Vitrification temperature

There have been isolated QSPR studies of a number of other polymer properties. These include the dielectric constant [144], the dielectric dissipation factor (tan 8) [168], the solubility parameter [169], the molar thermal decomposition function [170], the vitrification temperature of polyarylene oxides [171], and quantities relating to molecularly imprinted polymers [172, 173]. The interested reader is referred to the literature for further information. 

Toropov AA, Nurgaliev IN, Balakhonenko OI et al. (2004) QSPR modeling of vitrification temperatures for polyarylene oxides. 1 Struct Chem 45 706-712... 

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 values of Tv for some substances are summarized in Table 1 together with the melting points, Tm, for the same compounds. It can be seen from the table that the vitrification temperature is usually lower than the melting temperature by 40-70°. 

Coatings derived from cholesteric liquid crystalline polymers are used commercially as reflective sheets and polarisers. The liquid crystal is cooled below the vitrification temperature resulting in a solid polymer that is amorphous but contains large regions of frozen liquid crystalline order. Such structures are also found in nature in the iridescent, almost metallic colours of beetles and other insects, which result from helical cholesteric structures in the outer layer of the carapace. 

Figure 1 shows that the air and nitrogen environments do not change the vitrification temperature of the quinoxaline resin. 

The nitrogen scan indicates a vitrification temperature of 235°C. The corresponding air scan yields a vitrification temperature of 232°C, well within the limit of experimental uncertainty. 

However, the curves in Figure 2 shows that while the nitrogen cure shows a vitrification temperature of the sulfone resin at 208°C the corresponding air scan vitrification temperature is 222 >C. 

The higher uncured Tg of the quinoxaline resin requires a lower extent of reaction to reach the vitrification point during the scan. The cumulative effect at vitrification may not be sufficient to show different vitrification temperatures. 

The physico-mechanical properties of aminoplasts in the articles are determined by the degree of hardening and macrostructural defects. In the cooling of the articles down to room temperature reactive groups in the polymer are still retained, but their interaction is made difficult due to the loss of mobility caused by the molecules of the reticular polymer because of the latter s vitrification. Simultaneously a nonequilibrium supramolecular structure is recorded. Heat treatment of the articles does not alter the supramolecular structure, the latter remaining invariable. Heat treatment at a temperature below the vitrification temperature may only cause either a certain additional hardening of the binder or increase the... 

There are thus three fixed points to consider in the testing of fireclays or for that matter in the examination of refractories in general, viz., the vitrification temperature at which the mass becomes dense and practically impervious, the point at which it becomes overfired and vesicular and the softening temperature. The difference between the vitrification and overfiring temperature constitutes the vitrification range which is of considerable technical importance from the manufacturing standpoint. 

Figure 1. LFCM dynamic fluidity as a function of temperature 1 - the so-called multicoloured LFCM, 2 - brown LFCM. Vitrification temperature T9 corresponds to conventional viscosity value of 1 TPa-s.

LFCM are the alkali-earth silicate glasses, whose vitrification temperature belongs to 1000 K -F 1100 K temperature interval. 

Fig. 6.19. Free energies of crystal, (solid line), cluster, (dashed line) and cluster in an ensemble, (dash-dotted line) at different temperatures, T = 0.6 (a) and T = 0.55 (b). Kinetic vitrification temperature is close to 0.6. Temperature, free energies and radius are scaled by Tm, Hm, and a, respectively...

Fig. 6.21. Cluster fraction temperature dependences x(T) for LTS (1) and HTS (2). At thermodynamical vitrification temperature V, we have x(T) = x, and the percolated liquid cluster disappears at T < rgh...

Hot-melt thermoplastic elastomer systems (23. 24) are also effective coating materials. These materials are generally based on copolymers that are comprised of hard (crystalline or glassy) and rubbery (amorphous) segments contained in separate phases. The hard-phase regions form physical cross-links below their crystallization or vitrification temperature, and the system therefore has elastomeric properties. The moduli and low-temperature characteristics of these materials can be tailored to compare reasonably well with silicone rubbers at -40 C. However, they are limited in high-temperature applicability because of enhanced creep or flow due to softening. 

Substrates of different types influence the glass vitrification temperature of the polymer. The glass-transition temperature Tg was determined from the tiorning point of the dependence of the logarithm of the retention volume on the inverse temperature, logVq vs. 1/T. As Fig. 1.3 shows, the effect of the substrates of low (hydrophobicized... 

Figures 10 and 11 show two extreme morphologies developed for PP/EPR by the isothermal unmixing at Tx and the subsequent diffusion-limited crystallization (14). The morphology shown in Fig. 10 was developed by unmixing at 200 C for 5 min. and subsequent crystallization at 125 C for 5 min., followed by an eventual rapid cooling below the vitrification temperature of the mixture. On the other hand the morphology shown in Fig. 11 was developed by unmixing at 200 C for 20 min. and subsequent crystallization by quenching the mixture in an ice-water bath.

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