Amorphous phase equilibria

Solvent Resistance. At temperatures below the melting of the crystallites, the parylenes resist all attempts to dissolve them. Although the solvents permeate the continuous amorphous phase, they are virtually excluded from the crystalline domains. Consequently, when a parylene film is exposed to a solvent a slight swelling is observed as the solvent invades the amorphous phase. In the thin films commonly encountered, equilibrium is reached fairly quickly, within minutes to hours. The change in thickness is conveniently and precisely measured by an interference technique. As indicated in Table 6, the best solvents, specifically those chemically most like the polymer (eg, aromatics such as xylene), cause a swelling of no more than 3%. 

Pressure-induced amorphization of solids has received considerable attention recently in physical and material sciences, although the first reports of the phenomenon appeared in 1963 in the geophysical literature (actually amorphization on reducing the pressure [18]). During isothermal or near isothermal compression, some solids, instead of undergoing an equilibrium transition to a more stable high-pressure polymorph, become amorphous. This is known as pressure-induced amorphization. In some systems the transition is sharp and mimics a first-order phase transition, and a discontinuous drop in the volume of the substance is observed. Occasionally it is strictly not an amorphous phase that is formed, but rather a highly disordered denser nano-crystalline solid. Here we are concerned with the situation where a true amorphous solid is formed. 

Diffusion Rate Controlled Process If the rate of chemical reaction is much faster than the diffusion of water and EG through the solid amorphous phase, then the reaction can be considered to be at equilibrium throughout the pellet [21], The reaction rate is dependent upon the pellet size, the diffusivity of both water and EG, the starting molecular weight, and the equilibrium constants Ki and K5. In addition, the pellet can be expected to have a radial viscosity profile due to a by-product concentration profile through the pellet with the molecular weight increasing as the by-product concentrations decreases in the direction of the pellet surface [22-24],... 

For a number of applications, particularly those associated with conditions of continuous cooling or heating, equilibrium is clearly never approached and calculations must be modified to take kinetic factors into account. For example, solidification rarely occurs via equilibrium, amorphous phases are formed by a variety of non-equilibrium processing routes and in solid-state transformations in low-alloy steels much work is done to understand time-temperature-transformation diagrams which are non-equilibrium in nature. The next chapter shows how CALPHAD methods can be extended to such cases. 

Secondary phases predicted by thermochemical models may not form in weathered ash materials due to kinetic constraints or non-equilibrium conditions. It is therefore incorrect to assume that equilibrium concentrations of elements predicted by geochemical models always represent maximum leachate concentrations that will be generated from the wastes, as stated by Rai et al. (1987a, b 1988) and often repeated by other authors. In weathering systems, kinetic constraints commonly prevent the precipitation of the most stable solid phase for many elements, leading to increasing concentrations of these elements in natural solutions and precipitation of metastable amorphous phases. Over time, the metastable phases convert to thermodynamically stable phases by a process explained by the Guy-Lussac-Ostwald (GLO) step rule, also known as Ostwald ripening (Steefel Van Cappellen 1990). The importance of time (i.e., kinetics) is often overlooked due to a lack of kinetic data for mineral dissolution/... 

Fig. 19. Line shape analysis of the equilibrium spectrum of P420. The large dotted Lorentzians centered at 32.4 and 30.5 ppm and rather wide dotted Lorentzian centered at 32 ppm represent the orthorhombic crystalline and noncrystalline amorphous phases and crystalline-amorphous interphase, respectively. The dotted curve that is almost completely superimposed on the experimental spectrum indicates the composite curve of the component Lorentzians. dashed Weakly Lorentzians at 39, 34, 28, and 26 ppm represent the contributions from the methine and methylene carbons (a and p to the methine and methylene in the ethyl side group), respectively...

The resonance lines at 72.9 and 28.3 ppm are assigned to the crystalline components of a- and 3-methylene carbons because of their longer Tic values. These crystalline resonance lines are associated with two T1C values of ca. 209 and 9-10 s. This shows that both methylene carbons possess two components with different Tic >s> but this will not be discussed further, since the existence of plural TiC s is a normal finding for crystalline polymers as discussed in previous sections. On the other hand, the resonance lines at 70.9 and 27.0 ppm recognized for a-and (3-methylene carbons are assignable to the noncrystalline component, because these chemical shifts are very close to those in the solution. These lines are associated with only one Tic of 0.15 or 0-14 s and two T2c values of 7.95 s and 0.099 ms, or 8.22 s and 0.099 ms, respectively for the a- and (3 -methylene carbons. This suggests that the noncrystalline component involves two components, both associated with a same Tic and different T2c Js. The noncrystalline component with a T2c of 7.95 or 8.22 ms is thought to form an amorphous phase and that with a T2C of 0.099 ms comprises a crystalline-amorphous interphase. In order to confirm this, we examined the elementary line shapes of each component and performed the line shape decomposition analysis of the equilibrium spectrum. 

Similar line shape analyses for the equilibrium spectra at different temperatures were performed. At room temperature, where the amorphous phase is in a glassy state, the determination of the elementary line shape of the amorphous component was a little difficult. However, excellent line-decomposition analysis was obtained by introducing a broader Lorentzian centered at the same chemical shift as at higher temperatures. The result at room temperature is shown in Fig. 26-(b). Here the nature of the component line shapes A and B of the crystalline and crystalline-amorphous interphases is similar to that in the spectrum at 87 °C. However, the component line shape for the amorphous component is quite different from that at 87 °C that is distributed over a very wide chemical shift range centered at the same chemical shift to that at higher temperatures. This reflects the glassy state of the amorphous phase. In the glassy state, the molecular conformation in the amorphous phase will be distributed over all permitted conformations stationary in time and randomly in space. The wide component line shape of the amorphous component obtained here at room temperature well represents this molecular nature of the amorphous phase. 

Figure 30 shows the component analysis of the resonances of the methine and methyl carbons in the equilibrium DD/MAS 13C NMR spectrum. Here a Lorentz-ian function is assumed for each component. The rationality for this assumption was confirmed by examining the elementary line shapes for each component using the differences in the Tic and T2c values in a similar way to that described in preceding sections. The narrow Lorentzian components centered at 26.2 and 20.6 ppm, and 27.4 and 19.9 ppm are assignable to the methine and methyl carbons in the crystalline and amorphous phases, respectively, as discussed previously (see Table 13). In addition to these components, broad Lorentzian components are recognized centered at 26.6 and 21.1 ppm for the methine and methyl carbons. It was... 

In particular for cryogenic amorphous phase of ice share of physical intermolecular /-bonds is negligible component. In such case formulas for equilibrium share sizes come to expressions ... 

It should be mentioned that DSC and NMR do not measure the same parameters, and in this way, these techniques are complementary. DSC is a dynamic method, which gives information about the transitions between different phases of lipids, whereas NMR allows quantitation of liquid and solid phases at equilibrium. Indeed, NMR and DSC methods give different values for the solid fat index (SFI) (Walker and Bosin, 1971 Norris and Taylor, 1977) NMR values are much lower than those given by DSC below 20°C. For example, for milk fat at 5°C, DSC and NMR indicate 78.1% and 43.7% solid fat, respectively. The observed difference can be explained by the presence of an amorphous phase which, due to its melting enthalpy, is seen as a solid by the DSC method. Using time-domain NMR, Le Botlan et al. (1999) showed that in milk fat samples, an intermediate component exists between the solid and liquid phases, constituting about 6% of an aged milk fat. 

One of the characteristic of the nascent powder structure is the presence of an intermediate phase included with conventional crystalline and amorphous phases. The higher ductility of polyethylene nascent powders suggested the coexistence of less entangled amorphous phases located between crystalline and amorphous phases.24 4 46 This arises from the non-equilibrium crystallisation during polymerisation. Therefore, the polymerisation temperature affects the structure and the morphology of the nascent powder. 

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