Mimicking permafrost formation for the preparation of porous polymer membranes

Here, we demonstrate the fabrication of a microporous membrane and its surface modification via solvent crystallization of a polymer solution under a temperature gradient, similar to the natural process to form permafrost and the subsequent polygonal ground pattern produced during glacier formation. This system was prepared spreading a polyvinylidene fluoride (PVDF) solution on a silicon wafer, followed by crystallization. The solvent crystals were uniformly distributed by applying a directional temperature gradient over a PVDF solution layer. Finally, the microporous membranes were formed by removing solvent crystals via freeze-drying, similar to natural landform formation, such as the patterned ground that arises from permafrost. The introduction of co-solvent systems allowed for the systematic control of the pore morphologies of resulting polymer membranes including pore size, pore wall thickness, and surface topography. Overall, the composition of solvents in the crystallization of PVDF solution was the key factor affecting crystallization behavior, thus determining the morphology, crystallinity, and even polymorph. We believe this bio-inspired approach will open the door to a range of interesting studies related to micro/nano-materials fabrication.