Devitrification, mechanical

Hence, realization of the yielding process in amorphous glassy polymers requires clusters stability loss in the mechanical stress field, after which mechanical devitrification of the loosely packed matrix proceeds. Similar criterion was obtained for semicrystalline polymers [24]. 

If to consider the yielding process as polymer mechanical devitrification [36], then the same increment of fluctuation free volume is required for the strain achievement. This increment AY can be connected with as follows [50] ... 

The comparison of and values is adduced in Fig. 4.14, from which their satisfactory correspondence follows. This is confirmed by the conclusion, that crystallites partial meehanical melting (disordering) is necessary for fluctuation free volume growth up to the value, required for polymers mechanical devitrification realization [51]. 

As it is known [83], a glassy polymers behavior on cold flow plateau (part III in Fig. 4.17) is well described within the frameworks of the rubber high-elasticity theory. In Ref [39] it has been shown that this is due to mechanical devitrification of an amorphous polymers loosely packed matrix. Besides, it has been shown [82, 84] that behavior of polymers in rubber-like state is described correctly under assumption, that their structure is a regular fractal, for which the identity is valid ... 

Let us note, that the indicated temperatures also correspond well to loosely packed matrix glass transition temperature of PS and PPO K, which is approximately by 50 K below than macroscopic [26]. This assiunes, that the crazing process at high temperatures for PC and PPO is due to mechanically devitrificated loosely packed matrix drawing facility (in which instable clusters, restricting this process, are absent) in craze fibrils. The Eqs. (1.9), (1.12) and (4.66) indicate unequivocally the cause of PS tendency to... 

The rarely crosslinked epoxy polymer on the basis of resin UP5-181 has a low glass transition temperatnre T, which can be estimated according to shrinkage measurements data as being equal to 333 K. This means that the testing temperatures T = 293 K and Tg for it are close, which is confirmed by the small AOy valne for the native REP. This supposes (nanostructures) a small relative fraction of the nanoclusters [2, 3] and, since these nanoclusters have arbitrary orientation, an increase in results rapidly in their decay, which causes mechanical devitrification in the loosely packed matrix at > 0.36. The devitrificated loosely packed matrix gives an insignificant... 

Usually, the molecular strands are coiled in the glassy polymer. They become stretched when a crack arrives and starts to build up the deformation zone. Presumably, strain softened polymer molecules from the bulk material are drawn into the deformation zone. This microscopic surface drawing mechanism may be considered to be analogous to that observed in lateral craze growth or in necking of thermoplastics. Chan, Donald and Kramer [87] observed by transmission electron microscopy how polymer chains were drawn into the fibrils at the craze-matrix-interface in PS films [92]. One explanation, the hypothesis of devitrification by Gent and Thomas [89] was set forth as early as 1972. 

Here we are concerned with changes in the solid structure of the waste form, such as devitrification and phase separation in waste-containing glasses. Thus we need to understand the mechanisms and kinetics of the solid-state transformations and the effect of these changes on the solubility of the wastes. Besides the transformations that might occur in the dry state, we also need to know what hydrothermal changes might occur. 

The swelling mechanism is well established in rubbers (Flory, 1943). The expansion force generated by the solvent penetration is equilibrated by the entropic force linked to chain stretching. Little is known, in contrast, for the case of glassy polymers where plasticization effects are not sufficient to induce a devitrification. Swelling can be defined by... 

There are several ways to force glass to devitrify (whether devitrification is desired or not). One technique is to heat the glass until it begins to soften, then mechanically work, or flex, the glass while it cools. Eventually a whitish frost will appear on the surface in the region of compression (see Fig. 1.2). 

For high temperature and rubber-modified epoxy resins, thermal degradation events and the cloud point curve are included on the diagrams, respectively. Two degradation events have been assigned devitrification, or a glass-to-rubber event and revitrification, which is associated with char formation. The cloud points and depressions of Tg for different rubber-modified epoxies can be compared and related to volume fractions of the second phase and to the mechanical properties of the cured materials. 

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