Polymer nanocomposites containing polyhedral oligomeric silsesquioxane (POSS) molecules

The structure, morphology and properties of various polymer nanocomposites containing polyhedral oligomeric silsesquioxane (POSS) molecules were studied in this dissertation. Crystallization studies at quiescent and shear states in isotactic polypropylene (iPP)/POSS nanocomposites were performed by means of in situ small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). The addition of POSS molecules was observed to increase the crystallization rate of iPP in quiescent conditions, suggesting that the presence of POSS crystals acted as nucleating agents. The crystallization rate was significantly reduced at the POSS concentration of 30 wt%, which suggested the crystallization growth of iPP was retarded by the molecular dispersion of POSS. In situ SAXS experiment showed that the addition of POSS significantly increased the crystallization rate during shear. It is postulated that while POSS crystals have a limited role in shear-induced crystallization, molecularly dispersed POSS molecules behave as weak crosslinkers in the polymer melts and increase the relaxation time of iPP chains after shear. Therefore, the overall orientation of the polymer chains is improved and a faster crystallization rate is obtained. The addition of POSS was also found to decrease the average long period value of crystallized iPP after shear, which indicated that large numbers of iPP nuclei were probably initiated by the molecularly dispersed POSS molecules.; A phenomenon of physical gelation was observed in ethylene-propylene (EP)/POSS copolymers nanocomposites in the molten state. The small-amplitude oscillatory shear experiments showed that the addition of POSS molecules changed the melt-like rheological behavior in the EP homopolymer to pseudo-solid-like behavior in the nanocomposites. The frequency shift factors (a_T) at different temperatures were found to be independent of the POSS concentration, indicating that temperature-dependent relaxation processes were almost unaffected by the presence of the POSS molecules. WAXD results showed that the majority of POSS molecules were in the crystal form in the EP melt. The nanocomposite was also observed to have higher T_g and higher Young's modulus in the solid state, suggesting that the well-dispersed POSS molecules formed weak bonding with polymer chains. The stress-controlled shear experiments showed that higher POSS concentration led to an increase in the yield stress. It is concluded that there are two interactions that may attribute to the physical gelation in EP/POSS nanocomposites: the strong interactions between the POSS crystal (particle-to-particle), and the weak bonding between the POSS and the polymer chains (particle-to-matrix). We postulate that the particle-to-particle interactions dominate the rheological behavior of EP/POSS nanocomposites, whereas particle-to-matrix interactions may dominate other POSS polymer nanocomposites having POSS molecules well dispersed at the molecular level.; Nylon 6 samples containing 1 wt% and 5 wt% POSS (octamethyl-POSS and dodecaphenyl-POSS) were characterized by means of WAXD/SAXS, scanning electron microscopy (SEM), and tensile property tests. WAXD results showed that the majority of POSS molecules were also in the crystal form. The percentage of the POSS concentration was estimated by using a peak-fitting method to analyze the WAXD results, which is in good agreement with the concentration prepared experimentally. Both WAXD and SEM results implied that dodecaphenyl-POSS had better compatibility to the Nylon 6 chains than octamethyl-POSS. SEM showed that large POSS crystal aggregates were present containing multiple small crystals with an average diameter of approximately 200 nm. (Abstract shortened by UMI.)...