Differential Scanning Calorimetry (D.S.C.)
Differential scanning calorimetry (D.S.C.) is used to measure thermal transitions of materials. Glass transition may be identified by D.S.C., because it corresponds to a re-arrangement of the solid amorphous matrix involving therefore the breaking of bounds and creation of new ones.
The glass transition on heating is observed as an endothermic step in a D.S.C. thermogram. This change in the heat capacity (Cp) through Tg, occurs because the glassy and rubbery states have different physical properties, including Cp.
In a D.S.C., a heating system monitors the temperature of a sample and a reference pan, varying the heating rate to the pans such that the temperature of the sample and the reference pan remain the same during a scan of temperature.
The output of D.S.C. is a plot of energy versus temperature. The resulting thermogram relates the difference in energy supplied to the two pans which allows peak areas on a D.S.C. thermogram to correspond directly to changes in enthalpy.
From a D.S.C. thermogram the glass transition temperature is determined by the onset of the endothermic peak; a base line before and during the transition is used and the temperature corresponding to the intersection of these two lines is called the onset of glass transition.
Unfortunately, these changes sometimes are not very pronounced, the interference of other thermal transitions may mask the observation of the glass transition, what makes the interpretation of a thermogram difficult.