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Rheology and processing of nanoclay loaded polyethylene resins Devendra, Raghavendra

Abstract

Several compounds were prepared by melt mixing various formulations of a linear low density polyethylene, a graft modified polyethylene and an organically modified clay, in a twin screw extruder. X-ray diffraction, transmission electron microscopy and rheology were used to characterize the extent of intercalation of the silicate galleries. Incorporation of minimum weight fraction of 2% clay and 50% maleated polyethylene was found to introduce radical changes in the rheological behaviour of the nanocomposites. Broadening peaks in X-ray diffraction indicated increasing dispersion of silicate nanolayers inside polymer as a function of graft modification. With higher incorporation of maleated polyethylene, solid-like response begins to appear at low frequencies indicating possible networking. Flow activation energy was found to decrease with incorporation of clay. Dual flow behaviour of the nanocomposites beyond the Newtonian plateau in the form of a filler-like flow at low shear rates and polymer-like shear thinning flow at high shear rates was observed. Changes in flow energy of activation with respect to clay and fusabond concentration are analysed. Relaxation spectra determined using a parsimonious model were found to extend to higher relaxation time scales with exfoliation. The Cox-Merz rule was found to fail over the whole range of shear rates for exfoliated compounds, indicating increased interactions in the matrix resulting in highly anisotropic distribution of individual silicate layers. Fusabond induces higher elasticity into the composite and increases the tensile stress growth function during melt elongation. Fusabond exhibits highly ductile failure as opposed to the brittle failure of polyethylene. Hence, fusabond is observed to contribute towards the ductility of the nanocomposite. A combination of fusabond and clay was found to be complementary in initiating strain hardening. The mechanical properties of the nanocomposites did not show an impressive change at low clay concentrations. A unique dip at 0.1 wt% clay loadings to complement its role in processing was followed by a steady increase with increasing clay concentration. While fusabond and higher levels of clay accounted for the increased toughness of nanocomposites, 0.1 wt% of clay decreased the toughness. There exists a threshold level of clay concentration above which the intercalation and exfoliation phenomena play a significant role in enhancing the mechanical properties.

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