Title: Shear Induced Demixing in Polymer Melts and Solutions

Advisors: L. Gary Leal and Glenn Fredrickson

Abstract:

In most theoretical and experimental studies of multi-component complex fluids, it is implicitly assumed that if the fluid is well-mixed at rest, it will remain well mixed in flow. To leading order, this may be a reasonable first approximation for many materials and many applications. However, there is an abundance of experimental evidence to show that well-mixed polymer melts and solutions can fractionate or even demix in the presence of a sufficiently strong shear flow. To advance the current theoretical understanding of shear induced demixing phenomena, we have studied the linear and non-linear dynamics of shear induced demixing in two-fluid models of polymer melts and polymer solutions.
 
In a study of polymer blends a novel rheological model is introduced, combining the micromechanical picture of ‘double reptation’ with the non-linear stress relaxation kinetics of a ‘Rolie Poly’ fluid. In a comparison to experiments, this model is shown to capture the effects of polydispersity in both rheometric and non-rheometric flows of bi-disperse blends. Next, this rheological model is used to develop an improved two-fluid model of bi-disperse polymer blends. In the limit where a bi-disperse blend approximates an entangled polymer solution (i.e. self-entangled long chains mixed with much much shorter chains) the blend model qualitatively agrees with existing polymer solution models. More surprisingly, the blend model predicts that demixing instabilities persist even for blends of very similar chains; since the entropic mixing forces are small compared to the typical scale of elastic forces in the system, very little rheological contrast between chains is needed for a demixing instability to occur.