Crystalline phase nucleation

As supersaturation is increased, the entropy of phase transformation is lowered, thereby lowering AG for formation of the condensed crystalline phase (nucleation), as shown in Fig. 4-4. In the (3-carotene/THF/water example of Fig. 4-3, S > 8, the critical-sized embryo is about one or two molecules. 

Fig. 1. (a) Variations with temperature (relative to fusion temperature, 7)) of the relaxation times within the amorphous phase, T , and out of the amorphous phase into the crystalline phase (nucleation time), t ,. (h) Variations with temperature of the relaxation time ratio... 

Two types of phase equilibria are superimposed phase separation equilibria in amorphous and ciystalhne phases. The thermodynamic equilibrium of the system presented in Figure 6.2 corresponds to polymer crystallization. The amorphous phase is metastable. It can exist fi om the beginning of crystalline phase nucleation through subsequent crystallization imtil the equilibrium state for the crystalline polymer is reached. A system can degenerate into many simple systems for polymers that erystallize readily in dilute solutions where polymer segregates into separate crystallites. Polymers, capable of partial crystallization at the ejq)ense of the formation of local regular blocks of chains (e.g., PVC) can exist as a metastable amorphous-erystalhne system. In such cases, one phase repre-... 

It is supposed that geometry, morphology and crystalline phase nucleation and growth mechanism will be defined to a considerable degree by the nature of heterophase fluctuations in the amorphous state. So, the supposition of heterophase fluctuation from folded chains (Yech model [44]) results in the formation of crystallites with folded chains (CFC). In paper [43], an alternative model of heterophase fluctuation supposes the availability of parallel parts of different chains of macromolecules in it. The use of these models supposes the possibility of formation in the crystallisation process of CFC, crystallites with stretched chains (CSC) or some intermediate morphology... 

At a sufficiently low temperature, the phase nucleated will be crystalline rather than liquid. The theory is reviewed in Refs. 1 and 7. It is similar to that for the nucleation... 

Fig. 3. Curve ihustrating the activation energy (barrier) to nucleate a crystalline phase. The critical number of atoms needed to surmount the activation barrier of energy AG is n and takes time equal to the iacubation time. One atom beyond n and the crystahite is ia the growth regime.

This approach is an alternative to quantitative metallography and in the hands of a master gives even more accurate results than the rival method. A more recent development (Chen and Spaepen 1991) is the analysis of the isothermal curve when a material which may be properly amorphous or else nanocrystalline (e.g., a bismuth film vapour-deposited at low temperature) is annealed. The form of the isotherm allows one to distinguish nucleation and growth of a crystalline phase, from the growth of a preexisting nanocrystalline structure. 

They considered that cement formation was the result of an acid-base reaction leading to the formation of hydrates by a through-solution mechanism, by nucleation and precipitation from pore fluids. Two phases were found in the matrix, one amorphous and the other crystalline. The crystalline phase was newberyite. Finch Sharp concluded that the amorphous phase was a hydrated form of aluminium orthophosphate, AIPO4, which almost certainly contained magnesiiun. They ruled out a pure AlP04.nH20, for they considered that the reaction could not be represented by the equation... 

It is important to consider why H depends on the degree of order of the crystalline phase. Three different types of diffusion process act during the nucleation process. They are diffusion within the melt, within the interface between the melt and a nucleus (or crystal), and within the nucleus. It is obvious that the diffusion of chains within the melt can not be related to the dependence of H on the degree of order of the crystalline phase within the nucleus (or crystal). Therefore, the phase dependence of H should arise from... 

In semi-crystalline polymers the interaction of the matrix and the tiller changes both the structure and the crystallinity of the interphase. The changes induced by the interaction in bulk properties are reflected by increased nucleation or by the formation of a transcrystalline layer on the surface of anisotropic particles [48]. The structure of the interphase, however, differs drastically from that of the matrix polymer [49,50]. Because of the preferred adsorption of large molecules, the dimensions of crystalline units can change, and usually decrease. Preferential adsorption of large molecules has also been proved by GPC measurements after separation of adsorbed and non-attached molecules of the matrix [49,50]. Decreased mobility of the chains affects also the kinetics of crystallization. Kinetic hindrance leads to the development of small, imperfect crystallites, forming a crystalline phase of low heat of fusion [51]. 

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