Synthesis of an Epoxy Monomer from Bio-Based 2,5-Furandimethanol

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Synthesis of an Epoxy Monomer from Bio-based 2,5Furandimethanol and its Toughening via Diels-Alder Reaction Xiaobin Shen, Xiaoqing Liu, Jinggang Wang, Jinyue Dai, and Jin Zhu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01624 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 10, 2017

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Industrial & Engineering Chemistry Research

Synthesis of an Epoxy Monomer from Bio-based 2,5-Furandimethanol and its Toughening via Diels-Alder Reaction Xiaobin Shen,

†

†,‡

†

Xiaoqing Liu, *, Jinggang Wang,

†,‡

Jinyue Dai,

†,‡

and Jin Zhu

†

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China ‡

University of Chinese Academy of Sciences, Beijing, 100049, PR China * Correspondence to: Xiaoqing Liu, E-mail: [email protected], Tel: 86-574-86685925, Fax: 86-574-86685186,

Address: 1219 Zhongguan West Road, Zhenhai District, Ningbo 315201, China.

ACS Paragon Plus Environment


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ABSTRACT: A bio-based epoxy 2,5-bis[(2-oxiranylmethoxy)methyl]-furan (BOF) has been synthesized from 2,5-furandimethanol and toughened by its Diels-Alder (DA) reaction adduct DA-BOF, which contained flexible pendant chain. The chemical structures of the synthesized monomers were identified by Nuclear Magnetic Resonance (1H-NMR and

13

C-NMR). The thermal reversibility of DA-BOF was

investigated by Differential Scanning Calorimetry (DSC) and Ultraviolet-Visible (UV-Vis) spectroscopy. Based on the UV-Vis results, the degree of retro Diels-Alder (r-DA) reaction was determined to be 25-27% in different cured systems. The mechanical properties study indicated that when the content of DA-BOF reached 50%, an 85% increment in impact strength could be obtained without seriously sacrificing its tensile strength and modulus. The strategy described in this work could help us to achieve a bio-based epoxy resin with increased impact strength and nearly unaffected tensile properties.

Keywords: bio-based epoxy; toughen; furan; Diels-Alder reaction


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1. INTRODUCTION Epoxy resins have been widely used in adhesives, composites, coatings and structural materials et al, due to their excellent properties. However, the high brittleness severely limited their application fields. Up to now, significant efforts have been made to increase the toughness of epoxy resins. As we know, the toughened epoxy systems could be achieved by the addition of inorganic particles including alumina,1 silica 2 and graphene platelets.3 And also, the rubber particles, 4,5 block copolymers 6 and core/shell particles 7,8 were often employed to blend or copolymerize with the epoxy resins so as to increase their toughness. However, the addition of second organic phase will lead to the reduction in the modulus and strength. And the processing of epoxy resins will become more difficult when a large number of inorganic particles were introduced. Therefore, it is still a challenge for us to develop the new methods, which could toughen the epoxy networks efficiently without sacrificing their strength or process ability. Nowadays, the bio-based polymers have shown great potential to replace the traditional petroleum-based ones. Many kinds of thermoplastic polymers,9-14 such as polylactic acid (PLA), poly(hydroxyalkanoates) (PHAs), poly(butylene succinate) (PBS) and polyethylene (PE) et al have been synthesized from the bio-based feedstock and some of them have already been commercialized successfully. However, for the research on bio-based thermosetting resins, limited attention has been paid on them. Although, several renewable resources and their derivatives, such as soybean oil,15 tung oil,16 vanillin,17-19 lignin,20-23 rosin acid,24-27 starch,28,29 gallic acid30, 31 and isosorbide32 have been tried to prepare the bio-based epoxy resins, their performances still have great space and potential to be improved. For example, the cured resin derived from soybean oil or tung oil usually showed poor mechanical and thermal properties due to the presence of long soft aliphatic chains.33 The epoxies derived from lignin,20-23 rosin acid24-27 or gallic acid 30,31 had high glass transition temperature (Tg) and excellent tensile strength, but the high brittleness limited their



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application fields.21,

27

Although, the epoxy resin derived from diphenolic acid

demonstrated excellent properties, which was similar to those of diglycidyl ether of bisphenol A (DGEBA).34 It is still necessary for us to find out more ideal feedstock for high performance and functionalized bio-based epoxy synthesis. As we know, the chemical structure of building block plays an significant role in determining the properties of the synthesized materials. Recently, the bio-based furan derivatives have been considered as the promising alternatives to petroleum-based phenyl building blocks in polymer science, due to their aromatic characteristics.35 For example, Palmese GR and co-workers

36, 37

reported the furan-based epoxy systems

demonstrating high Tg and storage modulus. Liu XQ et al.38 synthesized the diglycidyl ester of 2, 5-furandicarboxylic acid and it showed higher Tg, similar or comparable mechanical properties after curing reaction when compared with diglycidyl ester of terephthalic acid. The bio-based epoxy resins with satisfied thermal properties could be synthesized from the renewable furanyl building blocks. However, how to toughen these highly crosslinked systems without sacrificing their modulus and strength is still a challenge. It is well known that the Diels-Alder (DA) cycloaddition reaction between furan ring and maleimide moieties is thermally reversible.39 The furan/maleimide based DA adduct will display decoupling characteristic through retro Diels-Alder reaction (r-DA) over 120 oC, while the DA adduct could be reformed again by the subsequent DA reaction after being cooled down. Using this thermally reversible reaction, many polymers demonstrating self-healing ability have been synthesized.40-48 In this work, a straightforward method to toughen the bio-based epoxy resin derived from 2,5-furandimethanol via DA reaction was presented. In Scheme 1, the synthetic route and chemical structures of the epoxy monomers (BOF and DA-BOF) as well as the curing agent were shown. After BOF, DA-BOF and their mixtures with different ratio were cured by isophorone diamine (IPDA), the properties of the different cured systems in terms of tensile properties, impact strength and thermal performance were


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studied. Especially, the toughening effect of soft aliphatic chain attached on BOF as well as the free maleimide monomers (HMI) homogeneously dispersed in the cured systems was investigated.

Scheme 1. Synthetic route and chemical structures of BOF, DA-BOF and IPDA

2. EXPERIMENTAL SECTION 2.1. Materials. All the reagents and solvents used in this work, including tetrabutylammonium bromide (TBAB), epichlorohydrin (ECH), isophorone diamine (IPDA, 99%), oxalyl chloride, triethylamine, ethyl acetate, hexane, dichloromethane, sodium hydroxide, hexylamine and maleic anhydride were supplied by Aladdin Reagent, China; 2,5-Furandimethanol (Purity: 98%; Melting point: 75-76°C; White to Pale yellow solid) was obtained from Yajie Zhang’s Group in Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences. All the chemicals were used as received.



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2.2. Synthesis of monomers 2.2.1. Synthesis of 2,5-Bis[(2-oxiranylmethoxy)methyl]-furan (BOF).36 Into a 1000 mL round-bottomed flask equipped with constant-pressure dropping funnel and thermometer as well as condenser, 313 mL of epichlorohydrin (4 mol) and 2.57 g of TBAB (0.008 mol) together with 51.25 g of 2,5-furandimethanol (0.4 mol) were charged under continuous stirring. After the reaction was performed at 65 °C for 5 h, the mixture was cooled down to 30°C, and 200 mL of sodium hydroxide aqueous solution (50% w/w) was added dropwise. Then, the mixture was extracted three times with dichloromethane. The organic phase was washed with deionized water and dried with anhydrous MgSO4 several times. A rotary evaporator was used to remove the solvent, leaving a brown-red liquid residue. The silica gel chromatography with the mixture of ethyl acetate/hexane (2:1 by volume) was used to purify the product and an overall yield of 64% was obtained (61.81 g). 1

H NMR (400 MHz, CDCl3-d6, δ, ppm): 6.27(s, 2H), 4.49(q, 4H), 3.78 (m, 2H), 3.52

(m, 2H), 3.13 (td, 2H), 2.77 (m, 2H), 2.59 (m, 2H). 13

C NMR (400 MHz, CDCl3-d6, δ, ppm): 151.8 (2C), 110.3 (2C), 70.7 (2C), 65.1 (2C),

50.7 (2C), 44.3 (2C).

2.2.2. Synthesis of N-hexyl maleimide (HMI).49 Into a 1000 mL round-bottomed flask, 53 mL of N-hexylamine (0.4 mol) was added dropwise to the solution of 43.15g maleic anhydride (0.44 mol) in 300 mL dichloromethane at 0 °C. The mixture was stirred at room temperature for 12 h before being washed with hydrochloric acid solution and dried with MgSO4. After filtration, the white solid was dissolved in 400 mL dichloromethane and cooled to 0 °C again, and then 5-10 drops of dimethyl formamide (DMF) together with 40 mL oxalyl chloride (0.48 mol) dissolved in 100 mL dichloromethane were added slowly. The solution was stirred at room temperature for 7 days before the solvent was removed in the rotary evaporator. After that, the residual dark red liquid was dissolved in 300 mL dichloromethane again and 84 mL


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triethylamine (0.6mol) was added. The mixture was stirred at room temperature for another 24 h before the precipitate was filtrated. At last, the rotary evaporator was employed to remove the solvent and the light yellow oil with the overall yield of 51% was obtained (36.71 g). 1

H NMR (400 MHz, CDCl3-d6, δ, ppm): 6.66 (s, 2H), 3.48 (t, 2H), 1.55 (m, 2H), 1.25

(s, 6H), 0.85 (t, 3H). 13


28.5 (1C), 26.4 (1C), 22.5 (1C), 13.9 (1C).

2.2.3. Synthesis of the epoxy monomer containing pendant chain (DA-BOF). Into a 500 mL round-bottomed flask equipped with a thermometer and mechanical stirrer, 28.8g of synthesized BOF (0.12mol) together with 21.72g of N-hexyl maleimide (0.12mol) and 200 mL chloroform were charged. After the mixture was stirred at 30 °C for 12 h, the solvent was removed using rotary evaporator and the rose-red liquid residue weighting 48.5 g was obtained. 1

H NMR (400 MHz, CDCl3-d6, δ, ppm): 6.30 (d, 2H), 4.17 (m, 4H), 3.86 (m, 2H),

3.53 (m, 4H), 3.28 (t, 2H), 3.17 (m, 2H), 2.78 (m, 2H), 2.61 (m, 2H), 1.39 (m, 2H), 1.23 (d, 6H), 0.85 (t, 3H). 13


69.2 (2C), 50.6 (2C), 47.4 (1C), 44.1 (2C), 38.7 (2C), 31.3 (1C), 27.4 (1C), 26.5 (1C), 22.4 (1C), 14.0 (1C).

2.3. Curing procedure. The predetermined BOF、DA-BOF and IPDA were dissolved in a small amount of chloroform and the mixture was stirred at room temperature for 15 min to get the homogeneous system. After the solvent was removed in the vacuum oven at 40 oC with the pressure of about 200Pa for 30 min, the gas free mixture was transferred into a mold with the cavity dimension of 3*10*100 mm3. The curing reaction was conducted at 60 oC for 2 h, 90 oC for 2 h and 120 oC for 10 h to achieve



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the fully cured resins. Then the temperature was decreased slowly from 120 oC to room temperature over a period of 2 h. The cured resin was carefully removed from the mold and ready for properties investigation. In this work, all of the samples were cured under the same conditions. The purity of epoxy and diamine monomer was high enough (＞99%). Therefore, the mole ratio of epoxy to IPDA in different systems could be determined by the amount of reactive groups (amino group and epoxide group). The stoichiometric ratio of primary amino group to epoxide group was 1:2, due to the fact that there are two active H in one primary amine. The composition for different systems was shown in Table 1. The nomination was based on the molar ratio of DA-BOF to BOF. For example, the sample DA-BOF 25% represents the cured system in which the molar ratio of DA-BOF to BOF is 1:3.

Table 1. Nomination and composition for the different cured systems (molar ratio) Samples

IPDA

DA-BOF

BOF

DA-BOF 0%

1

0

2

DA-BOF 25%

1

0.5

1.5

DA-BOF 50%

1

1

1

DA-BOF 75%

1

1.5

0.5

DA-BOF 100%

1

2

0

2.4. Measurements. NMR spectra were recorded on a 400 MHz Bruker AVANCE III spectrometer. It was conducted at 25 oC with the solvent of CDCl3. UV-Vis spectra were recorded on Lambda 950 (Perkin-Elmer, America). About 1g of each sample (different cured epoxy systems) was weighed and extracted in 300 mL dichloromethane thoroughly. Then the obtained solution was examined by UV-Vis spectra. Each sample was tested three times for accuracy. The DSC measurement was conducted on a Mettler-Toledo TGA/DSC I under the protection of nitrogen atmosphere. Approximately 5-10 mg of each sample (epoxy monomer mixed with curing agent) was weighed and sealed in aluminum crucibles.


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The samples were heated from 10 to 180 oC at a heating rate of 2 oC min-1. For the thermal properties investigation of the cured networks, the sealed sample was heated from 10 to 140 oC at a heating rate of 10 oC min-1 and the heating curves were recorded for analysis. In order to ensure the accuracy of the experiment, DSC measurement for each sample was repeated three times. The mechanical properties of the cured resins were carried out on an Instron 5567 Electric Universal Testing Machine (Instron, America). Tensile properties of the specimens with the gauge length of 40 mm were measured at a speed of 10 mm min-1 according to GB/T 1040.2-2006. The impact strength was evaluated by at a speed of 1 J and five specimens were tested for each sample to obtain an average value. The fracture surfaces of the impact fractured samples were examined by Scanning Electron Microscopy (SEM, Hitachi S-4800). Before examination, the fracture surfaces were cleaned with alcohol in order to eliminate any impurities. The fracture surfaces were then coated with a thin evaporated layer of platinum to improve the conductivity. The image with different magnification was taken for investigation and comparison.

3. RESULTS AND DISCUSSION 3.1. Synthesis and chemical structure characterization of DA-BOF. Scheme 1 illustrated the synthetic routes for epoxy monomers DA-BOF and BOF. The chemical structures of BOF, HMI and DA-BOF were confirmed by means of 1H-NMR and 13

C-NMR spectroscopy. The 1H-NMR spectra were presented in Figure 1. In Figure

1a, the peaks between 2.5 ppm and 4.0 ppm indicated that the epoxy groups had been successfully attached onto 2, 5-furandimethanol. The peak at 4.49 ppm represented the protons of methylene connected with the furan ring. And the peak at 6.27 ppm was attributed to the protons on furan ring. In Figure 1b, Hc, Hd, He showed the signals at 1.55, 1.25 and 0.85ppm, respectively. And Hb, protons of methylene adjacent with the nitrogen atom, displayed the characteristic chemical shift at 3.48 ppm. The peak



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showing at 6.66 ppm was assigned to the double bond in maleimide. In Figure 1c, the peaks between 2.5 and 3.2 ppm were attributed to He, Hf, Hg. The signals standing for Hb and Hc were shown from 3.4 ppm to 4.3 ppm. After the Diels-Alder reaction was completed, the characteristic peak for proton Ha was appeared at 6.30 ppm. And the signal for Hd was overlapped with that for Hc. The other peaks corresponding to the protons on maleimide were also assigned accordingly. Based on these analyses and the assigned 13C-NMR spectra (Supplemental Information S1), it could be concluded that the target products were synthesized successfully.

Figure 1. 1H-NMR spectra of BOF (a), HMI (b) and DA-BOF (c).


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3.2. Thermo-reversibility of DA-BOF. It is well known that the Diels-Alder (DA) cycloaddition reaction between furan ring and maleimide moieties is thermal reversible. 39 Therefore, the DA and r-DA reactions are inevitable during the curing process in the systems containing DA-BOF. In order to investigate this thermal reversible reaction clearly, the 1H-NMR spectra of DA-BOF after cyclic thermal treatment were collected (Figure 2). At first, the as-synthesized DA-BOF was applied to NMR measurement at room temperature and the 1H-NMR spectrum was collected (Figure 2a). Then the dissolved DA-BOF was heated to 100oC and kept at this temperature for 30 min, and subsequently quenched quickly to room temperature for the next 1H-NMR run (Figure 2b). Afterwards, the same sample was treated at 40oC for another 2h before cooling down to room temperature for 1H-NMR measurement again (Figure 2c). According to the 1H-NMR signals assignments in Figure 1, the characteristic peak 1 and peak 3 in Figure 3 were assigned to the protons in DA-BOF (DA adduct). Peak 1’ and peak 2 were attributed to protons in BOF (r-DA products) and peak 1’’ was standing for the protons in HMI (r-DA products). As clearly shown in Figure 2a, only peaks 1 and 3 were observed before heating treatment, indicating the formation of DA-BOF. However, after being treated at 100 oC for 30 min, the disappearance of peak 1 and peak 3 together with the newly appearance of peaks 1’, 1’’ and 2 indicated that most of the DA adducts (DA-BOF) was cleaved at 100 oC due to the r-DA reaction. After the r-DA product was treated at 40 oC for another 2 h, the reappearance of peaks 1 and peak 3 (Figure 2c) indicated the reformation of DA adduct (DA-BOF) again. Obviously, the thermal reversibility of DA-BOF was reflected by the 1H-NMR spectra clearly. Moreover, the DA and r-DA conversion of DA-BOF was quantitatively calculated to be about 84% based on the intensity variation of peak 2 and peak 3 before and after heat treatment.



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Figure 2. 1H NMR spectra for DA-BOF: (a) before heating, (b) after being heated at 100 oC for 30 min and quenched to room temperature, and (c) after being treated at 40 o

C for 2 h and quenched to room temperature again.

3.3. Preparation and characterization of epoxy networks. The synthesized epoxy monomer BOF and DA-BOF as well as IPDA were mixed together with the mole ratio of DA-BOF to BOF ranged from 0 to 100% (Table 1). The DSC curves with the heating rate of 2 oC /min for different reactive mixtures were shown in Figure 3. The initial curing temperatures for all the samples were as low as 40oC, indicating their high curing activity. The peak exothermic temperatures for DA-BOF 0%, DA-BOF 50% and DA-BOF 100% were 87oC, 85oC and 81oC, respectively. Usually, under the same curing conditions, the peak exothermic temperature can be taken as an indicator to evaluate the curing reactivity of the compound. The lower peak temperature usually indicates the higher curing reactivity. For the different systems, the higher content of DA-BOF resulted in a lower peak temperature, which indicated that the curing activity of DA-BOF was a little higher than that of BOF. It was worth noting that, for DA-BOF 50% and DA-BOF 100%, there was an endothermic peak appeared at 130oC.


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And the magnitude of this peak was increased with the increasing content of DA-BOF. Based on above results, this endothermic peak should be assigned to the r-DA reaction of DA-BOF during the heating process. Therefore, in order to inhibit the r-DA reaction during the curing process, the pre-curing should be carried out at a lower temperature and after a certain crosslinked system formed, the further curing reaction should be conducted at a higher temperature. When the full extent of curing reaction was achieved, the cured resins would be cooled down gradually, and most of the r-DA products could be converted into the DA adducts again. In this work, the following curing procedure was selected: heated at 60oC for 2 h, 90oC for 2 h and 120oC for 10 h, then slowly cooled down to room temperature over a period of 2 h. As the post-cure was operated at 120oC for a long time about 10 h, the full extent of all the curing reaction should be obtained. And at the same time, the r-DA reaction could be inhibited as more as possible during the slow-cooling process.

Figure 3. DSC heating curves of reactive mixtures at a rate of 2 oC min-1.

Huglin et al.50 calculated the content of maleimide (MI) groups based on UV-Vis spectra. Considering the fact that HMI also has a strong ultraviolet absorption at 301nm, the content of free HMI in different cured systems was calculated and the degree of r-DA reaction during the curing process was determined accordingly. As



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shown in Figure 4, the ultraviolet absorptions at 301nm for all the cured systems in turn were 0.44, 0.83, 1.10, 1.25. And a linear relationship between the ultraviolet absorption intensity and free HMI content was noted. The degree of r-DA reaction (DrDA) could be determined by the following equation:50 DrDA=A*V/(a*1cm*mMI)

(1)

here, A refers to the ultraviolet absorption at 301 nm, V is the volume of dichloromethane using as extraction solvent, a equals 3.937 (a constant shown in Figure 4), mMI is the total content of maleimide in cured epoxy resins. After calculation, the degrees of r-DA reaction for the cured systems were: 25.9% for DA-BOF 25%, 27.4% for DA-BOF 50%, 27.1% for DA-BOF 75% and 26.6% for DA-BOF 100%. These results indicated that the content of free HMI in the different cured systems was almost kept unchanged. In Scheme 2, the DA and r-DA reaction during the curing reaction was illustrated. Before the curing reaction, HMI was attached on furan ring via DA cycloaddition reaction (The blue ball in Scheme 2 represents DA-BOF and the red one represents BOF). When the temperature was increased, the crosslinked networks were formed accordingly. At the same time, due to the thermal reversibility of DA reaction at higher temperature, the r-DA reaction was happened and most of the DA adducts (DA-BOF) was cleaved. Therefore, almost all the HMI existed as free monomer instead of the pendant on furan ring (represented by the green ball in Scheme 2). After the curing reaction was finished and it was cooled down to room temperature gradually (Experiment Section), most of the DA adducts (DA-BOF) was formed again and the residual free HMI monomer (25-27%) was homogeneously dispersed in the cured systems.


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Figure 4. Ultraviolet absorption of different cured systems at 301 nm.

Scheme 2. Schematic illustration of DA/r-DA reaction during the curing process.

3.4. Thermal properties of different cured systems. DSC was employed here to investigate the thermal properties of different cured systems and the heating scan curves were shown in Figure 5. There were two main thermal transitions appeared during the heating scan. The first transition ranging from 30oC to 70oC was undoubtedly assigned to the glass transition. It was noted that the Tg of cured systems was decreased from 60oC for DA-BOF 0% to 40oC for DA-BOF 100% with the



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increasing content of DA-BOF. The reason was due to the presence of soft aliphatic chain attached on BOF as well as the free HMI dispersed in the cured systems, which increased the molecular motion and then the Tg was decreased in some extent. When the temperature was increased further, endothermic peaks were appeared above 110oC for the systems containing DA-BOF (DA-BOF 25%, DA-BOF 50%, DA-BOF 75%, DA-BOF 100%). However, for DA-BOF 0%, the endothermic peak after the glass transition was not detected. Based on above discussion, this endothermic peak was attributed to the r-DA reaction at higher temperature, which was ranged from 100 oC to 130 oC. And the enthalpy of r-DA reaction was measured to be from 0 to 0.18 W oC g-1 with the increasing content of DA-BOF. The increased enthalpy indicated that more r-DA reaction was happened in the DA-BOF richer systems (Table 2). These results were consistent with the conclusion obtained from UV-Vis spectra.

Figure 5. DSC heating curves for the cured epoxy networks.


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Table 2. Thermal properties of the cured systems Samples

Tg (oC)

TrDA (oC)

ΔHrDA (W oC g-1)

DA-BOF 0%

60

—

—

DA-BOF 25%

53

118

0.043

DA-BOF 50%

51

118

0.11

DA-BOF 75%

48

118

0.14

DA-BOF 100%

40

120

0.18

3.5. Mechanical performance. Figure 6a shows the tensile stress–strain curves for the different cured systems and the detailed data was summarized in Table 3. The stress–strain curves for DA-BOF 0%, DA-BOF 25% and DA-BOF 50% showed a nearly constant increase in stress with strain until a rupture occurred without yield. The elongation-at-break values were ranged from 2.7% to 2.9%. As for DA-BOF 75% and DA-BOF 100%, they showed a ductile fracture with yield. And it also noted that, when the content of DA-BOF was less than 50%, the tensile modulus and tensile strength of the crosslinked networks were decreased slightly with the increasing DA-BOF content. However, they were decreased dramatically when the content of DA-BOF exceeded 50%. Figure 6b shows the impact strengthen of the cured systems as a function of DA-BOF content. It was easy to notice that when the content of DA-BOF was less than 50%, the impact strength was increased from 2.7 KJ m-2 for DA-BOF 0% to 5 KJ m-2 for DA-BOF 50%, which meant an increment of 85% was achieved. As for DA-BOF 75% and DA-BOF 100%, their impact strength were conversely decreased. As we know, the incorporation of flexible units could result in a toughened thermosetting system. In this work, the flexible aliphatic chain (HMI) was attached on BOF via DA reaction and it was expected to act as a toughening agent. In addition, there were about 25-27% free HMI monomer dispersed in the cured systems due to the reversibility of DA reaction, which could serve as a plasticizer to soften the crosslinked networks and also lead to a decreased modulus as well as an increased



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impact strength. Therefore, it might be the toughening effect of DA-BOF together with the plasticizing effect of free HMI that influenced the mechanical properties of the cured systems. In the previous publications, the similar results were frequently reported.

51-53

For instance, Wool, RP 51 and Larock, RC 52 also found the plasticizing

effect of free un-reacted monomers or oligomers remained in the cured systems. The reason for the decreasing impact strength of DA-BOF 75% and DA-BOF 100% would be clarified in the following part.

Figure 6. (a) Tensile curves and (b) impact properties for the cured systems.

Table 3. Mechanical properties for different cured resins Tensile Samples

Impact (KJ m-2)

Strength (MPa)

Modulus (MPa)

Elongation at break (%)

DA-BOF 0%

68±3

3790±110

2.7±0.1

2.7±0.1

DA-BOF 25%

63±4

3540±130

2.7±0.3

4.0±0.3

DA-BOF 50%

63±3

3250±160

2.9±0.5

5.0±0.2

DA-BOF 75%

52±1

2600±110

4.8±0.3

2.6±0.2

DA-BOF 100%

38±1

2150±150

6.3±0.9

1.5±0.3


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3.6. Impact fracture surface morphology. The SEM micrographs could reveal more information about the impact fracture behavior of the cured systems, as depicted in Figure 7a–e. DA-BOF 0% displayed a homogeneous glassy fractured surface with cracks in different planes, indicating a single phase system as shown in Figure 7a, which usually means a poor impact strength. However, the fracture surfaces of the systems containing DA-BOF were relatively rough. Tortuous cracks, ridges and river marks could be observed on the fracture surfaces (Figure 7b, c, d and e). Moreover, with the increasing content of DA-BOF, the tortuous cracks, ridges and river marks became denser and more obvious. The rough fracture surface indicated the presence of a certain amount of domains or cavities, and usually more cracks represents more domains or cavities.

Figure 7. Impact fracture surface morphology of different cured systems: (a) DA-BOF 0%, (b) DA-BOF 25%, (c) DA-BOF 50%, (d) DA-BOF 75% and (e) DA-BOF 100%.

As we know, the impact strength depends on the strength of the resin matrix, the interface, especially the domains or cavities, which can induce the propagation of the cracks on the fracture surfaces to absorb energy. This was usually reported in other



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systems,

54-56

such as the epoxy composites toughened by silica nanoparticle

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55

or

graphene.56 And the uniformity in size and distribution of the domains were also responsible for toughness of the matrix.57 As shown in Figure 8a, nearly no domains were observed on the fracture surface of DA-BOF 0%. And some little domains were appeared in Figure 8b. As for DA-BOF 50%, more and more domains with increased size were detected. When the content of DA-BOF reached 75% or 100% for DA-BOF 75% or DA-BOF 100%, a large quantity of micron-sized domains could be seen (Figure 8d and e), which has seriously negative effects on the impact strength. As a result, when the content of DA-BOF was lower than 50%, the impact strength was increased from 2.7 KJ m-2 for DA-BOF 0% to 5 KJ m-2 DA-BOF 50%. However, when the content of DA-BOF was increased further, the impact strength was decreased obviously.

Figure 8. SEM of impact fracture morphology for different cured systems, with greater magnification: (a) DA-BOF 0%, (b) DA-BOF 25%, (c) DA-BOF 50%, (d) DA-BOF 75% and (e) DA-BOF 100%.


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4. CONCLUSIONS A bio-based epoxy monomer BOF has been synthesized from 2,5-furandimethanol and its Diels-Alder reaction adduct, DA-BOF was prepared to serve as the toughening agent. The DSC and UV-Vis spectra study revealed that there were about 25-27% N-hexyl maleimide was remained as free monomer in the cured systems due to the reversibility of DA reaction. When the content of DA-BOF reached 50%, the cured DA-BOF 50% system showed the highest impact strength of 5 KJ m-2 (increased 85% when compared with DA-BOF 0%) together with the slightly decreased tensile strength of 63MPa and modulus of 3250MPa. Both the toughening effect of DA-BOF and the plasticizing effect of free HMI in the systems influenced their mechanical properties.

ASSOCIATED CONTENT Supporting Information 13

C NMR spectra of BOF (a), HMI (b) and DA-BOF (c);

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]

ACKNOWLEDGEMENTS The authors greatly thank the financial support from National Natural Science Foundation of China (NSFC No.51373194), National Sci-Tech Support Plan (2015BAD15B08), the project co-funded by Chinese MIIT Special Research Plan on Civil Aircraft with the Grant No. MJ-2015-H-G-103 and European Union’s Horizon 2020 research and innovation program No.690638.



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A Table of Contents (TOC)


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Synthesis of an Epoxy Monomer from Bio-Based 2,5-Furandimethanol

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