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This paper presents dielectric relaxation data for organic glass-forming liquids compiled from different groups and supplemented by new measurements. The main quantity of interest is the "minimum slope" of the alpha dielectric loss plotted as a function of frequency in a log-log plot, i.e., the numerically largest slope above the loss peak frequency. The data consisting of 347 spectra for 53 liquids show prevalence of minimum slopes close to -1/2, corresponding to approximate square root(t) dependence of the dielectric relaxation function at short times. The paper studies possible correlations between minimum slopes and (1) temperature (quantified via the loss peak frequency); (2) how well an inverse power-law fits data above the loss peak; (3) degree of time-temperature superposition; (4) loss peak half width; (5) deviation from non-Arrhenius behavior; (6) loss strength. For the first three points we find correlations that show a special status of liquids with minimum slopes close to -1/2. For the last three points only fairly insignificant correlations are found, with the exception of large-loss liquids that have minimum slopes that are numerically significantly larger than 1/2. We conclude that--excluding large-loss liquids--approximate square root(t) relaxation appears to be a generic property of the alpha relaxation of organic glass formers.

We present a new model for dielectric data in the alpha-beta merging region. The model is constructed using electrical circuit analogies. It leads to an interpretation of the merging region as one where the total relaxation upon cooling separates in two relaxation processes, consistent with a view where the relaxing entities involved are the same for the two processes. We use this alpha-beta model to fit dielectric data in the merging region of two different molecular liquids. These fits are performed under the assumption that the intrinsic high-frequency behavior of the alpha relaxation is a -1/2 power law and that both the alpha and the beta process separately obey time temperature superposition. We get good quality fits in the entire frequency and temperature range studied. This supports the view that alpha relaxation high-frequency slopes that are found to be numerically smaller than 1/2 can be attributed to the influence of the beta relaxation.

To cite this version:Albena Ivanova Nielsen, Sebatian Pawlus, Marian Paluch, Jeppe Dyre. Pressure dependence of the dielectric loss minimum slope for ten molecular liquids. Philosophical Magazine, Taylor & Francis, 2008, 88 (33-35) We present a comprehensive study of data for the dielectric relaxation of ten glass-forming organic liquids at high pressure along isotherms, showing that the primary (α) high frequency relaxation is characterized well by the minimum slope and the width of the loss peak. The advantage of these two parameters is that they are model independent. For some materials with β processes in mHz and kHz range the high frequency slope tends to be −1/2 with pressure increase. Besides the two parameters captures the relaxation shape invariance at given relaxation time but different combinations of pressure and time [6].Glass may be regarded as the fourth state of conventional matter, isotropic as the liquid state, but solid as the crystalline state. With the notable exception of helium, any liquid may be turned into glass by cooling it fast enough to avoid crystallization [1][2][3][4][5].Physical systems usually relax following perturbations forced upon them. The relaxation of the systems consists of processes going on different time scales. The dominant and slowest relaxation process of a glass-forming liquid is the so-called α process. The α process defines the liquid relaxation time, an important quantity because the glass transition takes place when the relaxation time significantly exceeds the inverse relative cooling rate. Compression of supercooled liquid slows down the α relaxation (increases the characteristic relaxation time, τ ). On the other hand this effect can be compensated by heating up the liquid. Different combinations of p and T can result in the same relaxation dynamics at same τ or materials obey the temperature-pressure superposition at the same relaxation times (TTPS) [6][7][8][9][10][11]. There are differnet kinds of secondary (β) relaxation including the Johari-Goldstein (JG) and these of intra-molecular motions [11][12][13][14][15][16] or any excess wings.In a paper from 2001 it was shown that the high frequency slope of the dielectric loss for a group of materials tends to be −1/2 as the temperature approaches the glass transition temperature, T → T g , and this was linked to time-temperature superposition (TTS) [17], but there is found a general prevalence of the −0.5 for the slope in highly viscous liquids no matter if TTS is obeyed [18]. In those papers the dielectric scans were taken at ambient pressure. In the following is investigated: if this result holds for dielectric frequency scans for ten liquids along isotherms with increasing pressure (p → p g ) ; and if the used model-independent shape quantities, minimum slope and half loss peak width capture TTPS. The minimum slope, of course, may well be affected by secondary processes, the well expressed in the experimental frequency window as well the underlying low-frequency β relaxation * Corresponding author. Emai...