Eur. Phys. J. E (2015) 38: 91
https://doi.org/10.1140/epje/i2015-15091-6

Regular Article

The Modified VFT law of glass former materials under pressure: Part II: Relation with the equation of state

Physique des solides, Université de Paris-Sud, 91405, Orsay, France

^* e-mail: rault@lps.u-psud.fr
^** e-mail: jacques.rault@yahoo.fr

Received: 5 March 2015
Accepted: 16 July 2015
Published online: 31 August 2015

Abstract

The dynamical properties of glass formers (GFs) as a function of P, V, and T are reanalyzed in relation with the equations of state (EOS) proposed recently (Eur. Phys. J. E 37, 113 (2014)). The relaxation times τ of the cooperative non-Arrhenius α process and the individual Arrhenius β process are coupled via the Kohlrausch exponent n _S(T, P). In the model n _S is the sigmoidal logistic function depending on T (and P, and the α relaxation time τ _α of GFs above T _g verifies the pressure-modified VFT law: log τ _α ∼ E _β /nsRT, which can be put into a form with separated variables: log τ _α ∼ f(T)g(P). From the variation of n _S and τ _α with T and P the Vogel temperature T ₀ (τ _α → ∝, n _S = 0) and the crossover temperature (also called the merging or splitting temperature) T _B (τ _α ∼ τ _β, n _S ∼ 1) are determined. The proposed sm-VFT equation fits with excellent accuracy the experimental data of fragile and strong GFs under pressure. The properties generally observed in organic mineral and metallic GFs are explained: a) The Vogel temperature is independent of P (as suggested by the EOS properties), the crossover is pressure-dependent. b) In crystallizable GFs the T _B (P) and Clapeyron curves T _m(P) coincide. c) The α and β processes have the same ratio of the activation energies and volume, E*/V* (T- and P-independent), the compensation law is observed, this ratio depends on the anharmonicity Slater-Grüneisen parameter and on the critical pressure P* deduced from the EOS. d) The properties of the Fan Structure of the Tangents (FST) to the isotherms and isobars curves log τ versus P and T and to the isochrones curves P(T). e) The scaling law log τ = f(V ^Λ ) and the relation between Γ and γ. We conclude that these properties should be studied in detail in GFs submitted to negative pressures.