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