Spherulitic crystallization in poly(bis(trifluoroethoxy)phosphazene), PBFP

Abstract

Spherulite formation in poly(bis(trifluoroethoxy)phosphazene) has been investigated from the melt and from solution over a relatively wide crystallization and annealing range. Three polymorphic forms and one mesoform is found in this polymer. Several of these crystal modifications coexist in amounts which depend upon the crystallization conditions. Negatively birefringent spherulites increase in birefringence slightly upon being heated through the thermotropic T(1) transition. This change involves the formation of a chain extended morphology from a chain folded one. Microbeam X-ray analysis made within the spherulite shows that the unit cell [ a] direction is along the spherulite radius while the [ c] chain direction lies transverse to the spherulite radius. Moreover, below T(1) it has been established that the X-ray long period is invariant with annealing time and temperature, and above T(1) the periodicity disappears, or cannot be recorded. Whenever heating or cooling occurs through T(1), the spherulite birefringence appears to be invariant after initially heating through T(1). However, a substantial volume change (by dilatomery) of 6% occurs through T(1) and this is consistent with a change in crystal structure from a 3D orthorhombic structure below, to a 2D hexagonal form above T(1). From the molten state the transformation occurs rapidly from isotropic to the 2D form of the mesostate, not far below T m. The 2D hexagonal form reverts to 3D chain extended orthorhombic on cooling below T(1). This stable chain extended morphology also arises whenever PBFP is melted and then cooled below T(1). All transformations from the isotropic melt or the folded chain conformation must pass through the mesophase. All specimens are friable upon cooling below T(1), whereas the solution case spherulitic polymer film is ductile here.