Phase transitions in polyphosphazene films: Poly[bis(trifluoroethoxy)phosphazene]

Abstract

Abstract The morphology of cast and annealed films from poly[bis(trifluoroethoxy)phosphazene] \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm \{ PBFP,\rlap{--} [(CF}_{\rm 3} {\rm CH}_{\rm 2} {\rm O)}_{\rm 2} {\rm P = N\rlap{--} ]}_{\rm n} {\rm \} } $\end{document} has been studied by electron, scanning electron and optical microscopy. Several polymorphic forms and one mesophase (hexatic) have been established. From THF solution, PBFP has a chain folded morphology (α‐orthorhombic) which transforms into a chain extended 2D hexagonal mesophase at T (1). It remains in this state until final melting occurs at T m = 245°C approximately. Upon cooling the melt, it quickly reverts to the hexagonal mesophase again, but then passes into a chain extended (3D) γ‐orthorhombic modification. This material is much more stable thermodynamically than the α‐form, which is friable or brittle. PBFP cannot be quenched to a glassy form from the molten state, but it can be disordered as assessed by its reduced T (1) enthalpy and less well defined morphology assessed by electron microscopy. Another crystal modification (β‐monoclinic) has been found in solution cast low molecular weight PBFP films.