Effects of repeated wet/dry cycling on the structure and performance of sulfonated pentablock copolymer membranes.

Sulfonated block copolymers have shown potential as membranes for water purification. However, the performance of these materials under cyclic wet/dry conditions is not well understood. We measured the membrane structure, mechanical properties, and water vapor transport rates in a sulfonated pentablock copolymer as a function of the number of wet/dry cycles. The polymer is synthesized with an ABCBA block sequence, where A is poly(t-butyl styrene), B is poly(hydrogenated isoprene), and C is poly(styrene sulfonate). The ion exchange capacity is 2 meq, and membranes were prepared by coating from a solution. Using small angle X-ray scattering, we find the structure in as-prepared membranes resembles disordered micelles, and the characteristic length scale swells slightly with each wet/dry cycle. This lattice swelling is likely constrained by the glassy end-blocks. We also detect a lower yield point and less overall tensile strength with repeated cycling. Water vapor transport rates vary with the number of wet/dry cycle, however no specific trend was observed.