SHORT-TIME VISCOUS AND DENSITY RELAXATION IN GLYCEROL AND ORTHO-TERPHENYL

Coupling model analyses of short-time relaxation data of glycerol from quasielastic neutron scattering and ortho-terphenyl (OTP) by molecula r dynamic simulation are shown to account for both the time dependence of the correlation function and its temperature dependence. The latte r is expressible over a limited temperature range as an Arrhenius rela tion with an apparent activation energy for the fast alpha-relaxation process, with E(A) = 6.2 and 3.9 kcal/mol for glycerol and OTP, respec tively. The viscosities of these glass formers have also been measured up to temperatures sufficiently high that the mean viscosity relaxati on time, [tau(n)(T)], becomes short, approaching the crossover time, t (c) approximate to 2 ps, of the coupling model. When this condition is realized, [tau(n)(T)] becomes comparable to tau(D)(T) in both magnitu de and temperature dependence. Hence, the asymptotic high temperature activation energy of [tau(eta)(T)] approximately equals the apparent a ctivation energy of tau(D). The experimental data, supporting this con nection between macroscopic variables such as viscosity and microscopi c dynamics from neutron scattering and molecular dynamics simulation, indicate the significance of the fast cr-relaxation process in both fr agile and strong liquids.