Browsing by Author "Uan-Zo-li, Julie Tammy"
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- The Effects of Structure, Humidity and Aging on the Mechanical Properties of Polymeric Ionomers for Fuel Cell ApplicationsUan-Zo-li, Julie Tammy (Virginia Tech, 2001-12-04)The purpose of this work was to investigate the effects of structure, humidity and aging on the mechanical behavior of Nafion® and Dais® 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 Nafion® and sulfonated Dais® 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 Dais® 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 Dais® materials, by reducing the storage modulus and decreasing the structural integrity of the ionomers. The effect of pre-treatment of Nafion® 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 Nafion® and Dais® 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 Nafion® and 30% sulfonated Dais® 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 Nafion® and Dais® 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.
- Morphology, Crystallization and Melting Behavior of Propylene-Ethylene Statistical CopolymersUan-Zo-li, Julie Tammy (Virginia Tech, 2005-09-19)In this work the morphology, crystallization and melting behavior of novel Dow Chemical propylene-ethylene copolymers were investigated. The incorporation of ethylene units into a polypropylene chain resulted in the decrease in crystallization, melting and glass transition temperatures and overall crystallinity. Based on the shape of heat capacity curves and the dependence of the melting temperature offset on ethylene content, it was concluded that copolymers prepared using different catalyst systems exhibited different ethylene sequence length distributions. The behavior of Dow Chemical propylene-ethylene copolymers was compared to that of copolymers prepared using traditional metallocene and Ziegler-Natta catalysts. The catalyst system used in the preparation of these new copolymers is similar to a metallocene catalyst system. It was demonstrated that ethylene defects are partially included in the polypropylene crystal. The thermodynamic heat of fusion at the equilibrium melting temperature decreased by 44% with an increase in ethylene concentration from 0 mol% to 21.2 mol%. On the basis of calorimetric and density data, the inclusion model based on the Sanchez-Eby crystallization theory was shown to be applicable for the evaluation of the degree of crystallinity. At the same time, inadequacies were found in application of the rigid amorphous fraction model to these copolymers. The formation of gamma-phase crystals was shown to be favored by both an increase in the ethylene content and a decrease in the crystallization rate. Increase in the ethylene content was shown to lead to a decrease in the density, length and thickness of alpha-phase crystals. It was also demonstrated that the cross-hatching morphology is present in all propylene-ethylene copolymers.