EFFECT OF MOLECULAR-STRUCTURE ON RHEOLOGICAL AND CRYSTALLIZATION PROPERTIES OF POLYETHYLENES

This article describes the development of reliable techniques to measu re the isothermal crystallization rates (ICR) under quiescent as well as under small amplitude, oscillatory shear conditions. Quiescent crys tallization rates were obtained using a differential scanning calorime ter. Those under small amplitude shear were obtained using Rheometrics rheometers. It is shown how a small amount of long-chain branching in high-density polyethylene homopolymer (HDPE) dramatically influences rheological properties and enhances ICR. For these HDPEs, the rate inc reases with the increase in long-chain branching. The general applicat ion of isothermal crystallization studies, however, should be done wit h great caution. This is because the fundamentals of isothermal crysta llization require that it be done on the basis of a fixed undercooling with respect to the equilibrium melting temperature. Such a temperatu re is ill-defined for the commercial polymers having broad molecular w eight distribution (MWD). Nonetheless, a practical procedure is outlin ed wherein the melting curve of a previously isothermally crystallized sample is used as a substitute for judging the equilibrium melting po int and in deciding the selection of a proper crystallization temperat ure. Even this new procedure may not be applicable for polymers having heterogeneous short-chain branching distribution. (C) 1996 John Wiley & Sons, Inc.