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

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 2 wt% and less are reported. Viscosity reductions of similar to 93% are observed at 185 degreesC when the TLCP is fully nematic and similar to 89% at 220 degreesC when the TLCP is, initially, a nematic-isotropic biphase. HDPE extrudate distortion and melt fracture are completely eliminated for apparent shear rates up to 1000 s(-1) at 185 degreesC. Using a Mooney analysis, wall slip is shown to contribute to the viscosity reduction at a negligible level. At 220 degreesC, viscosity reductions are observed at a much higher critical wall shear stress than at 185 degreesC. This has been attributed to a flow induced phase transition from isotropic to nematic phase at 220 degreesC. 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. (C) 2001 Published by Elsevier Science Ltd.