Anomalous component dynamics of a binary mixture of associating glass-forming liquids

The binary mixtures of propylene glycol (PG) with 2-ethylhexylamine (EHA) show changes of the dynamics of the higher-T(g) component PG on increasing the concentration of the lower T(g) component EHA very different or even opposite in trend from those found in other associating and nonassociating binary mixtures. For example, the fragility index m increases and the coupling parameter n of the PG component increases on adding EHA, while T(g) of the PG component does not change up to 0.5 molar fraction of EHA. From the features present in the broadband dielectric spectra of the PG component at different compositions, the anomalies are identified to originate from the enhancement of intermolecular coupling in the primary relaxation of the higher-T(g) PG component by the presence of the EHA component. The enhancement can be traced to the fact that neat EHA has larger intermolecular coupling than neat PG, despite the former having a lower T(g) than the latter. The enhancement of intermolecular coupling of the PG component increases on increasing the concentration of EHA in the mixture, as evidenced by the appearance of the JG β-relaxation of the PG component well separated from the α-relaxation when the molar fraction of EHA has been increased to 0.75. In contrast, the JG β-relaxation of neat PG is too close to the α-relaxation and cannot be resolved. The anomalous dynamics of PG in the mixtures is challenging to all theories and models of glass transition to explain. The insight gained on the physical origin of the anomalous dynamics leads to a rationalization in the framework of the coupling model.