A NONLINEAR-REGRESSION APPROACH TO MODEL THE PHYSICAL AGING OF POLY(ETHER IMIDE)

Differential scanning calorimetry (DSC) was used to measure the relaxa tion enthalpy of glassy amorphous poly(ether imide) (PEI) at three hig h isothermal aging temperatures between 190 and 200 degrees C. A three -parameter model based on the Williams-Watts relaxation function was u tilized to describe the aging process of PEI, and nonlinear regression approach was used to estimate the model parameters. The prediction ac cording to this model was found to be more accurate in comparison with the two-parameter model based on the same Williams-Watts relaxation f unction. Furthermore, the peculiar aging behavior at the highest aging temperature of 200 degrees C was explained from the molecular view po int of transition from sub-T-g beta-relaxation to alpha-relaxation nea r T-g.