Graphene nanosheets decorated with ZnO nanoparticles: facile synthesis and promising application for enhancing the mechanical and gas barrier properties of rubber nanocomposites

Abstract

In this study, graphene nanosheets decorated with ZnO nanoparticles (NZG) were prepared from graphene oxide by a facile solvothermal method. Simultaneously, NZG as a substitute for conventional ZnO was incorporated into a rubber matrix. The NZG not only showed improved vulcanization efficiency but could also simultaneously play a reinforcing role for natural rubber (NR) nanocomposites. The results showed that the curing reaction of NR/NZG nanocomposites is significantly faster than that of neat NR. The dynamic and static mechanical properties and the gas barrier properties of NR/NZG nanocomposites were systematically investigated at different NZG contents. Compared with neat NR, the tensile strength, modulus at 300% strain and tear strength of the NR/NZG-2.5 nanocomposite are dramatically enhanced by 67.8%, 188.6% and 74.1%, respectively, at only 2.5 phr NZG content. The permeability of the NR/NZG-2.5 nanocomposite decreased by 53% compared to neat NR. The good dispersion of graphene and high vulcanization efficiency of ZnO nanoparticles, together with the enhanced interfacial interaction, is considered to be responsible for the significant synergetic effects in the improvement of the vulcanization characteristics, mechanical performance and gas barrier properties of the composites.

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1 Introduction

For decades, rubber nanocomposites have drawn enormous interest and extensive research owing to their attractive properties and promising applications. A notable concern is that the practical applications of rubber often require the use of cure activators in the vulcanization to obtain the desired reinforcement, further making rubber composites useful for wide applications. Nowadays, zinc oxide (ZnO), regarded as an indispensable cure activator, deserves more than passing notice in vulcanization. Its advantage lies in the fact that the network structure of rubber is explicitly related to crosslinking, more specifically the cure-activation effect of ZnO.^1,2

By now, the typical rubber vulcanization recipe contains a maximum of 5 phr (parts per hundred parts of rubber) conventional ZnO, because only thus can it ensure the vulcanization efficiency and enhance the mechanical performance. However, low vulcanization efficiency, consequent of applying it in rubber composites, has become a crucial issue. Moreover, ZnO has been designated as a hazardous chemical by some environmental groups and the European Union, as its toxicity seriously threatens aquatic species and causes unrecoverable damage to the ecological environment. Therefore, a reduction in the quantity of ZnO in rubber composites is clearly necessary.

To date, ZnO nanoparticles (nano-ZnO), have been a particularly good candidate when applied as an effective cure-activator for vulcanization. Due to its special characteristics such as the “small size effect”, “surface effect” and “quantum effect”,^3,4 the vulcanization efficiency of nano-ZnO can be significantly enhanced by broadening the contacts between the nano-ZnO and the rubber additives in the composites. Therefore, nano-ZnO may make a significant contribution to the reduction of ZnO levels in rubber composites and has aroused extensive research interest.^5–7 Nonetheless, the high surface energy and small size of nano-ZnO can prompt the irreversible aggregation of nanoparticles. This phenomenon can seriously impede the usage of nano-ZnO and impact the performance of the composites.

In order to obtain the desired performance of rubber composites by incorporation with nano-ZnO, a handy and effective method is required to weaken the aggregation of nano-ZnO. To achieve this objective, the decoration of metal oxide nanoparticles on layered nanomaterials has been exploited as a feasible approach.^8–10 Very recently, graphene (GE) has been examined as an ideal platform for supporting nanoparticles to exploit the metal oxide–GE nanocomposite,^11–14 an approach which mainly focuses on utilizing the GE sheets as carbon mats to anchor metal oxide materials to form new hybrid materials. Such an attachment of metal oxide nanoparticles (ZnO, TiO₂, SnO₂, etc.) onto the GE nanosheets can not only have a tendency to weaken the aggregation clusters of metal oxide, but also eliminate irreversible agglomerates or even restack to form graphite through van der Waals interactions during the synthesis process.^15,16 Also, it has been revealed that when incorporated into a polymer matrix,^17–19 GE with its unique structural, physical and chemical properties can remarkably improve the performance of polymer composites. Currently, some methods have proven to be useful to produce GE, such as mechanical exfoliation techniques,²⁰ chemical vapor deposition,²¹ solid-state carbon source deposition,^22,23 solution-based reduction of graphene oxide (GO),²⁴ and nanotube splitting and unzipping.^25,26 For all these processes, considerable interest in the practical application of GE has led to many theoretical and experimental efforts for the large-scale production of GE nanosheets, of particular concern is the most economic solution-based reduction of GO, which has been adopted by most researchers.^27,28 Recently, an effective one-pot method in which GO sheets and precursors of nano-ZnO are subjected to successive chemical reactions has been reported to prepare nano-ZnO–graphene (NZG) nanocomposites.^29,30 Noteworthily, few reports concern the incorporation of NZG into rubber for preparing rubber/NZG nanocomposites with high performance, thus this remains a considerable challenge. It is even possible to predict that NZG, as a promising reinforcement for rubber composites, is conducive to exploring the potential applications of nanoparticles/GE materials.

In this study, we reported a facile solvothermal method for the synthesis of NZG nanocomposites using GO and Zn(Ac)₂ as the precursors for GE and nano-ZnO, respectively. Subsequently, the resultant NZG was incorporated into a NR matrix to fabricate NR/NZG nanocomposites. The effects of NZG on the vulcanization characteristics, crosslink network, static and dynamic mechanical properties of NR nanocomposites were addressed. Simultaneously, the relationship between the dispersion state of GE and the overall performance of NR nanocomposites was illustrated and the influence of the GE morphology on the gas permeability of NR nanocomposites was further investigated.

2 Experimental

2.1 Materials

The chemicals used in the experiments are analytical grade reagents without further purification. Natural graphite, zinc acetate (Zn(Ac)₂), ammonia (NH₃·H₂O), hydrazine hydrate, ethanol, acetone and ethylene glycol (EG) were all provided by Guangzhou Chemical Reagent Factory.

All the rubber ingredients were industrial grade and used as received. NR was supplied by Guangzhou Rubber Institute. Zinc oxide, stearic acid, N-cyclohexylbenzothiazole-2-sulphenamide (CZ), 2,2′-dibenzothiazole disulfide (DM), 2-mercaptobenzimidazole (MB) and sulfur were provided by Guangzhou Longsun Technology Co., Ltd.

2.2 Synthesis of NZG nanocomposites

GO was synthesized by a modified Hummers method³¹ and ultrasonically-assisted exfoliation. Afterwards, NZG was fabricated by a simple solvothermal method. Typically, 100 mg of GO and 600 mg of Zn(Ac)₂ were dispersed in 80 mL of EG under sonication. 1.2 mL of NH₃·H₂O and 5 mL DI water were slowly added into the mixture with continuous stirring. The mixture was kept at 60 °C for 24 h. Subsequently, 50 μL of hydrazine hydrate was slowly added into the mixture and maintained at 90 °C for 12 h. The resulting product was isolated by centrifugation, repeatedly washed with water and ethanol, and finally dried in a vacuum oven at 60 °C for 24 h. Then, the as-synthesized NZG powders were obtained after heat-treatment in an air atmosphere at 600 °C for 6 h. For comparison, GE and nano-ZnO were synthesized through an identical procedure in the absence of Zn(Ac)₂ and GO, respectively. In order to measure the weight ratio of GE and nano-ZnO, a desired amount of NZG (m_a) was well dispersed in 1 M HCl solution to remove the nano-ZnO. The obtained product (m_b) was isolated by centrifugation, repeatedly washed with water and acetone, and finally dried in a vacuum oven at 60 °C for 24 h. Thereby, the mass fraction φ (wt%) of GE in the NZG powder can be calculated by the equation, φ = (m_b/m_a) × 100%. Herein, the weight content of GE in the NZG was calculated to be 25 wt%.

2.3 Preparation of NR/NZG nanocomposites

NR was compounded with rubber ingredients in an open two-roll mill and subjected to compression at 143 °C for the optimum curing time determined by a Rubber Process Analyzer. The formulation of the NR composite is listed as follows: NR, 100 g; ZnO, 5 g; stearic acid, 2 g; CZ, 1.5 g; DM, 0.5 g; MB, 1.0 g; sulfur, 1.5 g; NZG, variable. The NZG content in the composites was controlled to be 0.5, 1.0, 1.5, 2.0 and 2.5 phr. For comparison, a NR composite with 5 phr conventional ZnO (namely, “neat NR”), NR nanocomposites containing 0.125, 0.250, 0.375, 0.500 and 0.625 phr of GE, and NR nanocomposites with 0.375, 0.750, 1.125, 1.500 and 1.875 phr of nano-ZnO were also prepared in the same way. These nanocomposites are abbreviated as “NZG-x”, “GE-x” or “nano-ZnO-x” below. The unknown number x denotes NZG, GE or nano-ZnO content (phr) in the nanocomposites.

2.4 Characterization

X-ray diffraction (XRD) analysis was used to investigate the crystalline structures of NZG, GE and nano-ZnO. The XRD studies were performed with a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 0.1542 nm). The goniometer scanned diffracted X-rays at a scan speed of 0.15° s⁻¹. UV-vis absorption spectroscopy was carried out on a UNICAN UV-500 spectrophotometer in ethanol dispersion using a quartz cell with a 1 cm optical path. The evaluation of GE sheets dispersed in NR nanocomposites was carried out by field emission scanning electron microscopy (FESEM, Nova NANOSEM 430) and high resolution transmission electron microscopy (HRTEM, JEOL2100). The mechanical properties were all measured by a U-CAN UT-2060 instrument. Tensile tests were performed with dumbbell-shaped specimens according to ASTM D412 with a crosshead speed of 500 mm min⁻¹. The tear strength was determined with a right-angle tear die C according to ASTM D624. Crosslink density was recorded by an IIC XLDS-15 HT magnetic resonance crosslink density analyzer. A swelling test was performed by the method reported previously.³¹ The bound rubber content in the NR/NZG nanocomposites was determined by the same route as previously.³² Differential scanning calorimetry (DSC) measurements were performed on a TA Q20 instrument in a nitrogen atmosphere, at a heating rate of 20 °C min⁻¹. Dynamic mechanical analysis (DMA) was conducted on a DMA242C dynamic mechanical analyzer in tensile mode. The experiments were conducted in a temperature range of −135 to 75 °C at 3 Hz with a heating rate of 3 °C min⁻¹. The gas permeability of NR nanocomposites was measured using a gas permeability tester (VAC-V1, Labthink Instruments). The test gas used was nitrogen, and the samples used were circular-shaped specimens with 50 mm diameter and 1 mm thickness.

3 Results and discussion

3.1 Structural characterization of NZG nanocomposites

FESEM was performed to investigate the surface morphology and structure of the samples. As displayed in Fig. 1a, most of the GE nanosheets are stacked, curled and entangled together. And the synthesized nano-ZnO exhibits an obvious tendency for the nanoparticles to agglomerate, as shown in Fig. 1b. Fig. 1c and d show FESEM images of NZG nanocomposites. It should be noted that the surface of the curled GE nanosheets is covered by densely packed and irregularly shaped nano-ZnO in a large-scale, which implies a good combination between GE nanosheets and nano-ZnO. Interestingly, some nano-ZnO particles are grown on the edge of interlayers and inside interlayers of GE nanosheets, resulting in wrinkled textures which form a sandwich-like GE–ZnO structure to avoid the restacking of GE nanosheets. Simultaneously, the EDS spectrum (the inset of Fig. 1d) proves that Zn is present in the nanocomposites and no impurities have been introduced.

Fig. 1 (a–d) FESEM images of GE (a), nano-ZnO (b) and NZG nanocomposites (c–d). The inset in (d) is the EDS spectrum corresponding to the circle-marked region of the NZG nanocomposite in (d). (e–f) TEM images of NZG nanocomposites.

Fig. 1e and f show TEM images of the NZG nanocomposites. The GE nanosheets are decorated by nano-ZnO of 100 nm in diameter. Notably, some nano-ZnO particles are dispersed on the surface of the wrinkled GE nanosheets and some are covered or wrapped by thin GE nanosheets, in agreement with the FESEM observations.

The phase and structure of the synthesized samples were further detected by XRD. As shown in Fig. 2a, the XRD pattern of the synthesized NZG nanocomposites shows the diffraction peaks at 31.6°, 34.4°, 36.1°, 47.3°, 56.3°, 62.6°and 67.6° ascribed to the (100), (002), (101), (102), (110), (103) and (112) planes of ZnO, which are consistent with those of nano-ZnO. These results show that both the synthesized nano-ZnO and the nano-ZnO decorated onto the GE nanosheets can be mainly ascribed to hexagonal phase ZnO and appear to have excellent crystal structures. No obvious characteristic peaks corresponding to GO or graphite were observed. In contrast, it was found that the GE shows a new and broad (002) diffraction peak at 2θ = 24.02°, suggesting that the GE nanosheets were inclined to aggregate after reduction. This result indicates that the decoration of nano-ZnO on the surfaces of GE nanosheets can effectively inhibit the agglomeration of GE.

Fig. 2 XRD patterns (a) and UV-vis curves (b) of NZG, GE and nano-ZnO nanocomposites.

Fig. 2b displays the UV-vis absorption spectra of the synthesized NZG nanocomposites, together with GE and nano-ZnO for comparison. It can be clearly seen that the GE shows a strong absorption peak at 265 nm, generally regarded as the excitation of π-plasmon of graphitic structure.³³ And a strong absorption peak located at 376 nm is observed for the nano-ZnO.^33,34 Noteworthily, compared with GE and nano-ZnO alone, the absorption peaks assigned to GE and ZnO were shifted to 277 nm and 367 nm in the NZG nanocomposites, respectively. The shifts in the absorption peaks can be attributed to a coupling effect,^29,35 demonstrating a strong interaction between GE and nano-ZnO. This result can also be confirmed by XPS analysis (see Fig. S1 of the ESI†). The negative shifts in the binding energies for Zn2p3/2 and Zn2p1/2 clearly occurred because of the interaction between nano-ZnO and GE. Additionally, the difference of C1s spectra between GO and NZG suggests that most of the oxygen-containing functional groups have been removed after reduction.

3.2 Morphology of NR/NZG nanocomposites

The dispersion status of GE in the NR nanocomposites was evaluated by FESEM and HRTEM. The FESEM images of cryogenically fractured surfaces for neat NR and NR/NZG-2.5 nanocomposite are compared in Fig. 3a and b. The surface of neat NR is smooth. With the incorporation of NZG into the matrix, the sample exhibits a rough fractured surface with protuberances. These protuberances are derived from the introduced GE nanosheets wrapped by NR chains. HRTEM was used to directly visualize the dispersed GE nanosheets in the matrix. For the NR/GE-0.625 nanocomposite, the apparent stacking of GE nanosheets is observed in Fig. 3c. This may be because the aggregation of GE nanosheets can occur before latex coagulation. However, a comparison with the dispersion status of GE in the NR/NZG-2.5 nanocomposite (0.625 phr GE content) is displayed in Fig. 3d. It is clear that the GE nanosheets are finely dispersed throughout the matrix, and exist as individual layers without aggregates. These results provide clear evidence that the nano-ZnO covering the GE nanosheets effectively hinders the aggregation of GE during the mill-mixing process and vulcanization process. Additionally, the XRD analysis (see Fig. S2 of the ESI†) also reveals that nano-ZnO decorated on the GE facilitates the GE dispersion in the matrix, and NZG with a low content has a higher vulcanization efficiency compared with 5 phr conventional ZnO.

Fig. 3 FESEM images of neat NR (a) and NR/NZG-2.5 nanocomposite (b) after tensile tests, and HRTEM images of NR/GE-0.625 (c) and NR/NZG-2.5 (d) nanocomposites.

3.3 Vulcanization characteristics and network structure of NR/NZG nanocomposites

The effect of NZG on the vulcanization characteristics of NR nanocomposites has been analyzed, as well as a composite containing 5 phr conventional ZnO. The vulcanization characteristics of NR nanocomposites are expressed in terms of the scorch time (T₁₀), optimum curing time (T₉₀), minimum torque (M_L), and maximum torque (M_H). As depicted in Fig. 4, it can be seen that all vulcanization characteristics increase with increasing NZG content. The results indicate that the presence of NZG in the NR nanocomposites extends the incipient vulcanization and curing time of the vulcanization. The torque of NR nanocomposites increases with increasing NZG content, which is proportional to the formation of a crosslink network and the strong interfacial interactions between GE nanosheets and the matrix.^36,37 However, it is worth mentioning that NZG with a low content (0.5 or 1 phr) causes a reduction of vulcanization characteristics in comparison with 5 phr conventional ZnO, demonstrating that such NZG content cannot effectively facilitate the vulcanization. This phenomenon may be due to the fact that the low content of NZG cannot sufficiently promote the formation of the zinc salts of stearic acids, which play an important role in solubilizing those insoluble accelerators to form the actual catalyst for the vulcanization process. A higher content of NZG results in a significant increase in M_L and M_H. Meanwhile, the cure rate index (CRI) and the difference between M_H and M_L(ΔS) are presented in Fig. 4c. The CRI of NR/NZG nanocomposites are significantly higher than that of NR composite containing 5 phr conventional ZnO, suggesting that NR can be crosslinked rapidly with the inclusion of NZG. These results are due to the fact that nano-ZnO has a larger surface area than conventional ZnO, and therefore it has a tendency to accelerate the reactions between nano-ZnO, stearic acid and accelerators. This result prominently favors vulcanization, allowing sulfur insertion to occur more rapidly.³⁸ And ΔS values show an increase with increasing NZG content, which is closely associated with the degree of crosslink of the network structure of NR composites. The contribution of NZG to the crosslink network is also validated by swelling measurements in Fig. S3 of the ESI.† Therefore, these results convincingly show that NZG can behave as an effective cure activator in the vulcanization.

Fig. 4 Vulcanization characteristics of NR/NZG nanocomposites. (a) T₁₀ and T₉₀; (b) M_L and M_H; (c) ΔS and CRI.

To further investigate the effect of NZG on the network structure of NR nanocomposites, DSC was adopted to analyze the glass transition of NR/NZG nanocomposites as illustrated in Fig. 5a, and the results are listed in Table 1. An increase of T_g is clearly observed. This trend indicates that the good dispersion of GE and excellent vulcanization efficiency of nano-ZnO can lead to the enhanced network structure. Also, it is clearly seen that ΔC_p decreases systematically with increasing NZG content, which reflects a decrease of the chain mobility due to their interactions with the GE nanosheets. Similar results in polyurethane/clay composites³⁹ and poly(dimethylsiloxane)/silica composites⁴⁰ have been interpreted in terms of a fraction of polymer being immobilized on the surface of fillers. In order to follow that point, the weight fraction of immobilized NR (χ_im) was calculated by the following equations:⁴¹

ΔC_pn = ΔC_p/(1 − ω) (1)

χ_im = (ΔC_p0 − ΔC_pn)/ΔC_p0 (2)

where ΔC_p0 refers to the heat capacity jump at T_g of neat NR, ΔC_p is the heat capacity jump at T_g of NR/NZG nanocomposites, ΔC_pn represents the heat capacity jump at T_g of NR nanocomposites normalized to the rubber fraction, and ω is the weight fraction of GE in the nanocomposites.

Fig. 5 (a) DSC curves of neat NR and NR/NZG nanocomposites. (b) Weight fraction of immobilized NR (χ_im) as a function of weight fraction (ω) of GE. (c) Effect of NZG on the bound rubber content of NR/NZG compounds.

Table 1 DSC results for the glass transition of NR/NZG nanocomposites

Sample	ω^a (%)	Tg (K)	ΔTg (K)	ΔCp (J g^−1 K)	ΔCpn (J g^−1 K)	χim (wt%)
a ω, the weight fraction of GE in the nanocomposites; Tg, the glass transition temperature; ΔTg, the range of the glass transition; ΔCp, the heat capacity jump at Tg; ΔCpn, heat capacity jump normalized to the rubber fraction; χim, the weight fraction of immobilized NR in the nanocomposites.
Neat NR	0	213.65	3.3	0.466	0.466	—
NR/NZG-0.5	0.117	213.65	3.1	0.460	0.461	1.1
NR/NZG-1.0	0.233	213.85	3.2	0.452	0.453	2.8
NR/NZG-1.5	0.347	214.25	3.1	0.443	0.445	4.5
NR/NZG-2.0	0.461	214.45	3.2	0.436	0.438	6.0
NR/NZG-2.5	0.573	214.95	3.4	0.432	0.434	6.9

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DSC results are listed in Table 1 and χ_im is plotted as a function of ω in Fig. 5b. χ_im tends to increase with increasing ω, and this increase is moderate in view of the large variation of ω. The changed immobilized NR (χ_im) reflects the NR chains mobility affected by the interfacial interaction between the chains and the GE nanosheets. The higher the value of χ_im, the stronger the interfacial interaction between NR and GE. The underlying mechanism regarding the enhancement in the interfacial interactions is that GE bears –OH and –COOH groups even after reduction, which can be converted into thiol (–SH), thioester (–CSOR), and dithiocarboxylic ester (–CSSR) groups upon reaction with sulfur. These formed groups interact with the accelerator and sulfur, thus polysulfide species are generated to attack the unsaturated sites on the rubber chains and form a complex crosslink network.^42,43 Moreover, nano-ZnO decorated onto the GE nanosheets has a larger surface area than conventional ZnO, which prominently favors the vulcanization and efficiently enhances the interfacial adhesion between the GE nanosheets and NR matrix. Thus, the rubber chains in the interfacial layer are effectively anchored or attached to the surface of GE nanosheets, the result of which leads to the strong interfacial interactions between the GE nanosheets and NR matrix.

In order to verify the interfacial interactions, the results of bound rubber are illustrated in Fig. 5c, wherein the degree of rubber–GE interaction was measured by bound rubber content. It is apparent that bound rubber content significantly increases for the rubber compounds with higher NZG content, indicating that the rubber–GE interaction is enhanced by the increased NZG content. This variation is attributed to the good dispersion of GE in the rubber matrix which can amplify the contact areas between NR and GE, thereby making the rubber chains more efficiently attached to the GE.

3.4 Mechanical properties of NR/NZG nanocomposites

Following the excellent activator efficiency of NZG towards NR composites, the effect of NZG, GE and nano-ZnO on the mechanical properties of NR nanocomposites were experimentally investigated, emphasizing the superiority of NZG in the enhancement of nanocomposites. As depicted in Fig. 6, the results reveal that, as compared with the NR/nano-ZnO-1.875 nanocomposite, the tensile strength, modulus at 300% strain and tear strength of the NR/NZG-2.5 nanocomposite substantially increase by 18.3% (from 28.12 to 33.26 MPa), 68.6% (from 2.55 to 4.30 MPa) and 14.6% (from 30.27 to 34.68 kN m⁻¹), respectively. As a fascinating carbonaceous material, GE exhibits versatile characteristics for improving the performance of NR nanocomposites. An enhancement of the maximum strength is obtained as GE content is gradually increased. It is of interest to note that the NR/NZG-2.5 nanocomposite (0.625 phr GE content) provided better mechanical properties than those of the NR/GE-0.625 nanocomposite using 5 phr conventional ZnO as the cure activator. For example, the tensile strength of the NR/NZG-2.5 nanocomposite is up to 33.26 MPa, a 6.5% enhancement compared with that of the NR/GE-0.625 nanocomposite. For all the cases, the modulus increase is significant and is accompanied with a relatively noticeable decrease in elongation at break. Noteworthily, compared with neat NR filled with 5 phr conventional ZnO, the tensile strength, modulus at 300% strain and tear strength of NR/NZG-2.5 nanocomposite are dramatically enhanced to 33.26 MPa, 4.30 MPa and 34.68 kN m⁻¹, increasing by 67.8%, 188.6% and 74.1%, respectively. The mechanical properties of the composites are closely associated with the specific surface areas of the samples, which explains this phenomenon. As shown in Table S1 of the ESI,† the specific surface area of conventional ZnO is only 6.046 m² g⁻¹, while the specific surface area of NZG is up to 35.566 m² g⁻¹. The large surface area is conducive to promoting the formation of soluble sulfurating complexes for the vulcanization. So an increase in vulcanization efficiency affected by the specific surface area of the fillers results in increasing vulcanization reactions and improvements of the mechanical properties of the composites. Noteworthily, the specific surface area of NZG (35.566 m² g⁻¹) is larger than that of nano-ZnO (31.253 m² g⁻¹), directly indicating that ZnO nanoparticles decorated onto GE sheets can efficiently reduce their aggregation tendency. Therefore, NR/NZG nanocomposites filled with lower NZG content can exhibit superior mechanical properties than those of neat NR and NR/nano-ZnO nanocomposites. Additionally, to further show the reinforcement effect of NZG towards NR, the mechanical properties of NR composites prepared by physically adding rGO and ZnO (NR/rGO/ZnO composites) are provided in Fig. S4 of the ESI.† It is clearly seen that NR/NZG nanocomposites show improved mechanical properties compared to those of NR/rGO/ZnO composites at the same filler content. Accordingly, NZG has a much stronger reinforcement effect on the mechanical performance of NR nanocomposites compared with GE and nano-ZnO, and such enhanced mechanical properties make NZG very competitive for potential applications as an effective cure activator in vulcanization.

Fig. 6 Comparison of the mechanical properties of NR/NZG, NR/nano-ZnO and NR/GE nanocomposites. (a) Tensile strength; (b) modulus at 300% strain; (c) tear strength; (d) elongation at break. The symbol α represents 5 phr for conventional ZnO, and β, χ, δ, ε, and φ represent 0.5, 1.0, 1.5, 2.0, and 2.5 phr for NZG, or 0.375, 0.750, 1.125, 1.500, and 1.875 phr for nano-ZnO, or 0.125, 0.250, 0.375, 0.500, and 0.625 phr for GE, respectively.

The underlying mechanism regarding the increases in the mechanical properties of NR nanocomposites and the superiority of NZG in the enhancement of overall performance is attributed to the following: (a) the good dispersion of GE nanosheets in the matrix can preclude the stress concentration of cracks and dissipate energy during the loading situation, (b) the strong interfacial interaction between GE and NR can promote a very efficient load transfer from the matrix to GE nanosheets, and (c) the nano-ZnO decorated on the GE nanosheets possesses a large surface area, which can achieve the maximization of vulcanization efficiency by broadening the contacts between the nano-ZnO and rubber additives, and promote the formation of a crosslink network. Therefore, the impressive enhancement in the mechanical properties of NR/NZG nanocomposites should be ascribed to the incorporation of well-dispersed GE, the fine interfacial structure and the high vulcanization efficiency of nano-ZnO.

3.5 Reinforcing efficiency of NZG on NR nanocomposites

To further study the reinforcing efficiency of NZG as well as the structure of the nanocomposites, DMA measurement was carried on neat NR and NR/NZG nanocomposites. Fig. 7 shows the variation of storage modulus (E′) and loss factor (tan δ) at 3 Hz as a function of temperature (−135–75 °C) for NR nanocomposites. The E′ of NR nanocomposites is generally improved with increasing NZG content (>1.0 phr) at low temperature ranges, which shows that NZG can effectively reinforce the NR. With increasing temperature, the E′ suddenly drops down by 3 orders of magnitude, corresponding to the glass–rubber transition of NR chains. This drop can be assigned to an energy dissipation phenomenon.^19,44 It is apparent that the maximum value of tan δ shows a decrease with increasing NZG content and the location of the peak shifts to a higher temperature, which reflects the restricted rubber chains due to the strong interfacial interaction between GE and rubber.^36,44 Additionally, the decrease in the maximum value of tan δ was positively correlated with the enhanced interfacial interaction.⁴⁵ To further evaluate the interfacial interaction between GE and the NR matrix, the relationship between the tan δ values of neat NR and NR/NZG nanocomposites can be calculated by using the following equation.⁴⁶
where tan δ and tan δ_m represent the maximum value of tan δ for neat NR and NR/NZG composites, respectively. B is an interaction parameter, the larger the value, the stronger the interfacial interaction between NR matrix and GE. Φ is the volume fraction of GE.

Fig. 7 Storage modulus (a), loss factor (b) and interaction parameter (c) of neat NR and NR nanocomposites with different NZG contents.

According to the temperature dependence of tan δ of NR nanocomposites with different NZG contents shown in Fig. 7b, the maximum value of tan δ of the NR/NZG nanocomposites is decreased with increasing NZG content. And the evolution of B with NZG content is plotted in Fig. 7c. Noticeably, the B value is increased with increasing NZG content, suggesting consistently enhanced interfacial interactions.

Observations have shown that the GE nanosheets can be well dispersed through the NR matrix and a strong interfacial interaction between GE and NR matrix has been evidenced. We expected that NR/NZG nanocomposites should have potential applications in gas-resistant materials. The nitrogen permeability of NR/NZG and NR/GE nanocomposites as a function of GE content is shown in Fig. 8a. It clearly exhibits a significant decrease in the permeability of NR/GE nanocomposites with increasing NZG content. The permeability is decreased by 45% for the NR/GE-0.625 nanocomposite in comparison with that of neat NR. It is also worth mentioning that a more significant decrease in the permeability of the NR/NZG nanocomposites is observed with the inclusion of NZG. For example, the permeability of NR/NZG-2.5 at a GE content of 0.625 phr dramatically decreased by 53% compared to that of neat NR.

Fig. 8 (a) Nitrogen permeability of NR/NZG and NR/GE nanocomposites normalized by that of neat NR. (b–d) The model of gas flow for neat NR (b), NR/NZG (c) and NR/GE nanocomposites (d). The upper inset in (a) shows the absolute values of nitrogen permeability.

It is believed that the reductions in the gas permeability of NR nanocomposites can be largely associated with the dispersion of GE nanosheets and the interfacial interactions. Therefore, on the basis of the observed morphologies of the GE nanosheets (Fig. 3), a model was proposed to present the network for gas flow in NR nanocomposites, as shown in Fig. 8b–d. It clearly explained that the NZG exhibited higher enhancement in its gas barrier property in comparison with GE. As seen from Fig. 8a, neat NR has high gas flux across the matrix without the inclusion of layered materials. With the incorporation of NZG and GE into the matrix, a tortuous diffusion path is formed owing to the high aspect ratio of GE as shown in the schematic diagram of Fig. 8b and c. Nevertheless, the HRTEM results show that the GE in NR/NZG nanocomposites exhibits a better dispersion state in comparison with that in NR/GE nanocomposites. The good dispersion of GE is the most significant factor contributing to a reduced cross section and a tortuous diffusion path,^47,48 greatly interfering with the gas transport throughout the matrix. Importantly, the strong interfacial interaction also plays a key role in lessening the free volume at the interface, which is conducive to reduction in the gas permeability.⁴⁹ Additionally, the gas permeability of graphene, graphene oxide, and graphite derivative/elastomer nanocomposites is listed in Table 2. Compared with those values reported in the literature for elastomer–graphene nanocomposites, NZG is potentially a good candidate for gas-resistant materials with high performance.

Table 2 Gas permeability of graphene, graphene oxide, graphite derivative/elastomer nanocomposites

Elastomer	Filler	Content	Processing	Permeant	Relative reduction (%)
NR^47	SGO	0.3 (wt%)	Solution	Air	48
SBR^50	GE	3 (phr)	Latex compounding	Oxygen	67.2
NBR^51	EG	5 (phr)	Solution	Nitrogen	38
TPU^52	TRG	1.6 (vol%)	Melt	Nitrogen	52
XNBR^31	GO	1.9 (vol%)	Latex compounding	Nitrogen	55
NR^42	rGO	1 (phr)	Latex compounding	Oxygen	36

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4 Conclusions

Graphene nanosheets decorated with ZnO nanoparticles (NZG) were successfully synthesized by a facile solvothermal method in this study. Characterizations demonstrated that ZnO nanoparticles (nano-ZnO) are successfully anchored onto graphene (GE) sheets. Simultaneously, NZG as a substitute for conventional ZnO was incorporated into the NR matrix and the influence of NZG on the vulcanization characteristics, dynamic and static mechanical properties and the gas permeability of the obtained NR/NZG nanocomposites was investigated. The results showed that a low NZG content gives a higher vulcanization efficiency and much stronger reinforcement effect on the mechanical performance and gas barrier properties of NR nanocomposites compared with GE, nano-ZnO and 5 phr conventional ZnO, the results of which are positively correlated with the good dispersion of GE throughout the NR matrix, the enhanced interfacial interaction between the GE and the matrix, and the high vulcanization efficiency of nano-ZnO. Accordingly, NZG at a lower content is very competitive for preparing rubber composites with high performance, and may be regarded as a substitute for 5 phr conventional ZnO for application in rubber composites.

Acknowledgements

The authors gratefully acknowledge “National Basic Research Program of China (no. 2015CB654700 (2015CB654703))”, “National Natural Science Foundation of China” (U1134005), “Strategic New Industry Core Technology Research Project of Guangdong Province” (2012A090100017) and “National Undergraduate Training Programs for Innovation” (201410561037) for the financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07582c

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