Studies on the rheology and morphology of thermotropic liquid crystalline polymers
It is known that the physical properties of as-processed liquid crystalline polymers are highly dependent on the thermal and deformation histories of the materials experienced. The purpose of this study has been to examine the effect of thermal history on the rheology and morphological texture for several thermotropic liquid crystalline polymers. These include two copolyesters of para-hydroxybenzoic acid and polyethylene terephthalate and a copolymer of para-hydroxy benzoic acid and 6-hydroxy-2-naphthoic acid. For all three systems, it was found that the viscosities at temperatures below the normal flow temperatures were reduced as a result of preheating. Furthermore, these polymers were able to flow at temperatures as much - as 50°C below their normal flow temperatures if preheated and cooled rapidly. Also found was that it took a few minutes for the viscosities of the preheated samples to recover to a higher level at which the flow was ceased, during this period the materials were processable. These behaviors were attributed to the supercooling of the nematic state formed at preheating temperatures. This was supported by the lack of transition peaks in DSC traces obtained under the same cooling history as that in the Rheometric Mechanical Spectrometer. In the cold die extrusion experiment, a skin/core structure was observed for most extrudates. The thickness of the skin layer was directly related to the orientation and tensile properties of the extrudate. The thicker the skin layer was, the better the orientation and tensile properties were. The thickness of the skin layer was also found to increase with the extrusion speed. However, the orientation was only limited to the skin layer with the core region was relatively unoriented.
For the lubricated squeezing flow experiments, the newly designed device worked satisfactory and predicted the relationship that the transient biaxial extensional viscosities are very close to six times the transient shear viscosities for polystyrene at small strains. Loss of lubrication occurred at fairly low strains, 0.3 to 0.4, was the limit of this method. For all three liquid crystalline polymers, the transient biaxial extensional viscosity was found to decrease with the increase in squeezing rate and no steady state was reached. Yield stresses were observed for liquid crystalline polymers under the squeezing jump strain deformation and the magnitude of these yield stresses were of the order of several hundreds Pa. The existence of yield stresses could be due to the presence of some structure in the sample which prevented the stress from relaxing to zero. The constant stress lubricated squeezing flow method has been proved to not be suitable for estimating the biaxial extensional viscosity of liquid crystalline polymers because the constant slope region for the creep curve was too short and it was difficult to determine the extension rate accurately. Under a heavy stress of 70368 Pa and proper cooling, it was possible to generate some fibrous structure in the squeezed sample.