Structure and Properties of Sulfonated Pentablock Terpolymer Films as a Function of Wet–Dry Cycles

The structure and properties of poly(tert-butylstyrene-b-hydrogenated isoprene-b-sulfonated styrene-b-hydrogenated isoprene-b-tert-butylstyrene) (tBS-HI-SS-HI-tBS) films were investigated as a function of “wet–dry cycles”, where one “cycle” is defined as a 24 h soak in deionized water followed by a 24 h drying period in air. Films were characterized with a variety of complementary measurements that include X-ray scattering, infrared spectroscopy, water uptake, impedance spectroscopy, and tensile tests. We find that cycling drives a structural transition toward increasingly interconnected SS domains, which is favorable for water and ion transport. However, cycling can also induce mechanical deformations that reduce ductility, swelling, and water uptake. The significance of this trade-off is illustrated by comparing the properties for two film thicknesses as a function of cycle number: The ductility of thinner films (15 μm) is lost after four cycles, an effect that is correlated with the appearance of macro...