Flow induced chain alignment and disentanglement as the viscosity reduction mechanism within TLCP/HDPE blends

Abstract

Experimental observations on the use of a low transition temperature thermotropic liquid crystalline polymer, TLCP(1), as a processing aid for HDPE at low concentrations of 2wt% and less are reported. Viscosity reductions of ∼93% are observed at 185°C when the TLCP is fully nematic and ∼89% at 220°C when the TLCP is, initially, a nematic–isotropic biphase. HDPE extrudate distortion and melt fracture are completely eliminated for apparent shear rates up to 1000s−1 at 185°C. Using a Mooney analysis, wall slip is shown to contribute to the viscosity reduction at a negligible level. At 220°C, viscosity reductions are observed at a much higher critical wall shear stress than at 185°C. This has been attributed to a flow induced phase transition from isotropic to nematic phase at 220°C.A mechanism elucidating the viscosity reducing effects of the incorporation of TLCP into an HDPE matrix is also proposed. The TLCP droplets firstly deform into long fibrils during entry flow. This is followed by chain alignment of the nematic TLCP molecules within the nematic TLCP droplets. Such chain alignment forces the neighbouring PE molecules to align and disentangle, leading to a reduced bulk viscosity.