Studies of proton transport in confined thin polymer electrolytes are essential for providing additional information regarding the structure–property relationships of such materials.

The effect of molecular ordering and ordered domain size on molecular weight dependent proton conductivity in well-organized sulfonated polyimide (SPI) thin films has been investigated using both low molecular weight (LM w , M w = 1.3 × 10 4 ) and high molecular weight samples (HM w , M w = 2.6 × 10 5 ). Analyses of the films by grazing incidence small-angle X-ray scattering (GISAXS) revealed that the HM w SPI film exhibits the higher degree of inherent liquid crystalline-like lamellar ordering, and that it is consistent with a preferential in-plane main chain packing. Such molecular ordering and the in-plane oriented structure can have considerable influence on the proton transport characteristics in thin films. The larger ordered domains in the HM w SPI thin film with a thickness of approximately 500 nm shows the significant proton conductivity enhancement to a value of 2.6 × 10 −1 S/cm (at 25 °C and 95% relative humidity), which is more than an order of magnitude higher value than that of the smaller ordered domains in the LM w SPI thin film with less molecular ordering. The larger ordered domains in the HM w SPI thin film influence the conducting characteristics attributable to the fewer liquid crystalline-like domain boundaries relative to LM w SPI films because the smaller ordered domains in the LM w SPI thin film more obviously exhibit large-scaled domain boundaries, which can disrupt the fast ion transportation.

The organized structure and proton transport property in sulfonated polyimide (SPI) thin film have been studied, much less is known at the interfacial charge transport during confinement of such materials. The proton conductivity of SPI thin film is correlated well with the highly oriented polymer structures as determined by infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) and in situ RH dependent synchrotron grazing incidence X-ray diffraction (GI-XRD). The high proton conductivity of 2.8 × 10−1 S cm −1 at 80°C and 90% RH was achieved in the highly oriented SPI thin film. In contrast, the bulk SPI showed the drop of conductivity value at high relative humidity region, which is due to the significant structural disorder accompanied by the strong interaction between the sulfonic acid side chains and water molecules.