Atomistic mechanism of physical ageing in glassy materials

Abstract

PACS 81.05.Kf -Glasses (including metallic glasses) PACS 64.70.Q--Theory and modeling of the glass transition PACS 05.40.-a -Fluctuation phenomena, random processes, noise, and Brownian motion Abstract. -Using molecular simulations, we identify microscopic relaxation events of individual particles in ageing structural glasses, and determine the full distribution of relaxation times. We find that the memory of the waiting time tw elapsed since the quench extends only up to the first relaxation event, while the distribution of all subsequent relaxation times (persistence times) follows a power law completely independent of history. Our results are in remarkable agreement with the well known phenomenological trap model of ageing. A continuous time random walk (CTRW) parametrized with the atomistic distributions captures the entire bulk diffusion behavior and explains the apparent scaling of the relaxation dynamics with tw during ageing, as well as observed deviations from perfect scaling.

Deformation-induced accelerated dynamics in polymer glasses

“…The mean-squared displacement of the polymer beads increases with the number of hops as ∆x(n) 2 ∼ √ n (Rouse dynamics) rather than ∼ n for simple molecular glass formers due to the constraints of the covalent bonds. Alternatively, the aging behaviour of the relaxation time distributions were found to be qualitatively similar for the polymer model and the BMLJ glass [46]. This is a good indication that the results discussed here apply quite generally to glassy materials, and the polymer-specific aspects of the dynamics are seen primarily in the displacements rather than the relaxation times.…”

“…The hop displacements predictably become larger with increasing threshold. Using a continuous time random walk [46], we find that the mean squared displacement and van Hove function are self-consistently described by the hop statistics obtained using a range of thresholds near the shoulder of p(σ) (Fig. 12).…”

“…[46], we showed that aging can be understood as the direct result of the wide distribution of persistence times which, in the thermodynamic limit, has an infinite mean. Evolving the system of particles using a continuous time random walk parameterized with the initial condition p(t 1 , t w = 0), which reflects the quench, and the persistence time distribution p(τ ) is sufficient to predict the observed changes to the first hop time distributions Mean-squared displacement as a function of the number of hops n that a particle has experienced for the polymer model (•) and the BMLJ model ( ) (simulation details of the BMLJ results can be found in [46]). Lines have slopes of 1/2 (blue) and 1 (red).…”

Evolution of collective motion in a model glass-forming liquid during physical aging

“…Rottler and Robbins have shown how aging a polymer glass affects its yielding behaviors. 11 Additionally, Warren and Rottler [12][13][14] have provided a detailed analysis of the dynamics and creep response of an aging polymer glass. By providing a detailed analysis of the characteristic times of particle displacement in a polymer glass, they have shown how the mean of the distribution of hop times is divergent for an aging polymer, and the application of stress changes the shape of the distributions such that the mean hopping time becomes finite and relaxation is possible.…”

Computer simulations of glasses: the potential energy landscape

“…Warren and Rottler demonstrated the viability of CTRW as a variation of the "cage" or "trap" model by parameterising it using ultra-long term MD simulations of a binary Lennard-Jones glass. They observe that the key feature is that trap states are uncorrelated, and the trap energy is chosen from an exponential distribution at each step [188,189]. Helfferich et al detail the implementation of such a scheme, adding the further constraints that particles may not loop back to their starting positions, or oscillate between two states [190,191].…”