Dielectric and thermal relaxation in the energy landscape

Abstract

We derive an energy landscape interpretation of dielectric relaxation times in undercooled liquids, comparing it to the traditional Debye and Gemant-DiMarzio-Bishop pictures. The interaction between different local structural rearrangements in the energy landscape explains qualitatively the recently observed splitting of the flow process into an initial and a final stage. The initial mechanical relaxation stage is attributed to hopping processes, the final thermal or structural relaxation stage to the decay of the local double-well potentials. The energy landscape concept provides an explanation for the equality of thermal and dielectric relaxation times. The equality itself is once more demonstrated on the basis of literature data for salol.Comment: 7 pages, 3 figures, 41 references, Workshop Disordered Systems, Molveno 2006, submitted to Philosophical Magazin

An Eshelby model for highly viscous flow

“…Maybe the process which we characterize by τ D has another and faster mechanism than the one considered by Debye, which might account for the fact that it is accompanied by the relaxation of the energy and by the relaxation of the structure [15,18].…”

“…Here ǫ(0) is the static dielectric susceptibility, ǫ ∞ is the real part of ǫ(ω) in the GHz range (larger than n 2 , the square of the refractive index, because of vibrational contributions 17 ). These equations for the real and imaginary part of the dielectric constant contain one implicit assumption which might be wrong: That the same double-well potentials which allow the sample to flow are able to fully equilibrate the molecular orientation.…”

“…Note that this cut-off is not very sharp; therefore the β K -values one obtains tend to be larger than those of a Kohlrausch fit, between 0.5 and 0.7. If the glass former exhibits a pronounced secondary peak (Johari-Goldstein peak) one needs to add a Gaussian with three parameters to the form of equation (15). In this case, one has to reckon with different peak amplitudes in shear and dielectrics.…”

“…Here (0) is the static dielectric susceptibility and ∞ is the real part of (ω) in the GHz range (larger than n 2 , the square of the refractive index, because of vibrational contributions [15]).…”