凝固浴温度对PVDF铸膜液相分离过程和膜结构的影响

Influences of the Coagulation Bath Temperature on Phase Separation of PVDF Casting Solution and Membrane Structure

采用皮-亚两步凝胶成膜机理探讨了凝固浴温度对PVDF铸膜液的相分离行为及其膜结构的影响。采用浊度法测定不同温度时铸膜液体系的热力学性质,光透射仪测定不同凝固浴温度时沉淀速率对膜的形态和性能的影响。结果显示:皮层的生成主要受热力学性质控制,随着凝固浴温度的提高,皮层由液固分相转变为液液分相,延时时间缩短,皮层由相互融合的球粒致密结构转变为多孔结构。亚层的生成主要受皮层结构和溶剂/非溶剂相互扩散的影响,膜亚层主要发生液液分相;随凝固浴温度升高,分相速率加快,大孔发展更充分,膜的孔隙率和气通量提高,但结晶度降低。

The phase separation and the membrane formation of PVDF casting solutions in water bath at different temperatures were discussed with the skin-sublayer two steps precipitations mechanism. The casting solutions thermodynamics at different temperatures and the membrane-forming kinetic precipitation in water bath under different temperatures were investigated via cloud point determination and light transmittance measurement. Membrane morphology and performances were characterized. It is found that the skin layer formation is controlled by the casting solutions thermodynamics. With the coagulation temperature increasing, the solid-liquid demixing is changed to liquid-liquid demixing, the delayed time decrease, and the skin-layer morphology is transformed from a dense and spherulitic structure to a porous structure. The phase separation of the sublayer is dominated to occur liquid-liquid demixing by the skin layer structure and mutual diffusion of solvent and nonsolvent. With the coagulation temperature increasing, the phase separation is accelerated, the macrovoids develop gradually, the membrane porosity and gas flux are improved, but the cystallinity decreases.