  We develop a simple yet comprehensive nonlinear model to describe relaxation
phenomena in amorphous glass-formers near the glass transition temperature. The
model is based on the two-state, two-(time)scale (TS2) framework, and describes
the isothermal relaxation of specific volume, enthalpy, or shear stress via a
simple first-order nonlinear differential equation (the Trachenko-Zaccone [TZ]
equation) for local cooperative events. These nonlinear dynamics of
cooperatively rearranging regions (CRR) naturally arise from the TS2 framework.
We demonstrate that the solutions of the TZ equation comprehensively encompass
the Debye exponential relaxation, the Kohlrausch-Williams-Watts (KWW) stretched
and compressed relaxations, and the Guiu-Pratt logarithmic relaxation.
Furthermore, for the case of stress relaxation modeling, our model recovers, as
one of its limits, the Eyring law for plastic flow, where the Eyring activation
volume is related to thermodynamic parameters of the material. Using the
example of polystyrene (PS), we demonstrate how our model successfully
describes the Kovacs "asymmetry of approach" specific volume and enthalpy
experiments, as well as the stress relaxation. Other potential applications of
the model, including the dielectric relaxation, are also discussed. The
presented approach disentangles the physical origins of different relaxation
laws within a single general framework based on the underlying physics.
