Are rare, long waiting times between rearrangement events responsible for the slowdown of the dynamics at the glass transition?

Abstract

The dramatic slowdown of the structural relaxation at the glass transition is one of the most puzzling features of glass dynamics. Single molecule orientational correlation times show this strong Vogel-Fulcher-Tammann temperature dependence typical for glasses. Through statistical analysis of single molecule trajectories, we can identify individual glass rearrangement events in the vicinity of a probe molecule in the glass former poly(vinyl acetate) from 8 K below to 6 K above the glass transition temperature. We find that changes in the distribution of waiting times between individual glass rearrangement events are much less dramatic with temperature, the main difference being a small, but decisive number of increasingly long waiting times at lower temperatures. We notice similar individual, local relaxation events in molecular dynamics trajectories for a variety of glassy systems further from the glass transition, leading to waiting time distributions with similar features as those observed in the single molecule experiments. We show that these rare long waiting times are responsible for the dramatic increase in correlation time upon cooling.

Deeper penetration of surface effects on particle mobility than on hopping rate in glassy polymer films

“…6, R(τ ) decreases dramatically with τ in general. Detailed examinations of individual particle trajectories show that this is due to the abundance of back-and-forth hopping motions at low T widely studied in the literature [38,[45][46][47][48][49][50][51][52]. Since R(τ ) is a net hopping rate not registering the back-and-forth parts of the hops, at large τ , it underestimates the true hopping rate and is instead a better indicator of particle mobility describing long-time motions in the diffusive regime.…”

“…Surface effects penetrate up to z 5.5σ for τ = 20 and 40, but reach deeper to z 4.5σ for τ = τ max . To establish this more clearly, we numerically evaluate an integrated hopping event correlation defined by Particle back-and-forth motions have long been studied in glassy systems [38,[45][46][47][48][49][50][51][52] and cause the main peak at r = 0 in Fig. 11(b)-(c).…”

Aging in amorphous solids: A study of the first-passage time and persistence time distributions

“…Common approaches make use of (i) the collective motion of particles in the configuration phase space [18][19][20], (ii) the motion of a particle relative to its neighbors [21][22][23], or (iii) the single particle displacement [24]. It has recently been shown that these criteria provide similar results for the distribution of relaxation events [25]. Typical dynamics of a particle in a glassy state consists of in-cage high-frequency rattling motion accompanied by intermittent cage-breakage jumps (structural relaxation events).…”

Dynamic heterogeneity in an orientational glass

“…For the higher studied temperatures, the ratio is close to one, as expected for standard liquid conditions 61 , but for lower temperatures the ratio grows, as in the fluctuation dominated regime of supercooled liquids, where this is a signature of the breakdown of Stokes-Einstein relations 62,63 . Persistence times, defined as the average waiting time from any given point in the trajectory of a single molecule, until the next jump event, are comparable to the structural relaxation time t α in simple models of glass dynamics 64 . The probability distribution of persistence times is comparable to typical results of bulk experi- ments, due to the time-weighted nature of typical observations.…”