  Molecular dynamics simulations are used to investigate the connection between
thermal history and physical aging in polymer glasses, in particular the
effects of a temperature square step. Measurements of two-time correlation
functions show that a negative temperature step causes "rejuvenation" of the
sample: the entire spectrum of relaxation times appears identical to a younger
specimen that did not experience a temperature step. A positive temperature
step, however, leads to significant changes in the relaxation times. At short
times, the dynamics are accelerated (rejuvenation), whereas at long times the
dynamics are slowed (over-aging). All findings are in excellent qualitative
agreement with recent experiments. The two regimes can be explained by the
competing contributions of dynamical heterogeneities and faster aging dynamics
at higher temperatures. As a result of this competition, the transition between
rejuvenation and over-aging depends on the length of the square step, with
shorter steps causing more rejuvenation and longer steps causing more
over-aging. Although the spectrum of relaxation times is greatly modified by a
temperature step, the van Hove functions, which measure the distribution of
particle displacements, exhibit complete superposition at times when the
mean-squared displacements are equal.
