Production of high performance and innovative materials based on polybenzoxazine

Abstract

Nowadays, the rapid advancements in technology and industry encourage the production and development of high-performance, smart and innovative materials. Among these materials, phenolic resins as synthetic thermal-setting resins, exhibit versatile features like mechanical strength, solvent resistance, dimensional stability, chemical resistance and thermal resistance, thanks to their cross-linked structures. Phenolic resins, classified as synthetic thermostat resins, are widely utilized in numerous areas requiring high performance applications such as adhesives, surface and floor coatings, construction, space, robotics, aviation and automotive industry and sports vehicles. These resins have many good qualities, but when strong acids or bases are used as catalysts during synthesis, processing equipment corrodes and volatile are released during curing, which causes structural defects to form. Therefore, in recent years, numerous attempts have been undertaken to create new materials that can overcome the drawbacks of conventional phenolic resins while maintaining their mechanical and physical properties. Following various efforts, a novel kind of phenolic resin called polybenzoxazines (PBzs) has been improved, which can be compared to and even surpass traditional phenolic resins in terms of physical and mechanical properties. 1,3-Benzoxazine (Bz) monomers are subjected to thermally triggered catalytic or non-catalytic cationic ring-opening polymerization (CROP) to produce PBzs. Typically, Bz monomer production requires formaldehyde, a phenolic molecule, and a suitable primary amine. Bzs exhibit molecular design flexibility, enabling the synthesis of a wide range of monomers tailored to various application purposes. Therefore, PBzs are excellent candidates for producing smart and innovative materials with functionalities such as reprocessability, self-healing or recyclability by special design. The supramolecular interactions of PBzs can be used to build self-healing materials, or dynamic covalent bonding sites can be created in PBzs. Many studies have demonstrated that PBzs exhibit properties like shape memory, electrosensitivity, superhydrophobicity and self-healing. In the first chapter of the thesis, a new strategy is presented to build self-healing and recyclable PBz networks in soft conditions by utilising dynamic B-O bond exchanges. In this process, main chain PBz precursors are combined with phenyl boronic acid and then heated to 180°C for forming cross-linked PBz film. The addition of phenylboronic acid to PBz led to the formation of B-O bonds with phenoxy groups as well as B-N bonds with tertiary amines during the CROP of Bz, which prevented PBz from forming a permanently cross-linked structure. Therefore, the presence of dynamic B-O and B-N bonds caused the cross-linked PBz films to exhibit a very low Tg of about -1.5 °C. PBz films were successfully recycled multiple times at a relatively low temperature of around 110 °C and a pressure much lower than 1 MPa, without any extra ingredients to aid in the healing process. Tensile test was used to determine the recovery degree of the films. Besides, PBz films were investigated in detail by spectral characterisation using proton nuclear magnetic resonance (1H NMR) and Fourier transform infrared (FTIR) spectroscopies, rheological behaviour using rheometry, and thermal analysis using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The hydrolytic stability of cross-linked PBzs was investigated in addition to all of these analyses. Boronic ester based polymer compounds have low resistance to water, especially in humid conditions, which limits their use. In contrast to this problem, it was observed that the obtained cross-linked PBz films had much better hydrolytic stability compared to traditional boronic ester-based polymers, thanks to the intramolecular B-N bonds they possess. In the second chapter of the thesis, a simple and effective method is presented for synthesizing self-healing PBz networks using dynamic imine bond exchanges in soft conditions. In this process comprises the combination of polyethyleneimines (PEIs) with an vanillin functionalized bisbenzoxazine utilizing a Sc(OTf)3 catalyst, followed by heating at moderate temperatures (150 °C). The coupling of the amines of PEI with the free carbonyl groups of aldehyde-functionalized bisBz led to dynamic imine bond formation. The PBz films were exhibit self-healing abilities multiple times at wery low temperatures and pressures without any extra ingredients to aid in the healing process. The degree of recovery of the films was examined by tensile tests, and rheological analyzes were also used to calculate the activation energy of dynamic bonding, and to analysis stress relaxation. Additionally, due to the presence of Mannich bonds and inherent hydrophobic structure of PBz, PBz networks exhibited much higher hydrolysis stability than traditional imine-based systems. Consequently, our work effectively demonstrates how imine exchange methods may be used to make PBz networks into recyclable and self-healing polymers. Additionally, the approach is intriguing due to its simplicity. In the final chapter of the thesis, a new cross-linked ethylene-vinyl acetate-PBz (EVA-PBz) system with reprocessable dynamic silyl ether bonds was introduced, exhibiting enhanced toughness, thermal stability, and exceptional film-forming ability using both ethylene-vinyl acetate/ethylene-vinyl alcohol (EVA/EVOH) and Bz chemistry. The readily accessible and reasonably priced EVA copolymer was hydrolyzed to provide the precursor EVOH copolymer. In this context, the transetherification reaction between pendant hydroxyl groups on EVOH copolymers and siloxane monofunctional Bz produced cross-linked EVOH films. The obtained silyl ether-based cross-linked PBz films were analyzed for their spectroscopic properties by 1H NMR and FTIR spectroscopies, and their thermal properties by DSC and TGA, respectively. Additionally, the toughness and self-healing ability of the films were investigated by stress-strain analysis. The results proved that compared with pure EVOH film, the mechanical properties of cross-linked PBz films prepared by adding a small amount of specially designed Bz to EVOH were markedly enhanced. Furthermore, when subjected to pressure and heat, the films exhibited self-healing properties by initiating a silylether metathesis reaction that repaired damaged films. Throughout the process, commercially available EVOH and easily synthesized siloxane-based Bz, both of which are readily accessible and cost-effective materials, were used. Furthermore, the simplicity and efficiency of film formation via melt casting make this approach highly appealing for practical applications.