Effect of entropy on the dynamics of supercooled liquids: new results from high pressure data

Abstract

We show that for arbitrary thermodynamic conditions, master curves of the entropy are obtained by expressing S(T, V) as a function of TV ^{γ _G} , where T is temperature, V specific volume and γ_G the thermodynamic Grüneisen parameter. A similar scaling is known for structural relaxation times, τ = ℑ (TV ^γ); however, we find γ_G  < γ. We show herein that this inequality reflects contributions to S(T, V) from processes, such as vibrations and secondary relaxations, that do not directly influence the supercooled dynamics. An approximate method is proposed to remove these contributions, S ₀, yielding the relationship τ = ℑ₁(S − S ₀).

Acknowledgements

This work was supported by the Office of Naval Research. Inspiring conversations with S. Capaccioli and J.C. Dyre are gratefully acknowledged.

Notes

¹Secondary relaxations can be distinguished as one of two types: the so-called Johari–Goldstein (JG), which is coupled to the structural relaxation showing an activation volume , and non-JG which have , as found for example in the case of PPG oligomers 28, 29 and for other cases, in agreement with the extended coupling model of Ngai 30.