Crystallization and Melting Points

The present research group recently presented [130, 131] an automahc computer-controlled equipment which is able to record simultaneously temperature-time and conductivity-time funchons of up to 30 samples at very low heating and cooling rates down to 1.5 Kh. This equipment is also a valuable tool to determine reliable crystaUizahon and melhng points of IL [132] materials with a strong tendency to supercooling. 

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Crystallization and Melting Point. The abihty of PVA to crystallize is the single most important physical property of PVA as it controls water solubiUty, water sensitivity, tensile strength, oxygen barrier properties, and thermoplastic properties. Thus, this feature has been and continues to be a focal point of academic and industrial research (9—50). The degree of crystallinity as measured by x-ray diffraction can be directly correlated to the density of the material or the swelling characteristic of the insoluble part (Fig. 2). 

The effectiveness of random copolymerization in depressing crystallization and melting point can be further demonstrated by the polymer (3.21) (Jackson Kuhfuss, 1982). In comparison to the polymer (3.19), the polymer (3.21) has an even lower melting point (330 °C) even though the extent of the substitution is lower by 25%. 

As can be seen from the results presented above, the study of PEO-6-PCL has made evident the complexity of the crystallization and melting processes in double crystalline diblock copolymers that have the following characteristics their crystallization starts from a homogenous melt, crystallization and melting points of the blocks are very near to each other, and there is an absence of a nucleating effect of each block on the other. 

E. P. Donovan, F. Spaepen, D. Turnbull, J. M. Poate, and D. D. Jacobson, Heat of crystallization and melting-point of amorphous-silicon, Appl. Phys. Lett 42, 698-700 (1983). 

In the previous sections it was shown that the formation of lamellae with folded chains was essentially a kinetically controlled phenomenon. This section treats the free energy of polymer crystallization and melting point depression. 

The data within Table 2 suggest that both AFBi and AFG can be recovered from russet potato slices following both inoculation and incubation for 65 days with Aspergillus flavus strain ATCC 15548. However, the data are to be considered preliminary in nature since multiple solvent systems and approved confirmatory AOAC procedures combined with crystallization and melting points are required to definitively establish the presences of AFTs. 

The molecular and crystal stmctures of 2-phenylbenzotellurazole were determined by X-ray (89KGS1690). The dihedral angle between the planes of the five-membered heterocycle and 2-phenyl ring is 31.2°. In contrast to benzoisotel-lurazole, no shortened intermolecular Te- N contacts were found in the crystal stmcture of 2-phenylbenzotellurazole. Consequently, no anomalies in solubility and melting point were revealed for this compound as compared with its sulfur and selenium congeners. 

The mixture obtained is poured into 2 liters of water and the base is precipitated with 30% soda. The precipitated base is extracted with 400 cc of methylene chloride. After evaporation of the solvent, the N-(2-diethylaminoethyl)-2-methoxy-4-acetamino-5-chlorobenzamide formed crystallizes. The melting point is 86°C to 87°C and the yield is 95%. 

A mixture of 38.6 g (0.1 mol) of 3,4,3, 4 -tetramethoxy-6-(a-acetopropyl)-benzophenone, 5.5 g (0.11 mol) of 100% hydrazine hydrate or 3.52 g (0.11 mol) of hydrazine, and 500 ml of absolute ethanol is boiled for 5 hours. After adding 100 ml of benzene, 400 ml of solvent mixture Is distilled off from the reaction mixture by slow boiling for 3 hours. After cooling for 8 hours, 19 g of 5H-2,3-benzodiazepine derivative are separated from the residue as small, white crystals. The melting point is 133°C to 1 36°C (after recrystallizing from absolute ethanol, 136°C). 

Natural Occurrence of ( — )-proto-Quercitol. Although the dextrorotatory form (12) of proto-quercitol was discovered in acorns more than a century ago by Braconnot (5), who at first thought that it was lactose, the levorotatory form (13) remained unknown until 1961. In that year, Plouvier isolated it from leaves of the tree Eucalyptus populnea the yield was 0.55% (36). The optical rotation of the new compound was equal and opposite to that of the dextro enantiomer, and it was identical to the latter in its crystal form, melting point, solubilities, molecular formula and infrared spectrum. 

Table 1. Crystal forms, melting points, and vOH values of inclusion complexes with chalcones and analogues as guest molecules...

In a manner similar to that just described for differential thermal analysis, DSC can be used to obtain useful and characteristic thermal and melting point data for crystal polymorphs or solvate species. This information is of great importance to the pharmaceutical industry since many compounds can crystallize in more than one structural modification, and the FDA is vitally concerned with this possibility. Although the primary means of polymorph or solvate characterization s centered around x-ray diffraction methodology, in suitable situations thermal analysis can be used to advantage. 

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