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Polystyrene/Poly(2-vinylpyridine) Heteroarm Star Copolymer Micelles in Aqueous Media and Onion Type Micelles Stabilized by Diblock Copolymers+,‡ Costantinos Tsitsilianis and Dimitris Voulgaris Department of Chemical Engineering, University of Patras, 26500 Patras, Greece, and Institute of Chemical Engineering and High-Temperature Chemical Processes, ICE/HT-FORTH, P.O. Box 1414, 26500 Patras, Greece
Miroslav Sˇ teˇpa´nek, Kla´ra Podha´jecka´, and Karel Procha´zka* Department of Physical and Macromolecular Chemistry & Laboratory of Specialty Polymers, School of Science, Charles University in Prague, Albertov 2030, 128 40 Prague 2, Czech Republic
Zdeneˇk Tuzar Prague Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Square 2, 162 06 Prague 6, Czech Republic
Wyn Brown Department of Physical Chemistry, University of Uppsala, Box 532, 751 21 Uppsala, Sweden Received February 7, 2000. In Final Form: May 17, 2000 Micellization of polystyrene/poly(2-vinylpyridine) heteroarm star copolymers, PSnPVPn, in aqueous media has been studied by a combination of light scattering and fluorescence techniques. Three copolymer samples differing in numbers of arms and their length form micelles with compact hydrophobic cores and polyelectrolyte poly(2-vinylpyridine) shells. The association number decreases with increasing number of water-soluble arms and their length. Micellization equilibrium is kinetically frozen in aqueous media. The micellar cores are frozen and the behavior of the micellar solutions is controlled by the polyelectrolyte behavior of the polyelectrolyte shell. In the case of the present heteroarm stars with ca. 101 arms, the soluble and insoluble arms are strongly segregated and the micellar cores are formed of pure polystyrene as with micelles formed by linear diblock copolymers. As concerns the sample with several tens of arms, a significant steric hindrance, together with a low association number (ca. 4), does not allow for a complete segregation of polystyrene and the poly(2-vinylpyridine) arms. The micellar cores formed by the very hairy star copolymer with several tens of arms contain a fraction of poly(2-vinylpyridine) arms as shown by fluorescence measurements with solubilized pyrene. Alkalimetric titration of an acidic mixture of PSnPVPn micelles with a linear poly(2-vinylpyridine)-block-poly(ethylene oxide) copolymer, PVP-PEO, yields fairly monodisperse spherical onion-skin micelles. The onion-skin micelle has a compact spherical PS core deriving from the parent PSnPVPn micelle, a fairly thin and compact middle layer formed by PVP both from the parent micelle and PVP-PEO copolymer, and a protective PEO shell. The formation of onion-skin micelles is fully reversible and occurs suddenly and very rapidly at pH values higher than 4.8. The “onions” dissociate immediately into PSnPVPn micelles and PVP-PEO copolymer below pH 4.8. Evaluation of the alkalimetric titration data shows that only ca. 40% of the PVP units in the micellar shells are protonized (even in strongly acidic solutions that contain a large surplus of a strong acid, e.g., in 0.1 M HCl). This observation is in agreement with indirect fluorometric data which indicate that in the case of a weak polyelectrolyte, such as poly(2-vinylpyridine), the inner layer of the polyelectrolyte shell close to the core is not ionized.
Introduction Association of block copolymers in selective solvents (solvent for one type of block and a nonsolvent for the other type of block) may produce various structures ranging from nano- to microscopic dimensions. Diblocks, * To whom correspondence should be addressed. + The study is a part of the long-term Research Program of the School of Science of the Charles University in Prague, CEZ: J13/ 98: 113100001. ‡ Dedication: This paper is dedicated to Prof. Dr. Pavel Kratochvı´l, D.Sc., a recently retired Director of the Prague Institute of Macromolecular Chemistry of the Academy of Sciences of the Czech Republic, and one of the leading and world-recognized Czech polymer scientists, on the occasion of his 70th birthday.
AB, and triblocks, ABA, form fairly small spherical micelles with compact cores (B) and diffuse protective shells (A) in selective solvents for block A. Their hydrodynamic radii range usually from 20 to 50 nm. Triblocks, ABA (and higher multiblocks, ABAB, etc.), form flowerlike micelles with cores of block A in dilute solution and physical networks at higher concentration if they are dissolved in selective solvents for B. While micellization of diblock and triblock copolymers has been intensively studied both in selective organic solvents1 (the literature is so extensive that it is futile to give all relevant references) and aqueous media,2,3 micellization and related association phenomena in solutions of star-shaped copolymers have been studied only little.4
10.1021/la000176e CCC: $19.00 © 2000 American Chemical Society Published on Web 07/22/2000
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Table 1. Molecular Characteristics of PSnPVPn Heteroarm Star Copolymersa sample
10-5 × Mw b
10-4 × (Mw)core c
10-4 × (Mw)PS d
10-4 × (Mw)PVP e
wPVP, %f
ng
S1 S2 S3
5.07 (5.44) 3.30 (3.35) 4.25 (5.29)
1.6 0.75 2.3
2.00 2.45 0.30
5.65 2.90 2.28
66.0 47.7 84.5
6.9 6.1 19.6
a Measured in tetrahydrofuran and 1,4-dioxane (values in brackets). Details on characterization in ref 4c. b Weight-average molar mass, measured by SLS. c Weight-average molar mass of PDVB core, calculated from [DVB] per “living ends” (PS arms) molar ratio. d Weightaverage molar mass of PS arm, measured by SLS. e Weight-average molar mass of PVP arm, (Mw)PVP ) [Mw - (Mw)PSn]/n. f Weight fraction of PVP, measured by NMR. g Average number of PS or PVP arms, n ) [(Mw)PSn - (Mw)core]/(Mw)PS.
It has been found that the association number is considerably lower for heteroarm star copolymers containing an equal number of insoluble (A) and soluble (B) branches, type AnBn. The micellization equilibrium is shifted less in favor of micelles as compared with linear diblocks, AB, and triblocks, ABA. In contrast to diblock copolymers, the critical micelle concentration of heteroarm star copolymer micelles is several orders of magnitude higher even for high-molar mass samples (Mw ca. 104 - 105) and therefore is easily measurable, e.g., by static light scattering.5 (1) (a) Tuzar, Z.; Kratochvı´l, P. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15, p 1. (b) Riess, G.; Huertez, G.; Bahadur, P. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H. F., Bikales, N. M.; Overberger, C. G., Mengez, G., Eds.; Wiley: New York 1985; Vol. 2, p 324. (c) Webber, S. E., Munk, P., Tuzar, Z., Eds. Solvents and Self-Organization of Polymers; NATO ASI Series; Kluwer Academic Publishers: Dordrecht, 1996. (d) Gallot, Y.; Franta, E.; Rempp, P.; Benoit, H. J. Polym. Sci. C 1963, 4, 473. (e) Krause, S. J. Phys. Chem. 1964, 68, 1948. (f) Tuzar, Z.; Kratochvı´l, P. Makromol. Chem. 1972, 160, 301. (g) Price, C.; McAdam, J. D. G.; Lally, T. P.; Woods, D. Polymer 1974, 15, 228. (h) Krause, S.; Reismu¨ller, P. A. J. Polym. Sci., Polym. Phys. 1975, 13, 663. (j) Booth, C.; Naylor, T. D.; Price, C.; Rajab, N. S.; Stubbersfield, R. B. J. Chem. Soc., Faraday Trans. 1978, 74, 2352. (k) Procha´zka, K.; Baloch, M. K.; Tuzar, Z. Makromol. Chem. 1979, 180, 2521. (l) Price, C.; Hudd, A. L.; Booth, C.; Wright, B. Polymer 1982, 23, 650. (m) Tuzar, Z.; Stehlı´cˇek, J.; Konˇa´k, C.; Lednicky´, F. Makromol. Chem. 1988, 189, 221. (n) Plesˇtil, J.; Hlavata, D.; Hrouz, J.; Tuzar, Z. Polymer 1990, 31, 2112. (o) Procha´zka, K.; Bedna´rˇ, B.; Svoboda, P.; Trneˇna´, J.; Mukhtar, E.; Almgren, M. J. Phys. Chem. 1991, 95, 4563. (p) Gast, A. P. Langmuir 1996, 12, 4060. (2) (a) Riess, G.; Rogez, D. Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem. 1982, 23, 19. (b) Wilhelm, M.; Zhao, C.-L.; Wang, Y.; Xu, R.; Winnik, M. A.; Mura, J.-L.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 1033. (c) Tuzar, Z.; Webber, S. E.; Ramireddy, C.; Munk, P. Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem. 1991, 32, 525. (d) Procha´zka, K.; Kiserow, D.; Ramireddy, C.; Tuzar, Z.; Munk, P.; Webber, S. E. Macromolecules 1992, 25, 454. (e) Kiserow, D.; Procha´zka, K.; Ramireddy, C.; Tuzar, Z.; Munk, P.; Webber, S. E. Macromolecules 1992, 25, 461. (f) Ramireddy, C.; Tuzar, Z.; Procha´zka, K.; Webber, S. E.; Munk, P. Macromolecules 1992, 25, 2541. (g) Tian, M.; Quin, A.; Ramireddy, C.; Webber, S. E.; Munk, P.; Tuzar, Z.; Procha´zka, K. Langmuir 1993, 9, 1741. (h) Mortensen, K.; Brown, W. Macromolecules 1993, 26, 4128. (i) Astafieva, I.; Zhong, X. F.; Eisenberg, A. Macromolecules 1993, 26, 7339. (j) Astafieva, I.; Khougaz, K.; Eisenberg, A. Macromolecules 1995, 28, 7127. (k) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (l) Baines, F. L.; Armes, S. P.; Willingham, M. C.; Tuzar, Z. Macromolecules 1996, 29, 8151. (m) Krˇ´ızˇ, J.; Plesˇtil, J.; Tuzar, Z.; Pospı´sˇil, H.; Brus, J.; Jakesˇ, J.; Masarˇ, B.; Vlcˇek, P.; Doskocˇilova´, D. Macromolecules 1999, 32, 397. (n) Sˇ teˇpa´nek, M.; Podha´jecka´, K.; Procha´zka, K.; Teng, Y.; Webber, S. E. Langmuir 1999, 15, 4185. (3) (a) Procha´zka, K.; Martin, T. J.; Munk, P.; Webber, S. E. Macromolecules 1996, 29, 6518. (b) Procha´zka, K.; Martin, T. J.; Webber, S. E.; Munk, P. Macromolecules 1996, 29, 6526. (c) Sˇ teˇpa´nek, M.; Krijtova´, K.; Procha´zka, K.; Teng Y.; Webber, S. E.; Munk, P. Acta Polymer. 1998, 49, 96. (d) Sˇ teˇpa´nek, M.; Krijtova´, K.; Limpouchova´, Z.; Procha´zka, K.; Teng, Y. Webber, S. E. Acta Polymer 1998, 49, 103. (e) Teng, Y.; Morrison, M.; Munk, P.; Webber, S. E.; Procha´zka, K. Macromolecules 1998, 31, 3578. (f) Talingting, M. R.; Munk, P.; Webber, S. E.; Tuzar, Z. Macromolecules 1999, 32, 1593. Plesˇtil, J.; Krˇ´ızˇ, J.; Tuzar, Z.; Procha´zka, K.; Melnichenko, Y. B.; Wignall, G. D.; Talingting, M. R.; Munk, P.; Webber, S. E. Macromol. Chem. Phys. 2000. In press. (4) (a) Tsitsilianis, C.; Kouli, O. Makromol. Rapid. Commun. 1995, 16, 591. (b) Tsitsilianis, C.; Papanagopoulos, D.; Lutz, P. Polymer 1995, 36, 3745. (c) Tsitsilianis, C.; Voulgaris D. Macromol. Chem. Phys. 1997, 198, 997 and references therein. (d) Voulgaris, D.; Tsitsilianis, C.; Esselink, F. J.; Hadziioannou, G. Polymer 1998, 39, 6429. (e) Pispas, S.; Poulos, Y.; Hadjichristidis, N. Macromolecules 1998, 31, 4181. (f) Voulgaris D.; Tsitsilianis, C.; Grayer, V.; Esselink, F. J.; Hadziioannou, G. Polymer 1999, 40, 5879.
Ultracentrifugation experiments for a polyisoprenepolystyrene star-block copolymer, (PI-PB)4, i.e., for a star containing four polyisoprene-block-polystyrene branches joined together at the PI ends, have shown that the polymer chains exchange fairly rapidly between micelles and the nonmicellized stars (unimers) in organic selective solvents for PS.5 The above-mentioned findings are not surprising: the starlike unimer contains several stabilizing branches and a partial collapse of the insoluble blocks together with a slight stretching of the soluble ones may result in a conformation that is similar to a small multimolecular micelle. A series of polystyrene/poly(2-vinylpyridine) heteroarm star copolymers made up of a very dense poly(divinylbenzene) core, bearing a number of polystyrene branches and the same number of 2-vinylpyridine branches, PSnPVPn, have been recently synthesized and characterized at the University of Patras.4d,f The prepared samples differ not only in the number, but also in the length of the PS and PVP branches. The micellization behavior of these samples has been studied in toluene which is a selective solvent for PS as described in ref 4d,f. Nonprotonated poly(2-vinylpyridine) behaves as a typical hydrophobic polymer. It dissolves in organic solvents and is insoluble in water at pH higher than 4.8. However, in acid solutions (pH < 4.8), PVP is protonized and therefore becomes watersoluble, and behaves as a weak polyelectrolyte. We studied the micellization of linear block copolymers containing a long and strongly hydrophobic block, such as polystyrene or poly(tert-butyl acrylate) (PBA), and poly(2-vinylpyridine) in detail few years ago.3 We have also studied the onion-skin micelles that are formed during the alkalimetric titration of acidic mixtures of PBA-PVP or PSnPVPn micelles and linear poly(2-vinylpyridine)-block-poly(ethylene oxide) diblock samples.3b,f In this work, we extend our study to polyelectrolyte and onion-skin micelles formed by the novel PSnPVPn type of star-shaped samples in aqueous media. Experimental Section Copolymer Samples. The PSnPVPn star copolymers were prepared via anionic polymerization under an argon atmosphere using THF as solvent. A three-step sequential “living” copolymerization procedure was used.4c In the first step, the PS arms are synthesized a number of which are joined together in the second step by reacting the living PS chains with a small amount of divinylbenzene. A star-shaped polystyrene (PSn) is thus formed, bearing a number of active sites at the poly(divinylbenzene) (PDVB) core which is equal to the number of the attached PS arms. In the third step, a second generation of PVP arms grows from the cores on adding vinylpyridine (VP) to the reaction medium. All the samples are free from PS precursor residuals and have been characterized by static light scattering (SLS), 1H NMR, and gel permeation chromatography (GPC). Their molecular characteristics are collected in Table 1. Details of the synthesis and the characterization are reported elsewhere.4c (5) Procha´zka, K.; Glo¨ckner, G.; Hoff, M.; Tuzar, Z. Makromol. Chem. 1984, 183, 2521.
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Polymeric Micelles. Polymeric micelles containing PS cores and protonized PVP shells were prepared by stepwise dialysis as described in the previous papers.3 Samples were dissolved in an organic solvent mixture containing 70 vol % of 1,4-dioxane and 30 vol % of methanol. This mixture is a good solvent for all star copolymers studied and the samples dissolve as individual macromolecules. After full dissolution of the sample (we stir the solution overnight to ensure for a complete dissolution), methanol is slowly added up to 50 vol % under vigorous stirring to slowly decrease the solvent quality for PS. Then 0.1 M HCl is added, extremely slowly under stirring, until the total content of water is ca. 50 vol %. Micelles first appear in mixtures containing 2030 vol % of water, depending on the composition of the sample. A typical bluish tint due to intensive light scattering (observable by the naked eye) is a reliable and very sensitive indication of the first appearance of multimolecular micelles. The micellar solution is dialyzed against a solvent mixture containing 75 vol % of 0.1 M HCl and finally against aqueous 0.1 M HCl. The volume increases during dialysis to the aqueous medium by ca. 100%. The preparation procedure yields aqueous solutions of fairly monodisperse PSnPVPn micelles (containing about 3 mg/mL of the sample) with a fairly high reproducibility. Onion-Skin Micelles. Onion-skin micelles containing a compact inner PS core, a compact middle layer formed by collapsed and nonprotonized PVP chains and a protective watersoluble PEO shell were prepared by alkalimetric titration of mixtures of acidic PSnPVPn micelles with molecularly dissolved PVP-PEO linear copolymer. Titration was monitored potentiometrically. Formation of onion-skin micelles is accompanied by a sudden increase in pH and the intensity of scattered light (a sudden “deepening” of the bluish tint is well-observable by the naked eye). Further details of the preparation are given and discussed in the Results and Discussion section. Static Light Scattering. Molar masses of copolymer samples and polymeric micelles were measured using a photogoniodiffusiometer Sofica 42 000 equipped with a laser-light source. Experimental data were treated by the conventional Zimm technique. Values of the refractive index increment for copolymers in 1,4-dioxane were measured using a differential refractometer Brice-Phoenix V 5000 and for the aqueous solutions were calculated as the average values of increments for pure homopolymers weighted by the copolymer composition and compared with experimental values measured for the acidic micellar solutions.3a Details of the experimental setup, preparation and optical cleaning of solutions, measurement, and the data treatment are given elsewhere.5 Quasielastic Light Scattering. The light-scattering setup consists, as described previously,6a of a 488 nm Ar ion laser light source. The detector optics are coupled via a monomodal fiber to an ITT FW 130 photomultiplier. The ALV-PM-PD amplifierdiscriminator was connected to an ALV-5000 autocorrelator/ computer. The cylindrical scattering cells were sealed after filtration of solutions through 0.22 µm Millipore filters and immersed in a large-diameter thermostated bath containing Decalin placed at the axis of the goniometer. Measurements were made at different angles, sample concentrations and temperatures. Analysis of the data was performed by fitting the experimentally measured g2(t), the normalized intensity autocorrelation function, which is related to the electrical field correlation function g1(t) by the Siegert relation6b
g2(t) - 1 ) β|g1(t)|2
(1)
where β is a factor accounting for deviation from ideal correlation. For polydisperse samples, g1(t) can be written as the inverse Laplace transform (ILT) of the relaxation time distribution τA(τ):
g1(t) )
∫τA(τ) exp(-t/τ)d ln τ
(2)
where t is the lag time. The relaxation time distribution τA(τ) is obtained by performing the inverse Laplace transform (ILT) with (6) (a) Schille´n, K.; Brown, W.; Johnsen, R. M. Macromolecules 1988, 27, 4825. (b) Chu, B. Laser Light Scattering, 2nd ed.; Academic Press: New York, 1991. (c) Jakesˇ, J. Czech. J. Phys. 1988, B38, 1305.
the aid of a constrained regularization algorithm (REPES),6c which minimizes the sum of the squared differences between the experimental and calculated g2(t). The mean diffusion coefficient D is calculated from the second moments of the peaks as D ) Γ/q2, where q ) (4πno/λ) sinθ/2 is the magnitude of the scattering vector and Γ ) 1/τ is the relaxation rate. Here θ is the scattering angle, n0 the refractive index of pure solvent, and λ the wavelength of the incident light. Within the dilute regime, D varies linearly with the polymer concentration C, i.e.,
D ) D0(1 + kDC)
(3)
where D0 is the diffusion coefficient at infinite dilution, kD is the hydrodynamic “virial” coefficient related to the solute-solute, and solute-solvent interactions. The Stokes-Einstein equation relates the infinite dilution diffusion coefficient to the hydrodynamic radius (RH):
D0 ) kBT/6πηoRH
(4)
where kBT is the thermal energy factor and η0 is the temperaturedependent viscosity of the solvent. Steady-State Fluorometry. Steady-state fluorescence spectra were recorded with a SPEX Fluorolog 3 fluorometer. Time-Resolved Fluorometry. The time-correlated single photon counting technique was used for measurements of fluorescence lifetimes. The time-resolved fluorescence decays were recorded on a ED 299 T time-resolved fluorometer, Edinburgh Instruments, Inc., equipped with a nanosecond coaxial discharge lamp filled with hydrogen at 0.5 atm (half-width of the pulse ca. 1.2 ns).3c The apparatus allows for a multiplexing regime of the simultaneous acquisition of fluorescence and excitation profiles (SAFE). A reconvolution procedure was used to derive the true fluorescence decays that were further fitted to multiexponential functions using the Marquardt-Levenberg nonlinear least-squares method. Low values of the χ2 and random distributions of residuals were used as criteria of the fit. Potentiometric Measurements. The pH measurements were performed using a PHM 93 reference pH meter, Radiometer, Denmark, equipped with a combined glass microelectrode PHC 2406.
Results and Discussion Characterization of Polyelectrolyte Samples and Polyelectrolyte Micelles. In this work, we have studied two polystyrene-poly(2-vinylpyridine) samples containing 6 to 7 PS and the same number of PVP branches. The length of the PS branches is similar in both samples. The length of the PVP branches is significantly longer than that of PS in the first sample S1 and almost equal to that of PS in the second sample S2. These two samples represent typical examples of star copolymers with a fairly high number of branches of two chemically different types. To get a broader knowledge of the micellization of starlike copolymer samples, we have made further measurements on a highly branched sample, S3, that contains ca. 20 PS and 20 PVP blocks. The third sample represents a “very hairy star copolymer” that may behave differently from other star-shaped copolymers. Knowledge of the behavior of this sample is very important since it represents the limiting type of micellization behavior of an extremely highly branched micellizing copolymer which, in its unimeric form, mimics a micelle. The molar mass and composition of samples were determined in THF by static light scattering and NMR and have been reported earlier.4c In this work, we have remeasured only the weight-average molar masses in 1,4-dioxane using static light scattering in order to rule out any sample change over a long time scale (i.e., possible aging of the samples) that could have caused some irreversible cross-linking and prevented complete dissolution of the sample in the form of individual
Heteroarm Star Copolymer Micelles in Aqueous Media Table 2. Molecular Characteristics of Polyelectrolyte Micelles Based on Heteroarm Star Copolymersa sample S1 S2 S3 “tetramers” S3 “unimers”
RG, RH, A2, 10-6 × Mw b nav c nsfa d nme nmf mol mL/g2 g 4.11 9.53 1.43 0.42
8.2 56 45 29 200 34 4 80 39h 1 20
