TRANSIENT ELONGATIONAL FLOW BEHAVIOR OF THERMOTROPIC LIQUID-CRYSTALLINE POLYMERS

The transient elongational viscosity (eta(e)+BAR) behavior of two thermotropic liquid crystalline polymers, LCPs (hydroxypropylcellulose, HPC, and a copolyester of 2-hydroxy-6-naphthoic acid and para-hydroxybenzoic acid, Vectra A900), has been found to be similar to that determined for isotropic melts of linear polyolefins such as high-density polyethylene (HDPE). Measurements were made on a newly constructed rotary clamp extensional rheometer designed to handle low-viscosity materials at high temperatures. The values of eta(e)+BAR were observed to follow linear viscoelastic behavior at low strains and subsequently show mild strain hardening with increasing strain. The elongational flow behavior of HPC in the isotropic state was also determined and compared to that in the anisotropic state. Differences in the behavior appear to be related to differences in the structure of the melt and might indicate the influence of the melt state on the flow behavior of LCPs. However, the generality of these results to other LCP systems is uncertain because of the possible influence of residual crystallinity. The predictions of the Doi theory for elongational flow were also compared to the experimental behavior of the isotropic and anisotropic melts. It was found that the Doi theory could not predict quantitatively the elongational flow behavior of LCPs when the parameters in the model were determined from shear viscosity data. Additionally, while the Doi theory showed some qualitative agreement with the experimental behavior of both isotropic and anisotropic melts at low strains, it was unable to predict the strain-hardening behavior seen in those systems.