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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 subnanometre1,2 to 1 μm (refs 3, 4) influence bulk properties such as ion conductivity and water transport. Here we employ multi-axis pulsed-field-gradient NMR (ref. 5) to measure diffusion anisotropy, and 2H NMR spectroscopy5,6 and synchrotron small-angle X-ray scattering7 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 elastomers8, promises tailored membrane conduction and potentially allows understanding of tunable shape-memory effects in PEM materials9. This quantitative understanding will drive PEM design efforts towardsoptimal membrane transport, thus enabling more efficient polymeric batteries, fuel cells, mechanical actuators and water purification.

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Figure 1: NMR spectroscopy and synchrotron SAXS of a drawn PFSI.
Figure 2: Diffusion as a function of draw ratio and water content.
Figure 3: Linear relationship between channel ordering and diffusion anisotropy.
Figure 4: Hydrophilic channel domain alignment modes for uniaxially stretched membranes.

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Acknowledgements

This material is based on work supported by the National Science Foundation under award numbers DMR 0844933 and 0923107, and CBET 0756439. This material is further based on work supported in part by the US Army Research Office under Grant W911NF-07-1-0452 Ionic Liquids in Electro-Active Devices (ILEAD) MURI. Experiments at PAL were supported in part by the Ministry of Education, Science and Technology of Korea and POSTECH.

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J.L. and L.A.M. contributed major ideas to the project, collected NMR diffusion and alignment data, and wrote the primary text of the paper. J.K.P. and R.B.M. generated samples, collected SAXS data and provided interpretation and text editing.

Corresponding author

Correspondence to Louis A. Madsen.

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The authors declare no competing financial interests.

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Li, J., Park, J., Moore, R. et al. Linear coupling of alignment with transport in a polymer electrolyte membrane. Nature Mater 10, 507–511 (2011). https://doi.org/10.1038/nmat3048

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