THE EFFECT OF SKIN-CORE MORPHOLOGY ON THE HEAT-DEFLECTION TEMPERATUREOF POLYPROPYLENE

The relationship between solid-state morphology and heat-deflection te mperature (HDT) of nucleated polypropylenes was studied. Using optical microscopy to characterize the morphology and DMTA to determine the t emperature dependence of the tensile modulus, a composite model was ad apted to estimate the HDT. Both compression-molded films and injection -molded HDT bars were investigated. Compression-molded films were isot ropic except for a thin skin, and the temperature dependence of the te nsile modulus was very similar for all compression-molded films regard less of the nucleating agent. Results on isotropic specimens could not account for the higher ASTM HDT of nucleated samples. Injection-molde d HDT bars exhibited an anisotropic gradient in both the morphology an d the temperature-dependent modulus. A composite model was developed t o estimate the HDT. The model successfully predicted the ASTM HDT valu es relative to the HDT of unnucleated polypropylene. The increase in H DT was caused by the increased retention of melt orientation, due, in turn, to the higher crystallization temperature of the nucleated sampl es. (C) 1996 John Wiley & Sons, Inc.