Glasses quasi-equilibrium

There is no fundamental qualitative difference in mechanisms of low molecular weight (MW) penetrant diffusion in polymers above and below glass transition temperature, Tg, of the polymers [5,6]. The difference lies only in the fact that the movement of structural units of the macromolecule that are responsible for the transfer of penetrant molecules takes place at different supermolecular levels of the polymer matrix. At T > Tg the process of diffusion takes place in a medium with equilibrium or near-equUibrium packing of chains, and the fractional free volume, P(, in the polymer is equal to the fractional free volume in the polymer determined by thermal mobUity of strucmral units of macromolecules V((T), i e., V(= vut). At r< Tg the process of diffusion comes about under nonequihbrium packing conditions, although there exists a quasi-equilibrium structural organization of the matrix, where Vf> It is assumed that in this case Vf= where is the fractional free volume... 

The time dependence of some thermodynamic properties of amorphous materials below the glass transition is shown in Figure 4. The actual position of Tg depends on the cooling rate. For many practical purposes, energies and densities, along with most other properties of vitrified materials, are expressed in terms of their relaxation times. A fictive temperature Jf (T,/), is introduced, which represents the temperature at which the quasi-equilibrium enthalpy of the undercooled liquid equals that of the non-equilibrium glass Tf T,t) then accounts for the temperature and time dependence of the non-equilibrium state. The significance and usefulness of the fictive temperature is discussed in Chapter 11. 

The nanocrystals of such type form in various liquid media, such as organic solution [77, 81] or the softened quasi-liquid glass [82, 83], where there are no steric hindrances for the growth of equilibrium crystals without surface defects. At the same time, barriers for aggregation of clusters or atoms to metal nanocrystals in solid system that arises during the cryochemical solid-state synthesis favor the formation of crystals with structural defects,... 

Glass transitions involve mainly the onset or freezing of cooperative, large-amplitude motion and can be studied using thermal analysis. Temperature-modulated calorimetry, TMC, is a new technique that permits to measure the apparent, fiequency-dependent heat capacity. The method is described and a quasi-isodiermal measurement method is used to derive kinetic parameters of the glass transitions of poly(ethylene terephthalate) and polystyrene. A first-order kinetics expression can describe the approach to equilibrium and points to the limits caused by asymmetry and cooperativity of the kinetics. Activation energies vary from 75 to 350 kJ/mol, dependent on thermal pretreatment. The preexponential factor is, however, correlated with the activation energy. 

Id usually appears as a thin film on a glass surface, and until now we have not succeeded in obtaining a single crystal for the X-ray analysis. However, there are some reasons to speculate about its structure. For disilenes of the type RR Si=SiR R, photochemical cis-tram isomerization in solution was shown to occur [11], the trans isomer being predominant under equilibrium conditions. Of course, symmetrically substituted 1 crumot have real cis—trans isomers, but, by analogy, a similar equilibrium with predominance of a conformer, close to the tram one, seems likely in solution. This assumption is eonfirmed by Raman polarization measurements for a solution of 1 in hexane, because the selection mles observed for the conformer predominant in solution are consistent with C2h symmetry, that is, with a quasi-/ra s structure of this conformer [12]. As both the Raman and UV-Vis absorption and fluorescence spectra of solid Id are similar to those of 1 in hexane solution [1,12], we can suggest for Id also a quasi-/ra/is structure as shown in Fig. I. 

Recently, Chakraborty and coworkers have shown that, at strongly interacting pol5Tner-solid interfaces, the quasi two-dimensional glass-like structures are formed which are non-equilibrium, connected with non-equilibrium conformations of adsorbed chains. 

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