Langmuir 2002, 18, 6727-6729
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Nanoaggregate Formation of Poly(ethylene oxide)-b-polymethacrylate Copolymer Induced by Alkaline Earth Metal Ion Binding Yuan Li, Yong-Kuan Gong, and Kenichi Nakashima* Department of Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840-8502, Japan
Yoshio Murata Department of Chemistry, Faculty of Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka, 814-0180, Japan Received April 6, 2002. In Final Form: July 23, 2002 Dynamic light scattering, scanning electron microscopy, and turbidity measurements have been applied in the investigation of nanoaggregate formation of poly(ethylene oxide)-b-polymethacrylate (PEO-b-PMA) in which the PMA block is electrically neutralized with alkaline earth metal ions. They self-assemble to form micelle-like nanoaggregates comprised of a “core” of neutralized polyions surrounded by the PEO “corona”. The nanoaggregates have properties of both amphiphilic block copolymer micelles and polyelectrolyte complexes. The significance of these new types of nanoaggregates is that they can incorporate ionic compounds into their cores, in quite contrast to conventional polymer micelles in which only hydrophobic species can be incorporated into the core in aqueous solutions. The potential applications of these systems include a carrier for ionic drugs and a nanoreactor for chemical reactions of ionic species.
Introduction In the last two decades, many studies have shown that block copolymers with both hydrophobic and hydrophilic blocks can form micelles upon dissolution into water. Typical examples are studies on polystyrene-b-poly(ethylene oxide) (PS-b-PEO) by Winnik’s group,1-3 polystyrene-b-poly(acrylic acid) (PS-b-PAA) and polystyreneb-poly(methacrylic acid) (PS-b-PMA) by Eisenberg’s group,4-6 and poly(ethylene oxide)-b-poly(propylene oxide)b-poly(ethylene oxide) (PEO-b-PPO-b-PEO) by Chu’s group,7 Almgren’s group,8 and others.9-12 Recent development of the studies on block copolymer micelles is extensively reviewed by Tuzar et al.13 Although polymer micelles have several advantages compared with conventional low-molecular weight surfactant micelles, there is an important defect in earlier polymer micelles that ionic species cannot be incorporated into the core part because the core block is highly hydrophobic. Therefore, polymer micelles that incorporate * To whom any correspondence should be addressed. Fax: +81952-28-8548.Phone: +81-952-28-8850. E-mail:
[email protected]. (1) Zhao, C.-L.; Winnik, M. A.; Reiss, G.; Croucher, M. D. Macromolecules 1990, 6, 514. (2) Wilhelm, M.; Zhao, C.-L.; Wang, Y.; Xu, R. L.; Winnik, M. A.; Mura, J.-L.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 1033. (3) Xu, R. L.; Winnik, M. A.; Hallett, F. R.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 87. (4) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (5) Zhang, L.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777. (6) Moffitt, M.; Khougaz, K.; Eisenberg, A. Acc. Chem. Res. 1996, 29, 95. (7) Chu, B.; Zhou, Z. Nonionic Surfactants: Polyoxyalkylene Block Copolymers; Marcel Dekker: New York, 1996; Chapter 3. (8) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (9) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1. (10) Nakashima, K.; Takeuchi, K. Appl. Spectrosc. 2001, 55, 1237. (11) Nakashima, K.; Anzai, T.; Fujimoto, Y. Langmuir 1994, 10, 658. (12) Nakashima, K.; Anzai, T.; Fujimoto, Y.; Anzai, T. Photochem. Photobiol. 1995, 61, 592. (13) Tuzar, Z.; Kratochvil, P. Surface and Colloid Science; Plenum Press: New York, 1993; Chapter 1.
ionic species into the core have been anticipated. Recently, Kataoka et al. reported the formation of novel polymer micelles through electrostatic interaction between PEOb-poly(L-lysine) and PEO-b-poly(R,β-aspartic acid) in aqueous solutions.14 Their micelles are stable and monodispersive and can incorporate charged compounds (e.g., adriamycin and nucleic acids) into the core. Another type of block ionomer complexes was prepared by Kabanov et al. using surfactants and copolymers containing ionic and nonionic water-soluble blocks.15-18 They employed two kinds of systems. One consists of the diblock copolymer of poly(ethylene oxide)-b-polymethacrylate (PEO-b-PMA) and various single-, double-, and triple-tail cationic surfactants. The other comprises poly(ethylene oxide)-gpoly(ethyleneimine) (PEO-g-PEI) and alkyl sulfate surfactants. In the two systems, the surfactants are bound to the oppositely charged ionic blocks due to the electrostatic interaction. On the other hand, Bronstein et al.19-23 reported on micellar structure formation from block copolymer PEO-b-PEI where PEI block is coordinated with the transition metal compounds to form the micelle core. (14) Kataoka, K.; Harada, A. Science 1999, 283, 65. (15) Bronich, T. K.; Kabanov, A. V.; Kabanov, V. A.; Yu, K.; Eisenberg, A. Macromolecules 1997, 30, 0. 3519. (16) Kabanov, A. V.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941. (17) Bronich, T. K.; Popov, A. M.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2000, 16, 481. (18) Bronich, T. K.; Cherry, T.; Vinogradov, S. V.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 1998, 14, 6101. (19) Bronstein, L.; Antonietti, M.; Valetsky, P. Nanopart. Nanostruct. Films 1998, 145. (20) Bronstein, L.; Sedlak, M.; Hartmann, J.; Breulmann, M.; Colfen, H.; Antonietti, M. Polym. Mater. Sci. Eng. 1997, 76, 54. (21) Bronstein, L. M.; Sidorov, S. N.; Gourkova, A. Y.; Valetsky, P. M.; Hartmann, J.; Breulmann, M.; Colfen, H.; Antonietti, M. Inorg. Chim. Acta 1998, 280, 348. (22) Bronstein, L. M.; Sidorov, S. N.; Berton, B.; Sedlak, M.; Colfen, H.; Antonietti, M. Polym. Mater. Sci. Eng. 1999, 80, 124. (23) Sidorov, S. N.; Bronstein, L. M.; Valetskt, P. M.; Hartmann, J.; Colfen, H.; Schnablegger, H.; Antonietti, M. J. Colloid Interface Sci. 1999, 212, 197.
10.1021/la025811q CCC: $22.00 © 2002 American Chemical Society Published on Web 08/08/2002
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Langmuir, Vol. 18, No. 18, 2002
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This system is of interest from the standpoint of the development of novel hybrids with organic and inorganic components. In the present work, we have tried to prepare block ionomer nanoaggregates using alkaline earth metal ions (Ba2+ and Ca2+) and PEO-b-PMA in which PMA block is insolubilized by electric neutralization with the metal ions. In these water-soluble aggregates, the metal ions are simply bound to the PMA segment by electrostatic interaction, and the preparation of the aggregates is much easier compared to previous block ionomer complexes of PEO-b-PEI with transition metal compounds.19-23 Dynamic light scattering (DLS), scanning electron microscopy (SEM), and turbidimetry are used to observe the aggregate formation. From the data, we have evaluated the critical aggregation concentration (cac) of the polymer as well as size and stability of the aggregates. Experimental Section Materials. Poly(ethylene oxide)-b-poly(methacrylic acid) (Polymer Source Inc.) and poly(methacrylic acid) (Scientific Polymer Products Inc.) were used as supplied. PEO-b-PMA and polymethacrylate (PMA) were obtained by neutralization of the corresponding acidic form of the polymers with NaOH. The mean degree of polymerization of PEO-b-PMA in each block is 170 for PEO and 180 for PMA. Thus the molecular weights of each block are Mn (PEO) ) 7500 and Mn (PMA) ) 15 500. The molecular weight of PMA homopolymer is 150 000. Calcium chloride and barium chloride were employed for neutralizing PMA block. They were guaranteed grade and used without further purification. Water was purified with a Milli-Q purification system after ion exchanged and distilled. Sample solutions were prepared in the following way. Known amounts of PEO-b-PMA solutions were neutralized by titration with solutions of the metal ion to obtain desired values of the degree of neutralization (DN) under stirring with a magnetic stirrer for 5 min. Here, DN is defined by
DN (%) )
amount of added cations (mol) amount of COO- groups (mol)
Figure 1. Turbidity of PMA and PEO-b-PMA solutions as a function of DN with (a) Ba2+ and (b) Ca2+: (O) PMA (0.2 g L-1); (b) PEO-b-PMA (0.3 g L-1). The base-molar concentration of carboxylate group is the same for PMA and PEO-b-PMA solutions.
× 100
Turbidimetry. The turbidity was measured with a Jasco Ubest-50 Uv-vis spectrophotometer. The turbidity is calculated as (100 - T)/100, where T is transmittance (%) at 350 nm. DLS Measurements. DLS measurements were carried out with an Otsuka ELS-800 dynamic light scattering instrument at a fixed 90° scattering angle. Correlation functions were analyzed by a histogram method and used to determine the diffusion coefficient (D) of samples. Hydrodynamic radius (Rh) was calculated from D by using the Stokes-Einstein equation.3 Electron Microscopy. SEM measurements were carried out with Alpha1-25A electron microscope (Akashi Beam Technology). The samples were prepared by evaporating a drop of the sample solutions on a glass plate under vacuum.
Results and Discussion Turbidity Measurements. Figure 1 represents the turbidity change of the metal ion/PEO-b-PMA copolymer system as a function of DN. The data for the metal ion/ PMA homopolymer system are also shown for comparison. If we compare Figure 1a with Figure 1b, we note that the effects of Ba2+ and Ca2+ on the turbidity are similar to each other. In PMA homopolymer systems, the turbidity began to increase at about 60% of DN, and then phase separation occurred. In contrast, the Ba2+/PEO-b-PMA and Ca2+/PEO-b-PMA systems were transparent below 80% DN and became slightly opalescent above 100% DN. This suggests that the solubility of the block copolymer and homopolymer complexes is quite different. The complexes of PEO-b-PMA remain soluble because of the effect of a water-soluble nonionic PEO block when the
Figure 2. Diameter of nanoaggregate as a function of DN. Counterions used were (a) Ba2+ and (b) Ca2+. [PEO-b-PMA] ) 0.1 (2), 0.3 (O), and 0.5 g L-1 (b).
PMA block is neutralized by the metal ions, whereas homopolymer PMA precipitates under the same condition. These observations are coincident with those obtained by Kabanov et al. in the earlier studies on PEO-b-PMA and cationic surfactants systems.15-18 DLS Measurements. Figure 2 shows a plot of diameter (2Rh) of the aggregates as a function of DN with Ba2+ or Ca2+. The size of these aggregates was unchanged for several weeks. In Figure 2a, a constant diameter about 115 nm is obtained for Ba2+/PEO-b-PMA aggregates in
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Langmuir, Vol. 18, No. 18, 2002 6729
Figure 4. SEM micrograph of the aggregates prepared from PEO-b-PMA and Ba2+ under 100% DN.
Figure 3. Dependence of the diameter of the nanoaggregate on PEO-b-PMA concentration. Counterions: (a) Ba2+ and (b) Ca2+. DN ) 100% for both (a) and (b).
the DN range of 100-300% for 0.3 and 0.5 g L-1 polymer concentrations. For a PEO-b-PMA concentration of 0.1 g L-1, however, nanoaggregates do not seem to form below 200% DN. Figure 2b shows that the diameter of Ca2+/ PEO-b-PMA aggregates is about 130 nm at DN above 100% when the polymer concentration is 0.3 and 0.5 g L-1. For the polymer concentration of 0.1 g L-1, like the case of the Ca2+/PEO-b-PMA system, the nanoaggregates begin to form after DN reaches 175%. It is seen from Figure 2b that the diameters of Ca2+/ PEO-b-PMA aggregates are very large (200 × 250 nm) when DN ranges from 20 to 40% in solutions with a polymer concentration of 0.5 g L-1 (Figure 2b). This implies that the PMA cores of the aggregates are swollen with water due to the repulsion between unneutralized carboxylate groups. However, these large aggregates do not appear for the Ba2+/PEO-b-PMA system (Figure 2a). The difference between the two systems may be ascribed to the difference in the Stokes radii of the two metal ions: the radius of Ba2+ is 0.290 nm, while that of Ca2+ is 0.310 nm. As Ca2+ has a larger radius, the effective charge of the ion is smaller. Therefore, the neutralization of the carboxylate group with Ca2+ ion is less effective, resulting in formation of a larger PMA core. The dependence of the size of the complexes between PEO-b-PMA and Ba2+ or Ca2+ on the polymer concentration is presented in Figure 3. The diameter of Ba2+-based systems is constant and nearly 120 nm when the polymer concentration is above 0.2 g L-1 (Figure 3a). From this result, the cac is evaluated to be 0.2 g L-1 when the DN is 100%. For the case of Ca2+ with 100% DN, polymer concentration of 0.2 g L-1 is also regarded as the cac from Figure 3b. It should be noted here that the results in Figures 2 and 3 coincide with each other although the samples were prepared in different ways: the former by neutralization
titration and the latter by concentration titration. The diameter of the aggregates ranges from 90 to 150 nm in both cases. This cross-check experiment seems to give concrete evidence for the nanoaggregate formation. SEM Characterization. The morphology of the aggregates of the Ba2+/PEO-b-PMA system with different DN was investigated by scanning electron microscopy. A typical micrograph is presented in Figure 4 for the sample with 100% DN. The aggregates are close to spherical particles with an average diameter which are in agreement with DLS data. Since these systems are stable in solution and their sizes are close to those of other block copolymer micelles,13 it seems reasonable to assume that they are micelle-like aggregates with a core of PMA segments neutralized with metal ions and a corona of PEO segments. Conclusions We have shown the nanoaggregate formation from PEOb-PMA and the alkaline earth metal ions such as Ba2+ and Ca2+. The aggregate formation is based on electrostatic neutralization of carboxylate groups of PMA block with the metal ions, which results in insolubilization of this block. These complexes represent micelle-like aggregates with a water-insoluble core of neutralized PMA block and water-soluble corona of PEO block. Turbidity measurements indicate that the metal ionneutralized PEO-b-PMA copolymer systems are watersoluble while PMA-based systems cause precipitation under the same concentration of the carboxylate and DN values. The investigation by light scattering indicates that these complexes are the aggregates with sizes ranging from 90 to 150 nm. The images obtained by SEM give us a clear evidence of the nanoaggregate formation. However, details of the nature and structure of these aggregates still remain unrevealed. Further investigations are now in progress. Acknowledgment. The authors thank Dr. Hiroyuki Nakamura (AIST) for the measurement of SEM. The present work is partly defrayed by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan (No. 12640559). LA025811Q
