Synthesis of a novel benzoxazine monomer-intercalated montmorillonite and the curing kinetics of polybenzoxazine/clay hybrid nanocomposites

Abstract

Polybenzoxazine (PBZ), which has a structure similar to that of phenolic resin, is formed through the thermal self-curing of benzoxazine, that is through a heterocyclic ring opening reaction that requires no catalyst and releases no condensation byproducts. We have used the solvent blending method to prepare PBZ/clay nanocomposites possessing various clay contents. We synthesized a monofunctional benzoxazine monomer (MBM) and then treated the clay with this intercalation agent. The results of X-ray diffraction (XRD) analysis indicated that MBM intercalated into the galleries of the clay; the nanocomposite possessed an exfoliated structure at 3% clay content. To better understand the curing kinetics of the PBZ/clay nanocomposites, we performed dynamic and isothermal differential scanning calorimetry (DSC) measurements. We describe the thermodynamics of the curing process, using all three of the Kissinger, Ozawa, and Kamal models. The Kissinger and Ozawa methods gave fairly close results for the calculated activation energies, which decreased upon increasing the clay content. The Kamal method, based on an autocatalytic model, suggested a total reaction order of between 2.4 and 2.8. The glass transition temperature (T,) decreased upon increasing the clay content. Thermogravimetric analysis (TGA) indicated that the nanocomposites have higher decomposition temperatures than does the pristine PBZ; this finding suggests an enhancement in their thermal stability. (c) 2005 Wiley Periodicals, Inc.