Polyphosphazene Elastomers Containing Interdigitated Oligo- p -phenyleneoxy Side Groups: Synthesis, Mechanical Properties, and X-ray Scattering Studies

The range of polyphosphazene-based elastomers has been expanded through the use of phenoxy or oligo-p-phenyleneoxy minor cosubstituent side groups with majority 2,2,2-trifluoroethoxy side groups. Specifically, polymers with both trifluoroethoxy and low ratios of phenoxy, p-phenylphenoxy, p-diphenylphenoxy, or p-triphenylphenoxy cosubstituents, can generate noncrystalline, noncovalently cross-linked elastomers. These are formed through the steric interactions of the oligo-p-phenyleneoxy side groups. Small-angle X-ray scattering (SAXS) analysis of polymers containing p-diphenylphenoxy or p-triphenylphenoxy cosubstituents suggests that these macromolecules contain microdomains caused by the phase separation of the trifluoroethoxy and aryloxy groups, through stacking or agglomeration of the aryloxy units, and that those serve as noncovalent cross-linking points. Moreover, annealing of the polymers at elevated temperatures (150°C) causes a decrease in the average spacing between the aryloxy aggregates and has a direct effect on the mechanical properties, similar to the toughening caused by increases in the cross-link density in conventional elastomers.