Sphere-to-Rod Transition of Non-Surface-Active Amphiphilic Diblock

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Langmuir 2007, 23, 9162-9169

Articles Sphere-to-Rod Transition of Non-Surface-Active Amphiphilic Diblock Copolymer Micelles: A Small-Angle Neutron Scattering Study Ploysai Kaewsaiha,† Kozo Matsumoto,‡ and Hideki Matsuoka* Department of Polymer Chemistry, Kyoto UniVersity, Kyoto 615-8510, Japan ReceiVed February 8, 2007. In Final Form: June 2, 2007 Micellization behavior of amphiphilic diblock copolymers with strong acid groups, poly(hydrogenated isoprene)block-poly(styrenesulfonate), was investigated by small-angle neutron scattering (SANS). We have reported previously (Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Langmuir 2005, 21, 9938) that this strongly ionic amphiphilic diblock copolymer shows almost no surface activity but forms micelles in water. In this study, the size, shape, and internal structures of the micelles formed by these unique copolymers in aqueous solution were duly investigated. The SANS data were well described by the theoretical form factor of a core-shell model and the Pedersen core-corona model. The micellar shape strongly depends on the hydrophobic chain length of the block copolymer. The polymer with the shortest hydrophobic chain was suggested to form spherical micelles, whereas the scattering curves of the longer hydrophobic chain polymers showed a q-1 dependence, reflecting the formation of rodlike micelles. Furthermore, the addition of salt at high concentration also induced the sphere-to-rod transition in micellar shape as a result of the shielding effect of electrostatic repulsion. The corona thickness was almost constant up to the critical salt concentration (around 0.2 M) and then decreased with further increases in salt concentration, which is in qualitatively agreement with existing theories. The spherical/rodlike micelle ratio was also constant up to the critical salt concentration and then decreased. The micelle size and shape of this unique polymer could be described by the common concept of the packing parameter, but the anomalously stable nature of the micelle (up to 1 M NaCl) is a special characteristic.

Introduction Ionic amphiphilic block copolymers, which have polyelectrolyte chains as hydrophilic blocks, have been extensively studied experimentally during recent years1-15 because their applications, * To whom correspondence should be addressed. E-mail: matsuoka@ star.polym.kyoto-u.ac.jp. † Present address: Faculty of Science and Technology, Suan Sunandha Rajaphat University, Bangkok 10300, Thailand. ‡ Present address: Molecular Engineering Institute (MEI), Kinki University, 11-6 Kayanomori, Iizuka, Fukuoka 820-8555, Japan. (1) Zhung, L.; Eisenberg, A. Science 1995, 268, 1728. (2) Khougaz, K.; Astafieva, I.; Eisenberg, A. Macromolecules 1995, 28, 7135. (3) Amiel, C.; Sikka, M.; Schneider, J. W.; Tsao, Y. H.; Tirrell, M.; Mays, J. W. Macromolecules 1995, 28, 3125. (4) Mir, Y.; Auroy, P.; Auvray, L. Phys. ReV. Lett. 1995, 75, 2863. (5) (a) Guenoun, P.; Schlachli, A.; Sentenac, D.; Mays, J. W.; Benattar, J. J. Phys. ReV. Lett. 1995, 74, 3628. (b) Guenoun, P.; Delsanti, M.; Gaseau, D.; Auvray, L.; Cook, D. C.; Mays, J. W.; Tirrell, M. Eur. Phys. J. B 1998, 1, 77. (c) Guenoun, P.; Muller, F.; Delsanti, M.; Auvray, L.; Chen, Y. J.; Mays, J. W.; Tirrel, M. Phys. ReV. Lett. 1998, 81, 3872. (d) Weber, S. E. J. Phys. Chem. B 1998, 102, 2618. (6) (a) Fo¨ster, S.; Hermsdorf, N.; Leube, W.; Schnablegger, H.; Regenbreche, M.; Akari, S. J. Phys. Chem. B 1999, 103, 6657. (b) Fo¨ster, S.; Abetz, V.; Mu¨ller, A. H. E. AdV. Polym. Sci. 2004, 166, 173. (7) Gabaston, L. L.; Furlong, S. A.; Jackson, R. A.; Armes, S. P. Polymer 1999, 40, 4505. (8) Groenewegen, W.; Egelhaalf, S. U.; Lapp, A.; van der Maarel, J. R. C. Macromolecules 2000, 33, 3283. (9) Regenbrecht, M.; Akari, S.; Fo¨ster, S.; Mo¨hwald, H. J. Phys. Chem. B 1999, 103, 6669. (10) (a) Nakano, M.; Matsuoka, H.; Yamaoka, H.; Poppe, A.; Richter, D. Macromolecules 1999, 32, 697. (b) Nakano, M.; Matsumoto, K.; Matsuoka, H.; Yamaoka, H. Macromolecules 1999, 32, 4029. (11) Matsumoto, K.; Ishizuka, T.; Matsuoka, H. Langmuir 2004, 20, 72707282. (12) Matsuoka, H.; Matsutani, M.; Mouri, E.; Matsumoto, K. Macromolecules 2003, 36, 5321. (13) Matsuoka, H.; Maeda, S.; Kaewsaiha, P.; Matsumoto, K. Langmuir 2004, 20, 7412. (14) Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Langmuir 2004, 20, 6754.

which include nanoreactors, drug delivery, and DNA carrier systems, are broader than those of normal surfactants.16-17 The number of theoretical models describing the structure of charged polyelectrolyte micelles have been increasing. Many of the theories describe spherical micelles with a charged corona and an electrostatically neutral core.19-30 Recently, we found that the ionic amphiphilic diblock copolymers, which consist of hydrophobic and ionic polymer chains, are non-surface-active but form micelles in solution.12,13,15 Their solutions show no surface tension reduction and non-foam formation, in perfect contrast to low-molecular-weight ionic surfactants. This behavior has never been observed for nonionic amphiphilic block copolymers. Hence, the non-surface-active (15) Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Langmuir 2005, 21, 9938. (16) Winnik, M. A.; Yekta, A. Curr. Opin. Colloid Interface Sci. 1997, 2 (4), 424. (17) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, U.K. 1998. (18) Pincus, P. Macromolecules 1991, 24, 2912. (19) Wittmer, J.; Joanny, J. F. Macromolecules 1993, 26, 2691. (20) Dan, N.; Tirrell, M. Macromolecules 1993, 26, 4310. (21) Borisov, O. V. J. Phys. II 1996, 6, 1. (22) Biver, C.; Hariharan, R.; Mays, J.; Russel, W. B. Macromolecules 1997, 30, 1787. (23) Hariharan, R.; Biver, C.; Mays, J.; Russel, W. B. Macromolecules 1998, 31, 7506. (24) Zhulina, E. B.; Borisov, O. V. Macromolecules 1996, 29, 2618. (25) Israels, R.; Leermakers, F. A. M.; Fleer, G. J. Macromolecules 1994, 27, 3087. (26) Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Macromolecules 1995, 28, 1491. (27) Lyatskaya, Y. V.; Leermakers, F. A. M.; Fleer, G. J.; Zhulina, E. B.; Birshtein, T. M. Macromolecules 1995, 28, 3562. (28) Borisov, O. V.; Zhulina, E. B. Macromolecules 2002, 35, 4472. (29) Borisov, O. V.; Zhulina, E. B. Macromolecules 2003, 36, 10029. (30) Zhulina, E. B.; Adam, M.; L. R. Isaac; Sheiko, S. S.; Rubinstein M. Macromolecules 2005, 38, 5330.

10.1021/la7003672 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007

Sphere-to-Rod Transition of Copolymer Micelles

ionic amphiphilic block copolymer can be regarded as a novel substance. The origin of non-surface activity is thought to be image charge repulsion at the air/water interface.12,13,15 Because of its hydrophobicity, the block copolymer would be adsorbed at the air/water interface. However, near the interface, a strong electrostatic repulsion from the interface is induced by the image charge effect. Because the hydrophilic chain consists of polyions, this electrostatic repulsion is so strong that the polymers cannot be adsorbed but form micelles in bulk solution. So far, our investigation has been concentrated on the origin of the unique character (i.e., non-surface activity) and the examination of its difference from normal surfactant behavior with respect to the salt concentration and hydrophobic chain length dependence of the critical micelle concentration (cmc).12,13,15 In a previous study, we reported the non-surface activity of a hydrogenated polyisoprene/sodium poly(styrenesulfonate) (PIp-h2-b-PSSNa) diblock copolymer in aqueous solution.15 This polymer has an ionized segment (PSSNa) and a hydrophobic segment (PIp-h2) and thus resembles surfactant molecules where the polar head is replaced by an ionic chain. We confirmed the existence of micelles in aqueous solutions and changes due to polymer architecture and external conditions such as the addition of salt by a variety of different experimental methods, including dynamic light scattering (DLS) and hydrophobic dye solubilization by UV/vis and fluorescence spectroscopy. In this study, we focus on obtaining quantitative information on the structure, size, shape, and internal structure of these micelles in aqueous solution using small-angle neutron scattering (SANS). Because the non-surface-active phenomenon has been investigated recently, detailed micelle structure data is absolutely necessary for us to understand the non-surface-active behavior. The micellar conformation of the polyelectrolyte block copolymer is controlled by a number of contributing factors, which force the polymers to form another structure besides the simple spherical core-shell. For example, a cylindrical coreshell structure was reported by Cheng et al. for PS-PAA block copolymers.31 Schuch et al.32 and Chen et al.33 proposed the formation of vesicle-like structure for PIB-PMMA and PS-PAA in aqueous media. The transition of spherical surfactant micelles in solution to cylindrical geometry was described by Luzzati34 in 1964 and has been thoroughly studied since then by light scattering,35-36 SANS,37-40 and electron microscopy.41 In 2002, Fo¨rster et al. investigated the structure of charged spherical block copolymer micelles in aqueous solution with static and dynamic light scattering, small-angle neutron scattering, and cryo-electron microscopy as a function of added salt and found that the relation between shell ionic strength and added salt concentration follows a simple Donnan equilibrium.42 (31) Cheng, G.; Bo¨ker, A.; Zhang, M.; Krausch, G.; Mu¨ller, A. H. E. Macromolecules 2001, 34, 6883. (32) Schuch, H.; Klingler, J.; Rossmanith, P.; Frechen, T.; Gerst, M.; Feldthusen, J.; Mu¨ller, A. H. E. Macromolecules 2000, 33, 1734. (33) Cheng, L.; Shen, H.; Eisenberg, A. J. Phys. Chem. B 1999, 163, 9488. (34) Reiss-Husson, F.; Luzzati, V. J. Phys. Chem. 1964, 68, 3504. (35) Cirkel, P. A.; Koper, G. J. M. Langmuir 1998, 14, 7095-7103. (36) von Berlepsch, H.; Dautzenberg, H.; Rother, G.; Jaeger, J. Langmuir 1996, 12, 3613-3625. (37) He, L. Z.; Garamus, V. M.; Funari, S. S.; Malfois, M.; Willumeit, R.; Niemeyer, B. J. Phys. Chem. B 2002, 106, 7596-7604. (38) He, L. Z.; Garamus, V.; Niemeyer, B.; Helmholz, H.; Willumeit, R. J. Mol. Liq. 2000, 89, 239-248. (39) Stradner, A.; Glatter, O.; Schurtenberger, P. Langmuir 2000, 16, 5354. (40) Glatter, O.; Fritz, G.; Lindner, H.; Brunner-Popela, J.; Mittelbach, R.; Strey, R.; Egelhaaf, S. U. Langmuir 2000, 16, 8692-8701. (41) Bernheim-Groswasser, A.; Wachtel, E.; Talmon, Y. Langmuir 2000, 16, 4131. (42) Fo¨ster, S.; Hermsdorf, N.; Bo¨ttcher, C.; Linder, P. Macromolecules 2002, 35, 4096.

Langmuir, Vol. 23, No. 18, 2007 9163 Table 1. Characteristics of (Ip-h2)m-b-(SSNa)n Block Copolymers ma/nb

Mn

Mw/Mnc

degree of sulfonationd

25:40 38:50 62:40 131:54

7500 9000 11 100 19 000

1.08 1.09 1.06 1.12

0.62 0.71 0.82 0.99

a Number-average degree of polymerization of the PIp-h2 segment determined by 1H NMR for parent polymers in CDCl3. b Number-average degree of polymerization of the PSS segment determined by GPC for parent polymers (chloroform as an eluent with polystyrene standards). c Number-average molecular weight distribution of the block copolymer determined by GPC for parent polymers after hydrogenation (chloroform as an eluent with polystyrene standards). d Determined by elementary analysis.

In this study, SANS measurements were carried out for aqueous PIp-h2-b-PSSNa solutions at different PIp-h2 chain lengths under salt-free and salt-added conditions in order to investigate the structural transition of PIp-h2-b-PSSNa micelles as a function of hydrophobic chain length and salt concentration. As a result, the SANS curves could be described by the core-shell model, and the transition of the micellar shape from sphere to rod with increasing molar fraction of the hydrophobic part in a polymer was found. As a result of salt addition, the decrease in shell thickness and the sphere-to-rod transition occurred at almost the same point, when the added salt concentration exceeds the intrinsic ionic strength of the polyelectrolyte shell. The free ion concentration in the shell could be calculated to be over 0.2 M, and the micelles were stable against salt addition up to 1 M NaCl. The micellization behavior itself was not largely different from that of normal surfactant qualitatively, but anomalously high stability should be the special characteristic of this micelle. In addition, a comparison of the “critical salt density” with the PSS brush system is interesting. Experimental Section Materials. (PIp-h2-b-PSSNa) diblock copolymers were synthesized as previously reported.14-15 PIp-b-PS diblocks were synthesized by living anionic polymerization. After hydrogenation of the PIp blocks and sulfonation of the polystyrene (PS) block, the resulting poly(styrenesulfonic acid) blocks were neutralized using sodium hydroxide. Then the organic solvent (chloroform) was evaporated. The PIp-h2-b-PSSNa solution was dialyzed against ultrapure water, and the electric conductivity of dialyzed water was checked to determine the end point of dialysis. The degree of sulfonation was determined by elementary analysis. The characteristics of (PIp-h2b-PSSNa) are summarized in Table 1. Water used for polymer sample preparation and dialysis was ultrapure water obtained from a Milli-Q system (Millipore, Pittsburgh, PA). The resistance of the water was more than 18 MΩ cm. Sodium hydroxide and sodium chloride were obtained from Wako and used as received. Small-Angle Neutron Scattering (SANS) Measurement. Small angle neutron scattering experiments were conducted at the SANS-U system at the Institute for Solid State Physics, The University of Tokyo, at the JRR3-M reactor at Tokai, Ibaragi, Japan.43 For the SANS experiments, the copolymers were dissolved directly in D2O and NaCl D2O solutions (using 0.22 µm Millipore filters) and loaded into flat quartz cells with a 4 mm path length. Lupolen (polyethylene (PE) block) was used as the standard for absolute intensity calibration. The sensitivity of detector and q-resolution were corrected. The neutron wavelength distribution (∆λ/λ ) 20%) and collimation of the SANS machine were taken into account in the data evaluation. The data were corrected for background scattering in the usual way. (43) Ito, Y.; Imai, M.; Takahashi, S. Physica B 1995, 213, 889.

9164 Langmuir, Vol. 23, No. 18, 2007 The details of the measurement were described elsewhere.10 The effect of salt concentration on the SANS distribution was also investigated on each solution. In this case, the samples were directly dissolved in filtered NaCl solutions in D2O. A simple core-shell model and the Pedersen model44 were used to analyze the obtained SANS profiles for spherical micelles, but only the core-shell model was used in the sphere-rod mixture system. Fitting Models. Sphere and Rod Coexistence Core-Shell Model. If the contribution of interparticle interaction is negligible, then the neutron scattering cross section can be given by the equation dΣ(q) ) npP(q) dΩ where np is the number density of particles and P(q) is the particle form factor. The scattering vector q is given by q ) 4π sin θ/λ, where 2θ is the scattering angle and λ is the neutron wavelength. P(q) depends on the size, shape, and density distribution inside the scattering particles. The present study deals with two models describing the SAXS and SANS data (i.e., spherical and cylindrical models). In both cases, we assumed that the micelles consist of a core-shell structure. The form factor of a spherical core-shell model with the radii of the core RC and of the overall micelle RS can be written as follows45

Kaewsaiha et al. Fi )

i

where subscript i refers to D2O, H2O, Ip-h2, or SSNa. bz is the scattering length of atom z in the repeat unit or solvent molecule, and Vi is the corresponding volume. Thus, FD2O ) 6.406 × 1010 and FH2O ) -5.617 × 1019 cm-2 can be obtained. One can obtain the values of FIp-h2 and FSSNa after determining the densities of Ip-h2 and SSNa. For the calculation of FSSNa, the isotopic exchange of the hydrogen in the SO3H group of SSNa should be taken into account by assuming that this hydrogen is occupied by a deuterium or hydrogen in proportion to φD2O. In the case of a core-shell cylinder with radii of the core RC and the overall micelle RS and length L, its form factor is given by45

∫ {(F

FS(q)sphere )

3(sin(qRC) - qRC cos(qRc)) (qRC)3 3(sin(qRS) - qRS cos(qRS)) (qRS)3

where FC, FS, and F0 are the SLDs of the core, the shell, and the solvent, respectively. VC and VS are given as 4πRC3/3 and 4πRS3/3, respectively. The aggregation number (Nagg) is calculated from VC, n, and the volume of the Ip-h2 repeat unit (vIp-h2) Nagg )

φ PVC nνlp-h2

where φP is the volume fraction of polymer in the core and is equal to 1 for the close-packed core. The volume occupied by hydrophilic parts of polymers in the shell of a micelle is given by (NaggmVSSNa), where VSSNa is the volume of the SSNa repeat unit. Thus, the volume fraction of polymer in the shell (φS) is calculated as naggmνSSNa φS ) VS - V C and F0, FC, and FS are given by the SLDs of D2O (FD2O), H2O (FH2O), Ip-h2 (FIp-h2), and SSNa (FSSNa) F0 ) φD2OFD2O + (1 - φD2O)FH2O FC ) φpFIp-h2 + (1 - φP)F0 FS ) φSFSSNa + (1 - φS)F0 φD2O is the volume fraction of D2O in solvent. Here, the SLDs of repeat units and pure solvent can be calculated as (44) (a) Pedersen, J. S.; Posselt, D.; Mortensen, K. J. Appl. Crystallogr. 1990, 23, 321. (b) Pedersen, J. S.; Gerstemberg, M. C. Macromolecules 1996, 29, 1363. (c) Pedersen, J. S. J. Appl. Crystallogr. 2000, 33, 637. (d) Pedersen, J. S. J. Chem. Phys. 2001, 114, 2839. (d) Pedersen, J. S.; Gerstemberg, M. C. Colloids Surf., A 2003, 213, 175.

π

F(q)cylinder ) (1/2)

FC(q)cylinder )

FS(q)cylinder )

P(q)sphere ) {(FC - FS)VCFC(q)sphere + (FS - F0)VSFS(q)sphere}2 FC(q)sphere )

bz

∑ν

0

C

- FS)VCFC(q)cylinder + (FS - F0)VSFS(q)cylinder}2 sin β dβ

{ {

}{ }{

sin(q(L/2) cos β) L

q( /2)cos β

sin(q(L/2) cos β) L

q( /2)cos β

} }

2J1(qRC sin β) qRCsin β

2J1(qRS sin β) qRS sin β

where β is the angle between the axis of symmetry of the cylinder and the scattering vector q and J1 denotes the Bessel function of the first kind and of order 1. VC and VS are the volumes of cylinders with radii RC and RS, respectively, and length L. On the condition that q , 2π/L and L . RS, the form factor of the cylinder can be modified to a simpler form: P(q)cylinder )

{

(π/qL)

}

(FC - FS)VC2J1(qRC) (FS - F0)VS2J1(qRS) + qRC qRS

2

The neutron wavelength distribution (∆λ/λ ) 20%) and collimation of the SANS machine were taken into account in the data evaluation.43 Pedersen Model (Form Factor for Noninteracting Gaussian Chains).44 The form factor of a micelle contains four different terms: the self-correlation of the core, the self-correlation of the chains, the cross term between the core and chains, and the cross term between different chains. It can be written as Fmic(q) ) N2βcore2Fcore(q) + Nβchain2Fchain(q) + 2N2βcoreβchainScore-chain(q) + N(N - 1)βchain2Schain-chain(q) where q is the scattering vector, N is the aggregation number of the micelle, and βcore and βchain are the total excess scattering lengths of one hydrophobic block and one hydrophilic block, respectively. For a spherical homogeneous core with radius R and a smoothly decaying scattering-length density at the surface, the core self-term can be written as Fcore(q) ) Φ2(qR) exp(-q2σ2) where Φ(y) ) 3(sin y - y cos y)/y3 is the form factor amplitude of a sphere with a sharp surface. The last term takes into account the smoothly decaying density at the surface. σ describes the width of the interface. The chain self-correlation term for the Gaussian chains with a radius of gyration Rg is given by the Debye function (45) Guinier, A.; Fournet, G. Small-Angle Scattering of X-rays; John Wiley: New York 1955.

Sphere-to-Rod Transition of Copolymer Micelles Fchain(q) )

Langmuir, Vol. 23, No. 18, 2007 9165

2[exp(-x) - 1 + x] x2

where x ) q2Rg2. The core-chain term is {sin[q(d + R)] Score-chain(q) ) ψ(qRg) Φ(qR) exp(-q2σ2/2) q(d + R) where ψ(x) ) [1 - exp(-x)]/x2, again with x ) q2Rg2. The chainchain term is Schain-chain(q) ) ψ2(qRg)

(

)

sin[q(d + R)] q(d + R)

2

d ≈ Rg as this mimics nonpenetration of the corona chains. Small-Angle X-ray Scattering (SAXS) Measurement. SAXS was measured using a SAXS instrument in our laboratory. This instrument was composed of a Kratky U-slit optical system, a 1D position sensitive proportional counter (PSPC), and a 60 kV-200 mA rotating anode X-ray generator. The details of the instrument have been fully described elsewhere.46 A typical accumulation time was 6000 s. The obtained SAXS profiles were analyzed by a simple core-shell model.

Figure 1. Small-angle neutron scattering profiles for (Ip-h2)25-b(SSNa)40 D2O solutions. (a) The solid line is a fitting curve from a simple core-shell model. (b) The broken line is a fitting curve from the Pedersen model, which includes scattering from the corona region.

Results and Discussion Core-Shell and Pedersen Models. Figure 1a shows the SANS profiles for the 1% solution m:n ) 25:40 polymer in D2O. The solid line in the Figure is the fitting by the simple core-shell model. The profiles show the depression around q ) 0.01 Å-1. We do not know the reason for this depression, but it is probably due to the strong electrostatic interaction between micelle particles in the absence of salt.13 In fact, the interference peak actually disappeard by the addition of salt. However, because this is beyond the scope of this study, we will disregard these peaks in the analysis. As used in our recent studies of block copolymer micelles, the core-shell sphere model, which assumes centrosymmetry of the micelles, can well reproduce SANS profiles at q ) 0.02-0.05 Å-1. However, this model does not describe the observed scattering at high scattering vectors, which can be seen from the deviation of the calculated curve downward from the experimental curve for higher-q regions in Figure 1a. As in our previous studies,11,47 the reason for this observation is the “blob” scattering originating from the dissolved chains in the corona that surrounds the core of the micelles. This scattering contribution is included explicitly in models of the type described by Pedersen and Gerstenberg.44,48 In these models, the chains are assumed to obey Gaussian statistics and to be noninteracting, and this allows the form factor to be calculated analytically. This model has successfully been applied in the analysis of scattering data from block copolymer micelles.48-52 The calculated curve of the Pedersen model showed very good agreement with the data for the same sample (Figure 1b). It is (46) Ise, N.; Okubo, T.; Kunugi, S.; Matsuoka, H.; Yamamoto, K.; Ishii, Y. J. Chem. Phys. 1984, 81, 3294. (47) (a) Matsumoto, K.: Kubota, M.; Matsuoka, H.; Yamaoka, H. Macromolecules 1999, 32, 7122. (b) Matsumoto, K.; Mazaki, H.; Nishimura, R.; Matsuoka, H.; Yamaoka, H. Macromolecules 2000, 33, 8295. (c) Matsumoto, K.; Mazaki, H.; Matsuoka, H. Macromolecules 2004, 37, 2256. (48) Pedersen, J. S.; Hamley, I. W.; Ryu, C. Y.; Lodge, T. P. Macromolecules 2000, 33, 542. (49) Derici, L.; Ledger, S.; Mai, S.-M.; Booth, C.; Hamley, I. W.; Pedersen, J. S. Phys. Chem. Chem. Phys. 1999, 1, 2773. (50) Leclerc, E.; Calmettes, P. Physica B 1997, 241, 1141. (51) Plestil, J.; Kriz, J.; Tuzar, Z.; Prochazka, K.; Melnichenko, Y. B.; Wignall, G. D.; Talingting, M. R.; Munk, P.; Webber, S. E. Macromol. Chem. Phys. 2001, 202, 553. Plestil, J.; Pospisil, H.; Kadlec, P.; Tuzar, Z.; Kriz, J.; Gordeliy, V. I. Polymer 2001, 42, 2941. (52) Borbely, S. Langmuir 2000, 16, 5540.

Figure 2. Small-angle neutron scattering profiles for (Ip-h2)m-b(SSNa)n D2O solutions (1 wt %) without salt (O) and with 0.5 M NaCl (b). Solid and broken lines are fitting curves by a simple core-shell model and Pedersen model, respectively. m:n ) (a) 25: 40, (b) 38:50, (c) 62:40, and (d) 131:54.

obvious that the Pedersen model can reproduce an experimental curve better than the core-shell model including the high-q region. Hydrophobic Chain Length Dependence and Salt Effect. The effect of hydrophobic chain length and salt addition on the micellar size and shape becomes obvious when looking at the SANS data. Figure 2 shows the SANS profiles for 1% solutions of the four polymer samples in D2O with and without 1 M NaCl added. The solid and broken lines in the Figures are the fitting by the core-shell model and the Pedersen model, respectively. Figure 3 shows the SAXS profiles for 1% aqueous solutions of three samples (m:n ) 25:40, 38:50, and 62:40). The structural parameters evaluated in SAXS and SANS data analyses are summarized in Table 2. The fitting parameters were estimated with only reasonable accuracy and were not far from the

9166 Langmuir, Vol. 23, No. 18, 2007

Kaewsaiha et al.

Figure 3. Small-angle X-ray scattering profiles for (Ip-h2)m-b-(SSNa)n (m:n ) 25:40, 38:50, and 62:40) 1 wt % aqueous solutions. Solid lines are fitting curves from a simple core-shell model. Table 2. Structural Parameters for (Ip-h2)m-b-(SSNa)n Micelles in Solution Evaluated by SANS and SAXS m:n 25:40 38:50 62:40 131:54

Cs (M)a shape 0 1.0 0 1.0 0 1.0 0 1.0

sphere sphere sphere sphere rod sphere sphere rod sphere rod sphere rod

φb

RC (Å)c

RS (Å)d

Rg (Å)e

Naggf

1.00 1.00 1.00 0.85 0.15 1.00 0.75 0.25 0.60 0.40 0.20 0.80

31(30) 33 68(68) 70 70 60(55) 64 64 90 75 100 75

69(75) 71 136(135) 125 130 104(100) 90 90 120 120 130 110

14 18 28

35(33) 43 88(91) 155 150* 108(87) 78 92* 156 80* 166 70*

15

a C : salt concentration. b φ: volume fraction of spherical or rod-like S micelles, in parentheses for SAXS data. c RC: core radius. d RS: whole radius. e Rg: radius of gyration of the corona chain. f Nagg: aggregation number (* denotes aggregation number per 100 Å length for rod-like micelles).

hydrodynamic radius measured in our previous work.14 Note that almost the same size parameters were obtained by SAXS and SANS, indicating the reliability of the parameters. The profiles for the m:n ) 25:40 (with and without salt), 38:50, and 62:40 (without salt) polymers are well reproduced by the Pedersen model, which is the core-corona sphere model including blob scattering from the corona region, although a small upturn due to large aggregates is noticed. In the other profiles, we clearly see that forward scattering increases at low q (q < 0.001 Å-1). These increasingly negativeslopes at low q are indicative of a change from a 3D system (small spherical micelles) to 1D geometry (long thin cylinders).53 A detailed model evaluation of the scattering curves implies that they can be described in terms of the coexistence of spherical and rodlike micelles. A mixture of spherical and rodlike polymer micelles was already observed in our previous study.10 This kind of phenomenon was duly studied in lowmolecular-weight surfactant systems54 and the polymer micelle system6b,42 so now it is commonly accepted. Unfortunately, because we could not find any sphere and rod coexistence model including scattering from the corona region, the traditional coreshell model, which cannot reproduce profiles at high q, was adopted. In the data analysis for the rodlike micelles, we used same cylinder radius as spheres, and then we obtained satisfactory agreement. (53) (a) Higgins, J. S.; Benoit, H. C. Polymers and Neutron Scattering; Oxford University Press: NewYork, 1994. (b) Mittelbach, P. Acta Phys. Austriaca 1964, 19, 53. (c) Lindner, P. In Neutrons, X-rays and Light: Scattering Methods Applied to Soft Condensed Matter; Zemb, T., Ed.; Elsevier: Oxford, U.K., 2002; p 23. (54) (a) Zielinski, R.; Ikeda, S.; Nomura, H.; Kato, S. J. Colloid Interface Sci. 1989, 129, 175. (b) Hayashi, S.; Ikeda, S. J. Phys. Chem. 1980, 84, 744. (c) Ikeda, S.; Ozeki, S.; Tsunoda, M. J. Colloid Interface Sci. 1980, 73, 27.

The fraction of cylinders increased strongly with both longer PIp-h2 chain length and salt addition (Table 2). For longer hydrophobic chains, the transition behavior can be qualitatively understood from the concept of the “critical packing parameter” introduced by Israelachvili55 for low-molecular-weight surfactants using simple concepts such as the change in the critical packing parameter V/a0lc, where V is the volume of the hydrophobic tail of the surfactant, lc is the length of the tail, and a0 is the effective headgroup area. Increasing ionic strength reduces the effective area, which results in aggregates with less curvature. The effect of the number of carbons, nc, in the tail is more subtle because both the volume and the tail length are approximately proportional to nc. However, if the hydrophobic chain length keeps increasing in the spherical micelle, then the area occupied by the polymer at the core-shell interface increases, but the area that can be covered by a hydrophilic chain of a certain length is limited. Therefore, the transition to a rod shape will occur when the interfacial area per molecule of the spherical micelle exceeds this limit, so the longer tails will favor rodlike micelles. In other words, the micelles’ size may not increase very much if the transition to rod shape is available. That is why we used the same parameter to analyze SANS data of the sphere-rod coefficient system. In the same way, the packing parameter can be used in the salt-added systems. At high salt concentration when the repulsion between the charged headgroups is reduced, resulting in a reduction of a0, one could imagine similar packing in a rodlike micelle, which has a larger aggregation number with less curvature. For m:n ) 38:50, 62:40, and 131:54, the addition of salt enhanced the sphere-to-rod transition of the micelles’ structure (Figure 2b-d) Salt Concentration Dependence. Figure 4 completes the picture for the growing micellar aggregates of (Ip-h2)62-b-(SSNa)40 by SANS with increasing added salt concentration. The SANS profiles for 1% solutions of (Ip-h2)62-b-(SSNa)40 in D2O at salt concentration from 0 to 1 M NaCl are shown. The solid and broken lines in the Figures are the fittings by the core-shell and Pedersen models, and the structural parameters evaluated are summarized in Table 3. The profiles for 0 to 0.1 M NaCl are well reproduced by the Pedersen model, but the increasingly negative slope in the low-q region was observed at a salt concentration equal to and higher than 0.2 M NaCl. Those profiles were reproduced by the sphere-rod coexistence model. Note that we use the same cylinder radius as for spheres in the data analysis, and as a result, the fraction of cylinders (∼slope) increased with increasing salt concentration. (55) (a) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (b) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992.

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Figure 4. (a) Small-angle neutron scattering profiles for (Ip-h2)62-b-(SSNa)40 D2O solutions containing NaCl at various concentrations. Solid and broken lines are fitting curves from a simple core-shell model and the Pedersen model. (b) Magnification of the plot in part a in a small-q range showing the difference between two fitting models. Table 3. Structural Parameters for (Ip-h2)62-b-(SSNa)40 Micelles in D2O Solution at Various NaCl Concentrations as Evaluated by SANSa m:n

CS (M)

shape

φ

RC (Å)

RS (Å)

Rg (Å)

Nagg

62:40

0 0.01 0.1 0.2

sphere sphere sphere sphere rod sphere rod sphere rod

1.00 1.00 1.00 0.94 0.06 0.80 0.20 0.75 0.25

60 64 64 62 62 62 62 64 64

104 104 104 95 95 90 90 90 90

15 17 18

108 87 87 119 145* 120 145* 78 70*

0.5 1.0 a

Symbols are the same as in Table 2.

Figure 5 shows the salt concentration dependence of the shell thickness of (Ip-h2)62-b-(SSNa)40 micelles. Below 0.2 M NaCl, the thickness is constant, but at higher salt concentration, it decreases markedly as salt concentration increases. In Figure 5, the volume ratio of spherical and rodlike micelles was also plotted as a function of added salt concentration. The fraction of spherical micelles is also constant up to 0.2 M and then decreases. This behavior is similar to that of shell thickness, which suggests that the conformation of the corona chain is an important factor in determining the micellar shape. A similar phenomenon (i.e., the salt effect) occurring above a certain salt concentration was found in the PSSNa brush system.15 We have explained that this concentration (which we

Figure 5. Effect of salt concentration on the sphere/rod volume ratio of the micelle and corona thickness. (Inset) Magnification of the spherical micelle fraction at a higher salt concentration on a double logarithmic scale. The power law is about -1/5.

call the critical salt concentration) is the critical point where the concentration of salt ions in the bulk became higher than that of counterions in the brush layer. In the case of a polyelectrolyte brush in which one end is attached to the surface, the addition of salt beyond the critical concentration causes brush chain shrinkage. However, for a charged micelle system, the addition of salt to a micellar solution leads to both

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(i) an increasing aggregation number in the micelles and (ii) a decrease in the span of the micellar corona chains. When the electrostatic interaction between the polyelectrolyte chains is reduced by the addition of salt, (i) a larger number of chains can be involved in the formation of the micelle, increasing the aggregation number, and (ii) the extension of the corona chains becomes weaker. As shown in Figure 5, the presence of salt induced almost no structural change in polymer micelles below 0.2 M NaCl. As was reported before in the PS-b-PSSNa system,13 the micelle is surrounded by a corona of highly charged polyelectrolytes. Because the ion concentration in the corona region is very high, the small ions added to the bulk solution cannot go into the corona region of the particle, which results in no structural change in the micelles. In addition, it is the same phenomenon as in the PSS brush in the PIp-h2-b-SSNa monolayer at the water surface.14 The small ions in the bulk solution cannot enter the brush, which has a high ion concentration, resulting in no structural change in the PSS brushes. At a higher salt concentration, the small salt ions enter the corona region, there is a rapid decrease in shell thickness, and the number of rodlike micelles (increasing aggregation number in each micelle) increased as a function of salt concentration, which was noted. This phenomenon leads to a rapid decrease in shell thickness The aggregation number of the spherical particles and the aggregation number per unit rod length for rodlike particles were included in Tables 2 and 3. It is interesting that in this study the aggregation number per unit rod length for rod particles varied among different block copolymers (Table 2) but no striking change could be recognized between different salt concentrations (Table 3). The hydrophobic chain length dependence is easily explained by the packing parameter. The longer tail will favor rodlike micelles, so rodlike aggregates started to form and their volume fraction increased by the elongation of the hydrophobic chain. For different salt concentrations, as described in the previous section, the micelle size may not increase very much, and then we can see that core radii in Table 3 also did not change very much with increasing salt concentration. Because the electrostatic interaction was screened, the addition of salt may result in longer rodlike micelle formation. However, we were not able to determine the length distribution of rod because the consideration of the length distribution of the rod caused no change in the SANS profiles in the experimental q range. That is why we showed only the aggregation number per unit rod length in Tables 2 and 3. From the aggregation number, degree of polymerization, and micelle size parameters, the grafting density of PSSNa chain in the corona and the ion concentration in the corona could be calculated to be 0.19 chains/nm2 and 1.5 M, respectively. The ion concentration in the corona is much higher than the critical salt concentration, which suggests a high counterion condensation in the corona region. From a simple calculation using the critical salt concentration by which the external salt ion entered the corona, the degree of counterion condensation in the corona was found to be 87%. The brush layer system in the (Ip-h2)220-b(SSNa)50 monolayer at the air/water interface, which has a similar grafting density, showed a higher counterion concentration (2.04 M) and higher degree of ion condensation (95%).14 This can be explained by the concept that the curvature of the micelle core leads to lower congestion in the corona region and results in a lower degree of counterion condensation. The sphereto-rod transition of the polymer micelle itself may not be a new

Kaewsaiha et al.

discovery, but it plays an important role as compared with other systems such as polymer brushes with the same hydrophilic chain length. In the screening region (salted brush region, concentration higher than the critical salt concentration), the corona thickness decreased with increasing salt concentration. Exponent -1/5 for the corona thickness as a function of salt concentration was found (inset of Figure 5). The exponent close to (although a bit weaker than) -1/5 for the corona versus CS has also been obtained from our previous work on the (Ip-h2)220-b-(SSNa)50 monolayer system,14 by Guenoun et al. for the PtBS-b-PSSNa56 micelle, and by Fo¨rster et al. for the PEE-b-PSSNa micelle.42 Furthermore, it is in good agreement with Zhulina’s model of charged block copolymer micelles in salt-added solution.30 In addition to the characteristic dependence on salt addition, another interest is the unusually high salt resistance of PIp-h2b-PSSNa micelles. As is well known, even 10-3 M salt causes the aggregation of latex particles. Leemans et al. showed that colloidal particles grafted by ionic polymer chains showed a high added salt resistance.57 In our present case, the micelles stably exist even in a 1 M NaCl aqueous solution. The highest salt condition in this study, 1 M, is a very high salt concentration for colloid and micelle stability (seawater is ca. 0.6 M). The strong ionicity of PIp-h2-b-PSSNa micelles is considered to be an important factor in their strong resistance against salt in addition to the steric effect of corona chains. From these results, even though there is little difference in the concrete value, we have demonstrated that the corona region of polyelectrolyte micelles and the polyelectrolyte brush region in a monolayer at the air/water interface showed the same response to salt addition. This fundamental data, because of the inherent difficulty in synthesizing and adjusting the hydrophobicityhydrophilicity balance, has never before been reported, and we believe it will be very useful for future research on and applications of polyelectrolytes.

Conclusions We have presented results of small-angle neutron scattering experiments on aqueous solutions of PIp-h2-b-PSSNa. This polymer is non-surface-active but was found to form micelles composed of PIp-h2 cores with PSSNa coronas. In pure water, the polymer with a short hydrophobic chain length formed spherical micelles whereas rodlike micelles started to form and their volume fraction increased by the elongation of the hydrophobic chain. The effect of salt concentration on the micellar structure was investigated for solutions containing 0 to 1 M NaCl. The micelle size and shape were unaffected by the presence of up to 0.2 M NaCl. This was thought to be due to the very high ionic strength in the corona region, which was regarded as a high-density polyelectrolyte brush on the micelle core particle. The corona thickness was found to decrease beyond 0.2 M NaCl as a function of salt concentration, which can be clearly explained by the screening of the electrostatic interaction by the salt ions. Hence, this concentration can be regarded to be the critical salt concentration for this micelle and is strongly correlated to the effective counterion concentration in the corona. The decrease in corona thickness is also consistent with the observation that the volume fraction of rodlike micelles increased with increasing salt concentration. However, the core radius and the aggregate (56) Guenoun, P.; Davis, H. T.; Tirrel, M.; Mays, J. W. Macromolecules 1996, 29, 3965. (57) Leemans, L.; Fayt, R.; Teyssie´, Ph.; de Jaeger, N. C. Macromolecules 1991, 24, 5922.

Sphere-to-Rod Transition of Copolymer Micelles

number did not show a striking change with salt concentration. The direct observation by AFM was impossible becasue of the low Tg of the hydrophobic core polymer, but a special technique such as cryo-TEM might be possible, which should be the subject of a future study. The micelles were found to be anomalously stable against salt addition, at least up to 1 M NaCl. This is probably due to the electrostatic and steric effects of ionic corona chains around the micelle core. The concept of critical salt concentration and the dependence of PSSNa corona size on salt concentration is similar to what we previously found with a PSSNa brush system in a PIp-h2-b-PSSNa monolayer at the air/

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water interface, and it will be very useful for polyelectrolyte studies. Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan, to whom our sincere gratitude is due. This work was also supported by the Sasagawa Scientific Research Grant from The Japan Science Society and the 21st Century COE Program “COE for a United Approach to New Materials Science:. LA7003672