Sodium Humate Functionalized Graphene and Its Unique

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Sodium Humate-Functionalized Graphene and Its Unique Reinforcement Effects for Rubber Xuan Liu, Daqin Sun, Lanwei Wang, and Baochun Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 23 Sep 2013 Downloaded from http://pubs.acs.org on September 23, 2013

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Industrial & Engineering Chemistry Research

Sodium Humate-Functionalized Graphene and Its Unique Reinforcement Effects for Rubber

Xuan Liu a, Daqin Sun a, Lanwei Wang a, Baochun Guo*,a,b

a Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou, 510640, P, R, China.

b State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, P, R, China.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (BCG)

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ACS Paragon Plus Environment


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ABSTRACT: Sodium humate (SH), a cost-effective and environmentally friendly humic substance, was employed to non-covalently functionalize graphene. The functionalized graphene (SHG) could stably and individually be dispersed in water at very high concentration (up to 30 mg/ml). The stabilization mechanism of SHG colloid was revealed to be π-π interaction and hydrogen bonding between SH and graphene layer. SHG was incorporated into carboxylated nitrile rubber (XNBR) through latex co-coagulation to form XNBR/SHG composites. The composites were cured with sulfur and magnesium oxide (MgO). Unique reinforcement effect of SHG towards XNBR was found. For instance, with the incorporation of 1 phr of SHG, the fracture energy of the composite was doubled and the extensibility was improved, while the modulus was practically unchanged. The unique combination of high fracture energy, high extensibility and low modulus was correlated to the interactions between SHG and MgO which pronouncedly affected the crosslinking of the rubber.

KEYWORDS: graphene, sodium humate, concentration, reinforcement, rubber composite 2


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INTRODUCTION Graphene, a one-atom-thick sp2 carbon has drawn tremendous attention in recent years due to its exceptional high electrical conductivity1, thermal conductivity2 and unparalleled mechanical properties3. To fully transport its fascinating properties into polymeric materials, graphene or its derivatives (such as graphene oxide) have been incorporated into a wide variety of polymer matrix such as polystyrene4, poly(methyl methacrylate)5, epoxy resin6, poly(vinyl chloride)7, natural rubber8, styrene-butadiene rubber9 and so on. In such kind of systems, significant improvements on mechanical properties, electrical conductivity10 as well as thermal conductivity11 were reported.

The strong van der waal force and π-π interaction between graphene sheets have been demonstrated to be the persistent obstacles for scalable preparation and practical use of graphene, especially in the preparation of polymer/graphene composites. Many strategies for graphene functionalization have thus been developed to meet this challenge. Among the reported strategies, the chemical functionalization of chemically reduced graphene oxide (RGO) has been considered to be especially promising as the precursor, GO, exhibits extreme versatility in chemical derivatizations both covalently and noncovalently. Compared with covalent method, non-covalent method is less complicated in preparation and the functionalization process may better retain the inherent properties of graphene12. In addition, the functionalities introduced by the decorators offer the opportunities for the subsequent interfacial design in the composites. In recent years, a great variety of chemicals such as rhodamine B12a, florescent white agent13, hydrolysable tannin14, green tea polyphenol15, sulfonated polyaniline16, 1-pyrenebutyrate17 and ionic liquid18 have been utilized to 3



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non-covalently functionalize the RGO. Still, these reported functionalized RGO suffered from several undesired characteristics such as limited colloidal concentration and high cost. Consequently, exploitation of cost-effective and high concentration graphene colloid is still of great interests for the polymer/graphene composite.

Compared with other polymer/graphene composites, rubber/graphene composites have less been reported. Noticeably, although remarkably increased modulus and strength have been reported in many rubber/graphene systems, these composites were generally suffered from drastically deteriorated extensibility19. For instance, Potts et al.20 prepared RGO/NR composite through latex co-coagulation method followed by a milling process. Only 2 wt% of graphene led to 50% loss in elongation at break and the one without milling process even exhibited plastics-like tensile behavior. Bai et al.21 prepared GO/carboxylated nitrile rubber composites through latex co-coagulation process. In their system, about 20% decrease of break strain with only 0.44 vol% GO was observed. The strength and extensibility of rubber composite are determined by the dispersion state of filler and the interfacial interaction between filler and rubber chains. If the distance between two filler particles is smaller than the “critical distance” and the interaction between filler and rubber chains is strong enough so the external stress can be dissipated through alignment and slippage of rubber chains that adsorbed on the surface of filler22. Zhang et al.23 reported that high crosslink density reduced the conformational freedom and mobility of rubber chains which was detrimental to the orientation and slippage of the rubber chains which are essential in rubber reinforcement. Therefore, for better mechanical properties, over high crosslink density is usually not desired in the rubber composites. Unfortunately, the incorporation of graphene layers always 4


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introduce abundant physical crosslink24 and consequently largely reduced extensibility was usually observed25. As a consequence, introduction of strong graphene-rubber interaction for better dispersion state and regulation of the crosslink network structure for better extensibility are crucial for the rubber/graphene composites with combination of high strength and high extensibility.

Humic acid (HA), the degradation product of rotten biomass, is widely spread in nature. HA is non-toxic and has been widely used to prepare organic compound fertilizer with sodium, potassium, phosphorus etc. The compound fertilizer based on HA can significantly improve the quality of soil and promote the growth of crop. So, HA is an environmentally friendly material. Previously, HA has been demonstrated to be adsorbed stably on the surface of carbon nanotubes26 and GO27 via secondary forces. In this study, we used the cost-effective and environmentally friendly sodium humate (SH), a derivative from HA, as the stabilizer for graphene reduced from GO by hydrazine hydrate. The microstructures of SHG colloid were fully characterized and the stabilization mechanism was revealed. The SHG colloid was then co-coagulated with latex of carboxylated nitrile rubber to form XNBR/SHG composites. The effects of SHG on the structure and properties of the composites were investigated.

EXPERIMENTAL

SECTION

Raw Materials. Natural graphite powder (95%) was purchased from Shanghai Colloid Chemical Factory (China). Humic acid (organic substance >70%) was supplied by Jun Jue Chemical Co.Ltd (Guangzhou, China). Carboxylated nitrile rubber latex 5



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(44 wt% of solid content, 26 wt% ~ 28 wt% of acrylonitrile content) was obtained from Hong Tai rubber Co.Ltd (Shijiazhuang, China). The chemicals for oxidation of graphite are of analytic grade and used as received. Rubber additives are of industrial grade and used as received.

Preparation of functionalized graphene (SHG). The GO was prepared by oxidation of natural graphite powder with a modified Hummer’s method28. SHG was prepared by functionalizing GO with SH, followed by reduction process with hydrazine hydrate. Briefly, 1 g of GO was fully exfoliated in 500 ml of deionized water under sonication. Then the colloid was centrifuged at 8000 rpm for 3 min to remove any impurities. SH solution was prepared by neutralization of HA with sodium hydroxide. The SH solution containing 5 g of HA was then added to the as-prepared GO colloid. The mixture was treated with 1 ml of hydrazine hydrate (80 wt%) for 12 h at 80 oC. After the reduction, the suspension was centrifuged at 14000 rpm for 40 min for several times to remove the excessive SH. To characterize the microstructure of SHG, a piece of SHG paper was prepared by filtering the washed SHG colloid through a nylon membrane (0.22 µm) assisted by vacuum.

Preparation of XNBR/SHG composites. A designed amount of SHG colloid was added into the XNBR latex (solid content of 20 g) dropwise under vigorous stirring. The mixture subsequently stirred for at least 3 h before co-coagulating with calcium chloride solution (5 wt%) as the flocculant. The obtained compound was cut into small pieces and washed with deionized water several times until no chloride ions could be detected. The washed compound was dried under vacuum at 60 oC for 12 h. The obtained XNBR/SHG compound was then mixed with the ingredients on a conventional two-roll mill. The basic recipe for the 6


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compound is as follows, XNBR, 100 phr; sulfur, 1.5 phr; MgO, 5 phr; stearic acid, 1 phr; N-isopropyl-N'-phenyl-p-phenylene diamine (4010 NA), 2 phr; 2,2'-dibenzothiazole disulfide (DM), 1.5 phr; SHG, variable. These vulcanizates abbreviated as “GE-x” in the belowing. The unknown number x denotes SHG content (phr) in composites.

Characterizations. X-ray photoelectron spectroscopy (XPS) was received on a Kratos Axis Ultra DLD X-ray photoelectron spectroscope (UK) with Al Kα radiation of 1486.6 eV. X-ray diffraction (XRD) was carried out on a Bruker D8 Advanced X-ray diffractometer (Germany) with Cu-Kα radiation (λ = 0.1542 nm). The operated voltage and current is 40 kV and 40 mA, respectively. Thermogravimetric analysis (TG) was conducted on a TGA Q5000 instrument under nitrogen purging at a heating rate of 10 oC/min. Atomic force microscopy (AFM) were obtained from a Veeco Multimode V scanning probe microscope in a tapping mode. The sample was prepared by drop the dilute solution on a freshly cleaved mica. Field emission scanning electron microscopy (FESEM) was conducted on a Nova nano SEM 430 scanning electron microscope (Germany). Fourier transform infrared spectra (FTIR) was performed on a Bruker Vertex Fourier transform infrared spectrometer with attenuated total reflectance (ATR) mode. Raman spectra was collected on a LabRAM Aramis Raman spectrometer (HO RIBA Jobin Yvon, France) with an He-Ne ion laser (514.0 nm) as the source. UV-vis spectra was received from a Univo 4820 UV-vis dual beam spectrometer. High resolution transmission electron microscope (HRTEM) images were received from a JEOL2100 microscope. The sample was prepared by cutting with a ultramicrotome under cryogenically condition then placed on a copper grid. The curing parameters were determined by a U-CAN UR-2030 vulcameter (Taiwan) at 160 oC. The mechanical properties were all 7



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measured by U-CAN UT-2060 instrument (Taiwan). Dynamic mechanical analysis (DMA) was conducted on an EPLEXOR dynamic mechanical analyzer (Germany) in tensile mode. The frequency and heating rate were set as 5 Hz and 3 oC/min, respectively. Crosslink density was determined by equilibrium swelling experiment based on the Flory-Rehner equation using benzene as the solvent29. Briefly, the samples were first swollen to equilibrium in benzene to determine the total crosslink density (Ve1). The mixture consisting of benzene and dichloroacetate (weight ratio of 9:1) was employed to destroy the ionic crosslinks30. The sample was then swollen by benzene again to determine the remained crosslink density (Ve2). The ionic crosslink density can be calculated by subtracting Ve2 from Ve1. The fracture energy (K) of the composites is defined as the area surrounded by typical stress-strain curves and calculated using the following equation13: K= Where ε is the strain (%), σ is the stress (Pa), ρ is the density of composite (g/m3). RESULTS AND DISCUSSION Microstructures of SHG and the stabilization mechanism. To substantiate the individual dispersion of SHG in water, AFM experiments were performed. As depicted in Figure 1 (a) and (b), the GO sheets are fully exfoliated with lateral size ranging from several hundred nanometers to several micrometers and an average thickness about 1 nm, which in well agreement with other reports15, 31. Compared with GO, SHG sheets show similar lateral size, but possesses a larger thickness of about 1.74 nm. The increase in thickness is ascribed to the adsorbed SH molecules on both sides of graphene sheets. Considering that SH 8


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molecules are adsorbed onto the surface of RGO sheets and the intrinsic thickness of a naked graphene sheet, one may expect that most of these sheets are single layers. The SHG paper (Figure 1(d)) prepared by vacuum-assisted filtration exhibits densely arranged lamellar structure which also implies excellent dispersion state of the colloid. The comparison of XRD patterns of GO and SHG was made in Figure 1 (c). In the patterns of GO, the typical (002) diffraction peak at 26.5o (interlayer spacing of 0.335 nm) for natural graphite is completely disappeared and the (001) diffraction peak around 9.7o (interlayer spacing of 0.91 nm) characterizing GO stacking is found32. SHG exhibits two diffraction peaks. One peak is around 24.1o which is related to the interlayer space between SH and graphene17 (d-spacing of 0.37 nm). As SH molecules cover both the sides of graphene sheets in an offset face-to-face manner, considering the thickness of single-layer chemically reduced graphene (typically 1 nm)17, the thickness of single layer SHG can be estimated to be about 1.74 nm, which in accordance with the value obtained by AFM. The other peak locates around 3.8o is due to the intercalation of SH molecules among stacked graphene layers13, 33 (d-spacing of 2.32 nm). One of the most attractive characteristics of SHG colloid is the long-term stability and high concentration. We found SHG can be stably dispersed in water for long time (for example over 1 month). The photographs of GO, RGO (reduce directly without adding SH) and SHG suspensions are compared (inset of Figure 1 (d)). These suspensions with concentration about 5 mg/ml were stood for 1 month. As expected, GO can be stably dispersed in water due to the intrinsic hydrophilicity and RGO forms irreversible aggregates due to the strong interlayer π-π stacking. For SHG, however, no any aggregates are observed, suggesting long-term stability of SHG colloid. Surprisingly, the quantified maximum 9



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concentration for SHG colloid is up to about 30 mg/ml, which is significantly higher than the previously reported data34

Figure 1. AFM images of GO (a); and SHG (b); XRD patterns of GO and SHG (c); FESEM image of SHG paper (inset: suspensions of GO, RGO and SHG) (d)

TGA experiments were conducted to quantify the remained SH on SHG (Figure 2 (a)). It can be seen that GO losses 12% of its weight before 150 oC due to the evaporation of adsorbed water. The overall weight loss for GO is about 57%, which is mainly ascribed to the removal of the oxygenic moieties. For RGO, only 16% weight loss is observed, which is attributed to the removal of the residue oxygen-containing groups after treating with hydrazine hydrate. For SHG, the overall weight loss is about 40%. Considering the weight loss of RGO, it can be concluded that the SH content in SHG is about 24 wt%. It should be noticed that SHG also exhibits weight loss (about 10 wt%) before 150 oC similar to the 10


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weight loss of GO. As a consequence, it is reasonable to conclude that the quantity of SH molecules adsorbed on graphene sheets is less than 24 wt%. The very high concentration of SHG colloid (30 mg/ml) achieved with such low weight ratio of stabilizer/graphene is considered to be unusual. Such efficiency of SH in suspending graphene is definitely unprecedented compared with other reported non-covalent stabilizers12b,

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. XPS

measurement was conducted to further investigate the microstructure of SHG. As shown in Figure 2 (b), the deconvoluted C 1s curve of GO contains four types of carbon atom, namely sp2 hybridized carbon (284.5 eV), sp3 hybridized carbon (285.5 eV), carbonyl carbon (287.8 eV) and carboxyl carbon (289 eV)28, 32, 36. The peak intensities of these carbon atoms binding to oxygen in SHG are much weaker than those in GO, suggesting most of the oxygenic groups has been removed after reduction. In addition, a new peak with bind energy of 286 eV is clearly observed which assign to C-N as a result of reducing with hydrazine hydrate16, 28.

Figure 2. TGA curves of GO, RGO and SHG (a); XPS C1s spectra of GO and SHG (b)

Based on the individual dispersion of SHG colloid and its long-term stability as well as high concentration, SH is demonstrated to be an environmentally friendly and effective 11



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stabilizer for graphene sheets. Although there is no definite molecular structure for HA substance, the molecular model of SH could be illustrated (Scheme 1). Considering the conjugated structures bearing carboxyls and hydroxyls in SH molecule, it is proposed that the hydrogen bonding and π-π interaction between SH molecules and graphene sheets are responsible for the stabilization of SHG colloid. The UV-vis spectra of GO, RGO, SH and SHG are compared in Figure 3 (a). GO exhibits an obvious peak around 232 nm and a shoulder around 302 nm, which are corresponded to π-π* transitions of aromatic C=C bonds and n-π* transitions of C=O bonds, respectively. In the spectrum of RGO, the peak for the aromatic C=C bonds is red-shifted to 267 nm, indicating the electronic conjugation of the graphene sheets is restored after the reduction14. For SHG, a new peak is observed at 278 nm derived from the overlap of the absorption at 267 nm for RGO and 283 nm for HA. the n-π* transition locates at 283 nm for HA blue shift to 278 nm for SHG demonstrating the existence of π-π interaction between SH and graphene sheet14.

Raman spectra was employed to further characterize the microstructure of SHG .The D band concentrates about 1350 cm-1 is attributed to the vibration of sp3 hybridized carbon and disordered structure in graphite lattice. The G band locates about 1600 cm-1 is ascribed to the vibration of sp2 hybridized carbon atom. The intensity ratio of D band and G band (ID/IG) is often employed to reflect the defects of graphene and monitor functionalization of graphene. As illustrate in Figure 3 (b), ID/IG of RGO (1.06) and SHG (1.05) are higher than the value of GO which in consistent with other reports12a, 13. The increased ID/IG value for RGO and SHG implying the restoration of conjugated structure with smaller averaged area but more numerous in number comparing to GO17, 37. Compared with RGO, the G band of SHG is blue 12


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shifted to 1588 cm-1, providing additional indication of the π-π interaction between SH and graphene sheets13.

Figure 3. UV-vis spectra of GO, RGO, HA and SHG (a); Raman spectra of GO, RGO, and SHG (b)

Scheme 1. Molecular model of sodium humate.

Structures and performance of XNBR/SHG composites. The unprecedented high concentration and high stability of SHG colloid make SHG especially promising in the fabrication of polymer/graphene composites. Accordingly, we prepared a series of 13



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XNBR/SHG composites with different SHG content to investigate the reinforcement efficiency of SHG towards XNBR. XNBR was adopted as it is a commercially available polar rubber and was expected to be compatible with SHG which also possesses polar surface characteristics. For better dispersion, the combination of latex co-coagulation and two-roll milling process was utilized to prepare the XNBR/SHG composites19a, 20.

It is well known that the dispersion state of the reinforcement and the interfacial interactions are two crucial factors in governing the performance of the composite. FESEM and HRTEM were employed to evaluate the dispersion state of the XNBR/SHG composites. The FESEM images of cryogenically fractured surfaces for GE-1 and GE-5 are compared in Figure 4 (a) and (d). The observed white grains are the excessive rubber additives. Both samples exhibit rough fractured surfaces with convex structure. These convex structures are derived from the introduced SHG sheets entrapped by rubber chains, which is similar to the other reported systems of rubber/graphene composites21,

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Compared with GE-1, GE-5

possesses a coarser surface with obvious lamellar structures due to higher concentration of SHG in GE-5. Importantly, no aggregates are observed for both vulcanizates and the vague interface between SHG and XNBR indicates fairly strong interfacial interactions in these composites. HRTEM was used to directly visualize the dispersed SHG sheets in matrix. It can be seen that in the composite with low SHG content (GE-1, Figure 4 (b) and (c)), most of the dispersed SHG sheets remain as single layers, with the thickness about 2 nm, which is consistent with the AFM result. Obviously, with the incorporation of higher content of SHG (GE-5), the dispersed SHG sheets are much denser. In addition, apparent stacking of SHG sheets is observed (Figure 5 (e) and (f)). This is attributed to the decreased distance between 14


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SHG sheets in latex at high SHG content so the partial aggregation of SHG sheets can not be forbidden due to the faster aggregation rate of SHG sheets than latex particles when flocculant is added20. But it should be noticed that the stacked sheets exhibit a thickness less than 20 nm.

Figure 4. FESEM images and HRTEM images of GE-1 (a) (b) (c); GE-5 (d) (e) (f).

XRD was also employed to analyze the dispersion of SHG and the results are illustrated in Figure 5 (a). Considering XNBR exhibits a diffused peak around 20o for its amorphous structure which may overlap with the (002) diffraction of SHG. The peak concentrates at 3.8o characterizing SHG stacking is employed to characterize the dispersion state. It can be seen that no obvious diffraction is observed for all XNBR/SHG composites around 3.8o, suggesting excellent dispersion of SHG in XNBR21, 39. Due to the sampling depth of XRD (typically tens of nanometers), the stacking of SHG sheets in the sample with higher SHG loading, as reflected in HRTEM, is not observed. FTIR was conducted to investigate the interfacial interactions between SHG and XNBR. As illustrate in Figure 5 (b), the peak 15



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locates at 3412 cm-1 for XNBR and SHG is assign to stretching vibration of –OH. Obviously, this peak is red shifted to 3395 cm-1 in the spectrum of GE-7, which may be due to the hydrogen bonding between XNBR chain and SHG sheet. Due to the residue carboxyls on SHG, the absorption peaks locates at 1730 cm-1 in the spectrum of SHG is assigned to stretching vibration of C=O of -COOH. The peak around 1577 cm-1 could be attributed to the overlapped signals from C=O of –COO- and benzene skeleton vibration. In the spectrum of XNBR, the stretching vibration of C=O of COOH around 1730 cm-1 is also found. The carboxyls distributed along the XNBR chains exhibit reactivity toward MgO to form carboxylates which act as ionic crosslinks40. In addition, the carboxyls on SHG sheets which introduced by adsorbed SH molecules may also react with MgO to form carboxylates. The ionic bonding between XNBR chains and SHG sheets which connected through Mg2+ behave as the interfacial crosslinks. Both of the reactions can convert –COOH into –COO-41. Expectedly, After curing, the peak for C=O in –COOH is disappeared. the signal for carboxylate around 1577 cm-1 is stronger. The interfacial interactions between SHG sheet and XNBR chain are also considered to be originated from the chelation between catechol moieties of SH molecule and MgO. Actually the chelating capability of humic substances towards metal ions have been widely reported42

16


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Figure 5. XRD pattern of XNBR/SHG composites (a); FTIR spectrum of pure XNBR, SHG and GE-7 (b)

The interfacial interactions was further investigated by dynamic mechanical analysis (DMA).The temperature dependence of mechanical loss factor tan δ was plotted in Figure 6. two peaks are observed in all curves. The one locates at about -5 oC correspond to the glass transition process. The other locates about 55 oC is related to the ionic clusters, which derived from the aggregation of carboxylates. Such ionic clusters act as microphases lead to a relaxation above Tg called ionic transition43. Accord with most graphene/rubber composites, the decreased peak area of tan δ in Tg and the location of the peak shifts to higher temperature reflect the restricted rubber chains due to strong interfacial interaction between graphene and rubber22b, 38. Briefly, Tg of GE-7 is 2.7 oC higher than GE-0. At the local magnification images the peak correspond to ionic transition not only displace to higher temperature but also become broaden with the increase content of SHG. The large increment of Ti (the temperature correspond to the peak of ionic transition) between GE-0 and GE-7 (increase by 11.6 oC) indicates strong interfacial crosslinking and consequent restriction on 17



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the chain mobility by SHG sheets. The broaden transition for GE-7 in relative to GE-0 implies that incorporation SHG lead different units in wider relaxation time range. The aggregation of carboxylates lead to the formation of ionic clusters with different size in the matrix. Undoubtedly, the introduced interfacial crosslink by SHG and XNBR will influence the formation of ionic clusters, leading to much wider relaxation spectrum in GE-7.

Figure 6. Tan δ of XNBR/SHG composites as a function of temperature

Scheme 2. The crosslinking structures of XNBR/SHG composites.

As the interactions between SHG and MgO would consume part of MgO, it is believed that the crosslinking of the matrix will be influenced by the presence of SHG. Therefore 18


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equilibrium swollen experiment was adopted to determine the crosslink density. The value of total crosslink density and ionic crosslink density versus SHG content are plotted in Figure 7. The crosslinking structures of XNBR/SHG composites are schematically illustrated in Scheme 2. In general, improvement on crosslink density is observed by incorporating graphene into rubber matrix due to the introduced physical crosslink22c, 24, 44. But in this study, it is of interest to see both total crosslink density and ionic crosslink density exhibit a reversed parabola’s trend with the increase content of SHG. The interfacial interactions and filler dispersion should be taken into consideration to disclose the origins of such peculiar phenomenon. The combination of co-coagulation and milling process ensure the homogeneous and individual dispersion of SHG sheets when SHG content is low. The introduced SHG sheets can form hydrogen-bonding with XNBR that verified by the FTIR results. The formed hydrogen bond may “shield” the carboxyl groups on XNBR thus retard the formation of ionic clusters. There is another origin for the decreasing trend of the crosslinking. The well-dispersed SHG will consume part of MgO through neutralization by the residue carboxyls or through chelation by the catechol moieties. In another aspect, the introduction of SHG sheets will certainly increase the physical crosslinking with the XNBR chain. Consequently, when excessive SHG is included, the later effect will dominate and the crosslink density starts to increase with SHG content. The incorporation of SHG and its effects on crosslinking will undoubtedly affect the overall mechanical performance.

The modulus of rubber composites mainly depend on the crosslink density, filler networks and filler-matrix interactions. The illustrated unique trend in crosslink density is convinced to be responsible for the almost unchanged modulus for the composites with low SHG loading 19



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as shown in Figure 8 (b). In general, the high intrinsic modulus of graphene and the introduced physical crosslink lead to remarkable improvement on modulus of rubber22b, c However, in the present study, the modulus of XNBR/SHG composite is determined by the competition between SHG-induced increase in modulus and the retarded in chemical crosslinking. At low SHG content, The decreasing in chemical crosslinking of the rubber network could not be effectively compensated by the increase in modulus induced by the SHG sheets. Consequently a slight decrease in modulus is observed. At higher SHG content, the effects from interfacial crosslinking dominate and the modulus starts to rise with increasing SHG content. Both tensile strength and tear strength (Figure 8 (b) and (d)) are dramatically improved only with a low SHG content, for example, tensile strength of GE-1 increased by 120 % and the tear strength increased by 50 % in related to GE-0. After reaching a critical point, both tensile strength and tear strength decrease slightly with subsequent addition of SHG. As shown in Figure 8 (c), it is noticeable that the extensibility of XNBR/SHG composite also exhibits a similar trend to that for tensile strength, which is different from most reported rubber/graphene systems. Such an observation can also be interpreted by the strong interfacial interaction and the unique crosslink density. When the loading of SHG is low, the distance between neighboring SHG sheets is broaden enough and the strong interfacial interaction facilitate the alignment and slippage of adsorbed XNBR chains. In addition, the decreased crosslink density increase the conformational freedom and mobility of XNBR chains which also benefit for orientation and slippage of rubber chains when external stress is applied. Both consequences make contribution to the increase of extensibility. The concurrently observed increase in tensile strength and break strain are 20


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believed to be unusual as the calculated fracture energy (Figure 8 (c)), which is determined by the area below the typical stress-strain curves (Figure 8 (a)), would increase spectacularly with small amount of SHG. For example, with only 1 phr of SHG, the fracture energy is doubled compared with GE-0. The combination of high strength (or fracture energy), high extensibility and the low modulus is rarely realized. In most cases, the nanosized filler with huge specific area, such as graphene, may drastically increase the modulus and deteriorate the extensibility when compounding with rubbers22c, 39. Therefore, the obtained combination of high strength (fracture energy) high extensibility and low modulus in the present system is quite remarkable and providing unique insight in designing the rubber composites with high strength, high extensibility and low modulus.

Figure 7. Crosslink density of XNBR/SHG composites

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Figure 8. Typical stress-strain curves of XNBR/SHG composites (a); Tensile strength and modulus at 100% elongation of XNBR/SHG composites (b); Fracture energy and elongation at break of XNBR/SHG composites (c); Tear strength of XNBR/SHG composites (d)

CONCLUSIONS In this study, we used cost-effective and environmentally friendly sodium humate to non-covalently functionalize graphene for the first time. The individually dispersed SHG colloid with high concentration (up to 30 mg/ml) convincedly demonstrated SH as the very promising stabilizer for graphene. The stabilization mechanism was revealed to be hydrogen-bonding and π-π interaction between graphene sheets and SH molecules. Subsequently, XNBR/SHG composite was prepared through a combination of latex 22


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co-coagulation and two roll milling process followed by vulcanization with a mix system of sulfur and MgO. Significant reinforcement effect of SHG towards XNBR was found. For instance, the incorporation of 1 phr of SHG leads to about 120% increase in tensile strength, 104% increase in fracture energy and 50% increase in tear strength. Importantly, the rubber/SHG composites exhibited unique combination of high fracture energy, high extensibility and low modulus. Such outcome was correlated to the interactions between SHG and MgO which pronouncedly affected the crosslinking of the rubber. The present method provides unique insight in designing the rubber composites with high strength, high extensibility and low modulus by tuning the crosslinking of rubber through interaction between the reinforcement and the curative.

ACKNOWLEDGMENT

This work was supported by National Natural Science Foundation of China (5122230, 50933001 and 51333003), Program for New Century Excellent Talents in University (NCET-10-0393) and Fundamental Research Project for the Central Universities (2012ZG0002).

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REFERENCES (1) Gómez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K., Electronic transport properties of individual chemically reduced graphene oxide sheets. Nano Lett. 2007, 7, 3499-3503. (2) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N., Superior thermal conductivity of single-layer graphene. Nano Lett. 2008, 8, 902-907. (3) (a) Rao, C.; Sood, A.; Voggu, R.; Subrahmanyam, K., Some novel attributes of graphene. J Phys Chem Lett. 2010, 1, 572-580; (b) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J., Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science. 2008, 321, 385-388; (c) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S., Graphene-based ultracapacitors. Nano Lett. 2008, 8, 3498-3502. (4) Hu, H.; Wang, X.; Wang, J.; Wan, L.; Liu, F.; Zheng, H.; Chen, R.; Xu, C., Preparation and properties of graphene nanosheets–polystyrene nanocomposites via in situ emulsion polymerization. Chem Phys Lett. 2010, 484, 247-253. (5) Potts, J. R.; Lee, S. H.; Alam, T. M.; An, J.; Stoller, M. D.; Piner, R. D.; Ruoff, R. S., Thermomechanical properties of chemically modified graphene/poly (methyl methacrylate) composites made by in situ polymerization. Carbon. 2011, 49, 2615-2623. (6) Teng, C.-C.; Ma, C.-C. M.; Lu, C.-H.; Yang, S.-Y.; Lee, S.-H.; Hsiao, M.-C.; Yen, M.-Y.; Chiou, K.-C.; Lee, T.-M., Thermal conductivity and structure of non-covalent functionalized graphene/epoxy composites. Carbon. 2011, 49, 5107-5116.

24


Page 24 of 31

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Vadukumpully, S.; Paul, J.; Mahanta, N.; Valiyaveettil, S., Flexible conductive graphene/poly (vinyl chloride) composite thin films with high mechanical strength and thermal stability. Carbon. 2011, 49, 198-205. (8) Ozbas, B.; Toki, S.; Hsiao, B. S.; Chu, B.; Register, R. A.; Aksay, I. A.; Prud'homme, R. K.; Adamson, D. H., Strain-induced crystallization and mechanical properties of functionalized graphene sheet-filled natural rubber. J Polym Sci Pol Phys. 2012, 50, 718-723. (9) Ozbas, B.; O'Neill, C. D.; Register, R. A.; Aksay, I. A.; Prud'homme, R. K.; Adamson, D. H., Multifunctional elastomer nanocomposites with functionalized graphene single sheets. J Polym Sci Pol Phys. 2012, 50, 910-916. (10) (a) Kim, J. S.; Yun, J. H.; Kim, I.; Shim, S. E., Electrical properties of graphene/SBR nanocomposite prepared by latex heterocoagulation process at room temperature. J Ind Eng Chem. 2011, 17, 325-330; (b) Zhan, Y.; Lavorgna, M.; Buonocore, G.; Xia, H., Enhancing electrical conductivity of rubber composites by constructing interconnected network of self-assembled graphene with latex mixing. J Mater Chem. 2012, 22, 10464-10468. (11) Tang, Z.; Kang, H.; Shen, Z.; Guo, B.; Zhang, L.; Jia, D., Grafting of Polyester onto Graphene for Electrically and Thermally Conductive Composites. Macromolecules. 2012, 45, 3444-3451. (12) (a) Tang, Z.; Lei, Y.; Guo, B.; Zhang, L.; Jia, D., The use of rhodamine B-decorated graphene as a reinforcement in polyvinyl alcohol composites. Polymer. 2012, 53, 673-680; (b) Qi, X.; Pu, K.-Y.; Li, H.; Zhou, X.; Wu, S.; Fan, Q.-L.; Liu, B.; Boey, F.; Huang, W.; Zhang, H., Amphiphilic Graphene Composites. Angew Chem Int Edit. 2010, 49, 9426-9429.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(13) Tang, Z.; Zeng, C.; Lei, Y.; Guo, B.; Zhang, L.; Jia, D., Fluorescent whitening agent stabilized graphene and its composites with chitosan. J Mater Chem. 2011, 21, 17111-17118. (14) Lei, Y.; Tang, Z.; Liao, R.; Guo, B., Hydrolysable tannin as environmentally friendly reducer and stabilizer for graphene oxide. Green Chem. 2011, 13, 1655-1658. (15) Liao, R.; Tang, Z.; Lei, Y.; Guo, B., Polyphenol-Reduced Graphene Oxide: Mechanism and Derivatization. J Phys Chem C. 2011, 115, 20740-20746. (16) Bai, H.; Xu, Y.; Zhao, L.; Li, C.; Shi, G., Non-covalent functionalization of graphene sheets by sulfonated polyaniline. Chem Commun. 2009, 1667-1669. (17) Xu, Y.; Bai, H.; Lu, G.; Li, C.; Shi, G., Flexible graphene films via the filtration of water-soluble noncovalent functionalized graphene sheets. J Am Chem Soc. 2008, 130, 5856-5857. (18) Zhou, X.; Wu, T.; Ding, K.; Hu, B.; Hou, M.; Han, B., Dispersion of graphene sheets in ionic liquid [bmim][PF6] stabilized by an ionic liquid polymer. Chem. Commun. 2010, 46, 386-388. (19) (a) Potts, J. R.; Shankar, O.; Murali, S.; Du, L.; Ruoff, R. S., Latex and two-roll mill processing of thermally-exfoliated graphite oxide/natural rubber nanocomposites. Compos Sci Technol. 2013, 74, 166-172; (b) Hernández, M.; Bernal, M. d. M.; Verdejo, R.; Ezquerra, T. A.; López-Manchado, M. A., Overall performance of natural rubber/graphene nanocomposites. Compos Sci Technol. 2012, 73, 40-46. (20) Potts, J. R.; Shankar, O.; Du, L.; Ruoff, R. S., Processing–Morphology–Property Relationships and Composite Theory Analysis of Reduced Graphene Oxide/Natural Rubber Nanocomposites. Macromolecules. 2012, 45, 6045-6055. 26


Page 26 of 31

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21) Bai, X.; Wan, C.; Zhang, Y.; Zhai, Y., Reinforcement of hydrogenated carboxylated nitrile–butadiene rubber with exfoliated graphene oxide. Carbon. 2011, 49, 1608-1613. (22) (a) Wang, Z.; Liu, J.; Wu, S.; Wang, W.; Zhang, L., Novel percolation phenomena and mechanism of strengthening elastomers by nanofillers. Phys Chem Chem Phys. 2010, 12, 3014-3030; (b) Tang, Z.; Wu, X.; Guo, B.; Zhang, L.; Jia, D., Preparation of butadiene–styrene–vinyl pyridine rubber–graphene oxide hybrids through co-coagulation process and in situ interface tailoring. J Mater Chem. 2012, 22, 7492-7501; (c) Araby, S.; Zaman, I.; Meng, Q.; Kawashima, N.; Michelmore, A.; Kuan, H.-C.; Majewski, P.; Ma, J.; Zhang, L., Melt compounding with graphene to develop functional, high-performance elastomers. Nanotechnology. 2013, 24, 165601. (23) Liu, J.; Cao, D.; Zhang, L., Static and dynamic properties of model elastomer with various cross-linking densities: A molecular dynamics study. J Chem Phys. 2009, 131, 034903. (24) Zhan, Y.; Wu, J.; Xia, H.; Yan, N.; Fei, G.; Yuan, G., Dispersion and Exfoliation of Graphene in Rubber by an Ultrasonically-Assisted Latex Mixing and In situ Reduction Process. Macromol Mater Eng. 2011, 296, 590-602. (25) Yang, J.; Tian, M.; Jia, Q.-X.; Shi, J.-H.; Zhang, L.-Q.; Lim, S.-H.; Yu, Z.-Z.; Mai, Y.-W., Improved mechanical and functional properties of elastomer/graphite nanocomposites prepared by latex compounding. Acta Mater. 2007, 55, 6372-6382. (26) (a) Wang, P.; Shi, Q.; Liang, H.; Steuerman, D. W.; Stucky, G. D.; Keller, A. A., Enhanced Environmental Mobility of Carbon Nanotubes in the Presence of Humic Acid and Their Removal from Aqueous Solution. Small. 2008, 4, 2166-2170; (b) Wang, X.; Shu, L.; 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

Wang, Y.; Xu, B.; Bai, Y.; Tao, S.; Xing, B., Sorption of peat humic acids to multi-walled carbon nanotubes. Environ Sci Technol. 2011, 45, 9276-9283. (27) Hartono, T.; Wang, S.; Ma, Q.; Zhu, Z., Layer structured graphite oxide as a novel adsorbent for humic acid removal from aqueous solution. J Colloid Interf Sci. 2009, 333, 114-119. (28) Wang, L.; Liao, R.; Tang, Z.; Lei, Y.; Guo, B., Sodium deoxycholate functionalized graphene and its composites with polyvinyl alcohol. J Phys D Appl Phys. 2011, 44, 445302. (29) Ibarra, L.; Alzorriz, M., Vulcanization of carboxylated nitrile rubber (XNBR) by zinc peroxide. Polym Int. 1999, 48, 580-586. (30) (a) Yin, D.; Zhang, Y.; Zhang, Y.; Peng, Z.; Fan, Y.; Sun, K., Reinforcement of peroxide ‐cured styrene–butadiene rubber vulcanizates by mathacrylic acid and magnesium oxide. J Appl Polym Sci. 2002, 85, 2667-2676; (b) Kaang, S.; Jin, W.; Abdul Kader, M.; Nah, C., Effects of blend composition and mixing method on mechanical and morphological properties

of

zinc

dimethacrylate-reinforced

acrylonitrile-butadiene

copolymer

nanocomposites. Polym-Plast Technol. 2005, 43, 1517-1538. (31) Zeng, C.; Tang, Z.; Guo, B.; Zhang, L., Supramolecular ionic liquid based on graphene oxide. Phys Chem Chem Phys. 2012, 14, 9838-9845. (32) Yang, H.; Shan, C.; Li, F.; Han, D.; Zhang, Q.; Niu, L., Covalent functionalization of polydisperse chemically-converted graphene sheets with amine-terminated ionic liquid. Chem. Commun. 2009, 3880-3882. (33) Tang, Z.; Zhang, L.; Zeng, C.; Lin, T.; Guo, B., General route to graphene with liquid-like behavior by non-covalent modification. Soft Matter. 2012, 8, 9214-9220. 28


Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) (a) Jo, K.; Lee, T.; Choi, H. J.; Park, J. H.; Lee, D. J.; Lee, D. W.; Kim, B.-S., Stable Aqueous Dispersion of Reduced Graphene Nanosheets via Non-Covalent Functionalization with Conducting Polymers and Application in Transparent Electrodes. Langmuir. 2011, 27, 2014-2018; (b) Parviz, D.; Das, S.; Ahmed, H. T.; Irin, F.; Bhattacharia, S.; Green, M. J., Dispersions of non-covalently functionalized graphene with minimal stabilizer. ACS nano. 2012, 6, 8857-8867. (35) Yang, Q.; Pan, X.; Huang, F.; Li, K., Fabrication of high-concentration and stable aqueous suspensions of graphene nanosheets by noncovalent functionalization with lignin and cellulose derivatives. J Phys Chem C. 2010, 114, 3811-3816. (36) Wang, Y.; Shi, Z.; Yin, J., Facile Synthesis of Soluble Graphene via a Green Reduction of Graphene Oxide in Tea Solution and Its Biocomposites. ACS Appl Mater Inter. 2011, 3, 1127-1133. (37) Yang, D.; Velamakanni, A.; Bozoklu, G.; Park, S.; Stoller, M.; Piner, R. D.; Stankovich, S.; Jung, I.; Field, D. A.; Ventrice, C. A.; Ruoff, R. S., Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon. 2009, 47, 145-152. (38) Wang, J.; Jia, H.; Tang, Y.; Ji, D.; Sun, Y.; Gong, X.; Ding, L., Enhancements of the mechanical properties and thermal conductivity of carboxylated acrylonitrile butadiene rubber with the addition of graphene oxide. J Mater Sci. 2013, 48, 1571-1577. (39) Wu, J.; Huang, G.; Li, H.; Wu, S.; Liu, Y.; Zheng, J., Enhanced mechanical and gas barrier properties of rubber nanocomposites with surface functionalized graphene oxide at low content. Polymer. 2013, 54, 1930-1937. 29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(40) Ibarra, L.; Rodríguez, A.; Mora, I., Ionic nanocomposites based on XNBR-OMg filled with layered nanoclays. Eur Polym J. 2007, 43, 753-761. (41) Mandal, U. K., Ionic elastomer based on carboxylated nitrile rubber: infrared spectral analysis. Polym Int. 2000, 49, 1653-1657. (42) (a) Rashid, M., Absorption of metals on sedimentary and peat humic acids. Chem Geol. 1974, 13, 115-123; (b) Buffle, J.; Greter, F. L.; Haerdi, W., Measurement of complexation properties of humic and fulvic acids in natural waters with lead and copper ion-selective electrodes. Anal Chem. 1977, 49, 216-222. (43) Ibarra, L.; Alzorriz, M., Ionic elastomers based on carboxylated nitrile rubber and magnesium oxide. J Appl Polym Sci. 2007, 103, 1894-1899. (44) Li, C.; Feng, C.; Peng, Z.; Gong, W.; Kong, L., Ammonium-assisted green fabrication of graphene/natural rubber latex composite. Polym Composite. 2013, 34, 88-95.

BRIEFS: Sodium humate-functionalized graphene could be used as unique rubber reinforcement towards the combination of high strength and low modulus

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For TOC only

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Sodium Humate Functionalized Graphene and Its Unique

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