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

K L Ngai,Cristian Rodríguez-Tinoco,Marian Paluch

doi:10.1039/c7cp05357f

Abstract

Recently Fakhraai and coworkers measured surface diffusion in ultrastable glass produced by vapor deposition, ordinary glass with and without physical aging, and ultrathin films of the same molecular glass-former, N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD). Diffusion on the surfaces of all these glasses is greatly enhanced compared with the bulk diffusion similar to that previously found by others, but remarkably the surface diffusion coefficients DS measured are practically the same. The observed independence of DS from changes of structural α-relaxation due to densification or finite-size effect has an impact on the current understanding of the physical origin of enhanced surface diffusion. We have demonstrated before and also here that the primitive relaxation time τ0 of the coupling model, or its analogue τβ, the Johari-Goldstein β-relaxation, can explain quantitatively the enhancement found in ordinary glasses. In this paper, we assemble together considerable experimental evidence to show that the changes in τβ and τ0 of ultrastable glasses, aged ordinary glasses, and ultrathin-films are all insignificant when compared with ordinary glasses. Thus, in the context of the explanation of the enhanced surface diffusion given by the coupling model, these collective experimental facts on τβ and τ0 further explain approximately the same DS in the different glasses of TPD as found by Fakhraai and coworkers.

Highlights

Fakhraai and coworkers[9,10,11] have investigated the effect of variations in bulk dynamics on the surface diffusion of the molecular glass, N,N0-bis(3-methylphenyl)-N,N0-diphenylbenzidine (TPD) with its ordinary Tg = 330 K
The answer is complete if the mechanism can give quantitatively the size of the enhanced DS(T) with approximately the same value for all cases. We provide such a mechanism in the primitive relaxation of the coupling model[21,22,23,24,25] or its analogue the Johari–Goldstein (JG) b-relaxation[25,26,27,28,29] exemplified in experiment and simulations.[21]
The findings of practically no change of the surface diffusion coefficient DS(T) on ultrastable glass (SG), ordinary glass (OG) with and without physical aging, and nano-meter glassy thin films of TPD by Fakhraai and coworkers are interesting, and certainly have an impact on identifying the mechanism of enhanced surface diffusion. They suggest that fast surface diffusion is decoupled from the bulk dynamics, the exact mechanism of this decoupling merits further investigation

Summary

Introduction

It is generally believed that the high surface mobility or the much larger surface diffusion coefficients DS(T) rather than DV(T) plays a critical role in allowing the formation of highly stable glasses.[12,13] Glasses of higher stability will be formed in systems where surface diffusion is faster This is borne out by OTP2 having a larger DS(Tg) than IMC1 and having formed a more stable glass than IMC.[12,13] despite the large difference in the bulk dynamics of SG, OG, and annealed OG of TPD, the surface diffusion coefficients of these glasses measured[9,10,11] turn out to be nearly identical at two temperatures below the Tg of bulk OG. We use this result to support our explanation of the roughly invariant surface diffusion coefficient in SG, OG, and annealed OG as well as in nanometer thin films

Evidence from experiments

Aged OG

Nanometer thin films

Comparing SG at ambient pressure with OG at elevated pressures

Findings

Conclusions