Confinement of Functionalized Graphene Sheets by Triblock

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Confinement of Functionalized Graphene Sheets by Triblock Copolymers Laura Peponi,† Agnieszka Tercjak,‡ Raquel Verdejo,§ Miguel Angel Lopez-Manchado,§ In˜aki Mondragon,‡ and Jose` M. Kenny*,† European Centre for Nanostructured Polymers and UniVersity of Perugia-UdR INSTM, Pentima 4, 05100 Terni, Italy, “Materials + Technologies” Group, Departamento Ingenierı´a Quı´mica y M. Ambiente, Escuela Polite´cnica, UniVersidad Paı´s Vasco, Pza. Europa 1, 20018 Donostia-San Sebastia´n, Spain, and Institute of Polymer Science and Technology (CSIC), Juan de la CierVa 3, 28003-Madrid, Spain ReceiVed: August 3, 2009; ReVised Manuscript ReceiVed: September 17, 2009

The confinement of functionalized graphene sheets (FGS) in the polystyrene (PS) domains of a nanostructured poly(styrene-b-isoprene-b-styrene) (SIS) block copolymer is reported in this paper. The addition of FGS showed a drastic effect on the nanostructured morphology of the block copolymer (BC) matrix switching from longrange order cylinders for the neat BC to cylinders parallel and perpendicular to the free surface for the nanocomposite. The confinement of the FGS was confirmed by atomic and electrostatic force microscopy. AFM images showed that only the PS block was able to sequester and host the graphene sheets due to chemical affinity. Moreover, the average diameter of the PS domains suggested that the FGS presented a stable folded configuration. The sequestering of the FGS by the PS block is associated to the measured increase of 6 °C in the PS glass transition temperature. EFM experiments revealed superficial electrical conductivity of the nanocomposites. Introduction Graphene is a giant aromatic macromolecule with an enormous specific surface area and excellent mechanical, electrical, and thermal conductivity properties.1-4 Thus, the incorporation of individual graphene sheets into polymer matrices could lead to a new class of polymer composite materials with enhanced properties and new functionalities.5-9 Self-assembling of block copolymers (BCs) has been reported as a powerful method to generate well-ordered structures at the nanometer scale.10,11 BC can spontaneously self-organize into well-defined spherical, cylindrical, and lamellar morphologies with domain dimensions of 5-100 nm.12,13 This self-assembling is controlled by a reduction of the enthalpy as the blocks demix but also by the loss of entropy as the blocks arrange in ordered structures.14 The resulting equilibrium morphologies depend on the copolymer composition, the volume ratio of each block, and the experimental conditions, among other factors. These mesodomain structures of block copolymer, in fact, can act as hosts for sequestering nanoscopic inclusions of appropriate chemical affinity and geometry. Thus, while the properties of nanocomposite materials can be studied by either confining the dimensions of nano-objects or controlling the nanostructure of the matrix, the use of a BC matrix as a template for nanomaterials allows the combination of these two concepts: hybrid nanostructured BC materials and nanosized particles properly dispersed and distributed.15 * Corresponding author. E-mail: [email protected]. Phone: +39 0744 492939. Fax: +39 0744 492934. † European Centre for Nanostructured Polymers and University of Perugia-UdR INSTM. ‡ Universidad Paı´s Vasco. § Institute of Polymer Science and Technology (CSIC).

10.1021/jp9074527 CCC: $40.75

Over the last years, the production of nanocomposites and nanostructures based on the unique self-assembling properties of BC templates has been actively pursued,15-18 as it can yield a vast range of multifunctional properties for potential technological applications. The inclusion of nanofillers in the BC can affect the BC microstructure, providing new opportunities for scientific insights into the physics of self-organization. In fact, it has been recently reported that carbon nanotubes can be successfully dispersed in phase-separated block copolymers.19 In all cases, the dispersion of the carbon nanotubes was promoted by the BC nanostructuration but, at the same time, the BC nanostructure was affected by the nanoinclusions. In general, the block copolymer was designed in such a way that one block of the polymer forms a close interaction with the carbon nanotube (CNT) walls, providing solubility and driving force for the exfoliation, while the other block(s) forms a steric barrier or repulsion interaction between polymer-wrapped nanotubes, inducing the nanodispersion of the block phases with the nanotubes contained in one of the phases.20,21 Further studies of the effect of nanofillers in BC have shown that the addition of 2D nanoclay can lead to templating effects or disruption of long-range order.15 Although significant advances have been made in recent years, the prohibitive high cost of carbon nanotubes, the limited availability of high quality nanotubes in large quantities, and its difficult dispersion into the polymer matrix have limited their use. A plausible solution to overcome some of the difficulties encountered with CNTs can be the use of graphite-like nanoplatelets.1 Their excellent mechanical and electrical properties may be relevant at the nanoscale if graphite can be exfoliated into thin nanoplatelets, and even down to the single graphene sheet level. Graphene is a relatively new 2D nanomaterial  2009 American Chemical Society

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physically obtained from graphite by micromechanical cleavage2,3 or by explosive oxidation and successive reduction.8,22 Graphite oxide is heavily oxygenated, bearing hydroxyl and epoxide functional groups on their basal planes. The thermal exfoliation of graphite oxide at high temperatures gives rise to functionalized graphene sheets (FGS). The surface functionality of these graphene sheets can provide a better interaction with the host polymer, giving rise to intimate nanosheet-polymer interactions and a percolated interface essential to mechanical, thermal, and electrical enhancement.23-27 Moreover, graphene sheets have higher surface-to-volume ratios than single walled carbon nanotubes as a consequence of the inaccessibility of the inner nanotube surface to polymer molecules. The processing of polymer nanocomposites with designed mechanical, thermal, and electrical properties based on exfoliated graphene sheets is still a very complex and time demanding process, and few reports can be found on this topic in the scientific literature.9,28 The main aim of the research reported here is to investigate the morphological changes obtained in the self-organization of the block copolymer matrix used as the host of graphene nanosheets. The final purpose of this research is the development of transparent thin films for optoelectronic applications. Thus, we propose an easy method to obtain the dispersion of functionalized graphene sheets through their exfoliation and confinement in one of the blocks of a self-assembled poly(styreneb-isoprene-b-styrene) (SIS) BC matrix. The effects of the graphene confinement on both the morphology and the electrical properties of the BC based nanocomposite thin films were studied by atomic and electrostatic force microscopies (AFM and EFM, respectively). The analysis indicates that only one of the blocks (polystyrene, PS) is able to sequester and host the exfoliated graphene nanosheets. Experimental Section Preparation of Exfoliated Graphene Sheet Suspensions. Functionalized graphene sheets (FGS) can be produced by thermal expansion of graphite oxide.28-30 Graphite oxide was produced using natural graphite powder (universal grade, 200 mesh, 99.9995%) according to the Brodie method.31 The preparation and surface characterization of the FGS used in this research, including the detailed description of the functional groups on the graphene wall, have been reported elsewhere.8 FGS were then dissolved in 40 mL of toluene (0.025 g/mL) and sonicated with a 750 W Vibracell 75043 from Bioblock Scientific, with 30% amplitude for 10, 30, and 60 min. Each graphene solution was spin-cast (3000 rpm for 30 s) on a fluorine-doped tin-oxide (FTO)-coated glass. The FTO surface was first cleaned with ethanol, then cleansed sequentially with deionized water, and finally dried under a high purity nitrogen atmosphere. Film samples were analyzed by field emission scanning electron microscopy, FE-SEM ZEISS SUPRA 25, to monitor the degree of exfoliation reached by the graphene sheets. A homogeneous suspension with good graphene dispersion was obtained after 60 min of sonication time. Furthermore, to confirm the degree of exfoliation, a graphene thickness of about 1 nm has been measured by spectroscopic ellipsometry using an M-2000 ellipsometer from J. A. Woollam Co on the same films analyzed by FE-SEM. TEM images were obtained with a Philips Tecnai 20 TEM apparatus using an accelerating voltage of 200 kV. The nanocomposite samples were 40-50 nm thick and prepared with a LEICA EM UC6 ultracryomicrotome, cutting at -140 °C. Preparation and Characterization of Nanocomposite Thin Films. Poly(styrene-b-isoprene-b-styrene) block copolymer (SIS D 1165), kindly provided by Kraton Polymer, was dissolved in

Letters the homogeneous suspension of functionalized graphene sheets (0.5 wt %) and sonicated for a further 3 h in order to obtain a well-dispersed nanocomposite solution. This solution was spincast (3000 rpm for 30 s) on a glass support. Transparent thin films have been obtained and analyzed by atomic and electrostatic force microscopy (AFM and EFM), Nanoscope IVa Dimension 3100 from Digital Instruments. The same procedure was undertaken to produce neat SIS thin films. In all cases, the film thickness obtained was of the order of 300 nm measured by spectroscopic ellipsometry using an M-2000 ellipsometer from J. A. Woollam Co. Differential scanning calorimetry measurements were performed with a TA Instruments DSC Q200. All experiments were conducted under a nitrogen flow of 20 mL min-1, in a temperature range from -90 to 150 °C with a rate of 20 °C/ min after removing the thermal history of the samples. UV-vis measurements of the nanocomposite and neat SIS films were carried out with a Perkin-Elmer spectrometer Lambda 35. Results and Discussion A key challenge in the synthesis and processing of nanocomposites is the strong tendency of the nanofillers to selfaggregate. Hence, different sonication procedures were applied to achieve a well-dispersed suspension of functionalized graphene nanosheets in toluene. Figure 1 shows the dispersion state after sonication for 10 and 60 min observed by electron microscopy. After 10 min, the graphene sheets are characterized by wrinkled thin paper-like structures (Figure 1b,c), while, after 60 min, well dispersed and highly exfoliated nanosheets are obtained (Figure 1d and its inset and TEM image Figure 1e). The graphene thickness, of about 1 nm, has been calculated by ellipsometry (Supporting Information). Once a well-dispersed suspension of FGS in toluene was obtained, the block copolymer was dissolved in this suspension and the nanocomposite system was processed by spin-coating, obtaining transparent thin nanocomposite films with 0.5 wt % graphene, named FGS/SIS. A film thickness of 300 nm was measured by spectroscopic ellipsometry (Supporting Information). The transmittance (%T) at 550 nm of these nanocomposite films, measured with UV-vis spectrometer, was close to 96%, compared to 99% at 550 nm of the neat SIS (UV-vis spectra in Supporting Information). The nanostructured morphologies of the FGS/SIS samples were studied by AFM and compared with the neat block copolymer matrix prepared under the same experimental conditions. The particle size and shape, the chemical details, and the processing conditions are crucial when working with nanocomposite based on nanostructured block copolymers. The structural synergism between the nanoparticles and the self-assembled morphology of the block copolymer matrix plays an important role in the realization and understanding of well-ordered nanocomposite morphology. The compatibility of the nanoparticles with BC host matrix strongly depends on the symmetry of both of them. While 0D nanoparticles can be well-incorporated into any of the BC microdomains, i.e., sphere, cylindrical, double gyroid, and lamellar, 2D nanoparticles combine themselves with lamellar nanostructure.15 2D clay nanoparticles embedded in BC matrices have received relatively little attention from the morphological point of view, and based on the best of our knowledge, this is the first time where the confinement of 2D exfoliated graphene sheets into nanostructured block copolymers is reported. Considering the geometry of the nanoparticles and the morphology of the BC matrix, the “symmetry matching” concept plays an important role on the study of the self-assembled

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Figure 1. FE-SEM images for (a) neat GS and GS after (b) 10 min, (c) 30 min, and (d) 60 min of sonication time. The inset of part d shows the single graphene nanosheet of part d at higher magnification. TEM images of FGS before (e) and after (f) sonication.

nanostructured nanocomposite materials. The symmetry matching, in fact, could provide information about the formation of structures with long-range order in terms of space and orientation and also could be useful to locate and orient the nanoparticles in a specific position inside the BC host. Asymmetric particles with large surface area, as 1D and 2D, tend to influence the evolution of the BC self-assembling microstructures. Moreover, their shape and size influence the long-range order of the microdomains. Hence, it should be relevant to study the effect of sequestering the 2D graphene nanosheets into the selfassembled morphology of the BC host matrix even if it presents a nanostructured morphology different from the lamellar one. AFM analysis (Figures 2 and 3) revealed a clear change in the self-assembled microstructure of the nanocomposite based on the BC matrix. The poly(styrene-b-isoprene-b-styrene) (SIS) neat matrix had well-ordered cylindrical domains of PS blocks parallel to the free surface (Figure 2), while the FGS/SIS samples showed PS cylindrical domains disposed perpendicular and parallel to the free surface. Additionally, the cylindrical domains of the neat matrix had an average diameter of about 20 nm and long-range order, while the cylindrical domains of the nanocomposite BC had an average diameter of about 15-25 nm and no long-range order (Figure 3a and b). This loss of longrange order in the case of 2D clay particles had previously been reported by Lee et al. as they observed that BC cylinders tended to template around the clay stacks.32 Close inspection of the AFM images revealed the FGS were only present in one of the blocks, i.e., the PS block, as discussed

Figure 2. (3 × 3 µm) tapping-mode AFM phase image of neat SIS block copolymer.

below. As the lateral dimensions of the FGS can reach 1 µm (Supporting Information), the nanosheets appeared to have adopted a folded configuration within the cylinders. Stable folded states of the FGS have been reported by Schniepp et al.33 They carried out bending experiments with an AFM probe and suggested that the folding behavior of the sheets occurred at pre-existing lattice defects or wrinkles and/or functional groups, resulting from the adiabatic expansion. In our case, the

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Figure 3. (3 × 3 µm) tapping-mode AFM topography (a) and phase (b) images of FGS/SIS and the corresponding topography height profile (c). The arrows indicate some graphene nanosheets corresponding to the height profile reported in part c.

Figure 4. (5 × 5 µm) tapping-mode AFM phase image of FGS/SIS (Figure 4a) corresponding to the EFM phase images. (b) EFM phase image of FGS/SIS when 0 V has been applied. (c) EFM phase image of FGS/SIS when 5 V has been applied.

folded state can be considered as a result of the combination of two effects. First, the physical confinement within the PS blocks which led to the observed morphological change. Second, the chemical affinity of the FGS and PS due to the higher polarity of the PS blocks with respect to the PI ones and the presence of remaining epoxy, hydroxyl, and carboxyl side groups attached on the graphitic backbone of the FGS.8,22 The confinement of the graphene in the PS block was confirmed by DSC analysis (DSC thermograms in the Supporting Information). The glass transition (Tg) of the PI block remained invariant at about -58 °C for both SIS and FGS/SIS, while the Tg of the PS block shifted from 78 °C for neat SIS to 84 °C for FGS/SIS. This increment in the PS Tg indicated

the stiffening of the PS segments which is coherent with the reduced mobility of the PS domains as a consequence of the sequestering of graphene nanosheets. Moreover, this mobility reduction can also be responsible for the morphological changes in the nanocomposite. The topography height profile of the FGS/SIS samples (Figure 3c) showed a minimal number of “superficial” graphene sheets with a height profile of 1-10 nm corresponding to the FGS evidenced by arrows in Figure 3a. This fact confirmed that the FGS sheets were completely embedded within the BC domains during the sonication step, and it is an effective way to obtain dispersed graphene nanosheets and well dispersed nanocomposite films.

Letters In order to verify the good dispersion of the graphene sheets inside the self-assembled SIS matrix, we carried out electric force microscopy (EFM) measurements of the films (Figure 4). The EFM technique is a secondary imaging mode derived from tapping mode that measures the electric field gradient distribution above the sample surface and can be utilized to characterize surface electrical properties.34,35 We expected that, due to the different electrical response of the BC matrix and the graphene sheets, different areas of the surface would have different responses to the charged tip. As expected, the EFM image of the neat BC at 5 V applied voltage between the tip and the sample did not show any phase contrast, due to the nonconductive behavior of the matrix. Meanwhile, the EFM image of the FGS/SIS samples at 5 V (Figure 4c) showed evident phase contrasts, while no phase contrast was observed at 0 V (Figure 4b). The phase contrast observed at 5 V confirmed the presence of superficial electrical conductivity in the BC thin films, due to the sequestered graphene in the BC matrix. Conclusions The intrinsic ability of BC to self-assemble into different nanoscale structures has been used to successfully disperse functionalized graphene sheets in polymer nanocomposites. To the best of our knowledge, this is the first time that a block copolymer matrix has been used as a template to encapsulate graphene nanosheets into the PS phase. Furthermore, relatively large 2D graphene sheets tend to adopt a folded configuration as a result of both the chemical affinity and physical confinement in the PS phase of the SIS matrix. Finally, the good dispersion of graphene sheets in the SIS matrix was confirmed by EFM, where the contrast in the phase image has been attributed to the graphene electrical properties, with respect to the nonconducting behavior of the neat matrix. Further studies are required to control the properties and morphology of these new nanostructured materials. Finally, the study presents a successful approach to prepare graphene/block copolymer nanocomposites. The inclusion of the 2D particles within the BC provided the electrical properties that are required for optoelectronic applications. We expect that our approach will lead to the development of a new class of materials with enhanced properties. Acknowledgment. The work at ICTP-CSIC was supported by the Spanish Ministry of Science and Innovation under project MAT 2007-61116. R.V. acknowledges a Ramon y Cajal contract from MiCInn. Supporting Information Available: Ellipsometry results, UV-vis spectra, and DSC thermograms. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kotov, A. N. Materials science: Carbon sheet solutions. Nature 2006, 442, 254. (2) 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, 606. (3) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Twodimensional gas of massless Dirac fermions in graphene. Nature 2005, 438, 197. (4) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 6, 183. (5) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S.-B. T.; Ruoff, R. S. Graphene-based composite materials. Nature 2006, 442, 282.

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Letters (35) Tercjak, A.; Gutierrez, J.; Peponi, L.; Rueda, L.; Mondragon, I. Arrangement of conductive TiO2 nanoparticles in hybrid inorganic/ organic thermosetting materials using liquid crystal. Macromolecules 2009, 42, 3386.

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