Dynamics of the glass-forming liquid di-2-ethylhexyl phthalate (DOP) as studied by light scattering and neutron scattering

Abstract

Dynamic light scattering (depolarized Rayleigh and polarized Rayleigh–Brillouin) and quasielastic neutron scattering are employed to study the dynamics of the glass-forming liquid di-2-ethylhexyl phthalate (DOP) (Tg=184 K). The depolarized Rayleigh scattering measurements were made in the temperature range from 303 to 433 K, the polarized Rayleigh–Brillouin measurements in the range from 263 to 433 K, and the quasielastic neutron-scattering measurements in the range from 37 to 312 K and in the Q range from 0.33 to 1.84 Å−1. The orientation times for DOP, obtained from a single Lorentzian fit to the experimental depolarized spectra at high T, are in good agreement with recent dielectric data for the primary (α) relaxation. However, at lower T, the viscosity increases more strongly than the orientation times and the Stokes–Einstein–Debye equation which can adequately describe the dynamics in the high-T range is insufficient at temperatures close to Tg. The relaxation time obtained from the Rayleigh–Brillouin experiment is about 1 order of magnitude faster than the orientation times. In the neutron-scattering experiment we find a strong decrease of the elastic intensity and a corresponding increase of the quasielastic intensity around Tg. The data analysis with respect to the dynamics (from a two Lorentzian fit) revealed the existence of three processes affecting the high-frequency range: (i) a ‘‘fast’’ (τ2∼10 ps) Q-independent motion with weak T dependence (E2=1.54 kcal/mol), (ii) a ‘‘slow’’ Q-dependent motion, and (iii) a flat background increasing with T and Q. The fast process is discussed in terms of a very localized motion of the phenyl group (β relaxation) and, as such, as a precursor of the the primary (α) relaxation. The relaxation time of this process (τ2) was found to compare nicely with the time τmax from the Rayleigh–Brillouin (RB) experiment suggesting that the latter is caused by fast localized motions. The slow process is discussed in terms of the jump-diffusion model. The activation energy associated with the jump-diffusion times is 6.1 kcal/mol and it is associated with large-scale diffusional motion of the DOP molecule. The relaxation times obtained from this process are compared with the relaxation times obtained from the depolarized and dielectric techniques for the primary relaxation. Finally, the background can be identified with fast local motions and/or low-frequency excitations relaxing outside the energy window of our experiment.