Physical and mechanical properties of polyethylene for pipes in relation to molecular architecture. I. Microstructure and crystallisation kinetics

Ethylene/alpha -olefin copolymers having bimodal molar weight distribution are investigated in comparison with unimodal copolymers in order to underst and the incidence of the molecular architecture on the stress cracking resi stance. The preferred introduction of the co-units in the longest chains of bimodal copolymers is suggested to favour the occurrence of intercrystalli ne tie molecules during crystallisation. The more complex is the molecular architecture, the greater is the difficulty for crystallisation by regular chain folding. Intermolecular chemical heterogeneity resulting from preferr ed incorporation of the co-units in the long chains enhances the co-unit di sturbing effect on crystallisation without reducing crystallinity. Intra-mo lecular heterogeneity of the co-unit distribution is also suggested to be a n efficient means to generate tie molecules and random chain folding at the expense of regular chain folding. Isothermal crystallisation is used to pr obe the effect of molecular architecture on the crystallisation kinetics. I t appeared that the correlations between kinetics, molecular architecture a nd molecular topology of unimodal copolymers no longer hold when considerin g bimodal copolymers. In contrast, the crystal surface free energy proved t o be sensitive to topological changes resulting from molecular architecture modifications. (C) 2001 Elsevier Science Ltd. All rights reserved.