Lyotropic ordering for high proton conductivity in sulfonated semialiphatic polyimide thin films

Abstract

The influence of the semialiphatic backbone on molecular ordering and proton conductivity was investigated in comparison to the rigid aromatic backbone in highly proton-conductive organized polyimide thin films. We newly synthesized two alkyl-sulfonated semialiphatic polyimides (ASSPIs) with different molecular weights and investigated their molecular organized structure, proton conductivity, water uptake, and the dissociation state of protons from sulfonic acid groups in thin films by in situ measurements for grazing incidence small-angle X-ray scattering (GISAXS), quartz crystal microbalance (QCM), fourier transform infrared (FT-IR) spectra, and impedance spectra. Declining planarity in the semialiphatic backbone reduced the aggregative character and molecular ordering in the lyotropic liquid-crystalline (LC) structure. However, the higher-molecular-weight ASSPI exhibited the oriented lamellar structure despite the lower planarity of the main chain. The proton conductivity of the oriented lamellar thin film had more than half an order of magnitude higher value of 1.5 × 10-1 S cm-1 than did the nonoriented lamellar thin film (3.0 × 10-2 S cm-1) at 25 °C and 95% RH. These results indicate that in sulfonated polyimide thin films, the lamellar orientation greatly contributes to the high proton conductivity in ASSPI thin films. High-molecular-weight (HMw) sulfonated polyimide thin films with a semialiphatic main chain exhibited an oriented lamellar structure and high proton conductivity (1.5 × 10-1 S cm-1) under humidified conditions. Low-molecular-weight (LMw) sulfonated polyimide thin films with the same semialiphatic backbone exhibited a nonoriented lamellar structure and low proton conductivity (3.0 × 10-2 S cm-1) under humidified conditions. The results indicate that, in sulfonated polyimide thin films with semialiphatic main chain, the lamellar orientation greatly contributes to the high proton conductivity.