Formation of isotactic polypropylene membranes with bicontinuous structure and good strength via thermally induced phase separation method

Microporous membranes with bicontinuous structure and good strength were prepared via thermally induced phase separation (TIPS) method. Isotactic polypropylene (iPP) and diamyl phthalate (DAP) were used as a polymer and diluent, respectively. Based on Flory-Huggins theory, the phase diagram was obtained from the cloud point data. The effects of polymer concentration, quenching conditions and coarsening time on membrane morphology and mechanical properties were investigated. With the increase of polymer concentration, the morphology of the resulting membrane changes from a typical bicontinuous structure to a cellular structure gradually and the tensile strength increases as well. When quenching into water at different temperatures, a larger pore size was obtained at higher temperature. When quenching at 393 K, the pore size obviously increases with the increase of the coarsening time, however, when quenching at 333 K, the pore size changes little over the coarsening time.

Microporous membranes with bicontinuous structure and good strength were prepared successfully from the iPP–DAP system via TIPS method. The phase diagram was obtained for the iPP–DAP system. The polymer concentration at monotectic point is as high as about 60 wt.%. The polymer concentration, quenching conditions and coarsening effects are the major factors affecting the mechanism of phase separation and the final membrane morphology. With the increase of polymer concentration from 10 to 60 wt.%, the cross-sections of membrane change from a bicontinuous structure to a cellular structure gradually and the tensile strength increases from about 1.5 to 11.5 MPa, respectively. At different quenched conditions, a similar bicontinuous structure and smaller pore sizes were obtained by quenching into water at lower temperature. The pore size of final membranes, however, increases over the coarsening time obviously when quenched at 393 K as no solidification occurs to restrain the growth of polymer-lean droplets.