Thermal Behavior, Local-Scale Morphology, and Phase Composition of Spherulites in Melt-Crystallized Poly(Vinylidene Fluoride) Films

Synchrotron microbeam X-ray diffraction was employed to investigate the local-scale structure and solid-state phase transformation within individual spherulites of poly(vinylidene fluoride) (PVDF). In thin, non-oriented films, PVDF crystallizes into alpha and gamma-phases, forming distinct spherulitic morphologies: large, banded alpha-spherulites and smaller, irregular "mixed" spherulites dominated by the gamma-phase. For samples crystallized at high undercooling (160 degrees C), the mixed spherulites primarily consisted of the gamma-phase, with only a minor fraction of alpha-lamellae localized at the spherulite boundaries. At higher crystallization temperatures (165 degrees C), the alpha-phase was entirely absent from the mixed spherulites. High-temperature annealing induced a phase transformation from the alpha-phase to the gamma-phase, initiating at the interface between alpha- and gamma-spherulites. The transformation propagated radially along the b-axis of the alpha-spherulite, while its characteristic banded morphology remained intact. Radial scanning with an X-ray microbeam provided spatially resolved mapping of the structural transition within the alpha-spherulite at the micrometer scale, offering detailed insights into the transformation mechanism and its impact on the spherulitic structure. The fast crystal growth direction remained unaltered during the transition, suggesting minimal material transport and maintaining structural coherence.