Crystallization exotherms

Measurements of thermal analysis are conducted for the purpose of evaluating the physical and chemical changes that may take place in a heated sample. This requires that the operator interpret the observed events in a thermogram in terms of plausible reaction processes. The reactions normally monitored can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

Gatlin [1.30] measured not only Tg. for mannitol and Na-cefazolin by DSC, but also the dependence of the exothermic crystallization energy from the rewarming rate (Fig. 1.47). 

Figure 22.1. Typical DSC thermogram for milk fat. Ci and C2 are exothermic crystallization peaks obtained during cooling. h, h2 and h2 are endothermic peaks obtained during heating. (Reproduced with permission from ten Grotenhuis et al., 1999.)...

Exothermic Crystallization Precipitation Solidification Adsorption Solvent vapor induced crystallization of amorphous excipients Formation of salt forms of drug substances Melt granulation with semisolid excipients Solvent vapor sorption by drug substances... 

FIGURE 8.5 A micronized drug exposed to an RH ramp. The trace shows vapor wetting the powder, then a very large exotherm (crystallization) at 10 h, followed by a sharp endotherm (water loss). (Reproduced from Briggner, 1993. With permission of Thermometric Ltd.)... 

Another useful tool that the automotive chemist will use is the differential scanning calorimeter. The component will be measured against a reference as heat is applied to both samples. A constant temperature will be maintained and the difference in temperature (AT) required to raise the temperature of the sample in relation to the reference sample is measured. Because the reference sample is not undergoing phase transitions, the AT of the measured sample will show an endothermic (melting point) or exothermic (crystallization) peak as heat is applied [7-9],... 

Thermal reactions can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

SYNTHESIS A mixture of 7.4 g phthalic anhydride and 9.05 g of 2,5-dimethoxyphenethylamine (see the recipe for 2C-H for its preparation) was heated with an open flame. A single clear phase was formed with the loss of H20. After the hot melt remained quiet for a few moments, it was poured out into a crystallizing dish yielding 14.8 g of a crude solid product. This was recrystallized from 20 mL CH3CN, with care taken for an endothermic dissolution, and an exothermic crystallization. Both transitions must be done without haste. After filtration, the solids were washed with 2x20 mL hexane and air dried to constant weight. A yield of 12.93 g of N-(2-(2,5-dimethoxyphenyl)ethyl)phthalimide was obtained as electrostatic yellow crystals, with a mp of 109-111 deg C. A sample recrystallized from IPA was white, with a mp of 110-111 deg C. Anal. (C18H17N04) C,H,N. 

The TMDSC results are shown for the Coe-Flex polysulfide impression material and modulation condition (a) in Figure 19. There is a glass transition near -55 °C and an apparent weak crystalline polymer melting peak near 70 °C. No evidence of an exothermic crystallization peak can be seen on the bottom nonreversing heat-flow curve. The peak near 190 °C requires further study, but is not clinically relevant for the properties of the impression material. 

Chlorinated polymers/Copolyester-aniides Recent studies (5) of blends of chlorinated polyeAylenes with caprolactam(LA)-caprolactone(LO) copolymers have been able to establish a correlation between miscibiUty and chemical structure within the framework of a binary interaction model. In some of the blends, both components have the ability to crystallize. When one or both of the components can crystallize, the situation becomes rather more complicated. Miscible, cystallizable blends may also undergo segregation as a result of the crystallization with the formation of two separate amorphous phases. Accordingly, it is preferable to investigate thermal properties of vitrified blends. Subsequent thermal analysis also produces exothermic crystallization processes that can obscure transitions and interfere with determination of phase behavior. In these instances T-m.d.s.c has the ability to separate the individual processes and establish phase behavior. 

The frequent interruption in the crystallization of a semicrystalline/amorphous blend with the incorporation of salt not only elevates its T, but also reduces the crystallization kinetics and G of the crystallizable component in both miscible and immiscible blends (Rocco et al., 2002 Rocco et al., 2004 Chan and Rammer, 2008 Florjanczyk et al., 2004 Acosta and Morales, 1996). Salt-free miscible PEO/PBE 60/40 blend exhibits an exothermic crystallization peak at -31 °C, but... 

Next is the exothermic crystallization transition at T. Such crystallization on heating above the glass ttansition temperature is called cold crystallization [37]. It contrasts to crystallization on cooling from the melt, see also Figs. 4.122,4.136, and 4.138. After cold crystallization, the sample is semicrystalline with about 40%... 

Figure 1.23. Enlargement of raw data from Figure 1.2 illustrating the exothermic crystal perfection during melting when the heating rate is zero.

DSC provides a rapid and sensitive evaluation of isothermal crystallization behavior. The sample of interest is annealed at a sufficiently high temperature for a period long enough to remove the previous thermal history. The sample is then rapidly cooled to the crystallization temperature, and the time period of exothermic crystallization is measured. 

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