Short-term mechanical and structural approaches for the evaluation of polyethylene stress crack resistance

Abstract

Short-term mechanical behavior together with structural habits at various scale levels of polyethylene are studied in relation to molecular architecture in order to contribute to the understanding of the environmental stress crack resistance (ESCR). Three series of homopolyethylene and ethylene–hexene copolymers issued from 1st and 2nd generation chromium oxide catalysis as well as tandem-reactor Ziegler-Natta catalysis are investigated. Properties are discussed via the incidence on the chain topology of the co-unit concentration and distribution as well as the molar weight distribution. The creep compliance is mainly governed by density, i.e. stiffness. However, at similar density, the three polymers series displays a clear compliance drop that follows the increasing ESCR hierarchy. The natural draw ratio displays monotonous decrease with decreasing density that perfectly parallels the ESCR increase. This property also holds for materials of similar density. The creep compliance and the natural draw ratio are indicators of the macromolecular network strength via the intercrystalline tie molecules. Stepwise isothermal segregation (SIS) allows indirect assessment of the chain fraction that generates a high rate of tie molecules. Correlation is made between SIS and ESCR via the tie molecules that provide strength to the macromolecular network. SIS also enables discriminating polymers of similar density but different molecular architecture. Small-angle X-ray scattering affords quantitative evaluation of the stacking disorder of the semi-crystalline microstructure for which a correlation is established with the crystallization kinetics and the resulting chain topology.