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Graphene Oxide Monolayer as a Compatibilizer at the Polymer− Polymer Interface for Stabilizing Polymer Bilayer Films against Dewetting Tae-Ho Kim,† Hyeri Kim,† Ki-In Choi,† Jeseung Yoo,‡ Young-Soo Seo,‡ Jeong-Soo Lee,† and Jaseung Koo*,† †

Neutron Science Division, Korea Atomic Energy Research Institute (KAERI), Daejeon 34057, South Korea Department of Nano Science and Technology, Sejong University, Seoul 05006, South Korea

‡

S Supporting Information *

ABSTRACT: We investigate the effect of adding graphene oxide (GO) sheets at the polymer−polymer interface on the dewetting dynamics and compatibility of immiscible polymer bilayer films. GO monolayers are deposited at the poly(methyl methacrylate) (PMMA)−polystyrene (PS) interface by the Langmuir−Schaefer technique. GO monolayers are found to significantly inhibit the dewetting behavior of both PMMA films (on PS substrates) and PS films (on PMMA substrates). This can be interpreted in terms of an interfacial interaction between the GO sheets and these polymers, which is evidenced by the reduced contact angle of the dewet droplets. The favorable interaction of GO with both PS and PMMA facilitates compatibilization of the immiscible polymer bilayer films, thereby stabilizing their bilayer films against dewetting. This compatibilization effect is verified by neutron reflectivity measurements, which reveal that the addition of GO monolayers broadens the interface between PS and the deuterated PMMA films by 2.2 times over that of the bilayer in the absence of GO.

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INTRODUCTION Graphene and graphene derivatives have rapidly gathered interest as promising materials for polymer composites1−4 and graphene-based nanoelectronics4−9 because of their great potential for enhancing the electrical, thermal, and mechanical properties of polymeric materials. Graphene oxide (GO), which is a typical graphene derivative obtained from the chemical exfoliation of oxidized graphite, exhibits amphiphilic properties due to the hydrophobic basal carbon plane and the hydrophilic carboxylic group at the edges.10,11 These amphiphilic GO sheets have a strong interfacial interaction with polymers and improve the compatibility of immiscible polymer blends.12−14 The hydrophobic unoxidized basal plane contributes to the attachment of polymers by van der Waals forces, π−π stacking, and hydrophobic interaction, whereas the hydrophilic oxidized domain interacts with polymer chains through Coulomb interaction and hydrogen bonding.15 Hence, the polymer mobility can be restricted near the GO surfaces. GO sheets can tailor the interface between immiscible polymer blend films and stabilize them by reducing the interfacial tension between the entities.16 Even a small fraction loading of GO fillers can dramatically improve the mechanical properties of bulk polymer blends and enhance the miscibility of polymer pairs.17 Although considerable research has recently been devoted to investigating the enhanced mechanical properties and compatibilization effect of GO in bulk polymer-blend nanocomposites,18,19 the dynamics and morphology of polymer thin films near the surface of graphene or its derivatives have © 2016 American Chemical Society

rarely been investigated. The confinement of polymers in thin films with additives influences the dynamic aspects as well as their adhesion and wetting characteristics, hence confined polymers exhibit different physical properties from those of bulk polymers.20 For practical applications of organic electronic devices, polymer thin films should also be thermally stable against dewetting to achieve long-enough operating lifetimes with high performance during operation.21,22 To ensure the devices’ durability, it is important to elucidate the cause and mechanism of instability and dewetting phenomena of polymeric thin films. For graphene-based organic nanoelectronics, the effect of graphene or graphene oxide on the thermal stability of polymeric devices needs to be investigated as well. Although graphene-based organic photovoltaic devices with high energyconversion efficiency have received substantial attention,23 to the best of our knowledge the dewetting kinetics of polymer thin films with GO have not yet been reported. The effect of other carbon-based nanoparticles (i.e., fullerene and carbon nanotube) on the polymer dynamics and dewetting morphology of thin films has been previously investigated through dewetting and diffusion dynamics studies.24 For example, Barnes et al.25 and Bandyopadhyay et al.26,27 demonstrated that the addition of a small quantity of fullerene Received: August 19, 2016 Revised: November 2, 2016 Published: November 2, 2016 12741

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Quebec, Canada). All solvents were obtained from Sigma-Aldrich (St. Louis, MO, USA). Si Substrate Preparation. Polished Si(100) wafers were purchased from Shin-Etsu (Tokyo, Japan) and partitioned into 1 × 2 and 5 × 10 cm2 slices to be utilized as substrates. Their surfaces were rinsed with deionized (DI) water and then treated with UV/ozone to remove organic contaminants. Next, they were immersed in diluted hydrofluoric acid (HF) solution (H2O/HF 10:1) to etch the oxide layer and make the surface hydrophobic. The etched wafers were then thoroughly washed with DI water and dried with a stream of N2 gas. Thin-Film Polymer Bilayers for Dewetting Dynamics Study. The PS (Mw = 7 100 000 g/mol) solution in toluene was spun-cast at 2500 rpm on the prepared Si substrates; the film thickness was 75 nm. GO monolayers were deposited on the spun-cast PS substrates using the LS method, which has been described previously.31 The deposited film was characterized using atomic force microscopy (AFM, Nanoscope IIIa; Veeco Instruments Inc., Oyster Bay, NY, USA) with the noncontact tapping mode using a silicon nitride tip (size = 5 nm). Scanning electron microscopy (SEM; Magellan 400, FEI, Hillsboro, OR, USA) was also used with a low acceleration voltage (e.g., 1 kV) and a high current (e.g., 0.4 nA). For the deposition of the top polymer layer, the PMMA (Mw = 92 000 g/mol) solution in toluene was spun-cast on the UV-ozone-treated Si wafer, and the obtained film thickness was 63 nm. The spun-cast films were then carefully floated on the surface of DI water in a bath and were promptly transferred onto the sample. In addition, another sample of the PS (Mw = 221 000 g/mol) layer on the PMMA (Mw = 990 000 g/ mol) bottom layer with or without the GO monolayer was prepared by the same sample-preparation method. The thicknesses of the polymer films were measured by ellisometry (SE MG-1000 UV, Nano-View Co., Ltd., Ansan, Korea) and were confirmed by model scatteringlength-density profiles calculated from neutron reflectivity profiles. For the dewetting dynamics study, hole growth was monitored using an optical microscope (Olympus, Center Valley, PA, USA) as a function of annealing time at 178 °C (calibrated values) under vacuum conditions (10−4 Torr). Raman Spectroscopy and Neutron Reflectivity. Unpolarized Raman spectra were recorded for the polymer bilayer with and without the deposition of the GO monolayer at the polymer bilayer interface to confirm the GO monolayer’s existence. An InVia Raman microscope (Renishaw Ltd., Gloucestershire, U.K.) was used with an excitation source (incident power of 2 mW) of the 633 nm line from a He−Ne laser. The interfacial structure of the polymer bilayers with GO was also investigated using neutron reflectivity (NR). The specular NR experiments were conducted as a function of qz (≈ 4π/λ sin θ), where θ and λ were the grazing angle of the incident neutron beam and its wavelength, respectively. The REF-V reflectometer at the Cold Neutron Laboratory Building of HANARO at the Korea Atomic Energy Research Institute (Daejeon, Korea) was used, with a wavelength of λ = 4.75 Å and ΔQ/Q = 0.02−0.06. The neutron reflectivity data were corrected for the footprint, background, and dead time.32 The experimental reflectivity data were then fitted to the reflectivity profiles calculated from the model scattering-length-density profiles using the Parratt formalism.33 A Levenberg−Marquardt nonlinear least-squares method was used to find the best-fit values by adjusting the thickness, scattering-length densities, and interfacial width of the unknown layers with the least-squares statistic (χ2). Fourier Transform Infrared (FTIR) and X-ray Photoelectron Spectroscopy (XPS). To analyze functional species of GO, the FTIR spectrum of GO (bulk) was obtained using Spectrum 100FT-IR spectrometers (PerkinElmer Ltd., USA). The absorption spectrum in attenuated total reflectance (ATR) measurement mode was recorded via 32 scans in the 450 to 4000 cm−1 wavenumber range with 1 cm−1 resolution. The species were also confirmed by using spectra of C 1s recorded from X-ray photoelectron spectroscopy (K-ALPHA, ThermoFisher Scientific, Waltham, MA, USA).

(C60) (less than 5% by weight) affects the dewetting dynamics and structures of homopolymer and blend films. They found that the fullerene nanoparticles segregated at the polymer− polymer interface and the film−substrate interface and altered the interfacial tension by functioning as molecular surfactants. Previously, we also reported the effect of embedded multiwalled carbon nanotubes (MWNTs)28 and functionalized MWNTs29 on the dewetting dynamics of thin-film PS− PMMA polymer bilayers and the diffusion dynamics of PMMA−deuterated PMMA thin films through neutron reflectivity. In this case also, the interaction between fillers and polymer matrices was determined to play a crucial role in the dynamics of polymers in thin films. In this study, we investigated the influence of amphiphilic GO sheets on the dewetting of bilayer polymer thin films. The GO monolayer is deposited at the interface between immiscible PS and PMMA layers by the Langmuir−Schaefer (LS) technique, as illustrated in Figure 1a. We monitored the

Figure 1. (a) Schematic of the sample geometry for the dewetting experiments with and without the GO monolayer. For the GO sample, a solid PS layer (Mw = 7 100 000 g/mol) is spun-cast onto the substrate. Then, the GO monolayer is deposited on the PS layer by the LS technique. A liquid PMMA layer (Mw = 92 000 g/mol) is floated on top of the prepared sample layer. (b) AFM 3D scan image of a typical dewetting hole of bilayer PMMS/PS thin films. (c) The crosssectional traces correspond to the lines drawn across the figure.

dewetting hole growth of PMMA layers on solid-like PS substrates in the presence or absence of GO monolayers. The results indicate that GO sheets significantly stabilize the PMMA−PS bilayers against dewetting. This is due to the favorable interaction of GO with both PS and PMMA; GO sheets function as an amphiphilic compatibilizer at the polymer−polymer interface. Furthermore, this is verified by neutron reflectivity results, which reveal that GO addition broadens the PS−PMMA interface. Because dewetting is closely correlated with the performance of polymer-based nanoelectronic devices, this study can provide a strategic approach for stabilizing solar cell films against dewetting by using the GO sheet as an electrode.

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EXPERIMENTAL SECTION

Materials. GO was obtained by the modified Hummer’s and Offeman’s method30 from flake graphite (Aldrich; St. Louis, MO, USA) as previously reported in detail.31 PS (Mw = 7 100 000 g/mol, Mw/Mn = 1.15 and Mw = 221 000 g/mol, Mw/Mn = 1.08), PMMA (Mw = 990 000 g/mol, Mw/Mn = 1.80 and Mw = 92 000 g/mol, Mw/Mn = 1.08), and its deuterated polymer (dPMMA; Mw = 144 000 g/mol, Mw/Mn = 1.60) were purchased from Polymer Source Inc. (Dorcal, 12742

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Figure 2. (a) AFM image of deposited GO films on the bottom PS layer (PS, Mw = 7 100 000 g/mol) by the LS technique and its cross-sectional profiles. SEM image of the GO monolayer deposited under the same condition. (b) Raman spectra of bilayer PMMA/PS thin films with and without GO. (c) Typical NR profiles (best fitting in solid black line) of bilayer dPMMA/PS thin films with and without GO and (d) their corresponding scattering-length-density profiles.

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RESULTS AND DISCUSSION We investigated the influence of the GO monolayer on the dewetting dynamics of a thin-film polymer bilayer with a solidlike lower layer.34 In these systems, the PS bottom layer has higher viscosity than the PMMA top layer (ηPMMA ≪ ηPS). Figure 1a illustrates the cross-section of PMMA−PS bilayer thin films with and without GO monolayers at the interface, where we have selected a higher molecular weight (Mw = 7 100 000) of PS for the bottom layer and a lower Mw of PMMA (Mw = 92 000) for the top layer. A dewetting hole was formed in an unstable film with a dynamic contact angle (θd), surrounded by a rim, which gradually proceeded outward, as shown by the AFM three-dimensional (3D) image and its cross-section of the hole in Figure 1b,c. According to the theoretical expectation set by Wyart et al.,35 in this case, the dewetting hole opening was dominantly affected by the viscosity (ηPMMA) of the liquid-like top-layer PMMA and the contact angle (θe). For the dewetting sample with GO, in this regime, the PS for the bottom layer was spun-cast on HF-etched Si substrates. Next, the LS technique was applied to deposit the GO monolayer, and the floating technique was applied to deposit the top PMMA layer. Previously, we achieved assembled GO monolayer formation by assisting the positively charged octadecylamine at the gas−liquid interface.31 Here, we also obtained closely packed GO monolayers on the PS bottom

layer by LS deposition after monolayer compression up to 20 mN/m, as can be seen from the AFM results in Figure 2a. The thickness of the GO layer is 1.2 nm, indicating a single layer without coagulation and flocculation. The GO monolayer structure for a large area was obtained from an SEM measurement (Figure 2a). The Raman spectra of the PMMA−PS bilayer with GO also exhibited typical peaks of GO sheets in the G, D, G+D, and 2D bands (Figure 2b), indicating that the GO monolayers on PS substrates were stable without detachment even after the deposition of the top layer of the PMMA by the floating technique.36 However, this feature of the Raman spectrum was not shown for the control bilayer sample without GO. The film structure of bilayers with and without GO monolayers is also investigated by NR measurements. Because these measurements use the incident neutron beam, which can average over the entire length of the sample, we can obtain the overall film structure. Figure 2c shows the neutron specular reflectivity profile for bilayers of deuterated PMMA (dPMMA) and PS thin films spun-cast on Si substrates with and without GO monolayer deposition. We plotted the reflected scattering intensity as a function of the normal momentum transfer on the surface, qz. For the control sample without GO, the reflectivity profile shows several distinct fringes with a periodicity corresponding to the thickness of the bilayer. We can clearly see that, in the presence of the GO monolayer, the peak 12743

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It is known that the dewetting dynamics can be faster as the annealing temperature increases and also as the molecular weight of polymers decreases because of lower viscosity.34,38 However, the polymer thin film with GO monolayers did not dewet even at 203 °C for 20 h (Figure 4a), whereas the control

positions of Kiessig fringes shifted to a smaller momentum transfer (qz) because of the GO film thickness. The models used to fit the data are shown in Figure 2d, where the parameters to be fit are the thicknesses; the interfacial rms roughness; and the scattering-length densities (SLDs) of the PS layer adjacent to the silicon substrate, the GO layer at the interface, and the dPMMA for the top layer. According to the fitted SLD profile, the obtained thickness of the GO monolayer at the bilayer interface is 1.2 nm, which is in good agreement with the AFM results. To investigate the influence of the GO monolayer on the dewetting dynamics of bilayer polymer thin films, we monitored the dewetting hole growth of the PMMA layer on the PS substratewith and without the GO monolayer at their interface, using optical microscopyas a function of annealing time at 178 °C. Before being annealed, both samples exhibited very clear, flat surfaces without dewetting holes. After 4 h, dewetting begins from the PMMA layer on the PS substrate by the capillary wave as a result of the low compatibility between the polymer layers (Figure 3a).35,37 Figure 3b shows the optical

Figure 4. Typical optical microscope images of annealed bilayer samples of (a, c) 92 K PMMA and 32 K PMMA (b, d) floating on a 7100 K PS layer (a, b) with and (c, d) without GO monolayers at 203 °C for (a) 20, (b) 21, (c) 6, and (d) 3 h (scale bar = 20 μm).

samples readily reached equilibrium, forming PMMA droplets on the PS substrates by coalescing adjacent dewetting holes after annealing for 6 h at the same temperature (Figure 4c). Furthermore, we checked the dewetting for the lowermolecular-weight (Mw = 32 000 g/mol) PMMA and found that regardless of the molecular weight or temperature (below a polymer degradation temperature), dewetting did not occur in the presence of GO monolayers even after annealing for 21 h at the same temperature (Figure 4b). This implies that the GO monolayer at the interface between the PS and PMMA layers interrupts the dewetting hole growth to a large degree and significantly affects the stabilization of bilayer thin films of incompatible polymers against dewetting. We also investigated the dewetting of PS on the PMMA layer with GO sheets at the interface. Similarly, we prepared bilayer thin films of PS (Mw = 221 000 g/mol) floating on the PMMA (Mw = 990 000 g/mol) solidlike substrate with and without the GO monolayers by employing LS deposition. Figure 5 shows the optical microscopy images of dewetting holes from the PS films on the PMMA substrate with and without GO monolayers at the interface, annealed at 178 °C, as a function of time. The hole sizes gradually increased with annealing time. The diameters of eight holes are monitored under each annealing condition and plotted as a function of annealing time (Figure 5g). The hole growth rate decreased significantly in the presence of the GO monolayer at the PS−PMMA interface, compared to the control sample without GO. According to the theory of Wyart et al.,34,35 the dewetting velocity (V) of the polymer layer on the solid surface is mainly related to the viscosity of the top polymer layer (ηA) and the equilibrium contact angle (θe) γA 3 1 V= θe 12 ln 2 ηA (1)

Figure 3. Typical optical microscope images of dewetting holes from a bilayer sample of a 92 kDa PMMA floating on a 7100 kDa PS layer after annealing at 178 °C for (a) 4, (b) 62, and (c) 127 h. PMMA on PS films with GO monolayers at the interface of bilayers annealed at 178 °C for (d) 12, (e) 62, (f) 127, and (g) 315 h (scale bar = 20 μm).

microscope image of the control sample annealed for 62 h, exhibiting the usual holes surrounded by rims. The holes gradually grow as annealing progresses (Figure 3c) and maintain their original round shape. In the sample with GO monolayers, however, holes are hardly observed, even after annealing for 62 h at 178 °C. Although a very small hole can be seen in the GO sample after annealing for 127 h at 178 °C (Figure 3f), the hole size does not increase much even after annealing for 315 h (Figure 3g).

where γA is the surface tension of the top layer. In Figure 5g, the hole diameters for the control sample linearly increase with annealing time, indicating that V is constant. We also note that ηPMMA > ηPS/θe for PMMA and PS,35 hence expecting the lower 12744

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Figure 5. Typical optical microscope images of 221 kDa PS floated on 990 kDa PMMA films without GO monolayers at 178 °C for (a) 25, (b) 50, and (c) 75 min. The same component bilayer films with a GO monolayer at the bilayer interface at 178 °C for (d) 25, (e) 75, and (f) 130 min. (g) Dewetting hole diameter of 221 kDa PS on 990 kDa PMMA layers with and without GO monolayers as a function of annealing time at 178 °C.

demonstrates the reduction in the PS−PMMA interfacial energy caused by the GO sheet addition. This result is consistent with V ≈ θe3, as expected from eq 1. The ratio between the viscosity of the PS top layer in the samples with and without GO monolayers is 1.03. Thus, the viscosity remained constant regardless of the presence of GO sheets. Hence, the suppression of dewetting hole growth by GO sheet addition is due only to the reduction in the interfacial tension between the homopolymers with the addition of the amphiphilic GO monolayers. Both the morphology and the contact angle studies indicate a GO-induced compatibilization effect. The GO amphiphilic property was verified from the infrared absorbance spectroscopy and X-ray photoelectron spectroscopy (XPS) results where the peaks for C−C and CC bonds in the hydrophobic unoxidized basal plane and peaks for CO, C−O, and O−H bonds in the hydrophilic oxidized domains coexisted for the GO sheets (Figure S1). Therefore, they can act like a surfactant. Similar to the evolution of interfacial tension for immiscible polymer blends in the presence of a block copolymer at the interface, GO can also decrease the interfacial tension and improve the compatibility of the polymer blends. Russell et al., using the NR technique, showed that the symmetric block copolymer chains of PS-b-PMMA are segregated at the PS−PMMA interface and that the interfacial width broadens as the number of block copolymer chains is increased at the interface.45 However, if an excess amount of PS-b-PMMA is added, more than the critical micelle concentration, then diblock copolymer micelles are formed, which reduce the kinetics of segregation to reach equilibrium. This is a drawback in the use of block copolymers as an interfacial compatibilizer. In this study, however, the formation of closely packed GO monolayers at the PS−PMMA interface is easily achieved using the LS technique. The monolayer structure is maintained after annealing, and micellization does not occur for GO (Figure S2). In this regard, GO can alleviate the problems of conventional surfactants and copolymers. Using NR, we examined the compatibilization effect on the PS−dPMMA interface by adding the GO sheet. Figure 8 shows the NR profiles for a bilayer composed of PS and dPMMA with and without the GO monolayer at the interface after annealing for 3 h at 178 °C; in both samples, dewetting hole nucleation did not occur within this annealing time. The reflectivity results

layer to be solidlike. Because the hole diameter is much larger than the width of the rim, eq 1 for the regime of the constant dewetting velocity should hold. From the slope of the linear fits, V is 6.43 × 10−10 and 1.95 × 10−8 m/s for the PS layer on PMMA films with and without GO sheets at the interface, respectively. Thus, GO significantly suppresses the dewetting dynamics of the PS layers. To determine whether this slow hole growth for the GO sample was due to the interfacial tension change in the presence the GO sheet, we also measured the dewet droplet size and the equilibrium contact angle (θe) for the samples with and without GO by using an optical microscope and AFM, respectively. At equilibrium, the thin film bilayer deweted into large spherical droplets with diameters ranging from 6 to 14 μm (Figure 6a). Annealing the samples coalesced the PS layer into

Figure 6. Typical optical microscope images of PS on PMMA bilayers (a) without and (b) with the GO monolayer at the interface after annealing at 178 °C for 12 and 4 h, respectively. The annealing process has been treated until reaching the equilibrium stage of dewetting (scale bar = 20 μm).

large domains. With GO monolayers at the interface, the morphology of the phase-separated droplets was changed from a circular structure to an interconnected elongated structure (Figure 6b). In addition, the droplet size tended to decrease in the presence of GO. Similar phenomena have been reported by several groups after adding different types of compatibilizers, such as organoclays,40 fullerene nanoparticles,41,42 and block copolymers,43,44 to polymer blends. They attributed this morphology change of polymer blends to the increased miscibility. The AFM images of PS droplets on the PMMA substrates with and without the GO monolayer were also obtained after annealing at 178 °C up to equilibrium. The obtained Young contact angles are 17.6 ± 2.0 and 5.7 ± 0.9° for the control sample and the GO sample, respectively. This 12745

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contributions, namely, the intrinsic width, σin, and the roughness ⟨Δς2⟩ caused by capillary-wave broadening, yields the measured interfacial width, w = [σin2 + ⟨Δς2⟩]1/2.47 The mean square displacement of the interface from its average position due to capillary waves can be described by Δς 2 =

kBT ⎛ λmax ⎞ ln⎜ ⎟ 2πγ ⎝ λmin ⎠

(2)

where kB, T, and γ are the Boltzmann constant, absolute temperature, and surface tension of the film, respectively. λmax and λmin are the maximum and minimum wavelengths of fluctuations.47 Because the averaged amplitude of interfacial undulations is proportional to the reciprocal of the interfacial tension from this equation, we can conclude that the broadening of the interface is due to the decrease in the interfacial tension by GO sheet addition. This also implies that GO acts as a compatibilizer for immiscible polymer blend bilayers because of its amphiphilic property. Because of this compatibilization effect, the PMMA−PS bilayers are therefore stabilized against dewetting by adding the GO monolayer at the interface.

Figure 7. AFM images and its cross-sectional profiles of PS/PMMA bilayers (a) with and (b) without GO monolayers at the interface annealed at 178 °C for 12 and 4 h, respectively. Annealing treatment proceeded until approaching equilibrium stages of dewetting hole growth.

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are shown as a function of neutron momentum transfer, qz. The results indicate that the oscillations in the reflectivity profiles for the sample with the GO sheets were more rapidly damped in comparison to that for the control sample. Therefore, roughness changes at the surface and interface of the bilayer films. The AFM results, however, show that the values of surface roughness for both samples after 3 h of annealing are nearly the same (Figure S3). Thus, the only variable or unknown in the analysis is the width of the interface between the PS and dPMMA layers. Moreover, the scattering-length densities between GO and dPMMA are not significantly different (Figure 2d). At equilibrium, the GO layer cannot be distinguished from dPMMA for the neutron. Thus, we analyzed the NR data using a simple PS−PMMA bilayer model for both samples. The reflectivity profiles (Figure 8a, solid line) are calculated from the scattering-length-density (SLD) profiles in Figure 8b. The interfacial width of the PS and dPMMA layers with GO is 42.3 Å, which is 126% broader than that found between the PS and PMMA layers in the absence of the GO sheet (19.0 Å). Shull et al.46 have compared the theoretical and experimental widths of an immiscible polymer blend bilayer derived from the surface capillary wave model. The convolution of two

CONCLUSIONS

The NR technique was employed to examine the compatibilization effect on the PS−dPMMA interface in the presence of a GO sheet. The interfacial width of the PS and dPMMA layers expanded from 19.0 to 42.3 Å by adding the GO monolayers at the interface. This indicates the improved compatibility between PS and PMMA due to the amphiphilic property of GO. Because of this compatibilization effect, the PMMA−PS bilayers were also stabilized against dewetting by adding the GO monolayer at the interface. The dewetting behavior was then completely prevented, regardless of the annealing temperature (below the degradation temperature of PMMA) and the molecular weight of the PMMA in the top layer. The dewetting of PS on PMMA was also significantly suppressed by the GO monolayer at the interface, exhibiting a 30-fold-lower dewetting velocity than that of the control sample. The contact angle results of PS droplets on PMMA with GO reveal a reduction in the interfacial tension between PS and PMMA. This result agrees with V ≈ θe3, as expected from the theory of Wyart et al., also indicating the GO-induced compatilization and film stabilization of polymer blend bilayers.

Figure 8. (a) Typical NR profiles (best fit via solid black line) of bilayer PMMA/PS thin films with and without the GO monolayer at the interface of bilayer films after annealing at 195 °C for about 3 h and (b) their corresponding scattering-length-density profiles. 12746

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03095. Transmission infrared absorbance spectrum and C 1s Xray photoelectron spectroscopy spectrum of GO, AFM image of the GO monolayer after degradation of the polymers in the PMMA/GO/PS thin film annealed at 270 °C for 3 days, and AFM images to measure the rootmean-square roughness of bilayer samples of the PMMA (92 kDa) on the PS (7100 kDa) layer in the absence and presence of the GO monolayers at the interface after annealing at 178 °C for 3 h. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82 42 868 8436. Fax: +82 42 868 4629. Present Address

(T.-H.K.) Interdisciplinary Program in Creative Engineering, Materials Research Center, School of Energy, Materials, and Chemical Engineering, Korea University of Technology and Education, Cheonan 31253, South Korea. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Professor Miriam H. Rafailovich at Stony Brook University and Yujin Kim at the Korea Advanced Institute of Science and Technology (KAIST) for the discussion of polymer physics. This work was supported primarily by a grant from the National Research Foundation of Korea under contract no. 2012M2A2A6004260.

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REFERENCES

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