Anisotropic Proton Conductivity of Poly(aspartic acid) Thin Films

Abstract

Abstract Anisotropic proton conductivity was investigated in 60-nm-thick spin-coated thin films of partially protonated poly(aspartic acid)/sodium polyaspartate (P-Asp). The film showed large conductivity difference between in-plane and through-plane directions. The in-plane and through-plane conductivities to the substrate are 2.7 × 10−6 and 3.4 × 10−9 S cm−1, respectively, at the same relative humidity (RH) of 50% and 298 K. The proton conductivity for the in-plane direction was three orders of magnitude greater than that for the through-plane direction. The anisotropic origin is derived from the reported highly oriented structure of the P-Asp thin film. The proton conductivity exhibits no difference for the two in-plane directions between radial and circumferential directions, which can be expected as anisotropic main chain orientation by a centrifugal force during spin-coating.