PDF《Pure Appl. Chem., Vol. 72, No. 11,》

1 shows TG results for one of T. OZAWA ?

2000 IUPAC, Pure and Applied Chemistry 72, 2083C2099

2084 Fig.

1 TG curve of YBa2Cu3O7-δ by stepwise heating and cooling [18]. The symbols m, T, and t are respectively the mass gain, the temperature, and the time. the oxide superconductors, YBa2Cu3O7-δ, in ambient atmosphere by stepwise heating and then cooling [18]. Final mass at each step is plotted against the temperature (Fig. 2). Because the mass observed in heating is in good agreement with that in cooling, it can be concluded that the measured mass is the mass in equilibrium, and the nonstoichiometry is elucidated. This was a very important discovery, because the nonstoichiometry has much influence on the superconductivity. These results clearly illus- trate the advantage of isothermal run for observing temperature dependence of properties in equilibri- um. Similar stepwise heating was also made to obtain the relationship of molten fraction with the tem- perature for purity determination [19]. The stepwise heating and cooling could be applied to observe the temperature dependence of steady state. Furthermore, if the change in reacting species is negligible, it can also be applied to meas- urement of temperature dependence of reaction rate and hence the activation energy. This is used prac- tically for thermal endurance evaluation of polymeric insulating materials [20]. CONSTANT RATE HEATING The advantage of observation under constant rate heating over isothermal run can be shown by one typ- ical example. When we observe thermal decomposition of poly(methyl methacrylate) under high vacuum by evolved gas analysis (EGA) with a mass spectrometer, we can get results shown in Fig.

3 [21]. As clearly seen in Fig. 3, volatilization by the thermal decomposition of this material proceeds as a four-step process. However, the product in each step is all monomer, and other products were scarce- ly detected, so that the four steps of the thermal decomposition are all depolymerization (i.e., unzipping of monomer from the radical end). Observing the dependence of the amount of volatilized monomer upon the initial degree of polymerization and atmosphere in the polymerization, we could conclude the following mechanism of the four-step unzipping process [21]. The first unzipping step starts at weak bonds, such as copolymerized oxygen. The second and third unzipping steps are initiated at the poly- mer ends. There can be three types of the polymer ends (i.e., the initiator radical end, the saturated end, and the unsaturated end by disproportionation in the annihilation of polymerizing radicals). But we can- ?

2000 IUPAC, Pure and Applied Chemistry 72, 2083C2099 Temperature control modes in thermal analysis

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2 Equilibrium mass change of YBa2Cu3O7-δ observed by stepwise heating and cooling [18]. not recognize which of these three types of the polymer ends are the causes for these two unzipping processes initiated at the polymer end. The fourth and final unzipping step starts by random scission in the polymer main chain. Each unzipping step is terminated with a relatively short kinetic chain length by radical recombination. These are overall thermal decomposition behavior of poly(methyl methacry- late) under high vacuum. Imagining that we observed this process isothermally, as was done in the beginning of the poly- mer decomposition study, we can realize great advantages of constant rate heating over an isothermal run. When we observe the process isothermally at a relatively low temperature, we can observe the first- step reaction or the second-step reaction easily, but it is very hard to observe the third and the fourth steps, because they take a very long time and the rates of volatilization are very low. At a high temper- ature, the first and the second reactions proceed at a high rate, so that they would be completed in the time interval needed to heat up the sample to a desired isothermal temperature. Therefore, if we observe this process isothermally, we need a very long time to do numerous runs, otherwise it is very hard to reach a comprehensive, overall understanding of the process. Although kinetic analysis of data obtained by constant rate heating is somewhat complicated [17], this example clearly shows us the advantage of constant rate heating. SAMPLE-CONTROLLED THERMAL ANALYSIS One of advantages of constant rate heating is shown above, but its disadvantage is illustrated in the next example. When we observe the thermal decomposition of polyimide film in an ambient atmosphere by simultaneous TG-DTA at a constant rate, we get the results as shown in Fig.