Dendritic Crystals from Dilute Solution

Here 5 is the solvent concentration and n the (normal) distance from the advancing interface. The far field (bulk) solvent concentration is and for convenience the concentration of solvent at the crystal-solution interface is assigned the equilibrium value 5/, these can be described equivalently in terms of the polymer concentrations if desired. The interdiffusion coefficient 

The situation just described pertains to all crystals shown in Section 3.2.1 with weU-developed, straight 

The relation between interface curvature and local Se is captured by what is termed the capillary length  

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In the previous sections it was observed that when polymers are crystallized from dilute solutions, they form lamellar-shaped single crystals. These crystals exhibit a folded-chain habit and are of the order of 100 to 200 A thick. From somewhat more concentrated solutions, various multilayered dendritic structures are observed. 

Many polymers form more complex single crystals when crystalhzed from dilute solution including hollow pyramids that often collapse on drying. As the polymer concentration increases, other structures occur, including twins, spirals, and multilayer dendritic structures with the main structure being spherulites. 

Figure 5-26. Poly(ethylene) dendrites crystallized from a dilute xylene solution at 70° C (after...

When homopolymers are crystallized from very dilute solutions, either lozenge-shaped platelets or crystals that possess a dendritic habit are formed. Some typical electron micrographs of the crystals precipitated from dilute solution are shown in Figs. 1.12 and 1.13. The crystal habit that is observed depends on the molecular weight of the polymer and the crystallization conditions, such as the temperature and the nature of the solvent. A very striking feature is that the platelets are only about 100 to 200 A thick. In conjunction with selected-area electron diffraction studies, it is shown that the chains are again preferentially oriented normal, or nearly so, to the basal plane of the platelet. Considering the... 

Figure 33 Dendritic single crystals of high density polyethylene grown from dilute solution in xylene. (From Ref. 105.)...

Preliminary X-ray experiments indicate that the pendant chloro groups are organized in a highly ordered array and that the distance between these groups can be measured [37]. Additionally, several crystal habits, such as spherulites and dendrites, were observed by optical microscopy, upon precipitation of this polymer from a dilute toluene solution. 

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