Copolymerization and phase separation behaviors of benzoxazine–amine thermosets

Abstract

The ring-opening addition reaction of benzoxazine with amines offers advantages in terms of curing temperature and control of material characteristics. In the present article, a benzoxazine monomer, 3,3′-sulfonyl-bis(4,1-phenylene)-bis(3,4-dihydro-2H-1,3-benzoxazine) (BZS), was synthesized from 4,4′-diaminodiphenyl sulfone (DDS), paraformaldehyde and phenol. The copolymerization and phase separation behaviors of BZS/DDS curing systems were studied. Experimental results show that there are competitive copolymerization paths in the curing process, and, moreover, that these copolymerization paths may result in phase separation in the cured resin. These findings can be used for improving polybenzoxazine properties such as toughening, and thus have potential value for various applications such as in the development of coatings, adhesives, and composites.

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1. Introduction

Polybenzoxazines are a class of thermosetting resins that have unusual material properties.^1–3 Their mechanical performance and thermal stability are distinctive and offer significant advantages when compared with other thermosets.^4–12 In particular the molecular design flexibility of polybenzoxazines has gained increased interest in the field of high-performance polymers, because numerous benzoxazines can be synthesized using specially selected phenol derivatives and primary amines, and desired material properties can be incorporated into the final polybenzoxazine resin.^13–19

It has been widely recognized that the polymerization of benzoxazine monomers follows a cationic mechanism,^20–22 and several accelerators that can increase the polymerization rate have been successfully developed.^23–26 However, the ring-opening addition reaction, which is similar to curing epoxy with various hardeners, has recently attracted growing attention. Few compounds, including phenolic compounds,^27–31 thiols,^32–36 benzimidazoles,³⁷ benzoxazoles,^38,39 chitosan ammonium,⁴⁰ and some polymers containing aromatic structures,^41–43 have been reported to construct chemical linkages with benzoxazine.

In our previous work,⁴⁴ the ring-opening addition reaction of benzoxazine with an amine was demonstrated. The reaction mechanism of the benzoxazine–amine curing system is more complex than the polymerization of a benzoxazine monomer. Possible products include aminomethanaminium structures and aminomethylphenol structures. By addition of an amine, benzoxazine monomers can be cured at mild temperatures, and the material characteristics of the amine thermally-cured benzoxazine resin can be further tuned by variations in the amine structure.

To tailor the polymer network structures of the amine–benzoxazine resin precisely, we further studied the complex reaction mechanism of the amine/benzoxazine curing system at elevated temperatures. Experimental results showed that there are competitive copolymerization reactions in the curing process, and the resultant network structures depend on the reactivities of the amine and the benzoxazine monomer. Moreover, copolymerization may produce separated phases in the cured resin. Therefore, a suitable amine/benzoxazine curing system is an effective approach to improve some material properties like increasing toughness.

In the present article, a benzoxazine monomer, 3,3′-sulfonyl-bis(4,1-phenylene)-bis(3,4-dihydro-2H-1,3-benzoxazine) (BZS), is synthesized from 4,4′-diaminodiphenyl sulfone (DDS), paraformaldehyde and phenol. The BZS monomer is further reacted with DDS in the presence of an accelerator, methyl p-toluenesulfonate (PTSM), to synthesize a special amine cured polybenzoxazine resin. The copolymerization and phase separation behaviors during the curing process are investigated by analysis of the mechanical properties and thermal stability of the polybenzoxazine resin.

2. Experimental section

2.1 Materials

DDS, paraformaldehyde, and PTSM were purchased from Aladdin Reagent Co. Ltd (Shanghai, China). Bisphenol F based benzoxazine (BF-a, 75 wt% in butanone) was obtained from Huntsman investment Co., Ltd (Utah, USA). All other chemicals were of analytical grade and were purchased from Hangzhou Mike Chemical Agents Co. Ltd (Hangzhou, China).

2.2 Measurements

¹H NMR and ¹³C NMR spectra were recorded on an Avance AV-400 (400 MHz) NMR spectrometer (Bruker Co., Switzerland) in dimethylsulfoxide-D6 (DMSO-D6) with TMS as an internal standard. FTIR spectra were obtained using a Nicolet 5700 FTIR plus spectrometer (Nicolet Company, USA). Differential scanning calorimetry (DSC) data were obtained using a DSC-Q2000 (TA Instruments, USA) at a heating rate of 10 °C min⁻¹ in a nitrogen flow (35 mL min⁻¹) for all tests. Dynamic mechanical analyses (DMA) were performed with a DMA-1 analyser (Mettler-Toledo Corp., Switzerland) using the controlled single cantilever bending mode (1 Hz, 15 mm amplitude) at a heating rate of 3 °C min⁻¹ from 25 °C to 270 °C. The samples were of 8.0 mm width and approximately 0.6 mm thickness.

The surface morphology of the cured resins was observed by utilizing a JSM-6700F field emission scanning electron microscope (FE-SEM, JEOL, Japan). Samples that fractured in liquid nitrogen were coated with a gold coating (with a thickness of approximately 10 nm).

A simultaneous TGA/GC-MS coupling system was used to analyse the thermal stability and the products of the pyrolysis of the cured resins at a heating rate of 10 °C min⁻¹ from 35 °C up to 1000 °C under a helium gas flow of 20 mL min⁻¹. The TGA/GC-MS coupling system consists of four individual devices: a thermal gravimetric analyser (TGA, DSC1, Mettler-Toledo corp., Switzerland), sample storage instrument (SRA IST16, Mettler-Toledo corp., Switzerland), gas chromatograph (GC, 7890 GC, Agilent Technologies Inc., USA), and mass spectrometer (MS, 5975MSD, Agilent Technologies Inc., USA). The volatiles (250 μL) were collected simultaneously from the TGA device, stored temporarily in the SRA IST16 sample storage instrument, and injected one by one into the GC-MS device. The GC separation was carried out using an Agilent 19091s-433 capillary column (30 m, 0.25 mm i.d., 0.25 μm thickness). The injector temperature was set at 250 °C in the split mode with a split ratio of 10 : 1, and helium gas was used as the carrier gas at a constant flow of 1.2 mL min⁻¹. The MS transfer line and oven temperatures were set at 150 °C and 120 °C, respectively. The decomposition products were identified using the mass spectrum library attached to the GC/MS system.

2.3 Preparation of 3,3′-sulfonyl-bis(4,1-phenylene)-bis(3,4-dihydro-2H-1,3-benzoxazine) (BZS)

As shown in Fig. S1,† DDS (12.42 g, 0.05 mol), paraformaldehyde (6.00 g, 0.20 mol), and phenol (9.41 g, 0.10 mol) were mixed, heated at 100 °C for one hour, cooled to 25 °C, recrystallized in dimethyl formamide (DMF), washed with water (300 mL × 3), and dried under vacuum at 80 °C for 24 h to obtain a light yellow solid (BZS). ¹H NMR (DMSO-D6): 6.74–7.94, 5.47, 4.70 and 2.54 ppm; ¹³C NMR (DMSO-D6): 153.56, 151.09, 131.89–115.37, 76.94, 48.13 ppm; FTIR (KBr) 1589, 1501, 1384, 1293, 1144, 1104, and 954 cm⁻¹.

2.4 General formulation of the BZS/DDS curing systems

BZS (4.84 g, 0.01 mol), DDS (2.48 g, 0.01 mol), and PTSM in a certain ratio (Table 1) were mixed in tetrahydrofuran (THF) solvent (10 mL), dried at 60 °C overnight and cured at 120 °C, 160 °C, 200 °C, and 220 °C for 0.5 h, 0.5 h, 0.5 h, and 1 h, respectively. In total, four curing systems (MBZ, 1–4) were prepared from BZS/DDS mixtures and one control system (BZS1) was prepared from BZS without any amine (BZS/PTSM = 1/0.01).

Table 1 DSC data of the curing systems

Curing system	Molar ratio BZS : DDS : PTSM	Peak 1			Peak 2
		To (°C)	Tp (°C)	ΔH (J g^−1)	To (°C)	Tp (°C)	ΔH (J g^−1)
Pure BZS	1 : 0 : 0	—	—	—	210	243	105
MBZ1	1 : 1 : 0	172	196	13	234	259	19
MBZ2	1 : 1 : 0.01	165	188	7	230	257	25
MBZ3	1 : 1 : 0.05	139	174	20	219	253	47
MBZ4	1 : 1 : 0.08	148	175	5	212	246	51

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2.5 Preparation of BZS/DDS composite materials

BZS (4.84 g, 0.01 mol), DDS (2.48 g, 0.01 mol), and PTSM in a certain ratio (Table 1) were mixed in THF (5 mL) for three minutes and degassed for three minutes using a conditioning mixer (AR-100, Thinky, Japan). Four pieces of standard filter paper (having an average pore size of 18 μm) were dipped in the mixtures, squeezed to a weight of up to 200%, clamped in polyimide sheets, dried at 80 °C under vacuum to remove the solvent, and cured at escalated temperatures (120 °C/0.5 h, 160 °C/0.5 h, 200 °C/0.5 h, and 220 °C/1 h). The resultant composite samples (CMBZ, 1–4) were subjected to DMA tests (10 mm × 6 mm × 0.6 mm). As a control sample, the BZS1 sample was prepared using a similar process but DDS was not used (BZS : PTSM = 1 : 0.01).

3. Results and discussion

3.1 Polymerization of the MBZ mixtures

The curing processes of the BZS/DDS/PTSM mixtures were monitored by DSC at a heating rate of 10 °C min⁻¹ from 25 °C to 300 °C. As shown in Fig. 1 and Table 1, the neat benzoxazine, BZS, displays exothermic peaks in the temperature range from 210 °C to 243 °C. In contrast, addition of DDS results in three exothermic peaks at lower temperatures. The ΔH values (integration of the peaks) increase with the increase in the PTSM dosage. The multiple exothermic peaks suggest that complex reaction mechanisms are present in the curing process.

Fig. 1 DSC thermograms of the mixtures :(a) MBZ1, (b) MBZ2, (c) MBZ3, (d) MBZ4, and (e) pure BZS.

To further investigate the curing degree of the BZS/DDS mixtures under isothermal conditions, the curing mixtures of BZS1 and MBZ2 were cured progressively at 120 °C/0.5 h, 160 °C/0.5 h, 200 °C/0.5 h, 220 °C/0.5 h and 220 °C/1 h. The resulting samples that were cured in each cycle were characterized using DSC at a heating rate of 10 °C min⁻¹. As shown in Fig. 2, MBZ2 was completely cured in the 200 °C cure cycle, while BZS1 was cured in the 220 °C cycle. These results suggest that the addition of DDS promotes the curing process, i.e., lowers the curing temperature and shortens the curing time.

Fig. 2 DSC thermograms of the mixtures cured at (a) 120 °C/0.5 h, (b) 160 °C/0.5 h, (c) 200 °C/0.5 h, (d) 220 °C/0.5 h, and (e) 220 °C/1 h (dotted lines: BZS1; solid lines: MBZ2).

Moreover, the resin samples cured under different isothermal conditions were characterized by FTIR. As show in Fig. 3a, the characteristic absorption bands at 1039 cm⁻¹ and 1230 cm⁻¹ are assigned to the C–O–C symmetric and asymmetric stretching modes, respectively. The characteristic peak at 955 cm⁻¹ is attributable to the benzene with an attached oxazine ring.⁴⁵ The three peaks disappeared after the curing cycle of 200 °C/0.5 h, suggesting that the ring-opening polymerization of BZS1 had completely finished.

Fig. 3 FTIR spectra of the cured resins of (a) BZS1 and (b) MBZ2. Curing conditions: (1) 25 °C/one day; (2) 120 °C/0.5 h; (3) 120 °C/0.5 h, and 160 °C/0.5 h; (4) 120 °C/0.5 h, 160 °C/0.5 h, and 200 °C/0.5 h; (5) 120 °C/0.5 h, 160 °C/0.5 h, 200 °C/0.5 h, and 220 °C/0.5 h.

However, with the addition of DDS, the three peaks were clearly reduced when the MBZ2 mixture was stored at 25 °C. It is noteworthy that the absorptions at 1186 cm⁻¹ assignable to the C–N–C bond increased with the elevated curing temperatures. These results demonstrate the reversible ring-opening reaction of BZS with DDS, as demonstrated in our previous work.⁴⁴ After the curing cycle at 160 °C/0.5 h, the three peaks of the MBZ2 mixture had completely disappeared. Comparison of the FTIR spectra shown in Fig. 3a again supports the assertion that DDS has an acceleration effect on the BZS curing process.

3.2 Phase separation of the cured MBZ mixtures

The MBZ mixtures were cured to composite samples with standard filter paper (with an average pore size of 18 μm) as a reinforcement filler to reduce the errors generated in the forming process, and were evaluated using DMA. The DMA curves shown in Fig. 4a indicate that the addition of DDS can give the cured resins a high storage modulus (ranging between 4.4–5.3 GPa), meaning that there is a higher cross-linking density in the MBZ resins than in the BZS resins. The maximum value, 5.3 GPa, was obtained when the BZS : DDS : PTSM mol ratio was 1 : 1 : 0.01. The tan δ curves (Fig. 4b) show the viscoelastic transition of the cross-linked polymers, and further exhibit structural information about the cured resins. In particular, the glass transition temperatures (T_g), which can be reflected by the tan δ peaks, supply some interesting information about the phase separation in the cured resins. The cured MBZ resins exhibited multiple tan δ peaks, suggesting that the cured resins have separated phases. In contrast, the BZS1 resin without DDS has only a simple tan δ peak, suggesting that the resin is homogeneous. It can be concluded based on these DMA results that complex copolymerization mechanisms continued through the curing process.

Fig. 4 DMA spectra of the cured resins. Storage modulus (a) and tan δ (b).

The phase separation behaviors of the cured resins were further studied using SEM. The cured samples of BZS1, MBZ2, and MBZ4 were fractured in liquid nitrogen, coated with a thin gold layer, and examined by SEM. For the BZS1 resin, a smooth surface was observed by SEM, suggesting that the cured BZS1 resin has a homogeneous phase (Fig. 5a). In the cases of MBZ2 and MBZ4, the fractured interfaces are rough, indicating that the cured MBZ2 and MBZ4 resins have heterogeneous structures. These morphological results are in good agreement with the suggestions obtained from the DMA studies.

Fig. 5 SEM images of the fractured surfaces of the cured resins: (a) BZS1, (b) MBZ2, and (c) MBZ4.

Finally, the thermal stability and the thermally decomposed products of the cured resins were investigated using TGA/GC-MS techniques. The decomposed small chemicals in the volatiles were expected to offer further structural information on the network polymers.

Fig. 6 and Table 2 compare the thermograms of the cured resins in the 35–1000 °C range. By comparing with the BZS1 resin, the MBZ1 and MBZ2 resins showed higher decomposition temperatures at 5% weight loss (T_d5) but lower char yields. However, MBZ3 and MBZ4 resins showed lower T_d5 values and higher char yields than MBZ2.

Fig. 6 TGA curves of the cured resins.

Table 2 TGA data for the cured resins

Curing system	Molar ratio BZS : DDS : PTSM	Td5 (°C)	Td10 (°C)	CY at 800 °C (%)
BZS1	1 : 0 : 0.01	337	373	58.2
MBZ1	1 : 1 : 0	345	369	48.5
MBZ2	1 : 1 : 0.01	341	353	51.8
MBZ3	1 : 1 : 0.05	330	351	56
MBZ4	1 : 1 : 0.08	328	348	57.2

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By using the coupling GC-MS technique, small chemicals discharged from the decomposed resins of BZS1 (Fig. S2†), MBZ2 (Fig. S3†), and MBZ4 (Fig. S4†) were analysed. As shown in Fig. 7, and Table 3, the main decomposition products are toluene, aniline and phenol derivatives. Notably, aniline is the most important compound, released almost throughout the TGA processes of all the resin samples.

Fig. 7 Organic compounds detected from the decomposed volatiles using the TGA-GC/MS technique.

Table 3 Small chemicals detected from the decomposed volatiles using the TGA-GC/MS technique

Resins	Storage 1 (29.5 min)	Storage 2 (34 min)	Storage 3 (40 min)	Storage 4 (72 min)
BZS1	SO2, D1, D2, D3, and D4	SO2, D1, D2, D3, and D5	SO2, D1, D2, D3, and D5	CO2, D1, D3, and D6
MBZ2	SO2, D1, D3, D5, and D7	SO2, D1, D3, D5, and D7	SO2, D1, D3, and D8	CO2, D1, D3, and D6
MBZ4	SO2, D1, D2, D9, and D10	SO2, D1, D2, D9, and D10	SO2, D1, D2, D8, D9, and D10	CO2, D1

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The compositional differences between the volatiles discharged from the BZS1, MBZ2, and MBZ4 resins suggest that the MBZ resins have different polymer structures from the BZS1 resin. For example, compound D7 existed in the volatiles from MBZ2 and MBZ4 resins, but was not found in the volatiles from BZS1. The compound D3 was found in the volatiles from the BZS1 and MBZ2 resins, but not in the volatiles from MBZ4. However, the MBZ4 resin released the distinct compounds D9 and D10. These results are discussed with possible reaction mechanisms in the next section.

3.3 The proposed reaction mechanism

We found that the reaction behaviours of BZS and DDS are in good agreement with our previous report.⁴⁴ As shown in Fig. 8, the reaction of DDS and BZS produces multiple types of network structures by heating at an elevated temperature. These polymer networks form heterogeneous structures, which were reflected by the multiple T_g values, and were observed by SEM (Fig. 5).

Fig. 8 Possible mechanisms of the copolymerization of a BZS/DDS mixture.

As shown in Fig. 8, in the first reaction, the typical nucleophilic substitution of an amine onto the oxazine ring of BZS results in an intermediate polymer (IP). This linear polymer decomposes by heating, producing two iminium ions, IM1 and IM2. These intermediates react with other aromatic rings and give new polymer network structures. However, a few unreacted residues of the IP chains may remain in the cured resins, leading to the separated phases in the cured resins (phase 1). For the tan δ curves of the MBZ resins (Fig. 4b), the small peaks observed at low temperatures from 50 °C to 100 °C are attributable to the remaining IP chains.

Despite the electron donating effect of the methylene group of IM2, the iminium ion IM1 is more chemically reactive with aromatic rings than IM2. This suggestion has been confirmed by the DSC and FTIR results described above. The addition of DDS to BZS promotes the curing process, leading to shorter curing times at lower curing temperatures when compared with the curing process of pure BZS.

Possible mechanisms for the curing reaction of the DDS/BZS mixtures may involve electrophilic substitution reactions of the IM1 intermediate with different aromatic rings as shown in Fig. 8. These reactions produce network structures like PN1 and PN2, which form the separation phase (phase 2) corresponding to the T_g at 170 °C (Fig. 4b), and the small decomposition products D3, D5, and D7 (Fig. 7).

With an increase in the PTSM dosage, the reaction rate of IM2 with aromatic rings increases, leading to some advantages when in competition with the cross-linking reactions of IM1. It has been reported that PTSM is an initiator for the cationic ring-opening polymerization of benzoxazine.^23,46 The acceleration role of PTSM in the BZS/DDS mixtures is similar to these reports. Due to the steric hindrance of the tertiary amine structure in the IP, PTSM prefers to promote the IM2 elimination path. The reactions involved with IM2 produce new polymer network structures like PN3 and PN4, forming the new solid phases phase 3 and phase 4, which correspond to the T_g values of 150 °C and 200 °C, respectively (Fig. 4b). The MBZ3 resin has two T_g values of 150 °C and 170 °C, suggesting that the two competitive paths (IM1 and IM2) are almost on the same level. When the PTSM dosage was over 8% for BZS (MBZ4), the IM2 reactions become predominant, and produced two polymer network structures, PN3 and PN4, over different curing temperature ranges. The two tan δ peaks at 150 °C and 200 °C shown in Fig. 4b can be ascribed to phase 3 and phase 4, respectively.

4. Conclusions

In summary, we have described the copolymerization and phase separation behaviors of curing systems containing a benzoxazine monomer, 3,3′-sulfonyl-bis(4,1-phenylene)-bis(3,4-dihydro-2H-1,3-benzoxazine), and a diamine, 4,4′-diaminodiphenyl sulfone, heated at elevated temperatures. FTIR and DSC results showed that addition of DDS promotes the curing process of BZS, leading to curing temperature decreases and cross-linking density increases in the cured resins. DMA results and SEM images of the fractured interfaces showed that phase separation occurred during the curing process. TG-GC/MS analysis of the small decomposition products discharged from the cured resins indicated that addition of PTSM to the BZS/DDS mixtures determines the polymer network structures and the phase separation of the cured resins. The polymerization mechanism of the BZS/DDS mixtures consists of competitive copolymerization pathways after the production of a linear polymer. Two intermediate iminium ions are generated by the decomposition of the linear polymer, and further react with aromatic rings to form cross-linking structures. The copolymerization ratio between the competitive paths is varied by increasing the addition of PTSM, and can even cause separated phases in the cured resins. These results represent the first demonstration of the copolymerization and phase separation behaviors of a benzoxazine/amine curing system. These findings can be used for improving polybenzoxazine properties such as toughening, and thus have potential value for various applications such as in the development of coatings, adhesives, and composites.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51573167).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23509c

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