Innovative Hyperbranched Polybenzoxazine-Based Graphene Oxide-Poly(amidoamines) Nanomaterials

The covalent functionalization of graphene oxide (GO) surface with hyperbranched benzoxazine (BZ) structures has been achieved using poly(amidoamine) dendrimers (PAMAM) of different generations. By increasing the PAMAM generation, multiple benzoxazine rings were synthesized decorating the GO layers. The polymerization process and the exfoliation behavior were investigated. The novel BZ-functionalized GO hybrid materials were characterized by a combination of techniques such as FT-IR, XPS, and 1H-NMR for the confirmation of benzoxazine formation onto the GO layer surfaces. Raman and XRD investigation showed that the GO stacking layers are highly disintegrated upon functionalization with hyperbranched benzoxazine monomers, the exfoliation being more probably to occur when lower PAMAM generation (G) is involved for the synthesis of hybrid GO-BZ nanocomposites. The polymerization of BZ rings may occur either between the BZ units from the same dendrimer molecule or between BZ units from different dendrimer molecules, thus influencing the intercalation/exfoliation of GO. DSC data showed that the polymerization temperature strongly depends on the PAMAM generation and a significant decrease of this value occurred for PAMAM of higher generation, the polymerization temperature being reduced with ~10 °C in case of GO-PAMAM(G2)-BZ. Moreover, the nanoindentation measurements showed significant mechanical properties improvement in case of GO-PAMAM(G2)-BZ comparing to GO-PAMAM(G0)-BZ in terms of Young modulus (from 0.536 GPa to 1.418 GPa) and stiffness (from 3617 N/m to 9621 N/m).

Effect of acetylacetone metal salts on curing mechanism and thermal stability of polybenzoxazine

To overcome high water absorption of inorganic metal salts and their poor compatibility with resin, acetylacetone metal salts (M(acac) n) were selected as the catalysts of benzoxazine resin. Their effects on the catalytic activity, structure, and thermal stability of polybenzoxazine had been estimated by dynamic differential scanning calorimetry, in situ Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermal gravimetric analyzer. The results revealed that M(acac) n of iron (Fe3+), cobalt (Co3+ and Co2+), and copper (Cu2+) exhibited high catalytic activity and reduced evidently activation energy, especially acetylacetone iron salt. The addition of M(acac) n was beneficial to the formation of Ph–N–Ph structure, which was easy to form a denser carbon layer during thermal degradation, prevented heat transfer and further decomposition of the resin, and finally led to the increase of carbon residue at high temperature.

Synthesis and thermal properties of phenol- and amine-capped main-chain benzoxazine oligomers with multiple methyl substitutions

A series of main-chain benzoxazine oligomers with different methyl substitutions are successfully synthesized. Chemical structures are analyzed by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, and gel permeation chromatography. Effects of methyl substitutions on chemical shifts of protons in oxazine ring and thermal properties, including glass transition temperature, thermal stability, and char yield, are discussed. The influences of methyl substitutions on different positions are demonstrated: (i) substitution on phenols induces obvious increase in curing temperature while substitution on amine does not show apparent impact; (ii) substitution at different positions results in T g variation, following the sequence of none-substitution > substitution at end-capping > substitution on diamines in main-chain > substitution on bisphenols in main-chain; and (iii) substitution at end-capping would cause apparent deterioration in thermal stability while substitution on diamines in main-chain would benefit thermal stability and char yield. Experimental results and related explanations are provided in detail.

Synthesis and Characterization of Low-Cost Cresol-Based Benzoxazine Resins as Potential Binders in Abrasive Composites

A series of cresol-based benzoxazines were synthesized for potential application as a polymer matrix in abrasive composites. The chemical structures of the obtained benzoxazine resins were investigated in detail using Fourier transform infrared spectroscopy (FTIR) and hydrogen-1 as well as carbon-13 nuclear magnetic resonance spectroscopy (¹H NMR, ¹³C NMR) with an additional analysis using two-dimensional NMR techniques (2D NMR ¹H-¹H COSY, ¹H-¹³C gHSQC and gHMBC). Structural analysis confirmed the presence of vibrations of -O-C-N- at ~950 cm^-1 wavenumber, characteristic for an oxazine ring. The thermal properties of benzoxazine monomers were examined using differential scanning calorimetry (DSC) analysis. The polymerization enthalpy varied from 143.2 J/g to 287.8 J/g. Thermal stability of cresol-based benzoxazines was determined using thermogravimetry (TGA) analysis with additional analysis of the amount of volatile organic compounds (VOC) emitted from the synthesized benzoxazines during their crosslinking by static headspace coupled with gas chromatography technique (HS-GC). The amount of residual mass significantly differed between all synthesized polybenzoxazines in the range from 8.4% to 21.2%. The total VOC emission for benzoxazines decreased by 46-77% in reference to a conventional phenolic binder. The efficiency of abrasive composites with the benzoxazine matrix was evaluated based on abrasion tests. Performed analyses confirmed successful synthesis and proper chemical structure of cresol-based benzoxazines. All the experiments indicated that benzoxazines based on different cresol isomers significantly differ from each other. Good thermal performance and stability of the abrasive composites with the polybenzoxazine matrix and significantly lower VOC emission allow us to state that benzoxazines can be a promising and valuable alternative to the phenolics and a new path for the development of modern, eco-friendly abrasives.

Highly Crosslinked Polybenzoxazines from Monobenzoxazines: The Effect of Meta-Substitution in the Phenol Ring

It is possible to control the crosslink density of polymers derived from monobenzoxazines by switching the type of substituents in the phenolic ring and their relative position with respect to the phenol group. We prepared several substituted monobenzoxazines in the para and meta positions of the phenolic ring and studied how these substituents affected the polymerization temperature of monomers and the thermal stability of the final polymers and, more extensively, how they affected the crosslink network of the final polymers. Gel content and dynamic mechanical analysis confirm that ortho- and para-orienting substituents in the meta position generate highly crosslinked materials compared to para ones. This fact can lead to the design of materials with highly crosslinked networks based on monobenzoxazines, simpler and more versatile monomers than the commercial bisbenzoxazines currently in use.

Cyclo-oligomerization of hydroxyl-containing mono-functional benzoxazines: a mechanism for oligomer formation

The oligomerization of mono-functional benzoxazines is demonstrated with intermolecular cyclization via a Mannich bridge structure as the primary reaction pathway.

Stoichiometric analysis of competing intermolecular hydrogen bonds using infrared spectroscopy

We quantitatively analyze multiple hydrogen bonds in mixtures of two monomers: urethane dimethacrylate (UDMA) and triethylene glycol-divinylbenzyl ether (TEG-DVBE). The carbonyl stretching band in infrared (IR) absorption spectra is deconvoluted into free and hydrogen-bonded carbonyl groups. The amounts of the sub-components are determined for 21 mixture compositions and initially analyzed using a simple stoichiometric model (based on one dominant hydrogen acceptor group per monomer species) for the equilibrium state of hydrogen bond formation. However, our in-depth stoichiometric analysis suggests that at least two UDMA acceptor groups (carbonyl and alkoxy oxygens) and one TEG-DVBE acceptor group (ether oxygen) contribute to intermolecular hydrogen bonding interactions. This finding is further supported by a quantitative analysis of the hydrogen bonding effect on the N–H stretching band. Moreover, the equilibrium constants of these hydrogen bond formations confirm that the inter-association between UDMA and TEG-DVBE is non-negligible in comparison to the UDMA self-associations. Such quantitative information on intermolecular interactions provides insight into the effect of hydrogen bonding on the copolymerization kinetics of these monomer mixtures.

Stoichiometric analysis of infrared spectra from UDMA and TEG-DVBE mixtures provides quantitative information on competing hydrogen bonds and intermolecular interactions in equilibrium.

Polybenzoxazines: Thermal Responsiveness of Hydrogen Bonds and Application as Latent Curing Agents for Thermosetting Resins

This work aims at exploring the application of polybenzoxazines as thermal latent curing agents for epoxy resins. Thorough studies have shown that hydrogen bonds of polybenzoxazines block the reactivity of phenolic hydroxyl at ambient temperatures and break at elevated temperatures to release the free phenolic hydroxyl. On the basis of these findings, polybenzoxazines are used as thermal latent curing agents. Mixtures of polybenzoxazines and epoxy resins exhibit a long shelf life at room temperature, and the corresponding copolymers possess enhanced properties. This novel insight into using polybenzoxazines as thermal latent curing agents for epoxy resins is anticipated to help researchers explore novel latent curing agents and apply polybenzoxazines more widely.