Temperature cycling during the aging of a polymer glass

– Temperature cycling is used to study aging properties of plexiglass (PMMA) dielectric constant. If a negative temperature cycle is applied during the aging time the relaxation dynamics is just delayed for a time equal to the cycle period. In contrast this time shifting procedure cannot be applied to positive temperature cycles. The analogies and differences with similar experiments done in other aging systems, such as spin glasses, orientational glasses and supercooled liquids, are discussed. Introduction. – It is very well known that amorphous materials such as polymer solids present long relaxation time, when they are rapidly cooled from the liquid phase to any temperature below their glass transition temperature T g [1]. This phenomenon which is known as aging has been widely studied in spin-glasses (SG), where remarkable theoretical and experimental progress have been achieved [2, 3]. In these systems many features of the relaxation dynamics have been understood by imposing temperature cycles to the samples. The response of the systems to these cycles has been interpreted with a hierarchical structure of the free energy landscape, which evolves with temperature [3, 4], or in a mean-field theoretical approach [5]. Very recently such a kind of experiments have been repeated in supercooled liquids (SL) [6] and in orientational glasses (OG) [7] by measuring the dielectric susceptibility of the sample. The relaxation dynamics turned out to be rather different from that of spin-glasses, and a domain growth interpretation [8] seems to be rather appropriate for OG and SL. Temperature cycling has been also applied in the study of polymer mechanical properties [1, 9] but as far as we know no direct comparison to the previous systems has been done. The purpose of this letter is just to describe a series of experiments where temperature cycling has been applied to the measurement of the dielectric constant of polymers, specifically of PMMA. The dielectric constant measurement is a common method to analyze aging properties of polymers. After presenting the results of our measurements, we will discuss differences and similarities between polymers and other aging systems.