Isochronal superpositioning in the equilibrium regime of superpressed propylene carbonate to ∼ 1.8 GPa: A study by diffusivity measurement of the fluorescent probe Coumarin 1

We address the problem of glass-forming of liquids by superpressing. We study the pressure-induced dynamic change of the fragile van der Waals liquid propylene carbonate towards the glassy state in the equilibrium regime by measuring the diffusivity of the fluorescent probe Coumarin 1 embedded in the host liquid. The probe diffusivity is measured by the fluorescence recovery after photobleaching (FRAP) technique across a bleached volume generated by the near-field diffracted pattern of a laser beam. The recovered fluorescence intensity fits to a stretched exponential with the diffusive time $\tau$ and the stretched exponent $\beta$ as free parameters. In the pressure range [0.3-1.0]GPa the diffusivity decouples from the Stokes-Einstein relation. The decoupling correlates well to a decrease of $\beta$. The variation of $\beta$ is non-monotonous with $\tau$ showing a minimum at $\tau \sim {10}^3$ s. We evidence an isochronal superpositioning over about 3 decades of $\tau$ between ∼ 10 s and $\sim 3\times {10}^3$ s and a density scaling in the whole investigated pressure range. The pressure at which $\beta$ is minimum coincides to the dynamical crossover pressure measured by other authors. This crossover pressure is compatible with the critical point of MCT theory. As our studied pressure range encompasses the critical pressure, the non-monotonous variation of $\beta$ opens new insight in the approach to the critical point. Graphical abstract:

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