| This research focuses on the role of interphase regions in polymer composites and their role in the determination of performance and durability of polymer composites. When epoxy-amine systems are employed as matrix materials, the epoxy-amine stoichiometry can play a large role in the development of interphase property gradients. These property gradients include reductions in the glass transition temperature, T{dollar}sb{lcub}rm g{rcub}.{dollar} In this work, stoichiometry-microstructure-property relationships are developed for epoxy polymers. A two-phase microstructure, consisting of a high density microgel phase and a low density phase, is observed using two imaging modes of the atomic force microscope (AFM). Compositional changes cause changes in properties that are related to changes in the relative amounts and connectivities of the microstructural phases. Preferential adsorption of reacting species to the fiber surface will result in the formation of the low density phase near the fiber which lowers the T{dollar}sb{lcub}rm g{rcub}{dollar} of the interphase region. The presence of this phase is also observed to effect the moisture diffusion process, such that moisture saturation levels could be higher in the interphase region than in the bulk matrix. A high saturation level is related to an increased amount of swelling in the epoxies studied. Thus, the presence of moisture at the interphase is expected to alter the internal stress state local to the fiber-matrix interface.; To provide a direct quantitative probe of these property variations in interphase regions, a technique utilizing the indenting capabilities of the AFM is developed. This technique is used to evaluate local changes in response of single- and multi-component polymer systems due to changes in composition and/or microstructure. Elevated-temperature indentation response is also measured for several polymer composite systems using a heating stage developed specifically for this purpose. For sized fiber systems, unexpected property variations are observed, leading to the discovery of another possible interphase formation mechanism in these systems. Also, the size of the fiber-matrix interphase is determined to be smaller than the characteristic microstructure sizes. |
