Dynamic heterogeneities in glass-forming systems

We discuss the manifestation of dynamic heterogeneities in binary glass formers made of components with strongly different glass transition temperatures as revealed by 2H and 31P nuclear magnetic resonance spectroscopy (NMR) techniques such as line-shape, spin-lattice relaxation, and 2D exchange NMR. Polymer-plasticizer and non-polymeric mixtures are considered. Together with results from dielectric spectroscopy as well as from dynamic light scattering the component dynamics is probed separately. While the high-Tg component shows relaxation features similar to that of neat glass formers, the majority of the low-Tg component displays significantly faster dynamics and shows pronounced dynamic heterogeneities, i.e., an extremely broad distribution of correlation times G(ln τ), which may lead to quasi-logarithmic correlation functions and which are reflected in a characteristic NMR phenomenology. Consequently, two glass transition temperatures with non-trivial concentration dependences are identified. The dynamic heterogeneities are transient in nature as proven by 2D exchange NMR. Thus, below the upper Tg liquid-like (isotropic) reorientation as well as exchange within the distribution G(ln τ) is observed in an essentially rigid matrix formed by the high-Tg component. Yet, we find indications that a small fraction of the low-Tg component follows the dynamics of the high-Tg component. The results are compared with those collected for glass formers subjected to confining geometries. In both cases, the NMR features are very similar suggesting that also in binary glass formers (intrinsic) confinement effects may control the dynamics.