Shear stress relaxation and physical aging study on simple glass-forming materials

Relaxation and aging behaviors in three supercooled liquids: m-toluidine, glycerol, and sucrose benzoate have been studied by shear stress relaxation experiments in the time domain above and below their nominal glass transition temperatures. For the equilibrium state, the current study provides new data on the behavior of organic complex fluids. The shape of the relaxation function as characterized by the stretching exponent beta is discussed considering that a time-temperature master curve can be constructed even though the beta's for the individual response curves at each temperature vary systematically. In the nonequilibrium state, isothermal physical aging experiments at different glassy structures reveal that the effect of the aging process on the mechanical shear relaxation in these simple glass formers is similar to that observed in polymeric and other systems. Departure from the Vogel-Fulcher-Tamman behavior after the samples have aged back to equilibrium in the glassy state is observed for m-toluidine and, less strongly, for glycerol but not for sucrose benzoate. An inherent structure-based energy landscape concept is briefly discussed to account for the slow dynamics during the physical aging process.