Viscous Flow and Plastic Deformation of Glasses in the Model of Excited Atoms

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The critical displacement of an atom (a group of atoms) Δrmax, which corresponds to the interatomic interaction force at the maximum, is treated as the atomic excitation in liquids and glasses. The formation and migration of excited atoms are governed by local fluctuations of the configurational entropy. In amorphous linear polymers, the role of an excited kinetic unit is played by a small-sized macromolecular segment (an atomic group in a monomer). The atomic excitation in silicate glasses corresponds to the critical displacement of a bridging oxygen atom in the Si–O–Si bridge upon switching of adjacent bridging bonds. The activation energy e h = Pmaxνh and the activation volume νh = πd2Δrmax of the atomic excitation are introduced. The activation energy e h is determined by the work done against the forces of the internal pressure Pmax. The formation of an excited kinetic unit in polymers and glasses is a small-scale low-energy process (e h = 10–20 kJ/mol, νh = 5–70 A3). The total activation volume of the system Vf = νhNh is termed the fluctuation volume, and the model under consideration is referred to as the fluctuation volume model (or the model of excited atoms). The viscosity equation and other relationships of the hole theory are interpreted in the framework of the proposed model. This model is also applied to analyze the plasticity of amorphous polymers and glasses. A relationship between the glass transition condition and the Lindemann melting criterion is established.