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The present paper shows, by means of broadband dielectric measurements, that the primary α- and the secondary Johari–Goldstein (JG) β-processes in binary mixtures are strongly correlated. This occurs for different polar rigid probes dissolved in apolar glass-forming solvents, over a wide temperature and pressure range. We found that the coupling parameter n = 1 − βKWW and the ratio between α- and β-relaxation time reduce on increasing the size of the solute solved within the same apolar matrix. Moreover, such a ratio is invariant when calculated at different combinations of P and T maintaining either the primary or the JG relaxation times constant. Dielectric spectra measured at different T–P combinations but with an invariant α-relaxation time are well superposed in both the α- and β-frequency ranges. Experimental results can be rationalized by Coupling Model equation.

We investigated the microscopic origin of the excess wing through isothermal and isobaric dielectric relaxation measurements for the Quinaldine/tristyrene mixture. Our results show that the excess wing, characteristic of the high frequency side of the structural loss peak in neat Quinaldine, becomes a well resolved Johari-Goldstein secondary relaxation on mixing with the apolar tristyrene. Analyzing the temperature and pressure behavior of the two processes, a clear correlation has been found between the structural relaxation time, the Johari-Goldstein relaxation time and the dispersion of the structural relaxation (i.e. its Kohlrausch parameter). These results support the idea that the Johari-Goldstein relaxation acts as a precursor of the structural relaxation and therefore of the glass transition phenomenon. (c) 2007 Elsevier B.V. All rights reserved.

International audience; Dielectric loss spectra of pure quinaldine and tert-butylpyridine and their mixtures with tri-styrene are presented. The pure systems present an excess wing and no secondary peaks in the temperature interval from above to well below the glass transition. However, when mixed in low concentration with tri-styrene the excess wing is replaced by a distinct secondary peak. This distinct process can be identified as a Johari-Goldstein relaxation within the Coupling Model interpretation. In the frame of the Coupling Model the transition from the relaxation scenario with the excess wing to that with a distinct secondary peak is related to the increase of intermolecular constraints. In our case this increase of constraints is due to the low mobility component of the mixture (tri-styrene).

The present study demonstrates, by means of broadband dielectric measurements, that the primary α- and the secondary Johari-Goldstein (JG) β-processes are strongly correlated, in contrast with the widespread opinion of statistical independence of these processes. This occurs for different glass-forming systems, over a wide temperature and pressure range. In fact, we found that the ratio of the α- and β- relaxation times is invariant when calculated at different combinations of P and T that maintain either the primary or the JG relaxation times constant. The α-β interdependence is quantitatively confirmed by the clear dynamic scenario of two master curves (one for α-, one for β-relaxation) obtained when different isothermal and isobaric data are plotted together versus the reduced variable T g (P)/T, where T g is the glass transition temperature. Additionally, the α-β mutual dependence is confirmed by the overall superposition of spectra measured at different T-P combinations but with an invariant α-relaxation time.

"We report evidence from broadband dielectric spectroscopy that the dynamics of the primary alpha- and secondary Johari-Goldstein (JG) beta-processes are strongly correlated in different glass-forming systems over a wide temperature T and pressure P range, in contrast with the widespread opinion of statistical independence of these processes. The alpha-beta mutual dependence is quantitatively confirmed by (a) the overall superposition of spectra measured at different T-P combinations but with an invariant alpha-relaxation time; (b) the contemporary scaling of the isothermal-pressure and isobaric-temperature dependences of the alpha-and beta-relaxation times as plotted versus the reduced variable T-g(P)/T where T-g is the glass transition temperature. These novel and model-independent evidences indicate the relevance of the JG relaxation phenomenon in glass transition, often overlooked by most current theories."

The relationship between structural and secondary dynamics is one of the most recently considered and possibly important issues of the dynamics in glass formers. One relation of interest is the ratio between the activation energy of the secondary relaxation and RT g, where T g is the glass transition temperature, and R is the gas constant. This relationship has been recently rationalized by the Coupling Model in terms of many-body dynamics and was applied to the intermolecular Johari-Goldstein secondary relaxation. This article investigates the pressure dependence of this ratio and attempts to verify the validity of the Coupling Model predictions under such conditions.

In this paper, we study secondary relaxations in a neat glass former close and below the glass transition. The pressure and temperature dependences of the characteristic relaxation frequencies were investigated to find a connection between structural and secondary relaxations and the microscopic mechanism at the basis of the latter. We found that the ratio between the relaxation time of the structural and secondary processes is almost constant when considered at different values of temperature and pressure corresponding to the glass transition (same value of structural relaxation time). This result is also related to the finding of a constant broadness of the structural peak. We propose to use such dynamic relation between the two processes to distinguish between intramolecular secondary relaxation, reflecting the local motion of molecular subgroups, and intermolecular secondary relaxation, called also Johari-Goldstein process, originating by a local motion of the whole molecule. (c) 2007 Elsevier B.V. All rights reserved.

For over 10 decades, the dynamics of two glass-forming systems have been investigated by dielectric spectroscopy under cooling (from melting point to well below the glass transition temperature) and under compression (from atmospheric pressure up to 700 MPa). The alpha-relaxation time tau(alpha) was significantly affected by both thermodynamic variables, showing equal roles in slowing down the dynamics. Some similarities have been found; for instance, the dispersion of the alpha-process was shown to increase with decreasing temperature T and increasing pressure P. Furthermore, the same shape for relaxation dynamics over a broad time-scale was found by comparing two dielectric loss spectra obtained at different T and P but characterized by the same tau(alpha)(T, P). Additionally, it is noteworthy that the effect of T and P on slowing down the time scale of fast relaxation processes (beta-relaxation and excess wing), although less strong than in the case of alpha-process, was again comparable. The evidence demonstrates that in the investigated systems: (a) slow and fast relaxations are strongly related; (b) the shape of alpha-relaxation and (c) the separation between alpha- and alpha-relaxation time scale are controlled by tau(alpha)(T, P) and not by separate thermodynamic variables.

Dielectric spectra of several typical molecular glass-formers, showing one or more secondary processes resolved in the glassy state, have been measured at different temperatures. We found that the genuine Johari-Gold stein P-relaxation is connected to the structural relaxation. In fact, a clear correlation was found between the structural relaxation time, the Johari-Goldstein relaxation time and the dispersion of the structural relaxation (i.e. its Kohlrausch parameter). Moreover, for a group of epoxide oligomers the steepness index is correlated to the broadness of the structural peak and to the time separation between the structural and the Johari-Goldstein relaxation. These results support the idea that the Johari-Goldstein relaxation acts as a precursor of the structural relaxation. A rationale for our results is provided by the coupling model. (C) 2007 Elsevier B.V. All rights reserved.

The effect of isobaric cooling ( over the range 190 - 350 K) and isothermal compression (up to 700 MPa) on structural alpha- and secondary beta-relaxations has been studied for low molecular weight glass-forming systems. The shape of the a- loss peak was found to change with temperature T and pressure P but to be constant for a combination of T and P giving the same tau(alpha)(T, P). The invariance of shape at constant tau(alpha)(T, P) involved also the excess wing, i.e. the process showing up at the high-frequency tail of the alpha-loss peak in systems with no well-resolved beta-process. Likewise, systems where the excess wing evolved to a well-resolved beta-peak showed that the timescale of the beta-process was strongly related to that of the alpha-peak. Also in this case, once a given value tau(alpha)(T, P) was fixed, a corresponding value tau beta(T, P) was found for different T and P. Same results were found also for a binary mixture of a polar rigid molecule dissolved in an apolar solvent, i.e. a model system for Johari-Goldstein intermolecular relaxation. These evidences imply that a strong correlation exists between structural alpha- and Johari-Goldstein relaxation over a wide interval of temperature and density.

We investigated, by means of dielectric spectroscopy, the relaxation dynamics of glass forming binary mixtures composed by the quite rigid polar molecules tert-butylpyridine dissolved in the apolar solvent tristyrene. By changing the relative concentration of the components we observed a transition from a relaxation scenario with a structural process and an excess wing to that with a structural process and a well resolved secondary process. Another relaxation process, slower than the latter, was observed, well below T-g. Our detailed analysis evidenced that the secondary relaxation with shorter relaxation time can be identified as the Johari-Goldstein relaxation for all the mixtures, whereas the new relaxation process was attributed to a different type of motion of tert-butylpyridine needing a larger amount of free volume for the molecular rotation. (c) 2007 American Institute of Physics.

Broadband dielectric spectroscopy measurements were performed on glass-forming binary mixtures, composed of rigid polar molecules dissolved at low concentration in apolar viscous solvent (tristyrene). Dielectric spectra were dominated by the polar molecule contribution, so enabling the study of its dynamic behavior. A well resolved secondary relaxation, not attributable to internal degrees of freedom, was visible in both the liquid and glassy states: for its intermolecular nature, it can be called a 'genuine' Johari-Goldstein (JG) relaxation, in the sense that it is a local and non-cooperative process but entailing the motion of the molecule as a whole. Among our results, the following ones are noteworthy: (a) polar systems in the neat state showed an excess wing that became a well resolved JG-peak on mixing with the apolar solvent; (b) the time-scale distance between structural and JG loss peak increased with the apolar solvent fraction; (c) broader the structural loss peak was, larger was the separation in the frequency scale between structural and JG peak; (d) the JG relaxation time showed a non-Arrhenius temperature behavior above T, paralleling that of the structural relaxation time. All the results can be rationalized in the framework of Coupling Model. (c) 2006 Elsevier B.V. All rights reserved.