The evolution of material properties during physical aging

Abstract

Aging experiments using the NIST (National Institute of Standards and Technology) torsional dilatometer have been performed in which the temperature of an isothermally equilibrated epoxy glass was abruptly changed to a new temperature To and the evolution of the volume and torsional relaxation responses recorded. The results of down-jump and up-jump experiments were found to differ dramatically. Not only is the normal asymmetry of volume approach to equilibrium found, but the mechanical 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 after the T-jump. In the case of the down-jump the evolution of the modulus ceases prior to that of the volume of the sample. In the up-jump experiment, 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.

On the physics required for prediction of long term performance of polymers and their composites

“…The reason for this is our feeling that the effects of structure on the structural (volume, enthalpy) recovery response need not be the same as for the viscoelastic (creep, relaxation) response. Thus, although the changing thermodynamic state obviously impacts the kinetics of both structural recovery and creep or stress relaxation, its impact need not be the same for the different processes [14][15][16][17][18][19][20].…”

Evolution of collective motion in a model glass-forming liquid during physical aging

“…For samples equilibrated at a series of high temperatures and then subjected to a temperature jump to a common lower temperature, the approach to equilibrium is qualitatively different than if the samples each began at equilibrium at a lower temperature and then are subjected to a T jump to a common higher temperature. 8 It should be noted that molecular dynamics simulations cannot access the time scales of minutes, hours, days, and weeks relevant to many practical applications, but it is possible to perform temperature jump experiments in the supercooled regime to observe significant aging of thermodynamic and relaxation properties. This is the approach of the present paper and other recent studies of the aging effect in glass-forming materials.…”

Physical Aging of Photo-Crosslinked Poly(ethyl acrylate) Observed in the Nanometer Scales by Mach-Zehnder Interferometry

“…Among them, dilatometry has been widely used to monitor the changes in volume of polymers at different temperatures in the glassy state. [6][7][8] Recently, fluorescence method using probe molecules with the internal rotations sensitive to viscosity was also utilized to examine the aging phenomena of polymers.9 Furthermore, an X-ray diffraction method was recently proposed for monitoring the strain distribution in amorphous materials, 10 which would be able to provide a potential tool for studies on physical aging.Recently, we have developed a Mach-Zehnder interferometer (MZI) equipped with a high-pressure Mercury lamp (350 W, Moritex, Japan) to in situ monitor the time-evolution process of the strain in photoreactive polymers.11 In this study, the nanometer deformation was in situ observed for a poly(ethyl acrylate) cross-linked with 365 nm UV light of different intensities. The aging phenomena and the corresponding kinetics were observed and analyzed via the dependence of the deformation on irradiation time and cross-link density.…”