Physical aging of an amorphous polyimide: Enthalpy relaxation and mechanical property changes

The physical aging behavior of an isotropic amorphous polyimide possessing a glass transition temperature of approximately 239 degrees C was investiga ted for aging temperatures ranging from 174 to 224 degrees C. Enthalpy reco very was evaluated as a function of aging time following sub-T-g annealing in order to assess enthalpy relaxation rates, and time-aging time superposi tion was employed in order to quantify mechanical aging rates from creep co mpliance measurements. With the exception of aging rates obtained for aging temperatures close to T-g, the enthalpy relaxation rates exhibited a signi ficant decline with decreasing aging temperature while the creep compliance aging rates remained relatively unchanged with respect to aging temperatur e. Evidence suggests distinctly different relaxation time responses for ent halpy relaxation and mechanical creep changes during aging. The frequency d ependence of dynamic mechanical response was probed as a function of time d uring isothermal aging, and failure of time-aging time superposition was ev ident from the resulting data. Compared to the creep compliance testing, th e dynamic mechanical analysis probed the shorter time portion of the relaxa tion response which involved the additional contribution of a secondary rel axation, thus leading to failure of superposition. Room temperature stress- strain behavior was also monitored after aging at 204 degrees C, with the r esult that no discernible embrittlement due to physical aging was detected despite aging-induced increases in yield stress and modulus. (C) 1999 John Wiley & Sons, Inc.