In extrusion of polymer melts and solutions, flow instabilities occur that lead to
undesirable distortions of the surface; experiments link these
instabilities to slip of the polymeric liquid relative to the solid
surfaces of the extruder. Both
the mechanisms of slip at the molecular level (e.g. desorption and
disentanglement) and its consequences at
the macroscopic level have been active and controversial research
topics in the last decade. Recent work by our group in this area includes:
- Prediction of a new mechanism for flow-induced enhancement
of concentration fluctuations in concentrated polymer
solutions. We
have shown theoretically that in flows with slip, an instability
develops at high shear rate that leads to large concentration
fluctuations localized near the polymer/solid interface. This
result captures the main features of recent and previously
unexplained experimental observations in polystyrene solutions.
This new mechanism may be of practical importance in processing of
entangled polymeric materials.
- Establishment of criteria for determining
whether or not a given mathematical model of slip in polymer flow
will lead to small scale (short wavelength) flow instabilities,
and in particular, demonstrating that a particular model that we
developed leads to flow instability, agreeing qualitatively with
experimentally observed
trends.
- Demonstration of the role of stress-induced desorption of
polymer chains in the transition from weak to strong
slip.
