Study On Miscibility Of PVDF Blends And Formation Mechanism Of PVDF Blend Membranes

Due to the hydrophobicity of poly(vinylidene fluoride)(PVDF), the application of PVDF membrane was limited. In this paper, poly(methyl methacrylate)(PMMA) and polysulfonamide(PSA) were respectively introduced as the second polymer component to hydrophilized PVDF membrane. The miscibility of PVDF blends and the formation mechanism of PVDF blend membranes were investigated.The miscibility of PVDF blends in N,N-dimethyl acetamide(DMAC) were characterized by solubility parameter, glass transition temperature and viscosity, etc. The result showed PVDF/PMMA blend system is completely miscible while PVDF/PSA blend system is partially miscible. Based on the miscibility, hydrophilicity, polymer concentration etc.,the optimal composition and conditions for preparing PVDF membranes was obtained by normalization design method.According to the formation mechanism of single-polymer casting solution by immersion precipitation phase inversion process, we studied the formation mechanism of PVDF blend membranes under different conditions. Poly(ethylene glycol) (PEG) and (lithium chloride) (LiCl) were selected as additives ,and water, ethanol aqueous solution, sodium chloride aqueous solution and DMAC aqueous solution as coagulating bath, when the content of PEG in PVDF/PMMA casting solution rised, the phase separationiiwas delayed, which resulted in compacted membranes structure. For PVDF/PSA casting solution, PEG was the pore-former and LiCl was the cosolvent. With the decreasing of solvent-nonsolvent interaction, the phase separation time of PVDF blend casting solutions in different coagulation became longer and macrovoids in membrane structures smaller. The diffusion of solvent-nonsolvent in the gelation process of PVDF/PMMA accorded with Pick diffusion law, and PVDF and PMMA precipitated simultaneously because of the good miscibiiity between them. So PVDF crystal grain was not found in membranes. While the diffusion in the formation process of PVDF/PSA membrane did not obey Pick diffusion law, and the partial miscibiiity made PVDF and PSA precipitated respectively. Therefore, a few PVDF crystal grains could be observed in PVDF/PSA blend membranes.