Viscosity Vitrification

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-... 

The construction of a mold-filling model has been considered in the theory of thermoplastics processing. A rapid increase in viscosity also occurs in the flow of these materials, but the effect is different than in flow during reactive processing. The increase in viscosity of thermoplastic polymer materials is due to physical phenomena (crystallization or vitrification), while the increase in viscosity of reactive liquids occurs due to chemical polymerization reactions and/or curing. This comparison shows that the mathematical formulation of the problem is different in the two cases, although some of the velocity distributions may have similar features. 

Infinite viscosity is not an unambiguous indicator of gelation it can equally be caused by vitrification. This means that complementary methods, such as sol fraction measurements, are necessary to distinguish between both phenomena. 

In Section 4, we have examined, from a fundamental point of view, how temperature and cure affect the dielectric properties of thermosetting resins. The principal conclusions of that study were (1) that conductivity (or its reciprocal, resistivity) is perhaps the most useful overall probe of cure state, (2) that dipolar relaxations are associated with the glass transition (i.e., with vitrification), (3) that correlations between viscosity and both resistivity and dipole relaxation time are expected early in cure, but will disappear as gelation is approached, and (4) that the relaxed permittivity follows chemical changes during cure but is cumbersome to use quantitatively. 

Isoconversion curves, if shown, would approximately parallel the gelation line (as well as the full cure line) because gelation is considered to be an isoconversion state The extent of conversion after vitrification changes very slowly, but does not cease. Reference to a complete 11 1 diagram enables a time-temperature path of cure to be selected which will follow a desired viscosity-conversion path. 

Resin 5208 exhibits a second peak at longer times than the viscosity related maximum. Gillham (1) attributed a second peak observed by TBA to vitrification. Figure 5 displays the plots of the natural logarithm of t ,ax vs. the reciprocal of absolute temperature for the second peaks from OSA and TBA. The activation energies derived from the slope of a linear least squares fit of the results from both are somewhat different as indicated in Figure 5 and Table 1. 

The main feature of the nonequilibrium behavior of solutions dnring cryocrystallization is the appearance of amorphous solids. Generally vitrification of the liquid system depends on the rate of structural relaxation processes, which are substantially determined by the viscosity of the solution. At higher cooling rates and reduced temperatures, the cluster structure of the solution cannot follow the changes, predetermined by the equilibrium behavior of the system, so that even after solidification, the structure of the amorphous solid is very similar to the structure of the solution at low temperatnres. According to modem concepts, the amorphous state can be considered as a kind of snpercooled liqnid with an extremely high viscosity coefficient. 

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