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

Abstract

AbstractIsothermal physical ageing below the glass‐transition temperature (Tg) of a high‐Tg thermosetting difunctional epoxy/tetrafunctional aromatic amine system was investigated at different ageing temperatures (Ta) and chemical conversions (monitored by the Tg) 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 (Tβ) and the glass‐transition relaxation (Tg) (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 Tβ to above Tg, 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 Tg − Ta [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 Tg region and more localized motion associated with the glassy‐state relaxation Tβ. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2665–2675, 2003