Recovery of Polymer Glasses from Mechanical Perturbation

Abstract

Molecular dynamics simulations of a coarse-grained model of a polymer glass were used to study the recovery regime following deformation at constant stress or constant strain rate. We monitor dynamical as well as structural and energetic quantities to characterize the impact of deformation on the relaxation process. The α-relaxation times are reduced relative to an unperturbed sample immediately after deformation, and we observe a gradually increasing "erasure" of memory with increasing amount of deformation. Remarkably, the single deformation parameter that captures this continuous transition is the total strain at the end of deformation. For deformation up to of order the yield strain, the evolution of multiple measures of short-range order and the inherent structure energy closely track changes of the αrelaxation time, suggesting that the "rejuvenated" state is close to that of a younger glass. For larger strains accessible by constant strain rate deformation, however, plastic yield generates a distinctly different thermodynamic state.

Correlation between rearrangements and soft modes in polymer glasses during deformation and recovery

“…Even larger decreases in fact below the pre-aging value are seen in the pure shear case. This reversal of the aging effects is consistent with mechanical rejuvenation, by which deformation strains exceeding the yield strain erase the thermal history and return the material to a freshly quenched glassy state [21].…”

“…We study the polymer glass using molecular dynamics techniques and the well-known finitely extensible nonlinear elastic (FENE) bead-spring model [19], which is an excellent computer glass-former [20,21]. Each linear polymer consists of 50 identical monomers with covalent bonds along the polymer backbone that were modeled by a non-linear stiff spring-like interaction.…”

Distribution of local relaxation events in an aging three-dimensional glass: Spatiotemporal correlation and dynamical heterogeneity

“…It consists of a 6-12 Lennard-Jones (LJ) potential and a stiff non-linear spring-like interaction between bonded particles that prevents chain crossings. This model is used extensively in the study of glassy systems and exhibits key features like a power-law age dependence of the relaxation time and logarithmic aging of structural properties [4]. All results are reported in the usual LJ units, based on energy well depth ǫ, particle diameter σ, and mass m, as well as the characteristic time scale τ LJ = mσ 2 /ǫ 1/2 .…”

“…Beginning with the seminal studies of Struik 35 years ago on polymers [2], experiments consistently find that during aging, bulk quantities such as density and enthalpy increase logarithmically with the time elapsed since the glass was formed, while the principal structural relaxation time increases as a power law with age. This α-relaxation time, which can be directly observed from the decay of intermediate scattering functions [3] in computer simulations, ties the molecular scale dynamics to the non-equilibrium evolution and can be used as an "internal clock" of the glass [4]. The universal features of aging are an essential ingredient for the development of predictive models of the mechanics of glasses [5,6] and are intimately tied to the glass transition itself.…”

The UA?CG Workflow: High Performance Molecular Dynamics of Coarse-Grained Polymers

“…Unfortunately, most macromolecular processes of interest contain many orders of magnitude more particles and often bridge microsecond or even millisecond timescales or longer. These include phenomena like phase separation in polymer blends and composite materials [1], polymer crystallization, and glass formation and aging [2] to mention just a few. Despite our pervasive access to massively parallel computers, full unitedatom (UA) simulations do not come close to representing real-world polymer systems (see Figure 1), because they are too computationally expensive and slow.…”