Structural relaxation hysteresis

The kinetic character of the glass transition and the resulting non-equilibrium character of the glassy state are responsible for the phenomena of structural relaxation, glass transition hysteresis, and physical aging (Kovacs, 1963 Struik, 1978). 

Figure 4.4 shows a dilatometric or calorimetric experiment to show structural relaxation (physical aging) and glass transition hysteresis. The sample is cooled from T0 to T it is kept at Tj for a certain time and heated again to T0. During the cooling step, the material vitrifies at B, resulting in an abrupt decrease in both the expansion coefficient and the specific heat. 

Surface states exist on normal metals as well as on transition metals. Occupied and unoccupied surface states were recently studied experimentally and theoretically on single crystal surfaces of e.g. Ag (Reihl, 1985) Cu (Bartynski et al., 1985) and Ni (Borstel et al., 1985). Surfaces of transition metals are particularly interesting as they not only show a structural relaxation - an effect which is mostly weak on normal metals - but can also exhibit magnetic properties which differ from those of the bulk (Freeman, 1983). A surface enhanced magnetic order as well as magnetic surface reconstruction were observed on Gd(OOOl) (Weller et al., 1985). The magnetic hysteresis loop of the Fe 100) surface, very recently measured by means of the spin polarization of secondary electrons (Fig. 3), shows a softer behaviour within the outermost 5 8 due to reversed domain nucleation (Allens-pach et al., 1986). The structural and electronic differences of surfaces and bulk may manifest themselves as surface core level shifts (Eastman and Himpsel, 1982 Erbudak et al., 1983). In the case of rare earth metals and alloys they may even appear as surface valence transition (Netzer and Matthew, 1986 Kaindl et al., 1982). 

Widespread evidence for hysteresis and overshoot phenomena, such as displayed in Figure 13, indicates that structural relaxation is nonexponential even for small departures for which the linear approximation is adequate, and the time dependence may be represented by the function (7-10,36,113-115)... 

An important feature of filled elastomers is the stress softening whereby an elastomer exhibits lower tensile properties at extensions less than those previously applied. As a result of this effect, a hysteresis loop on the stress-strain curve is observed. This effect is irreversible it is not connected with relaxation processes but the internal structure changes during stress softening. The reinforcement results from the polymer-filler interaction which include both physical and chemical bonds. Thus, deforma-tional properties and strength of filled rubbers are closely connected with the polymer-particle interactions and the ability of these bonds to become reformed under stress. 

We should note that this article by Ya.B. apparently remained little noticed in its time. In any case, we are unaware of any reference to it in the works of other authors. This is explained by the fact that its ideas were far ahead of their time. Only in recent years, due to the wide application of physical methods in studies of adsorption and catalysis, have the changes in the surface (and volume) structure of a solid body during adsorption and catalysis been proved. Critical phenomena have been discovered, phenomena of hysteresis and auto-oscillation related to the slowness of restructuring processes in a solid body compared to processes on its surface. Relaxation times of processes in adsorbents and catalysts and comparison with chemical process times on a surface were considered in papers by O. V. Krylov in 1981 and 1982 [1] (see references at end of Introduction). 

Influence of Solvents. The stress-strain curves of untreated and ether-extracted corneum in water show marked differences (81). Untreated corneum, extended 5% and relaxed, shows hysteresis similar to that observed for other keratinaceous structures (Figure 35). The deformation mechanism is completely reversible, and hydrogen-bond breakdown and slow reformation may be the major factors determining the stress-strain relationships. With ether-extracted samples, complete recovery is observed from 5% extension but with little or no hysteresis. The more rapid swelling and lack of hysteresis of ether-extracted corneum in water may be related to the breakdown of hydrogen bonds normally shielded from the eflFects of water by the lipid-like materials removed by ether. 

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