Formaldehyde-Free Synthesis of Fully Bio-Based Multifunctional Bisbenzoxazine Resins from Natural Renewable Starting Materials

Bio-Based Thermosetting Resins: From Molecular Engineering to Intrinsically Multifunctional Customization

Recent years have witnessed a growing interest in bio-based thermosetting resins in terms of environmental concerns and the desire for sustainable industrial practices. Beyond sustainability, utilizing the structural diversity of renewable feedstock to craft bio-based thermosets with customized functionalities is very worthy of expectation. There exist many bio-based compounds with inherently unique chemical structures and functions, some of which are even difficult to synthesize artificially. Over the past decade, great efforts are devoted to discovering/designing functional properties of bio-based thermosets, and notable progress have been made in antibacterial, antifouling, flame retardancy, serving as carbon precursors, and stimuli responsiveness, among others, largely expanding their application potential and future prospects. In this review, recent advances in the field of functional bio-based thermosets are presented, with a particular focus on molecular structures and design strategies for discovering functional properties. Examples are highlighted wherein functionalities are facilitated by the inherent structures of bio-based feedstock. Perspectives on issues regarding further advances in this field are proposed at the end.

Robust Nitrogen-Doped Microporous Carbon via Crown Ether-Functionalized Benzoxazine-Linked Porous Organic Polymers for Enhanced CO2 Adsorption and Supercapacitor Applications

Nitrogen-doped carbon materials, characterized by abundant microporous and nitrogen functionalities, exhibit significant potential for carbon dioxide capture and supercapacitors. In this study, a class of porous organic polymer (POP) were successfully synthesized by linking Cr-TPA-4BZ-Br4 and tetraethynylpyrene (Py-T). The model benzoxazine monomers of Cr-TPA-4BZ and Cr-TPA-4BZ-Br4 were synthesized using the traditional three-step method [involving CH═N formation, reduction by NaBH4, and Mannich condensation]. Subsequently, the Sonogashira coupling reaction connected the Cr-TPA-4BZ-Br4 and Py-T monomers, forming Cr-TPA-4BZ-Py-POP. The successful synthesis of Cr-TPA-4BZ-Br4 and Cr-TPA-4BZ-Py-POP was confirmed through various analytical techniques. After verifying the successful synthesis of Cr-TPA-4BZ-Py-POP, carbonization and KOH activation procedures were conducted. These crucial steps led to the formation of poly(Cr-TPA-4BZ-Py-POP)-800, a carbon material with a structure akin to graphite. In practical applications, poly(Cr-TPA-4BZ-Py-POP)-800 exhibited a noteworthy CO2 adsorption capacity of 4.4 mmol/g, along with specific capacitance values of 397.2 and 159.2 F g-1 at 0.5 A g-1 (measured in a three-electrode cell) and 1 A g-1 (measured in a symmetric coin cell), respectively. These exceptional dual capabilities stem from the optimal ratio of heteroatom doping. The outstanding performance of poly(Cr-TPA-4BZ-Py-POP)-800 microporous carbon holds significant promise for addressing contemporary energy and environmental challenges, making substantial contributions to both sectors.

Immobilization of laccase on magnetic nanoparticles for enhanced polymerization of phenols

Laccase is an efficient biocatalyst for oxidative polymerization of organic substrates. However, cost of enzyme preparation, low stability and residual protein diminish the efficiency of laccase mediated polymerization. In this work, a series of silicon dioxide coated ferroferric oxide magnetic nanoparticles were modified by different functional groups including γ-methacryloxypropyltrimethoxy, succinic anhydride, glutaraldehyde and polyethylene imine. Infrared spectra indicated the magnetic carriers have been successfully modified. Vibrating sample magnetometer (VSM) analysis revealed that all of these carriers showed high magnetic responsiveness after the surface functionalization. Laccase from Cerrena sp. HYB07 was then respectively immobilized covalently on these functionalized magnetic carriers. All the immobilized laccases displayed higher thermostability than free laccase and glutaraldehyde functionalized support (named FSNG) immobilized laccase showed better performance. These immobilized laccases all showed higher efficiency than free laccase for oxidative polymerization of catechol and hydroquinone. The immobilized laccases could be separated from the water insoluble polymerization products. The polymerization product of hydroquinone by FSNG immobilized laccase showed the average polymerization degree of the poly(hydroquinone) was six (DP=6). This work provided a comprehensive exploration of laccase immobilization on magnetic carrier for catalyzing polymerization of phenols.

High-performance polyimine vitrimers from an aromatic bio-based scaffold

Bio-based vitrimers represent a promising class of thermosetting polymer materials, pairing the recyclability of dynamic covalent networks with the renewability of non-fossil fuel feedstocks. Vanillin, a low-cost lignin derivative, enables facile construction of polyimine networks marked by rapid exchange and sensitivity to acid-catalyzed hydrolysis. Furthermore, the aromatic structure makes it a promising candidate for the design of highly aromatic networks capable of high-performance thermal and dimensional stability. Such properties are paramount in polymeric thermal protection systems. Here, we report on the fabrication of polyimine networks with particularly high aromatic content from a novel trifunctional vanillin monomer prepared from the nucleophilic aromatic substitution of perfluoropyridine (PFP) on a multi-gram scale (>20 g) in high yield (86%). The trifunctional aromatic scaffold was then crosslinked with various diamines to demonstrate tunable viscoelastic behavior and thermal properties, with glass transition temperatures (Tg) ranging from 9 to 147 °C, degradation temperatures (5% mass loss) up to approximately 370 °C, and excellent char yields up to 68% at 650 °C under nitrogen. Moreover, the vitrimers displayed mechanical reprocessability over five destruction/healing cycles and rapid chemical recyclability following acidic hydrolysis at mild temperatures. Our findings indicate that vitrimers possessing tunable properties and high-performance thermomechanical behavior can be easily constructed from vanillin and electrophilic aromatic scaffolds for applications in heat-shielding materials and ablative coatings.

Construction novel polybenzoxazine coatings exhibiting corrosion protection of mild steel at different concentrations in a seawater solution

In this work, a new and effective polymeric coating is used to improve mild steel's corrosion resistance. The coating incorporates a Schiff base moiety into a benzoxazine (BZ) precursor, resulting in improved protection against corrosion. The SF-Tol-BZ polymerization behavior and thermal properties were studied using differential scanning calorimetry (DSC) and thermalgravimetric analysis (TGA), respectively, at different curing temperatures. The poly(SF-Tol-BZ) cured at 240 °C had a Td10 value of 604 °C and a Tg of 225 °C. The efficacy of poly(SF-Tol-BZ) coatings in protecting mild steel (MS) from corrosion in a NaCl (3.5%) solution at room temperature was evaluated using various corrosion measurements, including open circuit potential (OCP), and electrochemical impedance spectroscopy (EIS). The results showed that increasing the poly(SF-Tol-BZ) concentration led to a corresponding increase in its protective efficiency, reaching a maximum of 92% at a concentration of 300 g/L. The coatings also exhibited a 24-fold increase in Rct values and a one-order-of-magnitude reduction in CPE compared to the bare mild steel. Finally, the poly(SF-Tol-BZ) precursors demonstrated a CO2 uptake of 23 mg g-1 (measured at 298 K).

Benzoxazine-grafted-chitosan biopolymer films with inherent disulfide linkage: Antimicrobial properties

Synthesis and fabrication of naturally sourced biopolymers, especially chitosan, grafted with renewable small molecules have recently attracted attention as efficient antimicrobial agents and are highly desired for sustainable material development. Advantageous inherent functionalities in biobased benzoxazine extend the possibility of crosslinking with chitosan which holds immense potential. Herein, a low-temperature, greener facile methodology is adopted for the covalent confinement of benzoxazine monomers bearing aldehyde and disulfide linkages within chitosan to form benzoxazine-grafted-chitosan copolymer films. The association of benzoxazine as Schiff base, hydrogen bonding, and ring-opened structures enabled the exfoliation of chitosan galleries, and such host-guest mediated interactions demonstrated outstanding properties like hydrophobicity, good thermal, and solution stability due to the synergistic effects. Furthermore, the structures empowered excellent bactericidal properties against both E. coli and S. aureus as investigated by GSH loss, live/dead fluorescence microscopy, and morphological alteration on the cell surface by SEM. The work provides the benefits of disulfide-linked benzoxazines on chitosan, offering a promising avenue for general and eco-friendly usage in wound-healing and packaging material.

Synthesis of Daidzein and Thiophene Containing Benzoxazine Resin and Its Thermoset and Carbon Material

In this work, a novel bio-based high-performance bisbenzoxazine resin was synthesized from daidzein, 2-thiophenemethylamine and paraformaldehyde. The chemical structure was confirmed using nuclear magnetic resonance spectroscopy (NMR) and Fourier-transform infrared spectroscopy (FT-IR). The polymerization process was systematically studied using differential scanning calorimetry (DSC) and in situ FT-IR spectra. It can be polymerized through multiple polymerization behaviors under the synergistic reaction of thiophene rings with benzopyrone rather than a single polymerization mechanism of traditional benzoxazines, as reported. In addition, thermogravimetric analysis (TGA) and a microscale combustion calorimeter (MCC) were used to study the thermal stability and flame retardancy of the resulting polybenzoxazine. The thermosetting material showed a high carbon residue rate of 62.8% and a low heat release capacity (HRC) value of 33 J/gK without adding any flame retardants. Based on its outstanding capability of carbon formation, this newly obtained benzoxazine resin was carbonized and activated to obtain a porous carbon material doped with both sulfur and nitrogen. The CO₂ absorption of the carbon material at 0 °C and 25 °C at 1 bar was 3.64 mmol/g and 3.26 mmol/g, respectively. The above excellent comprehensive properties prove its potential applications in many advanced fields.

Bioinspired Natural Magnolol-Based Adhesive with Strong Adhesion and Antibacterial Properties for Application in Wet and Dry Environments

The development of environmentally friendly, green, and nontoxic adhesives with excellent dry and wet adhesion properties is of great attraction. In nature, barnacles and mussels exhibit strong adhesion by secreting a hydroxyl-rich dopa. Inspired by their adhesion mechanism, a simple biobased MAG-PETMP (MP) adhesive was prepared from magnolol (MAG) and pentaerythritol tetra (3-mercaptopropionate) (PETMP) by a thiol-ene click chemistry reaction. MP as an adhesive exhibits high bond strength with other substrates due to hydrogen bonds formed by the abundant hydroxyl groups at the interface and shows an inherent thermosetting network structure. Since MP has a thermosetting network, it exhibits excellent thermal stability, solvent resistance, and high mechanical strength, which make the adhesive stable in a humid environment. The cross-linking degree of MP can be easily controlled by adjusting the molar ratio of MAG and PETMP. Among the synthesized samples, the elongation at break of the MP 1 formulation is 174.27%, which makes it promising for use as a flexible adhesive. Moreover, the inherent antibacterial properties of MAG enable MP to exhibit antimicrobial properties and antibacterial adhesion to some extent. This work provides a simple biomimetic strategy that could enable the application of MAG for adhesives.

Low-Temperature Terpolymerizable Benzoxazine Monomer Bearing Norbornene and Furan Groups: Synthesis, Characterization, Polymerization, and Properties of Its Polymer

There is an urgency to produce novel high-performance resins to support the rapid development of the aerospace field and the electronic industry. In the present work, we designed and consequently synthesized a benzoxazine monomer (oHPNI-fa) bearing both norbornene and furan groups through the flexible benzoxazine structural design capability. The molecular structure of oHPNI-fa was verified by the combination characterization of nuclear magnetic resonance spectrum, FT-IR technology, and high-resolution mass spectrum. The thermally activated terpolymerization was monitored by in situ FT-IR as well as differential scanning calorimetry (DSC). Moreover, the low-temperature-curing characteristics of oHPNI-fa have also been revealed and discussed in the current study. Furthermore, the curing kinetics of the oHPNI-fa were investigated by the Kissinger and Ozawa methods. The resulting highly cross-linked thermoset based on oHPNI-fa showed excellent thermal stability as well as flame retardancy (T_d10 of 425 °C, THR of 4.9 KJg^-1). The strategy for molecular design utilized in the current work gives a guide to the development of high-performance resins which can potentially be applied in the aerospace and electronics industries.

Preparation and properties of benzoxazine precursors containing siloxane units and their epoxy copolymers

Four siloxane benzoxazines containing different rigid segments were successfully synthesized and characterized herein, including a benzene ring, a biphenyl, a naphthalene ring, and a diphenyl sulfone group. Different rigid segments had different effects on polymer properties. The introduction of the naphthalene ring and sulfone group considerably reduced the curing temperature of benzoxazine. Although the benzoxazine with the naphthalene ring exhibited low heat resistance, all the four samples showed a high char yield at 800°C under nitrogen atmosphere. In addition, during copolymerization with AG-80 epoxy, the introduction of epoxy promoted the curing of the benzoxazines containing the naphthalene ring and sulfone group. The heat resistance of all copolymers was considerably improved, especially for the copolymer containing the naphthalene ring, whose 5% thermal weight loss temperature ( Td5) increased from 248°C to 321°C under nitrogen atmosphere. The copolymer containing the biphenyl structure had the highest glass transition temperature, reaching 259.1°C. Copolymerization with epoxy also considerably improved the tensile strength and elongation at break of the copolymers, which were much higher than those of traditional bisphenol A-aniline based benzoxazine (BA-a). Compared with the neat benzoxazine prepared using siloxane and bisphenol A, the developed copolymers also had better tensile properties, and the copolymer containing the sulfone group showed the greatest improvement (from 49 to 69 MPa, from 3.1% to 9.12%).

Study on properties of copolymers based on different types of benzoxazines and branched epoxy resins

Previous studies on linear epoxy (bisphenol A epoxy) resin/benzoxazine composites showed that with the addition of epoxy (EP) resin, the resulting copolymers exhibited an increased glass transition temperature ( Tg) ( Tg reached a maximum value at a specific content), improved flexural strength, lower heat resistance, and reduced tensile strength. Herein, branched EP resin (AG-80)/benzoxazine copolymers featuring novolac (N-box) and siloxane (Si-box) chains were prepared without any external curing agent. In both systems, the EP resin endowed the copolymers with an increased crosslinking density; however, Tg continued to increase with increasing EP content. In addition, the heat resistance of the copolymers gradually enhanced. Different types of benzoxazines have various effects on the properties of copolymers. In terms of mechanical properties, AG-80/N-box copolymers exhibited brittle fracture characteristics; with increasing EP content, the flexural strength of the copolymer decreased while the tensile strength increased. AG-80/Si-box copolymers exhibited ductile fracture characteristics, with gradual increases in flexural and tensile strengths. Furthermore, with increasing EP content, the molecular chain migration ability and network homogeneity of the AG-80/N-box copolymers decreased gradually. Alternatively, in the case of the AG-80/Si-box copolymers, the molecular chain migration ability remained unchanged and network homogeneity improved. Hence, the developed copolymers can be used as resin matrices for the fabrication of advanced composites.