Effect of Number\nDensity of Epoxy Functional Groups\non Reaction Kinetics for Epoxy Resin

Improving the heat\nresistance of epoxy resins remains\nan important\nand challenging issue across a wide variety of applications. One strategy\nto address this challenge is the design of cured resins based on multifunctional\nepoxy. Compared to conventional difunctional epoxy resins, multifunctional\nepoxy resins are expected to easily form a highly cross-linking structure,\nwhich is anticipated to contribute to enhanced heat resistance. However,\nthere is a lack of information about the detailed mechanisms of the\nformation of such cross-linking structures and their effects on the\nphysical properties. We herein tracked the kinetics of curing reactions\nof difunctional and trifunctional epoxies with an amine hardener.\nDespite the identical reactivity of epoxy groups in both base monomers,\nthe trifunctional epoxy system cured faster. In addition, in the difunctional\nepoxy system, gelation was followed by vitrification, while in the\ntrifunctional epoxy system, gelation and vitrification occurred simultaneously.\nThe accelerated curing reaction observed in the trifunctional epoxy\nsystem could be explained in terms of the localized temperature increase\nfrom the reaction heat, which subsequently accelerated the following\nreactions. The resulting post-cured trifunctional epoxy resin did\nnot exhibit a clear glass transition. This, so-called <i>T</i><sub>g</sub>-less behavior was due to the glass transition temperature\nof the trifunctional epoxy unit being higher than its decomposition\ntemperature. This mechanism for the <i>T</i><sub>g</sub>-less behavior, combined with the accelerated curing reactions, provides\nvaluable insights for the design of thermosetting resins with enhanced\nthermal stability.