Critical Issues of Current Research on the Dynamics Leading to Glass Transition

Abstract

Glass transition is still an unsolved problem in condensed matter physics and chemistry. In this paper, we critically reexamine experimental data and theoretical interpretations of dynamic properties of various processes seen over a wide time range from picoseconds to laboratory time scales. In order of increasing time, the ubiquitous processes considered include (i) the dynamics of caged molecular units with motion confined within the anharmonic intermolecular potential and where no genuine relaxation has yet taken place; (ii) the onset of the Johari-Goldstein secondary relaxation involving rotation or translation of the entire molecular unit and causing the decay of the cages, to be followed by the cooperative and dynamically heterogeneous motions participated by increasing number of molecules or length scale; and (iii) the terminal primary alpha-relaxation with the maximum cooperative length-scale allowed by the intermolecular interaction and constraints of the glass former. Some general and important properties found in each of these processes are shown to be interrelated, indicating that the processes are connected, with one being the precursor of the other following it. Thus, a theory of glass transition is neither complete nor fundamental unless all of these processes and their inter-relations have been accounted. In addition to published data, new experimental data are reported here to provide a limited collection of critical experimental facts having an impact on current issues of glass transition research and servingas a guide for the construction of a complete and successful theory in the future.

Glassy dynamics in mono-, di- and tri-propylene glycol: From the α- to the fast β-relaxation

“…4 (solid lines). The presence of a constant loss in glassy matter has been considered since long (e.g., [14]) and quite recently found renewed interest due to its incorporation into the extended coupling model [12,13].…”

“…There are also other approaches to explain the excess intensity observed in this high-frequency region. Maybe the most prominent one is the extended coupling model [12,13], which involves an explanation in terms of a nearly constant loss, a phenomenon which is known since long [14]. So far, two molecular and two ionic glass formers have been investigated by dielectric spectroscopy in a sufficiently broad and continuous frequency range to allow for a meaningful analysis of the fast b-relaxation [5,[7][8][9][10].…”

“…Different techniques were used for characterization, among them neutron [15] and light scattering [16][17][18][19][20][21], optical Kerr-effect [22] as well as photothermal [23] and dielectric spectroscopy [3,[24][25][26][27][28][29][30][31] including elaborated studies at elevated pressure [27][28][29][30]. Moreover, different models were applied to the experimental data, especially the coupling model [12,13,32], the minimal model [33,34], a non-monotonic relaxation kinetic model [35] and an approach considering the hybridization of a slower unresolved JG b-with a faster c-relaxation [36]. However, in most dielectric experiments reported so far, the frequency range was restricted to frequencies below 10 MHz, except for [25] where spectra up to 1 GHz were reported.…”

Microscopic aspects of Stretched Exponential Relaxation (SER) in homogeneous molecular and network glasses and polymers

“…In molecular metallic, or colloidal glasses (such as orthoterphenyl, OTP) boson peaks are associated with cages around vacancies or miscoordinated (relative to the crystal) sites [37,38], while the boson peak in metallic glasses is modeled elastically in terms of local structural shear rearrangements [39]. These clusters are precursors of crystallization, and their structure involves weak interactions compared to the stronger interactions discussed in constraint theory (see above), which determine the composition dependence of the glass-forming tendency.…”

Recent progress in understanding relaxation in complex systems

“…Nevertheless, there are ordinary glassformers which shows a well resolved secondary relaxation but it bears no connection to the a-relaxation. Examples are benzophenone (BZP) and dibutylphthalate (DBP) studied by OKE, where the observed secondary relaxations do not shift on applying pressure [55]. The non-observation of the JG relaxation in these glassformers can be rationalized by their narrow a-loss peak (i.e., small n) and the aforementioned correlation of n with the separation between the two relaxations given by [logs a Àlogs JG ].…”