Thermally assisted self-healing behavior of anhydride modified polybenzoxazines based on transesterification

Deciphering the Self-Catalytic Mechanisms of Polymerization and Transesterification in Polybenzoxazine Vitrimers

The use of internal catalysts has emerged as a pivotal design principle to facilitate dynamic exchanges within covalent adaptable networks (CANs). Polybenzoxazines, specifically, have shown considerable potential in generating vitrimers through thermally induced transesterification reactions catalyzed internally by tertiary amines. This study aims to investigate the chemical complexities of transesterification reactions within benzoxazine vitrimers. To achieve this, model molecules using various phenolic acids and amino-alcohol derivatives were synthesized as precursors. The structure of these model molecules was fully elucidated by using nuclear magnetic resonance (NMR). Differential scanning calorimetry (DSC) and rheology experiments evidenced the accelerated network formation of the precursors due to the presence of aliphatic -OH groups. Thermogravimetric analysis coupled with microcomputed gas chromatography (TGA-μGC) was used to provide evidence of transesterification reactions. The results showed that the spatial proximity between tertiary amine and hydroxyl groups significantly enhances the rate exchange, attributed to a neighboring group participation (NGP) effect. Interestingly, kinetic experiments using complementary NMR techniques revealed the thermal latency of the tertiary amine of benzoxazine toward transesterification reactions as its opening is needed to trigger the dynamic exchange. The study highlights the crucial role of steric hindrance and tertiary amine basicity in promoting the dynamic exchange in an internally catalyzed system.

Application of Shape Memory and Self-Healable Polymers/Composites in the Biomedical Field: A Review

Shape memory-assisted self-healing polymers have drawn attention over the past few years owing to their interdisciplinary and wide range of applications. Self-healing and shape memory are two approaches used to improve the applicability of polymers in the biomedical field. Combining both these approaches in a polymer composite opens new possibilities for its use in biomedical applications, such as the "close then heal" concept, which uses the shape memory capabilities of polymers to bring injured sections together to promote autonomous healing. This review focuses on using shape memory-assisted self-healing approaches along with their respective affecting factors for biomedical applications such as tissue engineering, drug delivery, biomaterial-inks, and 4D printed scaffolds, soft actuators, wearable electronics, etc. In addition, quantification of self-healing and shape memory efficiency is also discussed. The challenges and prospects of these polymers for biomedical applications have been summarized.

Vitrimers trigger covalent bonded bio-silica fused composite materials for recycling, reshaping, and self-healing applications

In this work, a recycling, reshaping, and self-healing strategy was followed for polybenzoxazine through S-S bond cleavage reformation in vitrimers, and the supramolecular interactions are described. The E-ap benzoxazine monomer was synthesized through the Mannich condensation reaction using a renewable eugenol, 3-amino-1-propanol and paraformaldehyde. Furthermore, the E-3ap monomer was reinforced with various weight percentages (5, 10, and 15 wt%) of the thiol-ene group. Various weight percentages of functionalized bio-silica (BS) were also copolymerized with E-3ap (10%-SH) to increase the thermal stability. The structure of the monomers was confirmed by NMR and FT-IR analysis and the thermal properties of the cured materials were analyzed by DSC and TGA. Tensile test was used to study the mechanical property of the poly(E-3ap-co-SH)/BS material. The film was characterized by SEM and optical microscopy to investigate the self-healing properties of the poly(E-3ap-co-thiol-ene)/BS. Moreover, photos and video clips show the self-healing ability of a test specimen. The vitrimer-based renewable polybenzoxazine material exhibits a good recycling, reshaping, and self-healing abilities, and thus is a prime candidate for several industrial and engineering applications.

Reprocessable Polybenzoxazine Thermosets with High Tgs and Mechanical Strength Retentions Using Boronic Ester Bonds as Crosslinkages

In order to obtain reprocessable polybenzoxazine thermosets with high heat resistance and mechanical strength retentions, network structures without irreversible parts were constructed via crosslinking benzoxazine oligomers using boronic ester cross-linkers. Firstly, the benzoxazine monomer containing carbon–carbon double bonds was synthesized via the Mannich reaction. After thermal ring-opening polymerization, the benzoxazine oligomer containing carbon–carbon double bonds (OBZ) was yielded. Through the thiol-ene click reaction of the OBZ and dithiol cross-linker bearing boronic ester bonds, the polybenzoxazine thermosets using boronic ester bonds as crosslinkages (OBZ-BDB) were successfully synthesized. The structures of OBZ and OBZ-BDB were characterized by SEC, ¹H NMR, and FT-IR measurements. Reprocessing experiments showed that OBZ-BDB has remarkable reprocessability. The retention rates of the tensile strengths through three generations of reprocessing were 98%, 95%, and 84%, respectively. Meanwhile, OBZ-BDB cross-linked by boronic ester bonds had brilliant thermal properties. The T_g of the original OBZ-BDB was 224 °C. With the increase of the reprocessing generations, the T_gs basically remained unchanged.

Exploiting the Reversible Covalent Bonding of Boronic Acids for Self-Healing/Recycling of Main-Chain Polybenzoxazines

In this work, a new strategy for the synthesis of self-healable/recyable polybenzoxazine networks under mild conditions, by exploiting dynamic B–O bond exchanges is presented. The process is based on mixing...

Eco-Friendly Sustainable Poly(benzoxazine-co-urethane) with Room-Temperature-Assisted Self-Healing Based on Supramolecular Interactions

This work is an attempt to develop bio-based eco-friendly poly(benzoxazine-co-urethane) [poly(U-co-CDL-aee)] materials using cardanol-based benzoxazines (CDL) and hexamethylene diisocyanate (HMDI) to check their self-healing ability and thermal properties. CDL monomers were synthesized using cardanol, amino ethoxyethanol (aee) or 3-aminopropanol (3-ap), and paraformaldehyde through the Mannich reaction. Later, CDL-aee or CDL-3-ap monomers were copolymerized with a urethane precursor (HMDI), followed by ring-opening polymerization through thermal curing. The thermal properties of poly(U-co-CDL) were evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The self-healing behavior of the bio-based poly(U-co-CDL) was checked by applying a mild external pressure. The results revealed that the developed poly(U-co-CDL) showed repeatable self-healing ability due to supramolecular hydrogen-bonding interactions. Further, the self-healing ability of poly(U-co-CDL) was studied using density functional theory (DFT). From the above results, the developed material with superior self-healing ability can be used in the form of self-healing coatings and composites for various applications with extended shelf-life and reliability.

Dual-Layer Approach toward Self-Healing and Self-Cleaning Polyurethane Thermosets

There is an urgent need for coatings that exhibit both self-healing as well as self-cleaning properties as they can be used for a wide range of applications. Herein we report a novel approach toward fabricating polyurethane thermosets possessing both self-cleaning and self-healing properties. The desired coating was achieved via casting a bottom layer of self-healable polyurethanes comprised of reversible phenolic urethane bonds followed by a subsequent dip-coating of the prepared layer in a solution of bis(3-aminopropyl)-terminated polydimethylsiloxane (PDMS-NH2). The PDMS was used to impart self-cleaning properties to the coating. While the self-healing behavior of the bottom polyurethane layer is achieved through phenolic urethane chemistry, via the exchange of phenolic urethane moieties. The prepared coatings were tested for their optical, mechanical, self-healing, and self-cleaning properties using a variety of characterization methods, which confirmed the successful fabrication of novel self-cleaning and self-healing clear urethane coatings.

Unexpected Healability of an ortho-Blocked Polybenzoxazine Resin

Ring-opening polymerization of bifunctional benzoxazine has long been thought to produce a permanent network structure without reprocessing ability. Here, we demonstrate that surprising healability can be achieved by a controlled polymerization of an ortho-blocked bifunctional benzoxazine poly(oC-hda). The cured resin possesses a cross-linked structure, but can be deformed, remolded from crushed pieces or healed from mechanical damage. Based on a series of intensive experiments, we show that the healability can be explained by a dynamic bonding exchange mechanism between the phenoxy structures existing during the curing process. Moreover, we verify the possibility to heal the fatigue damaged poly(oC-hda) based composite to extend its service life. Our study provides another dynamic covalent bond to synthesize healable polymers, offering a broad platform for combining healability and desired thermosetting features together.

A High-Lignin-Content, Removable, and Glycol-Assisted Repairable Coating Based on Dynamic Covalent Bonds

Conventional thermoset coatings cannot be easily repaired and removed owing to their highly crosslinked structure. The investigation of repairable or removable coatings has been receiving extensive attention, but few reported coatings possess both features. In this work, a repairable and removable coating was developed through the curing of a modified Kraft lignin (L-COOH) with poly(ethylene glycol) diglycidyl ether (PEG-epoxy) in the presence of zinc catalyst. The L-COOH was prepared by functionalization of Kraft lignin with carboxylic acid groups. The cured material had a high lignin content (>47 wt %). At elevated temperatures (>140 °C), dynamic transesterification in the cured network was activated, which resulted in fast stress relaxation and imparted excellent repairability. If the vitrimer system was used as a coating for tin plates, it provided adequate hardness and adhesion properties. In addition, the lignin-PEG coating could be easily removed from the tin plate by using a mild (0.01-0.1 m) NaOH aqueous solution owing to the unique swelling ability of the coating in alkaline aqueous solution. With the assistance of ethylene glycol, this coating could achieve stress-free repairability in 15 min. This work demonstrates the first lignin-based repair- and removable epoxy coating based on vitrimer chemistry.

Combining polybenzoxazines and polybutadienes via simultaneous inverse and direct vulcanization for flexible and recyclable thermosets by polysulfide dynamic bonding

Simultaneous inverse and direct vulcanization of a benzoxazine and a polybutadiene gave recyclable films through dynamic S–S bonding.