Synthesis and thermally induced structural transformation of phthalimide and nitrile-functionalized benzoxazine: toward smart ortho-benzoxazine chemistry for low flammability thermosets

Characteristics of Polybenzoxazine Aerogels as Thermal Insulation and Flame-Retardant Materials

Polybenzoxazine-based aerogels are a unique class of materials that combine the desirable properties of aerogels-such as low density, high porosity, and excellent thermal insulation-with the outstanding characteristics of polybenzoxazines-such as high thermal stability, low water absorption, and superior mechanical strength. Polybenzoxazines are a type of thermosetting polymer derived from benzoxazine monomers. Several features of polybenzoxazines can be retained within the aerogels synthesized through them. The excellent thermal resistance of polybenzoxazines, which can withstand temperatures above 200-300 °C, makes their aerogel able to withstand extreme thermal environments. The inherent structure of polybenzoxazines, rich in aromatic rings and nitrogen and oxygen atoms, imparts flame-retardant property. Their highly crosslinked structure provides excellent resistance to solvents, acids, and bases. Above all, through their molecular design flexibility, their physical, mechanical, and thermal properties can be tubed to suit specific applications. In this review, the synthesis of polybenzoxazine aerogels, including various steps such as monomer synthesis, gel formation, solvent exchange and drying, and finally curing are discussed in detail. The application of these aerogels in thermal insulation and flame-retardant materials is given importance. The challenges and future prospects of further enhancing their properties and expanding their utility are also summarized.

Partially Bio-Based Benzoxazine Monomers Derived from Thymol: Photoluminescent Properties, Polymerization Characteristics, Hydrophobic Coating Investigations, and Anticorrosion Studies

In this work, four thymol-based benzoxazines were synthesized using four primary amines with different chain lengths, namely methylamine, ethylamine, 1-propylamine, and 1-butylamine, which are then named T-m, T-e, T-p, and T-b, respectively. The optical properties of the synthesized thymol-based benzoxazines were examined via the photoluminescent study of their solutions in acetone. The results show that all the prepared benzoxazines emitted blue light with the maximum wavelengths from 425 to 450 nm when irradiated by the excitation wavelengths from 275 to 315 nm. The maximum excitation wavelengths are found to be 275 nm. The polymerization of the thymol-based benzoxazines is triggered by heat treatments with different conditions (160, 180, and 200 °C for 1 h). According to the FTIR results, the heat-curing process introduces a presence of the OH peak, of which intensity increases as the curing temperature increases. Thermal decompositions of thymol-based benzoxazines regarding TGA analyses reveal the enhancement of thermal stability of the benzoxazines with respect to the N-substituent chain length, as significantly observed the change in the first thermal decomposition at temperature ranged from 253 to 260 °C. Synthesized benzoxazine derivatives are further employed to coat the substrate, e.g., the glass slides. The investigation of the water contact angle shows that the coating of the benzoxazines onto the surface improves the hydrophobicity of the substrate, resulting in the enlargement of the contact angle from 25.5° to 93.3°. Moreover, the anticorrosion performance of the polybenzoxazine coatings is examined using potentiodynamic polarization techniques. The results illustrate the anticorrosion efficiency of the thymol-based polybenzoxazine up to 99.99%. Both hydrophobic and electrochemical studies suggest the feasibility for employing benzoxazines in anticorrosion coating applications.

Microstructural Analysis of Benzoxazine Cationic Ring-Opening Polymerization Pathways

Herein, an evaluation of the initial step of benzoxazine polymerization is presented by mass spectrometry, with a focus on differentiating the phenoxy and phenolic products formed by distinct pathways of the cationic ring opening polymerization (ROP) mechanism of polybenzoxazine formation. The use of infrared multiple photon dissociation (IRMPD) and ion mobility spectrometry (IMS) techniques allows for differentiation of the two pathways and provides valuable insights into the ROP mechanism. The results suggest that type I pathway is favored in the initial stages of the reaction yielding the phenoxy product, while type II product should be observed at later stages when the phenoxy product would interconvert to the most stable type II phenolic product. Overall, the findings presented here provide important information on the initial step of the benzoxazine polymerization, allowing the development of optimal polymerization conditions and represents a way to evaluate other multifunctional polymerization processes.

Advances in Polymeric Neutron Shielding: The Role of Benzoxazine-h-BN Nanocomposites in Nuclear Protection

Given their substantial neutron capture cross-section, extreme hardness, and high chemical and thermal stability, boron-based materials are widely used as building blocks to protect against highly ionizing radiations such as gamma rays and neutrons. Indeed, uncontrolled nuclear radiation exposure can be highly hazardous to radiation workers and the public. In this sense, this work presents an extensive study and experimental evaluation of the nuclear shielding features of hexagonal-boron nitride (h-BN) based nanocomposite, where bisphenol-A based polybenzoxazine (BA-PBz) was used as matrix. The neutron shielding studies were carried out at the nuclear research reactor of Algeria NUR. The surface treatment of h-BN nanoparticles was confirmed by FTIR and XPS techniques. The curing behavior and the degradation phenomena of the nanocomposites were evaluated by DSC-TGA analyses. The distribution of h-BN nanoparticles within the polymer matrix was assessed by TEM and SEM. The results showed that the developed boron nitride-based nanocomposite exhibits intriguing shielding performances and good thermal stability. The DSC-TGA tests exhibit high degradation temperature that reach 279°C. The highest performances were obtained at an h-BN concentration of 7 wt%, where the macroscopic cross was found to be (Σ = 3.844 cm-1) with a screening ratio of (S = 96.12%), equivalent to a mean free path (λ) of 0.138 cm.

The Performance and Synthesis of Alkynyl-Functionalized Benzoxazine Containing Phthalide Side Groups and Cyano Groups with Different Molecular Weights

Benzoxazine resins are widely employed in a variety of applications due to their exceptional heat resistance and treatment properties. However, traditional benzoxazine resins still confront hurdles in today's engineering applications, such as their inability to provide long-term service in high-temperature settings and their inadequate toughness. In this study, four alkyne-functionalized benzoxazines with phthalide side groups and cyano groups of varying molecular weights were produced. Fourier transform infrared spectroscopy (FT-IR) and hydrogen nuclear magnetic resonance spectroscopy (1H-NMR) were used to characterize the resin structure, and differential scanning calorimetry (DSC) was used to investigate the thermal curing kinetics at different warming rates. The apparent activation energy was 116.9 kJ/mol. In-situ FT-IR was used to investigate the cure mechanism. Dynamic mechanical analysis (DMA) was used to evaluate the gelation time of BOZ series resins at various temperatures, and the curing process was designed by combining the results with DSC. The Tg of the composites made using BOZ-1N21 as the matrix was 336 °C, which was much higher than the Tg of the BP-a resin made with aniline, phenolphthalein, and formaldehyde (Tg = 251 °C). As a result, the resin system is expected to be employed in applications requiring high-temperature resistance and toughness.

Synthesis, characterization and protective efficiency of novel polybenzoxazine precursor as an anticorrosive coating for mild steel

In this study, 2-[(E)-(hexylimino)methyl] phenol (SA-Hex-SF) was synthesized by adding salicylaldehyde (SA) and n-hexylamine (Hex-NH₂), which was subsequently reduced by sodium borohydride to produce 2-[(hexylamino)methyl] phenol (SA-Hex-NH). Finally, the SA-Hex-NH reacted with formaldehyde to give a benzoxazine monomer (SA-Hex-BZ). Then, the monomer was thermally polymerized at 210 °C to produce the poly(SA-Hex-BZ). The chemical composition of SA-Hex-BZ was examined using FT-IR, ¹H, and ¹³C NMR spectroscopy. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and X-ray Diffraction (XRD), respectively, were used to examine the thermal behavior, surface morphology, and crystallinity of the SA-Hex-BZ and its PBZ polymer. Mild steel (MS) was coated by poly(SA-Hex-BZ) which was quickly prepared using spray coating and thermal curing techniques (MS). Finally, the electrochemical tests were used to evaluate the poly(SA-Hex-BZ)-coating on MS as anti-corrosion capabilities. According to this study, the poly(SA-Hex-BZ) coating was hydrophobic, and corrosion efficiency reached 91.7%.

Ambient-Dried, Ultra-high Strength, Low Thermal Conductivity, High Char Residual Rate F-type Polybenzoxazine Aerogel

The effect of the curing temperature (T _c) on the properties of PBO aerogel was investigated in this paper. The compressive strength of PBO aerogel prepared was much higher than that of PBO aerogel of the same density in other kinds of literature. With the robust F-type polybenzoxazine (PBO) aerogels with ultra-high Young's modulus (733.7 MPa at 0.48 g/cm³ and 1070 MPa at 0.57 g/cm³), excellent properties were obtained through a facile and scalable room-temperature HCl-catalyzed sol-gel method, followed by the ambient pressure drying technique. It is found that T _c plays a vital role in the polymerization process and the evolution of the microstructure of the 3D porous PBO network, where the necks between the nanoparticles become thick and strong when T _c is up to 150 °C, resulting in a pearl necklace-to-worm transformation in the micro-structure and significant growth in mechanical properties, but if T _c is higher than 180 °C, the pore volume and specific surface area will decrease sharply. Moreover, all synthetic PBO aerogels here possessed inherent flame retardancy and a high residual char rate in the volume density (0.32-0.57 g/cm³). These properties make the F-type PBO aerogels a candidate material in aerospace applications or other fields.

Synthesis of oligomeric phthalonitrile resins containing imide units and study of the methylene-cyano thermal synergistic polymerization effect

The meta- and para-catenated methylene-containing phthalonitrile oligomers were prepared from the reaction of an excess amount of 4,4′-(4,4′-isopropylidenediphenoxy) bis-(phthalic anhydride) (BPADA) with 4,4′-Methylenedianiline (MDA) in a N, N-dimethylformamide/ toluene solvent mixture, followed by end-capping agent with 4-nitrophthalonitrile or 4-phenoxyaniline in a two-step, one-pot reaction. Differential scanning calorimetry (DSC) showed that both PN oligomers exhibited low softening points. The self-catalyzed curing reactivity of the PN oligomers was confirmed by the isothermal rheological measurements. Fourier transform infrared spectroscopy (FTIR) and ultraviolet and visible spectrophotometry (UV–Vis) data of the pre-curing resins were employed to investigate the chemical structure of the pre-cured resins, suggesting that oligomers generated crosslinking sites, including triazine, isoindoline, and phthalocyanine. The results further confirmed the self-catalyzed curing reactivity of the oligomers. Thermal properties were investigated by dynamic mechanical analysis (DMA) and thermal gravimetric analysis (TGA), demonstrating good thermal properties of the cured resins. The glass transition temperatures (Tgs) of PIPN-1-325, PIPN-1-350, PIPN-1-375 were in the range of 285–345°C, the 5% weight loss temperature (T5%) was observed at 482°C. The PIPN-2-325, PIPN-2-350, PIPN-2-375 showed Tgs ranging from 293 to 370°C, and T5% of the resins were in the range of 481–501°C. Then the isothermal rheological results of model compound and PN oligomers implied that the curing process of PN oligomers was closely related to the methylene-cyano radical thermal synergistic polymerization (TSP) effect proposed in our previous research, and then a revised curing mechanism (radical TSP mechanism) was proposed.

Understanding the Adsorption Capacity for CO2 in Reduced Graphene Oxide (rGO) and Modified Ones with Different Heteroatoms in Relation to Surface and Textural Characteristics

Reduced graphene oxide is a material that has a variety of applications, especially in CO2 adsorption. The study of this research is the preparation of reduced graphene oxide with different heteroatoms and how the adsorption capacity is changed. The functionalization with other compounds bearing Si, S, N, and O was before reducing graphene oxide. Different monoliths were prepared by changing the ascorbic acid analogy and the temperature of reduction. The different porosity values, percentages of heteroatoms, and synthetic parameters show that the adsorption capacity is a complex procedure that can be affected by multiple parameters. Microporosity, different functionalities from heteroatoms, and high surface/volume of pores are the significant parameters that affect adsorption. All parameters should establish a balance among all parameters to achieve high adsorption of CO2.

Self-Polymerization Promoting Monomers: In Situ Transformation of Disulfide-Linked Benzoxazines into the Thiazolidine Structure

Polybenzoxazines obtained especially from green synthons are facing challenges of the requirement of high ring-opening polymerization (ROP) temperature of the monomer, thus affecting their exploration at the industrial front. This demands effective structural changes in the monomer itself, to mediate catalyst-free polymerization at a low energy via one-step synthesis protocol. In this regard, monomers based on disulfide-linked bisbenzoxazine were successfully synthesized using cystamine (biobased) and cardanol (agro-waste)/phenol. Reduction of the disulfide bridge in the monomer using dithiothreitol under mild conditions in situ transformed the oxazine ring in the monomer, via neighboring group participation of the -SH group in a transient intermediate monomer, into a thiazolidine structure, which is otherwise difficult to synthesize. Structural transformation of ring-opening followed by the ring-closing intramolecular reaction led to an interconversion of O-CH₂-N containing a six-membered oxazine ring to S-CH₂-N containing a five-membered thiazolidine ring and a phenolic-OH. The structure of the monomer with the oxazine ring and its congener with the thiazolidine ring was characterized by spectroscopic methods and X-ray analysis. Kinetics of structural transformation at a molecular level is studied in detail, and it was found that the reaction proceeded via a transient 2-aminoethanethiol-linked benzoxazine intermediate, as supported by nuclear magnetic resonance spectroscopy and density functional theory studies. The thiazolidine-ring-containing monomer promotes ROP at a substantially low temperature than the reported mono-/bisoxazine monomers due to the dual mode of facilitation of the ROP reaction, both by phenolic-OH and by ring strain. Surprisingly, both the monomer structures led to the formation of a similar polymer structure, as supported by thermogravimetric analysis and Fourier transform infrared study. The current work highlights the benefits of inherent functionalities in naturally sourced feedstocks as biosynthons for the new latest generation of benzoxazine monomers.

Review on the Accelerated and Low-Temperature Polymerization of Benzoxazine Resins: Addition Polymerizable Sustainable Polymers

Due to their outstanding and versatile properties, polybenzoxazines have quickly occupied a great niche of applications. Developing the ability to polymerize benzoxazine resin at lower temperatures than the current capability is essential in taking advantage of these exceptional properties and remains to be most challenging subject in the field. The current review is classified into several parts to achieve this goal. In this review, fundamentals on the synthesis and evolution of structure, which led to classification of PBz in different generations, are discussed. Classifications of PBzs are defined depending on building block as well as how structure is evolved and property obtained. Progress on the utility of biobased feedstocks from various bio-/waste-mass is also discussed and compared, wherever possible. The second part of review discusses the probable polymerization mechanism proposed for the ring-opening reactions. This is complementary to the third section, where the effect of catalysts/initiators has on triggering polymerization at low temperature is discussed extensively. The role of additional functionalities in influencing the temperature of polymerization is also discussed. There has been a shift in paradigm beyond the lowering of ring-opening polymerization (ROP) temperature and other areas of interest, such as adaptation of molecular functionality with simultaneous improvement of properties.

Elucidating the role of acetylene in ortho-phthalimide functional benzoxazines: design, synthesis, and structure–property investigations

Novel ortho-phthalimide-benzoxazines containing acetylene have been designed and their corresponding thermosets exhibit excellent thermal stability although the expected benzoxazole cyclization at a much higher temperature did not take place.

Corrosion Resistance of Mild Steel Coated with Phthalimide-Functionalized Polybenzoxazines

Herein, we synthesized two new phthalimide-functionalized benzoxazine monomers, pPP-BZ and oPP-BZ, through Mannich reactions of 2-(4-hydroxyphenyl)isoindoline-1,3-dione (pPP) and 2-(2-hydroxyphenyl)isoindoline-1,3-dione (oPP), respectively, with p-toluidine and paraformaldehyde. The structures of these two monomers were confirmed using Fourier transform infrared (FTIR) and nuclear magnetic resonance spectroscopy. We used differential scanning calorimetry, FTIR spectroscopy, and thermogravimetric analysis to study the polymerization behavior and thermal stability of the monomers and their corresponding polybenzoxazines. Poly(pPP-BZ) and poly(oPP-BZ) were formed on mild steel (MS) through spin-coating and subsequent thermal curing polymerization. We used various corrosion testing methods to examine the effect of the curing temperature on the corrosion resistance of the coated MS samples in 3.5 wt.% aqueous solution of NaCl. Among our tested systems, the corrosion rate reached a low of 2.78 µm·Y−1 for the MS coated with poly(pPP-BZ)180 (i.e., the coating that had been cured at 180 °C); this value is much lower than that (4.8 µm·Y−1) reported for a maleimide-based benzoxazine compound (MI-Bz)/33 wt.% ACAT (amine-capped aniline trimer) blend. Thus, the incorporation of the imide functional group into the PBZ coatings is an effective strategy for affording high-performance corrosion resistance.

Synthesis of ortho-methyltetrahydrophthalimide functional benzoxazine containing phthalonitrile group: Thermally activated polymerization behaviors and properties of its polymer

Two novel benzoxazine monomers, oHMTI-a and oHMTI-pn, have been obtained via the modified Mannich condensation from ortho-4-methyltetrahydrophalimide functional phenol, paraformaldehyde, and aniline/4-aminophthalonitrile, respectively. The chemical structures of both benzoxazine monomers have been studied by Fourier transform infrared (FT-IR) and 1H and 13C nuclear magnetic resonance (NMR) spectra. Their polymerization processes are investigated using in-situ FT-IR and different scanning calorimetry (DSC). Specifically, the phthalonitrile group in oHMTI-pn can react at a relatively lower temperature without adding any catalysts, which suggests the presence of the thermal synergistic polymerization effect in this benzoxazine monomer. In addition, the thermal and fire related properties of the resulting polybenzoxazines are evaluated by thermogravimetric analysis (TGA) and micro-scale combustion calorimetry (MCC). The polybenzoxazine derived from oHMTI-pn shows both high thermal stability and outstanding flame retardancy, with a T g of 350°C, a T d10 value of 417°C, a high char yield value of 65%, and a very low heat release capacity value of 35.2 J/(g·K).

Design and Effects of the Cinnamic Acids Chemical Structures as Organocatalyst on the Polymerization of Benzoxazines

This study focuses on the catalytic effect of the two geometric isomers of a cinnamic acid derivative, E and Z-forms of 3-methoxycinnamic acid (3OMeCA), analyzing the influence of their chemical structures. E and Z-3OMeCA isomers show very good catalytic effect in the polymerization of benzoxazines, decreasing by 40 and 55 °C, respectively, the polymerization temperatures, for catalyst contents of up to 10% w/w. Isothermal polymerizations show that polymerizations are easily realized and analyzed at temperatures as low as 130 °C and at much shorter times using Z-3OMeCA instead of E-3OMeCA. Thus, both cinnamic acids are good catalysts, with Z-3OMeCA being better. The molecular reasons for this difference and mechanistic implications in benzoxazine polymerizations are also presented.

An apigenin-based bio-benzoxazine with three polymerizable functionalities: sustainable synthesis, thermal latent polymerization, and excellent thermal properties of its thermosets

An apigenin-based benzoxazine exhibiting a thermal latent polymerization behavior, high thermal stability and low flammability has been synthesized from sustainable resources.