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Langmuir 2004, 20, 7412-7421
Micellization of Non-Surface-Active Diblock Copolymers in Water. Special Characteristics of Poly(styrene)-block-Poly(styrenesulfonate) Hideki Matsuoka,* Shin’ichi Maeda, Ploysai Kaewsaiha, and Kozo Matsumoto Department of Polymer Chemistry, Kyoto University, Kyoto 615-8510, Japan Received March 25, 2004. In Final Form: June 2, 2004 Strongly ionized amphiphilic diblock copolymers of poly(styrene)-b-poly(styrenesulfonate) with various hydrophilic and hydrophobic chain lengths were synthesized by living radical polymerization, and their properties and self-assembling behavior were systematically investigated by surface tension measurement, foam formation, hydrophobic dye solubilization, X-ray reflectivity, dynamic light scattering, small-angle neutron scattering, small-angle X-ray scattering, and atomic force microscope techniques. These copolymer solutions in pure water did not show a decrease of surface tension with increasing polymer concentration. The solutions also did not show foam formation, and no adsorption at the air/water interface was confirmed by reflectivity experiments. However, in 0.5 M NaCl aq solutions polymer adsorption and foam formation were observed. The critical micelle concentration (cmc) was observed by the dye solubilization experiment in both the solutions with and without added salt, and by dynamic light scattering we confirmed the existence of polymer micelles in solution, even though there was no adsorption of polymer molecules at the water surface in the solution without salt. By the small-angle scattering technique, we confirmed that the micelles have a well-defined core-shell structure and their sizes were 100-150 Å depending on the hydrophobic and hydrophilic chain length ratio. The micelle size and shape were unaffected by addition of up to 0.5 M salt. The absence of polymer adsorption at the water surface with micelle formation in a bulk solution, which is now known as a universal characteristic for strongly ionized amphiphilic block copolymers, was attributed to the image charge effect at the air/water interface due to the many charges on the hydrophilic segment.
Introduction Amphiphilic diblock copolymers, which consist of hydrophobic and hydrophilic chains, have unique properties such as self-assembly formation in water and at the air/water interface.1 The basic properties such as surface activity and self-assembly formation are important in polymer and surface sciences in addition to their practical use for medical (drug delivery, biocompatible materials, etc.) and surface modification (paints, coating, etc.) applications.2,3 We have been investigating amphiphilic block copolymers from synthesis, structure, and properties to selforganization (micelles, monolayers) for nonionic,4 weakly ionic,5 and fluorinated6 block copolymers from a fundamental scientific point of view and have found their unique properties and behaviors. Especially, ionic amphiphilic diblock copolymers have been hard to synthesize due to the large difference in solubility of each block, and there have been few systematic investigations on such poly* To whom correspondence should be addressed. (1) A part of this work was presented in the 51st Annual Meeting of the Society of Polymer Science, Japan, at Yokohama, May 2002: Maeda, S.; Matsumoto, K.; Matsuoka, H. Polym. Prepr. Jpn. 2002, 51, 577. (2) (a) Amphiphilic Block Copolymers: Self-Assembly and Application; Alexandridis, P., Lindman, B., Eds.; Elsevier: Amsterdam, 2000. (b) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, 1998. (3) Weber, S. E. J. Phys. Chem. B 1998, 102, 2618. (4) (a) Nakano, M.; Matsuoka, H.; Yamaoka, H.; Poppe, A.; Richter, D. Macromolecules 1999, 32, 697. (b) Nakano, M.; Matsumoto, K.; Matsuoka, H.; Yamaoka, H. Macromolecules 1999, 32, 4029. (5) Ishizuka, T.; Matsumoto, K.; Matsuoka, H. Polym. Prepr. Jpn. 2002, 51, 554. (6) (a) Matsumoto, K.: Kubota, M.; Matsuoka, H.; Yamaoka, H. Macromolecules 1999, 32, 7122. (b) Matsumoto, K.; Mazaki, H.; Nishimura, R.; Matsuoka, H.; Yamaoka, H. Macromolecules 2000, 33, 8295. (c) Matsumoto, K.; Mazaki, H.; Matsuoka, H. Macromolecules 2004, 37, 2256.
mers.7 Recently, we succeeded in synthesizing amphiphilic diblock copolymers having sulfonic groups, which were strongly ionized amphiphilic block copolymers, and found that they had very unique behavior: the polymers did not adsorb at the air/water interface, but they formed micelles in water solutions.8 This would be a totally new observation in the field of physical chemistry of surface and interfaces. It is currently accepted that the common surfactants or amphiphiles are first adsorbed at the water surface to form a Gibbs monolayer and form micelles in solution after saturation of the surface by molecules. The block copolymers with a carboxylic acid, that is, weakly ionized, were also studied previously.5,9 They also did not reduce the surface tension of the solution but they adsorbed at the air/water interface, which was confirmed by measurements of X-ray reflectivity and observation of foam formation, although in small amounts. Our previous observation, that is, micellization without adsorption, was considered to be a unique property only for strongly ionic amphiphilic diblock copolymers. However, to confirm the universality of this concept, a systematic investigation for other polymers having various molecular structures and/or architecture would be needed. In addition, the polymers used in our previous study were not purely strongly ionic since they had carboxylic acid groups in addition to sulfonic acid. (7) (a) Astafieve, I.; Khougaz, K.; Eisenberg, A. Macromolecules 1995, 28, 7127. (b) Keserow, D.; Prochazka, K.; Ramiredy, C.; Tuzar, Z.; Munk, P.; Webber, S. E. Macromolecules 1992, 25, 1197. (c) Fo¨ster, S.; Hermsdorf, N.; Leube, W.; Schnablegger, H.; Lindner, P.; Bo¨ttcher, C. J. Phys. Chem. B 1999, 103, 6657. (d) Ahrens, H.; Fo¨ster, S.; Helm, C. A. Macromolecules 1997, 30, 8447. (e) Gabaston, L. L.; Furlong, S. A.; Jackson, R. A.; Armes, S. P. Polymer 1999, 40, 4505. (8) Matsuoka, H.; Matsutani, M.; Mouri, E.; Matsumoto, K. Macromolecules 2003, 36, 5321. (9) Matsumoto, K.; Ishizuka, T.; Matsuoka, H. Langmuir, in press.
10.1021/la0492153 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/31/2004
Micellization of Non-Surface-Active Copolymers
Recently, it has become possible to synthesize a variety of novel polymers by rapid improvement of the living radical polymerization technique.10 In this study, we applied this contemporary synthetic technique to obtain a strongly ionic amphiphilic diblock copolymer, that is, poly(styrene)-b-poly(styrenesulfonic acid) with a wellcontrolled molecular weight, narrow molecular weight distributions, and block ratio (m:n), and studied systematically its fundamental properties, such as surface activity, foam formation, and self-assembling behavior in solution and at the air/water interface. As a result, we clarified that the polymer also forms micelles in solution above a certain polymer concentration (critical micelle concentration, cmc) without adsorption at the water surface in a no-salt condition.
Langmuir, Vol. 20, No. 18, 2004 7413 Scheme 1. Synthesis of Stm-b-SSNan
Experimental Section Materials. Hydrochloric acid, sodium hydroxide, 2,2′-azobisisobutyronitrile (AIBN), dichloromethane, methanol, and styrene were purchased from Wako Pure Chemicals (Osaka, Japan). AIBN and styrene were purified by standard methods. Trimethylsilyl iodide was purchased from Tokyo Kasei (Tokyo). Oil Orange SS was also a product of Tokyo Kasei. Deuterium oxide for neutron experiments, CDCl3, and DMSO-d6 were products of Cambridge Isotope Laboratory (U.K.). Molecular sieve 4A was purchased from Wako and dried under a vacuum at 150 °C for 2 h before use. Carbon tetrachloride was dried for at least 1 h in an eggplant flask with molecular sieves. Water used for sample preparation was ultrapure water obtained from a Milli-Q System (Millipore, Bedford, MA) whose resistance was more than 18 MΩ cm. Synthesis of DEPN and SSPen. The mediator for living radical polymerization, 2,2,5,5-tetramethyl-4-diethylphosphono3-azahexane-3-nidroxide (DEPN), was synthesized by the method proposed by Benoit et al.11 Neopenthyl p-styrenesulfonate (SSPen) was synthesized according to the procedure reported by Okamura et al.12 Synthesis of PS-b-PSSPen. Poly(styrene)-b-poly(styrenesulfonate) (PS-b-PSSPen) was synthesized basically according to the procedure reported by Okamura et al.12 DEPN end-capped PSt prepolymer was first synthesized as follows. To a 100 mL vacuum glass tube equipped with a Teflon screw cap, 1/200 equiv of AIBN and 2.5/200 equiv of DEPN were added to a styrene monomer, and the mixture was degassed with three freezethaw cycles. Then Ar was introduced into the tube. The mixture was stirred at 110 °C under an Ar atmosphere for 2 h. The resultant polymer was dissolved into dichloromethane and precipitated twice with methanol. The polymer was dried under a vacuum overnight. The number-averaged degree of polymerization (DP) and molecular weight distribution were determined to be 54 and Mw/Mn ) 1.15, respectively, by gel permeation chromatography (GPC). Polystyrene with DP ) 31 (Mw/Mn ) 1.09) was synthesized in a similar way but with different reaction time. Block copolymers were synthesized using the PSt prepolymer obtained above as a macroinitiator. The PSt was placed in a 100 mL vacuum glass tube equipped with a Teflon screw cap, and a 2-fold equivalent of SSPen to St unit was added. The mixture was heated under Ar until complete dissolution of PSt, and then the mixture was heated to 100 °C and stirred for about 1 h. After cooling to room temperature, the products were dissolved into chloroform and precipitated with a large amount of hexane. PSt-b-PSSPen was obtained after drying under a vacuum overnight. The block ratio was determined by 1H NMR with deuterated chloroform as a solvent (Scheme 1, step 2). (10) (a) Patten, T. E.; Matyjaszewski, K. Acc. Chem. Res. 1999, 32, 895. (b) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661. (c) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689. (11) Benoit, D.; Chaplinski, V.; Braslau, R.; Hawker, C. J. J. Am. Chem. Soc. 1999, 121, 3904. (12) Okamura, H.; Takatori, Y.; Tsunooka, M.; Shirai, M. Polymer 2002, 43, 3155.
Synthesis of PS-b-PSSNa. PSt-b-PSSPen thus obtained was dissolved into dried carbon tetrachloride. A 4-fold equivalent of trimethylsilyl iodide (TMSI) was added and heated at 50 °C overnight. The solution was concentrated under a vacuum, and unreacted TMSI and carbon tetrachloride were eliminated. Then, 1 M HCl/methanol (1/1 v/v) was added to the polymer. After vigorous stirring for about 4 h, a reddish-brown transparent solution was obtained. The obtained solution was neutralized by addition of 1 M NaOH; then finally uncolored PS-b-PSSNa solution was obtained. PS-b-PSSNa solution was dialyzed against ultrapure water for about 1 week. The electric conductivity of dialyzed water was checked to determine the end point of dialysis. The PS-b-PSSNa solution thus purified was lyophilized (Scheme 1, step 3). 1H Nuclear Magnetic Resonance (1H NMR). 1H NMR spectra were obtained by a JEOL GSX 270 (JEOL, Tokyo). Deuterated chloroform was used as a solvent for the polymer before hydrolysis, and deuterated dimethyl sulfoxide (DMSO-d6) was used for those after hydrolysis. Gel Permeation Chromatography. GPC experiments with chloroform as an eluent were performed using a JASCO system LC-900 with a UV-970 UV detector, a DG-980-50 refractive index detector, and polystyrene columns (K-802, K-803×2, K-804, and K-805). The eluent was chloroform for polystyrene, and the concentration of sample solution injected was ca. 12 mg/mL. For block copolymers, a Shodex KF-804L column was used with tetrahydrofuran as an eluent. Polystyrene standards (Pressure Chemical, Pittsburgh, PA) were used for calibration. The numberaveraged molecular weight (Mn) and the polydispersity index (Mw/Mn) were determined for polystyrene homopolymers and block copolymers. The increase of molecular weight for the block copolymer, that is, the block polymerization, was confirmed to proceed. Surface Tension Measurements. The surface tension of block copolymer aqueous solutions was measured with a FACE CBVP-Z Surface Tensitometer from Kyowa Interface Science Co.,
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Ltd. (Tokyo), using a Pt plate in full automatic mode. The highest polymer concentration solution was prepared first, and then the solutions of other concentrations were prepared by dilution with Milli-Q water or salt solution. The solution was prepared using only cleaned glassware. The surface tension was measured about 2 h after the solution was put into the glass cell without disturbance. Foam Formation and Stability. Pure water and 0.5 M NaCl aq solutions containing 1% polymer were shaken violently for 1 min. Then 1 and 30 min later, the state of the air/water interface was recorded with a digital camera. Solubilization Experiments. Hydrophobic dye solubilization experiments were carried out with a Hitachi UV/vis spectrophotometer U-3310 (Hitachi, Tokyo) with Oil Orange SS as a dye. The sample solutions with different polymer concentrations were prepared by the same procedure in the surface tension measurement using only glassware. The measurements were performed 1 week after Oil Orange SS addition into solutions. A 10 mm flat quartz cell (Hellma) was used, and the polymer solution without dye was used as a reference sample for spectrometry. The specific absorption by Oil Orange SS at 495 nm, which is distant from that of the benzene ring in the polymer unit (270 nm), was evaluated. Dynamic Light Scattering (DLS). DLS experiments were carried out using a BI-30 goniometer with a BI-3000AT correlator from Brookhaven Co. (Ronkonkoma, NY). The details of DLS measurements were as described previously.13 A DLS-7000 system (Otsuka Electronic, Osaka, Japan) was also used for some DLS experiments. The time correlation functions were analyzed by the double exponential and cumulant methods depending on the condition.14 The measurements were performed at five different scattering angles, and the diffusion coefficient was calculated from the slope of the straight line in the decay rate Γ versus q2 plot. q is the scattering vector. Small-Angle Neutron Scattering (SANS). The SANS experiments were performed using the SANS-U system of the Institute for Solid State Physics, The University of Tokyo, at the JRR3-M reactor at Tokai, Ibaragi, Japan.15 A 4 mm thick quartz flat cell was used as a sample container. D2O was used as the solvent for SANS measurement. The details of the measurement were described elsewhere.4b The obtained SANS profiles in absolute scale were analyzed by a simple core-shell model for sphere and sphere-rod mixture as in our previous studies.4a,6c Small-Angle X-ray Scattering (SAXS). The SAXS was measured using a SAXS instrument in our laboratory. This instrument was composed of a Kratky U-slit optical system, a 1D position sensitive proportional counter (PSPC), and a 60 kV-200 mA rotating anode X-ray generator. The details of the instrument have been fully described elsewhere.16 A typical accumulation time was 6000 s. The obtained SAXS profiles were analyzed by a simple core-shell model for sphere and sphere-rod mixture as in our previous studies.4a,6c X-ray Reflectivity (XR). XR was measured with a RINTTTR-MA reflectometer (Rigaku Corp.) with a Langmuir-Blodgett (LB) trough at the sample position. The details of the instrument, data collection, and data analysis were fully described elsewhere.17 All measurements were performed by specular geometry. Atomic Force Microscopy (AFM). Micelle particles were observed by AFM using an SPI3800 system with an SPA300 probe (Seiko Instruments, Tokyo, Japan). Slide glasses used as a substrate were kept in a saturated KOH solution of ethanol overnight. After rinses in water, they were sonicated in a chemical soap solution (SCAT). Sonication was repeated at least five times with changing solution; then the substrates were dried at 100 (13) Matsuoka, H.; Ogura, Y.; Yamaoka, H. J. Chem. Phys. 1998, 109, 6125. (14) Chu, B. Laser Light Scattering, Basic Principles and Practice, 2nd ed.; Academic Press: Boston, 1991. (15) Ito, Y.; Imai, M.; Takahashi, S. Physica B 1995, 213, 889. (16) Ise, N.; Okubo, T.; Kunugi, S.; Matsuoka, H.; Yamamoto, K.; Ishii, Y. J. Chem. Phys. 1984, 81, 3294. (17) (a) Yamaoka, H.; Matsuoka, H.; Kago, K.; Endo, H.; Eckelt, J.; Yoshitome, R. Chem. Phys. Lett. 1998, 295, 245. (b) Matsuoka, H.; Mouri, E.; Matsumoto, K. Rigaku J. 2001, 18, 54.
Matsuoka et al.
Figure 1.
1H
NMR spectra of St54-b-SSPen61 in CDCl3.
Figure 2.
1H
NMR spectra of St54-b-SSNa61 in DMSO-d6.
°C overnight. A 1 wt % water solution of polymers was dropped on the cleaned slide glass by a Pasteur pipet. After making the solution drop to the homogeneous water layer by tilting the slide, it was dried by aspirator evaporation. An SI-DF20 cantilever in dynamic force mode was used for the observation.
Results and Discussion Synthesis of Diblock Copolymers Having Strong Acid Groups. Figure 1 shows an example of the 1H NMR spectra for PSt-b-PSSPen. Formation of the desired block copolymer is obvious since block copolymer formation was already confirmed by GPC experiment. Since the degree of polymerization of the styrene block is already known by GPC measurement before the second polymerization, the block ratio could be calculated from the integrated intensity ratio of peak c to peak d. The ratio m:n ) 54:61 was obtained for the polymer shown in Figure 1. The m:n values of other polymers with various degrees of polymerization were estimated in the same way. Figure 2 shows the 1H NMR spectra for the m:n ) 54:61 polymer after hydrolysis of the neopenthyl group. The methyl peak (peak a in Figure 1 at ca. 1 ppm) had almost disappeared in Figure 2, which means that the hydrolysis reaction proceeded almost perfectly. In the original paper by Okamura et al.,12 the polymer was hydrolyzed at ambient temperature for 4 h by using methylene chloride (bp, ca. 39 °C) as a solvent. However, this treatment resulted in incomplete hydrolysis when the PSSPen chain was longer than PSt. Hence, in this study, we used carbon tetrachloride (bp, ca. 76 °C) as a reaction solvent and kept the polymer at 50 °C, which resulted in almost perfect hydrolysis even for polymers with longer PSSPen chains. The degree of hydrolysis was calculated from the peak area of these 1H NMR spectra. Table 1 shows the characteristics, that is, the molecular weight, the molecular weight distribution, the degree of polymerization of each block, and the degree of hydrolysis, for three block copolymers having sulfonic groups thus synthesized. All
Micellization of Non-Surface-Active Copolymers Table 1. Characteristics of Stm-b-SSNan Block Copolymers ma:nb
Mna,b
Mw/Mna
degree of sulfonationb
31:58 54:61 54:35
15 500 18 500 13 200
1.22 1.18 1.10
0.96 0.98 0.95
a 1H
Determined by GPC with THF as an eluent. b Determined by NMR.
Figure 3. Polymer concentration dependence of the surface tension of Stm-b-SSNan solutions in pure water and in 0.5 M NaCl aq: (left) m:n ) 31:58, (middle) m:n ) 54:61, (right) m:n ) 54:35. Open circles, in pure water; closed circles, in 0.5 M NaCl aq.
three dried polymer samples thus obtained could be dissolved directly into the water and salt solutions. Surface Inactivity of PSt-b-PSSNa. Figure 3 shows the polymer concentration dependence of the surface tension of PSt-b-PSSNa solution in water and 0.5 M NaCl aq for three samples with different block ratios, that is, m:n ) 31:58, 54:61, and 54:35. At first sight, it is obvious that the surface tension γ is not reduced with increasing polymer concentration and the cmc, which is always observed for low molecular weight surfactants and nonionic polymer surfactants as a point where γ becomes constant, is not detected. Furthermore, the absolute value of γ is almost equal to that for pure water (73 mN/m at 20 °C). From these surface tension measurements, it can be said that the PSt-b-PSSNa block copolymer is surface inactive; the polymer molecules were not adsorbed at the air/water interface although the polymer itself consists of hydrophilic and hydrophobic segments. This curious behavior is also observed even in 0.5 M NaCl aq. Interestingly, a similar phenomenon, that is, no reduction of surface tension, was also observed for weakly ionic amphiphilic diblock copolymer systems such as those having carboxylic acid groups5,9 and sulfonic acid groups at a very low concentration.18 Photos in Figure 4 show the foam formation and foam stability of these three polymer solutions in pure water and 0.5 M NaCl at a 1 wt % polymer concentration. For pure water solutions (indicated by “no salt” in the photos), no foam formation at all was observed for all these three polymers although just after vigorous shaking. On the other hand, 0.5 M NaCl solutions (indicated by “salt” in the photos) show slight foam formation. However, the foams were not so stable and the number of foams decreased 30 min after shaking. In principle, since foam formation results in an increase of surface area, a decrease of surface tension is necessary for foam formation.19 The mechanical energy by shaking changes into excess surface free energy for the surface area increase. The absence of surface tension change is consistent with the absence of (18) (a) Guenoun, P.; Davis, H. T.; Tirrell, M.; Mays, J. Macromolecules 1996, 29, 3965. (b) Fontaine, P.; Daillant, J.; Guenoum, P.; Alba, M.; Braslau, A.; Mays, J.; Petit, J.-M.; Rieutord, F. J. Phys. II France 1997, 7, 401. (19) For example: Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley: New York, 1997.
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foam formation for pure water systems. Foam stability is largely affected by molecules adsorbed at the foam surface. The water film of foam becomes thinner by the water draining effect of the Plateau border, but the water film was stabilized by steric and electrostatic repulsion between surfactant molecules adsorbed at both sides of the water film of foam.19 Hence, the observation of foam in the salt solutions means that some polymer molecules had been adsorbed at the water surface. However, the amount should be small since the surface tension was not markedly reduced even in 0.5 M salt solutions. The XR on the surface of liquids was directly measured using a theta-theta goniometer with the rotation plane in vertical geometry to detect the polymer molecules adsorbed at the air/water interface.17 XR profiles are shown in Figure 5. The reflectivity profiles of the pure water solutions were similar to those of pure water alone, but those of the salt solution clearly deviated from those of the pure solvent, that is, the 0.5 M NaCl aq solution showed an increase of reflectivity in the middle q range (q ) 0.1-0.4 Å-1; q is the absolute value of the scattering vector defined by q ) 4π sin θ/λ, where θ is the incident/ reflection angle and λ is the wavelength of X-rays), which means the existence of a film at the air/water interface. This fact is in good agreement with the observation of foam formation in the 0.5 M NaCl systems in Figure 4. However, since the reflectivity profiles of the salt systems do not show a Kiessig fringe, a well-defined monolayer or film is not formed. These XR profiles could not be quantitatively analyzed and the amount of polymers adsorbed could not be estimated since they are simple profiles without a Kiessig fringe, but only a small amount of polymer was adsorbed at the air/water interface in the 0.5 M NaCl solution. For the case of 31:58 polymer in 0.5 M NaCl solution, a very thin layer with 14 Å thickness and large roughness (say 7 Å) reproduced the experimental XR profile to some extent but we could not determine the density of the layer. This situation is consistent with the finding shown in Figure 3, that is, no decrease of surface tension. The findings mentioned so far, that is, the surface nonactivity, were similar to those reported previously for poly(diethylsilacyclobutane)-b-poly(sulfonated methacrylic acid) (PEt2SB-b-PSPMNa),8 which did not show reduction of surface tension, foam formation, or adsorption at the air/water interface. Those hydrophilic segments had not only sulfonic acid groups but also carboxylic acid groups. The amphiphilic diblock copolymer we synthesized in this study had only sulfonic acid groups in the hydrophilic chain. Hence, the peculiar behavior observed in this study might be universal for diblock copolymers having sulfonic acid groups on their hydrophilic segments. Interestingly, PEt2SB-b-PSPMNa showed micelle formation even without adsorption at the surface. In other words, a polymer micelle was formed without the Gibbs monolayer formation, which would be a totally new phenomenon in surface science. We performed hydrophobic dye solubilization experiments to confirm the micelle formation in the present case. Evidence for Micelle Formation without Gibbs Monolayer Formation and Existence of cmc. We performed solubilization experiments for St31-b-SSNa58 solutions at various polymer concentrations with addition of a water-insoluble hydrophobic dye. At low polymer concentrations, the solutions were clear and not colored. However, at certain polymer concentrations, the solution was colored by the dye and the color became deep with increasing polymer concentration. This apparent solubilization of water-insoluble hydrophobic dye means micelle
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Figure 4. Foam formation and stability for the solutions of Stm-b-SSNan in pure water and in 0.5 M NaCl aq at 1 wt % polymer concentration.
Figure 5. X-ray reflectivity profiles for the solutions of Stm-b-SSNan in pure water and in 0.5 M NaCl aq at 1 wt % polymer concentration. XR profiles for pure water and pure 0.5 M NaCl aq. The legend for the symbols in the figure is common for all three figures for 31:58, 54:61, and 54:35 (throughout this paper). Red lines are Fresnel curves (∼q-4).
Figure 6. Intensity of hydrophobic dye adsorption (495 nm) as a function of polymer concentration: (left) m:n ) 31:58, (middle) m:n ) 54:61, (right) m:n ) 54:35. Open circles, in pure water; closed circles, in 0.5 M NaCl aq.
formation: the hydrophobic dye molecules were solubilized in the hydrophobic core part of polymer micelles in solution.19 This color change was quantitatively measured by a UV/vis spectrometer, and the absorbance was plotted against polymer concentration in Figure 6. All three polymers showed a rapid increase in absorption at the polymer concentration of about 10-5 mol/L. This turning point
should be the cmc where unimers start to form micelles in solution. Thus, it was confirmed that the strongly ionized amphiphilic diblock copolymer Stm-b-SSNan also showed “polymer micelle formation without Gibbs monolayer formation” even at a higher polymer concentration (up to 1 wt %). This phenomenon is never observed for low molecular weight surfactants and amphiphilic polymers having carboxyl acid groups, which are not highly charged. The amphiphilic polymers having carboxyl acid groups also did not show surface tension reduction but showed foam formation in the absence of salt. Hence, it is fair to think that both of the conditions, that is, highly charged and strongly charged situations, are necessary for polymer micelle formation with no adsorption in the absence of salt. Guenoun et al.18 also observed no serious reduction of the surface tension of the solution at a very low polymer concentration (
