pubs.acs.org/Langmuir © 2010 American Chemical Society
Morphological Manipulation of Ionic Block Copolymer Micelles Using an Electric Field Sun Ju Lee† and Moon Jeong Park*,†,‡ †
Department of Chemistry and ‡Division of Advanced Materials Science (WCU), Pohang University of Science and Technology (POSTECH), Pohang, Korea 790-784 Received September 16, 2010. Revised Manuscript Received October 20, 2010
We present an electric-field-triggered sphere-to-cylinder transition of negatively charged block copolymer micelles with a radically low electric field of 30 V/cm. The system investigated is dilute solutions of strong polyelectrolyte containing ionic-b-neutral block copolymers (i.e., poly(styrenesulfonate-b-methylbutylene)). We have carried out in situ small-angle X-ray scattering experiments equipped with a dc power supply, combined with electron microscopy and atomic force microscopy. The application of small electrical fields across the solutions of spherical micelles results in the transient morphology of interconnected spheres, which are eventually transformed into a cylindrical shape with time. The E-field-induced cylindrical micelles revert to spherical micelles when the E field is switched off.
The self-assembly of block copolymers in dilute solutions has received a great deal of attention in the last few decades.1 When block copolymers are dissolved in selective solvents, they form socalled micelles. Intriguing micellar morphologies (i.e., spheres,2 toroids,3 wormlike cylinders,4 and vesicles5) have been discovered, and the morphologies have been found to be tunable by versatile external stimuli such as pH,6 temperature,7 and additives.8 The various micellar morphologies have been commonly understood on the basis of a packing model.9 Among a broad class of block copolymer micelles, ionic-bneutral block copolymers, hereafter referred to as ionic block copolymers, have been the subject of many studies10,11 in recent years because of their unique physicochemical properties such as temperature-dependent aggregation behavior,10b counterion-induced morphological change,10a,11a and pH-sensitive micellization/demicellization.11c We hope to categorize the ionic block copolymers into two categories. The first one is weak-polyelectrolyte-containing copolymers, such as poly(acrylic acid) (PAA),5,12 *Corresponding author. E-mail:
[email protected]. (1) (a) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267. (b) Savic, R.; Luo, L.; Eisenberg, A.; Maysinger, D. Science 2003, 300, 615. (c) Allen, T. M.; Cullis, P. R. Science 2004, 303, 1818. (d) Hadjichristidis, N.; Iatrou, H.; Pitsikalis, M.; Pispas, S.; Avgeropoulos, A. Prog. Polym. Sci. 2005, 30, 725. (2) Cao, L.; Manners, I.; Winnik, M. A. Macromolecules 2001, 34, 3353. (3) Chen, Z.; Cui, H.; Hales, K.; Li, Z.; Qi, K.; Pochan, D. J.; Wooley, K. L. J. Am. Chem. Soc. 2005, 127, 8592. (4) Jains, S.; Bates, F. S. Science 2003, 300, 460. (5) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (6) Na, K.; Lee, K. H.; Bae, Y. H. J. Controlled Release 2004, 97, 513. (7) (a) Dreiss, C. A. Soft Matter 2007, 3, 956. (b) Zakir, M. R.; Dincer, S.; Piskin, E. Prog. Polym. Sci. 2007, 32, 534. (8) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997. (9) (a) Won, Y.-Y.; Davis, H. T.; Bates, F. S. Science 1999, 283, 960. (b) Wang, X. S.; Guerin, G.; Wang, H.; Wang, Y. S.; Manners, I.; Winnik, M. A. Science 2007, 317, 644. (c) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1992. (10) (a) Joshi, J. V.; Aswal, V. K.; Goyal, P. S. J. Phys.: Condens. Matter 2007, 19, 196219. (b) Thibault, R. J.; Hotchkiss, P. J.; Grav, M.; Rotello, V. M. J. Am. Chem. Soc. 2003, 125, 11249. (11) (a) Cui, H. G.; Chen, Z. Y.; Zhong, S.; Wooley, K. L.; Pochan, D. J. Science 2007, 317, 647. (b) Rapoport, N. Prog. Polym. Sci. 2007, 32, 962. (c) Lee, E. S.; Na, K.; Bae, Y. H. Nano Lett. 2005, 5, 325. (12) (a) Perkin, K. K.; Turner, J. L.; Wooley, K. L.; Mann., S. Nano Lett. 2005, 5, 1457. (b) Korobko, A. V.; Jesse, W.; Lapp, A.; Egelhaaf, S. U.; van der Maarel, J. R. C. J. Chem. Phys. 2005, 122, 024902.
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where their charge density can be altered with changes in pH,13 temperature,14 or the amount of salt.8 They are becoming more popular in a wide range of emerging applications such as electrophoretic sensors15 and controlled release11 because they allow the sequestering of guest molecules and the release of guest molecules can be prompted by external stimuli.16 Another class of ionic block copolymers is strong-polyelectrolyte-containing copolymers, such as poly(styrenesulfonate) (PSS), which have been extensively studied for fuel cell applications in bulk form.17 However, not much is known about their dilute solution behavior, which in part is due to the difficulty encountered in synthesizing well-controlled materials.18 These copolymers can be highly charged in polar solvents; therefore, the micellization process is expected to be dominated by the delicate balance of electrostatic and steric interactions.19 In particular, those interactions are anticipated to be adjustable by an external electric field (E field), although how they influence micellar morphology is an open question. Herein we present the first experimental observation of an E-field-triggered sphere-to-cylinder transition of highly charged micelles with a radically small E field of 30 V/cm. We have synthesized a poly(styrenesulfonate-b-methylbutylene) (PSS-b-PMB) copolymer with PSS and PMB molecular weights of 2.6 and 1.8 kg/mol, respectively, where 50 mol % styrene is sulfonated, hereafter referred to as S18MB25(50). (See Supporting Information for detailed experimental conditions and procedures.) The molecular (13) (a) Xu, L.; Zhu, Z.; Borisov, O. V.; Zhulina, E. B.; Sukhishvili, S. A. Phys. Rev. Lett. 2009, 103, 118301. (b) Apostolovic, B.; Klok, H. A. Biomacromolecules 2008, 9, 3173. (14) Sundararaman, A.; Stephan, T.; Grubbs, R. B. J. Am. Chem. Soc. 2008, 130, 12264. (15) Komori, K.; Matsui, H.; Tatsuma, T. Bioelectrochemistry 2005, 65, 129. (16) (a) Kabanov, A.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941. (b) Bronich, T. K.; Keifer, P. A.; Shlyakhtenko, L. S.; Kabanov, A. V. J. Am. Chem. Soc. 2005, 127, 8236. (17) (a) Park, M. J.; Downing, K. H.; Jackson, A.; Gomez, E. D.; Minor, A. M.; Cookson, D.; Weber, A. Z.; Balsara, N. P. Nano Lett. 2007, 7, 3547. (b) Park, M. J.; Kim, S.; Minor, A.; Balsara, N. P. Adv. Mater. 2009, 21, 203. (c) Park, M. J.; Balsara, N. P. Macromolecules 2010, 43, 292. (18) (a) Matsumoto, K.; Hirabayashi, T.; Harada, T.; Matsuoka, H. Macromolecules 2005, 38, 9957. (b) Kim, T.-H.; Choi, S.-M.; Kline, S. R. Langmuir 2006, 22, 2844. (19) (a) Philip, D.; Stoddart, J. F. Angew. Chem., Int. Ed. 1996, 35, 1154. (b) Paul, S.; Patey, G. N. J. Phys. Chem. B 2007, 111, 7932.
Published on Web 10/28/2010
DOI: 10.1021/la103708d
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Figure 1. (a) Molecular structure of S18MB25(50) and (b) schematic illustration of the S18MB25(50) micelle structure when dissolved in methanol.
structure of S18MB25(50) is shown in Figure 1a. In methanol, spherical micelles consisting of a PMB core surrounded by a negatively charged PSS corona are the favored morphology for S18MB25(50), as illustrated in Figure 1b. To obtain information on the hydrodynamic radius of S18MB25(50) micelles in methanol, we first performed dynamic light scattering (DLS, Brookhaven BI-9000AT) experiments by varying the concentration. The time autocorrelation functions of the scattering intensity, g(2)(τ), were accumulated by a logarithmic digital correlator. Measurements were made at five different scattering angles (θ = 70, 90, 110, 130, and 170°), and the distribution of the decay rates was obtained by fitting to an exponential function as follows.20 gð2Þ ðτÞ - 1 ¼ ½A expð - ΓτÞ2
ð1Þ
where Γ is the characteristic decay rate. The diffusion coefficient, D, is characterized by the plot of the decay rate versus q2 based on the relationship ð2Þ
Γ ¼ q2 D θ
where q is the scattering vector, q = 4πn/λ sin( /2), taking into account of the scattering angle (θ) and the refractive index of the solvent (n). Figure 2a shows the normalized intensity correlation functions, g(2)(τ) - 1, for 0.1 and 1 wt % S18MB25(50) solutions obtained at a scattering angle of 170°. Solid lines are obtained by curve fitting using eq 1, and the dependence of the decay rates on q2 is presented in the inset of Figure 2a. For both solutions, the decay rates show a linear q2 dependency and the fitted lines go through the origin, implying diffusive characteristics. The hydrodynamic radii of a micelle, Rh, are calculated from the StokesEinstein equation20 to be 26.4 nm (0.1 wt %) and 6.1 nm (1 wt %). TEM images of 0.1 and 1 wt % solutions are shown in Figure S1 in the Supporting Information to confirm the spherical sphape of the micelles. The obtained micelle radii of 0.1 and 1 wt % solutions are 28 and 6 nm, respectively, which is in good agreement with solution DLS results. It should be noted that the Rh with 0.1 wt % S18MB25(50) is pronouncedly larger than that with 1 wt % S18MB25(50). This is distinguished from a vast body of results concerning uncharged micelles21 where an opposite trend is reported or negligible changes in micelle size are detected. Therefore, it is clear that the introduction of ionic moieties in one of the blocks results in (20) Chu, B. Laser Light Scattering, 2nd ed.; Academic Press: New York, 1991. (21) (a) Khougaz, K.; Zhong, Z. F.; Eisenberg, A. Macromolecules 1996, 29, 3937. (b) Poppe, A.; Willner, L.; Allgaier, J.; Stellbrink, J.; Richter, D. Macromolecules 1997, 30, 7462. (c) Bang, J.; Viswanathan, K.; Lodge, T. P.; Park, M. J.; Char, K. J. Chem. Phys. 2004, 121, 11489. (d) Zhang, H.; Xia, H.; Wang, J.; Li, Y. J. Controlled Release 2009, 139, 31.
17828 DOI: 10.1021/la103708d
Figure 2. (a) Normalized intensity correlation functions of 0.1 and 1 wt % S18MB25(50) in methanol measured at 170 o. The solid curves are fitted to eq 1. The dependence of the decay rates on q2 is shown in the inset. (b) Micelle radii obtained with different concentrations as indicated. The concentration-dependent zeta potential values are shown in the inset.
radically different micellization behavior. For an elaborate examination of this trend, the micelle radii and the zeta potential (ζ) values are recorded as a function of concentration. The ζ values were obtained from electrophoretic mobility values using the Smoluchowski approximation (Zetasizer Nano-ZS, Malvern). As shown in Figure 2b, a gradual decrease in the micelle size with increasing concentration has been observed. Interestingly, as plotted in the inset of Figure 2b, we observed a gradual decrease in the magnitude of ζ values with the decrease in concentration. This implies that strongly charged micelles with higher concentrations result in smaller micelles. Although the concentration dependence of the micelle size requires further investigation, our result is consistent with the limited data obtained thus far on charge-containing systems12b showing a decrease in micelle size with increasing ionization of micelle coronas. We carried out in situ small-angle X-ray scattering (in situ SAXS) with a dc power supply to investigate how the morphologies of charged micelles having strong ionic segments are influenced when a voltage is applied over the solutions. The solutions were placed in a home-built Teflon sample cell equipped with heavily doped Si-wafer electrodes, and a dc voltage in the Langmuir 2010, 26(23), 17827–17830
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Figure 3. In situ SAXS profiles of a 1 wt % S18MB25(50) solution obtained under an E field of 30 V/cm. The voltage-dependent zeta potential values are shown in the inset.
range of 5-30 V was applied using a Keithley power supply. A schematic illustrating the sample cell is shown in the Supporting Information (Figure S2). For SAXS experiments, a 2-mm-wide electrode was used to allow the transmission of an X-ray beam through the sample. We set the beam exposure area to be 1 mm away from the (þ) electrode, and the experiments were carried out at room temperature. Figure 3 shows in situ SAXS data from a 1 wt % solution upon switching the 30 V/cm E field on and off. Without the E field, the scattering profile indicates a featureless pattern, which is typical of dilute spherical micelles. After switching the E field on, however, we immediately observed a strong peak at q* = 0.78 nm-1 and a weak second-order peak with a ratio of 1:2. When the sample is annealed under an E field, we see 1q*, (3)1/2q*, (4)1/2q*, and (9)1/2q* Bragg reflections with q* = 0.75 nm-1, as shown by inverted solid triangles (1). This indicates the presence of a hexagonally packed cylinder with a domain spacing of d100 = 8.4 nm. The diameter of the cylinders is calculated to be ca. 10 nm (from D = 2d100/(3)1/2). To examine the stability of E-field-induced cylindrical micelles, the scattering intensity is continuously recorded after the E field is switched off. A small but continuous decrease in the main peak intensity is observed as a function of time, and after 20 min of annealing, remarkably, a new peak at (2)1/2q* is developed. Bragg reflections at 1q*, (2)1/2q*, (3)1/2q*, (4)1/2q*, (5)1/2q* (q* = 0.73 nm-1), as shown by inverted open triangles (r), indicate the formation of cubic-like structure that is presumably due to fluctuations in the cylindrical micelles.22 It is worthwhile to mention that we observed the different ζ values as a function of the applied dc voltage for a 1 wt % solution, as plotted in the inset of Figure 3. Whereas the ζ at a lower applied voltage is ca. -20 mV, the value decreases to -27 ( 1 mV upon increasing the voltage to above 20 V/cm. This implies that the corona of cylindrical micelles is more densely packed than that of spherical micelles, leading to higher surface charge densities in cylindrical micelles. It should be noted here that the (22) Ryu, C. Y.; Lodge, T. P. Macromolecules 1999, 32, 7190.
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Letter
E-field-triggered sphere-to-cylinder micelle transition was not observed in the presence of counterions. When equimolar NaCl is added to the 1 wt % solution, the morphological transition is hindered because the Naþ ions tend to stay near the S18MB25(50) micellar surface.23 (See Figure S3 in the Supporting Information.) In an attempt to understand the external E-field-triggered morphological changes of S18MB25(50) micelles, we have carried out atomic force microscopy (AFM) and transmission electron microscopy (TEM), combined with a dc power supply. The same Teflon cell illustrated in Figure S1 (but with a 4-mm-wide electrode) was employed, and the adsorption of negatively charged S18MB25(50) micelles on the (þ) electrode is expected as a result of electrophoresis. The samples were exposed to an E field for 3 min within the cell, and then the (þ) electrode was immediately taken out of the solutions while the E field was switched on. Figure 4a,b shows AFM images (tapping mode, Nanoscope III, Digital Instruments) obtained from 0.1 and 1 wt % S18MB 25(50) solutions, respectively. For the case of a 0.1 wt % solution (Figure 4a), the micelles are packed in short strings and the radius of the micelles is ca. 29 nm, which is in reasonably good agreement with the solution DLS data in Figure 2a,b. It is interesting that we found “bananalike” morphology coexisting with the spheres although the origin of the structure is not yet clearly understood. When the solution concentration is increased to 1 wt % (Figure 4b), the surface topology is distinctly dissimilar. The dominant population of wormlike cylindrical micelles is observed for the entire electrode area examined. The diameter of the cylinder is ca. 13 nm, and the length is 50-200 nm. It is therefore obvious that the effect of an applied E field on the morphology of S18MB25(50) micelles is becoming significant as the concentration of micellar solutions is increased. To elucidate further the nature of the E-field-induced micellar morphologies of S18MB25(50), we repeated the same protocols described above in the presence of a copper grid on top of an electrode for TEM experiments. To examine the core-shell structure of the micelles clearly, the PSS corona is darked by selective staining with RuO4 vapor. Imaging of stained samples was performed with a Philips FEI CM300 microscope operating at 200 kV. In Figure 4c-e, we show TEM images obtained from a 1 wt % solution in the absence or presence of an external E field. In the absence of an E field, as shown in Figure 4c, spherical micelles with a diameter of ca. 13 nm (1 wt %) are observed, which is in good agreement with the DLS results (Figure 2). When the E field is applied to the 1 wt % solution, cylindrical micelles with a ca. 15 nm diameter and 150 nm length are seen as shown in Figure 4d. As highlighted in the inset of Figure 4d, core-shell-type cylindrical micelles are also determined. In addition, it is worthwhile to mention that we were able to capture the transient morphology of merged spheres interconnected into a cylindrical shape as shown in Figure 4e. This implies that the formation of cylindrical micelles under an external E field originates from the local merging or rearranging of micelles with the retention of spherical symmetry. We schematically illustrate the E-field-triggered sphere-to-cylinder transitions of charged S18MB25(50) micelles in Scheme 1. In nonionic block copolymers (both in the bulk and in solutions), the E field has been known to be an efficient method (23) (a) Aswal, V. K.; Goyal, P. S. Phys. Rev. E 2000, 61, 2947. (b) Missel, P. J.; Mazer, N. A.; Carey, M. C.; Benedek, G. B. J. Phys. Chem. 1989, 93, 8354. (c) Konop, A. J.; Colby, R. H. Langmuir 1999, 15, 58. (24) (a) Boker, A.; Knoll, A.; Elbs, H.; Abetz, V.; Muller, A. H. E.; Krausch, G. Macromolecules 2002, 35, 1319. (b) Xu, T.; Zvelindovsky, A. V.; Sevink, G. J. A.; Gang, O.; Ocko, B.; Zhu, Y.; Gido, S. P.; Russell, T. P. Macromolecules 2004, 37, 6980. (25) (a) Giacomelli, F. C.; da Silveira, N. P.; Nallet, F.; Cernoch, P.; Steinhart, epanek, P. Macromolecules 2010, 43, 4261. (b) Giacomelli, F. C.; Riegel, I. C.; M.; St Petzhold, C. L.; da Silveira, N. P. Macromolecules 2008, 41, 2677.
DOI: 10.1021/la103708d
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Figure 4. AFM images taken from (a) 0.1 and (b) 1 wt % solutions where the (þ) electrode is examined after a 30 V/cm E field is applied for 3 min. TEM images of the 1 wt % solution (c) in the absence of the E field and (d, e) under the 30 V/cm E field. Scheme 1. Schematic Illustration of the E-Field-Triggered Sphere-to-Cylinder Transition of Charged Block Copolymer Micelles
for controlling morphology24-26 and orientation.27,28 Typically, a large electric field of 1-40 kV/mm is necessary to induce phase transitions because the driving force in the nonionic systems is intrinsically related to the dielectric contrast between microdomains. In contrast, the magnitude of the electric field required to induce a morphological change is greatly reduced to 30 V/cm (equivalent to 3 V/mm) in the present study, which is a comment on the ramifications of our work on the structure of widely studied neutral block copolymers or weak polyelectrolytes.24-26 To the best of our knowledge, there is no study showing a morphological transition under such a small E field. In the presence of electrostatic repulsion between likecharged micelles, it has been known that a nondensely packed corona is formed.29 Under a dc E field, charged micelles in polar solvents experience induced-charge electro-osmotic flow around them, which influences the surface tension of micelles and results in the distortion of micelle/solvent interfaces.25 Consequently, it is sensible that the deformation of the spherical micelles occurs toward the closest ones and that the interconnection of adjacent micelles occurs concomitantly.
Acknowledgment. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (project no. 2010-0018517) and the WCU (World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (project no. R31-2009-00010059-0). SAXS measurements were conducted on the 4C1 beamline at the Pohang Light Source (PLS). We gratefully acknowledge Prof. Taihyun Chang for providing access to the DLS instrument.
(26) Schmidt, K.; Pester, C. W.; Schoberth, H. G.; Zettl, H.; Schindler, K. A.; B€oker, A. Macromolecules 2010, 43, 4268. (27) (a) Xu, T.; Zhu, Y.; Gido, S. P.; Russell, T. P. Macromolecules 2004, 37, 2625. (b) Schmidt, K.; Schoberth, H. G.; Schubert, F.; Hansel, H.; Fischer, F.; Weiss, T. M.; Sevink, G. J. A.; Zvelindovsky, A. V.; B€oker, A.; Krausch, G. Soft Matter 2007, 3, 448. (28) Sakurai, S. Polymer 2008, 49, 2781. (29) (a) Nikolaides, M. G.; Bausch, A. R.; Hsu, M. F.; Dinsmore, A. D.; Brenner, M. P.; Gay, C.; Weitz, D. A. Nature 2002, 420, 299. (b) W€urger, A.; Foret, L. J. Phys. Chem. B 2005, 109, 16435.
Supporting Information Available: Synthesis of S18MB25(50). In situ small-angle X-ray scattering (in situ SAXS). TEM images of the 0.1 and 1 wt % S18MB25(50) solutions obtained in the absence of an external E field. Schematic illustration of a home-built Teflon sample cell. In situ SAXS profiles of a 1 wt % S18MB25(50) solution obtained under an E field of 30 V/cm at T = 25 °C with no salt and 1 equiv of NaCl. This material is available free of charge via the Internet at http://pubs.acs.org.
17830 DOI: 10.1021/la103708d
In conclusion, we present the electrokinetic control of the morphology of ionic micelles having hydrophobic cores and anionic coronas. The application of a very small E field of 30 V/cm to highly charged block copolymer micelles in a dilute solution results in the formation of cylindrical micelles, which revert to spherical micelles when the E field is switched off. This drastic conformational switching between spheres and cylinders makes the system a good candidate for electrophoretic sensors. Experiments to determine if similar effects are present in other ionic micelles are currently underway.
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