Vitrification processes liquid-glass transition

As opposed to the liquid-crystal transformation, the liquid-glass transformation is not a phase transition and therefore it can not be characterized by a certain transition temperature. Nevertheless, the term "the vitrification temperature , Tv, is widely used. It has the following physical meaning. As opposed to crystallization, vitrification occurs when the temperature changes continuously, i.e. over some temperature interval, rather than jump-wise. Inside this interval, the sample behaves as a liquid relative to some of the processes occurring in it, and as a solid relative to other processes occurring in it. The character of this behaviour is determined by the ratio between the characteristic time of the process, t, and the characteristic relaxation time of the matrix, x = t//G, where tj is the macroscopic viscosity and G is the matrix elasticity module. If t x, then the matrix should be considered as a solid relative to the process, and if t > x it should be considered as a liquid. The relation tjx = 1 can be considered as the condition of the matrix transition from the liquid to the solid (vitreous) state, and the temperature Tv at which this condition is realized as the temperature of vitrification. Evidently, Tv determined by such means will be somewhat different for the processes with different characteristic times t. However, due to the rapid (exponential) dependence of the viscosity rj on T, the dependence of Tw on t (i.e. on the kind of process) will be comparatively weak (logarith-... 

In contrast to the freezing point, Zg is not a true transition temperature, because the vitrification process occurs over a narrow temperature interval. Furthermore, Zg depends on the cooling rate. The slower a liquid is cooled, the longer the time available for configurational exploration, and hence the lower the temperature down to which the liquid can remain in equilibrium. Consequently, Zg increases with cooling rate (Moynihan etal, 1976 Brtining and Samwer, 1992). This means that the properties of a glass depend on the... 

The amorphous state is the characteristic of all polymers at temperatures above their melting points (except under special circumstances where liquid crystals may form). If a molten polymer retains its amorphous nature on cooling to the solid state, the process is called vitrification. In the vitrified amorphous state, the polymer resembles a glass. It is characteristic of those polymers in the solid state that, for reasons of structure, exhibit no tendency toward crystallization. The amorphous solid state is characterized by glass transition (Tg), which is described in a later section. We consider below only the behavior of polymer melt. 

See the later section in this article on Adhesive Processes for comments about storing premixed and frozen adhesives. Between Tgo and gei g, the liquid resin will react without gelation until its continuously rising glass-transition temperature becomes coincidental with the cure temperature, at which stage vitrification begins and the reaction may become diffusion controlled. Note that gei g is the temperature at which gelation and vitrification occur simultaneously. 

Fig. 13.5. Solid line thermogram for the heating of a rapidly quenched liquid that has gone through the glass formation process. Dashed line normal thermogram for the same material that has not gone through the vitrification process. Sample values for the glass transition temperature...

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