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 α and the β 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 α- and the β-relaxations to be studied altogether. The isobaric data show that the α-relaxation time ${\tau }_{\alpha }$ has temperature dependence that crosses over from one Vogel-Fulcher-Tammann-Hesse form to another at $T_B\approx 227\mathrm{K}$ and ${\tau }_{\alpha }\approx {10}^{-2}\mathrm{s}.$ The dc conductivity σ exhibits similar crossover at the same $T_B.$ At temperatures above $T_B,$ ${\tau }_{\alpha }$ and σ have the same temperature dependence, but below $T_B$ they become different and the Debye-Stokes-Einstein law breaks down. The breadth of the α relaxation is nearly constant for $T<\mathrm{}{T}_B,$ but decreases with increasing temperature for $T>\mathrm{}{T}_B.$ The time dependence of ${\tau }_{\beta }$ is Arrhenius, which when extrapolated to higher temperatures intersects ${\tau }_{\alpha }$ at $T_{\beta }$ nearly coincident with $T_B.$ Isothermal measurements at various applied pressures when compared with isobaric data show that the shape of the α-relaxation depends only on ${\tau }_{\alpha },$ and not on the T and P combinations. At a constant temperature, while ${\tau }_{\alpha }$ increases rapidly with pressure, the β-relaxation time ${\tau }_{\beta }$ is insensitive to applied pressure. This behavior is exactly the same as found in ${1,1}^{\prime }$-bis (p-methoxyphenyl) cyclohexane. The findings are discussed in the framework of the coupling model.

• Received 24 February 2003

DOI:https://doi.org/10.1103/PhysRevE.68.021503