We have measured the dielectric relaxation times of the α process of phenyl salicylate (salol) covering 11 decades in frequency. Being representative for the class of low molecular weight organic glass forming materials, the highly resolved temperature dependence of the dynamics in salol does not follow a particular function like the Vogel–Fulcher–Tammann (VFT) law over the entire accessible range of temperatures. In order to conduct a detailed and unambiguous analysis of the temperature dependence, we take advantage of the derivatives of the experimental log(fmax) values with respect to temperature, which allow us to either linearize the frequently used temperature laws or to resolve subtle changes in fmax(T) by decreasing the number of free parameters. In this manner we observe that none of the common routes for rationalizing the dynamics like Arrhenius, VFT, Souletie scaling, and idealized mode-coupling theory account for the experimental findings properly. However, we do observe a VFT behavior within the limits 265 K≤T≤320 K, i.e., for temperatures ranging from significantly above the glass transition at Tg=220 K to far above the melting point.

We have studied the temperature dependence of dielectric relaxation times in terms of the peak frequency f max (T) of dielectric loss ⑀Љ͑͒ and the dc-conductivity dc (T) of several glass-forming liquids, covering 12 decades in the peak frequency f max and 9 decades in dc . Although dc-conductivity samples the mobility of ionic tracers, its variation with temperature is similar to that of f max (T). The f max (T) and dc (T) are analyzed using the temperature-derivative method and compared to the viscosity data Ϫ1 (T). While most liquids reveal a common Vogel-FulcherTammann ͑VFT͒ behavior for f max , dc and Ϫ1 in an extended temperature range TуT m , some liquids deviate from this behavior by displaying a crossover at TϭT A to an Arrhenius regime. In these cases the quantity f max (T) decouples from the common curves for dc (T) and Ϫ1 (T) and attains activation energies in excess ͑ϳ40% for alcohols͒ of those related to translational processes. For many samples a departure from the VFT behavior occurs at lower temperatures T B ϽT m which tends to retard the glass transition. The onset of this qualitative change in the temperature dependence at T B turns out to be a characteristic temperature also in other experiments.

We have measured the dielectric relaxation of the glass-former 1-propanol for temperatures between 65 and 350 K in the frequency range 10−2 to 2⋅1010 Hz and the photon correlation spectro-scopy decays near Tg. Attributing the strong Debye-type process of 1-propanol to distinct -OH group effects leaves two faster processes related to the structural relaxation which can be identified as α-relaxation and Johari–Goldstein type β-relaxation characteristic of nonhydrogen-bonding supercooled liquids. From the temperature dependent relaxation times τ(T) regarding the three distinct loss peaks, we can specify an α-β-bifurcation temperature Tβ, which coincides with characteristic qualitative changes in the τ(T) behavior, as also observed for ortho-terphenyl and other glass-forming liquids. This assignment is confirmed by the correla-tion times derived from incoherent quasielastic light-scattering data obtained from the simultaneously measured photon-correlation spectroscopy.

No abstract

We have measured the dielectric relaxation of butylbenzene and of the glass-former propylbenzene in the frequency range 10−2 Hz to 2×1010 Hz in order to characterize the variation of relaxation times with temperature for these low loss liquids. Additionally, salol has been remeasured above 1 GHz with improved resolution. Using the sensitive data representation [−dlog10(fmaxHz)/d(1/T)]−1/2 vs 1/T we find demarcation temperatures TA, at which the temperature dependence changes from a Vogel–Fulcher type law within the limits TB⩽T⩽TA to Arrhenius behavior for T>TA, corresponding to a position of the loss peak fmax>2 GHz. The activation energies derived from dielectric relaxation data for T>TA are associated with the energy of vaporization, Eη∝ΔEvap. A comparison of dielectric relaxation times τD to viscosity data in this wide range of temperatures suggests the relation τD∝η/T rather than τD∝η.

No abstract

No abstract

Schonhals et al. Reply: The Comment by Cummins et al. [1] is concerned with the well-known fact that in low molecular glass forming liquids the temperature dependence of the n-relaxation times 7 (or the viscosities) cannot be universally described by the empirical Vogel-Fulcher-Tammann (VFT) equation 5tn 4 -I E O CA &I ( f = I/2m r; fo, B constants, To Vogel temperature). Assuming a low temperature VFT equation, the deviations at high temperature can be described by an Arrhenius behavior in good approximation within experimental accuracy [2,3]. This approach was suggested by the observation that the temperature-viscosity relationship for liquids well above the melting temperatures follows the Arrhenius law [4 -6]. The analysis of the temperature dependence of the relaxation time is complicated by the fact that the three parameters of the VFT equation cannot be uniquely determined from the activation plot. To overcome this difficulty Stickel [7] refined the data analysis by evaluating the difference quotient b, lnf/b, T. From Eq. (1) it follows, under the assumption that fo does not depend on temperature, that the plot of the inverse square root(5 lnf//JT) ' '-= (T -To)/B versus temperature shows a straight line. As a result of dielectric measurements of glycerol ( Fig. 1), we find a VFT behavior in the temperature range Ts~T~Tg + 100 K with ln(fo/sec ') = 14.2, B = 1040 K, and To = 127 K in good agreement with Fig. 1(b) of the Comment. Above the melting point of glycerol (T, "=290 K) we find deviations from this VFT law with the same tendency as indicated in this figure. This behavior is opposite to the previous conclusion of Schonhals et al [2] that .above Tq Arrhenius behavior is observed. The procedure described above was applied to a larger variety of glass forming liquids. Since salol was mentioned in the Comment, we show a similar plot for this 220 240 260 280 300 320 Temperature / K FIG. 2. Inverse square root of the difference quotient of the maximum position f, "vs te. mperature. substance (Fig. 2). In contrast to glycerol the figure shows deviations from the VFT behavior at low temperatures.For about ten glass forming liquids studied so far, a behavior similar to that plotted in Fig. 2 was observed.Since the activation plot lnf(T) is a continuously decreasing function in all cases, it is difficult to recognize characteristic temperatures by an unbiased evaluation of those data. The method of Stickel described above, however, shows a clear change of the temperature dependence of the difference quotient at certain temperatures. A detailed report about these results is in preparation [8].