A rheological study on the kinetics of hybrid formation in polypropylene nanocomposites

We present an experimental investigation on the creep behavior of molten polypropylene organically modified clay nanocomposites. The nanocomposite hybrids were prepared by melt intercalation in an extruder in the presence or absence of a compatibilizer. They were subsequently annealed and simultaneously characterized using high-temperature wide-angle X-ray diffraction and controlled stress rheometry. The creep resistance of compatibilized hybrids was significantly higher than that of uncompatibilized hybrids and also increased with annealing time. The microstructure of the nanocomposites as investigated by TEM and high-temperature WAXD showed the presence of clay crystallites dispersed within the polymer matrix. The creep data together with the microstructural investigation are probably indicative of a small amount of exfoliation from the edges of the clay crystallites during extrusion and annealing. The zero shear viscosity of the compatibilized nanocomposites containing greater than 3 wt % clay was at least 3 orders of magnitude higher than that of matrix resin and the uncompatibilized hybrids. Importantly. the large increase in zero shear viscosity was not accompanied by any increase in the flow activation energy compared to the matrix polymer. The compatibilized hybrids also showed an apparent "yield" behavior. We conclude that the solidlike rheological response of the molten nanocomposite originates from large frictional interactions of the clay crystallites. Compatibilizer has a significant influence in modifying the rheological behavior.