Dynamic correlation length scales under isochronal conditions

Abstract

The origin of the dramatic changes in the behavior of liquids as they approach their vitreous stateincreases of many orders of magnitude in transport properties and dynamic time scales -is a major unsolved problem in condensed matter. These changes are accompanied by greater dynamic heterogeneity, which refers to both spatial variation and spatial correlation of molecular mobilities. The question is whether the changing dynamics is coupled to this heterogeneity; that is, does the latter cause the former? To address this we carried out the first nonlinear dielectric experiments at elevated hydrostatic pressures on two liquids, to measure the third-order harmonic component of their susceptibilities. We extract from this the number of dynamically correlated molecules for various state points, and find that the dynamic correlation volume for non-associated liquids depends primarily on the relaxation time, sensibly independent of temperature and pressure. We support this result by molecular dynamic simulations showing that the maximum in the four-point dynamic susceptibility of density fluctuations varies less than 10% for molecules that do not form hydrogen bonds. Our findings are consistent with dynamic heterogeneity serving as the principal control parameter for the slowing down of molecular motions in supercooled materials. SIGNIFICANCEOn cooling or compressing a liquid, molecular motions become slower, effectively ceasing as the material becomes a glass. Coincident with this is development of reciprocity -neighboring species must make adjustments in order for a given molecule to move. Conceptually, slower motions and more cooperative movements would seem to be connected, as theoretical models often assume; however, experimental verification is lacking. We carried out nonlinear dielectric spectroscopy, which enables both the timescale and degree of cooperativity of molecular motions to be quantified. By obtaining results at elevated pressures and various temperatures, we determined that a given relaxation time is associated with a given degree of cooperativity, supporting the idea that cooperativity underlies the transition of a liquid to the glassy state.

Nonlinear dielectric spectroscopy in a fragile plastic crystal

“…1,2,3, 4,5,6,7,8). It was found that the response of glassy matter to high electrical fields goes far beyond the rather trivial polarization saturation effects investigated in some pioneering works on nonlinear dielectric properties (e.g., Refs.…”

Investigation of nonlinear effects in glassy matter using dielectric methods

“…The nonlinearity can be quantified, e.g., by the difference of high and low-field permittivity [9,16,21,24]. ii) Application of a high ac field and determination of the higher harmonics of the response, leading to higher-order permittivities [12,13,18,20,21,22,76,77]. iii) Application of a high dc bias voltage and measurement of the permittivity, usually with a smaller ac signal [78,79,80,81,82].…”

“…2(b)). Consequently, as mentioned in section 1, various nonlinear dielectric measurements were interpreted in terms of molecular cooperativity (e.g., [12,13,18,20,21,23,31]). However, aside of the saturation and chemical effects, several other mechanisms were also considered to give rise to dielectric nonlinearity.…”

“…However, in recent years a completely different approach is attracting increasing interest: In various works, both experimental and theoretical, it was revealed that valuable additional information on the glass transition can be gained by applying excessively high electrical fields (using voltages up to the kV range), thus driving the investigated material into a nonlinear regime (see, e.g., [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23]). This nonlinear dielectric response is monitored, e.g., by determining the dielectric permittivity at high fields and comparing it to * measured in the linear regime.…”

Communication: Effect of density on the physical aging of pressure-densified polymethylmethacrylate

“…A prominent example is the correlation of various properties with the time scale of molecular motions, as observed under isochronal conditions. In these studies, the primary relaxation time, τα, is maintained constant through simultaneous control of pressure and temperature [2], with consequent invariance for many liquids of the dynamic correlation length [3,4], the shape of the relaxation dispersion ("isochronal superpositioning") [5,6], the dynamic crossover [7,8] and for a few cases, the melting line [9]. An interpretation of isochronal invariance of properties comes from molecular dynamics (MD) simulations, which have shown that for a certain class of materials, the behavior, including τ, is governed by the existence of isomorphs [10,11,12].…”