Pyrene-Functional Copolymers

Nov 1, 2016 - Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States. ‡ Department of ...
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Gradient Films of Pristine Graphene/Pyrene-Functional Copolymers with Janus Electrical Properties Dorsa Parviz,† Ziniu Yu,‡ Stanislav Verkhoturov,§,∥ Micah J. Green,*,† and Ronald C. Hedden*,‡ †

Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409, United States § Materials Characterization Facility, Texas A&M University, College Station, Texas 77843, United States ∥ Department of Chemistry, Texas A&M University, College Station, Texas 77843,United States ‡

S Supporting Information *

ABSTRACT: We describe the first preparation of polymer-supported pristine graphene thin films with dramatically different electrical conductivities on the top and bottom surfaces. Pyrene-functional stabilizers based on polystyrene or poly(methyl methacrylate) were first synthesized by copolymerization of their monomers with 1-pyrenemethyl methacrylate. Stable dispersions of pristine graphene nanosheets were prepared by sonication of graphite in chloroform solutions of the pyrene-functional copolymers. Vacuum filtration of the polymer-stabilized graphene dispersions through a porous PTFE filter produces films with an internal gradient in nanosheet concentration. This gradient graphene concentration results in an electrically conductive, graphene-rich surface on one side of the composite film and a nonconductive, polymer-rich surface on its other side. Electrical conductivities ranging from 60 to 194 S/m are measured on the graphenerich surface, which are among the highest conductivities reported for pristine graphene/polymer composites. Through scanning electron microscopy and secondary ion mass spectrometry characterization, these films were found to contain three distinct layers: a polymer-rich top surface, a transition layer with a gradient in nanosheet concentration, and a buckypaper-like bottom layer consisting of densely packed, highly oriented graphene nanosheets. The gradient structure of these films and their Janus-like electrical conductivity has potential applications in graded coatings for radiofrequency and optical devices. KEYWORDS: pristine graphene, gradient composite film, poly(methyl methacrylate), polystyrene, pyrene

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layer-by-layer assembly,40 chemical vapor deposition transfer techniques,41 backfilling of graphene-based aerogels,42 and surface grafting with polymers or polymer brushes.43−51 Usually, GO is modified through covalent or noncovalent functionalization to provide stable dispersions in polymer matrixes. A reduction step is required to restore the electrical conductivity either before or after incorporation of GO into the polymer matrix. Pristine graphene offers advantages over GO in that it does not require reduction and potentially possesses higher conductivity compared to that of RGO.52 However, dispersions of pristine graphene in solid polymers stabilized by noncovalent interactions with functional polymers have been less commonly reported despite the simplicity of this approach. Recent work in our group has demonstrated that certain pyrene derivatives,53 including silicone polymers with pendant pyrene side groups,54 are effective stabilizers for pristine graphene. The pyrene groups adsorb onto the graphene surface

INTRODUCTION Nanocomposites of graphene or reduced graphene oxide (RGO) in polymers have received attention due to the potentially attractive combination of graphene’s electrical conductivity with the mechanical properties and processability of polymers. Nanocomposites of graphene or RGO with polystyrene (PS) and poly(methyl methacrylate) (PMMA) are among the most well-studied graphene nanocomposite systems.1,2 The abundance of experimental, theoretical, and computational studies exploring the properties of graphene and RGO dispersed in these commercially important, glassy thermoplastics underscores current interest in understanding and controlling their morphology and properties. Many strategies have been employed to disperse graphene or RGO in PS and PMMA, including melt processing methods,3−9 multilayer coextrusion,10 masterbatching approaches,11 foaming,12 in situ polymerization techniques,13−19 precipitation polymerization,20 methods based on emulsions or latexes21−24 and Pickering emulsions,25−28 attachment of small-molecule stabilizers or coupling agents to the nanosheets,29−31 reactive blending approaches,32−35 solution blending approaches,36−39 © 2016 American Chemical Society

Received: August 2, 2016 Accepted: November 1, 2016 Published: November 1, 2016 31813

DOI: 10.1021/acsami.6b09646 ACS Appl. Mater. Interfaces 2016, 8, 31813−31821

Research Article

ACS Applied Materials & Interfaces through π−π interactions, and the polymer chains provide steric shielding against aggregation of nanosheets.53 Using random copolymers of poly(dimethylsiloxane) and a pyrenefunctional siloxane unit, graphene-rich films with electrical conductivity up to 200 S/m were prepared by solution-casting dispersions of copolymer-stabilized graphene and leaching out the excess, unbound polymer on a porous substrate to concentrate the graphene nanosheets.54 Silicone elastomers containing stabilized graphene were also prepared by a similar route, but their electrical conductivity was far lower, as the leaching approach was not as readily applicable to the crosslinked networks.54 Elsewhere, pyrene-functional block copolymers55 and pyrene end-functional telechelic polymers56 have been employed as stabilizers of graphene oxide and graphene, respectively, for producing nanocomposites with polymers. Our previous work on pyrene-functional silicone stabilizers inspired the present study, in which pyrene-functional copolymers of atactic PS and PMMA are demonstrated to stabilize pristine graphene in solutions and permit formation of solid films with unusual electrical properties. A new approach for preparing thin films of glassy, amorphous polymers with an internal gradient in graphene concentration is presented herein. Copolymers of 1-pyrenemethyl methacrylate with either styrene or methyl methacrylate are prepared by simple, bulk, free-radical polymerizations. Graphene nanosheets are introduced into chloroform solutions of these polymers, after which cast films are prepared by a vacuum filtration approach. The resulting films are the first examples of graphene-containing polymer films having one electrically conductive surface and one electrically insulating surface. The continuous increase in the graphene nanosheet concentration across these composite films may enable one-step preparation of antireflective coatings for various radiofrequency (RF) and optical applications.57,58 In such coatings, the continuous increase in the relative permittivity of the dielectric composite with average conductivities (10−100 Ω/sq)59,60 minimizes the light or RF reflection from the surface. This report is the first to describe the preparation, structure, and electrical properties of these novel asymmetric gradient films.

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respectively. Table S1 summarizes the mole fractions of 1pyrenemethyl methacrylate monomer in the feed for each sample. Synthesis of Poly(styrene-co-1-pyrenemethyl Methacrylate) Copolymers (PSPMA). Procedures followed were similar to those for the synthesis of PMPMA copolymers, except styrene monomer was substituted for methyl methacrylate. PSPMA copolymers with 1pyrenemethyl methacrylate mass fractions of 0.01, 0.05, 0.10, and 0.15 were named PSPMA-1, PSPMA-5, PSPMA-10, and PSPMA-15, respectively. Preparation of Graphene Dispersions. To prepare graphene dispersions, 200 mg mL−1 of coplymer-1, 40 mg/mL of copolymer-5, 20 mg/mL of copolymer-10, and 13.33 mg mL−1 of copolymer-15 were dissolved in chloroform. These specific concentrations of the copolymers were chosen to yield 2 mg mL−1 of pyrene-functional repeat units in the solution. Subsequently, 30 mg mL−1 of expanded graphite was added to the solution, and the mixture was tip-sonicated (Misonix sonicator, XL 2000) for 1 h. To avoid solvent evaporation, an ice bath was used during sonication. To separate the exfoliated nanosheets from the graphitic aggregates, the resultant dispersions were centrifuged (Centrific Centrifuge 225, Fischer Scientific) for 4 h at 5000 rpm. The supernatant of the centrifuged dispersions was collected and used for characterization experiments and nanocomposite preparation. Preparation of Graphene/PMPMA and Graphene/PSPMA Films. To prepare the films, 10 mL of the graphene dispersions stabilized by PMPMA-10, PSPMA-10, PMPMA-15, and PSPMA-15 were vacuum filtered through a porous filtration membrane ((PTFE, 0.2 μm pores, 47 mm in diameter, Omnipore Membrane Filters). After removal of the solvent, the resultant graphene/polymer films were peeled off the membrane and air-dried at room temperature for 24 h to remove residual chloroform. For electrical conductivity comparison, films of graphene/polymer were prepared by drop-casting the same dispersions on a Kapton polyimide substrate. UV−Vis Spectroscopy. A Shimadzu 2550 spectrophotometer was used for UV−vis spectroscopy of 1-pyrenemethyl methacrylate, PMPMA-15, and PSPMA15 copolymers. The samples were dissolved in chloroform (0.01 mg/mL), and pure chloroform was used as the blank for the measurements. The pyrene-specific absorbance band at 348 nm was used to confirm the presence of pyrene groups in the copolymers. Gel Permeation Chromatography (GPC). The copolymers were characterized by GPC to calculate their molar masses and polydispersity indices (PDI). Four Phenomenex Phenogel columns (5 μm beads, pore sizes of 106, 105, 104, and 103 Å) in series were used, covering a molar mass range of 1 kg/mol to 10 000 kg/mol, and THF was used as the mobile phase. The calibration curves for PSPMA and PMPMA copolymers were obtained using PS and PMMA homopolymer standards, respectively. Elution profiles for PMPMA and PSPMA copolymers were recorded by a KNAUER Smartline 2300 Refractive Index detector. In addition, elution of pyrene groups was recorded by a Knauer V2.8 UV−vis detector at 348 nm, a wavelength which is specific to the pyrene groups. GPC experiments were performed with raw polymers before precipitation from THF with methanol to determine the fraction of pyrene groups that were incorporated into polymer chains. High-Resolution Transmission Electron Microscopy (HRTEM). The graphene dispersions in chloroform were deposited on 200 mesh carbon-coated copper grids (Electron Microscopy Sciences, CF200-Cu) and dried at room temperature for 1 min. Images were obtained on an FEI Tecani G2 F20 HRTEM at a voltage of 200 kV with a Gatan camera. Thermogravimetric Analysis (TGA). TGA was performed in a TA Instruments Q50 TGA to determine the graphene content of the cast an vacuum-filtered films. Dispersions were drop cast onto Kapton polyimide substrates, and the solvent was evaporated completely before the TGA experiments were conducted. Fifteen milligrams of each sample was heated from room temperature to 1000 °C at a rate of 10 °C/min in a nitrogen atmosphere. Similar procedures were followed to determine the overall graphene content of the vacuumfiltered films for comparison.

MATERIALS AND METHODS

Materials. Styrene (CAS no. 100-42-5, 99%) was purchased from Acros. Methyl methacrylate (CAS no. 80-62-6, 99%) was purchased from Alfa Aesar. 1-Pyrenemethyl methacrylate (99%), 2,2′-azobis(2methylpropionitrile) (98%), chloroform (HPLC grade), tetrahydrofuran (ACS grade), and methanol (ACS grade) were purchased from Sigma-Aldrich. Expanded graphite (grade 3806) was provided by Asbury Carbons. All chemicals were used as received. Synthesis of Poly(methyl Methacrylate-co-1-pyrenemethyl Methacrylate) Copolymers (PMPMA). Procedures for polymer synthesis are adapted from the Ph.D. thesis of Ziniu Yu.61 Various mass fractions of 1-pyrenemethyl methacrylate (0.01, 0.05, 0.10, and 0.15) were dissolved in methyl methacrylate (MMA) monomer to yield a mixture of a total mass of 5.0 g. To initiate bulk-free radical polymerization, 0.05 g of 2,2′-azobis(2-methylpropionitrile) (azobis(isobutyronitrile), AIBN) was dissolved in the monomer mixture, which was placed in a preheated dry-bath at 80 °C for 12 h in a sealed PTFE container. After being cooled to room temperature, the reaction product was a transparent, light yellow, glassy solid. Each polymer was purified by dissolving the material in tetrahydrofuran (THF) at a concentration of 100 mg mL−1, followed by precipitation from THF by slow addition of excess methanol (typical methanol:THF volume ratio of 2:1) with vigorous stirring. Copolymers synthesized with 1pyrenemethyl methacrylate mass fractions of 0.01, 0.05, 0.10, and 0.15 were named PMPMA-1, PMPMA-5, PMPMA-10, and PMPMA-15, 31814

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ACS Applied Materials & Interfaces Conductivity Measurements. The electrical resistance of the nanocomposites was measured using the four-point probe method. The four-point probe head (Signatone, SP4-40045TBY) was mounted on a resistivity measurement stand (Signatone, Model 302). The spacing between the probe tips was 1.5875 mm. The current was passed to the sample through the outer probes using a Keithley 6221 AC and DC current source. A Keithley 2000 multimeter was used to measure the voltage across the sample. The sheet resistance and electrical conductivity of the samples were calculated using the measured values of the voltage. The thickness of the graphene-rich section of the vacuum filtered films, measured on the cross-sectional SEM images of each film, was used for calculation of the electrical conductivity. Scanning Electron Microscopy (SEM). Graphene/copolymer nanocomposites were fractured into pieces and mounted on a doubleface carbon tape. An accelerating voltage of 2 kV was used to image the top surfaces and cross sections of the samples with a JEOL JSM7500F SEM instrument. Depth-Profiling Using Secondary Ion Mass Spectrometry (SIMS). A CAMECA 4F secondary ion mass spectrometer (Materials Characterization Facility, TAMU) was used for profiling. The crosssection surfaces were sputtered with a 5.5 keV O2+ beam with a current of 1.6 μA. The diameter of the beam was ca. 2 μm, and the raster was 250 × 250 μm2. The angle of incidence of the beam was 26° with respect to the sample’s surface. The measured secondary ions were positively charged. The ion signal was acquired for a 50 × 50 μm2 area. The time scale of the profile was recalculated to the depth using SRIM/TRIM simulation (http://srim.org/).

Figure 2. Absorbance spectra of 1-pyrenemethyl methacrylate and the synthesized copolymers. A pyrene-specific absorbance band appears for all samples at 348 nm.

at 348 nm. The percentage of 1-pyrenemethyl methacrylate groups incorporated into polymer chains ranged from 92 to 99% (Table S1). Because the reactivities of the monomers are unlikely to be equal, the respective copolymers are presumed not to have truly random sequence distributions. The possibility that pyrene groups could be concentrated in very short or very long chains must be considered. To characterize the molar mass distribution of the copolymers, GPC analysis was therefore performed using two independent detectors. A UV−vis detector set to 348 nm was used to monitor elution of the pyrene groups, while a refractive index (RI) detector was used to detect the elution of all polymeric species. The simultaneous elution of the pyrene groups and the polymer chains, shown in Figures 3 and S1, confirms the incorporation of the

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RESULTS AND DISCUSSION PMMA and PS copolymers with pyrene-functional side groups were prepared via the synthetic route described in Figure 1. 1-

Figure 1. Synthetic route for PMPMA and PSPMA copolymers. The labels n and m represent the mole fractions of MMA or styrene, respectively, and 1-pyrenemethyl methacrylate in the copolymer chain.

Figure 3. Comparison of GPC elution profiles of (a) PSPMA-15 and (b) PMPMA-15 obtained using the UV−vis and RI detectors.

Pyrenemethyl methacrylate was dissolved in either styrene or methyl methacrylate monomer, followed by bulk, thermally initiated free radical polymerization. Details of the synthesis are presented in the Supporting Information. 1-Pyrenemethyl methacrylate mass fractions of 0.01, 0.05, 0.10, and 0.15 were selected to yield four PSPMA copolymers with styrene (PSPMA-1 to PSPMA-15) and four PMPMA copolymers with methyl methacrylate (PMPMA-1 to PMPMA-15). Observation of the pyrene absorbance peak at 348 nm in the UV−vis traces from GPC analysis verified the presence of pyrene groups (Figure 2). This absorbance is specific to 1pyrenemethyl methacrylate and does not appear in the spectra of PMMA or PS homopolymers.62 The percentage of the 1pyrenemethyl methacrylate monomer that was incorporated into polymer chains was estimated from GPC elution profiles by comparing the integrals of the polymer elution peak and the monomer/oligomer elution peak(s) in the UV detector traces

1-pyrenemethyl methacrylate monomers into the polymer backbones. The UV−vis detector traces exhibited monomer/ oligomer elution peak(s) with a retention time of about 105 min, which indicates the presence of some residual low molar mass species containing pyrene groups. The number-average molar mass (Mn) and polydispersity index (PDI ≡ Mw/Mn) of each copolymer (Table S1) were calculated from both the UV− vis detector responses and the RI detector responses (Figure S2). In Figure S2, the RI detector traces are shifted to the right due to a difference in flow path length in the GPC system, so the retention times cannot be compared directly. Calibrations with polystyrene standards were therefore carried out independently for each detector for computation of Mn and Mw. The molar masses of PMPMA copolymers obtained from RI detector traces were slightly higher than those obtained from UV−vis detector traces, which suggests preferential incorporation of pyrene groups into the shorter PMPMA chains. On 31815

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nanosheets in graphene/PSPMA-15 (the inset of Figure 4a) and graphene PMPMA-15 after casting samples from chloroform dispersions and air-drying. The lateral size of the graphene nanosheets is on the order of 1 μm. Because of polymer coverage on the graphene surface and nanosheet folding, it was difficult to perform an edge count of the graphene sheets. To assess the graphene content of the dispersions, polymer films with and without added graphene were cast on an impermeable substrate (Kapton polyimide). TGA analysis was carried out after films were removed from the Kapton. Thermal degradation of the neat copolymers was complete when the samples reached 450 °C (Figure S3). The remaining mass of the composite film above 450 °C minus the remaining mass of a graphene-free control sample was taken as the graphene mass in the composite film. Figure S4a depicts the TGA results for cast films of graphene + PSPMA copolymers. Both cast films had graphene content of approximately 7.5 wt % (Table 1). Because the same pyrene

the other hand, the molar masses of PSPMA copolymers calculated from UV−vis detector traces were similar to or slightly higher than those obtained from RI detector traces, suggesting a slight preference for pyrene groups to be incorporated into the longer chains. The PDI of the PMPMA copolymers increased with increasing pyrene content, while the PDI of PSPMA copolymers showed the opposite trend. Although we did not measure reactivity ratios of the monomer pairs, a difference in reactivity ratios between the two systems could account for these observations. In a previous work, we grafted 1-pyrenemethyl methacrylate to poly(methylhydrogensiloxane) precursor chains via hydrosilylation reaction to obtain pyrene-functional silicone random copolymers.54 The material was separated into several fractions of relatively narrow molar mass distribution to study the effects of chain length on the polymers’ ability to stabilize graphene nanosheets in organic solvents. No clear trend in stabilizing efficiency was found with increasing copolymer molar mass. Because chains of any molar mass were able to stabilize substantial amounts of graphene in solution, we elected not to fractionate the PSPMA and PMPMA copolymers in this study, nor did we attempt to narrow their molar mass distributions by employing more sophisticated polymerization routes. The simple preparation route for the copolymers in this study is attractive from a scale-up and manufacturing standpoint. The PSPMA and PMPMA copolymers were subsequently used to prepare stabilized dispersions of graphene in chloroform. Graphene nanosheets can be exfoliated from graphite via sonication, but they reaggregate readily if they are not stabilized by a dispersant. The concentration of pyrene-functional repeat units, rather than total polymer concentration, was held constant at 2 mg/mL in all of the dispersions; thus, a larger copolymer concentration was chosen for those copolymers containing relatively little pyrene. The copolymer concentrations used for preparation of dispersions are reported in Table S2. The exfoliation of graphene nanosheets was carried out by tip sonication of graphite in the chloroform/copolymer solutions. Residual graphitic materials were removed by centrifugation, and the supernatants were collected. Centrifugation was performed to make sure that only stabilized pristine graphene nanosheets remain in the sample and contribute to the percolation in the composite. The graphitic sediment collected after centrifugation may be recycled to increase the graphene yield in this direct exfoliation process.63 Stable graphene dispersions were obtained for all copolymers except PSPMA-1. Figure 4 depicts HRTEM images of graphene

Table 1. Graphene Content of the Cast and Vacuum-Filtered Films of Graphene/PSPMA and Graphene/PMPMA Obtained from TGA Analysis and Measured Electrical Conductivities of the Filmsa graphene/ PSPMA-10 graphene content of cast film (wt %) graphene content of vacuum filtered film (wt %) electrical conductivity of cast film (S/m) electrical conductivity of vacuum filtered film (S/m)

graphene/ PSPMA-15

graphene/ PMPMA-10

graphene/ PMPMA-15

7.50

7.28

4.19

5.16

19.77

21.71

6.51

11.15

not conductive

not conductive

not conductive

not conductive

162.35 (± 18.40)

194.83 (± 67.96)

60.14 (± 5.37)

83.18 (± 4.69)

a Conductivities for the vacuum-filtered films were measured on the membrane-facing side of the films.

concentration was used in preparation of all the dispersions, while polymer mass fraction was allowed to vary, the similar graphene content in the cast films suggests that stabilization occurs primarily through the π−π interactions of pyrene and graphene. Similarly, Figure S4b shows the TGA results for the cast films prepared using graphene and PMPMA copolymers. All graphene/PMPMA films exhibited an extra degradation step compared to the neat PMPMA copolymers. Both cast films contained approximately 5 wt % graphene (Table 1), which further supports the idea that π−π stacking interactions between pyrene groups and graphene stabilize the dispersions. However, the lower graphene content compared to the graphene/PSPMA cast films illustrates that PSPMA copolymers are better dispersants for graphene nanosheets. While polystyrene homopolymer is not known to be a suitable stabilizer for pristine graphene nanosheets in chloroform, the pendant aromatic rings of polystyrene may provide a secondary stabilizing effect that increases the mass fraction of stabilized graphene after sonication. Despite the high graphene content in both PSPMA and PMPMA-based cast films, their electrical conductivities were below the measurable threshold for composite films (∼10−9 S/ m) with the equipment specified in the Supporting Information. The graphene content of the films is high enough that one might have expected the percolation threshold of the

Figure 4. HRTEM image of graphene nanosheets cast from a dispersion of (a) graphene/PSPMA-15 and (b) graphene/PMPMA-15 in chloroform. The inset shows the graphene/PSPMA-15 dispersion in chloroform. 31816

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Figure 5. (a and d) SEM images of the cross-section of graphene/PSPMA-15 and graphene/PMPMA-15 films, respectively. The inset of panel a shows the graphene-rich surface of graphene/PSPMA-15 film. (b and e) Magnified SEM images of the polymer-rich sections of graphene/PSPMA15 and graphene/PMPMA-15 films, respectively. (c and f) Magnified SEM images of the graphene-rich sections of graphene/PSPMA-15 and graphene/PMPMA-15 films.

results suggest that longer polymer chains become entangled during the filtration process and are less likely to pass through the filter. The pyrene mass fractions in the filtrates (mg pyrenemethyl methacrylate/mg of polymer) were measured from their UV absorbances at 348 nm without passing the material through the GPC columns. The PSPMA-15 filtrate contained about 82% as much pyrene as the original PSPMA15. The PMPMA-15 filtrate contained about 73% as much pyrene as the original PMPMA-15. These results are intuitive, as chains that are rich in pyrene-functional repeat units are more likely to interact with graphene and thus remain in the films. The filtration process preferentially removes shorter chains, especially those that are deficient in pyrene-functional repeat units. The electrical conductivities of the vacuum-filtered films were measured (Table 1), revealing, surprisingly, that all films were conductive only on the side that was in contact with the porous membrane. The conductivities of the membrane-facing surfaces ranged from 60.14 S/m for graphene/PSPMA-10 up to 194.83 S/m for graphene/PSPMA-15, whereas the conductivities of the air-facing surface were too low to be measured with the equipment used. The surface conductivities on the membranefacing side are among the highest values reported for graphene/ PMMA and graphene/PS nanocomposites in the literature.39,64−69 This work is the first report of asymmetric graphene-polymer composite films with Janus-like electrical properties on two sides. In this respect, our films differ from the hybrid buckypapers prepared previously by Liu et al.48 by vacuum filtration of polystyrene-grafted graphene oxide dispersion. Their films had impressive conductivities of up to 120 S/m, but there was no indication of a dramatic difference in conductivity between the top and bottom surfaces as in the present study. To investigate the distribution of graphene nanosheets within these asymmetric films, their cross sections were imaged by SEM (Figure 5a and d). The images revealed a morphological transition from a polymer-rich side (air interface) to a graphene-rich side (membrane interface), which is consistent with the conductivity measurements. In the polymer-rich side (Figure 5b and e), a few graphene sheets with random orientation are embedded in the polymer matrix. In contrast,

nanosheets to be exceeded. It is possible that slow chloroform evaporation causes formation of voids or drying-induced phase separation of polymer-stabilized graphene sheets from the free unbound polymer chains. The SEM images of graphene/ PMPMA-15 cast film in Figure S5 indicate the surface inhomogeneity of this film. In any event, electrical percolation of the nanosheets was not achieved in the cast films. Four of the PSPMA and PMPMA copolymers (PSPMA-10, PMPMA-10, PSPMA-15, and PMPMA-15) were subsequently used to prepare composite films by vacuum filtration of the chloroform dispersions over a porous PTFE membrane. Vacuum filtration was previously shown to be an effective strategy for obtaining conductive, graphene-rich films.48,54 The TGA characterization of vacuum-filtered films (Figure S4a) revealed a considerable increase in their graphene content, which reached about 20 wt % (Table 1). This increase in the graphene content indicates that some portion of the unbound polymer chains are removed during vacuum filtration. A similar increase in the graphene content was observed in the PMPMAbased vacuum filtered films (Figure S4b). The slight increase in the graphene content of the graphene/PMPMA-10 vacuum filtered film (6.51 wt %) suggests limited polymer removal in this sample (Table 1), but the polymer removal in the graphene/PMPMA-15 film was significant, and its graphene content rose to 11.15 wt %. Comparison between the graphene/PSPMA and graphene/PMPMA films indicates a more effective reduction in polymer content for graphene/ PSPMA during vacuum filtration. GPC analyses were repeated for the filtrates of the PSPMA15 and PMPMA-15 samples to characterize the molar mass and pyrene content of the polymer chains that passed through the filters. The filtrates were light yellow in color, suggesting that a negligible amount of graphene passed through the filter. For the PSPMA-15 filtrate, the UV detector trace at 348 nm provided Mn of 15 800 g mol−1 with a PDI of 3.0. The residual PSPMA in the filtrate had a lower number-average molar mass than the starting material, and its PDI was slightly lower. For PMPMA15, the UV detector trace at 348 nm provided Mn of the filtrate of 37 000 g mol−1 with a PDI of 1.7. The residual PMPMA in the filtrate had a number-average molar mass similar to that of the starting material, but its PDI was significantly lower. These 31817

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begins to dominate the removal of chloroform from the film. As a result, unbound polymer and some isolated nanosheets form a polymer-rich layer at the air interface. Some of these nanosheets are folded, much like those observed in the cast films formed by air-drying of the solvent. The nanosheets in the topmost layer are not subjected to compression against the PTFE surface and are thus less likely to exhibit orientation with respect to the substrate. The graphene/PSPMA-10 film (Figures S6a−c) and the graphene/PMPMA-10 film (Figure S6d−f) both exhibited asymmetric distributions of graphene nanosheets within their cross sections. However, the stacking of graphene sheets at the bottom surface was less efficient in graphene/PMPMA films compared to that in graphene/PSPMA films. Moreover, the thickness of the graphene-rich layer was higher in the PSPMA films compared to the PMPMA films. These observations are consistent with the TGA results, which revealed a larger enrichment in the graphene content of the PSPMA films compared to PMPMA films after vacuum filtration. This observation might be attributed to different mass fractions of bound vs unbound chains in the respective films or alternatively may be due to a difference in rheological properties of the polymer solutions. For both the PSPMA and PSPMA films, the densely packed nanosheet layer at the membrane interface approximates a buckypaper, whereas the polymer-rich layer at the air interface is essentially a plastic support layer that is expected to exhibit mechanical properties approximating those of the respective copolymers. To confirm the graded distribution of the graphene nanosheets across the composite film thickness, the depthprofiles of the vacuum filtered films were acquired using SIMS. The results for a graphene/PMPMA-10 film are presented in Figure 7a. The measured intensities of CH+, C2+, O+, and CH2+ ions were normalized with respect to measured intensity of C+ ions. Three specific regions were recognized within this film: (i) a graphene-rich region with in which the ratio of polymer/ graphene remains constant, (ii) a transition region in which the polymer/graphene ratio increases linearly with depth, and (iii) a polymer-rich region in which the polymer/graphene ratio remains constant. The presence of the middle (transition)

the graphene sheets in the graphene-rich side (Figure 5c and f) are highly aligned and are not separated by a substantial polymer layer. These observations confirm the depletion of polymer in the membrane-facing side of the film, meaning that the graphene concentration at this interface is even higher than the bulk values measured by TGA. SEM images for graphene/ PSPMA-10 and graphene/PMPMA-10 films are shown in Figure S6. The observed distribution of graphene nanosheets along the cross-section of the composite film results in a continuously increasing electrical conductivity throughout the film from the insulating side toward the conductive side. Figure 6 presents a schematic summarizing our postulated mechanism of formation of the gradient in graphene

Figure 6. Proposed mechanism of multilayer film formation via vacuum filtration and polymer leaching.

concentration during vacuum filtration. The accumulation of nanosheets at the membrane interface involves a flow-induced sedimentation process driven by strong hydrodynamic forces on the μm-sized graphene nanosheets. In the dispersions, a certain fraction of the copolymer chains adsorb onto the surface of graphene due to π−π interactions between pyrene and graphene; these chains sterically prevent aggregation of the dispersed nanosheets. The remainder of the polymer chains are unbound and are thus able to be drawn out through the pores of the filter when the vacuum is applied. As nanosheets migrate toward the porous PTFE membrane under pressure-driven flow, they settle on the surface, becoming aligned with the substrate. As nanosheets accumulate and stack at the interface, passage of liquid through the nanosheet layer presumably slows such that evaporation of chloroform from the air interface

Figure 7. Depth-profile analysis of (a) graphene/PMPMA-10 and (b) graphene/PSPMA-10 vacuum-filtered films obtained using SIMS. Three different zones of (i) graphene-rich, (ii) graphene/polymer mixture, and (iii) polymer-rich composite were detected in both films. 31818

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ACS Applied Materials & Interfaces region, which was not distinctive in the SEM images of the cross-section of the film, supports the proposed slowing of the polymer leaching as the accumulating graphene nanosheets restrict the flow of the unbound polymer solution toward the membrane. Similarly, the depth-profile of graphene/PMPMA15 film (Figure S7) indicates the existence of three distinct graphene-rich, transient, and polymer-rich regions in this film; however, the calculated thickness of graphene-rich region is higher relative to that of the graphene/PMMA-10 film. This observation is consistent with the TGA results (Table 1) showing a higher graphene concentration in the graphene/ PMPMA-15 vacuum filtered film. Figure 7b indicates the depth-profile of the graphene/ PSPMA-10 vacuum filtered film. The graphene-rich, transient, and polymer-rich regions are recognizable in the depth-profile of this composite film. The graphene-rich region covers a higher fraction of the film thickness compared to the graphene/ PMPMA films. Additionally, the continuous increase in the intensity values in the polymer-rich region confirms the presence of more graphene nanosheets in this region with respect to similar regions in graphene/PMPMA films. All these observations are fully consistent with the SEM and TGA results discussed earlier. The existence of three distinct layers with differences in graphene concentration and orientation suggests that vacuumfiltered polymer-graphene films have the potential to exhibit a wide range of structures depending on processing conditions. It may be feasible to create “programmed” graphene/polymer composite films with tunable graphene concentration vs depth profiles by controlling parameters such as polymer concentration, membrane pore size, and vacuum pressure differential. Such films could potentially be of utility as antireflective coatings for radiofrequency and optical devices where a continuously changing permittivity is required across the coating to reduce the electromagnetic reflections from the surface of these devices.57,58

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding was provided by the DuPont Young Faculty Award and by the United States National Science Foundation (NSF) under CAREER award CMMI-1253085. We acknowledge helpful advice from collaborators and colleagues, including Prof. Mustafa Akbulut of TAMU and Dr. Yordanos Bisrat from the Materials Characterization Facility at TAMU. We also thank the rest of the Green group at TAMU and the Hedden group at Texas Tech for their insight, particularly Dr. Lan Ma.

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REFERENCES

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CONCLUSIONS In this work, pyrene-functional copolymers of PS and PMMA were synthesized through a facile, one-step, free radical polymerization route. The attachment of pyrene groups to the polymer backbones and the molar mass distributions of the copolymers were characterized by UV−vis spectroscopy and GPC. The copolymers were used to exfoliate and stabilize pristine graphene nanosheets in chloroform solution. Through use of a vacuum filtration approach, asymmetric films with an internal gradient in graphene concentration were prepared, which exhibited a wide disparity in electrical conductivity between their top and bottom surfaces. These novel films have an electrically conductive side with conductivities up to ca. 194 S/m and a polymer-rich, electrically insulating side with an intervening layer having a gradient in graphene concentration. Asymmetric graphene/polymer films may have potential applications in sensors and specialized coatings, as polymer films with internal graphene concentration profiles could provide novel optical or electrical properties.

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GPC elution traces for PSPMA-1 and PMPMA-1 (S1); composition, molar mass, and PDI of each copolymer (Table S1); GPC elution profiles of all copolymers (S2); polymer content of each graphene/copolymer dispersion (Table S2); TGA results for the neat copolymers (S3); TGA results for graphene/copolymer cast and vacuum filtered films (S4); SEM images of the graphene/ PMPMA-15 cast film (S5); SEM images of graphene/ PSPMA-10 and graphene/PMPMA-10 vacuum filtered films (S6); and depth-profile of graphene/PMPMA-15 vacuum filtered film (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09646. 31819

DOI: 10.1021/acsami.6b09646 ACS Appl. Mater. Interfaces 2016, 8, 31813−31821

Research Article

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