pubs.acs.org/Langmuir © 2010 American Chemical Society
Strong Compression Rate Dependence of Phase Separation and Stereocomplexation between Isotactic and Syndiotactic Poly(methyl methacrylate)s in a Langmuir Monolayer Observed by Atomic Force Microscopy Naoyuki Aiba,†,§ Yuhtaro Sasaki,†,# and Jiro Kumaki*,†,‡ † Department of Polymer Science and Engineering, Faculty of Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan, and ‡Department of Polymer Science and Engineering, Graduate School of Science and Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan. §Present address: Material Technology Research and Development Laboratories, Tokai Rubber Industries, Ltd., 3-1 Higashi, Komaki 485-8550, Japan. #Present address: Miyako Branch, Tohoku Bank, 2-25 Aramachi, Miyako 027-0086, Japan.
Received May 7, 2010. Revised Manuscript Received June 18, 2010 The stereocomplex formation between isotactic and syndiotactic poly(methyl methacrylate) (it-PMMA, st-PMMA) in a Langmuir monolayer was studied by surface pressure-area isotherms and atomic force microscopy (AFM). We found that the stereocomplex formation was highly sensitive to the compression rate of the monolayer. At a normal compression rate of 0.5 mm/s provided by the moving barrier, the blend monolayer formed a clear phase separation of the it- and st-PMMA domains at 1 mN/m. Further compression to 15 mN/m resulted in a limited degree of stereocomplexation, mainly at the interface between the two domains. However, at a 1/50 slower compression rate of 0.01 mm/s, the blend did not form a clear phase separation at 1 mN/m and quantitatively formed a stereocomplex at 15 mN/m. This apparent immiscibility observed at the faster compression rate was found to be kinetically induced as a result of the rapid compression of the phase-separated mixture at the dilute state because it-PMMA and st-PMMA form expanded and condensed monolayer, respectively. On the other hand, at the slower compression rate, the blend formed a thermodynamically miscible phase, and as a result, the stereocomplex was quantitatively formed. This apparent phase separation of a mixed monolayer composed of an expanded and a condensed monolayer should be a common phenomenon for similar systems and might have caused misjudgment of the miscibility in such cases. The compression rate dependence should be carefully evaluated in order to determine the precise miscibility of blended monolayers in similar systems.
1. Introduction Polymer chains with an appropriate hydrophilic group in the repeating unit spread on a water surface as monolayers.1 Because such polymer monolayers are superior in mechanical and thermal stability in comparison with monolayers composed of small molecules, they have been extensively studied for possible application as functional thin films.1c Polymer monolayers are also an ideal model for the study of polymer chains in two-dimensional (2D) states, and the structure and properties in 2D states are also the subject of intense study. Frequently, polymer chains in monolayers are known to behave differently from those in 3D solids. For instance, the miscibility of polymer blends in a monolayer might be different from those in a 3D solid.2,3 Specific alignments of the monomer units of the polymer in the monolayer as a result of adsorption of a hydrophilic group in the monomer unit onto the water surface can lead to interactions between blended polymers in the monolayer that are different from those in 3D solid.2-4 Another example is acceleration of the crystallization rate in mono*To whom corresponding should be addressed. E-mail: kumaki@ yz.yamagata-u.ac.jp. (1) (a) Crisp, D. J. J. Colloid Interface Sci. 1946, 1, 49–70. (b) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (c) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (2) Wu, S.; Huntsberger, J. R. J. Colloid Interface Sci. 1969, 29, 138–147. (3) Gabrielli, G.; Puggelli, M.; Baglioni, P. J. Colloid Interface Sci. 1982, 86, 485–500. (4) Kawaguchi, M.; Nishida, R. Langmuir 1990, 6, 492–496.
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layers. The crystallization of isotactic poly(methyl methacrylate) (it-PMMA) is known to be extremely slow in 3D states and usually takes several days at a high temperature. In contrast, the it-PMMA monolayer on a water surface easily crystallizes by a simple compression of the monolayer,5,6 resulting in well-ordered 2D foldedchain crystals composed of double-stranded helices of it-PMMA within several minutes, as revealed by atomic force microscopy (AFM).7 Another unique feature of 2D states of polymer chains are supramolecule formations in multicomponent monolayers: for example, stereocomplex formations composed of it-PMMA/ syndiotactic (st) PMMA8-12 and poly(L-lactide)/poly(D-lactide) mixture13 and formation of st-PMMA/fullerene inclusion complex (5) Brinkhuis, R. H.G.; Schouten, A. J. Macromolecules 1991, 24, 1487–1495. (6) Brinkhuis, R. H. G.; Schouten, A. J. Macromolecules 1991, 24, 1496–1504. (7) Kumaki, J.; Kawauchi, T.; Yashima, E. J. Am. Chem. Soc. 2005, 127, 5788– 5789. (8) Brinkhuis, R. H. G.; Shouten, A. J. Macromolecules 1992, 25, 2725–2731. (9) Brinkhuis, R. H. G.; Shouten, A. J. Macromolecules 1992, 25, 2732–2738. (10) Liu, J.; Zhang, Y.; Zhang, J.; Shen, D.; Guo, Q.; Takahashi, I.; Yan, S. J. Phys. Chem. C 2007, 111, 6488–6494. (11) Kumaki, J.; Kawauchi, T.; Okoshi, K.; Kusanagi, H.; Yashima, E. Angew. Chem., Int. Ed. 2007, 46, 5348–5351. (12) Kumaki, J.; Kawauchi, T.; Ute, K.; Kitayama, T.; Yashima, E. J. Am. Chem. Soc. 2008, 130, 6373–6380. (13) (a) Bourque, H.; Laurin, I.; Pezolet, M.; Klass, J. M.; Lennox, R. B.; Brown, G. R. Langmuir 2001, 17, 5842–5849. (b) Klass, J. M.; Lennox, R. B.; Brown, G. R.; Bourque, H.; Pezolet, M. Langmuir 2003, 19, 333–340. (c) Pelletier, I.; Pezolet, M. Macromolecules 2004, 37, 4967–4973. (d) Lee, W.-K.; Iwata, T.; Gardella, J. A., Jr. Langmuir 2005, 21, 11180–11184. (e) Duan, Y.; Liu, J.; Sato, H.; Zhang, J.; Tsuji, H.; Ozaki, Y.; Yan, S. Biomacromolecules 2006, 7, 2728–2735. (f) Kim, Y. S.; Snively, C. M.; Liu, Y.; Rabolt, J. F.; Chase, D. B. Langmuir 2008, 24, 10791–10796.
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with fullerenes incorporated in the inner cavity of the st-PMMA helix14 by compression of the corresponding mixed monolayers. The stereocomplex formation of it- and st-PMMA occurs by simple compression of the mixed monolayers, while the stereocomplex formation in a 3D melt again requires significant time.15 The formation of the it-PMMA crystal, the PMMA stereocomplex, and the st-PMMA/fullerene inclusion complex, all of which are composed of the PMMA helices, indicate that the monolayer on the water surface is not a rigid film strongly adsorbed on the water surface. Rather, the monolayer is a flexible film ready to detach from the water surface and rearrange from a flat molecular structure adsorbed on the water surface into a thermodynamically stable helical structure by a simple compression. Study of a mechanism of supramolecular structure formations in 2D states, especially from multicomponent systems, should improve our understanding of the nature of polymer monolayers on the water surface. In this work, we investigated stereocomplex formation on a water surface from a mixed monolayer of it- and st-PMMA in detail using surface pressure-area measurements and AFM observations. The stereocomplex composed of complementary strands of it- and st-PMMA with an it/st stoichometory of 1:2 is a class of unique, polymer-based supramolecules formed in specific solvents and in melts as well as in monolayers on the water surface.15 The unique multistranded structure of the stereocomplex has been stimulating a wide range of researches, such as layer-by-layer films,16 template polymerizations,17 and nanostructure formation,18 as well as practical applications for an artificial dialyzer19 and thermoplastic elastomers.20 The structure of the stereocomplex has long been believed to be a double-stranded helix composed of a 91 it-PMMA helix surrounded by an 181 st-PMMA helix since the original proposal by Challa and co-workers based on X-ray diffraction of the stretched stereocomplex fiber in 1989.21 Recently, we successfully observed the molecular image of the stereocomplex prepared by the Langmuir-Blodgett (LB) technique on mica using high-resolution AFM.11 We found that the molecular image of the stereocomplex does not agree with the double-stranded helix model but is reasonably explained by a triple-stranded helix model composed of a double-stranded it-PMMA helix included in a singlestranded st-PMMA helix. We further studied the stereocomplex prepared from uniform it- and st-PMMAs (i.e., it- and st-PMMAs without molecular weight distribution) as a function of the molecular weight of the component PMMAs and confirmed that the sizes of the stereocomplex support the triple-stranded helix model and provided a molecular mechanism in the stereocomplex formation.12 During this series of studies, we found that the stereocomplex formation proceeded in a rather complicated way as mentioned below.22 The mixed monolayer of (14) Kawauchi, T.; Kumaki, J.; Kitaura, A.; Okoshi, K.; Kusanagi, H.; Kobayashi, K.; Sugai, T.; Shinohara, H.; Yashima, E. Angew. Chem., Int. Ed. 2008, 47, 515–519. (15) For reviews: (a) Spevacek, J.; Schneider, B. Adv. Colloid Interface Sci. 1987, 27, 81–150. (b) te Nijenhuis, K. Adv. Polym. Sci. 1997, 130, 67–81. (c) Hatada, K.; Kitayama, T. Polym. Int. 2000, 49, 11–47. (16) Serizawa, T.; Hamada, K.; Kitayama, T.; Fujimoto, N.; Hatada, K.; Akashi, M. J. Am. Chem. Soc. 2000, 122, 1891–1899. (17) (a) Buter, R.; Tan, Y. Y.; Challa, G. J. Polym. Sci., Part A1 1972, 10, 1031– 1049. (b) Buter, R.; Tan, Y. Y.; Challa, G. J. Polym. Sci., Polym. Chem. Ed. 1973, 11, 1003–1011. (c) Hamada, K.; Serizawa, T.; Akashi, M. Macromolecules 2005, 38, 6759– 6761. (d) Serizawa, T.; Hamada, K.; Akashi, M. Nature 2004, 429, 52–55. (18) Kawauchi, T.; Kumaki, J.; Yashima, E. J. Am. Chem. Soc. 2006, 128, 10560–10567. (19) (a) Sakai, Y.; Tanzawa, H. J. Appl. Polym. Sci. 1978, 22, 1805–1815. (b) Sugaya, H.; Sakai, Y. Contrib. Nephrol. 1998, 125, 1–8. (20) Kennedy, J. P.; Price, J. L.; Koshimura, K. Macromolecules 1991, 24, 6567– 6571. (21) Schomaker, E.; Challa, G. Macromolecules 1989, 22, 3337–3341. (22) See Figures 2 and S1 in ref 11.
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it-PMMA (number-average molecular weight (Mn) = 1.2 104) and st-PMMA (Mn = 1.3 104) macroscopically phase separated into two domains (it-PMMA and st-PMMA domains) under compression. However, further compression resulted in quantitative stereocomplex formation without residual phase separation. This rather complicated phenomena stimulated us to study details of the process, since the supramolecule formation in a multicomponent monolayer may provide an important clue for understanding the nature of a polymer monolayer on the water surface. In this study we used the same it-PMMA (Mn = 1.2 104) and a slightly higher molecular weight st-PMMA (Mn = 2.38 104) and studied the stereocomplex formation on the water surface as a function of the compression rate. We found that the slight increase in the molecular weight of st-PMMA prevented the stereocomplex formation. At a normal compression rate of 0.5 mm/s, the mixed monolayer composed of it- and st-PMMA phase separated, and as a result, stereocomplex formation was mainly limited to the interface of the phases under further compression. In contrast, at a 1/50 slower compression rate than normal, the monolayer was close to miscible, and further compression resulted in quantitative stereocomplex formation without residual phase separation. This apparent immiscibility at the faster compression rate was found to be kinetically induced as a result of the rapid compression between the expanded it-PMMA and condensed st-PMMA monolayers which, at a dilute state, are not miscible.
2. Experimental Section Materials. The it-PMMA and st-PMMA used in this work were prepared as reported previously.11 The Mn, molecular weight distributions (Mw/Mn), and tacticities (mm/mr/rr) were as follows: it-PMMA(12K): Mn = 12 000, Mw/Mn = 1.11, and mm/mr/rr = 97/3/0; st-PMMA(24K): Mn = 23 800, Mw/Mn = 1.10, and mm/ mr/rr = 0/4/96. The Mn and Mw/Mn values were measured by size exclusion chromatography (SEC) in chloroform using PMMA standards (Shodex, Tokyo, Japan) for the calibration. The tacticities were determined from the 1H NMR signals of the R-methyl protons. Highly purified chloroform (Infinity Pure, Wako Chemicals, Osaka, Japan) was used as the solvent for the spreading solutions without further purification. Water was purified by a Milli-Q system and used as the subphase for the LB investigations. Surface Pressure-Area (π-A) Isotherm Measurements and LB Film Preparations for AFM. The π-A isotherm measurements and LB film depositions of an it-PMMA, st-PMMA and their mixed monolayers (it/st = 1/2) were done as follows. An it-PMMA, st-PMMA, or their mixed solution (it/st = 1/2 in unitmolar ratio) in chloroform having a total polymer concentration of ca. 5 10-5 g/mL was spread on a water surface at 25 °C to produce an initial surface concentration of ca. 0.7 nm2/repeating unit (ru) in a commercial LB trough with an area of 60 15 cm2 and an effective moving barrier length of 15 cm (FSD-300AS, USI, Japan). The surface pressure was measured using filter paper as the Wilhelmy plate. Chloroform is not stereocomplex forming solvent, so a stereocomplex is not formed in the spreading solution containing it- and st-PMMA mixtures. Since we found that the stereocomplex formation significantly depended on the compression rates of the mixed monolayers, the monolayers were compressed at various compression programs shown as an inset in Figure 1: the compression rate from the start to 1 mN/m was 0.5 or 0.01 mm/s, and the compression rate from 1 mN/m to the end was 0.5 or 0.01 mm/s; thus, a monolayer was compressed by the four different compression programs (red, black, green, and blue in the diagram in Figure 1). Using a moving barrier speed of 0.5 and 0.01 mm/s corresponded to a compression rate of 3.610-2 and 7.2 10-4 nm2 Langmuir 2010, 26(15), 12703–12708
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Figure 1. π-A isotherms of it-PMMA(12K), st-PMMA(24K), and it-PMMA(12K) and st-PMMA(24K) mixture (it/st = 1/2 in unit-molar base) on water. The π-A isotherms of the it-PMMA and st-PMMA mixture were measured following the compression programs indicated in the inset; the compression rates from the start to 1 mN/m were 0.5 (red and black) and 0.01 mm/s (green and blue) and from 1 mN/m to the end were 0.5 (red and green) and 0.01 mm/s (black and blue), respectively. π -A isotherms of it-PMMA (dotted line) and st-PMMA (broken line) were measured at a constant compression rate of 0.5 mm/s. ru-1 min-1, respectively. A monolayer was deposited onto freshly cleaved mica by pulling it out of the water at a rate of 4.2 mm/min, while compressing the monolayer at the constant pressure during the compression programs (the vertical dipping method). AFM Observations. After drying the deposited monolayers in vacuo, they were observed by a commercial AFM (NanoScope IIIa or IV/multimode AFM unit, Veeco Instruments, Santa Barbara, CA) with standard silicon cantilevers (PointProbe, NCH, NanoWorld, Neuch^atel, Switzerland) in air in tapping mode. The typical settings of the AFM observations were as follows: a drive amplitude of 1.0-1.3 V, a set point of 0.951.25 V, and a scan rate of 2 Hz. The AFM images obtained are presented without any image processing except flattening.
3. Results and Discussion 3.1. π-A Isotherms under Different Compression Programs. Figure 1 shows π-A isotherms of it-PMMA(12K), stPMMA(24K), and their mixtures (it/st = 1/2 unit-molar base). The it-PMMA(12K) (dotted line) and st-PMMA(24K) (broken line) were compressed at a constant rate of 0.5 mm/s, while their mixture (red, black, green, and blue lines) was compressed following the four different compression programs indicated in an inserted diagram in the same color. The π-A isotherms indicated by red and blue lines were compressed at a constant rate of 0.5 and 0.01 mm/s, respectively, while those indicated by black and green lines were compressed at 0.5 (black) and 0.01 mm/s (green) until the surface pressure reached 1 mN/m, then the compression rate was switched, and further compressed at 0.01 (black) and 0.5 mm/s (green), respectively. The it-PMMA (dotted line) showed an expanded π-A isotherm with a transition at about 10 mN/m, which corresponds to its crystallization into two-dimensional folded-chain lamella crystals, whereas the st-PMMA (broken line) showed a condensed π-A isotherm without any apparent transition and forms amorphous films without any specific structures, as reported by the previous AFM studies.7,11 As Figure 1 shows, the π-A isotherms of mixtures of the it- and st-PMMA were completely different both from the π-A isoLangmuir 2010, 26(15), 12703–12708
therms of each component and that expected from the π-A isotherms of the it- and st-PMMA assuming an additive rule. Also, the π-A isotherms are found to strongly dependent on the compression programs. At the fastest but normal compression rate of 0.5 mm/s (red), at which it took about 18 min to complete a π-A isotherm measurement, the transition corresponding to the stereocomplex formation was not clear, but at the slowest compression rate of 0.01 mm/s (blue), at which it took about 13 h to finish a compression, a clear plateau transition at about 5 mN/m corresponding to the stereocomplex formation was observed. This transition of the it- and st-PMMA mixture at a surface pressure lower than that of the it-PMMA crystallization was previously identified to a stereocomplex formation by IR spectra8,11 and AFM10-12 of the resultant films. The smallest surface area (around 0.1 nm2/ru) where a steep increase of the surface pressure started after the plateau of stereocomplex formation indicated that the stereocomplex was most effectively formed at the slowest compression. The π-A isotherms under the intermediate compression programs, initially compressed at 0.5 mm/s and then reduced the rate to 0.01 mm/s at 1 mN/m (black) or vice versa (green), showed intermediate efficiency of stereocomplex formation. The surface pressures of the stereocomplex transition at the faster compression rate of 0.5 mm/s (red and green lines: 7 mN/m) were slightly higher than that at the slower compression rate of 0.01 mm/s (black and blue lines: 5 mN/m). 3.2. AFMs of Monolayers Compressed under Different Compression Programs. Figure 2 shows AFM height images of the monolayers of it- and st-PMMA mixtures deposited on mica after being compressed under the four different compression programs. A monolayer deposited on mica at 1 mN/m at the faster compression rate (0.5 mm/s) (Figure 2A) clearly showed a phase separated structure with thinner it-PMMA domains and thicker st-PMMA domains with distinguishing boundaries; the height difference between the two domains was around 0.36 nm.23 Further compression at the same faster compression rate of 0.5 mm/s (red arrow) to 10 and 15 mN/m (Figure 2C, center and right) resulted in formation of thicker new domains (about 1 nm high) mainly at the interface between the it- and st-PMMA domains as well as a small amount specifically only in the itPMMA domains. These higher domains correspond to a PMMA stereocomplex which formed by compression.10,11 The stereocomplex formation was strongly suppressed because of the phase separation of the it- and st-PMMA domains. The minor degree of formation of the stereocomplex at the higher compression rate corresponded well to the poorly defined transition in the π-A isotherm at the same compression rate (red line in Figure 1). A magnified image for the area indicated by the yellow square in Figure 2C (left) is shown in Figure 3. The residual it-PMMA domains were crystallized into 2D folded-chain lamella crystals, while the st-PMMA domains remained in a structure without any specific features.11,24 As shown in Figure 2D, further compression of the phaseseparated monolayer (Figure 2A) at the 1/50 compression rate of 0.01 mm/s (black arrow) resulted in more efficient stereocomplex formation. An increased amount of stereocomplex was formed, again, specifically in the it-PMMA domains, and no stereocomplex formation was visible in the st-PMMA domains. The (23) The height difference between the two domains was close to that of the phase-separated monolayer of the it-PMMA(12K)/st-PMMA(13K) mixture compressed at 0.5 mm/s in the previous study (0.31 nm).22 (24) As shown in the π-A isotherms in Figure 1, the surface pressure at the transitions of it-PMMA crystallization and it-PMMA/st-PMMA stereocomplexation were 10 and 5-7 mN/m, respectively. The PMMA stereocomplexation preferentially occurred at a lower surface pressure, in other words, with a lower energy barrier than the it-PMMA crystallization.
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Figure 2. Tapping-mode AFM height image of the monolayers of it-PMMA(12K) and st-PMMA(24K) mixture (it/st = 1/2 in unit-molar base) deposited on mica after being compressed at the different compression programs. Height profiles along the white lines for the images of the monolayers deposited at 1 and 15 mN/m are also shown. Magnified image for area indicated the yellow square in C (15 mN/m, right) is shown in Figure 3. The sizes of the images are indicated in each image as the width of the image.
specific stereocomplex formation in the it-PMMA domain is probably due to asymmetric diffusion of polymer chains into the domain of other component as a result of a difference in rigidity of the it- and st-PMMA domains. it-PMMA forms an expanded monolayer, which is more flexible than the condensed 12706 DOI: 10.1021/la1018289
st-PMMA monolayer. Thus, st-PMMA chains are expected to be able to more easily penetrate into the flexible it-PMMA domains; in contrast, the it-PMMA chains are difficult to penetrate into the rigid st-PMMA domains, resulting in the selective stereocomplex formation in the it-PMMA domains. Langmuir 2010, 26(15), 12703–12708
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Figure 3. Tapping-mode AFM height image of the monolayer of it-PMMA(12K) and st-PMMA(24K) mixture (it/st=1/2 in unitmolar base) compressed at 0.5 mm/s and deposited on mica at 15 mN/m (corresponding to the area indicated in Figure 2C (right)). The residual it-PMMA domain was crystallized into lamellas.
Figure 4. (A) Tapping-mode AFM height image of a monolayer of it-PMMA(12K) and st-PMMA(24K) mixture (it/st = 1/2 in unit-molar base) deposited at an area of 0.96 nm2/ru (0 mN/m) after keeping the monolayer on the water surface at the area for 13 h. The height profile along the white line is also shown. Scale: 1 μm 1 μm. (B) Magnified image of the yellow square in (A). A yellow line indicates a long it-PMMA chain among short itPMMA chains.
On the other hand, as shown in Figure 2B, the mixed monolayer compressed at the slower compression rate of 0.01 mm/s and deposited at a surface pressure of 1 mN/m did not showed clear phase separation, the larger domains with vague boundaries having smaller height difference between the domains (∼0.18 nm) than that compressed at 0.5 mm/s (0.36 nm). Further compression of the monolayer at the same slower compression rate (0.01 mm/s, blue line) resulted in no detectable phase separation, and the film was composed of a thick stereocomplex (∼1.36 nm) throughout the sample, indicating the almost quantitative stereocomplex formation throughout the monolayer (Figure 2F, right). On the other hand, compression switched to the faster rate (0.5 mm/m) at 1 mN/ m (green line) and also resulted in rather homogeneous stereocomplex formation (Figure 2E, right (15 mN/m)); however, on a closer inspection, some vague residual phase separation was Langmuir 2010, 26(15), 12703–12708
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Figure 5. Schematic representation of stereocomplex formation under fast (0.5 mm/s) and slow (0.01 mm/s) compressions. Blue, red, and yellow lines indicate it-PMMA, st-PMMA, and stereocomplex molecules, respectively.
recognizable. At 3 mN/m (Figure 2E, left), a circular homogeneous domain can be discerned in the bottom right portion of the image. In Figure 2E (center) at 10 mN/m two vertical streaks are apparent. The poor stereocomplex formation in the switched compression rate from 0.01 to 0.5 mm/s at 1 mN/m in comparison with the constant slow compression (0.01 mm/s) is also visible in the difference between the π-A isotherms shown in Figure 1 (green and blue lines). They show that slow compression at 0.01 mm/s up to 1 mN/m was not sufficient to attain complete miscibility, and slower compression was necessary to attain quantitative stereocomplex formation. 3.3. Origin of the Apparent Immiscibility of it- and stPMMA. As has already been mentioned, the miscibility of the mixed monolayer at 1 mN/m was significantly affected by the compression rate, and the miscibility strongly influenced the efficiency of the subsequent stereocomplex formation. The enhanced miscibility at the slower compression rate strongly indicated that the it- and st-PMMA mixture should be close to a thermodynamically miscible system at equilibrium, and the apparent phase separation between the two phases was formed kinetically. Then, why did apparent immiscibility appear for the thermodynamically miscible system under fast compression? In order to answer this question, we then investigated the equilibrium structure of the it- and st-PMMA mixture in a dilute state. Figure 4 shows AFM height images of a monolayer of the itPMMA and st-PMMA mixture (it/st = 1/2 in unit-molar base) deposited in a dilute state (0.96 nm2/ru, 0 mN/m) after keeping the monolayer on the water surface at the same area for 13 h. Even after this long time, the monolayer clearly phase separated into condensed circular st-PMMA domains with a submicrometer diameter and isolated single it-PMMA chains surrounding the domains. Since the molecular weight of the it-PMMA (Mn = 12 000) used was not sufficiently large to be seen as clear long strand chains, the chains appear as short dots scattered to fill the space outside the st-PMMA domains. However, a close look showed small numbers of longer chains among the short dot-like it-PMMA chains, which are high-molecular-weight it-PMMA chains contained in the it-PMMA due to the molecular weight distribution (see yellow line in Figure 4B). Thus, at a dilute state, it- and st-PMMA chains do not mix with each other. As previously reported, it-PMMA forms an expanded monolayer on a water surface and exists as isolated single chains. In contrast, st-PMMA forms a condensed monolayer on a water surface; thus, DOI: 10.1021/la1018289
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even in a dilute state, st-PMMA exists as condensed monolayers separately floating on the water surface.7 Figure 5 shows schematically how the stable structure of it- and st-PMMA mixtures in a dilute state (0 mN/m) is a phase-separated structure with condensed st-PMMA domains surrounded by isolated it-PMMA single chains. Fast compression (0.5 mm/s) of this phase-separated monolayer results in fixation of this phaseseparated structure, and further compression results in stereocomplex formation mainly at the interface of the it- and st-PMMA domains. On the other hand, slower compression (0.01 mm/s) of the phase-separated structure in a dilute state enables the monolayer to mix with each other and be close to a thermodynamically miscible structure at 1 mN/m. Then, further compression at the slower rate results in almost quantitative stereocomplex formation. The monolayer was thermodynamically miscible at 1 mN/m but apparently phase separated in the dilute state. This is analogous to a ternary blend of a miscible A-polymer and B-polymer pair with a selective solvent in the bulk. If we add a strongly selective solvent to a miscible A-/B-polymer blend, and assuming the solvent is a very good solvent for one polymer but a very poor solvent for the other polymer, the ternary blend should phase separate. In the present monolayer system, in the dilute state, the water surface is “a good solvent” for the it-PMMA; therefore, it-PMMA forms an expanded monolayer on the water surface, while the water surface is “a poor solvent” for the st-PMMA, and then st-PMMA forms a condensed monolayer on the water surface,7 resulting in a phase separation in the dilute state. 3.4. Stereocomplex Formation of it-PMMA(12K) and st-PMMA(13K) Mixture. The stereocomplexation efficiency of the it-PMMA(12K)/st-PMMA(24K) mixture studied here depended on the phase separation structure formed prior to the stereocomplexation at a low surface pressure around 1 mN/m. However, in the previous study, a mixture of the same it-PMMA(12K) and a st-PMMA with a slightly smaller molecular weight (Mn = 13 000, Mw/Mn = 1.13, and mm/mr/rr = 0/4/96), similarly phase separated at 1 mN/m under compression at the normal rate (0.5 mm/s), but further compression at the same rate resulted in the almost quantitative stereocomplex formation without any (25) (a) Labbauf, A.; Zack, J. R. J. Colloid Interface Sci. 1970, 35, 569–583. (b) Gabrielli, G.; Puggelli, M.; Faccioli, R. J. Colloid Interface Sci. 1971, 37, 213–218. (c) Gabrielli, G.; Puggelli, M.; Faccioli, R. J. Colloid Interface Sci. 1973, 44, 177–180. (d) Gabrielli, G.; Puggelli, M.; Ferroni, E. J. Colloid Interface Sci. 1974, 47, 145– 154. (e) Gabrielli, G.; Baglioni, P. J. Colloid Interface Sci. 1980, 73, 582–587. (f) Caminati, G.; Gabrielli, G.; Puggelli, M.; Farroni, E. Colloid Polym. Sci. 1989, 267, 237–245. (g) Kawaguchi, M.; Nagata, K. Langmuir 1991, 7, 1478–1482. (h) Kawaguchi, M.; Suzuki, S.; Imae, T.; Kato, T. Langmuir 1997, 13, 3794–3799. (i) Yamamoto, S.; Tsujii, Y.; Fukuda, T. Polymer 2001, 42, 2007–2013. (j) Aoki, H.; Kunai, Y.; Ito, S.; Yamada, H.; Matsushige, K. Appl. Surf. Sci. 2002, 188, 534–538. (k) Kawaguchi, M.; Suzuki, M. J. Colloid Interface Sci. 2005, 288, 548–552. (l) Ohkita, M.; Higuchi, M.; Kawaguchi, M. J. Colloid Interface Sci. 2005, 292, 300–303. (m) Lee, Y.-L.; Hsu, W.-P.; Lio, S.-H. Colloids Surf., A 2006, 272, 37–47. (n) Hsu, W.-P.; Li, H.-Y.; Chiou, M.-S. Colloids Surf., A 2009, 335, 73–79.
12708 DOI: 10.1021/la1018289
Aiba et al.
residual phase separation.22 This is in contrast to the stereocomplex formation of the it-PMMA(12K)/st-PMMA(24K) seen here at the same compression rate (0.5 mm/s) (Figure 2C). The slight decrease in the molecular weight of st-PMMA seems to be the sole reason for the strong acceleration of the stereocomplex formation in the PMMA(12K)/st-PMMA(13K) system. As shown in Figure 2C,D, the stereocomplex formation of the it-PMMA(12K)/st-PMMA(24K) occurred specifically in the flexible itPMMA domains and did not occur in the rigid st-PMMA(24K) domains, indicating that the rigidity of the st-PMMA(24K) domain mainly prevented the stereocomplex formation. Thus, the more quantitative stereocomplex formation using the stPMMA(13K) indicates that the cohesive force of the st-PMMA domain at this compression rate significantly decreased over this narrow molecular weight region and enabled the it-PMMA chains penetrate into the st-PMMA domains, resulting in the quantitative stereocomplex formation.
4. Concluding Remarks We have studied the stereocomplex formation between it- and st-PMMA in a monolayer and found that the miscibility of the two polymers and the resultant stereocomplexation strongly depended on the compression rate. The miscibility of polymer mixtures in Langmuir monolayers has been widely studied for many polymer blend systems,2-4,25 but generally, the apparent immiscibility we have observed here for a mixed monolayer of an expanded and a condensed monolayer are not well recognized. The apparent phase separation of such a mixed monolayer should be a common phenomenon in similar systems and may result in a misjudgment of the miscibility. In some cases, the miscibility of a mixture of an expanded and a condensed monolayer might need reevaluation, taking this effect into consideration.26 Acknowledgment. We appreciate the assistance of Dr. Takehiro Kawauchi, Toyohashi University of Technology, in the polymerization of the it-PMMA and st-PMMAs. This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (20106009) and Scientific Research (B) (21350059) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. (26) Recently, polymer thin films, which are thicker than a monolayer, but still in the ultrathin regime (
