Association Behavior of Fluorine-Containing and Non-Fluorine

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Association Behavior of Fluorine-Containing and Non-Fluorine-Containing Methacrylate-Based Amphiphilic Diblock Copolymer in Aqueous Media Kozo Matsumoto,* Tomokazu Ishizuka, Tamotsu Harada, and Hideki Matsuoka Department of Polymer Chemistry, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Received March 11, 2004. In Final Form: June 9, 2004 Fluorine-containing amphiphilic block copolymers, poly(sodium methacrylate)-block-poly(nonafluorohexyl methacrylate) (NaMAm-b-NFHMAn) (m:n ) 61:12, 72:33, 64:57), and the corresponding non-fluorinecontaining amphiphilic block copolymer, poly(sodium methacrylate)-block-poly(hexyl methacrylate) (NaMAm-b-HMAn) (m:n ) 64:10, 69:37, 67:50), were synthesized. Both polyNaMA-b-polyNFHMA and polyNaMA-b-polyHMA formed micelles above critical micelle concentrations, (cmc’s), around 3 × 10-5 to 1 × 10-4 mol/L, while neither polymer decreased surface tension of aqueous solutions. The size and shape of the micelles were examined by dynamic light scattering, small-angle neutron scattering, and smallangle X-ray scattering. PolyNaMA-b-polyHMA appeared to form only spherical micelles, while polyNaMAb-polyNFHMA with a long NFHMA segment formed both spherical and rodlike micelles. The micelles of fluorine-containing block copolymers were obviously larger than those of non-fluorine-containing block copolymers with the same chain length and the same hydrophilic/hydrophobic chain ratio. The fluorinecontaining block copolymer selectively solubilized fluorinated dye into the water phase when a mixture of decafluorobiphenyl and 2,6-dimethylnaphthalene was added to the micelle solution.

Introduction Fluorinated polymers have attracted much attention because of their unique characteristics such as high hydrophobicity, lipophobicity, chemical stability, and biocompatibility.1-3 A new class of polymeric amphiphiles may be produced by combinating fluorinated polymers with hydrophilic polymers.4 Recently, several studies on the association behavior of water-soluble fluorinated amphilphilic block copolymers have been reported.5-9 We have also reported the high surface activity, micelle formation, gel formation, and preferential solubilization of fluorinated compounds by using nonionic vinyl etherbased fluorine-containing amphiphilic block copolymers.10-13 In this study, we focused on a methacrylate* To whom correspondence may be addressed. E-mail: matsumo@ star.polym.kyoto-u.ac.jp. (1) Pittman, A. G. In Fluoropolymers; Wall, L. A., Ed.; Wiley: New York, 1977. (2) Sheirs, J. Ed. In Modern Fluoropolymers; John Wiley & Sons: New York, 1997. (3) Kissa, E. Ed. In Fluorinated Surfactants; Surfactant Science Series 50; Marcel Dekker: New York, 1994. (4) For review of amphiphilic block copolymers, see: Amphiphilic Block Copolymers: Self-Assembly and Applications; Alexandridis, P., Lindman, B., Eds.; Elsevier: Amsterdam, 2000. (5) Busse, K.; Kressler, J.; van Eck, D.; Ho¨ring, S. Macromolecules 2002, 35, 178. (6) Hussain, H.; Budde, H.; Ho¨ring, S.; Busse, K.; Kressler, J. Macromol. Chem. Phys. 2002, 203, 2103. (7) Hussain, H,; Busse, K.; Kressler, J. Macromol. Chem. Phys. 2003, 204, 936. (8) Soo, H.; Heo, J. Y.; Jeong, Y. T.; Jin, S.; Cho, D.; Chang, T.; Lim, K. T. Polymer 2003, 44, 5153. (9) Zhou, Z.; Li, Z.; Ren, Y.; Hillmyer M. A.; Lodge, T. P. J. Am. Chem. Soc. 2003, 125, 10182. (10) Matsumoto, K.; Kubota, M.; Matsuoka, H.; Yamaoka, H. Macromolecules 1999, 32, 7122. (11) Matsumoto, K.; Mazaki, H.; Nishimura, R.; Matsuoka, H.; Yamaoka, H. Macromolecules 2000, 33, 8295. (12) Matsumoto, K.; Nishimura, R.; Mazaki, H.; Matsuoka, H.; Yamaoka, H. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 3751. (13) Matsumoto, K.; Mazaki, H.; Matsuoka, H., Macromolecules 2004, 37, 2256.

based fluorine-containing amphiphilic block copolymer and examined its association behavior in aqueous media in detail. The methacrylate block copolymer was chosen for several reasons. First, a monomer of semifluorinated methacrylic ester is commercially available. Second, living anionic polymerization of semifluorinated methacrylic ester has been established, and a well-defined block copolymer with tert-butyl methacrylate can be synthesized.14 Third, the tert-butyl ester group in the block copolymer can be selectively hydrolyzed and an amphiphilic block copolymer bearing ionic poly(methacrylic acid) (polyMAA) can be precisely prepared. Fourth, the corresponding non-fluorine-containing block copolymer having the same chemical structure except fluorination can also be prepared so that we can directly compare their properties. Here, we synthesized sodium poly(methacrylate)-blockpoly(3,3,4,4,5,5,6,6,6-nonafluorohexyl methacrylate) (polyNaMA-b-polyNFHMA) and sodium poly(methacrylate)block-poly(hexyl methacrylate) (polyNaMA-b-polyHMA), examined their surface activity by surface tension measurements in the aqueous solutions, and evaluated the micelle structure by dynamic light scattering (DLS), smallangle neutron scattering (SANS), and small-angle X-ray scattering (SAXS). We also clarified the “fluorophilicity” of the fluorine-containing block copolymer micelles by solubilization experiments of water-insoluble fluorinated and nonfluorinated dyes. Experimental Section Materials. 3,3,4,4,5,5,6,6,6-Nonafluorohexyl methacrylate (NFHMA) (Daikin, Tokyo, Japan) and tert-butyl methacrylate (tBMA) (Wako Pure Chemical Industry, Osaka, Japan) were washed three times with 1 M NaOH (aqueous) and three times with water, dried over anhydrous Na2SO4, and distilled twice (14) Ishizone, T.; Sugiyama, K.; Sakano, Y.; Mori, H.; Hirao, A.; Nakahama, S. Polym. J. 1999, 31, 983.

10.1021/la049371+ CCC: $27.50 © 2004 American Chemical Society Published on Web 07/24/2004

Fluorine-Containing Block Copolymer over CaH2 under reduced pressure. Lithium chloride (purity 99.9%, Wako) was dried at 130 °C for 2 h under vacuum. 1,1Diphenylethylene (Tokyo Chemical Industry, Tokyo, Japan) was distilled over n-butyllithium under vacuum. n-Butyllithium (1.6 mol/L hexane solution) (Wako) and sec-butyllithium (1.0 mol/L cyclohexane/hexane solution) (Kanto Chemical, Tokyo, Japan) were titrated with 2-butanol and used as delivered. Lithium naphthalene tetrahydrofurane (THF) solution was prepared by treatment of lithium metal with naphthalene in THF. 1-Hexanol (Wako), methacryloyl chloride (Tokyo Chemical Industries), 1,4dioxane (Wako), and concentrated HCl (Wako) were used as delivered. THF and diethyl ether were freshly distilled over sodium benzophenone ketyl under an argon atmosphere. CD2Cl2 and THF-d8 used for 1H NMR measurement were purchased from Cambridge Isotope Laboratories (Cambridge, MA). D2O (99.9% atom D) used for SANS experiments was purchased from Aldrich. Synthesis of Hexyl Methacrylate (HMA). To a 500 mL round-bottomed flask equipped with a magnetic stirring bar, a three-way stopcock with a rubber balloon, and a rubber septa, 1-hexanol (152 mmol, 19.2 mL) and THF (200 mL) were added under an argon atmosphere. The mixture was cooled at 0 °C, n-butyllithium (1.60 mmol/L hexane solution, 152 mmol, 95.0 mL) was slowly added, and the mixture was stirred for 20 min. Then methacryloyl chloride (168 mmol, 16.2 mL) was slowly added and the whole was stirred overnight at room temperature. The resulting mixture was poured into water, and the products were extracted with diethyl ether. The organic phase was washed two more times with water, dried over anhydrous Na2SO4, and concentrated in a rotary evaporator. The residual oil was distilled twice over CaH2 under reduced pressure (74-75 °C/1.2 × 103 Pa) to give the title compound (8.26 g, 48.6 mmol) in 32% yield. Synthesis of PolyNFHMA. To a 100 mL two-necked roundbottomed flask equipped with a three-way stopcock, a rubber balloon, a rubber septum, and a magnetic stirring bar were added lithium chloride (20 mg, 0.47 mmol) and THF (16 mL) in an argon atmosphere. The solution was titrated with a THF solution of lithium naphthalene to eliminate a trace amount of reactive impurities. After the flask was cooled to -78 °C, 1,1-diphenylethylene (0.053 mL, 0.30 mmol) and sec-butyllithium (0.90 mol/L cyclohexane solution, 0.22 mL, 0.20 mmol) were added, and the mixture was stirred for 20 min to prepare the initiator solution (dark red solution). Then NFHMA (0.95 mL, 4.0 mmol) was added to the initiator solution, and the mixture was stirred for 2 h. The polymerization was terminated by addition of MeOH /H2O (v/v ) 5/1, 1.0 mL). Precipitation of the mixture into methanol followed by filtration and drying in vacuo gave polyNFHMA (1.33 g) in quantitative yield. Mn ) 8200 (determined by 1H NMR), Mw/Mn ) 1.10 (determined by gel permeation chromatography (GPC) using polystyrene as a standard). Synthesis of PolyHMA. To the initiator solution prepared as described above, HMA (0.73 mL, 4.0 mmol) was added, and the mixture was stirred for 2 h. The polymerization was terminated by addition of MeOH/H2O (v/v ) 5/1, 1.0 mL). Precipitation of the mixture into methanol followed by filtration and drying in vacuo gave polyHMA (0.68 g) in quantitative yield. Mn ) 5300 (determined by 1H NMR), Mw/Mn ) 1.08 (determined by GPC using polystyrene as a standard). Synthesis of Poly(tert-butyl methacrylate)-block-poly(3,3,4,4,5,5,6,6,6-nonafluorohexyl methacrylate) (polytBMAb-polyNFHMA) and Poly(tert-butyl methacrylate)-blockpoly(hexyl methacrylate) (polytBMA-b-HMA). tBMAm-bNFHMAn (m ) 72, n ) 33, where m and n represent number averaged polymerization degree of polytBMA segment and polyNFHMA segment, respectively) was synthesized. To a 100 mL two-necked round-bottomed flask equipped with a threeway stopcock, a rubber balloon, a rubber septum, and a magnetic stirring bar, lithium chloride (40 mg, 0.94 mmol) and THF (32 mL) were added in an argon atmosphere. The solution was titrated with a THF solution of lithium naphthalene to eliminate a trace amount of reactive impurities. After the flask was cooled to -78 °C, 1,1-diphenylethylene (0.11 mL, 0.60 mmol) and secbutyllithium (0.90 mol/L cyclohexane solution, 0.44 mL, 0.4 mmol) were added, and the mixture was stirred for 20 min to prepare the initiator solution (dark red solution). Then tert-butyl meth-

Langmuir, Vol. 20, No. 17, 2004 7271 Table 1. Characteristics of the Block Copolymer Synthesized in This Study polymer tBMAm-b-NFHMAn tBMAm-b-NFHMAn tBMAm-b-NFHMAn tBMAm-b-HMAn tBMAm-b-HMAn tBMAm-b-HMAn a

1H

m NMR 61 72 64 64 69 67

1H

n NMR 12 33 57 10 37 50

1H

Mn NMR

12900 21400 27700 11000 16300 18300

Mw/Mn GPC 1.11 a 1.03 1.10 1.11 1.05

The value could not be determined.

acrylate (3.9 mL, 24 mmol) was added to the initiator solution, and the mixture was stirred for 2 h. NFHMA (2.85 mL, 12 mmol) was added, and the mixture was stirred for 2 h. The polymerization was terminated by addition of MeOH/H2O (v/v ) 5/1, 1.0 mL). Removal of impurities by dialysis (Spectra/Pro 7, molecular weight cutoff 1000) in methanol for 1 week, followed by freezedrying from 1,4-dioxane in vacuo gave tBMA72-b-NFHMA33 (7.1 g) in 95% yield. Mn ) 19000 (determined by 1H NMR analysis). By tuning the molar ratio of the initiator, tBMA, and NFHMA, several samples of polytBMA-b-polyNFHMA with different m and n were prepared. Also by using HMA instead of NFHMA, we synthesized polytBMA-b-polyHMA. The samples prepared in this study are summarized in Table 1. Synthesis of Sodium Poly(methacrylate)-block-poly(3,3,4,4,5,5,6,6,6-nonafluorohexyl methacrylate) (polyNaMA-b-polyNFHMA) and Sodium Poly(methacrylate)block-poly(hexyl methacrylate) (polyNaMA-b-polyHMA). The tert-butyl ester groups of polytBMA-b-polyNFHMA and polytBMA-b-polyHMA were hydrolyzed by heating 1,4-dioxane (50 mL) solution of the block copolymers (ca. 5.0 g) in the presence of concentrated hydrochloric acid (35 wt % aqueous solution, 5.0 mL) at 80 °C for 3 h. The hydrolyzed block copolymers were precipitated in hexane, filtered out, washed thoroughly with hexane, and freeze-dried from 1,4-dioxane to give polyMAA-bpolyNFHMA and polyMAA-b-polyHMA having free carboxyl groups. Those copolymers were dissolved in diluted aqueous NaOH (ca. 1.1 equiv to carboxylic acid groups). The excess NaOH was removed by dialysis in water, and the copolymer was freezedried from the aqueous solution to give neutralized block copolymers polyNaMA-b-polyNFHMA and polyNaMA-b-polyHMA as powder samples in quantitative yield. Molecular Characterization. Gel permeation chromatography of the obtained polymers was carried out before hydrolysis in chloroform on a JASCO 880-PU chromatograph equipped with four polystyrene gel columns (Shodex K-802, K-803, K-804, and K-805) and a refractive index (RI) detector, JASCO 830-RI. The values of Mw/Mn were determined by GPC with a polystyrene standard calibration for polytBMA-b-polyNFHMA and polytBMA-b-polyHMA, where Mw and Mn are weight- and numberaverage molecular weights, respectively. Since the refractive index (RI) of fluorinated polymer is lower than that of chloroform, the signal of tBMA72-b-NFHMA33 became so weak that we could hardly detect the copolymer with the RI detector. The copolymer peak of tBMA64-b-NFHMA57, appeared in the opposite phase compared with the polytBMA homopolymer and had shifted to a higher molecular weight region. The other copolymer peaks clearly shifted to a high molecular weight region compared with the corresponding homopolymers keeping narrow molecular weight distributions. These facts clearly indicated the formation of the block copolymers. Proton NMR spectra were recorded by a JEOL GSX 270 spectrometer. Spectra of polytBMA-b-polyNFHMA and polytBMA-b-polyHMA in CD2Cl2 were similar to those of the analogous block copolymers reported previously.14 The polymerization degrees m and n of the block copolymers were determined by the 1H NMR end group analysis using phenyl proton signals (at 7.0-7.3 ppm) of the initiation end. Although the values were obtained for polytBMA-b-polyNFHMA and polytBMA-b-polyHMA precursor polymers, they can be applied for hydrolyzed polymers as well, since it was confirmed by 1H NMR that no subreaction had taken place during hydrolysis. The bulk densities of polyNFHMA, polyHMA, and polyNaMa were estimated as 1.63, 1.05, and 1.40 g/cm3, respectively using molecular mechanics simulation software Synthia in Polymer

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(BIOSYM, MSI). From these values, the scattering length densities of polyNFHMA, polyHMA, and polyNaMA were estimated as 2.82 × 1010, 4.01 × 109, 1.80 × 1010 cm-2, and the electron densities were estimated as 4.90 × 10-1, 3.50 × 10-1, and 4.36 × 10-1 Å-3, respectively. Surface Tension Measurements. The surface tension of an aqueous copolymer solution was measured on a CBVP-Z automatic surface tensiometer (Kyowa Interface Science Co., Ltd.) using a Pt plate in full automatic mode. The sample polymer solution (the concentration ranged from 1.0 × 10-6 mol/L to 6.3 × 10-4 mol/L) was prepared by direct dissolution of each neutralized polymer in pure water obtained by a Millipore Milli-Q system or 0.5 mol/L aqueous NaCl solution. The measurements were done at least 3 days after sample preparation. Solubilization Experiments. UV-visible spectra of dyesolubilized polymer solutions were taken on a U-3310 spectrophotometer (Hitachi, Ltd.) using the same concentration of the polymer solution as a reference. The dye-solubilized polymer solutions were prepared by adding an excess amount of a powdered dye (300 mg) to an aqueous solution of block copolymers (10 mL) and by stirring the mixture for at least 3 days at room temperature. In this study, the excess dyes were precipitated at the bottom of the sample containers, which indicated that the solutions were saturated with the dyes. Dynamic Light Scattering (DLS). The DLS measurements were performed on a dynamic light scattering apparatus of Photal SLS-6000HL (Otsuka Electrics, Osaka, Japan) equipped with a correlator (Photal GC-1000). A He-Ne laser (wavelength, 632.8 nm) was used for the measurements. Polymer solutions (2.5 × 10-4 mol/L) were prepared by directly dissolving the neutralized block copolymer into 0.5 mol/L NaCl (aqueous) and filtered through a membrane (Millex-VV, Millipore, pore size of 0.1 µm). The measurements were performed at 25 °C 3 days after sample preparation. Typical measuring angles were 60°, 75°, 90°, and 105°. Single-exponential fitting, cumulant fitting, and double exponential fitting were applied for the data analysis. Small-Angle Neutron Scattering (SANS). The SANS measurements were performed with the SANS-U at the Institute for Solid State Physics, the University of Tokyo, at the research reactor JRR-3M, Tokai, Japan. The wavelength (λ) of neutron beam was 7 Å (∆λ/λ ) 10%). Polymer solutions (2.5 × 10-4 mol/L) were prepared by direct dissolution of the neutralized block copolymer into D2O or D2O containing 0.5 mol/L NaCl and filtered through a membrane (Millex-GV, Millipore, pore size of 0.22 µm). Solutions were measured in quartz cells (Nippon Silica Glass Co., Tokyo) with a pass length of 4 mm at 25 °C at least 3 days after sample preparations. Scattering data measured by a twodimensional (2D) detector were circular averaged to 1D and then corrected for electronic background, and the scattering of the empty cell was subtracted. The data were transformed to absolute intensities using a Lupolen standard. From all scattering data of samples, we subtracted the scattering of solvent and the calculated incoherent scattering of the polymer. The SANS experiments were carried out at sample-detector distances of 1, 4, and 12 m, covering a range of the scattering vector (q) of 0.002 e q e 0.28 Å-1. The typical accumulation times of SANS runs were 1 h for a sample-detector distance of 1 m, 1 h for 4 m, and 3.5 h for 12 m. Small-Angle X-ray Scattering (SAXS). The SAXS measurements were performed using a Kratky type camera (Rigaku Corp., Tokyo) equipped with a rotating anode X-ray generator and a 1D position sensitive proportional counter (PSPC). The details of the apparatus and data treatment were as fully described elsewhere.15 Polymer solutions (3.2 × 10-4 mol/L) were prepared by directly dissolving the neutralized block copolymer into pure water or 0.5 mol/L aqueous NaCl and filtering through a membrane (Millex-GV, Millipore, pore size of 0.22 µm). Sample solutions were measured in glass capillaries (Mark, Berlin) with a diameter of 2 mm at 25 °C at least 3 days after sample preparation. The typical accumulation time of a SAXS run was about 3 h. In experimental data, the scattering from solvent was subtracted. After the sensitivity correction for PSPC, the SAXS data were desmeared.15 (15) Ise, N.; Okubo, T.; Kunugi, S.; Matsuoka, H.; Yamamoto, K.; Ishii, Y. J. Chem. Phys. 1984, 81, 3294.

Matsumoto et al. Data Analysis of SANS and SAXS Measurements. We analyzed SANS and SAXS data according to the reported procedures.13 When the contribution of interparticle interaction is negligible, the scattering intensity dΣ(q)/dΩ from an isolated particle is given by the equation

dΣ(q)/dΩ ) npP(q)

(1)

where np is the number density of particles and P(q) is the particle form factor. The scattering vector q is defined by q ) 4π sin θ/λ, where 2θ is the scattering angle and λ is the wavelength of the neutron or X-ray. Here we deal with the micelles having a coreshell structure with a homogeneous electron density in the core and the shell regions. In such a model, the hydrophobic segment (RMA) forms the core, and the shell contains hydrophilic segment (NaMA) hair and solvent water. The particle form factor of a core-shell model can be written as

P(q) )

∫

(1/2)

{(FC - FS)VCFC(q) + (FS - F0)VallFS(q)}2 sin β dβ

π 0

(2) where FC, FS, and F0 are the scattering length densities (for SANS) or electron densities (for SAXS) of the core, the shell, and the solvent, respectively. VC and Vall are the volumes of the core and the overall micelle, respectively. The scattering amplitude Fi(q) (i ) c or s, where c and s denote the core and the shell) depends on the size and shape of the scattering particles. For a monodisperse isolated sphere with a radius of Ri, Fi(q) is given by

Fi(q) ) 3{sin(qRi) - qRi cos(qRi)}/(qRi)3

(3)

Using the value of the electron density of the monomer unit FRMA and FNaMA, Fc and Fs can be represented by

Fc ) FRMA

(4)

Fs ) φsolF0 + (1 - φsol)FNaMA

(5)

where φsol is the volume fraction of the solvent in the shell, which can be calculated by the following equation with the degree of polymerization of NaMA (m), the volume of NaMA repeating units (νNaMA), the volume of the core (Vc), and the volume of the overall micelle (Vall)

φsol ) 1 - NaggmνNaMA/(Vall - Vc)

(6)

Nagg denotes the aggregation number of the micelles, which is calculated from Vc, the degree of polymerization of hydrophobic segment (n), and the volume of its repeating units (νRMA)

Nagg ) Vc/(nνRMA)

(7)

Assuming that all polymers contribute to the micelle formation, the number density np of the micelles is calculated as

np ) φpolym/(Nagg(nνRMA + mνNaMA))

(8)

where φpolym is the volume fraction of copolymer in solution. For a rodlike structure with a cross sectional core radius of RC, the cross sectional shell radius of RS, and rod length L, the scattering amplitude is given by

Fi(q) ) {(sin(q(Li/2) cos β)/(q(Li/2) cos β)}{2J1(qRi sin β)/ (qRi sin β)} (9) where β is the angle between the axis of symmetry of the rod and the scattering vector q. J1 denotes the Bessel function of the first order. If q . 2π/L and L . Rs, the form factor of rodlike micelles can be reduced to

Fluorine-Containing Block Copolymer

Langmuir, Vol. 20, No. 17, 2004 7273 Scheme 1. Synthesis of Block Copolymers

Figure 1. Magnified image of water droplet on a spin-coated (a) polyNFHMA film and (b) polyHMA film on a glass plate.

P(q) ) (π/qL){(Fc - Fs)Vc2J1(qRC)/(qRC) + (Fs - F0)Vall2J1(qRs)/(qRs)}2 (10) For SAXS data analysis, we introduced a shift factor f, since the intensity of the experimental data is obtained in arbitrary units because the values were not calibrated to an absolute scale. Therefore, the relative scattering intensity I(q) is given by

I(q) ) f np P(q)

(11)

In the core-shell model fitting, Rc, Rs, and f were variable, while the value of f was arranged to be the same in each SAXS measurement sequence. For the sphere-rod coexistence model, the scattering intensity is given by

dΣ(q)/dΩsphere-rod ) npφΝsphereP(q)sphere + npφΝrodP(q)rod (12) where φΝsphere and φΝrod are number fractions of the sphere and rodlike micelles in the total number of micelles in the solution and P(q)sphere and P(q)rod are form factors of the corresponding micelles. Equation 12 can be written as

dΣ(q)/dΩsphere-rod ) (mpφsphere/Νsphere)P(q)sphere + (mpφrod/Νrod)P(q)rod (13) where mp is the number density of polymer molecules, φsphere and φrod are volume fractions of the micelles in the total volume of micelles, and Νsphere and Νrod are the aggregation numbers of the sphere and rodlike micelles.

Results and Discussion Contact Angles of Water Droplet on Homopolymers. To check the hydrophobicity of polyNFHMA and polyHMA segments, we synthesized NFHMA and HMA homopolymers and measured contact angles of a water droplet on the homopolymer films (Figure 1) spin-coated on a slide glass. The static contact angles on polyNFHMA

and polyHMA were 117° and 88°, respectively. Since the chemical structures of the two polymers are the same except for fluorination of polyNFHMA, these results clearly indicated the high hydrophobicity of the fluorinated polymer. Synthesis of Amphiphilic Block Copolymers. To directly examine the effect of fluorination on the association behavior of an amphiphilic block copolymer, we synthesized a water-soluble fluorine-containing block copolymer, polyNaMA-b-polyNHFMA, and the corresponding non-fluorine-containing block copolymer, polyNaMA-b-polyHMA, having almost the same hydrophilic/ hydrophobic chain length ratio and almost the same total chain length. Precursor polymers, polytBMA-b-polyNFHMA and polytBMA-b-polyHMA, were synthesized according to the reported procedures14 (Scheme 1). Molecular characteristics of the obtained precursor polymers are listed in Table 1. Three sets of fluorine-containing and non-fluorine-containing block copolymers, whose polymerization degrees of hydrophobic segments were ca. 10, 35, and 50, were synthesized maintaining the polymerization degrees of the tBMA segments almost constant (61-72). Then, these precursor copolymers were treated with concentrated HCl in 1,4-dioxane to hydrolyze the tert-butyl ester groups in the polytBMA segment. Proton NMR spectra of the block copolymers in THF-d8 after hydrolysis are given in Figures 2 and 3. Signals of tertbutyl proton were not observed at all, while the signals

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Figure 2.

1H

NMR spectrum of MAA72-b-NFHMA33 in THF-d8.

Figure 3.

1H

NMR spectrum of MAA69-b-HMA37 in THF-d8.

of methylene protons adjacent to the ester oxygen remained intact even after the reaction. This indicated that the tertiary alkyl esters of the block copolymer were selectively hydrolyzed without affecting the secondary alkyl esters under this acidic condition. Neutralization of carboxyl groups with aqueous NaOH followed by dialysis in water and freeze-drying gave white powder samples of polyNaMA-b-polyNFHMA and polyNaMA-b-polyHMA. All the block copolymer samples synthesized here were directly soluble in water. Surface Tensions of the Block Copolymer Solutions. To examine the surface activities of the block copolymers in aqueous media, we measured the surface tension of the aqueous block copolymer solutions. Figures 4 and 5 show the surface tensions of aqueous solutions of fluorine-containing and non-fluorine-containing block

Matsumoto et al.

copolymers as a function of copolymer concentrations. From the contact angle of the NFHMA homopolymer, it was expected that the fluorinated block copolymer might possess high surface activity. However, the results of the measurement were completely different from this expectation. The surface tension of fluorine-containing block copolymer solution was almost constant in the concentration range studied regardless of hydrophobic chain length, and its absolute value was close to that of pure water (72.1 mN/m at 25 °C). The surface tension of the copolymer solution with NaCl was slightly lower than that of the pure water solution, but it was still close to the surface tension of pure water. The same phenomenon was observed also in the non-fluorine-containing block copolymer, which is completely different from the usual nonionic block copolymer. The surface tension of the usual


Langmuir, Vol. 20, No. 17, 2004 7275

Figure 4. Surface tension and UV absorbance (at 275 nm in the presence of DNM) of the aqueous solutions of fluorinecontaining block copolymers as a function of copolymer concentrations: (a) NaMA61-b-NFHMA12, (b) NaMA72-b-NFHMA33, (c) NaMA64-b-NFHMA57; (open circle) surface tension with 0.5 mol/L NaCl, (filled circle) surface tension without NaCl, (open triangle) UV absorbance with 0.5 mol/L NaCl, (filled triangle) UV absorbance without NaCl.

Figure 5. Surface tension and UV absorbance (at 275 nm in the presence of DMN) of the aqueous solutions of non-fluorinecontaining block copolymers as a function of copolymer concentrations: (a) NaMA61-b-HMA10, (b) NaMA72-b-HMA37, (c) NaMA64-b-HMA50; (open circle) surface tension with 0.5 mol/L NaCl, (filled circle) surface tension without NaCl, (open triangle) UV absorbance with 0.5 mol/L NaCl, (filled triangle) UV absorbance without NaCl.

nonionic block copolymer declines with increasing polymer concentration and reaches a constant value above a certain concentration (cmc), like low molecular weight surfactants. We previously observed this phenomenon in diblock copolymers with strong acid groups in the hydrophilic segment.16,17 In principle, ionic polymer chains having

many ions with a high charge density are repelled from the air/water interface, which prevails over the high hydrophobicity of the fluorinate polymer. Hydrophobic Dye Solubilization for Determination of cmc. A hydrophobic dye solubilization experiment with UV adsorption detection is a conventional technique to confirm micelle formation in solution and to estimate the critical micelle concentration (cmc). We applied this method to our block copolymer system with 2,6-dimethylnaphthalene (DMN) as a hydrophobic dye (Figures 4 and 5). In all cases under all conditions studied here the

(16) Matsuoka, H.; Matsutani, M.; Mouri, E.; Matsumoto, K., Macromolecules 2003, 36, 5321. (17) (a) Maeda, S.; Matsumoto, K.; Matsuoka, H. Polym. Prepr. Jpn. 2002, 51, 2350. (b) Matsuoka, H.; Maeda, S.; Kaewsaiha, P.; Matsumoto, K. Langmuir, in press.

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Figure 6. Time correlation functions of 1 wt % fluorinecontaining block copolymer aqueous solutions in the presence of 0.5 mol/L NaCl by DLS: (a) NaMA61-b-NFHMA12, (b) NaMA72b-NFHMA33, (c) NaMA64-b-NFHMA57. Solid lines are fitting curves calculated from (a) single exponential, (b) double exponential, and (c) double exponential functions. Scattering angle was 90°. Insets are the residuals of fitting curve (experimental data divided by fitting data plotted against τ).

onset of absorbance increase at 275 nm, which is a specific absorption band of DMN, was observed around 3 × 10-5 to 1 × 10-4 mol/L, which corresponded to cmc’s of those block copolymers. This indicates that the block copolymers form micelles, though the surface tensions of the solutions are constant at the value of pure water. Dynamic Light Scattering Analysis. The multimolecular micelle formation was confirmed by DLS experiments. Figures 6 and 8 show the time correlation functions and fitting curves for 2.5 × 10-3 mol/L block copolymer aqueous solutions in the presence of 0.5 mol/L NaCl. In

Matsumoto et al.

Figure 7. q2-Γ plot of (a) NaMA61-b-NFHMA12, (b) NaMA72b-NFHMA33 (filled circle, fast decay mode; open circle, slow decay mode), and (c) NaMA64-b-NFHMA57 (filled circle, fast decay mode; open circle, slow decay mode).

all cases, a slow decay was observed, which indicated the existence of multimolecular micelles. We analyzed the data first by single exponential fitting (one decay mode without decay rate distribution). If no good fitting was obtained by single exponential fitting, we used cumulant fitting (one decay mode with decay rate distribution). Only if the data could not be fit well by either fitting did we adopt double exponential fitting (two decay modes without decay rate distribution). The obtained decay rate Γ was plotted against q2 as shown in Figures 7 and 9. The linear relationship between q2 and Γ indicated that the dynamic mode was translational motion. The translational diffusion coefficients (D) were evaluated from the slope of the


Figure 8. Time correlation functions of 1 wt % non-fluorinecontaining block copolymer aqueous solutions in the presence of 0.5 mol/L NaCl by DLS: (a) NaMA61-b-HMA10, (b) NaMA72b-HMA37, (c) NaMA64-b-HMA50. Solid lines are fitting curves calculated from (a) cumulant, (b) single exponential, and (c) cumulant functions. Scattering angle was 90°. Insets are the residuals of the fitting curve (experimental data divided by fitting data plotted against τ).

straight line using the relation of Γ ) Dq2. The Rh values were calculated from the diffusion coefficients using the Stokes-Einstein relation Rh ) kBT/(6πη0D), where kB, T, and η0 represent the Boltzmann constant, absolute temperature, and solvent viscosity, respectively. The hydrodynamic radii (Rh) for all samples are summarized in Table 2. In the case of non-fluorine-containing block copolymers, the time correlation functions were well fitted by a singleexponential function or cumulant function, which resulted in the hydrodynamic radius (Rh) of 110, 180, and 300 Å,

Langmuir, Vol. 20, No. 17, 2004 7277

Figure 9. q2-Γ plots of (a) NaMA61-b-HMA10, (b) NaMA72-bHMA37, and (c) NaMA64-b-HMA50.

respectively. Also the correlation function for a fluorinecontaining block copolymer with the shortest hydrophobic chain, NaMA61-b-NFHMA12, can be reproduced by singleexponential fitting. On the other hand, the time correlation functions for fluorine-containing block copolymers with a longer fluorinated segment, NaMA72-b-NFHMA33 and NaMA64-b-HMA57, were fitted by double exponential functions, which resulted in the Rh of 260 and 1400 Å for NaMA72-b-NFHMA33 and 480 and 2500 Å for NaMA64b-NFHMA57. This means that besides normal small micelles, larger aggregates existed in these cases. The large aggregates are not obviously spherical micelles, because the Rh is too large. According to the concept of Israelachivili’s packing parameter in micelle,18 it might be reasonable to consider the large aggregates as rodlike

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Table 2. Hydrodynamic Radii of Block Copolymer Micelles in 0.5 mol/L NaCl Aqueous Solutions Analyzed by DLS

b

NaMAm-b-NFHMAn

analysis methoda

Rh (Å)

φb

m:n ) 61:12 m:n ) 72:33

single double

m:n ) 64:57

double

130 260 1400 480 2500

0.999 0.001 0.98 0.02

NaMAm-b-HMAn

analysis methoda

Rh (Å)

m:n ) 64:10 m:n ) 69:37

cum. single

110 180

m:n ) 67:50

cum.

300

a Key: single, analyzed by single-exponential fitting; double, analyzed by double exponential fitting; cum., analyzed by cumulant fitting. φ, volume fraction of the micelle.

micelles, which will be discussed further in the next section. When Rh values of the micelles were compared in the same copolymer system, they increased as the length of hydrophobic chain increased, which is analogous to usual block copolymer micelles. However, we found an interesting tendency here. That is, the Rh values of the fluorine-containing block copolymer micelles are always larger than those of non-fluorine-containing block copolymer micelles when copolymers having the same hydrophobic chain length are compared. This will be further discussed also in the next section. Small-Angle Neutron Scattering (SANS) Analysis. To evaluate the size and shape of the block copolymer micelle in detail, SANS measurements of the block copolymer solutions were performed. Figure 10 shows SANS profiles for fluorine-containing block copolymers and Figure 11 shows the profiles for non-fluorine-containing block copolymer in D2O. A strong scattering was observed in small angle regions, which is evidence of the existence of copolymer assemblies. In addition, a clear second maximum was observed around q ) 0.045 Å-1 for NaMA72-b-NFHMA33, 0.04 Å-1 for NaMA64-b-NFHMA57, 0.07 Å-1 for NaMA69-b-HMA37, and 0.05 Å-1 for NaMA67b-HMA50, indicating the formation of core-shell structure in the micelle. To further analyze the SANS profiles, we adopted spherical and rodlike core-shell micelle models and calculated the theoretical profiles. We considered that spherical and rodlike micelle coexistence model is the most plausible model at this stage, which was supported by DLS data as previously shown for the micelles of NaMA72b-NFHMA33 and NaMA64-b-NFHMA57 and by the concept of the micelle packing parameter. In fact, we have already reported coexistence of spherical and rodlike micelles in the case of a nonionic amphiphilic vinyl ether block copolymers.13,19 The existence of only spherical micelles was ruled out, since the slope of the SANS profile at small angle regions in Figure 10b (gradually decreasing dΣ(q)/ dΩ with increasing q) could not be reproduced at all by spherical particles even if the size distributions are considered, while it can be well-reproduced by adding a small fraction of rod micelles. In the micelle model here, we assumed that the density of micelle core and that of the micelle shell were homogeneous. We also assumed that the spherical and rodlike micelles have the same cross-sectional radius of the core (Rc) and the same crosssectional radius of the shell (Rs) and that they have no size distributions. Naturally the following facts should be noted. (1) The shell density is not constant and changes radially. (2) The micelle core and shell have their own size distributions. (3) The cross sectional radii of spherical and rodlike micelle might differ to some extent. However, to reduce the fitting parameters and discuss the results as simply as possible, we applied a rather simple micelle model here. For our present purpose, i.e., estimation of (18) Israelachivili, J. Intermolecular & Surface Forces; Academic: New York, 1992, Chapter 17. (19) Nakano, M.; Matsuoka, H.; Yamaoka, H.; Poppe, A.; Richter, D. Macromolecules 1999, 32, 697.

Figure 10. SANS profiles of (a) NaMA72-b-NFHMA33 and (b) NaMA64-NFHMA57 in D2O. Solid lines are the fitting curves. Fitting parameters are given in Table 3.

size and shape of micelles, this model is sufficient, which was proved by our previous studies.13,19,20 The solid lines shown in Figures 10 and 11 are the fitting curves obtained from the micelle models and they reproduced the SANS data quite well in both cases. The smearing of the scattering minimum at q ) 0.03-0.05 Å-1 was due to the (20) Matsuoka, H.; Yamamoto, Y.; Nakano, M.; Endo, H.; Yamaoka, H.; Zorn, R.; Monkenbusch, M.; Richter, D.; Seto, H.; Kawabata, Y.; Nagao, M. Langmuir 2000, 16, 9177.


Langmuir, Vol. 20, No. 17, 2004 7279 Table 3. Micelle’s Structural Parameters Obtained by SANS Model Fitting

polymer

added NaCl (mol/L)

shape

NaMA72-b-NFHMA33 NaMA72-b-NFHMA33 NaMA64-b-NFHMA57 NaMA69-b-HMA37 NaMA69-b-HMA37 NaMA67-b-HMA50

none 0.5 none none 0.5 none

sphere sphere/rod sphere/rod sphere sphere sphere

volume fraction Rca Rsb sphere/rod (Å) (Å) 1/0 0.96/0.04 0.85/0.15 1/0 1/0 1/0

100 100 122 65 70 88

145 140 160 100 100 120

a R , cross-sectional radius of the micelle core. b R , cross-sectional c s radius of the micelle shell.

Figure 11. SANS profiles of (a) NaMA69-b-HMA37 and (b) NaMA67-b-HMA50 in D2O. Solid lines are the fitting curves. Fitting parameters are given in Table 3.

polydispersity of the micelle size. The deviation at larger angles is partially due to the scattering dominated by individual fluctuation of polymer chains in the shell (blob scattering)19,20 and partially due to the volume change of shell-forming chains connected with the solvation effect, neither of which were considered in the present model. Additionally, the deviation at smaller angle (q < 0.008 Å-1) observed in the solution of NaMA69-b-HMA37 and NaMA67-HMA50 is due to the electrostatic micelle-micelle interparticle interactions (Figure 11). The structural parameters of the micelles obtained by the model fitting here are listed in Table 3. For comparison, analysis using core-corona micelle model reported by Pederson el al.22 was also performed for the data of NaMA72-b-NFHMA33 and NaMA69-b-HMA37 (21) Richter, D.; Schneiders, D.; Monkenbusch, M.; Willner, L.; Fetters, L. J.; Huang, J. S.; Lin, M.; Mortensen, K.; Farago, B. Macromolecules 1997, 30, 1053. (22) Pedersen, J. S.; Gerstenberg, M. C., Colloids Surf., A 2003, 213, 175.

Figure 12. SANS data and fitting curves (solid lines) obtained from the core-corona model: (a) NaMA72-b-NFHMA33 in D2O, fitting parameters Rc ) 98 Å, Rg-corona ) 35 Å); (b) NaMA69b-HMA37 in D2O, fitting parameters Rc ) 63 Å, Rg-corona ) 28 Å, where Rg-corona is the radius of gyration of corona.

in D2O. The fitting profiles are given in Figure 12. Rc and Rs (sum of Rc and 2Rg-corona) were estimated as 98 and 168 Å for NaMA72-b-NFHMA33 micelle, and those for NaMA69-

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Figure 13. SANS profiles of NaMA72-b-NFHMA33 in D2O in the presence of 0.5 mol/L NaCl. Solid line is the fitting curve. Fitting parameters are given in Table 3.

b-HMA37 were estimated as 63 and 119 Å. These values are very close to the Rc and Rs obtained from our coreshell models. This comparison again confirms that the simple core-shell model analysis can be used, at least for our discussion, on the micelle size here. Non-fluorine-containing block copolymers formed only spherical micelles. The fluorine-containing block copolymer with a medium hydrophobic chain length, NaMA72b-NFHMA33, also formed only spherical micelles. On the other hand, fluorine-containing block copolymers with a longer hydrophobic chain length, NaMA64-b-NFHMA57, formed a small amount of rodlike micelles along with the spherical micelles. This is because the block copolymer with longer hydrophobic chains has a larger packing parameter, and with NaMA64-b-NFHMA57, the parameter reached the critical point of the sphere-rod transition. Another important observation here is that fluorinecontaining block copolymers formed a larger micelle than the corresponding non-fluorine-containing ones. For example, Rc and Rs of NaMA72-b-NFHMA33 (100 and 145 Å) are significantly larger than those of NaMA69-b-HMA37 (65 and 100 Å). From DLS analysis in 0.5 mol/L NaClaq, Rh of smaller NaMA72-b-NFHMA33 micelle (260 Å) is larger than that of NaMA69-b-HMA37 (180 Å), which is in the same trend as the SANS results. This can be explained as follows. The aggregation number of a surfactant micelle is dependent both on the interfacial tension of core surface and on the repulsive interaction between hydrophilic headgroups. The aggregation number increases as the interfacial tension increases or as the headgroup repulsion decreases. In the micelle system here, the core-solvent interfacial tension of the fluorinated block copolymer is larger than that of the nonfluorinated block copolymer since the surface free energy of fluorinated polymer is lower (see Figure 1), whereas the headgroup repulsion in both copolymers has no difference. Thus, the micelle of the fluorine-containing block copolymer has a higher aggregation number and larger size than the corresponding non-fluorine-containing block copolymer. Figures 13 and 14 show the SANS profiles for NaMA72b-NFHMA33 and NaMA69-b-HMA37 in the presence of 0.5 mol/L NaCl, respectively. Peaks due to electrostatic

Matsumoto et al.

Figure 14. SANS profiles of NaMA64-b-HMA50 in D2O in the presence of 0.5 mol/L NaCl. Solid line is the fitting curve. Fitting parameters are given in Table 3.

micelle-micelle interaction observed around 0.008 Å-1 in Figure 11 disappeared because of the screening effect of the added salt. The SANS data were analyzed by model fitting, and the results are also listed in Table 3. The experimental data for non-fluorine-containing block copolymers were consistent with the theoretical profiles calculated from a spherical micelle model. However, the SANS profiles of fluorine-containing block copolymers could be reproduced by using a sphere-rod coexistence model, though the volume fractions of rodlike micelles are low. These results agreed well with those obtained by DLS analysis. That is a large aggregate (rodlike micelle, Rh ) 1400 Å) was observed in NaMA72-b-NFHMA33 in 0.5 mol/L NaCl aqueous solution, while no such an aggregate was seen in NaMA69-b-HMA37 in NaCl solution. The rodlike micelle formation by salt addition can be explained by the increase of packing parameters in the block copolymer molecules. The electrostatic repulsion at the headgroups is significantly reduced by the screening effect of the added salt, which increases the packing parameter, and the block copolymer can no longer maintain a spherical shape. Small-Angle X-ray Scattering (SAXS) Measurement. For further evaluation of the micelle structure, SAXS measurements of NaMA72-b-NFHMA33 and NaMA69b-HMA37 solutions were performed. Figures 15 and 16 show the SAXS profiles of 3.2 × 10-4 mol/L copolymer solutions measured in H2O and 0.5 mol/L NaCl(aqueous). The strong X-ray scattering in the small-angle regions confirmed the micelle formations by the block copolymers. We could reproduce the experimental data obtained with NaMA72-b-NFHMA33 solutions by theoretical profiles of core-shell spherical and rodlike micelle models using almost the same parameters as those obtained by SANS measurements. In SAXS profiles, we could not know much about the fraction of rodlike micelle because the q-range covered in SAXS is too high. However, we can detect strong scattering from the shell, so that we can confirm the accuracy of our core-shell model to some extent by reproducing the profiles with almost the same parameters obtained from SANS in the case of NaMA72-b-NFHMA33. On the other hand, we reasonably reproduced the ex-


Figure 15. SAXS profiles of NaMA72-b-NFHMA33 in H2O (filled circle) and H2O in the presence of 0.5 mol/L NaCl (open circle). Solid line is a fitting curve calculated using the core-shell sphere micelle model (Rc ) 105 Å, Rs ) 155 Å). Dotted line is a fitting curve calculated using the core-shell sphere and rodlike micelle model (Rc ) 105 Å, Rs ) 150 Å, φsphere ) 0.96). The profiles for 0.5 mol/L NaCl solution are shifted downward by 1 decade for better view.

Langmuir, Vol. 20, No. 17, 2004 7281

Figure 17. UV absorption spectra of NaMA72-b-NFHMA33 in the presence of DFB and DMN. Broken and dotted lines are spectra of 270 µmol/L DFB and 40 µmol/L DMN in n-octane, respectively.

Figure 18. UV absorption spectra of NaMA69-b-NFHMA37 in the presence of DFB and DMN. Broken and dotted lines are spectra of 110 µmol/L DFB and 140 µmol/L DMN in n-octane, respectively.

Figure 16. SAXS profiles of NaMA69-b-HMA37 in H2O (filled circle) and H2O in the presence of 0.5 mol/L NaCl (open circle). Solid and dotted lines are fitting curves calculated using hard sphere micelle model (Rs ) 140 Å). The profiles for 0.5 mol/L NaCl solution are shifted downward by 1 decade for better view.

perimental data for NaMA69-b-HMA37 by theoretical curves for a simple hard sphere as shown in Figure 16. X-ray scattering occurred from the simple hard sphere of the whole micelle, since the core and shell have similar electron densities. Thus the radius of the hard sphere evaluated here by SAXS analysis corresponds to the radius of the whole micelle. Selective Dye Solubilization. Since “like dissolves like”, it is conjectured that fluorinated micelles can preferentially encapsulate fluorinated compounds. We have already reported an example of selective solubili-

zation of fluorinated compound by fluorine-containing vinyl ether block copolymers,10,11,13 which we now call fluorophilicity. To check the generality of this property, we examined simultaneous solubilization of fluorinated and nonfluorinated hydrophobic dyes by the methacrylatebased block copolymers synthesized here. A mixture of decafluorobiphenyl (DFB) and DMN was added to the block copolymer solutions, and the amount of solubilized dye was evaluated by UV absorption spectroscopy. Figures 17 and 18 show the UV spectra of NaMA72-b-NFHMA33 and NaMA69-b-HMA37 aqueous solutions in the presence of the dye mixture. Those spectra were deconvoluted into a spectrum of each dye (also depicted in the figures), and the solubilized dye concentrations were independently estimated. The concentrations are summarized in Table 4 along with the results obtained from the other copolymer solutions. It is obvious that polyNaMA-b-polyNFHMA predominantly solubilized DFB, while polyNaMA-b-polyHMA predominantly solubilized DMN. In addition, [DFB] increased with the increase in core-forming segment length keeping [DMN] rather constant in the fluorine-containing

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Table 4. Dye Concentrations Solubilized in Aqueous Solutions of Block Copolymers polymer

[DFB] (µmol/L)

[DMN] (µmol/L)

[DFB]/[DMN]

NaMA61-b-NFHMA12 NaMA72-b-NFHMA33 NaMA64-b-NFHMA57 NaMA64-b-HMA10 NaMA69-b-HMA37 NaMA67-b-HMA50

43 270 300 10 110 150

15 40 30 30 140 200

2.9 6.7 10.0 0.33 0.77 0.77

Figure 19. Fluorinated dye selectivity as a function of polymerization degree of hydrophobic segment: filled circle, polyNaMA-b-polyNFHMA; open circle, polyNaMA-b-polyHMA.

block copolymer, while in the nonfluorinated block copolymer both [DFB] and [DMN] increased. As a result, fluorinated dye selectivity ([DFB]/[DMN]) of the fluorinecontaining block copolymer micelle increased with the increase of the length in the hydrophobic segment, while that of the nonfluorinated block copolymer micelle was

almost constant (Figure 19). Since a block copolymer with a longer hydrophobic segment forms micelles with a larger core size, we consider that fluorophilicity increases as the core size of the fluorine-containing block copolymer micelle increases. Conclusions Well-defined water-soluble fluorine-containing and nonfluorine-containing amphiphilic block copolymers, polyNaMA-b-polyNFHMA and polyNaMA-b-polyHMA, were synthesized. These copolymers formed micelles in an aqueous solution without decreasing the surface tension of the solutions, which we now consider a specific property of an amphiphilic copolymer having a long ionic chain. DLS and SANS analyses revealed that the fluorinated block copolymer forms larger micelles than the corresponding nonfluorinated block copolymer does. We think that it is due to a high interfacial tension between core and the solvent regions in the fluorinated block copolymer micelle. We also demonstrated by the selective solubilization experiment that the fluorine-containing block copolymer had fluorophilicity, and we found that the property was enhanced by elongation of the fluorinated segment. The simplicity of the polymer preparation and its unique properties should fascinate many scientists from both academic and industrial points of view. Acknowledgment. The present work was supported by Grant-in-aid (A15205017) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, and a Grant-in-aid for Young Scientists (No.15750104) from the Japan Society for the Promotion of Science. It was also supported by 21st century COE program, COE for a United Approach to New Materials Science, of Ministry of Education, Culture, Sports, Science, and Technology, Japan. We are deeply grateful to Professor Shibayama and Dr. Nagao (University of Tokyo) for their kind support of SANS measurements at Tokai, which is adopted as Proposal 02. 045. LA049371+

Association Behavior of Fluorine-Containing and Non-Fluorine

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