Syndiotactic polypropylene as potential material for the preparation of porous membranes via thermally induced phase separation (TIPS) process

This work reports the flat sheet membrane preparation from syndiotactic polypropylene (sPP) by thermally induced phase separation (TIPS) process. sPP obtained by polymerization using metallocene catalysts and isotactic polypropylene (iPP) also obtained via metallocene catalysis with similar molecular weight were used. The phase diagrams of sPP and iPP with diphenylether as diluent were obtained. The properties of three representative membranes from sPP and three from iPP, prepared using different PP concentrations were evaluated with respect to membrane pore structure, gas flow, liquid displacement (bubble point), and water permeability. Two selected membranes, one from sPP and one from iPP were analysed with regard to polypropylene bulk morphology (X-ray diffraction) and mechanical properties (tensile strength). Under the same formation conditions, membranes with less inter-connected pores and less porous surface were obtained from sPP compared with iPP. Overall, lower permeabilities had been found for the sPP membranes, which were attributed to the difference in pore morphology. The differences between sPP and iPP were also discussed in terms of different driving forces for liquid-liquid demixing as deduced from the phase diagrams. The X-ray analysis had shown that the sPP membranes had a higher amorphous phase content than that in iPP, and the mechanical test had revealed a pronounced ductile behaviour for sPP samples. These results helped to explain the lower permeabilities of the sPP membranes, and their pressure-dependency.