The role of thermodynamic stability in the characteristics of the devitrification front of vapour-deposited glasses of toluene

Abstract

Physical vapour deposition (PVD) has settled in as an alternative method to prepare glasses with significantly enhanced properties, providing new insights into the understanding of glass transition. One of the striking properties of some PVD glasses is their transformation into liquid via a heterogeneous mechanism that initiates at surfaces/interfaces. Here, we use membrane-based fast-scanning nanocalorimetry (10 4 K s À1) to analyse the variables that govern the transformation mechanism of vapourdeposited toluene glasses with different stabilities. Thin films ranging from 20 to 250 nm were prepared at deposition temperatures between 0.70 and 1.15 times the glass transition temperature. We show how a propagating growth front is the initial transformation mechanism in all the vapour deposited samples, revealing a clear tendency to faster front velocities for less stable samples. Contrary to other glassformers such as indomethacin, toluene shows a one-to-one relationship between limiting fictive temperature and front velocity. We associate this behaviour with the much simpler molecular geometry of toluene, which would prevent the presence of strong preferential molecular arrangements in the glass. However, the propagation distance of the growth front before the homogenous transformation mechanism dominates the transition is found to be dependent on the preparation conditions rather than on the thermal stability of the glass. Understanding the link between the growth variables and the properties of PVD glasses is crucial for finding and developing potential applications of this type of glass.

Communication: Surface-facilitated softening of ordinary and vapor-deposited glasses

“…15 The leading edge overlap in heat flow diagrams of films with distinct thicknesses strongly indicates that transformation rates are independent of sample size, i.e., kinetics are zero-order. Another group obtained similar results from FSC experiments, even in the case of ultrathin (20-200 nm) methylbenzene 17 and indomethacin 16 vapor-deposited films. In short, SF devitrification is now accepted as a general phenomenon, albeit unique to stable VD glasses.…”

Kinetic arrest of front transformation to gain access to the bulk glass transition in ultrathin films of vapour-deposited glasses

“…If a homogeneous mechanism is governing the transformation into the supercooled liquid, we should see, for same stability samples but different thicknesses, how the onset-and the entire peak-of the specific heat curves collapses. 4 For that purpose, films of different thicknesses of the low-T g material have been evaporated both as single and sandwiched layer. To evaluate if the transformation mechanism changes depending on the stability of the glass, the same strategy has been used for films obtained at different deposition temperatures.…”

“…3 For instance, a 20 nm thick layer of an ultrastable glass will take one-fifth less time to fully transform into supercooled liquid than a 100 nm thick layer at the same annealing temperature. 4 Thus, the heterogeneous transformation can reduce the high kinetic stability achieved for these glasses. This represents one of the major limitations for the conceivable applications of the ultrastable vapour deposited glasses.…”

Front-Mediated Melting of Isotropic Ultrastable Glasses

“…In thick films, the melting front propagates over a finite distance, c = c (T i , T m ), because deeper layers have homogeneously melted by a distinct bulk-like mechanism by the time the front reaches them. Available data suggest that c can vary from 20 to 2000 times the molecular size [15,21], smaller c being reported for less stable systems and higher T m [12,22]. Such a large length scale characterizing the dynamics of supercooled liquids is surprising in materials that are structurally homogeneous down to the molecular size.…”

“…To compare our observations with experiments, we consider toluene films [33][34][35]. For toluene, we take σ 0 ≈ 0.6nm [36], Ráfols-Ribé et al [12] report stabilitydependent c in the range 50-200nm, and Bhattacharya et al [37] find c ≈ 250nm. The largest length scale we measure for T i = 0.04 and T m = 0.095 gives c = 375σ 0 ≈ 225nm, which compares favorably with the measurements made using nanocalorimetry for samples vapor deposited around 0.95T g [12,22].…”