Evolution of properties with increasing cure of a thermosetting epoxy/aromatic amine system: Physical ageing

Isothermal physical ageing below the glass‐transition temperature (T_g) of a high‐T_g thermosetting difunctional epoxy/tetrafunctional aromatic amine system was investigated at different ageing temperatures (T_a) and chemical conversions (monitored by the T_g) using the torsional braid analysis freely oscillating torsion pendulum technique. In the absence of chemical reaction during an isothermal ageing process, the rate of isothermal physical ageing passes through a minimum with increasing conversion. The minimum is related to the minimum in mechanical loss between the secondary relaxation in the glassy state (T_β) and the glass‐transition relaxation (T_g) (the temperatures of both of which increase with increasing conversion). If isothermal ageing rates for all conversions (beyond gelation) would have been measured directly from temperatures below T_β to above T_g, it is concluded that two maxima in isothermal ageing rate would have been observed corresponding to the two relaxation processes. There exists a superposition in isothermal ageing rate versus T_g ? T_a [by shifting horizontally (and vertically)], which implies that the ageing rate is independent of the details of the changing chemical structure attributed to cure. Controlling mechanisms during physical ageing are segmental mobility associated with the T_g region and more localized motion associated with the glassy‐state relaxation T_β. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2665–2675, 2003