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Atomic mobility in strained glassy polymers: The role of fold catastrophes on the potential energy surface

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Abstract

Deformation is known to enhance the atomic mobility in disordered systems such as polymer materials. To elucidate the origin of this phenomenon, we carry out two types of simulations: molecular dynamics (MD) simulations, which determine the atomic trajectories at finite temperature, and quasi-static simulations, which determine the atomic trajectories in the limit of zero temperature (and in the limit of zero shear rate). The quasi-static simulations show discontinuous changes in properties, such as system energy and atomic mobility. We use a normal mode analysis to show that these discontinuous changes arise from fold catastrophes of the potential energy landscape, in which energy minima flatten out and the heights of energy barriers decrease to zero; this was demonstrated by normal mode frequencies following a power law with an exponent of 0.5 as the discontinuous change is approached. After the fold catastrophe, the system relaxes to a different energy minimum, giving rise to atomic displacements. These fold catastrophes are the only mechanism for diffusive atomic displacements in the quasi-static simulations, where there is no thermal energy. We compared the mean-squared displacements as a function of strain from the quasi-static simulations to those from MD simulations (which do include thermal effects)—the similarity of the values of the mean-squared displacements in these two types of simulations demonstrates that the fold catastrophes underlie the enhanced dynamics in strained polymer systems even at finite temperature. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

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