PVDF体系浸没沉淀相转化的两步成膜机理的探讨

Research on two-steps formation mechanism of PVDF membrane by immersion precipitation

研究了不同聚合物浓度铸膜液体系DMAc/H2O/PVDF的热力学性质、沉淀速度以及膜的结构,利用两步成膜机理探讨了PVDF体系的成膜机理.结果显示,皮层分相主要由体系热力学性质控制,当PVDF浓度逐渐增加时,PVDF结晶化作用越来越重要,皮层分相由瞬时液液分相逐渐转化为延时液固分相,膜上表面由多孔结构变成致密结构.亚层的分相由动力学扩散过程控制,分相时间由动力学扩散和体系热力学性质共同控制,随PVDF浓度的增加,结晶化作用增强,DMAc和H2O相互扩散速度减小,分相时间延长,使得亚层由瞬时液液分相转变为延时液液分相,膜亚层指状大孔结构减少海绵状孔结构增多,膜的结晶度提高.

Phase separation behaviors of the skin layer and sublayer were studied by the two - steps membrane - forming mechanism for PVDF/DMAc casting solutions with different concentrations immersing the harsh water coagulation bath. The results showed that the thermodynamics of casting solutions controlled the demixing of top layer. When PVDF concentration became higher gradually, the PVDF crystallization became more important, and the phase separation of the top layer was transformed from the instantaneous liquid- liquid （L- L） demixing to the delayed solid - liquid （S- L） demixing. So the membrane upper surface varied from a porous structure to a dense surface. In the control of kinetics, L- L demixing happened in the sublayer. The demixing time and sublayer structures were affected by the thermodynamics and diffusion. With PVDF concentration increasing, the PVDF crystallization became more and more dominant, and the diffusion rate of solvent and nonsolvent reduced, then the demixing time became longer, and the instantaneous L- L demixing was changed to the delayed L- L demixing in sublayer. Therefore, the macrovoid structures in membranes decreased, and the sponge - like structures increased with a increment of PVDF crystallinity.