TOPEM - The New Multi-Frequency Temperature-Modulated Technique

Introduction 

In a conventional DSC measurement, the heat flow in a sample is measured while the sample is heated or cooled at a constant rate of change of temperature or under isothermal conditions. The measured heat flow is the sum of the sensible and latent heat flow components. The sensible heat flow component (reversing heat flow) arises from the heat capacity, and the latent heat flow part (non-reversing heat flow) from chemical reactions, crystallization and vaporization processes.

φ_total(t) = φ_reversing(t) + φ_{non-reversing}(t) with

φ_reversing(t) = c p ⋅ m ⋅ β

where

c_p is the specific heat capacity,

m the mass and

β the heating rate

If the two events occur separately, the interpretation and evaluation of the DSC curve is usually straightforward. With complex processes, where the effects for example overlap, reliable interpretation and evaluation requires measurement techniques that can distinguish between the sensible and latent heat components.

Methods based on temperature-modulated DSC (TMDSC) facilitate the separation and provide additional information about the dynamics of the processes involved. 

TMDSC Development [1] 

The separation of heat flow into sensible and latent parts has been of interest ever since the first thermometric experiments in 1887 by H.L. Le Chatelier and the temperature difference measurements in 1899 by William C. Roberts-Austen. The first temperature-modulated experiments for the accurate determination of c p values can be traced back to the work of O.M. Carbino in 1910. He performed the experiments in a calorimeter. On the DSC side, the development was not quite so rapid. In 1955 S.L. Boersma introduced the technique of dynamic heat flow calorimetry, making quantitative heat flow measurement possible for the first time.

Scientists like S.C. Mraw and D.F. Naas (1979), H. Dörr (1980) and P.K. Dixon (1990) began to look into the separation of overlapping processes using DSC instrumentation. With the advent of modern computer technology, these techniques became available for an ever increasing number of users.

METTLER TOLEDO is proud to announce that with the invention of TOPEM^® , it has achieved another major breakthrough in the field of temperature-modulated DSC, in addition to the well-known IsoStep^® and ADSC methods.

Principles of the TOPEM^® Technique

To gain as much sample information as possible from one measurement, a pulse modulation is used that allows the measuring system to be completely characterized. In the TOPEM^® technique, the temperature pulses have a small pulse height and a stochastically (randomly) changing pulse duration (pulse width). Such pulses contain many different frequencies.

The principle of the measurement is shown in Figure 1. The DSC together with the crucible and sample comprise the measurement system to be analyzed. The sample information is derived by correlating the heat flow with the heating rate. A temperature profile and the corresponding heat flow are shown in Figure 2. 

TOPEM^® allows the dynamics of the system to be analyzed over a broad range of frequencies in one single measurement. The sensible heat flow is based on the quasi-static heat capacity

Features and Benefits 

• One measurement – simultaneous determination of sample properties as a function of time and temperature over a wide frequency range
• c_p calculation from the pulse response – very accurate determination of the quasi-static, specific heat capacity
• Simultaneous high sensitivity and high resolution – allows the measurement of low energy transitions or close-lying temperature-dependent effects
• Separation of sensible and latent heat flow – heat capacities can be determined even if the effects overlap
• Simplifies the interpretation of curves – frequency-dependent effects (e.g. glass transitions) can be easily distinguished from frequency-independent effects (e.g. loss of moisture)
• Extended PEM technique – eliminates instrumental influences and extends the measurable frequency range
• Automatic c_p adjustment – allows the determination of accurate frequency-dependent heat capacity values in one single measurement 

Application Example 

PET is an excellent example to illustrate the possibilities of the new modulation technique. In a first step the software calculates the following four curves from the modulated heat flow curve:

• Total heat flow
  
• Sensible heat flow
  
• Latent heat flow
  
• Quasi-static, specific heat capacity, c_p0
  

Both the glass transition and the cold crystallization are clearly visible in the modulated heat flow curve. For a more accurate interpretation, however, the calculated individual curves are more useful. During the glass transition at about 80 °C, the heat capacity increases, but decreases again slightly at the cold crystallization. This behavior is even more pronounced in the phase curve. In addition to the quasi-static c p0 curve, the diagram shows a c p curve with a measurement frequency of 16.7 mHz. The temperature shift to higher temperatures with increasing frequency at the glass transition can be clearly seen. In contrast, at the cold crystallization, no shift is observed because this effect depends only on the temperature.

TMDSC Method Comparison 

The three METTLER TOLEDO TMDSC methods differ with respect to the temperature program and the type of evaluation.

Conclusions

This new modulation technique has been patented by METTLER TOLEDO. The results obtained in the two methods described above are now obtained in one single measurement. In a TOPEM^® measurement, the heat capacity is determined under quasi-static conditions. The separation into sensible and latent heat components is very easy. Frequencies can be selected afterward allowing additional frequency-dependent information to be calculated without the need to perform additional measurements

_{TOPEM^® – The New Multi-Ffrequency Temperature-Modulated Technique | Thermal Analysis Application No. UC 222 | Application published in METTLER TOLEDO Thermal Analysis UserCom 22}