Resumen de New perspectives on the glass transition: crossover between cooperative relaxation and liquid growth dynamics
Since the discovery, in 2007, that physical vapor deposition constituted a powerful tool to produce glasses of unprecedent thermodynamic and kinetic stability, this technique has been extensively used as an alternative route to prepare glasses that span a broad range of stabilities. In particular, glasses prepared at substrate temperatures around 0.85Tg and slow deposition rates present the maximum stability, comparable to conventional glasses aged for thousands of years, hence the name ultrastable glasses. Due to their improved molecular packing, vapor deposited glasses transform into the supercooled liquid mainly via parallel growth fronts that initiate at regions with higher mobility (free surface and/or interfaces) and propagate into the glass, at least for the first stages of the transformation until the bulk mechanism is triggered. While the transformation via growth fronts is the dominating mechanism in highly stable glasses and it has been extensively studied, the homogeneous devitrification mechanism of highly stable glasses in the bulk is still far less understood.
In this work, we use quasi-adiabatic fast-scanning nanocalorimetry and microscopic techniques (mainly AFM) to study different characteristics of the bulk transformation mechanism behind the glass transition of thin film TPD glasses during annealing treatments well above their limiting fictive temperature. To gain access to the bulk transformation, we make use of the capping strategy, which consists on capping the all interfaces with a lower mobility layer (in our case TCTA), suppressing, in this way, the propagating fronts. The results show that the transformation into the liquid phase takes place via two parallel competing processes: a partial rejuvenation/softening of the stable glass and the emergence and growth of liquid patches within the glass. In the case of highly stable glasses, the transformation is dominated by the emergence of liquid regions that grow following a nucleation and growth like kinetics. Fittings using the KJMAE model indicate that the samples transform in a 2D geometry and with a certain nucleation rate. AFM measurements, strengthen this view by allowing a direct visualization of the equilibrated liquid regions appearing as a function of annealing time which grow propagating radially at a velocity compatible with the front velocity in non-capped samples. From the AFM measurements we also observe the presence of pre-existing liquid sites, where the transformation usually starts. Remarkably, the distance between equilibrated regions amounts to several micrometers, a value in close agreement with the crossover lengths found for many organic glass formers.
By growing glasses at different deposition temperatures, we can observe that this mechanism appears in glasses with lower stabilities as well, although in this case, the distance between equilibration sites is highly reduced (to only a few nanometers) and the partial softening of the glass matrix is significantly more important than in highly stable glasses, hinting that a totally cooperative glass relaxation is possible to observe depending on the stability of the glass.
We show that the transformation via the emergence of liquid regions does not depend only on stability but also on the ratio between the relaxation time of the glass, T_glass, and the alpha relaxation time of the equilibrated liquid, T_[Alfa], at a given temperature. If this ratio is high, we observe regions that transit directly into the equilibrated liquid and grow by dynamic facilitation. However, if this ratio is small, the glass transition proceeds by cooperative relaxation dynamics throughout the material. This behavior is found to be independent of the experimental procedure or protocol used to produce the glass. The results presented in this work contribute to deepen the existing knowledge in the field of glassy physics by providing new insights in the mechanisms followed during the glass transition.
