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

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.

Reversing Strain Deformation Probes Mechanisms for Enhanced Segmental Mobility of Polymer Glasses

“…Since then, many other models have been developed to understand the deformation behavior of polymer glasses. − Integral to the work of these later workers is the idea that during nonlinear deformation the segmental dynamics (local rearrangements involving a few repeat units along the chain) of a polymer glass becomes significantly faster, which then allows flow to occur at much lower stress than would be predicted by linear viscoelasticity. This interpretation is supported by recent simulations that have observed enhanced segmental dynamics during constant strain rate − and constant stress ,, deformation. Experimentally, changes in segmental dynamics during deformation have been measured by NMR, diffusion, dielectric spectroscopy, , and probe reorientation. ,,− The probe reorientation measurements show that the observed decrease in the average segmental relaxation time during deformation (up to 3 orders of magnitude) can roughly account for the observed flow stress, supporting the view that enhanced segmental dynamics is the key to understanding the nonlinearity of polymer glass deformation …”

Fundamental theories and basic principles of triboelectric effect: A review

“…(This helps explain why even the same polymers can be charged by tribo-contacts; it is because even identical polymers will have different material compositions at each depth, which account for the same material charge transfer [29].) Moreover, material transfer is possible if dynamical changes in the surfaces and tribological environments exist, because materials will tend toward a lower potential energy state and these dynamical changes can lead to the potential energy minimum shifts [53,54]. For example, one dynamical change (in quasistatic form) can be strain and its corresponding potential energy form (near the potential energy local maximum, i.e., the local energy barrier at interface) is…”

“…Wang et al [53] proved that the differences in the microstructure of chemically identical materials trigger distinct tribo-charging behavior. In this sense, as a strained surface will exhibit different microstructures due to voids and seams (which can be scaled from nano-to micrometers) and the different microstructures will trigger different surface potential energy minimum according to catastrophe theories [53,54], as shown in Fig. 5, the strained surface may exhibit different triboelectric behavior [20,53].…”

Multiscale analysis of friction behavior at fretting interfaces

“…According to Fig. 4(d), given the tight-bonding, the deformation is due to inter atomic potential barrier reduction, which frees the atoms/ions in the inner-material to move [47,48]. However, the attachment of the atom requires the energy barrier to be overcome at the interface, which is proportional to their bond dissociation energy.…”