Abstract
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
. A temporary stop of cooling produces a downward
relaxation of . Two main features are observed: i) when
cooling is resumed 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
keeps the memory of the aging history (Memory). The
analogies and differences with similar experiments done in spin-glasses are discussed.