Amide-Functionalized Ionic Liquids As Curing Agents for Epoxy Resin

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Amide functionalized ionic liquids as curing agents for epoxy resin: Preparation, characterization, and curing behaviors with TDE-85 Long Liu, Sheng Gao, Zhiyi Jiang, Yanqiang Zhang, Dayong Gui, and Suojiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01888 • Publication Date (Web): 08 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Industrial & Engineering Chemistry Research

Amide functionalized ionic liquids as curing agents for epoxy resin: Preparation, characterization, and curing behaviors with TDE-85 Long Liu,†§* Sheng Gao,†‡ Zhiyi Jiang,†Ⅱ Yanqiang zhang, † Dayong Gui, ‡* and Suojiang Zhang †* †

Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

‡

College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518055, China

§

Zhongke Langfang Institute of Process Engineering, Langfang 065001, China

Ⅱ

School of Chemical Engineering and Environment, China University of Petroleum, Beijing 102249, China

E-mail: [email protected], [email protected], [email protected]

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: A series of amide functionalized ionic liquids (ILs), e.g., 1-R-3-(2-amino-2oxoethyl)-imidazolium (R= –vinyl, –methyl –butyl) cations combined with Cl¯, N(CN)2¯, or NTf2¯anions, were synthesized and fully characterized as curing agent for diglycidyl-4,5-epoxycyclohexane-1,2-dicarboxylate (TDE-85). Studies of curing behaviors showed that ILs with N(CN)2¯anions exhibit moderate curing conditions. As to thermosets of TDE-85 cured by these curing agents, their tensile strength (95.1 to 111.7 MPa) and elongation at break (2.69 to 4.20%) are significantly improved compared with the traditional curing systems of DDS/TDE-85 (DDS, 4,4’-diamino diphenyl sulphone). These thermosets also exhibit good thermal mechanical properties with the glass transition temperature (Tg) > 200 °C, thermal degradation temperature (Td) > 300 °C. The possible mechanisms of curing reactions were analyzed by using IR, and results indicated that curing reactions of TDE-85 with amide functionalized ionic liquids may include three steps, –NH2 groups reacted with oxirane at lower temperatures, and then the anions further participated in the reaction to strengthen the cross-linking networks. This study provides informative guidelines for designing new curing agents.

1. INTRODUCTION Epoxy resins have been widely used for coatings, adhesives, electronic materials and composites due to their high adhesion strength, outstanding mechanical properties, good heat resistance, and high electrical resistance1-4. All these applications are based on the cross-linking reactions between epoxy resins and curing agents, and the curing agents play a key role in the crosslinking process. Generally, curing agents are divided into amine-type curing agents, alkali curing agents, anhydrides, and catalytic curing agents according to their chemical compositions 5-9. One kind of curing agent determined one kind of specific application of epoxy resin on composite


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materials. With the development of high temperature composite based on epoxy resin, some new curing agents that have lower curing temperatures and fast curing rates should be designed and synthesized. Ionic liquids (ILs) are composed of organic cations and organic or inorganic anions, which are liquids at temperatures below 100 °C. ILs show unique physical-chemical properties, such as thermal, chemical, and electrochemical stability, low volatility, nonflammability, excellent catalytic activity.10 These properties enabled ILs to be widely used as polymerizable monomers, plasticizers and amine-based curing agents. The uses of ILs as curing agents for epoxy resin are excellent ways to extend storage time, enhance mechanical stability as well as decrease the amount of curing agent usually required in case of amine-based hardeners.11 Table 1. Chemical Structures of ILs Curing Agents for Epoxy Resin ILs

Cations

Anions

Lit. Refs

[BMIM][BF4]

Ref. 12

[EMIM][N(CN)2]

Ref. 13

[BMIM][PF6]

Ref. 15

[DODIM][I]

Ref. 14

[APBIM][NTf2]

Ref. 11

[N4444][Leu]

Ref. 11

[THTP][EH]

Ref. 16


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The first curing agent based on ILs, i.e., 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4], for epoxy resin was described by Kowalczyk and Spychaj in 2003.12 After that, a series of work on ILs curing agents for epoxy resin have been reported. The reported ILs curing agents include 1-ethyl-3-methylimidazolium dicyanamide [EMIM][N(CN)2],13 N,N’dioctadecylimidazolium iodide [DODIM]I,14 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6],15 1-(3-Aminopropyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide [APBIM][NTf2],11 tetrabutylammonium leucine [N4444][Leu],11 trihexy(tetradecyl)-phosphonium 2-ethylhexyanoate [THTP][EH],16 and so on (Table 1). However, the types of ILs as curing agents for epoxy resins reported so far are limited to traditional structures. To enhance the mechanical properties of epoxy resin, additives that participate in cross-linking reactions should have enough functional groups. Importing functional groups in ILs has been considered as an effective method for tutoring the cross-linking reactions of epoxy resin. For example, alkenyl functionalized ILs of 1-allyl-3-methylimidazolium hexafluorophosphate significantly decreased the curing temperature of benzoxazine/epoxy thermosets and the strength toughness and thermal properties of benzoxazine/epoxy thermosets were also improved.17 The unique structural adjustable properties of ILs enable it is easy to import the specific functional groups based the structure of the epoxy resin. So, it is very necessary to design new functionalized ionic liquids to further meet the needs of epoxy resin composite materials modification. In this paper, a series of amide functionalized ILs of 1-R-3-(2-amino-2-oxoethyl)-imidazolium (R= –vinyl, –methyl –butyl) were synthesized with anions of Cl¯, N(CN)2¯and NTf2¯. These ILs were fully characterized by 1H and 13C NMR, IR spectroscopy, elemental analysis (EA), differential scanning calorimeter (DSC), and 1-Vinyl-3-(2-Amino-2-Oxoethyl)-Imidazolium


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Dicyanamide was further analyzed by X-ray diffraction. Additionally, curing systems of ILs/epoxy (TDE-85) were investigated by DSC, dynamic mechanical thermal analysis (DMTA), thermal gravimetric analysis (TGA) and tensile test. The curing behavior, mechanical properties and thermal stabilities of TDE-85/ILs thermosets were explored specifically. 2. EXPERIMENTAL SECTION 2.1 Materials Except AgN(CN)2 (made in our laboratory), all of the analytical reagents were purchased from commercial sources without further processing. 1-Vinyl imidazole (99%, Aladdin), 1-methyl imidazole (99%, Aladdin), 1-butyl imidazole (98%, Aladdin), 2-chloro acetamide (99%, Macklicn), NaN(CN)2 (96%, Aladdin), AgNO3 (99%, Aladdin), LiNTf2 (Shanghai D&B Diological and Technology Co. Ltd), 4,4’ - Diaminodiphenyl sulfone (98%, Sinopharm Chemical Reagent Co. Ltd), Acetonitrile (99.5%, Beijing Chemical Works), ethanol (99.7%, Beijing Chemical Works), and methanol (99.5%, Beijing Chemical Works) were used. TDE-85 epoxy resin with an epoxy value of 0.87 was purchased from Tianjin Jingdong Chemical Compound Material Co. Ltd (Tianjin, China). 2.2 Characterizations 1

H and 13C spectra were recorded on a Bruker 600 MHz spectrometer with d6-DMSO as the

locking solvent relative to Me4Si. IR spectra were recorded on a Thermo Nicolet 380 spectrometer using KBr pellets at room temperature. Elemental analyses were carried out on a Vario EL elemental analyzer. The decomposition temperatures of ILs were obtained on a TA Q5000 thermogravimetric analyzer at a heating rate of 10 °C min-1. Densities were measured at


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room temperature using a Micromeritics Accupyc 1340 instrument. Single crystal X-ray diffraction data were measured using a Bruker APEX-II CCD diffractometer. The melting point of the ILs and the curing process of epoxy/IL systems were investigated by using Mettler-Toledo DSC1 differential scanning calorimeter. The glass transition temperatures (Tg), loss factor (tan δ) and storage modulus were determined through TA Q800 dynamic mechanical thermal analysis, at a heating rate of 3 °C/min from 30 to 350 °C, frequency of 1Hz, and maximum displacement amplitude of 20 μm. Thermal gravimetric analysis (TGA) of the cured TDE-85 thermosets was conducted on SHIMADZU DTG-60H at a heating rate of 10 °C/min from 30 to 800 °C. The mechanical properties were obtained according to the China National Standard of GB/T 1040-2006 (or ISO 527-1:1993 Plastics—Determination of tensile properties) by using ShangHai XieQiang CTM8050 electronic universal material testing machine at a tensile rate of 2 mm/min. The gauge length of the sample is 25 mm and data of the tensile strength tests were calculated from a mean value of five specimens. 2.3 General procedure for the synthesis of the ionic liquids 2.3.1 Synthesize of compounds 1-3 Compounds 1-3 were synthesized according to the reference methods (Scheme 1).18 1-Vinylimidazole (17 mmol, 1.600 g), 1-Methyl-imidazole (23.8 mmol, 1.957 g), 1-Butyl-imidazole (17 mmol, 2.111 g) were mixed with 2-chloro acetamide (20 mmol; 1.870 g) in acetonitrile (40 mL) respectively at room temperature and then the mixture was refluxed at 70 °C for 64 h. The white precipitated materials were obtained and washed with acetonitrile for three times (3 15 mL). The pure products were obtained through recrystallization in ethanol.


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1-Vinyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Chloride (1). White solid (yield: 54.5%). 1H NMR (600 MHz, DMSO) δ = 5.10 (s, 2H; CH2), 5.44 (dd, 1H; CH2), 6.06 (dd, 1H; CH2), 7.46 (dd, 1H; CH), 7.61 (s, 1H; NH2), 7.90 – 7.94 (m, 1H; CH), 8.26 (s, 1H; NH2), 8.33 (t, 1H; CH), 9.75 ppm (s, 1H; CH). 13C NMR (151 MHz, DMSO) δ = 50.79, 108.78, 118.33, 124.72, 128.82, 136.70, 166.45 ppm; IR (KBr): ν = 597, 632, 835, 933, 1184, 1320, 1408, 1546, 1570, 1674, 3052, 3085, 3117, 3371 cm-1. EA (%) for C7H10N3OCl: C 44.81, H 5.37, N 22.40; Found: C 44.96, H 5.683, N 22.55. 1-Methyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Chloride (2). Colorless crystals (yield: 77.1%). 1H NMR (600 MHz, DMSO) δ = 3.92 (s, 3H; CH3), 5.02 (s, 2H; CH2), 7.53 (s, 1H; CH2), 7.73 (d, 2H; NH2), 8.16 (s, 1H; CH), 9.23 ppm (s, 1H; CH). 13C NMR (151 MHz, DMSO) δ = 35.73, 50.44, 122.89, 123.72, 137.73, 166.75 ppm. IR (KBr): ν = 600, 625, 651, 1168, 1306, 1398, 1574, 1627, 1690, 3112, 3283, 3395, 3454 cm-1. EA (%) for C6H10N3OCl: C 41.04, H5.74, N 23.93; Found: C 40.87, H 6.146, N 24.16. 1-Butyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Chloride (3). White solid (yield: 45.6%). 1H NMR (600 MHz, DMSO) δ = 0.87 (t, 3H; CH3), 1.22 (dq, 2H; CH2), 1.64 – 1.82 (m, 2H; CH2), 4.18 (t, 2H; CH2), 4.95 (s, 2H; CH2), 7.48 (s, 1H; CH), 7.73 (dt, 2H; NH2), 8.00 (s, 1H; CH), 9.20 ppm (s, 1H; CH). 13C NMR (151 MHz, DMSO) δ = 13.25, 18.71, 31.33, 48.51, 50.48, 121.64, 123.95, 137.28, 166.69 ppm. IR (KBr): ν = 602, 641, 761, 1174, 1203, 1282, 1303, 1400, 1568, 1697, 2870, 2952, 2989, 3118, 3253 cm-1. EA (%) for C9H16N3OCl: C 49.66, H 7.41, N 19.30; Found: C 49.83, H 7.381, N 19.34.


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2.3.2 Synthesis of compounds 4-6 Compounds 4-6 were obtained through anion exchange reactions between 1-3 and silver dicyanamide (AgN(CN)2) in methanol. Methanol (50 mL) solution of 1-Vinyl-3-(2-Amino-2Oxoethyl)-Imidazolium Chloride (40 mmol; 7.575 g), 1-Methyl-3-(2-Amino-2-Oxoethyl)Imidazolium Chloride (40 mmol; 7.034 g), and 1-Butyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Chloride (40 mmol; 8.724 g) were added dropwise into the suspension of MeOH (80 mL) and AgN(CN)2 (52 mmol; 9.043 g) respectively. After stirring at 40 °C for 6 h, the precipitate (AgCl) was filtered and the filtrate was evaporated under reduced pressure to give the crude products, then the pure products were obtained by recrystallizing in ethanol. 1-Vinyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Dicyanamide (4). Colorless crystals (yield: 78.0%). 1H NMR (600 MHz, DMSO) δ = 4.95 (s, 2H; CH2), 5.41 (dd, 1H; CH2), 5.94 (dd, 1H; CH2), 7.33 (dd, 1H; CH), 7.54 (s, 1H; NH2), 7.80 (t, 1H; CH), 7.83 (s, 1H; NH2), 8.14 (t, 1H; CH), 9.37 ppm (s, 1H; CH). 13C NMR (151 MHz, DMSO) δ = 51.31, 109.43, 118.82, 119.64, 125.36, 129.37, 137.21, 166.96 ppm. IR (KBr): ν = 522, 599, 920, 1187, 1284, 1319, 1412, 1555, 1580, 1655, 1678, 1706, 2140, 2196, 2244, 3095, 3147, 3342 cm-1. EA (%) for C9H10N6O: C 49.54, H 4.62, N 38.51; found: C 49.55, H 4.500, N 38.73. 1-Methyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Dicyanamide (5). White solid (yield: 93%). 1

H NMR (600 MHz, DMSO) δ = 3.85 (s, 3H; CH3), 4.90 (s, 2H; CH2), 7.48 (s, 1H; CH), 7.63 (p,

2H; NH2), 7.78 (s, 1H; CH), 9.00 ppm (s, 1H; CH). 13C NMR (151 MHz, DMSO) δ = 36.31, 50.94, 119.62, 123.46, 124.35, 138.26, 167.40 ppm. IR (KBr): ν = 625, 771, 1304, 1332, 1450, 1705, 2139, 2198, 2250, 3071, 3169, 3341 cm-1. EA (%) for C8H10N6O: C 46.60, H 4.89, N 40.76; found: C 46.69, H 4.933, N40.75.


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1-Butyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Dicyanamide (6). White solid (yield: 76.6%). 1

H NMR (600 MHz, DMSO) δ = 0.91 (t, 3H; CH3), 1.27 (dq, 2H; CH2), 1.78 (dt, 2H; CH2), 4.21

(t, 2H; CH2), 4.94 (s, 2H; CH2), 7.50 (s, 1H; CH), 7.72 (dt, 2H; NH2), 7.83 (s, 1H; CH), 9.11 ppm (s, 1H; CH). 13C NMR (151 MHz, DMSO) δ = 13.23, 18.71, 31.32, 48.56, 50.45, 119.05, 121.62, 123.96, 137.23, 166.67 ppm. IR (KBr): ν = 516, 642, 1170, 1306, 1337, 1402, 1567, 1679, 2145, 2201, 2256, 2964, 3098, 3144, 3202, 3263, 3369. EA (%) for C11H16N6O: C 53.21, H 6.50, N 33.85; found: C53.41, H 6.836, N 34.08. 2.3.3 Synthesis of compounds 7-9 7-9 were readily obtained from metathesis reactions of 1-3 and Lithium bis(trifluoromethanesulphonyl)imide (LiNTf2) according to the reference methods11,19 with minor modification. LiNTf2 (20 mmol; 5.742 g) were added into the deionized water (30 mL) solution of 1-Vinyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Chloride (22 mmol; 4.128 g), 1Methyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Chloride (22 mmol; 3.863 g) and 1-Butyl-3-(2Amino-2-Oxoethyl)-Imidazolium Chloride (22 mmol; 4.789 g), respectively. Two phases were formed after reaction for 36 h. Then oil phase was separated and washed with deionized water (6 10 mL), and colourless liquid was obtained after vacuum drying. 1-Vinyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Bis(trifluoromethylsulfonyl)imide (7). Light yellow liquid (yield: 20.2%). 1H NMR (600 MHz, DMSO) δ = 4.99 (s, 2H; CH2), 5.45 (dd, 1H; CH2), 5.98 (dd, 1H; CH2), 7.37 (dd, 1H; CH), 7.58 (s, 1H; NH2), 7.83 (s, 1H; CH), 7.87 (s, 1H; NH2), 8.18 (t, 1H; CH), 9.42 ppm (s, 1H; CH). 13C NMR (151 MHz, DMSO) δ = 50.74, 108.81, 116.25, 118.23, 118.39, 120.52, 122.65, 124.78, 128.78, 136.65, 166.40 ppm. IR (KBr): ν = 512,


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571, 741, 790, 1055, 1139, 1188, 1350, 1556, 1657, 1699, 3154, 3360, 3453 cm-1. EA (%) for C9H10N4O5S2F6: C 25.00, H 2.33, N 12.96; found: C 25.04, H 2.38, N 13.33. 1-Methyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Bis(trifluoromethylsulfonyl)imide (8). Colorless liquid (yield: 24.4%). 1H NMR (600 MHz, DMSO) δ = 3.89 (s, 3H; CH3), 4.94 (s, 2H; CH2), 7.52 (s, 1H; CH), 7.58 – 7.68 (m, 2H; NH2), 7.82 (s, 1H; CH), 9.04 ppm (s, 1H; CH). 13C NMR (151 MHz, DMSO) δ = 35.71, 50.38, 116.25, 118.39, 120.52, 122.65, 122.86, 123.78, 137.70, 166.70 ppm. IR (KBr): ν = 510, 571, 617, 654, 742, 791, 1056, 1195, 1350, 1405, 1579, 1614, 1699, 3166, 3361, 3454 cm-1. EA (%) for C8H10N4O5S2F6: C 22.86, H 2.40, N 13.33; found: C 22.52, H 2.552, N 13.29. 1-Butyl-3-(2-Amino-2-Oxoethyl)-Imidazolium Bis(trifluoromethylsulfonyl)imide (9). Colorless liquid (yield: 76.6%). 1H NMR (600 MHz, DMSO) δ = 0.91 (t, 3H; CH3), 1.19 – 1.35 (m, 2H; CH2), 1.69 – 1.84 (m, 2H; CH2), 4.21 (t, 2H; CH2), 4.94 (s, 2H; CH2), 7.51 (s, 1H; CH). 7.72 (dt, 2H; NH2), 7.84 (s, 1H; CH), 9.12 ppm (s, 1H; CH). 13C NMR (151 MHz, DMSO) δ = 13.15, 18.69, 31.31, 48.56, 50.45, 116.26, 118.39, 120.52, 121.60, 122.66, 123.96, 137.24, 166.66 ppm. IR (KBr): ν = 513, 571, 616, 741, 790, 1056, 1193, 1350, 1405, 1568, 1614, 1701, 2967, 3154, 3359, 3451 cm-1. EA (%) for C11H16N4O5S2F6: C 28.57, H 3.49, N 12.12; found: C 28.48, H 3.257, N 12.31. 2.4 Preparation of thermosets Thermosets were obtained with the following methods: TDE-85 was placed in a beaker and stirred at 90 °C, then the optimum amount of 4-6 were added respectively, and the mixtures were stirred till all the curing agent was dissolved. Then the mixture was injected into a Teflon mold that was pre-heated to 100 °C and coated with a release agent. Then the bubbles were removed


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and the mixture were cured with the prescribed procedures (Tgel/2h+Tcur/2h+Ttreat/2h). The thermoset of DDS/TDE-85 system for comparison was also obtained with the mass ratio of 52.6:100 and curing process of (Tgel/2h+Tcur/2h+Ttreat/2h). 3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization 3.1.1 Synthesis of ionic liquids The Amide functionalized ionic liquids were synthesized according to the typical methods of quaternization and ion exchange reactions as shown in Scheme 1.20-22 Compounds 1-6 are white solid at room temperatures, while 7 is light yellow liquid, and compounds 8 and 9 are colorless liquid.

AgN(CN)2

R

MeOH, 6h R

N

N 2-chloroacetamide MeCN, 64h

R

N

N 1-3

NH2 N

N

NH2 O

O

N(CN)2

4-6

Cl LiNTf2 H2O, 36h

R

N

N

NH2 O

NTf2

7-9

Scheme 1. Synthesis of Amide Functionalized Ionic Liquids (R = –Vinyl, –Methyl –Butyl) Structures of all compounds were confirmed by 1H and 13C NMR, IR spectroscopy, and elemental analysis methods. Compounds 2 and 4 were further characterized by single crystal Xray diffraction. In the 1H NMR spectra, the chemical shift of –CONH2 proton signals of cations were in the range from 7.58 ppm to 8.26 ppm. In the 13C NMR spectra, the weak signal around 119 ppm for 4-6 can be assigned to the carbon atom of the N(CN)2¯anion. The very weak


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signals ranging from 116 ppm to 123 ppm for 7-9 were attributed to the carbon atoms of the NTf2¯anion. In the IR spectra, the absorption bands of –CN groups are in the range of 2196– 2256 cm-1. The absorption bands of –CF3 groups are in the range of 1139–1350 cm-1. Absorption bands in the range of 3100–3500 cm-1 can be attributed to the vibration of N–H bonds of the – CONH2 group. 3.1.2 Single crystal analysis Single crystals of compounds 2 and 4, obtained through recrystallization in ethanol, were analyzed by X-ray diffraction. The crystallographic data and refinement details are summarized in Table S1, Supporting Information (SI). Both compounds 2 and 4 crystallized in the triclinic space group P21/c with the calculated densities of 1.38 g·cm-1 and 1.39 g·cm-1, respectively. Extensive hydrogen bonds can be found from packing diagram, and the data of some selected hydrogen bonds are listed in Table S2. The main hydrogen bonds interactions between the cation and anion of 4 are shown in Figure 1. For 4, five cations interact with two anions through seven kinds of hydrogen bonds, and several ring motifs are formed among these interactions. For example, hydrogen bonds of C(2)–H(2)…O(2) and C(12)–H(12)…O(2) form a ring of [R1, 2(6)]; Hydrogen bonds of C(3)–H(3)…N(1), N(5)–H(5B)…O(2) and N(5)–H(5A)…N(2) form a ring of [R3, 3(16)].


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Figure 1. View of the hydrogen bonds of two anions and five cations of 4. All ellipsoids are set at 30% probability. 3.1.3 Properties of ionic liquids The physicochemical properties of all ionic liquids were evaluated, including melting point (Tm), decomposition temperature (Td) and density (ρ). The properties of all the synthesized ionic liquids are listed in Table 2. Thermal behaviors of all ionic liquids were determined by TGA and DSC at a heating rate of 10 °C/min. As can be seen in Table 2, thermal behaviors mainly depend on the anion, with the melting points decreasing in the order of Cl¯> N(CN)2¯> NTf2¯. Compounds 1-3 have a higher melting points, ranging from 135.0 °C (3) to 185.0°C (1), which may lead to operational problems and not suitable for being used as epoxy resin’s curing agents.9 For compounds 4-9, moderate melting points (ranging from -41.2 °C (9) to 105.5 °C (4)) make them to be operable curing agents, especially for compounds 7-9 with NTf2¯anion, the melting points of which are in the range from -41.2 °C (9) to -29.7 °C (7). The NTf2¯based ILs are liquids and very easy to mix with epoxy resin at room temperature. As for thermal stabilities, decomposition temperatures increase with the trend of N(CN)2¯< Cl¯< NTf2¯, and these compounds are very stable below 200 °C with the decomposition temperatures ranging from 236.0 °C (4) to 409.9 °C (9).


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Table 2. Physicochemical Properties of the Synthesized ILs ILs

Tma (°C)

Tdb (°C)

ρ c (g·cm-3)

1

185.0

312.8

1.43

2

184.3

304.8

1.39

3

135.0

302.1

1.22

4

105.5

236.0

1.33

5

102.7

264.7

1.31

6

77.8

269.5

1.24

7

-29. 7

407.0

1.61

8

-34.2

401.1

1.63

9

-41.2

409.9

a

b

1.49 c

Melting point. Decomposition temperature. Density at 25 °C.

Densities of all compounds were measured with a gas pycnometer at 25 °C. Densities of solid state compounds 1-6 are in the range of 1.22 g·cm-3 (1) and 1.43 g·cm-3 (3). While the values for liquid state compounds 7-9 are higher, ranging from 1.49 g·cm-3 (9) to 1.63 g·cm-3 (8). 3.2 Curing behaviors As a type of alicyclic glycidyl ester epoxy resin, diglycidyl-4,5-epoxy-cyclohexane-1,2dicarboxylate (TDE-85, Scheme S1) possesses many outstanding properties, such as a higher epoxy value (0.85), low viscosity (~1500 cp at room temperature), good heat resistance, and high strength, which make it widely applied in composite materials, coatings and adhesives.23-25 Herein, the curing properties of amide functionalized ionic liquids of 4-9 were investigated by using TDE-85 as the resin matrix.


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3.2.1 Mass ratios of curing systems Table 3. Optimized Curing Parameters of ILs/TDE-85 System Composition

Optimum

Tgela

Tcurb

Ttreatc

acronym

mass ratio

(°C)

(°C)

(°C)

DDS/TDE-85

52.6:100

148

180

211

4/TDE-85

25:100

134

158

168

5/TDE-85

12:100

143

155

162

6/TDE-85

12:100

144

158

167

7/TDE-85

10:100

195

239

300

8/TDE-85

18:100

185

246

311

9/TDE-85 12:100 193 258 313 a Gel temperature. b Curing temperature. c Post-treatment temperature

The optimum mass ratios of ILs/TDE-85 were determined by using DSC analysis.26 Mixtures with different mass ratios for each curing agent were tested at the heating rate of 10 °C/min (Figure S3). Different mass ratios would have different exothermic peak areas. The optimum mass ratio for every curing system would have the maximum exothermic peak area (Figure S4). The optimized results are listed in Table 3, which indicates that there are no obvious trends for each kind of the curing agents, However, compound 6 (C11H16N6O, 248) has the same molecular weights with DDS (C12H12N2SO2, 248), the optimum mass ratio of 6/TDE-85 (12:100) is greatly decreased compare with DDS/TDE-85 (52.6:100) system,24 that means the multifunctional groups are involved in the curing reactions. 27 3.2.2 Non-isothermal analysis Non-isothermal DSC analysis were recorded at the heating rates of β = 5, 10, 15, and 20 °C/min (Figure S5). The onset temperature (To), peak temperature (Tp) and end temperature (Te) of the exothermic peak for each system’s curve are listed in Table S3. The fitting lines based on scatter plots of To, Tp and Te versus β are shown in Figure 2. The corresponding curing temperatures


15


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could be confirmed as the intercepts temperatures when the heating rate β was 0 °C/min.28,29 According to such method, the gel temperature (Tgel), curing temperature (Tcur) and posttreatment temperature (Ttreat) for each ILs/TDE-85 systems were calculated and listed in Table 3. The curing temperatures for ILs/TDE-85 systems mainly depended on the structure of the anions. Compounds 4-6 with the N(CN)2¯anion exhibit lower curing temperatures with the Tgel range from 134 to 144 °C, Tcur from 155 to 158 °C and Ttreat from 162 to 168 °C, which may be attributed to the catalytic properties of the N(CN)2¯anion.30 However, compounds 7-9 with NTf2¯anion have higher curing reaction temperatures, mostly more than 200 °C, which may be due to their higher thermal stability.10,13,26


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Figure 2. The fitting lines based on scatter plots of To, Tp and Te versus β for ILs/TDE-85 systems. (a) 4/TDE-85, (b) 5/TDE-85, (c) 6/TDE-85, (d) 7/TDE-85, (e) 8/TDE-85, (f) 9/TDE-85. Curing reaction kinetics is important in studying the relationship between the structure and properties of the thermosetting resin. The kinetic parameters, such as apparent activation energy (Ea) and pre-exponential (A), can be investigated with the aid of Kissinger and Ozawa methods.31-35 Kissinger’s equation is expressed as follows: ln

 Tp2

 ln

AR Ek  Ek RTp

(1)

where A is the pre-exponential; R is the ideal gas constant (8.314 J mol-1 K-1); Tp is the peak temperature; β is the heating rate; Ek can be obtained from the slope of ln(β/Tp2) versus 1/Tp plot (Figure 3), and A can be calculated from the intercept of the plot. Ozawa’s equation is given as follows: Eo 

R  ln   1.052 1/ Tp

(2)

where Eo can be obtained from the slope of ln β versus 1/Tp plots (Figure 4).


17


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For contrast, the curing reaction order (n) can be obtained with Crane method36 as shown in equation (3). d (ln  ) E     k  2TP  d (Tp1 )  nR 

(3)

where Ek is activation energy calculated by Kissinger method, and n can be derived from the slope of ln β versus 1/Tp plots when Ek/nR is much higher than 2Tp.

Figure 3. Plots for determination of the Ek and the A by Kissinger method in different curing systems: (a) 4/TDE-85, 5/TDE-85 and 6/TDE-85; (b) 7/TDE-85, 8/TDE-85 and 9/TDE-85.

Figure 4. Plots for determination of the Eo by Ozawa method in different curing systems: (a) 4/TDE-85, 5/TDE-85 and 6/TDE-85; (b) 7/TDE-85, 8/TDE-85 and 9/TDE-85.


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According to the above methods, curing kinetic parameters were calculated and the results are summarized in Table 4. Activation energy from Ozawa’s method are generally 3–4 kJ/mol higher than that from Kissinger method. For curing agent with the same cation, systems with N(CN)2¯anion have lower activation energy than that with NTf2¯anion. System of 7/TDE-85 has the highest activation energy (105.8 kJ/mol for Ek, 108.6kJ/mol for Eo), which means compound 7 exhibits the lowest curing reactivity. However, the activation energy value for DDS/TDE-85 system is lower than all ILs/TDE-85 system. Moreover, the values of curing reaction order are all less than 1, which means the curing reactions of these systems are very complicated due to their multiple functional groups.35 Table 4. Kinetic Parameters of the Curing Reaction for ILs/TDE-85 Systems Composition

a

Kissinger method

Ozawa method

acronym

Eka (kJ mol-1)

Ab (s-1)

nc

Eod (kJ mol-1)

4/TDE-85

84.2

3.03×109

0.919

87.1

5/TDE-85

81.8

1.72×109

0.917

84.8

6/TDE-85

83.9

2.68×109

0.918

86.9

7/TDE-85

105.6

9.64×10

9

0.924

108.6

8/TDE-85

87.8

8.97×10

7

0.906

92.1

9/TDE-85

87.0

4.40×10

7

0.903

91.6

DDS/TDE-85

66.1

5.23×10

6

0.892

70.4

b

c

Apparent activation energy from Kissinger method. Pre-exponential. Curing reaction order. d Apparent activation energy from Ozawa method.

3.3 Properties of thermosets In order to investigate the properties of the thermosets, compounds 4-6 were chose as the curing agents because of their moderate curing temperatures. Thermosets were prepared with the optimized curing conditions (Table 3), and the mechanical properties, thermomechanical properties and thermal behaviors were tested, as shown in Table 5.


19


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Mechanical properties of the proposed curing systems are greatly improved, with the tensile strength ranging from 95.1 MPa to 111.7 MPa, the elongation at break ranging from 2.69% to 4.20%, while these values for DDS/TDE-85 are 64.7 MPa and 1.79%, respectively. As for tensile modulus, the value of 4/TDE-85 system (4581.3 MPa) is slightly higher than DDS/TDE-85 system (4550.7 MPa). However, values of 5/TDE-85 (4130.3 MPa) and 6/TDE-85 (3517.9 MPa) systems are lower than that of DDS/TDE-85 system, which may be due to the effects on the side chain of the cations. Table 5. Thermomechanical, Thermal and Mechanical Properties of TDE-85/ILs Thermosets Tg a

Eb

ve c

Td d

(°C)

(MPa)

(mol/m3)

(°C)

0.91

220.4

87.7

1625.0

4/TDE-85

0.47

231.8

149.9

5/TDE-85

0.35

212.9

6/TDE-85

0.27

214.9

Composition acronym

tan δ

DDS/TDE-85

a f

b

Et f

ɛt g

(MPa)

(MPa)

(%)

367.7

64.7

4550.7

1.79

438.5

339.0

109.9

4581.3

2.69

477.6

4289.0

352.0

111.7

4130.3

3.94

390.7

5704.1

344.8

95.1

3517.9

4.20

c

d

σt

e

e

Glass transition temperature. Storage modulus. Cross-linking density. Decomposition temperature. Tensile strength. Tensile modulus. g Elongation at break.

Dynamic mechanical thermal analyses (DMTA) were carried out to analyze the dynamic elastic modulus and the glass transition of the cured ILs/TDE-85 systems. The cross-linking density was calculated using equation 4 as follows: 37,38

Er  3  ve  R  Tr

(4)

where Er is the storage modulus at ‘‘rubbery’’ state, i.e., at Tr = Tg + 30, ve is the cross-linking density (mol m-3), R represents the universal gas constant (8.314 Pa m3 mol-1 K-1). Glass transition temperature (Tg) and storage modulus are determined by temperature at the maximum tan δ.


20

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DMTA curves of ILs/TDE-85 thermosets are shown in Figure 5, and the calculated Tg and Er are listed in Table 5. Results of the compared sample of DDS/TDE-85 system are also listed in Table 5 and Figure 5. There are two steps of storage modulus and two tan δ peaks are formed for DDS/TDE-85 system with the temperature increasing, while there are only one step, one peak for the ILs/TDE-85 systems. Tg values for 4/TDE-85, 5/TDE-85 and 6/TDE-85 are 231.8 °C, 212.9 °C, and 214.9 °C respectively, which have no obvious trends compare with the DDS/TDE-85 system. The crosslinking densities of these thermosets are ranging from 438.5 mol/m3 to 5704.1 mol/m3. As for storage modulus, values of the thermosets are ranging from 87.7 MPa (DDS/TDE-85) to 390.7 MPa (6/TDE-85), which indicates that the thermosets of ILs/TDE-85 system are stiffer than the thermoset of DDS/TDE-85 system. The tan δ value at the curve maximum gives information on the ratio of energy E' stored and dissipated as heat E″ (tan δ = E″/E'). Lower tan δ values of 4/TDE-85, 5/TDE-85, and 6/TDE-85 systems are 0.47, 0.35, and 0.27 respectively, exhibiting high cross-linking efficiency and low loss of modulus values for them.10 Moreover, a broader tan δ peak of 5/TDE-85 system indicates a less uniform cross-linking structure.

Figure 5. Storage modulus (a) and tan δ (b) as a function of temperature of the cured TDE-85.


21


Page 22 of 30

The heat resistance of the thermosets was evaluated by TGA under nitrogen atmosphere at the heating rate of 10 °C/min. The TGA curves of TDE-85 cured by different curing agents are shown in Figure S6. They are thermally stable up to 300 °C. Thermal decomposition temperatures based on ILs are ranging from 339.0 °C (4/TDE-85) to 352.0 °C (5/TDE-85). Once the temperature was higher than 300 °C, the thermoset of ILs/TDE-85 systems began to decompose rapidly and most of the mass was lost within 300–600 °C. It is slightly lower than the value of the DDS/TDE-85 system (367.7 °C), which could be attributed to the thermal unstability of the N(CN)2¯anion. 3.4 Curing process for 4/TDE-85 system The thermoset of 4/TDE-85 system exhibits unique dynamic mechanical thermal properties, e.g, the highest glass transition temperature (231.8 °C), the lowest cross-linking density (438.5 mol/m3). To investigate the mechanism of its curing process, FT-IR analysis of 4/TDE-85 system at different curing steps was performed and results are shown in Figure 6. The band assignments are summarized in Table S5. As can be seen from Figure 6(a), IR spectrum at 3505 and 3342 cm1

can be assigned to the vibrations of –NH2 group in compound 4, the peak at 1678 cm-1 can be

assigned to the –CONH2 groups, while the vibration of N(CN)2¯anion bonds are assigned to the peaks in the range of 2139–2240 cm-1. As for TDE-85, the vibrations of oxirane ring can be assigned to the peak around 851–906 cm-1, the peak at 1736 cm-1 can be assigned to the –COOR group, while the broad peak in the range of 3300–3700 cm-1 was due to the presence of –OH group in TDE-85. When 4 and TDE-85 were mixed together at 90 °C, the peaks of –CONH2 and –NH2 disappeared and the intensity of the –OH peaks increased, which mean the –NH2 groups reacted with oxirane ring and more –OH groups were formed. The Figure 6(b) shows that with the temperature increasing to 134 °C, the peak intensity of N(CN)2¯and oxirane groups


22

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decreased simultaneously, which means anion of N(CN)2¯participates in the curing reaction at this temperature. When the mixture was further heated at 158 °C for 2 h, the signals of both groups disappeared completely and the spectrum of the mixture remains unchanged with the temperature further increasing, indicating that the curing processes were completed at 158 °C.

Figure 6. (a) IR spectra of 4, TDE-85, and 4/TDE-85 mixed at 90 °C. (b) IR spectra of 4/TDE-85 system under different curing schedules. The proposed curing reaction mechanism of 4/TDE-85 system is shown in Scheme 2. Curing reaction of 4/TDE-85 mainly includes three steps. In the first step, -NH2 group in cation reacts with oxirane at lower temperature (90 °C) to form a complex a, such reaction will be accelerated with the temperature increase. Isomerization reaction of complex a occurred according to step 2 at 134 °C, and compound b and c were formed. 37 In step 3, -CN group in compound c react with –OH group further to generate imine structure (d) and then the rearrangement reaction of imine structure to yield amide structure (e).39 Moreover, as a typical imidazole curing agent, compound b can also react with oxirane to form a crosslinked network structures, this reaction will incorporate with the above reactions to further enhance the mechanical properties of the thermosets.


23


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OH

Step 1: O N

NH2

N O

2 H2 C

N(CN)2

H2 C

N(CN)2 C H

R N

(TDE-85)

H C

H2C

O

OH H2C

N(CN)2

C H

H2C

O

H2 C

C N

N N

OH

N

R N

H C R OH

+

N

R

C H

N

C

O

N

a

R

OH

a

Step 2:

R

N N

4

N

C H

b

c

C CH R H2 OH

Step 3: OH

N H2C

C N

R

N

C N

C H

NH

O

H2C

CH R OH

c

2 HO

R1

R1 O

H2 C C H

C N

R1 O

C

R

N O

NH

OH

O

OH

H2C CH R OH

d

R1

N H

H2 C

C

N

N

R1 NH C

O

O

C R H

H C C R H2 OH

e

Scheme 2. Proposed curing reaction mechanism of 4/TDE-85 4. CONCLUSIONS A series of amide functionalized ionic liquids of 1-R-3-(2-amino-2-axoethyl)-imidazolium (R= –vinyl, –methyl –butyl) with Cl¯, N(CN)2¯, NTf2¯anions were synthesized and fully characterized. Ionic liquids with N(CN)2¯, NTf2¯anions exhibit good curing properties for TDE85. The amide functionalized dicyanamide ionic liquids could greatly decrease the curing temperatures and the amount of curing agents compared with the traditional curing agents DDS. The tensile strength (95.1 to 111.7 MPa) and elongation at break (2.69 to 4.20%) of the thermosets for ILs (N(CN)2¯)/TDE-85 systems greatly increased. These thermosets also exhibit good thermal mechanical properties with Tg higher than 200 °C, Td higher than 300 °C. Curing reactions for amide functionalized ionic liquids were analyzed by IR, which indicated that three steps were included: –NH2 groups reacted with oxirane firstly at low temperature, and then the


24

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anions further participated in the reaction to strengthen the cross-linking networks. These results may give some guideline for new curing agents design.

s Supporting Information ● Chemical Structure of diglycidyl-4,5-epoxy-cyclohexane-1,2-dicarboxylate (TDE-85) (Scheme S1); View of the hydrogen bonds of one anion and four cations of 2 (Figure S1); (a) TG and (b) DTG curves of ionic liquids (Figure S2); DSC curves of different ratios of TDE-85/ILs (Figure S3); Heat flow peak area of different curing systems (Figure S4); DSC curves of ILs/TDE-85 systems at heating rates of 5, 10, 15 and 20 °C/min under nitrogen atmosphere (Figure S5); The fitting lines based on scatter plots of To, Tp and Te versus β for DDS/TDE-85 (Figure S6); Plots for determination of the Ek and the A by Kissinger method (a) and Ozawa methods (b) (Figure S7); Stress-strain curves of DDS/TDE-85 and ILs/TDE-85 thermosets. (a) DDS/TDE-85; (b) 4/TDE-85; (c) 5/TDE-85; (d) 6/TDE-85; (e)7/TDE-85; (f) 8/TDE-85; (g) 9/TDE-85 (Figure S8); (a) TG and (b) DTG curves of ILs/TDE-85 thermosets (Figure S9); Crystallographic datas and structural determination of 2 and 4 (Table S1); Hydrogen bonds parameters of 4 (Table S2); Characterization parameters for the different non-isothermal ILs/TDE-85 systems at the heating rates of 5, 10, 15 and 20 °C/min (Table S3); Curing reaction data of 7-9/TDE-85 systems (Table S4); Thermomechanical, Thermal and Mechanical Properties of 7-8, DDS/TDE-85 Thermosets (Table S5); Thermomechanical, Thermal Properties of 4/TDE-85 Thermosets (Table S6); FT-IR band assignments of Figure 6 (Table S7). ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21676281, 21576270 and 21878278), Beijing Natural Science Foundation


25


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Amide-Functionalized Ionic Liquids As Curing Agents for Epoxy Resin

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