Abstract

The emergence of new single-reactor technologies has enabled the production of bimodal polyethylene at reduced capital costs, prompting a need for deeper investigations into the characterization and property-structure relationships of these materials. In this thesis, we are looking at how the molecular structure affects both the solid-state and the melt-state properties of polyethylene.
This study employs a material set featuring systematic variations in molecular weight distribution and short chain branching distribution. The initial step involves extracting the fundamental molecular parameters of the materials by utilizing statistical modelling to acquire the ethylene sequence length distribution and its features. Subsequently, we identify the key parameters governing our materials’ environmental stress crack resistance, a crucial concern, particularly for solid-state properties in pipe applications of bimodal polyethylene. We showed that many mechanical properties from strain hardening tests correlate with features of ethylene sequence length distribution, showing that this distribution is the most meaningful in describing the structure of bimodal polyethylene in terms of solid-state properties. An approach to studying the critical correlations between structure, crystallization, and mechanical properties is presented.
In the latter part of this study, we investigate the effect of molecular weight distribution on the wall-slip behavior of bimodal polyethylene melt under simple shear. The study under simple shear is important as it eliminates the confounding effects of pressure and shear rate gradient. We examine the impact of low molecular weight content on various slip regimes. We showed that the stress level at which the slip velocity becomes independent of molecular weight distribution decreases with an increase in short-chain content. The degree of entanglement between the interfacial and bulk chains drops sharply in the transition region based on the friction of coefficient investigations.
Throughout our investigations, we frequently observe melt rupture in our materials, prompting further exploration of this instability using visualization techniques. Melt rupture is critical as it limits the industrial applications of these materials. We showed that melt rupture is a time-dependent phenomenon that can occur after a stress and slip velocity plateau is reached, inviting additional studies on the effects of interfacial structural changes during slip.