Non-Surface Activity of Cationic Amphiphilic Diblock Copolymersiopscience.iop.org/1757-899X/24/1/012024/media/mse11_24... · Non-Surface Activity of Cationic Amphiphilic Diblock Copolymers

Non-Surface Activity of Cationic Amphiphilic Diblock Copolymers

Rati Ranjan Nayak1,# , Tasuku Yamada1 and Hideki Matsuoka1,* 1 Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan;

# Current address (R.R.N.): Bio-minerals Department, Institute of Minerals and Materials Technology,Council of Scientific & Industrial Research (CSIR), Bhubaneswar-751013, India; E-Mail: [email protected]

*Author to whom correspondence should be addressed; E-Mail: [email protected] (H.M.); Tel.: +81-75-383-2619; Fax: +81-75-383-2475.

Abstract. Cationic amphiphilic diblock copolymers containing quaternized poly (2-vinylpyridine) chain as a hydrophilic segment (PIp-b-PNMe2VP) were synthesized by living anionic polymerization. By IR measurement, we confirmed the quaternization of the polymer (PIp-b-PNMe2VP), and determined the degree of quaternization by conductometric titration. The surface tension experiment showed that the polymers are non-surface active in nature. The foam formation of the polymer solutions was also investigated with or without added salt. Almost no foam formation behavior was observed without added salt, while a little foam was observed in the presence of 1M NaCl. The critical micelle concentration (cmc) of the diblock copolymers with 3 different chain lengths was measured by the static light scattering method. The cmc values obtained in this study were much lower than the values obtained for anionic non-surface active diblock polymers studied previously. The hydrodynamic radii of the polymer micelle increased slightly in the presence of 1 M NaCl. The transmission electron microscopic images revealed spherical micelles in pure water. In the presence of salt, the cmc values increased as was the case for anionic polymers, which is unlike conventional surfactant systems but consistent with non-surface active anionic block copolymers. The microviscosity of the micelle core was evaluated using Coumarin-153 as a fluorescent anisotropy probe using steady-sate fluorescence depolarization. Non-surface activity has been proved to be universal for ionic amphiphilic block copolymers both for anionic and cationic. Hence, the origin of non-surface activity is not the charged state of water surface itself, but should be an image charge repulsion at the air/water interface.

Keywords: Diblock copolymers; anionic polymerization; non-surface activity, ionic polymers, polymer micelle, image charge, critical micelle concentration(cmc)

1. Introduction

Buried Interface Sciences with X-rays and Neutrons 2010 IOP PublishingIOP Conf. Series: Materials Science and Engineering 24 (2011) 012024 doi:10.1088/1757-899X/24/1/012024

Published under licence by IOP Publishing Ltd 1

The increasing interest in nanostructures for biomedical and material science applications has motivated the development of controlled methods for the fabrication and assembly of various molecular systems into functionalized nanostructures. One of the most successful strategies is the assembly of micelles and other supramolecular structures from amphiphilic block copolymers.[1-6] Amphiphilic diblock copolymers, which consist of hydrophobic and hydrophilic chains, have unique properties similar to small molecule surfactants, to form colloidal aggregates of a variety of different morphologies in dilute solution and at the air/water interface.[7-9]

From the fundamentals of colloids and surface chemistry, it is well understood that the common surfactants or amphiphiles are first adsorbed at the water surface to form a Gibbs monolayer and form micelles in aqueous solution after saturation of the surface by molecules. Recently, we found that strongly ionic amphiphilic copolymers have a very unique self assembling behavior, that is, micellization without adsorption at the air/water interface.[10-13] A schematic representation of this non-surface activity behavior of amphiphilic block polymer over surface activity of common surfactant is illustrated in Scheme 1. These polymers do not adsorb on the air/water interface, hence the surface tension of the solution does not decrease and the solution shows little foam formation. However, the critical micelle concentration (cmc) certainly exists and micelle aggregates can be found above cmc. This behavior is not common in surface and interface science, since a micelle is defined as a self-assembly of “surfactants (surface-active-agents)” above cmc. The origin of these unique properties has been thought to be an image charge effect at the air/water interface. [12,13]

The block copolymers with a carboxylic acid, that is, weakly ionic, were also studied previously [10,14]. If the polymers were neutralized, 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. Hence based on our previous observation, that is, micellization without adsorption was considered to be a unique property not only for strongly ionic amphiphilic diblock copolymers but also for weakly ionic amphiphilic diblock copolymers. However, to confirm the universality of this concept, a systematic investigation on other polymers having various molecular structures and/or architecture would still be needed. All our previous attempts to study this unique phenomenon i.e., non-surface activity, was based on anionic amphiphilic diblock copolymer systems. All the observations were consistent, but not perfect enough to conclude that the origin of non-surface activity is an image charge effect since the water surface has been often claimed to be negatively charged. [15,16] Hence, we report here the unique property of non-surface activity in the cationic amphiphilic diblock copolymer system.

Scheme 1: Schematic representation of common surfactant (upper part) and non-surface active amphiphilic diblock polymer (lower part) in aqueous solution.


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We synthesized cationic amphiphilic diblock copolymers containing a quaternized poly (2-vinylpyridine) group as a hydrophilic segment (PIp-b-PNMe2VP), having varying hydrophobic chain lengths to study the effect of hydrophobic chain length on adsorption behavior. Their fundamental properties, such as surface activity, self-assembling behavior in aqueous solution and at the air/water interface, and effect of salt, have been fully studied.

2. Experimental Section

2.1. Materials& Method 2-Vinyl pyridine (2VP), LiCl, calcium hydride, sodium, benzophenon, benzene, methanol, tetrahydrofuran (THF), triethylamine (TEA), dimethylformamide (DMF), and dimethylsulfoxide (DMSO) were purchased from Wako Pure Chemicals (Osaka, Japan). sec-Butyl-lithium was purchased from Kanto Chemical, Japan. Isoprene and Methyliodide (CH3I) were purchased from Tokyo Chemical Industry (TCI), Japan. Other chemicals and solvents were procured locally and where necessary distilled according to the reported procedure before use. Dialysis membrane (SPECTRUM Lab. Inc., Spectra/Por 6 MWCO:2000), anion exchange resin (Olgano Amberlite IRA-400) were used. We used Mili-Q water as ultra pure water. Isoprene and 2VP were distilled with calcium hydride two times respectively before use. Benzene and THF (the solvent for polymerization) were refluxed with sodium and benzophenone more than 1h. 1H NMR spectra were taken in a (JEOL GSX 400), FT-IR (SHIMADZU SSU-8000MCTMIRacle (with Ge prism)) was measured by total reflection infrared spectroscopy. GPC (gel column (Shodex KF804L), RI detector (JASCO RI-965), UV detector (JASCO UV2075 Plus), pump (PU-980), degasser (DG-980-50) column oven (CO-965, 40oC)) was used for polymer molecular weight determination. THF containing 5vol% of TEA was used as moving phase and calibration was done with a polystyrene standard. Surface tension was measured in FACE CBVP-Z surface tensiometer from Kyowa Interface Science Co., Ltd (Tokyo, Japan) using Pt plate. Conductivity was measured in DS-8M conductivity meter, HORIBA (Kyoto, Japan). For static light scattering (SLS) measurement, SLS-6000HL, Photal Otsuka Electronics Co., Ltd., Osaka, Japan optical system equipped with an He-Ne ion laser of λ0= 632.8 nm, a digital correlator, and a computer controlled and stepping-motor-driven variable angle detection system was used. For dynamic light scattering (DLS) measurements, GC-1000 correlator was equipped to Otsuka system. All the light scattering experiment performed at room temperature (25 oC). Steady-state fluorescence anisotropy (r) of Coumarin-153 was measured on a F-2500, Fluorescence spectrometer, HITACHI, Tokyo, Japan attached with polarizer and analyzer that uses the L-format configuration. The sample was excited at 430 nm and the emission intensity was followed at 550 nm. The r-value was calculated employing equation

r = (IVV – GIVH) / (IVV + 2GIVH) (1)

where IVV and IVH are the fluorescence intensities polarized parallel and perpendicular to the excitation light, and G is the instrumental correction factor (G = IVV/IVH). Transmission electron microscopy (TEM) measurements were kindly carried out by Hitachi High-Technologies Cooperation. Transmission electron microscopic images of the specimen quickly frozen with liquid ethane and embedded in vitreous ice were taken by a Hitachi H-7650 TEM with a cryo-transfer holder and observed at a temperature of about –170 oC. To check the concentration of specimen, the specimen were negatively stained with 2 % uranyl acetate solution and observed at room temperature.

2.2. Synthesis

2.2.1. Polymerization


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Polyisoprene-block-poly-2-vinylpyridine (PIp-b-P2VP) – a precursor of the cationic amphiphilic diblock copolymer was synthesized by living anion polymerization (Scheme 2) following the previous reported procedure [17-18]. In a typical reaction set up, 5.48 fold amount of LiCl with respect to the initiator (sec-BuLi) ([LiCl]/[BuLi]=5.48) was taken in a 100ml two-necked flask. The flask was fitted with a three-way cock with balloon and rubber septum. After the baking of LiCl, the system was filled with Ar gas (slightly high pressure). 7ml of benzene, 0.455ml (0.38mmol) of sec-BuLi, 1.85ml (18.5mmol) of isoprene were added to the system subsequently through the septum with constant stirring. Polymerization was carried out for 1 hr in a 45 oC oil bath. After this reaction, the flask was removed from the oil bath and 14ml of THF was added, which turned the color of the solution to bright yellow. The flask was cooled down to -78oC in a dry ice/ethanol bath and 2.0ml (18.5mmol) of 2VP was added with efficient stirring. The color of the solution was immediately changed to deep red. After 15 min., the reaction was terminated by adding 1ml of acidified methanol (methanol/acetic acid (10/1 v/v)) with efficient stirring. Termination of the polymerization was monitored by disappearance of the color. After removing the solvent, the residue was dissolve in methanol and precipitated into pure water. Obtained polymer was filtered out and dried in a vacuum oven. 1H NMR, GPC, FT-IR were taken for the polymer. 1H NMR (CDCl3, 400MHz ;ppm):δ0.5-1.0 (6H, H3C-CH2-CH(CH3)-PIp-),δ1.5-1.7 (3H, -CH2-C(CH3)=CH-CH2-),δ1.7-1.8 (1H, -CH2-CHPy-), δ4.5-4.7 (2H, -CH2-CH(-H3CC=CH2)- (from the side reaction;3,4-polymerization)),δ5.0 (1H, -CH2-C(CH3)=CH-CH2-),δ6.0-6.5, 6.5-7.0, 7.0-7.3, 8.0-8.5 (1H, Py-H (H of 6-,5-,4-,3-position respectively)), FT-IR: 1435-1600cm-1 (Py ring breathing mode).

Scheme 2: Synthesis of Block Copolymer.

2.2.2. Quaternization In a 100ml two-necked flask 0.20g of PIp-b-P2VP was dissolved in 10ml of DMF. The flask was fitted with a condenser and septum. After Ar gas exchange, 20-fold amount of CH3I to the number of Py rings was added to the solution through the septum with stirring. The reaction was carried out in a 40oC oil bath for overnight. After the reaction, the yellow solution was dialyzed against pure water till the conductivity of water almost became to that of pure water (about 1 month). The purified polymer was recovered by freeze drying. 1H NMR and FT-IR were taken for the quaternized polymer.

PIp-b-P2VP

PIp Isoprene

PIp-b-PNMe2VP


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2.3. Characterization Degree of polymerization (DP) of the polymer was determined by the end-group determination method using the 1H NMR chart of PIp-b-P2VP. In detail, DP was calculated from the following formula using the integration of a+b, d, d’ and i hydrogen peaks in the 1H NMR chart. (Figure 1) DPIP = (d+d’/2)/((a+b)/6), DP2VP = DPIP×(d/i). Polydispersity was determined with GPC. Degree of quaternization (D.Q.) of PIp-b-PNMe2VP was determined by conductometric titration. The counterionof quaternized pyridine of PIp-b-PNMe2VP was changed to OH- by running through a column filled with anion exchange resin. Then, 50ml of its 1mg/ml aqueous solution was titrated by 0.1M HCl aq. with measuring the conductivity. D.Q. was calculated from the formula: V1/V2, in which V1 is an added volume of HCl to first equivalent point as neutralization point of quaternized pyridine ring and V2 is a volume to second equivalent point as neutralization point of non-quaternized pyridine ring. The D.Q. values for each polymer are tabulated in Table 1 together with other parameters.

Figure 1. 1H NMR spectra of PIp-b-P2VP.

Table 1. Basic properties and parameters of the Pip-b-P2VP polymers and their micelles. cmc (mg/ml) Rh (nm) Sample

ID

m:n D.Q. Mn PDI

Water 1M NaCl Water 1M NaCl

Sample 1 14:40 0.76 5200 1.13 1.5 × 10-4 3.9 × 10-4 62 70

Sample 2 28:36 0.60 5700 1.31 0.8 × 10-4 1.98 × 10-4 60 70

Sample 3 46:54 0.57 8800 1.13 0.97 × 10-4 1.95 × 10-4 65 72

m,n : the degree of polymerization of hydrophobic and hydrophilic blocks, respectively, D.Q.: the degree of quaternization,

Mn: the number averaged molecular weight, PDI: polydispersity index (Mw/Mn), cmc: the critical micelle concentration, Rh:

the hydrodynamic radius by DLS.


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3. Results and Discussion PIp-b-P2VP was successfully synthesized using the living anionic polymerization technique. Water solubility of the polymer was introduced by quaternizing the 2-vinylpyridine unit of the polymer with methyliodide. Three polymers with different chain lengths were prepared, and their degrees of polymerization, degree of quaternization and polydispersity index are summarized in Table 1.

Figure 2 shows the concentration dependence of surface tension (γ) for aqueous solutions of 3 PIp-b-PNMe2VP polymers with different chain lengths (m:n=14:40, 28:36 and 46:54). Unlike low molecular weight surfactants and non-ionic polymeric surfactants, the surface tension of these polymers did not decrease with increasing concentration and also did not show any breakpoint like cmc. At a wide range of concentrations, the surface tension remained unchanged for all three polymers although a very slight decrease at a very high concentration is noticed for Samples 2 and 3. The absolute value of γ is almost equal to that of pure water i.e.,73mN/m. From this surface tension measurement, we can certainly conclude that our polymers here, which were cationic, are truly non-surface-active like anionic block copolymers studied previously

.

Figure 2. The concentration dependence of surface tension γ for aqueous solutions of three block copolymers with different chain lengths.

The non-surface activity of our cationic amphiphilic block copolymer, PIp-b-PNMe2VP, has been confirmed but micellization with non-surface activity should be examined. To check the existence of micelles in our polymer solutions static light scattering (SLS) measurements were carried out. For SLS measurements a stock solution of 0.5 mg/ml of each of the co-polymer solution was prepared separately in Milli-Q water and the different concentrations of polymer solution were made by dilution in Milli-Q water. Similar approach was made for the cmc determination in presence of 1M NaCl, in this case the stock as well as the different polymer concentrations of the polymer were made in 1M NaCl solution. The intensities of the scattered light at 90° over the incident light (Is/I0) was plotted against concentration (mg/ml), and the cmc values were determined at the point where Is/I0changed from a low slope linear zero to high slope line. (Figure 3) This stronger intensity is due to micelle formation since the scattering intensity is proportional to the square of the volume of scattering


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particles. The cmc values obtained for these polymers in water as well as in 1M NaCl are presented in Table 1. Lower cmc value was observed for longer hydrophobic block polymer, which is same as our previous anionic study and is quite understandable. However, surprisingly the cmc values obtained from this study were much lower than the values obtained for anionic non-surface active diblock polymers [13], although its origin is not yet clear at this moment. In the presence of salt, the cmc value is larger than that for pure water system for all three polymers, which is completely in opposite trend as described by Corrin-Harkins experimental law for low molecular weight ionic surfactant solutions. [19] This strange behavior is thought to be due to the adsorption of polymers at the air/water interface in salt-added systems[13], and was also confirmed for cationic polymers by the present study.

Figure 3. SLS plot for 3 different chain lengths in water and 1M NaCl.

The time correlation functions of the scattered field g(1)(q,τ) for three polymers in solutions (0.5 mg/ml) with and without added salt obtained by the DLS technique are shown in Figure 4a,c and e. The time correlation functions for all three samples could be well reproduced by double exponential fitting. The decay rate Γ values were evaluated and plotted against q2 (q = (4πn/λ0) sin (θ/2); where n, and λ0 the solvent refractive index, and the wavelength of light in a vacuum, respectively) in Figure 4b, d and f. Excellent linearity with the line passing through the origin guarantees that the dynamic modes detected by these measurements correspond to translational diffusion of the scatterers. From the slope of the straight line, the translational diffusion coefficients were calculated and converted into hydrodynamic radius (Rh) by using the Stokes- Einstein equation. The smaller Rh values (fast mode) were between 60-65 nm and the larger Rh, or the slow mode was between 165-222 nm for the pure water solutions, and the results are summarized in Table 1. Almost similar sized micelles (60 nm) were found from DLS measurement for the three polymers with different chain lengths. This is bigger than the polymer size, but we should notice that Rh is not the geometrical size and Rh for a polymer micelle is often larger probably due to the high friction effect of corona chains around hydrophobic micelle core. Almost the same size for the three polymers might be rather surprising, but the aggregation number of the micelle might be different, which should be further checked by small-angle scattering techniques. One should be noticed that the Rh of the micelle particles is not largely affected


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by the addition of salt. In this study, we observed almost the same Rh values both in the absence of salt and in the presence of 1 M NaCl(aq), which is an extremely high salt concentration. Micelle particles of low molecular weight ionic surfactants will change their shape and size or form large aggregates at a high salt concentration in the order of 10-1 M. However, similar phenomena were also observed for other strongly ionic amphiphilic diblock copolymers in our previous studies.[11-13,22]

Figure 4. DLS results for polymer solutions of sample 1, 2 and 3 in the presence and absence of 1 M NaCl. (a), (c), and (e), are the time correlation functions for the scattered field for the polymer

solutions in pure water and in 1M NaCl at 25 oC at the scattering angle of 90o for Sample 1, 2 and 3, respectively. (b), (d) and (f) are the decay rate Γvsq2 plot for the polymer solutions in pure water and

in 1M NaCl for Sample 1, 2 and 3, respectively. From the slope of the straight lines, the diffusion coefficient of the micelles was evaluated.

For core-cross-linked polymer micelles with a polyelectrolyte corona we also observed similar phenomena.[21] Hence, we may argue that the extremely high stability of micelles is a universal property of this type of polymer. We previously discussed the origin of this property, i.e., high


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stability of ionic polymer micelles. [22-23] The slightly larger Rh at 1M NaCl than no salt systems may mean formation of rod-like micelles, which was observed previously for anionic systems.[22]

As we observed micellization formation in all three polymers from SLS and the size of micelles could be estimated by DLS although no surface tension reduction and foam formation was observed. Further to check the nature of the micelle, especially of the hydrophobic core of non-surface active polymer micelles, we performed the steady state fluorescence depolarization experiment using Coumarin153 as an anisotropy probe. Fluorescence anisotropy is the most reliable and widely used technique to ascertain the location of the fluorophore in a heterogeneous environment such as polymer aggregates or micellar system.[24,25] When a fluorophore is excited with plane polarized radiation, the emission is partially depolarized. This depolarization has both intrinsic (static) and extrinsic (dynamic) causes. The former arises from photo selection and from the angular displacement of the absorption and emission dipoles of the fluorophore, while the major extrinsic cause of depolarization is its rotational diffusion during the lifetime of the emission. The degree of polarization of the fluorescence is conveniently expressed by the fluorescence anisotropy, r as given in equation 1. Figure 5 shows the steady state fluorescence anisotropy value as a function of solution temperature for all three samples. The anisotropy value around 20 oC for all three samples was 0.03~ 0.04 in aqueous solution. These values correspond to the typical micellar hydrophobic core.[26] In the presence of 1M NaCl, the anisotropy value increased to ~ 0.06 for all three polymers. The increase in anisotropy value indicates that the micellar core is affected by the addition of electrolyte. The steady state anisotropy value r, is related to the viscosity η around the probe molecule by well known Perin’s equation,

(2)

Where ro is the limiting anisotropy obtained in the absence of rotational motion, kB is the Boltzmann constant, T is the absolute temperature,Vh and τf are the molecular volume and fluorescence life time of the probe, respectively. Therefore, the high anisotropy value corresponds to a more rigid environment at a fixed temperature. Hence a higher anisotropy value indicates an increase in microviscosity induced by enhanced micellar hydration, as a result of the addition of 1M NaCl. The highly electrolytic medium shielded the repulsion of the charged PVP unit, thus by facilitating the fluorophore to have a more non- polar and rigid medium. A similar increase in anisotropy value was observed for bile salt micelles in the presence of 0.1M NaCl, but lowering of cmc was observed for this system.[27] In the present investigation using non-surface active cationic diblockpolymers, 1M NaCl increased the cmc value and the anisotropy.

With increase in temperature, the anisotropy value decreased for all three polymer samples from ~0.04 at 20 oC to ~0.01 at 60 oC. This decrease can be ascribed to a reduction of microviscosity at elevated temperatures, which increased Brownian motion and led to enhanced rotational diffusion. A similar trend was observed in the presence of 1M NaCl.

The micellar structure formed was further characterized through transmission electron microscopy. Figure 6 shows the negatively stained as well as freeze fractured TEM images for sample 3 in pure water. It shows spherical micelles of nearly 40 nm in diameter which is quite smaller than the value obtained from our DLS observations. Similar sized spherical micelles were obtained in the freeze-fractured TEM images. Some distorted images are also found may be due to the overlapping of few micelles.


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Figure 5. Temperature dependence of fluorescence anisotropy for Coumarin153 probe in the polymer solutions (Polymer concentration; 0.5 mg/ml).

Figure 6. Negatively stained TEM images (A,B) and vitreous ice-embedded TEM images (C,D) for sample 3 in pure water (Polymer concentration; 0.5 mg/ml). Acceleration voltage for image A and B

is 120 kV and for C and D it is 100 kV.(Scale Bar200 nm).


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Figure 7. Effect of salt on foam formation behavior for S2 polymer solutions (Polymer concentration; 0.5 mg/ml).

Further the foam formation ability of all three polymers was checked in the presence of 1M NaCl. No foam formation was observed in the absence of added salt. Surprisingly only sample 2 showed a little foam formation in the presence of added salt (Figure 7). This foam formation behavior in the presence of added salt was also observed in a previous study.[12] This observation may be explained by an image charge, i.e., electrostatic effect, as an origin of non-surface activity, since the polymer changed from non-surface active to surface active by salt addition.

4. Conclusions Cationic amphiphilic diblock copolymers (PIp-b-PNMe2VP) with different chain lengths were synthesized successfully by living anionic polymerization. We investigated the adsorption behaviour of polymers 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 the presence of salt, the cmc values increased as was the case for anionic polymers, which is completely unlike conventional surfactant systems. The foam formation of the polymer solutions was also investigated in the presence of added salt and a little foam formation behavior was observed in the presence of 1M NaCl. The hydrodynamic radius did not change at all even with the change in hydrophobic chain length. The transmission electron microscopic images revealed the formation of spherical micelles. The microviscosity of the micelle core was similar to that of common surfactant micelles. By these systematical studies, we confirmed that the cationic amphiphilic diblock copolymer, PIp-b-PNMe2VP, is also non-surface active in addition to anionic block copolymers, which certificates that non-surface activity is a universal phenomena for ionic amphiphilic block copolymers and that the origin of non-surface activity is an image charge effect at the air/water interface.

Acknowledgements This work was supported by a grant-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (20106006 and 19350058), 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. R.R.N. would like to express his sincere thanks to GCOE, which gave an opportunity to stay and perform research at Kyoto

No Salt

1M NaCl


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University in Japan for a year. H.M. would like to express his sincere thanks to Dr.Syuji Fujii, Osaka Institute of Technology, for drawing our attention to non-surface activity of cationic polymers and for fruitful discussions. TEM measurements were kindly carried out by Mr. Hiroaki Watanabe and Dr. Eiko Nakazawa, Hitachi High-Technologies Corporation, to whom our sincere thanks are due.

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Non-Surface Activity of Cationic Amphiphilic Diblock Copolymersiopscience.iop.org/1757-899X/24/1/012024/media/mse11_24... · Non-Surface Activity of Cationic Amphiphilic Diblock Copolymers