Physical aging of molecular glasses studied by a device allowing for rapid thermal equilibration

Abstract

Aging to the equilibrium liquid state of organic glasses is studied. The glasses were prepared by cooling the liquid to temperatures just below the glass transition. Aging following a temperature jump was studied by measuring the dielectric loss at a fixed frequency using a microregulator in which temperature is controlled by means of a Peltier element. Compared to conventional equipment, the new device adds almost two orders of magnitude to the span of observable aging times. Data for the following five glass-forming liquids are presented: dibutyl phthalate, diethyl phthalate, 2,3-epoxy propyl-phenyl-ether, 5-polyphenyl-ether, and triphenyl phosphite. The aging data were analyzed using the Tool-Narayanaswamy formalism. The following features are found for all five liquids: (1) The liquid has an "internal clock," a fact that is established by showing that aging is controlled by the same material time that controls the dielectric properties. (2) There are no so-called expansion gaps between the long-time limits of the relaxation rates following up and down jumps to the same temperature. (3) At long times, the structural relaxation appears to follow a simple exponential decay. (4) For small temperature steps, the rate of the long-time exponential structural relaxation is identical to that of the long-time decay of the dipole autocorrelation function.

Revealing the fast atomic motion of network glasses

“…Similar behaviour of the density fluctuations has been also reported in molecular dynamic simulations 39,40 and dielectric spectroscopy studies of polymeric glasses 41 . Differently from previous studies and from macroscopic measurements [4][5][6][7]15,29,[32][33][34][35] , we do not find any evidence of physical aging on our experimental time scale of about B4-6 h at any T, not even close to T g . While the absence of physical aging at very low temperatures has been already reported for polymeric glasses 42 , the observed stationary dynamics in the glass transition region is completely unexpected.…”

“…We show that contrary to the common idea of an almost arrested dynamics, structural glasses are able to relax and rearrange their structure on a length scale of a few Angstroms. On cooling the system in the glassy state, the structural relaxation time strongly departs from the supercooled liquid behaviour and displays a weak temperature dependence, with the absence of any detectable aging not even in the glass transition region, in disagreement with macroscopic studies and theoretical approaches [4][5][6][7]15,29,[32][33][34][35] . The relaxation time is found to be surprisingly fast, B100-1,000 s, even hundreds of degrees below T g , in contrasts with the general idea of an ultraslow dynamics in the deep glassy state.…”

“…On approaching the glass transition temperature T g , the structural relaxation time t of a supercooled liquid rises dramatically in a narrow temperature range, until the liquid eventually becomes a glass at T g 3 . Very little structural changes accompany this process, which however leads to an extremely complex nonequilibrium state, featuring in irreversible processes like rejuvenation, memory and time asymmetry effects [4][5][6][7][8] . In addition, the response of the glass to a given perturbation, like a temperature change, strongly depends on the composition of the system and on its previous thermal history [4][5][6][7][8][9] .…”

Compressed correlation functions and fast aging dynamics in metallic glasses

“…3,12,17 In addition, the long time decay of the correlation functions can be rescaled into a single master curve, leading to the validity of a time-waiting time superposition principle in the glassy state. 18,25 Recently, the aging of a Mg 65 Cu 25 Y 10 metallic glass has been studied in both the supercooled liquid phase and the glassy state by looking directly at the evolution of the dynamics at the interatomic length scale. 23 The glass transition has been found to be accompanied by (i) a crossover of the intermediate scattering function from the well characterized stretched exponential shape-thus described by a stretching exponent β < 1-in the supercooled liquid phase, to an unusual compressed exponential form-defined by a parameter β > 1-in the glassy state and (ii) to the presence of two very distinct aging regimes below T g .…”

Atomic-Scale Relaxation Dynamics and Aging in a Metallic Glass Probed by X-Ray Photon Correlation Spectroscopy

“…Although aging is known since centuries, a clear picture of the dynamics in the glassy state is still missing [3,4]. Most of the experimental information available on aging concerns macroscopic quantities, such as viscosity or elastic moduli [5][6][7][8][9][10][11], or focuses on dielectric relaxation [12][13][14][15][16][17], a quantity that is often difficult to relate directly to the particle-level dynamics. From these measurements, a characteristic time for the evolution towards equilibrium can be extracted, but no direct information on the connection between aging and the underlying microscopic dynamics is available.…”