Mannich Bridge Research Articles

Molecular dynamics simulations of hydrogen-bonded network structures of polybenzoxazines in the gas phase and aqueous solution

The crucial role of the amine functional group at the Mannich bridge of polybenzoxazines (PBZs) has been reported to be responsible for their hydrogen-bonded network structures. However, they have not been thoroughly studied in an aqueous solution and at the atomistic level. In this study, molecular dynamics simulations were applied to investigate the formation of hydrogen bond interactions of PBZs prepared from bisphenol A/methylamine (m-PBZ), bisphenol A/aniline-based (a-PBZ), and bisphenol A/2-(methylamino)ethylamine (e-PBZ). Based on the simulation results, the hydrogen-bonded network structures of the PBZs interfered with water molecules, leading to less compaction of the PBZ structure in the aqueous solution. The hydrogen bonding species of the m-PBZ and a-PBZ structures consisted of the –OH…N (Mannich) and –OH…O intramolecular interactions. However, for e-PBZ, the –OH…O species was not present, but the 2-(ethylamino)ethylamine substituent formed more hydrogen bonding species than those of m-PBZ and a-PBZ. Additionally, the intermolecular hydrogen bond interactions of the PBZs and water molecules were not detected in any of the aqueous solution simulations.

Synthesis and Polymerization of the Bio‐Benzoxazine Derived from Resveratrol and Thiophenemethylamine and Properties of its Polymer

AbstractResveratrol and 2‐thiophenemethylamine have been employed in the synthesis of a novel tri‐functional benzoxazine (RES‐th) to develop the bio‐benzoxazine monomer. The chemical structure of the synthesized monomer is confirmed by various characterization technics. The polymerization behavior is monitored by in situ Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The in situ FTIR results reveal distinct reaction mechanisms for the three oxazine rings presented in RES‐th, with both ether and phenolic Mannich bridge structures observed in the products. The activation energy values of RES‐th are calculated to be 119.05, 120.97, and 119.44 kJ mol−1 by Kissinger, Ozawa, and Starink methods, respectively, which are all based on the heat flow curves at various heating temperatures. The thermal stability and flame retardancy of the resulting polybenzoxazine (poly(RES‐th)) are investigated by thermogravimetric analysis (TGA) and microscale combustion calorimeter (MCC). The values of Td5 and Td10 of polybenzoxazine are found to be 356 °C and 399 °C, respectively, with a char yield of 66.3% at 800 °C. The prepared polybenzoxazine also demonstrates nonflammability characteristics with the values of heat release capacity (HRC) and total heat release rate (THR) of 18.65 J (g K)−1 and 2.69 kJ g−1, respectively. These findings suggest that the thermoset, poly(RES‐th), is a promising candidate for fire‐resistant applications.

Sensor Absorbents for Heavy Metal Ions By Low-Cost Functionalization of Porous Carbons

Porous carbons are key materials in electrodes for electrochemical energy storage as well as in water purification. Methods to modify porous carbons to enhance their properties as charge storing or water purification materials have been explored during the past decades [1,2]. The main application fields if porous carbon, water purification and energy storage, are highly cost sensitive. For this reason, there is a need to develop low-cost methods to modify porous carbons. Here, the use of benzoxazine chemistry to modify porous carbons is explored. Benzoxazine chemistry can be considered a protection group strategy for phenols, where the phenols from which the benzoxazines are synthesized, are deprotected upon benzoxazine polymerization. Further, benzoxazine polymerization produces Mannich bridges located near phenol groups, motifs that can be used to capture heavy metal ions. More specifically, in the first step, the porous carbons are partially impregnated with benzoxazine monomers, from melt or solutions to afford powders impregnated far below the liquid saturation level of the particles. That is, the particles retain porosity after the impregnation but still have a dry appearance. After impregnation, the particles are heated over the polymerization temperature of the benzoxazine to afford the porous carbon with immobilized functionalities. As phenols can initiate benzoxazine polymerization and be incorporated in the polymer, this method offers a simple route to produce polymerized and immobilized structure at a carbon interface. Here, results on metal ion absorption and the electrochemical response after exposure are presented for carbons modified by impregnation-polymerization of benzoxazines with or without incorporation of phenols. Inks were manufactured from the modified carbons. The electrochemical characterizations were performed on printed arrays of electrochemical cells on a substrate having dimensions of well plates and intended for automatized testing and where the carbon inks were deposited on the working electrodes. This methodology has a potential of simplifying the development of sensor materials and absorbents and can be automatized and potentially be supported with machine learning schemes. The electrochemical responses of exposure of the modified carbon electrodes to metal salt solutions show that impregnation-polymerization of functional benzoxazine can produce absorbents responding electrochemically to metal ion absorption with a scalable process starting from low-cost materials.

Construction of three-dimensional porous organic polymers with enhanced CO2 uptake performance via solid-state thermal conversion from tetrahedral benzoxazine-linked precursor

Porous organic polymers (POPs) with three-dimensional (3D) linkages have drawn great interest due to their potential applications in separation, adsorption, and sensing fields. Herein, a new 3D benzoxazine (BZ)-linked porous organic polymer (TPM-BZ-Py POP) was synthesized by linking a tetrahedral benzoxazine (TPM-BZ-Br4) with tetraethynylpyrene (Py-T) via Sonogashira coupling. Specifically, the synthesis of the tetrahedral benzoxazines with four oxazine rings in a single monomer involved the utilization of Schiff base formation, NaBH4 reduction as well as Mannich condensation reactions. To confirm the chemical structure of the benzoxazine monomers and their corresponding polybenzoxazines, Fourier transform infrared (FTIR), proton nuclear magnetic resonance (1H NMR), and carbon nuclear magnetic resonance (13C NMR) spectroscopy techniques were employed. The pore volume and specific surface area of the 3D TPM-BZ-Py POP were determined through N2 adsorption/desorption isotherms, and they were found to be 0.52 cm3/g and 185 m2/g, respectively. In addition, the solid-state chemical transformation occurred during thermal ring-opening polymerization (ROP), and the resulting 3D poly(TPM-BZ-Py) POP displayed higher CO2 capture ability (1.81 mmol/g) compared with TPM-BZ-Py POP (0.60 mmol/g) at room temperature. Such enhancement observed in poly(TPM-BZ-Py) POP was proposed to be caused by the formation of phenolic units and Mannich bridges within the polymer network by taking into consideration the structural transformation during the ring-opening polymerization (ROP). The functionalities in poly(TPM-BZ-Py) have great potential to interact with CO2 molecules through strong intermolecular hydrogen bonding or acid/base interactions, which further supports the benefit of incorporating oxazine rings into POP frameworks.

Melamine modified phthalonitrile resins: Synthesis, polymerization and properties

Phthalonitrile resin, as a thermosetting resin, possesses high chemical resistance, high thermal resistance, high thermo-oxidative stability and good mechanical properties. However, the extremely high curing temperature needed to achieve high curing degree severely limits its large-scale application. It is found that introduction of benzoxazine moities into phthalonitrile would increase the reactivity and decrease the curing temperature. In this work, phthalonitrile resin with melamine moiety (BAph-m) is synthesized by taking advantage of benzoxazine chemistry using melamine as amine source. Its chemical structure is confirmed by Fourier infrared spectroscopy (FTIR), 1H and 13C nuclear magnetic resonance (1H NMR) spectra. Its curing reactivity, polymerization mechanism and the properties of the resultant polymers are systematically studied. It is found that the introduction of melamine reduces the content of oxazine ring, leaving active groups like phenolic hydroxyls and secondary amines as build-in catalysts. Consequently, the modified resins show much higher reactivity. The exothermic peaks corresponding to ROP of benzoxazine ring and curing of phthalonitrile groups of BAph-30 m decreased to 197.4 °C and 278.2 °C, 77.7 °C and 30.1 °C lower than those of neat BAph. The gelation time shortened from 892 s for BAph-2m to 231 s of BAph-30 m. Moreover, it is found that the build-in active groups could catalyze the reaction of benzoxazine moities and phthalonitrile groups to form phenolic Mannich bridge structures, triazine rings, phthalocyanine rings and isoindoline. The modified resins show very high thermal stability with BAph-30m-280 showing initial decomposition temperature (Td5) of 492 °C and char yield (Yc) of 79.9%, which is 35 °C and 6.8% higher than those of BAph-280.

Solid state chemical transformations through ring-opening polymerization of ferrocene-based conjugated microporous polymers in host–guest complexes with benzoxazine-linked cyclodextrin

Herein, we describe two ferrocene-derived conjugated microporous polymers (FFCCMPs) prepared through Sonogashira couplings of 9-ferrocenylidene-2,7-dibromo-9H-fluorene (FFC) with tetraethynylpyrene (Py-T) and tetrakis(4-ethynylphenyl)ethene (TPE-T), respectively, and their properties determined using various spectroscopic techniques. These two FFC CMPs formed inclusion complexes (ICs) with a benzoxazine-linked β-cyclodextrin (CD-BZ), with host-guest interactions occurring between the ferrocene and β-CD units. We used Fourier transform infrared spectroscopy, X-ray diffraction, two-dimensional nuclear Overhauser enhancement NMR spectroscopy, solid-state 13C CP/MAS NMR spectroscopy, and thermogravimetric analysis to characterize these FFCCMP/CD-BZ ICs. The BZ units in the FFCCMP/CD-BZ ICs underwent thermal ring-opening polymerization to form new Mannich bridges featuring both intermolecular (OH···O) and intramolecular (OH···N) hydrogen bonds; the resulting TPE-FFCCMP/poly(CD-BZ) and Py-FFCCMP/poly(CD-BZ) ICs exhibited outstanding thermal stability, with the latter having the higher CO2 uptake ability (1.42 mmol g–1) and capacitance (46 F g–1 at 0.5 A g–1).Background: Host–guest complexes formed from macrocyclic hosts possessing hydrophobic cavities such as cyclodextrins (CDs) and various guest molecules have been studied widely for their fascinating characteristics. The interaction of ferrocene-containing CMPs with hydrophobic cavities of CDs to form inclusion complexes materials with excellent thermal properties and these materials can be used in different fields including energy storage and gas capture.Methods: Two ferrocene-based fluorene CMPs—TPE-FFC CMP and Py-FFCCMP—through Sonogashira couplings of 9-ferrocenylidene-2,7-dibromo-9H-fluorene (FFC), as the main block, with tetraethynylpyrene (Py-T) and tetrakis(4-ethynylphenyl)ethene (TPE-T), were successfully synthesized through Sonogashira couplings. Then, the two FFC CMPs were attached to a benzoxazine-functionalized β-cyclodextrin (CD-BZ) through host-guest complexation. The successful synthesis of two FFC CMPs and FFCCMP/CD-BZ ICs was investigated by using Fourier transform infrared spectroscopy, X-ray diffraction, two-dimensional nuclear Overhauser enhancement NMR spectroscopy, solid-state 13C CP/MAS NMR spectroscopy, and thermogravimetric analysis.Significant Findings: The present work offers a new and facile strategy for the preparation CMPs/CD inclusion complexes through supramolecular chemistry and using these materials as an electrode in the energy devices and CO2 uptake.

Method for producing catalysts having increased strength and decreased volume reduction

A series of four novel diamine-based benzoxazines containing aliphatic spacer groups in their structures were synthesized by the condensation of formaldehyde, phenol and aromatic diester diamine obtained by reduction of their precursor dinitro compounds. The four aromatic diamines synthesized are ethane-1,2-diyl bis (4-aminobenzoate), butane-1,4-diyl bis (4-aminobenzoate), hexane-1,6-diyl bis (4-aminobenzoate), and octane-1,8-diyl bis (4-aminobenzoate). The chemical structures of diamine benzoxazines are confirmed by Fourier-transform infrared (FTIR) and nuclear magnetic resonance spectroscopy (1H and 13C NMR). The curing properties with respect to the basic benzoxazine structure of the monomer were studied by differential scanning calorimetry (DSC). The softening points, onset of cure temperatures and the heat of polymerization have all decreased with increasing spacer length. DSC of polybenzoxazines indicated decrease of glass transition temperature (Tg) with increasing spacer length. The thermal behavior of polybenzoxazines was studied by thermo gravimetric analysis (TGA). Higher Td5 and Td10 values were obtained by polybenzoxazines with longer spacer groups but an inversion in thermal stability was observed after Td10. The derivative thermo gram of polybenzoxazine indicated a two stage degradation process due to chain cleavage at Mannich bridge as well as breakdown of polymer chain respectively. The char yield at 700 °C has decreased with increasing spacer length from 47% to 27%. The mechanical properties of glass cloth coated polybenzoxazines were evaluated with dynamic mechanical analysis (DMA). The storage modulus of the composite laminate of polybenzoxazine has increased with increasing spacer group which could be due to improvement in adhesion between fibers and resin with increasing flexibility of the resin.

Solid-State Chemical Transformations to Enhance Gas Capture in Benzoxazine-Linked Conjugated Microporous Polymers

We describe the first examples of benzoxazine (BZ)-linked tetraphenylethylene (TPE)- and pyrene (Py)-containing conjugated microporous organic polymers (CMPs)—TPE-TPE-BZ and Py-TPE-BZ, respectively—having high surface areas and pore volumes, prepared through multistep syntheses (Schiff base, reduction, Mannich, and Sonogashira–Hagihara coupling), and their solid-state transformations through ring-opening polymerizations (ROPs) to give BZ linkages have never been reported previously. The chemical structures and, particularly, the presence of BZ units in the TPE-TPE-BZ and Py-TPE-BZ CMPs were confirmed through Fourier transform infrared and solid-state nuclear magnetic resonance spectroscopic analyses. The TEP-TPE-BZ CMP had a high BET specific surface area and a high total pore volume (325 m2 g–1 and 0.69 cm3 g–1, respectively). Because BZ units can undergo ROP through simple thermal treatment without the need for a curing agent or catalyst, the CMPs readily underwent one-step chemical transformations in the solid state to form new CMPs featuring phenolic OH and Mannich bridges as functional groups capable of forming strong intermolecular hydrogen bonds and/or acid/base interactions with molecules of CO2 gas to enhance the gas uptake ability.

Implementation of meta-Positioning in Tetrafunctional Benzoxazines: Synthesis, Properties, and Differences in the Polymerized Structure

Implementation of meta-positioning in intermediate tetrafunctional benzoxazines instead of para-positioning was done in an attempt to obtain low curing and weight loss-free benzoxazine monomers before/during ring-opening polymerization. Three different meta-positioned tetrafunctional benzoxazine monomers (mBZ4s) were synthesized by the tandem reaction method using a starting phenol/amine, which has an aminomethylphenol backbone at the meta-position, aromatic diamines such as 4,4′-diaminodiphenyl methane (DDM-D) and m-phenylene diamine (mPDA) or bisphenols such as 4,4′-dihydroxydiphenyl methane (DDM-B), and paraformaldehyde. All mBZ4s showed a low curing temperature (mBZ4-mPDA (209 °C) < mBZ4-DDM-D (219 °C) < mBZ4-DDM-B (226 °C)), no weight loss before/during ROP and good thermal stability for resulting polybenzoxazines (initial degradation temperature at 10% weight loss (Td₁₀), 376–411 °C; char yield at 600 °C, 61–66%). The major structural differences between these monomers are the atoms present at the meta-positioning, N,O in mBZ4-DDM-D, N,N in mBZ4-DDM-B and a combination of m-N,O and m-N,N in mBZ4-mPDA. The ring-opening polymerization in these monomers proceeded via different pathways, intramolecular electrophilic substitution of iminium ion in m-N,O-positioned BZ4s (mBZ4-DDM-D and mBZ4-mPDA) due to presence of main active sites, ortho-position to the oxygen of oxazine ring within the molecule itself, and formed four-membered AZA cyclic rings along with phenolic Mannich bridges in the networked structure, whereas in m-N,N-type positioning, polymerization proceeded intermolecularly and resulted in traditional phenolic Mannich bridges. Solid-state ¹³C NMR analysis of poly(mBZ4)s obtained at 200 and 250 °C revealed the presence of AZA cyclic rings in the cured product of m-N,O-positioned tetrabenzoxazines and their further ring opening to form additional cross-links. The nature of cross-links was determined with the help of conducting a model study using N-phenylbenzazetine, which has a similar four-membered cyclic ring; the second part of this article is devoted to proving those aspects.

Bio-based, main-chain type polybenzoxazine precursor derived from sustainable furfurylamine and salicylaldehyde: Synthesis, characterization and properties

Main-chain type polybenzoxazine precursors (MCPPs) are a current focus in the polybenzoxazine field for their improved char yield, thermal stability and film-forming ability compared to monomeric benzoxazines resins. To improve sustainability, there is an urgent need to utilize renewable feedstocks instead of fossil-based chemicals in the preparation of MCPPs. Here, renewable furfurylamine and salicylaldehyde were used as staring materials to synthesize high biomass content MCPPs through a three-steps polymerization route. The number-average molecular weight of a bio-based MCPP is 6100 g/mol, and it has a polydispersity index of 1.7. The furan-containing MCPP shows two polymerization stages during heating; the main exothermic stage at approximately 250 °C is attributed to the ring-opening polymerization of benzoxazine units, and the secondary stage at higher temperature is due to a crosslinking reaction between furan rings, forming furfurylamine Mannich bridges, and an oxidization-induced coupling reaction. The average activation energy of an MCPP is approximately 173.8 kJ/mol. The 240 °C-cured film is transparent with a glass transition temperature of over 300 °C. The crosslinked film shows a high thermal stability and a char yield of greater than 66% in a nitrogen atmosphere. Scanning electron microscopy (SEM) images indicated that a defect-free film can be made by controlling the curing parameters.

In(NO3)3 catalyzed curing reaction of benzoxazine

Benzoxazine is a new kind of thermoset resin with excellent properties, but it suffers from high curing temperature and low char yield in the presence of catalyst without halogen. In(NO3)3 was herein used for the first time to efficiently catalyze the curing reaction of benzoxazine and to elevate the char yield at 800°C. The reaction of benzoxazine was catalyzed by In(NO3)3 after stirring at 35°C for 300 min, and the initial curing temperature decreased to 151°C. Polybenzoxazine/In(NO3)3 showed higher thermal stability and char yield at 800°C (increased by 7.5%) compared with those of polybenzoxazine. The possible pathway of coordination bonding between In3+ and benzoxazine was proposed. In the cross-linking process, two different structures, that is, the N, O-acetal bridge structure and arylamine Mannich bridge structure formed at 35°C, both existed, which ultimately affected the thermal stability of the cured product.

Cyclo-oligomerization of hydroxyl-containing mono-functional benzoxazines: a mechanism for oligomer formation

The oligomerization of mono-functional benzoxazines is demonstrated with intermolecular cyclization via a Mannich bridge structure as the primary reaction pathway.

Thermal responsiveness of hydrogen bonding and dielectric property of polybenzoxazines with different Mannich bridge structures

Variation of hydrogen bonding (HB) and dielectric properties of polybenzoxazines with different Mannich bridge structures at different temperatures are investigated by in-situ Fourier Transform Infrared (FTIR) Spectrometry, perturbation-correlation moving-window two-dimensional (PCMW2D) infrared spectroscopy and Broad Band Dielectric Spectrometer (BDS), respectively. It is found that in phenolic Mannich polybenzoxazine, –OH•••O HB tends to rearrange into thermally stable –OH•••N HB. In arylamine Mannich polybenzoxazine, –OH•••O HB is favorable at relatively low temperature and will dissociate at higher temperature. The PCMW2D infrared spectroscopy reveals that the massive dissociation temperature of –OH•••O HB occurs is generally higher than 100 °C and varies with different polybenzoxazines. The results from BDS suggest that the thermal responsiveness of dielectric properties is closely related to the HB and the thermal resistance of polybenzoxazines. The six-membered –OH•••N HB is beneficial for high frequency stability and thermal stability of dielectric properties, which makes the phenolic Mannich polybenzoxazine promising candidate as high-temperature dielectric material.

Exploring the thermal degradation mechanisms of some polybenzoxazines under ballistic heating conditions in helium and air

The degradation behaviour of five polybenzoxazines (PBZs) is studied using pyrolysis-GC/MS. Upon heating to 800 °C in helium the PBZs generate a variety of similar pyrolysis products including aniline (the major product in all cases), substituted phenols, acridine, and 9-vinylcarbazole. During the initial stages of heating (200–300 °C) aniline is the dominant pyrolysis product; from 350 °C onwards substituted phenols are released, particularly 2-methylphenol and 2,6-dimethyl phenol. The same major species are produced on heating in air, but in addition isocyanatobenzene is observed which results from the oxidation of Mannich bridges, along with a number of sulphurous species from the monomer containing a thioether bridge. This suggests that sulphur is more likely to be retained in the char in a helium atmosphere, but takes part in oxidative reactions to form pyrolysis fragments in air. During the ramped temperature cycles in both air and helium atmospheres the release of aniline was observed to rise, fall and then to rise again. This may be due a combination of the very high heating rate, poor thermal conduction of the polymer and the availability of the Mannich bridges to undergo breakdown.

Contribution of blocking positions on the curing behaviors, networks and thermal properties of aromatic diamine-based benzoxazines

To elucidate the contribution of blocking positions on aromatic diamine-based benzoxazines, we prepared several 4,4′-bis(3,4-dihydro-2H-1,3-benzoxazin-3-yl) diphenyl methane monomers with different phenols. The molecular chemical structures of the bi-functional monomers are characterized by FTIR and 1H-NMR. Particularly, the newly developed m-cresol based benzoxazine is further confirmed by 13C-NMR and ESI-MS and the oxazine ring position is verified. The curing behaviors are investigated by dynamic differential scanning calorimetry (DSC). Activation energies are analyzed by Kissinger, Ozawa and Starink methods at various heating rates. Due to structure difference in polymerized network, polymer with high amount of arylamine Mannich bridge shows low autocatalysis capability. The glass transition temperature (Tg) is closely related to the blocking position while thermal stability decreases comparatively regardless of the blocking position. High arylamine Mannich bridging leads to large amount of dangling groups, which reduces the decomposition peak temperature.

Effect of methyl substituent on the curing of bisphenol-arylamine-based benzoxazines

This work focuses on understanding the effect of methyl substituent on the curing of bisphenol-arylamine-based benzoxazines through investigating the curing kinetics of bisphenol A-based benzoxazines (BA-a and BA-35x) and bisphenol C-based benzoxazines (BC-a and BC-35x) using non-isothermal differential scanning calorimetry (DSC). Kissinger, Ozawa, Friedman, Starink and Málek methods were utilized to determine the kinetic parameters and built the kinetic models. BA-a, BC-a and BC-35x show only one dominant autocatalytic curing process, whereas, BA-35x exhibits two curing processes, i.e., the generation of arylamine Mannich at low-temperature (Reaction (1)) and the formation of phenol Mannich at elevated temperature (Reaction (2)). With the aid of Fourier transform infrared, the possible curing routes were also proposed. The results may contribute to a better understanding the curing of bisphenol-arylamine-based benzoxazines and the formation of arylamine Mannich bridge structures.

Research on thermal degradation process of p-nitrophenol-based polybenzoxazine

Effect of nitro group on thermal degradation process of polybenzoxazine was explored in this work through investigating p-nitrophenol-aniline-based benzoxazine (np-a) and its polymer (Pnp-a). The DSC and FTIR results show that the polymerization of np-a could complete even after a process at 160 °C for 1 h. TGA results in N2 and air atmosphere indicate that the char yield, thermal and thermo-oxidation stability of Pnp-a were better than those of phenol-aniline-based polybenzoxazine. And the similar DTG curves of Pnp-a in N2 and air were observed, implying that the degradation mechanism of Pnp-a in N2 and air was alike. These suggest that the essence of thermal degradation of Pnp-a in N2 was oxidation degradation. Furthermore, TGA-FTIR results show that the obvious volatilization of CO2 and H2O was detected during the almost whole thermal degradation process of Pnp-a. Py-GC-MS measurements and the structure evolution of Pnp-a during the thermal degradation process demonstrate that CO2 mainly resulted from methylene on Mannich bridge being oxidized by hydroxyl radical. Besides, LOI test shows that the flame retardance of polybenzoxazine copolymer increased with the addition of Pnp-a.

Thermal Degradation Behavior and Kinetics of Polybenzoxazine Based on Bisphenol-S and Allylamine

ABSTRACTThe thermal degradation behavior of polybenzoxazine based on bisphenol-S/allylamine was studied by Fourier transform–infrared spectroscopy and thermogravimetry–mass spectrometry. The reaction proceeded through detaching of Schiff bases and cleavage of aromatic C–S bonds and the Mannich bridge structures; the scission temperature of the aromatic C–S bond was lower than those of C–N and C–C bonds. The activation energy of the thermal degradation was evaluated with the Flynn–Wall–Ozawa method.

Synthesis and characterization of difunctional benzoxazines from aromatic diester diamine containing varying length of aliphatic spacer group: Polymerization, thermal and viscoelastic characteristics

A series of four novel diamine-based benzoxazines containing aliphatic spacer groups in their structures were synthesized by the condensation of formaldehyde, phenol and aromatic diester diamine obtained by reduction of their precursor dinitro compounds. The four aromatic diamines synthesized are ethane-1,2-diyl bis (4-aminobenzoate), butane-1,4-diyl bis (4-aminobenzoate), hexane-1,6-diyl bis (4-aminobenzoate), and octane-1,8-diyl bis (4-aminobenzoate). The chemical structures of diamine benzoxazines are confirmed by Fourier-transform infrared (FTIR) and nuclear magnetic resonance spectroscopy (1H and 13C NMR). The curing properties with respect to the basic benzoxazine structure of the monomer were studied by differential scanning calorimetry (DSC). The softening points, onset of cure temperatures and the heat of polymerization have all decreased with increasing spacer length. DSC of polybenzoxazines indicated decrease of glass transition temperature (Tg) with increasing spacer length. The thermal behavior of polybenzoxazines was studied by thermo gravimetric analysis (TGA). Higher Td5 and Td10 values were obtained by polybenzoxazines with longer spacer groups but an inversion in thermal stability was observed after Td10. The derivative thermo gram of polybenzoxazine indicated a two stage degradation process due to chain cleavage at Mannich bridge as well as breakdown of polymer chain respectively. The char yield at 700°C has decreased with increasing spacer length from 47% to 27%. The mechanical properties of glass cloth coated polybenzoxazines were evaluated with dynamic mechanical analysis (DMA). The storage modulus of the composite laminate of polybenzoxazine has increased with increasing spacer group which could be due to improvement in adhesion between fibers and resin with increasing flexibility of the resin.

Catalytic effect of trifluoroacetamido group on thermally induced ring-opening polymerization of 1,3-benzoxazine and formation of arylamine Mannich bridge structure

The autocatalytic ring-opening polymerization behavior of trifluoroacetamido group containing benzoxazine (FBA) is reported and its curing process was monitored by in situ proton nuclear magnetic resonance and Fourier transform infrared spectra analysis. The ring-opening polymerization temperature of FBA is significantly lower than that of typical 1,3-benzoxazine (BA), which can be attributed to higher stability of benzoxazine zwitterionic ion intermediate due to strong electron-withdrawing effect of trifluoroacetamido substituent. Arylamine Mannich bridge structure is found to form during polymerization, thereby enhancing the thermal stability of FBA homopolymer (poly-FBA). Differential scanning calorimetric analysis indicates that FBA effectively catalyzes the copolymerization of FBA and BA monomers at a temperature even lower than the polymerization temperature of pure FBA monomer, and the resultant copolymer exhibits higher glass transition temperature than either FBA or BA homopolymer, indicating promising possibility of FBA as reactive comonomer of polybenzoxazine systems for curing acceleration and performance improvement.