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Modeling the relaxation of polymer glasses under shear and elongational loads.

Fielding, S. M. and Moorcroft, R. L. and Larson, R. G. and Cates, M. E. (2013) 'Modeling the relaxation of polymer glasses under shear and elongational loads.', Journal of chemical physics., 138 (12). 12A504.

Abstract

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.

Item Type:Article
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Status:Peer-reviewed
Publisher Web site:http://dx.doi.org/10.1063/1.4769253
Publisher statement:© 2013 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of chemical physics, 138, 12A504 (2013) and may be found athttp://dx.doi.org/10.1063/1.4769253
Date accepted:No date available
Date deposited:03 June 2014
Date of first online publication:March 2013
Date first made open access:No date available

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