Calorimetric information

Adsorption microcalorimetry is probably the most direct method for describing in detail both the quantitative and the energetic features of surface sites. The ability of the microcalorimetric technique to readily reveal subtle differences among samples is worth stressing. Even though one has to be cautious about the nature of the acidic and basic sites, calorimetric information on the concentration and strength of the sites can be used with confidence for interpreting the catalytic behavior. 

Only a relatively small proportion of the data available in the literature is of such high precision. There are three types of calorimetric information available the modern high precision work, older work done before high precision work was undertaken, and modern work carried out for some reason to a lower standard of precision. A vast amount of thcrmochemical work was carried... 

In contrast, one finds many DSCs which are used only for qualitative DTA work on transition temperatures. The often-posed question of the difference between DTA and DSC is therefore easily answered DTA is the general term covering all differential thermal analysis techniques, while DSC must be reserved for scanning experiments that yield calorimetric information. 

Applications of DTA for Polymers. Table 2 (Ref 5, Chapt. l) describes some of the many applications of DTA and DSC. Both DTA and DSC can be used to determine the temperature of the transitions, cited in Table 2. However, the DSC peak area, in addition, gives quantitative calorimetric information (heat of reaction, transition, or heat capacity). DTA can only do so when calibration with a standard material allows the quantitative conversion of AT to heat flow and, ultimately, heat of transition (AH) or heat capacity (Cp). Also, the response of DTA with increasing temperature may be affected by poor heat transfer in the system, detector sensitivity, etc (4). For these reasons, when there is a choice between DSC and DTA, DSC is the preferred method. The illustrations shown below for applications of DSC in characterization of polymers also generally apply for DTA, with the limitations mentioned above. Therefore, DTA applications will not be considered here. Illustrations of polymer applications for DTA can be found in the Thermal Analysis section by Bacon Ke (6) in the previous edition of this encyclopedia. 

The state equation (Eq. (3.124)) written in this way is a system of first-order equations. Because of the available calorimetric information, the state variables should be transformed in such a way as to obtain a relationship between one input function P(t) and one output function. The relationship between the state variables and the output function has the following form ... 

Calorimetric information for very small sample sizes may be observed... 

Differential scanning calorimetry (DSC) is the most popular thermal analysis technique, the workhorse of thermal analysis. This is a relatively new technique its name has existed since 1963, when Perkin-Elmer marketed their DSC-1, the first DSC. The term DSC simply implies that during a linear temperature ramp, quantitative calorimetric information can be obtained on the sample. According to the ASTM standard E473, DSC is a technique in which the heat flow rate difference into a substance and a reference is measured as a function of temperature, while the sample is subjected to a controlled temperature program. As will be seen from this chapter, the expression DSC ... 

These examples of time-dependent DTA have shown that much information needed for modern materials analysis can be gained by proper choice of time scale. The thermal analysis with controlled cooling and heating rates has also been called dynamic differential thermal analysis (DDTA). Adding calorimetric information, as is described in Chapter 5, extends the analysis even further. All of this work is, however, very much in its early stage. No systematic studies of metastable crystal properties or information on hystereses in glasses have been made. 

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