One way to improve the mechanical properties of brittle glassy polymers is through the inclusion of an energy-dissipating rubbery component. This study explores the effect of poly(isoprene) (PI) content on the mechanical/fracture properties of the bulk polymer and the transport performance of the resulting phase-inversion membranes in an amphiphilic tetrablock poly(styrene)-b-poly(isoprene)-b-poly(styrene)-b-poly(4-vinyl pyridine) (SISV) system. Four block polymers with varying PI midblock content (0 to 36 wt%) were synthesized, keeping the P4VP content and overall molecular weights constant. Using the copper grid technique developed by Lauterwasser and Kramer1 and tensile tests, a two-step change in fracture behavior was observed - from brittle to rubber-toughened thermoplastic to thermoplastic elastomer - as PI content increased. This change in fracture mechanism was attributed to changes in the self-assembled morphology, observed with AFM imaging, which were brought about by the PI content. Membranes manufactured by SNIPS exhibited a transition from ordered to more disordered surface morphology with increasing PI, which affected the MWCO and PEG rejection profile, while maintaining high pure water permeance (~1,000 LMH/bar). Overall, these results highlight the delicate balance inherent to co-designing mechanical and transport performance in state-of-the-art membranes.
1. Lauterwasser, B.D. & Kramer, E.J. Philosophical Magazine A 39, 469-495 (1979)