Two-Dimensional Proton Conduction in Biomimetic Polymer Electrolytes Prepared the Polymer Nanosheet Multilayer Film
Authors: Tsukamoto Mayu , Matsui Jun , Yamamoto Shunsuke , Miyashita Tokuji , Mitsuishi Masaya 
Published: February 2020

1．Introduction
Organic materials which efficiently transport protons are used in various energy harvesting such as fuel cells and ATP production. Recently, it has been reported that proton transport in the organic materials can be controlled by electric field. [1] In general, protons are transported in two dimension in the protonic transistor, which suggest the importance of fabricating two dimensional proton conductive channel. Biological membrane is a good example for such two dimensional proton conduction. Thus we mimicked the proton conduction at the biological membrane by using polymer nanosheet multilayer film. [2] [3] In this study, we introduce amphoteric group, phosphonic acid to the polymer nanosheet multilayer film. The proton conductivity through the interlayer of the multilayer film was studied by impedance spectroscopy and the mechanism of two dimensional conduction was proposed.
2．Experiment

N-dodecylacryamide-co-phosphonic acid copolymers, [p(DDA/VPA)] (Fig. 1) were synthesized by free radical copolymerization of DDA with VPA in ethanol. The monolayer properties of p(DDA/VPA) were investigated by π−A isotherm measurements. The polymer was depositioned on a substrate using the Langmuir-Blodgett (LB) method and film was characterized by X-ray diffraction (XRD) measurement. p(DDA/VPA) was deposited on the substrate with comb-type Au electrode to study the proton conductivity by impedance measurements.
3．Result and discussion
The syntheses of p(DDA/VPA) copolymer were confirmed from 1H-NMR and FT-IR. The mole fractions of VPA in the copolymers (n) were determined to be 0.19〜0.52, by elemental analysis. π-A isotherms in all copolymers showed a steep rise in surface pressure and relatively high collapse pressure, indicating that the copolymers formed a densely packed polymer monolayer. The average limiting surface area of VPA suggested that the VPA units were located below the water surface. The lamellar structure of the nanosheet multilayer film prepared using the LB was confirmed by X-ray diffraction measurement. Because the interlayer space increased with n, p(DDA/VPA) formed a multilayer in which a phosphonic acid group is sandwiched between hydrophilic sites (Fig. 2). The proton conductivity parallel to the lamellar plane direction increased with VPA content until n = 0.41 and became 2.6 × 10-2 S cm-1 at 60 ˚C and 98% RH(Fig. 3). Interestingly, the conductivity of n = 0.45 decreased to 1.0×10-2 S cm-1 nevertheless the number of proton source was increased. From the result, we suggest that there is an optimal distance of acid group to achieve a high proton conduction in two dimensional nanospaces. [4]
5．Acknowledgments
This work was supported by the Research Program “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” within the “Network Joint Research Center for Materials and Devices”.
6．References
[1] C. Zhong, et al. Nat. Comm, 2011, 2, 476. [2] T. Sato et al., Langmuir,
2017, 33 (45), 12897–12902. [3] T. Sato et al., Langmuir,
2015, 31 (18), 5174–5180. [4] V. B. P. Leite et al., Phys. Rev. E, 1998, 57, 6835-6839

Figure 1