Pore size reduction in directional crystallization processing of porous polymeric membranes

Although various prep technologies of porous polymeric materials have evolved, the versatile control of pore morphology still remains as the most demanding subject. Herein, we applied the engineering principles of crystal habit by directional crystallization to the fabrication of porous PVDF (polyvinylidene fluoride) membranes. Pores of low tortuosity and relatively high porosity (80-90%) were successfully fabricated by the directional crystallization of DMSO (dimethyl sulfoxide) under a temperature gradient imposed by the controlled movement of a sample toward a liquid nitrogen reservoir followed by subsequent solvent removal. Pore size was controlled across a wide range by mixing DMSO with dioxane. Notably, nanoporous structures could be prepared at a 1:1 mixing ratio, in which the multi-step crystallization of solvents restricted by highly concentrated solutes enabled the preparation of nanomembranes. The homogeneous dispersion of TiO2 nanoparticles into PVDF, which are often used to improve hydrophilicity and antifouling performance, was easily achieved by this method. This novel prep technology based on the directional crystallization of solvent mixtures is a potentially viable solution to the limitations of current porous materials research.