PVDF Microporous Membrane Formation Via Thermally Induced Phase Separation

Poly(vinylidene) fluoride(PVDF) is the typical semi-crystal polymer with its good properties of chemical and aging resistance for membrane formation. The Millipore Company firstly develops"Durepore"microporous membrane with PVDF as membrane material in the middle of 1980s. At present, the PVDF membrane is successfully used in the oil-water separation, waste water disposal and gas filtration. There are several methods for PVDF membrane preparation, such as immerse precipitation, and thermally induced phase separation(TIPS). The TIPS broadens the membrane preparation materials, and easily makes the crystal polymer suitable for membrane materials.The effect of single and mixing diluents, the crystallization behavior of PVDF in PVDF/Dimethylphthalate(DMP) system, the different phase separation mechanisms in PVDF/benzophenone system, the additives polytetrafluoroethylene(PTFE) and clay, drawing technique on the membrane structure were studied.The four diluetns DMP, dibutyl phthalate(DBP), methyl salicylate(MS) and benzophenone with strong interactions with PVDF were chosen as the single diluent. The dioctyl sebacate(DOS), dioctyl adipate(DOA), dioctyl phthalate(DOP), and dioctyl terephthalate(DOTP) with weak interactions with PVDF were chosen as the mixing diluents. The different interactions between diluents and polymer resulted in the different membrane structure. As the interactions decreased, the spherulitic structure was more non-discernable and the porosity increased in the order of PVDF-DMP, PVDF-DMP-DOA, PVDF-DBP and PVDF-DMP-DOS. This order can be explained by the compatibility between PVDF and diluents.The crystallization rate and crystallinity were in the order of 80℃, 85℃and 95℃in the early stage of phase separation. Crystallinity of PVDF in PVDF-DMP system increased in the early stage of phase separation, and PVDF crystallized completely in the late stage of phase separation. In different quenching conditions, the greatest size and the mature of spherulites were obtained in air bath, the great amount of spherulites were obtained in ice water, and the irregular size of spherulites were...