The modification of polymeric materials using plasticizers or elastomers has been investigated in three research programs. The first describes epoxy resins modified with dimethylsiloxane, dimethyl-co-methyltrifluoropropyl siloxane, and dimethyl-co-diphenyl siloxane. The apparent compatibility between the epoxy and the siloxanes was enhanced by increasing methyltrifluoropropyl or diphenyl siloxane content or lowering molecular weight, resulting in profound changes in morphology and the resultant mechanical properties of the modified resins. Fracture toughness was most significantly improved using siloxanes containing at least 40% methyltrifluoropropyl siloxane or 20 and 40% diphenyl siloxane. Comparison of siloxane modifiers with butadiene acrylonitrile modifiers was valuable with regard to both property and morphological effects. The second research project considers the structure-property behavior of polyvinyl chloride (PVC) plasticized with low molecular weight diesters with emphasis on the contrasting effects of different plasticizers on the breadth of PVC's dynamic mechanical spectrum. It was clearly demonstrated that a less soluble plasticizer promoted a greater broadening at intermediate concentrations. Crystallization phenomena and static mechanical properties reflected the greater diluent effect of a more soluble plasticizer. The dynamic mechanical behavior as well as other critical experimental observations were explained using a model which postulates that the network junctions of plasticized PVC consist of "pockets" containing several small crystallites. These pockets are randomly dispersed in a matrix whose homogeneity is governed by the plasticizer's solubility and molar volume. The third research project describes the modification of high 1,4 polybutadiene (PB) with isopropyl azodicarboxylate (IAD) for potential .use as impact modifiers for polar polymers. A method for finding the extent of IAD modification of the PB has been developed using ¹³C nmr and UV spectroscopy. Solution blends of PVC with PB modified with up to 11 mol% IAD were found to be immiscible. Stress-strain testing suggested that IAD modification (11%) enhanced the apparent compatibility between PB and PVC at 25% rubber content. The relatively poor mechanical response of the blends was believed to be related to their somewhat porous morphology.