Nonsolvents Cause Swelling at the Interface with Poly(methyl

Dec 4, 2007 - Department of Applied Chemistry, Kyushu UniVersity, Fukuoka 819-0395, ... the dPMMA film was composed of a swollen layer and the interio...
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Langmuir 2008, 24, 296-301

Nonsolvents Cause Swelling at the Interface with Poly(methyl methacrylate) Films Keiji Tanaka,*,† Yoshihisa Fujii,† Hironori Atarashi,† Kei-ichi Akabori,† Masahiro Hino,‡ and Toshihiko Nagamura† Department of Applied Chemistry, Kyushu UniVersity, Fukuoka 819-0395, Japan, and Research Reactor Institute, Kyoto UniVersity, Osaka 590-0494, Japan ReceiVed July 16, 2007. In Final Form: October 1, 2007 Density profiles of a perdeuterated poly(methyl methacrylate) (dPMMA) film spin-coated on a substrate in water, hexane, and methanol, which are “nonsolvents” for dPMMA, were examined along the direction normal to the interface by specular neutron reflectivity (NR). The interfaces of dPMMA with the liquids were diffuse in comparison with the pristine interface with air; the interfacial width with water was thicker than that with hexane. Interestingly, in water, the dPMMA film was composed of a swollen layer and the interior region, which also contained water, in addition to the diffused layer. The interface of dPMMA with hexane was sharper than that with water. Although there were slight indications of a swollen layer for the dPMMA in hexane, the solvent molecules did not penetrate significantly into the film. On the other hand, in methanol, the whole region of the dPMMA film was strikingly swollen. To conserve mass, the swelling of the film by the nonsolvents is accompanied by an increase in the film thickness. The change in the film thickness estimated by NR was in excellent accord with the results of direct observations using atomic force microscopy (AFM). The modulus of dPMMA in the vicinity of the interfaces with liquids was also examined on the basis of force-distance curves measured by AFM. The modulus decreased closer to the outermost region of the film. The extent to which the modulus decreased in the interfacial region was consistent with the amount of liquid sorbed into the film.

1. Introduction In the future, the quantity of polymeric materials used for medical diagnosis and treatment will continue to increase.1 New tools for tailor-made diagnostics, such as DNA arrays and tips for micro-total-analysis systems, are generally made from polymers.2 Ultimately, we can expect that polymers will be buried in the human body as a part of organs and in-situ diagnostic or treatment equipment.3 In these applications, the polymer surface is in contact with a water phase. However, despite the importance of detailed knowledge of the fundamental interactions of polymer interfaces with liquids, such studies are very limited. Here, we describe the density profiles at the polymer/liquid interfaces with typical nonsolvents, including water. We find that the interface is diffuse, and anomalous swelling occurs beneath the interface. Consequently, the mechanical stiffness of the polymer is drastically altered, even at the nonsolvent interfaces. Poly(methyl methacrylate) (PMMA) is widely used in many technological applications because of its excellent mechanical, optical, and surface properties. In ophthalmology, for instance, PMMA is an important component in artificial lenses.4 As an initial benchmark for designing and constructing specialized biomedical surfaces containing this polymer, the adsorption behaviors of lipids and proteins onto PMMA in water should be systematically examined. Nevertheless, only a few reports dealing * To whom correspondence should be addressed. Phone: +81-92-8022879. Fax : +81-92-802-2880. E-mail: [email protected]. † Kyushu University. E-mail: [email protected] (Y.F.); [email protected] (H.A.); [email protected] (K.-i.A.); [email protected] (T.N.). ‡ Kyoto University. E-mail: [email protected]. (1) Castner, D. G.; Ratner, B. D. Surf. Sci. 2002, 500, 28. (2) Sun, Y.; Kwok, Y. C. Anal. Chim. Acta 2006, 556, 80. (3) Jagur-Grodzinski, J. Polym. AdV. Technol. 2006, 17, 395. (4) Trivedi, R. H.; Werner, L.; Apple, D. J.; Pandey, S. K.; Izak, A. M. Eye 2002, 16, 217.

with the issue have been published.5-7 A critical difference between other methacrylates, such as poly(2-hydroxyethyl methacrylate),8-10 poly(2-methoxyethyl acrylate)11-13 and poly(2-methacryloyloxyethyl phosphorylcholine),14-16 and PMMA is that the former can be dissolved, or at least swollen, with water on a macroscopic scale, whereas the latter cannot. Nevertheless, PMMA would be suitable as part of a functionalized biomedical surface if chains of PMMA at the water interface are dissolved into the bulk liquid. So far, several authors have studied the aggregation states of polymer brushes at the liquid interface.17-21 However, the interfacial aggregation states of liquids and polymers spin-coated (5) Gilchrist, V. A.; Lu, J. R.; Staples, E.; Garrett, P.; Penhold, J. Langmuir 1999, 15, 250. (6) Howse, J. R.; Steitz, R.; Pannek, M.; Simon, P.; Schubert, D. W.; Findenegg, G. H. Phys. Chem. Chem. Phys. 2001, 3, 4044. (7) El Khadali, F.; Helary, G.; Pavon-Djavid, G.; Migonney, V. Biomacromolecules 2002, 3, 51. (8) Wichterle, O.; Lim, D. Nature 1960, 185, 117. (9) Montheard, J. P.; Chatzopoulos, M.; Chappard, D. J. Macromol. Sci., ReV. Macromol. Chem. Phys. 1992, C32, 1. (10) Bajpai, A. K.; Kankane, S. J. Appl. Polym. Sci. 2007, 104, 1559. (11) Tanaka, M.; Motomura, T.; Kawada, M.; Anzai, T.; Kasori, Y.; Shiroya, T.; Shimura, K.; Onishi, M.; Mochizuki, A. Biomaterials 2000, 21, 1471. (12) Saito, N.; Motoyama, S.; Sawamoto, J. Artif. Organs 2000, 24, 547. (13) Gunaydin, S.; Farsak, B.; Kocakulak, M.; Sari, T.; Yorgancioglu, C.; Zorlutuna, Y. Ann. Thorac. Surg. 2002, 74, 819. (14) Ishihara, K.; Ziats, N. P.; Tierney, B. P.; Nakabayashi, N. J. Biomed. Mater. Res. 1991, 25, 1397. (15) Yoneyama, T.; Ishihara, K.; Nakabayashi, N.; Ito, M.; Mishima, Y. J. Biomed. Mater. Res. 1998, 43, 15. (16) Lewis, A. L.; Hughes, P. D.; Kirkwood, L. C.; Leppard, S. W.; Redman, R. P.; Tolhurst, L. A.; Stratford, P. W. Biomaterials 2000, 21, 1847. (17) Marzolin, C.; Auroy, P.; Deruelle, M.; Folkers, J. P.; Leger, L.; Menelle, A. Macromolecules 2001, 34, 8694. (18) Czeslik, C.; Jackler, G.; Hazlett, T.; Gratton, E.; Steitz, R.; Wittemann, A.; Ballauff, M. Phys. Chem. Chem. Phys. 2004, 6, 5557. (19) Huang, H. Q.; Penn, L. S. Macromolecules 2005, 38, 4837. (20) Mei, Y.; Wu, T.; Xu, C.; Langenbach, K. J.; Elliott, J. T.; Vogt, B. D.; Beers, K. L.; Amis, E. J.; Washburn, N. R. Langmuir 2005, 21, 12309. (21) Sanjuan, S.; Perrin, P.; Pantoustier, N.; Tran, Y. Langmuir 2007, 23, 5769.

10.1021/la702132t CCC: $40.75 © 2008 American Chemical Society Published on Web 12/04/2007

NonsolVent/Poly(methyl methacrylate) Interfaces

on substrates have not yet been probed. Also, the mechanical properties of polymers in liquids generally differ from the original bulk values if a propensity exists for the polymer and the liquid to combine.22-24 In such cases, the liquid molecules are simply sorbed into the polymer, so changes in these properties should depend on the actual extent to which the liquid molecules are sorbed. Although the amount of the sorbed liquid might be controlled by changing the molecular structure of the polymers, such a change will also affect the polymers’ bulk mechanical properties. With the substitution of a different type of liquid, the amount of liquid molecules sorbed into the polymers would be drastically altered. To elicit the most information regarding these issues, we believe that the mechanical properties of polymers in liquids should be discussed in conjunction with density profiles of the liquid/polymer interfaces. All these considerations have led us to examine, in this study, the density profiles and mechanical stiffness of PMMA at water, hexane, and methanol interfaces. 2. Experimental Section As a material, monodisperse perdeuterated PMMA (dPMMA) with a number-average molecular weight (Mn) of 296 kg/mol was purchased from Polymer Source Inc. A quartz block with the size of 60 × 60 × 8 mm was used as a substrate for neutron reflectivity (NR) measurements. The substrate was first immersed in pure toluene for a week. Then, it was ultrasonicated and was well-rinsed just prior to the film preparation. A film of dPMMA for NR was prepared from a toluene solution, spin-coated onto the quartz block, and annealed under vacuum for 24 h at 423 K. The film thickness, evaluated by ellipsometry, was 67.7 ( 0.5 nm. Atomic force microscopy (AFM, SPA300HV, SII NanoTechnology Inc., fitted with an SPI3800 controller) revealed that no pinholes existed at the film surface and that the root-mean-square roughness was 0.22 nm. In addition, X-ray photoelectron spectroscopy (XPS) revealed that the intensity ratio of neutral, ether, and carboxyl carbons for the dPMMA film was 3:1:1, a result meaning that the film surface was not contaminated by organic materials. When a density profile of a PMMA film is examined along the direction normal to the surface only in air, X-ray reflectivity should be good enough.25,26 However, the objective of this study is to study density profiles of PMMA in contact with liquids. Thus, NR was applied to the dPMMA films in liquids27,28 using the multilayer interferometer for neutrons (C3-1-2-2, MINE)29 at the Institute for Solid-State Physics, the University of Tokyo. A Teflon reservoir filled with a liquid was mounted onto the film. Purified and degassed water and spectroscopic grade hexane and methanol were used as liquids. The contact angles of water, hexane, and methanol were 73.2 ( 0.5, 10.0 ( 0.4, and 4.8 ( 0.5°, respectively. Prior to the measurement, the dPMMA film was aged in liquids for 2 h, which was apparently enough to cause swelling. Incident neutrons with a wavelength (λ) of 0.88 nm and a resolution of 5.1% were guided into the specimen from the quartz side, which was vertically mounted on a goniometer. All measurements were made using a single dPMMA film with an initial thickness of 67.7 ( 0.5 nm. Once the measurement in a liquid was completed, the film was taken away from the setup; then, it was well-dried and again annealed for 24 h at 423 K. At that point, the next measurement was (22) Shen, J.; Chen, C. C.; Sauer, J. A. Polymer 1985, 26, 511. (23) Hill, R. G.; Bates, J. F.; Lewis, T. T.; Rees, N. J. Mater. Sci. 1984, 19, 1904. (24) Kawagoe, M.; Nakanishi, M.; Qui, J.; Morita, M. Polymer 1997, 38, 5969. (25) van der Lee, A.; Hamon, L.; Holl, Y.; Grohens, Y. Langmuir 2001, 17, 7664. (26) Xu, T.; Goldbach, J. T.; Misner, M. J.; Kim, S.; Gibaud, A.; Gang, O.; Ocko, B.; Guarini, K. W.; Black, C. T.; Hawker, C. J.; Russell, T. P. Macromolecules 2004, 37, 2972. (27) Russell, T. P. Mater. Sci. Rep. 1990, 5, 171. (28) Wu, W. L.; Orts, W. J.; Van Zanten, J. H.; Fanconi, B. M. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 2475. (29) Ebisawa, T.; Tasaki, S.; Otake, Y.; Funahashi, H.; Soyama, K.; Torikai, N.; Matushita, Y. Physica B 1995, 213, 901.

Langmuir, Vol. 24, No. 1, 2008 297 Table 1. (b/V) Values for SiO2, dPMMA, and Liquids Used in This Study (b/V) × 104/nm-2 SiO2 dPMMA water

3.48 6.62 -0.561

(b/V) × 104/nm-2 hexane methanol

-0.575 -0.374

made. The liquids were tested in the following order: first water, next hexane, and finally methanol. The AFM and ellipsometric measurements revealed that the surface morphology and thickness of the dPMMA film had completely returned to their initial states after the above-mentioned procedure, except when the film was soaked in methanol. Thus, further experiments with a different liquid were abandoned after the methanol treatment. The reflectivity was calculated on the basis of the scattering length density (b/V) profile along the depth direction by means of Parratt32 software, which is a freeware program from the Hahn-Meitner Institute (HMI).30 The (b/V) values of SiO2, dPMMA, and liquids used for the calculation are collected in Table 1. The changes of the film thickness in liquids were independently examined by AFM. In this case, the dPMMA films used had been coated on silicon wafers with a native oxide layer, which should be chemically the same as the surface of the quartz block. This step was taken because the quartz block was too thick for our AFM setup. AFM observations of a height of the step produced in a part of the film cut by a blade showed that the initial thickness was in the range of 22-114 nm. The mechanical properties of the film in the vicinity of the film/ liquid interface were measured on the basis of force-distance curves derived from AFM observations.31-33 Cantilevers with spring constants (kc) of 26.4 ( 2.8 and 1.11 ( 0.22 N‚m-1 were used. The softer lever was applied only to the dPMMA film in methanol. The spring constants were calibrated following the method outlined by Butt et al.34 The tip radii (R) were estimated to be 41.6 ( 8.8 and 19.2 ( 2.7 nm, respectively, values determined by a spherical fit (SPIP, Image Metrology A/S).35 The approach-retract speed for a tip was set at 50 nm‚s-1. The data were averaged over 10 forcedistance curves at 10 randomly selected locations.

3. Results and Discussion 3.1. General Features of the NR Data. Panel a of Figure 1 shows the scattering vector, q [)(4π/λ)sin θ], dependence of NR of a dPMMA film contacting air, water, hexane and methanol phases. For clarity, each data set for the dPMMA film in liquids is off-set by a decade. Since just one film was used for all measurements, the four NR curves can be directly compared. As a general trend, once q exceeded a critical value, the reflectivity started to decrease. Then, it periodically rose and fell with increasing q, because of the interference between the reflected and refracted beams at the liquid (or air)/dPMMA and the dPMMA/quartz interfaces. These undulations in the reflectivity are the so-called Kiessig fringes. The NR curve for the dPMMA film in water showed two differences from that in air. The fringes were less visible in water than in air, especially with increasing q value. This result means that the water/dPMMA interface is less sharp than that of air/ dPMMA. In addition, in water, the Kiessig fringes shifted to the higher q, implying that the dPMMA film became thicker in water than in air, probably due to the swelling caused by the sorption of water molecules. However, the fringe positions returned to (30) http://www.hmi.de/bensc/instrumentation/instrumente/v6/refl/parratt_en.htm. (31) Chizhik, S. A.; Huang, Z.; Gorbunov, V. V.; Myshkin, N. K.; Tsukruk, V. V. Langmuir 1998, 14, 2606. (32) Domke, J.; Radmacher, M. Langmuir 1998, 14, 3320. (33) Sun, Y.; Akhremitchev, B.; Walker, G. C. Langmuir 2004, 20, 5837. (34) Butt, H. J.; Cappella, B.; Kappl, M. Surf. Sci. Rep. 2005, 59, 1. (35) Villarrubia, J. S. J. Res. Natl. Inst. Stand. Technol. 1997, 102, 425.

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Figure 2. Schematic illustration of the model used for a dPMMA film in a liquid.

Figure 1. (a) Neutron reflectivity for a dPMMA film in air, water, hexane, and methanol. Open symbols depict experimental data, and solid lines are reflectivity calculated on the basis of the scattering length density profiles shown in b. For clarity, each data set in liquids is off-set by a decade.

their initial locations at q > 0.75 nm-1. If the dPMMA film had been homogeneously swollen along the direction normal to the interface, the fringe positions should have simply shifted to higher q values. Hence, it is conceivable that the film's internal swollen structure was more complex. This issue is discussed at a later point. In contrast, the Kiessig fringes for dPMMA in hexane, which, like water, is a nonsolvent for PMMA, seem to be the same as those in air. Thus, it is likely that an indistinct interface and discernible swelling may not be universal characteristics of the interface between dPMMA and a nonsolvent. Interestingly, the NR curve for the dPMMA film in methanol was totally different from others. The three main differences are the following. First, the critical q position shifted to the lower side, a finding indicating that the scattering length density of the film was reduced. Second, the fringe period became much smaller than those in other cases. This is a sign that the film became thicker. Finally, the fringes became less visible in a high-q region. A striking, methanolinduced swelling of the dPMMA film might account for all of these results.

Figure 3. (a) Neutron reflectivity for the dPMMA film in water. Open symbols depict experimental data; broken and solid lines are calculated reflectivity on the basis of scattering length density profiles with and without a swollen layer shown in panel b.

3.2. NR Fitting Results. To discuss the density profile normal to the interface in quantitative terms, we attempted to fit the experimental NR data. Panel b of Figure 1 shows the model scattering length density (b/V) profiles used to obtain the best-fit reflectivity, and Table 2 summarizes the fitting parameters. The solid curves drawn in panel a of Figure 1, represent the best fits to the data, calculated on the basis of a layer model illustrated in Figure 2. In the latter, σ1, σ2, and σ3 are the standard deviations in Gaussian roughness for, respectively, the liquid (air)/dPMMA interface, the density depletion layer, and the quartz/dPMMA interface, while t denotes the total thickness of the film. In the case of the dPMMA in methanol, the methanol molecules were preferentially segregated at the substrate interface with the decay length (ξ), resulting in a decrease in scattering length density at that location. The decay length was defined as the distance at which the (b/V) at the substrate interface decreased by a factor of 1/e. Since the calculated and experimental curves were in good accordance, the model (b/V) profiles in Figure 1b appear

Table 2. Parameters Used to Fit Obtained Reflectivity Curves environment

t/nm

σ1/nm

air water hexane methanol

67.8 69.7 68.4 94.0

0.21 1.66 0.84 2.17

σ2/nm

σ3/nm

6.95 6.08

0.312 0.312 0.312

ξ/nm

χ2

t/tair,NR

t/tair,AFM

5.13

1.2 × 10-2 2.1 × 10-2 4.8 × 10-3 3.0 × 10-2

1.00 1.03 1.01 1.39

1.00 ( 0.01 1.03 ( 0.01 1.01 ( 0.01 1.40 ( 0.08

NonsolVent/Poly(methyl methacrylate) Interfaces

to accurately reflect the real composition changes along the direction normal to the interface. The σ1 for the dPMMA in air was 0.21 nm and was comparable to the AFM result that the dPMMA surface was quite flat with the root-mean-square roughness of 0.22 nm. The σ3 for the substrate interface was 0.312 nm, and this number was similarly used for the following cases with water and hexane. The interface between dPMMA and water was more diffuse than the comparable one between dPMMA and air, as shown in the panel b, and its σ1 value was 1.66 nm. Also, it is noteworthy that there exists a density depletion layer with σ2 ) 6.95 nm beneath the interface in this case. This seems to infer that the formation of a swollen layer in the vicinity of the water interface is induced by solvent molecules. The swollen layer did not evolve further even when the film was kept in water for up to 3 days. Furthermore, it is noteworthy that the (b/V) value in the interior region of the dPMMA film in water was lower than that of the bulk dPMMA, 6.62 × 10-4 nm-2, indicating that water molecules deeply penetrated into the film. The overall water content of the entire film was 3.4 vol %, a value close to the reported value of 2.3 vol %,36,37 which was estimated by weight change using a thick PMMA film. The water content in the interior region, namely, the constant density region, of the film was only 1.6 vol %. When the fitting process was forced to use a higher water content, the calculated reflectivity became poorer, being larger than the experimental data in the middle q regions and smaller at the higher regions. Since the dPMMA film contained water molecules after being immersed in the solvent, it became slightly thicker than before, the thickness increasing from 67.8 to 69.7 nm. If the film is in fact swollen by solvent molecules, the thickness must be increased to conserve the mass of dPMMA. Indeed, to obtain the best-fit curve, an increase in the film thickness by 3.4% was necessary. This is quite consistent with the mass balance of dPMMA. In addition, the thickness of the film was examined in water by AFM and was found to have increased from the original value by 3 ( 1%. Further discussion of the plausibility of a water-induced swollen layer of the dPMMA film is presented later. The interface between dPMMA and hexane was slightly broader than that between dPMMA and air, with a σ1 value of 0.84 nm. Interestingly, in the case of hexane, a density depletion layer with σ2 ) 6.08 nm had to be included to obtain a good fit, although the extent of swelling was trivial. The most remarkable case was with methanol. In this case, methanol molecules completely penetrated into the dPMMA film without forming a swollen layer, resulting in a constant (b/V) value that was smaller than that for the neat dPMMA. The volume fraction of methanol in the film was calculated on the basis of the integral of the (b/V) difference between observed and neat dPMMA profiles. The value so obtained was 37 vol % and was consistent with the reported value of 29 vol % for bulk PMMA.24 Thus, the film became thicker in methanol than air, its thickness increasing from 67.8 to 94.0 nm. The σ1 value for the dPMMA/methanol interface was 2.17 nm, which was the largest among the solvents employed. The most striking feature is that methanol was preferentially segregated at the substrate interface, a phenomenon not seen for dPMMA with water. The profile was well-represented by a single exponential with the characteristic length scale of ξ, 5.13 nm. 3.3. Swollen Layers Induced by Water. Interestingly, in water, the (b/V) reached a constant via a density depletion layer (36) Smith, L. S. A.; Schmitz, V. Polymer 1988, 29, 1871. (37) Ishiyama, C.; Higo, Y. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 460.

Langmuir, Vol. 24, No. 1, 2008 299 Table 3. Parameters for Models with, and without, a Swollen Layer to Fit the Neutron Reflectivity for the dPMMA Film in Water model

t/nm

σ1/nm

σ2/nm

σ3/nm

χ2

with swollen layer without swollen layer

69.7 64.8

1.66 0.95

6.95

0.312 0.312

2.1 × 10-2 5.6 × 10-2

with σ2 ) 6.9 nm, as shown in panel b of Figure 1. Here, we discuss the plausibility of the depletion layer, being a swollen layer. Figure 3a shows the NR profile for the dPMMA film in water. The dotted and solid curves in the figure depict the best fitting reflectivity with and without the swollen layer, respectively. Panel b illustrates the corresponding model scattering length density profiles, and Table 3 collects the parameters used for the fitting. The reflectivity curve calculated without the swollen layer deviated from the experimental data in the q range from 0.6 to 0.8. This leads to a χ2 value that was larger without the swollen layer than with it. It was possible to force a good fit to experiment without the swollen layer but only if the model used a b/V value of 7.30 × 10-4 nm-2 in the interior region. This is a value much larger than the corresponding value of dPMMA in the interior region, 6.62 × 10-4 nm-2, and thus, was unrealistic. Hence, it is plausible to include the presence of the swollen layer beneath the liquid interface. The role of the swollen layer on the mechanical properties of dPMMA in the interfacial region is discussed later. 3.4. Film Thickness. The NR fitting process indicated that it was most likely that the dPMMA film became thicker in liquids than in air due to liquid penetration. We tried to confirm this result using an AFM operating in the liquids. The t/tair in Table 2 is the thickness in a liquid normalized by the thickness in air and thus corresponds to the change in the film thickness in a liquid. The subscript of NR or AFM denotes how the value was obtained. The t/tair,AFM values were in good accordance with t/tair,NR. This gives us confidence that the fitting parameters used for the neutron reflectivity fitting in liquids were reasonable. We now turn to a visualization of the process by which the dPMMA film became thicker in liquids than in air. Figure 4 shows topographic images of the dPMMA film, in which a part of the film was cut by a blade, before and after the liquid immersion. The brighter and darker regions correspond to the film and the Si substrate, respectively. The observation area was the same before and after the liquid immersion. In the case of water and hexane immersion, the increase in thickness was quite small. Thus, thinner films were used so that the liquid-induced thickening behavior could be better seen. Comparing panels a and b, it was clear that the dPMMA film was thicker after the water immersion, its thickness increasing from 22.6 to 24.1 nm. Moreover, the extent by which the film thickened was more pronounced for this thinner film than for the 67.8 nm thick dPMMA film. Later, we shall explain this observation in terms of the swollen layer. Similarly, a thin dPMMA film was slightly thickened after immersion in hexane. On the other hand, even for a thick (66.6 nm) film of dPMMA film, methanol-induced film thickening was clearly observed, as shown in panels e and f. We here compare our thickness data with reported results. So far, the density profile of PMMA in deuterated water has been examined by two independent groups using NR.5,6 However, neither group observed the presence of the swollen layer seen here. Their experiments and ours differ in the deuterated species and film thickness. While they used deuterated water to confer the contrast between PMMA and water, we used deuterated PMMA. To examine the role of a different degree of hydrogen bonding in the two systems, we examined by AFM the thickness change of a corresponding PMMA film immersed in deuterated water. We found essentially no difference in behavior from

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Figure 4. Topographical images of dPMMA films in part scratched by a blade: (a and b) before and after water immersion; (c and d) before and after hexane immersion; (e and f) before and after methanol immersion.

Figure 5. Water-induced thickening for dPMMA films as a function of original film thickness.

dPMMA and water and therefore feel that we can discount the effect of the difference in deuterated species in this case, although isotopic labeling might induce changes to physical properties of polymers.38-41 The thicknesses of the PMMA films used by Lu and co-workers and Steitz et al. were 5.2 and 2.1 nm,5,6 respectively, being much thinner than ours. And, the overall water contents in their films were reported to be 5 and 8.4 vol %,5,6 respectively. Taking into account that the water content for our dPMMA was only 3.4 vol %, it is reasonable to assert that thinner films can apparently take up relatively more water. As the film becomes thinner, we envisage that the swollen layer merges into the interior region. In other words, for thinner films such as those used by Lu and co-workers and Steitz et al., the swollen layer becomes part of the interior region, where the water content is higher. To confirm this explanation, we plotted the variation of the t/tair,AFM (38) Bates, F. S.; Wignall, G. D. Phys. ReV. Lett. 1986, 57, 1429. (39) Jones, R. A. L.; Kramer, E. J.; Rafailovich, M. H.; Sokolov, J.; Schwarz, S. A. Phys. ReV. Lett. 1989, 62, 280. (40) Esker, A. R.; Grull, H.; Wegner, G.; Satija, S. K.; Han, C. C. Langmuir 2001, 17, 4688. (41) Tanaka, K.; Kajiyama, T.; Takahara, A.; Tasaki, S. Macromolecules 2002, 35, 4702.

value with decreasing film thickness in Figure 5. The values of t/tair increased with decreasing tair, indicating a corresponding increase in the water content. The solid line predicts the tair dependence of t/tair under the assumption that the swollen layer remains beneath the water interface, as shown in panel b of Figure 1, and the thickness of the constant-density interior region simply decreases without changing the structure of the swollen layer. The trend obtained from our experiments was similar to the prediction based on the presence of the swollen layer, and qualitatively agreed with the results published by Lu and coworkers and Steitz et al. 3.5. Interfacial Broadening. We have earlier presented evidence for the broadening of the interface in dPMMA/liquid compared to that of dPMMA/air, as shown in Table 2. There are four possible reasons that could explain the interfacial broadening in liquids. First, if the dPMMA is contaminated by small organic molecules, they will be preferentially segregated to the outermost region of the film,42 resulting in an apparent interfacial broadening. However, if that were the case, such a similar contaminated layer should also be present in air, so that there would be no additional broadening in liquid if contamination were the only source of broadening. In addition, the extent of the interfacial broadening increased in the order of hexane, water, and methanol. It seems very unlikely that, if a contamination layer at the interface was responsible for the interfacial broadening, the thickness would then depend on the nature of the overlying liquid in the abovementioned order. Furthermore, XPS revealed that the surface of the dPMMA film, which had been dried after taking it out from the liquid cell, was not contaminated. For all these reasons, we can discount the possibility of interfacial contamination being the source of interfacial broadening. The second possibility is that nanobubbles were present on the surface of the dPMMA film in liquids. When a hydrophobic surface is immersed in a hydrophilic liquid, it may be easy to form nanobubbles at the interface. Although nanobubbles have been observed for polystyrene (PS)43 and n-octadecyltrichlo(42) Seo, Y. S.; Satija, S. Langmuir 2006, 22, 7113.

NonsolVent/Poly(methyl methacrylate) Interfaces

Figure 6. Depth dependence of Young's modulus for PMMA near the liquid interface. The origin of the distance axis marks the location of the liquid (or air)/PMMA interface.

rosilane monolayers44 in water, it seems unlikely that this is the case for a hydrophilic dPMMA film.45 Also, the surface energy difference between dPMMA and liquid does not follow the order of hexane, water, and methanol. Finally, the signature of nanobubbles at the interface was not confirmed by AFM observations unlike those reported for PS films.43 Therefore, we can exclude the possibility that nanobubbles lower the (b/V) value in the interfacial region. A third possible mechanism of interfacial broadening is that the dPMMA surface became rougher after immersing it into liquids. If the surface is infinitely flat, the reflectivity before reaching the critical q is identically unity. The reflectivity in the total reflection region in water, hexane, and methanol were actually 0.90, 0.97, and 0.82, respectively. In other words, to the extent that the surface became rougher was in the order of hexane, water, and methanol. This ordering is consistent with the extent of swelling for the dPMMA film in the liquids. However, the formation of discernible holes and islands was not observed at the interface by AFM. Having dismissed the possibilities that contamination, nanobubbles, or roughening are responsible for interfacial broadening, we are left with the likelihood that dPMMA segments being at the outermost region of the film were dissolved into the liquid phase, resulting in a broader alteration of (b/V) in the interfacial region. For the moment, we do not have any experimental evidence to reject this hypothesis. 3.6. Swelling and Mechanical Properties. We have presented evidence that the dPMMA/liquid interface is more diffuse than that at dPMMA/air and that a swollen layer existed underneath the diffuse interface in the cases of water and hexane. Thus, it is probable that the mechanical properties of dPMMA near the liquid interfaces are consequently altered. Figure 6 shows the depth dependence of the Young's modulus (E) for PMMA in dried nitrogen gas, water, hexane, and methanol. E was calculated by the following equation based on force-distance curves measured by AFM.

kc dt 3 E(d) ) (1 - ν2) 1/2 3/2 4 R d (43) Steitz, R.; Gutberlet, T.; Hauss, T.; Klo¨sgen, B.; Krastev, R.; Schemmel, S.; Simonsen, A. C.; Findenegg, G. H. Langmuir 2003, 19, 2409. (44) Zhang, X. H.; Maeda, N.; Carig, V. S. J. Langmuir 2006, 22, 5025. (45) Agrawal, A.; Park, J.; Ryu, D. Y.; Hammond, P. T.; Russell, T. P.; McKinley, G. H. Nano Lett. 2005, 5, 1751.

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where ν, dt, and d are Poisson’s ratio, the deflection of a cantilever, and the indentation depth of a tip, respectively. Although the Poisson’s ratio should be dependent on the indentation of a tip, especially for water and hexane, it is almost impossible, at present, to estimate the depth dependence of the value experimentally. Thus, the ratio was fixed for the analysis: 0.33 for nitrogen gas, water and hexane and 0.41 for methanol.24 This assumption is probably reasonable because the modulus in the internal region, where the modulus is constant, is consistent with the bulk value. Since the measurement was quite sensitive to the ambient humidity, dried nitrogen gas was used as a substitute for air. As a general trend, E decreased closer to the interface. This was the case even for the film in the dried nitrogen. The differences of E among the three solvents in the vicinity of the liquid interface reflect the extent to which liquid molecules were sorbed. The interfacial layer in which E was lower was thicker in water than in hexane and methanol, in good accordance with the density profiles observed by NR. While the E values at more than 10 nm depth in nitrogen and hexane were 2.6 ( 0.3 and 2.5 ( 0.5 GPa, respectively, the corresponding bulk E in water decreased to 2.2 ( 0.5 GPa. This value matches the value of 2.25 GPa for the E of bulk PMMA containing water with an overall concentration of 2.3 vol %.37 The E value in a deeper region of the PMMA film swollen by methanol exhibited a drastically lower value of 0.22 ( 0.01 GPa. The methanol content in the interior region of the PMMA used here was 32 vol %. We can compare the value of the modulus we obtained with the Young's modulus of a comparable bulk PMMA containing 29 vol % methanol that was reported to be 0.5 GPa.24 It is clear that the interfacial mechanical properties of the polymers were dependent on which liquid was contacted and the extent to which liquid molecules were sorbed.

4. Conclusions Density profiles of a dPMMA film in water, hexane, and methanol were studied by NR. Although these liquids are typical nonsolvents for dPMMA, the liquid/polymer interfaces were diffuse in comparison with the air/polymer interface, probably due to interfacial roughening and the partial dissolution of segments at the outermost region of the film. In the case of water and hexane, the swollen layer was formed beneath the liquid/ polymer interface, and the extent was more pronounced for water than for hexane. On the other hand, such a swollen layer was not observed for dPMMA in methanol, simply because methanol molecules penetrated deeply into the film. Finally, the mechanical stiffness of PMMA in liquids was examined as a function of depth from the liquid (or air) interface using AFM. The modulus of PMMA in liquids generally decreased closer to the interfaces. The extent of the decrease and the depth at which a constant value of the modulus was recovered, were in good accordance with the density profiles of PMMA in the liquids found by NR. Acknowledgment. This research was partly supported by an Industrial Technology Research Grant Program in 2006 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, by a Grant-in-Aid for Young Scientists A (Grant No. 18685014), and by a grant for Science Research in a Priority Area “Soft Matter Physics” (Grant No. 19031021) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Also, this work was partially supported in the utilization of the reactor by the Inter-Univ. Program for common use JAEA facility. LA702132T