Rheological study of carbon nanofiber induced physical gelation in polyolefin nanocomposite melt

The rheological behavior of nanocomposites based on elastomeric ethylene-propylene (EP) random copolymer (84.3 wt% P) and well-dispersed modified carbon nanofibers (MCNFs) with concentrations from 0.5 to 20 wt% were studied by oscillatory-shear rheometry at varying temperatures. At relatively low temperatures, the entanglement density of the polymer chains appeared to increase with filler concentration, ensuring strong MCNF-polymer interactions. At elevated temperatures, pronounced deviations from the ideal melt behavior in the low frequency domain (i.e. positive slopes of the tan δ(ω) curves) were observed, indicating the formation of a three-dimensional percolated network. Following the Winter-Chambon criterion, the transition from pseudo-solid-like to liquid-like behavior (i.e. the critical gel point) upon cooling was determined. The physical gelation induced by MCNFs is a thermo-reversible phenomenon and its origin can be traced to the interactions of nanofillers, where the percolation temperature decreases with filler concentration. Abrupt changes in the critical gelation temperature, the stiffness of the gel and the relaxation exponent were observed in nanocomposites with MCNF loading above 10 wt%. This behavior indicates a change of the mechanism of physical gelation at high MCNF loadings that can be explained by the concept of bridge formation of polymer segments between two adjacent nanofillers in concentrated nanocomposites.