Investigation of the shear relaxation behavior of As-Se liquids within the framework of entropic and elastic models of viscous flow

Abstract

The shear relaxation behavior of supercooled arsenic selenide liquids is studied using small amplitude oscillatory parallel plate rheometry. Compositions with >80% Se, characterized by a low-dimensional network of selenium chain segments cross-linked by threefold coordinated arsenic atoms, display the coexistence of a nearly-Arrhenius, slow relaxation process and a strongly non-Arrhenius, fast relaxation process corresponding to bond scission/renewal and chain segmental motion, respectively. The coupling between the weakly non-Arrhenius bond scission and the strongly non-Arrhenius viscosity is achieved near Tg due to the pronounced temperature-dependence of the modulus corresponding to this relaxation process. The temperature dependence of this relaxation modulus is related to the conformational entropy of the chain segments. The large difference between the activation energy of viscous flow and that of the timescale of the relaxation associated with bond scission/renewal implies the need for a revision of the existing models of viscous flow in the literature.