The rheology and structure of thermotropic liquid crystalline polymers in extensional flow

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Virginia Tech

The transient shear and elongational flow behavior of HPC EF, G, and HBA/HNA (Vectra A900) have been measured in order to determine the behavior of liquid crystalline polymer systems and to establish whether this behavior is different from that for isotropic flexible-chain polymer melts and isotropic systems of rodlike polymers. In order to accomplish this a rotary clamp extensional rheometer was constructed which is capable of measuring the elongational flow behavior of polymer melts up to 320 °C and which can measure elongational viscosities as low as 1000 Pa's. Tests were conducted on HPC EF at 190 and 210 °C, on HPC G at 200 and 240 °C, and on HBA/HNA at 301 and 320 °C. It was determined that the transient shear stress behavior of the LCP systems scales with strain or alternately, reduced time and that the reduced stress is independent of shear rate over the range of rates investigated. This behavior is different from that for isotropic melts of flexible-chain and rodlike polymers in that the reduced stress for these systems is dependent on deformation rate. The transient elongational viscosity behavior of the LCPs was determined and found to follow linear viscoelastic response at very low strains and then shows mild strain hardening with increasing strain, which is qualitatively similar to the behavior of certain linear polyolefins such as PS and HDPE. From the elongational viscosity behavior determined for both isotropic and anisotropic HPC melts it was found that differences in the melt state can result in qualitative differences in the measured behavior. However, the presence of residual crystallinity in the systems studied makes it uncertain whether the results determined here are general for all LCP systems.

The prediction of the Doi theory for both steady and transient shear and elongational flow were calculated in order to establish whether the theory can predict or model the flow behavior of LCPs and to establish whether the elongational flow behavior of LCPs can be predicted from their shear flow behavior. The Doi theory was evaluated with the viscous drag or rod-solvent friction term retained in the equation for the stress tensor. It was found that the Doi theory cannot predict the shear or elongational flow behavior of LCPs with parameters determined only from molecular properties. At the same time, the Doi theory was found to be able to semi-quantitatively model the steady shear viscosity behavior of isotropic and anisotropic systems of rodlike polymers with retention of the viscous drag contribution to the stress tensor. Also, the Doi theory was able to qualitatively model the transient flow behavior of the same systems, though agreement for the anisotropic systems depended on the use of a multidomain average to represent the globally unoriented state of the material at equilibrium. However, the Doi theory was not able to predict the transient elongational flow behavior of either isotropic or anisotropic systems with parameters fit to shear flow data. Furthermore, the predictions of the Doi theory for elongational flow showed only marginal qualitative agreement with the experimentally determined behavior of both isotropic and anisotropic systems.