Chain Orientation-Dependent Polymorphic Crystallization of Poly(vinylidene fluoride): A Guide to Achieving Polar Phases

Utilizing flow fields to control the generation of polar phases holds great potential for the efficient fabrication of electroactive poly(vinylidene fluoride) (PVDF). Herein, we investigated the melt-stretch-induced polymorphic crystallization of PVDF and its underlying mechanism through various structural and morphological characterizations. Under controlled stretch conditions, we achieved rapid preparation of γ-PVDF with a 90% γ-phase fraction, reducing the processing time by over 2 orders of magnitude compared to standard static conditions. This improvement was attributed to the dominance of γ-phase nucleation and growth over α- and β-phases in a favorable flow environment. Based on experimental findings, a nucleation ability diagram was constructed to elucidate the strong chain orientation dependency of polymorphic crystallization. The diagram revealed that the optimal chain orientation degree required for flow-induced γ-, α-, and β-nucleation sequentially increased from 30 and 90 to over 90%, assuming that the chain orientation at the onset of crystallization aligns with the final crystal orientation. Consequently, stimulating the formation of the polar γ-phase through flow is comparatively easier than promoting the polar β-phase, which necessitates extreme chain extension. The competition among multiple phases under flow can be explained by chain conformational selection, which was semiquantitatively validated using stretch-induced conformational transition theory. This work provides valuable insights into the polymorphic crystallization behavior of PVDF under flow and can guide the design of flow parameters to achieve desired crystalline phases, particularly the polar γ- and β-phases.