The dynamics of binary mixtures of nonpolymeric viscoelastic liquids as studied by quasielastic light scattering

Abstract

In this paper we report quasielastic light‐scattering experiments of the compatible binary mixture of simple viscoelastic liquids made up of o‐terphenyl as one component and newly synthesized model materials, 1,1‐di(paramethoxyphenyl)‐cyclo‐hexan and 1,1‐di(paramethoxyphenyl, metamethyl)‐cyclohexan, as the other components. The measurements were done in a wide temperature range above T g (≂T g +100 K) of the mixtures and in a volume fraction range up to φ=0, 5 of bis‐cresol‐cyclohexane‐dimethylether and bis‐phenol‐cyclohexane‐dimethylether. We have found that the measured dynamics is governed by three effects: At short times we observe the dynamics of the density fluctuations of the system characterized by a broad distribution of relaxation times, which can be scaled to a masterplot taking the T g of the mixtures into account. At longer times we observe two translational diffusion processes whose decay is single exponential with a characteristic time proportional to q −2, q being the scattering vector. The faster one is attributed to the interdiffusion coefficient of the system. The second, slower mode is related to the motion of dynamic clusters which have their origin in a spatial inhomogeneity of the samples. The mean cluster diameter is about 1000 Å, thus in any case the occurrence of the slow mode is accompanied by a strong increase of the elastically scattered intensity towards small scattering angles. This mode also appears in pure components. We relate its intensity to the fluctuations of an order parameter, usually introduced to describe the glassy state.