Decoupling of surface diffusion and relaxation dynamics of molecular glasses

Abstract

Tobacco mosaic virus is used as a probe to measure surface diffusion of ultrathin films of ,-Bis(3-methylphenyl)-,-diphenylbenzidine (TPD) (12 nm [Formula: see text] 53 nm, where [Formula: see text] is the film thickness) at various temperatures below the glass transition temperature, [Formula: see text], of all films. As the film thickness is decreased, [Formula: see text] decreases rapidly and the average film dynamics are enhanced by 6-14 orders of magnitude. We show that the surface diffusion is invariant of the film thickness decrease and the resulting enhanced overall mobility. The values of the surface diffusion coefficient and its temperature dependence are invariant of film thickness and are the same as the corresponding bulk values ([Formula: see text]400 nm). For the thinnest films ([Formula: see text]20 nm), the effective activation energy for rearrangement (temperature dependence of relaxation times) becomes smaller than the activation energy for surface diffusion. These results suggest that the fast surface diffusion is decoupled from film relaxation dynamics and is a solely free surface property.

Discrete mobility on the surface of glasses

“…However, the most striking feature from the Zhang and Fakhraai investigation (8) was obtained when the T g measured by ellipsometry as a function of film thickness is compared with D s (8). In agreement with previous measurements, a reduction in the T g with decreasing film thickness was observed for TPD; the T g of a 12-nm film was reduced by ∼15 K. Thus, the state of affairs that emerged from this study was the following: despite a reduction in T g and molecular relaxation (17), D s remained invariant under the same conditions (Fig.…”

“…However, exactly how this enhanced mobile layer is correlated to the measured changes in T g and the corresponding underlying molecular dynamics has remained an intriguing, and a far-from-understood question (6,7). Reporting in PNAS, Zhang and Fakhraai found that the surface diffusivity of ultrathin films of a molecular glass former was enhanced compared with bulk and independent of film thickness, despite a concurrent dramatic reduction in the T g and enhancement in the relaxation dynamics within confinement (8).…”

“…Zhang and Fakhraai (8) sought to provide clarity regarding this question. The authors measured the surface diffusion coefficient as well as the T g of ultrathin glassy films of N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) prepared by spin coating.…”

“…They developed an ingenious approach to measure surface diffusion (D s ). By carefully placing a tobacco mosaic virus on the surface of glassy films, and subsequently monitoring the temporal surface tension-driven flow, they obtained D s by solving Mullins' equation (8). The one major assumption was that surface flow was solely a result of diffusion.…”

Why is surface diffusion the same in ultrastable, ordinary, aged, and ultrathin molecular glasses?

“…Several theoretical models were proposed to address this huge enhancement of surface mobility of small molecule OGs [5][6][7] and metallic glass. 8 Recently Fakhraai and coworkers [9][10][11] have investigated the effect of variations in bulk dynamics on the surface diffusion of the molecular glass, N,N 0 -bis(3-methylphenyl)-N,N 0 -diphenylbenzidine (TPD) with its ordinary T g = 330 K. Using the tobacco mosaic virus as a probe particle, they measured D S (T) on glasses of the same composition but with a large difference in bulk relaxation dynamics and in the glass transition temperature. The glasses of TPD include the ordinary glass (OG) obtained by liquid quenching, annealed glass after physical aging at 0.9T g for a week, ultrastable glass (SG) fabricated by physical vapor deposition at various substrate temperatures, and 12 to 30 nm thin films.…”