How Does the Hydrogen Bonding Interaction Influence the Properties

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How Does the Hydrogen Bonding Interaction Influence the Properties of Furan-based Epoxy Resins Xiaobin Shen, Xiaoqing Liu, Jinyue Dai, Yuan Liu, Yajie Zhang, and Jin Zhu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02901 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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Industrial & Engineering Chemistry Research

How Does the Hydrogen Bonding Interaction Influence the Properties of Furan-based Epoxy Resins Xiaobin Shen,

†,‡

†

†

Xiaoqing Liu, *, Jinyue Dai,

†,‡

Yuan Liu,

†,‡

†

Ningbo Institute of Materials Technology and Engineering,

Chinese Academy of Sciences, Ningbo, 315201, PR China ‡

†

Yajie Zhang and Jin Zhu

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) together with its petroleum-based analogues, 1,4-Bis[(2-oxiranylmethoxy)methyl]-benzene (p-BOB) and 1,3-Bis[(2-oxiranylmethoxy)methyl]-benzene (m-BOB) have been synthesized. After their chemical structures were identified by Nuclear Magnetic Resonance (1H-NMR and

13

C-NMR),

they were cured by the petroleum-based 4,4’-diaminodiphenylmethane (DDM) and bio-based 5,5’-methylenedifurfurylamine (DFDA), respectively. Thermal and mechanical properties of the cured resins were investigated by Differential Scanning Calorimetry (DSC), Thermogravimetric Analyzer (TGA) and Dynamic Mechanical Analysis (DMA). Especially, hydrogen bonding interactions in the different cured systems were studied by Fourier Transform Infrared Spectroscopy (FT-IR). Based on the structure-properties relationship investigation, it could conclude that it was the hydrogen bonding interaction playing a key role in determining the thermal properties of the furan-based epoxy resins.

Keywords: bio-based epoxy; furan; molecular rotation; hydrogen bonding interaction


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1. INTRODUCTION Nowadays, with the increasing concern on fossil resource consumption and environmental pollution, more and more attention has been paid on the polymeric materials synthesized from renewable resources. As for the epoxy resins, although the epichlorohydrin made from bio-based glycerol has been reported, bisphenol A, which accounts for more than 67% of the molar mass of diglycidyl ether of bisphenol A (DGEBA), is completely dependent on petroleum oil. In order to develop the epoxy resins with increased bio-based content, a large quantity of renewable resources, such as soybean oil, 1 tung oil, 2 vanillin, 3-5 lignin, 6-9 rosin acid,10-13 starch, 14, 15 gallic acid, 16, 17 diphenolic acid 18 and isosorbide, 19 have been tried as the starting materials for epoxy synthesis. However, compared with the traditional DGEBA systems, the epoxy resins derived from soybean oil or tung oil didn’t demonstrate desirable mechanical and thermal properties because of the long soft aliphatic chain.20 The rigid structures of lignin, 6-9 rosin acid

10-13

or gallic acid 16, 17could

endow the epoxy resins with high glass transition temperature (Tg) and mechanical properties. Unfortunately, their high performances were often compromised by the high brittleness. As we know, most of the furan compounds, such as furfurylamine, furoic acid, furfural, 2, 5-furandimethanol (2, 5-FDMO) and 2, 5-furandicarboxylic acid (2, 5-FDCA), could be produced from renewable resource. And due to the aromatic characteristics of furan derivatives, the bio-based furanyl building blocks have been regarded as the promising alternatives to petroleum-based phenyl building blocks for the synthesis of bio-based polymers with satisfied properties. Up to now, a string of furan-based polyesters, 21,22 polyamide 23, 24 and polyether ketone, 25, 26

demonstrating comparable or even better properties in contrast to petroleum-based analogues,

have been synthesized. It is well known that there is a close link between the chemical structures and properties of polymeric materials. For example, Koros WJ et al

21

investigated the chemical

structures of poly(ethylene furanoate) (PEF) and poly(ethylene terephthalate) (PET) in detail, and the better gas barrier properties and higher Tg of PEF were attributed to the hindrance in furan ring-flipping. Daan S. van Es and co-workers 2,5-furandicarboxylic

acid

(2,5-FDCA),

22

thoroughly compared the structures of

2,4-furandicarboxylic

acid

(2,4-FDCA),

3,4-furandicarboxylic acid (3,4-FDCA) and terephthalic acid (TA), isophthalic acid (IPA), phthalic acid (PA). Based on their results, the significant effects of molecular symmetry on the properties



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of polyesters were revealed. As for the furan-based epoxies, they also often demonstrated better mechanical and thermal properties when compared with their phenylic analogues.27-29 However, the deep reason hasn’t been explored systematically. Scheme 1 illustrated the chemical structures of 2,5-furandimethanol (2,5-FDMO), 1, 3-benzenedimethanol (1,3-BDMO) and 1,4-benzenedimethanol (1,4-BDMO) together with the projected angle between two –CH2OH groups attached on them. Apparently, with regard to the molecular symmetry, 2, 5-FDMO is more similar to 1, 3-BDMO rather than 1,4-BDMO, and the symmetry of 1,4-BDMO is highest. According to previous literatures,

21, 22

the symmetry of

monomers will significantly influence the properties of resulted polymeric materials. In addition, there is an oxygen atom in furan ring and the dipole moments of 2, 5-FDMO, 1, 3-BDMO and 1, 4-BDMO are different. As we know, the hydrogen bond is easy to be formed in the presence of oxygen atoms and –OH. Numerous works have demonstrated that the hydrogen bonding interaction could improve the properties of related polymers dramatically.30, 31 Therefore, we could propose that both the molecular rotation and hydrogen bond might be responsible for the higher performance of furan-based epoxy when compared with their phenylic analogues.

Scheme 1. Chemical structures and molecular symmetry of 2, 5-FDMO, 1,3-BDMO and 1,4-BDMO

In this work, three kinds of epoxy monomers with similar chemical structures (Scheme 2), including 1,4-benzenedimethanol derived 1,4-bis[(2-oxiranylmethoxy)methyl]-benzene (p-BOB), 1,3-bis[(2-oxiranylmethoxy)methyl]-benzene (m-BOB) based on 1,3-benzenedimethanol and 2,5-bis[(2-oxiranylmethoxy)-methyl]-furan (BOF) made from the bio-based 2,5-furandimethanol, were

synthesized.

In

addition,

4,

4’-diaminodiphenylmethane


(DDM)

and

5,

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5’-methylenedifurfurylamine (DFDA) were employed as the curing agents. Considering the similar chemical structures of these designed epoxy monomers and curing agents, we hoped that the influence of molecular rotation and hydrogen bonding interaction on the properties of cured epoxy resins could be revealed after chemical structure-properties relationship investigation, which could provide us important information on how to design and synthesize the thermosetting resins with high performance, especially the ones derived from bio-based furan derivatives. To the best of our knowledge, hardly any work concerning this issue was reported yet.

Scheme 2. Synthesis and structures of m-BOB, p-BOB, BOF, DFDA and DDM

2. EXPERIMENTAL SECTION 2.1. Materials and reagents. Reagents and solvents used in this work are as follows: 1,4-benzenedimethanol, formaldehyde, 1,3-benzenedimethanol, 4,4’-diaminodiphenylmethane (DDM), furfurylamine (FA), hydrochloric acid solution (HCl), sodium hydroxide (NaOH), tetrabutylammonium bromide (TBAB), epichlorohydrin (ECH) and chloroform. All of the mentioned chemicals were purchased from Aladdin Reagent, China and used as received. 2, 5-Furandimethanol (Purity: 98%; Melting point: 75-76°C) was supplied by Yajie Zhang’s Group in Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, and used as received.



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2.2. Synthesis of monomers 2.2.1 Synthesis of 5, 5’-Methylenedifurfurylamine (DFDA). DFDA was synthesized according to the procedure described in Holfinger’s work.32 After the round-bottomed flask containing FA (33.3 g, 0.34mol) was kept in the ice-water bath for 10min, 6 M HCl solution (200 mL, 1.21mol) was added dropwise in 30min. Then 37 %w/w formaldehyde solution (13.9 mL, 0.18mol) was added dropwise into the reaction mixture. In the following 5 h, the mixture was neutralized by 6 M NaOH solution slowly (200 mL, 1.21mol) and extracted with 500 mL chloroform for several times. After that the organic layer was washed with 500 mL salt solution several times and dried with anhydrous MgSO4. Then the rotary evaporator was used to remove the organic solvent and a reddish-brown oily liquid weighing 7.82g was obtained (Yield: 22.3%). 1

H-NMR (δ, ppm, 400 MHz, DMSO) 6.06 (t, J = 6.5 Hz, 1H), 6.02 (d, J = 2.9 Hz, 1H), 3.90 (s,

1H), 3.59 (s, 2H); 13C-NMR (δ, ppm, 400 MHz, DMSO) 156.52 (s), 149.76 (s), 106.91 (s), 105.51 (s), 38.80 (s), 26.87 (s); FT-IR (KBr): 790 cm-1 (mono-substituted furan ring), 1015 cm-1 (stretching of C-O-C in furan ring), 1566 cm-1 (stretching of C=C in furan ring), 3360 cm-1 (stretching of N-H).

2.2.2 Synthesis of 2,5-Bis[(2-oxiranylmethoxy)methyl]-furan (BOF).27 Into 63 mL of ECH (0.8mol) in a round-bottomed flask, 0.51g of TBAB (0.0016mol) together with 10.25 g of 2, 5-furandimethanol (0.08mol) were added under continuous stirring. After the mixture was reacted at 65°C for 5 h, 40 mL of NaOH solution (50% w/w) was added dropwise and the reaction was continued for another 5 h. Then the mixture was extracted with chloroform for three times, washed with deionized water and dried with anhydrous MgSO4 several times. After the residual solvent was removed via rotary evaporation, the crude product was purified by the silica gel chromatography with the mixture of ethyl acetate/hexane (2:1 by volume). At last, the final product with the weight of 11.81 g was obtained (Yield: 61.5%). 1

H-NMR (δ, ppm, 400 MHz, CDCl3) 6.29 (s, 1H), 4.51 (q, J = 12.9 Hz, 2H), 3.80-3.72 (m, 1H),

3.43 (dt, J = 10.4, 5.2 Hz, 1H), 3.15 (td, J = 6.3, 3.1 Hz, 1H), 2.79 (t, J = 4.6 Hz, 1H), 2.61 (dd, J = 4.9, 2.7 Hz, 1H); 13C-NMR (δ, ppm, 400 MHz, CDCl3) δ 151.84 (s), 110.34 (s), 70.74 (s), 65.20 (s), 50.74 (s), 44.31 (s); FT-IR (KBr): 927 cm-1 (stretching of C-O in oxirane group), 1254 cm-1 and 855 cm-1 (stretching of C-O-C in oxirane group), 760 cm-1 (mono-substituted furan ring), 1086


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cm-1 (stretching of C-O-C in furan ring), 1555 cm-1 (stretching of C=C in furan ring).

2.2.3 Synthesis of 1, 4-Bis[(2-oxiranylmethoxy)methyl]-benzene (p-BOB). 11.04g of 1, 4-benzenedimethanol (0.08mol), 0.97g of TBAB (0.003mol) and 40% w/w NaOH (100 mL) were mixed in a 500mL round-bottomed flask and stirred at 50 °C for 1 h. After the flask was put into the ice-water bath for 0.5 h, 63 mL of ECH (0.8mol) was added dropwise over a period of 0.5 h. Then the reaction temperature was increased to 25 °C and the reaction was maintained at this temperature for another 21 h. After the mixture was extracted with chloroform for three times, washed with deionized water, dried with anhydrous MgSO4 and concentrated using rotary evaporation, the colorless liquid weighting 17.24 g was obtained (Yield: 86.2%). 1

H-NMR (δ, ppm, 400 MHz, CDCl3) δ 7.34 (s, 1H), 4.58 (q, J=11.9 Hz, 1H), 3.77 (dd, J=11.4, 2.8

Hz, 1H), 3.50-3.36 (m, 1H), 3.24-3.15 (m, 1H), 2.80 (t, J=4.5 Hz, 1H), 2.62 (dd, J=4.8, 2.6 Hz, 1H); 13C-NMR (δ, ppm, 400 MHz, CDCl3) δ 137.86 (s), 128.30 (s), 73.45 (s), 71.19 (s), 51.28 (s), 44.69 (s); FT-IR (KBr): 902 cm-1 (stretching of C-O in the oxirane group), 1254 and 847 cm-1 (stretching of C-O-C in the oxirane group), 1621 cm-1 (stretching of C=C in the benzene ring), 1515 cm-1 (stretching of C-C in the benzene ring).

2.2.4 Synthesis of 1, 3-Bis[(2-oxiranylmethoxy)methyl]-benzene (m-BOB). m-BOB was synthesized following the similar procedure for the synthesis of p-BOB. The final product was a colorless liquid with the yield of 83.4%. 1

H NMR (δ, ppm, 400 MHz, CDCl3) 7.31 (dt, J=12.5, 7.7 Hz, 1H), 4.58 (q, J=11.9 Hz, 1H), 3.78

(dd, J=11.5, 3.0 Hz, 1H), 3.47-3.41 (m, 1H), 3.19 (ddd, J=6.7, 6.0, 2.9 Hz, 1H), 2.82-2.79 (m, 1H), 2.62 (dd, J=5.0, 2.7 Hz, 1H); 13C NMR (δ, ppm, 400 MHz, CDCl3) 138.13 (s), 128.54 (s), 127.12 (s), 127.03 (s), 73.17 (s), 70.92 (s), 50.83 (s), 44.25 (s); FT-IR (KBr): 905 cm-1 (stretching of C-O in the oxirane group), 1254 and 852 cm-1 (stretching of C-O-C in the oxirane group), 1613 cm-1 (stretching of C=C in the benzene ring).

2.3. Curing procedure. After predetermined BOF, p-BOB or m-BOB were mixed with DFDA at 25oC for 10min, respectively. The homogeneous system was applied to the vacuum oven at 25oC for another 15min to get rid of the dissolved gas. Then the gas free mixture was transferred into a



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stainless steel mold for curing reaction. The sample was cured at 1 h of 60oC, 90oC for 2 h, 120oC for 2 h, 140oC for 2 h and 180oC for 2 h to achieve the fully cured resins. Finally, it was slowly cooled down to room temperature and carefully removed from the mold for properties investigation. When DDM was used as the curing agent, it should be dissolved in a little acetone at first and then mixed with BOF, p-BOB or m-BOB. And the same degassing and curing procedures were conducted. The stoichiometric ratio of epoxy monomer to curing agent is fixed at 2:1 for all the systems. For different systems, their nomination and composition were listed in the following Table 1.

Table 1 Nomination for the different systems (molar ratio) Samples

Epoxy monomer (p-BOB, m-BOB or BOF)

Curing agent (DDM or DFDA)

p-BOB/DDM

2 (p-BOB)

1 (DDM)

p-BOB/DFDA

2 (p-BOB)

1 (DFDA)

m-BOB/DDM

2 (m-BOB)

1 (DDM)

m-BOB/DFDA

2 (m-BOB)

1 (DFDA)

BOF/DDM

2 (BOF)

1 (DDM)

BOF/DFDA

2 (BOF)

1 (DFDA)

2.4. Measurements. NMR spectra were recorded on a 400MHz Bruker AVANCE III spectrometer. It was conducted at 25oC using DMSO-d6 or CDCl3 as the solvents. Fourier transform infrared (FT-IR) spectra were conducted in absorption mode using a Thermo Nicolet 6700 Fourier transform infrared spectrometer. Chemicals were diluted with chloroform, dropped on a KBr matrix and cured resins (precursors or cured samples) were powdered and dispersed into a KBr matrix with the concentration of about 1% w/w. All of the samples should be dried in the vacuum oven for 4h before testing. Spectra were recorded from 400 to 4000 cm-1 and each sample was scanned for 32 times. DSC measurement was conducted on a Mettler-Toledo TGA/DSC I under a nitrogen atmosphere. Approximately 5-10 mg of each sample (curing precursors or polymers) was weighed and sealed in 40µL aluminum crucibles. The samples (epoxy monomer mixed with curing agent, or cured resins) were heated from 10oC to 200oC with different the heating rates for different purposes. TGA measurement was conducted on a Mettler-Toledo TGA/DSC1 thermogravimetric


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analyzer (Mettler Toledo, Switzerland) with dry air as the purge gas. The scanning rate was 20oC min-1 and the scanning range was from 50 to 800oC. For each measurement, about 5 mg sample was applied for test. DMA was performed on a Mettler-Toledo DMAQ800 under a tensile mode with the frequency of 1 Hz. The test samples were ramping from -100oC to 230oC at a heating rate of 3oC min-1.

3. RESULTS AND DISCUSSION 3.1. Synthesis and chemical structure characterization of BOF, p-BOB, m-BOB. The chemical structures of BOF, p-BOB and m-BOB 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 in the range of 2.5 ppm to 3.3 ppm represented the protons in oxirane groups. And the peak showing at 6.27 ppm was attributed to the protons on furan ring. The other peaks at 3.4ppm, 3.8ppm and 4.5ppm were also assigned accordingly. As for Figure 1b, the occurrence of peaks between 2.5 ppm and 4.0 ppm indicated that the epoxy groups had been attached onto terephthalyl alcohol. Hb, the protons of methylene connected with the benzene ring, appeared at 4.58 ppm. And all the protons in benzene ring, which were in the same chemical situation, showed the signal at 7.34 ppm. Figure 1c showed the 1H-NMR spectra of m-BOB and the characteristic peaks for epoxy groups were appeared at the same position in contrast to those of p-BOB. Due to the asymmetry of the X axis for m-BOB, the signals standing for the protons in benzene ring were split into multiple peaks, showing at around 7.31 ppm (a1, a2 and a3). Based on these analyses and the assigned 13

C-NMR spectra (Supplemental Information S1), the 1H-NMR and

13

C-NMR spectra of DFDA

(Supplemental Information S2), it could be concluded that the target products were synthesized successfully.



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Figure 1. 1H-NMR spectra of BOF (a), p-BOB (b) and m-BOB (c)

3.2. Curing behaviors of epoxy networks. The synthesized epoxy monomer p-BOB, m-BOB or BOF was respectively mixed with DDM or DFDA, following the stoichiometric ratio listed in Table 1. The curing behaviors of different reactive mixtures were monitored by DSC with the heating rate of 2oC min-1, as shown in Figure 2. It was noted that all of the different systems showed a single exothermal peak during the scanning. When DFDA was used as the curing agent, the peak exothermic temperatures for p-BOB/DFDA, m-BOB/DFDA and BOF/DFDA were 74oC, 80oC and 72oC, respectively. It is well known that the lower peak temperature usually means the greater possibility to take place the ring-opening curing reaction and higher curing reactivity under the same curing conditions.

31

The lower peak exothermic temperature of BOF/DFDA system

might indicate the higher curing activity of BOF when compared with BOB. When they were cured by DDM, the peak exothermic temperatures for p-BOB/DDM, m-BOB/DDM and BOF/DDM were 112oC, 116oC and 106oC, respectively. Obviously, the curing reactivity of DFDA was much higher than that of DDM when they were employed to cure the same epoxy monomer. In an effort to make sure each system has the same curing procedure and their properties


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comparison was reasonable, the curing process should be started at a temperature at which the curing reaction is relatively mild for all the systems. And the temperature for post-curing must be high enough to ensure the full curing reaction in a proper curing time. In addition, the possible degradation must be avoided during the curing process. Based on the DSC heating curves depicted in Figure 2, the curing procedures for all the samples were determined as follows: treated at 60oC for 1 h, 90oC for 2 h, 120oC for 2h, 140oC for 2 h and post-curing at 180oC for 2h. And this multistage heating procedure was usually employed in the literatures.13, 28, 29

Figure 2. DSC heating curves of different reactive mixtures (2oC min-1)

For a quantitative curing reactivity comparison of DDM and DFDA, Kissinger and Ozawa methods, which are considered to be a simple approach to quantify a complex thermoset curing process, were employed.33-35 The Kissinger method33 is based on a linear relationship between logarithm of (q Tp-2) and Tp-1 and can be explained by the following Equation (1): ln

ln 1

where Tp was the peak exothermic temperature (K), q was the heating rate (oC min-1), Ea was an average activation energy of the curing reaction (J mol-1). A was the pre-exponential factor (min-1) and R was the universal gas constant with a value of 8.314 J mol−1 K−1. Tp and its corresponding heating rate (q) could be obtained from Figure 3. Then the activation energy could be calculated from the slope of the plots of –ln (q Tp-2) versus Tp-1. In Figure 4a, typical plots for determination of Ea by Kissinger method for BOF/DDM and BOF/DFDA systems were shown.



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Figure 3. DSC heating curves of BOF/DDM (a) and BOF/DFDA (b) at different heating rate (2oC min-1, 5oC min-1, 10oC min-1, 20oC min-1).

The Ozawa method 34 is another way for the determination of Ea, based on Equation (2): ln ln

where

ln 5.331 1.052 2

ln1 was the integral conversion function.. For a constant

conversion, Ea could be derived from the slope of lnq against Tp-1 plots (Figure 4b). Tp and its corresponding heating rate (q), Ea obtained via Kissinger and Ozawa methods were summarized in Table 2. It was noticed that the activation energy of BOF/DFDA was lower than that of BOF/DDM, indicating the higher curing reactivity of DFDA in contrast to DDM, which was in good agreement with the results obtained from Figure 2.

Figure 4. Plots of –ln (q Tp-2) versus 1000Tp-1 (a) and lnq versus 1000Tp-1 (b)


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Table 2. Ea for BOF/DDM and BOF/DFDA determined by Kissinger and Ozawa method Heating rates(q) Samples

Tp (K)

(oC min-1)

Ea (kJ mol-1) Kissinger

2

379

5

400

10

417

20

436

2

345

5

366

10

379

20

395

BOF/DDM

BOF/DFDA

Ozawa

ln A * Kissinger

Ozawa

48.9

52.9

13.00

14.54

46.4

49.9

13.76

15.22

* The dimension of A is min-1, and was 0.5.

It is widely considered that the curing reaction between amine and epoxy monomer is essentially a step growth polymerization and the curing process can be monitored by FT-IR spectrum. Usually, the characteristic absorption bond attributed to oxirane ring is shown at about 910 cm-1 and the splitting peaks at 3200-3400 cm-1 represent primary diamine stretching vibration.36 If the curing reaction was completed, these characteristic signals would disappear. Figure 5 is the FT-IR spectra of all the samples before and after curing reaction. As for the p-BOB/DDM system (Figure 5a), the characteristic peak shown at 902 cm-1 was assigned to the stretching of C-O in the oxirane group. And the peaks in the range of 3200-3400 cm-1 demonstrated the primary amine stretching vibration of DDM. After the curing reaction was conducted following the above procedures, the band shown at 902 cm-1 was disappeared and the splitting peaks ranged from 3200 cm-1 to 3400 cm-1 were replaced by a much wider single peak centered at 3300 cm-1, which indicated that the epoxide group had been reacted with amine group to form the networks. In Figure 5b-5f, the similar changes were observed for p-BOB/DDM, m-BOB/DDM, m-BOB/DFDA, BOF/DDM and BOF/DFDA. These results clearly showed that the effective and full curing reactions between the epoxy monomers and the diamine cross-linkers had occurred.



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Figure 5. FT-IR spectra of all the different systems before and after curing reaction: (a) p-BOB/DDM, (b) p-BOB/DFDA, (c) m-BOB/DDM, (d) m-BOB/DFDA, (e) BOF/DDM, (f) BOF/DFDA.

3.3. Dynamic mechanical properties of different cured systems. DMA was employed to investigate the mechanical properties of the cured epoxy networks (Figure 6) and the related data was summarized in Table 3. Figure 6a showed the storage modulus of cured p-BOB/DDM, p-BOB/DFDA, m-BOB/DDM, m-BOB/DFDA, BOF/DDM and BOF/DFDA as a function of temperature. It was noted that the modulus of BOF/DFDA (2.8 GPa at 20oC) was higher than that of p-BOB/DDM (2.0 GPa at 20oC) at the same temperature and as for the other cured systems,


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their modulus was increased slightly with the increasing content of furan units. In Figure 6b, Tg was determined by the peak temperature in the Tan δ versus temperature graph. Obviously, when DDM was used as the curing agent, p-BOB/DDM, m-BOB/DDM and BOF/DDM demonstrated much higher Tg compared with those of p-BOB/DFDA, m-BOB/DFDA and BOF/DFDA. And when cured by the same curing agent (DDM or DFDA), the cured BOF systems showed higher Tg than that of cured p-BOB and m-BOB.

Figure 6. DMA curves of the cured epoxy networks: (a) storage modulus as a function of temperature and (b) tan δ as a function of temperature for the different cured resins.

As we know, the thermal properties of cured epoxy resins will be greatly affected by the crosslink density of the network, which should be discussed through the data from DMA. The crosslink density is defined as the number of moles of network chains per volume unit of the cured material.37,

38

And the theory of rubber elasticity is usually applied for the determination of



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thermosets’ crosslink density. According to this theory, the storage modulus in the rubbery plateau (E’R) will change with the crosslink density and it can be calculated by the following Equation (3):

′ 3 3

whereυc was the crosslink density (mol dm-3), E’R was the plateau modulus in the rubbery state (Pa), which was read at Tg+30 K, R was the gas constant (J mol-1 g-1), and T was the absolute temperature at Tg+30 K. The calculatedυc values for all the cured systems were listed in Table 3. All of the cured resins showed the similar crosslink density, which indicated that the thermal performance difference between the different systems was not caused by the crosslink density.

Table 3. Dynamic mechanical properties of the cured epoxy networks Storage modulus

Storage modulus

Crosslink density

Tg (oC) from

at 20 oC (MPa)

at Tg+30K (MPa)

(mol dm-3)

DMA

p-BOB/DDM

2000

15

1.5

86

p-BOB/DFDA

2600

12

1.4

47

m-BOB/DDM

2100

13

1.3

89

m-BOB/DFDA

2400

11

1.3

45

BOF/DDM

2400

17

1.5

149

BOF/DFDA

2800

13

1.4

59

Samples

3.4. Thermal properties of different cured systems. DSC heating curves for all the cured systems were shown in Figure 7 and the related data was summarized in Table 4. Obviously, except for a single thermal transition, no exothermic peaks were observed for all the cured samples during DSC heating scan, which indicated that all of the epoxy networks had been fully cured again and no further curing reaction was happened at higher temperature. This was in accordance with the results based on FT-IR. The thermal transition was undoubtedly assigned to the glass transition for all the cured systems. As for the p-BOB/DFDA and m-BOB/DFDA systems, they showed the same Tg at 41oC. And BOF/DFDA had a Tg at 47oC, 6oC higher than that of p-BOB/DFDA and m-BOB/DFDA. Compare the chemical structures of p-BOB/DFDA and m-BOB/DFDA with that of BOF/DFDA, the higher Tg might be attributed to the higher content of


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furan units in the BOF/DFDA system. When DDM was used as the curing agent, the p-BOB/DDM, m-BOB/DDM and BOF/DDM system showed Tg of 78oC, 76oC and 126oC, respectively, which were much higher than those of p-BOB/DFDA, m-BOB/DFDA and BOF/DFDA. It was easy to notice that it was –CH2NH2 connected with furan ring in DFDA, rather than the –NH2 group in DDM. The methylene group between furan ring and amine will provide more molecular flexibility and then decrease the Tg. This result was also reported in the previous literatures and the relatively lower Tg of the systems cured by DFDA was reasonable.28 In addition, the Tg values of different cured systems determined by DSC were lower than the ones obtained from above DMA results (Table 3). The similar result has been widely reported in the previous literatures and the reason should be corresponding to the different mechanism of DMA and DSC measurements. Nevertheless, the change tendency of Tg for different systems determined by means of DSC was in accordance to those obtained from DMA analysis. As we know, the introduction of furan ring is an effective way to improve the properties of polymeric materials, including thermosets and thermoplastics.21, 22, 29 However, the reason has not been investigated systematically. According to Palmese GR’s work and above analysis in the introduction part,

27

both the hard molecular rotation of furan ring and the hydrogen bonds originated from the extra oxygen atom in furan ring might be responsible for the higher thermal properties. Considering the projected angle between the two hydroxymethyl groups in m-BOB (120o) and p-BOB (180o), their molecular symmetries were different. This might influence the molecular rotation dramatically. However, in this work, the cured m-BOB systems showed similar thermal properties to that of cured p-BOB systems (Tg of p-BOB/DFDA and m-BOB/DFDA is 41oC; while p-BOB/DDM and m-BOB/DDM showed similar Tg at 78oC and 76oC, respectively), no matter which curing agent was used. These results indicated that the molecular rotation might not be the key factor in determining Tg of the cured systems. As for the furan ring, it showed the similar molecular symmetry to that of m-BOB (angles between –CH2OH are 129o and 120o).22 Therefore, the same conclusion to the BOB systems might be drawn.



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Figure 7. DSC heating curves of the cured epoxy networks. Table 4 Thermal properties of the cured epoxy networks Samples

Tg (oC) from DSC

Td5% (oC)

Td10% (oC)

Char yield at 800 oC

p-BOB/DDM

78

333

352

5%

p-BOB/DFDA

41

265

302

2%

m-BOB/DDM

76

331

349

4%

m-BOB/DFDA

41

273

309

2%

BOF/DDM

126

296

320

4%

BOF/DFDA

47

261

280

2%

Normally, the process and application of epoxy resins were conducted under air atmosphere. In this work, the thermal stability of different cured epoxy networks was investigated by TGA under dry air (Figure 8). And the important information, including the temperature at which 5% weight loss was occurred (Td5%), the temperature of 10% weight loss (Td10%) and char yield at 800oC, was listed in Table 4. From the degradation curves in Figure 8, it was noted that all the cured resins demonstrated similar thermal degradation behaviors. Compared with the DFDA cured systems, the DDM cured ones showed better thermal stabilities, which was indicated by the higher Td5% and Td10% of p-BOB/DDM, m-BOB/DDM and BOF/DDM. In addition, when the same curing agent was applied, the Td5% and Td10% of cured BOF was lower than those of m-BOB and p-BOB. As for the char yield, all the cured systems showed a very low char yield at 800oC under air atmosphere.


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Figure 8. TGA curves of the cured epoxy networks under air. 3.5. FT-IR of different cured systems. Based on above analysis, both the molecular rotation and crosslink density were not the key factors influencing the Tg of different cured systems. In order to investigate the hydrogen bonding interaction in the cured resins, FT-IR measurement was conducted and the related spectra were shown in Figure 9. As we know, the intramolecular -OH…N- hydrogen bond, -OH…O- hydrogen bond and intermolecular -OH…O- hydrogen bond usually show the characteristic absorption peaks at ~3180cm-1, ~3460cm-1 and ~3380cm-1, respectively. And the peaks above 3600cm-1 usually referred to the free –OH

39-42

. In Figure 9a,

after normalization, the intensity of the wide band ranged from 3000cm-1 to 3600cm-1 was much higher in the cured BOF/DDM system, which meant that more hydrogen bonds were formed in the cured BOF/DDM compared with that of cured p-BOB/DDM and m-BOB/DDM. And Figure 9b also indicated more hydrogen bonds in the cured BOF/DFDA system. In the cured epoxy resins, there is a large quantity of free –OH after the epoxide ring was opened, the introduction of furan unit will provide more proton acceptors due to extra oxygen atom in the furan ring. Therefore, the hydrogen bond is easy to be formed in the furan richer systems. Based on Palmese GR’s work,27 the characteristic absorption band of C−O−C in furan ring would shift to a lower frequency (red shift) after the hydrogen bond was formed, and this red shift was well correlated with the hydrogen bond density. In order to further investigate the hydrogen bonding interaction, the peak splitting of FT-IR spectrum was conducted (Figure 10). Via peak splitting, the proportion of H-bonded furan in the different networks could be roughly calculated. According to the literatures,27, 43 the H-bonded C−O−C in furan ring showed the characteristic peak at 1038cm-1, while the absorption band for free C−O−C in furan ring appeared at 1090cm-1.



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In Figure 10b, after peak splitting and area integral ranging from 980cm-1 to 1180cm-1, the area percentage for peak 1038cm-1 was about 11%, which indicated that the percentage of H-bonded furan ring was about 11% in the p-BOB/DFDA system. Based on Figure 10c and 10d, the percentage of furan ring involved in hydrogen bonding interaction was about 12% in the m-BOB/DFDA and BOF/DFDA systems. As the amount of furan ring in different epoxy resins was in accordance with the following order: p-BOB/DFDA≈m-BOB/DFDA < BOF/DFDA. Therefore, it was easy to conclude that more hydrogen bonds were formed between the C−O−C units in furan ring and the new generated –OH after curing reaction, which should be responsible for its relative higher Tg when compared with that of p-BOB/DFDA or m-BOB/DFDA. And in addition, the similar thermal properties of p-BOB/DFDA and m-BOB/DFDA were reasonable, as the hydrogen bonding interaction in them was same (Figure 10). As for the DDM cured systems, BOF/DDM, p-BOB/DDM and m-BOB/DDM, more hydrogen bonds were also found in BOF/DDM and the same conclusion could be drawn.

Figure 9. FT-IR spectra of all the cured epoxy networks: (a) the comparison of p-BOB/DDM, m-BOB/DDM and BOF/DDM, (b) the comparison of p-BOB/DFDA, m-BOB/DFDA and BOF/DFDA.


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Figure 10. (a) FT-IR spectra of BOF, p-BOB/DFDA, m-BOB/DFDA and BOF/DFDA; FT-IR spectra in the region of C-O-C stretching in furan ring, including samples: (b) p-BOB/DFDA, (c) m-BOB/DFDA and (d) BOF/DFDA.

4. CONCLUSIONS In order to investigate the effect of molecular rotation and hydrogen bonding interaction on the properties of furan-based epoxy, the epoxy monomer BOF derived from bio-based 2, 5-furandimethanol, and its petroleum-based analogues, p-BOB and m-BOB, have been designed and synthesized successfully. Results indicated that the cured systems containing more furan ring units showed relatively higher modulus and Tg. Based on DSC and DMA measurements, the harder molecular rotations of furan ring versus benzene ring was proved to be not the main reason for higher Tg. And FT-IR spectra analysis confirmed that it was the more hydrogen bonds formed between C−O−C units in furan ring and –OH that led to the better performance of furan-based epoxy.



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ASSOCIATED CONTENT Supporting Information: Figure S1. 13C NMR spectra of BOF (a), p-BOB (b) and m-BOB (c);

Figure S2. 1H NMR and 13C NMR spectra of DFDA. AUTHOR INFORMATION Corresponding Authors: E-mail: [email protected]

ACKNOWLEDGEMENTS The authors greatly thank the support from National Natural Science Foundation of China (NSFC No.51373194), National Sci-Tech Support Plan (2015BAD15B08), the project funded by Chinese MIIT Special Research Plan on Civil Aircraft with the Grant No. MJ-2015-H-G-103.

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