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摘要
Poly(vinyldiene fluoride) (PVDF) asymmetrical membranes using three solvents N, N-dimethylforamide (DMF), N, N-dimethylacetamide (DMAc), and dimethylsulfoxide ( DMSO) were prepared by phase inversion process. The thermodynamic properties of the casting solutions and the kinetic diffusion during membrane-forming processes were investigated via cloud point determination and light transmittance measurements. It was found that the thermodynamic stability of the casting solutions was enhanced gradually from DMSO/PVDF/water, DMF/PVDF/water to DMAc/PVDF/water, the delayed demixing due to the decrease of kinetic diffusion rates was easier to occur as the solvents were changed from DMF, DMAc to DMSO. The characterization of membranes showed that membranes containing a macrovoid structure were fabricated with higher porosity and lower crystallinity when DMSO and DMAc were used as solvent, but the honeycomb-structure membranes with higher crystallinity and lower porosity were prepared from DMF solvent, which indicated that the three membranes morphologies and properties were not controlled by the thermodynamic property or kinetic process uniformly. As a result, based on the theory of "the predominance on the growth of preformed nucleus" and the macrovoid development, the influences of the three solvents on the phase separation behavior of PVDF casting solutions, PVDF aggregate structure and membrane structure were discussed by the membrane formation mechanism of skin-sublayer two step precipitation. The results showed that for the three membrane-forming systems using DMF, DMAc and DMSO as solvent respectively, the delayed solid-liquid demixing took place in the skin layer under the control of the thermodynamics of casting solutions, and the dense skin layer formed with spherical particles. The delayed time was controlled by the kinetic diffusion. Mutual diffusion coefficient of solvent and non-solvent ( D) increased gradually in the order of DMSO, DMAc, DMF, therefore, the delayed time decreased following the sequence of DMSO, DMAc, DMF. The phase transformation behaviors of the sublayers of the three membrane-forming systems were dominated by the liquid-liquid demixing controlled by the kinetic diffusion processes. For the systems with DMSO as solvent, the instantaneous liquid-liquid demixing happened in the sublayer, and crystallization had little effect on the kinetic diffusion. It was found in the light transmission that the demixing time was short, and curve variation was steep. Correspondingly, the membrane formed with a fingerlike macrovoid structure and a small amount of crystal spherulite in the sublayer,thus the membrane displayed the lower crystallinity and the higher membrane thickness, higher porosities and higher gas fluxes. For the systems with DMAc as solvent, the instantaneous liquid-liquid demixing happened in the sublayer. Compared to the DMSO system, crystallization had a great influence on the kinetic diffusion, leading to the longer demixing time and the flat curve variation in the light transmission drawing. Accordingly, the membrane was fabricated with an amount of crystal spherulite and the concurrent structure of macrovoid and spongy-like pores in the sublayer, and the membrane displayed the high crystallinity and the low membrane thickness, porosities and gas fluxes. For the system with DMF as solvent, compared to the DMSO and DMAc systems, the delayed liquid-liquid demixing initiated in the sublayer, and the crystallization had a greatest effect on the kinetic precipitation process I resulting in the longer demixing tme and the flatter curve variation in the light transmission drawing. Finally, the membrane with a honeycomb-structure sublayer was prepared with the higher crystallinity and the smaller thickness, porosities and gas fluxes.
