differential scanning calorimetry

Differential scanning calorimetry, DSC, heats a tiny sample at a steady rate and measures how much heat it swallows or releases along the way — and from that heat flow it detects every thermal event inside: melting, freezing, the glass transition, curing, crystallisation. It is like carefully warming a pot and watching for the moments it briefly needs extra gas (to melt something) or suddenly gives heat back (as it sets).

Two little pans sit side by side on sensitive heaters: your sample and an empty reference. The instrument keeps both climbing in temperature at, say, 10 degrees C per minute, and records the DIFFERENCE in heat needed to keep them in step. When the sample melts it absorbs heat (an endothermic dip); when it crystallises it gives heat off (an exothermic peak); at the glass transition its heat capacity steps up, making a little jog in the baseline. The temperatures of these features give Tg (glass transition), Tm (melting), and Tc (crystallisation), and the peak areas give the heat of each transition.

DSC is the standard way to find a polymer's glass-transition temperature (the point where a plastic softens from glassy to rubbery) and its melting point, to check whether a thermoset is fully cured, or to measure how crystalline a polymer is. It works on metals and pharmaceuticals too. Caveats: only transitions with a heat change show up (a purely structural change with no enthalpy is invisible), the measured Tg shifts with heating rate, and sample mass and contact must be controlled or the numbers drift.