Invariant Fast Diffusion on the Surfaces of Ultrastable and Aged Molecular Glasses

Surface diffusion of molecular glasses is found to be orders of magnitude faster than bulk diffusion, with a stronger dependence on the molecular size and intermolecular interactions. In this study, we investigate the effect of variations in bulk dynamics on the surface diffusion of molecular glasses. Using the tobacco mosaic virus as a probe particle, we measure the surface diffusion on glasses of the same composition but with orders of magnitude of variations in bulk relaxation dynamics, produced by physical vapor deposition, physical aging, and liquid quenching. The bulk fictive temperatures of these glasses span over 35 K, indicating 13 to 20 orders of magnitude changes in bulk relaxation times. However, the surface diffusion coefficients on these glasses are measured to be identical at two temperatures below the bulk glass transition temperature $T_g$. These results suggest that surface diffusion has no dependence on the bulk relaxation dynamics when measured below $T_g$.

Figure 1

(a) Thermal stabilities of PVD and aged glasses post TMV measurements. Thickness changes of PVD glasses deposited at $0.85T_g$ (solid orange), $0.9T_g$ (solid blue), $0.95T_g$ (solid green), and glasses aged at $0.9T_g$ (open purple) were monitored upon heating and cooling with ellipsometry. The black-dashed line is the universal super-cooled liquid line for TPD. Arrows indicate the fictive temperature for each glass and bulk $T_g$. Inset shows TPD molecular structure. (b) Fictive temperatures versus the deposition or the aging temperatures.

Figure 2

(a) Representative AFM images of the surface response to TMV perturbation on a 400 nm stable glass ($T_{\text{dep}}=0.85T_g$) held isothermally at 303 K ($T_g\text{−}27\mathrm{K}$). Scale bars are 400 nm. (b) Temporal evolution of the profiles across the center of TMV shown in (a), each profile plotted was averaged from five near-center line profiles across the TMV. (c) Profiles in (b) collapse onto a universal curve after scaling $x$ with $t^{1/4}$, indicating that the profiles are self-similar and follow surface diffusion-controlled flow.

Figure 3

Half profile widths at a constant height $h=2\mathrm{nm}$ from the bottom of TMV plotted versus isothermal hold times at 296 and 303 K for five TPD glasses. At each temperature, the profile evolutions on different glasses all follow the $1/4$ power law indicated by the black dashed lines.

Figure 4

Surface diffusion coefficients on stable (filled), aged (half filled), and liquid-quenched glasses (open) measured at 303 (black) and 296 K (red), plotted versus fictive temperature. Dashed lines are the average $D_s$ at each temperature.