Breakdown of Debye-Stokes-Einstein and Stokes-Einstein relations in glass-forming liquids: An explanation from the coupling model

The breakdown of Debye-Stokes-Einstein and Stokes-Einstein relations in glass-forming liquids is shown to be a special case of a more general phenomenon. A general explanation is given in the framework of the coupling model, which is based on the fact that different dynamie variables may have different coupling parameters entering into the stretch exponents of their Kohlrausch correlation functions. In most of the applications, the explanation requires no adjustable parameter. An exception is the explanation of the breakdown of a Stokes-Einstein relation where an experimental technique has not yet been developed to determine the coupling parameter of translational diffusion. We can explain also the breakdown of the Debye-Stokes-Einstein relation of depolarized light scattering data of the liquid diglycidyl ether of bisphenol-A. This breakdown oceurs at high temperatures due to a reduction of the light scattering relaxation rate as compared with the shear stress relaxation rate. This behaviour is entirely opposite to the breakdown of the Stokes-Einstein relation of other glass-forming liquids, which oceurs at Iow temperatures due an enhancement of translational diffusion over rotational diffusion or shear stress relaxation rate.