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Molecular instabilities in capillary flow of polymer melts: Interfacial stick‐slip transition, wall slip and extrudate distortion
Author(s) -
Wang ShiQing,
Drda Patrick
Publication year - 1997
Publication title -
macromolecular chemistry and physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.57
H-Index - 112
eISSN - 1521-3935
pISSN - 1022-1352
DOI - 10.1002/macp.1997.021980302
Subject(s) - die swell , slip (aerodynamics) , capillary action , materials science , polymer , rheology , extrusion , mechanics , slippage , molecular dynamics , composite material , chemical physics , thermodynamics , chemistry , physics , computational chemistry
This article summarizes our recent experimental studies of an important class of polymer melt rheological phenomena including flow instabilities, stick‐slip transitions, interfacial melt/wall slip, gross extrudate distortion and sharkskin like surface roughening of extrudates. Our experimental results indicate that most aspects of the perplexing flow behavior of strongly entangled melts in polymer extrusion can be depicted in terms of melt/wall interfacial interactions at a molecular level. For high surface energy walls, a transition from no‐slip to slip hydrodynamic boundary condition (HBC) occurs through molecular disentanglement (coil‐to‐stretch) of adsorbed chains from unbound chains. For weakly adsorbing walls, interfacial slip takes place through a second molecular mechanism involving a stress‐induced polymer desorption process. Since the coil‐stretch transition of adsorbed chains is controlled by interfacial shear stress, the HBC can be unstable throughout the die wall under the condition of a constant flow rate, where the wall stress may fluctuate in time. Unstable HBC due to the interfacial molecular instabilities is shown to be the origin of a variety of global and local flow instabilities including the oscillating flow and the sharkskin like distortion of extrudate surface. With a set of new experiments to directly probe the interfacial molecular processes, we will be able to support the unified understanding of intriguing capillary melt flow characteristics summarized in this article.