The effect of TLCP melt structure on the bulk viscosity of high molecular mass polyethylene

A thermotropic liquid crystalline polyester (TLCP), denoted as TLCP3 and based on hydroxybenzoic acid, hydroquinone and sebacic acid, was used as a processing aid in the extrusion of high molecular mass polyethylene (HMMPE). This TLCP is in the nematic phase at 179.6-182 degrees C. Capillary rheometry experiments were carried out at two processing temperatures: 190 degrees C and 230 degrees C. At 190 degrees C TLCP3 is predominantly nematic and at 230 degrees C it is predominantly isotropic. It is an effective processing aid for HMMPE, particularly at 190 degrees C, with viscosity reductions in excess of 90% with a 1 wt% TLCP3/HMMPE blend. The rheological characteristics of the blends have been linked to the optical textures of the TLCP3 using hot-stage optical microscopy. From the experimental observations speculations are made about the mechanisms of viscosity reduction. Initial viscosity reductions are caused by TLCP3 structure effects (fibrillation and phase change), giving rise to fibrillation-induced molecular orientation in the neighboring HMMPE phase. This is only effective when nematic structures are present. Above a critical wall shear stress value the lubrication effect due to TLCP3 migration is dominant.