title: Linear coupling of alignment with transport in a polymer electrolyte membrane.
abstract: Polymer electrolyte membranes (PEMs) selectively transport ions and polar molecules in a robust yet formable solid support. Tailored PEMs allow for devices such as solid-state batteries,'artificial muscle' actuators and reverse-osmosis water purifiers. Understanding how PEM structure and morphology relate to mobile species transport presents a challenge for designing next-generation materials. Material length scales from subnanometre to 1 μm influence bulk properties such as ion conductivity and water transport. Here we employ multi-axis pulsed-field-gradient NMR to measure diffusion anisotropy, and (2)H NMR spectroscopy and synchrotron small-angle X-ray scattering to probe orientational order as a function of water content and of membrane stretching. Strikingly, transport anisotropy linearly depends on the degree of alignment, signifying that membrane stretching affects neither the nanometre-scale channel dimensions nor the defect structure,causing only domain reorientation. The observed reorientation of anisotropic domains without perturbation of the inherent nematic-like domain character parallels the behaviour of nematic elastomers, promises tailored membrane conduction and potentially allows understanding of tunable shape-memory effects in PEM materials. This quantitative understanding will drive PEM design efforts towards optimal membrane transport, thus enabling more efficient polymeric batteries, fuel cells, mechanical actuators and water purification.
authors.affiliation: Department of Chemistry and Macromolecules and Interfaces Institute, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA.
authors.forename: Jing
authors.initials: J
authors.lastname: Li
authors.forename: Jong Keun
authors.initials: JK
authors.lastname: Park
authors.forename: Robert B
authors.initials: RB
authors.lastname: Moore
authors.forename: Louis A
authors.initials: LA
authors.lastname: Madsen