The influence of acid-treated multi-walled carbon nanotubes on the surface morphology and thermal properties of alanine-based poly(amide–imide)/MWCNT nanocomposites system

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.

Development of an Atomic-Oxygen-Erosion-Resistant, Alumina-Fiber-Reinforced, Fluorinated Polybenzoxazine Composite for Low-Earth Orbital Applications

An atomic-oxygen-erosion-resistant fluorinated benzoxazine resin and composite were developed. The benzoxazine resin, abbreviated as “BAF-oda-fu,” consists of four benzoxazine rings, and was synthesized from bisphenol AF (BAF), 4,4′-oxydianiline (oda), furfurylamine (fu), and paraformaldehyde. The resin was characterized by infrared spectroscopy (FT-IR), proton nuclear magnetic resonance spectroscopy (1H NMR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). An analysis of the solvent-washed product showed a technical grade purity (>95%) and a yield of approximately 85%. Subsequent polymerization of the resin was successfully performed by heating step-wise and opening the benzoxazine rings to form a crosslinked network. Thermal analyses showed a melting temperature of 115 °C and polymerization temperature of 238 °C, both being characteristic values of benzoxazine monomers. The benzoxazine resin was also blended with polyoctahedral sisesquoxane (POSS) and reinforced with alumina fibers. The Tg of the resin, as determined by DMA of the composite, could reach as high as 308 °C when post-curing and the POSS additive were utilized. The low-Earth orbit atomic-oxygen erosion rate was simulated by an RF plasma asher/etcher. The atomic-oxygen resistance of poly(BAF-oda-fu) fell along an established trend line based on its fluorine content.

The synthesis and properties of isocyanate-based polyimide foam composites containing MWCNTs of various contents and diameters

Polyimide foams (PIFs) were synthesized using in situ polymerization from poly((phenyl isocyanate)-co-formaldehyde) (PAPI), pyromellitic dianhydride (PMDA), and multi-walled carbon nanotubes (MWCNTs) (0.05, 0.1, 0.2, 0.4, and 0.6 wt%) functionalized with –OH; the diameters were 10–20 nm, 20–30 nm, and >50 nm. The morphology, mechanical properties, and flame retardancy of the composites made from MWCNTs with different contents and diameters were studied. The effects of different contents of MWCNT on the properties of composites were compared. SEM results show that the pore morphology of PIF was not damaged when the content of the MWCNTs was low due to crosslinking between MWCNTs and amide bonds. When the content of the MWCNTs was high, the vacuoles of PIF became large and uneven. Compared to pure PIF, mwCNT-1 (0.2% MWCNT content) significantly increased the compressive strength (330%) and compression modulus (210%) of PI. Due to the significant thermal stability of PIF/MWCNTs, the degradation temperature of PIF/MWCNT-1 (0.2% MWCNT content) was increased from 302 °C to 321.5 °C upon addition of MWCNTs. The effects of different diameters of MWCNTs on the morphology and properties of the PIF/MWCNT composites were also compared. The morphology, thermal stability, and mechanical properties of the composites containing smaller MWCNTs were higher than those of composites containing larger MWCNTs. This is because MWCNTs act as nucleating agents to promote the formation and growth of bubbles. Smaller diameters of MWCNTs lead to higher MWCNT contents in the unit volume and more nucleation points of MWCNTs in the PIF. An increasing MWCNT diameter leads to a gradually decreasing number of bubbling nucleation centers. The LOI of PIF/MWCNTS increased with increasing MWCNT due to the nitrogen heterocyclic interaction between the PIF and MWCNTS. The diameter of MWCNTS had only a minor effect on the flame retardancy.

PIFs were synthesized using in situ polymerization from PAPI, PMDA, and MWCNTs (0.05, 0.1, 0.2, 0.4, and 0.6 wt%) functionalized with –OH; the diameters were 10–20 nm, 20–30 nm, and >50 nm.

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.

A naringenin-based benzoxazine with an intramolecular hydrogen bond as both a thermal latent polymerization additive and property modifier for epoxy resins

Epoxy resins are constantly attracting attention from industrial applications due to their excellent comprehensive properties. However, the traditional curing agents for liquid epoxy resins react with epoxides even at room temperature, which causes difficulties in processing since such mixtures cannot be directly used as single-component materials. In order to improve the shelf life of the mixtures, we have designed an intramolecular hydrogen bond-containing benzoxazine monomer as a smart thermal latent polymerization agent for epoxy resins. The newly obtained benzoxazine, NAR-a, has been synthesized from the Mannich condensation of naringenin, aniline and paraformaldehyde. In addition to playing the role of a curing agent, NAR-a has also performed as an excellent property modifier for epoxy thermosetting systems. The resulting thermoset based on the NAR-a/epoxy thermosetting system exhibits high thermal stability and good intrinsic flame retardance, with a T _g temperature of 201 °C, a T _d5 temperature of 349 °C, a low CET value (32.4 ppm per °C) and low heat release capacity (HRC of 159.1 J g^-1 K^-1). The combined long-term storage stability and versatility of the intramolecular hydrogen bond-containing benzoxazine/epoxy system provide a new strategy for the development of one-component epoxy-related thermosetting resins for application in high-performance areas.

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.

Optimization of a reduced enzymatic reaction cascade for the production of L-alanine

Cell-free enzymatic reaction cascades combine the advantages of well-established in vitro biocatalysis with the power of multi-step in vivo pathways. The absence of a regulatory cell environment enables direct process control including methods for facile bottleneck identification and process optimization. Within this work, we developed a reduced, enzymatic reaction cascade for the direct production of L-alanine from D-glucose and ammonium sulfate. An efficient, activity based enzyme selection is demonstrated for the two branches of the cascade. The resulting redox neutral cascade is composed of a glucose dehydrogenase, two dihydroxyacid dehydratases, a keto-deoxy-aldolase, an aldehyde dehydrogenase and an L-alanine dehydrogenase. This artificial combination of purified biocatalysts eliminates the need for phosphorylation and only requires NAD as cofactor. We provide insight into in detail optimization of the process parameters applying a fluorescamine based L-alanine quantification assay. An optimized enzyme ratio and the necessary enzyme load were identified and together with the optimal concentrations of cofactor (NAD), ammonium and buffer yields of >95% for the main branch and of 8% for the side branch were achieved.

Effects of End-Caps on the Atropisomerization, Polymerization, and the Thermal Properties of ortho-Imide Functional Benzoxazines

A new type of atropisomerism has recently been discovered in 1,3-benzoxazines, where the intramolecular repulsion between negatively charged oxygen atoms on the imide and the oxazine ring hinders the rotation about the C⁻N bond. The imide group offers a high degree of flexibility for functionalization, allowing a variety of functional groups to be attached, and producing different types of end-caps. In this work, the effects of end-caps on the atropisomerism, thermally activated polymerization of ortho-imide functional benzoxazines, and the associated properties of polybenzoxazines have been systematically investigated. Several end-caps, with different electronic characteristics and rigidities, were designed. ¹H and 13C nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) calculations were employed to obtain structural information, and differential scanning calorimetry (DSC) and in situ Fourier transform infrared (FT-IR) spectroscopy were also performed to study the thermally activated polymerization process of benzoxazine monomers. We demonstrated that the atropisomerization can be switched on/off by the manipulation of the steric structure of the end-caps, and polymerization behaviors can be well-controlled by the electronic properties of the end-caps. Moreover, a trade-off effect were found between the thermal properties and the rigidity of the end-caps in polybenzoxazines.

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

A novel ortho-phthalimide-functionalized benzoxazine monomer containing an ortho-nitrile group has been synthesized in order to further systematically evaluate the thermally induced structural transformation from benzoxazine resin to cross-linked polybenzoxazole. The chemical structure of the synthesized monomer has been confirmed by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. Also supporting the detailed structure is ¹H–¹³C heteronuclear multiple quantum coherence (HMQC), which identifies the local proton-carbon proximities. The polymerization behaviors, including the ring-opening polymerization of the oxazine rings and the cyclotrimerization of the nitrile functionalities, are studied by differential scanning calorimetry (DSC) and in situ FT-IR. In addition, the subsequent benzoxazole formation after polymerization has also been analyzed using thermogravimetric analysis (TGA) and magic-angle spinning (MAS) solid-state ¹³C NMR. The resulting cross-linked polybenzoxazole derived from the benzoxazine monomer exhibits exceptionally high thermal stability and low flammability, with an extremely high T_d5 temperature (550 °C), a high char yield value (70%) and an extraordinarily low total heat release (THR of 7.6 kJ g⁻¹).

A novel ortho-phthalimide-functionalized benzoxazine monomer containing an ortho-nitrile group has been synthesized in order to develop high-performance thermosets with high thermal stability and low flammability.