The so-called Direct and Sapphire methods are performed using a conventional DSC instrument and a linear temperature program. With traditional DSC, there is only one heat flow signal (Total). The heat capacity, however, consists of two components: the sensible heat capacity (reversing heat flow) and the latent heat capacity (non-reversing heat flow):
cp = cp, sensible + cp, latent
The latent heat capacity is related to physical or chemical transitions such as melting, crystallization, or chemical reactions. Such thermal events are observed on a DSC curve as endothermic or exothermic peaks. Consequently, the latent heat capacity is positive for endothermic events and negative for exothermic events.
The sensible heat capacity is related to the amount of heat absorbed due to rearrangements and overall movement of the molecules. This component is positive. The DSC curve in the diagram shows that the sensible heat capacity is directly related to the measured heat flow provided no thermal events occur. In many transitions, the sensible heat capacity defines the baseline of the related peak.
Temperature-modulated DSC (TMDSC) differs from conventional DSC in that it allows the total heat flow to be separated into reversing and non-reversing components. This is important for providing accurate cp data where different thermal effects overlap, e.g. glass transition (reversing heat flow component) and enthalpy relaxation (non-reversing heat flow component).
The temperature programs used in temperature-modulated DSC are more complex compared with those employed in conventional DSC experiments. METTLER TOLEDO offers three different techniques for performing temperature-modulated DSC measurements. Their most important features are summarized below.