Melt crystallization melting points, distributions

The question addressed here is whether the components crystallize separately or together. DSC is probably the most popular method. In this case the melting-point distribution w TjJ is measured and is transformed into a crystal thickness distribution w L ) using the Thompson-Gibbs equation. Let us select a sample with a broad, continuous molar mass distribution. The sample is crystallized under isothermal conditions for different periods of time. Each isothermal treatment is followed by a rapid cooling of the sample to a temperature at which crystallization is no longer active. The melting endotherm is recorded after each cycle of isothermal... 

Formyl and acety derivatives of aliphatic amines are, as a rule, oils or low-melting substances and they are therefore not very suitable for identification. On the contrary, acetylated aromatic amines are very suitable for identification because they crystallize well and have sharp melting points distributed over a broad temperature range. Identification constants of practically all acetyl derivatives are known, which is an additional advantage. Acetyl derivatives are usually crystallized from water or alcohol, or a mixture of both, as well as from benzene, cyclohexane (or a mixture of these), or ethyl acetate. 

The properties of fillers which induence a given end use are many. The overall value of a filler is a complex function of intrinsic material characteristics, eg, tme density, melting point, crystal habit, and chemical composition and of process-dependent factors, eg, particle-si2e distribution, surface chemistry, purity, and bulk density. Fillers impart performance or economic value to the compositions of which they are part. These values, often called functional properties, vary according to the nature of the appHcation. A quantification of the functional properties per unit cost in many cases provides a vaUd criterion for filler comparison and selection. The following are summaries of key filler properties and values. 

Strain-induced crystallization would presumably further improve the ultimate properties of a bimodal network. It would therefore obviously be of considerable importance to study the effect of chain length distribution on the ultimate properties of bimodal networks prepared from chains having melting points well above the very low value characteristic of PDMS. Studies of this type are being carried out on bimodal networks of polyethylene oxide) (55), poly(caprolactone) (55), and polyisobutylene (56). 

Modelling non-isothermal crystallization is the next important step in a quantitative description of reactive processing. This is particularly important, because crystallization determines the properties of the end product. Therefore, the development the spatial distribution of crystallinity, a, and temperature, T, with time throughout the volume of the reactive medium must be calculated. It is also noteworthy that crystallization and polymerization processes may occur simultaneously. This happens when polymerization proceeds at temperatures below the melting point of the newly formed polymer. A typical example of this phenomenon is anionic-activated polymerization of e-caprolactam, which takes place below the melting temperature of polycaproamide. 

Equimolecular proportions of theophylline and 2-amino-2-methyl-l-propanol are dissolved in water and the water is evaporated until crystallization is almost complete. The crystals are filtered off and dried. The product has a melting point of 254-256°C, softening at 245°C. It has a water solubility of about 55%. It may be compounded in the form of tablets, for oral administration, or may be prepared in solution for distribution in ampoules. For the manufacture of solutions for packaging in ampoules, it is more convenient to simply dissolve the theophylline and the butanolamine in water, without going through the intermediate step of separating the crystalline salt. 

Disturbances in a crystal can be brought about by the above mentioned or by other causes. These disturbances are arbitrarily distributed over the crystallites, so that a spread in AH occurs, resulting in a spread in Tm, a melting region, which has, as an upper limit, the melting point of the best crystal. 

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