Rheology of Unfilled and Filled Injection Moulding Grade Liquid Crystal Polymers

A liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. It has a particular kind of structural orientation in one or two dimensions. This structural orientation makes the rheological properties of Liquid Crystal Polymers (LCPs) often extraordinary and only partially understood. Furthermore, these properties vary from one LCP to another, which results in an unpredictable behaviour. It has been reported that accurate measurements of the shear rheological properties of LCPs are critical to allow appropriate prediction of LCP flows under pressure in a typical injection moulding machine. Whenever a thermotropic LCP is subjected to a shear field, there is a molecular reorientation process taking place resulting in the formation of a polydomain structure, which finally leads to behaviour like any other isotropic polymer. Rheological properties of four thermotropic LCPs were investigated from low to high shear rate regions. Out of these four LCPs, two were unfilled. The shear viscosities of the four LCPs show almost three zone dependency and the complex viscosities of the four LCPs show a typical shear-thinning behaviour. The effect of filler was investigated in terms of various rheological properties. The unusual temperature dependence of the viscosities can be explained by the nematic-isotropic transition, in which the rod-like molecules lose part of their orientation ordering and became partially isotropic with increased temperature.