Mechanisms of viscosity reduction in polymer blends containing thermotropic liquid crystalline polymer

The objective of this thesis is to elucidate the viscosity reduction mechanisms in polymer blends containing thermotropic liquid crystalline polymer (TLCP). We have observed drastic bulk viscosity reductions (> 95%) in PE/TLCP blend containing very small quantity of TLCP (< 1 wt%). In addition, a significant improvement in extrudate surface smoothness has also been observed coupled by the increase in processing window from 34 s^-1 to up to 1000 s^-1 of apparent shear rate. Systematic investigations using, rheo-optical, rheo-Raman, SIMS, XPS, SEM and TEM techniques lead to the conclusion that oriented TLCP induces PE configuration transition from random coils to extended chains that gives rise to such drastic viscosity reduction.

Firstly, systematic rheological measurements were performed in terms of effect of TLCP melt structure, matrix molecular structure and average molecular weight and wall slip. It is observed that the viscosity changes can be brought about only if the matrix and TLCP share similar molecular structures. The viscosity reduction is more significant for higher molecular weight system and a negative wall slip is observed in the Mooney analysis. Viscosity reduction ability of TLCP is stronger when the TLCP is in its fully nematic phase compared with the isotropic phase. Such ability is improved when the isotropic TLCP phase is undergone the shear induced isotropic-nematic transition at the higher apparent shear rate region.

Secondly, the melt interactions between TLCP and PE melt were investigated using the rheo-optical and rheo-micro-raman spectroscopy. It has been observed that the TLCP domain structure has a significant effect on PE crystallization kinetics. An elongated TLCP domain acts as a nucleating site for the row nucleated crystals in PE whereas an un-orientated TLCP domain shows no such effect.

Thirdly, the surface composition analysis on the extrudate surfaces using ToF-SIMS and XPS shows that the extrudate surface is depleted from TLCP as compared with the bulk. However, the surface is covered with a film of anti-oxidant rich PE film, explaining why wall slip is absent.

Finally, the SEM and TEM analysis show that the flow induced TLCP filaments have strong effects on the lamella orientation in PE phase. There is a global or long-range orientation of PE molecules on the extrudate surface which have molecular axis parallel to TLCP fibers. Additionally, TEM reveals inter- diffusion of PE chains to TLCP phase, suggesting strong compatibility between two phases.

The viscosity reduction is proposed and formulated into the binary flow pattern which is proved to be able to well model the experimental rheological responses.