Supercooling

In the case of very pure and homogeneous substances, supercooling is determined by the difference between the onset melting temperature and the nucleation temperature. It should be noted here that the nucleation is a stochastic process which, in contrast to the melting point, depends on the measuring conditions (e. g. cooling rate, sample volume). Therefore, this value only applies to these parameters. 

This definition is not applicable to blends of paraffins. Figure 1 shows the heat flow diagram of RT27 for different calculation of supercooling: peak-peak, offset-offset and onset-onset temperature.

Peak-peak (figure 2) and offset-offset (figure 3) are very much dependent on the heating rate, sample mass and the DSC device. Therefore, these differences are only comparable within the same DSC if the same heating rate and approximately the same thermal mass have been used. As the difference between onset temperature of the melting peak and  crystallisation peak can be  negative this is not an indicator for the degree of supercooling.

The challenge with blends or other inhomogeneous materials is that they have a melting and crystallization. So no nucleation point is visible.

Figure 1: Possibility offset-offset temperature

Figure 2: Possibility peak-peak temperature

Figure 3: Possibility offset-offset temperature