Glass Transition Phenomenology

Maxwell was the first to describe the viscoelastic response at the fluid-to-glass transition phenomenologically. He introduced a time-dependent shear modulus describing the response of a viscoelastic fluid to a time-dependent shear... 

In this chapter, we reviewed the phenomenology of water in the glassy state. Three amorphous ices were identified, namely LDA, HDA, and VHDA. The different recipes of preparation of these amorphous ices, the structure of these glasses, as well as the relationship between them were discussed in detail. A brief summary of the glass transition phenomenology of the different amorphous ices was also included. 

However, as anticipated above, the glass transition phenomenology involving structural processes in polymer materials rarely follows simple Arrhenius laws, and especially near Tg (where decreasing the temperature by 10 K can produce an increase of x and r of, say, three orders of magnitude), the dramatic increase of the viscosity and of the characteristic structural relaxation time is much better described by the Vogel-Fulcher-Tamman (VFT) equation ... 

The glass transition phenomenology of polymers gives a clear idea of how amorphous systems behave in terms of their temporal response, namely of the relaxation towards equilibrium occurring after an external (thermal, mechanical, electrical, etc.) stress. This picture is made even more evident... 

Goldstein, M. Phenomenological aspects of the glass transition. Submitted to "Modem Aspects of vitrous State , Editor J. D. McKenzie. London Butterworth Co. Ltd. 1963. 

Other mathematical forms may be used to describe the high frequency relaxation. These various equations, either phenomenological or based on diiiusion defect models lead to a characteristic relaxation time xhp of the glass transition (Ta) domain of the same order of magnitude. 

On the other hand, some phenomenological distributions of relaxation times, such as the well known Williams-Watts distribution (see Table 1, WW) provided a rather good description of dielectric relaxation experiments in polymer melts, but they are not of considerable help in understanding molecular phenomena since they are not associated with a molecular model. In the same way, the glass transition theories account well for macroscopic properties such as viscosity, but they are based on general thermodynamic concepts as the free volume or the configurational entropy and they completely ignore the nature of molecular motions. 

Where Tg is the glass transition temperature and C and are phenomenological parameters. In figure 8, we have arbitrarily chosen for Xb the value of Xj at 40.9 °C, i.e. 4.54 ns, and applied Equ. (12) together with the parameters C( and C2 given by Ferry from low frequency mechanical measurements. The corresponding WLF curve (dotted line) fits well the other measured values of Xi. Of course, a similar... 

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