Article pubs.acs.org/IECR
High Performance Natural Rubber Composites with Well-Organized Interconnected Graphene Networks for Strain-Sensing Application Bin Dong,† Sizhu Wu,†,‡ Liqun Zhang,†,‡ and Youping Wu*,†,‡ †
State Key Laboratory of Organic−Inorganic Composites and ‡Beijing Engineering Research Center of Advanced Elastomers, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *
ABSTRACT: High-concentration reduced graphene oxide (RGO) solution was produced using gelatin (Gel) as the stabilizer and subreductant and hydrazine hydrate (HHA) as the main reductant. The Gel-HHA-RGO nanosheets exhibited excellent colloidal dispersibility and stability in alkaline condition. The Gel-HHA-RGO filled natural rubber (NR) composites were prepared by water-based solution casting. Well-organized interconnected RGO networks were constructed throughout the NR matrix, which played an important role in determining the properties of composites. The tensile modulus and dynamic storage modulus were improved by several orders of magnitude with increasing RGO content. Meanwhile, a dramatic increase in electrical conductivity with a low percolation threshold of 0.21 vol % was perceived. Strain-sensing tests revealed that the RGO/NR composites exhibited outstanding strain sensitivity and repeatability, which could be used to detect the cyclic movements of human joints. The results are promising in the rubber industry to guide the fabrication of highly sensitive and stretchable strain sensors for engineering application. affinity to proteins, and biocompatibility.11 Nabeta et al. demonstrated that gelatin was an excellent dispersing agent for the stable dispersion of carbon nanotubes in water.12 Ge et al. prepared water-dispersible graphene by the direct sonication of pristine graphite using gelatin as assisted dispersing agent.13 Considering the π−π stacking interaction, hydrophobic− hydrophobic and hydrogen-bond interactions between gelatin and GO nanosheets,14 gelatin should also be an ideal surface modifier for the functionalization of GO nanosheets. More significantly, with the plentiful active amino groups (−NH2) on its backbone, gelatin was supposed to be a mild reductant for the synthesis of RGO nanosheets. Liu et al., An et al., and Chen et al. prepared biocompatible RGO using gelatin as the reductant and stabilizer for drug delivery application, however, without considering the reducing efficiency of gelatin to GO.11,15,16 In their research, reduction of GO by gelatin was carried out under neutral or acidic conditions. However, the reducing efficiency and dispersibility of RGO nanosheets were considerably dependent on pH values, and they would be highly improved under alkaline conditions.2,14 Due to its naturally imparted exceptional properties, graphene is undoubtedly a promising and multifunctional candidate to develop high-performance polymer composites,
1. INTRODUCTION Graphene, a one-atom-thick sp2 carbon, has attracted considerable attention in recent years because of its exceptional mechanical properties, high thermal and electrical conductivity, and large theoretical specific surface area.1 Graphene oxide (GO) has been well demonstrated to be an excellent precursor to massively produce chemically reduced graphene oxide (RGO) by ultrasonic exfoliation and chemical reduction.2 However, reduction of GO in aqueous solution often results in aggregation due to the π−π stacking and van der Waals interactions; this is the key challenge in synthesis and processing of bulk-quantity graphene nanosheets.3 To overcome this problem, surface modification of GO prior to chemical reduction becomes especially essential. In addition, the introduction of surface modifiers offers opportunities for further interfacial design in fabricating polymer nanocomposites. In recent years, a great variety of chemicals such as poly(sodium 4-styrenesulfonate),3 organosilane,4 sodium humate,5 fluorescent white agent,6 tea polyphenol,7 and amino acids8 have been used to covalently or noncovalently modify the RGO for obtaining well dispersed graphene nanosheets. Gelatin, a product of partial hydrolysis of collagen with different 18 amino acids residues, is widely used in food and medical fields.9 Gelatin is a kind of water-soluble ampholytic polyelectrolyte which shows inherent cationic nature at pH below its isoelectric point and anionic nature at pH above its isoelectric point.10 It offers some advantages, such as excellent solubility in water, environmental friendliness, remarkable © XXXX American Chemical Society
Received: January 16, 2016 Revised: March 15, 2016 Accepted: April 12, 2016
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DOI: 10.1021/acs.iecr.6b00214 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research after carbon nanotubes. Compared with other graphene filled polymer composites, however, graphene and its derivatives as nanofiller for rubber composites receive much less attention, especially for nonpolar species such as NR which are of important technological interest.17−22 For obtaining the maximum benefit, the graphene nanosheets should be dispersed in the rubber matrix in the form of a single sheet or only a few sheets.23 Meanwhile, an effective interfacial stress transfer between graphene and the rubber matrix is essential. Therefore, concerns are mainly concentrated on the realization of homogeneous dispersion and effective interfacial interaction and then the enhancement in mechanical, electrical and thermal conductivity, and gas barrier properties. For fabrication of uniformly dispersed graphene/rubber nanocomposites, solution blending is the most commonly employed method in academic studies.24 However, there exists a persistent problem that the use of organic solvent will cause serious pollution. Nowadays water-based latex compounding and cocoagulation has been proved to be a more promising and environmentally friendly way to realize homogeneous dispersion of nanofiller.17,22 Concerning the effective interfacial interaction, the surface chemistry of RGO nanosheets which is controlled by the reduction degree plays a critical factor in determining the overall performance of rubber composites.25 Lately, surface modifiers such as organosilane,26 polydopamine,27 and allylamine28 were used to functionalize graphene to enhance the interfacial interaction between graphene nanosheets and rubber molecules. Recently, Zhan et al., Scherillo et al., and Potts et al. constructed a segregated nanofiller network of RGO/NR composites by direct static hot pressing without the two-roll mill process, based on latex mixing and cocoagulation.20−22 Results suggested that the segregated RGO structure effectively decreased the electrical conductivity percolation threshold20 and enhanced the gas permeability and thermal conductivity,21,22 however, resulting in poor mechanical properties, compared with the uniformly dispersed RGO networks after the two-roll mill treatment. Moreover, the ultrasonic-assisted latex mixing and in situ reduction (ULMR) technique which involved hydrazine reduction of GO using latex particles as the stabilizer would cause flocculation and restacking of RGO platelets during the reduction process. Meanwhile, the RGO nanosheets would also inevitably agglomerate during the cocoagulation process.22 Owing to the limitations of latex cocoagulation, Yang et al. prepared RGO/NR composites with high nanofiller content via vacuum-assisted self-assembly (VASA).29 Similarly, the well-organized RGO networks were formed by VASA. In our research, after the successful preparation of gelatin-functionalized RGO with high concentration, solution casting should be more appropriate for fabricating high-nanofiller-content and bulk-quantity RGO/ NR composites without nanofiller restack. Herein, for fabrication of highly reduced and stable dispersed RGO aqueous solution with high concentration, gelatin was used as the surface modifier and subreductant and HHA as the main reductant. A relatively weaker reducing efficiency of gelatin than that of traditional reductant was discovered, which was inadequate to fabricate graphene/polymer composites with high electrical conductivity. To extend the application of graphene in the rubber industry, solution casting was employed to fabricate Gel-HHA-RGO reinforced NR composites. Due to the remarkable affinity to proteins of gelatin, a self-assembly process between NR latex particles (with a protein-protective
shell) and gelatin modified RGO nanosheets was observed during the solution casting. The interconnected weblike RGO networks segregated by latex particles were constructed due to the self-assembly. Results revealed that the interconnected RGO networks played a dominating role in influencing the properties of RGO/NR composites. As a practical application, the strain-sensing performance of RGO/NR composites was measured by detecting the electrical response under applied strain. This research provides new insights into the great potential application of graphene in the rubber industry.
2. EXPERIMENTAL SECTION 2.1. Materials. Gelatin type B from bovine skin with gel strength of about 225 g Bloom was provided by Sigma-Aldrich Co., Ltd. NR latex (NRL) with a total solid content of 60% was supplied by Hainan Natural Rubber Industry Group Co., Ltd., China. Natural flack graphite power with an average of 13 μm was purchased from Huadong Graphite Factory, China. The reagents used for the oxidation of graphite, including concentrated sulfuric acid (H2SO4), potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrochloric acid (HCl), and ammonia solution (NH3·H2O) were all provided by Beijing Chemical Factory, China. Reductant hydrazine hydrate (N2H4· H2O, HHA) was also provided by Beijing Chemical Factory, China. Nanodispersed sulfur suspension with a solid content of 10 wt % was provided by Shanghai Huzheng Nano Technology Co., Ltd., China. The other additives such as zinc oxide (ZnO, 2.0 wt %) and accelerator zinc diethyl dithiocarbamate (ZDC, 2.0 wt %) aqueous suspension were commercially available industrial products. 2.2. Reduction of GO by Gelatin and Gelatin/HHA. Graphite oxide was synthesized from natural graphite by the modified Hummers method.30 Exfoliated graphene oxide (GO) suspension with a concentration of 5 mg/mL was produced by ultrasonic exfoliation of graphite oxide with an ultrasonic cell crusher for 60 min. For the synthesis of gelatin functionalized GO (Gel-RGO), 2.0 g of gelatin was added to 100 mL of H2O in a flask, and the solution was stirred under 60 °C to obtain a clear solution. Before the solution blending, the pH values of both GO and gelatin solutions were adjusted to about 10 using ammonia solution. A mass ratio of gelatin to GO (5:1) was chosen. Aqueous GO suspension was dropwise added into gelatin solution with continuous stirring to obtain a uniform gelatin-GO suspension. Then, the blended suspension was stirred and maintained at 95 °C for 12 h. For the synthesis of gelatin-hydrazine hydrate-reduced graphene oxide (Gel-HHARGO) nanosheets, HHA in an amount equal to 10 mg per 1 mg of GO was added into the Gel-RGO suspension. The blended suspension was allowed to stir for another 12 h at 95 °C. The residual gelatin was removed by intensive centrifugation with abundant hot water for three times. The exfoliated Gel-RGO and Gel-HHA-RGO suspension with high concentration could be readily obtained by resonication under alkaline conditions. 2.3. Preparation of Gel-HHA-RGO Reinforced NR Composites. Gel-HHA-RGO/NR composites were fabricated by the simple solution-casting method. Typically, a designed amount of Gel-HHA-RGO aqueous suspension with a concentration of 5 mg/mL was incorporated into NRL under vigorous agitation for 30 min until the mixture became homogeneous. Then, a certain amount of vulcanizing agent aqueous dispersion including sulfur (2 phr), ZnO (0.5 phr), and ZDC (1 phr) was poured into the above-mentioned B
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2.7. Strain-Sensing Measurements. Strain-sensing tests were also conducted on the electrochemical workstation under a constant voltage of 5 V. The rectangular samples with a thickness of about 0.1 mm were stretched to a certain strain by a homemade stretching device. The computer recorded the changes of current signals with time. Under the constant voltage, the relative changes of resistance ΔR/R0, where ΔR = R−R0 and R0 was the initial resistance of samples, could be calculated based on Amperometric I-t curves. To monitor the bending movement of joints, the samples were attached to the back side of an index finger. With the cyclic movement of the finger under different blending amplitudes, the Amperometric I-t curves were recorded. Low motion amplitude (about 45° bending angle) and high motion amplitude (about 90° bending angle) were measured.
blending solution under mechanical stirring for another 30 min. After that, the homogeneous Gel-HHA-RGO/NRL blending solution including the vulcanizing agent was poured into Petri plates and then dried at 50 °C for film formation until a constant weight. After the solution casting, the obtained NR composites were vulcanized. The weight contents of RGO in the NR composites were designed to be 0, 0.3, 0.6, 1.8, 3.0, 4.8, 6.0, and 7.2 wt %, respectively. The volume fraction of RGO was determined by using the densities of 2.20 g/cm3 for RGO and 0.94 g/cm3 for neat NR. The corresponding volume fractions of RGO in the NR matrix were determined to be 0 vol %, 0.12 vol %, 0.25 vol %, 0.75 vol %, 1.25 vol %, 2.00 vol %, 2.50 vol %, and 3.00 vol %, respectively. 2.4. Characterization. UV−vis spectra were obtained from a UV-2600 (Shimadzu, Japan) UV−vis spectrophotometer in the wavelength range of 200−600 nm. Atomic force microscopy (AFM; Nano Scope Analysis, Bruker, Germany) was employed to analyze the morphology and thickness of GO and Gel-HHA-RGO nanosheets in tapping mode. The samples were prepared by dropping the dilute solution on freshly cleaved mica and drying under room temperature. The FourierTransform Infrared (FT-IR) spectra were obtained using a Bruker Tensor 27 spectrometer in transmission mode. Raman spectra were collected by confocal Raman spectroscopy (Renishaw inVia, England) from 800 to 2000 cm−1 at an excitation wavelength of 514 nm. Thermal gravity analysis (TGA) was carried out ranging from 30 to 800 °C under nitrogen atmosphere at a heating rate of 10 °C/min using a Mettler Toledo TGA1100SF (Switzerland). X-ray photoelectron spectroscopy (XPS) was obtained by an Escalab 250 XPS system (Thermo Electron Corporation, USA) with an Xray source of Al Kα radiation. The surface morphology of samples was observed on a Hitachi S-4800 scanning electron microscope (SEM) operated at an acceleration voltage of 5 kV. The exfoliated morphology of Gel-HHA-RGO and its dispersion morphology in the NR matrix were observed by transmission electron microscopy (TEM; Tecnai G2 20, Hong Kong FEI) operated at an accelerating voltage of 200 kV. The ζ-potential (zeta potential) of GO before and after reduction was measured with a Malvern Zetasizer (Nanoseries, Australia). 2.5. Mechanical Properties and Dynamic Mechanical Analysis. Tensile tests were performed using a CMT4104 electrical tensile instrument (SANS, Shenzhen, China) with a cross-head speed of 200 mm/min at 25 °C according to the standard ISO 37:2011. The storage modulus E′, loss modulus E″, and loss factor tan δ as a function of temperature were measured with a VA3000 dynamic mechanical thermal analyzer (DMTA; AREVA 01Db-Metravib Co., Ltd., France) under tensile mode. DMTA tests were performed under a frequency of 5 Hz and wide temperature range from −80 to 80 °C with a heating rate of 3 °C/min. 2.6. Electrical Conductivity and Tunneling Current− Voltage Measurements. Electrical conductivities of GO, RGO, and their NR composites with conductivity lower than 1 × 10−4 S/m were measured by a high resistance meter (EST121, Beijing Huajinghui Technology Co., Ltd., China), whereas that for materials with conductivity higher than 1 × 10−4 S/m were measured with the RTS-8 four-point probes resistivity systems. The tunneling current−voltage relationships were operated on an electrochemical workstation (CHI 660E, CH Instruments Inc., Shanghai, China) under a continuously changed voltage ranging from −5 V to +5 V.
3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of GelatinFunctionalized Graphene. A schematic diagram of the fabrication of gelatin functionalized graphene is presented in Figure 1. The chemical structure of gelatin is shown in Figure
Figure 1. Schematic diagram of gelatin functionalized graphene synthesis.
S1 in the Supporting Information (SI). First, the gelatin molecules could be adsorpted onto the surfaces of GO nanosheets through π−π, hydrophilic−hydrophobic, and hydrogen-bonding interactions.11,14 Once gelatin was attracted to the surfaces of nanosheets, further agglomeration was effectively prevented. After the noncovalent adsorption, gelatin was then covalently grafted onto RGO nanosheets through the ring-opening reaction between epoxy groups on GO and free amino groups on gelatin.31 For a further reduction of Gel-RGO, HHA was added subsequently. Considering the reaction between HHA and oxygen functional groups on gelatin molecules, the optimal mass ratio of HHA to GO was determined to be 10:1 (results are presented in Figure S2). It should be pointed out that the ammonia that we used to adjust the pH was also an effective reducing agent in the reduction of GO. However, considering the large addition content of gelatin (the weight ratio of gelatin to GO was 5:1), the content of ammonia was very little and could be ignored. The surface morphology of as-prepared Gel-HHA-RGO was first characterized by SEM and TEM (presented in Figure 2) for general view. The Gel-HHA-RGO displayed a typical flakelike morphology with obvious crumpled and undulating wrinkles on its surface. The flake-like morphology with dimensions changed from hundreds to thousands of nanometers was also illustrated by TEM. In addition, the appearance of separated RGO nanosheets clearly revealed that the Gel-HHAC
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Figure 2. Typical SEM (a), (b) and TEM (c), (d) micrographs of Gel-HHA-RGO nanosheets.
Figure 3. (a) FT-IR spectra of graphite, GO, gelatin, and Gel-HHA-RGO; (b) UV−vis absorption spectra and digital photographs of aqueous dispersions (inset) of GO, Gel-RGO, and Gel-HHA-RGO; (c) Measured electrical conductivities of GO, Gel-HHA, Gel-HHA-RGO, and HHARGO; (d) Raman spectra of GO, Gel-HHA, HHA-RGO, and Gel-HHA-RGO.
highly negatively charged and well dispersed in water;33 this was the premise for further chemical reduction and functionalization. As for gelatin, the broad peak around 3500 cm−1 was assigned to the stretching vibration of N−H coupling with hydroxy. The peaks at 1646 and 1535 cm−1 were attributed to the stretching vibrations of CO and C−N, respectively. Compared with the spectrum of gelatin, the presence of characteristic peaks at 1535 cm−1 (C−N stretching vibration) for Gel-HHA-RGO clearly indicated the functional-
RGO exhibited excellent stability in the form of a single nanosheet in water. The functionalization of graphene by gelatin was first confirmed by FT-IR spectroscopy which was shown in Figure 3a. Compared with that of graphite, the spectrum of GO showed the presence of alkoxy (C−O, 1052 cm−1), epoxy (C− O−C, 1226 cm−1), hydroxy (O−H, 1410 cm−1), carbonyl (CO, 1624 cm−1), and carboxyl (COOH, 1730 cm−1).32 The presence of abundant oxygen-containing groups made GO D
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Figure 4. Tapping-mode AFM images and height profiles of (a) GO and (b) Gel-HHA-RGO.
ization of graphene by gelatin.34 In addition, the presence of nitrogen element which was characterized by Energy Dispersive X-ray Spectra (EDX, in Figure S3) also implied that GO was functionalized by gelatin. The reduction of GO by gelatin and HHA was also monitored by UV−vis absorption spectroscopy which was presented in Figure 3b. The absorption peak of GO dispersion around 231 nm (attributed to π−π* transitions of aromatic CC bonds) gradually shifted to 265 nm, and the shoulder around 300 nm (attributed to n-π* transitions of aromatic C O bonds) disappeared, suggesting that the electronic conjugation was restored upon gelatin reduction.35 However, after the incorporation of HHA, the absorption peak at 231 nm was shifted to a much longer wavelength of 272 nm, suggesting a further reduction of RGO. In addition, the color of solution changed from yellow brown to dark after the reaction, and this could also confirm the reduction of GO.36 ζ-potential measurements were carried out to evaluate the stability of Gel-HHA-RGO aqueous suspension under alkaline conditions. For GO, an average ζ-potential of about −51 mV was observed, due to the presence of large numbers of hydroxy and carboxyl groups with a highly negative charge.37 After the reduction by gelatin and HHA, ζ-potential decreased to about −43 mV because of the partial removal of oxygen functional groups. According to the definition of colloid stability with ζpotential by the American Society for Testing and Materials (ASTM Standard D4187-82), the Gel-HHA-RGO aqueous dispersion exhibited excellent stability as its ζ-potential was lower than −40 mV. Therefore, the strong electrostatic repulsion among RGO nanosheets could ensure their stability for a long time. We found that with a concentration of 5 mg/ mL, the Gel-HHA-RGO dispersion could still keep its stability for several months without precipitate, which was favorable for the fabrication of highly filled graphene/polymer composites. Generally, electrical conductivity of RGO could directly reflect the restoration extent of electronic conjugation.38 The electrical conductivities of GO and its functionalized samples
were measured and presented in Figure 3c. It was noted that the conductivity of GO was improved from 2.4 × 10−7 S/m to 0.018 S/m after the reduction of gelatin, indicating that GO was reduced by gelatin, and the electronic conjugation was reestablished. However, the conductivity of RGO reduced by HHA could reach about 2510 S/m. Compared with that of HHA, the reducing capability of gelatin was considerably poor, and the conductivity of 0.018 S/m could not meet the demands for fabricating highly conductive graphene filled polymer composites. Therefore, HHA was added into Gel-RGO solution to obtain highly conductive RGO. Results showed that the conductivity of Gel-HHA-RGO reached 420 S/m; however, it was still slightly lower than that of HHA-RGO. The comparatively lower electrical conductivity of Gel-HHA-RGO was attributed to the gelatin that adsorbed and grafted on the surfaces of RGO. Raman spectroscopy, a useful tool to characterize carbonaceous materials such as carbon nanotubes and graphene, was employed to further monitor the microstructural changes before and after functionalization. The Raman spectra of GO and its functionalized samples are presented in Figure 3d. Two dominant bands, denoted as D band (located at about 1350 cm−1) and G band (located at about 1600 cm−1), were all observed for all the samples.39 The D band was ascribed to the vibration of sp3 hybridized carbon and disordered structure on the edges of GO, and the G band was derived from the vibration of sp2 hybridized carbon atoms in the graphite lattice.5 The intensity ratio of the D band and the G band (ID/IG) was often employed to reflect the defects of graphene and monitor the functionalization of graphene.40 Compared with that of GO, the ID/IG values of functionalized samples were all increased, implying the restoration of conjugated structure and more disordered structure with a smaller averaged area after reduction. The ID/IG value of Gel-HHA-RGO (1.20) was slightly higher than that of HHA-RGO (1.12), suggesting the higher defect concentration and more disordered structure after gelatin/HHA reduction. The RGO reduction was further E
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negatively charged in alkaline conditions. Therefore, the strong electrostatic repulsion between RGO nanosheets and NRL particles ensured the stable dispersion in aqueous solution. However, during the solution-casting process, with the gradual evaporation of water, the electrostatic repulsion became weakened. The Gel-HHA-RGO nanosheets could be selfassembled onto the surfaces of NRL particles due to the remarkable affinity between proteins and gelatin which was ascribed to the hydrogen-bond interactions. The self-assembly between RGO nanosheets and NRL particles was presented in Figure S5. Thanks to the self-assembly, the formed segregated structure became the original prototype of interconnected RGO networks in the NR matrix. SEM and TEM were simultaneously employed to examine the dispersion morphology of Gel-HHA-RGO nanosheets in the NR matrix, and the typical SEM and TEM micrographs are presented in Figure 6. Figure 6a showed the rugged and
characterized by XPS measurements. The C/O atom ratio of GO was measured to be 2.10; however, it shifted to 3.82 for Gel-RGO. Furthermore, the C/O ratios of HHA-RGO and Gel-HHA-RGO were 6.94 and 7.06, respectively, implying the similar reducing efficiency between gelatin/HHA and HHA. AFM was employed to characterize the thickness and morphology of graphene. Typical AFM images and its crosssection analysis of GO and Gel-HHA-RGO nanosheets are presented in Figure 4. The GO nanosheets exhibited lateral dimensions of several hundred nanometers and a thickness of about 0.9 nm, implying the complete exfoliation by sonication. After the modification, the thickness of obtained Gel-HHARGO nanosheets increased to about 3.2 nm. The increase in thickness was attributed to the attachment of gelatin on both sides of RGO. To roughly quantify the content of gelatin that was immobilized on RGO surfaces, TGA measurements were carried out (results are presented in Figure S4). Considering the overall weight losses of HHA-RGO and gelatin, it was reasonable to estimate that the content of gelatin immobilized on RGO nanosheets was about 66 wt %. It should be pointed out that this value was a little higher than other reported contents,5,6,41 which was mainly ascribed to the noncovalent adsorption and covalent grafting. The gelatin immobilized on the surfaces of RGO could keep the RGO independent from each other and prevent agglomeration in aqueous solution. Meanwhile, the gelatin was also supposed to act as an interfacial compatibilizer to improve the interfacial stress transfer between NR chains and RGO nanosheets. 3.2. Dispersion Morphology of Gel-HHA-RGO in the NR Matrix. The dispersion morphology of nanofiller in composites had a close relationship with the processing approach and caused a tremendous influence on the properties of composites. Gel-HHA-RGO filled NR composites were prepared by solution-casting method. The illustration of fabrication of Gel-HHA-RGO/NR composites is presented in Figure 5. Different from the latex cocoagulation which involved the inevitable restack of RGO once the addition of flocculant,22 solution casting could effectively prevent the restack by the obstruction of NRL particles. The NRL particles which were surrounded by a protein-protective shell were highly negatively charged due to the carboxylic acid ionization of surface protein.42 As mentioned above, the Gel-HHA-RGO were also
Figure 6. (a-d) SEM micrographs of freeze-fractured Gel-HHA-RGO/ NR composites sections; (e-h) TEM micrographs of Gel-HHA-RGO/ NR composites with varied RGO contents which showed that RGO nanosheets were wrapped around NRL particles to form an interconnected weblike networks.
undulated morphology of NR fractured surface, and the shiny parts represented the RGO nanosheets in the NR matrix. With the increase in RGO content, some obvious hemispherical holes appeared. As we know, the hemispherical holes were caused by the removal of spherical NRL particles that were surrounded by RGO nanosheets. The morphology reflected
Figure 5. Illustration of the preparation of Gel-HHA-RGO/NR composites with interconnected nanofiller structure by solutioncasting method. F
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Figure 7. (a) Representative stress−strain curves of Gel-HHA-RGO/NR composites with varied RGO content; (b) DMTA plots of storage modulus E′ versus temperature for Gel-HHA-RGO/NR composites with varied RGO content.
Figure 8. (a) Current−voltage characteristics of NR composites filled with different content of Gel-HHA-RGO; (b) Electrical conductivities of NR composites as a function of filler volume fraction. Inset: log σC plotted against log (φ−φC), where φC is the percolation threshold.
from SEM implied the formation of segregated RGO networks. TEM micrographs provided a more intuitive evidence of the construction of interconnected RGO networks. The spherical NRL particles were isolated from each other by RGO nanosheets, and the mutually connected RGO nanosheets contacted with each other to form three-dimensional weblike networks throughout the rubber matrix. Comparing the nanofiller structure that was constructed by latex cocoagulation,20−22 the weblike RGO structure in our solution-casted rubber matrix was much more pronounced and betterorganized. Due to the special structure which all nanofillers could participate in the construction of continuous and interconnected networks, it was expected that it would introduce a significant influence on the properties of NR composites. 3.3. Mechanical Behaviors of Gel-HHA-RGO/NR Composites. The typical stress−strain curves of Gel-HHARGO/NR composites are presented in Figure 7a. Results suggested that the incorporation of RGO greatly improved the mechanical properties of NR composites. The tensile strength reached its maximum when the volume fraction of RGO was 1.25 vol %. Results of Young’s modulus (initial tensile modulus) implied that the tensile stiffness of NR composites increased dramatically with increasing RGO loading. By adding only 2.00 vol % Gel-HHA-RGO, the Young’s modulus of the composite exhibited an increase of about 30-fold over neat NR. Surprisingly, different from the traditional stress−strain behaviors of graphene/rubber composites, an obvious stress
yielding phenomenon followed by a high elongation at break occurred when the RGO content reached 2.00 vol %. That was to say, the Gel-HHA-RGO/NR composites prepared by solution casting were possessed of a high stiffness, in combination with an excellent stretchability. This unique mechanical behavior was mainly determined by the special dispersion structure of Gel-HHA-RGO in the NR matrix and the effective interfacial interaction between Gel-HHA-RGO and NR chains. When the samples were deformed under stretching, the much tougher RGO networks should be first stretched and deformed, which induced a drastic increase in initial modulus, and then the soft rubber phase was deformed. Thanks to the effective interfacial stress transfer from the NR matrix to RGO sheets imparted by gelatin, the composites exhibited an excellent stretchability without rapid fracture. Zhan et al. and Potts et al. prepared RGO/NR composites with a weblike RGO structure by latex cocoagulation.20,22 Their prepared composites would be immediately ruptured without the following stretching process, and a much lower elongation at break was observed. In other words, the stress−strain behaviors of their samples trended toward that of a thermoplastic. The temperature dependence of storage modulus E′ is shown in Figure 7b. It could be observed that the E′, which provided insight into the stiffness of rubber, was drastically increased with increasing RGO content throughout the range of temperatures investigated, especially in the rubbery region. The neat NR owned an E′ of about 2 MPa under room temperature; however, the NR composites filled with 2.00 vol G
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Figure 9. (a) Relative changes of resistance ΔR/R0 versus applied strain of Gel-HHA-RGO/NR composites; (b) Gauge factors GF of Gel-HHARGO/NR composites under different applied strain.
Figure 10. (a) Photographs showing the strain sensors attached on index fingers for recording the bending movements of the finger; (b) Electrical response of strain sensor (filled with 1.25 vol % RGO) for motion detection of fingers.
% RGO possessed an E′ of 154 MPa. Even more strikingly, when the volume fraction of RGO was 3.00 vol %, the E′ dramatically reached about 846 MPa. Comparing the E′ of 3.00 vol % composites with that of neat NR, the modulus increased by a factor of 423there was a difference of about 3 orders of magnitude between them. The considerably increased E′ could be explained by the construction of interconnected RGO networks with much higher stiffness and the hydrodynamic effect associated with the strong interaction between RGO and NR chains.43,44 The loss modulus E″ and loss factor tan δ versus temperature for Gel-HHA-RGO/NR composites were presented in Figure S6. Results suggested that the local segmental dynamics of NR composites was not influenced by the incorporation of Gel-HHA-RGO nanosheets. 3.4. Electrical Conductivity of Gel-HHA-RGO/NR Composites. The dispersing morphology of nanofiller in rubber composites has a significant influence on the electrical properties. The tunneling current−voltage relationships of RGO/NR composites were presented in Figure 8a. The RGO/ NR composites exhibited a nonlinearity under low nanofiller content (0.75 vol %), suggesting the typical semiconducting behavior. Meanwhile, the increased linearity and tunneling current with increasing RGO content clearly indicated the enhanced electrical properties. The electrical conductivities of Gel-HHA-RGO/NR composites as a function of filler volume fraction are summarized in Figure 8b. It was noted that the conductivity of composites increased from 4.06 × 10−12 S/m to 0.23 S/m when the content of RGO was increased from 0.12 vol % to 3.00 vol %. The percolation threshold at which the conductivity dramatically increased with slightly increasing filler
content was around 0.21 vol %. Basically, highly conductive nanofillers were incorporated into polymers to construct threedimensional conductive pathways of nanofiller throughout the composites. The high electrical conductivity and lower percolation threshold of Gel-HHA-RGO/NR composites could be attributed to the dispersion morphology of RGO in the NR matrix. It was believed that the interconnected RGO networks could dramatically improve the effective volume fraction of nanofiller and the probability of interparticle contact, resulting in more pathways for conduction, compared with the traditional uniform nanofiller morphology after the two-roll mill processing.20 According to the percolation theory, the relationship between electrical conductivity and electrical percolation threshold can be determined by a power law relation45,46 σC ∝ (φ − φC )τ
where σC is the electrical conductivity of composites, φ represents the filler volume fraction, φC is the critical percolation threshold, and τ is the critical exponent. The best critical exponent τ to fit the log−log plot is about 7.85. It was proved that τ depends only on the dimensionality of composites.47 Especially, τ < 2.1 corresponds to the polymerbridged particle networks in the polymer matrix, whereas τ > 3.75 indicates the construction of direct particle−particle interaction networks.48,49 Therefore, the τ value of 7.85 clearly indicated the formation of conductive three-dimensional networks, which has been demonstrated by SEM and TEM micrographs. H
DOI: 10.1021/acs.iecr.6b00214 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research 3.5. Strain-Sensitive Performance of Gel-HHA-RGO/ NR Composites. Recently, the use of graphene filled conductive elastomer composites in sensing applications has caused great interests.50−53 The application of Gel-HHA-RGO/ NR composites in strain sensors was measured by detecting the electrical response under applied strain. Figure 9a presents the relative resistance change ΔR/R0 of Gel-HHA-RGO composites under varied strain up to 50%. We noted that the ΔR/R0 dramatically increased with increasing strain, suggesting an outstanding sensitivity for applied deformation. The increased ΔR/R0 was ascribed to the deformation and breakdown of interconnected RGO networks under applied strain. Meanwhile, the gauge factor (GF), defined as ΔR/(εR0), where ε was applied strain, was used to measure the responsible sensitivity of strain sensors. The GF under different strain ranged from 5% to 25% were calculated and presented in Figure 9b. It was noted noted that the strain sensitivity strongly depended on the content of RGO. The NR composite filled with 1.25 vol % RGO exhibited higher ΔR/R0 and GF than that filled with 0.75 vol % RGO under varied strain. However, the ΔR/R0 and GF highly decreased when the content of RGO exceeded 1.25 vol %, implying a decreased strain sensitivity. Compared with the conventional strain sensors which exhibited poor sensitivity (GF ≈ 2) and weak stretchability (maximum strain up to 5%),50 our prepared Gel-HHA-RGO/NR composites with three-dimensional interconnected RGO networks exhibited much higher stretchability and sensitivity in strain-sensing application. As a practical application of strain sensors, NR composite that was filled with 1.25 vol % RGO was chosen to moniter the bending movement of index fingers (Figure 10). It was noted that the ΔR/R0 was sensitively changed with joint movement. After repetition for about 50 times, it still exhibited discernible strain sensitivity, implying the excellent repeatability of this strain sensor. A further cyclic measurement (about 200 times) was presented in Figure S9. Meanwhile, the different motion amplitude of a finger caused a different electrical response: the high motion amplitude of a finger (about 90° bending angle) caused a stronger electrical response than the low motion amplitude (about 45° bending angle), implying the excellent sensitivity for applied strain. The results confirmed that the RGO/NR composites with interconnected conducting nanofiller networks possessed the potential application for monitoring the cyclic motions of human joints.
graphene with excellent solubility and stability in large-scale production and thus pioneers a new platform for development of high-performance RGO/NR composites for strain-sensing application.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00214. Chemical structure of gelatin. Reduction level of GelRGO by HHA. Energy Dispersive X-ray (EDX) spectra of GO and Gel-HHA-RGO. TGA curves. Self-assembly between Gel-HHA-RGO nanosheets and NRL particles. Dynamic viscoelastic properties of Gel-HHA-RGO composites. Electrical properties of Gel-HHA-RGO/ NR composites prepared by latex cocoagulation. Electrical response of bending movements of index finger joints (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: 86-10-64442621. Fax: 86-10-64456158. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to gratefully acknowledge the financial support of the State Key Program of National Natural Science of China (51333004), the Ministry of Science and Technology of China (2014BAE14B01), and the National Key Technology Support Program of China (2013BAF08B03).
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REFERENCES
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4. CONCLUSIONS In summary, gelatin functionalized RGO nanosheets with excellent colloidal dispersibility and stability were synthesized under alkaline conditions. Solution casting was employed to fabricate Gel-HHA-RGO/NR composites with well-organized interconnected RGO networks throughout the NR matrix. The weblike morphology of RGO networks played an important role in determining the properties of NR composites. The stiffness of NR composites was enhanced by several orders of magnitude with increasing RGO content. With interconnected RGO networks, the prepared RGO/NR composites exhibited excellent electrical properties with a low percolation threshold of about 0.21 vol %. The strain-sensing measurements revealed that the RGO/NR composites with interconnected nanofiller networks exhibited excellent strain sensitivity and repeatability under applied deformation, which could successfully detect movements of human joints. The present research provides a promising strategy for the preparation of highly reduced I
DOI: 10.1021/acs.iecr.6b00214 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.iecr.6b00214 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
