The yield of cross-Unking, G(X), varies from about 1 to 12 depending on the percentage of allyl groups in the polymer chain [68]. 

Properties. One of the characteristic properties of the polyphosphazene backbone is high chain dexibility which allows mobility of the chains even at quite low temperatures. Glass-transition temperatures down to —105° C are known with some alkoxy substituents. Symmetrically substituted alkoxy and aryloxy polymers often exhibit melting transitions if the substituents allow packing of the chains, but mixed-substituent polymers are amorphous. Thus the mixed substitution pattern is deUberately used for the synthesis of various phosphazene elastomers. On the other hand, as with many other flexible-chain polymers, glass-transition temperatures above 100°C can be obtained with bulky substituents on the phosphazene backbone. 

Applications. Polymers with small alkyl substituents, particularly (13), are ideal candidates for elastomer formulation because of quite low temperature flexibiUty, hydrolytic and chemical stabiUty, and high temperature stabiUty. The abiUty to readily incorporate other substituents (ia addition to methyl), particularly vinyl groups, should provide for conventional cure sites. In light of the biocompatibiUty of polysdoxanes and P—O- and P—N-substituted polyphosphazenes, poly(alkyl/arylphosphazenes) are also likely to be biocompatible polymers. Therefore, biomedical appHcations can also be envisaged for (3). A third potential appHcation is ia the area of soHd-state batteries. The first steps toward ionic conductivity have been observed with polymers (13) and (15) using lithium and silver salts (78). 

Morphological features for POPs can range from low-temperature elastomers (when aliphatic alkoxy substituents of different length are attached to the polyphosphazene skeleton), to crystalline, film- and fibre-forming materials... 

On this basis, five classes of different polyphosphazenes are considered as outstanding examples of this type of macromolecules, in which skeletal and substituent features overlap to the highest extent. The reported materials are elastomers, flame retardants and self-extinguishing macromolecules, polymeric ionic conductors, biomaterials, and photosensitive polymeric compounds all of them based on the polyphosphazene structure. 

Both types are hydrophobic materials that, depending on the side group arrangements, can exist as elastomers or as microcrystalline fiber- or film-forming materials. Preliminary studies have suggested that these two classes of polyphosphazenes are inert and biocompatible in subcutaneous tissue implantation experiments. 

Today commercial applications for polyphosphazene specialty elastomers exist In aerospace, marine, oil exploration and Industrial fields. This paper describes the properties and applications for this unique class of elastomers. 

The true value of the chloropolymer (I) lies in its use as an intermediate for the synthesis of a wide variety of polytorgano-phosphazenes) as shown in Figure 1. The nature and size of the substituent attached to the phosphorus plays a dominant roll in determining the properties of the polyphosphazene. Homopolymers prepared from I, in which the R groups are the same or, if different, similar in molecular size, tend to be semi-crystalline thermoplastics. If two or more different substituents are introduced, the resulting polymers are generally amorphous elastomers. (See Figure 1.)... 

JThe effect of the substituent on the properties of the polyphosphazenes is not fully understood. For instance, [NP(OCH ) ]n and [NP C CH. homopolymers are elastomers (8,29). Synthesis using lithium, in contrast to sodium, salts is claimed to produce rubber-like fluoroalkoxyphosphazene polymers (30). The presence of unreacted chlorine or low molecular weight oligomers can affect the bulk properties (31,32). Studies with phosphazene copolymers both in solution and in the bulk state (29,33-38) indicate a rather complex structure, which points out the need for additional work on the chain structure and morphology of these polymers. 

Polyphosphazene-based PEMs are potentially attractive materials for both hydrogen/air and direct methanol fuel cells because of their reported chemical and thermal stability and due to the ease of chemically attaching various side chains for ion exchange sites and polymer cross-linking onto the — P=N— polymer backbone. Polyphosphazenes were explored originally for use as elastomers and later as solvent-free solid polymer electrolytes in lithium batteries, and subsequently for proton exchange membranes. 

Some of the most useful polyphosphazenes are fluoroalkoxy derivatives and amorphous copolymers (11.27) that are practicable as flame-retardant, hydrocarbon solvent- and oil-resistant elastomers, which have found aerospace and automotive applications. Polymers such as the amorphous comb polymer poly[bis(methoxyethoxyethoxy)phosphazene] (11.28) weakly coordinate Li " ions and are of substantial interest as components of polymeric electrolytes in battery technology. Polyphosphazenes are also of interest as biomedical materials and bioinert, bioactive, membrane-forming and bioerodable materials and hydrogels have been prepared. 

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