Properties of mesogenic polyester binary systemscomposed of smectic and nematic mesogens

Abstract

Thermotropic liquid crystalline polyesters with two different mesogenic groups of smectic and nematic state were prepared in the form of random copolymer, block copolymer, and polymer blend. The thermal properties as well as the mesomorphic behavior were studied by differential scanning calorimetry and polarizing microscope. Crystallization kinetics of the homopolymers and their blends were analyzed using Avrami equation through isothermal DSC method. Rheological properties were studied by rheometrics dynamic spectrometer. The structural effects of the flexible spacer of the methylene group and the rigid mesogenic unit on the properties of the liquid crystalline polymer were also studied. The crystal melting temperatures of the random copolymers exhibited a eutectic point, and in the liquid crystalline state, the smectic to nematic transition was observed. In contrast to the random copolymer, the polymer blend showed coexisting nematic and smectic phases in the liquid crystalline state. The mesomorphic state of the polymer blends was very similar to that of low molecular weight binary mixtures. The thermal behavior of the block copolymers having long block segment was similar to that of the polymer blends, while block copolymers having short block segment resembled the random copolymers. It was found that the structure of the crystalline and liquid crystalline phases were strongly affected by components with different length of flexible spacer units in all of the two component polyester systems. The crystallization kinetic analysis showed that little reorganization occurred during the crystallization of the nematic polymer (NP6) from the liquid crystalline state, but massive structural rearrangement was observed for the smectic polymer. For the polymer blends, it was observed that the nematic polymer acted as a nucleating agent for the crystallization of the smectic polymer. The rheological behavior of the homopolymers in the liquid crystalline state was simila...