γ-Aminopropyl triethoxysilane functionalized graphene oxide for composites with high dielectric constant and low dielectric loss

Accepted Manuscript γ-Aminopropyl Triethoxysilane Functionalized Graphene Oxide for Composites with High Dielectric Constant and Low Dielectric Loss Xin Zhi, Yingyan Mao, Shipeng Wen, Yan Li, Liqun Zhang, Tung W. Chan, Li Liu PII: DOI: Reference:

S1359-835X(15)00169-4 http://dx.doi.org/10.1016/j.compositesa.2015.05.015 JCOMA 3936

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

28 February 2015 26 April 2015 18 May 2015

Please cite this article as: Zhi, X., Mao, Y., Wen, S., Li, Y., Zhang, L., Chan, T.W., Liu, L., γ-Aminopropyl Triethoxysilane Functionalized Graphene Oxide for Composites with High Dielectric Constant and Low Dielectric Loss, Composites: Part A (2015), doi: http://dx.doi.org/10.1016/j.compositesa.2015.05.015

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γ-Aminopropyl Triethoxysilane Functionalized Graphene Oxide for Composites with High Dielectric Constant and Low Dielectric Loss

Xin Zh i1 , Yingyan Mao 1 , Shipeng Wen1 , Yan Li1 , Liqun Zhang 1 , Tung W. Chan 3 , and Li Liu 1,2* 1. State Key Laboratory of Organic-Inorganic Co mposites, Beijing Un iversity of Chemical Technology, Beijing 100029, Ch ina; 2. State Key Laboratory of Chemical Resource Engineering, Beijing Un iversity of Chemical Technology, Beijing 100029, China; 3. Depart ment of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA. * Corresponding author: Li Liu Materials Science and Engineering, Beijing Un iversity of Chemical Technology 15 BeiSanhuan East Road, ChaoYang District, Beijing, 100029 (China) Tel.: (+)86-10-64443413 Fax: (+)86-10-64433964 E-mail: Liu [email protected]

Abstract: A facile and efficient approach was developed to simultaneously functionalize and tune the reduction state of graphene oxide (GO) with γ-aminopropyl triethoxysilane (APTES) aided by NH3 solution. X-ray diffraction, Fourier transform in frared spectroscopy, Raman spectroscopy, and X-ray 1

photoelectron spectroscopy indicated that many surface groups of GO sheets were removed, and APTES were successfully functionalized onto GO sheets. The APTES-functionalized GO sheets (GO-APTES) were d ispersed in water and further incorporated into nitrile butadiene rubber (NBR) by latex co-coagulation to form GO-APTES/ NBR composites. These composites featured high degrees of exfoliation and intercalation of GO-APTES sheets throughout the NBR matrix. More significantly, the GO-APTES/ NBR co mposites exhibited a relat ively high dielectric constant (~30.8) and a small loss factor (＜0.04) at 1.0 kHz, co mb ining with a good insulating property. The unique dielectric responses of GO-APTES/NBR co mposites open up the potential applicat ion s of these materials in resistive and capacitive field grad ing materials. Keywords: A. Nano co mposites; A. Coupling agents; B. Electrical properties; E. Surface treat ments

1.

Introduction With the rap id develop ment of electronic informat ion, electric industry, and power industry,

polymer-based dielectric materials with high d ielectric constant (high κ) and low dielectric loss have attracted significant interest.[1-3] Co mpared with conventional ceramic dielectrics, polymer-based dielectrics have an imp roved processability, chemical stability, and mechanical strength due to the nature of polymer.[3-6] Generally, the d ielectric constant of common poly mers is very low. Thus, a key issue is to enhance the dielectric constant of polymers while retaining other excellent properties. One co mmon approach is to introduce high κ ceramic fillers (such as BaTiO3 and CaCu 3 Ti4 O12 ) into polymers.[7-9] But ceramic filler loadings higher than 50 vol.% are often required to achieve the desired dielectric constant. Such high ceramic filler loadings can lead to processing difficulties, loss of poly mer-like behavior, excessive weight, and tendency to overheat at elevated electric fields. Another approach is to introduce 2

conductive fillers (such as metal particles and carbon nanotubes) into polymers and fabricate percolative composite capacitors.[2, 10] Conducting fillers can dramat ically enhance the dielectric constant

of a co mposite when the filler loading

gets close to the percolation threshold

, as

described by the well-known power law[11]: – where

………………………………………. (1)

is the dielectric constant of the polymer matrix and

is an exponent of about 1. However,

such a percolated composite also has a high dielectric loss due to the insulator-conductor transition near the percolation threshold. For the reduction of dielectric loss, conductive fillers are often coated with insulating shells or interlayered to prevent the fillers fro m d irectly contacting with one another. Shen[10] used core-shell hybrid particles with a metal core coated by organic dielectric shells as fillers, leading to consistently high κ and low tanδ of the poly mer dielectrics. Yang[12] coated mult i walled carbon nanotubes with polypyrrole by inverse microemu lsion polymerization , resulting in PS composites with enhanced dielectric properties . Recently, graphene oxide (GO)[13], a precursor for graphene, was produced by the oxidation and exfoliation of graphite. Unlike graphene, GO sheets are heavily o xygenated, bearing hydro xyl and epoxide functional groups on their basal planes, in addit ion to carbonyl and carbo xyl groups located at the sheet edges.[14] The presence of these functional groups introduces sp 3 defect sites to GO sheets, disrupting the intrinsic sp2 bonding orbitals and lowering the overall conductivity of graphene. The primary GO sheets are too insulating and exhib it no dielectric properties. Nevertheless, one can control the electrical propert ies of GO by various chemical o r thermal approaches to restore the sp2 network.[15, 16] Highly reduced GO (HRG) sheets with h igh C/O ratios always exh ibit good conductivity, and the HRG network in the poly mer co mposites can cause a large leakage current and 3

thus high dielectric loss. The surface modificat ion of HRG sheets is thus required for reduction of dielectric loss[17]. MnO2 /graphene nanosheets/MnO2 (MGM ) is fabricated as a dielectric filler to prepare M GM/polyvinylidene co mposites which exh ibit excellent d ielectric propert ies near the percolation threshold at 1 kHz.[18] Shang[19] fabricated graphene-polyvinylidene fluoride functional hybrid films with a polyaniline interlayer for enhanced dielectric properties. Li [20] prepared two-dimensional hybrid sheets by decorating insulating PANI on HRG sheets for PMMA co mposites with low d ielectric loss and high dielectric constant. If we can prepare chemically active reduced GO (CA RGO) sheets with C/ O ratios between those of GO and HRG, such CARGO sheets will have larger conductive areas between the insulating barriers . These CA RGO sheets may see use in preparing a composite with high dielectric constant and rather low loss .[21] Yousefi report a self-aligned RGO/epo xy nanocomposites with highly an isotropic mechanical and electrical properties which exh ibit exceptionally high dielectric constants.[22] Almadhoun show that nitrogen-doping by hydrazine along the edges of reduced graphene oxide embedded in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) results in a dielectric permittiv ity above 10 000 while maintaining a d ielectric loss below 2.[23] So me literatures report on chlorination of reduced GO for chlorine-doped GO/poly mer co mposites with enhanced dielectric p roperties.[24, 25] A mong various modifying strategies, silane coupling agents are frequently applied to modify the GO sheets. These silane functionalized GO sheets have shown promising to provide the covalently bonding of GO with the polymer matrix for co mposites with enhanced mechanical properties, but their report on dielectric properties is rare .[26-28] γ-aminopropyl trietho xysilane (APTES) has been widely applied as a coupling agent, while its amine groups interact with the functioal groups on GO sheets. In this study, a facile and efficient approach was developed to simu ltaneously functionalize and tune the reduction 4

state of GO with APTES aided by NH3 solution. Such an approach could remove many surface groups of GO sheets to increase the conductive areas between the insulating barriers. Additionally, APTES-functionalized GO (GO-APTES) sheets can form stable aqueous dispersion. The versatility of the dispersion of processable GO-APTES sheets is demonstrated by their incorporation into NBR composites that exhib it a relat ively high dielectric constant, a small loss factor, and good insulating property. The unique dielectric responses of GO-APTES/ NBR co mposites open up their potential applications in resistive and capacitive field grading materials.

2.

Experi mental

2.1 Materials Natural graphite flakes with an average diameter of 13 μm were purchased from Huadong Graphite Factory (China). Sulfuric acid (H2 SO4 , 95-98%), hydrochloric acid (HCl, 35-38%), potassium permanganate (KMnO4 ), sodium nitrate (NaNO3 ), calciu m chloride (CaCl2 ), hydrogen peroxide (H2 O2 , 30%), and ammonia (NH3 ·H2 O, 25-28%) were purchased from Beijing Chemical Factory (Ch ina). Acrylonitrile butadiene rubber latex (LNBR) was supplied by Shanghai Lishenxing Co mpany (Ch ina). γ-Aminopropyl trietho xysilane (APTES) (APTES) was obtained from Nan jing Shuguang Chemical Group Co., Ltd. (China). The dicu myl pero xide (DCP) was a commercial product. 2.2 Functionalization and Reduction of GO with APTES Graphite o xide was prepared fro m flake graphite by a modified Hu mmers method.[29] APTES (molecular formu la H2 NCH2 CH2 CH2 Si(OC2 H5 )3 ) is a coupling agent with a terminal group of amine. The graphite o xide (1.0 g) was dissolved and exfoliated in deionized water (500 ml) by ultrasonication to form stable graphene oxide (GO) dispersion. Subsequently, APTES (1 g) and NH3 ·H2 O solution (100 5

ml) were added. The mixture was reflu xed with mechanical stirring at 90 o C for 6 h. Then a rinsing-filt ration process was repeated for a few times to physically remove the APTES. 2.3 Preparation of GO-APTES/NBR Composites. A desired amount of GO-APTES dispersion was added into the NBR latex and stirred for 30 min. Then the mixture was co-coagulated by adding CaCl2 (1.0 wt%) aqueous solutions as the flocculating agents. The coagulated composites were washed with water and then dried in an oven at 50 o C for 24 hours. The dried composites were co mpounded with DCP (0.5 g DCP/100 g NBR) on a t wo-roll mill and the composites were subjected to compression in a standard mold at 150 o C and 15 MPa fo r an optimu m time that was determined by a d isc rheo meter (Model 750, Beijing Huan Feng Mechanical Factory). 2.4 Characterization

X-ray diffraction (XRD) measurements were carried out on a Rigaku RINT diffractometer with a Cu K radiation (40 kV, 200 mA) fro m the range of 1º to 30º at a scan rate of 5º/ min. Fourier transform infrared spectra (FTIR) were collected on a Bru ker Tensor 27 spectrometer. Raman spectra were recorded with a Renishaw in Via Raman microscope with an excitation wavelength of 514.5 n m. X-Ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCALA B 250 XPS system (Thermo Electron Co rporation, USA) with an Al Kα X-ray source (1486.6 eV photons).

A swelling test was determined through immersing the samples into toluene for 72 hours. The in itial samples were weighted to obtained m1 , and then immersed them into toluene for 72 hours; these sopped samples were wiped and weighted to obtained m2 . The samples were dried in vacuu m at 40 ºC for at least 24 hours and weighted to obtained m3 . The swelling ratio is determined by equation (2):

6

-

(2)

The crosslink density (e) is calcu lated using the Flory-Rehner equation:



 





(3)



where Vs , the molar volu me of solvent, is 106.3 cm3 / mol for toluene, and

is the Flory/Huggins

interaction parameter between toluene, which is taken as 0.43 for the elastomer. The rubber volume fraction (2 ) after swelling was calcu lated according to the equation:



where

is volu me fraction of the filler, and

(4)

is the ratio of elastomer to toluene density.

The exfoliation quality of GO-APTES sheets in the NBR matrix was observed with a CM12 transmission electron microscope (TEM ) at an accelerat ing voltage of 120 kV (Ph ilips, Netherlands). The fractured surfaces of the GO-APTES/NBR co mposites were investigated by a S-4700 scanning electron microscope (SEM) operating at a voltage of 5 kV (Hitachi, Japan). Tensile tests were performed on dumbbell-shaped specimens according to ASTM D412 by using a CMT 4104 Electrical Tensile Instrument (Shen zhen SANS Test Machine Co., Ltd. China) at a crosshead speed of 500 mm/ min at 23 ºC. Tear strength was determined with a right-angle tear die C according to ASTM D624 by the same tensile instrument. The dielectric properties of the samples were measured by an E4980A impedance analyzer (Agilent, U.S.A.) over the frequency range of 100 to 106 Hz under 220 V at roo m temperature. The volu me conductivities were measured by a ZC-90G resistivity meter (Shanghai Taiou Electronics, China).

7

3.

Results and Discussion

3.1 Functionalization of GO with APTES GO was functionalized with γ-aminopropyl triethoxysilane (APTES) aided by NH3 solution. Fig. 1 shows the FTIR spectra of GO, APTES, and GO-APTES. In the FTIR spectrum of GO, the peaks at 3427, 1726, 1691, 1401, and 1044 cm-1 are attributed to O-H stretching vibrations, C=O stretching vibrations, C=C stretching vibrations, O-H defo rmations, and C-O stretching vibrat ions, respectively. In the FTIR spectrum of GO-APTES, the two new peaks at 2924 cm-1 and 2854 cm-1 resulting fro m -CH2 stretching imply the existence of APTES on GO-APTES. Furthermore, the peak at 1030-1160 cm-1 corresponding to C-O-C is weakened, and a new peak at 1630 cm-1 (N-H stretching vibration) appears, indicating the formation of -C-NH-C- bonds due to the reaction between the epo xide group of GO and the amine group of APTES. A peak appears at 1097 cm-1 (Si-O-Si stretching vibrations) because the other terminal etho xy groups of APTES are easy to hydrolyze in water to form hydro xyl silane, and a condensation reaction will occur between the hydroxylsis, leading to Si -O-Si b ridging.

Insert Fig. 1 XPS was used to characterize the format ion of chemical bonds on the surface of GO before and after its functionalizat ion with APTES (Fig. 2). Typ ically, the C 1s peak reg ion of GO can be fitted with four curves. The binding energies at 284.8, 286.3, 287.0, and 288.4 eV are assigned to unoxidized graphite carbon skeleton (C-C), hydro xyl group (C-OH), epo xide group (-C-O-C-), and carbo xyl group (-O-C=O), respectively. Ho wever, in the XPS spectrum of GO -APTES, the peaks of the oxygen-containing groups, especially the peak of C−O−C in epoxide g roup, are greatly weakened in intensity. Accordingly, the ato mic rat io of carbon to o xygen (C/O) increases fro m 2.7 for GO to 3.3 for GO-APTES. Moreover, a new peak appears at 285.6 eV in the XPS spectrum of GO-APTES, 8

corresponding to C-N and demonstrating the reaction of GO with APTES.

Insert Fig. 2 XRD was fu rther used to characterize the structures of GO and GO-APTES. As shown in Fig. 3, a peak appears at 2θ=11.0° for GO, corresponding to a d-spacing of 0.79 n m, as a result of the intercalation of the functional groups into the interlayers of graphene sheets . With the functionalizat ion of exfo liated GO with APTES, GO-APTES exh ib its no diffraction peak, which may be suggest a 3-D disordered structure. The format ion of such a 3-D disordered structure may be interpreted as following reactions. On one hand, the terminal amine group of APTES has strong association with GO she ets. On the other hand, the terminal etho xy groups of APTES are easy to hydrolyze in water to deliver hydro xyl silane during reflu xing. A quick condensation will occur between the Si-OH bonds of hydroxy l silane, leading to Si-O-Si b ridging. In addit ion, the condensation reactions will occur not only between the hydroxyl silane mo lecules themselves or between these hydroxyl groups from different molecules.[30] These reactions could bridge-up different GO-APTES sheets, leading to a 3-D disordered structure.

Insert Fig. 3 Raman spectroscopy is a widely used tool for the characterizat ion of graphene.[31] As shown in Fig. 4, GO exh ib its a D band at 1356 cm-1 and a G band at 1592 cm-1 , while these two bands shift to 1351 and 1596 cm-1 for GO-APTES, respectively. As in the case of our Raman spectra, the band shifts can be due to the formation of topological defects with sp 3 structure, indicating the successful functionalization of the GO sheets.[31, 32] The intensity ratio o f D- to G-bands, ID/IG, co rrelating to the d isordered and ordered crystal structures of graphene, is opposite to the average size of sp 2 domains. Put it differently: ID/IG also correlate with the mean distance of two defects in graphene as it was found by Ar-ion bo mbard ment on 9

graphene.[33] The ID/IG ratios of GO and GO-APTES are respectively 0.874 and 1.046, wh ich means a decreased average size of sp 2 domains. This is because GO to GO-APTES transformation leaves behind topological defects and vacancies with a high quantity of structural defects. However, some functional groups are also removed during the reflu x station, leading to numerous small sp 2 domains. In a word, although the average size of sp2 decrease, numerous small sp 2 domains will lead to more conductive areas. This observation is consistent with the literature. [34, 35]

Insert Fig. 4 All these characterizat ions above confirm the intercalation and chemical reaction of APTES with GO. The reaction mechanis m of GO by APTES and NH3 solution is shown in Fig. 5. GO to GO-APTES transformat ion is through amino mo ieties of APTES and epo xy groups of GO. Moreover, with the addition of NH3 solution, some unreacted functional groups are removed during the reflu x station, leading to nu merous small sp2 do mains, which tune the reduction state of GO. Interestingly, there is a color change from the brown GO to the black GO-APTES in water (inset of Fig. 5), a direct evidence that reduction has occurred during the reflu xing of GO with APTES in NH3 solution. As reported, the addition of NH3 solution imp roves the deoxygenation of GO.[36] Owing to the reduction, the electrical conductivity of GO-APTES is 9.3×10-5 S/ m, mo re than 2 orders of magnitude higher than that of GO (2.3×10-7 S/ m). The reduction and functionalizat ion of GO with APTES are schematically shown in Fig. 5(a).

The XRD and FTIR results also suggest that the GO-APTES sheets are

bridged-up through hydrolysis condensation into GO-APTES powders with a three-dimensional disordered structure (Fig. 5(b)).

Insert Fig. 5

3.2 Preparation and structure of GO-APTES/NBR composites 10

The dried GO-APTES sheets are chemically b ridged; thus, it is hard to d isperse them into poly mers. After several rinsing-filtrat ion processes to remove the physically APTES, we dispersed the colloidal GO-APTES into water to form stable aqueous dispersion by a mild ultrasonic treat ment. Then we prepared GO-APTES/NBR co mposites, using a simple and environ ment-friendly latex co-coagulation method.

The microstructure of the GO-APTES/ NBR co mposite was first studied by XRD, and the XRD patterns are shown in Fig. 6. A broad d iffraction peak is observed for neat NBR, an indication of amorphous structure of the NBR matrix. The GO-APTES/NBR co mposite with 2.0 vol.% of GO-APTES displays an extra diffraction peak at 2θ=6.5°, corresponding to a basal spacing of 1.2 n m, which is larger than the interlayer spacing of GO sheets. The increase in interlayer spacing is an indication that, unlike the disordered structure of GO-APTES powders, some intercalated microstructures exist in GO-APTES/NBR co mposites . The formation of the intercalated microstructure can be ascribed to the latex co-coagulation process. When a GO-APTES aqueous dispersion is latex-co mpounded with NBR latex, the GO-APTES sheets are highly isolated by the NBR co llo idal particles. With the addition of flocculants, the NBR collo idal part icles will be co-coagulated with the GO-APTES sheets, which p revent the format ion of a three-d imensional d isordered GO-APTES network. Instead, some intercalated microstructure appeared in the composite.

Insert Fig. 6 The interfacial interactions and the dispersion state of GO-APTES in the NBR matrix at different GO-APTES loadings were characterized by SEM. SEM images in Fig. 7 show no obvious GO-APTES aggregates, indicating a fine dispersion state of GO-APTES sheets in NBR. Furthermore, we can see

11

that the surface of neat NBR is s mooth and flat. Co mpared with that of neat NBR, the fractured surface of the GO-APTES/NBR co mposite with a GO loading of 2.0 vol.% is rougher and has more protuberances. These protuberances are GO-A PTES sheets wrapped with NBR molecu les. Because of the strong interaction between the GO-APTES sheets and the NBR matrix, the GO-APTES sheets are deeply embedded in the elastomer and are not pulled out during tensile fracture.

Insert Fig. 7 3.3 Properties of GO-APTES/NBR co mposites

The vulcanization characteristics of neat NBR and GO-APTES/NBR co mposites are shown in Fig. 8 (a). The min imu m torque (ML ) and maximu m torque (MH ) both increase with the addit ion of GO-APTES sheets because of the strong interfacial interactions between the GO-APTES sheets and the NBR matrix. The difference between maximu m and min imu m torques △M, corresponding to the degree of crosslinking, increases with increasing GO-APTES loading as shown in Fig. 8 (b). We further quantitatively analyze the effect o f GO-APTES on the crosslink density by swelling measurement in the following discussion.

The swelling ratio and crosslink density of GO-APTES/ NBR co mposites were calculated and shown in Fig. 8 (c). With increasing GO-APTES loading, the swelling ratio reduces while the crosslink density increases. The reduction in swelling ratio may be due to the laminated structure of GO -APTES, which creates a tortuous path, leading to the reduction of the solvent diffusion. As the concentration of the curing package relat ive to NBR is identical in all the samples, the GO -APTES sheets must have provided physical crosslink points, contributing to the increase in crosslink density.

Typical stress-strain curves of neat NBR and GO-APTES/NBR co mposites are shown in Fig. 8 (d). 12

With increasing GO-APTES loading, the mechanical properties of GO-APTES/ NBR co mposites improve because of the marked reinfo rcement of nanodispersed GO-APTES. However, as the loading of GO-APTES exceeds 2.0 vol.%, the tensile strength of NBR no longer rises and the elongation at break significantly decreases, since high GO-APTES loadings restrict the movement of poly mer chains.

Insert Fig.8 The dielectric constant and loss factor of GO/NBR co mposites and GO-APTES/NBR co mposites at different filler loadings as a function of frequency at room temperature are su mmarized in Fig. 9(a-b). Functionalization of GO with APTES removes more surface groups and leads to larger conductive areas between the insulating barriers. As a result, co mparing to GO/NBR co mposite, the dielectric constant of GO-APTES/ NBR co mposite increase at the same filler loading. The d ielectric constant and factor loss of the GO-APTES/NBR co mposites hardly change with frequency at low frequencies, while the dielectric constant decreases and the loss factor increases with frequency at frequencies above 10 kHz. As shown in Fig . 9(c), at 1 kHz, the dielectric constant of NBR increases more than doubles after the addition of 2.0 vol.% GO-APTES, a lo w filler loading. Meanwhile, the loss factor increases moderately fro m 0.03 to 0.04 at 1 kHz. As reported by Wang et al.[37], fillers with higher aspect ratio can increase the dielectric constant more efficiently. The increase of dielectric constant in percolative composites can be explained by a microcapacitor network model[38]: neighboring high aspect ratio conductive fillers can be considered as electrodes and a thin layer of poly mer between them acts as the dielectric, forming nu merous microcapacitors in the composite. On the other hand, the conductive filler network in the composite can cause a large leakage current and thus a high dielectric loss. Coating an insulating material on the surface of the conductive fillers is a generally way to avoid the high loss. 13

Insert Fig.9 In the case of GO-APTES, the functionalization of GO with APTES removes more surface groups and leads to larger conductive areas between the insulating barriers, wh ich increases the effective aspect ratio of GO-APTES fillers and therefore the dielectric constant of the composite. On the other hand, the dielectric constant increases with only a slight increase in the loss factor because the intrinsic barriers limit the leakage current to avoid a high loss . The loss factor can be described by

tan   where



and

 

conductivity, and

      2f 

………………………………. (5)

are the real and imaginary parts of the permittivity, respectively, σ is the DC

f is the frequency. The frequency independence of the dielectric loss of the GO

composites at low frequencies indicates a low DC conductivity. Fig. 9(d) shows that the DC conductivities of the GO-APTES/ NBR co mposites are lower than 3 × 10-13 S/m, ind icating that GO-APTES sheets have good insulating properties even though the total number of surface groups has been reduced. Conductive paths are not formed in both GO/ NBR co mposites and GO-APTES/NBR composites even with increased filler loadings because GO to GO-APTES transformation leaves behind topological defects and vacancies. The major contributions to the dielectric loss of the composites are the dipolar or interfacial relaxat ion processes, instead of the leakage current as conductive fillers. In summary, the excellent dielectric p roperties of the GO-APTES/NBR co mposites can be explained by the special structure of GO-APTES sheets, with large conductive areas between insulating barriers, leading to imp roved dielectric constant but low loss factor.

4.

Conclusions We successfully functionalized and tuned the reduction state of GO with APTES aided by NH3 14

solution. The APTES-functionalized GO (GO-APTES) sheets have large conductive areas between insulating barriers. In the conductive regions where surface groups are reduced, the high aspect ratio of GO-APTES sheets leads to an increase in the dielectric constant of the composite, while the intrinsic barriers of GO, which limit the leakage current, avoid a h igh dielectric loss. At 1 kHz, the d ielectric constant of the GO-APTES/ NBR co mposite with a GO-APTES volu me fraction of 2.0 vol.% is 30.8, whereas the dielectric loss tangent is as low as 0.04. With novel d ielectric properties and mechanical properties, the composites have potential applications as field controlling materials or insulation materials in high voltage power systems and electronic devices.

Acknowledgements

The research was supported by the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT0807), the Nat ional Natural Science Foundation of China (51073008 and 51103005).

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Figure captions Fig. 1. FTIR spectra of GO, APTES, and GO-APTES. Fig. 2. XPS spectra: (a) C 1s spectrum of GO, (b) C 1s spectrum of GO-APTES, and (c) general spectra of GO and GO-APTES. Fig. 3. XRD patterns of GO and GO-APTES. Fig. 4. Raman spectra of GO and GO-APTES. Fig. 5. (a) Schemat ic of react ion of GO with APTES. (b) Hydrolysis condensation between different GO-APTES sheets to form three-dimensional disordered structure. The insets are the photos of aqueous dispersions of GO and GO-APTES after functionalizat ion (concentrations of GO and GO-APTES in H2O = 0.05 mg/ mL). Fig. 6. XRD patterns of neat NBR and GO-APTES/NBR co mposite at GO-APTES loading o f 2.0 vol.%. Fig. 7. SEM images of cross section of (a) neat NBR and (b-f) GO-APTES/NBR co mposites with GO-APTES fractions of 0.2 vol.%, 0.4 vol.%, 0.8 vol.%, 1.2 vol.%, 2.0 vol.%, respectively. Fig. 8. (a) Vu lcanizat ion curves of GO-APTES/ NBR co mposites; (b) △M of GO-APTES/NBR composites with different GO-APTES loadings; (c) swelling ratio and crosslink density of GO-APTES/ NBR co mposites with different GO-APTES loadings; (d) stress-strain curves of GO-APTES/ NBR co mposites. Fig. 9. (a) Dielectric spectroscopy of GO/NBR co mposites showing comparison of d ifferent GO loading; (b) Dielectric spectroscopy of GO-APTES/NBR co mposites showing comparison of different GO-APTES loading; (c) Dielectric constant and loss factor of GO/ NBR co mposites and GO-APTES/ NBR co mposites as function of filler loading at 1 kHz; (d) Electrical conductivit y of GO/ NBR co mposites and GO-APTES/ NBR co mposites as function of filler loading. 18

Fig. 1. FTIR spectra of GO, APTES, and GO-APTES.

Fig. 2. XPS spectra: (a) C 1s spectrum of GO, (b) C 1s spectrum of GO-APTES, and (c) general spectra of GO and GO-APTES.

Fig. 3. XRD patterns of GO and GO-APTES.

Fig. 4. Raman spectra of GO and GO-APTES.

Fig. 5. (a) Schematic of reaction of GO with APTES. (b) Hydrolysis condensation between different GO-APTES sheets to form three-dimensional disordered structure. The insets are the photos of aqueous dispersions of GO and GO-APTES after functionalization (concentrations of GO and GO-APTES in H2O = 0.05 mg/mL).

Fig. 6. XRD patterns of neat NBR and GO-APTES/NBR composite at GO-APTES loading of 2.0 vol.%.

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 7. SEM images of cross section of (a) neat NBR and (b-f) GO-APTES/NBR composites with GO-APTES fractions of 0.2 vol.%, 0.4 vol.%, 0.8 vol.%, 1.2 vol.%, 2.0 vol.%, respectively.

Fig. 8. (a) Vulcanization curves of GO-APTES/NBR composites; (b) △M of GO-APTES/NBR composites with different GO-APTES loadings; (c) swelling ratio and crosslink density of GO-APTES/NBR composites with different GO-APTES loadings; (d) stress-strain curves of GO-APTES/NBR composites.

Fig. 9. (a) Dielectric spectroscopy of GO/NBR composites showing comparison of different GO loading; (b) Dielectric spectroscopy of GO-APTES/NBR composites showing comparison of different GO-APTES loading; (c) Dielectric constant and loss factor of GO/NBR composites and GO-APTES/NBR composites as function of filler loading at 1 kHz; (d) Electrical conductivity of GO/NBR composites and GO-APTES/NBR composites as function of filler loading.