UHMWPE网络增强PVDF中空纤维疏水膜的微结构设计与调控

Polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) were the main membrane materials in membrane distillation. The stretching pores in PTFE membrane were difficult to control, while hydrophobic PVDF membrane has low strength, due to the particle packing structure. There is an urgent need to develop hydrophobic membrane with high strength and convenient controlled pore size. Based on the thermally induced phase separation (TIPS) and immiscible polymer blend theory, a membrane forming system containing PVDF, UHMWPE and diluent was put forward in this study. The rough and porous structure, with UHMWPE as the continuous phase and PVDF as the dispersed phase, was designed. Then the membrane performances including hydrophobic, strength and permeability were improved. The crystallization, phase separation and fiber forming behavior were explained, mainly for the PVDF segregation, PVDF spherulitic, UHMWPE lamellae and fibril, diluent droplet, polymer interface development. The control methods of PVDF microspheres connected by UHMWPE network, PVDF enriching membrane surface and multiple microporous were explored. The relationship between membrane structure and properties was discovered. The hollow fiber hydrophobic membranes with 0.1-0.4µm pores and good strength were obtained at last. This study provided theoretical guidance for the membrane material used in membrane distillation.

膜蒸馏常用膜有聚四氟乙烯膜和聚偏氟乙烯(PVDF) 超疏水膜，前者为孔径不易控制的拉伸孔结构，后者多为低强度的粒子堆积结构，迫切需要开发强度高、孔径可控分布的超疏水膜。依据热致相分离(TIPS)成膜理论与不相容聚合物共混相关理论，本项目提出PVDF/超高分子量聚乙烯(UHMWPE)/稀释剂不相容成膜体系，设计并构建以UHMWPE为连续相、PVDF为分散相的粗糙多孔结构，实现超疏水膜强度和渗透性能协同提高。阐述以PVDF离析、PVDF球晶聚集体、UHMWPE片晶及原纤、稀释剂液滴、聚合物界面发展为重点的结晶、相分离及纤维成形行为，确定TIPS纺丝成膜过程中UHMWPE网络连接PVDF微球、膜表面PVDF富集、多重微孔等膜结构的调控方法，揭示膜结构与膜疏水性能、力学性能、渗透性能的关系规律，获得孔径易于调控、且分布在0.1-0.4µm的增强型中空纤维疏水结构，为膜蒸馏用膜材料制备提供理论指导。

以矿物油/邻苯二甲酸二丁酯(DBP)为复合稀释剂，研究了成膜体系的粘度、相图、结构，发现共混成膜体系冷却过程中，依次发生聚偏氟乙烯(PVDF)与DBP之间的液-液(L-L)相分离，PVDF优先结晶引起的固液(S-L)相分离，超高分子量聚乙烯(UHMWPE)与矿物油之间的L-L相分离，以及UHMWPE结晶引起的S-L相分离，最终呈现典型的“海-岛”结构，其中UHMWPE/矿物油为连续相，PVDF/DBP为分散相，该研究内容进一步丰富了TIPS法共混成膜理论。在成膜体系研究基础上，通过TIPS法，制备具有UHMWPE原纤网络增强结构的UHMWPE/PVDF共混中空纤维膜。利用UHMWPE原纤网络状结构增强PVDF球晶之间的连接性，提高膜的力学性能。加入两种物质的良稀释剂，可以进一步改善UHMWPE的流变性，提高共混膜的可纺性。同时，UHMWPE/矿物油与PVDF/DBP体系为不相容体系，在制膜过程中不相容体系可产生膜的界面孔，利用这一特性丰富膜孔结构。通过调节共混配比、纺丝温度、冷却温度等因素，控制了膜的微观结构(原纤网络连接微球、膜表面 PVDF 富集、多重微孔)，获得平均孔径在0.7 μm，孔径分布在0.36-6.38 μm，拉伸强度达11.8 MPa，接触角始终大于90 º。膜的疏水性能、力学性能与渗透性能协同提高，这使UHMWPE/PVDF共混中空纤维膜有望在水处理领域应用成为可能。