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

Abstract

The transient shear and elongational flow behavior of HPC EF, G, and HBA/HNA (Vectra A9W) 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 OC 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 OC, and on HBNHNA at 301 and 320 OC. 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.