Correlation of the melting behavior and copolymer composition distribution of Ziegler-Natta-catalyst and single-site-catalyst polyethylene copolymers

The multimodal differential scanning calorimetry melting endotherms observed for commercial linear low-density polyethylenes are due to broad and multimodal short-chain-branching distributions. Multiple peaks, observed in melting endotherms of isothermally melt-crystallized and compositionally homogeneous polyethylene copolymers are due to intrachain heterogeneity. This intrachain heterogeneity is quantified by the distribution of ethylene sequence lengths within the chains. These compositionally homogeneous copolymers undergo a primary crystallization, which produces a population of thicker lamellae, creating a network that places severe restrictions on segment transport in subsequent secondary crystallization, which produces a population of thinner crystals. The restrictions on segment transport imposed by the initial network created by the primary crystallization of thicker lamellae severely limits the total crystallinity achieved in the random copolymers studied. The solution crystallization of such copolymers produces a continuous distribution due to more facile segment transport in a dilute solution, in contradistinction to the multimodal distribution produced in the melt crystallization. (C) 2001 John Wiley & Sons, Inc.