Pressure-Induced Glass Transition Probed via the Mobility of Coumarin 1 Fluorescent Molecule

Abstract

The route to form a glass is generally achieved upon cooling where the slowing down might be interpreted as the trapping of molecules in potential wells. On the other hand, isothermal compression induces a glassy state by modifying the molecular packing ending in jamming. Here, we focus on how isothermal compression perturbs the mobility of a probe molecule in three different host liquids up to the pressure-induced glass transition. By use of the fluorescence recovery technique, the diffusion of the fluorescent molecule Coumarin 1 (C1) is measured in poly(propylene glycol) (PPG-1000M and -2700M), in the fragile van der Waals propylene carbonate (PC), and in hydrogen-bonded methanol and ethanol. High pressures up to 6 GPa are obtained with a diamond anvil cell. In PC at a pressure ∼1.3 GPa close to the glass-transition pressure, the diffusion coefficient of C1 follows an Arrhenius behavior with an ∼5 orders of magnitude increase of the diffusive time. No decoupling from the Stokes-Einstein equation is noticed. A similar exponential behavior is measured in ethanol and methanol but extended to different pressure ranges up to 2.5 and 6.2 GPa, respectively. In PPG-1000M a decoupling from the Stokes-Einstein relation is observed between 0.3 and 0.8 GPa that could be related to a modification of the interaction between polymer segments and the probe molecule. These results might indicate that interaction between probe and dynamic heterogeneities become less important under applied pressure, unlike in the temperature-induced glass transition.

Ion Dynamics in a Highly Compressed Solution of Tetra-Ethylammonium – Tetrafluoroborate Salt in Propylene Carbonate: A Study in the Equilibrium and Glassy States

“…60,61 The large diamond anvil culets used in our setup give a limiting pressure of 6 GPa at room temperature. 58,59 This limiting pressure is well above the glass transition pressure of propylene carbonate.…”

“…The pressure in the sample chamber is determined by the ruby fluorescence method. 58,65 Ruby chips with 0.5 wt% Cr 2 O 3 , of several tens of microns, are deposited at the bottom of the chamber (see Fig. 2).…”

“…5,8,10,41,56,57 We show that the anomalous increase in dc-conductivity occurs at approximately the same crossover pressure P cross at which a dynamic change in the diffusivity of uncharged fluorescent molecules in propylene carbonate is observed. Indeed, for P  P cross , from the time-recovery of the fluorescent intensity in photobleaching experiments, 58,59 the molecular diffusivity has been shown to exhibit a more Arrhenius-like behavior for increasing pressure. This dynamic change could be related to the anomalous behavior of the ion transport observed between P cross and P G .…”