Associating Behavior of Sulfonated Polyisoprene Block Copolymers

Self-association of highly asymmetric triblock copolymers of sulfonated isoprene (SI) with short styrene (St) blocks at both chain ends (St−SI−St)...
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Langmuir 2000, 16, 2083-2092

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Associating Behavior of Sulfonated Polyisoprene Block Copolymers with Short Polystyrene Blocks at Both Chain Ends 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 Co. Ltd., 34-1, Tohwada, Kamisu-machi, Kashima, Ibaraki 314-02, Japan Received August 5, 1999. In Final Form: October 13, 1999 Self-association of highly asymmetric triblock copolymers of sulfonated isoprene (SI) with short styrene (St) blocks at both chain ends (St-SI-St) was studied. The studies were carried out using fluorescence probe, quasielastic light scattering, and static light scattering (SLS) techniques. Three block copolymers having different degrees of polymerization of the St block (DPSt) and SI block (DPSI) were employed, i.e., polymers with DPSt ) 6 and DPSI ) 350 (St-SI-St-5), DPSt ) 11 and DPSI ) 320 (St-SI-St-10), and DPSt ) 19 and DPSI ) 240 (St-SI-St-20). The formation of hydrophobic microdomains in water was indicated by fluorescence probe experiments using 1,6-diphenyl-1,3,5-hexatriene and pyrene. The presence of a critical polymer concentration for the onset of the hydrophobic domain formation (cmc) was indicated by excitation spectra of pyrene fluorescence. Apparent cmc values, obtained by a method that does not take into account the partition of pyrene between the aqueous and polymer phases, are on the order of 10-110-2 g/L. The existence of cmc suggests that micelles are formed via a closed association in equilibrium. The formation of micelles with increasing polymer concentration (Cp) is preceded by the formation of oligomeric aggregates in solutions of all the three polymers as suggested by analysis of pyrene fluorescence spectra. The hydrodynamic radii (Rh) of unimers and the oligomeric aggregates are on the order of a few nanometers. Values of Rh for micelles are 40-50 nm for St-SI-St-5 (Cp ) 1-4 g/L) and 28-33 nm for St-SI-St-20 (Cp ) 0.1-2.25 g/L), being independent of Cp in these Cp ranges. In the St-SI-St-20 system at Cp higher than 2.25 g/L, Rh of micelles grows as a result of bridging. In the case of St-SI-St-10, however, the presence of both micelles (Rh ) 20-30 nm at Cp ) 0.25-2.0 g/L) and bridged micelles (Rh ) ca. 100 nm at Cp ) 0.75-1.75 g/L) was found above cmc. The size of these bridged micelles showed a strong dependence on Cp. Taken together with weight-average masses of the micelles and aggregation numbers estimated from SLS, flower-type micelles were proposed as a hypothetical model for the triblock copolymer aggregates.

Introduction Water-soluble amphiphilic block copolymers, consisting of hydrophilic and hydrophobic blocks, have been extensively studied in recent years because of their potentials in practical applications as well as academic interest in their self-assembling phenomena.1 A large number of studies reported so far have dealt with AB-type diblock or ABA-type triblock copolymers, where A and B represent hydrophilic and hydrophobic blocks, respectively. In the case of amphiphilic ABA-type triblock copolymers with relatively short hydrophobic middle blocks and longer hydrophilic side blocks, spherical micelles are usually observed in aqueous solutions, with the hydrophobic blocks in the cores and the hydrophilic blocks in the coronas. The †

Osaka University. On leave from the Jagiellonian University, Faculty of Chemistry, 30-060 Krako´w, Ingardena 3, Poland. § Japan Synthetic Rubber Co. Ltd. ‡

(1) See for example (only papers published during last 2 years are included): (a) Alexandridis, P. Macromolecules 1998, 31, 6935. (b) Borovinskii, A. L.; Khokhlov, A. R. Macromolecules 1998, 31, 1180. (c) Quintana, J. R.; Ja´nez, M. D.; Herna´ez, E.; Garcia, A.; Katime, I. Macromolecules 1998, 31, 6865. (d) Rager, T.; Meyer, W. H.; Wegner, G.; Winnik, M. A. Macromolecules 1997, 30, 4911. (e) Creutz, S.; van Stam, J.; Antoun, S.; de Schryver, F. C., Je´roˆme, R. Macromolecules 1997, 30, 4078. (f) Ding, J.; Liu, G. Macromolecules 1998, 31, 6554. (g) Harada, A.; Kataoka, K. Macromolecules 1998, 31, 288. (h) Jo¨rgensen, E. B.; Hvidt, S.; Brown, W.; Schille´n, K. Macromolecules 1997, 30, 2355. (i) Mondescu, R. P.; Muthukumar, M. Macromolecules 1997, 30, 6358. (j) Tuzar, Z.; Pospı´sˇil, H.; Plesˇtil, J.; Lowe, A. B.; Baines, F. L.; Billingham, N. C.; Armes, S. P. Macromolecules 1997, 30, 2509.

micellar cores may be glassy or liquidlike, depending on the type of the hydrophobic block sequence. The micelle formation of such amphiphilic ABA-type block copolymers has been studied in detail,1c,2 large part of attention being devoted to PEO-PPO-PEO block copolymers,3 where PEO and PPO represent poly(ethylene oxide) and poly(propylene oxide) blocks, respectively. Various AB- and ABA-type amphiphilic block copolymers where A is a polyelectrolyte block have been also investigated, which include 4-vinylpyridinium alkyl halide,2a,b,4 sodium acrylate,5 sodium methacrylate,6 and sulfonated (2) See for example: (a) Gauthier, S.; Eisenberg, A. Macromolecules 1987, 20, 760. (b) Gouin, J.-P.; Williams, C. E.; Eisenberg, A. Macromolecules 1989, 22, 4573. (c) Ten Brinke, G.; Hadziioannou, G. Macromolecules 1987, 20, 486. (3) See for example: (a) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (b) Brown, W.; Schille´n, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991, 95, 1850. (c) Glatter, O.; Scherf, G.; Schille´n, K.; Brown, W. Macromolecules 1994, 27, 6046. (d) Marinov, G.; Michels, B.; Zana, R. Langmuir 1998, 14, 2639. (e) Mortensen, K.; Brown, W. Macromolecules 1993, 26, 4128. (f) Schille´n, K.; Brown, W.; Kona´k, C. Macromolecules 1993, 26, 3611. (g) Zhou, Z.; Chu, B. Macromolecules 1988, 21, 2548. (4) (a) Gao, Z.; Varshney, S. K.; Wong, S.; Eisenberg, A. Macromolecules 1994, 27, 7923. (b) Gauthier, S.; Duchesne, D.; Eisenberg, A. Macromolecules 1987, 20, 753. (5) Astafieva, I.; Zhong, X. F.; Eisenberg, A. Macromolecules 1993, 26, 7339. (6) (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.

10.1021/la991056d CCC: $19.00 © 2000 American Chemical Society Published on Web 12/10/1999

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styrene7 blocks covalently linked to hydrophobic blocks such as styrene blocks. When a dry sample of an amphiphilic block copolymer is directly dissolved in water, polymer micelles may be formed spontaneously. The micellization may be a reversible equilibrium between micelles and free (nonmicellized) polymer molecules (i.e., unimers), which may involve the entry and exit of unimers in to and out of micelles (i.e., an exchange between unimers and micelles). The micellization of this type, atypical of closed association, is thermodynamically governed, and the net growth of micelles stops when a free energy minimum is attained.8 Thus, aggregation numbers (numbers of polymer molecules comprising a single micelle) are determined by a minimum free energy of the system, resulting in micelles that are monodisperse in mass and size with a narrow distribution of aggregation numbers around the most stable state.9 When hydrophilic blocks are not long enough to enable direct dissolution of a block copolymer in water, micelle solutions may be obtained by first dissolving the polymer in an organic solvent miscible with water, followed by dialyzing the solution against water to remove the organic solvent. The micelles thus obtained are not “equilibrium micelles” but are “kinetically-frozen micelles”.4a,6a,d,10 In the case of amphiphilic BAB-type triblock copolymers where the middle block is soluble in water while two side blocks are insoluble, associative properties should be completely different from those of ABA-type block copolymers. However, the association behavior of amphiphilic BAB-type triblock copolymers is still much less understood than that of ABA-type block copolymers.11 In particular, while a closed association model1h,f,3c,6c is generally accepted for the association of ABA-type block copolymers, yielding core-corona type micelles in dilute solutions, it is an interesting question whether the association of BAB-type block copolymers follows a closed or open association model. If the two hydrophobic side blocks associate strongly in the same polymer chain and all the unimers exist as looped chains, the block copolymers may undergo a closed association forming “flower-type micelles”12 because the looped unimers may behave as if they were AB-type block copolymers as a building block for a micelle. However, to our best knowledge, such flowertype micelles of BAB-type block copolymers are not known to date. We previously investigated the association behavior in water of amphiphilic diblock copolymers comprising short styrene (St) blocks and long sulfonated isoprene (SI) blocks and the degrees of polymerization of the St block (DPSt) and SI block (DPSI) ranging from 40 to 130 and from 210 (7) Amiel, C.; Sikka, M.; Schneider, J. W.; Tsao, Y.; Tirrell, M.; Mays, J. W. Macromolecules 1995, 28, 3125. (8) (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; Matijevic, E., Ed.; Plenum: New York, 1993; Vol. 15(1). (9) Tian, M.; Qin, A.; Ramireddy, C.; Webber, S. E.; Munk, P.; Tuzar, Z.; Procha´zka, K. Langmuir 1993, 9, 1741. (10) (a) Yu, K.; Eisenberg, A. Macromolecules 1998, 31, 3509. (b) Xu, R.; Winnik, M. A.; Riess, G.; Chu, B.; Croucher, M. D. Macromolecules 1992, 25, 644. (c) Chan, J.; Fox, S.; Kiserow, D.; Ramireddy, C.; Munk, P.; Webber, S. E. Macromolecules 1993, 26, 7016. (d) Eckert, A. R.; Webber, S. E. Macromolecules 1996, 29, 560. (e) Karymow, M. A.; Procha´zka, K.; Mendenhall, J. M.; Martin, T. J.; Munk, P.; Webber, S. E. Langmuir 1996, 12, 4748. (f) Munk, P.; Ramireddy, C.; Tian, M.; Webber, S.; Procha´zka, K.; Tuzar, Z. Makromol. Chem. Macromol. Symp. 1992, 58, 195. (11) Balsara, N. P.; Tirrell, M.; Lodge, T. P. Macromolecules 1991, 24, 1975. (12) (a) Halperin, A. Macromolecules 1991, 24, 1418. (b) Semenov, A. N.; Joanny, J. F.; Khokhlov, A. R. Macromolecules 1995, 28, 1066.

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to 810, respectively.13 We have found that the St-SI diblock copolymers undergo micellization in aqueous solutions and form micelles with a well-defined critical micelle concentration (cmc) on the order of 10-2 g/L, the hydrodynamic radii (Rh) of the micelles ranging from 35 to 85 nm in 0.1 M NaCl aqueous solutions independent of the polymer concentration (Cp) (up to 1.0 g/L). The distributions of the relaxation times in quasielastic light scattering (QELS) were found to be unimodal and independent of the Cp (up to 1.0 g/L), suggesting that the micelle formation is due to closed association. As an extension of our prior work on the St-SI diblock copolymers, this paper focuses on the St-SI-St triblock copolymers to assess the effect of block copolymer architecture on the association behavior in water. The StSI-St block copolymers employed in this work are highly asymmetric BAB-type block copolymers where A is a long SI block and B is a very short St block, DPSI being in the range 240-350 with DPSt in the range 6-19. We chose these asymmetric triblock copolymers because dry samples of these polymers were able to be dissolved directly in water. Here, we report on the association behavior of these St-SI-St triblock copolymers in water as studied by various fluorescence techniques using 1,6-diphenyl-1,3,5hexatriene (DPH) and pyrene as probes solubilized in polymer aggregates. Evidence is given for the formation of micellar structures. Static light scattering (SLS) and QELS techniques were also employed for the characterization of the polymer micelles. On the basis of the characterization data, we propose a model of flower-type micelles with different degrees of intermicellar bridges depending on the relative lengths of the St and SI blocks. Experimental Section Styrene-Isoprene-Styrene Triblock Copolymers. The block copolymers employed in the present work were synthesized at the Specialty Materials Laboratory of Japan Synthetic Rubber Co., Ltd. Styrene-isoprene-styrene triblock copolymers were prepared by living anionic polymerization in the sequential order of styrene, isoprene, and styrene as follows: To a stirred solution of a predetermined amount of styrene, ranging from 15 g (0.14 mol) to 120 g (1.15 mol), in 1200 g of dried cyclohexane and 0.6 g of dried tetrahydrofuran (THF) in a 5-L glass autoclave, 20 mL of a cyclohexane solution of n-butyllithium (15 wt %) was added, and the mixture was stirred for 2 h at 60 °C. A small portion of the reaction mixture was sampled out for the measurement of size-exclusion chromatography (SEC). To the polystyrene living anion solution, a predetermined amount of isoprene, ranging from 360 g (5.28 mol) to 570 g (8.34 mol), was added to initiate anionic polymerization of isoprene. The reaction mixture was stirred for 2 h at 60 °C. To this polyisoprene living anion solution, styrene of the same amount as employed for the first sequence polymerization was added to proceed with the polymerization of styrene initiated by the living polyisoprene anion. The reaction mixture was stirred for 2 h at 60 °C. The polymerization was stopped by adding 1 g of 2-propanol. Block copolymers were recovered by evaporating the solvents and residual monomers under reduced pressure and subjected to SEC measurements. Sulfonation of Styrene-Isoprene-Styrene Triblock Copolymers. A solution of sulfur trioxide/1,4-dioxane complex was first prepared as follows: To 600 g of 1,4-dioxane, 60 g of anhydrous sulfuric acid was added dropwise with stirring. The reaction mixture was kept at room temperature (about 25 °C) and stirred for 2 h. A predetermined amount of the prepared sulfur trioxide/1,4-dioxane complex solution was added dropwise to a 600 g 1,4-dioxane solution of 60 g of the styrene-isoprenestyrene block copolymer with stirring. The temperature of the reaction mixture was kept below 25 °C during the reaction by water-cooling. After the mixture was stirred for an additional 2 (13) Szczubialka, K.; Ishikawa, K.; Morishima, Y. Langmuir 1999, 15, 454.

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Langmuir, Vol. 16, No. 5, 2000 2085 Table 1. Characteristics of St-SI-St Copolymers sulfonationa

wt ratio of the St and I blocks before av molecular mass before sulfonationa av molecular mass after sulfonation av mass of the St block av mass of the I block DPStc DPSId DPSt/DPSI Rh of the micelle (nm) apparent micelle wt av mass in 0.1 M NaClh estimate of the aggregation no.j “apparent” cmc (g/L) in waterk “apparent” cmc (g/L) in 0.1 M NaClk

St-SI-St-5

St-SI-St-10

St-SI-St-20

2.5/95/2.5 2.5 × 104 6.1 × 104 a 6.2 × 102 2.4 × 104 6 350 0.017 40-50e 1.5 × 106 25 0.37 0.35

5/90/5 2.4 × 104 5.2 × 104 a 1.2 × 103 2.2 × 104 11 320 0.034 20-30f 1.4 × 107 i 270i 0.35 0.088

10/80/10 2.0 × 104 4.6 × 104 b 2.0 × 103 1.6 × 104 19 240 0.081 28-33g 9.7 × 105 21 0.098 0.072

a Based on SEC. b Calculated on the basis of molecular mass of the parent copolymer obtained from SEC. c Average degree of polymerization of the St block. d Average degree of polymerization of the SI block. e In the Cp range 1-4 g/L. f In the Cp range 0.25-2.0 g/L. g In the Cp range 0.1-2.3 g/L. h Roughly estimated by SLS in 0.1 M NaCl. i Value obtained for solutions containing both micelles and bridged micelles. j Obtained by dividing micelle molecular weight by polymer molecular weight. k Determined from excitation spectra of pyrene probe.

h, a 15 wt % NaOH aqueous solution was added to neutralize the reaction mixture, followed by the addition of 300 g of methanol to the mixture. The mixture was heated to 80 °C for 4 h with stirring. Aqueous solutions of sulfonated block copolymers were obtained after evaporating organic solvents under reduced pressure. The aqueous solutions of the polymers were dialyzed against deionized water. Dry samples of the polymers were recovered by freeze-drying. Solvents and Reagents. Milli-Q water was used to prepare polymer solutions. Analytical-grade solvents were obtained from Wako Pure Chemical. Pyrene, purchased from Nacalai Tesque, was recrystallized from ethanol. DPH was obtained from Nacalai Tesque and used as received. Analytical-grade NaCl from Wako Pure Chemical was used without further purification. Preparation of Polymer Solutions. Polymer solutions were prepared by direct dissolution of dry polymer samples into pure water. The ionic strength of the polymer solutions was adjusted by adding NaCl. The polymer solutions were then heated at 6070 °C until complete dissolution, sonicated for about 1 h, and allowed to stand for at least 1 day for equilibration. Solutions of polymer micelles with pyrene solubilized were prepared by dissolving dry polymer samples into a pyrene-saturated aqueous solution, followed by heating at 60-70 °C until dissolution and sonication for 1 h. A pyrene-saturated aqueous stock solution was prepared as reported in our previous paper.13 Solutions containing DPH solubilized in polymer micelles were obtained as follows: One milliliter of a 1 mM DPH solution in THF was added slowly to 1 L of Milli-Q water with vigorous stirring, and the solution was stirred for an additional 10 min. An aliquot of this solution was added to solutions of polymers of predetermined concentrations, and the solutions were stirred overnight in the dark. Measurements. a. Size-Exclusion Chromatography (SEC). 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. Static Light Scattering (SLS). 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° with 10° increment. The values of refractive index increment, dn/dc, were obtained using an Otsuka DRM-1020 double-beam differential refractometer. Sample solutions were filtered with a 0.2-µm (for solutions of St-SI-St-5 and -20) or 0.45-µm (for solutions of St-SI-St-10) pore size disposable membrane filter prior to measurement. The Zimm plots, obtained using at least four solutions of different concentrations, were used to assess micelle mass according to the equation

K(Cp - cmc) 1 ) + 2A2(Cp - cmc) Rθ Mw

(1)

where K ) 4π2n02(dn/dc)2/NAλ04 is an optical constant with NA,

n0, and λ0 being Avogadro’s number, the refractive index of the solvent, and the wavelength of light in vacuo, respectively. Rθ is the Rayleigh ratio, c is the Cp, and cmc is the critical micelle concentration found using excitation spectra of the pyrene probe. The Rayleigh ratio was determined using the known Rayleigh ratio of toluene. For block copolymers, which are heterogeneous in chemical composition, eq 1 yields an apparent molecular weight rather than a real weight-average molecular weight.14 c. Quasielastic Light Scattering (QELS). The apparent hydrodynamic radius (Rh) and the distribution of the relaxation times were measured with an Otsuka Electronics Photal DLS7000 light scattering spectrometer equipped with a 75-mW Ar laser operating at λ ) 488 nm. Data were collected using an ALV-5000 wide-band multi-τ digital autocorrelator allowing characterization of relaxation time distributions over 8 decades. The “smoothing” parameter used in the program was selected to be 0.5 in all cases. All measurements were performed at 25 °C. The micelle Rh and the distributions of relaxation times were measured as a function of the Cp and scattering angle. Sample solutions were filtered prior to measurements using a 0.20- or 0.45-µm pore size disposable membrane filter. All the solutions were prepared in 0.1 M NaCl. Mathematical treatment of the data obtained has been described elsewhere.13 d. Steady-State Fluorescence Spectra. A Hitachi F-4500 fluorescence spectrophotometer was employed for the measurement of fluorescence emission and excitation spectra. Emission spectra were measured in the range from 350 to 550 nm, and excitation spectra were measured in the region from 285 to 365 nm, both with a 0.2-nm resolution. The width of both excitation and emission slits was 2.5 nm. All spectra were measured using air-equilibrated solutions at room temperature. The fluorescence spectra of pyrene solubilized in polymer micelles were measured using excitation wavelength of λex ) 339 nm, while excitation spectra were measured with emission wavelength of λem ) 373 nm.

Results Characterization of the Triblock Copolymers. The water-soluble St-SI-St triblock copolymers employed in this work were synthesized by selective sulfonation of the middle isoprene block of the parent St-I-St block copolymers. The quantitative sulfonation of the isoprene block was confirmed by the complete disappearance of IR bands associated with olefinic bonds in the isoprene block.13 The triblock copolymers employed in the present work are coded as St-SI-St-x, where x designates the weight percent of St blocks in their parent block copolymers (i.e., the St-I-St block copolymers). The parent block copolymers contained 5, 10, and 20 wt % of styrene divided into two side blocks of the same length (Table 1). The molecular masses of the sulfonated block copolymers, (14) Bushuk, W.; Benoit, H. Can. J. Chem. 1958, 36, 1616.

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

calculated from the molecular masses of the parent block copolymers estimated by SEC, are in the range (4.6-6.1) × 104 (Table 1). The degrees of polymerization of the styrene blocks, DPSt, range from 6 to 19 while the degrees of polymerization of the sulfonated isoprene blocks, DPSI, range from 240 to 350, DPSt/DPSI ratios falling in the range 0.017-0.081 (Table 1). The asymmetry in the St and SI block lengths is even larger taking into account that the isoprene monomer unit introduces four carbon atoms into the chain while the styrene unit introduces two carbon atoms. Thus, a difference in the associative properties of the St-SI-St block copolymers may arise from a combined effect of the differences in DPSt (increasing in the order St-SI-St-5, -10, and -20) and in DPSI (increasing in the opposite order). The chemical structure of the polymers assumed based on the literature data15 on the sulfonation of olefins with sulfur trioxide/1.4-dioxane complexes is presented in Chart 1. The relative lengths of the SI and St blocks, assuming the same length of the two St blocks in the same polymer chain, are also illustrated in Chart 1. It is to be noted here that, besides these three block copolymers, we prepared two more block copolymers with higher St block contents (30 and 40 wt % before sulfonation) but we found that only the former three polymers were soluble in water, although their solubility was quite limited. Block copolymers St-SI-St-5, -10, and -20 form optically clear solutions only up to Cp about 4.0, 2.0, and 2.5 g/L, respectively. On the other hand, all five block copolymers were well-soluble in a 50/50 (v/v) mixture of acetonitrile and water (St-SI-St-5, -10, and -20 are freely soluble while St-SI-St-30 and -40 are soluble up to about 21.0 and 8.5 g/L, respectively). By dialysis of acetonitrile/ water solutions against pure water, optically clear aqueous solutions of all the five block copolymers can be prepared. For St-SI-St-5, -10, and -20, much higher concentrations of the polymers can be reached by this protocol than those obtained by direct dissolution of dry samples in water. These findings suggest that the St-SI-St block copolymers may undergo different types of self-associations depending on the protocol for the preparation of polymer solutions. In many cases, a given amphiphilic copolymer offers only one way of micelle formation, i.e., either by direct dissolution of dry samples in a selective solvent or by transfer from a good to selective solvent through dialysis. The highly asymmetric St-SI-St block copolymers with very short St blocks are an interesting example of copolymers whose self-association can be induced in (15) Nagayama, M.; Okumura, O.; Noda, S.; Mori, A. J. Chem. Soc., Chem. Commun. 1973, 21, 841.

both ways. This paper only focuses on aqueous solutions of St-SI-St-5, -10, and -20 prepared by direct dissolution of the block copolymers in water. Fluorescence Probe Studies. First, we performed fluorescence probe experiments to answer a fundamental question as to whether the St-SI-St block copolymers undergo self-association in aqueous solutions accompanied by the formation of hydrophobic microdomains. One of the probes used for this purpose is DPH. The use of DPH in micellization studies is based on strong dependence of its UV absorption and fluorescence intensities on the polarity of solvent, being strong in nonpolar solvents and very weak in polar ones. This probe has been used in studies of uncharged, zwitterionic, and cationic surfactant micelles.16 Appearance of hydrophobic microdomains, when the concentration of a micelle-forming surfactant or polymer exceeds a cmc in an aqueous solution in the presence of DPH, is therefore accompanied by a strong increase in the absorption and fluorescence intensities of this probe.3a The dependence of the integral intensity of DPH fluorescence on the concentration of the St-SI-St block copolymers is shown in Figure 1. Large increases in the fluorescence intensity by 2 orders of magnitude were found for all the three polymers with increasing Cp. This increase in the fluorescence intensity is clearly related to the St content in the polymer; i.e., it is largest for St-SI-St-20 and smallest for St-SI-St-5. These observations indicate the formation of hydrophobic microdomains resulting from the association of St blocks, which, in turn, suggests the formation of a micellar structure with St blocks in the interior core and SI blocks in the exterior corona. If this is the case, the micellization may commence to occur abruptly at a certain critical Cp (i.e., at a cmc). However, because of the absence of a clear break in the plots in Figure 1, we could not confirm the presence of cmc from the DPH fluorescence data, if any. The occurrence of the hydrophobic microdomain formation and micellization may be also conveniently detected using molecular pyrene as a fluorescence probe. Its application in studying hydrophobic aggregation is based on the fact that the vibrational fine structure of pyrene fluorescence undergoes significant changes on going from nonpolar to polar media.17,18 The intensity ratio of the third to the first vibronic bands, known as the I3/I1 ratio, has been shown to be a good indicator of the polarity of (16) Chattopadhyay, A.; London, E. Anal. Biochem. 1984, 139, 408. (17) Nakajima, A. J. Mol. Spectrosc. 1976, 61, 467. (18) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.

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Figure 1. Integral fluorescence intensity of DPH solubilized in the aqueous solutions of (O) St-SI-St-5, (×) St-SI-St-10, and ()) St-SI-St-20 without added salt.

microenvironments surrounding pyrene molecules. The 0-0 transition from the lowest excited state, giving rise to the I1 peak, is symmetry forbidden. In polar media this forbiddance is released due to distortion of the π-electron cloud, while I3 is allowed and thus is solvent insensitive. The values of I3/I1 range from 0.53 in water to 0.90 in toluene and 2.0 in perfluoromethylcyclohexane.18 The use of pyrene in the studies of amphiphilic polymers and surfactants is based on the solubilization of pyrene molecules within hydrophobic cores of polymeric or surfactant micelles. Therefore, appearance of hydrophobic microdomains and micelles, when Cp is increased, is accompanied by a more or less fast increase in the I3/I1 ratio. The dependence of the I3/I1 ratio on the concentration of the St-SI-St block copolymers is shown in Figure 2. The Cp range examined is from 1.0 × 10-3 g/L up to the solubility limit of each polymer. It can be found that an increase in the Cp results in an increase in the I3/I1 ratio for all three polymers. This is clear evidence for the aggregation of St blocks yielding hydrophobic microdomains. In the plots for each polymer, there seem to be three distinctive regions with respect to the Cp. In the low Cp regime, the I3/I1 ratio grows gradually with increasing Cp. In the medium concentration regime, the I3/I1 ratio increases more significantly, and there are clear breaks in the plots between these two Cp regimes. This suggests the presence of an onset Cp for the formation of hydrophobic microdomains. With a further increase in the Cp, the I3/I1 ratio tends to saturate, suggesting complete solubilization of pyrene probes. The I3/I1 ratios were examined both in the absence and in the presence of 0.1 M NaCl. The addition of NaCl causes lowering of the Cp at which the fast growth of the I3/I1 ratio begins. This is an indication of an increasing tendency for the hydrophobic microdomain formation with increasing ionic strength, arising from the shielding of repulsive Coulombic forces between SI blocks. Also, the plots of the I3/I1 ratios in the absence and presence of NaCl for StSI-St-20 almost overlap in the high concentration regime while for St-SI-St-10 the saturated value of I3/I1 is lower in 0.1 M NaCl than in water. This difference is even larger for St-SI-St-5. These observations may suggest that, in the case of St-SI-St-20 micelles formed in the absence of NaCl, the cores are large enough to completely protect solubilized pyrene from the bulk aqueous phase while the cores of St-SI-St-5 micelles may be too small to completely protect pyrene. It should be noted that the plots of the I3/I1 ratios for all the three polymers exhibit a positive slope in the lowconcentration regime, indicating that pyrene probes are

Figure 2. I3/I1 for pyrene solubilized in the micelles of (a) St-SI-St-5, (b) St-SI-St-10, and (c) St-SI-St-20 in aqueous solutions in the absence (O) and presence (0) of 0.1 M NaCl. Arrows show the cmc values found from excitation spectra.

gradually solubilized in a hydrophobic phase as Cp is increased even before a cmc is reached. These observations suggest the presence of a premicellization process, i.e., the formation of di-, tri-, tetrameric aggregates, etc. at Cp < cmc. These small aggregates should have some ability to solubilize pyrene and therefore increase I3/I1. Similar observations were reported by Almgren et al.19 for hydrophobically end-capped poly(ethylene oxide). This polymer forms micelle-like aggregates in aqueous solution in semidilute or concentrated regimes, whereas at very low concentrations the polymer exists in the form of unimers, some of which are closed to loops, or in small oligomeric aggregates. The formation of micelles can also be conveniently traced using analysis of excitation spectra of pyrene.5,20 When pyrene is solubilized in a hydrophobic phase, the 0-0 absorption band, whose maximum in water lies around 334 nm, is gradually replaced by a band that lies around 340 nm. This shift can be more sensitively observed in excitation spectra than in absorption spectra. Thus, one can use this spectral shift to estimate a cmc of micelleforming compounds in aqueous media. To estimate cmc values for St-SI-St block copolymers, excitation spectra of pyrene solubilized in block copolymer solutions were analyzed in the range 285-365 nm. The excitation spectra were deconvoluted using Spectracalc (19) Alami, E.; Almgren, M.; Brown, W.; Franc¸ ois, J. Macromolecules 1996, 29, 2229; (20) Wilhelm, M.; Zhao, C.; Wang, Y.; Xu, R.; Winnik, M. A.; Mura, J.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 1033.

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Figure 3. Deconvoluted fluorescence excitation spectra of pyrene solubilized in aqueous solutions of St-SI-St-20 at polymer concentrations of (a) 2.0 × 10-3, (b) 0.15, and (c) 2.5 g/L in the absence of NaCl.

software after transforming all the spectra to zero baseline. The deconvolution technique applied, which allowed mixed Gaussian/Lorentzian peaks to be fitted, yielded the areas of the 334- and 340-nm bands for each spectrum. The ratios of the area of the 340-nm band to the area of the 334-nm band, I340/I334, were calculated for various Cp. An example of this analysis is presented in Figure 3. The I340/I334 ratios are plotted as a function of Cp for all the polymers in Figure 4. In the regime of low concentrations of polymer, there is almost no contribution of the 340-nm band and the plots are almost horizontal on the baseline. However, starting from a certain concentration, a fast increase in I340/I334 begins, the plots showing a well-defined break. Therefore, the plot of the I340/I334 ratio indicates cmc more clearly than the plot of the I3/I1 ratio. Values of cmc estimated from the excitation spectra are only “apparent” values because they do not take into account the partition of pyrene molecules between the bulk phase and micellar cores. Moreover, an additional advantage of the plot of the I340/I334 ratio is a much wider range of I340/ I334 values compared to I3/I1 values. The values of the cmc, both in the absence and presence of 0.1 M NaCl, determined from the interception of the two linear sections of the plots in Figure 4, are given in Table 1. It was found that the cmc decreases with an increase in the length of the St block. The value of the cmc in the absence of NaCl is largest for St-SI-St-5 (0.37 g/L) and much smaller for St-SI-St-20 (0.098 g/L) (Table 1). In both cases the cmc does not change substantially after addition of 0.1 M NaCl; the cmc drops to 0.35 and 0.072 g/L, respectively. The behavior of St-SI-St-10 is, however, different from that

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Figure 4. I340/I334 ratios for the fluorescence excitation spectra of pyrene solubilized in the solutions of (a) St-SI-St-5, (b) St-SI-St-10, and (c) St-SI-St-20 as a function of polymer concentration in the absence (O) and presence (0) of 0.1 M NaCl.

of the former two polymers in that the cmc of the latter changes substantially upon addition of NaCl. Moreover, in the absence of NaCl, the cmc of St-SI-St-10 is similar to that of St-SI-St-5 and in the presence of NaCl it is close to that of St-SI-St-20. The values of cmc for the present triblock copolymers were found to be similar to those for the St-SI diblock copolymers13 with the same molecular weight and the same length of St blocks. From the comparison of the plots of the I3/I1 and I340/I334 ratios as a function of Cp, one can see that these plots are complementary. Namely, I3/I1 increases with increasing Cp until the cmc is reached and then saturates. In the case of the plots of I340/I334 ratios, however, the situation is opposite; the I340/I334 ratio is constant and close to zero at Cp up to the cmc, and when the concentration exceeds the cmc, it starts increasing. Quasielastic Light Scattering (QELS) Studies. The analysis of the distribution of the relaxation times in QELS was performed using inverse Laplace transform (ILT).13 Thus calculated distributions of relaxation times for different Cp are shown in Figure 5. For St-SI-St-5 and -20 the distributions of the relaxation times are bimodal (except for the lowest Cp case where the distributions are unimodal). Large peaks are located in a regime of longer relaxation times (0.1-0.4 ms), and very small peaks are found in a regime of much shorter relaxation times (0.010.04 ms). Magnified peaks in the shorter relaxation time regime are presented in the inserts in Figure 5. These relaxation time ranges correspond to entities whose Rh values are on the order of tens of nanometers for the slow

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Figure 5. QELS relaxation time distributions for (a) St-SISt-5 at Cp ) 0.05 (- - -), 0.25 (‚ ‚ ‚), and 4.00 g/L (s); (b) StSI-St-10 at Cp ) 0.05 (- - -), 0.50 (‚ ‚ ‚), and 2.00 g/L (s); and (c) St-SI-St-20 at 0.05 (- - -), 0.50 (‚ ‚ ‚), and 2.25 g/L (s) in 0.1 M NaCl at 25 °C measured at 90°. (inserts) Magnified presentations for the distributions of the fastest relaxation mode.

(2)

Figure 6. Hydrodynamic radii of unimers (O), micelles ()), and aggregates (0) in 0.1 M NaCl for (a) St-SI-St-5, (b) StSI-St-10, and (c) St-SI-St-20 calculated from the relaxation times at the maxima of the distributions shown in Figure 5 using eq 2.

where T is the Kelvin temperature, η is the viscosity of the solution, and D is the diffusion coefficient. Thus, the fast mode may be interpreted as originating from both unimers and oligomeric aggregates while the slow mode may be interpreted as originating from micelles. In the case of St-SI-St-10, the presence of three relaxation modes was found (also except for the lowest Cp case). Small peaks were found in the range of 0.01-0.02 ms (insert in Figure 5b). This range of the relaxation times corresponds to objects whose Rh values are on the order of nanometers and thus can be interpreted as originating from unimers and/or oligomeric aggregates. Peaks found in the range of 0.1-0.2 ms correspond to entities as large as tens of nanometers. A characteristic feature for StSI-St-10, comparing to the other two polymers, is that it shows large peaks in a longer relaxation time regime (0.2-1 ms) corresponding to objects whose Rh values are on the order of tens to hundreds of nanometers. As will be discussed in the later section in detail, we interpret the intermediate mode (0.1-0.2 ms) and slow mode (0.2-1 ms) as resulting from micelles and bridged micelles (e.g., micelles bridged by some block copolymer chains), respectively. The appearance of the fairly symmetrical peaks in the slow relaxation time regime for St-SI-St-5 and -20 and in the intermediate relaxation time regime for St-SISt-10 suggests that polymer micelles are formed according to a closed association model. However, the slow mode

peaks for St-SI-St-20 shift toward longer relaxation times at higher Cp (i.e., >2.25 g/L), suggesting that micelles of this polymer have a tendency to become bridged at higher Cp. The Rh values for all the modes in the distribution of relaxation times were calculated from eq 2 and plotted in Figure 6 as a function of Cp. It can be found that the Rh values for the unimers and oligomeric aggregates (the fast mode for all the three polymers) are on the order of a few nanometers. The Rh values of micelles are fairly constant in the intermediate and high concentration regions for St-SI-St-5 (slow mode), in the whole Cp range for St-SI-St-10 (intermediate mode), and in the low and intermediate Cp regions for St-SI-St-20 (slow mode). The values of Rh for St-SI-St-5, -10, and -20 are 40-50 nm (at Cp ) 1-4 g/L), 20-30 nm (at Cp ) 0.25-2 g/L), and 30-35 nm (at Cp ) 0.1-2.25 g/L), respectively. The values for St-SI-St-10, however, should be considered only as approximates, because the two distributions of the relaxation times for discrete (nonbridged) micelles and bridged micelles are not fully resolved (Figure 5b), for which case ILT does not work well. In the low Cp regime one can see a sharp increase in Rh with increasing Cp only in the case of St-SI-St-5 (Figure 6a). This Cp region agrees with the cmc value determined from pyrene excitation spectra for St-SI-St-5 solutions in 0.1 M NaCl (Figure 4). The fast growth of Rh and subsequent plateau would suggest that an increase in Cp initially brings about the

mode and nanometers for fast mode relaxation, as can be calculated from the Einstein-Stokes relation

Rh ) kT/6πηD

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Figure 8. Plot of the relaxation rate, Γ, as a function of the cube of the scattering vector, q3, for aqueous solutions of (O) St-SI-St-10 at Cp ) 2.0 g/L and (0) St-SI-St-20 at Cp ) 2.5 g/L in 0.1 M NaCl at 25 °C.

Figure 7. Plot of the relaxation rate, Γ, as a function of the square of the scattering vector, q2, for aqueous solutions of (a) St-SI-St-5 at Cp ) 0.05 (O), 1.0 (0), and 4.0 g/L ()); (b) StSI-St-10 at Cp)0.05 (O) and 1.0 g/L (0); and (c) St-SI-St-20 at Cp ) 0.05 (O) and 1.75 g/L (0) in 0.1 M NaCl at 25 °C.

growth of polymer aggregates in size while a further increase in Cp results in an increase of the number of micelles of well-defined Rh. In contrast, no such growth in micelle size at a low Cp regime was observed for StSI-St-10 (intermediate mode) and St-SI-St-20 (slow mode) in parts b and c of Figure 6, respectively, because their cmc values are below the lowest Cp limit for QELS measurements. The micelles of St-SI-St-20 show a tendency to associate forming bridged micelles at Cp higher than 2.25 g/L, as can be seen from an increase in Rh at this Cp (Figure 6c). In the case of St-SI-St-10, bridged micelles (slow mode in Figure 6b) show an increase in size over the entire range of Cp studied except for a small plateau in a Cp region 1-1.7 g/L. Because the micelles of St-SISt-10 have a constant size (intermediate mode in Figure 6b), a reason for the growth in size of the bridged micelle seems to be an increase in the number of bridges with increasing Cp. The dependence of the decay rate of the autocorrelation function, Γ, at the maxima of the slow modes in the distributions of the relaxation times on the square of the scattering vector, q2, is shown in Figure 7. It can be seen that for low and intermediate concentrations for all three polymers and for the highest concentration of St-SISt-5, Γ is linearly dependent on q2 (with correlation coefficients, R, ranging from 0.98 to 0.99) passing through the origin. These observations are indicative of the QELS relaxation being due to the translational diffusion of scattering particles. In the case of the highest Cp of St-

SI-St-10 (2.0 g/L) and -20 (2.5 g/L), however, Γ values show a linear dependence on q3 (R ) 1.00) (Figure 8). These observations support the conclusion on the formation of bridged micelles, as will be discussed in the Discussion section. Static Light Scattering (SLS) Studies. The QELS results indicate that the size of micelles formed from StSI-St-5 and -20 is fairly constant in a Cp regime 1.0-4.0 g/L for St-SI-St-5 and in a Cp regime 0.1-2.25 g/L for St-SI-St-20 (Figure 6a,c). Thus, it may be possible to estimate the micelle mass in the respective Cp regimes by SLS. In St-SI-St-10 solutions, however, both discrete micelles and bridged micelles are present in the whole Cp range studied. Therefore, the obtained value of the micelle mass for St-SI-St-10 is an average value for the discrete and bridged micelles. In SLS measurements, Cp was changed within the range of 0.75-1.75 g/L for St-SI-St-5 and 0.5-2.25 g/L for StSI-St-20 and the angle was changed from 30° to 150° with a 10° increment. The Zimm plot (data not shown) obtained for St-SI-St-20 is curved, indicating that micelle masses are not exactly constant within the concentration range examined, and thus the value of the micelle mass obtained should be treated only as a rough estimate of the real mass. Moreover, the molecular weight of the polymer micelle determined by SLS may by somewhat overestimated because of water molecules bound to SI blocks. The values of weight average mass of the micelle obtained by SLS measurements are listed in Table 1. The values of apparent molecular weights of St-SI-St-5 and St-SISt-20 micelles correlate well with the values of Rh from QELS (Table 1). From the weight average molecular weights of the polymer (unimer) and its micelle, aggregation numbers, i.e., the number of unimers in one micelle can be roughly assessed (Table 1). The aggregation number of St-SISt-20 is slightly smaller than that of St-SI-St-5, but in terms of the number of St units, the core of the St-SISt-20 micelle is much larger than that of the St-SI-St-5 micelle. The large aggregation number obtained for StSI-St-10 is indicative of a mixture of micelles and bridged micelles. Discussion In the case of BAB-type triblock copolymers, two basic types of associations are possible.1h, 3b,c,11,19,21 If both the hydrophobic end blocks have a tendency to be incorporated in the same micellar core, a closed association may take (21) Nguyen-Misra, M.; Mattice, W. L. Macromolecules 1995, 28, 1444.

Triblock Copolymer Micelles

place with the formation of so-called “flower-type micelles” with a well-defined radius and aggregation number. On the other hand, if the hydrophobic side blocks form the cores of different micelles, a spatially extended structure may be formed, which may eventually result in gelation or precipitation. One can also think of a series of intermediate models between these two extreme models. An important issue in our understanding of the association of the St-SI-St block copolymers is to answer a question as to which model is applicable for aqueous solutions of these BAB-type copolymers. The presence of a cmc for St-SI-St solutions suggests that the micelle formation takes place under equilibrium conditions where unimers and oligomeric aggregates coexist with micelles at Cp > cmc. This is also suggested by the presence of the small fast mode peaks (unimers and oligomers) and large slow mode peaks (micelles) in the distributions of the relaxation times in QELS for StSI-St-5 and -20 (Figures 5a,c and 6a,c). For solutions of St-SI-St-10, the presence of a third mode at very long relaxation times suggests the presence of bridged micelles (Figures 5b and 6b). At high Cp (2.0 g/L) of St-SI-St-10 and -20, Γ is linearly dependent on q3 rather than on q2 (Figure 8). Both theoretical22 and experimental23,24,25 studies imply that a linear dependence of Γ on q3 is indicative of an internal motion of scatterers. A plausible cause of the internal motion may be the dynamic character of the bridged micelles, e.g., arising from simultaneous formation and dissociation of micelle bridges. The QELS results suggest that all three St-SI-St triblock copolymers form flower-type micelles with differing extents of the micelle bridges depending on Cp (Figure 6). The Rh values for the micelles from St-SISt-5, a block copolymer with the shortest St blocks and longest SI block, are constant once micelles are formed (Cp > 1.0 g/L), indicating that micelles show no tendency for bridging at Cp < 4.0 g/L (Figure 6). The Rh values for the micelles formed by St-SI-St-20, a block copolymer with the longest St blocks and shortest SI block, are smallest and fairly constant independent of Cp up to 2.25 g/L, although Rh increases at Cp > 2.5 g/L. Thus, micelles of this polymer tend to become bridged only at Cp > 2.5 g/L. In the case of St-SI-St-10, however, micelles always coexist with bridged micelles in the whole Cp range studied, a manifestation of the strongest tendency for micelle bridging. An explanation for the different micellization behavior of these three block copolymers is as follows. If the two hydrophobic side blocks in BAB-type block copolymers associate in the same polymer chain, the unimer exists as a looped chain. On the other hand, if the two side blocks exist as free chain ends, the unimer would exist as an extended or coiled chain with open chain ends. In aqueous solutions of BAB-type block copolymers, these two types of unimers may exist together in equilibrium, and the population of the looped unimers relative to the openchain unimers may depend on the relative length of the hydrophilic and hydrophobic blocks in the copolymers. In the present St-SI-St block copolymer case, the relative population of looped unimers may increase in the order St-SI-St-5 < St-SI-St-10 < St-SI-St-20 because the DPSt/DPSI ratio increases in this order. If most of the (22) Han, C. C.; Akcasu, A. Z. Macromolecules 1981, 14, 1080. (23) Takada, A.; Nemoto, N. Prog. Colloid Polym. Sci. 1997, 106, 1183. (24) Koike, A.; Nemoto, N.; Inoue, T.; Osaki, K. Macromolecules 1995, 28, 2339. (25) Takada, A.; Nishimura, M.; Koike, A.; Nemoto, N. Macromolecules 1998, 31, 436.

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Figure 9. Proposed model for the micellization of St-SI-St block copolymers in water.

unimers are in the loop form, the triblock copolymers may undergo a closed association and yield flower-type micelles because the two St blocks in the same polymer chain would be incorporated together in the same micellar core and loops of the SI middle blocks would be located in the micellar exterior as corona. This situation is expected to occur in the case of St-SI-St-20 solutions. In contrast, if most of the unimers are in the open-chain form, the micelles may be formed where most unimers have only one chain end incorporated into a micellar core while the other is freely dangling. This situation is expected to occur in the case of St-SI-St-5 solutions. An intermicellar bridge for St-SI-St-5 may be unstable because St blocks are too short, and thus hydrophobic interactions are too weak to be able to tie two negatively charged micelles and hold them together in a close proximity against electrostatic repulsions between the micelles. However, in the case where the unimers are a mixture of the loop and open-chain conformations, both the closed and open associations may occur concurrently, and bridged flower micelles may be formed where the two St blocks in the same polymer chain occupy cores in the same or different micelles, the latter case leading to a bridge between two micelles. In this case, the size of the bridged micelles may increase with increasing Cp because the bridge between micelles is more likely to be formed at higher Cp. This may be the case of the micellization of St-SI-St-10. The postulated associative behavior of the St-SI-St triblock copolymers emerging from the above discussions is schematically illustrated in Figure 9. It should be interesting to compare Rh values of triblock St-SI-St copolymers with those of diblock St-SI copolymers. The Rh values for triblock St-SI-St copolymers are markedly smaller than Rh values for diblock St-SI copolymers.13 It seems that St-SI-20 and St-SI-St-20 are best for comparison because of the same St content and molecular weight of the unimers. It can be seen that Rh for St-SI-St-20 (about 28-33 nm at Cp ) 0.1-2.25 g/L) is about half that for St-SI-20 (about 60 nm at Cp ) 0.1-1.0 g/L13). This is in accordance with a flower-type micelle model assumed for BAB-type triblock copolymers. According to this model the backfolding of polymer chains with both chain ends immersed in the same micellar core should cause a roughly 50% decrease in the micelle size compared to a star-type micelle formed by the same

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polymer chains, if the core size is much smaller than that of the corona, which should be the case for both St-SI-20 and St-SI-St-20. Conclusions The St-SI-St triblock copolymers were found to form micelles when dry samples of the polymers were directly dissolved in water. The existence of cmc (on the order of Cp ) 10-1-10-2 g/L) found from fluorescence probe experiments indicated the occurrence of closed-type association, where micelles are in equilibrium with unimers. The formation of oligomeric aggregates below cmc was suggested by pyrene fluorescence spectra. In St-SI-St-5 solutions, the presence of unimers, oligomeric aggregates, and micelles (above cmc) was found. Values of Rh for the St-SI-St-5 micelles are fairly constant at 40-50 nm in the Cp range 1.0-4.0 g/L. In St-SI-St-10 solutions, however, besides unimers, oligomeric aggregates, and discrete micelles, bridged-micelles are present. Discrete micelles had constant Rh of 20-30 nm in the Cp range

Szczubiałka et al.

0.25-2.0 g/L, while the Rh for the bridged micelles increases up to 150 nm at Cp ) 2.0 g/L. In St-SI-St-20 solutions, unimers, oligomeric aggregates, and micelles (nonbridged) were found at Cp < 2.5 g/L. The values of Rh for the micelles were almost constant (30-35 nm) in the Cp range 0.1-2.25 g/L but grew steeply at Cp > 2.25 g/L, indicating the occurrence of micelle bridging at higher Cp. On the basis of experimental data, flower-type micelles, a mixture of nonbridged and bridged, were proposed as a hypothetical model. The different extents of the micelle bridging are explained as resulting from the ratio of openchain and looped conformations of unimeric block copolymer molecules existing in aqueous solutions. Acknowledgment. This work was supported in part by a Grant-in-Aid for JSPS fellow and a Grant-in-Aid Scientific Research No. 10450354 from the Ministry of Education, Science, Sports, and Culture, Japan. LA991056D