| 词条 | Johari-Goldstein relaxation |
| 释义 |
}} Johari-Goldstein relaxation, also known as the JG β-relaxation, is a universal property of glasses and certain other disordered materials. Initially posited in 1969 by Martin Goldstein, its existence was proved experimentally by Gyan P. Johari and Martin Goldstein in 1970 by experiments on glasses made from rigid molecules. The relaxation, a peak in mechanical or dielectric loss at a particular frequency, had previously been attributed to a type of molecular flexibility. The fact that such a loss peak shows up in glasses of rigid molecules lacking this flexibility demonstrated its universal character. The JG relaxation is regarded as the precursor of the α-relaxation, i.e., its occurrence facilitates viscous flow, and it must occur before the latter can occur. Accordingly, the narrow ε″ peak observed at GHz frequencies in the spectra of polar liquids at high temperatures is associated with processes similar to the JG relaxation that is observed at lower temperatures. As the liquid is cooled, the ε″ peak at GHz frequency broadens and shifts to lower frequencies. A new peak in ε″ emerges from the low frequency side of this peak once some value of higher density and viscosity are reached. The relaxation time at which it emerges is typically 0.1 to 1 μs. As the liquid is further cooled, this peak broadens and grows in strength at the expense of the original peak and rapidly moves to very low frequencies at T < Tg from above. This is the α-relaxation peak. Its relaxation rate changes more rapidly on cooling than the rate of JG relaxation and the motions that produce it become kinetically frozen at Tg while the motions due to the JG relaxation persist. In the glassy state, its mechanism is seen to be thermally activated hindered reorientation, the same as that in a low viscosity liquid at high temperatures. The discovery was made by Johari and Goldstein in the period 1970-1976 and these observations have now been confirmed by a number of authors on many types of systems. The relative amplitudes of the JG process and the α processes in terms of dielectric strength have also been given in some cases. It is known that viscosity and density of a liquid also increase as a liquid polymerizes at a fixed T and macromolecular structure grows in the liquid as covalent bonds replace weaker Van der Waals interactions. More recently, it has been shown by dielectric measurements that during polymerization, the α-relaxation process emerges from the JG relaxation at a fixed T as monomeric liquids polymerize under isothermal conditions, i.e., the viscosity of the liquid increases irreversibly until it becomes a glass isothermally. As polymerization progresses, the ε″ peak observed in the GHz frequency range for the monomeric liquid begins to decrease in strength while its position does not change significantly. A new ε″ peak that corresponds to the α-relaxation evolves on the low frequency side, increases in strength, and shifts to lower frequency as the viscosity increases. As polymerization occurs, the decrease in Δε for the high frequency relaxation with time has the same form as the decrease in Δε_JG as structural relaxation occurs during isothermal annealing. References1. ^J. K. Vij and G.Power, Physical ageing and the Johari–Goldstein relaxation in molecular glasses, Journal of Non-Crystalline Solids 357 (2011) 783–792
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2 : Phase transitions|Critical phenomena |
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