Films Exposed to Supercritical Carbon Dioxide - ACS Publications

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Swelling of Poly(n‑butyl methacrylate) Films Exposed to Supercritical Carbon Dioxide: A Comparative Study with Polystyrene Jayanta Kumar Bal,*,† Thomas Beuvier,‡,§ Guillaume Vignaud,∥ Mohamed Souheib Chebil,‡,∥ Soumaya Ben-Jabrallah,‡ Ikbal Ahmed,† Yves Grohens,∥ and Alain Gibaud*,‡ †

Centre for Research in Nanoscience and Nanotechnology, University of Calcutta, Technology Campus, Block JD2, Sector III, Saltlake City, Kolkata, 700098 India ‡ LUNAM Université, IMMM, Faculté de Sciences, Université du Maine, UMR 6283 CNRS, Le Mans, Cedex 9 72000 France § European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, 38043 Grenoble Cedex 9, France ∥ Laboratoire d’Ingénierie des MATériaux de Bretagne, Centre de Recherche, Rue de Saint Maudé, BP 92116, 56321 Lorient Cedex, France S Supporting Information *

ABSTRACT: We report here the swelling and relaxation properties of confined poly(n-butyl methacrylate) (PBMA) films having thicknesses of less than 70 nm under supercritical carbon dioxide (scCO2) using the X-ray reflectivity technique. Swellability is found to be dominant in thinner films compared to thicker ones as a consequence of the confinement-induced densification of the former. Swellability is proportionately increased with the density of the film. PBMA films exhibit a more significant swelling than do PS films, and their differences become more prominent with the increase in film thickness. A comparison between the results obtained for polystyrene (PS) and PBMA ultrathin films reveals that the swellability is dependent upon the specific intermolecular interaction between CO2 and the chemical groups available in the polymers. Owing to strong Lewis acid−base interactions with scCO2 and the lower glass-transition temperature (bulk Tg ≈ 29 °C), PBMA films exhibit a greater amount of swelling than do PS films (bulk Tg ≈ 100 °C). Though they reach to the different swollen state upon exposition, identical relaxation behavior as a function of aging time is evidenced. This unprecedented behavior can be ascribed to the strong bonding between trapped CO2 and PBMA that probably impedes the release of CO2 molecules from the swollen PBMA films manifested in suppressed relaxation.

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INTRODUCTION Supercritical CO2 (scCO2) has been known for years to be an environmentally friendly solvent and a reaction medium for processing and modifying materials. It is commonly used in the extraction, fractionation, and impregnation of polymers as well as in polymerization and other reactions.1−7 The ability of CO2 to swell polymers, e.g., biocompatible polymers, and to lower their glass-transition temperatures, thereby facilitating the diffusion of small drug molecules into such polymers, has also led to much interest in the utilization of CO2 in drug delivery applications.8−10 In addition, the precipitation of polymer particles from scCO2 and the coating of surfaces with fluoropolymer films deposited from CO2 solutions have also proved to be of considerable practical interest. The affinity of CO2 for a polymer has been studied by various methods in the past, such as Fourier transform infrared (FTIR) spectroscopy, NMR, creep compliance, neutron scattering, ellipsometry, and simple optical boundary observation.11−15 A major effect observed under exposure of a polymer to pressurized CO2 is plasticization.3 The plasticization of © 2016 American Chemical Society

polymers is characterized by an increase in segmental mobility, chain mobility, and interchain distance. The plasticizing effect of CO2 has been suggested to be of the Lewis acid−base type.16 Kazarian et al.16 reported that polymers containing carbonyl groups act as an electron donor and exhibit a specific intermolecular interaction with CO2 acting as an electron acceptor rather than as an electron donor. Johnston et al.17 also suggested that the interaction of CO2 with polymers possessing acrylate groups (containing carbonyl groups) may have a Lewis acid−base nature. Specific interactions were proposed to exist between CO2 and the dipoles of C−F bonds or fluorine to explain the increased solubility of CO2 in fluorine-containing polymers. PS does not possess carbonyl groups and thus does not have strong Lewis acid−base interactions with CO2. Therefore, the solubility of CO2 in a polymer might vary considerably with the Received: December 4, 2015 Revised: January 25, 2016 Published: January 26, 2016 1716

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Figure 1. XRR data (different symbols) and analyzed curves (solid line) of (a) S4.2 nm, (b) S5 nm, (c) S6.6 nm, and (d) S12 nm PBMA films before and after scCO2 exposure (curves are shifted vertically for clarity). Insets: corresponding EDPs. The z scale of all EDPs is kept the same for clarity. Arrows indicate the swelling of the films. Length of the arrows remains the same in order to discriminate the amount of swelling.

addition, we report the relaxation mechanism of superswollen PBMA films. A comparison is made with PS ultrathin films. The possible molecular origins behind their observed differences are discussed here.

nature of the polymer. One of the objectives of this work is to compare the swelling properties of confined polymers with or without CO groups under pressurized CO2 exposure. The swelling is monitored by X-ray reflectivity (XRR) because it is the most suitable technique for measuring the thickness of films less than 200 nm thick with a very high precision (on the order of angstroms) due to the wide range of accessible qz wave vector transfer and the high dynamical range of X-ray instruments.18 It is then possible to determine the swellability of polymer films by measuring the thickness before and after CO 2 exposition, as reported in our previous study.18 Furthermore, quantitative analysis of the XRR data yields the electron density profile (EDP) in the direction normal to the surface of the film. While the effect of CO2 has long been studied extensively in thick films of PS and poly(methyl methacrylate) (PMMA) and more recently in thin films, to the best of our knowledge there is no report on the swelling behavior of ultrathin confined PBMA films under scCO2 in the thickness range of 4 to 65 nm. Recently, Shinkai et al.19 reported higher in situ swelling of very thick (thickness ≈ 150−300 nm) films of PBMA compared to that of PS and PMMA using the ellipsometric technique. A detailed explanation of observed differences for these polymers was not mentioned in their report. Recently, we have shown that confined ultrathin films of PS exhibit some modifications in the conformation of chains which allow the densification of thinner films.20 Furthermore, the density of polymer films was found to govern the swelling in scCO218 and in water vapor21 environments. In this work, we demonstrate the superswelling behavior of PBMA ultrathin films on a Si surface along with their relaxation kinetics by monitoring the film thickness using the XRR technique. A correlation between confinement and swellability is well established. Deviation from the bulk behavior, manifested in the densification with confinement, leads to a higher swellability of polymers confined on the nanoscale. In

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EXPERIMENTAL SECTION

PBMA thin films (Polymer Source Mw = 150k) of different thicknesses (varied from 4 to 65 nm) were prepared by adjusting the concentration of PBMA−toluene solutions. Films were deposited by spin-coating (Karl Suss and APEX spinNXG-M1) these solutions at 2000 rpm for 1 min onto RCA-cleaned Si(100) substrates. In RCA cleaning, the Si surfaces (of size ∼20 × 20 mm2) were made hydrophilic by immersing them in a mixed solution of ammonium hydroxide (NH4OH, Sigma-Aldrich, 25%), hydrogen peroxide (H2O2, Acros Organics, 35%), and Milli-Q water (H2O/NH4OH/H2O2 = 2:1:1 by volume) for 10 min at 100 °C. Then the substrates were dried prior to spin-coating. As a result, a fresh 2 nm silicon oxide layer is formed on the top surface of Si.22 Those prepared samples are labeled by their initial thickness as S4.2 nm, S5 nm, S6.6 nm, S12 nm, S19.2 nm, S29.5 nm, S50.8 nm, and S65.4 nm. The XRR technique was used to characterize the EDPs of the spin-coated films. The samples were then loaded into a pressure cell (Separex) having a small volume of 60 mL. In order to make sure that no impurity or air was present in the cell, we purged the cell two or three times with CO2 gas at ∼5 bar. We subsequently sealed and pressurized the cell with CO2 (Air Liquide, 99.99%) using a manual pressure generator (Separex). The cell was heated to the desired temperature using a jacket in which water was flowing at a fixed and controlled temperature with a precision of 0.1 °C. The pressure was measured with a precision of ±0.6%. Pressure and temperature were kept fixed at 80 bar and 35 °C, respectively. The cell was depressurized within 2 min by opening a valve in the cell after 1 h of exposition of the samples in scCO2. XRR measurements were carried out using a versatile X-ray diffractometer (Empyrean Panalytical) setup to investigate the structure of PBMA films before and after CO2 exposure. The diffractometer was equipped with a Cu source (sealed tube) followed by a W/C mirror to select and enhance the Cu Kα radiation (λ = 1.542 Å). All measurements were carried out in θ−θ geometry for which the sample was kept fixed during the measurements. The 1717

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Figure 2. XRR data (different symbols) and analyzed curves (solid line) of (a) S19.2 nm, (b) S29.5 nm, (c) S50.8 nm, and (d) S65.4 nm PBMA films before and after scCO2 exposure (curves are shifted vertically for clarity). Insets: corresponding EDPs. Arrows indicate the swelling of the films. The z scale of all EDPs and the length of the arrows remain the same in order to discriminate the amount of swelling easily.

Figure 3. (a) Swellability S and (b) absolute swelling ΔH of PBMA and PS films as a function of initial film thickness Hi. Dashed−dotted lines through S and ΔH are the analyzed curves using eqs 1 and 2, respectively. intensity was measured with a Pixel 3D detector using a fixed aperture of three channels (0.165°) in the 2θ direction. Under such conditions at a given angle of incidence θ, a nonvanishing wave vector component qz is given by (4π/λ)sin θ with a resolution of 0.0014 Å−1. The XRR technique essentially provides an EDP, i.e., average electron density (ρ) in plane (x−y) as a function of depth (z). From the EDP, it is possible to estimate the film thickness along with its electron density and interfacial roughness. An analysis of XRR data has been carried out using the matrix technique.23,24 For the analysis, the film was divided into a number of layers including roughness at each interface.

absolute swelling ΔH = Hf − Hi by the thickness of the film before exposure, we obtain the swellability S(%) = (Hf − Hi)/ Hi × 100%. Here, Hi and Hf indicate the thickness of the film before and after CO2 exposure, respectively. It is found that the thicknesses of the S4.2 nm, S5 nm, S6.6 nm, S12 nm, S19.2 nm, S29.5 nm, S50.8 nm, and S65.4 nm films increase to 8.4 nm, 10 nm, 11.7 nm, 18.5 nm, 26.6 nm, 38.5 nm, 64.9 nm, and 81.2 nm, respectively after exposition, yielding swellabilities of S ≈ 100 ± 5%, 100 ± 4%, 77 ± 3%, 54 ± 2%, 38 ± 1%, 30 ± 1%, 28 ± 0.4%, and 24 ± 0.3%, respectively. It is thus straightforward that S decreases and ΔH increases with Hi. In Figure 3a,b, we have respectively plotted S and ΔH for all the films as a function of the initial thickness Hi. The data for PS films, which are taken from our previous study,18 are also shown in this figure. As shown in Figure 3a, the swellability tends toward a constant value designated as bulk swellability Sbulk. On the contrary, the swellability exhibits a hyperbolic increase when Hi tends to 0. One can thus separate the swellability into two terms as

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RESULTS AND DISCUSSION Swelling Behavior. Observed and calculated XRR data of the pre- and postexposed films having different thicknesses are shown in Figures 1 and 2. Kiessig fringes are evident in all of the curves. They display a much shorter period after exposition than before. This observation proves that films exposed to scCO2 exhibit a large amount of swelling which is preserved even after depressurization. The swelling is also obvious in the EDPs shown in the insets of Figures 1 and 2. Note that for clarity the X scale, i.e., depth z of EDPs (shown in the insets of Figures 1 and 2), was kept the same in each figure. Dividing the 1718

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Langmuir Table 1. Parameters before and after scCO2 Exposure Obtained from the Analysis of the X-ray Reflectivity Data amount of material within the film (e−/Å2) films S4.2 S5 S6.6 S12 S19.2 S29.5 S50.8 S65.4

before 16 18 23 43 57 88 176 177

± ± ± ± ± ± ± ±

2 2 2 4 6 9 18 18

after 27 30 35 56 70 105 206 210

± ± ± ± ± ± ± ±

top surface roughness (Å) before

3 3 3 6 7 10 21 21

3.7 3.8 4.5 5.0 5.4 5.3 5.7 5.9

± ± ± ± ± ± ± ±

interface roughness (Å)

after

0.3 0.4 0.3 0.4 0.2 0.3 0.5 0.6

5.5 5.0 5.5 6.5 6.2 6.5 6.3 6.2

± ± ± ± ± ± ± ±

0.3 0.3 0.3 0.4 0.4 0.5 0.3 0.3

before 3.0 4.0 4.0 3.9 3.7 3.2 3.7 3.8

± ± ± ± ± ± ± ±

0.3 0.4 0.4 0.4 0.4 0.3 0.3 0.3

after 4.0 4.8 4.3 4.5 4.5 4.6 4.7 4.2

± ± ± ± ± ± ± ±

0.4 0.4 0.3 0.4 0.3 0.4 0.3 0.4

Figure 4. (a) Initial electron density ρi of PBMA and PS films as a function of initial film thickness Hi. “Initial” means before scCO2 exposure. (b) Variation of swellability S with the normalized density of polymer ρnorm = ρi/ρbulk, which is a function of film thickness because ρi varies with thickness as shown in panel a. S can be scaled with ρnorm by linear curves (dashed−dotted lines) for both PBMA and PS films following eq 3.

S(Hi) = S bulk +

H0 × 100(%) Hi

Hi ≪

(1)

The first term, Sbulk, corresponds to the bulk swellability that governs the relative increment of thickness in thicker films. The second term, H0/Hi, describes the hyperbolic increase of the swellability when the initial thickness Hi of the film becomes close to a specific value H0. An alternative expression of this equation can be obtained by multiplying eq 1 by Hi. This provides an expression of the absolute swelling ΔH = Hf − Hi as a function of the initial film thickness Hi as ΔH(Hi) = H0 + S bulkHi

H0 = Hthreshold S bulk

This threshold value is found to be ∼94 nm for PS and ∼21 nm for PBMA. The big difference between these two values for these two polymers is attributed to their swellabilties. PBMA is much more swellable than PS. The threshold value is also a good indicator of the limit at which a thin film behaves as the bulk. It is common to model a supported PS film (Hi) by three distinct layers consisting of a mobile surface layer (Hitop) at the top of the film hydrodynamically coupled to the bottom layer (Hibottom) in contact with the substrate, with a bulklike layer (Hibulk) in between. By decreasing the film thickness, the middle layer in a bulklike state proportionately decreases, finally vanishing when the thickness approaches the radius of gyration Rg (∼15 nm) of the polymer. This implies that the swelling of ultrathin films is mainly governed by the layer in contact with the substrate and the surface layer. In addition the swellability of these layers is much higher than the swellability of a bulk layer. The increment in thickness ΔH can be expressed in terms of the increment of three sublayers, ΔH(Hi) = ΔHibottom + ΔHitop + ΔHibulk. Assuming that the first two terms, ΔHibottom and ΔHitop, remain constant throughout this thickness range and comparing with eq 2, H0 can be thought of as the sum of the swelling of the bottom and top layers. In connection with the increase in film thickness, we also find that both roughnesses, i.e., top surface and film−substrate interface roughnesses, for all of the films are increased slightly (Table 1). It suggests that the films undergo some morphological changes or changes in the entanglement of the polymer chains associated with swelling due to the strong interaction of scCO2 with PBMA films. A trend in the increment in the amount of material within the film can be seen

(2)

The validity of this equation is shown in Figure 3b, where it is clear that ΔH(Hi) evolves linearly with Hi. Equation 2 shows that H0 is the intercept of this function with the ordinate axis when Hi tends toward zero. Note that Hi = 0 is not physically acceptable in such a model because this would mean that no polymer film would be attached to the substrate. The slope of the linear function directly gives the value of Sbulk. The dashed− dotted lines plotted in Figure 3 are fits to the measurements according to eqs 1 and 2. It is straightforward to extract from these plots the values of H0 which are found to be around 3.8 and 4.7 nm for PBMA and PS, respectively. Their respective bulk swellability Sbulk is on the order of 18% and 5%. These latter values are quite close to the reported in situ bulk swellability values of ∼25% for PBMA and ∼7% for PS at 80 bar by Shinkai et al.19 During the depressurization cycle, a minor relaxation may take place. It is remarkable that eq 2 evidences that the swelling is a cooperative phenomenon involving the entire film except when the initial thickness of the film is small enough to neglect the bulk swelling. This occurs whenever 1719

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Langmuir in Table 1. This implies that some amount of CO2 remains trapped after exposure, although a large error in the measurements is possible. Hence we could not be confident about the quantity of trapped CO2. In addition to measuring the thickness of the film, XRR also allows us to access the electron density. Figure 4a illustrates how the initial electron density ρi varies with the initial film thickness Hi. The density of thinner PBMA films increases up to ∼6% in comparison with the bulk density ρbulk ≈ 0.33 e/Å3. Such densification of ∼10% (shown in Figure 4a) is also encountered in the case of PS as reported in our previous work.18 Notably, these films were not annealed. Annealing will essentially densify the films by compensating for their thicknesses. In the annealed PS film of thickness ∼5 nm grown on oxide-free Si, an increase to 30% was observed by Vignaud et al.20 This was explained in detail by considering the concept of free volume. However, the variation of density with Hi seems to be identical to that of S (shown in Figure 3a). For this reason, we have plotted S as a function of the normalized density ρnorm = ρi/ρbulk with respect to that of the bulk as shown in Figure 4b. ρnorm is calculated by considering ρbulk = 0.33 e/Å3 (for PS and PBMA). Please note that ρnorm is less than 1, which means that the coverage of the as-prepared bulk film is not 100%. This can be attributed to the lack of annealing. The annealing of a thick (∼53 nm) PBMA film at 100 °C for 24 h leads to a higher ρnorm value leaning toward 1 (Supporting Information, Figure S1). Interestingly, S behaves linearly with ρnorm for both polymers in the following way S = a + bρnorm

Figure 5. Swellability S of PBMA films having different thicknesses as a function of log t. Inset: relaxation rate β as a function of initial film thickness Hi of PBMA and PS films.

state and PBMA has CO2-philic carbonyl groups while PS does not. It is well known that PBMA is in a rubbery state under ambient conditions as its bulk Tg is ∼29 °C while PS is in a glassy state as its bulk Tg is ∼100 °C. The presence of flexible aliphatic chains in PBMA (Figure 6) tends to limit the packing of the chains. Because of the rotational motion of these groups, the free volume is likely higher than in PS, and this explains why Tg is lower for PBMA. On the other hand, bulky pendant groups such as a benzene ring of PS can grab the neighboring chains like a “fish hook” and restrict rotational freedom, which raises Tg, contrary to the behavior for PBMA.25 This in turn means that PBMA molecules can vibrate more easily and can exhibit segmental motion in which large portions of the molecule move around. When polymers are in a glassy state, they generally are hard, rigid, and brittle, but in a rubbery state, they are mobile and flexible. In the case of PBMA films, unlike PS, we surprisingly observed that drastic remnant swelling is present in the exposed films even after complete depressurization. This can be attributed to the higher mobility of PBMA chains which favors the superior swelling of films. When the initial thickness is in the strong confinement regime (Hi < 15 nm ≈ Rg, radius of gyration), the interaction is dominated by the substrate−polymer interface, but when the thickness is higher (Hi > 15 nm ≈ Rg), the interaction of the bulk counterpart which is the characteristics of a particular polymer starts contributing. Thus, in this regime the swelling is governed by polymer−polymer interactions. The swelling of PS and PBMA in the confinement regime is nearly similar for both polymers. This suggests that there is no appreciable difference in interfacial interactions. The difference becomes prominent with increasing initial thickness as PBMA chains are more mobile due to the lower Tg. Furthermore, the solubility of CO2 in polymers such as PBMA that contains CO groups is greater than the solubility of CO2 in polymers without CO groups, such as PS. The bond electron density of CO2 is more polarized toward the oxygen atoms in the molecule, leaving a partial positive charge on the carbon atom and negative charges on the oxygen atoms. This makes the carbon atom an electron acceptor in a Lewis acid−Lewis base interaction with groups such as the CO group in the backbone of PBMA which acts as a Lewis bases. There is also evidence from microwave and radio frequency spectroscopy, ab initio calculations, and infrared spectroscopy that CO2 acts as a Lewis acid in the presence of Bronsted and Lewis bases such as water, amines, amides, and basic polymers.26−28 Recently, Teja and co-workers29,30 calculated

(3)

where b is the slope of the curves that governs the densitymediated swelling associated with the confinement of the films. This characteristic appears to be a general phenomenon for all kinds of polymers. Thus, fundamentally it is of paramount interest. Because of the greater amount of swelling as observed from XRR analysis, the linear curve (dashed−dotted line) for PBMA is shifted upward. Relaxation Behavior. As a result of swelling, the films are no longer in the equilibrium state but rather reside in the outof-equilibrium state. Thus, they would be assumed to relax with time provided that they have enough activation energy for this process to take place. The time-dependent relaxation or deswelling of the swollen films was addressed under ambient conditions by measuring the XRR curves over a few days (Supporting Information S2). To quantitatively understand the relaxation behavior, we have plotted S as a function of the logarithm of aging time t for films of different thicknesses under ambient conditions in Figure 5. Linear behavior is encountered where the slope β = dS/d log(t) determines the relaxation rate. It is evident from this figure that there is a difference in the slopes of the different films. However, β values obtained for different films are plotted as a function of initial film thickness Hi in the inset of Figure 5. A slight difference is observed in β values which are dominant in thinner compared to thicker films, which demands that the thinner films try to reach their equilibrium state earlier than the thicker ones. Furthermore, β values of PBMA films are also compared with the PS films. Surprisingly, there is no appreciable difference in β. Interpretation. The enormous difference in the swelling behavior of PBMA and PS is the clear signature that the former has a much stronger affinity for CO2 than does the latter. This is probably related to the facts that PBMA is in a rubbery state at the exposition temperature (35 °C) while PS is in a glassy 1720

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Figure 6. Schematic illustration of chemical structures and swelling mechanism of PBMA and PS films.

of a polymer thin film is established. A significant difference in swellability and absolute swelling, which are dominant in PBMA, is encountered. Lower Tg and the presence of specific interactions between CO2 and the carbonyl groups of PBMA, unlike PS, allow a large amount of mobility of polymer chains in scCO2, which results in highly swollen films. Owing to the strong affinity of the CO group with CO2, it is feasible to consider that some of the CO2 molecules remain trapped within the polymer chains even after complete depressurization that slows down the relaxation of the swollen films.

the enthalpy (ΔHa) for the Lewis acid−base interaction of CO2 with the PBMA CO group from the shift of FTIR spectra upon CO2 exposure. This yields ΔHa = −8.3 kJ/mol. In contrast to PBMA, PS does not show any shift in FTIR spectra that corresponds to the absence of the acid−base interaction.11,16 The same logic was used to explain the higher swelling of PMMA compared to that of PS.31 Notably, they have comparable Tg values. On the other hand, though PBMA and PMMA contain a CO group, the former exhibits higher swelling than the latter.15 These indicate that both parameters, such as Tg and chemical bonds, are equally important in the context of swelling. However, it is difficult to understand the good swelling behavior of PS, although it is less than for PBMA, under scCO2. Raveendran et al.32 have suggested that the oxygen atoms in CO2, having partial negative charges, could also exhibit weak electrostatic interactions with properly placed electron-deficient C−H bonds of the π system (phenyl ring) in PS via cooperative C−H···O bonds that help stabilize the Lewis acid−base complex. These observations suggest that the Lewis acid character of CO2 probably leads to a significant interaction between the carbon atom in CO2 and the CO oxygen atom. Consequently, some of the CO2 molecules may remain trapped within the PBMA polymer chains (as schematically illustrated in Figure 6), perhaps inhibiting the relaxation behavior. It is found that the swelling of a polymer film has a direct consequence on its own electron density. Furthermore, in the present study and in previous studies18,20 we have mentioned that the confinement effect could change the conformation of the chains, which results in densified thinner films. Such a confined denser film could be considered to be a system with polymer chains that are more closed-packed than in the thicker films. Upon scCO2 exposure, the chains would expand like a suppressed spring that releases its stress. Hence, a larger amount of swelling is observed in thinner films than in thicker ones.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04436. XRR and EDP of as-prepared and annealed PBMA films. Relaxation of swollen PBMA films under ambient conditions. Time-evolution of XRR and EDP of swollen PBMA films under ambient conditions. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thankfully acknowledge the CEFIPRA/IFCPAR program for financial support. J.K.B. thankfully acknowledges the Department of Science and Technology (DST), Government of India, for providing a research grant through an INSPIRE Faculty Award (IFA13-PH-79).

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CONCLUSIONS We have carried out XRR measurements to investigate the swelling and relaxation characteristics of PBMA films having different thicknesses exposed to scCO2 and compared with PS. In both cases, thinner films exhibit a large swellability in opposition to thicker films due to densification of the former. A general relation among the density, thickness, and swellability

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