Changes in dynamic crossover with temperature and pressure in glass-forming diethyl phthalate S. Pawlus, 1 M. Paluch, 1 M. Sekula, 1 K. L. Ngai, 2 S. J. Rzoska, 1 and J. Ziolo 1 1 Institute of Physics, Silesian University, Uniwersytecka 4, 400-07 Katowice, Poland 2 Naval Research Laboratory, Washington, D.C. 20375-5320, USA ~Received 24 February 2003; published 12 August 2003! Dielectric relaxation measurements have been used to study the crossover in dynamics with temperature and pressure, onset of breakdown of the Debye-Stokes-Einstein law, and the relation between the a and the b relaxations in diethyl phthalate. The measurements made over 10 decades in frequency and a broad range of temperature and pressure enable the dc conductivity and the a- and the b-relaxations to be studied altogether. The isobaric data show that the a-relaxation time t a has temperature dependence that crosses over from one Vogel-Fulcher-Tammann-Hesse form to another at T B ’227 K and t a ’10 22 s. The dc conductivity s exhibits similar crossover at the same T B . At temperatures above T B , t a and s have the same temperature dependence, but below T B they become different and the Debye-Stokes-Einstein law breaks down. The breadth of the a relaxation is nearly constant for T ,T B , but decreases with increasing temperature for T .T B . The time dependence of t b is Arrhenius, which when extrapolated to higher temperatures intersects t a at T b nearly coincident with T B . Isothermal measurements at various applied pressures when compared with isobaric data show that the shape of the a-relaxation depends only on t a , and not on the T and P combinations. At a constant temperature, while t a increases rapidly with pressure, the b-relaxation time t b is insensitive to applied pressure. This behavior is exactly the same as found in 1,18 -bis ~p-methoxyphenyl! cyclohexane. The findings are discussed in the framework of the coupling model. DOI: 10.1103/PhysRevE.68.021503 PACS number~s!: 64.70.Pf, 77.22.Gm INTRODUCTION Many molecular liquids can be supercooled to avoid crys- tallization and eventually transformed to the glassy state @1–3#. For those that possess permanent dipole moment, broadband dielectric spectroscopy can be used to probe dif- ferent molecular motions over a wide range of time scales from picoseconds in the liquid to hundreds of seconds in the vicinity of glass transition @4–9#. Typical relaxation pro- cesses observed by means of this spectroscopy method are the cooperative a-relaxation and the local, noncooperative secondary b relaxation @4,5,7,10–15#. The secondary b re- laxations are either of intramolecular or intermolecular in origin. The latter is best exemplified by the secondary relax- ations in rigid molecules and often referred to as the Johari- Goldstein ~J-G! relaxation. However, at the present time, there is no general agreement on the precise definition of a J-G relaxation. In addition, there is the dc conductivity s, which originates from mobile ions commonly present in di- polar liquids. As dc conductivity is related to viscosity of the liquid by the combination of Nernst-Einstein and Stokes- Einstein equations, it also provides useful information about the a relaxation @4,16–23#. In recent years, the change in relaxation dynamics at a temperature T B ~about 1.2T g for fragile liquids and even higher for intermediate liquids! from a simpler one at higher temperatures to a more complex one at lower temperatures has drawn considerable interest @15,24,25#. The temperature dependence of a-relaxation time t a is well described by a Vogel-Fulcher-Tammann-Hesse ~VFTH! equation for T .T B , but conforms to another VFTH equation for T ,T B @24,26#. Above T B the temperature dependences of the self- diffusion coefficient and viscosity are the same, but below T B they differ leading to a breakdown of the Stokes-Einstein relation @26–28#. From extrapolation, T B seems to be the temperature below which the secondary relaxation emerges and splits off from the a relaxation. T B is also the tempera- ture above which the dispersion of the a relaxation is narrow and below which the dispersion becomes broader @29,30#. Up to now, most experimental works on the dynamics of supercooled liquids are based on temperature variation at at- mospheric pressure @4 – 6,11,12,19,20,24,25#. As an alterna- tive to temperature change, the dynamics of the relaxation processes also can be investigated by compression of a liquid at constant temperature. The effect of pressure on the dynam- ics is determined by the activation volume, in analogy to activation energy in temperature variation @8,14,21–23,31– 39#. Of course, our understanding of the dynamics of various processes can be improved by measurements by varying both temperature and pressure. In this paper, we present an experi- mental study of dynamic glass transition at different pres- sures and temperatures in diethyl phthalate ~DEP!, a low molecular liquid. This liquid has a resolved secondary relax- ation at lower temperatures and Arrhenius temperature de- pendence for t b , which when extrapolated to higher tem- peratures seems to indicate that t b merges with t a at a temperature T b , nearly the same as T B . This behavior sug- gests that of a typical Johari Goldstein. However, the pres- sure dependence of t b indicates otherwise, as we shall see from the results to be presented. EXPERIMENTAL The molecular structure of the diethyl phthalate is dis- played in inset of Fig. 2. The sample was supplied by Aldrich Chemicals. The temperature-dependent dielectric measure- ments were carried out using the experimental setup made by Novo-Control GmbH. This system was equipped with a PHYSICAL REVIEW E 68, 021503 ~2003! 1063-651X/2003/68~2!/021503~7!/$20.00 ©2003 The American Physical Society 68 021503-1