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 shearrate
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.