Dynamic heat capacity and reaction enthalpy during the two-stage network growth in diepoxide–diamine compositions

Abstract

The real and imaginary components of the dynamic heat capacity, C p ′ and C p ″, respectively, have been measured for a fixed frequency of 5 mHz during the polymerization of various compositions of a diepoxide–diamine, molecular liquid mixture to a network structure. The heat evolved during the polymerization was measured simultaneously. C p ′ decreased in two steps as the covalent bonds formed and the network structure grew. The steps became more separated when the amount of the already excess diepoxide was further increased. C p ″ showed a peak in its plot against the polymerization time, but only in the region where C p ′ showed a second step. This is attributed to the increase in the relaxation time leading to vitrification of the liquid. For the diepoxide-rich compositions, the enthalpy release also occurred in two steps and it was more for the second stage of the network's growth than for the first. Combined measurements of the exothermic effects and C p ′ and C p ″ thus delineated two stages of the network's growth by two chemical reactions. The nature of the second-stage network growth that ultimately vitrifies the stoichiometric liquid mixture is discussed. It is concluded that the second-stage growth is mass-controlled and occurs by an etherification reaction whose thermodynamic consequences have been elusive in past studies.