Transient shear flow properties of carbon fiber-filled liquid crystalline polymer

Abstract

Unsteady flow of filled polymer systems can cause some difficulties in their processing, such as injection or extrusion molding, in which flow in the molten state is involved. In order to determine the optimum conditions and thus improve the performance of the end products, it is necessary to understand the transient/unsteady flow behavior of these systems. In a previous work, we found remarkable non-Newtonian properties and shear thickening flow behavior of liquid crystalline polymers (LCP) and their filled systems. This article deals with the dependence of transient flow properties, including shear thickening, of pure and carbon fiber-filled LCP upon the shear rate and shear strain history; the effect of the filler content is also discussed. The results indicate that the abnormal flow behavior of the tested materials may be caused by gradual disintegration of the domain structure formed in LCP (materials exhibit apparent yield stress) and then continual orientation of molecules under shear flow. The shear thickening behavior of the materials seems to disappear with increasing fiber content. This behavior suggests that it is necessary to measure the flow properties under a sufficient ramp point delay time and equilibration time in order to obtain reliable data under stable conditions.