Permeability-Strength Trade-off in Nanoporous Polyethylene Membranes Derived from Etchable Triblock Polymer...

Nanoporous polymeric membranes are utilized in applications that include ultrafiltration and battery separators. However, the strength and toughness of these porous materials can limit their implementation in high performance applications. One way to increase membrane strength without using supports is to reduce the membrane void fraction (f_void); however, this can negatively impact permeability. We explored this permeability-strength trade-off using a library of polylactide-block-polyethylene-block-polylactide triblock polymers, which served as precursors to nanoporous membrane materials. The triblock polymers were processed using a solvent casting technique followed by selective polylactide (PLA) removal and oxygen plasma etching to yield nanoporous polyethylene (PE) membranes. The volume fraction of the etchable block (f_PLA) allowed for precise control of f_void by modifying the PLA content, determination of the compositional window for network connectivity, and elucidation of relationships between membrane porosity, permeability, and membrane strength. At f_PLA < 0.27, isolated PLA domains were unable to be completely hydrolyzed, making these compositions unsuitable for nanoporous membrane generation, and at f_PLA > 0.74, we observed a transition from percolating network into isolated PE domains that were also not useful for membrane applications. Between f_void = 0.27 to 0.74, we observed a clear permeability-strength trade-off, where lower void fraction membranes had high yield stresses (σ_y = 11-14 MPa) and elastic moduli (E = 400-700 MPa) but low air permeability (<6,000 L m² hr^-1 bar^-1 at 0.28 bar). In contrast, high porosity membranes exhibited lower yield stresses (σ_y = 2-8 MPa) but higher air permeabilities (up to 8,000 L m² hr^-1 bar^-1 at 0.28 bar). These findings enable future research to strategically tailor block polymer compositions to achieve the desired mechanical properties and permeability in nanoporous membranes derived from block polymers.