Taming the Phase Transition Ability of Poly(vinylidene fluoride) from α to γ′ Phase

The transition of poly(vinylidene fluoride) (PVDF) α crystals to γ′ phase is, in general, unpredictable. In the present work, the related α–γ′ phase transition has been regulated through adjusting the thermal treatment protocol of the sample. The enhanced α–γ′ phase transition ability has been achieved through stepwise melt crystallization of PVDF. It consists of three steps, including (i) isothermal crystallization at T c1 for t c1, (ii) quick cooling from T c1 to room temperature at 25 °C, and (iii) isothermal annealing at a relative higher temperature of T c2 for t c2 . The obtained results demonstrate that the insufficient melting at T c2 of the α-PVDF microcrystallites created during cooling from T c1 to 25 °C promotes the γ-PVDF crystallization during t c2 with a curled lamellar structure. These γ-PVDF crystals accelerate the transition of α-PVDF spherulites formed at T c1 into its γ′ counterpart simultaneously during annealing at T c2 . The α–γ′ conversion efficiency is confirmed to be dependent on the cooling rate from T c1 to 25 °C as well as the annealing temperature of T c2 . A synergistic adjustment of the cooling rate from T c1 to 25 °C and the T c2 can result in a full transformation of the α-PVDF spherulites formed at T c1 into γ′-PVDF spherulites. The findings provide important hints for guiding the fabrication of PVDF thin films with high content of the electroactive polar γ phase.