Langmuir 2008, 24, 13863-13865
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Micelles of a Diblock Copolymer of Styrene and Ethylene Oxide in Mixtures of 2,6-Lutidine and Water Z. Tuzar, P. Kadlec, P. Sˇteˇpa´nek,* and J. Krˇízˇ Institute of Macromolecular Chemistry, HeyroVsky´ Sq. 2, 16206 Prague 6, Czech Republic
F. Nallet CNRS, Centre de Recherche Paul-Pascal, 115 AVenue du Docteur-Schweitzer, 33600 Pessac, France
L. Noirez Laboratoire Léon Brillouin, CEA Saclay, 91191 Gif-sur-YVette, France ReceiVed October 14, 2008. ReVised Manuscript ReceiVed NoVember 5, 2008 We studied the micelle formation of a diblock copolymer of styrene and ethylene oxide in mixtures of 2,6dimethylpyridine (2,6-lutidine) and water. Micelles are formed in a broad solvent composition range with a volume fraction of water ranging from 0.05 to 0.85, where neither polystyrene nor polyethylene oxide homopolymers are soluble. The diffusion behavior of pure solvent mixtures and in solutions of copolymer micelles is reported. In LTD/water mixtures, two diffusive processes corresponding to self-difusion and two modes belonging to mutual diffusion and diffusion of solvent clusters have been found. In copolymer solutions, the mode of copolymer micelle diffusion replaces the mode of solvent cluster diffusion. Quasielastic light scattering, small-angle neutron scattering, and pulsed-field gradient NMR have been employed in our study.
Introduction Block copolymers in selective solvents (i.e., solvents that are thermodynamically good for one block and poor for the other) self-assemble into multimolecular micelles, the cores of which are formed by insoluble blocks and the shells by soluble blocks. During the last 50 years, hundreds of experimental and theoretical studies1-5 have described the detailed structure of copolymer micelles, and their dynamics, kinetics, transport properties, higher organization, etc. From an experimental point of view, mixed solvents are frequently employed because their thermodynamic quality is readily tunable and micelles with the required structure and properties can be prepared. In our present study, we describe the micelle formation of polystyrene-b-poly(ethylene oxide) (PS-PEO) in mixtures of lutidine (2,6-dimethylpyridine) and water (LTD/water). In a wide range of solvent composition, neither PS nor PEO homopolymers are soluble, but their diblock copolymer dissolves easily and forms micelles with a narrow size distribution.
Experimental Section Materials. Copolymer Sample. Polystyrene-b-poly(ethylene oxide) (PS-PEO) was synthesized using the living anionic copolymerization technique and was employed in our previous work.6 The weight-average molecular weight is 2.1 × 104, and the weight fraction of PS is 0.46. SolVents. Lutidine (2,6-dimethylpyridine, p.a.) and dimethylformamide (DMF) were purchased from Sigma-Aldrich, and deuterated water was purchased from Chemotrade GmbH, Leipzig. * Corresponding author. E-mail:
[email protected]. (1) Tuzar, Z.; Kratochvíl, P. AdV. Colloid Interface Sci. 1976, 6, 201. (2) Krause, S. J. Phys. Chem. 1964, 68, 1948. (3) Halperin, A.; Tirrell, M.; Lodge, T. P. AdV. Polym. Sci. 1992, 100, 31. (4) Tuzar, Z.; Kratochvı´l, P. Surf. Colloid Sci. Ser. 1993, 15, 1. (5) Letchford, K.; Burt, H. Eur. J. Pharm. Biopharm. 2007, 65, 259. (6) Sˇteˇpa´nek, M.; Podha´jecka´, K.; Tesarˇova, E.; Procha´zka, K.; Tuzar, Z.; Brown, W. Langmuir 2001, 17, 4240.
Experimental Techniques. Quasi-elastic Light Scattering (QELS). The commercial instrument ALV (Langen, Germany) consists of a CGE photogoniometer equipped with a Uniphase 22 mW HeNe laser, an ALV6010 correlator, and a pair of avalanche photodiodes operated in pseudo-cross-correlation mode using an optical fiber splitter. The instrument and data treatment are described in more detail elsewhere.7 The distributions of relaxation times τ, calculated from the measured correlation functions using the program REPES,8 are shown in this study in the equal area representation9 τA(τ) versus log(τ). The relaxation time τ is related to the diffusion coefficient D by the relation D ) (1/τq2), where q is the scattering vector q ) (4πn/λ)sin(θ/2) with n being the refractive index of the solvent mixture, λ being the wavelength of light, and θ being the scattering angle. The hydrodynamic radius of the copolymer particles RH was calculated from the Stokes-Einstein relation RH ) kBT/6πηoD, where kB is the Boltzmann constant, T is the absolute temperature, and ηo is the viscosity of the solvent mixture. When fluctuations in solvent composition are observed, the Stokes-Einstein relation yields their hydrodynamic correlation length10 ξ instead of RH. Small-Angle Neutron Scattering (SANS). The experiment was performed at the Laboratoire Leon-Brillouin at CEA-Saclay on the spectrometer PAXY.11,12 Measurements were performed with a 128 × 128 multidetector (pixel size 0.5 × 0.5 cm2) using a nonpolarized, monochromatic incident neutron beam collimated with circular apertures at a sample-to-detector distance of 7 m (with wavelength λ ) 8 Å). The 2D scattering patterns were isotropic so that they were azimuthally averaged to yield the dependence of the scattered intensity I(q) on the scattering vector q. (7) Sˇteˇpa´nek, P.; Tuzar, Z.; Kadlec, P.; Krˇízˇ, J. Macromolecules 2007, 40, 2165. (8) Jakesˇ, J. Collect. Czech. Chem. Commun. 1995, 60, 1781. (9) Sˇteˇpa´nek, P. In Dynamic Light Scattering: The Method and Some Applications; Brown, W., Ed.; Oxford Science Publications: Oxford, U.K.,1993. (10) Sˇteˇpa´nek, P.; Lodge, T. P.; Kedrowski, C.; Bates, F. S. J. Chem. Phys. 1991, 94, 8289. (11) Sˇteˇpa´nek, P.; Tuzar, Z.; Nallet, F.; Noirez, L. Macromolecules 2005, 38, 3426. (12) http://www-llb.cea.fr/spectros/spectro/paxy.html.
10.1021/la803397g CCC: $40.75 2008 American Chemical Society Published on Web 11/14/2008
13864 Langmuir, Vol. 24, No. 24, 2008
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Pulsed-Field-Gradient NMR. Self-diffusion PFG NMR experiments were measured with an upgraded Bruker Avance DPX300 spectrometer using a z-gradient inverse-detection probe connected to a BGU2 gradient unit. The instrument and the determination of the self-diffusion coefficient of the mixed-solvent components are described in ref 7. All QELS, SANS, and NMR measurements were performed at 25 °C.
Results and Discussion Diffusion Processes in Binary Mixtures of Neat Solvents. Two main diffusion processes can be expected13,14 in binary solvent mixtures: the self-diffusion of solvent molecules (Ds ≈ 10-9 m2/s) and mutual diffusion (Dm ≈ 10-9-10-10 m2/s). Many calculations have been made that relate self-diffusion and mutual diffusion to thermodynamic parameters of the system,14-17 but successful comparison to experimental data is usually obtained only in the case of thermodynamically ideal mixtures. Experimental data for nonideal systems (such as those containing an associating component, e.g., alcohol) are not easy to fit with theoretical models. Strongly nonideal behavior has been described for many mixtures (e.g., ethyl acetate/cyclohexane,13 methanol/ benzene,18 ethanol/benzene,17 and ethanol/carbon disulphide.19 For such mixtures, a pronounced minimum is observed in the composition dependence of Dm. When the lower or upper critical solution temperature LCST or UCST is accessible, Dm reflects the critical fluctuation behavior in the vicinity of these temperatures.20,21 An LTD/water mixture has been thoroughly studied, especially in the vicinity of the LCST. The critical point is localized by a temperature Tc of around 33 °C and a volume fraction of water φw of around 0.7. The literature data for these parameters exhibit small variations.22,23 In this study, all measurements are performed at 25 °C, safely below the coexistence curve (i.e., in the onephase region of the phase diagram). A large difference in the refractive indices of LTD (n ) 1.50) and water (n ) 1.33) enabled the measurement of mutual diffusion both in pure LTD/water and in the presence of PS-PEO by QELS. Diffusion coefficients in pure LTD/water without the copolymer as measured by QELS and NMR are shown in Figure 1. For the QELS experiments, all of the observed processes are diffusive (i.e., their relaxation rate is Γ ) Dq2 as established by measurements at several scattering angles). Curves 1-3 in Figure 1 belong to self-diffusion coefficients of neat solvents and to the mutual diffusion coefficients of solvent mixtures. All three curves show pronounced minima indicating nonideal behavior, as for the reported systems containing an associating (clustering) component.13,15 It can be assumed that the existence of clusters (never quantitatively reported before) is also responsible for the given behavior in our system. The slow diffusion process observed in our QELS measurements (see the lowest curve in Figure 1 and the slow mode in Figure 2a) confirms the existence of these clusters; the cluster diffusion Dcl is two orders of magnitude slower than mutual diffusion. By applying the Stokes-Einstein relation to this cluster component, we obtain a typical cluster size of about 90 nm. (13) Gulari, E.; Brown, R. J.; Pings, C. J. AIChE J. 1973, 19, 1196. (14) Aminabhavi, T. M.; Munk, P. J. Phys. Chem. 1980, 84, 442. (15) Li, J.; Liu, H.; Hu, Y. Fluid Phase Equilib. 2001, 187, 193. (16) Sanni, S. A.; Hutchison, P. J. Chem. Eng. Data 1973, 18, 317. (17) McCall, D. W.; Douglass, D. C. J. Phys. Chem. 1967, 71, 987. (18) Krahn, W.; Schweger, G.; Lucas, K. J. Phys. Chem. 1983, 87, 4515. (19) McKeigue, K.; Gulari, E. J. Phys. Chem. 1984, 88, 3472. (20) Park, I. H. Polymer 1999, 40, 2003. (21) Park, I. H.; Kim, M. J. Macromolecules 1997, 30, 3849. (22) To, K.; Choi, H. J. Phys. ReV. Lett. 1998, 80, 536. (23) Grattoni, C. A.; Dawe, R. A.; Seah, C. Y.; Gray, J. D. J. Chem. Eng. Data 1993, 38, 516.
Figure 1. Self-diffusion coefficients (Ds from PFG NMR) of water (1) and LTD (2), mutual diffusion (3) (Dm from QELS), and the diffusion coefficient of solvent clusters (4) (Dcl from QELS) as a function of the volume fraction of water φw in the mixed solvent LTD/water. Data in the composition range of φw ) 0.20-0.80 were obtained at θ ) 90°, and data below φw ) 0.20 and above φw ) 0.80 were obtained at θ ) 30°.
Figure 2. Distribution of relaxation times τ for four mixtures of LTD/ water (a) and solutions of the PS-PEO sample (c ) 5 × 10-3 g/ml) (b). The curves are labeled by φw. The angle of measurement is θ ) 90°. Peaks are identified by labels corresponding to mutual diffusion (τm), copolymer micelle diffusion (τcop), and solvent cluster diffusion (τcl).
Diffusion Processes in Polymer Solutions of PS-PEO in LTD/Water Mixtures. The PS homopolymer is soluble in neat LTD and in aqueous mixtures with compositions of up to φw ) 0.05. The PEO homopolymer is soluble in the range of φw ) 0.83-1. In the composition range of φw ) 0.05-0.83, both homopolymers are insoluble. QELS measurements of four LTD/water mixtures and for the corresponding solutions of PS-PEO copolymer (c ) 5 × 10-3 g/mL) are shown in Figure 2a,b (mixtures with smaller and higher water content are not shown because the amplitude of the mutual diffusion mode is very small at θ ) 90°). Distributions of relaxation times for the PS-PEO solutions in LTD/water mixtures with φw ) 0.2-0.95 and at copolymer
Letters
Figure 3. Hydrodynamic radius RH (b) and intensity of scattered light (O) of PS-PEO in LTD/water mixtures with φw ) 0.2-0.95, c ) 5 × 10-3 g/mL, T ) 25 °C, and θ ) 90°.
concentrations of 1 × 10-3 to 2 × 10-2 g/mL (well below the polymer overlap concentration in neutral solvent, c* ≈ 0.15 g/mL) show the presence of particles with a narrow size distribution. The data show two modes with relaxation times τm corresponding to the mutual diffusion coefficient Dm and a new one with a relaxation time τcop belonging to copolymer particles. The first mode describes fluctuations in solvent composition usually characterized by the dynamic quantity Dm; the spatial extent of the fluctuations can be characterized10 by a correlation length ξ obtained from the Stokes-Einstein equation. For our data, the resulting correlation length ξ is below 2 nm assuming that the macroscopic viscosity is applicable on this length scale. Our measurements show that whereas Dm and Ds (NMR data in the presence of the copolymer are identical to curves 1 and 2 in Figure 1 and not shown here) are not influenced by the presence of the copolymer the solvent cluster mode disappears. Although we cannot explain this disappearance theoretically, it must be related to the presence of the copolymer in the mixture. Figure 2a shows that the amplitudes of modes (relative areas below the curves) belonging to mutual diffusion and cluster diffusion are comparable in the four mixtures. After the addition of the copolymer (Figure 2b), the relaxation time of mutual diffusion τm is unchanged, but its relative amplitude is smaller. The slower mode in Figure 2b belongs to polymeric particles. Its relaxation time (corresponding to the dimension of the particles) is smaller by almost an order of magnitude compared to that of the cluster mode in Figure 2a. Values of the hydrodynamic radius RH of the copolymer particles, obtained from τcop, are plotted together with the integral intensities of scattered light in Figure 3. Values of composition-dependent solvent viscosities and refractive indices were taken from ref 23. The dramatic increase by two orders of magnitude of the intensities of scattered light at concentrations above φw ) 0.6 can be explained by the increasing contrast between the refractive index of the copolymer, ca. n ) 1.47, and that of a given solvent mixture varying from n ) 1.47 at φw ) 0.25 to n ) 1.34 at φw ) 0.95. (The contrast is essentially given by the square of the refractive index increment, (dn/dc)2, and that in the zeroth-order approximation is proportional to the difference in these refractive indices.) The refractive index increment is strongly influenced by the selective sorption, as we have shown previously.24 Surprisingly, RH changes very little in this broad composition interval, increasing from 20 nm at φw ) 0.20 to 27 nm at φw ) 0.95. This indicates the similar nature of the copolymer particles. (24) Tuzar, Z.; Kratochvı´l, P. Macromolecules 1977, 10, 1108.
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Figure 4. SANS scattering curve of the solution of PS-PEO (c ) 2 × 10-2 g/mL) in an LTD/D2O mixture (φD2O ) 0.85).
Because the RH values are much higher than the RH of molecularly dissolved PS-PEO in a good solvent (RH ) 3.8 nm in DMF as measured by QELS), we assume that the polymeric particles are multimolecular associates, most probably micelles. Whereas in mixtures with higher water content we intuitively expect micelles with a PEO shell and a PS core preferentially swollen with LTD, in mixtures poor in water inverse micelles can be expected with PEO cores preferentially swollen with water. The nature of the PS-PEO particles in the mixed solvent with excess water has been investigated by SANS. The scattering curve of PS-PEO solution (c ) 2 × 10-2 g/ml) in LTD/D2O (φw ) 0.85) is shown in Figure 4. The best fit to this curve is obtained with a concentric sphere model with an inner radius of 11.8 nm and a whole radius of 24.3 nm. It can be assumed that the particles are spherical micelles with a core formed by PS blocks and shells formed by PEO blocks. Because the neutron scattering length densities25 F of PS (F ) 1.41 × 10-6 Å-2) and PEO (F ) 0.62 × 10-6 Å-2) are small and comparable to that of LTD (F ) 1.16 × 10-6 Å-2), the contrast of the core is given by LTD preferentially sorbed to PS. Indeed, the bulk solvent with φD2O ) 0.85 has F ) 5.55 × 10-6 Å-2, which is substantially larger than F of LTD. A detailed SANS study in a broad range of φw is under way.
Conclusions Two relaxation times belonging to mutual diffusion and the diffusion of clusters in LTD/water mixtures have been described by QELS. In the presence of the PS-PEO copolymer, mutual diffusion can still be observed in the solutions, but the diffusion of solvent clusters disappears. Diblock copolymer PS-PEO in LTD/water mixtures forms multimolecular particles with a narrow size distribution. Their core/shell structure has been found with SANS for mixture LTD/d-water with φD2O ) 0.85. The cores are formed by PS blocks preferentially solvated by LTD, and their shell, by PEO blocks. Surprisingly, the polymeric particles are formed spontaneously in mixtures with φw from 0.05 to 0.83 in which either PS or PEO homopolymers are insoluble. It can be assumed that the strong selective sorption of LTD on PS and water on PEO controls the process. Acknowledgment. We acknowledge support by the grant agency of the Academy of Sciences of the Czech Republic (grant 4050403). LA803397G (25) King, S. M. Chapter 7 In Modern Techniques for Polymer Characterisation; Pethrick, R. A., Dawkins J. V., Eds.; Wiley & Sons: New York, 1999.
