Endothermic peaks in DSC

Make a list of the types of changes that can give exothermic peaks in DSC and DTA. Make a list of the types of changes that can give endothermic peaks in DSC and DTA. 

TGA and DSC indicate that the residual cure stage of the phenolic thermoset hes between 100 and 200°C and releases water and other volatile by-products. The cure stage is observed as an exothermic peak in the DSC curves (Siegmann and Narkis, 1977). The condensation stage involving two phenolic hydroxyls may occur around 300°C and form diphenyl-ether type bonds with consequent mass loss (Pilato, 2010a Vazquez et al., 2002 El Mansouri et al., 2011). This event corresponds to an endothermic peak in DSC curves (Siegmann and Narkis, 1977). 

Areas and positions of distinct endothermic peaks in DSC thermograms for trypsin—ovoinhibitor—chymotrypsin complexes (Zahnley,... 

The SCB distribution (SCBD) has been extensively studied by fractionation based on compositional difference as well as molecular size. The analysis by cross fractionation, which involves stepwise separation of the molecules on the basis of composition and molecular size, has provided information of inter- and intramolecular SCBD in much detail. The temperature-rising elution fractionation (TREE) method, which separates polymer molecules according to their composition, has been used for HP LDPE it has been found that SCB composition is more or less uniform [24,25]. It can be observed from the appearance of only one melt endotherm peak in the analysis by differential scanning calorimetry (DSC) (Fig. 1) [26]. Wild et al. [27] reported that HP LDPE prepared by tubular reactor exhibits broader SCBD than that prepared by an autoclave reactor. The SCBD can also be varied by changing the polymerization conditions. From the cross fractionation of commercial HP LDPE samples, it has been found that low-MW species generally have more SCBs [13,24]. 

Generally, the crystalline melting point of a polymer corresponds to a change in state from a solid to a liquid and gives rise to an endothermic peak in the DSC curve [3,9], The equilibrium melting point may be defined as... 

As a demonstration of the use of DSC in the characterization of zeolites and related materials, Figure 4.37 illustrates the DSC profile of the LECA zeolite [53], The LECA zeolite has a phase composition of 13 4 [%], in wt %, of gismondine and 87 4 [%] of quartz, anorthite, and glass. The DSC profile presented two endothermic peaks at 112°C and 195°C, and an exothermic peak at 312°C. As is very well known, zeolites evolve adsorbed water in the course of heating [97]. Consequently, the endothermic peaks in the DSC profile of LECA zeolite sample were clearly related to adsorbed water located in different sites of the LECA zeolite surface and the framework of the gismondine contained in the LECA zeolite [53],... 

Partially crystalline behavior at low temperature (endothermic melting peaks in DSC at approximate -50 °C) is observed for weakly crosslinked siloxane particles and the corresponding graft copolymers. 

Differential scanning calorimetry Samples of gels (8 to 11 mg) were placed in hermetically sealed stainless steal pans and analyzed with a DSC QlOO (TA Instruments, New Castle, DE, USA). An empty pan was used as reference. Samples were heated at 10°C/min from -40 to 120°C. At least triplicate samples were analyzed for each gel. The endothermic melting peak around 0°C was taken as an evidence of ice melting. Endothermic peaks in the 50 to 70°C range were taken as evidence of starch retrogradation (stored samples). 

A differential scanning calorimeter (DSC 1-B, Perkin-Elmer Corporation) was used to determine the extent of cure 10-mg to 20-mg specimens were tested at a scanning rate of 10°C/min. An exothermic peak on the thermograph indicates the heat of reaction whereas an endothermic peak in the amorphous polymer indicates the presence of residual stresses or the occurrence of a transition such as the glass transition. The presence of an exothermic peak in the DSC-scan of a pre-cured sample is an indication of incomplete curing. 

The amorphous state of the previous drug has been obtained by quenching the system from the molten state. In this case, the DSC profile exhibits a single thermal event, namely, a heat capacity step at 69°C associated with a glass transition. The absence of an endothermic peak in the melting zone (180°C) proves that the amorphous drug does not crystallize in the time scale of the experiment. Complementary studies have shown that partial crystallization occurs after 20 h at room temperature. 

Thermogravimetry measures the weight change of a sample as a function of temperature. A total volatile content of the sample is obtained, but no information on the identity of the evolved gas is provided. The evolved gas must be identified by other methods, such as gas chromatography, Karl Fisher titration (specifically to measure water), TG-mass spectroscopy, or TG-infrared spectroscopy. The temperature of the volatilization and the presence of steps in the TG curve can provide information on how tightly water or solvent is held in the lattice. If the temperature of the TG volatilization is similar to an endothermic peak in the DSC, the DSC peak is likely due or partially due to volatilization. It is usually necessary to utilize multiple techniques to determine if more than one thermal event is responsible for a given DSC peak. 

In order to shed more light on the Interaction of water with these mucopolysaccharides, we employ the DSC method to follow the melting behavior of aqueous solutions of the polysaccharides cooled to -50 C. The polymers chosen here are chondroitin sulfate A (Chn S-A), chondroitin sulfate C (Chn S-C), chondroitin (Chn), heparin (Hpn), and hyaluronic acid (HyA). Their chemical structures are shown in Figure 1. The DSC curves allow us to determine the amount of the non-freezing water In highly concentrated solutions, since any endothermic peak Is not observed for such solutions over a wide temperature range (17. 18). This paper will also describe the presence of more than one endothermic peak In the DSC curves for solutions of relatively low polymer concentrations. 

Oil shales were characterized using DSC by Rajeshwar et al. (235) a typical DSC curve of a Green River oil shale is shown in Figure 7J4. The endothermic peak in the 250-450°C temperature range corresponds to the thermal decomposition of oil shale kerogen. The area under this peak, the enthalpy value, was found to be directly proportional to the amount of kerogen in the shale in the absence of minerals that are thermally active in this temperature range. A least-squares fit of the AH obtained experimentally follows the equation... 

Pitches were characterized by DSC andTMA by Barr and Lewis (238). The DSC curves of a typical petroleum pitch, which had a Mettler softening point of 152°C, are illustrated in Figure 7.15. An endothermic peak in the vicinity of the expected Tg point was found in the initial DSC curve. A second run of the sample, after cooling it rapidly from 240 to — 10°C, showed a well-defined Tg with an onset temperature of 83°C. The endothermic peak was no longer visible in the curve. Glass transition temperatures and softening... 

In contrast, in DSC and WAXD studies on HDPE/LDPE blends cooled both rapidly and slowly, Datta and Birley (61) found that both the rapidly and slowly cooled blend samples exhibited two distinct endothermic peaks in the DSC measurements, indicating that the HDPE and LDPE components in the blends crystallize separately. However, no data were given on the molecular characteristics of LDPE. 

List the types of physical changes that can yield exothermic and endothermic peaks in DTA and DSC. 

Differential scanning calorimetry (DSC) curves of levofloxacin hemihydrate measured under various conditions showed different thermograms. This behavior was attributed to the dehydration process that resulted in a multiple-phase transition. Dehydration at higher temperatures (above 70° C) gave a sharp endothermic peak in the DSC thermogram due to the melting of the y-form, and at a lower temperature (50°C) it led to the observation of a sharp endothermic peak due to the... 

In DSC, the extent of reaction is obtained by first determining the total area of the peak that corresponds to the complete reaction. The required information from the experiment is the fraction of the complete reaction at a series of temperatures so that nonisothermal kinetics procedures can be applied. At a specific temperature, the partial peak area is determined, and the fraction of the complete reaction at that temperature is determined by dividing the area of the peak up to that temperature by the total peak area. A typical endothermic peak in a DSC trace is shown in Figure 8.2. In this case, the temperatures at which a is to be determined are indicated as... 

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