Multi-layer graphene oxide synergistically modified by two coupling agents and its application in reinforced natural rubber composites

Multi-layer graphene oxide (MGO) was co-modified with bis-(P,P-bis-ethylhexyldiphosphato)-ethanediolato titanate triethanolamino chelate solution (NDZ-311w) and bis-(γ-triethoxysilylpropyl)-tetrasulfide (Si-69). Then the co-modified MGO was incorporated into natural rubber (NR) by conventional two-roll mill mixing to prepare MGO/NR composites. The large macromolecule of NDZ-311w is able to efficiently intercalate the layers and increase the interlamellar space of MGO, subsequently resulting in the exfoliation of MGO into thinner sheets with better dispersity. Moreover, the oxygen-containing polar groups of MGO can be largely consumed by Si-69, which enhances the interfacial interaction between MGO and the NR matrix and improves the mechanical properties of the MGO/NR composites. Compared to pure natural rubber, the tensile strength, the stress at 300% strain, and tear resistance of co-modified MGO/NR composites are increased by 26%, 98% and 15%, respectively.


Introduction
Natural rubber (NR), as an important biopolymer, has excellent chemical and physical properties, for example, high elasticity at room temperature. However, NR usually needs to be reinforced with enhancing llers to improve its shortcomings, such as poor strength, low modulus, and poor wear resistance. Graphene oxide (GO) is one of the new two-dimensional reinforcing and functional llers for rubber nanocomposites, due to ultrahigh mechanical strength, large surface area, low density, and high thermal conductivity. [1][2][3][4] The performances of GO/NR composites can be inuenced by many factors. The dispersion of GO and the compatibility of GO with a natural rubber matrix are the two important research directions for the preparation of GO/NR nanocomposites.
GO/NR nanocomposites have gained wide attention. Many scholars have used various kinds of surfactants or coupling agents to improve the dispersion of GO [5][6][7][8][9] and the interface interaction between GO and the NR matrix. [10][11][12][13][14] Ma et al. 15 modied GO with a silane coupling agent by solution blending, which improved the dispersion of GO in silicone rubber. The results showed that the mechanical properties and thermal properties of GO/silicone rubber composites were greatly improved. Zhan et al. 16 prepared natural rubber/graphene (NR/GE) composites by an ultrasonically-assisted latex mixing and in situ reduction process.
GO was dispersed in natural rubber latex using an ultrasonic eld and then in situ reduced, followed by latex coagulation to obtain the NR/GE masterbatch. This process produced much better dispersion and exfoliation of GE in the matrix and contributed to an increase in the tensile strength. Compared with pure rubber, the tensile strength and tear strength for NR/GE composites were increased by 47% and 50%, respectively. Li et al. 17 incorporated GO that was modied with two differently terminated silane coupling agents into an epoxy resin to prepare nanocomposites. The results showed that the Young's modulus and tensile strength of amino-functionalized GO/epoxy (APTS-GO/epoxy) composites were greatly improved, and the fracture toughness and fracture energy of epoxy-functionalized GO/epoxy (GPTS-GO/ epoxy) composites were nearly doubled at 0.2 wt% epoxyfunctionalized GO loading.
In this paper, multilayer graphene oxide (MGO) was co-modied by NDZ-311w and Si-69, which takes advantage of the two coupling agents to create a synergistic modication method. The oxygen-containing functional groups of MGO are depleted by the coupling agents, and the hydrophobicity is largely enhanced. MGO synergistically modied by NDZ-311w and Si-69 can be more effectively stripped and the compatibility between the co-modied MGO and NR matrix is largely improved. Then, the MGO/NR masterbatch was fabricated by mixing the modied MGO aqueous dispersion with NR latex, followed by coagulation. Aer that, MGO/NR composites were obtained by introducing MGO/NR masterbatch into NR matrix and then vulcanization. The test results indicate that the tensile strength, modulus at 300% strain and tear resistance of co-modied MGO/NR composites are greatly improved by the introduction of MGO.

Preparation of modied MGO
Firstly, the ethanol solutions of coupling agents with a 5 wt% concentration were prepared at room temperature, and the pH value of the solutions was adjusted to 5.0 by using glacial acetic acid. Then, a certain amount of MGO was added to the 90 vol% ethanol solution to prepare the MGO suspension through ultrasonic treatment for 45 min. Aerwards, the ethanol solution of coupling agent was added into the MGO suspension, followed by magnetic stirring at 60 C for 4 h. Finally, the solid content was separated from ethanol solution by vacuum ltration and then washed with ethanol solution at least three times to remove excess reactants. The modied MGO powder was obtained by drying in a vacuum oven at 80 C for 12 h. MGO-N, MGO-S and MGO-N-S were modied with NDZ-311w, Si-69, and both coupling agents, respectively.

Preparation of modied MGO/NR nanocomposites
The modied MGO powder was dispersed in water by ultrasonic treatment to produce the modied MGO suspension and then a certain amount of the aforementioned suspension was dropped in NR latex with stirring for 1 h, followed by the coagulation with adding 2 wt% CaCl 2 solution. The coagulation isolated by ltration was further dried under oven at 60 C for the constant weight. Subsequently, the masterbatch, NR matrix and all other agents were mixed by an open two-roll mill at room temperature, according to Table 1. As control experiments, unmodied MGO/NR and pure NRL/NR composites were prepared by the same process. All the samples were cured at 143 C up to their optimum cure time (t 90 ) with 10 MPa, and the vulcanized samples were stored at room temperature for at least 24 h before testing.

Characterizations
Fourier transform infrared spectrometer (FTIR, Nicolet 6700, Thermo Scientic Co. Ltd., USA) was used to analyze the functional groups of the modied MGO. X-ray photoelectron spectrometer (XPS, K-Alpha, Thermo Scientic Co. Ltd., USA) was applied to analyze the chemical elements of the modied MGO. The crystalline structure of the modied MGO was analyzed by a X-ray diffractometer (XRD, PANalytical B.V Co. Ltd., Netherlands) with Cu Ka radiation under a voltage of 40 kV and a current of 30 mA. Atomic force microscope (AFM) images was taken by a Nanoscope III D Multimode scanning probe microscope (Dimension ICON, Bruker Co. Ltd., USA) in a tapping mode. The modied MGO dispersion was coated onto a freshly exfoliated mica substrate and dried at room temperature to prepare the testing samples. Dynamic mechanical performance analysis was carried out on a rubber process analyzer (RPA 2000, Alpha Technologies, USA) with mixed rubber. All tests were conducted by using a strain sweep test with monitoring strain from 0.7% to 400% at 1 Hz frequency with 60 C. A universal testing machine (Hegewald & Peschke Co. Ltd., Germany) was used to measure the tensile and tear properties of the MGO/NR composites at a uniform crosshead speed of 500 mm min À1 according to on GB/T 528-1998 (tensile property) and GB/T 529-1999 (tear resistance), respectively. The reported values including tensile strength, elongation, stress at 300% strain, stress at 100% strain and tear strength were recorded as the averages of ve tests. The morphologies of modied MGO and MGO/NR composites samples were observed using a JSM-7500F scanning electron microscope (SEM) (JEOL Ltd., Japan).

Characterizations of modied MGO
To shed light on the interaction between MGO and coupling agents, FTIR spectra were used to detect the functional groups of the modied MGO, which is shown in Fig. 1. In the spectrum of MGO, the broad peak at 3408 cm À1 results from the stretching vibration of -OH, while sharp peaks at 2977 cm À1 , 1715 cm À1 , 1620 cm À1 , 1384 cm À1 and 1048 cm À1 can be  In addition, the stretching vibration of -OH at 3408 cm À1 has been reduced, which demonstrates that the hydrophobicity of MGO-N-S is greatly improved. In order to further understand the interactions between coupling agents and MGO, XPS has been used to detect the surface chemical changes of MGO. Fig. 2 shows XPS survey spectra of the MGO samples. As shown in Fig. 2 The XPS C 1s peaks were tted by a multipeak Lorentzian tting program (XPS peak) which are shown in Fig. 2    The MGO sheet is difficult to disperse because of the strong van der Waals force and electrostatic force between the sheets of graphite. 29,30 The MGO-N shows a weak and broad diffraction peak at 2q ¼ 8.9 , assigned to the basal spacing of 0.99 nm. This phenomenon illustrates that the huge molecules of NDZ-311w destroy the crystal structure of MGO and insert the MGO interlayer. An additional peak at 19.6 suggests that the MGO cannot be fully exfoliated by NDZ-311w. 31,32 The sharp diffraction peak around 9.8 for MGO-S shows that the basal spacing of MGO was 0.90 nm, which illustrates that Si-69 can not effectively exfoliate MGO. MGO-N-S does not shown any obvious diffraction peak, indicating that MGO-N-S is in an exfoliated state due to the synergistic modication of NDZ-311w and Si-69. 33,34 The morphology of MGO and MGO-N-S has been analyzed by AFM. Fig. 4 presents the tapping-mode AFM photos and the corresponding height proles. The results show that the average thickness of MGO and MGO-N-S is 370 nm (Fig. 4(a)) and 54 nm (Fig. 4(b)), respectively, which indicates that MGO can be exfoliated into thinner GO sheets by the synergistic modication of NDZ-311w and Si-69. Moreover, the size of MGO is largely decreased from more than 10 mm to about 4 mm. In combination with the FTIR, XPS and XRD results, this phenomenon can be explained by that large macromolecule of NDZ-311w is able to efficiently intercalate the layers and increase the interlamellar space of MGO, and Si-69 can consume the oxygencontaining functional groups of MGO and reduce the interaction between MGO lamellas. In consequence, the exfoliation of MGO into relatively thin GO sheets is achieved as expected.
SEM has been applied to assistantly characterize the morphology of MGO and modied MGO. As shown in Fig. 5(a), MGO sheets are accumulated as a chunk due to the strong interaction between MGO sheets, and hence, the dispersity of   Fig. 5(b-d), respectively. Obviously, the MGO is exfoliated into thinner and smaller GO sheets, which is consistent with the results of AFM.
All these above results conrm that we have obtained an expected exfoliated dispersion structure of MGO. The reinforcing efficiency of MGO in composites depends on not only the dispersion of the MGO sheets in the matrix but also the interface interaction between the MGO sheets and the matrix. Considering that the majority of the functional groups of MGO are carbonyl and carboxyl groups at the sheet edges and the sulde groups of Si-69 can be reacted with NR molecules, the Si-69 is introduced. And organic modier NDZ-311w is also introduced because its huge molecule can insert into the  Paper interlayer of MGO and entangle with NR molecular chains. The interface bridges are thus built between the MGO sheets and the NR matrix by chemical bonding and physical entanglement points, as illustrated in Fig. 6.
To reveal the reinforcing mechanisms, the morphology of the tensile fracture surface of vulcanized unmodied-and modied-MGO/NR composites was investigated by SEM. As shown in Fig. 7(a), MGO is aggregated in NR matrix and extracted from NR matrix, which states that the dispersity of MGO and the interface interaction between MGO and NR matrix are rather poor. As shown in Fig. 7(b), the thickness of MGO stacks is reduced due to the exfoliation of MGO into small MGO sheets aer the modication by NDZ-311w. However, NDZ-311w lacks chemical crosslinking points to react with NR molecular chains which can provide powerful interface interaction between MGO and NR matrix. As shown in Fig. 7(c), there are some stacks on the tensile fracture surface, but there is no obvious MGO pulled out of the tensile fracture surface. The reason is that MGO can not be effectively exfoliated into fewsheet by using Si-69, but Si-69 can provide chemical crosslinking points to react with NR molecular chains. As shown in Fig. 7(d), there is no stacked texture and obvious extracted MGO on the tensile fracture surface. This appearance suggests that the synergistic modication by NDZ-311w and Si-69 improves the dispersity of MGO and enhances the interface interaction between MGO and NR matrix. All the results indicate that NDZ-311w and Si-69 co-modication MGO can achieve perfect exfoliation and dispersion in MGO-N-S/NR composites, which is consistent with the results of XRD and AFM analyses.  composites show the minimal tan d, due to the synergistic modication of MGO by NDZ-311w and Si-69. When MGO is co-modied by NDZ-311w and Si-69, more coupling agents can be bonded onto the surface of MGO, which can provide larger numbers of entanglement and crosslinking points to restrict the mobility of the NR molecular chains. Fig. 9(a) displays the stress-strain curves of cured MGO/NR composites and pure NR. We notice that the tensile strength of MGO/NR composites is greatly increased due to a reinforcing effect of the MGO. However, there is a decrease in elongation at break of the composites, compared to pure NR material. MGO sheets well exfoliated by both NDZ-311w and Si-69 own pretty dispersion in NR matrix and enhanced interfacial interactions, which can cause the maximum stress transfer and the highest tensile strength. Mechanical properties including elongation at break, tensile strength, stress at 100% strain, stress at 300% strain and tear strength of MGO/NR composites and pure NR material are shown in Fig. 9(b). NDZ-311w can improve the elongation at break and tensile strength of the composites, and Si-69 can improve the stress at 100% strain, stress at 300% strain and tear strength of the composites. It is worth noting that the combining effect of NDZ-311w and Si-69 is higher than that of either NDZ-311w or Si-69 alone. The tensile strength, the stress at 100% strain, the stress at 300% strain and tear strength of MGO-N-S increase by as much as 26%, 38%, 98%, and 15%, respectively, over those of pure NR.
The excellent reinforcement by MGO sheets is not only due to the high degree of MGO sheets exfoliation in the rubber matrix, which allows a large contact area between the MGO sheets and the NR matrix, but also related to the strong interfacial interaction between MGO and NR matrix. To sum up, the improved mechanical properties are correlated with MGO homogenous dispersion, interfacial adhesion between MGO and NR matrix as well as synergistic modication of MGO by NDZ-311w and Si-69.

Conclusions
In this study, the synergistic reinforcing effect of NDZ-311w and Si-69 can effectively exfoliate MGO sheets, improve the dispersity of MGO in the NR matrix, and enhance the interfacial interaction between MGO and NR matrix. The modied MGO/ NR composites were prepared by the conventional two-roll mill mixing method. In the MGO-N-S/NR composites, the chemical bridge between MGO and the NR matrix is built through NDZ-311w and Si-69, and the interfacial strength of the composites is signicantly improved. The fabricated MGO-N-S/ NR composites exhibit signicantly better mechanical properties than the pure matrix. The tensile strength, the stress at 300% strain and tear strength of MGO-N-S are increased by 26%, 98%, and 15%, respectively.

Conflicts of interest
There are no conicts of interest to declare.