Micelle Formation of Diblock Copolymers of Styrene and Sulfonated

Dry samples of these St−SI block copolymers could be directly dissolved in water, undergoing intermolecular self-association to form core−corona t...
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Langmuir 1999, 15, 454-462

Micelle Formation of Diblock Copolymers of Styrene and Sulfonated Isoprene in Aqueous Solution Krzysztof Szczubiałka,†,‡ Katsuhiro Ishikawa,§ and Yotaro Morishima*,‡ Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan, and Specialty Materials Laboratory, Japan Synthetic Rubber Company Ltd., 34-1 Tohwada, Kamisu-machi, Kashima, Ibaraki 314-02, Japan Received June 24, 1998. In Final Form: November 12, 1998 Asymmetric diblock copolymers of styrene (St) and sulfonated isoprene (SI) with a short St block and a much longer SI block were synthesized by sulfonation of diblock copolymers of St and isoprene, and their micellization in aqueous solution was investigated. Three block copolymers having different degrees of polymerization of St block (DPSt) and SI block (DPSI) were employed, which include polymers with DPSt ) 40 and DPSI ) 240 (St-SI-20), DPSt ) 60 and DPSI ) 210 (St-SI-30), and DPSt ) 130 and DPSI ) 820 (St-SI-207). Dry samples of these St-SI block copolymers could be directly dissolved in water, undergoing intermolecular self-association to form core-corona type micelles. The micellar mass, hydrodynamic size and its distribution, critical micelle concentration (cmc), and aggregation number were estimated by light scattering and fluorescence techniques. It was suggested that the micelles and unimers coexist at equilibrium with a well-defined cmc on the order of 10-2 g/L, with St-SI-207 (with the longest SI block) showing a higher cmc than the other two polymers. The aggregation number was strongly dependent on the DPSt/ DPSI ratio, with St-SI-30 (with the highest DPSt/DPSI ratio) showing a largest aggregation number of 480. The hydrodynamic size of the micelle was almost independent of the polymer concentration up to 1.0 g/L, ranging from 61 nm for St-SI-30 to 210 nm for St-SI-207 in 0.01 M NaCl. On increasing ionic strength, the hydrodynamic size of St-SI-207 decreases markedly because of a collapse of the SI blocks in the micelle corona. The equilibrium constant for the partition of pyrene probes between the aqueous bulk and micellar phases was found to be on the order of 105 M-1, reflecting effective solubilization of pyrene molecules in the cores of the St-SI block copolymer micelles. Protection of pyrene fluorophores from a bulk quencher (acrylamide) was found to be more effective in the St-SI-30 and St-SI-207 micelles than in the St-SI-20 micelle, attributable to a larger size of the micelle core.

Introduction Various physicochemical properties of amphiphilic polymers in aqueous solutions have been extensively studied in recent years partly because of a growing interest in potential applications of such polymers.1-3 It is wellknown that block copolymers, when dissolved in a selective solvent, a solvent good for one block and poor for the other, undergo self-assembly, leading to the formation of various morphologies, e.g., spheres, rods, or lamellae. Water can be a selective solvent for hydrophobic/hydrophilic block copolymers, allowing for the formation of spherical micelles, in a typical case, having hydrophobic cores and hydrophilic outer layers (i.e., coronas). The micelle formation of amphiphilic block copolymers in aqueous solutions, †On leave from the Faculty of Chemistry, Jagiellonian University, 30-060 Krako´w, Ingardena 3, Poland. ‡ Osaka University. § Japan Synthetic Rubber Co. Ltd.

(1) See, for example: (a) Kiserow, D. J.; Itoh, Y.; Webber, S. E. Macromolecules 1996, 29, 7847. (b) Kiserow, D. J.; Itoh, Y.; Webber, S. E. Macromolecules 1997, 30, 2934. (c) Procha´zka, K.; Martin, T. J.; Munk, P.; Webber, S. E. Macromolecules 1996, 29, 6518. (d) Procha´zka, K.; Martin, T. J.; Webber, S. E.; Munk, P. Macromolecules 1996, 29, 6526. (2) See, for example: (a) Kobayashi, A.; Matsuzaki, F.; Yanaki, T.; Morishima, Y. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1997, 46, 4080. (b) Morishima, Y.; Aota, H.; Saegusa, K.; Kamachi, M. Macromolecules 1996, 29, 6505. (c) Morishima, Y.; Sato, T.; Kamachi, M. Macromolecules 1996, 29, 3960. (d) Yamamoto, H.; Mizusaki, M.; Yoda, K.; Morishima, Y. Macromolecules 1998, 31, 3588. (3) See, for example: (a) Barros, T. C.; Adronov, A.; Winnik, F. M.; Bohne, C. Langmuir 1997, 13, 6089. (b) Winnik, F. M.; Adronov, A.; Kitano, H. Can. J. Chem. 1995, 73, 2030. (c) Yamazaki, A.; Song, J. M.; Winnik, F. M.; Brash, J. L. Macromolecules 1998, 109, 31. (d) Yamazaki, A.; Song, J. M.; Winnik, F. M.; Brash, J. L. Macromolecules 1998, 109, 31.

in particular, has been the focus of increasing interest in the past decade, and a number of excellent reviews have appeared.4 The size, aggregation number, critical micelle concentration (cmc), and dynamics of micelle formation and dissociation depend on various parameters such as the overall molecular weight of amphiphilic block polymers, relative length of hydrophobic and hydrophilic blocks, solvent composition, ionic strength and pH of solutions, and temperature. Among amphiphilic block copolymers, a wide variety of polyelectrolyte block copolymers, consisting of hydrophobic and ionic blocks, have been investigated, which include 4-vinylpyridinium alkyl halide,5 sodium acrylate,6 and methacrylate7 and sulfonated styrene8 blocks covalently linked to hydrophobic blocks such as styrene blocks. Ionic (4) For a recent review, see: (a) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 31, (b) Solvents and Self-Organization of Polymers; Webber, S. E., Munk, P., Tuzar, Z., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1996. (c) Webber, S. E. J. Phys. Chem. B 1998, 102, 2618. (5) (a) Gao, Z.; Varshney, S. K.; Wong, S.; Eisenberg, A. Macromolecules 1994, 27, 7923. (b) Gouin J.-P.; Williams C. E.; Eisenberg A. Macromolecules 1989, 22, 4573. (c) Gauthier, S.; Duchesne, D.; Eisenberg, A. Macromolecules 1987, 20, 753. (d) Gauthier, S.; Eisenberg, A. Macromolecules 1987, 20, 760. (6) Astafieva, I.; Zhong, X. F.; Eisenberg, A. Macromolecules 1993, 26, 7339. (7) (a) Eckert, A. R.; Martin, T. J.; Webber, S. E. J. Phys. Chem. 1997, 101, 1646. (b) Qin, A.; Tian, M.; Ramireddy, C.; Webber, S. E.; Munk, P.; Tuzar, Z. Macromolecules 1994, 27, 120. (c) Kiserow, D.; Chan, J.; Ramireddy, C.; Munk, P.; Webber, S. E. Macromolecules 1992, 25, 5338. (d) Cao, T.; Munk, P.; Ramireddy, C.; Tuzar, Z.; Webber, S. E. Macromolecules 1991, 24, 6300. (8) Amiel, C.; Sikka, M.; Schneider, J. W.; Tsao, Y.; Tirrell, M.; Mays, J. W. Macromolecules 1995, 28, 3125.

10.1021/la9807558 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/30/1998

Sulfonate Block Copolymer Micelles

block copolymers with relatively short hydrophobic blocks normally form spherical micelles with cores that are glassy or liquidlike, depending on the type of monomer units constituting hydrophobic blocks. The micelle formation of such ionic block copolymers may occur under equilibrium conditions. Namely, when ionic block copolymers are directly dissolved in water, micelles may be formed spontaneously in a reversible equilibrium process between micelles and single free polymer molecules (i.e., unimers). This equilibrium is a closed association process,9 and characteristics of such equilibrium micelles are nearly monodisperse in mass and size. At a thermodynamic equilibrium, aggregation numbers (numbers of polymer molecules that consist of a single micelle) may be determined by the minimum free energy of the system, and therefore the distribution of aggregation numbers around the most stable state is quite narrow.10 When the length of an ionic block relative to a hydrophobic block is decreased, micelle solutions may not be able to be prepared by direct dissolution of polymers into water. Instead, the polymers may be first dissolved in an organic solvent that is miscible with water, and then water may be added slowly to the polymer solution, followed by dialysis of the solution to remove the organic solvent. This treatment may result in aqueous solutions of kinetically frozen micelles. A variety of morphologies of such kinetically frozen micelles include spheres (a typical example may be crew-cut micelles), rods, lamellae, and vesicles, depending on the relative lengths of hydrophobic and ionic blocks.5a,11 Effects of hydrophobic and ionic block lengths on the critical micelle concentration (cmc) have been reported on block copolymers of styrene and sodium acrylate.6,12 A small increase in the length of the styrene block brings about a large decrease in cmc, whereas the influence of the length of the sodium acrylate block on cmc is more complicated. The influence of the length of styrene and sodium acrylate block lengths on the aggregation number has been studied.13,14 The length of the styrene block has a larger effect on the aggregation number than the length of the sodium acrylate block. The dependence of the radius of the micelle core on the length of both the soluble and insoluble blocks has also been studied.15 The dimensions of the core are positively correlated with the length of the insoluble block, while an increase in the length of the soluble block brings about a decrease in the core size. It is well-known that an increase in the ionic strength leads to a decrease in the solubility of nonionic species in water and to a decrease in electrostatic interactions, thus influencing the associative behavior of amphiphilic polyelectrolytes in water. In the case of the styrene-sodium acrylate block copolymers, the cmc decreases linearly with the square root of the ionic strength and the aggregation (9) (a) Elias, H.-G. J. Macromol. Sci. 1973, A7, 601. (b) Tuzar, Z.; Kratochvı´l, P.; Procha´zka, K.; Munk, P. Collect. Czech. Chem. Commun. 1993, 58, 2362. (c) Tuzar, Z.; Kratochvı´l, P. Surface and Colloid Science Series; Matijevic, E., Ed.; Plenum: New York, 1993; Vol. 15, Number 1. (10) Tian, M.; Qin, A.; Ramireddy, C.; Webber, S. E.; Munk, P.; Tuzar, Z.; Procha´zka, K. Langmuir 1993, 9, 1741. (11) See, for example: (a) Yu, K.; Eisenberg, A. Macromolecules 1998, 31, 3509. (b) Yu, K.; Eisenberg, A. Macromolecules 1996, 29, 6359. (c) Yu, K.; Zhang, L.; Eisenberg, A. Langmuir 1996, 12, 5980. (d) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (e) Zhu, J.; Lennox, R. B.; Eisenberg, A. J. Phys. Chem. 1992, 96, 4727. (12) Astafieva, I.; Khougaz, K.; Eisenberg, A. Macromolecules 1995, 28, 7127. (13) Khougaz, K.; Astafieva, I.; Eisenberg, A. Macromolecules 1995, 28, 7135. (14) Zhong, X. F.; Eisenberg, A. Macromolecules 1994, 27, 1751. (15) Zhang, L.; Barlow, R. J.; Eisenberg, A. Macromolecules 1995, 28, 6055.

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number increases with an increase in the ionic strength.12 These observations are a consequence of shielding electrostatic repulsions within a micellar corona.16 A strong influence of pH can be observed in block copolymers consisting of a weak electrolyte block, e.g., carboxylic group.17,18 With these types of block copolymers, micellization and micelle properties may be controlled by pH,19 suggesting a potential application as a chemicaldelivery system. The influence of pH on the associative behavior of amphiphilic weak polyelectrolytes depends on the pK of dissociating groups.20 In the case of micelles formed from amphiphilic polycarboxylate block copolymers, a decrease in pH below their pK results in the protonation of the carboxylate blocks, leading to a collapse of coronas and to a decrease in the micelle diameter. On the other hand, the block copolymers consisting of strongly dissociating groups, such as sulfonate groups, are fully dissociated in a much wider range of pH, and thus their response to a pH change is much smaller. Among many studies of amphiphilic block copolymer micelles in aqueous systems, true equilibrium micelle systems may be very rare except for highly asymmetric hydrophilic/hydrophobic block copolymers with a long hydrophilic block and a very short hydrophobic block. To obtain an equilibrium micellar system with block copolymers consisting of comparable lengths of hydrophilic and hydrophobic blocks, the selective solvent must be poor enough for the hydrophobic block, but at the same time, it must be good enough to sufficiently swell the core and allow unimers to be exchanged between micelles and the bulk phase and also between different micelles. The best example for such equilibrium systems is the micellization of block copolymers of styrene and methacrylic acid in a 1,4-dioxane/water (80/20, v/v) mixture.9b,17 When water/ 1,4-dioxane solutions of the block copolymers are dialyzed against pure water, an equilibrium state is shifted according to the solvent composition during the dialysis up to a point where an equilibrium state is kinetically frozen. Thus, aqueous solutions of such kinetically frozen micelles of the styrene-methacrylic acid block copolymers can be prepared. To our best knowledge, systematic studies of the micellization of strong electrolyte block copolymers have not been reported so far. Recently, one of the present authors and his collaborators at Japan Synthetic Rubber Co. have succeeded in the quantitative sulfonation of block copolymers of styrene (St) and isoprene (I). Because St-I block copolymers can be synthesized by living anionic polymerization, the degree of polymerization (DP) of each block sequence can be easily controlled. Therefore, we can prepare block copolymers of St and sulfonated isoprene (SI) with well-defined block lengths. We have been investigating the associative behavior of these St-SI block copolymers with different St and SI block lengths in aqueous solution. In the present study, we chose to employ asymmetric St-SI block copolymers, with the DP of the St block ranging from 38 to 135 and that of the SI block ranging from 210 to 820. Dry samples of these block copolymers can be directly dissolved in water. In this paper we will report on the micelle formation of these block copolymers and some properties of the block copolymer (16) Dan, N.; Tirrell, M. Macromolecules 1993, 26, 4310. (17) Munk, P.; Ramireddy, C.; Tian, M.; Webber, S. E.; Procha´zka, K.; Tuzar, Z. Makromol. Chem., Macromol. Symp. 1992, 58, 195. (18) Chan, J.; Fox, S.; Kiserow, D.; Ramireddy, C.; Munk, P.; Webber, S. E. Macromolecules 1993, 26, 7016. (19) Martin, T.; Procha´zka, K.; Munk, P.; Webber, S. E. Macromolecules 1996, 29, 6071. (20) McCormick, C. L.; Elliott, D. L. Macromolecules 1986, 19, 542.

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micelles as investigated by various fluorescence techniques with the use of pyrene probes solubilized in the micelle cores. Light scattering techniques, including static light scattering (SLS) and quasielastic light scattering (QELS), were also employed for the characterization of the polymer micelles. Experimental Section Block Copolymers of Styrene and Isoprene. The block copolymers of styrene and isoprene were prepared by living anionic polymerization as follows: To a stirred solution of a predetermined amount of isoprene, ranging from 420 g (6.17 mol) to 480 g (7.05 mol), in 1200 g of dried cyclohexane and 0.6 g of dried tetrahydrofuran (THF) in a 5-L glass autoclave was added at 60 °C a 30-mL cyclohexane solution of n-butyllithium (15 wt %), and the mixture was stirred for 2 h. A small portion of the reaction mixture was sampled out and subjected to sizeexclusion chromatography (SEC) measurements to determine the molecular weight of the isoprene block. A predetermined amount of styrene, ranging from 120 g (1.15 mol) to 180 g (1.73 mol), was added to the polyisoprene living anion solution to proceed block copolymerization. The reaction mixture was stirred at 60 °C for 2 h. The polymerization was stopped by adding 1 g of 2-propanol. Block copolymers were recovered by evaporating solvents and residual monomers under reduced pressure and subjected to SEC measurements. Block Copolymers of Styrene and Sulfonated Isoprene. A solution of sulfur trioxide/1,4-dioxane complex was first prepared as follows: To 600 g of 1,4-dioxane was added dropwise with stirring 60 g of anhydrous sulfuric acid. The reaction mixture was kept at room temperature (about 25 °C) and stirred for 2 h. A predetermined amount of a thus-prepared sulfur trioxide/1,4dioxane complex solution was added dropwise to a 600-g 1,4dioxane solution of 60 g of the styrene-isoprene block copolymer with stirring. The temperature of the reaction mixture was kept below 25 °C during the reaction. After the solution was stirred for additional 2 h, 198 g of a 15 wt % NaOH aqueous solution and 300 g of methanol were added to the reaction mixture. The mixture was heated to 80 °C for 4 h with stirring. Aqueous solutions of sulfonated block copolymers were obtained after evaporation of organic solvents under reduced pressure. The aqueous solutions of the polymers were dialyzed against deionized water. Solvents and Reagents. Deionized water was used to prepare polymer micelle solutions. Analytical-grade solvents were obtained from Wako Pure Chemical Industries, Ltd., and used as received. Pyrene, purchased from Nacalai Tesque, Inc., was recrystallized from methanol. Analytical-grade NaCl, from Wako Pure Chemical Industries, Ltd., was used without further purification. Acrylamide was recrystallized from ethanol prior to use. Preparation of Polymer Micelle Solutions. Polymer micelle solutions were prepared by direct dissolution of dry polymers into pure water or into aqueous solutions of NaCl with predetermined ionic strengths. The polymer solutions were then sonicated for about 1 h until complete dissolution. Solutions of polymer micelles with pyrene solubilized were prepared by dissolving dry polymers into a pyrene-saturated aqueous solution, which was followed by sonication for 1 h. A pyrene-saturated aqueous stock solution was prepared as follows: Molecular pyrene was first dissolved in acetone in a flask at a concentration of 1 mg/mL, and acetone was evaporated on a rotary evaporator to dryness in the form of a thin film on the inner wall of the flask. Deionized water was then poured into the flask, and the aqueous layer was mechanically stirred overnight in the dark. The resulting pyrene solution was filtered through a 0.2-µm PTFE filter. The thus-prepared stock solution of pyrene was stored in the dark. Measurements. (a) Size-Exclusion Chromatography. SEC analysis was performed with a Tosoh HCL-8020 system equipped with an UV-8010 UV detector. A combined column of MXL-L and G4000XL was employed with THF as an eluent. Standard polystyrene samples were used to calibrate the molecular weight. (b) Quasielastic Light Scattering. The apparent hydrodynamic radii and their distribution were measured with an Otsuka

Szczubiałka et al. Electronics Photal DLS-7000 light scattering spectrometer equipped with a 75-mW Ar laser operating at λ ) 488 nm. Data were collected using an ALV wide-band multi-τ digital autocorrelator. All measurements were performed at 25 °C. The micelle diameters (at θ ) 30°, 60°, and 90°) and micelle size distributions (at θ ) 90°) were measured as a function of polymer concentration. The solutions were filtered prior to measurements using a 0.20or 0.45-µm disposable membrane filter. The intensity autocorrelation function g(2)(t) was measured experimentally, which is related to the normalized autocorrelation function g(1)(t) by the relation

g(2)(t) ) B[1 + β|g(1)(t)|2]

(1)

where β is a parameter of the optical system and B is a baseline. The hydrodynamic radii of micelles were calculated from a cumulant analysis of the autocorrelation function, which was performed by fitting a polynom of the first order to the function ln(g(2)(t) - 1). The polynomial coefficients are converted into the coefficients of the cumulant expansion of the normalized autocorrelation function

ln(g(1)(t)) ) ln(A) - Γt

(2)

where A is the amplitude and Γ is the decay rate. Γ is used to calculate the apparent diffusion coefficient, D, from the relation

Γ ) Dq2

(3)

where q represents the scattering vector, (4πn/λ) sin(θ/2), where n is the refractive index of the solution, λ is the wavelength of the scattered light, and θ is the scattering angle. The hydrodynamic radius (Rh) is calculated from the Stokes-Einstein relation

Rh ) kT/6πηD

(4)

where T is the Kelvin temperature and η is the viscosity. To obtain the relaxation time distribution, τA(τ), the inverse Laplace transform (ILT) of the normalized intensity correlation function, g(2)(t), was performed using the algorithm REPES according to the equation

g(1)(t) )

∫ τA(τ) e ∞

0

-t/τ

d ln τ

(5)

where τ is the relaxation time. The values of η and n for each micelle solution were assumed to be equal to those of pure water. (c) Static Light Scattering. For SLS measurements an Otsuka Electronics Photal DLS-7000 light scattering spectrometer was employed. The measurements were performed at 25 °C. The scattering angle was varied from 30° to 150°. The values of refractive index increment, dn/dc, were obtained using an Otsuka DRM-1020 double-beam differential refractometer. (d) Steady-State Fluorescence. A Hitachi F-4500 fluorescence spectrophotometer was employed for the measurements of fluorescence and excitation spectra. The spectral resolution was 2.5 nm for both excitation and emission. All spectra were measured using air-equilibrated solutions at room temperature. The fluorescence spectra of pyrene solubilized in polymer micelles were measured using an excitation wavelength of λex ) 334 nm. Excitation spectra of pyrene solubilized in polymer micelles were monitored at λem ) 374 nm. (e) Fluorescence Decay. The fluorescence lifetime of pyrene solubilized in polymer micelles was measured with a timecorrelated single-photon-counting technique using a Horiba NAES 550 system equipped with a flash lamp filled with hydrogen. The excitation wavelength was set to 343 nm. Pyrene fluorescence was detected at λ > 370 nm using an L-37 cutoff filter. Measurements were performed using air-equilibrated solutions of a polymer concentration of 1.0 g/L and a pyrene concentration of 2.0 × 10-7 M. (f) Viscosity. Reduced viscosities of polymer solutions were measured using a modified Ubbelohde-type viscometer at 30 °C. The polymers were dissolved in pure water or in a 0.1 M NaCl aqueous solution.

Sulfonate Block Copolymer Micelles

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Chart 1. Proposed Structures of St-SI Copolymers and Their Relative Block Lengths

Table 1. Characteristics of the Diblock St-SI Copolymers polymer symbol weight ratio of the St and I blocksa weight average molecular weighta average mass of the St blocka average mass of the I blocka,b DPStc DPSId DPSt/DPSI weight average mass of micelle aggregation number “apparent” cmc (10-2 g/L) “real” cmc (10-2 g/L) Kv (×105)

St-SI-20 St-SI-30 St-SI-207 20/80

30/70

20/80

2.0 × 104 2.0 × 104 7.0 × 104 4.0 × 103 1.6 × 104 38 235 0.16 3.2 × 106 75 7 4 2.1

6.0 × 103 1.4 × 104 58 206 0.28 1.9 × 107 480 5 3 1.6

1.4 × 104 5.6 × 104 135 824 0.16 1.7 × 107 110 8 8 3.1

a Before sulfonation. b Isoprene block consists of about 90% of 1,4, 10% of 3,4, and traces of 1,2-structures. c Average degree of polymerization of the St block. d Average degree of polymerization of the SI block.

(g) Infrared (IR) Spectra. IR spectra of dry polymers were recorded on a Jasco FT/IR-3 spectrometer using a KBr pellet method.

Results and Discussion Characterization of Polymers. Chart 1 shows a chemical structure for the St-SI block copolymers proposed on the basis of the literature21 on the sulfonation of olefins with sulfur trioxide/1,4-dioxane complexes. The sulfonation of the isoprene block in the parent block copolymer was practically quantitative as indicated by the complete disappearance of IR bands due to olefinic groups at 910 and 1000 cm-1. The St-SI block copolymers were most soluble in water and dimethyl sulfoxide (up to about 1.0 g/L), slightly soluble in methanol (up to about 0.1 g/L), and insoluble in chloroform, toluene, acetonitrile, acetone, N,N-dimethylformamide, tetrahydrofuran, 2,2,2trifluoroethanol, and 1,1,1,3,3,3-hexafluoro-2-propanol. Relative lengths of the St and SI blocks for the three block copolymers employed in the present study are schematically illustrated in Chart 1. Some characteristic data for these block copolymers and their parent St-I block copolymers are collected in Table 1. The block copolymer chains are highly asymmetric with a short St block and a long SI block. From these asymmetric block sequences, one can anticipate that these block copolymers form spherical “star-like” micelles in aqueous solutions. At a polymer concentration of about 1.0 g/L, St-SI-20 gives an optically clear aqueous solution, whereas aqueous solutions of St-SI-30 and St-SI-207 are slightly opaque (21) Nagayama, M.; Okumura, O.; Noda, S.; Mori, A. J. Chem. Soc., Chem. Commun. 1973, 21, 841.

Figure 1. Zimm plot for St-SI-30. This and other SLS measurements were carried out at 25 °C at angles from 30° to 150° with a 10° increment. The polymer concentration was varied from 0.2 to 1.0 g/L with a 0.2 g/L increment.

(see Chart 1 for the abbreviations of the polymers). The solubilities of these polymers in 0.2 M NaCl are lower than those in pure water. Similar tendencies have been reported for hydrophilic/hydrophobic block copolymers.22,23 Micellar Mass. Figure 1 shows an example of Zimm plots for St-SI-30 in a 0.1 M NaCl aqueous solution. The plots are reasonably linear within the ranges of the concentration from 0.2 to 1.0 g/L and the angle from 30° to 150°. The apparent weight-average molecular weights for all of the St-SI block copolymers in 0.1 M NaCl, calculated from Zimm plots, are listed in Table 1. The apparent weight-average molecular weights for the StSI block copolymers are much higher than those calculated on the basis of the molecular weights of the parent St-I block copolymers before sulfonation (Table 1). This is a manifestation of the micelle formation of the St-SI block copolymers in aqueous solution. The aggregation numbers of the St-SI block copolymer micelles in aqueous solution were roughly estimated by dividing the micellar mass (i.e., the apparent weightaverage molecular weight in 0.1 M NaCl) by the unimer mass (i.e., the molecular weight calculated from the molecular weight of the parent St-I block copolymer). The results are presented in Table 1. The weight-average molecular weight of the polymer micelle determined by SLS may be somewhat overestimated because of water molecules bound to the SI blocks. Therefore, the aggregation numbers thus calculated may also be overestimated and thus regarded as an upper limit of the true values. The aggregation number of St-SI-30, thus estimated to be 480, is much larger than those of the other two polymers (i.e., 75 for St-SI-20 and 110 for St-SI-207). The degree of polymerization for the St block (DPSt) in St-SI-30 is 58, which is larger than DPSt ) 38 in St-SI-20 and smaller than DPSt ) 135 in St-SI-207. On the other hand, the ratio of DPSt to the degree of polymerization of the SI block (DPSI) for St-SI-30 is 13/87, which is larger than those for St-SI-20 and St-SI-207 (DPSt/DPSI ) 8/92 for both of the polymers). From these comparisons, it seems that the DPSt/DPSI ratio has a much larger effect on the aggregation number than DPSt and the overall molecular weight of the block copolymer. The finding that St-SI-30 (with the highest DPSt/DPSI ratio) has the largest aggregation number is consistent (22) Rager, T.; Meyer, W. H.; Wegner, G.; Winnik, M. A. Macromolecules 1997, 30, 4911. (23) Selb, J.; Gallot, Y. Makromol. Chem. 1980, 181, 809.

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Figure 2. Micelle diameters measured by QELS (a) at 0.01 M NaCl and (b) at 0.1 M NaCl: (O) St-SI-20; (0) St-SI-30; (4) St-SI-207.

with a theoretical prediction.24 A similar effect of the relative length of a core-forming block on the aggregation number has been reported.7b,25,26 In the micellization of block copolymers of St and methacrylic acid (MA), aggregation numbers are a strong function of the St block length.7b,25 The overall molecular weight of the St-MA block copolymer also has a pronounced effect on the aggregation number.7b,25 In the case of the present St-SI block copolymers, by contrast, aggregation numbers are less dependent on both the St block length and the overall molecular weight than they are in the St-MA block copolymer case. This difference between the St-MA and St-SI block copolymers may be explained by a difference in the extent of electrostatic repulsions between electrolyte blocks. Namely, the ionization of the MA block can be suppressed within a corona layer, whereas the SI block can be almost completely ionized. Therefore, repulsive interactions can be much weaker in St-MA block copolymer micelles than in St-SI block copolymer micelles, allowing a higher molecular weight St-MA block copolymer with a longer MA block to form a micelle of a larger aggregation number. In contrast to the St-MA block copolymer case, the aggregation number of a higher molecular weight St-SI block copolymer, St-SI-207, is limited to a smaller value than that of St-SI-30, although DPSt for the former is larger than that for the latter. Hydrodynamic Size of the Micelle. The hydrodynamic sizes for the St-SI block copolymer micelles were estimated by QELS. In Figure 2, the cumulant diameters of the micelles are plotted as a function of the polymer concentration in 0.01 and 0.1 M NaCl aqueous solutions. The diameters are nearly independent of the polymer concentration up to 1.0 g/L (except for St-SI-207 in 0.01 M NaCl, which shows a slight increase in the diameter (24) Procha´zka, K.; Tuzar, Z.; Kratochvı´l, P. Polymer 1991, 32, 3038. (25) Munk, P.; Qin, A.; Tian, M.; Ramireddy, C.; Webber, S.; Tuzar, Z.; Procha´zka, K. J. Appl. Polym. Sci. 1993, 52, 45. (26) Fo¨rster, S.; Zisenis, M.; Wenz, E.; Antonietti, A. J. Chem. Phys. 1996, 104, 9956.

Figure 3. Relaxation time distributions for the St-SI copolymer micelles at varying polymer concentrations (g/L) at 0.1 M NaCl: (O) 0.1; (0) 0.3; (]) 0.7; (4) 1.0.

in a low concentration region). The diameters in 0.01 M NaCl are approximately estimated to be 67, 61, and 210 nm (on average) for St-SI-20, St-SI-30, and St-SI-207, respectively. The hydrodynamic size for the St-SI-207 micelle is much larger than those for the other two polymer micelles because of a much larger corona formed by longer SI blocks that are extended because of strong electrostatic repulsions in the corona. When the NaCl concentration is increased to 0.1 M, the micelle diameter for St-SI-207 (with the longest SI block) markedly decreases to 160 nm, resulting from a collapse of SI blocks in the corona because of screening of electrostatic repulsions between SI blocks. By contrast, the micelle size for St-SI-20 (with the shortest SI block) does not change significantly upon an increase in the salt concentration. On the other hand, the diameter of the St-SI-30 micelle shows an increase from 61 to 79 nm, which may be a consequence of an increase in the aggregation number. The shielding of electrostatic repulsive forces in the corona, brought about by increasing salt concentration, may result in an increase in hydrophobic interactions in the core, which may be relatively stronger in St-SI-30 than in St-SI-20 because of larger DPSt in the former. This situation may lead to an increase in the aggregation number, and thus the hydrodynamic diameter for the St-SI-30 micelle may be increased. Figure 3 shows relaxation time distributions obtained from the autocorrelation functions measured at a scattering angle of 90° at four different concentrations in 0.1 M NaCl. All of the distributions are unimodal and independent of the polymer concentration in the range 0.1-1.0 g/L, suggesting that micelle formation is due to closed associations.9 Plots of the average relaxation rates (Γ) as a function of the square of the scattering vector (q2),

Sulfonate Block Copolymer Micelles

Figure 4. I3/I1 ratios in the fluorescence spectra of pyrene solubilized in aqueous solutions of the St-SI block copolymers at varying polymer concentrations: (O) St-SI-20; (0) St-SI30; (4) St-SI-207.

based on QELS data obtained at different measuring angles for all three polymers at four different concentrations, presented approximately linear lines passing through the origin (data not shown), indicating that the relaxation mode is virtually due to a diffusive process. Approximate values of the hydrodynamic diameters, calculated from the Stokes-Einstein relation along with the viscosity of water using approximate values of the diffusion coefficients (D) estimated from the slopes of the Γ-q2 plots, were in good agreement with the diameters calculated by the first cumulant method using data obtained at a measuring angle of 90°. Solubilization of Pyrene in the Micelle. The vibrational fine structure of pyrene fluorescence spectra undergoes significant perturbations on going from nonpolar to polar media.27,28 The intensity ratio of the third to the first vibronic bands, known as an I3/I1 ratio, has been shown to correlate well with the microenvironmental polarity.28 The 0-0 transition from the lowest excited electronic state, giving rise to the I1 peak, is symmetry forbidden. Its intensity may be increased because of distortion of the π-electron cloud in polar media, while the I3 peak is not forbidden and solvent insensitive. Thus, the I3/I1 ratio can be used as a measure of the polarity (hydrophobicity) of the microenvironment around pyrene molecules. The values of I3/I1 range from 0.53 in water to 0.90 in toluene and 2.0 in perfluoromethylcyclohexane.28 Pyrene, a hydrophobic compound, is poorly soluble in water (up to 7 × 10-7 M),29 but in a system composed of both hydrophobic and hydrophilic phases, such as an aqueous solution of a block copolymer composed of hydrophobic and hydrophilic blocks, pyrene is preferentially solubilized in the hydrophobic phase. Thus, if a micelle-forming block copolymer is dissolved in an aqueous solution of pyrene, pyrene is solubilized in hydrophobic micellar cores, leading to an increase in the I3/I1 ratio. The St-SI block copolymers were dissolved in a pyrenesaturated water at varying polymer concentrations, and fluorescence spectra of the aqueous solutions were measured at room temperature. The I3/I1 ratios estimated from the pyrene fluorescence spectra are plotted as a function of the polymer concentration in Figure 4. The I3/I1 ratios are practically the same as that of pyrene in water at polymer concentrations on the order of 10-3 g/L, but the ratios begin to increase significantly near a polymer concentration of (2-4) × 10-2 g/L, indicating that pyrene molecules begin to be solubilized in the hydrophobic cores (27) Nakajima, A. J. Mol. Spectrosc. 1976, 61, 467. (28) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (29) Schwartz, F. P. J. Chem. Eng. Data 1977, 22, 22.

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of the block copolymer micelles at this polymer concentration. These increases in the I3/I1 ratio with increasing polymer concentration indicate that more pyrene molecules are solubilized with increasing polymer concentration. It is to be noted that the I3/I1 ratios are slightly different for different polymers. For example, St-SI-30 shows larger I3/I1 ratios than St-SI-207 does in a polymer concentration region less than 0.1 g/L, presumably because, at the same polymer concentration in units of grams per liter, StSI-30 solutions contain a larger number of St units than St-SI-207 solutions. At higher polymer concentrations (>0.2 g/L), however, these two polymers show almost the same I3/I1 ratio, presumably because all pyrene molecules are completely solubilized in micelle cores. In this high polymer concentration region, the I3/I1 ratios for St-SI20 are slightly smaller than those for the other two polymers. We will come back to this point in a later subsection. It is also to be noted that the polymer concentrations at which the I3/I1 ratio begins to increase are quite similar among the three block copolymers. Decay measurements of pyrene fluorescence in aqueous solutions containing the St-SI block copolymers also indicated the solubilization of pyrene molecules in micelle cores. In a control experiment, the lifetime of pyrene fluorescence in water was confirmed to be about 210 ns. This pyrene lifetime increased to 300-360 ns in 1.0 g/L aqueous solutions of the St-SI block copolymers (χ2 ) 1.21-1.74). Critical Micelle Concentration (cmc). The fact that the dry polymer samples could be directly dissolved in water to give solutions of micelles with a unimodal relaxation time distribution (Figure 3) suggests that polymer micelles are formed at equilibrium between micelles and unimers with a well-defined cmc. Therefore, we attempted to estimate the cmc by fluorescence methods reported by Astafieva et al.6 and Wilhelm et al.30 using pyrene as a fluorescent probe. When pyrene is solubilized in a hydrophobic phase, the wavelength of the 0-0 absorption band, whose maximum in water lies around 334 nm, shifts to around 340 nm. This shift in the 0-0 band can be more sensitively observed by excitation spectra than by absorption spectra. With increasing micelle concentration, the intensity of the 334-nm band decreases relative to the intensity of the 340 nm band. Thus, one can use this spectral shift of pyrene to estimate a cmc for micelle-forming compounds in aqueous media. In the present study, we analyzed pyrene excitation spectra in the wavelength region 315-350 nm to estimate cmc. Excitation spectra in this region were deconvoluted with Spectracalc software after adjustment to a common zero baseline. The deconvolution technique applied allowed mixed Gaussian/Lorentzian peaks to be fitted. The ratios of the area of the 340-nm band to the area of the 334-nm band, I340/I334, were calculated for various polymer concentrations. An example of this spectral analysis is presented in Figure 5 where deconvoluted excitation spectra are shown for three different concentrations of St-SI-20. At very low polymer concentrations there is no contribution of the 340-nm band, but as the polymer concentration is increased, the 340-nm band appears at a certain polymer concentration, which is considered to be an “apparent” cmc for the polymer. The I340/I334 ratios for all three block copolymers are plotted as a function of the polymer concentration in Figure 6. The I340/I334 ratio begins to grow rather abruptly at polymer concentrations (30) Wilhelm, M.; Zhao, C. L.; Wang, Y.; Xu, R.; Winnik, M. A. Macromolecules 1991, 24, 1033.

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Figure 6. I340/I334 ratios for the fluorescence excitation spectra of pyrene solubilized in the solutions of the St-SI block copolymers as a function of the polymer concentration: (O) St-SI-20; (0) St-SI-30; (4) St-SI-207. The ratios were calculated from deconvoluted excitation spectra.

Figure 5. Deconvoluted fluorescence excitation spectra of pyrene solubilized in aqueous solutions of St-SI-20 at polymer concentrations of (a) 9.8 × 10-6, (b) 6.3 × 10-2, and (c) 1.0 g/dL.

of approximately 7 × 10-2, 5 × 10-2, and 8 × 10-2 g/L for St-SI-20, St-SI-30, and St-SI-207, respectively. It should, however, be borne in mind that the estimation of cmc by this method neglects the partition of pyrene molecules between the aqueous phase and micelles. Because saturated values of the I340/I334 ratios are not well-defined in the plots shown in Figure 6, the I3/I1 ratios shown in Figure 4 seem to be more suitable for the estimation of the partition of pyrene. The ratio of the pyrene concentrations in the aqueous phase and in the micellar phase, [Py]m/[Py]w, was calculated using the following equation:

()

I3 I3 [Py]m I1 I1 min ) I3 I3 [Py]w I1 max I1

()

(6)

On the other hand,

[Py]m [Py]w

)

Kvχs(c - cmc) 1000Fs

(7)

where Kv is the equilibrium constant for the partition of pyrene molecules between the aqueous and micellar phases, c is the total polymer concentration in grams per liter, χs is the weight fraction of the St block in the polymer, which was estimated to be 0.09 for St-SI-20 and StSI-207 and 0.15 for St-SI-30, and Fs is the density of the St core of the micelle, which is assumed to be the same as that in the bulk polystyrene ()1.04 g/mL). When [Py]m/[Py]w is plotted as function of the polymer concentration, it seems that the plots can be fitted with two intersecting straight lines (Figure 7). The cmc values

Figure 7. [Py]m/[Py]w estimated from I3/I1 for pyrene solubilized in aqueous solutions of the St-SI block copolymers plotted against the polymer concentration: (O) St-SI-20; (0) St-SI30; (4) St-SI-207.

can be obtained from the intercept between sloping lines and the X axis, while Kv can be calculated from the slope of the respective lines. Thus, estimated values of cmc and Kv are listed in Table 1 together with “apparent” cmc values estimated from excitation spectra. In polymer micelle solutions at equilibrium, a constant concentration (i.e., cmc) of unimers should coexist with micelles. However, there is no peak due to such unimers in the relaxation time distribution data shown in Figure 3. The cmc values for all three block copolymers are on the order of 10-2 g/L, which are lower by more than 1 order of magnitude than the polymer concentrations employed for the QELS measurements. Considering that micelles scatter much more strongly than the corresponding unimers, it may be reasonable that only micelle peaks can be observed in the plot of τA(τ) as a function of τ (Figure 3). The largest values of cmc, both “apparent” and “real” ones, are found for St-SI-207. This finding can be rationalized by the fact that this polymer has the longest SI blocks, and therefore repulsive forces inside the micelle corona are stronger than those in St-SI-20 or St-SI-30 micelle coronas. Furthermore, St-SI-207 is more soluble in water because the SI block length relative to the St block length is much longer than those in the other two block copolymers. On the other hand, the cmc of St-SI-30 is lowest for the opposite reason; i.e., longer St blocks make hydrophobic interactions more favorable, leading to the formation of micelles at a lower polymer concentration. For all three polymers, Kv values are quite large, reflecting a strong tendency of pyrene molecules to be solubilized in micelle cores of the St-SI block copolymers. Protection of Solubilized Pyrene from a Bulk Quencher. Pyrene fluorescence was found to be quenched

Sulfonate Block Copolymer Micelles

Langmuir, Vol. 15, No. 2, 1999 461

Figure 8. Fraction of pyrene fluorophores not accessible to the quencher (acrylamide) in aqueous solutions of the St-SI block copolymers plotted as a function of the polymer concentration: (O) St-SI-20; (0) St-SI-30; (4) St-SI-207.

by acrylamide in aqueous solution, with both steady-state and time-dependent quenching data following the SternVolmer equation:

I0 ) 1 + KSV[Q] ) 1 + kqτf[Q] I

(8)

where I0 and I are the intensities of pyrene fluorescence in the absence and presence of the quencher, respectively, KSV is the Stern-Volmer constant, τf is the fluorescence lifetime in the absence of the quencher, and kq is the second-order quenching rate constant. The kq value for pyrene in aqueous solution was determined from the steady-state Stern-Volmer plot to be 1.07 × 109 M-1 s-1. In the presence of St-SI block copolymers, however, the Stern-Volmer plot shows a downward curvature, suggesting that some fractions of pyrene molecules are not accessible to the quencher. If that is the case, a two-state model may be applied. Equation 8 can be modified as

I0 1 1 ) + I0 - I fa KSVfa[Q]

(9)

where fa is the fraction of chromophore molecules accessible to the quencher. All of the steady-state fluorescence quenching data in the present study were found to follow eq 9, plots of I0/(I0 - I) against 1/[Q] showing linear relationships (data not shown). From intercepts of the plots, fa values were estimated for all of the block copolymers. The fraction of pyrene fluorophores not accessible to the quencher, 1 fa, is plotted as a function of the polymer concentration in Figure 8. The extent of the pyrene protection increases with an increase of the polymer concentration in a low polymer concentration region, reaching a maximum extent at a polymer concentration of 0.4-0.6 g/L. This maximum of 1 - fa reflects the saturation of pyrene solubilization in micelle cores, with complete solubilization of pyrene being effected at a polymer concentration of 0.4-0.6 g/L. It should be noted that pyrene fluorophores are better protected in the micelles of St-SI-30 and St-SI-207 than in the micelle of St-SI-20. This observation may be explained by a difference in the size of the micelle core. Although we do not know the real size of the micelle core, we can roughly compare the core sizes in terms of the number of styrene monomeric units existing in a micelle core. The numbers of the styrene monomeric units in the St-SI-30 and St-SI-207 micelle cores are approximately 10 and 5 times larger than that in the St-SI-20 micelle core, respectively, as calculated from the aggregation number and DPSt. Thus, the core volume for the St-SI-30

Figure 9. Reduced viscosities of aqueous solutions of the StSI block copolymers in the absence (a) and presence of 0.1 M NaCl (b) plotted as a function of the polymer concentration: (O) St-SI-20; (0) St-SI-30; (4) St-SI-207.

micelle may be twice as large as that for the St-SI-207 micelle, although the mass of the St-SI-207 micelle is about the same as that of the St-SI-30 micelle (Table 1). It is also suggested that the core volume for the St-SI-30 micelle is approximately 10 times larger than that of the St-SI-20 micelle. The less effective protection of pyrene fluorophores in the St-SI-20 micelle observed in Figure 8 may be attributed to the smaller core size for this polymer micelle. This is consistent with the smaller I3/I1 ratios for the St-SI-20 micelle at polymer concentrations greater than 0.2 g/L (Figure 4). Pyrene molecules solubilized in the larger cores in the St-SI-30 and St-SI-207 micelles would experience a greater hydrophobicity of the interior of the cores, as indicated by the I3/I1 ratios at polymer concentrations (>0.2 g/L) that are high enough to solubilize all pyrene molecules. In such a situation, it would be more difficult for polar acrylamide molecules to penetrate into the micelle core to encounter pyrene fluorophores. Viscosity of Micelle Solutions. The reduced viscosities of aqueous solutions of the St-SI block copolymers, with and without added salt, are plotted against the polymer concentration in Figure 9. Without added salt, the viscosities for St-SI-207 are much higher that those for St-SI-20 and St-SI-30, arising from the largest corona size in the St-SI-207 micelle. The viscosity of St-SI-207 significantly decreases with increasing polymer concentration in a concentration range between 0.1 and 1.0 g/L, typical of many polyelectrolytes. In contrast, the viscosities of St-SI-20 and St-SI-30 show only a slight decrease with increasing polymer concentration. When NaCl is added up to 0.1 M, the reduced viscosities of these polymers are markedly decreased. This decrease is brought about by the shielding of electrostatic repulsions between SI blocks in the micelle corona, leading to a collapse of the SI blocks. The largest decrease in the viscosity for StSI-207 results from the fact that the corona of the micelles formed by this polymer is very large and an increase in the ionic strength brings about a significant decrease in its size.

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The reduced viscosity for St-SI-20 is slightly higher than that for St-SI-30, although the mass of the St-SI20 micelle is smaller than that of the St-SI-30 micelle (Table 1). The core size for the St-SI-30 micelle is much larger than that of the St-SI-20 micelle, as discussed in the previous subsection, but the length of the SI block in the St-SI-30 micelle corona is shorter than that in the St-SI-20 micelle corona. The longer SI block in the StSI-20 micelle corona may be responsible for the slightly higher viscosity of St-SI-20 than that of St-SI-30 in the whole range of the polymer concentrations in Figure 9. These considerations lead to a conclusion that the reduced viscosity depends more strongly on the length of the SI block in the corona than on the micellar mass. As we discussed in a previous subsection, the cumulant diameter of the St-SI-30 micelle was found to increase from 61 to 79 nm with an increase of the NaCl concentration from 0.01 to 0.1 M (Figure 2), suggesting an increase in the aggregation number. This finding seems to be in disagreement with the finding that the reduced viscosity for St-SI-30 is lower in 0.1 M NaCl than in deionized water. However, this apparent disagreement may be explained by considering that the reduced viscosity strongly depends on the length of the SI block in the corona and is less sensitive to the aggregation number or micelle mass.

St-SI-207 (i.e., block copolymer with the highest molecular weight and the longest SI block), an increase in the NaCl concentration from 0.01 to 0.1 M results in a large decrease in the micelle size. The size of St-SI-20 micelles does not change with an increase in the ionic strength, whereas the size of St-SI-30 micelles increases, probably because of a change in the aggregation number caused by an increase in the ionic strength. The relaxation time distributions in QELS were found to be unimodal and independent of the polymer concentration in the range 0.1-1.0 g/L, suggesting that micelle formation is due to closed associations. The highest value of the aggregation number was found for the copolymer with the highest St content, while it seems that the overall molecular weight does not have a great influence on the aggregation number. It was found, by measuring I3/I1 ratios for pyrene molecules solubilized in aqueous solutions of the block copolymers and by quenching of pyrene fluorescence by acrylamide, that hydrophobicity of the cores of St-SI-30 and St-SI207 micelles is greater than that of St-SI-20 micelles which seems to be related to the size of the micelle core in terms of the number of hydrophobic monomeric units forming the core. “Real” cmc values are on the order of a few milligram per deciliter and are lowest for St-SI-30, i.e., the polymer with the highest St content.

Conclusions

Acknowledgment. This work was supported in part by a Grant-in-Aid for JSPS fellow and a Grant-in-Aid on Priority-Area-Research, “New Polymers and Their NanoOrganized Systems” (No. 277/08246236), from the Ministry of Education, Science, Sports, and Culture, Japan.

The diblock copolymers of styrene and sulfonated isoprene were shown to undergo micellization in aqueous solutions with a well-defined cmc. The diameters of micelles were found to be quite large and independent of the polymer concentration up to 1.0 g/L. In the case of

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