This research is focused on the investigation of phase morphology and blend stability within ionomeric perfluorocyclobutane (PFCB)/poly(vinylidene difluoride) (PVDF) copolymer blend membranes. The morphologies of these unique materials, designed as proton exchange membranes (PEMs) for proton exchange membrane fuel cells (PEMFCs), have been examined not only in the as-cast/as-received state, but also as a function of exposure to various ex-situ aging environments. The morphological investigations used to probe the response of these ionomer blends have been designed to mimic the environment within a PEMFC and will therefore enhance our understanding of the implications of morphological changes which may occur during fuel cell operation. Thermal annealing of the membranes has been conducted to determine the materials' morphological response to various temperatures in the absence of hydration. The results of these thermal annealing studies have facilitated the isolation of morphological contributions stemming from thermal exposure. Immersion of the blend membranes in liquid water has allowed for singular identification of the role of hydration in the blend membranes' morphological rearrangement and phase stability. However, as the typical fuel cell environment to which these membranes will be exposed is complicated by the presence of both temperature and humidity, our ex-situ investigations have also included the exposure of PFCB/PVDF copolymer blend membranes to simultaneous thermal annealing and hydration conditions – a treatment we refer to as "hygrothermal aging." This unique procedure serves as a simplified method whereby the complex fuel cell environment may be simulated, and the resultant morphological response researched. While the work presented herein has enhanced our understanding of the blend stability of the specific membranes investigated, we have also advanced the fundamental knowledge of the role of morphology with respect to the fuel cell performance of blend materials and the corresponding implications of morphological rearrangements. Such an understanding is essential in the development of morphology-property relationships and eventual optimization of membrane materials designed for use in fuel cells. / Ph. D.