Sustainable Graphene Suspensions: A Reactive Diluent for Epoxy

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Sustainable graphene suspensions: A reactive diluent for epoxy composite valorization Jiheng Ding, Obaid ur Rahman, Qiaolei Wang, Wanjun Peng, and Haibin Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01282 • Publication Date (Web): 06 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017

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ACS Sustainable Chemistry & Engineering

Sustainable graphene suspensions: A reactive diluent for epoxy composite valorization Jiheng Ding*, Obaid ur Rahman, Qiaolei Wang, Wanjun Peng, Haibin Yu* Key Laboratory of Marine Materials and Related Technologies, Key Laboratory of Marine Materials and Protective Technologies of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China Corresponding Author *E-mail: [email protected] (J.-H. DING), Ningbo Institute of Industrial Technology, CAS, 1219 Zhongguan Xi Road, Zhenhai District Ningbo, Zhejiang, 315201,P. R. China; [email protected] (H.-B. YU), Ningbo Institute of Industrial Technology, CAS, 1219 Zhongguan Xi Road, Zhenhai District Ningbo, Zhejiang, 315201,P. R. China; Keywords: graphene; furan diepoxide monomer; suspension; reactive diluent; epoxy composites

Abstract. Graphene, a novel nanofiller with excellent mechanical properties and high thermal and electrical conductivity, has the potential to enhance the value-added properties of polymer composites. In this study, we synthesized furan diepoxide (FdE) monomer and used it as a graphene dispersant. Graphene suspensions with concentrations of 0.5, 1.0, and 2.0 mg/mL were used as epoxy reactive diluents to improve the properties of graphene/epoxy nanocomposites. The UV and Raman spectra show that π–π interactions between the graphene nanosheets and FdE stabilized the graphene in the FdE suspension. Dispersion of 2.0 mg/mL of nanosheets into

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an epoxy resin enhanced the tensile strength by 26.8%, tensile modulus by 40.4%, flexural strength by 21.6%, flexural modulus by 16.8%, and glass transition temperature by 7.2%.

Introduction In recent decades, carbon-based materials have been widely used as nanofillers to enhance the performance of polymers for different value-added applications.1–5 Graphene consisting of an atomic monolayer is among the stiffest and strongest nanomaterials and is a building block of graphite.6–9 Along with its excellent mechanical properties, the high thermal conductivity and excellent chemical stability of graphene make it an ideal precursor for many engineering and high-tech applications.10–13 For some applications, graphene has to be dispersed in a polymeric matrix to obtain a uniform and well-dispersed polymer nanocomposite; the quality of the composite depends largely on the affinity of the nanofiller to the polymer matrix.14 Graphene has a high surface area and strong van der Waals forces between nanosheets, which tend to form irreversible aggregates in most known polymers during fabrication, degrading the mechanical properties of polymer composites.15–18 Depending on the application, better mechanical, thermal, and electrical properties may be required; modifications of graphene to improve the dispersibility, compatibility, and interfacial interactions between nanosheets and the polymer matrix are particularly important. Among various techniques, chemical modification of graphene is the most common strategy for this purpose, as reflected in the literature. For example, Chang et al.19 reported enhanced graphene dispersibility by polyimide grafting via covalent functionalization. Fang et al.20 prepared reinforced polystyrene composites by grafting polystyrene chains onto the surface of a single-layer graphene nanosheet using covalent bonding by diazonium addition and adopting atomic transfer radical polymerization. Chen et al.21 studied the effect of well-dispersed graphene-loaded epoxy coatings on the corrosion resistance and friction resistance of materials. Gu et al.22 found that effective dispersion of graphene in a waterborne epoxy resin by noncovalent functionalization significantly increased the corrosion resistance of materials. Cao et al.23 demonstrated notable enhancement of the tensile modulus and flexural modulus at 2.0 wt% modified graphene content. According to these studies, modification of polymer fragments or reactive sites on graphene could result in better dispersion and enhanced properties.24,25 However, covalent functionalization of graphene commonly


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destroyed desirable properties and required harsh reaction conditions. In contrast, noncovalent functionalization of graphene was deemed to be promising, as it can preserve almost all the original characteristics of graphene. Biobased materials have received increasing attention recently owing to diminishing oil reserves, serious environmental pollution issues, and the increasing cost of petroleum products. Biomass conversion into useful polymers or composites has considerable economic and environmental benefits. Here, a type of commercial graphene suspension was prepared using a biobased furan diepoxide (FdE) monomer solution. It was found that strong absorption occurred between the graphene and FdE owing to π–π interactions (Figure 1). On the basis of these advantages, the suspension was used as a novel stabilizer, or reactive diluent, to improve the performance of graphene/epoxy composites. Various studies have suggested that FdEfunctionalized graphene played multiple roles in composites, such as (1) reducing the viscosity of the epoxy matrix in the absence of organic solvents, (2) promoting the dispersion of graphene in the epoxy matrix, and (3) acting as a novel nanofiller or cross-linking agent in epoxy composites to enhance their properties. Experimental Section Materials Commercial graphene (3–10 layers; thickness, 1–5 nm, diameter, 1–10 µm; purity, ~99.5%) was obtained from Xiamen Knano Graphene Technology Corp., Ltd. (Xiamen, Fujian, China). Further, 2,5-furan dimethanol (FDM) was supplied by Hangzhou Dingyan Chemical Co., Ltd. and recrystallized from an ethyl acetate/hexane mixture before use (98%). Epichlorohydrin (EPC), sodium hydroxide (NaOH), tetrabutylammonium bromide (TBAB), anhydrous magnesium sodium (MgSO4), acetone, hydrochloric acid (HCl), tetrahydrofuran (THF), ethyl acetate (EA), petroleum ether (PE), and 2-ethyl-4-methylimidazole (2,4-EMI) were all obtained from Aladdin Chemical Reagents and used without further purification. Industrial-grade bisphenol A epoxy resin (EP) (E-44, epoxy value of 0.464 eq/100 g) was provided by Zhejiang Anbang Coating Co. Ltd. A curing agent (10,10′-oxybisphenoxarsine, OBPA) was prepared according to the method described by Ding et al.26 All other reagents were purchased from Aladdin and used as received. Synthesis of furan diepoxide (FdE)


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FdE was synthesized according to a reported method.26 Briefly, EPC (50.51 g, 546 mmol), 50% w/w aqueous NaOH (19.2 g, 468 mmol), and tetrabutylammonium hydrogen sulfate (1.25 g, 3.9 mmol) were added to a three-necked round-bottom flask (250 mL) at room temperature. The system was stirred vigorously for 30 min. Next, FDM (5.0 g, 39 mmol) in THF (30 mL) was added dropwise to the system over a period of 30 min under nitrogen atmosphere. Subsequently, the reaction was continued for 2.5 h at 50 °C, and the progress of the reaction was monitored by thin-layer chromatography. The reaction mixture was washed with ice-water and extracted using EA (50 mL × 2). The organic phase was dried using MgSO4, filtered, and evaporated to dryness. The FdE was purified and collected by flash chromatography (EA:PE = 1:2) on silica. The yield of the clear liquid was up to 75%. 1

H NMR (400 MHz, d6-DCl3): (d, ppm) 6.28 (m, 1H), 4.60 (m, 2H), 3.75 (m, 1H), 3.55 (m,

1H), 2.90 (m, 1H), 2.62 (m, 1H), 2.35 (m, 1H); 13C NMR (400 MHz, d6-DCl3): 151.60, 109.51, 70.72, 65.30, 50.67, 44.35. Preparation of graphene suspensions (FdE-G)

Figure 1. Different graphene suspensions and the possible adsorption mechanism of FdE on the graphene surface. The typical procedure for obtaining an FdE-stabilized graphene suspension was as follows: Graphene sheets were dispersed into 10 mL of FdE (without any solvents) in a 250 W ice-water bath sonicator for 2 h to obtain a stable graphene suspension. The suspension was centrifuged at 1000 rpm for 15 min to remove larger sheets; no obvious precipitation (Figure 1d) was observed after 24 h of storage. In contrast, direct suspension of graphene in water or in ethanol solution


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without FdE resulted in agglomeration and precipitation (Figure 1a,b). The concentrations of the resulting suspensions were 0.5±0.01, 1.0±0.01, and 2.0±0.01 mg/mL. Preparation of FdE-G/epoxy composites The FdE-G/epoxy (FdE-G/EP) composites were fabricated according to the procedure shown in Figure 2. The viscosity of the EP was found to be effectively reduced with 10 wt% loading of FdE-G, which critically favored subsequent experiments using the epoxy resins. Thus, we used 10 wt% of FdE as the diluent to formulate our FdE-G/EP composites. To prepare the specific FdE-G/EP composites, 1.0 g of the graphene suspensions synthesized as described above was added to 10.0 g of EP and stirred vigorously for 15 min at 1000 rpm to yield FdE-G/EP hybrid suspensions. The curing reaction of EP was performed using the curing agent OBPA (the epoxy and curing agent were used in a 1:0.75 molar equivalent ratio) in the presence of 2, 4-EMI as the catalyst (1 wt% of epoxy). The materials were mixed and degassed under vacuum at 50 °C for 10 min. The reaction mixture was placed into a mold, and the reaction was conducted at 65 °C for 3 h, 145 °C for 3 h, and 195 °C for 5 h; the final cured samples were held at room temperature for characterization. According to the graphene concentration, the FdE-G/EP composites are denoted as pure PE, FdE/EP, FdE-G0.5/EP, FdE-G1.0/EP, and FdE-G2.0/EP. For example, “pure PE” represents the EP system without added G and FdE. “FdE/EP” denotes the EP system with 10 wt% FdE. “FdE-G0.5/EP” represents the EP system containing 10 wt% FdE-G, where the concentration of G was 0.5 mg/mL.

Figure 2. Schematic showing the preparation of FdE-G/EP composites.

Characterization


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The 1H NMR and

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C NMR spectra were obtained using a Bruker AVANCE III 400M

NMR spectrometer. The UV–visible (UV–vis) spectra were recorded by a computer-controlled spectrophotometer (PerkinElmer Lambda 950). Raman spectroscopy was performed using a LabRam I confocal Raman spectrometer (France). The morphology of the graphene sheets was investigated using transmission electron microscopy (TEM) (Tecnai G2 F20) and atomic force microscopy (AFM) (Dimension 3100 V scanning probe microscope). The morphology, microstructure, and fracture surfaces of the composites were observed by field-emission scanning electron microscopy (SEM) (FEI Quantum 250FEG). The glass transition temperature (Tg) of the composites was analyzed on a Netzsch DSC 214 differential scanning calorimetry (DSC) instrument under a protective nitrogen atmosphere at 5 °C/min. The weight losses of the composites with respect to the temperature were recorded by thermogravimetric analysis (TGA) using a Diamond TG/DTA machine. The viscosity was measured according to a standard method (GB/T 22314-2008). Tensile and flexural tests were conducted using ASTM D638 with a crosshead speed of 5 mm/min and ASTM D790 with a rate of 1.4 mm/min at 25 °C, respectively, to determine the effect of the FdE-G-reinforced EP on the mechanical properties. The thermal conductivity was measured following ASTM DE1461-92 with a Netzsch LFA 457 laser flash thermal diffusivity apparatus. The surface water contact angles of the samples were tested with a contact angle meter (OCA20, DataPhysics, Germany), and the presented values are averages of three readings at different locations.

Results and Discussion Synthesis of furan diepoxide (FdE) The two-step reaction between an organic acid and excess EPC is generally almost impossible to use for preparation of products with a satisfactory yield under complex reaction conditions and post-treatments. Therefore, the direct di-glycidylation route was employed to synthesize FdE with a yield of 75% and a purity of 96.0%. The epoxy equivalent weight of the final as-tested product was as high as 135 g/eq. The chemical structure of the FdE was confirmed by 1H NMR and

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C NMR. In the 1H

NMR spectrum (Figure 3a), the signal at 6.28 ppm corresponds to the proton in the furan ring of FdE (designated as proton 1), and the signals at δ = 4.60 ppm were assigned to the protons of the –OCH2 connecting the glycidyl ether moiety to the furan ring (labeled 2). The signals at


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3.55–3.75 ppm indicated protons of –CH2 adjacent to the oxirane ring (labeled 3).27 The signals at 2.35–2.62 ppm and 2.90 ppm were attributed to proton 5 of –CH and proton 4 of –CH2 in the oxirane ring.28 Interestingly, the integrated area of these signals matches well the theoretical ratio of numbers for the corresponding protons. Figure 3b shows the 13C NMR spectrum of FdE. The signals at 151.60 and 109.51 ppm are clearly attributable to C2 and C1 on the furan ring.29 The peaks at 70.72 and 65.30 ppm represent C3 and C4, which correspond to the connecting carbon atom of the furan ring and the oxirane ring, respectively. Further, the peaks at 50.67 and 44.35 ppm are associated with the C5 and C6 carbon atoms in the oxirane ring, respectively. These results proved that the target compound of FdE was synthesized successfully and that direct diglycidylation was an efficient technique to obtain FdE.

Figure 3. (a) 1H NMR and (b) 13C NMR spectra of FdE. Dispersion and characterization of FdE-functionalized graphene Figure 1 shows the dispersed state and possible mechanism of the graphene suspensions. After 24 h of storage without a stabilizer, obvious precipitation and floating of sheets occurred at the bottom of the water and ethanol solutions. In contrast, the graphene sheets were highly stable in the FdE solution, and no precipitation was observed in the suspension. This was attributed mainly to the strong π–π interaction between graphene and FdE.


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Figure 4. (a) UV–vis absorption spectra and (b) Raman spectra of G and FdE-G. In this work, commercial graphene sheets were dispersed in FdE by sonication. To study the interaction between the FdE and graphene sheets, the FdE-G suspensions were characterized by UV–vis and Raman spectrometry. Figure 4a shows the UV–vis spectra of the commercial graphene, FdE solution, and FdE-G suspension. The two characteristic peaks of FdE at 274 and 323 nm are related to the π–π* transition of C=C and the p–π* transition of =C–O in the furan unit, respectively. However, in the FdE-G suspension, the absorption peaks for the π–π* and p– π* transitions were blue-shifted to 271 and 313 nm, respectively. This result provides direct evidence for the strong π–π interaction between FdE and graphene.24 Figure 4b shows the Raman spectra of graphene and FdE-G. Graphene materials generally exhibit a G peak (at 1580 cm−1) and a D peak (at 1350 cm−1) in their Raman spectra. The G peak represents the in-plane bonding stretching motion of sp2 hybrid carbon atoms, whereas the D peak represents the breaking mode near the Brillouin zone boundary K and is related to the edge of the sheets.26,28 The weak D peak at 1350 cm−1 implies that the FdE-functionalized G sheets have few defects. The red shift observed in the G peak of FdE-G (from 1582 to 1587 cm−1) confirmed the electron transfer between the graphene sheets and FdE.30-32 The variations in the UV–vis and Raman spectra proved the presence of π–π interactions between FdE and graphene.


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Figure 5. (a) Low-resolution and (b) high-resolution TEM images of FdE-G nanosheets; (c) SAED pattern and (d),(e) AFM images of FdE-G nanosheets. Owing to its large specific surface area and the strong intermolecular force between the sheets, commercial graphene is easily agglomerated in most polymer matrixes. In this study, a stable FdE-G suspension was obtained via ߨ–ߨ interaction, as verified by the UV–vis and Raman spectra. The morphology and textural structure of graphene in the suspensions were investigated by TEM. As shown in Figure 5a, the graphene sheets resemble a typical exfoliated transparent thin film with wrinkled surface structure, indicating that agglomeration of commercial graphene was effectively prevented by the FdE molecules. The average thickness of the FdE-G was 4–5 layers, as shown in Figure 5b, which further demonstrates exfoliation of the commercial graphene. The selected-area electron diffraction (SAED) pattern in Figure 5c shows relatively strong {1100} spots, which is characteristic of single-layer and few-layer FdE-functionalized graphene sheets.29 Figure 5d,e show typical TEM images of FdE-G. The lateral size of the nanosheets was about 1.0 µm, and the average thickness was 2 to 3 nm, implying that the graphene was made of 3–5 stacked single-layer sheets. Note that the FdE-G was thicker than a single graphene sheet, owing mainly to multilayer adsorption of FdE on the graphene surface.


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Figure 6. (a) Liquid FdE-G/EP mixtures before curing (left to right): FdE-G0.5/EP, FdE-G1.0/EP, and FdE-G2.0/EP; (b) digital photographs of cured EP composite samples (top to bottom): pure EP, FdE/EP, FdE-G0.5/EP, FdE-G1.0/EP, and FdE-G2.0/EP; (c),(d) optical photos of the liquid EP systems before curing; numbers indicate the number of graphene layers in various areas of the flake. Figure 6a shows digital photographs of the liquid FdE-G/EP mixtures before curing. The addition of FdE-G to the colorless and transparent EP produced a fully homogeneous and transparent FdE-G/EP mixture. After curing, the as-prepared solid FdE-G/EP samples were absolutely homogeneous and transparent, suggesting that no agglomeration occurred during the curing process (Figure 6b). To investigate the dispersion condition of graphene in the EP matrix, we obtained optical microscopy images of the uncured FdE-G/EP mixture. Figure 6c,d show optical micrographs, in which numerous single- and few-layer graphene sheets could be observed at the acquisition spot. The images clearly show that the graphene has a thickness of 1– 3 layers, indicating complete exfoliation and homogenous distribution of graphene sheets in the EP matrix. The efficient exfoliation obtained here is very similar to that in the work reported by Amirova et al.31 Mechanical properties of the FdE-G/EP composites


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Figure 7a shows the viscosity of the EP containing different mass fractions of FdE. The EP system containing FdE-G has a lower viscosity than pure EP, whereas the 10 wt.% FdE/EP mixture is less viscous (

Sustainable Graphene Suspensions: A Reactive Diluent for Epoxy

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