Heat capacity and thermodynamic properties of p-terphenyl: Study of order–disorder transition by automated high-resolution adiabatic calorimetry

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Abstract
The heat capacity of a sample of zone‐refined, high‐purity p‐terphenyl has been determined from 4 to 370 K in a fully automated high‐resolution vacuum adiabatic calorimeter and from 320 to 580 K in a differential scanning calorimeter. The melting point of p‐terphenyl is 487.0 K and the enthalpy of fusion is 35.3 kJ/mol. A λ‐type solid–solid transition occurs with a peak temperature of 193.55 K. The transition is highly reproducible without observable hysteresis, even after various thermal treatments. In the transition region which spans from 140 to 240 K, the sample reaches a state of thermal equilibrium within a period of one‐half to one hour, as normally required in adiabatic calorimetry. These characteristics are desirable for the application of the equilibrium λ transitions as a calibration standard for use in dynamic calorimetry. The behavior of the λ transition at equilibrium is mapped in high temperature resolution, with small temperature increments of measurement down to 0.01 K by adiabatic calorimetry. In the temperature region between the transition and the fusion, the heat capacity of the high‐temperature form of the crystal is proportional to the temperature Cp =0.94 T J/K/mol to within 1%. The heat capacity of p‐terphenyl in the liquid state above its melting point of 487.0 K appears to be an extension of the heat capacity of the liquid, and the supercooled liquid, state of o‐terphenyl above its glass transition temperature of about 243 K.

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