W H E A T O N a single thermocouple measurement. Detailed quantitation of the thermal trace, as is done in differential scanning calorimetry and differential thermal analysis measurements, is difficult because of the complexity of the heat transfer phenomena at these high heating rates. Nevertheless, much qualitative information is produced, and we are attempting to move toward more quantitation. Returning to the description of the thermal decomposition of EDD, we see that Figure 7 shows the quantified gas products from EDD superimposed on the difference thermal trace (19). This permits chemical and physical events to be attached to the temperature deflections. The initially negative slope of the thermal trace results from the additional heat capacity of the filament with the sample present. The melting endotherm occurs at about 175-180 °C. At about 200 °C, melting is complete (not isothermal because of the rapid heating rate) and the liquid phase continues to heat to about 275 °C without evidence of decomposition off-gassing. : At 275 °C, the first gas products are detected. These are HN03(g) formed by proton transfer and desorption, and a small quantity of NCMg), probably from thermal decomposition of HNO3. NH3(g) then appears, perhaps from C-N bond heterolysis. The lag in the thermal trace shows that this stage of decomposition is, as expected, overall endothermic. Howev-
er, above 330 °C, CO2 from backbone oxidation and the more reduced nitrogen oxide products, NO and N2O, can be detected. These products are created by exothermic reactions in the condensed phase, as evidenced by the increased heating rate of the filament. Thus the combination of real-time temperature record and near real-time observation of the gas products helps map the overall reaction sequence during the fast thermal decomposition of a complex material. Pressure as a variable
Apart from its practical value for suppressing sublimation and evaporation of the sample when needed, pressure is a useful research variable. Because the initial pressure in the cell can be set as desired, we have found that performing thermolysis with the initial pressure as the major variable gives additional insight (20). Pressure differences affect gas diffusion rates; that is, the decomposition gases are forced to remain in contact with the condensed phase for different lengths of time. At lower pressure the most reactive gases (NO2, HONO) usually are detected in high relative concentrations because they are able to diffuse away from the reaction zone. When the applied pressure is increased, these gases remain in the reaction zone longer and react to the extent that products having intermediate stability dominate (NO, HCN). At the highest pressures studied, the most
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WHEATON Figure 7. Difference thermal trace superimposed on the quantified gas products from EDD. H20, NH4NO3 aerosol, and any IR-inactive gases are excluded.
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989 · 903 A
