Sub-T_g molecular relaxation and enthalpy relaxation behavior in amorphous glassy poly(ether imide)

Physical aging behavior of glassy, amorphous poly(ether imide) (PEI) at a high aging temperature range (15 ∼ 25 °C below the polymer T_g of 215 °C) was investigated. A quantitative modeling approach was utilized to describe the aging process of PEL The experimental data of enthalpy of relaxation for constructing the models were obtained using differential scanning calorimetry (DSC). At relatively low aging temperatures, the enthalpy of relaxation simply increases linearly with the logarithm of time. At high temperatures another parameter accounting for distribution is needed for better description. Furthermore, the sub-T_g molecular chain relaxation of aged PEI was characterized using dynamic mechanical analysis (DMA). Physical aging significantly suppressed the β chain relaxation but not the γ-relaxation, suggesting that the chain segmentai motion for the β-relaxation is the one responsible for chain conformation changes reaching an equilibrium glassy state.