Local viscosity of supercooled glycerol near T _g probed by rotational diffusion of ensembles and single dye molecules

Abstract

We probe the rotational diffusion of a perylene dye in supercooled glycerol, 5-25 K above the glass-transition temperature (T g ‫؍‬ 190 K) at the ensemble and the single-molecule level. The singlemolecule results point to a broad distribution of local viscosities that vary by a factor of five or more for different individual fluorophores at a given temperature. By following the same single molecules at various temperatures, we find that the distribution of local viscosities itself broadens upon approaching T g. This spatial heterogeneity is found to relax extremely slowly, persisting over hours or even days. These results convey a picture of heterogeneous liquid pockets separated by solid-like walls, which exist already well above the viscosimetric glass transition.fluorescence ͉ glass transition ͉ anisotropy ͉ correlation G lycerol is an archetypal molecular glass former. Although glycerol crystals can be grown from solution, the pure liquid readily supercools because of its high viscosity [10 3 times that of water at room temperature (1)], which is caused by the formation of a highly branched network of intermolecular hydrogen bonds. The viscosity of supercooled glycerol increases by 10 orders of magnitude between 291.8 K, the melting temperature of crystalline glycerol, and its viscosimetric glass-transition temperature T g ϭ 190 K. Throughout this range of temperatures, macroscopic rheology measurements seem consistent with the expected behavior of a Newtonian liquid, although they also reveal highly nonexponential kinetics (1-3).The necessity for cooperative motion at larger and larger scales on approaching the glass transition was recognized a long time ago by Adam and Gibbs (4). In their view, glass formation arises from dynamical arrest of cooperative domains, and their spread in sizes leads to an overall nonexponential relaxation. Indeed, spatially inhomogeneous dynamics (5) have been observed with a variety of techniques in many glass formers, including simple liquids (6-10) and polymers (11-15). Inhomogeneities have been found in the particular case of supercooled glycerol by dielectric hole burning (16), NMR (17), x-ray (18), light scattering (19), and stimulated Brillouin gain spectroscopy experiments (20, 21). These observations have to be reconciled with the common view of a supercooled liquid, which is supposed to remain a normal ergodic liquid until the glass transition. Heterogeneous regions with different relaxation dynamics are therefore expected to exchange with one another to restore ergodicity, by processes called environmental exchanges (6,9,10,12,(14)(15)(16). The typical mechanisms, length scales, and time scales of environmental exchanges remain largely unclear. NMR and dielectric relaxation point to times comparable to the molecular rotation times (16,22), whereas light scattering and fluorescence show inhomogeneities with exceedingly slow relaxation (9,12,(19)(20)(21).Single-molecule fluorescence is a well established method for studying rotational diffusion (23-26). Linear dichroism...

Communication: Crystallite nucleation in supercooled glycerol near the glass transition

“…9 However, because of limited neutron beam time, it was impossible to reproduce the same preparation conditions, with aging times of several weeks at 205 K. Instead, we opted for a shorter aging time at higher temperatures, 240 K and 220 K. Under these conditions, our experiments revealed a crystallization process in supercooled glycerol near the glass transition, consistent with earlier rheological observations. 14 The neutron scattering results indicate the crystalline nature of the solid-like structures.…”

“…35 This is smaller than at least a few nanometers revealed by singlemolecule microscopy. 9 These different experimental results may be due to the different thermal histories applied on the samples. Therefore, heterogeneity should be probed under full control of the thermal history.…”

“…glycerol. 9,[12][13][14] We have evidence that a nucleation process takes place during aging at a temperature near T g . Nonetheless, crystallization strongly depends on the thermal history that the sample was submitted to.…”

“…[7][8][9][10][11] Zondervan et al observed that these variations can be surprisingly long-lived, even at temperatures well above the glass transition temperature (T g ). 9 This may indicate the existence of a nearly static solid-like network at temperatures above T g , which was also revealed by rheological measurements. 12 Besides heterogeneity, aging and structural changes were also observed in supercooled liquids near the glass transition.…”

“…Hence, the structural origin of the solid-like structure developed in supercooled glycerol near T g becomes significantly important for a better understanding on previous experimental observations. 9,[12][13][14] To get more insight into the structural organization of the solid-like network and on the solid-like fraction of the sample, we performed neutron diffraction and inelastic neutron scattering.…”