Nonequilibrium crystal

In simple liquids studied by computer simulation the problem of glass formation through structural arrest cannot be dissociated from the problem of nonequilibrium crystallization (homogeneous nucleation), since under many conditions the time scales for the two processes are comparable. A broad review emphasizing phase changes has recently been given by Frenkel and McTague, to which we refer the interested reader for a more... 

Point 2 On the other hand, far from equilibrium there appears a variety of mechanisms corresponding to the possibility of occurrence of various types of dissipative structures. For example, far from equilibrium we. .. may also have processes of self-organization leading to nonhomogeneous structures to nonequilibrium crystals. " ... 

The problems of nonequilibrium crystallization and melting discussed in Chap. 6 and Sect. 7.1 for single-component systems, are even more pronounced for multi-component systems. The additional problem of separating the components by diffusion into phases of different concentration complicates the description of multi-component systems even further. Usually equilibrium concentrations cannot be achieved at the moment of crystallization or melting, i.e., the processes of component diffusion and crystallization/melting are not molecularly coupled. One must thus always consider two deviations from equilibrium. One is the formation of non-... 

No such statement can be made for the volume or enthalpy changes on transition, although similar crystal structures and molecules may lead to similar changes (93). Further general observations on fusion are the lowering of Tm by solvents (as a second, noncrystallizable component) and the lowering of I m for small crystals (11). The last effect has also been used to describe the fusion of small, nonequilibrium crystals as commonly found in metastable, semicrystalline polymers (11,94). 

When a nonequilibrium crystal is mixed with water, the solution concentration attained may momentarily exceed the equilibrium solubility. Also, the rate of dissolution of nonequilibrium crystals is typically faster than that of the equilibrium crystal. Advantage is often taken of these properties of nonequilibrium crystals in drug formulations to provide a form of the drug compound that dissolves more rapidly, and even to a greater extent, than the equilibrium crystal [50]. 

For many crystals (but not all see Sects. 4.7.1-4.7.2) the superheating on melting is much less serious than the supercooling on crystallization, so that melting may approach equilibrium as long as one starts with an equilibrium crystal. For nonequilibrium crystals it is, at best, possible to analyze the zero-entropy-production melting described in Sect. 4.7.1. 

The higher ductility of polyethylene nascent powders suggested the coexistence of less entangled amorphous phases located between the crystalline and amorphous phases [26,47]. This arises from the nonequilibrium crystallization during polymerization. Therefore, the polymerization temperature affects the structure and the morphology of the nascent powder. 

With the help of measurements of the lamellar thickness, /, by dUatometry, AFM or X-ray, and the melting temperature of the nonequilibrium crystal by DSC, it is possible to obtain the equilibrium melting temperature, T, by extrapolation to infinite lamellar thickness. Next, with Equation (9.7), it is also easy to calculate the fold surface free energy. Usually, the large surface area of the polymer crystal explains irreversibility of the melting process. 

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