Dewetting of Thin Polymer Films on Wrinkled Graphene Oxide


Mar 31, 2019 - Dewetting of Thin Polymer Films on Wrinkled Graphene Oxide ... (PMMA) thin films on the wrGO monolayers was perfectly prevented, wherea...
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Dewetting of Thin Polymer Films on Wrinkled Graphene Oxide Monolayers Kyoung-Il Jo, Tae-Ho Kim, Ki-In Choi, Hoyeon Lee, Jae Hak Choi, Joona Bang, Tae-Hwan Kim, Guangcui Yuan, Sushil K. Satija, and Jaseung Koo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00141 • Publication Date (Web): 31 Mar 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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Dewetting of Thin Polymer Films on Wrinkled Graphene Oxide Monolayers

Kyoung-Il Jo,∥,1,2 Tae-Ho Kim,∥,3 Ki-In Choi,1,3 Hoyeon Lee1, Jae-Hak Choi,3 Joona Bang,2 TaeHwan Kim,4 Guangcui Yuan,5 Sushil K. Satija,5 Jaseung Koo3,*

1Neutron

Science Division, Korea Atomic Energy Research Institute (KAERI), 989-111 Daeduk-

daero, Yuseong-gu, Daejeon, 34057, Korea 2Department

of Chemical and Biological Engineering, Korea University, 145 Anam-ro,

Seongbuk-gu, Seoul, 02841, Korea 3Department

of Organic Materials Engineering, Chungnam Nation University, 99 Daehak-ro,

Yuseong-gu, Daejeon, 34134, Korea 4Department

of Quantum System Engineering, Chonbuk National University, Baekje-daero,

Deokjin-gu, Jeonju, 54896, Korea 5Center

for Neutron Research, National Institute of Standards and Technology, 100 Bueau Dr.

Gaithersburg, MD 20899 USA

Corresponding Author: (J. K.) Email: [email protected] 1

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ABSTRACT We investigated the effect of the morphological structure of a graphene oxide (GO) monolayer on the dewetting dynamics of the upper polymer thin films. The Langmuir-Schaefer (LS) technique was used to prepare a wrinkled GO (wrGO) structure with a root mean square (rms) roughness of 22.7 Å. The dewetting behavior of polymethylmethacrylate (PMMA) thin films on the wrGO monolayers was perfectly prevented, whereas the PMMA thin films on a flat GO monolayer were dewetted at 203 °С. This wrinkle effect of the GO can be also obtained when the GO monolayers are intercalated to PMMA/polystyrene (PS) interface. In this multilayer, the flat GO monolayer at the interface between the PS and PMMA layers was spontaneously roughened with the rms roughness of 46.9 Å after annealing, and also prohibited the dewetting behavior. From the results, we found that in order to improve the compatibility of polymer blends by adding the 2D nanosheets, it is important to control the morphological structure of the sheets at the interface, along with the manipulation of the GO-polymer interactions.

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INTRODUCTION The dewetting of polymer films1 must be essentially prevented to achieve the desired performance in a variety of applications such as packages,2-3 biofilm coatings,4-5 and polymerbased nanoelectronics.6-8 It is well known that a dewetting hole forms in an unstable film with a dynamic contact angle (θd), surrounded by a rim, which gradually proceeds outward.9 Previous computer simulations and experimental studies have shown that the dewetting dynamics of polymer thin films are controlled by several approaches.10 For example, the viscosity of polymer layers can be tuned by varying the molecular weight of polymers11 and by adding nanofillers into the polymer matrix,12-15 which significantly affects the dewetting dynamics.9,

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liquid–liquid dewetting,17 the thickness of the bottom polymer layer is particularly important for strong interactions between the polymer and the substrate.18 Near the polymer–substrate interface, the polymer mobility is reduced and the dewetting dynamics are suppressed.19 Finally, the interfacial energy between two polymer layers is also a key parameter to prevent dewetting.20 By adding various compatibilizers at the polymer–polymer interface, the miscibility of the two immiscible polymers can be improved, thereby increasing the stability of the polymer bilayer against dewetting.21-22 Graphene oxide (GO)23-26, obtained from the chemical exfoliation of oxidized graphite, has attracted attention as a nanofiller for nanocomposites27-31 and as a compatibilizer for incompatible polymer blends32-34 or liquid emersions owing to the amphiphilic nature of hydrophilic oxidized groups and hydrophobic carbon basal planes.35-36 Therefore, the oxidized groups of GO show a strong interaction with polymethylmethacrylate (PMMA) through hydrogen bonding,37 and the conjugated carbon plane of GO also interacts with polystyrene (PS) 3

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through π–π and van der Waals interaction.37 Taguet et al. found that GO is located at the interface between immiscible polymers in bulk blends.38 Furthermore, GO dramatically reduces the domain size of polymers due to its function as a compatibilizer.34 We also reported that GO monolayers at the interface between PS and PMMA improve the compatibility of this PS– PMMA blend film.39 Neutron reflectivity (NR) results showed that the interfacial width between PS and deuterated PMMA (dPMMA) became broader after adding the GO monolayer at the interface, which also led to inhibition of the dewetting dynamics and reduction in the interfacial tension between them.39 Although it is clear that the interaction between GO and polymers plays an important role in the hindrance of the dewetting dynamics of the upper polymer layers, a further question still arises: whether the morphology change in the GO during the interfacial broadening can also affect the dewetting dynamics. According to the Wenzel model,40 the rough surface of the solid interface in contact with the polymer can change the interfacial energy. In terms of wetting and surface roughness, the mechanisms of super-hydrophobicity and super-hydrophilicity are well established.41 This change in interfacial energy is essentially related to the dewetting dynamics of the polymer. Furthermore, a theoretical model for the relationship between surface roughness and polymer-surface interaction was established by Douglas et al.42 They found that changes in surface roughness could alter the favorability of the surface for polymers. In this regard, we can suppose that in our case, the wettability of the films is governed by the GO surface structure. As long as the interaction between the polymer and GO is attractive, a rough interface can be expected to enhance the interaction more and prevent dewetting of the liquid polymer layer more effectively. 4

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In this study, to clarify the effect of the GO surface roughness on the dewetting dynamics, the flat GO monolayer and wrinkled GO (wrGO) sheets were prepared for comparison using Langmuir-Schaefer technique. After depositing these GO sheets at the polymer-solid substrate interface and at the polymer-polymer interface, the dewetting dynamics of the polymer films on the GO sheets were investigated at high annealing temperature, but below degradation temperature. The structure of the GO sheets at the polymer-solid interface and polymer-polymer interface was measured using X-ray and neutron reflectivity techniques. Previously, we reported the effect of the polymer-GO interaction on dewetting and diffusion dynamics of PS and PMMA thin films.43 From these results, we can conclude that the film stabilization against dewetting can be interpreted as the result of the interplay between the GO–polymer interaction and the morphological structure of the GO.

EXPERIMENTAL SECTION Materials. GO was obtained by the modified Hummer’s and Offeman’s method44 from flake graphite (Aldrich, St. Louis, Missouri, USA), as reported in detail previously.45 PS (Mw = 7,100,000 g/mol, Mw/Mn = 1.15 and Mw = 221,000 g/mol, Mw/Mn = 1.08), as well as PMMA (Mw = 32,000 and 92,000 g/mol, Mw/Mn = 1.06 and 1.08, respectively) and its deuterated polymer (dPMMA; Mw = 144,000 g/mol, Mw/Mn = 1.60) were purchased from Polymer Source Inc. (Dorcal, Quebec, Canada). All solvents were obtained from Sigma Aldrich (St. Louis, Missouri, USA).

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Si substrate preparation. Polished 3’ and 4’ Si (100) wafers were purchased from Shin-Etsu (Tokyo, Japan). For dewetting analysis, some 4” wafers were partitioned to 1 × 2 cm2 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, the substrates were immersed in diluted hydrofluoric acid (HF) solution (H2O: HF, 10:1) to etch the oxide layer and render the surface hydrophobic. The etched wafers were then thoroughly washed with DI water and dried with a N2 gas stream. Flat GO monolayer and wrinkled GO monolayer. The LS technique was employed to make a flat GO layer and wrGO layers. The GO aqueous solution used in the LS method was diluted to 40 ppm, and the ODA solution was prepared at a concentration of 1 mg/ml in chloroform and sprayed on the surface in a suspension (total area of 367 cm2) in Langmuir trough (KSV 2000, KSV NIMA, Espoo, Finland). After 20 min, the barrier was closed at a constant speed of 5 mg/ml to densify the GO sheets. Using this method, it was possible to obtain samples with constant GO monolayer roughness according to the degree of surface compression (π) (for details, see reference No. 44). Once the desired surface compression was reached, the prepared Si sample approached the surface of the layer where the layer is formed. Then, when the GO layer was adsorbed on the Si surface, it was regenerated and dried in the vacuum chamber. Thin-film polymer bilayers for dewetting dynamics study. If the sample requires a PS layer, the PS (Mw = 7,100,000 g/mol) solution in toluene was spun-cast at 2,500 rpm on the prepared Si substrates. The GO monolayers were deposited on the spun-cast PS substrates using the LS method, which we reported previousely.45 For dewetting experiment using the high-rigidity PS 6

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layer, the spun-cast PS films were crosslinked via UV irradiation in vacuum. The GO layer is then stacked as above. For the deposition of the top (upper) polymer layer, the PMMA (Mw = 92,000 g/mol) solution in toluene was spun-cast on the UV/ozone-treated 4” Si wafer. The spuncast films were then carefully floated on the surface of DI water in a bath, and promptly transferred onto the sample. The samples are stored in a vacuum chamber for 24 hrs. The thicknesses of the polymer films were measured by ellipsometry (SE MG-1000 UV, Nano-View Co., Ltd., Ansan, Korea), and were confirmed by model scattering-length-density (SLD) profiles calculated from NR and XRR profile. For the dewetting dynamics study, the hole growth was monitored using an optical microscope (Olympus, Center Valley, PA, USA). Samples were annealed using vacuum oven at vacuum conditions (10-4 Torr) at 203 ° C (measured value) for each desired time, followed by cooling at room temperature before measuring light microscopy. In addition, we investigated the dewetting dynamics using in situ optical microscopy system (Linkam scientific instrument Ltd, THMS600, UK) capable of annealing the samples on specially-designed hot plate under nitrogen atmosphere and measuring the surface structure of the thin films. For the measurement, the completed sample is placed inside the in situ measuring equipment and fudging is performed with nitrogen gas. After that, the hot plate was heated to the target temperature and the sample was continuously observed at a certain position (Figure S1). Neutron and X-ray reflectivity. NR was used to investigate the interfacial structures of annealed dPS-b-PMMA thin films and their multilayer confined between GO surfaces. The specular NR measurements were performed with an NG7 reflectometer at the Cold Neutron Facility of the National Institute of Standards and Technology (Gaithersburg, MD, USA) with a wavelength (λ) of 4.76 Å and Δλ/λ of ∼0.025. The NR data were measured as a function of qz 7

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(~4π/λsinθ), where θ was the grazing angle of the incidence neutron beam via specular NR experiments. In order to fix the resolution with a constant value of Δqz/qz ≈ 0.03, the width of the vertical slits was set to be tunable as a function of qz, whereas the horizontal slit width was retained as 30 mm. The obtained NR data were corrected for foot prints and background, and an analysis thereof was conducted based on the computational reflectivity profiles by using a Parratt formalism.46 These computational profiles were calculated by using a Levenberg–Marquardt nonlinear least-squares method by adjusting the thickness, SLD, and interfacial width of the unknown layers with a least-squares statistic (χ2). The structural properties of flat GO and wrGO layers were also investigated by using high-resolution X-ray reflectivity (XRR, Bruker, D8 DISCOVER) measurements. The results of the XRR profiles were fitted to obtain electron density profiles via the same formalism. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Unpolarized Raman spectra were recorded for the GO monolayer. An InVia Raman microscope (Renishaw Ltd., Gloucestershire, U.K.) was used with an excitation source (incident power of 2 mW) of the 633nm line from a He–Ne laser. An analysis of the chemical functionalities of GO was conducted by using the C 1s spectra recorded from X-ray photoelectron spectroscopy (XPS, K-ALPHA., Thermo Instrument Inc., USA). The surface morphology was confirmed by atomic force microscopy (AFM) with a piezo scanner (Nanoscope IIIa; Veeco Instruments Inc., Plainview, NY, USA). This analysis was performed using a silicon nitride tip in noncontact tapping mode.

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RESULTS AND DISCUSSION We have previously reported that the assembled GO monolayer formation was achieved with assistance by positively charged surfactant (i.e., ODA) at the gas–liquid interface. Different morphological structures of the GO monolayers (i.e., flat GO and wrGO) were obtained by using the LS technique at various surface pressures.45 In the present study, we used this technique to investigate the effect of the wrGO structure on the dewetting dynamics of PMMA thin films (the samples are illustrated in Figures 2a and 2e). At the surface pressure of 20 mN/m, the closely packed flat GO monolayer could be clearly observed, as shown in the AFM results (Figure 1a). The cross-section profile was measured along the black solid line in the figure. From the height difference between the red arrows, the thickness of GO was obtained to be 1.0 ± 0.2 nm. This corresponds to a single layer of the GO, which is in a good agreement with previous reports.45 By monitoring surface pressure–area (π–A) isotherms of the GO Langmuir monolayer at the interface (Figure S2), we obtained a reproducible monolayer structure at the consistent pressure. With further compression of the GO monolayers (50 mN/m), the wrGO morphology could be obtained. After transferring them onto Si substrates and the spun-cast PS thin films, the surface morphological structure was measured using AFM (Figure 1b). The topography AFM results and corresponding cross-section profiles clearly show a rough surface morphology of the GO monolayer upon over-compression. The rms roughness values of the flat GO and wrGO were 5.3 and 22.7 Å, respectively from AFM results.

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Figure 1. AFM images of (a) flat GO monolayers and (b) wrGO layers on spun-cast PS layer, which were transferred by LS technique at the surface pressures of 20 mN/m and 50 mN/m, respectively. (c) Raman spectra of PMMA/PS films with flat GO monolayers and wrGO layers at the interface, and without the GO layers. (d) C 1s spectra of GO obtained from the narrow-mode XPS measurement. After the LS deposition, the Raman spectra of flat GO and wrGO monolayer at the polymer–polymer interface were measured (Figure 1c). The results showed typical peaks of GO components, namely G, D, G + D, and 2D bands.47 These observations implied that both GO layers were stably attached at the interface of PMMA/PS, regardless of their morphological structure, without detachment even after transferring of PMMA thin films on the LS layers of GO by using floating techniques. In contrast, these features derived from the GO layers were not 10

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observed from PMMA/PS sample without GO. The amphiphilicity of the GO was verified from XPS measurements, where the peaks for the C-C and C=C bonds in the hydrophobic unoxidized basal plane and the peaks for C=O, C-O, and O-H bonds in the hydrophilic oxidized domain coexisted for the GO sheets, as shown in Figure 1d, which indicated that the chemical functionalities of the GO layers maintained after conducting the LS techniques.48 Using these two different morphologies of GO, namely, flat GO and wrGO, we investigated the effect of the morphological structure of GO on the dewetting dynamics of the top polymer layer. We prepared PMMA thin films (551 Å) on the GO monolayer deposited on the Si substrates (PMMA/flat GO/Si), as illustrated in Figures 2a and e, where the bottom Si substrate acts as an extremely rigid surface. Furthermore, this rigid surface of silicon oxide on the Si substrate has been known to be strongly interactive with the GO monolayer.49 Hence, the morphology of the deposited GO on the SiO2 would not be changed as GO is strongly held by SiO2–GO interactions. We monitored the dewetting hole growth of the PMMA/flat GO/Si by using optical microscopy as a function of annealing time at 203 °C. The size of the sample was 1× 2 cm2 for optical analysis. Before annealing, the sample exhibited flat top surfaces without defects. After annealing for 2 hrs, holes surrounded by rims were observed and their diameters were 4.7 ± 1.4 µm. This annealing temperature was found to be lower than the degradation temperature of PS and PMMA from thermogravimetric analysis (Figure S3). Furthermore, the holes gradually grew as the annealing progressed, as shown in Figures 2c and d, while maintaining their original shapes.

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Figure 2. Schematics of sample geometry for dewetting experiments (a) PMMA/flat GO/Si and (e) PMMA/wrGO/Si samples. Typical optical microscope images of surface from PMMA/flat GO/Si after annealing at 203 °C for (b) 2, (c) 9, and (d) 24 hrs, and from PMMA/wrGO/Si after annealing at 203 °C for (f) 2, (g) 14, and (h) 24 hrs. For rough surface of GO on the dewetting dynamics, we deposited the wrGO sheets on the Si substrate using the LS method. After deposition of the PMMA layer (612 Å) by a floating technique, the PMMA/wrGO/Si was obtained, as illustrated in Figure 2e. We monitored the dewetting growth of the PMMA layer on the wrGO/Si surface by optical microscopy as a function of annealing time. Interestingly, the PMMA/wrGO/Si did not exhibit any dewetting hole, even after 10 and 24 hrs annealing at 203 C (Figures 2g and h), unlike the dewetting results of the PMMA/flat GO/Si (Figures 2c and d). In order to confirm the structure of the flat GO and wrGO layers in the composite films after annealing for 24 hrs, the PMMA layer was removed using acetone. We then investigated the GO surface structures using the AFM. The structure of the flat GO and wrGO was maintained after annealing. The rms roughness value of wrGO after annealing (2.07 nm, Figure S4) was similar to that before annealing (2.27 nm, Figure 1b). The roughness of top PMMA surface on wrinkled GO monolayer was also maintained even after 12

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annealing for 24 hrs. This indicates that the wrinkled GO monolayer deposited on the bottom does not initiate dewetting phenomenon although previous studies have reported that embedded fillers in polymer films can act even as defects and generate dewetting holes50-51. The only difference between PMMA/flat GO/Si and PMMA/wrGO/Si is that the different morphology of the sandwich GO sheets was applied. Therefore, the inhibition of dewetting for the PMMA/wrGO/Si is also probably due to the GO wrinkle structure. In addition to the effect of the roughness of the GO, the interaction between the polymer and the GO has a more dominant influence on the dewetting dynamics. In this regards, we have reported that the interaction between the GO and the polymer plays an important role in the diffusion dynamics and dewetting dynamics of the polymer39,

43.

Our previous diffusion

dynamics study showed that the closer the PMMA is to the GO surface, i.e., the thinner the thin film, the slower the mobility of the polymer. This is due to the attractive interaction between PMMA and GO. Similarly, in this study, the thickness dependence of the polymer film on the flat GO surface on the dewetting dynamics was investigated to confirm the interaction between PMMA and flat GO. We prepared PMMA thin films with various thicknesses ranging from 18 to 65 nm on the flat GO surface. Figure S5 shows that the PMMA/flat GO/Si sample forms a dewetting hole in the entire film thickness range, but the average dewetting hole diameter decreases as the film thickness of the PMMA decreases (Figures S5b, c, and d); for example, 11.1 ± 0.8, 14.9 ± 0.9, and 21.3 ± 1.2 µm for PMMA film thicknesses of 28, 40, and 65 nm, respectively, after annealing for 9 hrs. We also obtained the dewetting velocity from the hole growth as a function of annealing time. In all cases, the hole growth exhibits a linear relationship with the annealing time (Figure S6). This is in agreement with the theory of Brochard-Wyart et 13

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al.52 in the case of a liquid–solid regime with Young’s construction. In the figure, the slopes of the linear fits correspond to the dewetting velocity. The velocity of the 28-nm-thick layer (1.08 × 10−10 m/s) was half that of the 67-nm-thick layer (2.05 × 10−10 m/s) due to the attractive interaction between GO and PMMA. The interfacial structure of the dPMMA/flat GO/Si was measured using in situ NR technique. The sample were fabricated using 3” Si wafer. Figure 3a shows the NR profiles of dPMMA/flat GO/Si annealed at 173 °C for 3 hrs. The reflectivity profiles are plotted as a function of neutron momentum transfer, qz. We analyzed the reflectivity of the dPMMA/flat GO/Si sample using a three-layer model. Based on SLD profiles for this model (Figure 3b), the reflectivity fits were obtained as shown by the solid lines in Figure 3a. The results showed that the interface between dPMMA and GO was sharp after annealing. The interfacial rms roughness was maintained as 2.8 Å. Because the structure of the planar GO layer was unchanged, the thickness of the GO layer was obtained as 1.1 nm.45 This is consistent with the thickness of the GO single layer from the AFM results. In addition, the structure of the PMMA/wrGO/Si was measured using XRR, using in situ NR technique. The sample were fabricated using 5 × 10 cm2 Si wafer. using in situ NR technique. fits to the reflectivity profiles and AFM results, the SLD profiles of PMMA/wrGO/Si before and after annealing were determined (Figure 3d). The interfacial widths (σ) between PMMA and wrGO before and after annealing were 20.7 and 20.1 Å, respectively. From the results, we found that the wrGO structure at the interface of the PMMA and Si layers was also maintained during annealing. The rough surface of wrGO in

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PMMA / wrGO / Si will play an important role in suppressing dewetting of the upper PMMA. No dewetting holes are seen in Figures 2f, g, and h.

Figure 3. (a) Neutron reflectivity profiles (best fit in black line) of the dPMMA/flat GO/Sisamples before and after annealing at 203 oC and (b) the corresponding SLD profiles. (c) X-ray reflectivity profiles (best fit in black line) of PMMA/wrGO/Si sample before and after annealing at 203 oC and (d) the corresponding normalized electron density profiles. Despite the obvious effect of the GO on the polymer dewetting, another question still arises: whether underlying Si substrate may affect the dewetting dynamics. The mobility of PMMA near the Si substrates is known to significantly decrease owing to the polymer pinning effect on the substrates53. Because the thickness of the GO monolayer in the PMMA/flat GO/Si 15

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is only 1.01 nm, the attractive interaction between PMMA and Si may affect the dewetting dynamics. The long-range dispersion force between the PMMA and Si surface across the GO can be calculated as54, W(𝑥) = ―𝐴 12𝜋𝑥2, where A is the effective Hamaker constant that describes the interaction between interfaces of PMMA and Si across the GO monolayer and x is the thickness of the GO, assuming that the PMMA layer is thick enough to ignore the effective Hamaker constant between air and the substrate across the PMMA layer. However, one can see that if x is small, the long-range dispersion force between the PMMA and Si can not be ignorable. Hence, to eliminate the influence of Si substrates, we added a UV-crosslinked PS (crPS) layer (denoted PMMA/GO/crPS/Si), as illustrated in Figure 4a. After UV irradiation for 2.5 hrs under vacuum, the crPS thin films were obtained. We used it as a solid-like substrate for Young’s construction. They are not washed even by the PS-good solvents such as chloroform (Figure S7). We deposited the flat GO monolayer on the crPS film surface by the LS method. Similar to the previous sample preparation, the PMMA/flat GO/crPS/Si was obtained after depositing the top PMMA layer. We annealed the sample at 183 C for 24 h. From the dewetting results, we found that the sample was stable against dewetting by GO (Figure 4b, c, and d). No indication of the nucleation of dewetting holes was apparent in the figure. However, the PMMA/crPS/Si exhibited dewetting holes after 150 min of annealing (Figures S8b, c, and d). These dewetting dynamics were faster than those of the PMMA layers on Si substrates (denoted as PMMA/Si, Figures S8f, g, and h). This indicated that the crPS layer reduced the interaction between PMMA and Si. Although the additional crPS layer lowers the interaction between the PMMA and the Si substrate, the flat GO monolayers intercalated at the interface between PMMA and crPS of the PMMA/GO/crPS/Si (Figures 4b, c, and d) suppress the dewetting hole more significantly 16

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compared to that of the PMMA/flat GO/Si (Figures 2b, c, and d) and the PMMA/crPS/Si (Figure S8). This tendency is unexpected, considering the weaker interaction between PMMA and crPS than that between PMMA and SiO2 substrate. As explained above, if the PMMA/GO/crPS/Si sample has a long-range dispersion force between PMMA and crPS across the GO monolayer, this repulsive interaction takes place due to the immiscibility of PMMA and crPS. On the other hand, for the PMMA/GO/Si sample, a strong attractive interaction between PMMA and SiO2 can be considered across a flat GO. It is well known that the PMMA mobility on SiO2 surfaces is considerably slow, like a dead layer, due to pinning effect53. Also Shin et al. reported that Si substrates contain a few nanometer thick SiO2 layer even after HF etching55. In this regard, the dewetting dynamics of the PMMA/GO/Si can be expected to be slower than that of the PMMA/GO/crPS/Si sample. However, from results in Figure 4, we obtained the opposite of what one can expect. This indicates that in addition to the interfacial interaction enhanced by GO, there is another effect on stabilization against dewetting.

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Figure 4. Schematics of sample geometry of the PMMA/GO/crPS/Si samples. Typical optical microscope images of surface from PMMA/GO/crPS after annealing at 203 oC for (b) 2, (c) 14, and (d) 24 hrs. (e) The neutron reflectivity profiles of PMMA/GO/crPS/Si samples before and after annealing at 203 oC for 3 hrs. (f) The corresponding SLD profiles. In order to check the structural change during annealing, we measured the neutron reflectivity. Figures 4e and f show the NR and the corresponding SLD profile of the PMMA/GO/crPS/Si, respectively. From the results, one can see that the interfacial rms roughness between the PMMA and GO was changed from 5.3 Å to 13.5 Å. This result implies that the morphological structure of flat GO in the PMMA/GO/crPS/Si has changed and that the GO sheets are probably intermingled with the PMMA layer in the annealing process becasue the interaction between GO and the crPS layer is relatively weak. In contrast, the flat morphology of the GO in the PMMA/flat GO/Si was maintained after annealing due to the strong interaction between the GO and SiO2. Previously, we have reported that GO functions as a compatibilizer at the interface because it has an amphiphilic nature that allows it to interact with both PS and PMMA39. The GO exhibit an amphiphilic nature because the hydrophobic unoxidized basal plane and peaks and the hydrophilic oxidized domains coexisted. The GO at the PS−PMMA interface altered the interfacial tension by functioning as molecular surfactants39. In addition, the structure of the GO sheets was roughened after annealing by the function of the GO as a compatibilizer of the GO at the PS-PMMA interface for the PMMA/GO/PS/Si. We believe that the two polymers are intermixed by the enhanced compatibility of PS and PMMA, which causes the GO structure undulate at the interface. The rough structure of GO seems to be able to also improve the interfacial energy between PS and PMMA more effectively. From the optical microscopy results, no dewetting hole was observed for the PMMA/GO/PS/Si, even after 18

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annealing for 24 hrs at 203 °C (Figure 5). Hence, this film stabilization against dewetting can be interpreted as the result of the interplay between the GO–polymer interaction and the morphological structure of the GO.

Figure 5. Schematics of sample geometry for dewetting experiments of PMMA/PS/Si and PMMA/GO/PS/Si samples. For the PMMA/GO/PS/Si sample, the LS technique was conducted to deposit the GO layer on the PS (Mw = 7,100,000 g/mol) layer; then, the PMMA layer (Mw = 92,000 g/mol) was transferred as a top layer. Typical optical microscope images of dewetting holes from PMMA/PS/Si sample after annealing at 203 °C for (b) 10, (c) 30 min, and (d) 1 hr, and from the PMMA/GO/PS/Si after annealing at 203 °C for (f) 2, (g) 14, and (h) 24 hrs. In order to clarify the GO structure effect on dewetting at the polymer-polymer interface, we characterized the interfacial structure of the dPMMA/GO/PS/Si and dPMMA/ PS/Si using the NR technique. Figure 6a shows reflectivity profiles of the thin films after annealing at 173 °C for 14 hrs. The oscillations of the profile for the dPMMA/GO/PS/Si were more rapidly damped than those for the dPMMA/PS/Si sample. This indicated that the interfacial roughness increases in the presence of the GO after annealing. From the SLD profile, we could clearly see that a broader interface between dPMMA and PS layers was obtained (Figure 6b). The interfacial width (σ) values of dPMMA/GO/PS/Si and dPMMA/PS/Si were obtained as 46.9 and 19.0 Å, respectively. 19

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The overall interfacial roughness of immiscible polymers is the result of interplay of intrinsic width, σin and the roughness .

caused by capillary-wave broadening, that is,

is the mean square displacement of the interface from its average

position due to capillary waves, described by

where kB, T, and γ are the

Boltzmann constant, absolute temperature, and surface tension of the film, respectively, and and relation,

are the maximum and minimum wavelengths of fluctuations56. From this is inversely proportional to the interfacial tension, indicating that improvement of

compatibility with the better interfacial interaction increase the roughness at the polymerpolymer interface. Hence, the difference in σ between dPMMA/GO/PS/Si and dPMMA/PS/Si is due to the function of GO as a compatibilizer, which may cause a change in the roughness of the GO monolayer at the interface between dPMMA and PS. Also note that the coefficients of thermal expansion of PS (5×10−4 /K) and PMMA (5×10−4 /K) are similar57-59, so that the interfacial roughness may not be affected by thermal expansion coefficient during sample cooling. In the case of the PMMA/GO/PS/Si, the roughness of GO monolayers increases after annealing (σ = 46.9 Å), which also results in that the gap between the GO platelets may become larger. In this gap, immiscible two polymers, i.e., PS and PMMA, directly contact, leading to no GO effect on suppression of dewetting in the respect of molecular local dynamics. However, since the dewetting behavior corresponds to long-range motion, the size of the dewetting hole is relatively much larger than the area of the gap. Therefore, the effect of this gap on dewetting dynamics may be ignorable. On the other hand, the wrinkle structure of the GO, which occurs 20

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spontaneously between the PS and PMMA interfaces, more effectively prevents the dewetting behavior. This effect is much dominant over the effect of the gap between the GO platelets.

Figure 6. (a) Neutron reflectivity profiles (best fit shown by black line) of two different types of polymer multilayer films, the dPMMA/PS/Si and dPMMA/GO/PS/Si bilayer after annealing at 203 oC for 3 hrs, and (b) the corresponding SLD profiles. Previous theoretical studies have shown the effect of surface roughness on polymersurface interaction.42 As GO surface roughness increases, not only does the probability of polymer-GO surface contact increase owing to the larger “GO surface area,” but the “polymerGO surface interaction” is also enhanced in terms of the fundamental physical surface chemistry. The polymer adsorption on the GO surface occurs when the interaction of the attracting forces with each other overcomes the loss of configurational entropy caused by the polymer being confined on the GO surface. That is, the polymer adsorption on a rough GO surface is associated with smaller loss of configurational entropy, compared to the adsorption on a flat GO. Thus, even in the case of relatively small enthalpy contributions of polymer-surface interactions, the rough surface can adsorb the polymer without surface chemical modifications. For this reason, in

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our case, the roughness of wrGO can lead to effective polymer–GO interactions and thus suppress dewetting.

CONCLUSION We have studied the dewetting dynamics of polymer thin films on wrinkled GO layers. We used a LS method to deposit the wrGO layers and flat GO monolayers from the liquid–gas interface onto solid substrates. The flat GO monolayers were added to PMMA/Si substrate interface and PMMA/PS interface. Although the dewetting hole growth of the PMMA/PS is much faster than that of the PMMA/Si in the absence of the GO, the intercalation of an approximately 1-nm-thick GO monolayer improves the film stability more effectively at the polymer-polymer interface for the PMMA/GO/PS/Si than the polymer-solid substrate interface for the PMMA/GO/Si. Neutron reflectivity results show the increase in the GO roughness (46.9 Å) in the case of the PMMA/GO/PS/Si, as opposed to the flat GO structure for the PMMA/GO/Si (5.6 Å) after annealing. This morphological change of GO monolayers at the interface prevents dewetting by improving the wettability of PMMA. This is also evidenced by studying the dewetting dynamics of polymer films on wrinkle structures of GO sheets on Si substrates fabricated by the LS technique. From these results, we found that beside the GO–polymer interaction, the formation of the rough structure of the GO sheet at the interface plays a crucial role in the GO-induced film stabilization in immiscible polymer blend films. 22

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the Internet at http://pubs.acs.org. Optical microscope images of dewetting holes at in situ system from the PMMA/flat GO/Si and PMMA/wrGO/Si Surface; Surface pressure–area (π–A) isotherms of GOODA; TGA graph of the PMMA and PS; AFM image of the wrGO after annealing; Optical microscope images of dewetting holes from PMMA/flat GO/Si samples with various PMMA layer thicknesses; Dewetting hole diameters of PMMA/flat GO/Si samples; Confirmation of the crosslinking of UV-irradiated crPS thin films; Optical microscope images of dewetting holes from the PMMA/crPS/Si and PMMA/Si. (PDF)

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AUTHOR INFORMATION Corresponding Author *(J. K.) Email: [email protected]; Tel.: +82 42 821 6619; Fax: +82 42 821 8870 ORCID Jaseung Koo: 0000-0002-3646-0805 Author Contribution ∥

K.-I. Jo and T.-H. Kim contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors thank to the deep cooperation of Korea Atomic Energy Research Institute for their offers of better experimental environment. This work was supported by a grant from by research fund of Chungnam National University and the National Research Foundation of Korea under Contract 2017M2A2A6A01019911. The identification of commercial products does not imply endorsement by the National Institute of Standards and Technology nor does it imply that these are the best for the purpose.

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