Endothermic transition

The polybutadienes prepared with these barium t-butoxide-hydroxide/BuLi catalysts are sufficiently stereoregular to undergo crystallization, as measured by DTA ( 8). Since these polymers have a low vinyl content (7%), they also have a low gl ass transition temperature. At a trans-1,4 content of 79%, the Tg is -91°C and multiple endothermic transitions occur at 4°, 20°, and 35°C. However, in copolymers of butadiene (equivalent trans content) and styrene (9 wt.7. styrene), the endothermic transitions are decreased to -4° and 25°C. Relative to the polybutadiene, the glass transition temperature for the copolymer is increased to -82°C. The strain induced crystallization behavior for a SBR of similar structure will be discussed after the introduction of the following new and advanced synthetic rubber. 

An example of monotropic behavior consists of the system formed by anhydrous ibuprofen lysinate [41,42], Figure 4.12 shows the DSC thermogram of this compound over the temperature range of 20-200°C, where two different endothermic transitions were noted for the substance (one at 63.7°C and the other at 180.1°C). A second cyclical DSC scan from 25 to 75°C demonstrated that the 64°C endotherm, generated on heating, had a complementary 62°C exotherm, formed on cooling (see Fig. 4.13). The superimposable character of the traces in the thermograms demonstrates that both these processes were reversible, and indicates that the observed transition is associated with an enantiotropic phase interconversion [41]. X-ray powder (XRPD) diffraction patterns acquired at room temperature, 70°C, and... 

Often the character of materials in a mixture undergoes solid-state rearrangements due to the pressures associated with compaction, which may or may not be polymorphic in nature. Consider the pre-compression powder blend, whose DSC thermogram is shown in Fig. 4.16, and which features the presence of four endothermic transitions. In the postcompression, ground tablet sample whose DSC thermogram is shown in Fig. 4.17, the endotherms having maxima at 86.5 and 106°C remain relatively constant (maxima at 85.3 and 104.2°C). On the other hand, the third endotherm in the pre-compression thermogram shows considerable attrition in the post-compression sample, and an additional endotherm (not previously observed in the pre-compression sample)... 

Part of the difficulty in interpretation of the PTA XPS data is uncertainty in the extent of hydration of some of the samples. The commercial samples are typically marked nl FO." making no commitment as to its true water content. Consequently, it was necessary to investigate water content on our own. In Figure 3, the thermogravimetric curve associated with Sigma PTA is shown. Two endothermic transitions are shown at 45 and 160 °C. The mass loss during the scan out to 210 C was 7% of the original amount. This corresponds to -13 waters of hydration. 

Starch gelatinization carmot be fully explained using DSC as the only experimental tool. As discussed in previous sections, DSC measures the net endothermic transitions that take place during starch gelatinization. It is known that the DSC parameters obtained for the same sample depend on heating rate (Fig. 5.20 Shiotsubo and Takahashi, 1984) transition... 

The nature of the phase formed in the slow coagulation experiment can be deduced on the basis of its thermal transitions, and from its X-ray diffraction pattern. In the DSC trace of the slowly coagulated PBT-MSA-water system shown in Figure 6, three endothermic transitions are observed between 90 °C and 240 °C. Corresponding transitions were observed by optical microscopy using a hot stage. Solid PBT fibers or films do not exhibit thermal transitions below about 650°C (1). 

Enthalpy relaxation is often responsible for two endothermic transitions in the initial dsc thermograms, and a single, broad transition is observed in most second scans and in dmta thermograms. 

We have observed such a transition in intact membranes of M. laidlawii which occurs at the same temperature as in the membrane lipids dispersed in water (77). Figure 11 shows representative endothermic transitions of membranes and lipids in water. Membranes were prepared for calorimetry by sedimenting at high speed, then 90-100 mg. of packed pellet were sealed in a stainless steel sample pan. The material was neither dried nor frozen before examination. Total membrane lipids were extracted with chloroform-methanol 2 1 v/v then dried and suspended in water. Lipids from the membranes of cells grown in the usual tryptose medium without added fatty acids are shown in a, while b and c are scans of intact membranes from the same cells. In b the membrane preparation had not been previously exposed to temperatures above 27 °C. The smaller transition at higher temperature probably arises from... 

With heating from 5 to 45°C, thermal changes in conformation in the major /3-casein are observed by spectral methods (Garnier 1966). From measurements of the optical density at 286 nm and of the specific optical rotation at 436 nm, a rapidly reversible endothermic transition (AH 30 kcal/mole) with a half-transition temperature of 23-24°C is observed. The optical rotatory dispersion data suggest a decrease in the poly-L-proline II structure (12 to 5%) and a slight increase in a-helix (11 to 16%) with increasing temperature. This transition probably occurs prior to association, since it is rapid, and the carboxyacyl derivative of the monomer, which does not polymerize with increasing temperature, also demonstrates the optical rotatory disperson thermal transition. 

Fig. 7.7 Differential thermal analysis (DTA) in N2 flow of C60H36 (top trace) showing a broad endothermic transition at 412°C, just at the onset of the decomposition.C(j0D36 shows two endothermic transitions at 379°C and 465°C. The second transition being the onset of the decomposition...

Thermal transitions which are commonly observed in frozen systems are illustrated in Figure 7 where, for the sake of this discussion, a deflection upward indicates an endothermic transition. The glass transition is a shift in the baseline toward higher heat capacity. Crystallization during the DSC experiment is observed as an exothermic event, and eutectic melting is an endothermic transition which preceeds the melting of ice. 

As detailed by Koradia et al. [16], the DSC and TGA studies were run at a heating rate of 10 °C / min, and a nitrogen purge was set at 80 ml / min for the DSC work and at 20 ml/min for the TGA work. The DSC thermogram of Form-I consisted of three endothermic transitions, with temperature maxima at 181, 186, and 190 °C. The DSC thermogram of Form-II also consisted of three endotherms, characterized by temperature maxima at 177,179, and 182 °C. During the TGA analysis, no temperature-induced loss of mass was observed for either polymorph, indicating that the two polymorphic crystal forms were anhydrous and nonsolvated. 

Alam et al. [21] reported that the endothermic transition of Form-I was observed at 179.78 °C, and that the endothermic transition of Form-II was observed at 177.34 °C. 

A DuPont TA-9900 thermal analyzer system, attached to a DuPont Data Unit, was used to obtain the DSC thermogram of EDTA over a range of 50 to 350°C. The thermogram shown in Figure 2 is fairly complicated, showing possible water loss at low temperature. The melting endotherm exhibited a maximum at 250.9°C, and the noise in the baseline following the endothermic transition is indicative of the thermal decomposition of the compound. 

Differential thermal analysis (DTA) thermograms of ( )-etodolac sodium salt exhibited endothermic transitions around 80,120, and 297°C and an exothermic transition around 83°C [12]. The exothermic phase change was observed after exposure of the sample to moisture, indicating conversion of the amorphous form of ( )-etodolac sodium salt to a crystalline phase. In contrast, the thermogram of (+)-etodolac sodium salt, after exposure to moisture, showed endotherms at 60, 80, 120, and 297°C, indicating that the salt contained methanol, acetonitrile, and water. There was no sign of degradation product formation. 

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