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 temperature. From the figure of phase diagram and vitrification line, we found small amount of water (10-20 wt %) can induce the vitrification of polymer solution in PSf/NMP/water system at 15℃ and more amount of water is needed to induce the vitrification at higher temperature.
We observed the cross-sectional membrane morphologies with SEM (Scanning electron microscope) varying the parameters including polymer concentration and bath compositions. When we increase solvent content in water bath, we can observe the occurrence of decreasing number of macrovoid and morphology is fully changed from finger-like type to cellular structure in case of 20/80 mixture of water/NMP by weight. It is believed that macrovoid are formed via spinodal decomposition mechanism and cellular structures are formed via nucleation and growth of polymer poor phase.
Atomic force microscopy (AFM) is an effective instrument with which to investigate the surface morphologies of membranes. The present work has shown that the tapping mode AFM is able to image the original, unprepared surface of polysulfone membranes and image analysis can provide detailed information on the surface pore structure and allows quantitative determination of the pore size distribution. Membrane prepared from 100wt% water as a nonsolvent shows nodular structures of 15 ~ 25nm nodule size and it is believed to be a result from spinodal decomposition mechanism. Membrane prepared from mixed nonsolvent bath (water : NMP ratio is 20 to 80 in weight) has a porous structures of mean radius of 146nm and this morphology is a result of nucleation and growth of the polymer poor phase. The pore diameters obtained from AFM are more accurate than those derived from SEM because the potential of altering the membranes pore structure during sample preparation is eliminated.
