相转化耦合成膜方式下膜微结构的形成机制与调控

In order to overcome the difficulty in hydrophilic modification, ultrafiltration pore size and network structure for PVDF microporous membrane prepared via thermally induced phase separation process (TIPS). Inspired by the principles of the confined crystallization, surface segregation and surface deposition, PVDF was used as the substrate polymer, TIPS technique and blend were used to prepared microporous membrane. A new idea of controlling membrane microstructure was proposed as follow: Phase separation process and crystallization behavior were controlled by the differences of diluent in water-solubility of and solvency for PVDF and amphiphilic copolymers, which in turn led to different phase inversion process for PVDF and amphiphilic copolymers. That is, the coupling phase inversion was used to control mass transfer and crystallization process. The influences of water-solubility, Species and additive amount for diluents on phase diagram, crystallization and phase separation form were explained systematically from thermodynamics and dynamics, models of phase separation and confined crystallization dynamics were erected, membrane and hydrogel layer formation mechanisms were revealed. The formation and evolution rules of microstructure with thin and controllable pore size separation layer, network structure supporting layer and stronger antifouling were clarified. The investigations enriched the theory of thermally induced phase separation, provided reference to prepare functional membrane.

为有效解决热致相分离技术制备PVDF膜存在亲水改性难、获得超滤孔径和网状孔结构不易的难题，受聚合物受限结晶、两亲性共聚物表面偏析与表面沉积及反应增容的启发，本项目以PVDF为基材，将热致相分离和共混技术相结合，提出利用稀释剂水溶性的不同以及对PVDF和两亲性共聚物溶解能力的差异，使PVDF和两亲性共聚物在降温过程中发生不同相转化成膜方式之间的有机耦合来调节铸膜液体系的相分离过程和结晶行为，即通过调节体系的相容性、传质以及结晶实现对中空纤维膜微结构调控的新研究思路。从热力学和动力学两方面阐释稀释剂水溶性、两亲性共聚物种类和添加量对铸膜液体系相图、结晶和相分离形态的影响，建立相分离和受限结晶动力学模型，揭示体系成膜机理和水凝胶层形成机制，弄清S-L相分离条件下具有分离层薄且孔径可调、支撑层为网状孔、抗污染能力强的膜微结构形成和演变规律。研究丰富了热致相分离技术制膜理论，为膜功能化制备提供借鉴。

以聚偏氟乙烯（PVDF）为原料、以水溶性和非水溶性的有机物为稀释剂，将PVDF与和PVDF相容性不同的无定形聚合物（SMA、PVB）以及结晶性聚合物（EVOH、PA6、PET）进行共混，采用热致相分离技术制备聚合物膜。研究了共混体系的成膜机理，表征了膜的微结构、测试膜的性能。研究表明，SMA的加入使发生S-L相分离的“PVDF/稀释剂”体系依然发生S-L相分离，膜截面可因稀释剂水溶性的不同而分别形成网状孔或者是有一定贯通性的蜂窝孔结构，深化了SMA表面沉积机理，揭示了膜表面水凝胶层的形成机制以及SMA在耦合相转化成膜过程中的致孔、影响传热、传质以及结晶等多重作用；PA6的加入不仅也可经S-L相分离获得薄皮层、表面开孔、截面为贯通的蜂窝孔结构，而且可在不同共混量的条件下形成晶粒、短纤维以及网状结构，从而增强膜的力学性能；揭示了EVOH在成膜过程中的原位成纤、水分子内增塑提高膜渗透性能和增强增韧的机理以及共混PVB形成“榫-铆”连接结构的增强机理；PET阻碍PVDF结晶的生长，最终形成球状粒子之间的网状孔，动力学研究结果与膜结构和性能有较好的对应关系。首次揭示了具有高临界共溶温度的二元混合稀释剂体系的成膜机理。因此，利用稀释剂水溶性的不同以及对PVDF和两亲性共聚物溶解能力的差异，使PVDF和两亲性共聚物在降温过程中发生不同相转化成膜方式之间的有机耦合来调节铸膜液体系的相分离过程和结晶行为，即通过调节体系的相容性、传质以及结晶实现对膜微结构和性能的调节，深化和发展了热致相成膜理论。.　　共混适量的SMA、PVB、PA6以及PET均可降低皮层厚度、改善膜孔贯通性和亲水性，提高膜的通量、截留率以及抗污染性能。尤其是SMA及SMA钠盐可在PVDF表面发生沉积可形成厚度在20-220nm的薄且连续的沉积层，得到膜孔径小于10nm的超滤膜。共混适量的PVB在大幅度提高膜的渗透性能的同时可显著提高膜的力学性能。这两类膜不仅适合于水处理，还可在膜表面规模化引入羟基、酸酐或荷负电的官能团，为后续膜的进一步规模化的功能化改性奠定基础。