Influence of Molecular Weight on Molecular Ordering and Proton Transport in Organized Sulfonated Polyimide Thin Films

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, <i>M</i>_w = 1.3 × 10⁴) and high molecular weight samples (HM_w, <i>M</i>_w = 2.6 × 10⁵). 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