![]() ![]() With these data, we construct a spatio-temporal, microscopic map of the relaxation of a molecular glass, delivering a clear picture of the heterogeneous devitrification dynamics of glasses with enhanced stability. The observed patterns are compatible with a time-dependent number of initiation sites that appear to propagate radially. We address the continuum mechanics finite element modelling (FEM) of the wrinkling phenomenon to demonstrate that the local surface wrinkles appearing at T ann > T g1 (TPD) are induced by the equilibration of the intermediate TPD layer at localized spots. The mechanical instabilities produced during isothermal treatments above T g1 are measured in situ using atomic force microscopy (AFM) and optical microscopy. Here, we take advantage of a previously established protocol to induce bulk ‘melting’ in thin-film glasses 33, 34, 35 by capping the organic film ( N, N′-bis(3-methylphenyl)- N, N′-diphenylbenzidinem, TPD, T g1 = 333 K measured at 10 K min −1) with two ultrathin layers of an organic glass with a higher T g (tris(4-carbazoyl-9-ylphenyl)amine or TCTA, T g2 = 428 K, 10 K min −1). Wrinkling can be locally induced at specific sites of the film by localized surface modifications using focused ion beams 30, swelling-induced stress through toluene absorption 31 or local heating with an external source, such as a laser 32. The instability can be initiated by annealing the organic material into the rubbery state where wrinkling across the whole surface appears due to the development of compressive stresses induced by differences in thermal expansion coefficients 28, 29. One approach to induce surface wrinkling is by capping a soft thin-film material (typically a polymer, but also a small-molecule organic glass) spin-coated on a rigid substrate by a thinner film of a metal layer 24 or by another organic layer with a higher T g (ref. Well-defined surface undulations can be produced in several ways 23, 24, leading to self-organized wrinkling patterns with a rich variety of morphologies depending on the particularities of the system 25, 26. Since direct spatial visualization of equilibrated regions above T on is extremely challenging due to the subtle structural changes over very small distances, we follow a different strategy based on the local mechanical instabilities that the liquid regions generate on a rigid ultrathin layer grown on top of the glass. Vapour-deposited stable glasses appear as model systems to explore this scenario 18, 19, 20, 21 and recent simulations point in this direction 22. However, we have recently shown this view is not unique and can be changed by accessing a temperature and/or time regime above T on where glasses transform into the liquid by the formation of localized regions of liquid within a glassy matrix 14, even though the associated length scale has not been directly measured by experiments yet 15, 16, 17. It is generally accepted that the relaxation of the glass into the supercooled liquid (SCL) at the devitrification temperature on heating, T on, happens through a gradual softening of the glass across its entire volume with a correlation length of a few nanometres. The existence of these clusters of mobility has been inferred from previous experiments 1, 3, 4, 5, 6, 7, 8, 9, 10, 11, but it is experimentally challenging to identify them or their length scales, although it is recognized that the broad distribution of time scales in a glass can be associated with the presence of mobile and immobile regions 12, 13. Dynamic heterogeneities are characterized by clusters of regions of atoms or molecules with correlated mobility that grow as the temperature is decreased. A hallmark in liquid and glassy dynamics is the recognition that dynamic heterogeneities are at the core of the slowing down of the dynamics and are responsible for the glass transition and its temperature dependence 1, 2. The temperature that marks the transition from the ergodic state, the liquid, to the non-ergodic one, the glass, is known as the glass transition temperature, T g. ![]() Glasses are non-equilibrium materials arrested during cooling in metastable configurations. ![]()
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