Tunable Functionalization of Graphene Oxide Sheets through Surface

Feb 10, 2015 - Surface functionalization of graphene oxide (GO) sheets using polymers has emerged as a subject of enormous scientific interest due to ...
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Tunable Functionalization of Graphene Oxide Sheets through Surface-Initiated Cationic Polymerization Baopeng Li,† Wenpeng Hou,† Jinhua Sun,† Shidong Jiang,† Linli Xu,† Guoxing Li,† Mushtaque A. Memon,† Jianhua Cao,† Yong Huang,† Christopher W. Bielawski,‡,§ and Jianxin Geng*,† †

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Haidian District, Beijing 100190, China ‡ Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States § Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea S Supporting Information *

ABSTRACT: Surface functionalization of graphene oxide (GO) sheets using polymers has emerged as a subject of enormous scientific interest due to the wide applications of GO in polymer composites and functional graphene-based materials. In this study, we grafted GO sheets with polystyrene (PS) and poly(styrene−isoprene) (PSI) using GO itself as a cationic initiator for homopolymerization of styrene and copolymerization of styrene and isoprene. The resultant GOg-PS and GO-g-PSI composites displayed enhanced dispersibility in common organic solvents. With increasing the loading of isoprene in the copolymerization reaction, the glass transition temperature of the obtained products gradually decreased, combining the increased roughness of the GO-g-PSI sheets compared with the GO-g-PS sheets, which indicated the increased phase separation between the PS and PI segments in the PSI layer. Therefore, the packing of the GO-g-PS sheets, as well as the GO-g-PSI sheets, was not as compact as that of the GO sheets, leaving gradually increased quantity of pores in the films prepared with GO-g-PS and GO-g-PSI. Capitalizing on these tunable characters, hybridized membranes prepared by depositing GO sheets, GO-g-PS sheets, and the GO-g-PSI sheets obtained with gradually increased loading of isoprene in the copolymerization on the surfaces of commercially available polytetrafluoroethylene membranes displayed gradually increased gas permeability.

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polar organic solvents.23 On the other hand, the π−π interactions between the basal planes lead to the stacking of GO sheets, which often hampers dispersibility of GO in less polar organic solvents and polymer matrices. Since the incorporation of GO and RGO into various polymer matrices as fillers affords access to high-performance polymer composites, extensive efforts have been devoted to enhancing the dispersibilities of GO and RGO. While the noncovalent functionalization of GO with surfactants,24 polyelectrolytes,25 or organic conjugated systems affords stable suspensions of GO or RGO,11,26 the covalent functionalization of GO with polymers effectively provides access to graphene sheets, which exhibit not only miscibility with the polymer matrices27,28 but also the useful properties of the polymers.19,21,26 For example, our group grafted the conjugated polymer, poly(3-hexylthiophene) (P3HT), onto the surfaces of GO sheets through a grafting-to approach that utilized the functional groups present on the GO and the terminal groups

INTRODUCTION Graphene has attracted great attention due to its excellent electrical, optical, and mechanical properties since the report by Geim and co-workers in 2004.1 Graphene can be prepared by various methods including mechanical exfoliation,1 micromechanical cleavage of natural graphite,2−4 chemical vapor deposition,5−9 and epitaxial growth.10 The quality of the graphene prepared by these methods is high, but the yield is typically low, which poses an intrinsic limitation for many potential applications. Fortunately, a chemical method, which involves the oxidation of natural graphite powders followed by the reduction of the resultant graphene oxide (GO), is a highyield approach for production of reduced graphene oxide (RGO).11 Indeed, the graphene materials prepared using the aforementioned chemical method have been employed in a variety of applications, such as transparent conducting films,11,12 energy storage,13−17 and polymer composites.15,18−21 GO displays a broad range of functional groups including unsaturated double bonds, epoxy and hydroxyl groups on the basal planes, and carboxylic acid groups at the edges.22 Because of the hydroxyl and carboxylic acid groups, GO sheets are strongly hydrophilic and can be dispersed in water and some © XXXX American Chemical Society

Received: December 31, 2014 Revised: February 2, 2015

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Macromolecules of P3HT chains19 and found that the resultant RGO/P3HT composite displayed interesting dual-fluorescence and enhanced photothermal properties.20,21 Previously, GO was functionalized with various monomers using atomic transfer radical polymerization (ATRP), which followed covalent attachment of ATRP initiators to the GO.29,30 In addition, GO has also been modified via grafting-through methodologies that involve free radical polymerization,31 polycondensation polymerization,27,28 or Ziegler−Natta polymerization.32 Recently, GO was shown to function as a heterogeneous catalyst for various synthetic and polymerization reactions.33−35 For instance, GO catalyzed the cationic polymerization of butyl vinyl ether34 and the cationic ring-open polymerization of εcaprolactone.35 The aforementioned catalytic activities derive from the acidic functionalities present on the surfaces of GO sheets, which effectively initiate the cationic polymerizations. Moreover, these polymerization reactions often afford polymer composites containing GO as the filler. Different from traditional composite forming processes, in which polymers are first prepared separately and then blended with carbon fillers, the GO-initiated polymerization method merges effectively these two steps into one: the GO initiates the polymerization reaction and functions as the filler simultaneously. Regardless, the previously reported examples of functionalizing GO utilized only homopolymers, which places intrinsic limitations on the surface properties of the resulting composites. Herein, GO was used as cationic initiator for the homopolymerization of styrene as well as the copolymerization of styrene and isoprene. As a result, the surfaces of GO were effectively grafted with the homopolymer of styrene (PS) and the copolymer of styrene and isoprene (PSI). The grafted GO sheets, designated as GO-g-PS and GO-g-PSI, showed enhanced dispersibility in common solvents, including tetrahydrofuran (THF), chloroform, and toluene. The roughness of the films of the aforementioned materials was found to increase with PI content, which resulted in a number of interesting and potentially useful surface properties. Finally, membranes with tunable gas permeabilities were prepared by depositing the polymer-grafted GO sheets on the surfaces of commercially available polytetrafluoroethylene (PTFE) membranes.

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the reaction. To investigate the impact of the ratio between styrene and isoprene on the polymerization and the properties of the resultant composites, various ratios of styrene and isoprene, including 10:1, 10:2, 10:5, and 10:10, were explored. The grafted GO sheets were designated as GO-g-PSI(10:1), GO-g-PSI(10:2), GO-g-PSI(10:5), and GO-g-PSI(10:10), respectively. Preparation of GO-g-PS and GO-g-PSI Films. Films of GO-g-PS or GO-g-PSI were prepared by using a previously reported vacuum filtration method.12,37 The GO-g-PS and the GO-g-PSI obtained with different styrene/isoprene ratios were dispersed in THF to obtain suspensions with concentrations of 0.1 mg mL−1. Afterward, 1.5 mL of the suspension was applied to vacuum filtration using an anodic aluminum oxide (AAO) membrane (25 mm diameter and 0.1 μm pores) which led to the deposition of a composite film on the membrane’s surface. Note that the effective diameter of the films prepared using the 25 mm diameter membranes was 20 mm. After being dried in an oven set to 60 °C for at least 4 h, the AAO membrane was exposed to an aqueous solution of NaOH (3 M), which led to the dissolution of the AAO membrane and deposition of the composite film on the surfaces. After the basic solution was fully replaced with deionized (DI) water, the composite film was transferred to a substrate (e.g., a glass slide) and then dried at 60 °C overnight in an oven. As a control, GO films were prepared using GO suspension in DI water by following a similar procedure. The films were characterized using goniometry and scanning electron microscopy (SEM). The gas permeability measurements utilized hybrid membranes that were prepared by depositing the GO-g-PS or GO-g-PSI onto commercially available PTFE membranes (25 mm diameter and 0.1 μm pores). Different quantities of the GO-g-PS or GO-g-PSI suspensions in THF (0.1 mg mL−1) were used to control their respective film thicknesses. Film thicknesses were evaluated by measuring the corresponding films deposited onto glass substrates. Characterization. FT-IR spectra were recorded on an Excalibur 3100 spectrometer with a resolution of 0.2 cm−1 using KBr pellets. Raman spectra were collected on a Renishaw inVia-Reflex confocal Raman microscope with an excitation wavelength of 532 nm. Thermogravimetric analysis (TGA) was performed with a Q50 TGA at a scanning rate of 5 °C min−1 under the protection of N2. Differential scanning calorimetry (DSC) data were collected on Mettler Toledo DSC1 Star System. The scanning temperature ranged from −80 to 140 °C, and the scanning rate was 20 °C min−1 for the first cycle and 10 °C min−1 for the second cycle. Atomic force microscopy (AFM) data were recorded on a Bruker Multimode 8 atomic force microscope in the tapping mode with a cantilever having a resonance frequency of 128−197 kHz. AFM samples were prepared by spin-coating suspensions of GO, GO-g-PS, or GO-g-PSI onto freshly cleaved mica slides. Transmission electron microscope (TEM) observations were performed on a JEOL JEM-2100 TEM operated at 200 kV. TEM samples were prepared by spin-coating suspensions of GO, GO-g-PS, or GO-g-PSI onto carbon-coated mica slides and transferring the carbon film supported samples onto 400 mesh Cu grids. SEM observations were performed on a Hitachi S4300 field emission SEM operated at acceleration voltage of 10 kV. Contact angles of the membranes were measured with the sessile drop method, using a goniometer (Contact Angle System OCA20, DataPhysics Instruments). The thicknesses of the GO, GO-g-PS, and GO-g-PSI films deposited on glass substrates were measured using a surface profiler (Bruker DektakXT). Gas permeability of the hybrid membranes was measured using a capillary flow porometer (CFP1500A, Porous Materials Inc.). The diameter of the gas path was 6 mm.

EXPERIMENTAL SECTION

Materials. GO was synthesized by following a modified Hummers’ method.11,36 The monomers including styrene and isoprene (extra purity grade) were purchased from Aladdin Industrial Corp. The polymerization inhibitor in styrene was removed using basic alumina column before polymerization. All the other chemicals were purchased from Sinopharm Chemical Co. Ltd. and used as obtained. GO-Initiated Cationic Polymerization. Prior to being used in a polymerization reaction, the GO was first ground into a fine powder. In a typical polymerization, styrene (3 mL) and GO powder (1 wt % of the styrene) were added to a 25 mL Schlenk flask. After sonication for 1 h, the reaction flask was placed into a 65 °C oil bath and stirred for 24 h. The reaction vessel was then cooled to room temperature and diluted with THF so that the resulting product, including free PS and PS-grafted GO sheets (GO-g-PS), could be precipitated from methanol. The product was obtained via filtration and dried in a vacuum oven. The GO-g-PS composites were isolated by redispersing the material in THF (200 mL) with the aid of sonication followed by vacuum filtration. For the copolymerization of styrene and isoprene, polymerization and separation conditions similar to those described above were used with the exception that a mixture of styrene and isoprene was used in

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RESULTS AND DISCUSSION Compared with other surface-initiated approaches such as ATRP and Ziegler−Natt--type polymerizations,29,30,32 the use of GO as cationic initiator for surface modification of GO sheets simplifies the overall process because no additional surface modification of GO sheets with initiators is required. B

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Scheme 1. (a) Representative Molecular Structure of GO, (b) Schematic Illustration of the Formation of GO-g-PS and GO-gPSI, and (c) a Possible Reaction Mechanism of Using GO as the Cationic Initiator for Homopolymerization of Styrene and the Copolymerization of Styrene and Isoprene

The acidic functionalities of GO sheets function as Brφnsted acids and facilitate the surface-initiated cationic polymerization reaction (Scheme 1a). Building off of such concepts, we describe herein a series of styrene homopolymerizations and styrene/isoprene copolymerizations as a means to continuously adjust the surface properties of the resulting materials (Scheme 1b). The polymerization is believed to start with the formation of an ion pair through proton transfer from GO to a monomer and propagates through repeated insertion of monomers that ultimately result in new ion pairs (Scheme 1c). The optical images of powders of the GO initiator as well as the products obtained from the GO-initiated polymerization of styrene and GO-initiated copolymerization of styrene and isoprene (10:1) are shown in Figure 1. Compared to GO, which is black, the colors of products of the homopolymerization and the copolymerization reactions were relatively exhibited pale, indicating good dispersion of the polymer modified GO sheets in the polymer matrixes. Similar results were obtained from the copolymerization of styrene and isoprene at different ratios (i.e., 10:2, 10:5, and 10:10), although we noted that in our hands the isoprene could not be homopolymerized using GO. The yields of the GO initiated homopolymerization and copolymerization reactions are summarized in Table S1. Next, efforts were directed toward

elucidating the structure and morphology of the GO sheets grafted with the polymers. The residual polymers which were not covalently attached to the GO were removed by dissolving the product powders in THF (a good solvent of PS and PSI), filtering through a PTFE membrane (0.02 μm porosity), and finally washing with THF. As shown in Figure 1, the GO precipitated after standing in THF for 6 weeks, whereas the GO-g-PS and GO-g-PSI(10:1) materials were still well dispersed. Collectively, these observations suggested to us that the surface modification of GO with PS or PSI enhanced the dispersibilities of the resultant materials. The grafting of PS and PSI to the surfaces of GO sheets was verified using FT-IR spectroscopy (Figure 2a). The IR spectrum of GO exhibited a broad signal from ca. 3000 to 3650 cm−1 and featured signals at 1721, 1624, 1393, 1223, and 1049 cm−1. The broad peak and the peak at 1393 cm−1 were ascribed to the stretching and bending vibrations of O−H bonds. The peaks at 1223 and 1049 cm−1 were consistent with the stretching vibrations of C−O bonds that may have formed through linking O to sp2 C and sp3 C, respectively. The peak at 1624 cm−1 was attributed to the vibrations of the framework of graphene domains dispersed in the GO sheets. Finally, the peak at 1721 cm−1 may stem from the stretching vibrations of CO bonds. Upon grafting of PS, the GO-g-PS exhibited peaks from C

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Figure 1. Optical images of (a) GO as a powder, (b) a powder of the product obtained from the GO-initiated polymerization of styrene, (c) a powder of the product obtained from the GO-initiated copolymerization of styrene and isoprene (10:1), (d) a suspension of GO in THF, (e) a suspension of GO-g-PS in THF (0.2 mg mL−1), and (f) a suspension of GO-g-PSI(10:1) in THF (0.2 mg mL−1). Note: the images shown in panels d−f were taken after the suspensions had been standing for 6 weeks.

Figure 2. (a) FT-IR spectra of GO, GO-g-PS, and GO-g-PSI(10:1) composites. (b) Raman spectra of GO, GO-g-PS, and GO-g-PSI(10:1) composites.

2820 to 2980 cm−1, characteristic of the stretching vibrations of methyl C−H bonds, and from 3000 to 3100 cm−1, which were assigned to the stretching vibrations of the aryl C−H bonds. Collectively, these results suggested to us that the GO was covalently modified with PS. We speculate that the PS chains were grafted onto GO sheets through two possible routes: (1) the incorporation of the double bonds found within the basal plane of GO into the cationic polymerization, which might restrict the propagation of the polymer chain due to the large steric effect of the GO sheets, and/or (2) the reaction of the growing chain end with the corresponding counterion. The first route is similar to previously reported grafting-through approaches employed to modify the surfaces GO sheets using, for example, free radical polymerization.27 Polymer brushes have been grafted onto the GO sheets through the involvement of double bonds of GO sheets into the polymerization reaction. On the other hand, although the spectrum of GO-g-PSI(10:1) was similar to that of the GO-g-PS due to the similar functional groups contained in the polymer backbones, a peak that may originate from C−C and CC bonding modes was detected at 1581 cm−1, consistent with the copolymerization of the isoprene and styrene. In addition, 13C NMR data also supported the involvement of isoprene in the copolymerization reaction (Figure S1). Raman spectra were also used to characterize GO as well as the aforementioned GO-g-PS and the GO-g-PSI(10:1) composites (Figure 2b). The Raman spectrum of GO exhibited characteristic peaks at 1599 and 1354 cm−1, corresponding to the G and D bands of material’s carbon framework. The D band reflected the presence of the sp3 amorphous carbon

generated during the synthesis of the GO. Regardless, a new peak was detected at ca. 1096 cm−1 in the Raman spectra of the GO-g-PS and GO-g-PSI(10:1) materials and was attributed to the vibration of C−CH3 bonds,38 which exist at the ends of the PS chains and on the branches of the PI fragments in the PSI chains. In support of this assessment, the GO-g-PSI(10:1) exhibited a stronger Raman peak at 1096 cm−1 than the GO-gPS. Furthermore, the D bands displayed by GO-g-PS and GOg-PSI(10:1) materials were detected at 1344 cm−1, a relatively lower value when compared to that displayed by GO (1354 cm−1), indicating the chemical modification of GO surfaces or electron transfer between the polymers and GO. As shown in the inset of Figure 2b, the grafting of PS and PSI onto the surfaces of GO sheets resulted in increased intensity ratio between D band and G band (ID/IG) in the following order: GO-g-PSI(10:1) > GO-g-PS > GO. In agreement with previous reports,11,39 the increased ID/IG ratio reflected decreased average size of the sp2 domains in the corresponding GO-gPS and GO-g-PSI composites. Next, the morphologies of the aforementioned materials were analyzed using AFM. As shown in Figure 3, the GO sheets were relatively flat and thin, with a thickness of ca. 1 nm. They are thicker than ideal sheets of graphene (0.34 nm) due to the presence of the oxygen-containing functional groups on the surfaces and on the edges of the GO.11 In contrast, the GO-gPS sheets were ca. 3−4 nm thick, which was thicker than the GO sheets due to the attachment of PS chains onto the surfaces of the GO. It can also be seen from its cross section image that the surface of a GO-g-PS sheet was rougher than that of a GO D

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Figure 3. AFM images of (a) GO, (b) GO-g-PS, and (c) GO-g-PSI(10:1). TEM images of (d) GO, (e) GO-g-PS, and (f) GO-g-PSI(10:1).

gradually decreased Tgs. For example, increasing the content of isoprene from 10:1 to 10:10, the Tg decreased from 48.4 to 7.9 °C. This phenomenon can be ascribed to the increased content of the softer segments of PI. Therefore, the DSC data indicated that the composition of the grafted polymer brushes could be effectively tuned by changing the feed ratio of the two monomers in the GO-initiated copolymerization. Figure 4b displays TGA data for GO, GO-g-PS, and various GO-g-PSI composites obtained with different styrene/isoprene ratios. One can see that GO underwent a significant weight loss of ca. 31 wt % in a temperature range from ca. 125 to 210 °C, corresponding to the removal of oxygen-containing groups.19 In contrast, the TGA curves of the GO-g-PS and GO-g-PSI materials contained two marked weight losses at ca. 200 and 390 °C, respectively, which corresponded to the decomposition of oxygen-containing groups on GO and the degradation of the polymers. The significant weight loss observed at high temperature indicated that there were polymers grafted onto the surface of GO sheets, and this finding is consistent with the AFM and TEM data. Furthermore, the GO-g-PSI materials obtained with various styrene/isoprene ratios displayed a greater weight loss at ca. 390 °C than the GO-g-PS sheets (Figure S2), which may reflect a greater grafting density of the PSI on GO than that of PS. With the polymer-modified GO materials in hand, subsequent efforts were directed toward gaining a deeper understanding of their surface properties. The GO-based films were prepared via vacuum filtration using stable suspensions of GO-g-PS and GO-g-PSI.12,37 SEM images of the GO-based

sheet. The grains on the surfaces of the GO-g-PS sheets might originate from the aggregation of the PS chains on the GO surfaces. The GO-g-PSI(10:1) sheets were even thicker (ca. 4 nm) than the aforementioned materials and exhibited an increased roughness. We speculate that the increased roughness in the GO-g-PSI material may be due to phase separation between the PS and PI fragments, as phase separation was readily observed in analogous copolymers containing PS and PI.40 Moreover, the reactivity ratios of styrene and isoprene indicate that the PSI is not expected to be an alternating copolymer,41 but one which contains segments of PS and PI. The morphologies of the aforementioned materials were further investigated using TEM. Since the GO sheets are very thin, the contrast is very low. However, due to their increased thicknesses, the GO-g-PS and the GO-g-PSI(10:1) materials exhibited enhanced contrasts and roughness. Collectively, these results were in agreement with observations made using AFM. Moreover, these results indicate that the surface and chemical properties of GO may be modified and tuned through homopolymerization and copolymerization reactions. To evaluate the thermal properties of the products obtained from various GO-initiated polymerizations, the corresponding powders were measured with DSC and TGA. As shown in Figure 4a, the product prepared through the homopolymerization of styrene displayed a glass transition temperature (Tg) of ca. 56.6 °C, which is lower than that typically displayed by PS and consistent with a low degree of polymerization. Upon the addition of isoprene to the aforementioned polymerization reaction, the corresponding product powders exhibited E

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Figure 4. (a) DSC curves of the product powders of GO-initiated homopolymerization of styrene and GO-initiated copolymerization of styrene and isoprene with different ratios. (b) TGA curves of GO sheets, GO-g-PS composite, and GO-g-PSI composites obtained with different styrene/isoprene ratios.

Figure 5. SEM images of the films prepared with (a) GO, (b) GO-gPS, (c) GO-g-PSI(10:1), (d) GO-g-PSI(10:2), (e) GO-g-PSI(10:5), and (f) GO-g-PSI(10:10).

films were recorded in order to ascertain the effect of the polymer grafts on the surfaces of the GO (Figure 5). Compared with a pristine film of GO, the film of the GO-g-PS exhibited a rougher surface, which could be ascribed to the PS aggregations on the GO sheets. SEM images of the films prepared with the GO-g-PSI composites composed of various monomer ratios were also obtained. From these data, the GO-g-PSI films appeared to contain rougher surfaces than analogous films composed of the GO-g-PS composite and the roughness of the former films increased with the content of the PI. As a result, the packing of the GO-g-PS sheets and the GO-g-PSI sheets was not as compact as that of the GO sheets, leaving pores between the GO-g-PS and the GO-g-PSI sheets in their corresponding films. Thus, the quantity of the pores in the GOg-PSI films may be expected to increase with isoprene content. Since the polymer grafted GO sheets including the GO-g-PS and the GO-g-PSI all have good dispersity in THF (Figures 1d,f and 3), the impact of the dispersity of the materials in solvent can be ruled out. The contact angle of water was measured in order to further ascertain the surface properties of the films (Figure 6). It is well-known that there are polar functionalities on the surfaces of GO and that the films of GO typically exhibit a small contact angle of water. In contrast, the GO-g-PS film showed an increased contact angle of water due to the hydrophobic nature of the PS chains grafted on the surfaces of GO sheets. Moreover, a film of GO-g-PSI displayed an even higher contact

angle than that of GO-g-PS, and the contact angle became larger and larger as the PI component increased in the GO-gPSI composites. Based on the SEM data which revealed that the GO-g-PSI films displayed a gradually increased roughness as the content of PI increased (Figure 5), the increased contact angle could be ascribed to the different surface morphologies of the films as it was reported that rough surfaces could enhance the contact angle of water.42,43 By comparing the different contact angles measured for the aforementioned materials, it appears that surface wettability of GO can be tuned by grafting PS or PSI with different ratios of styrene to isoprene. Graphene has been shown to exhibit good gas barrier properties.44 Recent studies have indicated that polymer− graphene nanocomposites show reduced gas permeabilities when compared with the corresponding polymer films.45−47 As described above, we found that GO exhibited tunable surface roughness by grafting PSI with different ratios of styrene to isoprene; as a result, the GO, GO-g-PS, and GO-g-PSI composites displayed different packing structures in their corresponding films. As such, the gas permeability of graphene films can be tuned by grafting polymers using different ratios of styrene to isoprene in the surface-initiated polymerization. The membranes used for gas permeability measurement were prepared by depositing the aforementioned materials on the surfaces of commercial PTFE membranes. Varying the concentration or the volume of the respective suspensions F

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Figure 7. (a) Summary of the gas permeabilities displayed by various hybrid membranes prepared by depositing GO, GO-g-PS, or GO-g-PSI onto the surfaces of PTFE membranes. (b) Plots of the gas permeability of the hybrid membranes (left axis) versus the thickness of the GO-g-PSI(10:1) layer (right axis) as a function of the volume of the GO-g-PSI(10:1) suspension used (0.1 mg mL−1). Figure 6. Contact angle images of films composed of (a) GO, (b) GOg-PS, (c) GO-g-PSI(10:1), (d) GO-g-PSI(10:2), (e) GO-g-PSI(10:5), and (f) GO-g-PSI(10:10). (g) A summary of the contact angles measured.

The thickness of the GO-g-PSI(10:1) layer increased linearly as a function of the volume of the suspension used, whereas the gas permeability displayed an inverse effect.

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CONCLUSIONS GO can be used as cationic initiator for homopolymerization of styrene as well as the copolymerization of styrene and isoprene. The resultant GO-g-PS and GO-g-PSI composites showed good dispersibility in common solvents such as THF, chloroform, and toluene. AFM and TEM data indicated that the GO, GO-gPS, and GO-g-PSI materials displayed gradually increased surface roughness. As a result, the packing between the GO-gPS sheets and the GO-g-PSI sheets was not as compact as that of GO sheets, leaving pores in their corresponding films. SEM data indicated that the roughness of the films prepared with GO-g-PSI increased with PI content, which ultimately resulted in larger water contact angles. Likewise, hybrid membranes prepared by depositing the aforementioned materials on the surfaces of commercial PTFE membranes displayed gas permeabilities that increased with PI content. We expect that the results described herein will lead to new methods for controlling the structures of GO-based composites in twodimensional films. Depending on the chemical properties of such materials, which may be conveniently tuned through

enabled control over the thicknesses of the GO-based layers in the corresponding hybrid membranes. The gas permeability was evaluated using a N2 transmittance rate (NTR), which is the time needed for 100 cm3 of N2 to pass through 1 cm2 of the membrane under a pressure of 1 Pa. The NTR values of the hybrid membranes prepared using GO, GO-g-PS, and GO-gPSI obtained with different ratios of styrene to isoprene are summarized in Figure 7a. Among these samples, the hybrid membrane containing GO showed the lowest gas permeability since it yielded the largest NTR value. Consistent with the SEM data, the hybrid membranes containing GO-g-PS or GO-g-PSI showed improved gas permeability, as the NTR values became smaller with increasing isoprene content. While each material analyzed displayed the same thickness, ca. 110 nm, in their corresponding hybrid membranes, the gas permeabilities of the hybrid membranes could also be tuned by changing the thickness of the GO-based layer. Figure 7b shows the NTR values of membranes prepared by depositing different volumes of a suspension of GO-g-PSI(10:1) in THF (0.1 mg mL−1). G

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copolymerization chemistry, the corresponding membranes could offer utility in selective filtration and chemical separation.

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ASSOCIATED CONTENT

S Supporting Information *

13

C NMR spectra of free PS and free PSI, TGA and differential TGA analysis of the polymer grafted GO sheets, yields of the GO initiated homopolymerization and copolymerization reactions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Hundred Talents Program” of Chinese Academy of Sciences, the National Natural Science Foundation of China (21274158 and 91333114), and the Chinese Academy of Sciences Visiting Professorships for Senior International Scientists (2013T1G0019).

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REFERENCES

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DOI: 10.1021/ma5026237 Macromolecules XXXX, XXX, XXX−XXX