Enhanced αγ' Transition of Poly(vinylidene fluoride) by Step Crystallization and Subsequent Annealing

Poly(vinylidene fluoride) (PVDF) exhibits pronounced polymorphs. Its gamma phase is attractive due to the electroactive properties. The gamma-PVDF is however difficult to obtain under normal crystallization condition. In a previous work, we reported a simple melt-recrystallization approach for producing gamma-phase rich PVDF thin films through selective melting and subsequent recrystallization. We reported here another approach for promoting the alpha gamma' phase transition to prepare gamma-phase rich PVDF thin films. To this end, a stepwise crystallization and subsequent annealing process was used. The idea is based on a quick generation of a large amount of alpha-PVDF crystals with some of their gamma-PVDF counterparts at suitable crystallization temperature and then annealing at a temperature above the crystallization temperature for enhancing the molecular chain mobility to overcome the energy barrier of phase transition. It was found that crystallizing the PVDF melt first at 152 A degrees C for 4 h, then quenching to room temperature and finally annealing the sample at 160 A degrees C for 100 h was the most efficient to produce gamma-PVDF rich films. This is related to the melting and recrystallization of the alpha-PVDF crystals produced during quenching in the annealing process at 160 A degrees C, which favors the formation of gamma-PVDF crystals for triggering the alpha gamma' phase transition.