Polybenzoxazine-montmorillonite hybrid nanocomposites: synthesis and characterization

Polybenzoxazine-clay hybrid nanocomposites were prepared from a polybenzoxazine precursor (B-a) and organically modified-montmorillonite (OMMT) as a type of layered silicates. OMMTs were prepared by surface treatment of montmorillonite (MMT) by octyl, dodecyl or stearyl ammonium chloride. The curing behavior of B-a in the presence of dispersed OMMT was followed by IR and DSC. DSC showed that the onset of the ring opening polymerization of pristine B-a started at 223 degrees C. The ring opening polymerization of B-a in the presence of OMMT started at 177-190 degrees C, however, suggesting the catalytic effect of the OMMT surface on the ring opening polymerization. DSC and IR indicated that the curing of B-a was completed by the end of the 230 degrees C cure cycle. The dispersion of OMMT in the polybenzoxazine matrix was confirmed by XRD, which indicated the collapse of the registry of the OMMT. The absence of basal spacings diffractions of OMMTs from the XRD patterns suggests the dispersion of OMMT layers on the molecular level when the surface was pretreated with long chain surfactants like dodecyl or stearyl ammonium chloride. However, in case of OMMT, which was pretreated with short chain surfactants like octyl ammonium chloride, the intercalation of polybenzoxazine into the clay galleries occurred with a regularly stalked layered structure. Viscoelastic measurements showed that the T-gS of the hybrid materials were higher than that of the pristine resin. In addition, the storage modulii of the hybrid materials were maintained up to higher temperatures suggesting the reinforcement attained by OMMT. Isothermal and dynamic TGA showed that nanocomposites have delayed decomposition temperatures when compared with pristine polybenzoxazine indicating the enhancement in the thermal stability. (C) 2000 Elsevier Science Ltd. All rights reserved.