The use of DSC to characterize structural relaxation in thermosetting polymers

Structural relaxation in different epoxy-anhydride and epoxy-diamine resins has been investigated by differential scanning calorimetry using annealing and cooling rate experiments. The annealing experiments lead to the determination of enthalpy loss, Delta H, at an equivalent annealing temperature T-a= T-g - 20, and for periods of annealing time, t(a), between 1 h and 4 months. The variation of Delta H with logt(a), defines a relaxation rate per decade, rrpd, which is very sensitive to changes of the epoxy network. The cooling rate experiments allow the determination of the apparent activation energy, Delta h*. The effect of the degree of crosslinking, the addition of a reactive diluent, which acts as flexibilizer, and the length of cross-link on rrpd and Delta h* was studied.