Relaxation of Proton Conductivity and Stress in Proton Exchange Membranes Under Strain

[+] Author and Article Information
Dan Liu

 Macromolecular Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061danl@vt.edu

Michael A. Hickner

Department of Chemical and Biological Systems, Sandia National Laboratories, Albuquerque, NM 87123mahickn@sandia.gov

Scott W. Case

Engineering Science and Mechanics, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061scase@exchange.vt.edu

John J. Lesko

Engineering Science and Mechanics, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061jlesko@exchange.vt.edu

J. Eng. Mater. Technol 128(4), 503-508 (Jun 06, 2006) (6 pages) doi:10.1115/1.2345441 History: Received July 29, 2005; Revised June 06, 2006

The stress relaxation and proton conductivity of Nafion 117 membrane (N117-H) and sulfonated poly(arylene ether sulfone) copolymer membrane with 35% sulfonation (BPSH35) in acid forms were investigated under uniaxial loading conditions. The results showed that when the membranes were stretched, their proton conductivities in the direction of the strain initially increased compared to the unstretched films. The absolute increases in proton conductivities were larger at higher temperatures. It was also observed that proton conductivities relaxed exponentially with time at 30°C. In addition, the stress relaxation of N117-H and BPSH35 films under both atmospheric and an immersed (in deionized water) condition was measured. The stresses were found to relax more rapidly than the proton conductivity at the same strains. An explanation for the above phenomena is developed based on speculated changes in the channel connectivity and length of proton conduction pathway in the hydrophilic channels, accompanied by the rotation, reorientation, and disentanglements of the polymer chains in the hydrophobic domains.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

A schematic of the screw-driven stainless-steel stretching fixture and the two-point conductivity cell. The whole apparatus was put into deionized water to measure the proton conductivity of the stretched sample at specific strains and temperatures.

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Figure 6

Relaxation of proton conductivity and stress of N117-H film at 25% strain, 30°C (SR denotes stress relaxation)

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Figure 7

Relaxation of proton conductivity and stress of BPSH35 film at 7.5% strain, 30°C

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Figure 8

A comparisons between relaxations of proton conductivities of N117-H films at 25% and 50% strain, 30°C

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Figure 9

Temperature hysteresis of proton conductivity of N1135-H film

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Figure 10

Temperature hysteresis of proton conductivity of NE1035-H film

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Figure 2

A schematic of the setup for measuring the stress relaxations of N117-H and BPSH35 samples immersed in deionized water

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Figure 3

The proton conductivity of a N117-H film measured before, immediately after stretching to 7.5% strain and after 1h45min relaxation at 30°C, 50°C, and 70°C

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Figure 4

The sketching of Nafion under low and high strains based upon the elongated polymer aggregates model by Rubatat, Heijden, Gebel and Diat (12) (permission of reproduction from ACS publications)

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Figure 5

A schematic of the proposed bundle-cluster model (14) for PEMs. The boundaries of hydrophobic bundles define the pathway of proton conduction



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