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

Temperature control modes in thermal analysis* Takeo Ozawa? Chiba Institute of Technology, Tsudanuma 275-0016, Japan Abstract: Now we can use several temperature control modes, for example, isothermal run including stepwise heating and cooling, constant rate heating (or cooling), temperature con- trol for sample thermal history, sample-controlled thermal analysis (SCTA or controlled-rate thermal analysis, CRTA), temperature jump, rate jump, temperature modulation, and repeat- ed temperature scanning.

Their advantages and drawbacks are reviewed with some illustra- tive examples, especially for application to kinetic analysis. The combined use of these vari- eties of temperature control mode is recommended by showing examples. Temperature mod- ulation and repeated temperature scanning are discussed in comparison with temperature- modulated differential scanning calorimetry (DSC), and common and analogous points are elucidated. In relation to this, the possibility of the imaginary part of the overall reaction rate constant in complex reactions is postulated. Finally, these modes are classified and tabulated from two viewpoints, and other possible modes are shown. INTRODUCTION For a recent few decades, we have observed remarkable development in thermal analysis [1], espe- cially in new techniques by which chemical and structural changes in the sample can be directly observed. Thermal analysis by Fourier transform infrared spectroscopy is one example [2]. In other examples, X-ray diffraction is observed simultaneously during differential scanning calorimetry (DSC) [3], and volatilized products are analyzed by mass spectrometry as evolved gas analysis (EGA) together with thermogravimetry (TG) [4]. By these new microscopic techniques, we can directly observe chemical and physical changes in the sample. Thus, we can learn what is going on in the sam- ple. However, observing bulk physical property changes by classic thermal analysis (e.g., enthalpy changes by DSC, mass changes by TG, and dimension changes by thermodilatometry), we can only learn something is going on in the sample, but we cannot learn what is going on. Comparing the clas- sic thermal analysis with the new microscopic thermal analysis, great progress in thermal analysis is clearly realized. Another aspect of the development in thermal analysis is diversification of temperature control modes, and nowadays we have the following modes of temperature control;

? isothermal run, including stepwise heating and cooling ? constant rate heating (or cooling) ? temperature control for sample thermal history [5] ? sample-controlled thermal analysis (SCTA or controlled-rate thermal analysis: CRTA) [6] ? temperature jump [7] and rate jump [8] ? temperature modulation [9C11] ? repeated temperature scanning [12] *Lecture presented at the 12th International Congress on Thermal Analysis and Calorimetry, Copenhagen, Denmark, 14C18 August 2000. ?E-mail: [email protected]. Pure Appl. Chem., Vol. 72, No. 11, pp. 2083C2099, 2000. ?

2000 IUPAC

2083 The last two are different in data processing. In the temperature-modulated TG oscillating change of the physical property we observe is analyzed by Fourier analysis, and the amplitude of the oscillat- ing rate of conversion is compared with that of temperature modulation [11], while equivalent isother- mal curves of the conversion versus its rate are extracted in the latter. Details will be described below. Each mode has its suitable applications, and the advantages and drawbacks are discussed in this paper by using illustrative applications, mainly in applications to kinetics analysis of polymer decom- position. Appropriate choice of these modes and combined use should be made. To make full use of these modes, methods for kinetic analysis is also examined, and FriedmanCOzawa plot [13,14] is con- cluded to be the most suitable because of the most wide applicability. It is also postulated that an over- all rate constant may have an imaginary part due to transient change to steady state, and it would be observed as phase shift in tm-TG (or tm-DSC) and delayed change in TG (or nonmodulated DSC) by repeated temperature scanning. These points are reviewed in this paper. The third mode of temperature control is somewhat different from the other. To elucidate the nature of an observed physical transition, researchers used to observe the effect of heat treatment of the sample on the transition. One typical example is described in ref. 5, in which differential thermal analy- sis (DTA) was used in combination with an adiabatic calorimeter, and the effect of annealing was first observed by DTA prior to the calorimetry. In another example, the effect of heat treatment on polypropylene crystallization was observed [15,16]. In this mode of temperature control, a thermal analysis instrument, such as DSC and DTA, is used as a tool for sample heat treatment to give a desired thermal history to the sample, as well as a tool for observation. In TG, similar heat treatment is also made within a thermobalance, for instance, to completely dry the sample prior to the thermal decom- position to diminish the effect due to residual water [17]. Since this mode of temperature control is dif- ferent from the other modes, it will not be discussed any more in this paper. ISOTHERMAL RUN Isothermal run is an old method utilized for many decades to observe reactions, and its results are very simple for us to kinetically analyze the data, but it takes a long time when we observe reactions and their temperature dependence. This is clearly shown in one example in the next section. However, stepwise heating and cooling, which is one of the modifications of isothermal observa- tion, is suitable to observe equilibrium as shown in an example. Figure