Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 353

Whether in the form of affinity, fictive temperature, or structural order parameter, additional variables must be introduced to deal with the nonequilibrium thermodynamics of glass‐forming systems and, in particular, with the time dependence of their properties in relaxation regimes. Phenomenological advances now make it possible to predict these properties as a function of time and temperature or to determine accurately the entropy irreversibly produced, but the mechanisms involved at the atomic or molecular level generally remain to be deciphered. The physical nature of the glass transition is a case in point, as are the origins of Kauzmann catastrophe, of the strong variations of the PD ratio, of the diversity of relaxation timescales or of, as illustrated by the well‐known memory effects, the complex nonlinear coupling of the parameters of the differential equations with which these processes are described.

Not only could highly sensitive calorimetric experiments yield valuable original data in this respect but coupling of different techniques such as dielectric spectroscopy and temperature‐modulated calorimetry should bring new insights on the dynamics and thermodynamics of the glass transition. Recent experiments on ultra‐stable organic glasses obtained by vapor deposition techniques are, for instance, promising [25, 26]. And whereas very long aging performed well below T_g should also give new clues on the laws driving complex relaxation processes in the glassy state [24], experiments made at extremely rapid timescales (e.g. spectroscopy) are in contrast needed to investigate relaxation in supercooled liquids where equilibrium is quickly achieved. To give a single example, ultrastable organic glasses obtained by vacuum‐deposition techniques should be of special interest in view of their internal stability that is equivalent of that of hyper‐aged glasses (with aging time of millions of years) obtained by conventional melt cooling [25]. For this particular class of glasses, the aforedefined T_M values are so much lower (by a few tens of degrees) than the standard glass transition temperatures that T_M and T_g cannot be indiscriminately used in Eq. (9b) [25, 26]. Among other consequences, new insights should then be gained on the non‐unity of the PD ratio. Finally, such experiments should of course be firmly complemented by fundamental work. Microscopic theories and atomistic simulations must be developed and, as stringent tests of their value, their predictions checked in terms of macroscopic physical properties.