PHYSICAL AGING OF A POLYETHERIMIDE - CREEP AND DSC MEASUREMENTS

Creep and differential scanning calorimetry (DSC) measurements have be en used to study the physical aging behavior of a polyetherimide. Isot hermal aging temperatures ranged from 160 degrees C to T-g with aging times ranging from 10 min to 8 days. The only measurable effect of phy sical aging on the short-time creep curves is a shift of the creep com pliance to longer times. Andrade plots of the compliance versus the cu be root of time are linear at short times with the slope beta decreasi ng with increasing aging time to a constant value once equilibrium is reached. Log beta(3) is related directly to the degree to which the cr eep curves shift to longer times with physical aging, and is used in t his work as a measure of physical aging. A reduced curve of log beta(3 ) versus log aging time is obtained for the aging temperatures investi gated by appropriate vertical and horizontal shifts. The enthalpy chan ge during aging increases linearly with the logarithm of the aging tim e, t(alpha), leveling off at equilibrium at values which increase with decreasing aging temperature. Hence, both nonequilibrium and equilibr ium temperature shift factors can be calculated from the DSC data. Goo d agreement is observed between the equilibrium temperature shift fact ors obtained from the creep and DSC data. The temperature dependence o f the nonequilibrium temperature shift factors is found to be an order of magnitude smaller than that of the equilibrium shift factors. The time scales to reach equilibrium for enthalpy and far mechanical measu rements are found to be the same within experimental error. (C) 1995 J ohn Wiley & Sons, Inc.