Asymmetric porous membranes from binary polymer solution by physical gelation induced phase separation

Abstract

Phase separation induced by physical gelation has been identified, based on systematic experimental data, as the mechanism responsible for the generation of porous structures in elastomeric polydimethylsiloxane (PDMS) assisted drying of binary polymer solution. Comparative experiments of natural drying have enabled one to clarify the similarities and differences in those different drying processes. Based on experimental observations, qualitative explanatory drying models have been suggested for those drying processes. Slow, equilibrated solvent removal in natural drying has led to the formation of gel-phase skin, and this gel-phase skin undergoes wrinkling instability due to large volume shrinkage. The gelation-induced phase separation has occurred at the very last stage of natural drying, leading to nanoscale pores in very thin bottom-most layer. On the other hand, rapid, instantaneous absorption of solvent into PDMS in PDMS-assisted drying has yielded glassy skin at the initial stage of drying. Then the underlying, trapped solution has been separated, induced by physical gelation, into solvent-rich sub-phases embedded into polymer-rich matrix, leading finally to microporous support layer in asymmetric polymeric membrane structure. Physical gelation-induced phase separation, ahead of vitrification of amorphous polymer, has been found to be the responsible mechanism for the generation of porous structure in elastomeric PDMS-assisted drying process. The elucidated phase separation mechanism involved in PDMS-assisted drying, when combined with quantitative kinetic models for drying processes, can pave the way for the fabrication of asymmetric polymeric membranes using other amorphous polymers.