Configurational specific heat of molecular liquids by modulated calorimetry

The specific heat of a liquid varies as its structure and molecular vibrational frequencies vary with the temperature. We report the magnitude of the structural or configurational part Cp,conf for five molecular liquids by measuring their dynamic and the apparent specific heats, and find that the unrelaxed or vibrational specific heat, of the equilibrium liquid, is not greatly different from that of the nonequilibrium glass. Therefore, the vibrational part of the specific heat Cp,vib does not change substantially when a glass becomes an ultraviscous liquid. This contradicts the inference that there is a large sigmoid-shape discontinuous increase in Cp,vib as the structure of a glass kinetically unfreezes on heating above its Tg, and further that Cp,conf is 20%-50% of the net Cp change at the glass transition.