Effect of polypropylene molecular weight on porous membrane formation by thermally induced phase separation

The effect of the polymer molecular weight on the phase diagram and the phase separation rate was investigated in the polypropylene membrane formation via thermally induced phase separation (TIPS). The cloud point curve was shifted to the lower temperature region as the polymer molecular weight decreased, while the dynamic crystallization temperature did not change so much. The phase separation rate was measured by the light scattering method. The system with lower polymer molecular weight initially showed the smaller interphase periodic distance Λ. However, the growth rate of Λ was much larger and finally Λ exceeded the value in the case of higher molecular weight polymer. The membrane prepared with the higher molecular weight polymer showed the smaller inter-connected structure, which was in the contrast with the larger cellular pore structure in the case of the lower molecular weight polymer. The larger cellular structure was attributable to the faster phase separation rate. The polymer molecular weight can influence not only the pore size but also the pore shape.