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

The transition of poly(vinylidene fluoride) (PVDF) alpha crystals to gamma' phase is, in general, unpredictable. In the present work, the related alpha-gamma' phase transition has been regulated through adjusting the thermal treatment protocol of the sample. The enhanced alpha-gamma' 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 degrees 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 alpha-PVDF microcrystallites created during cooling from T-c1, to 25 degrees C promotes the gamma-PVDF crystallization during t(c2) with a curled lamellar structure. These gamma-PVDF crystals accelerate the transition of alpha-PVDF spherulites formed at T-c(1) into its gamma' counterpart simultaneously during annealing at T-c2. The alpha-gamma' conversion efficiency is confirmed to be dependent on the cooling rate from T-c(1), to 25 degrees C as well as the annealing temperature of T-c2. A synergistic adjustment of the cooling rate from T-c1 to 25 degrees C and the T-c(2) can result in a full transformation of the alpha-PVDF spherulites formed at T-c1, into gamma'-PVDF spherulites. The findings provide important hints for guiding the fabrication of PVDF thin films with high content of the electroactive polar gamma phase.