Tuning the Surface Properties of Graphene Oxide by Surface-Initiated

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Tuning the Surface Properties of Graphene Oxide by Surface-Initiated Polymerization of Epoxides: an Efficient Method for Enhancing Gas Separation Yu Wu, Pan Jia, Linli Xu, Zhangyan Chen, Linhong Xiao, Jinhua Sun, Jun Zhang, Yong Huang, Christopher W. Bielawski, and Jianxin Geng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14895 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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ACS Applied Materials & Interfaces

Tuning the Surface Properties of Graphene Oxide by Surface-Initiated Polymerization of Epoxides: an Efficient Method for Enhancing Gas Separation

Yu Wu,† Pan Jia,† Linli Xu,† Zhangyan Chen,‡ Linhong Xiao, Jinhua Sun,† Jun Zhang,‡ Yong Huang,† Christopher W. Bielawski,#,⊥ Jianxin Geng*,† †

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Haidian District, Beijing 100190, China, E-mail: [email protected] ‡

Institute of Chemistry, Chinese Academy of Sciences, 2 Zhongguancun North First Street, Haidian District, Beijing, 100190, China

#

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea

⊥

Department of Chemistry and Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

Abstract: Here we describe an in situ approach for growing polyepoxides from the surfaces of graphene oxide (GO) using a surface-initiated polymerization reaction. The polymerization methodology is facile and general as a broad range of epoxides carrying various functional groups have been successfully polymerized by simply adding GO powders in the epoxide monomers. The resultant polyepoxide grafted GO are found to show enhanced dispersibility in various common solvents and to exhibit increased d-spacing between the basal planes. In particular, grafting poly(2,3-epoxy-1-propanol) (PEP) to GO results in a composite (i.e., GO-g-PEP) that is dispersible in water and miscible with polyether block amide, i.e., Pebax MH 1657. Preliminary studies have indicated the membranes prepared using Pebax/GO-g-PEP composites exhibit enhanced CO2 permeabilities and selectivities, when compared to 1 ACS Paragon Plus Environment


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H2, O2 or N2. The excellent performance in gas separation is attributed to the layered structure of the GO-g-PEP sheets with enlarged d-spacing and the functional groups present on the PEP chains grafted to the surfaces of GO sheets.

Keywords: Graphene oxide, graphene oxide-initiated polymerization, polyepoxides, membranes, gas separation

Introduction Graphene is a two-dimensional nanomaterial that displays high electrical and thermal conductivities,1 excellent mechanical properties,2,3 and large specific area.4 Since Geim’s pioneering report,5 the material has attracted significant attention in the fields of chemistry,6,7 physics,8 electronic engineering9,10 and material science.11−16 Graphene oxide (GO), the precursor of chemically converted graphene, is prepared by exfoliating oxidized graphite and displays properties that are unique from graphene due to the presence of oxygen-containing functional groups.17 These functional groups render GO strongly hydrophilic and dispersible in many solvents, including water.18 In addition, the functional groups that react easily provide viable routs to modification of GO for various applications and have been utilized to facilitate chemical transformations.19−24 For example, it was demonstrated that GO catalyzes the oxidation of various alcohols and olefins, facilitates the hydration of alkynes under mild conditions,25 and is used as a solid acid catalyst to polymerize olefins.26,27 Most recently, we demonstrated that GO as a cationic polymerization initiator grows a homopolymer of styrene or copolymers of styrene and isoprene directly from its surfaces.28 While graphene has been shown to exhibit excellent gas barrier properties,29 the incorporation of nanopores enables the material to be utilized in gas separation applications.30,31 Such pores have been 2 ACS Paragon Plus Environment

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introduced using techniques that include electron beam nanosculpting,32 focused ion beam lithography,33 and ultraviolet-induced oxidative etching.34 Unfortunately, it remains challenging to create high-density and uniform nanopores on graphene membranes since the aforementioned methods are costly and often inefficient. While gas diffusion paths in few-layered GO membranes can be engineered for gas separation, where gas molecules pass through the structural defects35 or spaces formed between the layers,36−38 such membranes are rarely freestanding and often prepared on porous supports. By contrast, membranes consisting of polymer/GO composites are often freestanding, straightforward to scale and can be tuned by varying the functionality of the polymer host. Xu et al.39 reported a series of polymer/GO composite membranes wherein the combination of the laminar structures of GO sheets and the networks of hydrogen-bond interactions between the GO sheets and the polymer matrix facilitates gas separation. Moreover, modifying GO with certain functionalized polymers is expected to enhance the interactions with various gases and afford membranes with improved gas separation properties.40,41 However, to the best of our knowledge, achieving selective gas permeation by tailoring the surface functionalities of GO sheets has not yet been reported. Here we describe an in situ method for tuning the surface properties of GO sheets through a GO initiated ring-opening polymerization of epoxides and show that the resultant GO as filler in polymer membranes enhances gas separation. Compared to conventional approaches, this method is more efficient as it combines the polymerization of epoxides and the covalent grafting of the resultant polyepoxides to GO sheets into one step. The polymerization methodology is also general as a broad range of epoxides carrying various functional groups are successfully polymerized. Of the various products (GO-g-polyepoxide), poly(2,3-epoxy-1-propanol) (PEP) grafted GO (GO-g-PEP) shows high dispersibility in water, which facilitates the preparation of composite membranes using a water-soluble polyether block amide (i.e., Pebax MH 1657). Gas permeation experiments reveal that the Pebax/GO-g-

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PEP composite membranes exhibit an improved selectivity of CO2, when compared to N2, O2 or H2, as well as an enhanced permeation of CO2. The methodology developed is expected to facilitate the preparation of graphene-based composites with tunable functionalities for various applications of separation techniques.

Experimental Section Materials: GO was prepared by a modified Hummer’s method.42 Before used, GO was dried in a vacuum oven and ground into a powder. Pebax MH 1657 was purchased from Arkema Inc. Propylene oxide (PO), butyl glycidyl ether (BGE), epichlorohydrin (ECH) and 2,3-epoxy-1-propanol (EP) were purchased from Aladdin Industrial Corp. Before used, they were dried over anhydrous MgSO4. Polymerization: GO initiated polymerization of PO was carried out at 40 °C. In detail, GO (83 mg) and PO (20 mL) were added into a 25 mL flask. After refluxing for a designated period (i.e., 12, 24, 36 or 48 h), the mixture was added to 400 mL diethyl ether to obtain the sediment. The sediment included GO-g-PPO and free PPO and was obtained by centrifugation. Free polymer was removed by THF washing to obtain GO-g-PPO. Polymerization reactions of the other monomers were performed following a similar approach at different temperatures by refluxing for 48 h. Polymerization temperature for each polymerization reaction was determined on the basis of the boiling point of the respective monomer, i.e., 40, 80, 60 and 80 for PO, BGE, ECH and EP, respectively. For the cases of ethylene glycol being used in the polymerization, it was added into the flask after the monomers were added. Preparation of gas separation membranes: Pebax MH 1657 (0.8 g) was dissolved in a mixture of ethanol and water (14 and 6 g of ethanol and DI water, respectively) at 80 °C. A pre-determined amount of GO-g-PEP was dispersed in DI water (20 mL). The solution of Pebax and the aqueous suspension of 4 ACS Paragon Plus Environment

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GO-g-PEP were then combined and stirred using a high speed mixer at a speed of ca. 2000 revolutions min−1 for 10 min. The mixture was then cast in a petri dish. A dense Pebax/GO-g-PEP membrane was obtained after allowing the solvent to evaporate at room temperature for 24 h, followed by being subjected to a vacuum for 8 h. Afterwards, the membrane was peeled off and the thickness was measured using a gauge. Pebax/GO-g-PEP membranes with various quantities of GO-g-PEP were prepared by using different amounts of GO-g-PEP in the aforementioned procedure. The thicknesses of the Pebax/GO-g-PEP membranes were controlled to range from 20 to 70 µm by varying the quantity of the composite suspension used in the preparation method. Pristine Pebax and Pebax/GO membranes containing various quantities of GO were prepared using a similar procedure as described above. Gas permeation measurements: Gas permeation experiments were conducted on a Labthink VAC-V2 Gas Permeability Tester using a differential pressure method.43 The membranes were measured under 25 °C and 0% relative humidity. Gas permeability data for H2, CO2, O2 and N2 were recorded separately. Gas permeability was expressed in term of Barrer (1 Barrer = 1×10−10 cm3 (STP) cm cm−2 s−1 cmHg−1). General characterization: Thermogravimetric analysis (TGA) was carried out with a Q50 TGA at a scanning rate of 10 °C min−1 under the atmosphere of N2. 3−5 mg sample was used for each TGA measurement. FT-IR spectra were recorded on an Excalibur 3100 spectrometer with a resolution of 0.2 cm−1 using KBr pellets. Raman spectra were recorded on a Renishaw inVia-Reflex confocal Raman microscope with an excitation wavelength of 532 nm. The X-ray diffraction (XRD) patterns from 3 to 50 degrees were collected on a Bruker D8-advance diffractometer with an incident wavelength of 0.154 nm (Cu Kα radiation) and a Lynx-Eye detector. The XRD patterns from 1 to 3 degrees were collected on a D/max 2500 X diffractometer with an incident wavelength of 0.154 nm. Transmission electron microscope (TEM) observations were performed on a JEOL JEM-2100 TEM operated at 200 kV. TEM 5 ACS Paragon Plus Environment


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samples were prepared by casting the suspensions of the GO-g-polyepoxide composites in THF on 400 mesh copper grids with supporting carbon films and drying in air. Atomic force microscopy (AFM) images were recorded on a Bruker Multimode 8 microscope in the tapping mode with a cantilever having a resonance frequency of 128−197 kHz. AFM samples were prepared by spin-coating the suspensions of the GO-g-polyepoxide composites onto freshly cleaved mica slides.

Results and Discussion

Scheme 1. (a) Schematic illustration of grafting polyepoxides to GO sheets through a surface initiated polymerization. (b) Schematic illustration of elongated paths for gases passing through Pebax/GO-gPEP composite membranes.

A schematic illustration of grafting various polyepoxides to GO sheets via a surface initiated polymerization is shown in Scheme 1. The monomers that were utilized include PO, BGE, ECH and EP, and the corresponding products were respectively denoted as GO-g-PPO, GO-g-PBGE, GO-g-PECH and GO-g-PEP. Polymerization time was optimized to be 48 h using PO on the basis of the quantity of 6 ACS Paragon Plus Environment

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total PPO formed (Table S1). Polymerization temperature for each polymerization reaction was determined on the basis of the boiling point of each of the monomers. Under optimized polymerization conditions, the contents of grafted polyepoxides were measured to be ca. 25, 31, 47 and 73 wt% via TGA (Table 1), respectively, an outcome that might be related to the reactivities of the corresponding monomers. The TGA data revealed two weight loss stages (Figure 1): the first occurred lower than 250 °C and was attributed to thermal cleavage of the oxygen-containing groups on GO sheets; the second occurred at temperatures higher than 250 °C and was ascribed to the thermal degradation of the grafted polyepoxides. The successful polymerization of the epoxides carrying various functional groups indicated the generality of this approach, if only the monomers were liquids at the temperatures for the polymerization reaction. Table 1. Summary of GO initiated polymerization of epoxides. Entry

[a]

Monomer

Ethylene

Polymerization

Polymerization

Quantity of total

Quantity of grafted

glycol[b]

time[c] (h)

temperature[d] (oC)

polymer formed[e] (wt%)

polymer[f] (wt%)

GO[a] (wt%)

1

PO

0.5

—

48

40

26

25

2

BGE

0.5

—

48

80

34

31

3

ECH

0.5

—

48

60

52

47

4

EP

0.5

—

48

80

81

73

5

PO

0.5

1:30

48

40

19

17

6

BGE

0.5

1:30

48

80

34

27

7

ECH

0.5

1:30

48

60

32

20

8

EP

0.5

1:30

48

80

78

59

The weight percentage of GO with respect to the monomers;

[b]

Ethylene glycol was used in the

polymerization for entries 5−8 and the molar ratio of ethylene glycol to monomer was 1:30; [c]

Polymerization time was optimized on the basis of the quantity of total polymer formed (Table S1);

[d]

Polymerization temperature was determined on the basis of the boiling points of the monomers; [e]The 7 ACS Paragon Plus Environment


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quantities of polymers were calculated from the TGA data (Figure S1). [f]Free polymers were removed by repeated cycles of centrifuging and washing with THF.

Figure 1. TGA data recorded for GO, GO-g-PPO, GO-g-PBGE, GO-g-PECH and GO-g-PEP.

The covalent attachment of the polyepoxides to GO sheets was also supported by a series of FTIR and Raman spectroscopy measurements (Figure 2). The FT-IR spectrum recorded for GO contained characteristic signals at 3429, 1736, 1624, 1385, 1226 and 1057 cm−1, which corresponded to the stretching vibrations of the O−H bonds, the stretching vibrations of the C=O bonds, the stretching vibrations of unoxidized C=C bonds in the carbon lattice, the in-plane bending vibrations of the C−OH bonds, the stretching vibrations of the C−O−C bonds in the epoxy groups and the stretching vibrations of the C−O bonds in the alkoxy groups, respectively. In contrast, the polyepoxides modified GO exhibited signals from 2850 to 2950 cm−1, which were assigned to the stretching vibrations of methylene C−H bonds. The spectrum recorded for GO-g-PECH exhibited a characteristic signal at 750 cm–1, which was assigned to the stretching vibration of the C–Cl bonds. The aforementioned differences in the FT-IR spectra of GO and GO-g-polyepoxide composites were consistent with the grafting of the corresponding polyepoxides to GO. Figure 2b shows the Raman spectra of GO and the GO-g8 ACS Paragon Plus Environment

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polyepoxide composites. The spectrum recorded for GO featured with two characteristic bands at 1603 and 1350 cm−1, which correspond to the G and D bands, respectively. The G band was ascribed to the domains of sp2 hybridized carbons, whereas the D band was attributed to structural defects. The intensity ratio of D band to G band (ID/IG) is often used as an indication of the relative disorder in graphene-derived materials. From the Raman data, it was determined that GO and the modified GO materials displayed ID/IG ratios that increased in the following order: GO < GO-g-PPO < GO-g-PBGE < GO-g-PECH < GO-g-PEP. The increase in ID/IG ratio may be due to the grafted polyepoxides, which led to decreased average size of the sp2 domains,28 and the variation of the ID/IG ratio was consistent with the TGA data.



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Figure 2. (a) FT-IR spectra and (b) Raman spectra recorded for GO and GO-g-polyepoxide composites.

The optical images of GO-g-PPO powder and its suspensions in various solvents are shown in Figure 3. In contrast to the black color of the GO starting material (Figure 3a), the GO-g-PPO composite exhibited a uniform grey color (Figure 3b), consistent with homogeneous grafting of the PPO chains to GO sheets. As a result, the GO-g-PPO composite exhibited excellent dispersibility in common organic solvents, including tetrahydrofuran (THF), N,N-dimethylformamide (DMF), dichloromethane (DCM) and toluene (Figure 3c). Similar results (i.e., the uniform appearance and good dispersibility in commonly used solvents) were also observed for the resultant GO sheets prepared by grafting the other 10 ACS Paragon Plus Environment

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polyepoxides, i.e., PBGE, PECH and PEP (Figures S2−S4). It is noteworthy that GO-g-PEP exhibited good dispersibility in water and water-miscible solvents due to hydrophilic properties of the polymeric grafts (Figure S4).

Figure 3. Optical images of (a) a GO powder and (b) a GO-g-PPO composite powder. (c) Suspensions of GO-g-PPO in THF, DMF, DCM and toluene (0.2 mg mL−1). The images were taken after the suspensions had been standing for one week.

Because GO has been reported to initiate the cationic polymerization of vinyl monomers such as styrene and isoprene,28 we suggest that the aforementioned epoxides were polymerized in an analogous manner (Scheme S1). Protons released from the carboxylic acid groups of GO result in protonation of the epoxides while the GO sheets act as counter anions. Chain propagation then occurs following a conventional cationic ring-opening mechanism. The growing polymer chains may graft onto GO sheets due to the incorporation of the epoxides on GO basal planes into the chain propagation, the reaction of the growing chain ends with the corresponding counterions, and/or the termination of the polymer cations by the hydroxyl groups on GO, whereas termination of the growing polymer cations by residual 11 ACS Paragon Plus Environment


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water leads to free polyepoxides. The proposed polymerization mechanism was supported by the relatively higher quantities of free polymers obtained for polymerizing PO, BGE, ECH and EP in the presence of ethylene glycol (Table 1) due to termination of the growing polymer cations by ethylene glycol. Control test indicated that GO did not lead to polymerization of ethylene glycol. Compared with the previous report,28 the results of this research show advantages in high grafting quantities of the polyepoxides and a general applicability of various epoxide monomers. The structures of GO-g-polyepoxide sheets were analyzed using XRD (Figure 4). Pristine GO sheets exhibited a XRD pattern that contained an intense Bragg reflection cantered at 2θ = 11.65°, which corresponded to a d-spacing of 0.76 nm. By contrast, the GO-g-polyepoxide sheets displayed relatively large d-spacing values, which were correlated with the quantities of the polymers grafted to GO (i.e., 1.09, 1.84, 2.22 and 4.33 nm for GO-g-PPO, GO-g-PBGE, GO-g-PECH and GO-g-PEP, respectively). Moreover, broad reflection peaks were observed around 20° and attributed to the grafted polyepoxides. Different frameworks of the grafted polymers might be attributable to slight the shifts of the broad reflection peaks. The morphologies of the individual GO-g-polyepoxide sheets were characterized using TEM and AFM. In contrast to the relatively smooth surfaces of GO sheets (Figure 5a and 5b), the GO-g-polyepoxide sheets exhibited wrinkled surfaces with areas of high contrast (Figure 5c and 5d), consistent with the presence of grafted polymers. Statistical analysis indicated that the thicknesses of GO-g-polyepoxide sheets were correlated with the quantities of the grafted polyepoxides, i.e., ca. 2.44, 4.00, 5.16 and 8.11 nm measured for GO-g-PPO, GO-g-PBGE, GO-g-PECH and GO-gPEP, respectively (Table S2). Therefore, a conclusion can be drawn that the grafted polyepoxides resulted in larger interlayer distances between the basal planes of GO-g-polyepoxide sheets than the value measured for pristine GO. As discussed below, such result was found to facilitate the gas permeation of the Pebax-based composite membranes containing GO-g-PEP sheets. 12 ACS Paragon Plus Environment

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Figure 4. XRD patterns of GO, GO-g-PPO, GO-g-PBGE, GO-g-PECH and GO-g-PEP recorded at various ranges: (a) 1−3 degrees and (b) 3−50 degrees.



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Figure 5. (a) TEM and (b) AFM images of GO sheets. (c) TEM and (d) AFM images of GO-g-PEP sheets.

Pebax MH 1657 is one of the most widely used polymers for CO2 selective permeation because its PEO fragments display a high affinity toward CO2,44 while GO sheets with high aspect ratios in polymer matrices have been reported to increase the tortuous channels for gas molecules.45 Thus, we hypothesized that the membranes comprised of Pebax and the GO sheets modified with functional polymer chains should improve the permeability of certain gases. A series of Pebax-based composite membranes containing various loadings of GO-g-PEP was prepared and denoted as Pebax/GO-g-PEP (x wt%), where x wt% referred to the weight percentage of GO-g-PEP in the membranes. The homogeneous appearance of each of the membranes was consistent with a uniform distribution of the GO-g-PEP in the Pebax matrix (Figure 6a). Among the GO-g-polyepoxide sheets obtained, GO-g-PEP was used for gas separation application due to its good miscibility with Pebax and the functionalities in PEP chains.


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Figure 6. (a) Images of Pebax/GO-g-PEP membranes with various contents of GO-g-PEP (indicated). (b) Gas permeability and (c) gas selectivity data measured for Pebax/GO-g-PEP membranes with various contents of GO-g-PEP.

The gas permeabilities (P) of the Pebax/GO-g-PEP membranes were measured using differential pressure methodology43 in conjunction with H2, CO2, O2 and N2. The P values of the Pebax/GO-g-PEP membranes containing various quantities of GO-g-PEP are shown in Figure 6b. While the CO2 15 ACS Paragon Plus Environment


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permeability of a membrane prepared with pristine Pebax was measured to be 40 Barrer (1 Barrer = 1×10−10 cm3 (STP) cm cm−2 s−1 cmHg−1), incremental addition of GO-g-PEP in the Pebax-based membranes gradually enhanced CO2 permeability, with a maximum P value (110 Barrer) measured at a GO-g-PEP content of 2 wt%. Increasing the addition of GO-g-PEP in a Pebax-based membrane to 5 wt% resulted in a significantly decreased CO2 permeability (22 Barrer) due to the gas barrier property of graphene materials.29 While the impact of adding GO-g-PEP on the P values for the other gases (i.e., H2, O2 and N2) through the Pebax/GO-g-PEP membranes followed a similar trend as that observed for CO2, the P values measured for CO2 were larger than those measured for the other gases. In control tests, addition of GO resulted in decreased P values for the aforementioned gases (Figure S7). The different impacts of GO-g-PEP and GO sheets on the gas permeability of the respective Pebax-based membranes can be attributed to the different d-spacings between the GO-g-PEP sheets and GO sheets (Figure 4): the enlarged d-spacing between GO-g-PEP sheets provided larger channels for the permeation of the gases measured. The PEP brushes grafted on GO sheets resulted in elongated gas diffusion paths (Scheme 1b). Moreover, the functionalities such as C−O−C and C−OH groups in PEP chains favor the permeation of CO2 compared with H2, O2 and N2. Therefore, addition of GO-g-PEP in the Pebax membranes resulted in a high CO2 selectivity to H2, O2 and N2 (Figure 6c). The apparent CO2 selectivity can be described by ideal selectivity (α) values, i.e., the ratio of the P values of two gases (e.g., αCO2/N2 = PCO2/PN2). The Pebax/GO-g-PEP membranes exhibited the highest CO2/N2 and CO2/O2 selectivities (i.e., αCO2/N2 = 67 and αCO2/N2 = 26) at a GO-g-PEP content of 0.5 wt% and the highest CO2/H2 selectivity (αCO2/H2 = 11) at a GO-g-PEP content of 2 wt%.


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Figure 7. (a) Ideal selectivity and (b) gas permeability data obtained for the membranes prepared using pure Pebax, Pebax/GO (0.5 wt%) and Pebax/GO-g-PEP (0.5 wt%).

To further demonstrate the advantages of GO-g-PEP sheets in separation of CO2, the gas permeability and selectivity of Pebax/GO-g-PEP (0.5 wt%) membranes were compared with that of a pure Pebax membrane and the Pebax/GO (0.5 wt%) membranes (Figure 7). A Pebax/GO-g-PEP (0.5 wt%) membrane exhibited higher CO2 selectivity to H2, O2 and N2 than a Pebax/GO (0.5 wt%) membrane (Figure 7a). Meanwhile, in contrast to a decreased CO2 permeability measured for the Pebax/GO (0.5 wt%) membrane with respect to that measured for a pure Pebax membrane, the Pebax/GO-g-PEP (0.5 wt%) membrane exhibited an increased CO2 permeability (Figure 7b). GO sheets 17 ACS Paragon Plus Environment


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are commonly regarded as isolated domains of sp2 hybridized C dispersed in a sp3 hybridized C–O matrix. The conjugated π bonds of the isolated sp2 domains and the polar bonds of the sp3 hybridized C– O matrix make the GO sheets show intimate affinity to CO2 and thereafter improve the diffusion of CO2 in the Pebax/GO membranes with very low GO contents (0.1 wt%, Figure S7).38,46,47 But, higher GO contents resulted in decreased CO2 permeabilities because of the gas barrier property of GO (Figure S7).29 By contrast, the GO-g-PEP sheets exhibited an increased d-spacing between the GO basal planes (Figure 4), and thus CO2 molecules could readily pass through the spaces between the GO-g-PEP sheets. As a result, the Pebax/GO-g-PEP membranes with even relatively higher GO-g-PEP contents (up to 2 wt%, Figure 6b) could exhibit enhanced CO2 permeabilities. Meanwhile, the grafted PEP chains provided the GO-g-PEP sheets more functionalities (i.e., C−O−C and C−OH bonds) that exhibited intimate affinity to CO2 and selectively improved the diffusion of CO2 in the Pebax/GO-g-PEP membranes. Therefore, the GO-g-PEP sheets not only own the two-dimensional structural feature of pristine GO sheets, which increase the length of gas diffusion paths, but also provide additional functional fragments, which promote the permeation and selectivity for separation of CO2. Moreover, the permeability and selectivity of the Pebax/GO-g-PEP membranes were successfully tuned by varying the GO-g-PEP content in the membranes (Figure S8).

Conclusions In summary, GO was found to initiate the polymerization of various epoxides and resulted in grafting polyepoxides to the surfaces of the GO sheets. The resulting GO-g-polyepoxide sheets exhibited larger d-spacing values than pristine GO sheets and displayed good dispersibility in a wide range of solvents. In particular, GO-g-PEP exhibited good dispersibility in water-miscible solvents and good miscibility with functional polymers such as Pebax. A series of Pebax/GO-g-PEP membranes 18 ACS Paragon Plus Environment

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containing various quantities of GO-g-PEP (from 0.1 to 2 wt%) showed enhanced CO2 permeabilities and selectivities when compared to H2, O2 or N2. The excellent gas separation performance can be attributed to the layered structure of the GO-g-PEP sheets with an enlarged d-spacing and the affinity to CO2 due to the functional groups in the PEP chains grafted. The method described is expected to facilitate the preparation of graphene-based composites with tunable functionalities for various applications, including gas separation, sea water desalting and heavy metal separation.

Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: …. Structural characterization of the GO-g-polyepoxide composites, optical images of various GO-gpolyepoxide powders and their suspensions, proposed polymerization mechanisms, morphologies of GO and GO-g-polyepoxide composites, gas permeability and gas selectivity data (PDF)

Acknowledgment: This research was supported by the “Hundred Talents Program” of Chinese Academy of Sciences and the National Natural Science Foundation of China (21274158, 91333114). C.W.B. is grateful to the IBS (IBS-R019-D1) and the BK21 Plus Program as funded by the Ministry of Education and the National Research Foundation of Korea.



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References: (1) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385−388. (2) Wang, X.; Zhi, L. J.; Mullen, K. Transparent, Conductive Graphene Electrodes for Dye-Sensitized Solar Cells. Nano Lett. 2008, 8, 323−327. (3) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity of Single-Layer Graphene. Nano Lett. 2008, 8, 902−907. (4) Wu, Z.-S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Muellen, K. 3D Nitrogen-Doped Graphene Aerogel-Supported Fe3O4 Nanoparticles as Efficient Eletrocatalysts for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134, 9082−9085. (5) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (6) Xu, L.; Jiang, S.; Li, B.; Hou, W.; Li, G.; Memon, M. A.; Huang, Y.; Geng, J. Graphene Oxide: A Versatile Agent for Polyimide Foams with Improved Foaming Capability and Enhanced Flexibility. Chem. Mater. 2015, 27, 4358−4367. (7) Huang, X.; Qi, X.; Boey, F.; Zhang, H. Graphene-Based Composites. Chem. Soc. Rev. 2012, 41, 666−686. (8) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (9) Evlashin, S.; Dyakonov, P.; Khmelnitsky, R.; Dagesyan, S.; Klokov, A.; Sharkov, A.; Timashev, P.; Minaeva, S.; Maslakov, K.; Svyakhovskiy, S.; Suetin, N. Controllable Laser Reduction of Graphene Oxide Films for Photoelectronic Applications. ACS Appl. Mater. Interfaces 2016, 8, 28880−28887. (10) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S. Graphene-Based Ultracapacitors. Nano Lett. 2008, 8, 3498−3502. 20 ACS Paragon Plus Environment

Page 21 of 26

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


(11) Sun, J.; Xiao, L.; Jiang, S.; Li, G.; Huang, Y.; Geng, J. Fluorine-Doped SnO2@Graphene Porous Composite for High Capacity Lithium-Ion Batteries. Chem. Mater. 2015, 27, 4594−4603. (12) Li, G.; Sun, J.; Hou, W.; Jiang, S.; Huang, Y.; Geng, J. Three-Dimensional Porous Carbon Composites Containing High Sulfur Nanoparticle Content for High-Performance Lithium-Sulfur Batteries. Nat. Commun. 2016, 7, 10601−10601. (13) Rao, C. N. R.; Sood, A. K.; Subrahmanyam, K. S.; Govindaraj, A. Graphene: The New TwoDimensional Nanomaterial. Angew. Chem., Int. Ed. 2009, 48, 7752−7777. (14) Ding, Y. B.; Bai, W.; Sun, J. H.; Wu, Y.; Memon, M. A.; Wang, C.; Liu, C. B.; Huang, Y.; Geng, J. X. Cellulose Tailored Anatase TiO2 Nanospindles in Three-Dimensional Graphene Composites for High-Performance Supercapacitors. ACS Appl. Mater. Interfaces 2016, 8, 12165−12175. (15) Memon, M. A.; Bai, W.; Sun, J. H.; Imran, M.; Phulpoto, S. N.; Yan, S. K.; Huang, Y.; Geng, J. X. Conjunction of Conducting Polymer Nanostructures with Macroporous Structured Graphene Thin Films for High-Performance Flexible Supercapacitors. ACS Appl. Mater. Interfaces 2016, 8, 11711−11719. (16) Jia, W.; Tang, B.; Wu, P. A Novel Composite PEM with Long-Range Ionic Nanochannels Induced by Carbon Nanotube/Graphene Oxide Nanoribbon Composites. ACS Appl. Mater. Interfaces 2016, 8, 28955−28963. (17) Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906−3924. (18) Paredes, J. I.; Villar-Rodil, S.; Martinez-Alonso, A.; Tascon, J. M. D. Graphene Oxide Dispersions in Organic Solvents. Langmuir 2008, 24, 10560−10564. (19) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228−240.



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

Page 22 of 26

(20) Su, C.; Loh, K. P. Carbocatalysts: Graphene Oxide and Its Derivatives. Acc. Chem. Res. 2013, 46, 2275−2285. (21) Kim, M.; Lee, C.; Seo, Y. D.; Cho, S.; Kim, J.; Lee, G.; Kim, Y. K.; Jang, J. Fabrication of Various Conducting Polymers Using Graphene Oxide as a Chemical Oxidant. Chem. Mater. 2015, 27, 6238−6248. (22) Toth, P. S.; Ramasse, Q. M.; Velicky, M.; Dryfe, R. A. W. Functionalization of Graphene at the Organic/Water Interface. Chem. Sci. 2015, 6, 1316−1323. (23) Meng, D.; Yang, S.; Guo, L.; Li, G.; Ge, J.; Huang, Y.; Bielawski, C. W.; Geng, J. The Enhanced Photothermal Effect of Graphene/Conjugated Polymer Composites: Photoinduced Energy Transfer and Applications in Photocontrolled Switches. Chem. Commun. 2014, 50, 14345−14348. (24) Meng, D.; Yang, S.; Sun, D.; Zeng, Y.; Sun, J.; Li, Y.; Yan, S.; Huang, Y.; Bielawski, C. W.; Geng, J. A Dual-Fluorescent Composite of Graphene Oxide and Poly(3-Hexylthiophene) Enables the Ratiometric Detection of Amines. Chem. Sci. 2014, 5, 3130−3134. (25) Dreyer, D. R.; Jia, H.-P.; Bielawski, C. W. Graphene Oxide: A Convenient Carbocatalyst for Facilitating Oxidation and Hydration Reactions. Angew. Chem., Int. Ed. 2010, 49, 6813−6816. (26) Dreyer, D. R.; Bielawski, C. W. Graphite Oxide as an Olefin Polymerization Carbocatalyst: Applications in Electrochemical Double Layer Capacitors. Adv. Funct. Mater. 2012, 22, 3247−3253. (27) Ji, J. Y.; Zhang, G. H.; Chen, H. Y.; Wang, S. L.; Zhang, G. L.; Zhang, F. B.; Fan, X. B. Sulfonated Graphene as Water-Tolerant Solid Acid Catalyst. Chem. Sci. 2011, 2, 484−487. (28) Li, B.; Hou, W.; Sun, J.; Jiang, S.; Xu, L.; Li, G.; Memon, M. A.; Cao, J.; Huang, Y.; Bielawski, C. W.; Geng, J. Tunable Functionalization of Graphene Oxide Sheets through Surface-Initiated Cationic Polymerization. Macromolecules 2015, 48, 994−1001.


Page 23 of 26

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


(29) Bunch, J. S.; Verbridge, S. S.; Alden, J. S.; van der Zande, A. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Impermeable Atomic Membranes from Graphene Sheets. Nano Lett. 2008, 8, 2458−2462. (30) Jiang, D.-e.; Cooper, V. R.; Dai, S. Porous Graphene as the Ultimate Membrane for Gas Separation. Nano Lett. 2009, 9, 4019−4024. (31) O'Hern, S. C.; Boutilier, M. S. H.; Idrobo, J.-C.; Song, Y.; Kong, J.; Laoui, T.; Atieh, M.; Karnik, R. Selective Ionic Transport through Tunable Subnanometer Pores in Single-Layer Graphene Membranes. Nano Lett. 2014, 14, 1234−1241. (32) Fischbein, M. D.; Drndic, M. Electron Beam Nanosculpting of Suspended Graphene Sheets. Appl. Phys. Lett. 2008, 93, 113107. (33) Celebi, K.; Buchheim, J.; Wyss, R. M.; Droudian, A.; Gasser, P.; Shorubalko, I.; Kye, J.-I.; Lee, C.; Park, H. G. Ultimate Permeation across Atomically Thin Porous Graphene. Science 2014, 344, 289−292. (34) Koenig, S. P.; Wang, L.; Pellegrino, J.; Bunch, J. S. Selective Molecular Sieving through Porous Graphene. Nat. Nanotechnol. 2012, 7, 728−732. (35) Li, H.; Song, Z.; Zhang, X.; Huang, Y.; Li, S.; Mao, Y.; Ploehn, H. J.; Bao, Y.; Yu, M. Ultrathin, Molecular-Sieving Graphene Oxide Membranes for Selective Hydrogen Separation. Science 2013, 342, 95−98. (36) Chi, C.; Wang, X.; Peng, Y.; Qian, Y.; Hu, Z.; Dong, J.; Zhao, D. Facile Preparation of Graphene Oxide Membranes for Gas Separation. Chem. Mater. 2016, 28, 2921−2927. (37) Kim, H. W.; Yoon, H. W.; Yoon, S.-M.; Yoo, B. M.; Ahn, B. K.; Cho, Y. H.; Shin, H. J.; Yang, H.; Paik, U.; Kwon, S.; Choi, J.-Y.; Park, H. B. Selective Gas Transport through Few-Layered Graphene and Graphene Oxide Membranes. Science 2013, 342, 91−95. (38) Schrier, J. Fluorinated and Nanoporous Graphene Materials as Sorbents for Gas Separations. ACS Appl. Mater. Interfaces 2011, 3, 4451−4458. 23 ACS Paragon Plus Environment


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

Page 24 of 26

(39) Shen, J.; Liu, G.; Huang, K.; Jin, W.; Lee, K.-R.; Xu, N. Membranes with Fast and Selective GasTransport Channels of Laminar Graphene Oxide for Efficient Co2 Capture. Angew. Chem., Int. Ed. 2015, 54, 578−582. (40) Li, X.; Cheng, Y.; Zhang, H.; Wang, S.; Jiang, Z.; Guo, R.; Wu, H. Efficient CO2 Capture by Functionalized Graphene Oxide Nanosheets as Fillers to Fabricate Multi-Permselective Mixed Matrix Membranes. ACS Appl. Mater. Interfaces 2015, 7, 5528−5537. (41) Shen, Y.; Wang, H.; Liu, J.; Zhang, Y. Enhanced Performance of a Novel Polyvinyl Amine/Chitosan/Graphene Oxide Mixed Matrix Membrane for CO2 Capture. ACS Sustainable Chem. Eng. 2015, 3, 1819−1829. (42) Geng, J.; Jung, H.-T. Porphyrin Functionalized Graphene Sheets in Aqueous Suspensions: From the Preparation of Graphene Sheets to Highly Conductive Graphene Films. J. Phys. Chem. C 2010, 114, 8227−8234. (43) Berean, K.; Ou, J. Z.; Nour, M.; Latham, K.; McSweeney, C.; Paull, D.; Halim, A.; Kentish, S.; Doherty, C. M.; Hill, A. J.; Kalantar-zadeh, K. The Effect of Crosslinking Temperature on the Permeability of Pdms Membranes: Evidence of Extraordinary CO2 and CH4 Gas Permeation. Sep. Purif. Technol. 2014, 122, 96−104. (44) Li, P.; Wang, Z.; Qiao, Z.; Liu, Y.; Cao, X.; Li, W.; Wang, J.; Wang, S. Recent Developments in Membranes for Efficient Hydrogen Purification. J. Membr. Sci. 2015, 495, 130−168. (45) Xin, Q.; Li, Z.; Li, C.; Wang, S.; Jiang, Z.; Wu, H.; Zhang, Y.; Yang, J.; Cao, X. Enhancing the CO2 Separation Performance of Composite Membranes by the Incorporation of Amino AcidFunctionalized Graphene Oxide. J. Mater. Chem. A 2015, 3, 6629−6641.


Page 25 of 26

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


(46) Burress, J. W.; Gadipelli, S.; Ford, J.; Simmons, J. M.; Zhou, W.; Yildirim, T. Graphene Oxide Framework Materials: Theoretical Predictions and Experimental Results. Angew. Chem., Int. Ed. 2010, 49, 8902−8904. (47) Shan, M.; Xue, Q.; Jing, N.; Ling, C.; Zhang, T.; Yan, Z.; Zheng, J. Influence of Chemical Functionalization on the CO2/N2 Separation Performance of Porous Graphene Membranes. Nanoscale 2012, 4, 5477−5482.



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Suggested TOC graphic


Tuning the Surface Properties of Graphene Oxide by Surface-Initiated

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