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

Abstract

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.

High Proton Conduction of Organized Sulfonated Polyimide Thin Films with Planar and Bent Backbones

“…In our previous study of the GI-SAXS measurements (Figure 4b), a self-assembled lamellar structure parallel to the substrate surface has been determined in the planar ASPI-2 thin film. 22,24 This lamellar distance expands linearly to the out-ofplane direction by water uptake. Furthermore, molecular ordering improves by water uptake based on the lyotropic LC property (Figure S5).…”

“…An earlier study demonstrated that the degree of molecular ordering strongly affects proton conductivity. 24 To reveal the origin for the high proton conductivity in both planar and bent ASPI thin films, POM, GISAXS, and pMAIR measurements were carried out.…”

“…Our previous study revealed that the molecular weight of polymer affects the LC optical domain size. 24 Therefore, in this study, similar molecular weight (Table 1) was chosen for planar and bent ASPIs for comparison. Results suggest that the LC-like morphology and domain size depend on not only the molecular weight but also the polymer backbone.…”

“…To achieve high proton conductivity of polymer electrolytes, various polymer electrolyte membranes such as perfluorosulfonated polymers, − sulfonated block copolymers, and graft copolymers have been examined based on a concept of the well phase-separated hydrophilic channels composed of hydrophobic backbones and hydrophilic parts. The proton conductivity of polymer electrolyte membranes is closely related to several parameters such as acidity, number of sulfonic acid groups, main chain and side chain structures, and membrane morphology. − From a different perspective, structural confinement effects of polymer electrolyte thin films have been reported. − Structure and transport properties in confined polymer thin films differ drastically from those of the bulk materials because of interactions between the polymer and substrate surface. , For example, a study of Nafion thin films demonstrated that the degree of the phase segregation, water uptake, and proton conductivity decreased concomitantly with decreasing film thickness. ,, …”

Progress on highly proton-conductive polymer thin films with organized structure and molecularly oriented structure

“…Molecular weight is another useful parameter to enhance ASPI thin film proton conductivity [100]. In general, the molecular weight of proton-conductive polymers contributes to mechanical strength, but it does not contribute to proton conductivity.…”