Highly Permeable and Robust Responsive Nanoporous Membranes by Selective Swelling of Triblock Terpolymers with a Rubbery Block

The selective swelling of amphiphilic block copolymers has been demonstrated to be extremely facile and efficient in producing nanoporous membranes. However, all previous works are limited to diblock copolymers composed of two glassy blocks, suffering from inherent mechanical weakness. Here we elucidate the selective swelling-induced pore generation of triblock terpolymers with a rubbery polyisoprene (PIP) block, polyisoprene-block-polystyrene-block-poly(2-vinylpyridine) (PIP-b-PS-b-P2VP). A short exposure to ethanol turns the initially dense films to nanoporous membranes with well-defined interconnected porosity. We fabricate composite membranes with the nanoporous terpolymer thin films as the selective layers deposited on macroporous substrates. Using PS-b-P2VP diblock copolymer without a rubbery third block for comparison, we identify the role of the rubbery PIP blocks in determining the mechanical properties as well as the swelling behaviors of the terpolymer. The rubbery PIP blocks enhance the mechanical robustness of the nanoporous membranes as revealed by nanoindentation tests on one hand and evidently accelerate the swelling process because of their softening effect to the PS matrix on the other hand, thus leading to 2-3-fold improved permeability. Moreover, the membranes exhibit a fast stimuli-responsive function as well as enhanced hydrophilicity because of the preferential aggregation of P2VP chains on the pore walls.