Tuning morphology of siloxane bond-based polybenzoxazines by controlling the sol–gel and curing processes

Abstract

Novel polybenzoxazine/polysilsesquioxane nanocomposites were synthesized with uniform and reproducible structural morphology, high Tg, and good thermal stability. Polysilsesquioxane was oriented in a nanometer-size lamellar structure. Benzoxazine monomer based on silane (BA-3aptms) was synthesized by reaction between bisphenol A, 3-(trimethoxysilyl)propylamine, and paraformaldehyde in a 1:2:4 molar ratio. Four experimental conditions of the two stage sol–gel process for polysilsesquioxane were studied, and their effect on the material characteristics investigated. In the first stage, hydrolysis was favored, while the second stage mostly consisted of condensation. The reaction condition with the longest hydrolysis time allowed the increase of the concentration of silanol units and the selective condensation, favoring the formation of a higher amount of cage-like cyclic structures. In addition, this experimental procedure was well controlled so that the sol–gel and oxazine-ring opening reactions were extremely reproducible from batch to batch. All processes were monitored by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis and dynamic mechanical thermal analysis, scanning electron and transmission electron microscopies, and X-ray diffraction. A chemometric approach based on a multivariate curve resolution model was applied to the FTIR spectra, which allowed the identification of four reaction components associated with the sol–gel and the curing processes.