Stepwise DSC

Differential scanning calorimetry (DSC) Modulated or stepwise DSC Dielectric thermal analysis (DETA)... 

The lower limit of purity determination using the stepwise DSC method is about 95 mole-% without linearization and 92 mole-% with linearization (57). However, the time required is approximately I -2 hours compared to about 30 min for the DSC method. Using only two peaks in the stepwise DSC method, the investigators concluded that this method could be used down to 90 mole-%. Palermo and Chiu (57) do not think that it can be used below this value because the van t Hoff approximation becomes invalid. The two-step method is independent of the heat of fusion and less time-consuming. It was cautioned that other analytical methods should always be used in conjunction with DSC methods, whenever possible. Once the applicability of the method is established, DSC may prove to be the most convenient one to use. 

Developments in differential scanning calorimetry (DSC) for the characterisation of semicrystalline thermoplastics, including mixed recyclates are discussed. The techniques include stepwise DSC for the accurate determination of specific heat capacity, rapid determination of crystallinity using the Gray-Mathot total enthalpy technique, and analysis of crystallisation kinetics. The techniques show the influence of impurities on the glass transition temperature, crystallinity and crystallisation. 6 refs. 

Temperature-risiag elution fractionation (tref) is a technique for obtaining fractions based on short-chain branch content versus molecular weight (96). On account of the more than four days of sample preparation required, stepwise isothermal segregation (97) and solvated thermal analysis fractionation (98) techniques usiag variatioas of differeatial scanning calorimetry (dsc) techniques have been developed. 

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]. 

Comparison of chemiluminescence isothermal runs with oxygen uptake and DSC measurements has been at the centre of interest since practical industrial applications of the chemiluminescence method were attempted. It is a fact that the best comparison may be achieved when studying polymers that give a distinct induction time of oxidation typical for autoaccelerating curves of a stepwise developing oxidation. This is the particular case of polyolefins, polydienes and polyamides. The theoretical justification for the search of a mutual relationship between the oxidation runs found by the various methods follows directly from the kinetic analysis of the Bolland-Gee scheme of polymer oxidation. 

Thermal data where reactions are detected at lower temperatures are obtained from test runs on an ARC or other more sensitive calorimeters. In the ARC, the temperature is raised stepwise and at a much slower effective rate than with the DSC. The ARC is nearly adiabatic and, thus, more nearly approaches plant reactor conditions. Another important advantage is the fact that the reaction pressure is monitored and recorded in the ARC. 

Wissing S, Craig DQM, Barker SA, Moore WD. An investigation into the use of stepwise isothermal high sensitivity DSC as a means of detecting drug-excipient incompatibility. Int J Pharm 2000 199 141-150. 

Systems copolymer SEO/diethyl phthalate of different concentration have been studied by DSC and low-angle X-ray diffraction as a function of the crystallization temperature. It has been found that for a fixed concentration the characteristic parameters of the LC structure vary stepwise. For instance, for a diethyl phthalate concentration of 10%, the thickness dB of the PEO layer increases suddenly when the crystallization temperature reaches 19°, 29° and 42 °C and each jump of the dB parameter corresponds to a decrease by one of the average number v of folds of PEO chains265. An example of the phase diagram crystallization temperature/solvent concentration is given in Fig. 36 for the copolymer SEO 4 whose molecular parameters are summarized in Table 8. The degree of crystallinity of the PEO chains increases with both the crystallization temperature and solvent concentration265. ... 

More complex temperature programmes are sometimes useful. These might combine periods of variable heating and cooling rates with isothermal periods. For example, stepwise heating can be used to detect the onset of melting under quasi-isothermal conditions (Laye, 2002). Modulated temperature DSC (MTDSC), in which the linear temperature scan is perturbed by a sinusoidal, square or saw-tooth wave, or other modulation of temperature, has a number of potential advantages over the conventional linear scan. These include increased sensitivity and resolution, and the ability to separate multiple thermal events (Laye, 2002). 

DSC, middle IR and near IR data suggest that the crosslinking reaction of an epoxy resin with phthalic anhydride is a stepwise process as follows. 

The dehydration of inorganic salt hydrates is usually stepwise but not always predictable. For copper sulfate pentahydrate, three DSC peaks are apparent corresponding to the loss of 2, 2, and 1 waters of hydration. Further, there is an increase in dehydration enthalpy of this salt with increasing numbers of hydration waters removed. This type of study is of importance in the estimation of the purity of the sample since the dehydration enthalpy is directly proportional to the water content of the sample. Similarly, DSC can be used to determine the moisture content of materials generally, since the measured endothermic dehydration DSC peaks are directly proportional to the water content of the material. 

Charrier et al. accurately measured the equilibrium temperature of the solid-solid phase transition of high purity caesium nitrate by stepwise heating and by the method of extrapolation to zero heating rate. The authors reported a mean value of 154.3 0.1 C, that was obtained by using two different heat-flux DSC instruments. Because the transition temperature of caesium nitrate is close to that of the fusion of indium, indium can be used as a single-point calibrant. This approach decreases the error that may arise when the calibration value for the material of interest is determined by interpolation between two-point or multiple-point calibrations [66],... 

Cell Loading. Sample and reference buffer are loaded into the cells using the same procedure described above for DSC to eliminate air bubbles. Erratic performance usually indicates either bubbles or leaking cells. The cells are overfilled outside the calorimeter to about 2 mm above the tip of the loading capillary and then inserted stepwise into the calorimeter to gradually equilibrate the cells and minimize perturbation of the calorimeter. The syringe is put into place at the end of the equilibration to minimize the effects of leakage from the tip of the injection needle. Total equilibration requires approximately 2 hr. 

Polymers with flexible spacer groups. Polymers obtained by precipitation or by casting films from solutions were found to be amorphous as shown by X-ray and microscopical observations. The DSC data revealed that on heating up the samples for the first time a stepwise increase of the specific heat occurred which could be attributed to a glass transition. Thus at room temperature the as received samples were in the isotropic glassy state, apparently no liquid crystalline solutions exist. 

The determination of phase separation in partially miscible polymer blends by means of thermal analysis is often difficult because of the small demixing enthalpy and the slow rate of the diffusion-controlled process. Dreezen et al. (2001) smdied the phase separation of PEO/PES and PEO/Aramid blends by optical microscopy, conventional DSC and MTDSC. The onset of phase separation from optical microscopy corresponds very well to the onset of a small stepwise increase in the MTDSC heat capacity (Fig. 10.31). 

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