(A) study on the mechanism of membrane formation from Polysulfone/NMP/water system by means of phase inversion

Abstract

Thermodynamics and kinetics of membrane forming polysulfone (PSf)/NMP/Water system was studied and membrane morphologies and related phase separation behavior was also investigated. The cloud point curves for ternary systems of PSf/THF/water and PSf/NMP/water were determined by a titration method at 15℃, 30℃, 45℃ and 60℃. A small amount of water (3~10 wt% water) was needed to achieve liquid-liquid phase separation in both systems and the temperature effect was small. From numerical calculation, it was found that the binary interaction parameters for the PSf/solvent/water system enlarges the homogeneous region in phase diagram with a smaller $\chi_{13}, a greater $\chi_{12}$, and a smaller $\chi_{23}$ and the effect of polymer molecular weight was negligible except in the range of low molecular weight. The slope of the tie lines indicated that demixing of the ternary system occurred at relatively similar nonsolvent concentration in both phases. We obtained a value of 2.7 for the water-PSf interaction parameter by fitting the experimental cloud point curve with the calculated binodal lines. We formulated ternary diffusion equations relating chemical potential gradients in the polymer film with friction coefficients between components, and obtained the diffusion trajectory in the phase diagram. In the present calculation, instead of equilibrium interfacial boundary conditions, we use new boundary conditions including mass transfer coefficient. By doing this, we could demonstrate the possible spinodal decomposition mechanism in case of instantaneous demixing. The vitrification line for the ternary system of PSf/NMP/water was determined by a DSC measurement with varying the compositions. The vitrification composition was 72.0 wt % of polysulfone and 28.0 % of solvent at 15℃ and 79.8 wt % of polysulfone and 20.2 % of solvent at 60℃, respectively. We also found that the slope of the vitrification line changed with temperature and it was steep in the case of higher...