Impact of aspect ratio of carbon nanotubes on shear and extensional rheology of polyethylene nanocomposites

This paper reports the results of studies on the effect of aspect ratio of multiwall carbon nanotubes (CNT) on the shear and extensional rheological behavior of low density polyethylene (LDPE) nanocomposites. Up to a CNT loading of 2wt%, as used in the present study, the shear rheological data suggest no network formation in the nanocomposites, irrespective of the aspect ratio of the nano filler. Dynamic shear viscosity η' increases with increase in loading and aspect ratio of CNT. However, at low CNT loadings (0.1 wt%) and with CNT of high aspect ratio, η' for the nanocomposites is found to be lower than that of neat polymer. Steady shear rheology results show negative values for the normal stress for the high aspect ratio CNT which is believed to be due to the tumbling of CNT with high aspect ratio. Results of extensional viscosity measurements show that extent of strain hardening is dependent on the CNT aspect ratio and follows the order, high aspect ratio >medium aspect ratio > short aspect ratio, while the time of break follows the reverse order. The effect of aspect ratio on critical extensional stress becomes prominent only at the high aspect ratio, but the stress increases with the increase in CNT loading, irrespective of the aspect ratio.