Entropy basis for the thermodynamic scaling of the dynamics ofo-terphenyl

Structural relaxation times and viscosities for non-associated liquids and polymers are a unique function of the product of temperature, T , times specific volume, V , with the latter raised to a constant, γ τ . Similarly, for both neat o -terphenyl (OTP) and a mixture, the entropy for different T and pressure, P , collapses to a single curve when expressed versus TV γ S , with the scaling exponent for the entropy essentially equal to the thermodynamic Grüneisen parameter. Since the entropy includes contributions from motions such as vibrations and secondary relaxations, which do not affect structural relaxation, γ S <γ τ . We show herein that removal of these contributions gives a satisfactory account of the magnitude of γ τ . Moreover, the relaxation times of OTP are found to be uniquely defined by the entropy, after subtraction from the latter of a V -independent component.