Regulating the performance of polybenzoxazine via the regiochemistry of amide substituents

Polymer 181 (2019) 121807

Contents lists available at ScienceDirect

Polymer journal homepage: www.elsevier.com/locate/polymer

Regulating the performance of polybenzoxazine via the regiochemistry of amide substituents

T

Jingkai Liua,b, Lijun Caoa,b, Jinyue Daia,b, Dasha Xiac, Yunyan Penga,b, Shuaipeng Wanga, Yuan Liua,b, Xiaoqing Liua,∗ a Key Laboratory of Bio-based Polymeric Materials Technology and Application of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang, 315201, PR China b University of Chinese Academy of Sciences, Beijing, 100049, PR China c School of Environmental and Chemical Engineering, Jiangsu University of Science & Technology, Zhenjiang, Jiangsu, 212003, PR China

H I GH L IG H T S

formation of inter- or intramolecular H-bonding in PBZ was achieved through the benzoxazine isomers design. • Selective on the regiochemistry of amide substituent and its effect on the properties of PBZ were investigated. • Manipulation is the first time the effect of H-bonding on the polybenzoxazole formation temperature from o-amide PBZ was reported. • ItH-bonding was proved to have relationship with the fire resistance of o-amide PBZ due to the formation of polybenzoxazole. •

A R T I C LE I N FO

A B S T R A C T

Keywords: Polybenzoxazine Hydrogen bonding Curing reaction Polybenzoxazole Flame retardancy

A new strategy for the property regulation of polybenzoxazine via the regiochemistry of amide substituents was reported in this work. After the benzoxazine isomers, 2,6BAP-abz and 3,5BAP-abz, were synthesized, their chemical structures were confirmed by NMR, FT-IR and MS spectra. Then, how does the regiochemistry of amide substituent regulate the curing behaviors of benzoxazine monomers, as well as the thermal properties and flame retardancy of cured resins, poly(2,6BAP-abz) and poly(3,5BAP-abz), were investigated. Results showed that the intramolecular H-bonding involved with the ortho-CONH and O in oxazine ring in 3,5BAP-abz was much stronger than that in 2,6BAP-abz, which was responsible for its higher curing reactivity. As for the cured resin, poly(2,6BAP-abz) demonstrated higher glass transition temperature (Tg) and lower surface free energy (SFE) due to the formation of more intramolecular N–H⋯N interaction between pyridine and ortho-CONH. In addition, this H-bonding was also related with the oxazole ring formation temperature and fire retardancy of poly(2,6BAP-abz) and poly(3,5BAP-abz). Overall, a deeper insight into the effect of regiochemistry on properties of polybenzoxazine was achieved.

1. Introduction Polybenzoxazine has been an attractive thermosetting resin in recent years, not only because of its molecular design flexibility, but also excellent properties including near-zero shrinkage upon curing, low water absorption, high glass transition temperature (Tg) and thermal stability, outstanding mechanical and electrical properties [1,2]. In addition, no hardeners or catalysts is required and no byproducts is generated during the curing process [1]. All of these advantages make polybenzoxazine attract increasing attention and it has been widely used in the field of electronic packaging, aviation and aerospace.

∗

Compared with other typical thermosetting resins, polybenzoxazine enjoys richer and more diverse H-bonding systems, including O–H⋯O, O–H⋯π, O–H⋯N and O⋯H+ N [3]. It is well known that, besides the chemical structures of benzoxazine monomers, H-bonding plays an essential role in determining the performance of polybenzoxazine, including the curing reaction, thermal and mechanical properties as well as other unique characteristics [3]. For example, the crosslinking network might be interfered with H-bonding during curing reaction, which is related to near-zero shrinkage during curing reaction [4]. The formation of H-bonding will restrain the segment motion and lead to improved Tg [5]. In addition, due to the existence of a large quantity of H-

Corresponding author. E-mail address: [email protected] (X. Liu).

https://doi.org/10.1016/j.polymer.2019.121807 Received 2 July 2019; Received in revised form 10 September 2019; Accepted 14 September 2019 Available online 16 September 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.

Polymer 181 (2019) 121807

J. Liu, et al.

pyridinedicarboxylic acid (3,5-PC) (99%), triethylanmine (99%), sulfoxide chloride (SOCl2) (99%), dimethylformamide (DMF) (99.5%), oaminophenol (98%), 1,4-dioxane (99%), CHCl3 (99%), n-hexane (99%) and paraformaldehyde (99%) were purchased from Aladdin Reagent, China. All the chemicals were used as received without treatment.

bonding, the interaction between Ar-OH and H2O molecule will be shielded, where polybenzoxazine is endowed with low water absorption [6,7]. And most recently, it was recognized that the H-bonding involved with O in oxazine ring can promote the ring-opening reaction and then lower the curing temperature as well [7]. Therefore, the incorporation of extra hydrogen donors or acceptors has been of much concern for the enhancement of H-bonding in polybenzoxazine [6]. For instance, Ishida and his co-workers once lowered the curing temperature of benzoxazine by 60 °C through the introduction of o-amide group, which formed a five-membered H-bonding with the O atom in oxazine ring [8]. And the Tg of polybenzoxazine could be significantly increased after the addition of extra hydroxyl groups [9]. Meanwhile, the aromatic rings bearing heteroatoms, such as furan [10,11], imidazole [12], and pyridine [13], have also been taken as the hydrogen acceptors to improve the interaction with Ar-OH. On the other hand, blending with other polymers be capable of H-bonding interaction with polybenzoxazine is also an effective approach. Kuo et al. once prepared low-surface-energy poly(vinyl phenol)/polybenzoxazine copolymers by minimizing the intermolecular H-bonding [14]. Qutubuddin obtained chitosan/polybenzoxazine films with improved thermal and mechanical properties due to the increased H-bonding between them [15]. Poly(β-caprolactone) rich in O atom was proved to be able to enrich the H-bonding in polybenzoxazine [16]. However, in some systems, the different regiochemistry might be more appropriate to explain their property differences [17] and H-bonding might be a strong contributing factor [18,19]. In this work, two axial symmetric benzoxazine isomers containing pyridine ring and amide group, 2,6BAP-abz and 3,5BAP-abz shown in Scheme 1, were synthesized. Obviously, the regiochemistry of amide substituents in 2,6BAP-abz and 3,5BAP-abz were different from each other, which will undoubtedly affect their properties. In addition, 2,6BAP-abz and 3,5BAP-abz possess the same hydrogen donor and receptor, the selective formation of H-bonding in both the monomers and cured resins is expectable, due to the different position of amide group with respect to the N atom in pyridine ring. Based on which, the features of H-bonding can also be characterized and the properties of cured resins can be compared. Thus, a new strategy for regulating the properties of polybenzoxazine will be gained.

2.2. Synthesis of precursors and monomers The synthesis route of 2,6BAP is shown in Scheme 1. 2,6-PC (450 mg, 3.66 mmol), NEt3 (0.50 mL, 3.60 mmol) and catalytic amounts of DMF (~3 drops) were dissolved in CHCl3 (10 mL) under an atmosphere of argon. Then SOCl2 (0.27 mL, 3.71 mmol) was added dropwise and the solution was stirred at room temperature for 4 h. After that, the mixture was slowly transferred into a solution of o-aminophenol (346 mg, 1.83 mmol) and NEt3 (0.50 mL, 3.60 mmol) dissolved in CH2Cl2 (5 mL) at 0 °C. The mixture was stirred for another 20 h and then poured into distilled water. At last, the precipitate was dried in a vacuum oven at 60 °C for 6 h and the brown powder was obtained. (Yield: 56%) 1H NMR (400 MHz, CDCl3, δ, ppm): 10.42 (s, 2H), 10.03 (s, 2H), 8.44–8.21 (m, 3H), 8.02 (d, J = 7.3 Hz, 2H), 7.04 (dd, J = 11.2, 4.1 Hz, 3H), 6.96 (d, J = 7.0 Hz, 2H), 6.88 (t, J = 7.6 Hz, 2H). Anal. Calcd for C19H15N3O4: C, 65.32; H, 4.33; O, 18.32; N, 12.03. Found: C, 65.23; H, 4.24; O, 18.43; N, 12.11. 3,5BAP was obtained following the similar procedure to that for 2,6BAP synthesis, except for 3,5-PC using as acid. The overall yield of final product was 57%. 1H NMR (400 MHz, CDCl3, δ, ppm): 9.94 (s, 2H), 9.71 (s, 2H), 9.25 (s, 2H), 8.81 (s, 1H), 7.62 (d, J = 7.8 Hz, 2H), 7.07 (dd, J = 11.0, 4.4 Hz, 2H), 6.94 (d, J = 8.0 Hz, 2H), 6.85 (t, J = 7.6 Hz, 2H). Anal. Calcd for C19H15N3O4: C, 65.32; H, 4.33; O, 18.32; N, 12.03. Found: C, 65.21; H, 4.25; O, 18.51; N, 12.05. 2,6BAP-abz was prepared by formaldehyde (200.1 mmol), aniline (70.2 mmol) and 150 mL of 1,4-dioxane. The mixture in 250 mL round flask was stirred at 70 °C for 2 h before 2,6BAP (34.6 mmol) was added. Then, the reaction temperature was gradually increased up to 100 °C and maintained at this temperature for 24 h. After that, the mixture was treated by rotary evaporator to remove solvent before it was dissolved in acetone and added dropwise into n-hexane. The precipitate was formed and filtrated to get the crude products. After recrystallization in ethyl acetate, the product was dried in a vacuum oven and a tangerine powder was obtained (yield: 42%). FT-IR (KBr, cm−1): 923, 1340, and 1362. 1H NMR (400 MHz, CDCl3, δ, ppm): 10.01 (s, 2H), 8.47 (s, 2H), 8.39 (s, 2H), 7.31 (d, 6H), 6.97–6.69 (m, 8H), 5.21 (s, 4H), 4.67 (s, 4H). 13 C NMR (400 MHz, CDCl3, δ, ppm): δ 161.24 (s), 149.29 (s), 148.07 (s), 144.29 (s), 139.38 (s), 129.30 (s), 126.25 (s), 125.34 (s), 122.09 (s),

2. Experimental 2.1. Material 2,6-pyridinedicarboxylic

acid

(2,6-PC)

(99%),

3,5-

Scheme 1. Chemical structures and synthesis of 2,6BAP-abz and 3,5BAP-abz. 2

Polymer 181 (2019) 121807

J. Liu, et al.

120.85 (s), 119.28 (s), 118.59 (s), 81.02 (s), 49.34 (s, 8H). Anal. Calcd for C33H29N5O4: C, 70.83; H, 5.22; O, 11.43; N, 12.52. Found: C, 70.51; H, 5.18; O, 11.65; N, 12.67. 3,5BAP-abz was synthesized following a similar procedure to that used for 2,6BAP-abz. The as-synthesized 3,5BAP (5.29 g, 15.2 mmol) was used instead of 2,6BAP. After purification, an orange powder was obtained (yield: 45%). FT-IR (KBr, cm−1): 925, 1340 and 1363. 1H NMR (400 MHz, CDCl3, δ, ppm): 9.86 (s, 2H), 9.20 (s, 2H), 8.73 (s, 1H), 7.55 (d, 2H), 7.29–6.99 (m, 10H), 6.97–6.75 (m, 4H), 5.53 (s, 4H), 4.71 (s, 4H). 13 C NMR (400 MHz, CDCl3, δ, ppm): 162.36 (s), 150.69 (s), 148.01 (s), 143.54 (s), 133.64 (s), 129.39 (s), 126.27 (s), 122.14 (d), 120.77 (d), 118.64 (d), 80.88 (s). Anal. Calcd for C33H29N5O4: C, 70.83; H, 5.22; O, 11.43; N, 11.43. Found: C, 70.43; H, 5.16; O, 11.72; N, 12.69.

scope of 5 μm × 5 μm, the root-mean-square surfaces roughness (Sq) was direct produced by instrument calculation.

2.3. Preparation of polybenzoxazines

3.1. Synthesis and characterization of 2,6BAP-abz and 3,5BAP-abz

2,6BAP-abz or 3,5BAP-abz was added into a stainless steel mold and put pressure on it. The monomer was cured by a programmed temperature rising procedure: 2 h at 180 °C, 2 h at 200 °C, and 4 h at 220 °C. After curing was finished, in order to prevent cracking, the cured resin was cooled down to room temperature slowly. 2,6BAP-abz and 3,5BAPabz were cured using the same procedure.

Scheme 1 illustrates the synthesis of 2,6BAP-abz and 3,5BAP-abz, and related NMR and MS spectra were offered in Supporting Information (Figs. S1–S4). The signals for Mannish bridge in 2,6BAP-abz and 3,5BAP-abz, Ar-CH2-N- and O–CH2–N-, which are the indicators for oxazine ring formation, are observed at 4.62–4.70 ppm and 5.22–5.52 ppm (Supporting Information, Fig. S2a and Fig. S3a). It is worth pointing out that the chemical shift of N–H in amide group are quite different for 2,6BAP-abz (9.86 ppm) and 3,5BAP-abz (9.25 ppm), which indicates their various acidity [24]. As for the FT-IR spectra (Fig. 1), besides the similar characteristic bonds showing at 1224 (C–O–C) and 925 cm−1 (benzoxazine) [25]. Some other slight differences are noteworthy. The signals for pyridine ring are shown at 1620 (C]N stretching), 1446 and 775 cm−1 (pyridine group). However, all of these bonds standing for 3,5BAP-abz are exhibited in sharp shapes, while they are only small shoulder peaks for 2,6BAP-abz. The reason might be that, as for 3,5BAP-abz, the N heteroatom in pyridine ring possess higher electron cloud density, leading to stronger absorption peaks. While for 2,6BAP-abz, the N atom in pyridine ring participates in the formation of intramolecular H-bonding with N–H in amide group, whose lone pair electron is shared by the protons, which results in the weaker IR signals. For the sake of further investigating the difference between 2,6BAPabz and 3,5BAP-abz, density functional theory (DFT) calculation was employed. Both the simulated 3D structures for 2,6BAP-abz and 3,5BAP-abz were optimized, and their electrostatic potential distribution were displayed in Fig. 2 with gradual colors, where the darker color represents the higher negative electrostatic potential. It was noticed that both 2,6BAP-abz and 3,5BAP-abz showed similar butterflylike structure, and the most evident difference lies in pyridine ring, especially the N atom. In general, the higher negative electrostatic potential means a stronger nucleophilic capacity and higher ability to draw protons. In 2,6BAP-abz, the electron cloud density of N was shared after the H-bonding was formed, which had an agreement with

2.5. Computation details The geometry optimization was carried out using DFT method with ORCA package [20]. The hybrid M062X functional and TZVP basis set was employed [21]. Electrostatic potential (ESP) analysis was employed to compare the properties of 2,6BAP-abz and 3,5BAP-abz. Based on statistical analysis, Murray and co-workers found a set of functions, which connects ESP in molecular surface and macroscopic properties [22,23]. 3. Results and discussion

2.4. Characterization 1 H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a 400 MHz Bruker AVANCE III spectrometer using CDCl3 as solvent with tetramethyl silane as internal standard reagent. For 2D 1 H–1H NOESY NMR spectra, the parameters were: 2048 data points along the f2 dimension, 256 free induction decays for the f1 dimension, 2 s relaxation delay and mixing time of 700 ms. Mass spectra (MS) were measured with a LC-Q-TOF (AB Sciex, America) at 500 °C with an ionization voltage of 5500 V. Elementary analysis was conducted at Elementar (Elementar Germany), the samples were tested for three times respectively, in which the final results derived from the average. Fourier transform infrared (FT-IR) spectra were collected on a Thermo Nicolet 6700 Fourier transform infrared spectrometer (Thermo-Fisher Scientific) and the transmission mode was taken. Sample was pressed into disk by grounding with KBr powder. Spectra were collected from 400 to 4000 cm−1 with 32 scans for each sample. The in-situ FT-IR measurement was conducted at various temperature (25, 150, 180, 210, and 240 °C). Differential scanning calorimetric (DSC) measurements were conducted with a METTLER TOLEDO-TGA/DSC I under a nitrogen atmosphere flow rate of 20 mL min−1. The sample was scanned from 50 to 300 °C using various heating rates of 5, 10, 15 and 20 °C min−1. Dynamic mechanical analysis (DMA) was conducted on a Mettler-Toledo DMAQ800 under a tensile mode at a frequency of 1 Hz with the amplitude of 5 μm. The testing samples in the shape of bar were scanned from 0 to 360 °C at a heating rate of 3 °C min−1. Each sample was tested at least three times to ensure the accuracy. Thermogravimetric analysis (TGA) and Differential thermal gravity (DTG) were performed on a Mettler-Toledo TGA/DSC1 thermogravimetric analyzer (METTLER TOLEDO, Switzerland) with high purity nitrogen as purge gas (50 mL min−1) at a scanning rate of 20 °C min−1 from 50 to 800 °C. Thermogravimetric infrared gas chromatograph/ mass spectrometer (TG-IR-GC/MS) was carried out by a PerkinElmer TGA 8000-specturm Two-Clarus SQ8T with the same conditions used for TGA testing and the IR spectra were scanned from 400 to 4000 cm−1. Microscale combustion calorimeter (MCC) was recorded on MCC-2. Cured samples were heated from 100 to 900 °C at a heating rate of 1 °C s−1 in an 80 cm3 min−1 stream of nitrogen. Each sample was weighted about 5 mg and dried at 100 °C. Surface tension/dynamic contact angle were determined by a DCAT21. Each sample was tested at least three times to ensure accuracy as well. Scanning probe microscopy (SPM) was collected using a Vecco Dimension 3100 with the scanning

Fig. 1. FT-IR spectra of 2,6BAP-abz and 3,5BAP-abz. 3

Polymer 181 (2019) 121807

J. Liu, et al.

related to the presence of amide groups [27]. It is well known that the onset and peak curing temperatures can be taken to evaluate the curing activity of thermosets under the same curing conditions. The higher onset or peak curing temperature usually means lower curing reactivity. Apparently (Fig. 4), compared with 3,5BAP-abz, 2,6BAP-abz showed lower curing activity under the same condition. The activation energy (Ea) of the curing reaction for 3,5BAP-abz and 2,6BAP-abz were calculated using Kissinger and Ozawa equation, respectively [28,29]. And the Kissinger method can be explained by the following Equation (1):

AR ⎞ E ⎛q ⎞ ln ⎜ 2 ⎟ = ln ⎛ − a E RT T a p q ⎝ ⎠ ⎝ ⎠ ⎜

⎟

(1)

where Tp was the peak curing temperature (K), q was the DSC heating rate (oC/min), A served as the pre-exponential factor (min−1) and R represented the universal gas constant (8.314 J/mol.K). Ea was the average activation energy of the curing reaction (J/mol). From the linear relationship between logarithm of (qTp−2) and Tp−1 (Supporting Information, Fig. S5a), the Ea was calculated to be 182.3 kJ/mol for 2,6BAP-abz and 172.6 kJ/mol for 3,5BAP-abz (Table 1). As for the modified Ozawa method, it was expressed by the following Equation (2):

Fig. 2. Electrostatic potential distribution map of 2,6BAP-abz and 3,5BAP-abz.

above FT-IR results. Therefore, we initially believe that in 2,6BAP-abz, there is H-bonding interaction between the N atom in pyridine and N–H in amide. Meanwhile, the N–H in ortho-CONH will promote the ringopening reaction and lower the curing temperature in the way of forming H-bonding with the O in oxazine ring [26]. It can be easily inferred that N–H in amide group interfered by the pyridine ring will exhibit a weaker catalytic effect on the curing reaction. To verify this conjecture, in the following section, the influence on their curing behaviors was discussed in detail.

AE E ln q = ln ⎛ a ⎞ − ln g (α ) − 5.331 − 1.052 ⎜⎛ a ⎞⎟ RT ⎝ R ⎠ ⎝ p⎠

(2)

α

Where g(α ) =

∫ fdα(α) = −ln(1 − α ) and Ea could be obtained from the 0

slope of ln q against Tp−1 (Supporting Information, Fig. S5b). They were 180.9 and 171.7 kJ/mol for 2,6BAP-abz and 3,5BAP-abz, respectively (Table 1). Based on the calculated curing activation energy (Ea), 3,5BAP-abz possessed higher curing activity compared with 2,6BAP-abz. This was in line with the results obtained from Fig. 3. As we know, the ringopening reaction of oxazine ring plays an essential role in determining the polymerization of benzoxazine monomer. The easier the ring opening is, the higher the activity is. And usually, it is the structure features of benzoxazine monomers, including the varied functional groups [30,31] and their position or arrangement [32,33], determine the initiation of polymerization. As for the synthesized 2,6BAP-abz and 3,5BAP-abz in this work, they are isomers and the only difference between them is the location of N atom. In another word, the regiochemistry of amide groups is different, i.e. ortho-position for 2,6BAPabz and meta-position for 3,5BAP-abz. In order to investigate the effect of regiochemistry of amide groups on molecular level, the DFT calculation was considered again. The geometric features of 2,6BAP-abz and 3,5BAP-abz were shown in Fig. 5 and related values were listed in

3.2. Curing behaviors of 2,6BAP-abz and 3,5BAP-abz FT-IR measurement was carried out to monitor the curing process of 2,6BAP-abz and 3,5BAP-abz. In Fig. 3, the intensity of absorption band at 925 cm−1 (oxazine ring) showed a decreasing tendency along with the increased curing temperature, and it was completely disappeared when the temperature was increased up to 210 °C, suggesting the complete ring-opening reaction of oxazines. What should be mentioned was that the intensity of this peak was decreased more rapidly for 3,5BAP-abz, which indicated its relative higher curing activity compared with that of 2,6BAP-abz. In order to further investigate the curing behaviors of 2,6BAP-abz and 3,5BAP-abz, their curing profiles studied by DSC are shown in Fig. 4. Despite the high purity of synthesized 2,6BAP-abz and 3,5BAPabz proved by NMR and mass spectra (supporting information, Figs. S1–S4), there is no melting endotherm observed in the DSC curves. The same result was also reported in previous literature and the reason was

Fig. 3. FT-IR spectra of 2,6BAP-abz and 3,5BAP-abz after thermal treatment at the noted temperature for 1 h. 4

Polymer 181 (2019) 121807

J. Liu, et al.

Fig. 4. DSC heating curves for 2,6BAP-abz (a) and 3,5BAP-abz (b) at different heating rates (5, 10, 15, 20 °C min−1). Table 1 Calculated Ea and lnA for 2,6BAP-abz and 3,5BAP-abz according to Kissinger and modified Ozawa methods. Samples

2,6BAP-abz

3,5BAP-abz

a

Heating rates (oC min−1)

5 10 15 20 5 10 15 20

Tp (K)

476.2 482.1 486.2 490.5 470.8 477.2 481.6 485.5

Ea (kJ mol−1)

ln Aa

Kissinger

Ozawa

Kissinger

Ozawa

182.3

180.9

38.5

40.6

172.6

171.7

36.5

38.7

The dimension of A is min−1, and α was 0.5.

Table 2. As we know, the oxazine ring is nonplanar and the torsional strain, which can be characterized by the torsional angle (ϕ) of C–O–CH2–N flat and the CH2–O bond distance (γ) [34], represents the degree of difficulty in ring opening reaction in a certain extent. As for 2,6BAP-abz, it showed the C–O–CH2–N flat torsional angle of −53.4° and CH2–O bond distance of 0.136 nm. While they were −40.4° and 0.137 nm for 3,5BAP-abz. These results meant that 3,5BAP-abz had a higher torsional strain compared with 2,6BAP-abz, which led to an accelerated ring-opening reaction. And this is in a good agreement with above FT-IR and DSC study. Obviously, the only difference between 2,6BAP-abz and 3,5BAP-abz is regiochemistry, in which the H-bond donors and acceptors involved will cause the formation of different H-bonds. As the research points out, the polymerization of ortho-amide benzoxazine could be accelerated by the five-membered H-bonding between CONH and O in oxazine ring, and the higher strength would lead to higher reactivity [35]. Looking deep insight into the chemical structures of 2,6BAP-abz and 3,5BAP-abz, the existence of similar intramolecular H-bonding was expectable, which was validated by the 1H NMR spectra. In Fig. 6a and b, the chemical shift of both -NH- and O–CH2–N showed no dependence on the solution concentration, which is the major feature of intramolecular H-bonding in ortho-amide benzoxazine monomer [36]. It is well known that, during the formation of H-bonding, the elements with high electronegativity, such as N, O and F, will act as the proton acceptor. For 2,6BAP-abz, as depicted in Scheme 2, the NH tended to participate in the formation of intramolecular H-bonding with O in oxazine ring and N in pyridine ring, and the proton drawing competition between them was inevitable. As for 3,5BAP-abz, only the NH and O in oxazine ring had the possibility to construct intramolecular H-

Fig. 5. The geometric features of oxazines and intramolecular H-bonds in 2,6BAP-abz (a) and (c); in 3,5BAP-abz (b) and (d).

bonding. Therefore, in 3,5BAP-abz, the strength of intramolecular Hbonding involved with O in oxazine ring should be higher. In Fig. 6c and (d), the 1H–1H nuclear overhauled effect spectroscopy (NOESY) 5

Polymer 181 (2019) 121807

J. Liu, et al.

Table 2 The geometric values for 2,6BAP-abz and 3,5BAP-abz. Samples

2,6BAP-abz 3.5BAP-abz

Geometric properties γ(CH2–O) (nm)

Φ(C–O–CH2–N) ()̊

0.136 0.137

−53.40 −40.37

offered us a feasible method to prove this fact [37]. Theoretically, the chemical environment of proton acceptor will be changed after the formation of H-bonding, which will affect the through-space interaction involved with the hydrogen. In Fig. 6c, the proton in -CONH- showed two obvious signals marked as dots a and b, which was corresponded to its coupling with the hydrogen at position a and b [35,36]. However, only one signal standing for the interaction between -CONH- and the proton at position-c was observed in Fig. 6d. In addition, the intensities of signal a and b in Fig. 6c were higher than that of signal a in Fig. 6d. As we know, the signal intensity reflects the strength of through-space interaction and the higher signal intensity means stronger or more interaction [38]. Thus, it concluded that the intramolecular H-bonding involved with -CONH- and O in oxazine ring in 3,5BAP-abz was much stronger than 2,6BAP-abz, which endowed 3,5BAP-abz with superior catalytic effect. Desiraju and Steiner divided H-bonds into three types, considering their diverse interatomic distance, bonding angle and so on [39]. In Table 3, the main parameters for strength evaluation of intramolecular H-bonds in 2,6BAP-abz and 3,5BAP-abz, including bond length, bond angle and the distance between the proton donor and acceptor, were collected based on the DFT results in Fig. 5c. As we know, the H-bond can be expressed as X–H⋯Y. In Table 3, d1 and D1 together with θ1 were employed to evaluate the bond of N1–H⋯O, while d2, D2 and θ2 were for N1–H⋯N2 (Fig. 5c and d). In Desiraju and Sreiner's theory [39], for

Scheme 2. The intramolecular H-bonding exist in 2,6BAP-abz and 3,5BAP-abz.

a strong H-bond, d should be within the range of 1.5–2.2 Å, D of 2.3–3.2 Å and θ of 130–180°, while for a weak H-bond, d is usually between 2.0 and 3.0 Å, D is between 3.0 and 4.0 Å, and θ varies in the range of 90–180°. As shown in Table 3, for 2,6BAP-abz, d1 and D1 together with θ1 were all out of ranges that strong H-bonds needed, and the H-bond of N1–H⋯ N2 was stronger than that of N1–H⋯O, indicating by the values of d, D and θ, which was not good for the ringopening of oxazine. Whereas, in 3,5BAP-abz, the H-bond of N1–H⋯O showed much stronger features than those of N1–H⋯N2, that was beneficial to open the oxazine ring at a lower temperature. Obviously, this result was also in consistent with above chemical structure analysis and DSC measurement. In summary, the different H-bonding interaction in 2,6BAP-abz and 3,5BAP-abz are shown in Scheme 2 and for easy identification, the Hbonding involved N, H and O were highlighted in varied color. Obviously, in 2,6BAP-abz, besides the O atom in oxazine ring, the N atom in pyridine also participated in the formation of intramolecular Hbonding. While in 3,5BAP-abz, the N atom in pyridine did not compete

Fig. 6. Concentration dependence of 1H NMR for 2,6BAP-abz (a) and 3,5BAP-abz (b); 1H–1H nuclear overhauled effect spectroscopy (NOESY) for 2,6BAP-abz (c) and 3,5BAP-abz (d). 6

Polymer 181 (2019) 121807

J. Liu, et al.

Table 3 Parameters for the intramolecular H-bonds in 2,6BAP-abz and 3,5BAP-abz. Samples

2,6BAP-abz 3,5BAP-abz

H-bonds angle ()̊

Interatomic distance (Å)a d1(O⋯H)

d2(N2⋯H)

D1(O⋯N1)

D2(N1⋯N2)

θ1(N1–H⋯O)

θ2(N1–H⋯N2)

2.29 2.09

2.12 4.69

3.31 3.10

3.13 5.70

97.96 109.52

107.43 Disregardb

a d is the distance between the proton and acceptor; D is the distance between H-bond acceptor and H-bond donor heteroatom; Where D1 is the sum of d1 and N1–H, D2 is the sum of d2 and N1–H. b D2 for 3,5BAP-abz exceeds the range of hydrogen bond formation, so θ2 is disregarded.

IUPAC's definition, the H-bonding could be formed between atoms, anions, molecules and molecular fragments [41]. In polybenzoxazine systems, benzene and phenolic hydroxyl group are ubiquitous and the interaction of O–H⋯π are widely existed, and they were not illustrated in Scheme 3 for a concise expression. Besides the general H-bonding in polybenzoxazine, the worth noting fact was that, in poly(2,6BAP-abz), the N atom in pyridine was more incline to form the intramolecular Hbonding with the ortho-CONH (marked by yellow circle in Scheme 3). While in poly(3,5BAP-abz), more intermolecular H-bonding between the N atom in pyridine and the -OH/NH in near molecular fragments were in evidence. This was reasonable considering the structural difference between poly(2,6BAP-abz) and poly(3,5BAP-abz). Normalized FT-IR spectra were used to investigate the different Hbonding proportion and change in poly(2,6BAP-abz) and poly(3,5BAPabz) at different temperatures (Fig. 7). Based on literature results [42–44], the FT-IR spectra for polybenzoxazines in the range of 2650 and 3700 cm−1 could be divided into four characteristic absorption bonds, respectively standing for the intra- N–H⋯O (2925 cm−1), intraO–H⋯N (3120 cm−1), inter-/intra- O–H⋯O and inter- O–H⋯N (3350 cm−1), and intra- O–H⋯π H-bonding (3600 cm−1) by peak splitting method. Fig. 7 illustrates the split FT-IR spectra comparison between poly(2,6BAP-abz) and poly(3,5BAP-abz) at 25 °C (Fig. 7a and c) and 250 °C (Fig. 7b and d) (the R2 values of the fitting curves are higher than 0.9). It was noted that, when the temperature was increased from 25 to 250 °C, the intensity of all the split peaks were decreased, and especially the bond centered at 3600 cm−1 completely disappeared. This result was reasonable because of the decreased Hbonding strength at higher temperature. And the strength of intramolecular H-bonding is higher than the intermolecular ones, therefore it can endure higher temperature. IR extinction coefficient will

with the O atom in oxazine ring, which led to higher strength or density of N–H⋯O intramolecular H-bonding, and then a superior catalytic effect on curing reaction. These results were all proved and discussed in above section. 3.3. H-bonding features in the cured resins Generally speaking, the ortho-amide polybenzoxazines will convert into polybenzoxazole (PBO) when the temperature is higher than 250 °C [26,40]. Therefore, in this work, the curing temperature should be high enough to ensure full curing reaction of 2,6BAP-abz and 3,5BAP-abz. Meanwhile, the possible rearrangement into PBO should be avoided, for the sake of H-bonding investigation on poly(2,6BAPabz) (cured 2,6BAP-abz) and poly(3,5BAP-abz) (cured 3,5BAP-abz). In the experimental section, the mild curing procedures were employed, under which polybenzoxazine would not convert into PBO according to our earlier work [35]. In order to make a conformation, the FT-IR spectra of as-cured resins before and after being post-heated at 250 °C for 1 h were shown in Fig. S6 (Supporting Information). In which, the complete disappearance of absorption bands at 925 cm−1, standing for the C–O–C antisymmetric stretching, suggested the full curing reaction. And the unchanged signals showing at 1683 cm−1 (C]O- absorption band) could be taken as an indicator for the failure formation of PBO. What should be emphasized here is that, although poly(2,6BAP-abz) and poly(3,5BAP-abz) possessed the same hydrogen donors and acceptors in both type and quantity, the proportion of inter- and intramolecular H-bonding in the cured resins will be varied due to their different regiochemistry, which will generate different H-bonding forming conditions. In Scheme 3, the H-bonding existed in poly (2,6BAP-abz) and poly(3,5BAP-abz) were proposed. According to

Scheme 3. Possible H-bonding interaction in poly(2,6BAP-abz) and poly(3,5BAP-abz) before and after heating at 250 °C. 7

Polymer 181 (2019) 121807

J. Liu, et al.

Fig. 7. Different types of H-bonding in poly(2,6BAP-abz) (a) and poly(3,5BAP-abz) (b) after peak splitting in the range of 2650 and 3700 cm−1 at 25 °C and 250 °C respectively; Comparison of split peaks' integrated areas in poly(2,6BAP-abz) and poly(3,5BAP-abz) at 25 and 250 °C (c).

3.4. Wettability and surface free energy (SFE) of cured resins

vary with temperatures, but the change will be very slight and the temperature-dependent IR is still a frequently used method for Hbonding strength comparison. For a semiquantitative investigation, the integrated areas of split peaks shown in Fig. 7a and b were taken to compare the intensity of different H-bonding at varied temperatures, and their ratios were illustrated in Fig. 7c. In Fig. 7c, the different colored bars represent varied content of different type of H-bonding in poly(2,6BAP-abz) and poly(3,5BAP-abz). Compared with poly(3,5 BAPabz), both the intensity and percentage of intramolecular H-bonding in poly(2,6 BAP-abz), i.e. intra- N–H⋯O/N (red bar) and intra- O–H⋯N interaction (blue bar), were relatively higher at 25 °C. When the temperature was increased to 250 °C, the intensity of intra- N–H⋯O/N and O–H⋯N H-bonding in poly(2,6 BAP-abz) was slightly decreased. However, in poly(3,5BAP-abz), the intensity of intra- N–H⋯O/N interaction was less than in poly(2,6 BAP-abz) and obviously reduced, while the proportion of inter-/intra- O–H⋯O and inter- O–H⋯N (plum red bar) was higher than in poly(2,6 BAP-abz) both at 25 °C and 250 °C despite the obvious sliding. This is because the different relative position of the N atom in pyridine give rise to different inclination of forming H-bonding, a N–H⋯N H-bonding with high strength is more easily to appear in poly(2,6 BAP-abz) while the N atom in pyridine in poly(3,5BAP-abz) is away from CONH which was mainly responsible to the formation of intermolecular O–H⋯N H-bonding with low strength. This result agreed well with the proposed H-bonding features of poly (2,6BAP-abz) and poly(3,5BAP-abz) illustrated in Scheme 3. And the conclusion that more intramolecular H-bonding was formed in poly (2,6BAP-abz), could be drawn.

For further investigation on the H-bonding features of poly(2,6BAPabz) and poly(3,5BAP-abz), their wettability was evaluated by the static contact angle. Three liquids with different polarities, namely water (H2O), ethylene glycol (EG) and diiodomethane (DIM), were took as the probe solutions for this testing. As shown in Fig. 8, no matter what probe solution was used, the contact angle (CA) was lower than 90°. Compared with poly(2,6BAP-abz), poly(3,5BAP-abz) showed a better surface wettability when the same probe solution was used, indicating by the larger CA. As we know, the surface wettability has a very close relationship with the surface roughness and surface free energy (SFE) [45]. In order to rule out the influence of surface roughness, the scanning probe microscopy (SPM) was employed, and the measured surface roughness was described by root-mean-square surfaces roughness (Sq). As depicted in Fig. 8d and h, Sq was 22.6 nm for poly(2,6BAPabz) and 22.0 nm for poly(3,5BAP-abz), respectively, which indicated their similar surfaces roughness. Therefore, the different surface wettability between poly(2,6BAP-abz) and poly(3,5BAP-abz) should be attributed to their different SFE. Based on Owens-Wendt-Rabel-Kaelble (OWRK) theory [46,47], the SFE could be calculated by the following Equation (3):

0.5(1 + cosθ) γl γld

=

γsd +

γsp

γl p γld

(3)

where γld and γsd represent the dispersive parts of liquid and solid respectively, while γl p and γsp embody the polar parts of the them. The SFE and two component values of water, EG and DIM can be obtained 8

Polymer 181 (2019) 121807

J. Liu, et al.

Fig. 8. Contact angle (CA) under different solution for poly(2,6BAP-abz) ((a), (b), (c)) and poly(3,5BAP-abz) ((e), (f), (g)). (a) and (e) for H2O; (b) and (f) for EG; (c) and (g) for DIM. The surface roughness of poly(2,6BAP-abz) and poly(3,5BAP-abz) is measured by SPM, (d) for poly(2,6BAP-abz) and (h) for poly(3,5BAP-abz).

in Table 4, poly(2,6BAP-abz) and poly(3,5BAP-abz) showed different wettability and SFE, which should be attributed to their different content of intra- and intermolecular H-bonding. It is widely recognized that if more H-bonding interaction occurred within one molecule, the intramolecular energy will increase and inevitably lead to the decreased intermolecular force. In this work, as discussed above, poly(2,6BAPabz) possessed more intramolecular H-bonding, especially the N–H⋯N, which meant a less dipole moment structure. And this was the reason for the lower SFE of poly(2,6BAP-abz) compared with poly(3,5BAPabz). 3.5. Thermal and mechanical properties of polybenzoxazines Fig. 10a shows the DMA curves of poly(2,6BAP-abz) and poly (3,5BAP-abz). Determined by the peak temperature in tan δ graph, Tg of poly(2,6BAP-abz) was 256 °C and it was 230 °C for poly(3,5BAP-abz), which was much higher than the commercial bisphenol A-based benzoxazine of 215–220 °C [51]. As previously reported, the intra- Hbonding in polybenzoxazine could limit the rotation of chain segments and then lead to a higher Tg [5]. In this work, the higher Tg of poly (2,6BAP-abz) was also attributed to the presence of more intra- Hbonding involved with the N atom in pyridine ring and ortho-positioned -CONH- groups. In Fig. 10b and c, both poly(2,6BAP-abz) and poly(3,5BAP-abz) showed two weight loss stages, where the first degradation occurred in the temperature range of 250–450 °C and the second weight loss was observed above 450 °C. Poly(2,6BAP-abz) demonstrated higher thermal stability, indicating by its higher Td5% of 332 °C and Tmax of 343 °C. As for poly(3,5BAP-abz), the Td5% and Tmax were only 318 and 322 °C, respectively. Normally, polybenzoxazines possess Td5% higher than 350 °C. The relatively lower Td5% and Tmax in this work might be related with the formation of oxazole ring, during which H2O was generated [52]. This phenomenon was also supported by the DSC curves in

Fig. 9. Fitting curve of 0.5(1 + cosθ ) γl/ sqrt γld vs sqrt (γl p/ γld ).

directly from literature [48], whose specific values and their polarity are listed in Table S1 (Supporting information). Based on the linear fitting graph of 0.5(1 + cosθ) γl/ sqrt γld and sqrt (γl p/ γld ) (Fig. 9), the slope (sqrt γsp ) and intercept (sqrt γsd ) could be obtained. The sums of γsp and γsd , which serves as the SFE of poly(2,6BAP-abz) and poly(3,5BAP-abz) (i.e. γs ), were collected in Table 4. Fowkes suggested that the intermolecular forces donated to SFE could be divided into two parts, i.e. dispersion force and polar force, and the latter one was mainly caused by the H-bonding [49]. Furthermore, the intramolecular H-bonding is more easier to reduce the SFE of polymers when compared with the intermolecular one [50]. As shown

Table 4 Wettability and SFE of poly(2,6BAP-abz) and poly(3,5BAP-abz). Samples

Poly(2,6BAP-abz) Poly(3,5BAP-abz)

Contact angle

Physical parameters (mN ⋅m−1)

Water

Ethylene glycol

Diiodomethane

γs

γsp

γsd

75.20 82.54

64.18 75.77

44.80 48.92

70.18 78.94

48.86 55.35

21.32 23.59

9

Polymer 181 (2019) 121807

J. Liu, et al.

Fig. 10. (a) DMA curves, (b) TGA profile, (c) differential curves of TGA (DTG) and (d) DSC curves (under N2 with the heating rate of 20 °C/min) for poly(2,6BAP-abz) and poly(3,5BAP-abz).

formed by the intramolecular cyclization between phenolic hydroxyl and ortho-amide group through a nucleophilic attack process [54]. Therefore, a favorable attack position will result in facile conditions for this reaction. In this work, poly(2,6BAP-abz) and poly(3,5BAP-abz) converted into PBO at different temperature. In poly(2,6BAP-abz), as shown in Scheme 4, the intramolecular N–H⋯N interaction hindered the rotation of N–CO bonds [55], making it difficult for the hydroxyl to turn to a favorable attack position. And most importantly, the strength/ intensity of N–H⋯N intramolecular H-bonding was high enough and it could exist at high temperature, supported by above FT-IR spectra in Fig. 7. As for poly(3,5BAP-abz), the N–CO bond could rotate more easily, and then led to a lower oxazole ring formation temperature. Consequently, it concluded that the features of H-bonding played an essential role in determining the conversion reaction from polybenzoxazine to PBO. In this work, the intramolecular H-bonding resulted in a higher conversion temperature, while the intermolecular one led to a lower conversion temperature.

Fig. 10d. The endothermic peaks in the temperature range of 260–330 °C indicated the conversion of polybenzoxazine into polybenzoxazole (PBO). In order to make a further confirmation, poly(2,6BAP-abz) and poly (3,5BAP-abz) were isothermally heated at 260 °C for 1 h and the produced volatile substances were in-situ detected by TG-FTIR-GCMS. As shown in Fig. 11a and b, both cured resins showed a certain percentage of weight loss. Poly(2,6BAP-abz) showed a weight loss of 3.34%, while poly(3,5BAP-abz) demonstrated a higher weight loss of 5.05%. In Ishida's work, the synthesized poly(oTFA-ddm) also showed a weight loss of 5.54 wt% when it was isothermally treated at 260 °C for 1 h, and it was attributed to the loss of water during the formation of PBO [35,52]. However, in our study, there is a value closed to it (poly(3,5BAP-abz)) while another is lower, implying different processes regarding weight loss took place. For more details, the fragments released from poly (2,6BAP-abz) and poly(3,5BAP-abz) at 260 °C was detected by the FT-IR spectra in the range of 3200–4000 cm−1, as shown in Fig. 11c and d. In Fig. 11c, the characteristic absorption bonds standing for O–H and N–H stretching were observed in poly(2,6BAP-abz), indicating cleavage of the groups containing O and N, it's reasonable because the C–N–C bond is prone to break under this such high temperature [53]. While in Fig. 11d, the obvious signals for water were detected, which supported the formation of oxazole ring. Also, there is no obvious group fragment signal, indicating that the formation of PBO protect the groups taking part in this reaction from cleavage and increases the thermal stability of the material, and the release of less fragment may reduce the probability of combustion, which will be discussed in next part. All the results above indicated that poly(3,5BAP-abz) could convert into PBO at a lower temperature (< 260 °C). The extremely high mechanical and thermal properties of aromatic PBO have gained extensive attention. However, the traditional methods for PBO synthesis suffer from harsh reaction conditions. Ishida and his coworkers once reported their pioneer work about producing PBO from ingeniously designed benzoxazine resins [26]. The oxazole ring was

3.6. Flame retardant properties of poly(2,6BAP-abz) and poly(3,5BAPabz) Microscale combustion calorimetry (MCC), measuring the combustion and heat release of decomposed gases at a constant heating rate, has been widely used for the flame retardancy investigation in polymer science. Specific heat release rate (HRR), heat release capacity (HRC) and total heat release (THR) determined by this method are related to the flame resistance of materials. In general, the lower value of HRC or THR indicates better flame retardancy. Based on Fig. 12, the HRC and THR values for poly(2,6BAP-abz) were calculated to be 108.3 J g−1K−1 and 18.6 kJ g−1. While for poly(3,5BAP-abz), they were 87.2 J g−1K−1 and 13.3 kJ g−1, respectively, much lower than those of poly(2,6BAPabz), indicating its relatively higher flame resistance. Compared with previous literatures [56], the HRC of poly(2,6BAP-abz) and poly (3,5BAP-abz) was much lower than most of bisphenol A-based 10

Polymer 181 (2019) 121807

J. Liu, et al.

Fig. 11. Thermogravimetric analysis of poly(2,6BAP-abz) (a) and poly(3,5BAP-abz) (b) isothermally treated at 260 °C for 1 h under a N2 atmosphere; FT-IR spectra of TG-FTIR-GCMS at 260 °C for poly(2,6BAP-abz) (c) and poly(3,5BAP-abz) (d).

should be slightly different [58]. However, in this work, poly(3,5BAPabz) showed much lower HRC compared to its analogue poly(2,6BAPabz). As discussed above, due to the different position of N atom in pyridine ring with respect to the amide group, the features of Hbonding in poly(2,6BAP-abz) were different from those in poly(3,5BAPabz). In poly(2,6BAP-abz), the intramolecular N–H⋯N interaction restricted the rotation of N–CO bond, hindering the attack of hydroxyl group and leading to a higher PBO formation temperature (Scheme 4). When the temperature increased above 260 °C, the C–N–C bonding in

polybenzoxazines. Ishida and his co-workers recently reported the enhanced thermal properties and flame retardancy of polybenzoxazines containing ortho-amide group, in which the low HRC was attributed to the conversion of oxazine into oxazole ring upon heating [57]. In this work, the low HRC of poly(2,6BAP-abz) and poly(3,5BAP-abz) was also caused by the formation of PBO structures. According to Walters and Lyon's theory, the HRC of different chemical moieties could be theoretically calculated and the contribution of aromatic rings with different substitution positions to flame retardancy

Scheme 4. Illustration for the conversion of poly(2,6BAP-abz) and poly(3,5BAP-abz) into PBO structure influenced by H-bonding. 11

Polymer 181 (2019) 121807

J. Liu, et al.

flame retardancy was reported, and a more detailed investigation will be published elsewhere. 4. Conclusions Two amide-functional benzoxazine isomers, 2,6BAP-abz and 3,5BAP-abz, were skillfully designed and successfully synthesized. Due to the different regiochemistry of amide substituents, the selective formation of intra- or intermolecular H-bonding between N atom in pyridine ring and -CONH was achieved. Besides the different regiochemistry, the intramolecular N–H⋯N interaction in 2,6BAP-abz was a strong contributor to its lower curing activity. While in 3,5BAPabz, a higher strength or density of N–H⋯O (in oxazine ring) was also responsible for its higher ring-opening activity. As for the cured system, poly(2,6BAP-abz) demonstrated a higher Tg and lower SFE thanked to the higher percentage of intramolecular H-bonding. The different regiochemistry and H-bond were proved to have relationship with flame retardancy of polybenzoxazines. In poly(2,6BAP-abz), the rotation of N–CO bond was hindered and then generated a higher oxazole ring formation temperature, which should be the reason for its lower flame retardancy compared to poly(3,5BAP-abz). It was the first time that the effect of regiochemistry and H-bonding on the PBO formation temperature from o-amide polybenoxazine was reported. Overall, the manipulation on the regiochemistry of amide substituents and its effect on the properties of polybenzoxazines were investigated, and H-bonding was proved to be a strong contributing factor in this work. Acknowledgment The authors are grateful for the financial support from National Ten Thousand Talent Program for Young Top-notch Talents, Ten Thousand Talent Program for Young Top-notch Talents of Zhejiang Province, “Science and Technology Innovation 2025” Major Project of Ningbo (2018B10015), National Natural Science Foundation of China (Grant No.51373194) and Chinese Postdoctoral Science Foundation (2018M642499). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymer.2019.121807. References [1] H. Ishida, Handbook of Benzoxazine Resins, Copyright © 2011 Elsevier B.V., 978-0444-53790-4, 2011. [2] N.N. Ghosh, B. Kiskan, Y. Yagci, Polybenzoxazines-new high performance thermosetting resins: synthesis and properties, Prog. Polym. Sci. 32 (11) (2007) 1344–1391. [3] S. Zhang, Q. Ran, Q. Fu, Y. Gu, Preparation of transparent and flexible shape memory polybenzoxazine film through chemical structure manipulation and hydrogen bonding control, Macromolecules 51 (17) (2018) 6561–6570. [4] H. Ishida, D.J. Allen, Gelation behavior of near-zero shrinkage polybenzoxazines, J. Appl. Polym. Sci. 79 (3) (2001) 406–417. [5] H. Ishida, D.J. Allen, Physical and mechanical characterization of near-zero shrinkage polybenzoxazines, J. Polym. Sci. B Polym. Phys. 34 (6) (1996) 1019–1030. [6] P. Yang, X. Wang, H. Fan, Y. Gu, Effect of hydrogen bonds on the modulus of bulk polybenzoxazines in the glassy state, Phys. Chem. Chem. Phys. 15 (37) (2013) 15333–15338. [7] L. Chun-Syong, W. Chih-Feng, L. Han-Ching, C. Hsin-Yi, C. Feng-Chih, Fabrication of patterned superbydrophobic polybenzoxazine hybrid surfaces, Langmuir 25 (6) (2009) 3359–3362. [8] M. Baqar, T. Agag, R. Huang, J. Maia, S. Qutubuddin, H. Ishida, Mechanistic pathways for the polymerization of methylol-functional benzoxazine monomers, Macromolecules 45 (2012) 8119–8125. [9] C.H. Lin, Y.R. Feng, K.H. Dai, H.C. Chang, T.Y. Juang, Synthesis of a benzoxazine with precisely two phenolic OH linkages and the properties of its high-performance copolymers, J. Polym. Sci. A Polym. Chem. 51 (12) (2013) 2686–2694. [10] Y.L. Liu, C.I. Chou, High performance benzoxazine monomers and polymers containing furan groups, J. Polym. Sci. Part A Polymer Chemistry 43 (21) (2005) 5267–5282.

Fig. 12. MCC test DTG curves for poly(2,6BAP-abz) (a) and poly(3,5BAP-abz) (b), and their THR vs temperature (c).

poly(2,6BAP-abz) began to break and more combustible fragments would be released at the initial stage of combustion [53]. The distinction between the MCC and DTG curves in the first stage was a good evidence for this result. As shown in Fig. 12a, the similar peak strength of MCC and DTG curves indicated that most of the generated fragment was combustible. And in Fig. 12b, the lower MCC peak showed that some of the released fragments were hard to be oxidized and then less heat was detected. Those were the reasons for the different flame-retardant properties between poly(3,5BAP-abz) and poly(2,6BAP-abz). In previous work, the conversion of o-amide containing polybenoxazine into PBO, and then led to an increased thermal stability was often reported [26,59]. But how did the PBO formation temperature influence its degradation behavior was never noticed. Herein, it was the first time that the effect of regiochemistry and intramolecular Hbonding in polybenoxazine on the PBO formation temperature and its 12

Polymer 181 (2019) 121807

J. Liu, et al.

78071–78080. [35] X. Shen, L. Cao, Y. Liu, J. Dai, X. Liu, J. Zhu, S. Du, How does the hydrogen bonding interaction influence the properties of polybenzoxazine? An experimental study combined with computer simulation, Macromolecules 51 (13) (2018) 4782–4799. [36] P. Froimowicz, K. Zhang, H. Ishida, Intramolecular hydrogen bonding in benzoxazines: when structural design becomes functional, Chem. Eur J. 22 (8) (2016) 2691–2707. [37] K. Zhang, P. Froimowicz, L. Han, H. Ishida, Hydrogen-bonding characteristics and unique ring-opening polymerization behavior of ortho-methylol functional benzoxazine, J. Polym. Sci. A Polym. Chem. 54 (22) (2016) 3635–3642. [38] C. Li, A. Strachan, Molecular simulations of crosslinking process of thermosetting polymers, Polymer 51 (25) (2010) 6058–6070. [39] G.R. Desiraju, T. Steiner, The Weak Hydrogen Bond: in Structural Chemistry and Biology, International Union of Crystal, 2001. [40] W. Zhang, P. Froimowicz, C.R. Arza, S. Ohashi, Z. Xin, H. Ishida, Latent catalystcontaining naphthoxazine: synthesis and effects on ring-opening polymerization, Macromolecules 49 (19) (2016) 7129–7140. [41] E. Arunan, G.R. Desiraju, R.A. Klein, J. Sadlej, S. Scheiner, I. Alkorta, D.C. Clary, R.H. Crabtree, J.J. Dannenberg, P. Hobza, Definition of the hydrogen bond (IUPAC Recommendations 2011), Pure Appl. Chem. 83 (8) (2011) 1637–1641. [42] J. Wang, F. Xue, M.Q. Wu, X.Y. He, W.B. Liu, X.D. Shen, Synthesis, curing kinetics and thermal properties of bisphenol-AP-based benzoxazine, Eur. Polym. J. 47 (11) (2011) 2158–2168. [43] H.D. Kim, H. Ishida, A study on hydrogen-bonded network structure of polybenzoxazines, J. Phys. Chem. A 106 (14) (2002) 3271–3280. [44] Q. Wu, X. Liu, L.A. Berglund, FT-IR spectroscopic study of hydrogen bonding in PA6/clay nanocomposites, Polymer 43 (8) (2002) 2445–2449. [45] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (8) (1936) 988–994. [46] C.J.V. Ossb, L. Jub, M.K. Chaudhuryc, R.J. Goodb, Estimation of the polar parameters of the surface tension of liquids by contact angle measurements on gels, J. Colloid Interface Sci. 128 (2) (1989) 313–319. [47] D. Kaelble, Dispersion-polar surface tension properties of organic solids, J. Adhes. 2 (2) (1970) 66–81. [48] B. Jańczuk, T. Białopiotrowicz, W. Wójcik, The components of surface tension of liquids and their usefulness in determinations of surface free energy of solids, J. Colloid Interface Sci. 127 (1) (1989) 59–66. [49] F.M. Fowkes, Attractive forces at interfaces, Ind. Eng. Chem. 56 (12) (1964) 40–52. [50] C.F. Wang, Y.C. Su, S.W. Kuo, C.F. Huang, Y.C. Sheen, F.C. Chang, Low-surface-freeenergy materials based on polybenzoxazines, Angew Chem. Int. Ed. Engl. 45 (14) (2006) 2248–2251. [51] X. Ning, H. Ishida, Phenolic materials via ring-opening polymerization: synthesis and characterization of bisphenol‐A based benzoxazines and their polymers, J. Polym. Sci. Part A Polymer Chemistry 32 (6) (1994) 1121–1129. [52] K. Zhang, L. Han, P. Froimowicz, H. Ishida, A smart latent catalyst containing otrifluoroacetamide functional benzoxazine: precursor for low temperature formation of very high performance polybenzoxazole with low dielectric constant and high thermal stability, Macromolecules 50 (17) (2017) 6552–6560. [53] Y.L. Hong, H. Ishida, Mechanistic study on the thermal decomposition of polybenzoxazines: effects of aliphatic amines, J. Polym. Sci. B Polym. Phys. 36 (11) (1998) 1935–1946. [54] X. Shen, X. Liu, J. Dai, Y. Liu, Y. Zhang, J. Zhu, How does the hydrogen bonding interaction influence the properties of furan-based epoxy resins, Ind. Eng. Chem. Res. 56 (38) (2017) 10929–10938. [55] F. Hu, J.J.L. Scala, J.M. Sadler, G.R. Palmese, Synthesis and characterization of thermosetting furan-based epoxy systems, Macromolecules 47 (10) (2014) 3332–3342. [56] J. Yue, C. Zhao, Y. Dai, H. Li, Y. Li, Catalytic effect of exfoliated zirconium phosphate on the curing behavior of benzoxazine, Thermochim. Acta 650 (2017) 18–25. [57] J. Liu, N. Safronava, R.E. Lyon, J. Maia, H. Ishida, Enhanced thermal property and flame retardancy via intramolecular 5-membered ring hydrogen bond-forming amide functional benzoxazine resins, Macromolecules 51 (23) (2018) 9982–9991. [58] R. Sonnier, B. Otazaghine, L. Dumazert, R. Menard, A. Viretto, L. Dumas, L. Bonnaud, P. Dubois, N. Safronava, R. Walters, R. Lyon, Prediction of thermosets flammability using a model based on group contributions, Polymer 127 (2017) 203–213. [59] H. Ishida, Z. Kan, L. Jia, An ultrahigh performance cross-linked polybenzoxazole via thermal conversion from poly(benzoxazine amic acid) based on smart o-benzoxazine chemistry, Macromolecules 47 (24) (2014) 8674–8681.

[11] X. Shen, J. Dai, Y. Liu, X. Liu, J. Zhu, Synthesis of high performance polybenzoxazine networks from bio-based furfurylamine: furan vs benzene ring, Polymer 122 (2017) 258–269. [12] P. Yang, Y. Gu, A novel benzimidazole moiety-containing benzoxazine: synthesis, polymerization, and thermal properties, J. Polym. Sci. A Polym. Chem. 50 (7) (2012) 1261–1271. [13] Y. Shibayama, T. Kawauchi, T. Takeichi, Synthesis and properties of polybenzoxazines containing pyridyl group, High Perform. Polym. 26 (1) (2013) 60–68. [14] S.W. Kuo, Y.C. Wu, C.F. Wang, K.U. Jeong, Preparing low-surface-energy polymer materials by minimizing intermolecular hydrogen-bonding interactions, J. Phys. Chem. C 113 (48) (2009) 20666–20673. [15] A.A. Alhwaige, T. Agag, H. Ishida, S. Qutubuddin, Biobased chitosan/polybenzoxazine cross-linked films: preparation in aqueous media and synergistic improvements in thermal and mechanical properties, Biomacromolecules 14 (6) (2013) 1806–1815. [16] H. Ishida, Y.H. Lee, Study of hydrogen bonding and thermal properties of polybenzoxazine and poly(caprolactone) blends, J. Polym. Sci. B Polym. Phys. 39 (7) (2001) 736–749. [17] L. Dumas, L. Bonnaud, M. Olivier, M. Poorteman, P. Dubois, High performance biobased benzoxazine networks from resorcinol and hydroquinone, Eur. Polym. J. 75 (2016) 486–494. [18] B.J. Holland, J.N. Hay, The thermal degradation of PET and analogous polyesters measured by thermal analysis–Fourier transform infrared spectroscopy, Polymer 43 (6) (2002) 1835–1847. [19] J. Zhao, H. Xiao, G. Qiu, Y. Zhang, N. Huang, Z. Tang, Solid-state polycondensation of poly(ethylene terephthalate) modified with isophthalic acid: kinetics and simulation, Polymer 46 (18) (2005) 7309–7316. [20] F. Neese, The ORCA program system, Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2 (1) (2012) 73–78. [21] Y. Zhao, D.G. Truhlar, The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06 functionals and 12 other functionals, Theor. Chem. Acc. 119 (2008) 525-525. [22] A. Schäfer, C. Huber, R. Ahlrichs, Fully optimised contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr, J. Chem. Phys. 100 (8) (1994) 5829–5835. [23] A. Schäfer, H. Horn, R. Ahlrichs, Fully optimized contracted Gaussian basis sets for atoms Li to Kr, J. Chem. Phys. 97 (4) (1992) 2571–2577. [24] F.T. Huque, J.A. Platts, The effect of intramolecular interactions on hydrogen bond acidity, Org. Biomol. Chem. 1 (8) (2003) 1419–1424. [25] J. Dunkers, H. Ishida, Vibrational assignments of 3-alkyl-3,4-dihydro-6-methyl-2H1,3-benzoxazines in the fingerprint region, Spectrochim. Acta Part A Molecular & Biomolecular Spectroscopy 51 (6) (1995) 1061–1074. [26] T. Agag, J. Liu, R. Graf, H.W. Spiess, H. Ishida, Benzoxazole Resin: a novel class of thermoset polymer via smart benzoxazine resin, Macromolecules 45 (22) (2012) 8991–8997. [27] M.G. Mohamed, C.H. Hsiao, K.C. Hsu, F.H. Lu, H.K. Shih, S.W. Kuo, Supramolecular functionalized polybenzoxazines from azobenzene carboxylic acid/azobenzene pyridine complexes: synthesis, surface properties, and specific interactions, RSC Adv. 5 (17) (2015) 12763–12772. [28] H.E. Kissinger, Reaction kinetics in differential thermal analysis, Anal. Chem. 29 (11) (1957) 1702–1706. [29] T. Ozawa, A new method of analyzing thermogravimetric data, Bull. Chem. Soc. Jpn. 38 (11) (1965) 1881–1883. [30] J. Dai, N. Teng, X. Shen, Y. Liu, L. Cao, J. Zhu, X. Liu, Synthesis of biobased benzoxazines suitable for vacuum-assisted resin transfer molding process via introduction of soft silicon segment, Ind. Eng. Chem. Res. 57 (8) (2018) 3091–3102. [31] J. Dai, S. Yang, N. Teng, Y. Liu, X. Liu, J. Zhu, J. Zhao, Synthesis of eugenol-based silicon-containing benzoxazines and their applications as bio-based organic coatings, Coatings 8 (3) (2018) 88–95. [32] A. Chernykh, T. Agag, H. Ishida, Effect of polymerizing diacetylene groups on the lowering of polymerization temperature of benzoxazine groups in the highly thermally stable, main-chain-type polybenzoxazines, Macromolecules 42 (14) (2009) 5121–5127. [33] S.N. Kolanadiyil, M. Minami, T. Endo, Synthesis and thermal properties of difunctional benzoxazines with attached oxazine ring at the Para -, Meta -, and Ortho -position, Macromolecules 50 (9) (2017). [34] S. Shukla, A. Mahata, B. Pathak, B. Lochab, Cardanol benzoxazines-interplay of oxazine functionality (mono to tetra) and properties, RSC Adv. 5 (95) (2015)

13