Rheological property and stretching characteristics of thermotropic LC polyesters and their blends with PET

Abstract

The thermotropic liquid crystalline polyesters(LCPs) with two kind of methylene groups in a main chain were synthesized by melt polycondensation. One molecule has repeating units in the following composition; ◁수식 삽입▷(원문을 참조하세요) where Ph means p-phenylene unit and x was controlled to have values of 1, 0.7, 0.5, 0.3 and 0. The random copolyesters(x=0.7, 0.5 and 0.3) revealed lower melting temperatures and wider nematic ranges between melting and isotropic transition temperatures than the two homopolyesters (x = 1 and 0). And the lowest melting temperature was shown in the copolyester with x =0.5. The spinnability of the LCPs with x=1, 0.7, and 0.3, was very good. The spun fiber had good tensile properties comparable to commercialized thermotropic LC polyesters. And the morphology for the tensile fractured and split surfaces of the LCP fibers showed fibrillar structure to prove the good tensile properties. It was well understood from the rheological properties such as a heavy shear thinning behavior with increasing shear rate, apparent yield stress, and a long relaxation time. Also these LCP fibers showed more or less plastic deformation, to insist relatively high flexibility of the fiber, in the tensile test. Especially the copolyester having x=0.7 showed the highest flexibility among them. The blends of poly(ethyleneterephthalate) and LCPs with x = 1, 0.7, and 0.3 were prepared by twin screw extrusion. The melt viscosity for the blend with PET was sharply decreased by the fibrillation of the LCP domain under a strong shearing force near the capillary entrance. A sheet of the blend was prepared by single screw extrusion and sheet casting; and a film was made by stretching biaxially and quenching of the sheet. The LCP domains did not form a fibrillar structure because of relatively low shearing force in the processing of sheet and film. The effect of LCP on crystallization rate was dominant in the sheet while LCP addition itself did not contribute much to the cr...