Experimental phase diagrams, crystalline

Figure 7.30 illustrates the limits from equilibrium to cold crystallization. They are based on a computer calculation for three-dimensional cold crystallization. It is assumed in this case that sequences of less than three A-units do not crystallize, and that at least two out of the four neighboring chains must match the sequence of three repeating units to make the central sequence crystalline. If all four lateral neighbors are crystalline, the repeating unit is in the interior of the crystal, if only three are crystalline, it is on the surface, and if only two are crystalline, it is part of an edge. With this critical sequence length and two surface free energies, experimental data of poly(ethylene terephthalate-co-sebacate) can be discussed, as is shown in Fig. 7.31. The experimental phase diagram of the same copolymer, but after slower cooling, is displayed in Fig. 7.26.

Figure 1.17 Experimental phase diagram of the Genamin TO2O/H2O/CUSO4 system La, lamellar Li, micellar L2, crystalline. The dashed lines are rough boundaries. Reprinted with permission from Longww/r 1999, 75, 3738-47 1999 American Chemical Society.

Prinsen et al. [23] and Warren et al. [31] used dissipative particle dynamics to simulate dissolution of a pure surfactant in a solvent. Tuning surfactant-surfactant, surfactant-solvent, and solvent-solvent interactions to yield an equilibrium phase diagram similar to Fig. 1 at low temperatures except for the absence of the V i phase, they found that the kinetics of formation of the liquid crystalline phases at the interfaces was rapid and that the rate of dissolution was controlled by diffusion, in agreement with the above experimental results. 

In order to analyze the dependence of the liquid crystalline transition properties on temperature (i.e. on the solvent quality), it is necessary to introduce the attraction of rods parallel to their steric repulsion. This has been done by Rory9 . The classical phase diagram of Rory for the solution of rods (see Fig. 2) agrees well with experimental results from the qualitative point of view1 . However, the Rory theory cannot give adequate answers to all the questions connected with the orientational ordering in the system of rigid rods. Indeed ... 

In some cases the transition looks like a continuous glass transition, but at temperatures and pressures that are far from the glass region in the bulk phase diagram. In fact, the molecules may not readily form glasses in the bulk. In other cases the transition from liquid to solid behavior of the film appears to occur discontinuously, as if the film underwent a first-order freezing transition [200,205]. Simulations suggest that this is most likely to happen when the crystalline phase of the film is commensurate with the solid walls and when the molecules have a relatively simple stmcture that facilitates order [178 180,191,209]. Unfortunately, no direct experimental determination of the stmcture of confined films has been made to determine whether they are crystalline or amorphous, and different behavior has been reported for the same... 

In some reactant mixtures as many as six crystalline phases were detected, e.g. as in the case of the composition Ce Mo Te 7 8 5 after calcination at 550 C for 8 hours. The presence of more phases than those permitted under thermodynamic equilibrium is a consequence of the incompleteness of the reactions between the components under our experimental conditions. It is also noticed that various areas of the phase diagram are highly sensitive to the preparative conditions e.g. the composition... 

Water in the supercooled state has been studied less thoroughly than in the superheated one. Experimental data mainly refer to pressures close to atmospheric. Ice exists in different crystalline forms, and the water phase diagram has an elaborate form if one does not restrict oneself to the region of low pressures p < 200 MPa). The pol5miorphism of ice may manifest itself at a low pressure too. It has been found that during crystallization of amorphous layers of light and heavy water there forms a mixture of hexagonal and cubic ice. 

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