Macromolecules, Vol.32, No.17, 5581-5593, 1999
Dynamics and shear orientation behavior of a main-chain thermotropic liquid crystalline polymer
The dynamics and shear orientation behavior of a model thermotropic liquid crystalline polymer (DKMS-7,9) were characterized by mechanical rheometry and in situ wide angle X-ray diffraction (WAXD). In the nematic phase (between 120 and 192 degrees C), DHMS-7,9 shows a three-region flow curve accompanied by a strong and relatively constant orientation over the entire shear rate range. A mysterious mesophase occurs between 92 and 120 degrees C that has much higher viscosity than that of the nematic phase and a monotonically shear thinning flow curve. In this phase, a striking flip of the orientation from the flow direction to the vorticity direction occurs below a critical shear rate. This orientational flipping is reversible in response to step changes of temperature and/or shear rate. Examination of the linear viscoelastic behavior suggests certain rheological similarities between the x-phase and layered fluids.
Keywords:NORMAL STRESS DIFFERENCE;RHEOLOGICAL DIFFERENCES;MOLECULAR-ORIENTATION;NEUTRON-SCATTERING;PHASE-TRANSITION;X-RAY;FLOW;RHEOMETRY;POLYETHER;MODEL