Melt crystallization definition

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

From all these data analyses, we can definitely say that the D and H chain stems are distributed statistically randomly in the crystalline lamellae of the D/H cocrystallized blend. This conclusion is quite important in relation with the chain-folding problem, a controversial research theme that had been discussed for a long time (30). The random distribution of the D and H chain stems naturally supports the idea that the D and H chains reenter randomly into and out of the crystalline lamellae as shown in Fig. 5.7. The regular adjacent reentry model is impossible to apply at all as for as the melt-crystallized sample is concerned. 

Thus, in this section we describe melt crystallization, in which the crystal formation takes place from the polymer melt. Melt crystallization of polymers can be subdivided into (1) isothermal crystallization, a crystallization process that takes place at a definite, constant temperature and (2) nonisothermal melt... 

It is a white, deliquescent solid, very powdery, which exhibits polymorphism on heating, several different crystalline forms appear over definite ranges of temperature -ultimately, the P4O10 unit in the crystal disappears and a polymerised glass is obtained, which melts to a clear liquid. 

Compound 6 crystallizes from cyclohexane as colorless needles which have no definite melting point there is a change of color to yellow at 128-134 C and the compound then melts sharply at 187-189 r C. When the colorless form is kept for a long time or recrystallized from pyridine or dimethyl sulfoxide it is changed into the yellow modification of mp 187-189 C recrystallization from cyclohexane reverses the process. It has been suggested that the yellow stable form has structure 6A and that the colorless metastable compound is the tautomer 2-methyl-l//-pyrido[2,3-6][l, 4]diazepin-4(5//)-one (6B). There is evidence from 1H NMR spectroscopy that the isomeric pyridodiazepin-2-one, yellow crystals, mp 195—197 " C, exists as an inseparable mixture of the tautomers 4-methyl-l//-pyrido[2,3-6][l,4]diazepin-2(3//)-one (7 A) and 4-methyl-l H-pyrido[2,3-6][l, 4Jdiazepin-2(5//)-one (7B) in the ratio 1 3. 

Many papers deal with the crystallization of polymer melts and solutions under the conditions of molecular orientation achieved by the methods described above. Various physical methods have been used in these investigations electron microscopy, X-ray diffraction, birefringence, differential scanning calorimetry, etc. As a result, the properties of these systems have been described in detail and definite conclusions concerning their structure have been drawn (e.g.4 13 19,39,52)). 

With decreasing temperature, the density oscillation becomes very pronounced and grows into a deeper melt region. At 300 K, for example, we can see at least 5 layers after 1.28 ns. Within the layers, as will be shown later, definite order in chain orientation and chain packing is observed suggesting the growth of polymer crystals. 

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