On the Origin of Slow Changes in Ionic Conductivity of Model Block Copolymer Electrolyte Membranes in Contact with Humid Air

Abstract

Proton conductivity (σ) and degree of hydration (λ) of poly(styrenesulfonate−methylbutylene) (PSS−PMB) block copolymers in contact with humid air were studied as a function of temperature under high-humidity conditions (relative humidity between 90 and 98%). The volume fraction of the hydrophilic PSS block in the dry state was 0.27 ± 0.01 in all of the samples, and the size of the hydrophilic channels was varied by varying the overall molecular weight of the samples. All of the samples have a lamellar structure in the dry state. The water uptake data were unremarkable, and a degree of hydration of 14 ± 2 H2O molecules per sulfonic acid group was obtained, regardless of temperature, thermal history, and hydrophilic channel size. In contrast, measured values of σ were highly dependent on thermal history and sample molecular weight. Equilibrated values of σ, obtained only after heating the samples to 90 °C for 48 h, were significantly lower than those obtained after initially hydrating the polymer films duri...