Vitrified region

For the epoxy resins studied, the mobility factor based on heat capacity coincides very well with the diffusion factor, calculated from the nonreversing heat flow via chemical kinetics modelling, and describing the effects of diffusion control on the rate of conversion of the cure reaction. Although the two resins behave quite differently, this coincidence between the mobility factor and diffusion factor is valid for both systems. Therefore, the mobility factor can be used for a quantitative description of then-altered rate of conversion in the (partially) vitrified state for the decrease in rate during vitrification, the increase in rate during devitrification and the diffusion-controlled rate in the (partially) vitrified region in between both processes. 

The slow mode s properties correspond to the predicted properties of a Kivelson glass, in which the glass transition is driven by the formation of local clusters that cannot for some reason grow into space-filling lattices. By this interpretation, as supported by the near-equality of the longest relaxation times of the slow mode and the viscoelastic moduli, at least some polymer viscoelastic properties would seem to be driven by the interaction of a dispersion of vitrified regions within polymer solutions. 

As discussed in Section 9.10, an additional very slow mode is observed in hydroxypropylcellulose water, even in the absence of probe particles. The mode appears to correspond to the diffusion of long-lived local vitrified regions through the solution. The regions, which are correlated with the solutionlike-meltlike transition, appear to be substantially larger in spatial extent than a single polymer chain. 

A detailed study of the effect of temperature on the reaction kinetics of etr with a set of acceptors over a broad time interval of 10 5-l s in the region of ultralow temperatures (4.2-100 K) was performed in ref. 79. For the acceptors CrO, Fe(CN)jF, and N02, the decay curves for electrons et stabilized in deep traps of a water-alkaline (8M NaOH) matrix were found to vary only slightly with variation of temperature. The same result was obtained for the reactions of these acceptors with e stabilized in deep traps of vitrified mixtures of water with ethylene glycol [105]. Thus, at temperatures of 4-100 K, the main contribution of the reaction of et with the above acceptors in both matrices is made by a temperature-independent channel of electron tunneling. 

Butyl chloride (BC) + Cl2 On being cooled to 77 K, a solution of Cl2 in BC (mole ratio 1 3) completely vitrifies (Tg 100 K). The irradiation of the system by 60Co y rays leads to the accumulation of stabilized active centers. The chain chlorination reaction, not observed during radiolysis, is initiated only on heating up the irradiated system, starting from the devitrification region. The heat of reaction is 138 + 12kJ/mole.27... 

Methylbutadiene-l,3 (MB) -I- S02. A solution of S02 in MB (1 1) was prepared at T 170 K. On freezing, such a solution vitrifies. The thermoacti-vated copolymerization in the system, containing active centers stabilized during the low-temperature radiolysis, also begins in the devitrification temperature region at 100K.17... 

When an amorphous polymer is gradually cooled from above the glass transition temperature Tg its volume decreases (see Fig. 13.32) according to its thermal expansion coefficient aj. In the region around the Tg the volume decrease will lag behind, starting at temperature Tel because the rate of reorganisation process becomes too small. The polymer starts to vitrify and a temperature Tel will be reached where the reorganisation completely stops and where the vitrification process is completed. Decrease of volume is only the result of normal volume contraction with expansion coefficient ag. The relationship between both thermal expansion coefficients is... 

One difference between allylamine and a mixture of allylamine with H3P04 is that, at 77 K, the former is crystalline while that latter is vitreous. To check if the different polymerization is due to the different phase state, a mixture of allylamine with glycerin was also irradiated at 77 K and defrosted. Although a 2 5 v/v mixture of allylamine and glycerin vitrifies in the same temperature region as the allylamine-H3P04 mixture, it was found that polymerization does not take place in the allylamine-glycerin mixture. 

Figure 6.21 Vitreous Ice Freezing Device A thin (60-200 nm) carbon support film perforated with holes is placed on a standard EM carbon grid and then a sample of biological macromolecule is applied and rapidly frozen in liquid ethane (<138 K). After this, the vitrified sample is transferred into a cryoholder (under liquid nitrogen conditions) and then transferred to the microscope for visualisation. The holes in the thin film allow for the formation of monolayers of biological macromolecules in a range of orientations embedded within a thin (20-60 pm) layer of vitreous ice. The visualisation of such monolayer regions gives the best possible cryo-EM images of the embedded biological macromolecules.

In summary, electrostatic repulsion stabilizes lamellar phases in ionic systems, whereas entropy reduction stabilizes lamellar phases in nonionic systems or in ionic systems in apolar solvents or in high ionic strength water. Also, the presence of suitable cosurfactants (generally alcohols), which increase the flexibility of the membranes, leads to the formation of dilute lamellar phases, for example, in the system brine-SDS-pentanol [133] or brine-SDS-pentanol-dodecane [134]. Recently, it was shown [135] that two distinct lamellar phases coexisted in the dilute region of the system cetylpyridinium chloride-hexanol-brine. The two phases differ in turbidity, viscosity, density, and some other physical properties. One of these lamellar phases is classically stabilized by the competition between van der Waals, hydration, and electrostatic forces. The other phase is entropically stabilized. The difference between electrostatically and sterically stabilized lamellar phases was demonstrated by transmission electron microscopy on thin vitrified... 

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