Rubbery Midblock Content Controls Formation and Mechanical-Separation Trade-offs in Block Polymer Membranes

Nonsolvent-induced phase separation (NIPS) is widely used to fabricate ultrafiltration (UF) membranes; however, the non-equilibrium nature of this process often yields disordered structures that limit separation performance. Self-assembly of amphiphilic block polymers can impose surface order; yet, most block polymer platforms reported to date rely on brittle polystyrene-based structural matrices. Here, we examine an amphiphilic tetrablock polymer, poly(styrene-b-cis-4,1-isoprene-b-styrene-b-4-vinylpyridine) (SISV), in which a rubbery poly(cis-4,1-isoprene) (PI) midblock is introduced between two glassy poly(styrene) (PS) blocks to enhance toughness. We consider four block polymers with PI contents ranging from 0 to 36 wt %, a fixed poly(4-vinylpyridine) (P4VP) fraction of 25 wt %, and an overall number-average molecular weight of Mn ≈ 100 kDa. Bulk mechanical testing of notched and unnotched specimens reveals a two-step transition in mechanical response with increasing PI content: from brittle to rubber-toughened thermoplastic, and ultimately to thermoplastic elastomer. Membranes fabricated via self-assembly assisted NIPS (SNIPS) exhibit a concomitant shift from ordered to increasingly disordered surface morphologies with increasing PI content; this shift alters the molecular-weight cutoff (MWCO) and poly(ethylene glycol) (PEG) rejection profiles while maintaining high pure-water permeance (ca. 1300 L m–2 h–1 bar–1). Together, these results delineate a clear trade-off between separation performance and mechanical robustness, and identify block polymer architecture and phase-inversion conditions as practical levers for manufacturing membranes that balance separation performance with large-scale processability.