Molecular motion at the experimental glass transition

Abstract

We propose a novel computational strategy to study the glass transition of molecular fluids. Our approach combines the construction of simple yet realistic models with the development of Monte Carlo algorithms to accelerate equilibration and sampling. Inspired by the well-studied ortho-terphenyl glass-former, we construct a molecular model with an analogous triangular geometry and construct a `flip' Monte Carlo algorithm. We demonstrate that the flip Monte Carlo algorithm achieves a sampling speedup of about $10^9$ at the experimental glass transition temperature $T_g$. This allows us to systematically analyze the equilibrium structure and molecular dynamics of the model over a temperature regime previously inaccessible. We carefully compare the observed physical behavior to earlier studies that used atomistic models. In particular, we find that the glass fragility and the departure from the Stokes-Einstein relation are much closer to experimental observations. We characterize the development and temperature evolution of spatial correlations in the relaxation dynamics, using both orientational and translational degrees of freedom. Excess wings emerge at intermediate frequencies in dynamic rotational spectra, and we directly visualize the corresponding molecular motion near $T_g$. Our approach can be generalized to a \rev{broad range of molecular geometries and paves the way to a deeper} understanding of how molecular details may affect more universal physical aspects characterizing molecular liquids approaching their glass transition.

Figures (13)

• Figure 1: (a) Schematic representation of the studied molecule with atoms $A$, $B$ and $C$ connected by non-rigid bonds. (b) Representative snapshot of the bulk simulated molecular fluid with $N=4000$ molecules.
• Figure 2: The three Monte Carlo moves of the flip Monte Carlo algorithm. For a selected molecule three flips can be proposed, corresponding approximately to $180^\circ$ rotations around one the three axis shown with lines. Each flip corresponds to an intramolecular swap of two atoms exchanging their identity.
• Figure 3: Temperature evolution of two-point density correlations. (a) Radial distribution function of atoms. (b) Structure factor of atoms. No significant change in structure with temperature is observed.
• Figure 4: Molecular rotational and translational correlation functions for flip MC dynamics (left column) and physical MD dynamics (right column). The temperatures are the same in both columns, from left to right: $T=3$, $2$, $1.6$, $1.4$, $1.25$, $1.2$, $1.15$, $1.1$, $1.05$, $1$, $0.97$, $0.95$, $0.92$, $0.85$, $0.8$, and $0.75$.
• Figure 5: Temperature evolution of three relaxation times defined from different time correlation functions for the physical MD dynamics (full symbols) and the flip MC dynamics (open symbols). Relaxation times are rescaled by the onset time $\tau_o$, independently defined for each dynamics. For physical dynamics, $\tau_o \approx 1.4$, and for the flip dynamics $\tau_o \approx 1.3 \times 10^3$. On this scale, $\tau_2$ and $\tau_\alpha^{\rm cm}$ are almost undistinguishable. The dashed segment indicates the onset temperature $T_o$.