Thermograms appearance

DSC appears to be a more precise indicator than TG when comparing degradation routes. Thus many TG thermograms appear similar in air and under an inert atmosphere yet the DSC thermograms are markedly different. 

Triethanolamine salts of alcohol sulfates form white crystals when obtained in pure form after recrystallization. At their melting point they are semisolid with gelatinous appearance and the transition is difficult to detect. Melting points, determined through thermograms obtained by differential scanning calorimetry, gave 72, 76, 80, and 86°C for dodecyl, tetradecyl, hexadecyl, and octadecyl sulfates, respectively [63]. 

It must be noted that the heat capacity of the calorimeter cell and of its contents p, which appears in the second term of Tian s equation [Eq. (12)], disappears from the final expression giving the total heat [Eq. (19)]. This simply means that all the heat produced in the calorimeter cell must eventually be evacuated to the heat sink, whatever the heat capacity of the inner cell may be. Changes of the heat capacity of the inner cell or of its contents influence the shape of the thermogram but not the area limited by the thermogram. It is for this reason that heat-flow microcalorimeters, with a high sensitivity, are particularly convenient for investigating adsorption processes at the surface of poor heat-conducting solids similar in this respect to most industrial catalysts. 

This simplified equation is equivalent to Tian s equation [Eq. (16)], and it appears that n is indeed the time constant r of the calorimeter. Thence, the successive coefficients n in Eq. (29) may be called the calorimeter time constants of 1st, 2nd,. .., ith order. When the Tian equation applies correctly, all time constants r, except the first r may be neglected. Since the value of the coefficients n of successive order decreases sharply [the following values, for instance, have been reported (40) n = 144 sec, r2 = 38.5 sec, r3 = 8.6 sec, ri 1 sec], this approximation is often valid, and the linear transformation of many thermal phenomena produced by the thermal lag in the calorimeter may actually be represented correctly by Eqs. (16) or (30). It has already been shown (Section IV.A) that the total heat produced in the calorimeter cell is then proportional to the area limited by the thermogram. 

From Tian s equation [Eq. (30)3, it appears that in order to transform the calorimeter response g(t) into a curve proportional to the thermal input f(t), it is sufficient to add, algebraically, to the ordinate of each point on the thermogram g(t), a correction term which is the product of the calorimeter time constant n, by the slope of the tangent to the thermogram at this particular point. This may be achieved manually by the geometrical construction presented on Fig. 10. 

All heat evolutions which occur simultaneously, in a similar manner, in both twin calorimetric elements connected differentially, are evidently not recorded. This particularity of twin or differential systems is particularly useful to eliminate, at least partially, from the thermograms, secondary thermal phenomena which would otherwise complicate the analysis of the calorimetric data. The introduction of a dose of gas into a single adsorption cell, containing no adsorbent, appears, for instance, on the calorimetric record as a sharp peak because it is not possible to preheat the gas at the exact temperature of the calorimeter. However, when the dose of gas is introduced simultaneously in both adsorption cells, containing no adsorbent, the corresponding calorimetric curve is considerably reduced. Its area (0.5-3 mm2, at 200°C) is then much smaller than the area of most thermograms of adsorption ( 300 mm2), and no correction for the gas-temperature effect is usually needed (65). 

A DTA thermogram of dobutamine hydrochloride, at a heating rate of 5°C per min. in a nitrogen atmosphere of 40 cc per min., shows (figure 1) an endotherm at 196°C which appears to indicate a melt. 

The dynamic mechanical thermal analysis also indicated molecular mixing of the two components. The Tg of the thermoplastic was lower than the composite, as determined from the inflection point of the E curve seen in Figures 13 1. The PEOX glass transition temperature appeared at 62°C, while that for PVP showed up at ll5°C. The discrepancy in the Tg of PVP had already been addressed. The Tg extrapolated from the DMTA thermogram was reported since no artifacts were to be introduced. The samples exemplified for all DMTA thermograms were processed in a normalized fashion. Any thermal or hygroscopic Influences were eliminated. Some water may be trapped in the neat thermoplastics but this was minimized since they are hot pressed at 150°C and stored under calcium... 

The calorimetric measurement per se can detect additional intermediates during transitions. There are clear-cut cases where two or more phases are seen in melting curves, or thermograms, showing that intermediates are appearing. The difficult case is when only one transition is seen. Whether or not there is a single transition can be tested by measuring A/fD N(cal) and A/fD H(vh) for the transition.13 (This does not require two separate experiments because the bell-shaped... 

The thermogram of this sample is shown in Figure 8. The ammonium nitrate peak and the first peak of ammonium sulfate match closely the positions of standard compounds. However, the peak at 270°C is noticeably shifted from the second ammonium sulfate peak, although the overall appearance of the doublet is similar to that of ammonium sulfate. A tentative explanation of this observation is that the sample does not contain pure ammonium sulfate, but rather that some of the ammonium ions are replaced by a charged organic nitrogen complex. This complex should produce the right chemical shift (relative to ammonium) in the ESCA spectrum, decompose at a lower temperature than ammonium bisulfate, and hydrolyze to ammonium in water solution. 

In a dynamic experiment, the temperature and the conversion vary with time. Since the temperature is forced to follow the imposed scan rate, by varying the scan rate, the peak appears at different times, that is, at different temperatures (Figure 11.10). This allows for kinetic analysis of the thermograms. The principles of such evaluations can be demonstrated on a single first-order reaction, as an example. The temperature varies linearly with time ... 

As a conclusion, it appears clear that measures must be taken to avoid omitting the charging of compound (A) the omission of (B) is not as critical, and requires no special measure. Thus, based on two dynamic DSC thermograms, important conclusions for the safety of the studied process can be drawn. 

The thermograms with Co content x < 1.5 are characterized by one maximum about 375 K. The shift of this maximum to higher temperature and appearing furthers maximums at higher and lower temperature is observed with the rise of... 

The reference material is chosen to be one that exhibits no thermally induced transitions within the temperature range of interest. Thus, transitions that occur in the sample during the applied temperature programme appear as peaks or troughs, depending on whether they are exothermic or endothermic, on the plot of differential heat flow versus temperature (the thermogram) that is the output of the instrument (Figure 22.1). Commonly used reference materials are listed by Hatakeyama and Quinn (1994). 

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