When many semicrystalline polymers are used as matrix materials in composites, a morphology known as the transcrystalline region is formed on the surface of the reinforcing material. This region introduces a new crystalline structure to the system that is different from that of the bulk matrix material. Whether this region is advantageous or detrimental to the mechanical performance of the composite has been debated. Therefore, efforts were made to control the size and structure of this region for a specific composite system, i.e., nylon 66 reinforced with high modulus carbon fibers. In many systems this additional phase can be avoided simply by altering the crystallization history of the matrix polymer. In this study, the interphase region is removed not by changing the crystallization history of the matrix, but by altering the crystallization kinetics of the matrix material by introducing a diluent which is known to induce such changes in blends of itself and the host polymer. The diluent in this study is poly(vinyl pyrrolidone) (PVP) which is a highly polar, uncrystallizable polymer, and the host polymer is nylon 66 which is a highly polar crystallizable polymer. Initially, microscopy studies were performed on blends of nylon and two molecular weights of PVP at very low diluent concentrations, i.e., < 7% by weight. Next, commercial high modulus carbon fibers were unsized by exposure to a benzene wash. In addition, sets of the unsized fibers were sized with various amounts and molecular weights of the diluent by exposure to dilute solutions. Both unsized and sized fibers were then embedded in the previously made blends, and an optical study of the morphological changes in the interphase is performed. Furthermore, preliminary studies of the fiber surfaces using x-ray photoelectron spectroscopy (XPS) were conducted. PVP dramatically reduced the nucleation density of spherulites and modified the lamellar organization in the spherulites (as evidenced by the occurrence of banding which is the twisting of lamella as they grow radially). Furthermore, in the presence of unsized fibers, the addition of small amounts of diluent to the matrix increased the size of the transcrystalline region. At slightly higher diluent concentrations, the nucleation density on the fiber surface was reduced. Only with sizing of the fibers with the diluent along with adding the diluent to the matrix was there a complete removal of the transcrystalline region. / Master of Science