Secondary relaxation processes in binary glass formers: Emergence of islands of rigidity

In addition to the primary alpha-process, some neat glass formers show a well resolved secondary beta-process (type-B) or solely an excess wing (type-A). We investigate two binary glass forming systems composed of a type-A and a type-B component. H-2 nuclear magnetic resonance (NMR) spectroscopy is selectively applied to the type-B component in order to characterize the beta-process over a large range of mole fractions x in the glassy state. We demonstrate that for x greater than or similar to 0.75 the apparent relaxation strength is constant, i.e., all molecules of type-B participate in the beta-process and the time constant tau(beta)(T) is independent of concentration. For x < 0.75, however, the apparent relaxation strength decreases abruptly, which we interpret in terms of population: below this concentration a fraction xi of type-B molecules still exhibits essentially the beta-process of the neat system (in terms of time scale and mechanism), while others have been immobilized. The arise of such a scenario is verified by 2D and spin-lattice relaxation H-2 NMR techniques. In selective H-2 NMR experiments on the type-A component we observe a contribution to the beta-process of the type-B molecules at medium concentrations. The latter finding and the rather sharp threshold occurring at x approximate to 0.75 may indicate that the beta-process is a cooperative process. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4798655]