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 Tc1 for tc1, (ii) quick cooling from Tc1 to room temperature at 25 °C, and (iii) isothermal annealing at a relative higher temperature of Tc2 for tc2. The obtained results demonstrate that the insufficient melting at Tc2 of the α-PVDF microcrystallites created during cooling from Tc1 to 25 °C promotes the γ-PVDF crystallization during tc2 with a curled lamellar structure. These γ-PVDF crystals accelerate the transition of α-PVDF spherulites formed at Tc1 into its γ′ counterpart simultaneously during annealing at Tc2. The α–γ′ conversion efficiency is confirmed to be dependent on the cooling rate from Tc1 to 25 °C as well as the annealing temperature of Tc2. A synergistic adjustment of the cooling rate from Tc1 to 25 °C and the Tc2 can result in a full transformation of the α-PVDF spherulites formed at Tc1 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.