Thermal analysis melting point determination

Gehenot, A. Rao, R.C. Maire, G. Value of thermal analysis in the critical evaluation of classical methods of melting point determination. Int. J. Pharm. 1988, 45, 13-17. 

Alexander and Carlson ( ) proposed a phase diagram for the V-VO system based on melting point determinations, differential thermal analysis, metallographic observations, and x-ray parametric measurements. These authors ( ) reported that VO(cr) melts congruently at 2063 10 K. We adopt this value for In a phase diagram proposed by Stringer (2) a melting temperature of... 

Differential thermal analysis (DTA) represents an improvement to the melting point determination in that the difference in temperature between the sample and a reference is monitored as a function of temperature. As long as no thermal transitions take place, the temperature of the sample and the reference will be the same because the heat capacities of the two are roughly equivalent. However, differences in temperature between the sample and reference are manifested when changes occur that require a finite heat of reaction. If AH for the transition is positive (endothermic reaction), the temperature of... 

In the Phillips process, polyphenylene sulfide (PPS) is obtained from the polymerization mixture in the form of a fine white powder, which, after purification, is designated Ryton V PPS. Characterization of this polymer is complicated by its extreme insolubility in most solvents. At elevated temperatures, however, Ryton V PPS is soluble to a limited extent in some aromatic and chlorinated aromatic solvents and in certain heterocyclic compounds. The inherent viscosity, measured at 206°C in 1-chloronaphthalene, is generally 0.16, indicating only moderate molecular weight. The polymer is highly crystalline, as shown by x-ray diffraction studies (9). The crystalline melting point determined by differential thermal analysis is about 285°C. 

C. Equilibrium between Solid and Liquid Phases only. I. The Components are Completely Miscible in the Liquid State. a) The pure components only occur as solid phases. Polymorphism of components. Determination of the equilibrium curve. Example, b) Compounds are formed with a congruent meltings point. The indifferent point. Determination of the composition of a compound by thermal analysis. Examples, (c) Compounds are formed with an %ncongruent melting-point. Determination of the composition of the coinpound by thermal analysis. Example. (d) Solid solutions or " mixed crystals are formed, i) The two components can form an unbroken series of solid solutions. Examples. Melting-point curve. Example. Fractional crystallisation of solid solutions, h) The two components do not form a continuous series of solid solutions. Examples. Changes in solid solutions with the temperature. II. The Components are not Completely Miscible in the Liquid State. Suspended transformation. 

In some cases the difference in crystal lattice energy between two polymorphs is relatively small reflecting only a small difference between the unit structure. Hakanen and Laine [15] illustrated the use of thermal analysis to the determination of polymorphs. Using terfenadine, two polymorphs and a solvate were identified using x-ray diffraction (XRD), DSC and TG to determine kinetic parameters for the structural change of the methanol solvate by desolvation on heating. Laine et al [16] examined the polymorphic structures within ibopamin which exists in two monotropic forms. The melting points of Form I and II were 134.8 0.4° and 130.2 0.5° respectively. 

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). 

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). 

Fusible mixtures, with a composition having a suitable melting point, are selected by the thermal analysis of the two- and three-component system. Thermal analysis determines either the beginning and end of solidification or the beginning and end of the melting of the mixture. 

Three to 4 mg of extracted biopolymers was encapsulated in aluminum pans for the measurements. Each sample was first annealed at 200°C for 3 min. The melting point was determined using a Mettler DSC 30 Thermal Analysis System. Dry nitrogen was used as the flow gas with a flow rate of 30 mL/min, calibrated with indium and mercury. 

CHARACTERIZATION. Melting points were determined on an E. I. DuPont Series 99 Thermal Analyzer at 20°C/min. Inherent viscosities of polyamic acid solutions were obtained at a concentration of 0.5% (w/w) in DMAc at 35°C. Glass transition temperatures (T ) of the fully cured polymer films were measured by thermomechanical analysis (TMA) on a DuPont 943 Analyzer in air at 5°C/min. Films fully-cured at 300°C were tested for solubility at 3-5% (w/w) solids concentration in DMAc,N,N-dimethylformamide (DMF), and chloroform (CHCl-j). Solubilities at room temperature were noted after periods of 3 hours, 1 day and 5 days. Refractive indices of 1 mil thick films were obtained at ambient temperature by the Becke line method (11) using a polarizing microscope and standard immersion liquids obtained from R. P. Cargille Labs. 

Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and infrared spectroscopy are the common techniques used in the characterization of the structure of the congealed solid. Thermal analytic methods, such as DSC and differential microcalorimetric analysis (DMA), are routinely used to determine the effect of solutes, solvents, and other additives on the thermomechanical properties of polymers such as glass transition temperature (Tg) and melting point. The X-ray diffraction method is used to detect the crystalline structure of solids. The infrared technique is powerful in detecting interactions, such as complexation, reaction, and hydrogen bonding, in both the solid and solution states. 

A melting point of 1221 K was determined by thermal analysis (cooling curves) of the K-Kg system by Dworkin and Bredig (7). This is considerably higher than earlier values of 1108 K ( ) and 1185 K (L4), but it is confirmed by their drop calometric measurements (6). A melting point of 1221+10 K is adopted with a- H" = 3.86 0.04 kcal raol" (6). 

A significant part of the physical and chemical property analyses is being carried out with x-ray diffraction and x-ray spectrographic equipment and thermal analysis techniques. Thermal analyses have been used to determine melting point, decomposition temperature, and sinterability of waste sludge from APCS operations (Figure 6). These properties are needed to determine applications of the material. 

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