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. 2025 Mar 7;10(10):10662–10674. doi: 10.1021/acsomega.4c11620

Exploration of Poorly Recognized Advantages of Supply Chain Sustainable, Various Aliphatic Multiamine-Based Polybenzoxazines

He Xia †, Faiza ‡, Pablo Froimowicz §, Maria Laura Salum §, Mitchell Smith ‡, Jennifer Carter †, Hatsuo Ishida §,*
PMCID: PMC11923697  PMID: 40124007

Abstract

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Aliphatic amine-based viscous benzoxazine resins that are liquid at or near room temperature have great potential for their ease of processability and raw material accessibility, which will play a significant role in large-scale industrial applications. In this study, eight aliphatic benzoxazines are synthesized by the traditional Mannich reaction under solventless conditions using linear diamines, branched diamines, or triamine and cycloaliphatic diamines with eugenol and paraformaldehyde. A recycling sample preparative size-exclusion chromatography (SEC) is used to obtain the purified products. 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy are employed to study the molecular structure of all the aliphatic amine-based benzoxazines. Two exothermic peaks are commonly observed by differential scanning calorimetry (DSC) and are assigned to the oxazine ring-opening and eugenol allyl group polymerization as well as degradation of the allyl group in the eugenol part. The glass transition temperatures (Tg) of all of the aliphatic amine-based polybenzoxazines are evaluated by DSC analysis of the polymers. The activation energy of the polymerization is determined by DSC at different heating rates from 2 to 20 °C/min and calculated by the Kissinger and modified Ozawa methods to be in the range of 102–134 kJ/mol. In addition, the thermal degradation and flammability properties of the polymer are monitored by thermogravimetric analysis (TGA) and microcombustion calorimetry (MCC), respectively. All the monomers showed initial degradation after 250 °C and low flammability with limiting oxygen index (LOI), heat release capacity (HRC), and total heat release (THR) values of 44.8, 77–153 J/g K, and 7.58–20.50 kJ/g, respectively. Among all of the aliphatic amine-based benzoxazines, the cycloaliphatic amine-based benzoxazine (EU-1,2dch) polymers show the best thermal stability and the highest Tg, while tris-functional amine-based benzoxazine (EU-tapa) shows the smallest heat release capacity and the lowest Tg due to their unique structures.

1. Introduction

Polybenzoxazine, a class of thermosetting, addition-polymerizable phenolic resins, has attracted significant attention from researchers in various areas due to its outstanding mechanophysical properties. These remarkable characteristics include excellent flame retardancy, low shrinkage during polymerization, high mechanical strength, low dielectric constant and loss, ease of polymerization, and excellent chemical resistance. Their applications cover a vast range of diverse applications in the aerospace, automotive, electronic as well as biomedical industries.1−14 However, as researchers increasingly focus on their excellent mechanical and thermal properties, benzoxazines with ordinary strengths but unique properties tend to be ignored.

Benzoxazine monomers are synthesized from amines, phenols, and formaldehydes.6 In the past, multiple kinds of aromatic amines have been used with various phenols, such as aniline, diaminobenzene, 1,4-methylenedianiline, and 1,4-diaminoether, which contribute to high thermal stability due to their rigidity. Since 2011, benzoxazine synthesis based on naturally sourced phenols and amines has been extremely active, and biobased benzoxazine papers occupy roughly one-third of the benzoxazine publications.10 Benzoxazines based on a large number of phenols, such as cardanol, vanillin, diphenolic acid, lignin, guaiacol, 7-hydroxycoumarin, eugenol, umbelliferone, coumarin, chavicol, phloretic acid, pyrogallol, daidzein, phytophenol, resveratrol, 4-vinylguiacol, apigenin, urushiol, phloroglucinol, naringenin, pterostilbene, sesamol, thymol, and primary amines, such as stearylamine, furfurylamine, chitosan, difuran diamine, dopamine, tyramine, octadecylamine, and dehydroabietylamine, have been reported to date.10 While these natural renewable phenols and amines are considered representative sustainable materials, in this study, we will expand the definition of sustainability to materials that are relatively stable to supply chain fluctuations rather than being simply made by nature. In this category, one can find many aliphatic amines that are produced in very large quantities and used widely in relation to epoxy resins and other mass-produced resins.

However, it can be seen from the literature that the use of aliphatic amines as an amine source for benzoxazine synthesis is rather scarce since aliphatic amine-based benzoxazines are generally considered to have low performance. This is because the modern trend in benzoxazine research is to seek stronger and more thermally resistant materials. To date, some studies on aliphatic diamine-based benzoxazines have been reported, though no attempts have been made to generalize the advantages of aliphatic amine-based benzoxazines.15−22 Furthermore, the advantages of diamine-based benzoxazines in comparison to those of bisphenol-based benzoxazines have not been evaluated.

In this work, aliphatic amines were simply divided into three categories: linear aliphatic amines, branched aliphatic amines, and cycloaliphatic amines. Linear aliphatic diamine-based benzoxazine was systematically studied by Chernykh et al.15 and Allen and Ishida,16,17 who focused on the chain length, rate of polymerization, and network properties of polybenzoxazine. Also, in some other studies, Gungor and Bati showed the use of aliphatic diamines for the naphthoxazine synthesis.23 Puozzo et al. synthesized a novel trifunctional benzoxazine based on tris(3-aminopropyl)amine, which was intended for elastic resins for soft pressure sensing due to its low viscosity monomer.24 Cycloaliphatic amine-based benzoxazine also possesses its own advantages. Shah et al. focused on the polymerization behavior and comparison between cycloaliphatic and aromatic amine-based benzoxazines as well as different amounts of the functional group benzoxazines, clarifying that the thermal properties of cycloaliphatic amine-based benzoxazine are as good as those of benzoxazine based on aromatic amines.25

In 2021, Midhina and others reported on the thermodynamic properties of the branched aliphatic amine-based benzoxazines, and their research revealed a large gap between the melting and polymerization temperatures, providing a wide processing window and ease of manufacturing of products.26 Viscous liquid monomers at or near room temperature are also a prominent focal point in benzoxazine research recently, showcasing distinctive properties and exceptional potential for diverse applications.27,28

Keeping the lack of literature on aliphatic multiamine-based benzoxazines as a group in mind, rather than individual examples, this study is designed to introduce a series of aliphatic multiamine-based benzoxazines. We will attempt to adopt ecofriendly and cost-effective strategies whenever appropriate. Eugenol, also known as clove oil, is widely used in biobased benzoxazine synthesis and is employed as a phenolic component in this work.

This paper reports the synthesis of eight aliphatic diamine- or triamine-based benzoxazines. Emphasis was placed on aliphatic multiple amines that are available inexpensively and in very large quantities. Since some of these compounds do not crystallize, purification to the usual single-crystal level purities has been very difficult. However, the purity achieved ranges from reasonable to good for benzoxazines with two linear aliphatic amines, four branched derivatives, and two cyclic ones. The replacement of bisphenol with aliphatic diamines offers several benefits, including (i) low cost and higher supply chain stability, (ii) high reactivity that helps energy conservation by lowering the activation energy, and (iii) less toxicity. Through a comparative analysis of the discernible outcomes, their cost-effectiveness, easier handleability, as well as thermal properties will be discussed. The polymerization behavior of the monomers has been studied by differential scanning calorimetry (DSC), and the presence of extra exothermic peaks in the DSC has been discussed and demonstrated. The thermal properties and the polymerization behavior of monomers and polymers are the main aims of this work, which are monitored by DSC, TGA, and MCC.29,30

2. Experimental Section

2.1. Materials

The following compounds were used as received: eugenol (≥99%), ethylene diamine (eda) (99%), hexamethylenediamine (hda), isophrone diamine (ipda) (98%), 1,3-diaminopropane (1,3peda) (99%), 1,2-diaminocyclohexane (1,2dch), (99%), 2-methylpentane-1,5-diamine (2mpda) (97%), 1,3-pendanediamine (1,3peda), tris(3-aminopropyl) amine, (tapa) (98%), paraformaldehyde (98%, Acros), chloroform (≥99.5%), acetone (≥99.5%), toluene (anhydrous, 99.8%), methanol (absolute, ≥ 99%), 1-butanol (absolute, ≥99%), sodium hydroxide (pellets, ≥98.0), and anhydrous sodium sulfate (≥99.0). All of the compounds, except for paraformaldehyde from Acros, were obtained from Sigma-Aldrich and used without further purification.

2.2. Instruments

Recycling preparative HPLC was obtained from the Japan Analytical Industry Co., Ltd. (JAI) (LaboACE LC-7080 Plus series) and was used in the recycling preparative size-exclusion chromatography (SEC) mode for purification of liquid or low-temperature melting amorphous solid products with a 10 mL/min flow rate using 99.8% pure chloroform at room temperature. The SEC columns used were HR series that were optimized for high speed and multiple organic solvents, and the molecular weight ranged from a few hundred to a few thousand. Thus, this technique is termed recycling preparative SEC in the following sections. The dilute solution of each resin was injected at room temperature and circulated in the column, the separation was monitored, and separated portions were collected and analyzed by running several (3–5) cycles after ensuring that the peak of the targeted fraction had little overlap with the peaks of other fractions.

1H and 13C nuclear magnetic resonance (NMR) spectroscopic spectra were obtained using a Bruker Avance NMR spectrometer, which was operated at 500 MHz for 1H and 125.721 MHz for 13C. Deuterated chloroform (CDCl3) and dimethyl sulfoxide d6 (DMSO-d6) were used as NMR solvents to dissolve the benzoxazine samples. Tetramethylsilane was used as an internal standard in the NMR solvent.

Fourier transform infrared (FTIR) spectroscopic spectra were obtained using an Agilent spectrometer instrument model Cary 630 FTIR, employing a single-reflection attenuated total reflectance (ATR) accessory with a germanium (Ge) crystal and spectral range of 4000–650 cm–1 at a spectral resolution of 4 cm–1. A deuterated triglycine sulfate detector was used.

Differential scanning calorimetric (DSC) analysis was conducted using a TA Instruments DSC Q2000 instrument (New Castle, Delaware), under a N2 atmosphere at a flow rate of 60 mL/min. To characterize the monomers, nonisothermal measurements were carried out at a heating rate of 10 °C/min within a temperature range of 30–330 °C utilizing approximately 1–3 mg of samples.

Thermogravimetric analysis (TGA) was carried out using a TA Instruments TGA500 (New Castle, Delaware). The experiments were run under an N2 atmosphere at a flow rate of 60 mL/min with a heating rate of 10 °C/min within a temperature range spanning from 30 to 800 °C. The char yield was determined as the percentage of residual weight at 800 °C under a nitrogen atmosphere. Platinum crucibles were employed, each containing approximately 5 mg of the samples.

Microcombustion calorimetry (MCC) analysis was conducted using a Microscale Combustion Calorimeter (Model MCC-4, Deatak Company, Chicago) under a 20 mL/min oxygen flow at a constant 1 °C/s heating rate over the temperature range from 75 to 800 °C. Method A (anaerobic pyrolization) in this instrument is employed for all polymers, which means that the data of heat release capacity, maximum specific heat release, heat release temperature yield of pyrolysis residue, and specific heat of combustion of fuel gases can be obtained from the results. Platinum crucibles were employed, each containing approximately 2–5 mg of the samples. Each sample was tested at least 3 times, and the final data are the average of three tests.

2.3. Synthesis of Linear Aliphatic Diamine-Based Benzoxazine Monomers

Due to the high reactivity of these linear aliphatic amines, named ethylene diamine-based benzoxazine (abbreviated as EU-eda), and 1,6-hexane diamine-based benzoxazine (abbreviated as EU-hda), a modification of the solventless synthesis was used in this aliphatic amine-based benzoxazine synthesis,31 Eugenol (3.28 g, 0.02 mol), ethylene diamine (0.6 g, 0.01 mol), and paraformaldehyde (1.2 g, 0.04 mol) were dissolved in 20 mL of chloroform and heated under stirring for 1.25 h at the reflux temperature. All of the samples were washed with 1 N sodium hydroxide solution three times, and then the pH was adjusted to neutral with water. After the solvent was evaporated, the samples were washed several times with methanol, and a white powder was obtained. Subsequently, chloroform and toluene were used at a ratio of 2:1 for crystallization. Finally, colorless crystals were collected after recrystallization and completely dried in a vacuum oven. Yield: 81%.

1,6-Hexane diamine (1.16 g, 0.01 mol), eugenol (3.28 g, 0.02 mol), and paraformaldehyde (1.2 g, 0.04 mol) were added to a round bottom flask under solvent-free conditions at 105 °C for 1.5 h. The crude product was dissolved in toluene, washed with 1 N sodium hydroxide solution three times, and neutralized with an additional wash with water. The organic layer was dried over anhydrous magnesium sulfate. A yellow powder was collected after drying under air and then washing several times with 1-butanol. Finally, a white powder was collected after recrystallization in chloroform and toluene (3:1) and dried under vacuum. Yield: 73%

2.4. Synthesis of the Branched and Cyclic Aliphatic Diamine-Based Benzoxazine

All of the branched and cyclic aliphatic multiamine-based benzoxazine, named 1,3-diaminopentane-based benzoxazine (abbreviated as EU-1,3peda), 1,2-diaminopropane (abbreviated as EU-1,2pda), isophorone diamine-based benzoxazine (abbreviated as EU-ipda), 1,2-diaminocyclohexane-based benzoxazine (abbreviated as EU-1,2dch), 2-methylpentane-1,5-diamine-based benzoxazine (abbreviated as EU-2mpda), and Tris(3-aminopropyl) amine-based benzoxazine (abbreviated as EU-tapa) were synthesized by the solventless method. Due to the liquid nature of most of the branched benzoxazines, purification via recrystallization was difficult and, thus, was carried out using preparative recirculating SEC, yielding around 96% pure resins. 0.01 M aliphatic diamines (different weights were measured for each amine), eugenol (3.28 g, 0.02 mol), and paraformaldehyde (1.2 g, 0.04 mol) were mixed in a flask and stirred for a certain time. All the conditions are listed in Table 1. After synthesis, all of the samples were dissolved in toluene and washed in the same manner as the linear aliphatic amine-based benzoxazine. After that, purification was carried out using recycling preparative SEC, followed by drying in a vacuum oven. The conditions of the different products are summarized in Table 1. The products obtained were either viscous liquids or low-temperature liquefying waxy solids.

Table 1. Synthesis Conditions and Final Status of the Branched and Cyclic Aliphatic Amine-based Benzoxazine.

monomers approach temp (°C) duration (h) final product status
EU-1,2pda solventless 110 1.05 pale orange viscous liquid
EU-1,3peda solventless 105 1.15 yellowish viscous liquid
EU-2mpda solventless 110 1.5 orange viscous liquid
EU-1,2dch solventless 110 2.5 brown waxy solid
EU-ipda solventless 105 1.25 yellowish waxy solid
EU-tapa solventless 110 1.5 orange waxy solid
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2.5. Polymerization of the Aliphatic Amine-Based Benzoxazine

The polymerization conditions of the monomers were determined by DSC analysis. All the monomers were heated in a vacuum oven from the initial temperature to the polymerization temperature; then, the compounds were maintained at the polymerization temperature for several hours. The complete polymerization of benzoxazine was confirmed by FTIR and DSC analyses. The polymerization process is discussed in the DSC analysis section. All of the specific conditions adopted are shown in Table 2. In order to ensure complete polymerization so that the ultimate properties could be determined, the duration of heating was adopted to be longer than the ordinary conditions used in the literature.

Table 2. Polymerization Conditions of all the Aliphatic Amine-based Benzoxazine.

benzoxazine mononers initial temperature (°C) polymerization temperature (°C) duration (h)
EU-eda 60 205 7
EU-hda 60 210 7
EU-1,2pda 60 190 7
EU-1,3peda 60 195 7
EU-2mpda 60 185 7
EU-1,2dch 60 190 7
EU-ipda 60 190 7
EU-tapa 60 205 7
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3. Results and Discussion

3.1. Synthesis of the Aliphatic Multiamine-Based Benzoxazine Monomer

Linear and branched multiamine-based monomers were synthesized successfully by the Mannich condensation reaction, as shown in Scheme 1. The chemical structures of all of the aliphatic multiamine-based benzoxazines are listed in Figure 1. Most of the branched aliphatic multiamine-based benzoxazine are liquid in nature at or near room temperature. Thus, the standard recrystallization method cannot be adopted for purification other than for linear diamine-based benzoxazines. The solvent purification method was not effective, probably because the impurities were asymmetric benzoxazines with closed and open Mannich bases as terminals. This molecule acts as a weak surfactant and disturbs the efficient separation of monomer molecules. However, up to 96% monomer purification was achieved using the recycling preparative SEC technique. This was not the case for linear aliphatic amine-based benzoxazines, as they are readily crystallized, and crystalline solids are obtained. To fulfill the goal of greener synthesis, the solventless method and modification of the solventless method have been used to avoid or minimize the use of organic solvents.31

Scheme 1. Synthesis of Aliphatic Amine-Based Benzoxazine Monomers.

Scheme 1

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Figure 1.

Figure 1

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Chemical structures of all of the aliphatic diamine- or triamine-based benzoxazines studied.

3.2. NMR Spectra of the Monomers

1H and 13C NMR analyses were employed to confirm both the molecular structures and purities of the monomers. Representative 1H and 13C NMR spectra of EU-1,3peda are shown in Figure 2, while the spectra of the rest of the monomers, EU-eda, EU-hda, EU-1,2pda, EU-2mpda, EU/-ipda, EU-1,2dch, and EU-tapa, are shown in Figures S1–S7, respectively.

Figure 2.

Figure 2

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1H NMR spectra of EU-1,3peda (a) and 13C NMR spectra of EU-1,3peda (b).

The chemical structures of all of the aliphatic multiamine-based benzoxazine monomers were confirmed by 1H and 13C NMR and FTIR spectroscopy. Figure 2(a) shows the 1H NMR spectrum of EU-1,3peda. The characteristic resonances in the 1H NMR spectrum that are attributed to the oxazine ring in benzoxazine are located at 3.96 ppm (Ph–CH2-N) and 4.91 ppm (O–CH2-N).18 The presence of the aliphatic portion of the amine component, which is connected to the oxazine ring, was confirmed by the resonances at 0.86, 1.49, 1.67, 2..39, and 2.73 ppm.17 The doublets at 5.08 and 3.25 ppm and the multiplet around 5.87 ppm are assigned to the allyl groups in the eugenol moiety.32 A distinctive resonance at 3.71 ppm is due to the –OCH3 protons that are part of the substituent to the benzene ring. Moreover, the resonances at 6.49 and 6.60 ppm suggest the presence of aromatic protons. Besides, the absence of hydroxyl groups implies high purity of the final product. It should be noted that the sharp resonance at 3.30 ppm is due to water in the NMR solvent (DMSO-d6) rather than in monomers. The purities of the monomers are listed in Table 3.

Table 3. 1H NMR Chemical Shifts of All Aliphatic Amine-Based Benzoxazines.

monomers EU-eda hda (ppm) EU-1,2pda (ppm) EU-1,3peda (ppm) EU-2mpda (ppm) EU-ipda (ppm) EU-1,2dch (ppm) EU-tapa (ppm)
oxazine ring (O–CH2–N) 3.89 3.86 3.94 3.96 3.93 4.04 3.95 3.85
oxazine ring (N–CH2–C) 4.78 4.76 5.03 4.91 4.89 4.74 4.93 4.74
aliphatic backbone 2.82 1.28, 1.47, 2.58 1.12, 1.93, 2.08 0.86, 1.49, 1.67, 2.39, 2.73 0.90, 1.09, 1.48–1.64 2.52, 2.73 0.76–1.21, 1.57–1.66 2.4, 0.79–1.12 1.50, 2.22 2.08, 2.60, 2.67
allyl group (−CH2–CH=CH2) 3.89, 5.07, 5.91 3.23, 5.06, 5.91 3.25, 5.08, 5.92 3.19, 5.00, 5.87 3.28, 5.10, 5.93 3.27, 5.07, 5.94, 3.22, 5.07, 5.92 3.22, 5.03, 5.89
methoxy group (−O–CH3) 3.70 3.69 3.74 3.71 3.84 3.84 3.84 3.70
aromatic protons 6.38, 6.60 6.38, 6.59 6.55 6.66 6.49, 6.60 6.39, 6.57 6.30–6.53 6.26–6.59 6.33, 6.59
purity 99% 98% 98% 96% 96% 95% 95% 94%
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All of the structures and purities of the monomers were identified and confirmed by 1H NMR spectra, as shown in Figure 2 as well as in Figures S1–S7. The characteristic resonances near 3.90 and 4.80 ppm suggest protons from the oxazine rings, providing clear evidence of the successful production of all the benzoxazine monomers. To conveniently elucidate all the NMR resonances of the different compounds, Table 3 presents all the chemical shifts in the 1H NMR protons from oxazine rings, aliphatic cores, allylic substitution groups, and methoxy groups in the benzoxazine molecules.

Figure 2(b) shows the 13C NMR spectrum, which further supports the successful synthesis of EU-1,3peda. The presence of characteristic peaks at around 83.0 and 49.8 ppm reveal the typical benzoxazine structure due to the O-CH2–N and Ph-CH2–N groups of the oxazine ring, respectively.17 Other distinguished peaks at 10.9, 23.8, 24.7, 39.7, and 57.8 ppm confirm the alkyl group from the multiamine part.33

The singlet at 56.2 ppm is due to the –OCH3 carbon. Furthermore, the characteristic carbon resonances at 115.6, 137.9, and 39.7 ppm are assigned to the allyl carbons (−CH2–CH=CH2) from eugenol.32 The aromatic carbons are found at 110.4 121.3, 130.8, 144.9, and 147.7 ppm. All of the carbons from the EU-1,3peda structure were identified. Figures S1(b)–S7(b) show the 13C NMR spectra of EU-hda, EU-1,3peda, EU-1,2pda, EU-2mpda, EU-ipda, EU-1,2dch and EU-tapa. Combined with the 1H NMR spectra, the chemical structures of all of the benzoxazine monomers were verified.

3.3. FTIR Analyses

Figure 3(a) shows the FTIR spectra of all of the aliphatic multiamine-based benzoxazines from 3800 to 2400 cm–1, whereas Figure 3(b) shows those in the 1750–750 cm–1 region. As highlighted in Figure 3(a), the bands observed at 2924 and 2854 cm–1 are due to the antisymmetric and symmetric stretching vibrations of both the methylene groups within the oxazine ring, unsaturated CH2 groups of eugenols, and aliphatic chains of the amine moieties. A very weak band slightly above 3000 cm–1 is due to the CH and CH2 stretching modes of the allyl group of eugenol,34 which heavily overlap with the aromatic CH stretching modes. Furthermore, it is worth mentioning that the absence or the extremely weak hydroxyl band centered around 3400 cm–1 further demonstrates the good purity of the monomer.17,35 A strong band near 918 cm–1 can be readily observed in the FTIR spectrum of each monomer, which is due to the oxazine ring mode. This band has traditionally been considered the CH out-of-the-plane mode of a benzene ring to which an oxazine ring was attached. However, this mode was reassigned to the oxazine ring mode with a small amount of potential energy contribution from the CH out-of-the-plane mode by more recent detailed analysis using isotopes and theoretical analysis.36 Also, another significant band is at 1224 cm–1, which is due to the antisymmetric stretching mode of the oxazine C–O–C group.37 Both bands are highlighted in Figure 3(b), which support the oxazine ring formation.

Figure 3.

Figure 3

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(a) FTIR spectra of all of the aliphatic amine-based benzoxazine monomers from 3800 to 2400 cm–1. (b) FTIR spectra of aliphatic amine-based benzoxazines ranging from 1750 to 750 cm–1.

3.4. Thermal Properties of Benzoxazine Monomers

The polymerization behavior of all of the aliphatic multiamine-based benzoxazines was studied by differential scanning calorimetry (DSC). Figure 4 shows the DSC thermograms of all of the benzoxazine monomers at a heating rate of 10 °C/min under a nitrogen atmosphere from 30 to 350 °C. The DSC results are summarized in Table 4. As shown in Figure 4, strong and sharp melting peaks of EU-eda and EU-hda at 156 and 120 °C, respectively, are observed, reflecting a high degree of crystallinity and purity.

Figure 4.

Figure 4

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DSC spectra of all of the aliphatic amine-based benzoxazine monomers under a N2 atmosphere.

Table 4. DSC Data of All Aliphatic Amine-Based Benzoxazine Monomers.

sample Tm (°C) Tonset (°C) Tmax (°C) processing window (°C) enthalpy of polymerization (J/g)
EU-eda (linear) 156 205 235 49 156.4
EU-hda (linear) 120 215 245 95 38.9
EU-1,2pda (branched) / 167 215 167 78.1
EU-1,3peda (branched) / 195 228 195 18.8
EU-2mpda (branched) / 200 223 200 30.7
EU-1,2dch (cyclic) / 200 262 200 36.8
EU-ipda (cyclic) / 195 264 195 48.5
EU-tapa (triamine) / 205 235 205 89.5
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As mentioned previously, all of the branched aliphatic multiamine-based benzoxazines are viscous liquids. The absence of melting endotherms in the DSC thermograms of EU-1,2pda, EU-1,3peda, EU-1,2dch, EU-2mpda, EU-ipda, and EU-tapa is consistent with the liquid states of those compounds at room temperature. It is worth mentioning that EU-1,2pda is unique and can become solid or even crystals under certain specific conditions, for example, by extremely slow evaporation of the solvent. However, while it can behave as a solid under certain conditions, it can melt and transform into a liquid with slight heating (65 °C). Having polymerization exotherms in the range of 220–260 °C, the processing windows of these compounds are as wide as 200 °C or more.37 Thus, even if an initiator or catalyst is added, it can still provide a sufficiently wide processing window. Having a liquid state at or near room temperature and a wide processing window are definite advantages when manufacturing composite materials.

Moreover, it can be seen that the DSC thermograms of all the aliphatic multiamine-based benzoxazines have two obvious exothermic peaks from 223 to 337 °C. Also, it has been widely reported that the ring-opening polymerization of benzoxazine typically shows an exothermic peak from 200 to 275 °C. The first exothermic peaks of all aliphatic multiamine-based benzoxazines were also located in the same temperature range. Thus, these exotherms are safely attributed to the ring-opening polymerization of the oxazine groups. The second exothermic peaks are not observed in other benzoxazines but are common in eugenol-based benzoxazines. However, their origins remain controversial. Thirukumaran believed that the appearance of a second exotherm is due to the degradation of aliphatic chains.33,34 Dumas et al. believed that the emergence of the second exotherm is due to the degradation of the eugenol part. It is worth mentioning that Dumas could be the first person to discuss this problem by comparing the DSC and TGA thermograms of the same eugenol-based benzoxazine monomer.38−40 Besides, Ramesh et al. believed that the second exotherm was due to the cross-linking of the allyl group in the eugenol moiety.32

In light of previous studies, the possible reasons for the presence of a second exothermic peak may be either the degradation of the aliphatic group or the eugenol part or the cross-linking of the allyl group. To observe the polymerization process and verify the reasons mentioned above, FTIR and DSC are used. It is important to ensure that the temperature and time are sufficient to complete the ring-opening polymerization in benzoxazine and ensure that all oxazine rings have been opened and cross-linked after polymerization. To achieve this, all aliphatic multiamine-based benzoxazines were heated near the onset temperature of polymerization for at least 5 h, as shown in Table 2. Subsequently, the properties and structures of the polymers were monitored by DSC and FTIR spectroscopy. Figures 5 and 6 show the DSC thermograms and FTIR spectra of all of the polymers after polymerization, respectively. In the FTIR spectra, all polymers lack peaks near 1244 cm–1, which is due to the asymmetric vibration of C–O–C on the oxazine ring,41 verifying that all oxazine rings in the benzoxazines are fully cross-linked after polymerization. The near disappearance of the peaks at 918 cm–1 supports this conclusion. Furthermore, the C=C band at approximately 1634 cm–1 due to the allyl group disappeared, indicating that the allyl group was fully cross-linked at a temperature well below this second DSC peak.

Figure 5.

Figure 5

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DSC thermograms of the polymers after polymerization under a nitrogen atmosphere.

Figure 6.

Figure 6

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FTIR spectra of all of the aliphatic amine-based benzoxazine polymers after polymerization.

However, from the DSC thermograms in Figure 5, it can still be observed that there is an increasing trend in the baseline from 270 to 350 °C after complete polymerization, which has been confirmed by FTIR spectroscopy. To explore the reason for this phenomenon, a comparison between the TGA and DSC thermograms of the EU-eda monomer is displayed in Figure 7. EU-eda was chosen as an example since the purity of this monomer was the highest, and the interpretation of the results will not be strongly influenced by the presence of impurities. It is apparent that the degradation of monomer parallels its polymerization, initiating with the polymerization process but spanning a broader temperature range compared to polymerization, demonstrating the same results as those in the Dumas work.38 In other words, after the full cross-linking of the oxazine ring and allyl group, the polymer is continuously degraded, thereby contributing to the manifestation of the second exothermic peak. The absence of a window between the two exothermic peaks also verifies this explanation. Due to the extremely short carbon chain of EU-eda, the possibility of degradation of the aliphatic part is rather low.32 Therefore, the main reason for the increasing trend in the DSC of the polymers and the second peak in the DSC baseline of the monomers can possibly be the degradation of the eugenol part, which is most likely the allyl cross-linked part as the degradation and polymerization occur almost simultaneously. The proposed polymerization process is shown in Scheme 2.

Figure 7.

Figure 7

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DSC and TGA thermograms of the EU-eda monomer under nitrogen ranging from room temperature to 400 °C.

Scheme 2. Proposed Polymerization Mechanism of the Aliphatic Amine-Based Benzoxazine.

Scheme 2

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In summary, the combination of DSC, FTIR, and TGA results during the polymerization of the monomers further verified that the degradation temperature range of the allyl group on the side of eugenol was higher than that of the open polymerization of the benzoxazine ring, as well as its own polymerization. In other words, the first exothermic peak in DSC is due to the polymerization of the oxazine ring and allyl group.38,40 The second exothermic peak is dominated by the degradation of the allyl cross-linked portion in the eugenol part.

After determining the reason for the two exothermic peaks, the enthalpies of polymerization of all aliphatic multiamine-based benzoxazines were determined, which are provided in Table 4. The values of the enthalpies of polymerization are 156.4, 38.9, 78.1, 18.8, 30.7, 36.8, 48.5, and 89.5 J/g for EU-eda, EU-hda, EU-1,2pda, EU-1,3peda, EU-2mpda, EU-1,2dch, EU-ipda, and EU-tapa, respectively. All of the results of benzoxazines were much lower than those of the traditional benzoxazines, except for EU-eda. The main reason is probably that the degradation of the eugenol part in the molecules coincides with the polymerization process, which affects the measurement of the enthalpy of polymerization. In addition, two exothermic peaks heavily overlapped, especially EU-1,3peda, EU-2mpda, EU-1,2dch, and EU-ipda, which made the results imprecise by the calculation of the integration of the first exothermic peak. Further detailed studies are required to understand this complex behavior.

A comparison of the polymerization temperatures of different monomers in the same category leads to additional results. As mentioned previously, all compounds in this work can be divided into three categories: linear aliphatic diamine-based benzoxazines (EU-eda and EU-hda), branched aliphatic diamine- or triamine-based benzoxazines (EU-1,2pda, EU-1,3pda, EU-2mpda, and EU-tapa), and cyclic aliphatic diamine-based benzoxazines (EU-1,2dch and EU-ipda). In the comparison of linear aliphatic benzoxazine, EU-eda exhibits a lower polymerization temperature at 235 °C than that of EU-hda (245 °C).18 Cycloaliphatic diamine-based benzoxazines, EU-1,2dch and EU-ipda, exhibit exothermic peaks at 262 and 264 °C, which are apparently higher than the other benzoxazine monomers, possibly due to the rigidity of the cyclic structure and the subsequent lower molecular mobility. The fact that EU-ipda has a slightly higher polymerization temperature than EU-1,2dch is possible because EU-ipda has more complex branched alkyl groups than EU-1,2dch, making it more sterically hindered and difficult to polymerize. The linearly branched aliphatic amine-based benzoxazines (EU-1,3pda and EU-2mpda) exhibit a lower temperature than the other categories. The exothermic peak of EU-tapa is observed at 235 °C, which is slightly higher than that of linear aliphatic benzoxazines but lower than that of cycloaliphatic benzoxazines, possibly due to the steric hindrance caused by the branch point in the amine moieties. It is likely that the molecular mobility of the amine moiety has a direct and significant influence on the polymerization temperature of aliphatic multiamine-based benzoxazines. Further detailed analysis is needed to understand the quantitative influence of this subject.

3.5. Glass Transition Temperatures of Polybenzoxazines

The glass transition temperatures (Tg) of all of the aliphatic multiamine-based polybenzoxazines are studied by DSC thermograms of the polymers in the temperature range from 80 to 220 °C. The results are shown in Figure 8. It can be observed that the Tg values of poly(EU-eda), poly(EU-hda), poly(EU-1,2pda), poly(EU-1,3peda), poly(EU-2mpda), poly(EU-1,2dch), poly(EU-ipda), and poly(EU-tapa) are 143, 99, 188, 175, 145, 157, 140, and 92 °C, respectively. Compared to the Tg value of the traditional aromatic amine-based polybenzoxazine (poly(BA-a)) at around 170 °C,42 it is obvious that most of the aliphatic amine-based benzoxazines exhibit lower glass transition temperatures. It is possibly because the aromatic units have a higher rigidity in the chain backbone, which can limit the motion and rotation within or between the molecules. Among cyclic aliphatic diamine-based benzoxazine polymers, poly(EU-1,2dch) shows a higher Tg than poly(EU-ipda) because multiple linear alkyl branches are connected to the cyclic backbone in the EU-ipda molecules. In the linear category, poly(EU-hda) shows a lower Tg because longer chains in the molecular structure increase the possibility of thermal movement of the molecular chains in the polymer. The shorter chains in the molecular structures potentially cause tighter packing and/or greater steric hindrance of the polymers.43 Likewise, poly(EU-1,2pda) shows the highest Tg among all of the linear branched aliphatic mutiamine-based polybenzoxazines because of its shortest carbon chain in the backbone. Therefore, it is reasonable that trifunctional aliphatic amine-based polybenzoxazine (poly(EU-tapa)) displays a low glass transition temperature, possibly due to the poor packing of the three alkyl branches connected to the nitrogen (Figure 9).

Figure 8.

Figure 8

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DSC thermograms of the aliphatic multiamine-based benzoxazine polymers in the temperature range from 80 to 220 °C.

Figure 9.

Figure 9

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Kissinger (a) and modified Ozawa (b) method plots for EU-1,3peda.

3.6. Polymerization Kinetics of All Benzoxazines

The kinetic parameters of all of the benzoxazines are evaluated by DSC at different heating rates. Figures S8–S15 show the DSC thermograms of all the monomers recorded at heating rates from 2 to 20 °C/min under a 60 mL/min N2 flow rate. As expected, the onset temperature of polymerization and exothermic peak temperature shift to higher temperatures as the heating rate increases.

The activation energy was calculated using the well-known Kissinger and modified Ozawa methods.43,44 According to the Kissinger method, the activation energy can be obtained by a linear correlation between the logarithms and the reciprocal of the exothermic peak temperature. The calculation of the equation is as follows

3.6.

where β is the heating rate, A is the frequency factor, Tp is the exothermic peak temperature, Ea is the activation energy, and R is the gas constant. From the linear plot of ln (β/T2p) against 1/Tp, the activation energy can be calculated from the slope of the line.

According to the modified Ozawa method, the activation energy can be calculated using the following equation

3.6.

where C is a constant.

Figure 9 shows a representative Kissinger and modified Ozawa plots for EU-1,3peda. The slopes of all the lines in the plot were determined, and the activation energies (Ea) were calculated and are shown in Table 5.

Table 5. Activation Energies of All Benzoxazines Obtained by the Kissinger Method and Modified Ozawa Method.

samples Ea (kJ/mol) by Kissinger Ea (kJ/mol) by Ozawa
EU-eda (linear) 125.7 127.5
EU-hda (linear) 140.4 142.0
EU-1,2pda (branched) 120.9 122.6
EU-1,3peda (branched) 133.0 134.4
EU-2mpda (branched) 99.8 102.8
EU-ipda (cyclic) 148.3 149.6
EU-1,2dch (cyclic) 141.3 142.7
EU-tapa (triamine) 129.9 131.4
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From these results, it can be observed that cyclic aliphatic diamine-based benzoxazines (EU-ipda and EU-1,2dch) show the highest activation energies among all the monomers, which is probably due to the cyclic alkyl units exhibiting poorer mobility and, thus, lower reactivity than the linear or branched alkyl parts in the backbone. Between the linear aliphatic amine-based benzoxazines (EU-eda and EU-hda), EU-hda shows a higher activation energy, possibly due to the dilution effect of the benzoxazine moiety by the long chains. The trifunctional aliphatic amine-based benzoxazine (EU-tapa) still exhibits a very low activation energy among all the benzoxazines, possibly because the polar and nucleophilic nitrogen proton activates the ring-opening process during polymerization.45

3.7. Thermal Properties of the Polybenzoxazines

3.7.1. TGA Analysis

The thermal properties of polybenzoxazines are studied by thermogravimetric analysis (TGA). Figure 10 shows the TGA thermogram of aliphatic multiamine-based polybenzoxazines under an N2 atmosphere at a heating rate of 10 °C/min over the temperature range from room temperature to 800 °C. All the TGA data, including the initial degradation temperature (Ti), 5% weight loss degradation temperature (Td5), 10% weight loss degradation temperature (Td10), 20% weight loss degradation temperature (Td20), and char yield at 800 °C of the polymers, are summarized in Table 6. Among them, poly(EU-1,2dch) shows the highest thermal stability with Ti at 286 °C, Td5 at 351 °C, Td10 at 415 °C, and Td20 at 498 °C because the cyclic alkyl groups may impart more thermal stability than the linear or branched alkyl structures. However, the poly(EU-ipda) polymer (Ti at 295 °C, Td5 at 334 °C, Td10 at 374 °C, and Td20 at 398 °C) shows lower thermal stability, probably because the linear side groups substituted to the cycloaliphatic ring may rather readily degrade.

Figure 10.

Figure 10

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TGA thermograms of all of the aliphatic amine-based polybenzoxazines.

Table 6. TGA Data of All Aliphatic Amine-Based Benzoxazine Polymers.
polymers Ti (°C) Td5 (°C) Td10 (°C) Td20 (°C) char yield (%) calculated LOI value
poly(EU-eda) (linear) 256 338 401 418 56.0 39.8
poly(EU-hda) (linear) 297 361 382 416 45.8 35.7
poly(EU-1,2pda) (branched) 312 359 373 402 52.4 38.5
poly(EU-1,3peda) (branched) 290 393 423 460 62.0 42.2
poly(EU-2mpda) (branched) 309 345 402 453 35.0 31.4
poly(EU-1,2dch) (cyclic) 286 351 415 498 68.7 44.8
poly(EU-ipda) (cyclic) 287 352 385 409 27.0 28.3
poly(EU-tapa) (triamine) 283 354 397 416 58.1 40.6
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In the linear aliphatic mutiamine-based polybenzoxazines group, poly(EU-eda) (Ti at 256 °C, Td5 at 338 °C, Td10 at 401 °C, and Td20 at 418 °C) exhibited slightly higher thermal stability than poly(EU-hda) (Ti at 297 °C, Td5 at 361 °C, Td10 at 382 °C, and Td20 at 416 °C) from Ti and Td20 points of view, which is probably because the shorter chain in the skeletons is more stable at higher temperature than the long chain, possibly due to the steric hindrance from the terminal portion of the aliphatic amine chain.

The linear branched aliphatic amine-based benzoxazine polymers (poly(EU-1,2pda), poly(EU-1,3peda), poly(EU-2mpda)) also show impressive thermal stability, though slightly inferior to the cyclic aliphatic amine-based benzoxazine polymers. The results of the comparison between them are similar to those for linear aliphatic amine-based benzoxazine polymers, indicating that the compound with a shorter carbon chain structure displays a modestly higher level of thermal stability.

Although not very obvious, tris-functional aliphatic amine-based benzoxazine (EU-tapa) possesses one of the lowest thermal stabilities among them (Ti at 282 °C, Td5 at 354 °C, Td10 at 397 °C, and Td20 at 416 °C, which are very close to those of poly(EU-1,2pda) and poly(EU-ipda)). This is probably because the C–N bond energy is marginally smaller than the C–C bond energy, and thus, the structure of the center of tris-amine may degrade at a lower temperature.

3.7.2. Flame Retardancy of the Polymers

The flame retardation of the polymers is studied from the results of the char yield and limiting oxygen index (LOI). LOI is defined as the minimum percentage of oxygen in an oxygen/nitrogen mixture required to support continuous polymer combustion and is often used to evaluate the flame retardancy of the polymers. The LOI can be generally calculated by the char yield from the TGA results according to the van Krevelen and Hoftyzer equation.46

3.7.2.

where LOI is the limiting oxygen index and CY is the char yield.

The LOI results are calculated and are summarized in Table 6. Generally, the polymer’s limiting oxygen index (LOI) should exceed the threshold value of 26 to impart self-extinguishing properties, meeting the criteria for various applications that demand superior flame retardance.46,47 From Table 6, it is evident that all the aliphatic multiamine-based polybenzoxazines exhibit a high LOI from 28 to 45. Since aliphatic multiamine compounds usually exhibit high flammability, the current results are rather unexpected. Among them, poly(EU-1,2dch) has the highest char yield and LOI value of 68.7% and 44.8, respectively, due to its stable cycloaliphatic core in the benzoxazine monomer. In addition, trifunctional aliphatic amine-based polybenzoxazine, poly(EU-tapa), is also considered to have excellent flame-retardant properties due to its high LOI value of up to 40.6. It is expected that trifunctional benzoxazine leads to a higher cross-link density. Furthermore, the nitrogen functional molecules should provide greater flame retardance since the molar contribution of the antiflammability proposed by Lyon and Walter48,49 is greater than that of many organic functional groups, such as aliphatic groups. Consequently, poly(EU-tapa) is expected to exhibit good flame retardance, as is supported by the observed result.

3.7.3. Microcombustion Calorimetry Analysis

Microcombustion calorimetry was used to quantitatively study the flammability of polybenzoxazines, which can exhibit the exact data of the flame-retardant properties rather than qualitative estimates from TGA. Figure 11 shows the microcombustion calorimetric thermograms of all the polymers under a 20 cc/min oxygen flow rate at a constant heating rate of 1 °C/s from 75 to 800 °C. In general, the heat release rate (HRR) is a measure of the amount of heat released per unit of time by a combustion process. The heat release capacity (HRC) is one of the most quantitative parameters used to evaluate the flammability of a material, which can be calculated by dividing the maximum HRR by the heating rate.50 The total heat release (THR) can be represented by the area under the heat release rate curve. In general, the lower the HRC and THR are, the less flammable the materials are.51 The HRC values of multialiphatic amine-based polymers are listed in Table 7. Typically, when the HRC is lower than 300 J/g K, materials are deemed self-extinguishing, while materials with values less than 100 J/g K are considered nonignitable.51 It is evident that all the aliphatic amine-based polybenzoxazines synthesized in this study show excellent antiflammability. Among them, poly(EU-eda), poly(EU-1,2pda), and poly(EU-tapa) exhibited outstanding antiflammability and can be nonignitable polymers since their HRC values are 85, 86, and 77 J/g K, respectively. Compared to poly(BA-a), an extensively studied benzoxazine based on bisphenol A and aniline with an HRC value of 320 J/g K,52 aliphatic multiamine-based benzoxazines containing eugenol exhibit more prominent thermal properties. Therefore, incorporating a eugenol moiety into aliphatic multiamine-based polybenzoxazine leads to excellent flame retardancy, making this series of polymers attractive for applications with high thermal requirements.

Figure 11.

Figure 11

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Microcombustion calorimetric thermograms of all the polymers studied.

Table 7. Heat Release Properties of All of the Aliphatic Amine-Based Polybenzoxazines.
polymers HRC (J/g K) THR (kJ/g)
poly(EU-eda) 85 7.58
poly(EU-hda) 161 13.50
poly(EU-1,2pda) 86 12.35
poly(EU-1,3peda) 129 11.00
poly(EU-2mpda) 104 11.30
poly(EU-1,2dch) 153 20.50
poly(EU-ipda) 110 14.10
poly(EU-tapa) 77 7.88
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Conclusions

In this work, to explore the potential advantages of different kinds of aliphatic multiamine-based benzoxazines, eight aliphatic multiamine-based benzoxazines were synthesized successfully from biobased phenol (eugenol) and paraformaldehyde via a solventless synthesis method. To achieve relatively high purity, recycling preparative SEC was used for further purification. The chemical structures and purities of all of the benzoxazine monomers were confirmed by 1H NMR, 13C NMR, and FTIR spectroscopy. Moreover, in the DSC thermograms of all of the aliphatic multiamine-benzoxazine monomers, the presence of an extra exothermic peak from 290 to 350 °C other than the ring-open polymerization peak from 220 to 260 °C was studied by FTIR, DSC, and TGA and discussed. The Tg values of all of the polymers were examined by DSC, which exhibited a temperature range from 92 to 188 °C. The activation energy of polymerization was investigated by DSC of the monomer at different heating rates and was calculated to be between 99–148 kJ/mol (Kissinger) and 102–148 kJ/mol (modified Ozawa). In addition, the thermal stability and flame retardation of the polymers were studied using the TGA and MCC results. It is inferred that all of the polymers in this work have very good thermal stability and excellent flame retardancy with LOI values from 28 to 45 and HRC values from 77 to 161 J/g K.

The aliphatic amine-based polybenzoxazines have their own advantages, in addition to being economical and readily accessible. First, all of the branched aliphatic amine-based benzoxazines are liquid benzoxazines, even in high purity at room temperature for composite manufacturing. Second, most of them reveal a lower glass transition temperature and slightly lower activation energy compared to the traditional aromatic amine-based benzoxazines. Lastly, most of them still have good thermal stability and outstanding flame retardation, which provide unique potential as a replacement for other ordinary performance polymers, such as epoxies and vinyl esters, with the unique advantages of benzoxazine chemistry.

Acknowledgments

The authors are indebted to the financial support of MRA Systems, LLC., Baltimore, MD.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c11620.

  • 1H NMR spectra of EU-eda (a) and 13C NMR spectra of EU-eda (b); DSC curves of EU-eda at different heating rates; the Kissinger(a) and modified Ozawa(b) methods plots for EU-eda (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao4c11620_si_001.pdf (899.9KB, pdf)

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