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Stepwise Stereocomplex Assembly of Isotactic Poly(methyl methacrylate) and Syndiotactic Poly(alkyl methacrylate)s on Surfaces Ken-ichi Hamada,† Takeshi Serizawa,† Tatsuki Kitayama,‡ Nobutaka Fujimoto,‡ Koichi Hatada,‡ and Mitsuru Akashi*,† Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, Korimoto 1-21-40, Kagoshima 890-0065, Japan, and Graduate School of Engineering Science, Osaka University, Machikaneyama-cho 1-16, Toyonaka 560-0043, Japan Received February 5, 2001. In Final Form: May 16, 2001 Isotactic (it-) poly(methyl methacrylate) (PMMA) and syndiotactic (st-) poly(ethyl methacrylate) (PEMA) and poly(propyl methacrylate) (PPMA) were synthesized by anionic polymerization in toluene at -78 °C with t-C4H9MgBr and t-C4H9Li/(C2H5)3Al, respectively, as initiators/coordinators. Combined it- and stpolymers were assembled by the alternate immersion of a 9 MHz quartz-crystal microbalance (QCM) as a substrate into their acetonitrile solutions at ambient temperature. QCM analysis showed constant growth of the assembly except in the first two steps. The assembled ratios between st-PEMA and st-PPMA with it-PMMA (st-/it-) were 2.5 × 0.4 and 1.8 × 0.5, respectively, indicating stepwise stereocomplex formation. The static contact angle of the assembly at each step and its reflection absorption spectrum also implied complex formation. The amount of stereocomplex assembled was significantly affected by the st-polymer concentration and its molecular weight. Atomic force microscopy observation showed the assemblies to have a molecularly smooth surface.
Introduction Recently, the systematic design by self-organization of individual macromolecules with a highly ordered nanostructure in solutions or solids has been of great interest. Self-organized structures exist naturally, especially occurring as biopolymers such as proteins, polysaccharides, and DNA, which maintain specific dimensional ordering in aqueous phase, to perform various functions. It is difficult, however, to regulate molecular orientation and/or an ordered structure using synthetic polymer solid materials, except in polymers with high crystallinity such as these. Although a Langmuir-Blodgett (LB) technique is a promising method to obtain molecularly layered structures,1 this method requires polymers to have amphiphilic properties and suitable apparatus. Therefore, preparation of regulated polymer assembly on substrates using other simpler methodologies is of great interest not only because of potential applications in the synthesis of polymeric materials but also because of scientific significance in the field of polymer surface chemistry.2 Detailed studies of the molecular assembly of certain polymers should lead to new scientific insights regarding the molecular events involved in the assembly. Our research group has succeeded in preparing binaryblended and ultrathin polymer films with a molecularly * To whom correspondence should be addressed. Tel: +81-99285-8320. Fax: +81-99-255-1229. E-mail:
[email protected]. † Kagoshima University. ‡ Osaka University. (1) (a) Blodgett, K. B.; Langmuir, I. J. Am. Chem. Soc. 1934, 56, 495. (b) Kuhn, H.; Mo¨bius, D. Angew. Chem., Int. Ed. Engl. 1971, 10, 620. (2) (a) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (b) Polymer Surface Dynamics; Andrade, J. D., Ed.; Plenum Press: New York, 1988. (c) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Harcourt Brace Jovanovich: Boston, 1991. (d) Tsukruk, V. V. Prog. Polym. Sci. 1997, 22, 247. (e) Grainger, D. W. Prog. Colloid Polym. Sci. 1997, 103, 2243.
regulated assembly structure by using stereoregular polymers of methacrylates.3 Isotactic (it-) and syndiotactic (st-) poly(methyl methacrylate)s (PMMAs) are known to form a stereocomplex with a double-stranded helical structure, in which it-PMMA is surrounded by twice the molar amount of st-PMMA, as proposed by Challa et al. and other researchers.4 The films were prepared based on stepwise stereocomplex formation by alternate immersion of a quartz-crystal microbalance (QCM) substrate in acetonitrile solutions of each of the stereoregular PMMAs.3a The assembly was also performed from a combination of it-PMMA and st-poly(methacrylic acid) using a suitable selection of each of the solvents.3b This methodology was comprised of a novel application of an alternate adsorption technique, which was originally developed by Decher et al.5 and utilized by other researchers in order to prepare polyelectrolyte multilayers.6 It is interesting to note that the resulting stereocomplex assembly has nanoscale film thickness and a molecularly regulated assembly structure and that the formation of the stereocomplex on the ultrathin film surface includes penetration of a st-polymer chain into the layer of it-PMMA that is physically adsorbed at the interface. On the other hand, the morphology of nanoscale aggregation of the PMMA stereocomplex that (3) (a) Serizawa, T.; Hamada, K.; Kitayama, T.; Fujimoto, N.; Hatada. K.; Akashi, M. J. Am. Chem. Soc. 2000, 122, 1891. (b) Serizawa, T.; Hamada, K.; Kitayama, T.; Katsukawa, K.; Hatada, K.; Akashi, M. Langmuir 2000, 16, 7112. (4) (a) Bosscher, F.; Brinke, G. T.; Challa, G. Macromolecules 1982, 15, 1442. (b) Brinke, G. T.; Schomaker, E.; Challa, G. Macromolecules 1985, 18, 1925. (c) Schomaker, E.; Challa, G. Macromolecules 1989, 22, 3337. (d) Spevacek, J.; Schneider, B.; Straka, J. Macromolecules 1990, 23, 3042. (e) Watanabe, W. H.; Ryan, C. F.; Fleischer, P. C., Jr.; Garrett, B. S. J. Phys. Chem. 1961, 65, 896. (f) Liquori, A. M.; Anzuino, G.; Coiro, V. M.; D’Alagni, M.; Santis, P. D.; Savino, M. Nature 1965, 206, 358. (g) Spevacek, J.; Schneider, R. Adv. Colloid Interface Sci. 1987, 27, 81. (h) Feitsma, E. L.; De Boer, A.; Challa, G. Polymer 1975, 16, 515. (i) De Boer, A.; Challa, G. Polymer 1976, 17, 633. (5) (a) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (b) Decher, G.; Hong, J.-D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (c) Decher, G. Compr. Supramol. Chem. 1996, 9, 507. (d) Decher, G. Science 1997, 277, 1232.
10.1021/la0101898 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/04/2001
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Table 1. Stereoregular Poly(alkyl methacrylate)s Synthesized in This Study tacticity (%) it-PMMA st-PMMA st-PEMA st-PPMA
Mn
Mw/Mn
mm
mr
rr
20 750 22 700 10 600 21 340 10 880 23 800 12 340
1.26 1.26 1.24 1.11 1.14 1.09 1.07
97 0 1 1 0 2 0
2 11 11 8 9 7 6
1 89 88 91 91 91 94
is physically adsorbed at interactive surfaces has been clarified by direct observation using atomic force microscopy (AFM).7 Syndiotactic polymers in the stereocomplex are not necessarily formed by PMMA, because the methyl ester group of st-PMMA is oriented toward the outside of the stereocomplex. Therefore, the combinations of it-PMMA and other st-poly(alkyl methacrylate)s also form similar stereocomplexes.8 It is attractive to analyze the similar thin film formation with st-polymers having bulky side chains. In this study, we analyzed the stepwise assembly of it-PMMA with st-poly(ethyl methacrylate) (PEMA) and st-poly(propyl methacrylate) (PPMA) onto surfaces from acetonitrile solutions. QCM was used as a substrate to quantify the amount of adsorbed polymer to the nanogram level and to facilitate the analysis of static contact angles and reflection absorption spectra (RAS). Experimental Section Materials. Isotactic and st-poly(alkyl methacrylate)s were synthesized by anionic polymerization in toluene at -78 °C over a period of 24 h with t-C4H9MgBr9 and t-C4H9Li/(n-C4H9)3Al,10 respectively, as initiators. Characteristics such as the numberaverage molecular weight, which were measured using the size exclusion chromatography with poly(methyl methacrylate) standard in THF, are listed in Table 1. The tacticities (mm:mr:rr), which were measured using the 1H NMR signals of the R-methyl protons, are also shown in Table 1. The distribution of the molecular weights (Mw/Mn) was approximately 1.1, indicating a narrow distribution. Acetonitrile which was used to prepare the polymer solutions, was purchased from Nacalai Tesque (Japan). Ultrapure distilled water was provided by the MILLI-Q laboratory (MILLIPORE). (6) (a) Kaschak, D. M.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 4222. (b) Advincula, R.; Aust, E.; Meyer, W.; Knoll, W. Langmuir 1996, 12, 3536. (c) Hoogeveen, N. G.; Stuart, M. A. C.; Fleer, G. J. Langmuir 1996, 12, 3675. (d) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3422. (e) Delcorte, A.; Bertrand, P. Langmuir 1997, 13, 5125. (f) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78. (g) Cheung, J. H.; Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2712. (h) Lvov, Y.; Onda, M.; Ariga, K.; Kunitake, T. J. Biomater. Sci., Polym. Ed. 1998, 9, 345. (i) Lvov, Y.; Haas, H.; Decher, G.; Mo¨hwald, H.; Mikhailov, A.; Mtchedlishvily, B.; Morgunova, E.; Vainshtein, B. Langmuir 1994, 10, 4232. (j) Lvov, Y.; Ichinose, I.; Ariga, K.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (k) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3427. (l) Ariga, K.; Lvov, Y.; Onda, M.; Ichinose, I.; Kunitake, T. Chem. Lett. 1997, 125. (b) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Langmuir 1997, 13, 6195. (m) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (n) Yonezawa, T.; Onoue, S.; Kunitake, T. Adv. Mater. (Weinheim, Ger.) 1998, 10, 414. (o) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Lett. 1996, 831. (7) Grohens, Y.; Castelein, G.; Carriere, G.; Spevacek, J.; Schultz, J. Langmuir 2001, 17, 86. (8) (a) Bosscher, F.; Keekstra, D.; Challa, D. Polymer 1981, 22, 124. (b) Kitayama, T.; Fujimoto, N.; Terawaki, Y.; Hatada, K. Polym. Bull. 1990, 23, 279. (9) Hatada, K.; Ute, K.; Okamoto, Y.; Kitayama, T. Polym. J. 1986, 18, 1037. (10) Kitayama, T.; Shinozaki, T.; Sakamoto, T.; Yamamoto, M.; Hatada, K. Makromol. Chem 1989, 15, 167.
Quartz-Crystal Microbalance. The stepwise fabrication of films was analyzed quantitatively using a QCM as a substrate. An AT cut QCM with a parent frequency of 9 MHz was purchased from USI (Japan). A part of the crystal (9 mm in diameter) was coated on both sides with gold electrodes (4.5 nm in diameter). The frequency was monitored by an Iwatsu frequency counter (model SC7201). The leads of the QCM were sealed and protected by silicon rubber gel in order to prevent degradation of the solvent contents when the QCM was immersed in the various solutions. The amount of assembled polymer was calculated from the decrease in frequency of the QCM, ∆F, using Sauerbrey’s equation11 as follows
-∆F )
2F02 A(Fqµq)1/2
∆m
(1)
where F0 is the parent frequency of the QCM (9 × 106 Hz), A is the electrode area (0.159 cm2), Fq is the density of the quartz (2.65 g cm-3), and µq is the shear modulus (2.95 × 1011 dyn cm-2). This equation was reliable when frequencies were measured in air, as described in this study, because the mass of the solvents is never detected as a frequency shift, and the effect of the viscosity of the absorbent on the frequency can be ignored. Stepwise Assembly. Before assembly measurements were made, the QCM electrodes were treated three times with a piranha solution (H2SO4/H2O2 ) 3:1) for 1 min each time, followed by rinsing with pure water and drying with N2 gas in order to clean the electrode surface. Following cleaning, the QCM was immersed in the solvent used in this study for 30 min as pretreatment. Then, the QCM was immersed in an it-PMMA solution of adequate concentration (0.85 × 10-2-2.6 × 10-2 unit M) for adequate time (15 min in most cases) to achieve equilibrium adsorption at ambient temperature. The QCM was taken out, thoroughly rinsed with the same solvent for approximately 20 s, and dried with N2 gas. The frequency decrease was subsequently measured. The QCM was then immersed again into a st-poly(alkyl methacrylate) solution, and the same washing procedure was repeated. The above procedure was repeated for stepwise assembly of the polymers. The film thickness of the resulting poly(alkyl methacrylate) assemblies was estimated from the frequency shift, assuming that the film density was the same as that of solid PMMA (1.188 g cm-3)12 and that the film surface was flat. The assembly ratio between it-PMMA and st-poly(alkyl methacrylate) was obtained by calculating the mean value of the ratio at the step from it-PMMA to st-poly(alkyl methacrylate), because stereocomplex formation occurs similarly during this step, as discussed in our previous paper.3 Other Measurements. The static contact angle of the stepwise poly(alkyl methacrylate) ultrathin films assembled on the QCM was measured by the sessile drop method with a goniometer type G-1 (Erma Co., Tokyo, Japan) with distilled water at 25 °C. Contact angles were determined at 40 and 60 s after application of the drop. The volume of water in the drop was 3 µL. All reported values represent the average of at least six measurements taken at different locations on the film surface. Infrared spectra were collected from both RAS and attenuated total reflection (ATR) measurements. The RAS were obtained with a Herschel FT/IR-610, Jasco (Japan) equipped with a mercury-cadmium-telluride (MCT) detector. One side of a glass slide (13 × 26 mm), which was used as a substrate, was coated with gold to obtain a reflective surface. Films were deposited by stepwise immersion of the substrate into polymer solutions; the formation procedure was essentially the same as that on the QCM. ATR spectra were obtained using the same apparatus attached to a Zn-Ge internal reflection element (60 × 10 × 3.75 mm3) with an incidence angle of 45°. In both measurements, the interferograms were co-added 50 times and Fourier transformed at a resolution of 8 cm-1. AFM images were obtained in a Digital Instruments NanoScope III that was operated in a tapping mode in the air at ambient temperature. We have not performed any image processing other than flat leveling. The mean square (11) Sauerbrey, G. Z. Phys. 1959, 155, 206. (12) Spevacek, J.; Schneider, B. Makromol. Chem. 1974, 175, 2939.
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Figure 1. Frequency shift of QCM with a stepwise assembly from acetonitrile solutions at a concentration of 1.7 × 10-2 unit M of (a) it-PMMA (Mn ) 20 750) and st-PMMA (Mn ) 22 700), (b) it-PMMA (Mn ) 20 750) and st-PEMA (Mn ) 21 340), and (c) it-PMMA (Mn ) 20 750) and st-PPMA (Mn ) 23 800). Results from the combination of it-PMMA and st-PMMA have been already reported.3a roughness (Ra) in given observed areas was estimated from the following equation
Ra )
1
Ly Lx
∑∑|F(x,y)| dx dy LxLy 0
(2)
0
where F(x, y) is the surface relative to the center plane that is a flat plane parallel to the mean plane and Lx and Ly are the dimensions of the surface.
Results and Discussion Stepwise Assembly of Poly(alkyl methacrylate)s. Stereocomplex formation between stereoregular PMMAs in solutions is well-known to be dependent on the solvent species. In general, stereocomplex formation is not determined by the polarity of the solvent, which can solubilize the polymers. In our previous paper, we selected acetonitrile, acetone, and N,N-dimethylformamide (DMF), which are known to be strongly complexing solvents, and concluded that acetonitrile was suitable for the assembly of stereoregular PMMAs on surfaces.3a Acetonitrile is a theta solvent for it- and st-PMMAs at 28 °C, while acetone and DMF are good solvents. The poor solubility of the PMMAs in acetonitrile was suitable for the stepwise polymer assembly. Therefore, we selected acetonitrile as a solvent in this study also. Figure 1 shows the dependence of the frequency shift on the assembly steps when the QCM was alternately immersed in the combination of it-PMMA (Mn ) 20 750) and st-PMMA (Mn ) 22 700), st-PEMA (Mn ) 21 340), or st-PPMA (Mn ) 23 800) solutions for 15 min at ambient temperature at a concentration of 1.7 × 10-2 unit M. In all cases, the frequencies decreased with increasing assembly steps, indicating stepwise polymer deposition possibly based on stereocomplex formation. In the two initial steps, we observed larger frequency shifts due to the direct influence of the gold substrate of the QCM on the assembly process, similar to observations in a previous paper.3a When we immersed the QCM in it- or st-polymer solution alone for a much longer time, the frequency shift saturated at a level obtained in one step of the assembly process. These observations imply stereocomplex formation between these stereoregular polymers on QCM surfaces. The frequency shifts after a 20-step assembly
Figure 2. Static contact angle of the stepwise a assembly from acetonitrile solutions at a concentration of 1.7 × 10-2 unit M of (a) it-PMMA (Mn ) 20 750) and st-PMMA (Mn ) 22 700), (b) it-PMMA (Mn ) 20 750) and st-PEMA (Mn ) 21 340), and (c) it-PMMA (Mn ) 20 750) and st-PPMA (Mn ) 23 800).
(∆F20 steps) between it-PMMA and st-PMMA, st-PEMA, and st-PPMA were 416, 239, and 402 Hz, respectively. The apparent film thicknesses were estimated to be 9.7, 5.5, and 9.3 nm, respectively. These values were also consistent with the values that were obtained by the scratching of these assemblies on a QCM substrate using an AFM tip (9.7 ( 0.3, 5.4 ( 0.5, and 9.4 ( 0.6 nm, respectively). In other words, the density of these films was determined to be the same as that of solid PMMA (1.188 g cm-3).12 It is difficult to explain the reason for which the amount of assembly with the combination of it-PMMA and st-PEMA was the smallest. In the present study, the same it-PMMA was utilized for all assembly procedures. In addition, the stepwise assembly is performed by complex formation of the st-polymers with the physically adsorbed it-PMMA (see the following section). From these observations, the physical adsorption ability of it-PMMA on the complex surface prepared from it-PMMA and st-PEMA seems to be smallest. Further analysis concerning the difference in assembled polymer amount will be needed. In the quantitative QCM analysis described above, we showed the possibility of stereocomplex formation on QCM surfaces. In each stereocomplex, the stoichiometry between it- and st-polymers is significant and hence should be discussed. It is possible to analyze the stoichiometry because the frequency shift was monitored at each step of the assembly. Other research groups have experimentally and theoretically studied the ratio between stereoregular PMMAs in mixed solutions and have estimated it to be 1:2 (it-PMMA/st-PMMA).3a It is possible to discuss stereocomplex formation between it-PMMA and st-poly(alkyl methacrylate)s, because, in these cases, the st-/itratio has the same value. In the case of the present assemblies, the mean ratios (st-/it-polymers) determined from the QCM measurements were 2.0 ( 0.4, 1.8 ( 0.5, and 2.5 ( 0.5, respectively, which were estimated from the it-PMMA to st-poly(alkyl methacrylate) step, except for the two initial steps, and indicated stereocomplex formation. Static Contact Angle Measurement. The static contact angles on the air side of the film surface of stereoregular PMMAs are significantly different from each other because of the selective accumulation of functional
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Figure 3. Dependence of the frequency shift, after a 20-step assembly, and the ratio of st-/it-polymers on the concentration of (a) it-PMMA (Mn ) 20 750) and st-PEMA (Mn ) 21 340) and (b) it-PMMA (Mn ) 20 750) and st-PPMA (Mn ) 23 800).
groups at the surface.13 We used this characteristic to monitor the stepwise assembly of the stereoregular polymers on the substrate. In the system using stereoregular PMMAs,3a the angle of the ultrathin PMMA film was altered by the stepwise assembly. Here, we analyzed the static contact angle of the assemblies prepared from it-PMMA and other st-poly(alkyl methacrylate)s. Figure 2 shows the dependence of the angles, at the QCM surface, on each assembly step. The angle alternately changed with each step, indicating stepwise assembly on the substrate. The mean angles at the odd steps (it-PMMA assembly) were 63.3 ( 0.3°, 63.7 ( 0.5°, and 63.0 ( 0.3° for the it-PMMA assembly with st-PMMA, st-PEMA, and st-PPMA, respectively. These values were essentially the same, within experimental error, and were consistent with 63.0 ( 0.3°, which is the angle of a bulk it-PMMA film cast on a bare QCM substrate. In addition, the surface roughness does not seem to affect the angle (see AFM analysis). Therefore, we considered the surface following immersion in it-PMMA to be completely covered by itPMMA molecules alone. Significantly, the it-PMMA layer was not replaced by st-polymer in the complex film, although the interfacial free energy is higher than that of st-polymer (see below). This replacement may preferably have occurred to minimize the interfacial free energy. These results imply stable stereocomplex formation under the physically adsorbed it-PMMA layer. On the other hand, the mean angles at the even steps (st-poly(alkyl methacrylate) assembly) were 71.2 ( 0.4°, 75.7 ( 0.5°, and 78.0 ( 0.6°, respectively, which were different from each other. In addition, these values were slightly smaller than 73.2 ( 0.8°, 78.7 ( 0.7°, and 80.3 ( 0.6°, which are the angles of the bulk cast st-PMMA, stPEMA, and st-PPMA films, respectively. These values suggest that stereocomplex formation occurred at each even step, although the real contact angle of the stereocomplex surface has not been previously reported. Significantly, the angle for the present assemblies increased with increasing alkyl chain length of the st-polymers. Note that the alkyl ester groups of the st-poly(alkyl methacrylate) point outward in the assembly structure of the doublestranded stereocomplex, as Challa et al. have reported.4c Our observation also supports stereocomplex formation between it-PMMA and st-poly(alkyl methacrylate)s on the QCM surface. We concluded that the surface composition
of each ultrathin film was altered by the stepwise assembly, possibly by physisorption of it-PMMA and by stereocomplex formations between it-PMMA and st-poly(alkyl methacrylate)s on the substrate. At the point of polymeric coating with ultrathin films on various materials, it is useful to control the delicate wettability, which is possible using the present system. The obtained films would be also thermally stable, compared to the corresponding homopolymers.14 Effect of Concentration and Molecular Weight on Assembly. Either clear solutions, precipitates, or gels are obtained by mixing acetonitrile solutions of it- and st-PMMAs, depending on the concentration.4g In the stepwise assembly of stereoregular PMMAs,3 the amount assembled was increased and could be saturated by increasing the PMMA concentration, although the assembly ratio (st-/it-PMMA) remained constant at 2, based on complex stoichiometry. We analyzed the effect of polymer concentration on stepwise assembly. The combination of stereoregular PMMAs has been described previously.3a Here we demonstrated combinations between it-PMMA and st-PEMA or st-PPMA. The frequencies were shifted by stepwise assembly between it-PMMA (Mn ) 20 750) and st-PEMA (Mn ) 21 340) or st-PPMA (Mn ) 23 800), similar to Figure 1, even though the concentrations were altered (original data not shown). Figure 3 shows the dependence of -∆F20steps and the ratio of st-/ it-polymers assembled on the concentration. In both cases, the ratio had a somewhat large experimental error because of the detection limitations of the 9 MHz QCM at each step, although the -∆F20steps was reproducible within a 5% error. The -∆F20steps clearly increased and was saturated with increased concentration, indicating that the assembly was based on concentration dependent adsorption. On the other hand, as the st-/it- ratios were still around 2 at all concentrations, complex formation was suggested even when the concentration was altered. The ratios will be more accurately studied using a highly sensitive QCM soon. The molecular weights of it- and st-PMMAs are known not to affect stereocomplex formation if the PMMAs have sufficient stereoregularity and high enough molecular weights.4g Here, st-PEMA (Mn ) 10 880) and st-PPMA (Mn ) 12 340) with smaller molecular weights were also applied to stepwise assembly. The frequency shift was
(13) (a) Tretinnikov, Qleg. N. Langmuir 1997, 13, 2988. (b) Tretinnikov, Qleg. N.; Ohta, K. Langmuir 1998, 14, 915.
(14) Kitayama, T.; Fujimoto, N.; Hatada, K. Polym. Bull. 1991, 26, 629.
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Figure 4. RAS spectra of a 30-step assembly of it-PMMA (Mn ) 20750) and st-PEMA (Mn ) 21340) at a concentration of 1.7 × 10-2 unit M from (c) acetonitrile and ATR spectra of the cast films of (a) it-PMMA, (b) st-PEMA, and (d) sum spectra of a and b.
Figure 5. RAS spectra of a 30-step assembly of it-PMMA (Mn ) 20750) and st-PPMA (Mn ) 23800) at the concentration of 1.7 × 10-2 unit M from (c) acetonitrile and ATR spectra of the cast films of (a) it-PMMA, (b) st-PPMA, and (d) sum spectra of a and b.
greater as the number of assembly steps increased, regardless of molecular weight, indicating stepwise complex formation of it-PMMA and st-PEMA or st-PPMA (original data not shown). Moreover, the frequency shifts after a 20-step assembly were 198 and 291 Hz, respectively. The values are clearly smaller than those in Figure 1, although the st-/it- ratios were still around 2. Although a detailed discussion of the molecular weight effect is difficult, the physisorption of it-PMMA seemed to be facilitated by the increasing molecular weight of the stpolymer. If one recalls the stereocomplex stoichiometry, the it-PMMA utilized here might assemble with stpolymers with around twice the molecular weight by way of 1:1 molecule assembly. As the st-polymers did not have the corresponding molecular weight, st-PEMA and stPPMA should be cross-linked in the assembly. Absorption Spectra Measurement. Infrared spectroscopy is one of the most useful tools for evaluating stereocomplex formation of stereoregular PMMAs, as previously reported by other researchers.15 The absorption band of main chain CH2-rocking vibrations at around 840860 cm-1 in addition to CdO-stretching vibrations at around 1700-1800 cm-1 is available for evaluating PMMA stereocomplex formation. We have utilized RAS/Fourier transform infrared spectroscopy (FT-IR), which can detect (15) Tretinnikov, Qleg. N.; Nakao, K.; Ohta, K.; Iwamoto, R. Macromol. Chem. Phys. 1997, 197, 753.
weak absorption by the PMMA ultrathin film.3a We observed that the main peak in CH2-rocking absorption for the PMMA assemblies was approximately at 860 cm-1 but not at 840 cm-1, indicating stereocomplex formation between it- and st-PMMAs on the substrate. The CdOstretching vibration band for the stepwise assembly was also shifted to a higher wavenumber (1750 cm-1) than that of the cast films (1725 cm-1). In this paper, we measured RAS for each assembly. As the assembling amount for the assembly between itPMMA and st-PEMA was too small to measure using RAS, the stepwise assemblies from water-mixed solutions of these polymers were applied instead. In our previous study, the amount of PMMA stereocomplex assembled was significantly increased by the addition of water to the acetonitrile solutions.3a Even in the present system, the amount of assembly between it-PMMA and st-PEMA or st-PPMA after 20 immersion steps was increased to be 601 and 892 Hz, respectively. Polymer thicknesses were calculated to be 14 and 21 nm, respectively. In addition, the st-/it- ratio of these assemblies from water-mixed solutions was 2.0 ( 0.3 in both cases, indicating stereocomplex formation (QCM data not shown). Figure 4 shows RAS data for the assembly between it-PMMA and st-PEMA that was prepared from the acetonitrile/water (10/1, v/v) mixed solution, together with the ATR/FT-IR spectra of cast films obtained from
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shifted to a higher wavenumber (1732 cm-1) compared to values for cast films (1725 cm-1), similar to observation in the PMMA system.3a Accordingly, stereocomplex formation between it-PMMA and st-PEMA was suggested from the RAS measurement. Figure 5 shows RAS data for the assembly between it-PMMA and st-PPMA that was prepared from the acetonitrile/water (10/0.4, v/v) mixed solution, together with ATR/FT-IR spectra of cast films obtained from itPMMA and st-PPMA, as well as a computer-simulated sum of their spectra in the unit molar ratio 1:2.16 The CH2-rocking peak of the PPMA assembly was also different from the sum spectrum. In addition, CdO-stretching vibration bands for the stepwise PPMA assembly was also shifted to a higher wavenumber (1750 cm-1) compared to those of the cast films (1725 cm-1). Therefore, these observations also suggested stereocomplex formation between it-PMMA and st-PPMA in the assembly. AFM Image of Ultrathin Film. AFM is a useful tool for the analysis of the surface topology of ultrathin films. In this study, we directly observed the surface of the assemblies that were deposited on a QCM by AFM using a tapping mode. First, we observed the surface of a bare QCM gold substrate and analyzed the roughness to facilitate comparisons with assembled ultrathin films. Figure 6a shows the AFM image of a bare QCM surface. As reported previously,3 we observed the domain-like surface structure of the spattered gold film on the QCM surface. The mean square roughness (Ra) was 1.8 nm. These values show that the surface of a QCM electrode is relatively smooth. Parts b and c of Figure 6 show the AFM images of the assemblies between it-PMMA (Mn ) 20 750) and st-PEMA (Mn ) 21 340) or st-PPMA (Mn ) 23 800), respectively, which were prepared from the acetonitrile/water mixed solutions (10/1 and 10/0.4, v/v, respectively). Both of the images also showed the domain-like structures that were apparently different from that of the bare QCM surface (Figure 6a). The Ra of the assemblies was 3.0 and 5.1 nm, respectively, and the surfaces seemed to be smooth enough. We found that the polymers resulting from stepwise assembly between it-PMMA and st-poly(alkyl methacrylate)s have a domain-like and molecularly smooth surface. Conclusions
Figure 6. AFM image of (a) a bare QCM electrode and a 20step assembly between it-PMMA (Mn ) 20750) and (b) st-PEMA (Mn ) 21340) or (c) st-PPMA (Mn ) 23800).
it-PMMA and st-PEMA, as well as a computer-simulated sum of their spectra in the unit molar ratio 1:2.16 Analysis of the PMMA stereocomplex formation from the RAS measurement was relatively easy, because only one CH2 group is included in the molecule. In the present case, because the CH2 group is included in the lateral side chain, the analysis was complicated. In fact, the peak at 860 cm-1, which is a key peak for stereocomplex formation, was observed even for the cast film of st-PEMA. However, complex formation was suggested on the basis of the difference in spectra between the assembly and the sum of it-PMMA and st-polymer spectra. In addition, the CdO-stretching vibration band for the assembly was (16) The sum spectra is the mere sum of it-PMMA and st-PEMA which were unified to obtain the intensity of the CdO-stretching vibration band.
We demonstrated the stepwise assembly of it-PMMA and st-poly(alkyl methacrylate)s on a QCM substrate by alternate immersion in acetonitrile solutions. The st-/itpolymer ratio calculated from the adsorption amount in each step, the static contact angle, and spectroscopic studies confirmed stepwise stereocomplex assembly. The static contact angle of the assembly surface suggested that complex formation occurred during the step from itPMMA to st-poly(alkyl methacrylate)s. The assembly includes a molecular rearrangement that was initialized by the penetration of st-polymer with bulky branched chains into the it-PMMA layer. The contact angle at stereocomplex surface increased with increasing alkyl chain length of st-polymers. On the point of polymeric coating by ultrathin films on various materials, it is useful to control the delicate wettability by using the present system. The assembly was achieved by altering conditions such as the polymer concentration, molecular weight of the st-poly(alkyl methacrylate), and the type of solvent. The surface topography of these assemblies was analyzed by AFM, revealing a domain-like structure, that was molecularly smooth. We remain interested in the mechanical properties and thermal stability of assemblies
Stereocomplex Assembly of Methacrylate Polymers
between it-PMMA and st-poly(alkyl methacrylate) with nanoscale thickness. It is conceivable that there is generality for stepwise preparation of ultrathin polymer films using stereocomplex formation at the liquid-solid interface. Acknowledgment. We acknowledge Dr. A. Kishida (Kagoshima University, Japan) for useful discussions. This
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work was financially supported in part by a Grant-in-Aid for Scientific Research in the Priority Area of “Molecular Synchronization for Design of New Materials System” (No. 404/11167270) and by a Grant-in-Aid for Scientific Research (No. 12750802) from the Ministry of Education, Science, Sports and Culture, Japan. LA0101898
