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

We present an experimental investigation on the creep behavior of molten po lypropylene organically modified clay nanocomposites. The nanocomposite hyb rids were prepared by melt intercalation in an extruder in the presence or absence of a compatibilizer. They were subsequently annealed and simultaneo usly 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 in creased with annealing time. The microstructure of the nanocomposites as in vestigated by TEM and high-temperature WAXD showed the presence of clay cry stallites dispersed within the polymer matrix. The creep data together with the microstructural investigation are probably indicative of a small amoun t of exfoliation from the edges of the clay crystallites during extrusion a nd annealing. The zero shear viscosity of the compatibilized nanocomposites containing greater than 3 wt % clay was at least 3 orders of magnitude hig her than that of matrix resin and the uncompatibilized hybrids. Importantly . the large increase in zero shear viscosity was not accompanied by any inc rease in the flow activation energy compared to the matrix polymer. The com patibilized hybrids also showed an apparent "yield" behavior. We conclude t hat the solidlike rheological response of the molten nanocomposite originat es from large frictional interactions of the clay crystallites. Compatibili zer has a significant influence in modifying the rheological behavior.