Anisotropic Proton Conduction in Aligned Block Copolymer Electrolyte Membranes at Equilibrium with Humid Air

Abstract

The effect of alignment of proton-conducting domains in hydrated poly(styrenesulfonate-b-methylbutylene) copolymer films on conductivity was studied by impedance spectroscopy. Pressing isotropic samples obtained by casting results in lamellae aligned in the plane of the film. Application of electric fields and flow fields on the isotropic samples results in lamellae aligned perpendicular to the plane of the film. The alignment of lamellae, quantified by a combination of two dimensional small angle X-ray scattering (SAXS), birefringence, and transmission electron microscopy (TEM), was much better in the pressed samples than in the field-aligned samples. Conductivity was measured in the plane of the film (σ∥) and normal to the plane of the film (σ⊥). Only the pressed sample showed highly anisotropic proton conduction with σ∥/σ⊥ = 75. In this case, the value of σ∥ increased by 30% after alignment, relative to that obtained from the as-cast samples. The values of σ∥/σ⊥ obtained after electric field and shear field alignment were 1.2 and 1.4, respectively, in spite of partial alignment of the domains, and the increase in σ⊥ after alignment was less than 20%.