  The aging behavior of polymer glass, poly(methyl methacrylate), has been
investigated through the measurement of ac dielectric susceptibility ata fixed
frequency after a temperature shift $\Delta T$ ($\le $ 20 K)between two
temperatures, $T_1$ and $T_2$. A crossover from cumulative aging to
non-cumulative aging could be observed with increasing $\Delta T$ using a twin
temperature ($T$-) shift measurement. Based on a growth law of a dynamical
coherent length given by activated dynamics, we obtained a unique coherent
length for positive and negative $T$-shifts. The possibility of the existence
of temperature chaos in polymer glasses is discussed.
