Physical gelation in ethylene-propylene copolymer melts induced by polyhedral oligomeric silsesquioxane (POSS) molecules

The rheological behavior of ethylene-propylene (EP) copolymers containing polyhedral oligomeric silsesquioxane (POSS) molecules was investigated by means of wide-angle X-ray diffraction (WAXD), oscillatory shear, stress and strain controlled rheology in the molten state and dynamic mechanical analysis (DMA) in the solid state. WAXD results showed that the majority of POSS molecules in the EP melt were present in the crystal form. Oscillatory shear results showed that the EP/POSS nanocomposites exhibited a solid-like rheological behavior compared with the liquid-like rheological behavior in the neat resin, i.e. POSS caused physical gelation in EP. While POSS exhibited only a minimum effect on the flow activation energy of EP, the high POSS concentration samples were found to induce higher yield stress than the neat resin. This behavior was similar to the Bingham rheology, indicative of a structured fluid. DMA results indicated that the presence of POSS increased the Young's modulus as well as the T_g of the EP copolymer. These results suggested that two types of interactions contributed to the physical gelation in EP/POSS melts were present: the strong particle-to-particle interactions between the POSS crystals and the weak particle-to-matrix interactions between the POSS crystals and the EP matrix.