We investigated binary low-molecular-weight glass formers as model systems for mixtures of small and large molecules. Tricresyl phosphate (TCP) in oligomeric styrenes (OS), benzene in OS and polystyrene (PS), and benzene in TCP were studied by applying dielectric spectroscopy as well as 1H, 2H, and 31P NMR spectroscopy. Temperatures above and below the glass-transition temperature (T G) are covered. The dielectric loss of the small component appears broader the higher the molecular ratio M/m is, and the lower the TCP concentration and the lower the temperature are chosen. Close to T G, extremely broad distributions of correlation times G(log τ) result, which are similar to those reported in the cases of polymer−plasticizer systems, although for our systems the motional heterogeneities are already established at similar M and m. By applying 1H and 2H (1D and 2D) NMR on benzene in OS and PS, we can demonstrate that the large molecules basically behave as in neat glass formers. However, the small molecules exhibit an isotropic reorientation also well below T G, and the dynamics is rather characterized by a random jump process than by rotational diffusion, the latter being found in neat systems. Furthermore, we can prove that within G(log τ), exchange processes take place, even below T G, which essentially occur on the same time scale as reorientation.

We review ' H and also some 'H nuclear magnetic resonance (NMR) results on mobile guest molecules dissolved in organic high and low molecular weight glasses. The guest molecules such as benzene exhibit high symmetry and perform a rotational jump process between equivalent orientations in the solid state. Characteristic two-phase NMR spectra and a pronounced non-exponential 2H spin-lattice relaxation are observed. The 'H spin-lattice relaxation is exponential, and shows a non-Arrhenius temperature dependence. All these features can be well described over a large temperature range (lo-2OOK) by assuming a broad distribution of activation energies and entropies. The distribution of activation energies displays significant differences for the various guest/host systems. We think the approach chosen here can also be applied to describe 8-processes or internal processes in neat glasses.