Introduction

Graphene, a monolayer with sp2-bonded carbon atoms arranged in a honeycomb lattice, has attracted much attention since the first report of its successful separation from graphite by Novoselov et al. in 2004 [1]. It is the thinnest and hardest nanomaterial in the world [2, 3]. Because of its individual nanostructures and remarkable physical and chemical properties [4, 5], it has been considered as a filler candidate to improve the integrated properties of polymers [6]. The nanomaterials, such as layered clay silicate (montmorillonite) [7, 8], SiC [9], SiO2 [10], and carbon nanotubes (CNTs) nanoparticles [11, 12], etc., were investigated according to the literature in the previous works. Interestingly, graphene and its derivatives are frequently used in polymers, such as, poly (methyl methacrylate) (PMMA) [13], polyurethane (PU) [14], polyamide (PA) [15], polystyrene (PS) [16], polypropylene (PP) [17], bismaleimide [18–20], epoxy [21–23], cyanate ester [24, 25] and so on. However, two problems with the polymers prevail which are as follows: (1) graphene is hard to disperse uniformly in the matrices; (2) the interfacial bonding between graphene and matrices is weak. Therefore, it is necessary to explore the effective methods for graphene functionalization [26, 27].

In this study, cyanate ester (CE) resin was selected as the basic matrix because it has remarkable properties, such as good dielectric [28–30] and mechanical properties [31], excellent thermal stability [32, 33] and good processing characteristics [34], thus it can be a good alternative to other thermosetting resins used in the fields of aviation and aerospace, microelectronics, surface mount technology (SMT) and electronic printed circuit board (PCB), etc.

In this work, the functional graphene nanoflakes (FGNs) were prepared by chemical method. The FGNs/CE/EP nanocomposites were prepared via solution intercalation method, and then the comprehensive properties of the nanocomposites were investigated.

Experimental

Materials

Bisphenol A dicyanate (BADCy) ester, a white crystal powder, (purity ≥99 %, cyanate equivalent of 139 g/eq), was purchased from Jiangdu Wuqiao Resin Factory (Yangzhou, China). Bisphenol A epoxy resin, (viscosity: 12,000–15,000 cp, EEW: 0.51), a colorless, transparent and syrupy liquid, was obtained from Xi’an Resin Factory (Xi’an, China). Natural graphene flakes were provided by Xinghe Graphite Co., Ltd., (Qingdao, China). γ-(2,3-Epoxypropoxy) propyltrimethoxysilane was purchased from Sinopharm Chemical Reagent Co., Ltd., (Xi’an, China). The potassium permanganate (KMnO4), sulfuric acid (H2SO4, 98 %), absolute ethyl alcohol and acetone were obtained from Xi’an Chemical Reagent Factory (Xi’an, China).

Preparation of graphene oxide (GO)

Graphene oxide (GO) was synthesized from natural graphene through Hummers’ method according to the literature procedure [35]. The steps were as follows: 3.0 g natural graphite flakes, 3.0 g NaNO3 and 147 mL concentrated H2SO4 were mixed with stirring in an ice-water bath. Then, 18 g KMnO4 was added into the mixture slowly. In the next step, the solution was stirred for 3 h at 35 °C to form a thick paste. Deionized water of 120 mL was injected into the thick paste slowly and the resulting solution was stirred for 30 min while the temperature increased to 85 °C. Finally, 750 mL deionized water containing 30 mL H2O2 was added until the color turned into brownish yellow, followed by filtering the solution before cooling. After that, the filter cake was washed with deionized water several times until its pH was neutral and dried in a vacuum oven at 60 °C for 24 h. The GO was sealed and stored at room temperature for further use.

Preparation of the functional graphene nanoflakes (FGNs)

100 mg GO was heated in a tube furnace at 1050 °C for 30 s, and then cooled at room temperature. The resultant product was denoted as graphene (GN).

100 mg GN was dispersed in 100 mL absolute ethyl alcohol (1 mg/mL), and sonicated for 1 h. Next, 10 mL of 95 % ethanol solution with 0.5 g γ-(2,3-epoxypropoxy) propyltrimethoxysilane was added slowly with continuous stirring at 65 °C for 24 h. Finally, deionized water was used to filter and wash the filter cake several times to remove the unreacted coupling agent. The filter cake was dried in an oven to obtain functional graphene nanoflakes, denoted as FGNs. Then, xFGNs (x = 0.5, 1.0, 2.0, and 3.0 wt%) were dispersed in acetone (1 mg/mL) and sonicated for 5 h for further use.

Preparation of CE/EP copolymer

The cyanate ester (CE) monomer was stirred at 130 °C for 1 h, and then appropriate epoxy (EP) was incorporated into the liquid and stirred at 100 °C for 2 h to form a homogeneous prepolymer, which was denoted as CE/EP prepolymer. After that, the mixture was degassed to remove entrapped air at 100 °C for 30 min in a vacuum oven. Finally, the mixture was poured into a mold, the inner surface of which was coated with Teflon release paper. The cure and post-cure processes followed the protocol: 180 °C/2 h + 200 °C/6 h and 220 °C/4 h, successively.

Preparation of FGNs/CE/EP nanocomposites

The pre-weighed xFGNs were added into the CE/EP prepolymer, prepared according to the above procedure, and stirred at 100 °C for 1 h. The residual acetone was evaporated with vacuum distillation. The mixture was degassed to remove entrapped air at 100 °C for 30 min in a vacuum oven, and then the mixture was poured into a mold, the inner surface of which was coated with Teflon release paper. The cure and post-cure processes followed the protocol: 180 °C/2 h + 200 °C/6 h, and 220 °C/4 h, successively.

Characterizations and measurements

Differential scanning calorimeter (DSC)

Differential scanning calorimeter (DSC) 700 (A Mettler Toledo Instruments, Switzerland) was used to character the prepolymer. Each sample weighed 5–10 mg and was placed in sealed aluminum crucible under N2 atmosphere at a 60 mL/min flow rate. The samples for non-isothermal scans were heated from room temperature to 350 °C at different heating rates of 5, 10, 20, and 30 °C/min, respectively.

Mechanical properties

The mechanical properties of the samples including unnotched impact (Izod) and flexural strength were tested using an Instron Universal Testing Apparatus (USA), according to GB/T2571-1995 and GB/T2570-1995 and the dimensions of impact and flexural test specimens were 80 × 10 × 4 mm3 and 80 × 15 × 4 mm3, respectively. Meanwhile, at least five samples needed to be prepared for each measurement.

Scanning electron microscopy

The flexural and impact fractured surfaces of the specimens were observed using a Quanta 200 SEM (USA) at an accelerated voltage of 20 kV. All the specimens were dried at 65 °C for 24 h, and then they were coated with a thin layer of Au for SEM imaging.

Dielectric properties

The dielectric properties of the specimens were tested with a S914 broadband dielectric loss spectrometer (QBG-3D, China). Each specimen had the thickness of 2 mm and diameter of 27 mm.

Thermogravimetry analysis (TGA)

Thermal analysis of the specimens was performed with a Q50 TGA (TA Instrument Co., USA) from room temperature to 800 °C with heating rate of 20 °C/min, in N2 atmosphere.

Results and discussion

Dynamic DSC analysis

Figure 1 presents the detail of non-isothermal DSC curves including the CE/EP and CE/EP/FGNs prepolymers at the heating rates of 5, 10, 20, and 30 °C/min, respectively. The exothermic peaks are upwards. In Fig. 1, the temperatures corresponding to the peak of CE/EP/FGNs prepolymer are lower than those of CE/EP at the same heating rate. The result of the comparison clearly indicates that the FGNs have catalytic effect on the curing process of the CE/EP/FGNs prepolymer.

Fig. 1
figure 1

The comparison of DSC curves between the CE/EP and the CE/EP/FGNs prepolymers at different heating rates, a 5 °C/min, b 10 °C/min, c 20 °C/min and d 30 °C/min

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Mechanical properties of the FGNs/CE/EP nanocomposites

The mechanical properties of the FGNs/CE/EP nanocomposites with various FGNs contents are shown in Fig. 2. It can be seen that the flexural and impact strength increase in the initial phase. The flexural and impact strength with 2.0 wt% FGNs reach up to the maximum values, 170.8 MPa and 15.5 kJ/m2, i.e., by 1.17 times and 1.21 times, respectively. However, the curves slightly decrease by further increasing of the FGNs content. It is believed that the incorporation of appropriate FGNs content in the FGNs/CE/EP nanocomposites improves their mechanical properties. A detail polymerization mechanism analysis conducted to examine the results is that: the silane coupling agent, as a “bridge” group, can react with the hydroxyls of the graphene surface and isocyanate groups of the matrix, forming strong interfacial bonding between different components [36]. The reaction mechanism of the CE and EP as well as the structure of the nanocomposites is shown in Scheme 1, respectively.

Fig. 2
figure 2

Mechanical properties of FGNs/CE/EP nanocomposites

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Scheme1
scheme 1

The reaction mechanism of CE and EP as well as the structure of the FGNs/CE/EP nanocomposites a the reaction mechanism of CE and EP and b the structure of the FGNs/CE/EP nanocomposites

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The variation of the flexural modulus versus the content of FGNs and the variation of deflection versus the content of FGNs at break are shown in Figs. 3 and 4, respectively. It can be seen that for the FGNs/CE/EP nanocomposites, the flexural modulus and deflection achieve the maximum value with 2.0 wt% FGNs content, corresponding to the result of the flexural strength in Fig. 2. Figure 5 shows the strain–stress curves of the FGNs/CE/EP nanocomposites at various FGNs contents. From Fig. 5, it can be seen that for the FGNs/CE/EP nanocomposites, the stress-at-break increases with the FGNs content (2.0 wt% FGNs), while the strain-at-break decreases initially and then it increases with increasing the FGNs content.

Fig. 3
figure 3

Variation in flexural modulus versus the FGNs content

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Fig. 4
figure 4

Variation in deflection versus the FGNs content

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Fig. 5
figure 5

Stress–strain curves

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SEM micrographs of the FGNs/CE/EP nanocomposites

Typical images of the flexural and impact fracture surfaces on the FGNs/CE/EP nanocomposites with different FGNs contents are displayed in Fig. 6. The flexural fracture surfaces of the FGNs/CE/EP nanocomposites with 0, 1.0 and 3.0 wt% FGNs contents are shown in Fig. 6a–c, respectively. The impact fracture surfaces of the FGNs/CE/EP nanocomposites with 0, 1.0 and 3.0 wt% FGNs contents are shown in Fig. 6d–f, respectively. The CE/EP copolymer exhibits river, tongue and scales patterns (Fig. 6a, d), suggesting a typical brittle feature. Meanwhile, rougher surfaces are observed in the fracture surfaces of the FGNs/CE/EP nanocomposites with various FGNs contents. Dimple zones like sea waves, smaller scales and fibrous patterns are observed in the fracture surfaces of the FGNs/CE/EP nanocomposites, indicating that the fracture surfaces have a typical tough feature.

Fig. 6
figure 6

SEM images of the flexural fracture surfaces and the impact fracture surfaces corresponding to the FGN/CE/EP system with various FGNs contents. Flexural fracture surfaces: a 0 wt% FGNs, b 1.0 wt% FGNs and c 3.0 wt% FGNs; Impact fracture surfaces: d 0 wt% FGNs, e 1.0 wt% FGNs and f 3.0 wt% FGNs

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Dielectric properties of the FGNs/CE/EP nanocomposites

It is well known that the remarkable dielectric property is one of the most desirable properties for the cyanate resin and its derivates. The dielectric properties of the FGNs/CE/EP nanocomposites are shown in Fig. 7. For the FGNs/CE/EP nanocomposites, the dielectric constant increases and the dielectric loss factor enhances significantly with 2.0 wt% FGNs content. The reasons are that the dielectric loss of the FGNs and the interface polarization between the FGNs and the matrix are large.

Fig. 7
figure 7

Dielectric constant and dielectric loss of the FGNs/CE/EP nanocomposites with different FGNs content from 10 to 60 MHz, respectively; a dielectric constant versus FGNs content and b dielectric loss versus FGNs content

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TG analysis

The thermal degradation behaviors of the pure CE resin, CE/EP copolymer and FGNs/CE/EP nanocomposites with different FGNs contents were evaluated by TG in combination with DTG analyses. The curves of TG and DTG are presented in Fig. 8a, b, respectively. The characteristic temperatures T 5 %, T max and T final of the FGNs/CE/EP nanocomposites obtained from TG analysis are shown in Table 1. From Fig. 8a, the weight loss increases gradually from 300 to 600 °C and the nanocomposites show higher char residue than that of the CE/EP copolymer, but lower than that of pure CE resin. Thus, the addition of FGNs increases the thermal stability of the FGNs/CE/EP nanocomposites. This is because the macromolecular chain stability of the FGNs/CE/EP nanocomposites is enhanced by the interfacial bonding between the FGNs and the matrix. Figure 8b shows that the thermal degradation temperatures corresponding to the peaks enhance slowly with increasing the FGNs content. The thermal degradation temperatures of the FGNs/CE/EP nanocomposites are higher than that of the CE/EP copolymer, but lower than that of pure CE resin as well. These phenomena indicate that the addition of FGNs improves the thermal stability of the FGNs/CE/EP nanocomposites.

Fig. 8
figure 8

TG-DTG curves of pure CE resin, CE/EP copolymer and the CE/EP/xFGNs system (x = 0.5, 1.0, 2.0, and 3.0 wt%); a TG curves and b DTG curves

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Table 1 Characteristic temperatures of FGNs/CE/EP nanocomposites obtained from TG analysis
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Conclusion

Functional graphene nanoflakes were employed as fillers to improve the integrated properties of the FGNs/CE/EP nanocomposites. The results described that the FGNs/CE/EP nanocomposites with appropriate FGNs contents had excellent comprehensive properties. The flexural and impact strength of the FGNs/CE/EP nanocomposites with 2.0 wt% FGNs content reached 170.8 MPa and 15.5 kJ/m2, respectively. The dielectric and thermal properties of the FGNs/CE/EP nanocomposites with 2.0 wt% FGNs increased as well. Therefore, the incorporation of the FGNs into the CE/EP copolymer had a significant effect in improving the physical and chemical properties of the nanocomposites. The FGNs acted not only as a catalyst in the whole curing process, but also as a reinforcing agent in the FGNs/CE/EP nanocomposites.