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

Proton conductivity (sigma) and degree of hydration (lambda) of poly(styrenesullonate-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 sigma were highly dependent on thermal history and sample molecular weight. Equilibrated values of sigma, obtained only after heating the samples to 90 degrees C for 48 h, were significantly lower than those obtained after initially hydrating the polymer films during the heating runs. In addition, the low molecular weight samples were more sensitive to thermal history than the high molecular weight samples. Small-angle neutron scattering and transmission electron microscopy studies on the humidified samples revealed that the low molecular weight samples undergo a transition to hexagonally perforated lamellae upon hydration while the highest molecular weight sample did not. We speculate that the slow changes in sigma are due to the formation of less connected ion transporting channels or ionic clusters that impede ion motion. Equilibrated ionic conductivities increase as the hydrophilic channel size decreases,