A molecular theory for the sub-t_g plastic mechanical response of amorphous polymers

Abstract

A theory based on the concepts of quasi-punctual defects and hierarchically constrained molecular dynamics is extended to account for the effects of large stresses on the rate of elementary molecular motions. The extended theory provides a physical interpretation for the general features of the experimentally observed sub-T _g mechanical response of amorphous polymers. Sub-T _g plastic deformation is accordingly associated with the creation of defects, whose concentration increases towards a constant, stationary value, which is characteristic of the kinetic equilibrium occurring between the creation of more defects to accommodate further deformation and the recombining of defects due to structural relaxation. The stationary value of the concentration of defects is found to be independent of temperature in the low-temperature regime and to decrease more rapidly as the temperature is increased above a certain threshold value and as structural relaxation becomes a prominent factor. The results are discussed in the light of previously proposed formalisms such as thermodynamic treatments.