Short-Time Relaxational Dynamics of the “Strong” Glass-Former Methanol

Abstract

According to the coupling model of relaxation, structural relaxation in glass-forming liquids is comprised of an intermolecularly uncorrelated step ("fast R-relaxation process") in the picosecond time range followed by a slowed, intermolecularly cooperative, "slow R" process. Molecular dynamics simulation data [Sindzingre, P.; Klein, M. J. Chem. Phys. 1992, 96, 4681] have shown that for the "strong" liquid methanol, the fast relaxation step is absent. This finding is in contrast to the prominent fast relaxation appearing in fragile liquids about the glass transition temperature. The differing behavior of methanol can be accounted for from an analysis of the self part of the intermediate scattering function, F S (k,t) according to the coupling model. The latter relates the magnitude of the fast R-relaxation to the relaxation time, τ*, and to the exponent of the slow R-process described by the stretched exponential function exp[-(t/τ*) ]. This function fits the experimental F S (k,t) for t longer than 2 ps. The apparent absence of a fast relaxation step in methanol is shown to be a consequence of the weak intermolecular constraints governing the dynamics in "strong" liquids, a result consistent with the prominence of the fast process in polymers and other fragile glass-formers. This conclusion is supported by dielectric relaxation data (frequencies up to 90 GHz) and far-infrared data (>150 GHz) on methanol.

9 Dielectric relaxation in solutions

“…161 For octanol the OPLS potential also yields considerably enhanced dynamics, but in contrast to ethanol three relaxation times are found, in agreement with the experimental data. 162 Ngai and Roland 163 discussed the apparent lack of a fast relaxation process in MD simulations 164 of the incoherent structure factor of methanol above the glass transition. They showed that the coupling model for glassforming liquids 165 predicts that the amplitude of the 'fast a' relaxation (the 'b process' of the mode coupling theory 166 ) of 'strong liquids' like alcohols is an order of magnitude smaller than the relaxation strength of the 'slow a' process, which is in line with the experimental complex permittivity data at room temperature.…”

Glassy dynamics in supercooled-liquid and glassy ethanol: A molecular dynamics study

“…9 Molecular simulations have often been used to study the behavior of supercooled liquids and glasses, going from hard sphere or Lennard-Jones systems 10 to molecular liquids. In particular, methanol has been widely investigated by means of computer simulations [11][12][13] and therefore our results can also be used to establish a comparison between the dynamics of an almost-rigid molecule such as methanol ͑characteristic frequencies are far too high to be relevant at the temperatures of interest͒ and that of a system that has one internal ͑mo-lecular͒ degree of freedom, which has a frequency enabling mode hybridization with the ''lattice'' motions. It is suspected that the presence of this internal mode that leads to the appearance in the stable crystal of two molecular conformers in a 50:50 proportion is the main reason for the appearance of an stable plastic phase in the case of ethanol.…”

Probing the Debye Dielectric Relaxation in Supercooled Methanol

“…The α-relaxation time of methanol at temperature around the melting point was analyzed using dynamic simulation by Ngai and Roland (1997). The data are represented in Figure 9 by the open diamond, and the data are comparable with the time of the faster relaxation determined experimentally in the study by Barthel et al (1990), as denoted by the open hexagon.…”

“…It should be noticed that the relaxation time in the study by Denney and Cole (1955) was determined using two or three additive Debye relaxations to analyze the CC (ε′ vs. ε″) plots of the dielectric spectra, while the data reported in the latter publications are obtained from the frequent-dependent isothermal dielectric spectra measurements. The fits to the Debye relaxation Barthel et al (1990) (experimental, open hexagon) and Ngai and Roland (1997) (MD, open diamond).…”