PhD Oral Exam - Mohammadali Sattari, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The properties of polyethylene are determined by its molecular structure. Bimodal polyethylene, in comparison to conventional unimodal polyethylene, shows superior mechanical properties. 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, that determine its final properties and processability.

This study employs a material set featuring systematic variations in molecular weight distribution and short chain branching distribution, which are critical factors in polyethylene structure. 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. Our findings demonstrate that the ethylene sequence length distribution highly influences the structure-property relations. Furthermore, we expand our research to find the main molecular parameters affecting Young’s modulus and other mechanical properties from strain hardening tests.

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 and demonstrate that while short chains influence the slip behavior in weak and strong regimes, the molecular weight distribution does not affect the slip behavior in the strong slip regime, which is consistent with findings from capillary studies.

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 note that melt rupture can occur after a stress and slip velocity plateau is reached, inviting additional studies on the effects of interfacial structural changes during slip.