Dynamic Heterogeneities in Binary Glass-Forming Systems

Abstract

Starting with an overview of major results of the main (α) and secondary (β) relaxation in neat glass formers as compiled by dielectric and nuclear magnetic resonance (NMR) spectroscopy as well as by light scattering, the contribution deals with elucidating the component dynamics in binary glass formers. Dynamically asymmetric mixtures with high-Tg contrast of their components are in focus. In addition to polymers, specially synthesized non-polymeric glass formers are considered as high-Tg component and mixed with a low-Tg simple liquid. While the high-Tg component in the mixtures shows relaxation features similar to that of neat glass formers, the low-Tg component displays significantly faster dynamics and pronounced dynamic heterogeneities, i.e., an extremely broad distribution of correlation timesDistribution of correlation times G(lnτ), which may lead to quasi-logarithmic correlation functions. Two glass transition temperatures with non-trivial concentration dependences are identified. The dynamic heterogeneities are transient in nature as proven by 2D2D exchange NMRExchange NMR . Thus, liquid-like (isotropic) reorientationLiquid-like (isotropic) reorientation of the low-Tg additive as well as exchange within its distribution G(lnτ) is observed in an essentially rigid high-Tg matrix. The results show similarity with those collected for glass formers in confining geometries, suggesting that in asymmetric binary glass formers (intrinsic) confinement effects may control the dynamics either. We also investigate the β-process in the mixed glasses introduced by the low-Tg additive. It is rediscovered for all concentrations with virtually unchanged time constants. NMR identifies the β-relaxations as being similar to those of neat glasses. A spatially highly restricted motion with an angular displacement below ±10° encompasses all molecules. Very similar spectral features are observed for the high-Tg component in NMR. Apparently, the (small) additive molecules “enslave” the large molecules to perform a common hindered reorientation. At lowest additive concentrations, one finds indications that the β-process starts to disintegrate. We conclude that the β-process is a cooperative process.