The relaxation spectrum and physical ageing of polyetherimide

Abstract

The development of physical ageing in polyetherimide with time and at various temperatures up to 30 K below the glass transition temperature, T g, has been studied using differential scanning calorimetry (d.s.c.). The value of the exotherm which developed on heating an aged specimen through T g with time was used to analyse the kinetics of the process and so determine the time for the glass to reach equilibrium. The T g was determined calorimetrically for glasses prepared at different cooling rates from 0.32 to 160 K min −1 and an activation energy for glass formation was found to be 1150 ± 150 kJ mol −1. This compares with that of physical ageing of 1470 ± 300 kJ mol −1. In this analysis the specific heat of polyetherimide (PEI) has been measured over a temperature range either side of T g and the change in heat capacity, ΔC p( T g), determined as 210 J kg −1 K −1. Dynamic mechanical and dielectric studies have also been carried out in order to determine the effect of physical ageing on the relaxation spectra of PEI, and the monomeric friction coefficient was also calculated. Tan δ and the flexural modulus have been measured over a range of frequencies and temperature. A composite curve incorporating the measured values of T g by various techniques including dynamic mechanical and dielectric thermal analyses, d.s.c. and physical ageing covering a relaxation timescale of 20 decades highlights the kinetic nature of the glass transition and clearly associates physical ageing with the glass forming process.