Extending the Anomalous Dilation in CO2 to Thick Polymer Blend

Apr 22, 2016 - We investigated the effect of density fluctuation of supercritical carbon dioxide (scCO2) on anomalous swelling of multilayer polymer t...
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Extending the Anomalous Dilation in CO2 to Thick Polymer Blend Films: A Neutron Reflectivity Study Jaseung Koo,*,† Tadanori Koga,‡,§ Bingquan Li,∥ Sushil K. Satija,⊥ and Miriam H. Rafailovich‡,§ †

Neutron Science Division, Korea Atomic Energy Research Institute (KAERI), Daejeon 305-353, South Korea Department of Materials Science and Engineering and §Chemical and Molecular Engineering Program, State University of New York at Stony Brook, Stony Brook, New York 11794-2275, United States ∥ Dow Chemical, Collegeville, Pennsylvania 19426, United States ⊥ Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ‡

ABSTRACT: We investigated the effect of density fluctuation of supercritical carbon dioxide (scCO2) on anomalous swelling of multilayer polymer thin films on the ridge in the pressure− temperature phase diagram of CO2. In order to measure the swelling ratio along the film depth, we alternately deposited hydrogenated poly(methyl methacrylate) (PMMA) and deuterated polystyrene (dPS) thin films and performed the neutron reflectivity measurements as a function of CO2 pressure at 36 °C. The results showed that, in contrast to previous studies, CO2 was to penetrate deeply throughout the multilayer thin film where the magnitude of swelling along the density fluctuation ridge of CO2 was independent of film thickness. Block copolymer thin films of dPS-b-PMMA with a parallel lamellar orientation also showed similar swelling behavior in scCO2. However, it is well-known that single-layer polymer thin films exhibit anomalous swelling behavior only near the film surface. This difference is probably due to the fact that the multilayer thin films have the CO2-philic PMMA layer sandwiched between dPS layers, which can function as a CO2 reservoir, thereby transferring the CO2 molecules from the PMMA layers to the adjacent dPS layers. Furthermore, we found that the interaction between polymers and substrates was not significant in scCO2 from diffusion dynamics results using neutron reflectivity, thereby facilitating anomalous dilation of polymers near the substrates without a pinning effect.

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in polymer thin films, even when the bulk polymers have very low solubility in CO2. From in situ neutron reflectivity results, Koga et al. showed an anomalous swelling, as much as 30%− 60%, in a wide variety of polymer thin films along the ridge.17,18 The sorption of CO2 at the ridge also significantly improves the compatibility of immiscible polymer blend films due to its cosolvent behavior21 and induces the plasticization effects for glassy polymers, thereby increasing the polymer chain mobility below the glass transition temperature even outside of the density fluctuation ridge.22−25 This plasticization effect for polymer thin films was reported to enhance the polymer interdiffusion dynamics in the vicinity of the ridge through the in situ neutron reflectivity measurement.26−28 In the case of polymers that are soluble in scCO2, dilation is linear with pressure, and only a small excess is observed along the density fluctuation ridge.18,29 In the case of polymers where CO2 is a poor solvent, penetration is enthalpically unfavorable. However, it has been previously shown that excess sorption of CO2 on multiple polymer substrates, which include flat films,17 particles,30 and

INTRODUCTION Supercritical carbon dioxide (scCO2) has gathered substantial interest in recent years1−5 as an alternative solvent for a wide range of important technological applications, such as organic photovoltaics,6 supercapacitors,7 reactor cleanup,8 pharmaceutical product synthesis,9,10 and solvent-free coating11−13 due to its unique advantages of inert, nonflammable, nontoxic, and environmentally benign properties. CO2 also has a moderate critical temperature (Tc = 31.3 °C) and critical pressure (Pc = 7.38 MPa), facilitating access to the supercritical state where CO2 exhibits gas-like viscosities and liquid-like density with tunable physical properties by small changes in temperature and pressure.14 Large density fluctuations in CO2 within narrow temperature and pressure regimes in the scCO2 region, known as the “density fluctuation ridge”, can be obtained from the continuous line across the Tc, where the more liquid-like and more gas-like regions can be separated, as shown in Figure 1. Although scCO2 is a good solvent for many small molecules, unfortunately, only a limited class of polymers, such as amorphous fluoropolymers15 and polysiloxanes,16 are soluble in scCO2, which represents the major drawback in polymer process and polymer synthesis.17 Koga et al.17,18 and Li et al.19,20 reported that large density fluctuations at the ridge can significantly improve the solubility © XXXX American Chemical Society

Received: September 18, 2015 Revised: April 10, 2016

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DOI: 10.1021/acs.macromol.5b02070 Macromolecules XXXX, XXX, XXX−XXX

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the chain/surface correlation, which damps out the density fluctuations and hence driving force of penetration. In the case of the CO2-philic polymer, the enthalpic interactions drive penetration, but the question still remains whether the fluctuation persist with increasing total film thickness In order to address this question, we produced multilayer films of alternating CO2-phobic dPS and CO2-philic PMMA layers using both thin film floating and deposition as well as self-assembly of a dPS-b-PMMA thick diblock copolymer film, as illustrated in Figures 4a and 4d, respectively. We then measured the isothermal swelling ratios in each layer as a function of depth and pressure, using in situ neutron reflectivity. Since the CO2-phobic polymer could only be swelled when the scCO2 is in its density fluctuating state, we could determine in this manner whether this state can be preserved for large distances within a polymer film and whether direct molecular coupling at the interface between the CO2-philic and CO2phobic polymers was required for propagation to occur.

Figure 1. Schematic phase diagram of CO2 near the critical point. Critical temperature and pressure are denoted as Tc and Pc, respectively.

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blends,31 occurs along the density fluctuation ridge. Sumi et al.32 and Koga et al.18 have shown that in dilute systems scCO2 will swell both CO2-philic and CO2-phobic polymer chains, where the polymer enhances the density fluctuation correlation lengths, thereby causing an increase in entropic component of the equation of state. In the case of the CO2-phobic polymer, this increase may compensate for some of the unfavorable entropy of mixing, resulting in the anomalous dilation of the chain.17,18,33−36 Hence, it was also postulated that the dilation seen in the films may be a result of penetration by the excess scCO2 where the large density fluctuations can now couple with fluctuations in the swelling of the polymer chains. Grest et al. and Meredith et al.37,38 estimated the thickness of the adsorbed CO2, but ellipsometric measurements by multiple groups indicated that the swelling region exceeded the predicted adsorbed thickness.34,39 Sirard et al.29 and Koga et al.18 found that in the case of CO2-philic polymers pressure was the predominant factor in isothermal swelling with only a small anomalous spike in the vicinity of the density fluctuation ridge. In the case of the CO2-phobic polymers, the density fluctuation conditions were the primary drivers, and the amount of dilation decreased rapidly with increasing film thickness. Koga et al.17 and Chebil et al.40 showed that the amount of dilation was decreased with penetration depth into the film. The region of maximum dilation at the interface of the polymer and the scCO2 was a function of the polymer molecular weight, scaling with a radium of gyration (Rg), confirming that the anomalous dilation was a function of the polymer structure, as predicted by Sumi et al.,32 where fluctuations in the chain structure coupled to those in the supercritical fluid, effectively extending their correlation length. Furthermore, Koga et al.17 also showed that the dilation was rapidly damped within 2Rg of the surface, which is also correlated to probability if direct contact between segmental regions of the chains and the film surface occurs.41 Hence, the fluctuations in the solvent only affect chains in direct contact with it. As the film thickness increases, the contribution to the dilation of the film from the fluctuations at the interface becomes smaller. Most polymeric materials consist of polymer mixtures. Here, we want to explore the penetration of scCO2 in a layered system of CO2-phobic and CO2-philic polymers, where the mechanism differs among the layers. In the case of the CO2phobic polymers, penetration of the scCO2 is clearly limited by

EXPERIMENTAL SECTION

Materials. Polystyrene (PS), poly(methyl methacrylate) (PMMA), and their deuterated analogues were purchased from Polymer Source Inc. (Dorval, Quebec, Canada). All other solvents and reagents were obtained from Sigma-Aldrich (St. Louis, MO). All purchased chemicals were used without further purification. Preparation of Thin Polymer Films. Polished 10 mm thick Si (100) wafers (3 in. diameter) were purchased from Wafer World (West Palm Beach, FL). The surfaces were treated with a modified Shiraki technique: the substrates were immersed in a piranha solution of H2O:H2O2:H2SO4 (3:1:1 vol) mixture for 10 min at 80 °C and rinsed in deionized (DI) water. To create a hydrophobic surface, they were immersed in H2O:HF (5:1 vol) for 30 s at room temperature.42 For the multilayer sample, PMMA was chosen as the bottom layer in consideration of its preferentiality for segregation to the Si oxide substrate due to its polarity,43 while PS was deposited as the top layer. The sample geometry is shown in Figure 4a. PMMA (Mw = 235K, Mw/Mn = 1.10) was dispersed at the concentration of 5 mg/mL in toluene. After syringe filtration with a PTFE membrane (0.5 μm, Millipore, Billerica, MA), the PMMA solution was rapidly spun-cast onto HF etched Si wafers at 2500 rpm for 30 s. The thickness of the layers, 245 ± 7 Å, was measured using ellipsometry. To provide a good contrast between the polymer−polymer interface for neutron, deuterated PS (dPS, Mw = 250K, Mw/Mn = 1.15) was used for the multilayer thin film. The dPS dissolved in toluene (5 mg/mL) was spun-cast onto the piranha-treated hydrophilic Si wafer (the film is 270 ± 8 Å thick) and carefully floated from DI water onto the PMMA substrate. PMMA and dPS for the third and fourth layers, respectively, were also deposited in the same manner. The sample was then annealed at 130 °C in a vacuum of 10−3 Torr for 3 h to remove the residual solvent and relax strains induced by the spinning process. For the sample preparation for the diffusion dynamics studies, the PS (Mw = 650K, Mw/Mn = 1.11) was first dissolved in toluene at the various concentrations to obtain the bottom layer with different thicknesses. This solution was then spun-cast onto the surface of the piranha-treated hydrophilic Si wafer, and the second layer of the dPS (Mw = 250K, Mw/Mn = 1.09) with the constant thickness (34 nm) was also spun-cast onto another piranha-treated hydrophilic Si wafer and then carefully floated from DI water onto the polymer-coated substrates. Neutron Reflectivity. The neutron reflectivity (NR) experiments were performed with the NG7 reflectometer at the Cold Neutron Facility of the National Institute of Standards and Technology (Gaithersburg, MD) with a wavelength (λ) of 4.76 Å and Δλ/λ ∼ 0.025. The samples were mounted in a high pressure, temperaturecontrolled chamber, specially designed for NR experiments. Since the neutron beam has a large penetration depth, incident neutrons can pass through the silicon substrates and reflect from the solid−liquid B

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nonlinear least-squares method was used to find the best fit values by adjusting thickness, scattering length densities, and interfacial width of the unknown layers with least-squares statistic (χ2). The physical quantities to fit the data are the thicknesses, the interfacial root-meansquare (rms) roughness (σ), and the SLD corresponding to the elastic coherent scattering per unit volume, which is crucial for the study of the multilayer system.

interface, as illustrated in the scattering geometry (Figure 2a), facilitating determining the in situ thickness, composition, and

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RESULTS In order to determine the penetration of scCO2 across immiscible polymer interfaces, we first have to establish the penetration of scCO2 within the CO2-phobic layer and determine whether interactions with the solid substrate influence the results. We therefore prepared bilayer samples where a layer of hydrogenated PS (hPS) was first spun-cast on a native oxide Si substrate and cleaned in piranha solution, and then a thin layer of dPS, approximately 2.5Rg thick, was floated on top from DI water. The samples were then placed in a vacuum oven at T = 50 °C, which is T ≪ Tg, to remove adsorbed water but prevent interfusion and reactions with the substrate.45 The neutron scattering geometry for data acquisition is shown in Figure 2a, where we see that the neutron beam penetrates through the native oxide covered Si substrate and probes the interfaces between the two polymer layers as well as those between the entire polymer bilayer film and the scCO2 and native silicon oxide interfaces. The SLD at each of these interfaces for a bilayer sample with the initial hPS and dPS film thicknesses of 0.7Rg and 2.5Rg, respectively, is shown in Figure 2c. From the figure, we see that prior to exposure, the interfaces are sharp and the Si wafer is covered with a native oxide layer (1.3 nm thick). This SLD profile was the one which provided the best fit to the spectra shown in Figure 2b and plotted as a solid line. The profiles obtained after exposure to scCO2 for various times are also shown. From the Figure 2b, we find that two major events are occurring. First, the frequency of the oscillations changes almost immediately, and the amplitude of the oscillations decreases with increasing exposure time. The change in frequency corresponds to the dilation of the sample, as shown. From the Figure 2c, we find that the dilation is uniform and the SLD value is decreased throughout the sample, indicating uniform penetration of the scCO2. The decrease in amplitude corresponds to the increase in the interfacial width between the two layers, as shown in Figure 2b. No segregation of scCO2 to the substrate interface is observed. Furthermore, no increase in the off-specular (diffuse) scattering is observed following exposure to scCO2. This is consistent with formation of a uniform polymer/CO2 layer and miscibility of the CO2 in the polymer film. This is in contrast to the behavior at pressures and temperature off the ridge where phase-segregated domains of polymer and CO2, such as those reported in ref 46, are formed, where large off-specular scattering would result from the multiple interfaces. In Figure 3a, we show an expanded plot of the interfacial region, and in Figure 3b, we plot the interfacial width for the thickest and thinnest hPS samples studied vs t1/2, from which we can extract the diffusion coefficient using the Fickian diffusion formula, Δσ = 2(Dt)1/2, where D represents a diffusion coefficient, and Δσ = (σ2 − σ02)1/2, where σ is an interfacial root-mean-square (rms) roughness and σ0 is an initial rms roughness of the interface. In Figure 3c, we plot the diffusion coefficients obtained as a function of the hPS film thickness, where we find no significant difference. This is in contrast to the recent report of Koga et al.,28 who found that

Figure 2. (a) Scattering geometry for in situ neutron reflectivity measurement of the polymer thin film in CO2. (b) Representative neutron reflectivity data from the dPS/PS bilayer film (160 Å thick for the bottom layer) exposed to CO2 at T = 36 °C and P = 8.2 MPa. The solid lines are the best fits to data. Consecutive reflectivities have been offset from each other for clarity. (c) Corresponding SLD profiles as a function of CO2 exposure time. interfacial structure of polymer thin films immersed in fluids or gases. Because of the high contrast between deuterated polymers and hydrogenated polymers in terms of scattering length density (SLD), the neutron reflectometry is a powerful tool for the investigation of multilayer thin films with a subnanometer resolution. In the figure, the z-axis corresponds to the normal direction to the sample surface, and the x- and y-axes are the in-plane and out-of-plane directions perpendicular to the z-axis, respectively. The NR data were collected as a function of the wavevector transfer, q⃗ = kf⃗ − ki⃗ , where kf⃗ and ki⃗ are the incident and scattered wave vectors, by changing the incident (θi) and exit (θf) angles with qz = (4π/λ sin θi). The specular reflectivity was measured from the intensity at qx = 0 while maintaining θi = θf. The vertical slits gradually opened as a function of qz to fix the resolution at a constant value of Δqz/qz ≈ 0.03 while the size of the horizontal slits was set to 30 mm. The neutron reflectivity data were first corrected for footprint and background. The experimental reflectivity data were then fitted to reflectivity profiles calculated from model scattering length density profiles using a Parratt formalism.44 A Levenberg−Marquardt C

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copolymer thin film of dPS-b-PMMA with a parallel lamella orientation on HF etched silicon substrates as illustrated in Figures 4a and 4d. The neutron reflectivity data for the (dPS/ PMMA)2 multilayer and dPS-b-PMMA film, shown in Figures 4b and 4e, respectively, were collected as a function of CO2 pressure at 36 °C. We plotted the reflected scattering intensity as a function of the momentum transfer normal to the surface, qz. From the figure, one can clearly see that the peak positions of Kiessig fringes gradually shifted to smaller momentum transfer (qz) with increasing the pressure up to 8.2 MPa (ridge condition) in both cases of (dPS/PMMA)2 multilayer and dPSb-PMMA film, indicating that overall thickness of the film increases due to the CO2 sorption as the CO2 pressure is approached to the density fluctuation ridge. The reflectivity curves were fit with model density profiles for (dPS/PMMA)2 multilayer and dPS-b-PMMA film (Figures 4c and 4f, respectively) at the corresponding pressures, where one can see that each polymer layer was swollen without preferential segregation of CO2, resulting in exhibiting uniform SLD values for the polymer−CO2 mixture obtained through each layer and the similar SLD numbers between PMMA1 and PMMA2 layers and between dPS1 and dPS2 layers. This indicates that the sorption of CO2 in the film can reach equilibrium after exposure to scCO2 at desired pressure and temperature for 30 min prior to NR data collection. Hence, the CO2 concentration through the each dPS or PMMA layer was uniform for the layer thickness of the (dPS/PMMA)2 multilayer and dPS-b-PMMA lamella samples. From Figures 4c and 4f, one can see that good fits can be obtained without concentration gradient and preferential adsorption of CO2 at the silicon substrates. The linear dilation was calculated from the equation Sf = (L − L0)/L0, where L and L0 are the measured thickness of the swollen and unswollen polymer thin films, respectively. We tabulated the L, L0, and Sf before and after exposure to scCO2 at T = 36 °C and P = 8.2 MPa in Table 1 for (dPS/PMMA)2 multilayer and dPS-b-PMMA film, respectively. From the results, one can see that the Sf values were similar through the entire depth of the polymer film, indicating that CO2 deeply penetrated into the lower layers (i.e., PMMA1 and dPS1 layers). These results are, however, different from those previously reported from swelling behavior of single-layer polymer thin films in CO2. We had previously reported that the Sf value at the ridge decreased as the dPS film thickness was increased and concluded that the anomalous swelling of polymer thin films is a surface effect induced by density fluctuation of CO2 at the ridge.17 We also compared the swelling features between thin films and bulk polymers and found that the anomalous swelling only occurred in the polymer thin films at the polymer−CO2 interface. Studies of the Rg dependence of the swelling behavior of dPS or deuterated polybutadiene (dPB) thin films showed that the most pronounced swelling effect was obtained when the film thicknesses were less than 4Rg. Hence, for films thicker than 4Rg, we showed that CO2 could not penetrate throughout the CO2-phobic polymer films even at the ridge condition.17 However, we found that a different swelling behavior occurred in the case of the multilayer polymer thin films in CO2. Figures 5a and 5d show the linear dilation (Sf) of the dPS layers for (dPS/PMMA)2 multilayer and dPS-b-PMMA film, respectively, in the direction normal to the surface. Here we can see that although the dPS1 layer was positioned 776 Å (corresponding to 5.5Rg) below from the top surface of the film, this layer showed the excessive swelling at the density fluctuation ridge (P = 8.2 MPa) of CO2. We also found that the

Figure 3. (a) SLD profiles as a function of CO2 exposure time. Expanded plot of the interfacial region. (b) The rms roughness plotted as a function of the square root of the time at T = 36 °C and P = 8.2 MPa for the PS/dPS bilayer films (160 and 780 Å thick for the bottom layer). (c) Diffusion coefficients plotted as a function of the bottom layer thickness.

the interfacial diffusion was a function of the film thickness. In their case, the native oxide was removed, and hence the surfaces are not comparable to those used here. In this case, no Guyslain brush-type layer is formed which was shown to trap subsequent chains, thereby no hindering diffusion. Hence, in the presence of CO2, the polymer chains can freely move without interactions with the substrate due to CO2 screening of the interaction between polymer layer and the silicon oxide substrates. This indicates that the reduced swelling behavior of the single-layered dPS thin films in the CO2 is not due to the interaction between polymer−substrate. Instead, the limited sorption of CO2 into PS films is more likely to hinder anomalous swelling of the film. In order to investigate the effect of the CO2 sorption into the thin films on anomalous dilation, we prepared the multilayer consisting of alternate layer of CO2-phobic (i.e., dPS) and CO2philic polymers (i.e., PMMA), namely dPS−PMMA-dPS− PMMA (designated as (dPS/PMMA) 2) and the block D

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Figure 4. (a, d) Schematic illustrations of the sample configuration ((dPS/PMMA)2 multilayer and dPS-b-PMMA thin film on HF etched silicon substrate). (b, e) Neutron reflectivity data for (dPS/PMMA)2 multilayer and dPS-b-PMMA thin film as a function of CO2 pressure at 36 °C. Consecutive reflectivities have been offset from each other for clarity. Solid lines represent the reflectivity calculated from corresponding scattering length density (SLD) profiles shown in (c) and (f). CO2 SLD profiles on the ridge are plotted in the inset of the figure.

Table 1. Thickness Change of Each Layer of the (dPS/PMMA)2 and dPS-b-PMMA Thin Films before and after Exposure to scCO2 and Their Corresponding Linear Dilation (Sf) at T = 36 °C and P = 8.2 MPa dPS-b-PMMA

(dPS/PMMA)2 thickness (Å) Sf

no CO2 CO2 (8.2 MPa)

PMMA1

dPS1

PMMA2

dPS2

PMMA1

dPS1

PMMA2

dPS2

274 328 0.20

236 277 0.17

266 311 0.17

250 289 0.16

108 131 0.21

175 192 0.10

221 269 0.22

89 98 0.10

swelling features of the dPS1 and dPS2 layers were similar in both cases of (dPS/PMMA)2 multilayer and dPS-b-PMMA film, indicating deep sorption of CO2 in these multilayer and copolymer thin films. The PMMA1 and PMMA2 layers also exhibited similar swelling features (Figures 5b and 5e for (dPS/ PMMA)2 multilayer and dPS-b-PMMA film, respectively) regardless of the film thickness above these PMMA layers. One can also see that each layer for both dPS and PMMA has uniform SLD values after exposure of scCO2, which indicates that CO2 had penetrated deeply all the way down to the bottom layer. In this case, the anomalous swelling did not occur at the ridge for both of the PMMA1 and PMMA2 layers. Sf for

PMMA layers gradually increased with increasing CO2 pressure to 13.7 MPa for both the (dPS/PMMA)2 multilayer and the dPS-b-PMMA films. Similar results were also observed with single-layer PMMA films using neutron reflectivity results, where the PMMA thin films showed the better sorption of CO2 compared to those in the dPS, dPB, and dSBR thin films at pressures far above the density fluctuation ridge, which were attributed to specific intermolecular interaction between the PMMA carbonyl oxygen and the carbon atom of CO2.29 The dilation of the PMMA layer occurs due to the favorable interaction energy between the scCO2 and the polymer. Hence, the penetration is enthalpically favored, and the scCO2 behaves E

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Figure 5. (a) Pressure dependence of Sf for dPS layers (dPS1 and dPS2) at T = 36 °C. (b) Pressure dependence of Sf for PMMA layers (PMMA1 and PMMA2) at T = 36 °C. (c) Interfacial widths for each polymer layer interface in the multilayer as a function of pressure at 36 °C. (d) Pressure dependence of Sf for dPS blocks (dPS1 and dPS2) at T = 36 °C. (e) Pressure dependence of Sf for PMMA blocks (PMMA1 and PMMA2) at T = 36 °C. (f) Interfacial widths for each copolymer block interface in the assembled lamella as a function of pressure at 36 °C.

polymers along the density fluctuation ridge, thereby promoting the compatibility between polymer blends.21 From the best fits to the in situ NR data of (dPS/PMMA)2 multilayer, we obtained the interfacial width as shown in Figures 5c and 5f for (dPS/PMMA) 2 multilayer and dPS-b-PMMA film, respectively, where one can see that the interfacial width broadened by the excess sorption of CO2, indicative of improving the miscibility between dPS and PMMA. The interfacial broadening was also affected by the density fluctuation ridge, therefore showing a drastic pressure dependence. In addition, we found that the pressure dependence of the interfacial width between PMMA1 and dPS1 was similar to that for other layer interfaces, such as dPS1/PMMA2 and PMMA2/ dPS2 interfaces. We can therefore conclude that the interfacial width broadening in CO2 for this multilayer was also unaffected by the film depths from the surface to the corresponding interfaces. This indicates that the excess CO2 molecules were capable of deep penetration into the film, thereby enhancing the miscibility of polymer layers. The sorption of CO2 in both

like a good solvent, stretching the polymer, independent of other constraints. In the case of dPS, the enthalpic interactions with scCO2 are unfavorable. Penetration only occurs on the ridge, where the system tries to decrease the fluctuation intensity, and the penetration is very sensitive to entropic and enthalpic penalties. In the case of the spun-cast film, the dPS chains are in their Gaussian configuration and hence easy to swell. In the block copolymer, the dPS chains are confined at the PMMA interface, forming a brush, making it more difficult to impose additional stretching. As a result, the swelling ratio of dPS chains for the block copolymer thin films was smaller than that for spun-cast films. The SLD of CO2 in the dPS block was also lower than that in the PMMA block, as shown in the inset of Figure 4f, in contrast to results of the multilayer where the SLD values of CO2 in dPS and PMMA layers were obtained to be similar on the ridge. Furthermore, the interfacial width between the dPS and PMMA layers was investigated in the presence of CO2. scCO2 is known to function as a cosolvent for two immiscible F

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with the similar swelling property within the entire film thickness. The adoption of CO2 in this reservoir system makes it possible to maximize CO2 sorption within the thick multilayer films, in contrast to single layer of dPS thin film where the surface effect on the anomalous dilation can happen due to limitation of CO2 penetration into the films.

PS and PMMA layers facilitates the function of CO2 as a cosolvent for the thick polymer blend films. The CO2 molecules at the polymer−polymer interface may screen unfavorable interactions, thereby broadening the interface between them.21

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DISCUSSION Supercritical CO2 has a well-defined ridge away from the critical point where the amplitude of the density fluctuations is maximal.47−49 These fluctuations drive the fluid into any polymer surface where their amplitude can be damped. The range will depend on the correlation of the fluctuations and the interactions between the polymer and the fluid.18,29,32,34 In the case of unfavorable interactions, the favorable interactions are quickly balanced by the unfavorable enthalpy term between the solvent and polymer. This was demonstrated by Koga et al.,17,18,21,35 who observed dilation, diffusion, and interfacial broadening only along the locus of points defining the ridge. In the case of a favorable enthalpic interaction, there are no limits as to the solvent penetration. In our case, penetration into the entire film is possible since it is a combination of CO2-phobic and CO2-philic layers. The fluid penetrates the phobic layer, which is thinner than the correlation length of the fluctuations, and is then absorbed through the philic layer to the phobic layer beneath. In this manner, the CO2 can be “wicked” throughout the film, regardless of the film thickness. From these results, it is evident that the anomalous swelling at the maximum density fluctuation of CO2 is caused by the scCO2 absorption into the entire layer of the film along the depth, rather than simply by the excess adsorption of CO2 at the polymer−scCO2 interface as theoretically predicted by Wang and Sanchez.50 Our finding is consistent with the previous report by Li et al. where they also showed that anomalous swelling occurred in the middle layer of the thin films as well as at the interface of polymer/CO2 and polymer/substrate, which could not be simply accounted the excess surface adsorption.34 Herein, we postulate the model for the swelling behavior of the polymer multilayer in the presence of CO2 as illustrated in Figure 6. The thickness of each layer in this multilayer film was

CONCLUSION We investigated the effect of density fluctuation of CO2 in the supercritical region on the swelling ratio of the multilayer polymer thin films consisting of alternate layers of dPS and PMMA. We found that each layer in the multilayer thin film had a similar degree of swelling for pressure and temperature on the density fluctuation ridge regardless of its distance from the surface of the film. This result is different from that obtained in single layer polymer thin films exposed to pressure and temperature on the density fluctuation ridge of scCO2, where the degree of swelling was scaled with the distance from the film surface/Rg. The magnitude of the anomalous dilation therefore decreased rapidly in the interior of the film or for distances larger than 3Rg from the polymer−CO2 interface. This difference is probably due to the fact that the CO2-philic PMMA layer sandwiched between dPS layers can function as a CO2 reservoir. The excess sorption of CO2 in the PMMA layer can allow deeper penetration of CO2 to the next dPS layer. The absorption of CO2 in this reservoir system permits continuous CO2 sorption within the thick multilayer films, which in turn enables the CO2 to improve the miscibility between PS and PMMA even in the bottom layers near substrates. Furthermore, we found that the diffusion dynamics of polymer thin film in scCO2 was not reduced at the polymer−substrate interface, indicating that the interaction between polymers and substrates was not significant in scCO2. Polymer chains can freely move without the interaction near the substrate in the CO2, thereby facilitating anomalous dilation without the pinning effect.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +82 42 868 8436; Fax +82 42 868 4629 (J.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dilip Gersappe at the SUNY Stony Brook for the discussion on mechanism of anomalous swelling behavior of polymer thin films in scCO2. This work was supported primarily from a grant from by the National Research Foundation of Korea under Contract 2012M2A2A6004260.

Figure 6. Schematic representation for anomalous dilation of (a) (dPS/PMMA)2 multilayer and (b) dPS single layer.

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less than 2Rg. Upon the CO2 exposure, the excess CO2 molecules can penetrate into the film and top dPS layer (LdPS2 < 2Rg) can have a uniform sorption of CO2 within this thin layer. The CO2 molecules dissolved within the top dPS layer can be transferred to the PMMA layers due to the PMMA−CO2 interaction. The excess sorption of CO2 in the PMMA layer can allow the uniform swelling with the constant SLD value within this layer, which also facilitates more deeply penetrating of CO2 to the next dPS layer. In other words, the CO2-philic PMMA layer sandwiched in dPS layers can act thus as a CO2 reservoir, resulting in maintaining the sorption of CO2

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