Crystal growth faces

Doubts about the impact on crystallization of such processes have already been raised by the present author in a paper that, very gracefully, Gert Strobl allowed to be published in parallel with his own contribution that presented a different viewpoint [8]. In that paper, the preeminence of a more classical nucleation and growth scheme (Fig. 1) was advocated crystallization is viewed as a more sequential process in which incoming stems probe the crystal growth face and are accepted if they fulfill the correct criteria. If not, the incoming stems are rejected or must undertake conformational adjustments. In other words, the classical nucleation and growth process can be seen as dominated or controlled by the crystal (substrate structure, or... 

Abstract Recent extensive experimental work and the limited theoretical studies of the phenomenon of self-poisoning of the crystal growth face are reviewed. The effect arises from incorrect but nearly stable stem attachments which obstruct productive growth. Experimental data on the temperature and concentration dependence of growth rates and... 

V Rate of step propagation on a crystal growth face (often also referred to as g)... 

Figure 6. Schematic representation of molecular deposition on the crystal growth face (a, d), the NIF form (b, e), and the F2 form (c, f) for a linear alkane (a—c) and the branched alkane Cg6Hig3CH(CH3)C94Hi89 (d—f). (From 59 with permission of American Chemical Society).

Figure 51. A series of fluctuations at the crystal growth face. The squares represent crystallized chain segments, (a) Steps allowed in the Frank-Tosi model,226 (b and c) some other possible steps (after ref 4).

It is assumed in the Lauritzen-Hoffman theory that, once a chain segment has been laid down on the growing surface parallel to the chains already forming the crystal growth face, the next units in the chain continue to be laid down to form a straight stem. When a lamellar face is reached the chain folds and the following units are laid down adjacent to... 

Figure 6.1. Velocities of crystal growth faces (a) invariant crystal, b) overlapping...

The crystallization process is controlled by the nucleation density and by the diffusion of chains to the crystal growth face. ... 

The observable growth rate, G, is a combination of the secondary nucleation rate, I, and the size of crystal growth face and is described by basic relationships ... 

Polymer crystallization is dominated by the process of tmtangling molecules and then straightening them onto the crystal growth face. Because of this, it is a comparatively slow process compared with crystalhzation of simple molecular species, and it may require supercooling by tens of Kelvins to occur at a significant rate. 

For example, consider the cross sections of a growing crystal as in Fig. 18-58. The polygons shown in the figure represent varying stages in the growth of the crystal. The faces marked A are slow-growing... 

Rate of Growth Crystal growth is a layer-by-layer process, and since growth can occur only at the face of the crystal, material must be transported to that face from the bulk of the solution. Diffusional... 

Crystal growth is a diffusion and integration process, modified by the effect of the solid surfaces on which it occurs (Figure 5.3). Solute molecules/ions reach the growing faces of a crystal by diffusion through the liquid phase. At the surface, they must become organized into the space lattice through an... 

Fig. 2.2. A diamond-shaped crystal showing four sectors and the folds lying parallel to the (110) growth faces...

Fig. 4.2a, b. Configurations of molecules which are unfavourable to further growth, a The growth face of a lamella is shown, on which a molecule has deposited but is prevented from reaching the length required for stability by other attachments elsewhere, b Two examples of possible cross-sections perpendicular to the growth front. The outermost depositions must be removed before further growth of the stable crystal... 

A very recent application of the two-dimensional model has been to the crystallization of a random copolymer [171]. The units trying to attach to the growth face are either crystallizable A s or non-crystallizable B s with a Poisson probability based on the comonomer concentration in the melt. This means that the on rate becomes thickness dependent with the effect of a depletion of crystallizable material with increasing thickness. This leads to a maximum lamellar thickness and further to a melting point depression much larger than that obtained by the Flory [172] equilibrium treatment. 

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