Tailoring the pore size and permeability of isoporous membranes through blending with poly(ethylene glycol): Toward the balance of macro- and microphase separation

Abstract

Isoporous membranes prepared from the combination of self-assembly and non-solvent induced phase inversion (SNIPS) were known for their asymmetric structures that were characterized with an isoporous top layer and porous/interconnected sublayer. This unique feature provides an opportunity to tailor the structure on toplayer and sublayer separately or synchronously by adjusting the membrane formation. In the current work, poly(ethylene glycol)s with molecular weights (MWs) ranging from 550 to 20,000 g/mol were blended with polystyrene-block-poly(acrylic acid) and their effects on pore formation were carefully investigated. It was firstly inspected that the loading amount of PEGs that preserved isoporous structure should be lower than 40 wt%, which was found to depend slightly on their MWs. However, the surface morphologies indicated that PEGs of higher MWs had endowed membranes with larger pore size. Solutions for membrane fabrication were thus in-situ analyzed by small angel X-ray scattering (SAXS), which indicated that PEGs of higher MWs led to stronger microphase separation for the block copolymer. The flow resistances of the isoporous top layer and porous sublayer were finally calculated through a combination of Darcy and Hagen-Poiseuille equation. Typically, the smallest flow resistance and largest permeability were obtained for the PEG10K which had achieved the optimal balance of pore opening in toplayer and sublayer. The result found herein will help us to learn how PEGs modify the membrane formation from the surface to internal when they are used as additives for the SNIPS process.