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.

Synthesis and characterization of polybenzoxazine/silica‐based hybrid nanostructures for flame retardancy applications

“…This difference is associated with the high Si–O–Si bond dissociation energy (460 kJ/mol), giving excellent thermal resistance to materials. [ 49,54 ] In addition, it can be seen that the thermal stability of the materials is better under oxidizing atmosphere. According to previous investigations, [ 61 ] this behavior can be due to the formation of thermally stable chemical structures by oxidation reactions (decomposition products), such as quinone, amide, and imide bonds.…”

“…In order to obtain the optimal conversion of siloxane structures in short reaction times the procedures were performed by following the procedure reported in previous study of Forchetti et al, reaching a compromise solution between the sol–gel process (hydrolysis and condensation reactions) and the thermally induced curing by oxazine‐ring opening. [ 49 ] The BA‐3aptms precursor synthesis procedure was divided into three stages. In the first stage, BA‐3aptms monomer and isopropanol (isopropanol/Si molar ratio = 4.5) were added to a flask provided with magnetic stirring at 30 °C to favor the hydrolysis.…”

“…Particularly, the sol–gel process from organosilane precursors, such as triethoxysilyl‐ or trimethoxysilylsilanes [–Si–(O–CH 3 ) 3 ] turned out to be an interesting technique for the preparation of silicon‐based polymers, since it is possible to generate a network of siloxane bonds (–Si–O–Si–) of different configurations, generally known as polysilsesquioxanes. [ 44–51 ] In our previous study, [ 49 ] we synthesized a trimethoxysilyl‐based benzoxazine monomer (BA‐3aptms) and investigated the sol–gel and oxazine‐ring opening processes, obtaining a novel polybenzoxazine/polysilsesquioxane nanocomposite. The polysiloxanes were oriented in a nanometer‐size laminar structure of sheets of phenolic crosslinked network sandwiched between layers of polysilsesquioxanes, which gave an excellent balance of mechanical and thermal properties, and results in great interest in the high‐performance materials field.…”

“…One of main the interests of this laminar nanostructure in thermosetting polymers is that it can provide a morphology stabilization and reduce or eliminate micro‐cracking during curing, as well as to improve the impact resistance, since the polybenzoxazines frequently lack of toughness and tend to be very brittle. The present study is an extension of such article, [ 49 ] where the influence of SiO 2 spherical nanoparticles on the properties (laminar morphology, thermal, dynamic‐mechanical, hydrophobicity, and flame retardancy) of the polybenzoxazine/polysilsesquioxane material is investigated with the aim of obtaining valuable materials for flame retardancy applications. Also, nanocomposites from the conventional benzoxazine (BA‐a) and SiO 2 nanoparticles were synthesized for comparative purposes.…”

Tailoring thermal and thermomechanical properties of biobased blends by hybridization of difuran benzoxazine with polysiloxane‐based benzoxazine and epoxy monomers

“…This behavior is related to different siloxane‐bonded networks formed by the sol–gel process 32,33 . In our previous article, the influence of the sol–gel process conditions on the structural morphology and the thermal and mechanical performance of the hybrid benzoxazine resin was studied 34 . The results indicated that the morphology can be tailored by modifying the sol–gel conditions and thereby the thermal and mechanical properties of the final crosslinked materials.…”