The Effect of Thermotropic LC Polyesters on the Crystallization Rate and Mechanical Properties of Poly(ethylene terephthalate) Film

Abstract

The thermotropic liquid crystalline polyesters (TLCPs) with two kind of methylene groups in a main chain were synthesized by melt polycondensation. One molecule has repeating units in the following composition; [GRAPHICS] 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). The blends of poly(ethylene terephthalate) and TLCPs with x = 1, 0.7, and 0.3 were prepared by twin screw extrusion. A sheet of the blend was prepared by single screw extrusion and sheet casting, and a film was made by stretching biaxially and annealing the sheet. The effect of TLCP on crystallization rate was dominant in the sheet while TLCP addition itself did not contribute much to the crystallization when compared with the stress induced crystallization in the cold stretching of a stretched and quenched film. The strength and modulus were improved in transverse direction (TD) as well as in machine direction (MD) by adding TLCP. In particular the blend film with random copolyester having x = 0.3 showed high improvements in strength and modulus. The dimensional stability of the film was also improved by the addition of the TLCPs.