PhD Oral Exam - Shadrach Kwakye-Nimo, 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

Polymer melt exhibits complex relaxation and flow behaviors when subject to some form of deformation. The ensued stress experienced by the melt depends on the entire deformation history and not just the deformation rate or the magnitude. To better anticipate the response of melts to deformations, an understanding of their molecular structure is essential. This understanding helps not only in optimizing the process conditions, which turn to be big cost savers, but also enables the production of end-products with controlled and desirable properties.

The research presented here establishes correlations between the features of the molecular weight distributions (MWD) and the rheological behavior of broadly distributed bimodal HDPE and well tailored distributions of polybutadiene. We start by characterizing a set of high-density polyethylene (HDPE) materials which possess very high levels of polydispersity. Then, using statistical learning, the most important features influencing the rheological behavior are identified. We show that by using just the shape features of the MWD the relaxation behavior of the melt can be inferred. We proceed to investigate the behavior of polymer melt at the interface. Using the simple shear flow experimental setting, we eliminate the possibility of contributions from flow induced fractionation and show for the first time that surface segregation could occur in the absence of bulk shear gradient. We additionally show that apart from the surface enrichment being shear-rate dependent, the enrichment-depletion transition point also occurs at a much lower molecular weight value than predicted in literature. We then proceed to show the key role played by the short chain in surface fraction. By using bi-disperse polybutadiene designed to vary in both short chain length and volume, an enhanced relaxation of the melt was shown to positively improve the polymer surface fraction. We finally show the dependence of non-linear rheological behavior of the materials on the features of their molecular weight distributions.