[15a-416-1~10] 12.3 Functional Materials and Novel Devices

[15a-416-9] water sorption and proton conductivity in alkyl sulfonated polyimide thin films

〇Ryosuke Goto¹, Mitsuo Hara¹, Yutaro Ono², Yuki Nagao², Shusaku Nagano³ (1.Grad. Sch. of Eng. Nagoya Univ., 2.JAIST, 3.Nagoya Univ. VBL)

Keywords:proton conductive polymer, alkyl sulfonated polyimide, in-situ humid controlled FT-IR

Alkyl sulfonated aromatic polyimide (ASPI) was synthesized. The ASPI exhibited high proton conductivity (10^-2 S cm^-1) in thin films at the relative humidity (RH) 95 %. Under humid conditions, the ASPI bulk exhibited new strong scattering peak in the small angle region. The scattering peak indicates formation or enhancement of the structural order by water uptake. For further insights into the structure, structural analysis for the ASPI thin film was performed by in-situ humid controlled grazing incidence small angle x-ray scattering (GI-SAXS) measurement. A characteristic scattering was observed in out of plane direction in small angle region under dry condition. The scattering was assigned to the periodical plane structure parallel to the substrate plane. Hence, the ASPI forms the lamella structure parallel to the substrate in the film. Moreover, the small angle peak was enhanced and shifted to the small angle region with humidity increase. The peak intensity enhancement and shift denote that the ASPI exhibits kinds of lyotropic liquid crystal phase due to their hygroscopic property. In other words, the highly ordered lamellar structure was formed parallel to the substrate plane by the polymer and water uptake layers and the lamellar spacing was enhanced with humidity. To analyze proton-dissociation process from sulfo group, we conducted in-situ humid controlled fourier-transform infrared spectroscopy (FT-IR) measurement on the ASPI thin film. Two representative absorptions for sulfo group were observed at 1030 cm^-1 and 915 cm^-1 at 0% RH. The absorption at 1030 cm^-1 was corresponding to the v(S=O) symmetric stretching mode of the dissociated sulfo group. The absorbance of –SO₃^- species drastically increased from 0% to 30% RH and was essentially unchanged more than 60% RH. On the other hand, the absorption at 915 cm^-1 was assigned to the v(S-O) stretching mode of the undissociated sulfo group with reflecting the same trend. The absorbance of –SO₃H species decreased with humidity in contrast to one of –SO₃^-. These results indicate that proton-dissociation from sulfo group was almost completed at 60% RH and the protons migrate into hydrophilic layers. In this presentation, we discuss the high proton conductivity of ASPI in thin film with humid condition based on water uptake and polymer orientation structure.