Making Benzoxazine Greener and Stronger: Renewable Resource

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Making Benzoxazine Greener and Stronger: Renewable Resource, Microwave Irradiation, Green Solvent and Excellent Thermal Properties Na Teng, Shimin Yang, Jinyue Dai, Shuaipeng Wang, Jun Zhao, Jin Zhu, and Xiaoqing Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00607 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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ACS Sustainable Chemistry & Engineering

Making Benzoxazine Greener and Stronger: Renewable Resource, Microwave Irradiation, Green Solvent and Excellent Thermal Properties

Na Teng 1, 2, Shimin Yang 4, Jinyue Dai 1, 3, Shuaipeng Wang 1, Jun Zhao 4, Jin Zhu 1, 3, Xiaoqing Liu *, 1, 3

1 Ningbo

Institute of Materials Technology and Engineering, Chinese Academy of

Sciences, 1219 Zhongguan West Road, Zhenhai District, Ningbo, Zhejiang 315201, P. R. China 2

University of Chinese Academy of Sciences, No.19 A, Yuquan Road, Shijingshan

District, Beijing 100049, P. R. China 3

Key Laboratory of Bio-based Polymeric Materials Technology and Application of

Zhejiang Province, 1219 Zhongguan West Road, Zhenhai District, Ningbo, Zhejiang 315201 , P. R. China 4 Shanghai Space Propulsion Technology Research Institute, No. 3888, Yuanjiang Road, Minhang District, Shanghai 201100,P. R. China *To whom all correspondences should be addressed E-mail addresses: [email protected]

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ABSTRACT: In order to fulfill the principles of green chemistry, renewable magnolol and furfurylamine were taken to synthesize benzoxazine under microwave irradiation using poly(ethylene glycol) (PEG) as the solvent. The reaction was monitored by the yield of target compound under varied conditions. Only in 5 min, the yield of target benzoxazine monomer (M-fa) could be high up to 73.5% in PEG 600 system, indicating the desired advantage of microwave irradiation for benzoxazine synthesis. After the chemical structure of M-fa was confirmed, its polymerization behavior, processability and thermo-mechanical properties were carefully evaluated. The cured resins presented a high glass transition temperature (Tg) of 303 °C and exceedingly good thermal stability with 5% and 10% weight loss temperature higher than 440 and 463 °C, respectively, and a char yield of 61.3%. In addition, M-fa demonstrated viscosity lower than 1 Pa·s during the temperature range from 100 to 194 °C, which indicated its good processability. This work provided us a strategy for the synthesis of high bio-based content high performance thermosets under environmentalfriendly conditions, i.e. microwave-assisted heating method and green solvent.

Keywords: Magnolol, Furfurylamine, Bio-based polybenzoxazine, Microwave irritation, Green chemistry


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INTRODUCTION Polybenzoxazine, a relatively new kind of phenolic resin, not only inherits the merits of traditional phenolic resin, such as superior thermal and mechanical properties, but also possesses low surface energy, good dielectric, excellent dimensional stability and molecular design flexibility. It has been a promising candidate in various high valueadded applications, including electronic packing and aerospace.1 Benzoxazine can be readily synthesized via Mannich reaction from varied phenolic derivatives and primary amines. Currently, bisphenol A-based benzoxazine might be the most widely used product. However, the effect of bisphenol A on human endocrine system has attracted more and more attention.2 In addition, concerns on the depletion of crude oil and environmental issues are driving us to develop polymeric materials from renewable feedstock. Therefore, a large quantity of naturally occurring phenolic compounds, such as cardanol,3-5 eugenol,6,

7

vanillin,8-10 sesamol,11 catechol,12

chavicol,13 urushiol,14 daidzein15 and guaiacol

16

have been tried as the raw materials

for benzoxazine synthesis. However, most of them suffered from low crosslink density, low glass transition temperature (Tg) and poor thermal stability.3-9, 16-18 For instance, the benzoxazine derived from cardanol had a very low Tg due to the presence of long flexible alkyl chain.3-5 The mono-functional benzoxazines synthesized from guaiacol16 and arbutin17 also showed low Tg because of the limited crosslinking sites. In order to increase the crosslink density, some multifunctional petroleum-based compounds are usually introduced and it is the mono-bio-based (either phenol or amine is from biobased feedstock), rather than fully bio-based benzoxazine has been developed.13 Based on the literatures survey results, it concluded that significant efforts should be dedicated to the properties and bio-based content improvement of benzoxazine. And the exploration of biomass-derived phenols with multi-functional is worthy of expectation. Magnolol, also known as 4-allyl-2-(5-allyl-2-hydroxy-phenyl) phenol, can be isolated from the stem bark of Magnolia officinalis. As a bioactive compound, magnolol has been used as neurologically active agent in traditional Chinese and Japanese medicine for centuries.19,

20

Besides that, magnolol is a highly functional 3

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compound containing both phenolic and allyl groups, which allows for high performance polymers synthesis. However, it was seldom used for polymer synthesis 21

and never taken as the phenolic resource to prepare benzoxazine. Furfurylamine is

one of the few readily available bio-based amines that can be obtained from agriculture by-products. It has been reported that the furan ring in furfurylamine could participate in the ring-opening polymerization of benzoxazine, and thus improve the crosslink density of cured resins.22-28 That makes furfurylamine a promising amine resource for high performance benzoxazine synthesis. In order to fulfill the principles of green chemistry, microwave-assisted heating method has been widely used in organic synthesis, and it has been regarded as an energy-efficient technique to achieve high yield and selectivity in a short reaction time. Recently, Lomonaco

12, 29

and Lochab

6

tried microwave irradiation to prepare

benzoxazine and satisfied results have been achieved. Unlike the traditional method, this microwave-assisted reaction could be achieved in poly(ethylene glycol) (PEG) and the commonly used toxic solvents, such as toluene and dioxane, were avoided. In this study, a high bio-based content benzoxazine was synthesized using magnolol, furfurylamine and paraformaldehyde as raw materials in PEG with the help of microwave irritation (Scheme 1). In order to investigate this reaction in detail, three kinds of PEG with different molecular weights (PEG 200, PEG 400 and PEG 600) were tried and the yield of obtained benzoxazine (M-fa) along with reaction time was detected by Nuclear magnetic resonance (NMR). Before properties study, M-fa was structurally characterized by NMR, Fourier transform infrared spectroscopy (FT-IR), high-resolution mass spectrometry (HR-MS) and elemental analysis. Then, the polymerization behavior and processing properties of M-fa and the thermal stability as well as dynamic mechanical properties of cured M-fa were investigated.


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Scheme 1. Synthetic scheme for bis-benzoxazine M-fa.

EXPERIMENTAL SECTION Raw Materials. Magnolol (98%), furfurylamine (99%), paraformaldehyde (96%) and poly(ethylene glycol) (PEG 200, PEG 400, PEG 600) were obtained from Aladdin Reagent, China. Chloroform (99%), sodium hydroxide (96%), petroleum ether (6090°C), ethyl acetate (99%) and magnesium sulfate (98%) were obtained from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as received without any further purification.

Synthesis of M-fa. 5.33 g magnolol (0.02 mol), 2.52 g paraformaldehyde (0.084 mol, 4.2 equiv.), 3.88 g furfurylamine (0.04 mol) and a certain amount of PEG (PEG 200, PEG 400 or PEG 600) were added into a round bottom flask equipped with a magnetic stirrer and a condenser. The flask was then exposed to microwave irradiation at a frequency of 2.45 GHz in a microwave reactor (MWave-5000, Shanghai Xinyi). The reaction temperature was increased to 120 °C in 3 min and then the reaction was lasted 5 ACS Paragon Plus Environment


for an extra 1, 3, 5 and 7 min, respectively. In order to keep the temperature at 120 °C, the cooling system was always on during the reaction. After that, the mixture was cooled down to room temperature and 150 mL of chloroform was added. The organic layer was washed several times with 1N sodium hydroxide solution and deionized water until colorless. Then it was dried over sodium sulfate and filtered, followed by drying under reduced pressure. The residue was further purified by recrystallizing from petroleum ether: ethyl acetate (10:1) and light yellow crystals was obtained finally. For comparison, the traditional oil bath heating method was also applied to synthesize Mfa, and the overall yield was calculated.

1H

NMR (CDCl3, δ, ppm): 7.43 (s, 2H), 7.02 (s, 2H), 6.82 (s, 2H), 6.36-6.35 (t, 2H),

6.27-6.26 (d, 2H), 6.06-5.96 (m, 2H), 5.17-5.08 (m, 4H), 4.91 (s, 4H), 4.08 (s, 4H), 3.98 (s, 4H), 3.38, 3.36 (d, 4H). 13C

NMR (CDCl3, δ, ppm): 151.93 (s, 1C), 149.84 (s, 1C), 142.50 (s, 1C), 137.62 (s,

1C), 131.60 (s, 1C), 129.63 (s, 1C), 126.87 (s, 1C), 126.18 (s, 1C), 119.32 (s, 1C), 115.73 (s, 1C), 110.18, 108.77 (s, 1C), 82.10 (s, 1C), 49.78 (s, 1C), 48.35 (s, 1C) FT-IR (KBr), cm−1: 1467, 1361, 1225, 1076, 933, 911, 736. Elemental Analysis: (calculated) C (75.57%), H (6.34%), N (5.51%); (found) C (75.65%), H (6.45%), N (5.65%). HR-MS (m/z): 509.2482 [M+H] +

Curing of M-fa. M-fa was added into a stainless steel mould with the dimension of 80 mm × 80 mm × 0.5 mm, melted and degassed under vacuum at 160 °C for 5 min, and then step-cured in an air-circulating oven using the following heating process: 160 °C (2 h), 180 °C (2 h), 200 °C (2 h), 220 °C (2 h), 240 °C (2 h), 250 °C (1 h). After that, the cured resins were allowed to slowly cool down to room temperature. In order to calculate the conversion rate of M-fa by FT-IR, it was cured at specific temperature (160 °C, 180 °C and 200 °C) for various lengths of time. And the FT-IR spectra for the resins after each stage curing was recorded for analysis. 6 ACS Paragon Plus Environment

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Measurements. Fourier transform infrared spectra (FT-IR) were recorded on a NICOLET 6700 (a, America) spectrometer in the range of 400 to 4000 cm-1 at a resolution of 4 cm-1. 400 MHz AVANCE III Bruker NMR spectrometer (Bruker, Switzerland) was used to record the NMR spectra for M-fa at 25 °C, in deuterated chloroform with tetramethysilane (TMS) as an internal standard. High-resolution mass spectrometry (HR-MS) was acquired using a LC-Q-TOF (AB Sciex, America) at 500 °C with an ionization voltage of 5500 V. Elementary analysis (EA) measurement was performed on Elementar (Elementar, Germany). Differential scanning calorimetric (DSC) analyses were conducted on a Mettler-Toledo MET DSC (METTLER TOLEDO, Switzerland) under high purity nitrogen with a flow rate of 20 mL/min. To determine the activation energy of M-fa polymerization, the samples were scanned at different heating rates of 5, 10, 15 and 20 °C/min from 25 to 350 °C. All the analysis were performed in aluminum crucibles. The thermal stability of cured M-fa, poly(M-fa) was determined with a Mettler-Toledo TGA/DSC1 (METTLER TOLEDO, Switzerland). As for the non-isothermal measurements, approximate 10 mg of poly(M-fa) were heated from 50 to 800 °C at a rate of 10 °C/min under a nitrogen and air atmosphere, respectively. The isothermal stability measurements were performed at temperatures of 300, 350 and 400 °C, respectively. The weight loss as a function of time for poly(Mfa) were recorded for analysis. Dynamic mechanical analyses (DMA) were performed on a TA Instrument (TA Q800, USA) with the sample size of 20 mm × 5 mm × 0.5 mm. The measurements were conducted in tension mode with amplitude of 10 mm, using 3 °C/min heating rate at a frequency of 1 Hz in the temperature range of 25 to 350 °C. To evaluate the melt processability of the benzoxazine, dynamic rheological analyzer (Anton Paar Physica MCR-301) was employed. The tests were performed between parallel upper and lower plates (25 mm and 50 mm in diameter, respectively) at specific temperature (120, 140 and 160 °C) in a steady shear mode, and with a frequency and strain of 1 Hz and 0.1%, respectively.



RESULTS AND DISCUSSION Synthesis and Characterization of M-fa PEG-400, as a kind of green solvent with moderate polarity, can convert microwave irradiation into heat efficiently. In addition, it does not interfere with the ring closure reaction of benzoxazine. Thus, microwave irritation has been tried to synthesize several benzoxazine monomers with good yields in much short reaction times.6, 12, 29 In order to further validate the universal applicability of microwave-assisted heating for benzoxazine synthesis, it was also employed for the synthesis of M-fa from magnolol and furfurylamine in this work (Scheme 1). As expected, microwave irradiation was proved to be really effective, and high-purity M-fa with the yield of 68.6% could be obtained in only 5 min, while the traditional oil bath heating method afforded M-fa with yield of 59.7% in 24 h. Generally, the dielectric constant of solvent has a profound effect upon the ring closure of open Mannich base, and low dielectric constant favors ring closure.30 However, the higher dielectric constant of solvent is conducive to microwave-promoted reaction.31 Therefore, it is essential to investigate the effect of dielectric constant of solvent on the yield of benzoxazine under microwave irradiation. In this work, PEG with different molecular weight, including PEG 200, PEG 400 and PEG 600, were respectively taken as the reaction solvent. And their effects on the yield of M-fa along with reaction time are shown in Figure 1(a). It was noticed that, in the first 1 min, magnolol presented high reactivity with furfurylamine and paraformaldehyde in all kinds of PEG, and the highest yield was obtained in the system of PEG 200, following by PEG 400 and PEG 600. As we know, PEG with low molecular weight displays high dielectric constant and the dielectric constant of solvent has negative effect on ring closure, which will result in complex effect on the formation of benzoxazine. The relatively higher M-fa yield obtained in PEG 200 at first 1 min might be caused by the higher heat conversion efficiency of PEG 200, compared with PEG 400 and PEG 600. As the reaction proceeded, the M-fa yield first increased and then decreased and the optimal reaction time in PEG 200, PEG 400 and PEG 600 was within 3, 5 and 5 min, 8 ACS Paragon Plus Environment

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respectively. What should be emphasized here was that the M-fa yield obtained in PEG 400 and PEG 600 increased much faster than that in PEG 200, and the highest yield for M-fa was 67.9% in PEG 200, 68.6% in PEG 400 and 73.5% in PEG 600. These results indicated that the solvent with low dielectric constant was more favorable to the formation of benzoxazine ring at a certain temperature even under microwave radiation, and the solvent with higher polarity would more easily initiate the ring opening reaction of benzoxazine.32 As shown in Figure 1(b), the effect of reactant concentration (the molar ratio of magnolol : furfurylamine : paraformaldehyde is fixed at 1 : 2 : 4.2) on M-fa yield in PEG 600 was studied. All the reactions were performed at 120 °C and the reaction time was 5 min. The yield of M-fa increased rapidly from 44.6% to 73.5%, when the reactant concentration increased from 0.16 to 0.39 g/mL. However, further increase of reactant concentration to 0.59 g/mL resulted in a reduced yield of 70.0%. Therefore, the optimal reactant concentration was 0.39 g/mL.



Figure 1. The yields of M-fa in different solvents as a function of reaction time (a) and yields of M-fa in PEG 600 as a function of reactant concentration (b).

The chemical structure of M-fa was confirmed by NMR. As shown in Figure 2(a), the chemical shift showing at 4.91 and 3.98 ppm corresponded to the methylene units in the oxazine rings (O-CH2-N and Ph-CH2-N). The characteristic signals for allyl group were observed at 5.08-5.17 (CH2-CH=CH2) and 5.96-6.06 ppm (CH2-CH=CH2). In addition, the resonance peaks appeared at 7.43 and 6.26-6.36 ppm were assigned to furan protons. The

13C

NMR spectrum shown in Figure 2(b) also identified the

characteristic signals of oxazine rings and allyl group at 82.10, 49.78 and 137.62, 115.73 ppm, respectively. And the signals showing at 149.84, 142.50, 110.18 and 108.77 ppm were attributed to carbons in furan ring. To further verify the structure of M-fa, FT-IR analysis has been conducted (Figure S1). The absence of peak between 3200 and 3600 cm-1 indicated that phenolic hydroxyl was not present in the samples. The characteristic peak showing at 1225 and 1076cm-1 were assigned to the C-O-C stretching of oxazine ring, confirming the formation of benzoxazine. The band appeared at 933 cm-1 stood for the mixture virbrations of O-C stretching of oxazine and phenolic ring. 33 Besides, the presence of allylic bond in M-fa was indicated by the band observed at 911 cm-1. These characteristic peaks were all consistent with the published studies and confirmed the successful synthesis of M-fa. 10 ACS Paragon Plus Environment

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The purity of benzoxazine monomer is crucial to determine its curing behavior and properties of cured resin. Accordingly, before properties investigation, the purity of Mfa was carefully confirmed by mass spectrum and elemental analysis. As exhibited in Figure S2, the molecular ion [M+H]+ peak in mass spectrum appeared at 509.2482, which was in good accordance with the theoretical value of 509.6028. Furthermore, elemental analysis of M-fa revealed the content of C, H and N were 75.65%, 6.45% and 5.65%, respectively, which agreed well with the theoretical compositions (C, 75.57%; H, 6.34% and N, 5.51%).

Figure 2. (a) 1H-NMR and (b) 13C-NMR spectra of M-fa.



Curing Behaviors of M-fa Recently, furfurylamine has been increasingly incorporated into benzoxazine monomers due to its naturally occurring and reactive nature.7, 16, 34-36 In addition, allyl in benzoxazine monomers can also be involved in the formation of polymer network.3742

As we know, the network structure of polybenzoxazine is crucial to its properties. In

this study, M-fa monomer, with special molecular structure, not only contains furan and allyl groups, but also is blocked at ortho- and para- positions with respect to the phenolic hydroxyl. Therefore, it is interesting and important to study the curing behaviors of M-fa. Figure 3 showed the DSC curves of M-fa at varied heating rates. The sharp endothermic peak at 100 °C indicated the high purity of M-fa. Moreover, compared with previously reported values shown in Table 1, the melting point of M-fa was lower than most of bio-based bis-benzoxazines, and even lower than many monobenzoxazine, which indicated its good processability. Interestingly, only one exothermic peak centered at 218 °C was observed when the heating rate was 5 °C/min, even though the upper temperature of DSC was set to be 350 °C. Based on literature survey,6, 44-46 the ring-opening polymerization (ROP) of ortho- and para- blocked furan containing benzoxazine takes place below 270 °C, and the exothermic peak for oxazine and allyl thermal polymerization reaction usually overlaps.40, 42, 47 Therefore, the single exothermic peak during DSC heating scan in Figure 3 should be attributed to the reaction of oxazine ROP and allyl radical polymerization, which was lower than that of some previously reported bio-based benzoxazines, such as 240 °C for guaiacol-based benzoxazine 16 and 256 °C for cardanol-based benzoxazine.5


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Figure 3. Non-isothermal DSC exothermic curves of M-fa at different heating rates.

Table 1.Thermal characterization of bio-based benzoxazine monomers and cured resins reported in previous literatures Structure

Name

Tm (°C)

Tpolymerization (°C)

Td5 (°C)

Char Yield (%)

U-fa

138

198

420

60

IE-fa

99

238

375

E-fa

74

222

S-fa

120

G-fa

V-fa

Tg (°C) n.r.[a

Ref

]

28

60

179‡

6

361

53.8

148‡

7

236

374

64

n.r.[a]

240

352

125

205

351


n.r.[a ]

11

56

148*

16

65

270*

9


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CV-pda

112

240

413

n.r.[a]

353‡

13

CD-a

n.r.[a]

265

355

13

58‡

5

CDddm

n.r.[a]

256

369

32

94‡

5

V-CDeda

101

227

373

n.r.[a]

129‡

43

CT-fa

198

220

n.r.[a]

n.r.[a]

A-fa

107

207

n.r.[a]

MU-dm

133

228

M-fa

100

229

n.r.[a ]

12

n.r.[a]

190*

17

362

32

207*

18

440

61

303‡

this work

[a] n.r.: not reported. [b] n.s.: not shown. Tg values are reported from DSC* or tanδ‡

To further understand the curing mechanism of M-fa, FT-IR spectra at different curing stages were obtained to monitor the structure evolution during the curing process. As shown in Figure 4, all the absorbance was normalized using the band at 1467 cm-1 corresponding to tetra-substituted benzene ring as the internal standard. After being heated at 180 °C for 2 h, the characteristic peaks for oxazine ring at 933, 1225 and 1361 cm-1 disappeared or broadened, indicating the complete ring-opening. Moreover, a new absorption peak showing at 1261 cm-1 appeared, which was attributed to the bridge model of -CH2-NR-CH2- and confirmed the cross-link formation.15 Additionally, a 14 ACS Paragon Plus Environment

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concomitant decrease in the intensity of absorption peak at 736 cm-1, which was assigned to C-H out-of-plane wagging vibrations of furan ring, suggested its involvement in the ROP reaction.28 Meanwhile, along with the curing reaction, the increasing absorption band for phenolic hydroxyl at 3200-3600 cm-1 was observed, further indicated the cross-link formation by electrophilic substitution of iminium ion on furan ring as shown in Scheme 2. More carefully analyzing the variation in Figure 4, the intensity of band for allyl group showing at 911 cm-1 began to decrease after curing at 160 °C for 2 h, confirming the participation of allyl double bond in forming cross-link networks (Scheme 2), which was coincident well with above DSC analysis. Such participation would undoubtedly play a vital role in leading to high thermal stability and glass transition temperature, which will be illustrated in the following part.

Figure 4. FT-IR spectra of M-fa during the step by step curing reaction (cured at 160 °C for 2 h, 180 °C for 2 h, 200 °C for 2 h, 220 °C for 2 h, 240 °C for 2 h and 250 °C for 1 h).



Scheme 2. Proposed curing mechanism of M-fa.

To obtain further information about the curing behavior of M-fa, the ROP kinetic was studied. In suit FT-IR analysis (Figure S3) was utilized to calculate the rate of oxazine ring-opening by following the normalized reducing intensity of absorption band near 933 cm-1. Figure 5 displayed the conversion curves of M-fa to ring-opening structure at 160, 180 and 200 °C, respectively. As noticed, when it was cured at a relatively low temperature, the oxazine ring-opening rate reduced rapidly with the curing time, and even after being cured at 160 °C for 180 min, the conversion could only reach 50.8%. However, once the curing temperature was increased to 200 °C, the oxazine ring opened rapidly, and the conversion reached near 100% only within 75 min as illustrated in Figure 5.

Figure 5. Conversion of oxazine ring-opening at 160, 180 and 200 °C for 180 min. 16 ACS Paragon Plus Environment

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The overall polymerization kinetic of M-fa was studied using non-isothermal DSC measurement at different heating rates of 5, 10, 15 and 20 °C/min in a nitrogen atmosphere (Figure 3). The apparent activation energy (Ea) of this curing system can be calculated based on the following well-known Kissinger (equation 1) and the modified Ozawa (equation 2) methods. 48, 49

― ln (𝛽 𝑇2𝑃) = 𝐸𝑎 𝑅 𝑇𝑝 ― ln (𝐴𝑅 𝐸𝑎)

(1)

ln 𝛽 = ―1.052 × 𝐸𝑎 𝑅 𝑇𝑝 +∁

(2)

where β is the heating rate, Tp is the exothermic peak temperature, Ea is the activation energy, R is the gas constant, A is the pre-exponential factor and C is a constant. By plotting ln (β/Tp2) and ln β against 1/Tp (Figure S4), the Ea for M-fa curing reaction was calculated to be 122.9 and 125.8 kJ/mol, respectively. This value was equal to or lower than those of some previously reported allyl functionalized benzoxazines, such as 197 kJ/mol (Kissinger) for E-fa.6 The reason for this result might be the special structure of M-fa. According to Liu’s previous work, 50 the ortho-ortho benzoxazine usually showed higher polymerization rate and superior properties when compared with the paracounterparts. In this work, magnolol has the phenolic hydroxyl groups at ortho-ortho placement and experiences the same situation as the one reported by Liu et al. The deceleration by the allyl group and acceleration by the ortho-ortho placement offset each other, and finally such an activation energy was obtained.

Thermal Stability of poly(M-fa) After curing reaction, the resulted polybenzoxazine named as poly(M-fa) was developed. The thermal stability of poly(M-fa) was evaluated by TGA under nitrogen and air atmosphere. As shown in Figure 6(a), no matter under nitrogen or air atmosphere, negligible weight loss was observed below 360 °C for poly(M-fa). When the temperature was further increased, the 5% and 10% weight loss (Td5 and Td10) occurred at 440 and 463 °C, respectively. These values were higher than or comparative 17 ACS Paragon Plus Environment


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with that of almost all previously reported bio-based polybenzoxazines (Table 1). For example, a umbelliferone-furfurylamine-containing polybenzoxazine (poly(U-fa)) reported by Froimowicz exhibited Td5 and Td10 values of 420 and 467 °C, respectively. 28

To the best of our knowledge, 467 °C is the highest Td10 for all of ever reported bio-

based polybenzoxazines, which was only 4 °C higher than that for poly(M-fa) in this work. In addition, isothermal TGA under nitrogen were performed to gain further insight into the thermal stability of poly(M-fa). In Figure 6(b), less than 1 wt% weight loss was observed for poly(M-fa) after being heated at 300 °C for 120 min, suggesting its superior stability toward thermal degradation. With the incremental testing temperature, the residual weight at 350, 400, 450 and 500 °C were 97.9%, 93.9%, 90.5% and 84.3%, respectively. Meanwhile, it was worth noting that the weight loss rate of poly(M-fa) decreased dramatically with the prolongation of heating time at certain temperature, and the residual weight displayed little variation after 100 min. All these results indicated that poly(M-fa) has remarkably high thermal stability and probably be exploited for the next generation of thermosets with high thermal stability. As the thermal decomposition of polybenzoxazines usually begin with the cleavage of amino part, the excellent thermal stability of poly(M-fa) may be ascribed to the participation of furan ring into cross-linking reaction, which suppressed segmental decomposition into gaseous fragments.26, 51 In addition, cross-linked allyl groups caused higher crosslinking density, which might be another reason for the excellent thermal stability of poly(M-fa).37-39 The char yield, typically defined as the residual weight at 800 °C under nitrogen atmosphere, was not only associated with the degree of aromaticity, but also could be enhanced by the higher cross-linking density.16,

52

Therefore, it was not

surprising that poly(M-fa) exhibited high charring ability, showing a char yield of 61.3% obtained from the curve in Figure 6(a), which again implied the excellent thermal stability of poly(M-fa).


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Figure 6. Non-isothermal and isothermal TGA curves for the poly(M-fa).

Dynamic Mechanical Properties of poly(M-fa) The dynamic mechanical properties of poly(M-fa) was evaluated by DMA. Figure 7(a) displays the temperature dependence of storage modules (E’) and dissipation factor (Tan δ), from which the viscoelastic transition of poly(M-fa) could be seen and thus structural information could be further explored. It was observed that the E′ of poly(Mfa) at 25 °C, an important parameter to evaluate material stiffness, was high up to 2460 MPa. Tg of poly(M-fa), determined by the tan δ peak temperature, was found to be 303 °C. That was much higher than that of traditional bisphenol A-based polybenzoxazines with Tg at about 180 °C.53-56 In addition, as shown in Table 1, although poly(M-fa) had a lower Tg than the mono-bio-based poly(CV-pda), it exhibited higher Tg compared to 19 ACS Paragon Plus Environment


the vast majority of bio-based polybenzoxazines, possibly due to its high cross-link density and stiff molecular structure. Besides, from the DMA spectrum, the β-transition around 100 oC was obviously observed, which was attributed to the incomplete network structure of polybenzoxazines according to Ishida's previous work.57 To perform a graphical evaluation and comparison, Td5 and Tg of bio-based polybenzoxazines presented in Table 1 were collected in Figure 7(b). It was evident that poly(M-fa) exhibited outstanding thermal stability and reasonably high Tg.

Figure 7 DMA curves of poly(M-fa) (a); Td5 and Tg of poly(M-fa) compared with literature results (b).


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Processability of M-fa Rheological behavior is a key factor for the processability evaluation of high performance thermosetting resins. Normally, the processing temperature is defined as the temperature range between liquefying point (Lp, the transition temperature of solid to liquid) and gel point (Gp, the transition temperature of liquid to solid at high temperature).58 In addition, the viscosity required for resin transfer molding (RTM) at the processing temperature is roughly lower than 1 Pa·s.59 Therefore, the dynamic viscosity and isothermal rheology analysis of M-fa were studied. As shown in Figure 8(a), M-fa demonstrated viscosity lower than 1 Pa·s at the temperature range from 100 to 194 °C, and thus it had a wide temperature window available for processing. The inserted image in Figure 8(a) visually illustrated the ultra-low viscosity of M-fa at 150 °C. Moreover, M-fa could keep stable viscosity as low as 0.2 Pa·s at 120 °C for at least 120 min shown in Figure 8(b), which was quite suitable for RTM process. Besides, even at 160 °C, the gelation time, corresponding to the point at which the shear viscosity reached 103 Pa·s, was found to be as long as 60 min, which was further suggestive of a good processability of M-fa.



Figure 8. Viscosity as a function of temperature with the heating rate of 3 °C/min (a) and viscosity as a function of time at different temperatures (b) for the M-fa.

CONCLUSIONS In this article, we have designed and successfully synthesized a high bio-based content bis-benzoxazine from magnolol, furfuryamine and formaldehyde in PEG via microwave heating protocols. Using low dielectric PEG 600 as a solvent, the yield of M-fa reached 73.5% only within 5 min. The DSC and FT-IR results revealed relatively high reactivity of M-fa toward polymerization. M-fa also showed excellent processability with wide processing window. Both the non-isothermal and isothermal TGA results showed exceptionally good thermal stability of poly(M-fa). In particular, the Td5 of poly(M-fa) was as high as 440 °C, which was higher than almost all the previously reported value for bio-based polybenzoxazines. The Tg of poly(M-fa) was also remarkable, up to 303 °C. In summary, we have demonstrated the high possibility of developing natural renewable resources for the generation of high-performance resins under environmental friendly conditions. The principles of green chemistry were fulfilled by renewable feedstock, green solvent and energy-efficient heating technique in this work.


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ASSOCIATED CONTENT Supporting Information FT-IR and HR-MS spectrum of M-fa; In suit FT-IR spectra of M-fa; Kissinger and Ozawa plots for determination of M-fa activation energy.

NOTES The authors declare no competing financial interest. *Corresponding authors: Xiaoqing Liu, E-mail address: [email protected]

ACKNOWLEDGMENTS The authors are grateful for the financial support from National Natural Science Foundation of China (Grant No.51373194), National Key Technology Support Program (2015BAD15B08), Chinese Postdoctoral Science Foundation (2018M642499) and the project co-funded by Chinese MIIT Special Research Plan on Civil Aircraft with the Grant No. MJ-2015-H-G-103.

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For Table of Contents Use Only

A bio-based high performance polybenzoxazine was prepared by exploiting natural renewable raw materials under environmental friendly conditions


Making Benzoxazine Greener and Stronger: Renewable Resource

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