Anomalous normal stress behaviour for polymer nanocomposites and liquid crystalline polymer composites

First normal stress difference (N1) behaviour of polymer nanocomposites and liquid crystalline polymers is a measure of elasticity and is affected by shear stress due to the change in the morphology at the molecular and nanostructure level. Generally for macromolecules the first normal stress difference increases with increase in stress level. However, in polymeric nanocomposites, N1 is affected by the level of loading and the dispersion of nanofillers. The relationship between N1 and filler contents could be used as a scale to verify the level of dispersion as N1 behaves independent of filler contents in intercalated systems but is dependent in exfoliated systems. In this study, the steady shear rheological behaviours of polylactide (PLA) and nanographite platelets (NGP) bionanocomposites with 1, 2, 3, and 5 wt% filler contents have been investigated. In addition the shear rheological properties of glass filled liquid crystalline polymers (LCPs) have also been examined. The objective has been to obtain a correlation between N1, filler contents, and shear rate of the measurements. The results suggest that the N1 in PLA/NGP bionanocomposites is dependent on the level of loading and on shear rate beyond a critical value. However, a similar correlation between N1 and shear rate for liquid crystalline polymer composites could not be obtained in both high and low rate regions. The anomalous behaviour of N1 for liquid crystalline polymers will be discussed.