Insights into Polymorphic Transition of PVDF during Nonsolvent-Induced Phase Separation

Nonsolvent-induced phase separation (NIPS) is a facile and cost-effective technique for the efficient fabrication of multifunctional polyvinylidene fluoride (PVDF) membranes. However, the polymorphic transition of PVDF during this process is not well understood, although different interpretations have been proposed. Here, the polymorphism of solution-cast PVDF films and the corresponding NIPS membranes has been investigated. Results show that the use of polar aprotic solvents is normally unable to induce the α → β phase transformation in the cast films, whereas immersing the cast wet films in a nonsolvent water bath (forming the NIPS membranes), the β phase emerges, revealing that both solvent and nonsolvent are crucial for polymorphic transition during NIPS. Further experiments show that (1) a low-temperature bath, a long immersion duration, and a suitable solution concentration can promote the β phase formation; (2) solvents having large dipole moment (μ), hydrogen-bond accepting power (βHBA), Hansen solubility parameter disparity (δt,P–S), and viscosity (ηS) tend to produce a high fraction of the β phase, F(β), otherwise producing medium or low F(β); and (3) self-polarized piezoelectric PVDF membranes can be directly fabricated via NIPS, the piezoelectricity of which is closely related to the solvent used. Finally, a new understanding of polymorphic transition for the NIPS membranes was proposed from the viewpoint of solvent–nonsolvent–polymer interactions that produce the "pulling" effects on the polymer chains to induce conformational change from gauche to trans (α → β/γ).