Glassy polymers show “strain hardening”: at constant extensional load, their flow first accelerates, then arrests. Recent experiments under such loading have found this to be accompanied by a striking dip in the segmental relaxation time. This can be explained by a minimal nonfactorable model combining flow-induced melting of a glass with the buildup of stress carried by strained polymers. Within this model, liquefaction of segmental motion permits strong flow that creates polymer-borne stress, slowing the deformation enough for the segmental (or solvent) modes then to re-vitrify. Here, we present new results for the corresponding behavior under step-stress shear loading, to which very similar physics applies. To explain the unloading behavior in the extensional case requires introduction of a “crinkle factor” describing a rapid loss of segmental ordering. We discuss in more detail here the physics of this, which we argue involves non-entropic contributions to the polymer stress, and which might lead to some important differences between shear and elongation. We also discuss some fundamental and possibly testable issues concerning the physical meaning of entropic elasticity in vitrified polymers. Finally, we present new results for the startup of steady shear flow, addressing the possible role of transient shear banding.
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28 March 2013
Research Article|
January 02 2013
Modeling the relaxation of polymer glasses under shear and elongational loads
S. M. Fielding;
S. M. Fielding
1Department of Physics,
Durham University
, Science Laboratories, South Road, Durham DH1 3LE, United Kingdom
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R. L. Moorcroft;
R. L. Moorcroft
1Department of Physics,
Durham University
, Science Laboratories, South Road, Durham DH1 3LE, United Kingdom
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R. G. Larson;
R. G. Larson
2Department of Chemical Engineering,
University of Michigan
, Ann Arbor, Michigan 48109-2136, USA
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M. E. Cates
M. E. Cates
3SUPA, School of Physics and Astronomy,
The University of Edinburgh
, JCMB Kings Buildings, Edinburgh EH9 3JZ, United Kingdom
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J. Chem. Phys. 138, 12A504 (2013)
Article history
Received:
September 06 2012
Accepted:
November 08 2012
Citation
S. M. Fielding, R. L. Moorcroft, R. G. Larson, M. E. Cates; Modeling the relaxation of polymer glasses under shear and elongational loads. J. Chem. Phys. 28 March 2013; 138 (12): 12A504. https://doi.org/10.1063/1.4769253
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