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.

中文翻译：

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驯化聚偏二氟乙烯从α到γ'相的相变能力

通常，聚偏二氟乙烯（PVDF）α晶体向γ'相的转变是不可预测的。在目前的工作中，已经通过调节样品的热处理方案来调节相关的α-γ'相变。通过逐步熔融PVDF可以实现增强的α-γ'相变能力。它包括三个步骤，包括（i）在t _c1进行t _c1的等温结晶，（ii）从T _c1快速冷却到25°C的室温，以及（iii）在相对较高的T _c2温度下进行等温退火。Ť _C2。所得结果表明，熔融温度不足。在从T _c1冷却至25°C的过程中产生的α-PVDF微晶的T _c2促进了t _c2期间具有卷曲层状结构的γ-PVDF结晶。这些γ-PVDF晶体在T _c2退火期间同时加速了在T _c1处形成的α-PVDF球晶向其γ′对应物的转变。可以确定α-γ'的转换效率取决于从T _c1到25°C的冷却速率以及T _c2的退火温度。从冷却速率的协同调整Ť _C1至25℃和Ť_c2可以导致在T _c1处形成的α-PVDF球晶完全转变为γ'-PVDF球晶。这些发现为指导制备具有高电活性极性γ相含量的PVDF薄膜提供了重要提示。