Khadra Kessairi

Interdependence of Primary and Johari-Goldstein Secondary Relaxations in Glass-Forming Systems

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 Tg(P)/T where Tg 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.

Correlation of structural and Johari–Goldstein relaxations in systems vitrifying along isobaric and isothermal paths

The effect of isobaric cooling (over the range 190-350 K) and isothermal compression (up to 700 MPa) on structural α- and secondary β-relaxations has been studied for low molecular weight glass-forming systems. The shape of the α-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 T α (T, P). The invariance of shape at constant T α (T, P) involved also the excess wing, i.e. the process showing up at the high-frequency tail of the α-loss peak in systems with no well-resolved β-process. Likewise, systems where the excess wing evolved to a well-resolved β-peak showed that the timescale of the β-process was strongly related to that of the α-peak. Also in this case, once a given value T α (T, P) was fixed, a corresponding value Tp(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 α- and Johari-Goldstein relaxation over a wide interval of temperature and density.

Relaxation dynamics in tert-butylpyridine/tristyrene mixture investigated by broadband dielectric spectroscopy.

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 Tg. 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.

Relationship between structural and secondary relaxation in glass formers: Ratio between glass transition temperature and activation energy

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.

Effect of pressure on relaxation dynamics at different time scales in supercooled systems

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 α-relaxation time τα 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 α-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 τα(T, P). Additionally, it is noteworthy that the effect of T and P on slowing down the time scale of fast relaxation processes (β-relaxation and excess wing), although less strong than in the case of α-process, was again comparable. The evidence demonstrates that in the investigated systems: (a) slow and fast relaxations are strongly related; (b) the shape of α-relaxation and (c) the separation between α- and β-relaxation time scale are controlled by τα(T, P) and not by separate thermodynamic variables.

Evidences of a Common Scaling Under Cooling and Compression for Slow and Fast Relaxations: Relevance of Local Modes for the Glass Transition

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.

Excess wing and Johari–Goldstein relaxation in binary mixtures of glass formers

Dielectric loss spectra of pure quinaldine and tert-butylpyridine and their mixtures with tristyrene 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 tristyrene 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 (tristyrene).

Interdependence of Primary and Secondary Relaxations in Glass‐Forming Systems Undergoing Compression and Cooling

Dynamics of the primary and the Johari‐Goldstein (JG) secondary relaxations for polar molecules as a component in mixtures and also neat glass‐forming systems have been investigated by means of broadband dielectric measurements at ambient pressure and elevated pressures P and various temperatures T. The ratio of their relaxation times was found invariant to different combinations of P and T keeping the primary relaxation time constant. In addition, isochronal spectra, obtained with different T‐P combinations, superimposed. Several other properties of the JG relaxation were also found to be related to or correlated with the primary relaxation. Our results indicate the fundamental role played by the JG relaxation in glass transition.