Materials Science

A Model for Aging

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Science  10 Feb 2012:
Vol. 335, Issue 6069, pp. 636
DOI: 10.1126/science.335.6069.636-d

When cooled below the glass transition temperature, amorphous polymers show a sudden and rapid increase in viscosity, which macroscopically manifests as a hardening or solidifying of the material. Nonetheless, even at these temperatures, local segmental motion is possible, and the polymers thereby age with time as the chains relax into a lower-energy state. Recent experiments have shown that when deformed at a constant load, glassy polymers initially flow rapidly but then suddenly stop, with a striking decrease in the segmental relaxation time, but the basis for this behavior has not been well understood. Fielding et al. tackle this problem by developing a simple model of elastic polymer dumbbells immersed in a glassy solvent, in effect. Under a constant load, rejuvenation of the glassy segments occurs as initial fluidization leads to an accelerated rate of motion. However, the associated increase in the stress borne by the polymer segments then causes an arrest in the strain, allowing the glassy segments to resolidify. The authors note that the unloading behavior of the glassy chains is not well captured by the simple model, as the collapsing polymer chains carry enough stress to cause the glassy solvent to flow. By including a simple modification of the effective modulus of the polymer to reflect more realistic conditions, they are able to capture much of the behavior seen experimentally.

Phys. Rev. Lett. 108, 48301 (2012).

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