Effect of temperature and volume on structural relaxation time: Interpretation in terms of decrease of configurational entropy Daniele Prevosto a,b, * , Simone Capaccioli a,b , Mauro Lucchesi a , Pierangelo Rolla a a INFM e Dip. di Fisica, Universita ` di Pisa, Largo Pontecorvo 3, I-56127 Pisa, Italy b INFM CRS-SOFT, I-00185 Roma, Italy Available online 2 August 2005 Abstract We analyze the slowing down of structural relaxation dynamics of two small molecular glass formers and one polymer: o-terphe- nyl, triphenylchloromethane, and poly(methylmethacrylate). Considering the literature data of expansivity and heat capacity we cal- culate configurational entropy using a previously proposed relation between the configurational and the excess entropy, and we directly check the Adam and Gibbs theory for glass transition. In particular, we clearly show that using such expression for con- figurational entropy the predicted linear dependence between the logarithmic of structural relaxation time and the product of tem- perature with configurational entropy is well satisfied and it does not depend on pressure. Moreover, we also derive an equation for calculating the pressure dependence of the glass transition temperature, which in these systems is in good accordance with the experi- mental values. Ó 2005 Elsevier B.V. All rights reserved. PACS: 64.70.Pf; 77.22.Gm; 05.70.Ce 1. Introduction Several investigations performed by varying pressure, P, and temperature, T, revealed that in many systems thermal energy and density variations play an equally important role in the vitrification process [1]. The Adam and Gibbs (AG) theory includes both these contribu- tions since it relates the increase of structural relaxation time, s, to the reduction of configurational entropy, S c , by [2], sðT ; P Þ¼ s 0 exp C AG TS c   ; ð1Þ where s 0 is the value of s in the limit of infinite (TS c ), and C AG is assumed as a constant. This theory is based on the assumption of an ÔidealÕ second order transition occurring at a temperature T 2 [2]. Such transition has never been experimentally observed because of the huge slowdown of molecular motions near the glass transi- tion. Such kinetic phenomenon prevents systems from undergoing a transition towards a different thermody- namic state. However, despite this unresolved point, the AG theory has been applied with success to repro- duce the dynamics of supercooled liquids above the glass transition in many cases (see [3–7] and references there- in). Moreover, since it includes the contributions of the thermal energy and of the density, it appears as a suit- able theory for the interpretation of the glass transition. Moreover, in the last few years some works showed that, in the dynamic region s < 10 8 s, glassy systems whose isochronal dielectric spectra e(m) for different tempera- tures and pressures superpose [8], also verify a scaling relation between the isobaric relaxation-time curves measured at different pressure values [3,8]. In these sys- tems usually a relation log[s(T, P)] proportional to [TS exc (T, P)] 1 is also fulfilled [4–7], which in the case 0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.03.057 * Corresponding author. Tel.: +39 05 0221 4322; fax: +39 05 0221 4333. E-mail address: prevosto@df.unipi.it (D. Prevosto). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 351 (2005) 2611–2615