Nucleation and Growth of the Supercooled Liquid Phase Control Glass Transition in Bulk Ultrastable Glasses

We report the anomalous bulk transformation of vapor deposited stable glasses into the liquid state. The transformation proceeds through two competing parallel processes: partial rejuvenation of the stable glass and nucleation and growth of liquid patches within the glass. The kinetics of the transformation extracted from heat capacity curves after isothermal runs is dominated by the heterogeneous nucleation and growth process that initiates at preexisting seeds and propagates radially at a velocity proportional to the alpha relaxation time. Remarkably, the distance between the activation seeds is independent of temperature within experimental uncertainty and amounts to several micrometers, a value in close agreement with the crossover length for TPD glasses. We speculate the initiation sites for the transformation of the glass into the supercooled liquid are localized regions of lower stability (or density).

Figure 1

(a) Specific heat as a function of temperature for as-deposited stable glasses grown at $T_{\mathrm{dep}}=285K$ (continuous black line) and glasses undergoing partial transformations at $T_g+29\mathrm{K}$. Annealing times range from 3-to-5 s between 0 and 65 s. The red curve represents a TPD glass totally transformed into supercooled liquid. (b) Calculated transformed fraction (left axis, blue open circles) and the ratio between the peak temperature shift of the stable TPD glass over time $\mathrm{\Delta }{T}_{\text{peak}}(t)$ and the maximum $T_{\text{peak}}$ variation, $\mathrm{\Delta }{T}_{\max }=T_{\text{peak}}(t=0)-T_{\text{peak}}(\mathrm{FC})$ (right axis, black solid squares) as a function of annealing time.

Figure 2

(a) Transformed fraction vs the logarithm of the transformation time at four different temperatures. Dashed line, fittings obtained by the KJMAE equation. (b) $\ln [-\ln (1-{\mathit{x}}_{\mathit{l}})]$ vs logarithm of the transformation time normalized by $t_{\max }$ for both thick and capped thin films. (c) Growth front velocity vs temperature for TPD stable glasses. TPD data from Ref. [24]. The red symbols represent data used for the KJMAE fitting. (d) Specific heat measured by nanocalorimetry at $3.5\times {10}^4\mathrm{K}/\mathrm{s}$ of a thin film (orange open circles) that transforms by front and a capped layer (open green squares) that transforms by bulk together with the calculated $c_{\mathrm{sp}}$ (continuous lines) using values derived from isothermal measurements.