Non-surface Activity and Micellization Behavior of Cationic

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Non-surface Activity and Micellization Behavior of Cationic Amphiphilic Block Copolymer Synthesized by Reversible Addition
Fragmentation Chain Transfer Process Arjun Ghosh,† Shin-ichi Yusa,‡ Hideki Matsuoka,*,† and Yoshiyuki Saruwatari§ †

Department of Polymer Chemistry, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan § Osaka Organic Chemical Industries Ltd., 7-20 Azuchi-Machi 1-Chome, Chuo-Ku, Osaka 541-0052, Japan ‡

ABSTRACT: Cationic amphiphilic diblock copolymers of poly(n-butylacrylate)-b-poly(3-(methacryloylamino)propyl)trimethylammonium chloride) (PBA-b-PMAPTAC) with various hydrophobic and hydrophilic chain lengths were synthesized by a reversible addition
fragmentation chain transfer (RAFT) process. Their molecular characteristics such as surface activity/nonactivity were investigated by surface tension measurements and foam formation observation. Their micelle formation behavior and micelle structure were investigated by fluorescence probe technique, static and dynamic light scattering (SLS and DLS), etc., as a function of hydrophilic and hydrophobic chain lengths. The block copolymers were found to be non-surface active because the surface tension of the aqueous solutions did not change with increasing polymer concentration. Critical micelle concentration (cmc) of the polymers could be determined by fluorescence and SLS measurements, which means that these polymers form micelles in bulk solution, although they were non-surface active. Above the cmc, the large blue shift of the emission maximum of N-phenyl-1-naphthylamine (NPN) probe and the low micropolarity value of the pyrene probe in polymer solution indicate the core of the micelle is nonpolar in nature. Also, the high value of the relative intensity of the NPN probe and the fluorescence anisotropy of the 1,6-diphenyl-1,3,5hexatriene (DPH) probe indicated that the core of the micelle is highly viscous in nature. DLS was used to measure the average hydrodynamic radii and size distribution of the copolymer micelles. The copolymer with the longest PBA block had the poorest water solubility and consequently formed micelles with larger size while having a lower cmc. The “non-surface activity” was confirmed for cationic amphiphilic diblock copolymers in addition to anionic ones studied previously, indicating the universality of non-surface activity nature.

’ INTRODUCTION There are several reports on the surface activity and selfassembly formation at the air/water interface and in the water by amphiphilic diblock copolymers, which consist of hydrophobic and hydrophilic chains.1
3 Hydrophobic chains coagulate in aqueous solution and precipitate, whereas the hydrophilic chain prevents precipitation by forming a core shell micellar structure, which have several applications including nanoreactors, drug delivery, DNA carrier systems, and surface modifications (paints, coating, etc.).4
6 It is well-known that the common surfactants or amphiphiles are first adsorbed at the water surface to form a Gibbs monolayer, which results in reduction of surface tension of the solution, and then form micelles in solution after saturation of the surface by molecules. We first reported the unique behavior of amphiphilic diblock copolymers; that is, they do not adsorb at the air/water interface (hence they do not reduce the surface tension of the solution) with increase of polymer concentration, but they form micelle in aqueous solution above critical concentration at a particular hydrophobic and hydrophilic chain ratio and/or solvent condition.7
10 This is a totally new observation in the field of physical chemistry of surface and interfaces. The block r 2011 American Chemical Society

copolymers with a neutralized carboxylic acid7,8 and quaternary pyridinium groups9 that are weakly ionized, and a sulfonic acid group10 that is strongly ionized, have also been studied. They also did not reduce the surface tension of the solution, but they formed micelles in aqueous solution. Almost no adsorption was confirmed by measurements of X-ray reflectivity and observation of foam formation, although in small amounts. Before our report, Wagner et al. observed no reduction of surface tension of the solutions of the ionic block copolymers in their emulsion polymerization study, but they did not give a detailed discussion.11 After our report, M€uller et al. also demonstrated that low glass transition temperature polymers, ionic PBA-b-PAA block copolymers, were not adsorbed in air/water interface, but they formed micelles in aqueous solution.12 Laschewsky et al. also studied the effect of hydrophilicity and hydrophobicity of block copolymer on the surface activity and nonactivity.13 It was also recently reported that a strong polyacid/weak polyacid diblock copolymer did not show a surface tension reduction in Received: April 27, 2011 Revised: June 12, 2011 Published: June 13, 2011 9237

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Langmuir water even at a low pH condition.14 Amiel et al. observed no adsorption on the silica surface in pure water with their charged amphiphilic diblock copolymer of PtBS-b-PSS, which is known to form micelles in aqueous solution.15 The origin of non-surface activity is thought to be image charge repulsion at the air/water interface.7
10 Because of its hydrophobicity, the block copolymer would be adsorbed at the air/water interface.16
18 However, near the interface, a strong electrostatic repulsion from the interface is induced by the image charge effect.16
18 Because the hydrophilic chain consists of polyions, this electrostatic repulsion is so strong that the polymers cannot be adsorbed but form micelles in bulk solution. So far, our investigation has been concentrated on the origin of this unique character (i.e., non-surface activity), and the examination of its difference from normal surfactant behavior with respect to the salt concentration and hydrophobic chain length dependence of the critical micelle concentration (cmc).7
10 A systematic investigation for other polymers having various molecular architecture would be needed to confirm the universality of this concept. In addition, the cationic polymers used in our previous study were not completely quaternized and behaved as weakly ionizable polymers.9 Recently, it has become possible to synthesize polymer with well-defined and controlled molecular weight by controlled/ “living” radical RAFT polymerization. 19 In this study, we applied this contemporary synthetic technique to obtain a strongly cationic amphiphilic diblock copolymer with fully ionized hydrophilic chain, that is, poly(n-butylacrylate)-b-poly(3-(methacryloylamino)propyl)-trimethylammonium chloride (nBA)m-b-(MAPTAC)n with a well controlled molecular weight, narrow molecular weight distributions, and tuned block ratio (m:n), and studied systematically its fundamental properties, such as surface activity, foam formation ability of solution, and self-assembling behavior in solution and at the air/water interface.20 As a result, we clarified that these polymers also form micelles in solution above a certain polymer concentration (critical micelle concentration, cmc) without adsorption at the water surface, supporting the image charge as a primary origin of no-surface activity.

’ EXPERIMENTAL SECTION Materials. 4,40 -Azocyanovaleric acid (ACVA), 2,20 -azobisisobutyronitrile (AIBN), dimethyl formamide (DMF), methanol, tetrahydrofuran (THF), n-butyl acrylate (BA), fluorescence probe, pyrene, NPN, and DPH were purchased from Wako Pure Chemicals (Osaka, Japan). 3-(Methacryloylamino)propyl)-trimethylammonium chloride was received from Osaka Organic Chemicals Co. Ltd. AIBN, n-butyl acrylate, pyrene, NPN, and DPH were purified by standard methods. (4-Cyanopentanoic acid)-4-dithiobenzoate was used as chain transfer agent (CTA), which was synthesized according to literature.21 D2O and CD3OD were products of Cambridge Isotope Laboratory (U.K.). Water used for sample preparation was ultrapure water obtained from a Milli-Q System (Millipore, Bedford, MA) whose resistance was 18 MΩ cm. 1 H Nuclear Magnetic Resonance (1H NMR). 1H NMR spectra were obtained by a JEOL 400WS (JEOL, Japan). Deuterated water was used as a solvent for the macro-MAPTAC, and deuterated methanol (CD3OD) was used for block copolymer. Gel Permeation Chromatography. GPC experiments with aqueous buffer as an eluent were performed using a JASCO system LC-2000 with a UV-2075 UV detector, a RI-2031 refractive index detector, and Shodex OH pack (SB-804 HQ). The eluent was 0.3 M Na2SO4 and 0.5 M CH3COOH, pH 3 buffer solutions for PMAPTAC, and the concentration of the sample solution injected was ca. 2 mg/mL.

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The number averaged molecular weight (Mn) and the polydispersity index (Mw/Mn) were determined for macro-MAPTAC. Surface Tension Measurements. The surface tension of each block copolymer aqueous solution was measured with a FACE CBVP-Z surface tensiometer from Kyowa Interface Science Co., 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 12 h after the solution was put into the glass cell without disturbance. Foam Formation and Stability. Pure water and 1 M NaCl aqueous solutions containing 0.1 and 1 mg/mL polymer were shaken violently for 1 min. Next, 1 and 30 min later, the state of the air/water interface was recorded with a digital camera. Fluorescence Measurement. Fluorescence spectra of NPN, DPH, and pyrene probe (0.2
1.0 μM) were measured with a F-2500, fluorescence spectrometer, HITACHI, Tokyo, Japan attached to a polarizer and analyzer that uses the L-format configuration. For NPN, the samples were excited at 340 nm, and emission spectrum was recorded between 360 and 500 nm, and for pyrene, the samples were excited at 335 nm, and emission spectrum was recorded between 350 and 450 nm. For DPH, the sample was excited at 350 nm, and the emission intensity was followed at 450 nm using excitation and emission slits with band-pass of 2.5 and 2.5
5.0 nm, respectively. The r-value was calculated employing the equation: r ¼ ðI VV
GI VH Þ=ðI VV + 2GI VH Þ

ð1Þ

where IVV and IVH are the fluorescence intensities polarized parallel and perpendicular to the excitation light, and G is the instrumental correction factor (G = IHV/IHH). The temperature (25 ( 0.1 °C) of the water-jacketed cell holder was controlled by use of a Thermo Neslab RTE-7 circulating bath. Because fluorescence probes are insoluble in water, a 0.5 mM stock solution of these probes in methanol was prepared. The final concentration of the probe was adjusted to 0.2
1.0 μM by addition of an appropriate amount of the stock solution. All fluorescence measurements were started 24 h after sample preparation. Light Scattering (SLS and DLS). Static and dynamic light scattering measurements were performed at 25 °C with a Photal DLS7000DL light scattering spectrometer (Otsuka Electronic, Osaka) equipped with a He
Ne laser (632.8 nm, 15 mW). For dynamic light scattering (DLS) measurements, GC-1000 correlator was attached to the Otsuka system. The typical accumulation times were 30 min for 45° and 30 min for 105°. The time correlation functions were analyzed by the double exponential and Cumulant methods depending on the condition.22 The measurements were performed at four 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. Hydrodynamic radii were calculated by Stokes
Einstein equations: Rh ¼ K B T=6πηDapp

ð2Þ

q ¼ ð4πn=λÞ sinðθ=2Þ

ð3Þ

Synthesis of PBA-b-PMAPTAC. Poly(n-butylacrylate)-b-poly(3-(methacryloylamino)propyl)-trimethylammonium chloride) (PBAb-PMAPTAC) was synthesized by RAFT polymerization. Macro-MAPTAC was synthesized fast as follows: In a 100 mL Schlenk flask were dissolved 25.4 mg (0.09 mmol) of ACVA and 45.5 mg (0.16 mmol) of CTA in 2 mL of DMF. To this mixture were added 2 g (9.0 mmol) of MAPTAC and 8 mL of water. Next, the mixture was degassed with three freeze
pump
thaw cycles. Finally, the tube was filled with Ar gas, and 9238

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Scheme 1. Synthesis of the Block Copolymer

Figure 1. 1H NMR spectra of block copolymer in CD3OD. the mixture was stirred at 70 °C for 2 h. The crude product was dialyzed for 3 days in Milli-Q water, and water was changed twice a day. The dialyzed solution was lyophilized to make a solid. The number-averaged degree of polymerization (DP) and molecular weight distribution were determined to 40 and Mw/Mn 1.05, respectively, by gel permeation chromatography (GPC) using five standard poly(2-vinylpyridine) (Scheme 1). Block copolymers were synthesized using the macro-MAPTAC homopolymer obtained above as a macro initiator. In 2 mL of methanol in a 25 mL Schlenk flask was dissolved 0.2 g (0.022 mmol, Mn 9000) of PMAPTAC, and 1 mL (7.08 mmol) of n-BA and 1.8 mg (0.011 mmol) of AIBN were added to the PMAPTAC unit. The mixture was degassed with three freeze
pump
thaw cycles and heated under Ar to 70 °C. Reaction was monitored by 1H NMR measurement of a small amount of reaction mixture. The product was precipitated from THF, and the solid product was dialyzed with a 12 000 MWCO tube for 7 days and lyophilized. The end of dialysis was judged by conductivity measurement of the water used for dialysis. The block ratio was determined by 1H NMR with deuterated methanol as solvent (Figure 1).

’ RESULTS AND DISCUSSION Non-surface Activity of PBA-b-PMAPTAC. As reported previously, an ionic amphiphilic block copolymer is universally nonsurface active; that is, polymers are not adsorbed at the air/water interface but form micelles in solution when the required conditions related to molecular weight, block ratio, and salt concentration are satisfied. In this work, we further investigate the non-surface activity of strongly cationic amphiphilic block copolymer by surface tension measurement and foam formation observation. Figure 2 shows the polymer concentration

dependence of the surface tension (γ) of PBA-b-PMAPTAC aqueous solution for three polymer samples with different block ratios, that is, m:n = 42:40, 60:40, and 105:40. It was observed that the surface tension did not decrease with increasing polymer concentration, and the cmc could not be detected as a point where γ becomes constant, unlike the cases for low molecular weight surfactants and nonionic polymer surfactants. Furthermore, the absolute value of γ is almost equal to that for pure water (73 mN/m at 25 °C), which means that there is almost no surface excess and also no surface depletion. From these surface tension measurements, it can be concluded that the PBA-bPMAPTAC block copolymer is non-surface active; the polymer molecules were not adsorbed at the air/water interface, although the polymer itself consists of hydrophilic and hydrophobic segments. 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 (neutralized) groups,7,8 and strongly ionic sulfonic acid groups at a very low concentration.10,23 Theodoly et al. showed the effect of hydrophobicity (styrene, butylacrylate, and diethyleneglycolethyletheracrylate) with the same hydrophilic block (acrylic acid) on the surface activity of the block copolymer; in that report, it showed that with strong hydrophobic chain containing block copolymer forms micelles without adsorption at the air/water interface, whereas weak hydrophobic block containing polymer adsorbed at the air/water interface.24 They also reported that PBA-b-PAA adsorbed little at the air/water interface. In the present work, PBA-b-PMAPTAC with strong cationic block did not adsorb at the air/water interface. This also suggested that the hydrophobicities are not the only factor, but also charged of the block copolymer play an important role in the surface activity/nonactivity of their solution. Figure 3 presented the foam formation and foam stability of three polymer solutions in pure water at 0.1 and 1 mg/mL polymer concentrations. Even after vigorous shaking, their solutions did not show any foam formation. In principle, the mechanical energy generated by shaking changes the excess surface free energy, which leads to an increase in surface area and results in foam formation. 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 on both sides of the water film of foam.25 Hence, the absence of foam formation in the present system means that almost no polymer is adsorbed at the air/water interface. The absence of surface tension change is consistent 9239

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Figure 2. Plot of surface tension (γ) with the concentration of PBA-bPMAPTAC polymer.

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Figure 4. SLS scattering intensity at 90° with respect to polymer concentration at 25 °C for three cationic amphiphilic block copolymers. Each arrow shows the cmc position.

Table 1. Basic Properties and Parameters of the PBA-bPMAPTAC Polymers and Their Micellesa cmc (10
3 mg/mL)

with the absence of foam formation for pure water systems. This curious behavior was also observed even in 1 M NaCl solution. No polymer adsorption at the air/water interface was confirmed directly by X-ray reflectivity measurements for anionic systems in our previous works.10 Micelle Formation. SLS Measurement. From the results above, it was confirmed that the PBA-b-PMAPTAC copolymers are non-surface active; we further examined whether these polymers form micelles by different techniques. This is an important point because there is a possibility that polymers are just dissolved in water due to lack of amphiphilic nature. To check the existence of micelles in our polymer solutions, static light scattering measurements (SLS) were carried out. The intensity of the scattered light at 90° over the incident light (Is/I0) was plotted against the concentration (mg/mL), and the cmc values were determined at the point where Is/I0 changed from a low slope line near zero to a high slope line (break point) (Figure 4). This stronger intensity at a higher concentration is evidence of micelle formation because the scattering intensity is proportional to the square of the volume of scattering objects. The cmc values obtained for these polymers are presented in Table 1. CMC values of the block copolymers are significantly low as compared to micellar forming amphiphilic molecules. We also studied the effect of salt concentration on the scattering intensity of the polymer solution. It was observed that even at high concentration of salt (1 M), there was no change of scattering intensities of the polymer solutions. In case of micelle forming low molecular weight surfactants, their micelles may change their shape and size or form large aggregates in the presence of salt with high concentration (∼0.1 M). However, in the present case, high charge density in the corona prevents change in the shape and size of the micellar structure. This “extremely high stability against salt” is due to ionic (cationic in the present case) corona around the micelle, which was also observed and discussed in our previous works.10

m:n

Mn

PMAPTAC

40

9000 1.05

PBA-b-PMAPTAC1 PBA-b-PMAPTAC2

SLS

NPN

pyrene

42:40 14 500

4.20

3.40

3.96

60:40 16 800

1.80

1.90

2.30

PBA-b-PMAPTAC3 105:40 22 500

0.82

1.10

0.90

polymer

Figure 3. Foam formation observation of PBA-b-PMAPTAC polymer in water (after 1 min).

PDI

a

m,n: The degree of polymerization of hydrophobic and hydrophilic blocks, respectively. Mn: The number averaged molecular weight. PDI: Polydispersity index (Mw/Mn). cmc: The critical micelle concentration by different techniques. m was calculated from 1H NMR spectra, and n was calculated from GPC measurement.

Fluorescence Probe Studies. To examine the formation of micelles with a core
shell structure, the fluorescence probe studies were performed using NPN as a probe molecule. Fluorescence properties of NPN are frequently used in biology26,27 for estimation of membrane permeability induced by different biological events as well as in the characterization of the aggregates formed by synthetic surfactants28 or amphiphilic polymers.29 In aqueous medium, NPN exhibits a very weak fluorescence with emission maximum (λmax) at 460 nm. The position of the emission maximum of NPN exhibits a blue shift in going from water to less polar solvent. While emission maximum is sensitive to polarity change, the intensity rise is a function of viscosity of the medium. The NPN probe being hydrophobic in nature normally becomes solubilized in the hydrophobic core of micelles, which is recognized by the large blue shift of the λmax accompanied by a huge enhancement of fluorescence intensity. Thus, the nature of the hydrophobic core of aqueous polymer solutions for the copolymers can be probed by measuring the change in relative intensity at different polymer concentrations relative to that in water. The variation of wavelength shift of emission maximum (Δλ) and relative fluorescence intensity (I/I0) of NPN probe with a polymer concentration is depicted in Figure 5. For all three copolymers, Δλ and I/I0 increased substantially as the polymer concentration increased, reaching a plateau at a concentration above ca. 1.0 mg/mL. This suggests incorporation of NPN molecules in the hydrophobic core of the micelle. The large blue shift of emission maximum of NPN probe 9240

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Figure 5. Effect of PBA-b-PMAPTAC polymer concentration on wavelength shift of emission maximum (Δλ) and relative fluorescence intensity (I/I0) in water at 25 °C.

Figure 6. Variation of micropolarity parameter (I1/I3) with PBA-bPMAPTAC polymer in water at 25 °C.

Figure 7. Variation of fluorescence anisotropy (r-value) of DPH probe with the concentration of PBA-b-PMAPTAC polymer.

suggested that the environment of the core of the micelle is nonpolar in nature, and the high value of relative fluorescence intensity also suggested the high viscosity of the core of micelle. The intersection point of the plot indicates the cmc values of the polymers. The cmc of the copolymers was 0.0034, 0.0019, and 0.0011 mg/mL for BA42-b-MAPTAC40, BA60-b-MAPTAC40, and BA105-b-MAPTAC40, respectively. These cmc values are consistent with those by the SLS technique. The cmc value of the block copolymer increased with the decrease of hydrophobic chain length. The relatively larger I/I0 value of NPN fluorescence in the presence of BA105-b-MAPTAC40 is indicative of higher viscosity of the microenvironment, as compared to that of the other two block copolymers. Micropolarity of Copolymer Aggregates. The intensity of the vibronic peaks of pyrene fluorescence spectrum strongly depends on the polarity of the pyrene environment. The ratio of the first (I1) and third (I3) vibronic peak reflects the microenvironment of the probe position, which is known as micropolarity index.30
33 Figure 6 showed that above a certain concentration of polymer, the micropolarity index decreases with the increase of polymer concentration and reaches a minimum value. The cmc values obtained from the intersection point of these plots are closely equal to those obtained from fluorescence titration using NPN probe and SLS technique. Above the cmc, the value of I1/I3 is very low as compared to those in water (1.82), which indicates that the microenvironment of the probe is highly nonpolar. The highly nonpolar microenvironment of the self-assemblies is also indicated

by the large blue shift in the emission maximum (Δλ) of NPN spectrum. The low polarity of the local environment of the probe suggests that the fluorophore is solubilized in the hydrophobic sites of the micelle, and this also suggests more ordering at the interface. The ordering of the aggregate interface is a result of reduced degree of water penetration in the hydrocarbon layer, in accordance with the reduction observed in micropolarity sensed by the probe molecules. These results also support the results for other polymers reported previously.34 Microviscosity in Block Copolymer Micelle. To further verify the microenvironment of the polymer micelle, we carried out fluorescence anisotropy measurements using DPH probe. The DPH molecule is a well-known membrane fluidity probe and has been used for studying many lipid bilayer membranes.35
37 The molecule is almost insoluble in water and thus weakly fluorescent. Upon solubilization in the hydrophobic core of micelles, its fluorescence intensity is enhanced. The steady-state fluorescence anisotropy (r) of DPH probe reflects the microviscosity (more appropriately microfluidity) of surfactant and polymer micelles core. Therefore, we measured r of DPH in the presence of the copolymers in aqueous solution. Figure 7 shows an effect of fluorescence anisotropy (r) as a function of polymer concentration, suggesting polymer micelle formation and thus substantiate the results obtained from the studies with NPN probe. Above the cmc, the anisotropy value increased with the polymer concentration, which is due to strong hydrophobic interaction.38 This suggests that the microenvironments of the hydrophobic 9241

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Figure 8. DLS results for 0.2 mg/mL polymer solutions of BA42-b-MAPTAC40, BA60-b-MAPTAC40, and BA105-b-MAPTAC40 in water. Parts (a), (c), and (e) are the time correlation functions for the scattered field for the polymer solutions in pure water at 25 °C at the scattering angle of 90°, and (b), (d), and (f) are the decay rate Γ versus q2 plot for the polymer solutions in pure water for BA42-b-MAPTAC40, BA60-b-MAPTAC40, and BA105-bMAPTAC40, respectively. From the slope of the straight lines, the diffusion coefficient of the micelles was evaluated.

core of the copolymers micelle are more viscous than lowmolecular weight surfactant micelles. The increased microviscosity might be due to lower mobility of the butyl chains, which are covalently bound to the polymer backbone. It should be noted that the r-value increases with increase of hydrophobic chain length as the relative fluorescence intensity of NPN increased with chain length. The highest value of r in the case of BA105-b-MAPTAC40 copolymer micelle is reasonable because the r-value increases with the increase of hydrophobe content of the copolymer. Size of Copolymer Micelle. The hydrodynamic radii of the copolymer micelle were estimated from decay rate (Γ) obtained by DLS measurements. The decay rates were first measured at various scattering angles in the range of 60
105° for a given concentration of copolymers. Figure 8 shows the plots of Γ as a function of the square of the scattering vector (q). The linearity of the plots that pass through the origin clearly suggests that the decay rate reflects the translational diffusion of the scattering object. The DLS measurements were performed at different concentrations of the copolymers. From the slope of the straight line in Figure 8, the translational diffusion coefficients were

calculated and converted into hydrodynamic radius (Rh) by assuming the Stokes
Einstein equation. The smaller Rh values (fast mode) were between 40 and 100 nm and the larger Rh, or the slow mode was between 160 and 220 nm for the pure water solutions. The small Rh values correspond to polymeric micelle size. The excellent linearity observed in the slow mode (Figure 8) suggested that this corresponds to the translational diffusion of larger particles in these system. The end-to-end distances of fully stretched BA42-b-MAPTAC40, BA60-b-MAPTAC40, and BA105b-MAPTAC40 are 20.5, 25.0, and 37.5 nm, respectively. The larger value of Rh of the polymer micelles as compared to length of stretched chain is due to the fact that Rh is nongeometrical in size and that the friction effect of corona chains around hydrophobic micelle core is high.39 The variations of hydrodynamic radii (Rh) with polymer concentration are shown in Figure 9. With the increase of polymer concentration, the average size of all three polymers increases as the fluorescence intensity of NPN and anisotropy of DPH probe increase with concentration. Also, with the increase of hydrophobic chain length, hydrodynamic radii as well as viscosity increased due to contour length increase. This observation is consistent with our previous findings and can 9242

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of polymers is destabilized by image charge effect. Frozen-micelle formation is the secondary factor to make the non-surface active nature stronger. In fact, we observed very strong non-surface activity for polystyrene-b-poly(styrene sulfonate).10a We will discuss this point more in detail in a forthcoming paper, which is now underway.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +81-75-383-2619. Fax: +81-75-383-2475. E-mail: matsuoka@ star.polym.kyoto-u.ac.jp.

Figure 9. Variation of hydrodynamic radius (Rh) with the concentration of PBA-b-PMAPTAC polymers in aqueous solutions.

be interpreted by the critical packing parameter for micelle proposed by Israellativili.40 The hydrodynamic radii of these polymers do not change by addition of high concentration of salt (the results are not shown here) as reported in our previous results.10 This is due to a high charge density in the corona region of the micelles. This high charge density prevents penetration of the small ions in bulk; hence added salt ions cannot go into the corona region, which is the origin of the lack of change of micelle structure and high stability.

’ ACKNOWLEDGMENT This work is supported by a grant-in-aid for Scientific Research on Innovative Areas “Molecular Soft-Interface Science” (20106006) and 19350058 from the Ministry of Education, Culture, Sports, Science and Technology of Japan, to which our sincere gratitude is due. This work was also supported by the Global COE Program, GCOE for International Center for Integrated Research and Advanced Education in Material Science, and by the Japan Society for the Promotion of Science (JSPS). A.G. would like to express his sincere thanks to GCOE and JSPS. ’ REFERENCES

’ CONCLUSIONS Cationic amphiphilic diblock copolymers (PBA-b-PMAPTAC) with different chain lengths were synthesized successfully by controlled/“living” radical RAFT polymerization. We investigated the adsorption behavior of polymer at the air/water interface using surface tension measurement. Light scattering measurements were carried out to detect the micelle formation. All polymers formed micelles in the solution without adsorbing at the air/water interface. In addition, the cmc values of the polymer and microenvironment were measured by the fluorescence probe technique. The hydrodynamic radii increased with increase of hydrophobic chain length. The microviscosity of the micelle core was higher than that of common surfactant micelles. Thus, our newly synthesized cationic amphiphilic diblock copolymers proved to be non-surface active. In other words, polymer molecules do not adsorb at the air/water interface (hence, no surface tension reduction), but they form micelles in solution, like the other anionic block copolymers we studied previously. This experimental fact strongly supports our argument on the origin of this very unique property, which might be out of common sense of surface and interface chemistry, that the image charge effect at the air/water interface is the key factor of this property. Although some authors have reported that the water surface is negatively charged,41 this might not be essential for nonsurface activity because both anionic and cationic block copolymers show this unique property. Recently, it was proposed that frozenmicelle formation is the main factor of non-surface active nature.24 However, we already reported that poly(hydrogenated isoprene)-bpoly(styrene sulfonate), which forms nonfrozen micelle, shows non-surface activity.10b The non-frozen nature of the micelle was confirmed by sphere to rod transition of micelle shape by SANS.42 Furthermore, we found a very interesting result for cmc, for example, the “negative” Corrin
Harkins coefficient, which should be a special characteristic of non-surface active polymers. What is important is “stable” micelle formation, whereas the adsorption

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