© Copyright 1999 American Chemical Society
APRIL 13, 1999 VOLUME 15, NUMBER 8
Letters Shear-Induced Formation of Multilamellar Vesicles (“Onions”) in Block Copolymers Johannes Zipfel,†,‡ Peter Lindner,‡ Marina Tsianou,§ Paschalis Alexandridis,*,| and Walter Richtering*,† Albert-Ludwigs-Universita¨ t Freiburg, Institut fu¨ r Makromolekulare Chemie, Stefan-Meier-Strasse 31, D-79104 Freiburg i. Br., Germany, Institut Laue-Langevin, B.P. 156, F-38042 Grenoble, France, Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-22100 Lund, Sweden, and Department of Chemical Engineering, University at Buffalo, State University of New York, Buffalo, New York 14260-4200 Received December 2, 1998. In Final Form: January 26, 1999 The use of shear to control the microstructure in the lyotropic lamellar region of a ternary isothermal poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (Pluronic P123)/water/butanol system was investigated by means of small angle neutron and light scattering (SANS, SALS), birefringence, and rheology. A transition from parallel to perpendicular alignment of lamellae, with a corresponding decrease in the viscosity, was found with increasing shear rate at high block copolymer concentrations. At low polymer content, however, the samples were shear thickening, and a shear-induced formation of multilamellar vesicles (“onions”) was observed for the first time in block copolymer systems.
Introduction The influence of shear flow on the structure of complex fluids (such as surfactants in solution or block copolymer melts) with lamellar morphology has attracted growing interest recently. Controlled shear offers the opportunity to create distinct alignments of microstructures and thus lead to novel material properties.1-13 The shear-induced alignment of the lamellae can be * To whom correspondence should be sent. E-mail:
[email protected]. † Albert-Ludwigs-Universita ¨ t Freiburg. ‡ Institut Laue-Langevin. § Lund University. | University at Buffalo. (1) Larson, R. G. The Structure and Rheology of Complex Fluids; Oxford University Press: Oxford, U.K., 1998. (2) Safinya, C. R.; Sirota, E. B.; Bruinsma, R. F.; Jeppesen, C.; Plano, R.; Wenzel, L. Science 1993, 261, 588. (3) Fredrickson, G. H.; Bates, F. S. Annu. Rev. Mater. Sci. 1996, 26, 501. (4) Wiesner, U. Macromol. Chem. Phys. 1997, 198, 3319.
characterized by the orientation of the layer normal. We denote here the real-space orientation with the layer normal pointing along the neutral direction as perpendicular (a), and that with the layer normal pointing along the velocity gradient direction as parallel (c). In addition to the orientation of lamellae, shear flow can induce the formation of multilamellar vesicles (MLVs), sometimes (5) Chen, Z.-R.; Kornfield, J. A.; Smith, S. D.; Grothaus, J. T.; Satkowski, M. M. Science 1997, 277, 1248. (6) Diat, O.; Nallet, F.; Roux, D. J. Phys. II 1993, 3, 1427. (7) Mang, J. T.; Kumar, S.; Hammouda, B. Europhys. Lett. 1994, 28, 489. (8) Bergenholtz, J.; Wagner, N. Langmuir 1996, 12, 3122. (9) Mortensen, K. J. Phys.: Condens. Matter 1996, 8, A103. (10) Penfold, J.; Staples, E.; Tucker, I.; Tiddy, G. J. T.; Kahn Lodhi, A. J. Appl. Crystallogr. 1997, 30, 744. (11) Bergmeier, M.; Gradzielski, M.; Hoffmann, H.; Mortensen, K. J. Phys. Chem. B 1998, 102, 2837. (12) Berghausen, J.; Zipfel, J.; Lindner, P.; Richtering, W. Europhys. Lett. 1998, 43, 683. (13) Zipfel, J.; Berghausen, J.; Lindner, P.; Richtering, W. J. Phys. Chem. B, in press.
10.1021/la981666y CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999
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called “onions”, in lamellar lyotropic mesophases formed by surfactants.6-13 The formation of such vesicles is very important from the fundamental aspect of creating discrete and higher-order self-assembled structure starting from the “open-ended” lamellae, and also from the applied aspects of modification of the rheological properties and of encapsulation of active ingredients. The potential for encapsulation, in particular, is of great interest to pharmaceutical formulations, given the ease of preparation, the controlled size (by the shear conditions6,8), and the multiple lamellar layers that protect the contents of the vesicle and provide for their sustained and controlled release. It would be desirable, given the fundamental and applied aspects outlined above, to extent the principle of the shearinduced formation of multilamellar vesicles to systems based on block copolymers. Block copolymers offer great flexibility in setting the desired morphology and allow a larger variation of the length and time scales characteristic of the microstructure as compared to typical surfactants.14 Moreover, block copolymers can be synthesized that are biocompatible or functionalized with specific chemical groups.15 Hydrophilic poly(ethylene oxide) (PEO) segments, for example, have been shown to increase the blood circulation time of lipid-based vesicles.16 The case of forming vesicles from PEO-containing block copolymers would be advantageous in this context. However, vesicle formation from sheared lamellar block copolymer solutions or melts has not been reported (although vesicular structures have been observed in block copolymer systems17,18). Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-b-PPO-b-PEO) block copolymers in the presence of selective solvents (such as water, selective for the PEO block) have proven to be very versatile systems in terms of phase behavior and microstructure. We have established that the type and composition-temperature stability range of the self-assembled microstructure are functions of the block copolymer molecular weight and PEO/PPO block ratio, the solvent type, and, more importantly, the block copolymer/solvent composition.14,15,19 The ability to control the microstructure, together with the many applications of PEO-PPOPEO block copolymers (they are commercially available as Poloxamers or Pluronics, and a number of them are approved for pharmaceutical use), renders this class of block copolymers a promising system for investigation. In this letter we present results obtained by birefringence and small angle light and neutron scattering (SALS, SANS) from lamellar block copolymer solutions under shear. The shear-induced formation of multilamellar vesicles (“onions”) is reported for the first time in block copolymer systems. Experimental Section The poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) block copolymer, Pluronic P123, was kindly supplied by BASF Corp. The nominal molecular weight of Pluronic P123 is 5750 g mol-1 with a PEO content of 30%. The ternary sample compositions were selected to fall in the lamellar region of the isothermal (25 °C) Pluronic P123-D2O-butanol phase (14) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1998, 14, (10), 2627. (15) Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1996, 1 (4), 490. (16) Lasic, D. D. Angew. Chem. 1994, 106, 1765. (17) Kinning, D. J.; Winey, K. L.; Thomas, E. L. Macromolecules 1988, 21, 3502. (18) Hoffmann, H. Personal communication. (19) Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1997, 2, 478.
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Figure 1. Viscosity (b) and flow birefringence (0) versus shear rate for the sample with 30 wt % block copolymer. diagram reported recently.20 This system is interesting in that it forms a lamellar structure even at relatively low (20%) polymer concentrations. Neutron scattering experiments have been performed on the instrument D11 at the Institut Laue-Langevin (ILL) in Grenoble, France.21 The scattering data were collected with a twodimensional detector covering a range of momentum transfer q from 1.1 × 10-2 to 7.5 × 10-2 Å-1 and corrected for background and empty cell scattering. The incoherent scattering of H2O was used for absolute calibration according to standard procedures. A Couette-type shear cell with a 1 mm gap was used,22 and rectangular apertures were employed to reduce the beam size (0.25 mm × 15 mm or 1 mm × 15 mm). Two scattering configurations were used: (i) in the so-called radial position, the neutron beam passes the sample along the gradient direction; (ii) in the tangential position the neutron beam passes along the flow direction through the side of the cell. We aligned the beam in the middle of the gap using a diaphragm of 0.25 mm. Measurements in the middle of the gap lead to asymmetric scattering patterns due to higher transmission at the outer side of the gap. For rheo-optical studies, a Bohlin CS-10 rheometer equipped with a quartz glass cone/plane shear geometry was used. For birefringence (∆n) measurements, the incident He-Ne laser beam passes through the sample along the direction of the velocity gradient. The retardance was determined using the method described by Lim and Ho.23 The actual experimental setup is given elsewhere.24 A slightly modified setup was used for depolarized (HV) small angle light scattering (SALS) measurements under shear.25 All experiments were performed at 25 °C.
Results and Discussion We studied a number of samples within the lamellar region of the ternary isothermal PEO20-b-PPO70-b-PEO20/ butanol/D2O phase diagram.20 Here we report on results obtained with two samples at a butanol/water weight ratio of 0.3 and polymer concentrations of 30 and 21 wt %, respectively. Shear-Induced Orientation of Lamellae. Results for a sample containing 30 wt % block copolymer, obtained from rheo-optical and SANS experiments, are presented in Figures 1 and 2, respectively. As seen in Figure 1, the (20) Holmqvist, P.; Alexandridis, P.; Lindman, B. J. Phys. Chem. B 1998, 102 (7), 1149. (21) Lindner, P.; May, R. P.; Timmins, P. A. Physica B 1992, 180&181, 967. (22) Lindner, P.; Oberthuer, R. C. Rev. Phys. Appl. 1984, 19, 759. (23) Lim, K.-C.; Ho, J. T. Mol. Cryst. Liq. Cryst. 1978, 47, 225. (24) Schmidt, J.; Weigel, R.; Burchard, W.; Richtering, W. Macromol. Symp. 1997, 120, 247. (25) Berghausen, J.; Fuchs, J.; Richtering, W. Macromolecules 1997, 30, 7574.
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Figure 2. SANS patterns observed for the sample with 30% polymer at shear rates of 50 s-1 (top) and 1200 s-1 (bottom): left, radial beam; right, tangential beam.
sample was shear thinning and a transition was observed at a shear rate of about 200 s-1. The viscosity decreased sharply, and the birefringence curve also showed a pronounced transition indicating a change in the lamellae orientation. SANS experiments in the tangential beam configuration provide a powerful means to distinguish between different types of layer orientation because the Bragg scattering is detected for lamellae with the layer normal either along the velocity gradient direction or along the neutral direction. With the radial beam, lamellae aligned parallel to the walls (c) cannot be detected. The SANS data in Figure 2 thus provide further information on the type of shear-induced transition. At low shear rates, a strong Bragg peak is found along the velocity gradient direction with the tangential beam configuration. This clearly demonstrates that the bilayers were aligned parallel to the walls, that is, with their layer normal along the velocity gradient direction. The radial beam configuration only probes a very small amount of residual layers aligned perpendicularly. At higher shear rates, however, the layers reoriented with their normal along the neutral direction, as can be clearly seen from the results of the tangential beam experiment. Thus, from SANS and rheo-optical experiments one can conclude that the sample at a polymer concentration of 30% shows a reorientation from parallel (c) at low shear rates to perpendicular (a) at high shear rates. Shear-Induced Formation of Vesicles. However, a very different behavior was observed when the block copolymer content was reduced to 21%. Figure 3 shows results from rheo-SALS. The viscosity of the 21% sample was much higher than that of the (more concentrated) 30% sample and increased with shear. A four-lobe scattering pattern, characteristic of vesicles, was observed in depolarized light scattering. At shear rates higher than 1.5 s-1 the viscosity of the sample decreased. Upon further
Figure 3. Viscosity versus shear rate for the sample with 21 wt % block copolymer. The inset shows the depolarized SALS pattern at the shear rate 1 s-1.
increase of the shear rate, the sample became inhomogeneous, thus not allowing us to investigate the behavior at higher shear rates. The viscosity increase at low rates and the four-lobe SALS patterns are well-known from lamellar phases of low-molecular-weight surfactants and are indicative of a shear-induced formation of multilamellar vesicles.6,8,26,27 This result is confirmed by SANS. Figure 4 displays the intensity distributions obtained with radial and tangential beam configurations. In both cases, the Bragg peak was (26) La¨uger, J.; Weigel, R.; Berger, K.; Hiltrop, K.; Richtering, W. J. Colloid Interface Sci. 1996, 181, 521. (27) Weigel, R.; La¨uger, J.; Richtering, W.; Lindner, P. J. Phys. II 1996, 6, 529.
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Figure 4. SANS patterns for the sample with 21% polymer at the shear rate 1 s-1: left, radial beam; right, tangential beam.
observed along the entire azimuthal trace of the twodimensional multidetector. From the isotropic Bragg scattering in the sheared sample, one can deduce the presence of MLVs because the radial symmetry of the “onions” gives rise to an isotropic distribution of layer normals. Thus both SANS and SALS data provide evidence for the shear-induced formation of multilamellar vesicles from an aqueous triblock copolymer solution. The behavior of the ternary lamellar system PEOPPO-PEO/water/butanol under shear resembles the general behavior observed in lyotropic lamellar phases of common surfactants. With surfactants a reorientation from parallel to perpendicular alignment of planar bilayers has been reported for different systems and also vesicle formation has been observed in several cases.6-13,26,27 Block copolymer systems, however, usually display different behavior. Transitions between different types of lamellae orientation have been observed, and mostly a transition from perpendicular to parallel orientation was found with increasing frequency, reflecting the longer relaxation times of the macromolecules.2-5 There are only very few studies on lamellar block copolymer solutions. Zyrd and Burghardt recently investigated polystyrenepolyisoprene block copolymer solutions and observed a reorientation from perpendicular to parallel alignment with increasing shear rate.28 Mortensen and co-workers reported on the structure of aqueous mixtures of a PEOpolyisobutylene-PEO triblock copolymer and observed shear alignment of the lamellar phase.9,29 To the best of our knowledge, a shear-induced transition from a lamellar phase with planar layers to multilamellar vesicles has not been observed in block copolymer systems. The lyotropic lamellar block copolymer phase investigated (28) Zyrd, J. L.; Burghardt, W. R. Macromolecules 1998, 31, 3656. (29) Mortensen, K.; Talmon, Y.; Gao, B.; Kops, J. Macromolecules 1997, 30, 6764.
in this study thus behaves more like a lyotropic surfactant mesophase than as a typical block copolymer system. One can speculate whether the vesicle formation observed in the block copolymer/solvent system studied here is caused by the relatively high solvent concentration and, in particular, by the presence of butanol. We found butanol to decrease the lamellar spacing (compared to water) and concluded that butanol participates at the polar/apolar interface (acting as a cosurfactant) and increases the effective interfacial area per block copolymer molecule.20 In some surfactant systems, it has been observed that the cosurfactant/surfactant ratio influences the topological transition from planar bilayers to vesicles. This was attributed to alterations in splay and saddle splay moduli when the cosurfactant content was changed.30 Thus, butanol can facilitate defect formation and the mobility of molecules between different layers. These properties seem to be important for vesicle formation. Obviously, ternary mixtures of amphiphilic block copolymers offer a great potential for a variation of mesophase structure both at rest and under shear. Detailed studies on the influence of solvent composition and polymer molecular weight are in progress and will be reported later. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. We also thank the Deutscher Akademischer Austausch Dienst and the Svenska Institut for travel support. J.Z. acknowledges a Marie-Curie research fellowship of the European Commission. LA981666Y (30) Boltenhagen, P.; Kleman, M.; Lavrentovich, O. D. J. Phys. II 1994, 4, 1439.
