THE EVOLUTION OF MATERIAL PROPERTIES DURING PHYSICAL AGING

Aging experiments using the NIST (National Institute of Standards and Technology) torsional dilatometer have been performed in which the tem perature of an isothermally equilibrated epoxy glass was abruptly chan ged to a new temperature T-0 and the evolution of the volume and torsi onal relaxation responses recorded. The results of down-jump and up-ju mp experiments were found to differ dramatically. Not only is the norm al asymmetry of volume approach to equilibrium found, but the mechanic al responses are found to evolve differently from the volume response, contrary to simple free volume models of the physical aging process. It is found that the torsional modulus changes with increasing time af ter the T-jump. In the case of the down-jump the evolution of the modu lus ceases prior to that of the volume of the sample. In the up-jump e xperiment, the contrary is true, viz., the modulus continues to evolve after the volume has attained its equilibrium value, The implications of this for the description of material behavior are discussed.