Substrate Dependence of the Proton Transport and Oriented Structure in Oligo[(1,2-propanediamine)-alt-(oxalic acid)] Thin Films

Abstract

Three oligo[(1,2-propanediamine)-alt-(oxalic acid)] thin films were prepared on SiO 2 , R-plane sapphire, and MgO(100) substrates, and their proton conductivities were investigated. Conductivity depended on the substrate and varied from 4 © 10 ¹3 to 1 © 10 ¹4 S cm ¹1 under the same conditions (80% relative humidity and 298 K). The thin films between the Rplane (1102) sapphire and MgO(100) substrates showed different molecular oriented structures. The thin film on the sapphire substrate, which shows higher proton conductivity, has a structure oriented parallel to the substrate. The molecular orientation in the thin film could be correlated with the proton-transport properties.The design of highly proton-conductive solid electrolytes is necessary for many applications in the field of solid-state ionics. One fundamental method to create highly proton-conductive material is chemical modification by strong acid groups.1 When using chemical modification such as sulfonation, the resultant chemical stability and mechanical strength are often unfavorable. However, excellent proton conductivity is exhibited. Because of this tradeoff between proton conductivity and mechanical strength and chemical stability, a new method to produce a highly proton-conductive material has been sought for a long time.In a Nafion membrane, protons are known to be transported through nanochannels made of sulfonic acid groups and water clusters, a point that has been discussed intensively in the literature.2 Nanochannels are created by phase separation with the amphiphilic character of Nafion. For a Nafion membrane, the high proton conductivity is known to result from phase separation. An earlier study by the author revealed that the proton conductivity of Nafion thin film (400 nm) is much less than that of a commercial membrane. 3 In the thin film, sulfonic acid groups are highly oriented and mutually isolated from each other, producing poor conductive channel formation. Therefore, the proton-conductivegroup orientation is important to improve proton conductivity.A recent report has described anomalous proton conductivity by thin films based on oligomeric amides. 4 Thin films (ca. 200 nm) of oligomers on SiO 2 substrate exhibit about 10-fold higher proton conductivity than pelletized material (thickness ca. 0.5 mm). Although the reason for this enhancement in proton conduction remains unknown, application of this phenomenon shows promise as a new strategy to produce highly proton-conductive materials. Our group previously reported that this origin for the proton-conductivity enhancement by thin film can result from structural effects such as molecular orientation by thin film on the substrate. This report explains the substrate dependence of the proton-transport property in thin films, which had different molecular-oriented structure.Proton-conductive oligomeric amide (oligo[(1,2-propanediamine)-alt-(oxalic acid)]) was synthesized as described elsewhere.4 Details related to synthesis and characterization of the oligomeric amide are presented ...

Enhancement of Proton Transport in an Oriented Polypeptide Thin Film

“…Recently, we have found the substrate dependence between MgO and sapphire of the proton transport in the thin film and this was originated from the oriented structure at the interface. 46 These results suggest to us a possible route to fabricate thin films of different proton transport properties by substrate selection. However, the preexponential factor (σ 0 ) in the Arrhenius eq 1 for the SiO 2 substrate is higher than that for the MgO substrate.…”

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

“…Results showed that proton conductivity is changed by molecular orientation of the polymer according to an interaction between the substrate and the polymer interface [21,22]. In 2008, the first report was made of a study in which the proton conductivity of oligo[(1,2-propanediamine)-alt-(oxalic acid)] thin films was improved by the molecular orientation [23][24][25]. As the oriented structure of other research groups, Tamura and Kawakami presented composite membranes containing uniaxially aligned sulfonated polyimide nanofibers by an electrospinning process (Figure 1(f)) [26].…”

“…In combination with X-ray scattering technique, it is useful to discuss the structure of crystalline and amorphous parts in other materials [43,44,47,48,[52][53][54]. It also makes it possible to discuss the substrate dependence in greater detail [25,36,37] or casting solvent dependence [43,54] of the interfacial structure.…”

Proton-Conductivity Enhancement in Polymer Thin Films

“…In recent polymer research fields, taking advantage of the interface extends the molecular design such as the oriented structure and organized structure for the highly proton conductive materials. In this Feature Article, I proposed the concept of molecular orientation to improve proton conductivity for various proton conductive polymers using the interface of substrate surfaces. − The interface can modify the degrees of freedom for polymer structures through interaction with functional groups, wettability, and the surface charge between the substrate surface and polymers. Some proton conductive polymers exhibit an oriented structure by the fabrication of thin films.…”

“…Copyright 2009 Elsevier. Plots c–e were adapted with permission from ref . Copyright 2013 The Chemical Society of Japan.…”