Memory in the aging of a polymer glass

Low-frequency dielectric measurements on plexiglass (PMMA) show that cooling and heating the sample at constant rate give a hysteretic dependence on temperature of the dielectric constant ε. A temporary stop of cooling produces a downward relaxation of ε. Two main features are observed: i) when cooling is resumed ε goes back to the values obtained without the cooling stop (i.e. the low-temperature state is independent of the cooling history); ii) upon reheating ε keeps the memory of the aging history (Memory). The analogies and differences with similar experiments done in spin-glasses are discussed.