Department of Chemistry and Macromolecules and Interfaces Institute, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA.
Nat Mater. 2011 Jun 19;10(7):507-11. doi: 10.1038/nmat3048.
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.
聚合物电解质膜 (PEM) 在坚固但可成型的固体支撑中选择性地传输离子和极性分子。定制的 PEM 允许制造固态电池、“人造肌肉”致动器和反渗透水净化器等设备。了解 PEM 结构和形态如何与移动物种传输相关是设计下一代材料的挑战。从亚纳米到 1 微米的材料长度尺度影响离子电导率和水传输等体性质。在这里，我们采用多轴脉冲梯度 NMR 测量扩散各向异性，并采用 (2)H NMR 光谱和同步加速器小角 X 射线散射研究水含量和膜拉伸对取向有序的影响。引人注目的是，传输各向异性与对齐程度呈线性关系，这意味着膜拉伸既不影响纳米级通道尺寸也不影响缺陷结构，只会导致畴重排。各向异性畴的定向排列而不破坏固有向列相畴特性的观察结果与向列弹性体的行为相似，有望实现定制的膜传导，并可能有助于理解 PEM 材料中的可调形状记忆效应。这种定量理解将推动 PEM 设计努力实现最佳的膜传输，从而使更高效的聚合物电池、燃料电池、机械致动器和水净化成为可能。
