Strengthened epoxy resin with hyperbranched polyamine-ester anchored graphene oxide via novel phase transfer approach
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This work investigated the mechanical properties of epoxy resin composites embedded with graphene oxide (GO) using a novel two-phase extraction method. The graphene oxide from water phase was transferred into epoxy resin forming homogeneous suspension. Hyperbranched polyamine-ester (HBPE) anchored graphene oxide (GOHBPE) was prepared by modifying GO with HBPE using a neutralization reaction. Fourier transform-infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM) showed that the HBPE was successfully grafted to the GO surface. The mechanical properties and dynamic mechanical analysis (DMA) of the composites demonstrated that GOHBPE played a critical role in mechanical reinforcement owing to the layered structure of GO, wrinkled topology, surface roughness and surface area ascending from various oxygen groups of GO itself, and the inarching of HBPE and the reaction among GO, HBPE, and epoxy resin. The transferred GOHBPE/epoxy resin composites showed 69.1% higher impact strength, 129.1% more tensile strength, 45.3% larger modulus, and 70.8% higher strain compared to that of cured neat epoxy resin. The glass transition temperature (Tg) of GOHBPE/epoxy resin composites was increased from 135 to 141 °C and their damping capacity was also improved from 0.71 to 0.91. This study provides guidelines for the fabrication of strengthened polymer composites using phase transfer approach.
KeywordsGraphene oxide Epoxy resin Hyperbranched polyamine-ester Composites Two-phase extraction Mechanical properties
Graphene serves as the basic cell of other carbon materials (such as fullerene, carbon nanotubes, and graphite) and has attracted a great deal of attention in recent years due to its unique physical and chemical properties such as high thermal heat conductivity (5300 W (m K)−1 ), mechanical elasticity modulus (1.0 TPa ) and large specific surface (2600m2 g−1 [3, 4, 5]). Owing to these outstanding properties, graphene sheets have been widely applied in different fields such as lithium ion batteries [6, 7, 8, 9], supercapacitors [10, 11, 12], biomedical materials [13, 14, 15], and composite materials [16, 17].
However, the weak interactions between pure graphene and other media as well as strong van der Waals forces between graphene sheets make it prone to aggregation and therefore limit its potential application. On the other hand, graphene oxide (GO) platelets have similar structures in two-dimensional space compared to graphene. They also possess chemically reactive functionalities, such as hydroxyl and epoxy groups located on the basal plane and carbonyl and carboxyl groups located mainly at the edge, which can introduce further functionalization of the GO platelets and improve the dispersion and compatibility of GO within the matrix resin. These properties allow for a wider range of applications for graphene and graphite oxide materials [18, 19].
Carboxylic acids have been a key feature in many organic reactions applied to GO. The acid groups can be activated using thionyl chloride, and then a subsequent acylation reaction using propargyl alcohol can be used to obtain the alkynyl GO (GO–C≡CH). The well-defined immobilization of polystyrene (PS) onto GO subsequently takes advantage of click chemistry between the alkyne GO sheets and azido-terminated polystyrene . The poly (styrene-b-ethylene-co-butylene-b-styrene) (SEBS) triblock copolymers can be covalently attached onto GO by taking advantage of a similar click reaction, which incorporates PS into the platelets as the reinforcing fillers. In addition, the epoxy and hydroxyl groups located on the basal plane can be easily modified through ring-opening reactions and esterification. For example, Yang et al.  reported functionalized GO platelets via a nucleophilic SN2 displacement reaction between the epoxide and amine groups of 3-aminopropyltriethoxysilane (APTS) and the covalent incorporation of functionalized GO into silica matrix. In 2011, Yang et al.  demonstrated the azide GO via the reaction among 2-bromoisobutyryl bromide, triethylamine, and GO. The azide-functionalized GO and alkynyl PS can be coupled via a Cu catalyzed [3 + 2] Huisgen cycloaddition between the alkyne and azide end groups, yielding polymer-functionalized GO, which exhibits excellent solubility in organic solvents such as tetrahydrofuran (THF), dimethyl formamide (DMF), and trichloromethane (CHCl3). Adjusting the length of the PS chain allows the control over the distance between various layers of GO.
In addition to modifications via covalent grafting, GO can also undergo non-covalent functionalization via π-π stacking or van der Waals interactions. This type of reaction has the added benefits of being relatively simple and also maintaining the original conjugated structure, mechanical properties and electrochemical properties of the GO platelets. However, non-covalent bonds are relatively weak, which makes the grafting density distribution of the molecule on the GO less. For example, Lu et al.  reported that GO bonds dye-labeled ssDNA via strong non-covalent interactions between nucleobases and aromatic compounds to obtain DNA sensors, which can be used as platforms for the fast, sensitive, and selective detection of biomolecules.
Polymer composites with GO have attracted great interest due to their unique properties, which are derived from extended interactions between the GO and the matrix and their wide potential applications such as electromagnetic wave shielding and sensors [24, 25, 26, 27, 28, 29, 30, 31, 32]. For example, Cao et al.  found that the tensile strength and young’s modulus of GO composites increased by 78 and 73%, respectively, when compared with pure polystyrene. The N-doped graphene oxides composited with Co3O4 nanoparticles enhanced activation energy for the low temperature region by 81% compared to pure methylsilicone resin . Yang et al.  demonstrated that the compressive failure strength and the toughness of APTS monoliths improved by 19.9 and 92%, respectively, when 0.1 wt% functionalized GO sheets were added. In general, nanofillers are directly dispersed in a resin matrix via thermal exfoliation at high temperature [35, 36, 37, 38] or are dispersed in an organic solvent which is later evaporated [9, 39, 40, 41], resulting in polymer composites. However, Yang et al.  recently described a new economically viable method where GO was dispersed through a two-phase extraction resulting in GO/epoxy resin composites. The compressive failure strength and toughness of composites were drastically improved. In order to improve the dispersion in the matrix and increase their extraction efficiency, we treated GO by hyperbranched polyamine-ester (HBPE) in this work. HBPE with tertiary amine functionalities were grafted onto GO to obtain well dispersed GO in water. The functionalized GO (named as GOHBPE) was then transferred to the epoxy matrix through a two-phase extraction. This green method is different from the traditional hot melt dispersion method and organic solvent based methods while maintaining the structural integrity of the GO during the process. Finally, the GO/epoxy resin composites were prepared by the casting method. DMA measurements, tensile and impact testing were utilized to elucidate the mechanical properties of composites.
2 Experimental section
Natural graphite (CP, ignition residue ≤ 0.15%, granularity ≤ 30 μm), hydrochloric acid (HCl, 36–38%) and concentrated sulfuric acid (H2SO4, 98%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Potassium permanganate (KMnO4, AR) and sodium nitrate (NaNO3, AR) were purchased from Shanghai Su Yi Chemical Reagent Co. Ltd. The epoxy resin E-51 was obtained from Wuxi Blue Star resin factory and was composed of the diglycidyl ether of bisphenol A resin with an average epoxy value Ev = 0.51 mol 100 g−1. Methyltetrahydrophthalic anhydride (MeTHPA) was used as the curing agent and purchased from Pu Yang Huicheng Electronic material Co, Ltd. 2,4,6-Tri(dimethylaminomethyl) phenol (DMP-30) was used as an accelerant and was purchased from Aladdin Chemistry Co. Ltd.
The hyperbranched poly(amine-ester)s (HBPE) was synthesized using pentaerythritol as the core molecules and N,N-diethylol-3-amine methylpropionate as the AB2 branched monomer based on our previous work . Briefly, the N,N-diethylol-3-amine methylpropionate was prepared via Michael addition of methyl acrylate and diethanolamine as the AB2 monomer. Then pentaerythritol, AB2 monomer, and p-toluene sulfonic acid were stirred and processed at 115 °C for 2.5 h. The residual unreacted monomers and by-product methanol were removed to obtain HBPE.
2.2 Synthesis of GO
The GO was obtained by pressurized oxidation . NaNO3, natural graphite and H2SO4 (mass ratio 1:1: 50) were added to a hydrothermal reactor, followed by the slow addition of KMnO4 under ice water. The kettle was then tightened quickly. The hydrothermal reactor was frozen for 2.5 h at 0 °C and then heated for 3 h at 110 °C. After cooling, the kettle was opened and the mixture was poured into deionized water and stirred to dilute the reaction. A proper amount of H2O2 and HCl were then added until the solution turned yellow. The reaction mixture was then centrifuged for 5 min at 8000 rpm. Dialysis of the under layer was deposited for 3 days in a dialysis bag followed by ultrasonic dispersion for 30 min to obtain the dispersion liquid. The dispersion liquid was dried in an oven at 80 °C to obtain GO.
2.3 Preparation of modified GO (GOHBPE)
Hyperbranched polyamine-ester (HBPE, 5 g) and GO (1.0 g) were added to a beaker. The mixture was dispersed using a high shear dispersing emulsifier for 15 min. The suspension was then poured into three separate flasks. An adequate amount of toluene sulfonic acid as the catalyst was added and reacted for 24 h at 60 °C. The products were filtered with a mixed fiber film with 0.22 μm in diameter and rinsed with deionized water. The final solid powder (referred to as GOHBPE) was dried at 80 °C for 24 h before further use.
2.4 Preparation of GOHBPE/epoxy resin composites
A homogenous dispersion of GOHBPE in epoxy resin was obtained using a two-phase extraction. First, 2 g GOHBPE was dispersed in 4 mL deionized water in an ultrasonic bath for 1 h to prepare GOHBPE suspension. Next, 40 g epoxy resin was mixed with a portion of the GOHBPE suspension in three 250-mL flasks. The mixtures were stirred at 50 °C for overnight in order to completely remove water resulting in a homogenous mixture. The contents of GOHBPE were 0, 0.1, 0.2, 0.5, and 1 wt%.
The mixtures were degassed at 90 °C for 10 min to remove bubbles before adding hardener. The hardener and accelerant were then added to the mixture at ambient temperature and the mixture was quickly poured into a preheated steel mold (epoxy resin/hardener/accelerant (weight ratio) = 100:75:1). The mold was heated at 80 °C for 1 h, 140 °C for 3 h, and 180 °C for 3 h. After curing, the samples were cooled in the stove and were incised using standard procedures.
The surface functional groups were characterized with a Fourier transform-infrared (FTIR) spectroscopy (Digilab FTS3000). X-ray diffraction (XRD) was carried out using a XRD-6000 diffractometer with CuKa radiation (λ = 1.54 Å). Transmission electron microscopy (TEM) was performed on ultra thin films using a JEM-2100 instrument and an accelerating voltage of 100 kV. A few drops of aqueous sample were placed on a copper grid and dried for the TEM measurement. Raman spectroscopy was carried out at room temperature using a Renishaw InviaReflex spectrometer equipped with a 532-nm semiconductor laser. All the samples were powders, which were deposited directly on the quartz substrate.
Dynamic mechanical analysis (DMA) was performed using a DMAQ800 dynamic analyzer and a heating rate of 5 °C min−1. The samples were heated from 25 to 200 °C using 5 °C min−1 rate at 1 Hz frequency. The mechanical properties of the GO/epoxy composites were tested on the universal testing machine (CMT-5105) according to GB/T 2567–2008. A crosshead speed of 5 mm/min*** was used.. Ten specimens of each composite were tested and the mean values of the mechanical properties were reported. Scanning electron microscope (SEM) examinations were completed using a JMS–6480 at 5.0 kV. The SEM samples were coated with gold to make them conducting.
3 Results and discussion
The test results revealed that stress-strain curves of the addition of GOHBPE and pure epoxy resin exhibit a similar tendency, as shown in Fig. 6b. All samples broke immediately after the stress reached the maximum value. In the whole tensile curves, no yield points appear for any sample attributed to the brittle fracture of epoxy resin. It is notable that the stress-strain curve reveals a strong nonlinear feature different from the description in ref. , which illustrate the elastic module decreasing first and then rising continuously until the sample was fractured. As we know, the modulus (E) is the ratio of stress and strain in the low strain region, namely, the tensile modulus decreases first and then increases in the tensile process. Figure 6c illustrates the stress-strain curve and the calculated value for neat epoxy resin. “A” region on curve in Fig. 6c displays the modulus decreasing from 473 to 295 MPa for a short period of time because the macromolecular disentanglement of epoxy resin in a small space leads to the lower stress induced deformation. B region reveals the modulus increasing from 295 to 826 MPa. For brevity, enhancing orientation degree under tensile force causes the rising intermolecular force which results in the ability of resisting deformation. The modulus of neat epoxy resin reaches its highest (826 MPa) at tensile strength (12 MPa).
The tensile test results of GOHBPE/epoxy resin composites
neat epoxy resin
Tensile strength (MPa)
Tensile modulus (MPa)
Tensile strain (%)
The loss factor (tan θ) is usually used to evaluate the damping property of materials [53, 54, 55, 56]. The temperature at which the loss factor curve reaches a maximum is often recorded as the T g. It can be seen from Fig. 8c that the T g of pure epoxy resin, according to this measurement, is 135 °C with a loss factor of 0.71. The T g and the loss factor both increase reaching a maximum of 141 °C and 0.91, respectively, when the GOHBPE content is 0.2 wt%. This indicates that while increasing the toughness of the epoxy resin, GOHBPE can also improve its damping properties. In the mixture, the curing agents not only solidify the epoxy resin but also react with the GO sheets through the epoxide and amine groups. Simultaneously, hydroxyl and carboxyl groups of GO and the epoxy resin, amine groups of HBPE and the epoxy resin also may participate in the chemical reactions. In short, GOHBPE is embedded in the epoxy resin via covalent bonding, resulting in a stronger interface and increased cross linking, and higher T g. Moreover, the flexibility of the GO ameliorates the damping properties of the composites.
The HBPEs were successfully grafted on GO via covalent bonding forming a stable suspension in water. The GOHBPE was homogeneously transferred into epoxy resin matrix from water using a two-phase extraction. In addition, GOHBPE was found to improve the mechanical properties, heat stability, and damping properties of the epoxy resin matrix owing to the strong interfacial bonding strength, covalent cross-linking, and 2-D GO structure. The tensile strength, the modulus, and the strain had been improved by 129.1, 45.3, and 70.8%, respectively at 0.2% transferred GOHBPE sheets. At the same fraction, the highest impact strength was 15.9 KJ m−2 and had been improved by 69.1%. Their T g and loss factor were increased from 135 °C and 0.71 to 141 °C and 0.91, respectively. This method may have broader applications in the future for graphene and other polymer matrix composites.
We gratefully acknowledge the supports from the Priority Academic Program Development of Jiangsu Higher Education Institution; the Key Laboratory Funded by Jiangsu advanced welding technology, National Natural Science Foundation of China (No. 51402132), Jiangsu Provincial Natural Science Foundation of China (Grant No. BK2012279 and No. BK20140505), and US National Science Foundation under grants of CMMI-1560834 and IIP-1700628.
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