Thermotropic liquid-crystalline copolyester/thermoplastic elastomer in situ composites. I. Rheology, morphology, and mechanical properties of extruded strands

Abstract

A thermotropic liquid-crystalline polymer (TLCP), a copolyester with a 60/40 molar ratio of p-hydroxy benzoic acid and poly(ethylene terephthalate), was blended with a styrene/ethylene butylene/styrene thermoplastic elastomer with a twin-screw extruder. The rheological behavior, morphology, and mechanical properties of the extruded strands of the blends were investigated. The rheological measurements were performed on a capillary rheometer in the shear rate range of 5-2000 s-1 and on a plate-and-plate rheometer in the frequency range of 0.6-200 rad s-1. All the neat components and blends exhibited shear thinning behavior. Both the shear and complex viscosities of all the blends decreased with increasing TLCP contents, but the decrease in the shear viscosity was more pronounced. The best fibrillar morphology was observed in the extruded strands of a blend containing 30 wt% TLCP, and a lamellar structure started to form at 40 wt% TLCP. With an increasing concentration of TLCP, the tensile modulus of the blends was greatly enhanced, whereas the tensile strength was almost unchanged. The elongation at break of the blends first slightly decreased with the addition of TLCP and then sharply dropped at 40 wt% TLCP. The tension set measured at 200% deformation slightly increased with increasing TLCP contents up to 30 wt%, over which the set value was unacceptable for a thermoplastic elastomer. A remarkable improvement in the dynamic mechanical properties of the extruded strands was observed in the blends with increasing amounts of TLCP.