Thermal analysis melting point

The targeted production of dendroisomers and their exact analysis raise hopes that it will become possible to design such large molecules rich in inner workings with a view to obtaining specific properties, such a solubility, melting point, thermal stability, cavities. 

Thermal Resistance and Flammability. Thermal analysis of PVA filament yam shows an endothermic curve that starts rising at around 220°C the endothermic peak (melting point) is 240°C, varying afitde depending on manufacture conditions. When exposed to temperatures exceeding 220°C, the fiber properties change irreversibly. 

Thermodynamic Properties. The thermodynamic melting point for pure crystalline isotactic polypropylene obtained by the extrapolation of melting data for isothermally crystallized polymer is 185°C (35). Under normal thermal analysis conditions, commercial homopolymers have melting points in the range of 160—165°C. The heat of fusion of isotactic polypropylene has been reported as 88 J/g (21 cal/g) (36). The value of 165 18 J/g has been reported for a 100% crystalline sample (37). Heats of crystallization have been determined to be in the range of 87—92 J/g (38). 

The heavy metal salts, ia contrast to the alkah metal salts, have lower melting points and are more soluble ia organic solvents, eg, methylene chloride, chloroform, tetrahydrofiiran, and benzene. They are slightly soluble ia water, alcohol, ahphatic hydrocarbons, and ethyl ether (18). Their thermal decompositions have been extensively studied by dta and tga (thermal gravimetric analysis) methods. They decompose to the metal sulfides and gaseous products, which are primarily carbonyl sulfide and carbon disulfide ia varying ratios. In some cases, the dialkyl xanthate forms. Solvent extraction studies of a large number of elements as their xanthate salts have been reported (19). 

HIPS in combination with stabilizers. Four basic types of HBCD are produced by all the HBCD producers, i.e. low melt, medium range, high melt and thermal stabilized HBCD. Analysis of the HBCD manufactured by several producers in given in Table 1 (ref. 3). The melting point of HBCD varies with changes in isomer ratio and composition of impurities. 

In a study on the thermal and UV ageing of two commercial polyfoxymethy-lene) (POM) samples, one of which was a copolymer (see related study discussed later under Section 4.3, thermogravimetric analysis (TGA)), used in car interior applications, involving both DSC and TGA, isothermal OIT measurements were made at several different temperatures [8]. One conclusion from this study was that "extrapolation of the OIT data from high temperatures (molten state) to ambient temperatures in the solid state does not reflect effective antioxidant performance at room temperature", and thus measurements close to the melting point are not appropriate for reliable lifetime estimations. 

Differential. thermal analysis can also be used to construct binary phase diagrams on the basis of observed melting points. This information is of importance since the nature of the phase diagram as would exist for an enantiomeric pair can be instrumental in choosing a resolution strategy [23,24]. When a drug candidate contains one or more chiral centers, it is frequently... 

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. 

Most workers in the pharmaceutical field identify thermal analysis with the melting point, differential thermal analysis, differential scanning calorimetry,... 

Applications in polymer analysis are illustrated in Figures 11.13 and 11.14. The former shows schematically the typical thermal processes which can occur on heating a polymer, and the latter an analysis of a seven-component mixture based upon melting points. 

Differential thermal analysis of sodium valproate shows a large endotherm beginning at 100°C and ending at 118°C which is possible due to the loss of water. A sharp endothermic peak at 450°C is indicative of the melting point of sodium valproate. 

The calibration of DTA systems is dependent on the use of appropriate reference materials, rather than on the application of electrical heating methods. The temperature calibration is normally accomplished with the thermogram being obtained at the heating rate normally used for analysis [20], and the temperatures known for the thermal events used to set temperatures for the empirically observed features. Recommended reference materials that span melting ranges of pharmaceutical interest include benzoic acid (melting point 122.4°C), indium (156.4°C), and tin (231.9°C). 

Most workers in the pharmaceutical field identify thermal analysis with the melting point, DTA, DSC, and TG methods just described. Growing in interest are other techniques available for the characterization of solid materials, each of which can be particularly useful to deduce certain types of information. Although it is beyond the scope of this chapter to delve into each type of methodology in great detail, it is worth providing short summaries of these. As in all thermal analysis techniques, the observed parameter of interest is obtained as a function of temperature, while the sample is heated at an accurately controlled rate. 

Plots of melting point against optical purity are commonly referred to as phase-composition diagrams. The direct proportionality of melting point with heat of fusion has also been employed to construct similar plots based on thermal analysis data. The classification of racemic modifications into three different types with regard to their crystal packing (32) can be made based on the overall shape of these plots as follows ... 

Differential thermal analysis (DTA) has provided a wealth of information regarding the thermal behavior of pure solids as well as solid mixtures [10]. Melting points, boiling points, transitions from one crystalline form to another, and decomposition temperatures can be obtained for pure materials. Reaction temperatures can be determined for mixtures, such as ignition temperatures for pyrotechnic and explosive compositions. 

Ionic liquids are a class of solvents and they are the subject of keen research interest in chemistry (Freemantle, 1998). Hydrophobic ionic liquids with low melting points (from -30°C to ambient temperature) have been synthesized and investigated, based on 1,3-dialkyl imidazolium cations and hydrophobic anions. Other imidazolium molten salts with hydrophilic anions and thus water-soluble are also of interest. NMR and elemental analysis have characterized the molten salts. Their density, melting point, viscosity, conductivity, refractive index, electrochemical window, thermal stability, and miscibility with water and organic solvents were determined. The influence of the alkyl substituents in 1,2, 3, and 4(5)-positions on the imidazolium cation on these properties has been scrutinized. Viscosities as low as 35 cP (for l-ethyl-3-methylimi-dazolium bis((trifluoromethyl)sulfonyl)amide (bis(triflyl)amide) and trifluoroacetate) and conductivities as high as 9.6 mS/cm were obtained. Photophysical probe studies were carried out to establish more precisely the solvent properties of l-ethyl-3-methyl-imidazolium bis((trifluoromethyl)sulfonyl)amide. The hydrophobic molten salts are promising solvents for electrochemical, photovoltaic, and synthetic applications (Bon-hote et al., 1996). 

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