The Effects of Structure, Humidity and Aging on the Mechanical Properties of Polymeric Ionomers for Fuel Cell Applications
Uan-Zo-li, Julie Tammy
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The purpose of this work was to investigate the effects of structure, humidity and aging on the mechanical behavior of NafionÂ® and DaisÂ® ionomers. It was determined that the majority of the properties of these membranes were controlled by the formation and growth of the ionic clusters that were the direct result of the ionic nature of these materials. In the process of this study, the properties of NafionÂ® and sulfonated DaisÂ® polymers were investigated by dynamic mechanical analysis and thermal gravimetric analysis and their water uptake and sorption and desorption isotherms were measured. A mastercurve and a shift factor plot were constructed for 60% sulfonated DaisÂ® membrane. It was determined that an increase in the degree of sulfonation raised the glass transition temperature of these materials by facilitating the formation of the ionic clusters which acted as physical crosslinks, thereby reducing the mobility of polymeric chains. Water was found to effectively plasticize the membranes, especially in the case of DaisÂ® materials, by reducing the storage modulus and decreasing the structural integrity of the ionomers. The effect of pre-treatment of NafionÂ® was investigated and the glass transition temperature was found to increase as a function of the severity of the treatment procedure. The maximum water uptakes were determined for virgin and aged NafionÂ® and DaisÂ® membranes and their vapor phase water sorption diffusion coefficients were calculated. The sorption process was found to follow pseudo-Fickian behavior, while the movement of water out of the membranes during the desorption process was determined to be controlled by mechanisms other than diffusion. Lastly, the effect of exposure of NafionÂ® and 30% sulfonated DaisÂ® membranes to the saturated environment at elevated temperatures was investigated and found to result in the increase in the glass transition temperature of the materials. Results of the exposure effects on the diffusion properties of NafionÂ® and DaisÂ® were inconclusive. Preliminary findings attributed the changes in the properties of the materials to the counteractive actions of physical aging and the growth of the ionic clusters.
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