Semiflexible thermotropic liquid crystalline polymers with a mesogenic unit of 4,4``-diphenoxy terephthalate and two flexible spacers of decamethylene and hexamethylene were synthesized changing the ratio of two aliphatic units in a main chain. The polymers had the following schematic molecular structures:
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where x was controlled to have a value of 0, 0.25, 0.5, 0.75, and 1. The texture of all synthesized polymers observed by a polarized microscopy showed a typical schlieren texture in the range between melting and isotropization temperatures, suggesting nematic liquid crystalline polymers.
The rheological properties of the polyesters was measured by Rheometrics dynamic spectrometer (RDS) to determine the condition of fiber spinning. Melting transition temperatures measured by RDS (i.e., rheological transition temperature) and polarized optical microscope (POM) were higher than those by differential scanning calorimetry (DSC). The polyesters showed the dependence of rheological properties on the thermal history at a temperature between the melting temperature measured by DSC and the rheological transition temperature by RDS but above the rheological transition temperature they showed almost the same properties affected negligibly by thermal history. The dependence on thermal history was caused by unmelted crystallites which were confirmed by the measurement of yield stress with temperature.
Below the rheological transition temperature spin-drawing of a fiber was difficult because of fiber breakage and the free-fall fiber showed low orientation, tensile strength, and tensile modulus. On the other hand, above it the free-fall fiber had better orientation and higher tensile strength and tensile modulus, and the fiber showed a better orientation and higher modulus with increase of draw ratio. It was found, therefore, the rheological transition temperature measured by rheometer played more important role for the determination of processing tempe...