PHYSICAL AGING OF A POLYETHERIMIDE - VOLUME RECOVERY AND ITS COMPARISON TO CREEP AND ENTHALPY MEASUREMENTS

Volume recovery measurements have been used to study the physical agin g behavior of a polyetherimide. Isothermal aging temperatures near T-g ,were studied with aging times ranging up to several days. The volume decreases during physical aging and levels off at equilibrium. For co mparison purposes, the data are normalized to yield the departure from equilibrium which varies from unity at very short aging times to zero when equilibrium is reached. As the aging temperature decreases, the normalized curves are shifted to longer times without a significant ch ange in shape. Hence, the data can be reduced by aging time-temperatur e superposition. The temperature dependence of the shift factors used to reduce the volume recovery data and the times to reach equilibrium for the volume recovery follow the WLF equation and agree within exper imental error with the values from enthalpy and creep measurements obt ained in previous work. However, the approach to equilibrium for volum e appears to differ from that of enthalpy, with volume recovery being faster than the enthalpy recovery at short times. (C) 1997 John Wiley & Sons, Inc.