Polyphosphazenes thermal stability

This effect has also been observed in polyphosphazenes containing alkyl- or phenyl-carborane as pendent groups.12 A typical synthetic route to poly(phenyl-carboranyl-di-trifluoroethoxy-phosphazene) having pendent phenyl-carborane groups is shown in scheme 4. A substantial improvement in the thermal stability of the polymer was observed. This is attributed to a retardation of the ring-chain de-polymerization mechanism due to steric hindrance effects of the carborane units, inhibiting helical coil formation. 

The final class of polymers containing carboranyl units to be mentioned here is the polyphosphazenes. These polymers comprise a backbone of alternating phosphorous and nitrogen atoms with a high degree of torsional mobility that accounts for their low glass-transition temperatures (-60°C to -80°C). The introduction of phenyl-carboranyl units into a polyphosphazene polymer results in a substantial improvement in their overall thermal stability. This is believed to be due to the steric hindrance offered by the phenyl-carborane functionality that inhibits coil formation, thereby retarding the preferred thermodynamic pathway of cyclic compound formation (see scheme 12). 

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. 

Among inorganic heterocycles, perhalogenated cyclic phosphazenes occupy a very prominent place as the precursors for polyphosphazenes whose properties can he tuned by changing the substituents on the phosphorus sites (J). The heterocycles, as such, provide a robust framework for building a variety of novel molecules that have high thermal stability and flame retardancy, and have also been used recently to prepare a variety of dendrimers (2). The most widely studied among these are the perchlorinated cyclophosphazenes. Their syntheses reactions, properties, and application potential have been well documented a, 3-7). 

For halogen or alkoxy-substituted polymers Tg values are low (between -60 and —100° C). Alkoxy poly phosphazene with Ci - C3 substituents are elastomers. Higher alkoxy- or aryloxy-substituted polyphosphazenes are thermoplastics. Fluoroalkoxypolyphosphazenes exhibit a good stability toward diluted acids and bases. Some of them have outstanding thermal stability and good flame-retardant properties. 

Figure 2 Properties in polyphosphazenes are determined hy (1) the backbone bonds that control the inherent flexibility of the polymer via their influence on bond torsional freedom, and also provide photo-and thermo-oxidative stahihty (2) the side groups control polymer solubility, reactivity, thermal stability, crystallinity, cross-linking, and (indirectly) polymer flexibility (3) free volume between the side groups affects polymer motion, solvent penetration, membrane behavior, and density (4) functional groups (usually introduced hy secondary reactions) affect soluhihty, biological behavior, proton conduction, cross-hnking, and many other properties...

Polyphosphazene has good chemical and thermal stability. Its polyphosphazene backbone is highly flexible. Various side chains can be introduced to this backbone readily. Cross-linking is needed in order to reduce the dimensional changes in the presence of methanol or water. The membranes have shown reasonable proton conductivity and low methanol crossover. However, an improvement in mechanical strength is needed for practical fuel cell applications. 

In general, polyphosphazenes are not very resilient to heating, but their thermal decomposition occurs with a high degree of crosslinking and the formation of a dense structure that acts as a flame retardant [3], For some copolymers, the presence of phosphazene sequences enhances the thermal stability. Some other literature reports regarding thermal decomposition of these compounds are given in Table 17.2.1. 

These depolymerization reactions are complicated by side reactions. For example, if both P—Cl and P—OCH2CF3 or P—OCgHj units are present in the same reaction system, thermolysis causes elimination of CF3CH2CI or C5H5CI with simultaneous crosslinking of the chains . Clearly, depolymerization and elimination reactions are important in the context of the practical thermal stability of polyphosphazenes. Thus,... 

Polyphosphazenes, owing to their inorganic backbone, are characterized by properties that are not common to organic pol5miers, namely low temperature flexibility, nonflammability, good thermal stability, and biocompatibility. 

Crystallinity. Polyphosphazenes with a single substituent that is either small or rigid are semicrystalline. Their melting behavior consists of two first-order transitions, T(l) and Tm, separated by a 150-200°C gap. Optical microscopy showed that the crystalline structure was not lost at T(l), but rather existed as a mesophase until Tm was reached (2). Most fluoroalkoxy and aryloxy derivatives exhibited very high thermal stability with decomposition temperatures of 300-400°C. 

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