One of the most surprising recent developments in DSC is the discovery that it is possible to scan at hundreds of degrees centigrade per minute and still obtain excellent data. For some applications, this leads to much more accurate and meaningful data than information obtained at much slower traditional heating rates. Since the introduction of DSC (differential scanning calorimetry) in the early 1960s the scan rate used for most DSC measurements has been 10◦C/min. This rate has provided good data for many applications, though sometimes slower rates have been employed to give improved resolution of events, e.g. polymorphism, or time for a reaction to occur. Occasionally, faster rates of 20◦C/min may have been used, but seldom anything faster than this; the fear being that thermal lags may cause inaccuracy of temperature measurement or that thermal gradients across a sample would make the data meaningless. However, this has proved not to be the case.

Pijpers and Mathot first reported the use of fast scan rates whilst based at the DSM laboratories in Holland and this has given rise to a range of applications using scan rates of up to 500◦C/min (at the time of writing the maximum available for commercially available analyzers). Particularly significant are applications in the fields of polymers and pharmaceuticals. This approach was called high-performance DSC (HPer DSC).

Some DSC analyzers, particularly power compensation systems, have been capable of heating at high rates for many years. Part of the reason how such high rates are achievable is the small mass of the furnaces in a power compensation DSC. They are just less than a gram in weight, and being of very low mass can be readily heated at high scan rates. But the first issue that springs to mind is whether the sample itself can also be heated at high rates or whether thermal lags will be increased so as to make the data meaningless. For example, how much is the onset of melting of a standard such as indium affected by increasing scan rate, and what happens to the peak width and the heat of fusion measurement? These questions can be readily checked by running an indium standard at increasing rate. To the very great surprise of those of us who have used DSC for many years, the fact is that the effects on indium and other standards are relatively small; for example, if a system is heated at 500◦C/min the onset of melting is shifted by only 6–8◦ when compared to the value obtained when heating at the slow rate of 10◦C/min, which is easily corrected for by normal calibration procedures. Following calibration, results are accurate both for temperature and energy; see Figure 1 and Table 1.

Figure 1. Indium after calibration, heated at rates of up to 500◦C/min.

Table 1. Onset and heat of fusion data from indium scanned at rates of up to 500◦C/min after calibration showing that measurement of both temperature and energy is quantitative

This indicates that measurement is not only practical but also quantitative, and there is no mathematical treatment of the data involved to enhance the appearance at high rates; it is literally the direct heat flow measurement of the analyser. Power compensation circuitry responds at twice mains frequency, typically 100–120 Hz, and so the analyser can not only heat at high rates, but is also fully capable of making sensible and accurate measurements whilst heating at these rates so as to correctly characterise the events that occur. This leads to a definition of HyperDSC which refers to the ability to make meaningful measurements whilst heating at high rates.

There are other practical issues to be aware of when using fast scan rates, and the duration of the initial transient is one of them. This is the time it takes for the analyser to control the scan at the required rate and shows as a period of instability at the start of a trace before flat baseline appears. The duration of the transient will depend to some extent on the size of sample, the type of pan and instrument set-up, and significantly upon the purge gas chosen. It must be fairly short if practical measurements are to be made since, for example, if the transient was of 1 min and the scan rate 500◦C/min then the run would need to begin at least 500◦C below the transition to be measured, which would make the technique of little value. With power compensation DSC the transient is typically of 6- to 8-s duration when using helium as the purge gas, independent of the scan rate (see Figure 2) which is an acceptably short period for sensible measurements to be made. Figure 3 shows the Tg of polyisobutylene at −70◦C measured at 400◦C/min. This would be impossible to measure if the instrument had a long transient, but is easily measured when using a DSC equipped with a liquid nitrogen system.

Figure 2. Transient duration is less than 8 s at rates of up to 500◦C/min. At 500◦C/min this means that the transient takes only about 60◦C, which allows meaningful sub-ambient measurement.

Figure 3. Tg of polyisobutylene scanned at 400◦C/min. The analyser is controlled at −100◦C allowing this Tg to be observed beginning around −70◦C. Short initial transient times are essential for this type of measurement.

Fast controlled cooling rates are also of great significance for DSC measurements, particularly with respect to crystallinity or glassy morphology within a material. Figure 4 shows an example of cooling rates that can be achieved using a power compensation DSC. The effect of different cooling rates on morphology is easily demonstrated with PET, which shows increasing amounts of amorphous material if cooled at increasingly fast rates from the melt; see the comparison in Figure 5. If cooled quickly PET remains completely amorphous, and when reheated slowly it shows a large glass transition from the amorphous material, followed by crystallisation which will then occur with slow heating rates. This is termed cold crystallisation when it occurs in this way. If cooled slowly from the melt the PET will be partially crystalline to start with, so will not show such a significant Tg nor a cold crystallisation event.

Figure 4. Cooling rates achieved with power compensation DSC equipped with liquid nitrogen cooling. The figure shows program temperature with the actual scan rate, which shows deviation at the lower temperatures. Rates of 200◦C/min are achievable through the ambient region.