Self-seeded crystallization and optical changes of polymorphism poly (vinylidene fluoride) films

Poly (vinylidene fluoride) (PVDF) films with polymorphic crystal structure, α, γ, and γ′, were prepared at 155 °C. The melting points of the three crystals increase progressively in the order of α, γ, and γ′, and the partially melting and recrystallization of the polymorphic PVDF was revealed. During the crystallization process, the α-to-γ′ transition takes place in solid state, result in a great deal of γ tiny crystals or seeds which can survive above melting temperature of α crystals. when heat-treated at 178 °C and cooled, the density of γ self-seeds is so high that no γ spherulites can grow, but only bundles of tube-like γ lamellae, distribute irregularly in the film, without birefringence. With increasing heat-treated temperature (Th), the amount of γ self-seeds decreases. The lamellae after annealing can organize into γ-PVDF spherulites with negative birefringence, and the sizes increase gradually with Th. The second melting one is γ spherulite, which can recrystallize into γ one again at low Th, for example, 180 °C. The γ′-PVDF crystal melt finally, owing to its highest melting point. At low Th, it just loses the birefringence, but can reappear and induce γ crystals to grow from the surrounding melt. At higher Th, 186 °C, it can recrystallize into γ crystals, keeping its original outline, also induce γ crystals, but the nucleation density decreases. At 188 °C, the crystal history of the film is removed completely, the kinetically favored α crystallization reoccurs after cooling. These results can provide a useful way to induce the formation of polar PVDF phases.