Gelation behavior of near-zero shrinkage polybenzoxazines

A new class of phenolic thermosetting resins is developed that is based on the ring-opening polymerization of a benzoxazine precursor. These new mater ials seek to combine the thermal properties and flame retardance of phenoli cs with the mechanical performance and molecular design flexibility of adva nced epoxy systems. These materials overcome many of the traditional shortc omings of conventional novolac and resole-type phenolic resins, while retai ning their benefits. The viscoelastic behavior of the polybenzoxazines duri ng isothermal cure is monitored by dynamic mechanical analysis. Isochronic measurements show that although the aniline-based benzoxazine has a lower a ctivation energy for the gelation process than the methylamine-based resin, it has a slower rate of reaction. The purified monomer and as-synthesized precursor for each benzoxazine are found to polymerize by the same mechanis m, despite the absence of an initiating species in the purified resins. The chemical gelation phenomenon of the methylamine-based resin is probed by a multifrequency dynamic cure analysis that allows determination of the inst ant of chemical gelation, as well as the network relaxation exponent, n. Th e constant value of the exponent regardless of cure temperature demonstrate s that chemical gelation is, in fact, an isoconversion event for the methyl amine-based benzoxazine. The multifrequency and isochronic analyses are sho wn to produce very similar gel times and activation energies for the gelati on process. (C) 2000 John Wiley & Sons, Inc.