Effect of the polypropylene type on polymer-diluent phase diagrams and membrane structure in membranes formed via the TIPS process

Syndiotactic polypropylene (sPP) and isotactic polypropylene (iPP) were used to investigate the effect of the polypropylene microstructure (tacticity) on phase diagrams for PP-diphenyl ether systems and the morphology of membranes made from these systems. The syndiotactic and isotactic polypropylenes were both produced via metallocene catalysts and had similar average molecular weights. It was found experimentally that polypropylene microstructure affects the location of the binodal curve and the morphology of membranes produced via the thermally induced phase separation (TIPS) process. The influence on the binodal curve was attributed to stereochemistry effects, chain rigidity, and local chain packing. In isothermal experiments, droplet size was similar for the two types of PP, while in non-isothermal experiments, the sPP sample yielded larger droplets than the iPP sample. These results were attributed mainly to viscous effects and to differences in the growth period (resulting from a shift in the cloud point and dynamic crystallization curves). The final morphology of the sPP membrane was related to polymer chain flexibility and to shrinkage of the sample during diphenyl ether extraction and sample drying.