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 a nd in situ wide angle X-ray diffraction (WAXD). In the nematic phase (betwe en 120 and 192 degrees C), DHMS-7,9 shows a three-region flow curve accompa nied 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 monotonical ly shear thinning flow curve. In this phase, a striking flip of the orienta tion from the flow direction to the vorticity direction occurs below a crit ical shear rate. This orientational flipping is reversible in response to s tep changes of temperature and/or shear rate. Examination of the linear vis coelastic behavior suggests certain rheological similarities between the x- phase and layered fluids.