THE DERIVATION OF THERMODYNAMIC PROPERTIES BY DSC - FREE-ENERGY CURVES AND PHASE-STABILITY

The derivation of enthalpies, entropies and free energies of fusion and transition from DSC curves is discussed. Although the apparent location of a thermal event is much influenced by rate effects, isothermal enthalpy changes are readily obtained by simple extrapolations. The corresponding entropies are always flawed by departures from reversibility and data must be corrected by forcing data to obey the condition that DELTAG = 0 at a first-order transition. Free energy curves may then be used to define the relative stabilities of different structures. Procedures are illustrated by examples from a series of liquid crystal-forming materials that show widespread solid-state polymorphism.