Aging under stress in polymer glasses

We investigate plastic yield in glassy polymers subsequent to physical aging in the presence of a nonvanishing stress. Molecular dynamics simulations show that the applied stress does not alter the physical aging rate, but instead causes an apparent modification of the aging rate of the shear yield stress with a concomitant anisotropic mechanical response. A molecular level analysis identifies the primary cause of this behavior as the orientation of covalent bonds, which is proportional to strain and logarithmic aging time. We also observe a Bauschinger effect, which amplifies or reduces the pressure dependence of shear yielding. Control simulations with a monovalent Lennard-Jones glass offer further evidence that these effects require chain connectivity and are distinct from rejuvenation and overaging behavior reported for a broad class of amorphous solids.