Molecular interaction between acidic sPPSU and basic HPEI polymers and its effects on membrane formation for ultrafiltration

The concept of acid/base polymer blends with the aid of electrostatic interaction has been employed to fabricate membranes for fuel cell and pervaporation, but not for ultra-filtration. This study aims to investigate the fundamentals of fabricating flat-sheet ultrafiltration (UF) membranes from acid/base polymer complexes via non-solvent induced phase inversion and to explore their potential for ultrafiltration. Basic hyperbranched polyethyleneimine (HPEI) polymers and acidic sulfonated polyphenylenesulfone (sPPSU) polymer are blended together as casting solutions. It is found that the molecular interaction between them is greatly influenced by the HPEI amount, changing from ionic crosslinking with chain entanglement to polymer aggregation. As a result, the phase inversion mechanism and polymer precipitation rate during membrane formation as well as membrane morphology vary significantly as a function of HPEI content. With an increase in HPEI concentration, membrane morphology transforms from an initially fully sponge-like structure to a finger-like macrovoid cross-section, then back to a totally macrovoid-free structure again. In addition, the as-prepared UF membranes demonstrate great variations in swelling ratio, surface charge, roughness, hydrophilicity, mechanical strength and UF performance. At the optimal HPEI concentration of 0.3 wt%, the as-cast UF membranes exhibit the highest mechanical stability and swelling resistance with an optimal water flux of 724 LMH/bar and a reduced molecular weight cut-off of 86.4 kDa.