Clustered melt crystals

In quiescent homogeneous melts, crystallization proceeds by the formation of isotropic quasi-spherical clusters known as spherulite. Figure 1 illustrates the final structure of a crystallized polymer (PHB), showing clearly the different spherulites originating from nucleation, whose growth competing to fill the space was stopped by impingement. 

In addition to showing that solvent-free melt and sublimation crystallization conditions offer an attractive route to new polymorphs, a CSD survey of these methods of crystallization and the frequency of Z was performed (Table 3-5). There is a dramatic increase in the occurrence of Z > 3 crystal structures when melt or sublimation crystallization conditions are used [20]. The occurrence of high Z in melt crystallization and sublimation methods is ascribed to the rapid cooling of the hot liquid or vapor (100-300° C) in the open flask or on the cold finger (kinetic phase), conditions under which hydrogen-bonded clusters are likely to condense in a pseudo-symmetric crystalline arrangement. On the other hand, the slower nucleation process of solution crystallization gives the frequent situation of Z < 1 (88% hits). 

Orange-colored clusters of crystals, not melted at 360+ Shows mutatotation [ajg1 — 20 (after 15 hrs in 0.051V... 

A second mechanism for increasing disorder on melting which cannot be conveniently represented by a quasi-crystalline model for the melt involves the formation of association complexes. Quite generally, these can be defined as clusters of the units of structure (e.g., molecules or ions) in the crystal which have approximately the same distance between nearest neighbours as in the crystal lattice, but which need not have the full regularity of crystal packing. As already stated, only one particular form of cluster, the crystal nucleus can normally be extended indefinitely... 

Hoffman, J. D. (1983) Regime III crystallization in melt-crystallized polymers the variable cluster model of chain folding, Polymer, 24, 3-26. 

Hoffman JD, Lauritzen JI (1961) Crystallization of bulk polymers with chain folding theory of growth of lamellar spherulites. J Res Natl Bur Stand 65A 297-336 Hoffman JD, Guttman CM, DiMarzio EA (1979) On the problem of crystallization of polymers from the melt with chain folding. Faraday Discuss Chem Soc 68 177-197 Hoffman JD (1983) Regime in crystallization in melt-crystallized polymers The variable cluster model of chain folding. Polymer 24 3-26... 

Solidification First order phase transformation, in which a liquid (melt) crystallizes Sol-gel transition Transition of a colloidal solution into a solid network extending through the whole sample space. The network is made up of the colloidal particles attached to each other mechanically and might be bonded by various potentials. At the point of contact so-called necks establish. The sol-gel transition might be considered as exactly that point when a percolating net of particles (a cluster) spans through the wet fluid. See also gelation... 

Hikosaka M, Okada H, Toda A, Rastogi S, Keller A (1995) Dependence of the lamellar thickness of an extended-chain single crystal of polyethylene on the degree of supercooling and the pressure. J Chem Soc Faraday Trans 91(16) 2573-2579 Hoffman JD (1983) Regime III crystallization in melt-crystallized polymers the variable cluster model of chain folding. Polymer 24(l) 3-26... 

The product, Si4(SiBu 3)4, forms intensely orange crystals that are stable to heat, light, water and air, and do not melt below 350°. The Si-Si distances within the closo-Si4 cluster are 232-234 pm and the exo Si-Si distances are slightly longer, 235-237 pm (cf. Si-Si 235.17 in crystalline Si). Comparison with the closo-amon Si4 , which occurs in several metal silicides (p. 337) and is isoelectronic with the P4 molecule, is also appropriate. 

The mixture of the all trans and mainly 9,10-cis vitamin A acid may be separated by fractional crystallization from ethanol. All trans vitamin A acid has a melting point of 180° to 182°C and 9,10-cis vitamin A acid, which crystallized in the form of pale yellow fine needles collected into clusters, has a melting point of 189° to 190°C. 

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