In situ composites from blends of polycarbonate and a thermotropic liquid-crystalline polymer: The influence of the processing temperature on the rheology, morphology, and mechanical properties of injection-molded microcomposites

摘要

This work was aimed at understanding how the injection-molding temperature affected the final mechanical properties of in situ composite materials based on polycarbonate (PC) reinforced with a liquid-crystalline polymer (LCP). To that end, the LCP was a copolyester, called Vectra A950 (VA), made of 73 mol % 4-hydroxybenzoic acid and 27 mol % 6-hydroxy-2 naphthoic acid. The injection-molded PC/VA composites were produced with loadings of 5, 10, and 20 wt % VA at three different processing barrel temperatures (280, 290, and 300 degrees C). When the composite was processed at barrel temperatures of 280 and 290 degrees C, VA provided reinforcement to PC. The resulting injection-molded structure had a distinct skin-core morphology with unoriented VA in the core. At these barrel temperatures, the viscosity of VA was lower than that of PC. However, when they were processed at 300 degrees C, the VA domains were dispersed mainly in spherical droplets in the PC/VA composites and thus were unable to reinforce the material. The rheological measurements showed that now the viscosity of VA was higher than that of PC at 300 degrees C. This structure development during the injection molding of these composites was manifested in the mechanical properties. The tensile modulus and tensile strength of the PC/VA composites were dependent on the processing temperature and on the VA concentrations. The modulus was maximum in the PC/VA blend with 20 wt % VA processed at 290 degrees C. The Izod impact strength of the composites tended to markedly decrease with increasing VA content. The magnitude of the loss modulus decreased with increasing VA content at a given processing temperature. This was attributed to the anisotropic reinforcement of VA. Similarly, as the VA content increased, the modulus and thus the reinforcing effect were improved comparatively with the processing temperature increasing from 280 to 290 degrees C; this, however, dropped in the case of composites processed at 300 degrees C, at which the modulus anisotropy was reduced. Dynamic oscillatory shear measurements revealed that the viscoelastic properties, that is, the shear storage modulus and shear loss modulus, improved with decreasing processing temperatures and increasing VA contents in the composites. Also, the viscoelastic melt behavior (shear storage modulus and shear loss modulus) indicated that the addition of VA changed the distribution of the longer relaxation times of PC in the PC/VA composites. Thus, the injection-molding processing temperature played a vital role in optimizing the morphology-dependent mechanical properties of the polymer/LCP composites. (c) 2007 Wiley Periodicals, Inc.