Understanding the non-solvent induced phase separation (NIPS) effect during the fabrication of microporous PVDF membranes via thermally induced phase separation (TIPS)

The thermally induced phase separation (TIPS) method is regaining momentum as a competitive platform to fabricate highly porous microporous membranes. In membrane technology, there has been an active search for more sustainable ways to fabricate polymeric membranes using green solvents. Rhodiasolv PolarClean® is a recently identified environmentally friendly TIPS solvent that shows high potential for the preparation of microporous PVDF membranes. Interestingly, its high miscibility with water induces a nonsolvent-induced phase separation (NIPS) effect on the membrane surface and this simultaneous NIPS-TIPS effect is referred to as the combined NIPS-TIPS (N-TIPS) method. In this work, a thorough investigation was carried out to understand the underlying phenomena in the membrane formation kinetics during the N-TIPS process. It was found that the NIPS and TIPS morphology can be tailored to control the mechanical properties, pore size distribution, and flux of the prepared membranes. For instance, increasing the coagulation bath solvent concentration facilitated the formation of a spherulitic morphology, whereas increasing the bath temperature induced the formation of a bicontinuous morphology free of macrovoids. It was determined that by controlling the phase separation kinetics, the mechanical properties of the prepared PVDF membranes could be remarkably improved from 0.9 MPa to 6.1 MPa. Several pore-forming additives including polyvinylpyrrolidone, Pluronics F-127, LiCl, and glycerol were employed to induce surface pores and their effects were thoroughly characterized. The membranes prepared with Pluronic additives exhibited high water permeabilities up to 2800 L m−2 h−1 bar−1 with narrow pore size distributions.