Proton conductivity enhancement in oriented, sulfonated polyimide thin films

Studies of proton transport in confined thin polymer electrolytes are essential for providing additional information regarding the structure-property relationships of such materials. Using a combination of proton transport measurements and structural characterization, we explored the effect of proton conductivity in sulfonated polyimide (SPI) under both bulk and nanostructured thin film systems. A SPI film confined to a thickness of approximately 530 nm shows significant proton conductivity enhancement to a value of 2.6 x 10(-1) S cm(-1) (95% RH at 298 K), which is almost one order of magnitude more proton conductive than the bulk system (3.0 x 10(-2) S cm(-1) at 90% RH and 298 K). In thin films, the preferred chain packing along the in-plane direction can have considerable influence on the charge transport characteristics, which leads to the enhanced proton conductivity. Infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) and in situ grazing-incidence small-angle X-ray scattering (GISAXS) were used to investigate the direction of polymer orientation and the changes in the internal polymeric structure under various humidity conditions, respectively. Under high-humidity conditions, the strong interaction between the side-chain sulfonic acid groups and water molecules causes an abrupt change in the internal structure in bulk SPI. Such a structural rearrangement results in a liquid-crystal-like ordered polymer structure. The RH dependent FTIR-ATR studies reveal that the accumulation of a large fraction of water molecules with stronger hydrogen bonding at high humidity regions causes the huge deviation in the internal morphology of the bulk samples.