Curing Reaction of Benzoxazine Under High Pressure and the Effect on Thermal Resistance of Polybenzoxazine

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1 FULL PAPER Curing Reaction Curing Reaction of Benzoxazine Under High Pressure and the Effect on Thermal Resistance of Polybenzoxazine Ka Zeng, Jiayue Huang, Junwen Ren, and Qichao Ran* Two benzoxazine monomers, bisphenol A/aniline type benzoxazine (BA-a) and bisphenol A/tert-butylamine type benzoxazine (BA-tb), are prepared with high purity. Their curing behaviors and cure kinetics are studied by high pressure differential scanning calorimetry (HP DSC) at normal pressure and high pressure, respectively. Meanwhile, the apparent activation energy (E a ) is calculated by Kissinger and Ozawa methods. The results show that the curing reaction temperature and E a of BA-a are little influenced by high pressure, but for BA-tb, its curing reaction temperature and E a increase greatly. FTIR is used to analyze the difference in the hydrogen bonds in cross-linking structures of cured BA-a and BA-tb. Also, molecular simulations are used to figure out the effect of the high pressure on chemical structures. It is found that the content of hydrogen bonds in polybenzoxazines cured at high pressure increases, resulting in the decrease of cross-linking densities. Moreover, DSC and thermogravimetric analysis tests are carried out to study the thermal resistance of these polybenzoxazines, and the results indicate that their glass transition temperatures (T g s) and thermal stability decrease after curing at high pressure. 1. Introduction Polybenzoxazine is a kind of high-performance thermosetting resin with excellent properties, such as high thermal stability and T g, high char yield, low water absorption, near zero volumetric shrinkage during curing, and extraordinary molecular design flexibility. [1] The ring-opening reaction of benzoxazine can proceed by heating or catalysts to form cross-linking network structures. [2 6] Taking bisphenol A/aniline type benzoxazine (BA-a) as an example, its cross-linking structures may contain phenolic Mannich bridge structure, arylamine Mannich bridge structure, and methylene bridge structure. [7 10] Besides these basic covalent cross-linking structures, there are many intramolecular and intermolecular hydrogen bonds in polybenzoxazine. [11 13] These hydrogen bonds make polybenzoxazines possess good mechanical and heat-resistant properties in spite K. Zeng, J. Huang, Dr. J. Ren, Dr. Q. Ran College of Polymer Science and Engineering State Key Laboratory of Polymer Materials Engineering Sichuan University Chengdu , China ranqichao@scu.edu.cn The ORCID identification number(s) for the author(s) of this article can be found under DOI: /macp of low cross-linking density. [14] In fact, the formation of the hydrogen bonding during polymerization may affect the process of the curing reaction and the cross-linking structures of resulted polymers. Chirachanchai et al. found that some monofunctional benzoxazines were more likely to form oligomers rather than polymers during the ring-opening poly merization. They attributed it to the difficulties of further polymerization obstructed by strong intramolecular hydrogen bonding between the hydroxyl groups of the phenolic rings and the nitrogen atoms in the dimer. [15] The research from Phongtamrug et al. showed that the backbone molecules could get self-stabilized with the hydrogen bonding network, and the activity of further cross-linking reaction was decreased. [16] Laobuthee et al. believed that the formation of intramolecular hydrogen bonding would result in the deactivation of the phenolic ring, which made the electrophilic substitution reaction of immonium ions only happen in the neighborhood of other benzene rings. [17] Bai et al. found that the OH N hydrogen bond hindered further polymerization of benzoxazines and inhibited the increase of the cross-linking density. [18] The published studies mainly focused on the curing mechanism and curing kinetics of benzoxazines at normal pressure. [19 26] However, the effect of the pressure on the formation and types of hydrogen bonds in polybenzoxazine has been rarely reported. Thermosetting resins are often used as matrices for composites. [27 30] During the forming process of composites, the matrix usually cures at a certain pressure. Therefore, it is necessary to study the curing reaction and the properties of cured resin under the pressure. Lee et al. studied the effect of the pressure on the curing behavior of unsaturated polyester resin with initiator. They found that the curing reaction rate became slow and the final curing degree was limited under the pressure. They thought that the free volume effect that can block the curing reaction became predominant as the pressure was high enough. [31] The dependence between the pressure and curing behavior of epoxy resins was studied by Nakamae et al., who found that the T g and mechanical properties of the resin decreased with the increase of curing pressure. [32] They attributed it to the limiting effect of high pressure which hinders the diffusion and collision of molecules, and leads to low curing rate and incomplete curing reaction. Benzoxazine resin, as a matrix with good performance, has been used for preparing (1 of 9)

2 composites, but little is known about the effect of the pressure on its curing reaction. [33,34] In this paper, two bisphenol-a type benzoxazine monomers based on aniline and tert-butyl amine with high purity were synthesized, respectively. Both monomers were cured under normal pressure and high pressure. The effects of the pressure on their curing behaviors, polymer cross-linking structures, cross-linking densities, and thermal resistance were investigated. 2. Experimental Section 2.1. Materials Bisphenol-A, aniline, tert-butylamine, and formalin solution were used as received from Sigma-Aldrich. Sodium hydroxide, toluene, isopropanol, alcohol, and acetone were purchased from Fisher Scientific and used as received Syntheses of Benzoxazine Monomers Synthesis of Bisphenol A/Aniline-Based Benzoxazine Toluene (45 ml) and bisphenol A (0.1 mol) were first added into formalin solution (38%, 0.4 mol) with a ph of 8 (adjusted by NaOH solution) at room temperature. Then, aniline (0.2 mol) was dropped into the mixture solution at 50 C. The mixture was refluxed at 80 C for 5 h. The resin solution was washed with NaOH solution and then rinsed with water for several times. The solvent was removed under vacuum and followed by washing with alcohol to get white powder. Finally, the crude product was recrystallized in a mixture solvent of toluene and acetone. After several days, some colorless rod-shaped crystals were obtained from the solution. The experimental procedure is shown in Scheme S1a, Supporting Information. 1 H NMR (600 MHz, CDCl 3, δ): (m, 16H, Ar H), 5.34 (s, 4H, N CH 2 -O), 4.59 (s, 4H, O CH 2 Ar), 1.56 (s, 6H, CH 3 ); IR (KBr) (v, cm 1 ): 1292, 1035 (vs, Ar O C), 1116 (m, C N C), 948 (m, oxazine ring), 690, 750 (m, monosubstituted benzene). IR and 1 H NMR spectra of BA-a can be seen in Figures S1a and S2a, Supporting Information, as well as the picture of its crystal (Figure S3a, Supporting Information) Synthesis of Bisphenol A/Tert-butylamine-Based Benzoxazine Similar procedure was conducted to synthesize bisphenol A/ tert-butylamine (BA-tb) as shown in Scheme S1b, Supporting Information. The product of BA-b was recrystallized in isopropanol and white snowflake crystals were obtained. 1 H NMR (600 MHz, CDCl 3, δ): (w, 6H, Ar H), 4.92 (s, 4H, N CH 2 O), 4.02 (s, 4H, O CH 2 Ar), 1.58 (s, 6H, CH 3 ), 1.16 (s, 18H, CH 3 ); IR (KBr) (v, cm 1 ): 1218, 1025 (vs, Ar O C), 1367, 1134 (C N C), 922 (m, oxazine ring). IR and 1 H NMR spectra of BA-tb can be seen in Figures S1b and S2b, Supporting Information, as well as the picture of its crystal (Figure S3b, Supporting Information) Preparation of Polybenzoxazine BA-a and BA-tb were cured under normal and high pressure in a HP DSC instrument, respectively. The monomers cured under normal pressure were denoted as BA-a-n and BA-tb-n, respectively. The cured samples were denoted as poly(ba-a-n) and poly(ba-tb-n). The monomers for curing under pressure were recorded as BA-a-p and BA-tb-p, respectively, and the cured samples were poly(ba-a-p) and poly(ba-tb-p). BA-a was cured at 200 C for 2 h while BA-tb was cured at 180 C for 2 h. A high pressure of 750 psi was used for curing Characterization The 1 H NMR spectra were recorded on a Varian Oxford AS600 (600 MHz) using chloroform (CDCl 3 ) as a solvent with tetramethylsilane (TMS) as an internal reference. FTIR spectra were measured by a Bomem Michelson MB100 FTIR spectrometer, which was equipped with a deuterated triglycine sulfate (DTGS) detector and a dry air purge unit. Thirty-two scans were coadded to obtain spectrum at a resolution of 4 cm 1. DSC measurements were conducted with a TA Instruments DSC Model 2920 under a constant flow of nitrogen (60 ml min 1 ) at different heating rates of 5, 10, 15, 20, and 25 C min 1. High pressure DSC tests were done using a TA Instruments HP DSC Model The high pressure was supplied by a nitrogen cylinder. The air in the cell was substituted with the nitrogen three times before test. The curing of the samples (2.0 ± 0.5 mg) was sealed using hermetic aluminum pans and covered with lids. Modulated DSC (MDSC) was made with a TA Instruments Q20 under a constant flow of nitrogen (50 ml min 1 ) at a heating rate of 2 C min 1. The modulation mode was +/ 2 C 60 s 1. Thermogravimetric analysis (TGA) was performed by a TA Instruments Q500 TGA under nitrogen at a heating rate of 10 C min 1 up to 800 C. Molecular simulation was conducted with Material Studio 4.0. An amorphous cell was built optimized with NPT dynamics optimization to simulate the supposed curing system, and the cross-linking bonds were built manually. Hydrogen bonds of each system were calculated after complete curing. 3. Results and Discussion 3.1. Curing Behavior of Benzoxazine The polymerization of BA-a and BA-tb under normal pressure and high pressure was studied by DSC, and the results are shown in Figure 1 and Table 1. The results showed that the curing temperatures of both benzoxazines increased under the pressure. The exothermic peaks of BA-a and BA-tb increased by 4 and 9 C, respectively, which indicates the pressure has a significant influence on the curing reaction of benzoxazine. Owing to the existence of the tertbutyl structure, BA-tb is more sensitive to the pressure than BA-a with rigid phenyl-ring structure. In addition, the total exothermic reaction enthalpies of both pressurized curing systems, BA-a-p and BA-tb-p, were lower than those of BA-a-n and BA-tb-n, respectively, (2 of 9)

3 Figure 1. DSC curves of a) BA-a-n, b) BA-a-p, c) BA-tb-n, and d) BA-tb-p at a heating rate of 20 C min 1. suggesting that the polymerization of benzoxazine monomers can be restrained by high pressure. Moreover, it can be found that the enthalpies of BA-a system were bigger than those of BA-tb system, which may be attributed to more reactive sites on aniline ring of BA-a Curing Kinetics of Benzoxazine The effect of the pressure on curing reaction kinetics of BA-tb and BA-a was further studied by non-isothermal DSC. DSC curves of BA-tb and BA-a under atmospheric and high pressure at different heating rates are shown in Figure 2 and Figure S4, Supporting Information, respectively. It can be observed that the exothermic peaks shifted to a higher temperature with higher heating rate under normal pressure or high pressure as summarized in Table 2 and Table S1, Supporting Information. The apparent activation energy (E a ) is an important parameter to the reactivity of the curing reaction. The curing reaction cannot take place unless the molecules involved in the reaction get greater energy than the activation energy. Generally, the larger the E a is, the more difficult the reaction will be, Table 1. DSC data of benzoxazine monomers. Sample T i [ C] T p [ C] T f [ C] H [kj mol 1 ] BA-a-n BA-a-p BA-tb-n BA-tb-p Figure 2. DSC curves of BA-tb-n and BA-tb-p at different heating rates. and the higher the reaction temperature is required. According to Kissinger and Ozawa methods, E a can be calculated by Equations (1) and (2), respectively. [35] β = AR T E Ea ln ln 2 p a RT p Ea lnβ = C RT p where β is the heating rate, A is the frequency factor, T p is the temperature of the exothermic peak, E a is the activation energy, R is the gas constant, and the C is a constant. Figure 3 shows the plot of ln(β/t p 2 ) and ln(β) against 1/T p for BA-a and BA-tb according to Kissinger and Ozawa methods. E a (1) (2) (3 of 9)

4 Table 2. DSC data of BA-tb-n and BA-tb-p at different heating rates. Sample Heating rate [ C min 1 ] T i [ C] T p [ C] T f [ C] H [kj mol 1 ] BA-tb-n BA-tb-p Table 3. Average activation energies of BA-a and BA-tb obtained from Kissinger and Ozawa methods. Kinetic methods BA-a-n [kj mol 1 ] BA-a-p [kj mol 1 ] BA-tb-n [kj mol 1 ] BA-tb-p [kj mol 1 ] Kissinger Ozawa of each system as shown in Table 3 was obtained by the slope. The E a s calculated by Kissinger and Ozawa methods are similar to each other, which verifies the reliability of the results. For BA-a, its E a at atmospheric pressure was 114 to 119 kj mol 1, and its E a decreased slightly after pressurization. For BA-tb, the E a value increased significantly from 96 to 99 kj mol 1 under normal pressure to 114 to 116 kj mol 1 under high pressure, which meant that the pressure had little effect on the polymerization of BA-a, but it obviously inhibited the polymerization of BA-tb, which is consistent with the results of previous curing behaviors. Comparing with the results of BA-a and BA-tb, it can be concluded that the effect of the pressure on curing behaviors of BA-tb and BA-a is different. The chemical structure of BA-a contains more benzene rings that can form stiff chains. This kind of chain structure is difficult to move during further curing. Therefore, the pressure has little effect on the movement of the chain segments. However, BA-tb contains relatively flexible alkane chains. When suffering from the high pressure, the movement of the segments is easy to be restrained, which makes the collision and diffusion of molecules more difficult and requires higher energy to polymerization Cross-Linking Structures of Polybenzoxazines Figure 3. Ln(β/T p 2 ) and ln(β) versus 1/T p plots of BA-a and BA-tb. In order to study the effect of the pressure on the cross-linking structures of BA-a and BA-tb, FTIR analyses of the samples cured by isothermal DSC under normal and high pressure were carried out, respectively, and the spectra are shown in Figures 4 and 5. FTIR spectra were normalized based on the intensity of aliphatic methylene absorption at 2938 cm 1. As shown in Figure 4b, the characteristic absorption band of the oxazine ring at 948 cm 1 and the 1,2,4-trisubstituted benzene ring in 1492 cm 1 disappeared under normal pressure. In addition, the mono-substituted benzene ring at 692 and 750 cm 1 remained unchanged, and 1,2,3,5-tetrasubstituted benzene ring appeared at 1482 cm 1, which indicates the formation of phenolic Mannich bridge. [36,37] The FTIR spectrum of poly(ba-a-p), as shown in Figure 4c, was almost the same as that of poly(ba-a-n). The results show that the pressure has little effect on the cross-linking structure of poly(ba-a). As can be seen from Figure 5b, the intensity of the characteristic band due to oxazine ring for BA-tb-n at 922 cm 1 obviously reduced. At the same time, the 1,2,4-trisubstituted benzene ring at 1500 cm 1 disappeared, and 1,2,3,5-tetrasubstituted benzene ring at 1480 cm 1 appeared. These changes on bands indicated that phenolic Mannich bridge structure was also formed for BA-tb after normal polymerization. Comparing Figure 5b with 5c, we can find that FTIR spectra of poly(ba-tb-p) and poly(ba-tb-n) (4 of 9)

5 The difference in FTIR spectra of BA-a and BA-tb curing under different pressure is mainly concentrated in the phenolic hydroxyl groups. As we know, lots of hydrogen bonds involved in phenol hydroxyl groups exist in cross-linking structures of polybenzoxazines, so we further studied the cases of the hydrogen bonds in all cured samples. In polybenzoxazine, the hydrogen bond donor is the OH produced by the ring-opening reaction, and the hydrogen bonding acceptors can be O and N atoms. As a result, OH N, O +HN, and OH O hydrogen bonds are formed.[38] Among them, OH O belongs to intermolecular hydrogen bond while OH N belongs to intramolecular hydrogen bond or intramolecular hydrogen bond. The band in the range of 3600 to 2000 cm 1 in the FTIR spectra of cured BA-a and BA-tb in Figures 4 and 5 were processed by Gaussian fitting, and the results are shown in Figures 6 and 7. The hydrogen bonds can be fitted into three Figure 4. FTIR spectra of a) BA-a, b) poly(ba-a-n), and c) poly(ba-a-p). were similar except a broad band around 3433 cm 1 due to the absorption of phenolic hydroxyl groups. Its intensity became higher after being cured under high pressure. The above results indicate that the pressure hardly changes the bridge structures from ring-opening reactions of BA-a and BA-tb, but makes BA-tb form more phenolic hydroxyl groups. Figure 5. FTIR spectra of a) BA-tb, b) poly(ba-tb-n), and c) poly(ba-tb-p). Figure 6. The analyses of FTIR spectra at 4000 to 2000 cm 1of poly(baa-n) and poly(ba-a-p) (5 of 9)

6 Figure 8. The relative contents of different hydrogen bonds of polybenzoxazines. hydrogen bonds. The possible cross-linking structures and hydrogen bonds of BA-tb cured under normal and high pressure are shown in Scheme Molecular Simulation Figure 7. The analyses of FTIR spectra at 4000 to 2000 cm 1 of poly(batb-n) and poly(ba-tb-p). To analyze the curing process of each benzoxazine monomer, we define the cross-linking density as N c /N t, where N c and N t are the consumed number of active atoms and the total number of active atoms, respectively. It is supposed that there is no decomposition while heating. The cross-linking structures of four polybenzoxazines based on molecular simulation are shown in Figure S5, Supporting Information, and the results are listed in Table 4. Two kinds of hydrogen bonds, OH N and OH O, were monitored in each system. For poly(ba-a-n), the occupancy of OH N in all hydrogen bonds was up to 90%, different types, that is, O + HN, OH N, and OH O. The relative contents of different types of hydrogen bonds were calculated based on their peak areas, and the results are shown in Figure 8. The relative content of O + HN of poly(ba-a-p) increased slightly, while the relative content of OH N decreased slightly. Moreover, the total relative contents of O + HN and OH N of poly(ba-a-p) was 81% that is slightly higher than 80% of poly(ba-a-n). However, the case of poly(ba-tb) was very different. The contents of intramolecular hydrogen bonds O + HN and OH N decreased obviously in poly(ba-tb-p). At the same time, the content of the intermolecular hydrogen bond OH O was greatly increased from 16% to 71%. This is because tertbutyl amine structure with a smaller volume in BA-tb deforms more easily under the pressure, which leads to the formation of more intermolecular OH O that may be transformed from intramolecular Scheme 1. The possible cross-linking structures and hydrogen bonds of poly(ba-tb-n) and poly(ba-tb-p) (6 of 9)

7 Table 4. Calculated hydrogen bond quantities and cross-linking densities of polybenzoxazines. Sample OH N OH O Cross-linking density [%] poly(ba-a-n) poly(ba-a-p) poly(ba-tb-n) poly(ba-tb-p) while this ratio rose to 96% when the pressure went up to 750 psi, even though the total number of all hydrogen bonds was almost the same in both systems. For poly(ba-tb), it can be observed that the ratio of OH O became high after being cured under high pressure, which is different from the situation of poly(ba-a). The fact is that the increase of the number of OH O for poly(ba-tb-p) agrees well with the above FTIR results. Furthermore, when the pressure increased from atmospheric pressure to 750 psi, the cross-linking densities of both poly(ba-a-p) and poly(ba-tb-p) decreased T g s of Polybenzoxazines Figure 9 shows modulated DSC curves of four polybenzoxazines cured under atmospheric pressure or high pressure. It can be observed that the T g of poly(ba-a-p) was 176 C, which is slightly lower than that of poly(ba-a-n). Similarly, the T g of cured BA-tb was also reduced from 102 to 94 C after being cured under high pressure. The results imply that the curing at high pressure could decrease the T g of polybenzoxazine. According to the analysis of the FTIR spectra, for the BA-a system, the total hydrogen bonds O + HN and OH O increased slightly, and these two kinds of hydrogen bonds may hinder polymerization to reduce cross-linking density, which is also proved by molecular simulation. It should be noted that, for BA-a, the reduction of the cross-linking density was not significant, so the T g of poly(ba-a-p) decreased slightly. But for BA-tb, the content of intermolecular OH O in poly(ba-tb-p) was greatly increased. It is also found that the cross-linking density of poly(ba-tb-p) has greater reduction than that of poly(ba-a-p) based on molecular simulation, so the decrease extent of the T g of poly(ba-tb-p) is higher than that of poly(ba-a-p). Additionally, it should be noted that the T g s of poly(ba-tb) were unobvious in the MDSC curves. So, we just used a change trend to give an imprecise T g. Even so, a transformation range can be found between 90 and 100 C for poly(ba-tb-p), and there was a small vibration between 100 and 110 C for poly(ba-tb-n) Thermal Stability of Polybenzoxazines TGA thermograms of these polybenzoxazines are shown in Figure 10, and the relevant data are summarized in Table 5. Obviously, the 5% weight-loss temperature (T d5 ), the 10% weight-loss temperature (T d10 ), and char yield at 800 C of poly(ba-a) decreased after curing under high pressure. Meanwhile, more obvious trend can be observed for BA-tb system. Figure 9. The modulated DSC curves of polybenzoxazines. Compared with poly(ba-tb-n), T d5 and T d10 of poly(ba-tb-p) decreased as much as 35 C. In addition, it can be seen from the DGA curves that the degradation rate of the first degradation stage of poly(ba-a-p) was higher than that of poly(ba-a-n). For poly(ba-tb-p), not only the degradation rate of the first degradation stage greatly increased, but also the degradation temperature significantly shifted to a lower temperature. These phenomena indicate that the thermal stability of polybenzoxazines decreases when curing under pressure. The main reason is that the cross-linking density decreases, and the hydrogen bonding rapidly cleaves at high temperature. 4. Conclusions Two benzoxazine crystals, BA-a and BA-tb, were obtained by careful recrystallization. Their exothermic peak temperatures (7 of 9)

8 Keywords benzoxazine, cure kinetics, high pressure, thermogravimetric analysis Received: August 3, 2018 Revised: September 30, 2018 Published online: October 24, 2018 Figure 10. TGA curves of polybenzoxazines. Table 5. TGA results of polybenzoxazines. Sample T d5 [ C] T d10 [ C] Char yield [%] poly(ba-a-n) poly(ba-a-p) poly(ba-tb-n) poly(ba-tb-p) increased under high pressure, and the apparent activation energy of BA-tb increased obviously, which should be attributed to the restraint of the movement of the segments under high pressure. The effect of the high pressure on the curing reaction of BA-tb was greater than that of BA-a, because BA-a contains more benzene rings that make the chains stiff. Furthermore, the high pressure made BA-tb form more intermolecular OH O hydrogen bonds that decreased the cross-linking density of the network. As a result, the T g s, thermal stability, and char yields of both polybenzoxazines cured under the high pressure decreased. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This study was supported financially by the National Natural Science Foundation of China (Project No ). Some chemical materials and test results were obtained from Prof. Ishida s labs. The authors want to appreciate Prof. Ishida s kind assistance. Conflict of Interest The authors declare no conflict of interest. [1] A. Tuzun, G. Lligadas, J. C. Ronda, M. Galià, V. Cádiz, (Eds: H. Ishida, P. Froimowicz), in Advanced & Emerging Polybenzoxazine Science & Technology Elsevier B. V. Amsterdam, Netherlands 2017, Ch. 5. [2] K. Zhang, R. Cai, Q. Zhuang, X. Liu, G. Yang, Z. Han, J. Polym. Sci., Part A: Polym. Chem. 2014, 52, [3] K. Zhang, Q. Zhuang, Y. Zhou, X. Liu, G. Yang, Z. Han, J. Polym. Sci., Part A: Polym. Chem. 2012, 50, [4] K. Zhang, Q. Zhuang, X. Liu, R. Cai, G. Yang, Z. Han, RSC Adv. 2013, 3, [5] X. Liu, R. Zhang, T. Li, P. Zhu, Q. Zhuang, ACS Sustainable Chem. Eng. 2017, 5, [6] S. Shukla, B. Lochab, Polymer 2016, 99, 684. [7] Q. C. Ran, D. X. Zhang, R. Q. Zhu, Y. Gu, Polymer 2012, 53, [8] X. Li, Y. Xia, W. Xu, Q. Ran, Y. Gu, Polym. Chem. 2012, 3, [9] K. Zhang, Q. Zhuang, X. Liu, G. Yang, R. Cai, Z. Han, Macromolecules 2013, 46, [10] H. J. Kim, Z. Brunovska, H. Ishida, Polymer 1999, 40, [11] S. Zhang, P. Yang, Y. Bai, T. Zhou, R. Zhu, Y. Gu, ACS Omega 2017, 2, [12] P. Yang, X. Wang, H. Fan, Y. Gu, Phys. Chem. Chem. Phys. 2013, 15, [13] S. Gemma, C. Camodeca, M. Brindisi, S. Brogi, G. Kukreja, S. Kunjir, E. Gabellieri, L. Lucantoni, A. Habluetzel, D. Taramelli, J. Med. Chem. 2012, 55, [14] H. D. Kim, H. Ishida, Hydrogen Bonding of Polybenzoxazines, Elsevier B. V, Amsterdam, Netherlands 2011, p [15] S. Chirachanchai, A. Laobuthee, S. Phongtamrug, J. Heterocycl. Chem. 2009, 46, 714. [16] S. Phongtamrug, S. Chirachanchai, K. Tashiro, Macromol. Symp. 2006, 242, 40. [17] A. Laobuthee, S. Chirachanchai, H. Ishida, K. Tashiro, J. Am. Chem. Soc. 2001, 123, [18] Y. Bai, P. Yang, Y. Song, R. Zhu, Y. Gu, RSC Adv. 2016, 6, [19] J. Wang, M. Q. Wu, W. B. Liu, J. W. Bai, Q. Q. Ding, Y. Li, Eur. Polym. J. 2010, 46, [20] J. Wang, H. Wang, J. T. Liu, W. B. Liu, X. D. Shen, J. Therm. Anal. Calorim. 2013, 114, [21] X. Y. He, J. Wang, N. Ramdani, W. B. Liu, L. J. Liu, L. Yang, Thermochim. Acta 2013, 564, 51. [22] X. Y. He, J. Wang, Y. D. Wang, C. J. Liu, W. B. Liu, L. Yang, Eur. Polym. J. 2013, 49, [23] T. Zhang, J. Wang, T. Feng, H. Wang, N. Ramdani, M. Derradji, X. Xu, W. B. Liu, T. Tang, RSC Adv. 2015, 5, [24] S. Xu, Y. Han, Y. Guo, Z. Luo, L. Ye, Z. Li, H. Zhou, Y. Zhao, T. Zhao, Eur. Polym. J. 2017, 95, 394. [25] V. García-Martínez, M. R. Gude, A. Ureña, React. Funct. Polym. 2018, 129, 103. [26] J. Yue, C. Zhao, Y. Dai, H. Li, Y. Li, Thermochim. Acta 2017, 650, 18. [27] C. C. Yang, Y. C. Lin, P. I. Wang, D. J. Liaw, S. W. Kuo, Polymer 2014, 55, [28] S. Devaraju, M. R. Vengatesan, A. A. Kumar, M. Alagar, J. Sol-Gel Sci. Technol. 2011, 60, 33. [29] H. Zhang, W. Gu, Q. Ran, Y. Gu, J. Macromol. Sci., Part A 2014, 51, (8 of 9)

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