How polymers lose memory with age.

Uniquely in the world of materials, polymers deformed at high temperature and subsequently quenched at low temperature, memorize the temperature at which they have been processed. Polymers can even memorize multiple temperatures. This temperature memory is reflected by a maximum of residual stress restored at the temperature of initial processing. It has been speculated that this capability could arise from the presence of dynamical heterogeneities in glassy domains of polymers. Processing the material at a given temperature would result in the selection of certain heterogeneities that participate in the storage of mechanical stress. Because dynamical heterogeneities are associated with particular relaxation times, the temperature memory of polymers should depend on the time, for example, the glass transition temperature depends on the frequency. The first experimental study of temporal effects on the temperature memory of polymers is presently reported. It is found that aging at high temperature shifts the maximum of residual stress towards greater temperatures. The corresponding loss of memory is explained by the relaxation of dynamical heterogeneities with short characteristic times. The present results clarify the origin of the temperature memory and provide insights into their efficient exploitation in applications.

Thermoelectrical Memory of Polymer Nanocomposites.

The inclusion of nanoparticles improves the behavior of shape-memory polymers and allows new functionalities. It is shown in the present work that polyamide fibers loaded with carbon nanotubes (CNTs) exhibit novel memory functions associated to their electrical conductivity. Similar to classical shape memory polymers, the materials are predeformed at high temperature and then quenched down to room temperature and subsequently reheated. Their resistivity is recorded during the process and is found to decrease with temperature during the last heating stage. The rate of resistivity decrease exhibits a well-defined maximum at the temperature of predeformation. This unique response clearly shows an accurate thermoelectrical material memory. Temperature memory extended to electrical properties could serve for future sensing applications coupled to shape changes.