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Langmuir 2008, 24, 13376-13382
pH/Potential-Responsive Large Aggregates from the Spontaneous Self-Assembly of a Triblock Copolymer in Water Jun Hu,†,‡ Xiuli Zhuang,† Lihong Huang,†,‡ Le Lang,†,‡ Xuesi Chen,*,† Yen Wei,*,†,§ and Xiabin Jing† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, Graduate School of Chinese Academy of Sciences, Beijing 100039, China, and Department of Chemistry, Drexel UniVersity, PennsylVania 19104 ReceiVed August 13, 2008. ReVised Manuscript ReceiVed September 18, 2008 A simple triblock copolymer, mPEG750-aniline pentamer-mPEG750, was prepared by condensation polymerization. The solubility of aniline pentamer in this kind of copolymer was improved in common solvents especially in aqueous solution, and the electroactivity of this copolymer was confirmed by UV-vis and CV in aqueous solution. When aniline pentamer was in its emeraldine state, the copolymer spontaneously self-assembled into large spheres (with diameters up to 1000 nm) in acidic aqueous solution (pH < 3), and into microspheres (with diameters of about 300 nm) in alkali aqueous solution, while the size of the aggregates decreased with the increase of pH. For reversible transition between the large spheres and microspheres under the change of the pH and potential, which changed the doping state and the oxidation state, respectively, the copolymer has potential applications in sensors, controlled drug release, and so forth.
Introduction Amphiphilic block copolymers can form aggregates and selfassemble into micelles.1-4 In block copolymer solutions, spherical micelles, rods, cylinders, tubules, and vesicles have been identified in many studies.5-9 However, large spherical directly selfassembled species (with diameters up to 1000 nm), especially species obtained by spontaneous self-assembly of small molecules in water, have hardly been observed.5,6,8 As a model of polyaniline, oligoaniline also has three different oxidation states, which are fully reduced leucoemeraldine (LM), half-oxidized emeraldine (EM) and fully oxidized pernigraniline (PNA). Among these states, only the EM salt shows high electrical conductivity. Polyaniline and oligoanilines have received considerable attention because of the ease in changing the oxidization states by chemistry or electrochemistry methods, and the ease in changing the doping state by adding acid or alkaline.9 More importantly, the changes of oxidization states and doping states offer the electroactive polymer new applications. Lire et al.10 synthesized polyaniline-polyacrylamide composites, which showed potential application of conducting semi-interpenetrating networks for the controlled release of safranin depending on the * Corresponding author. Tel.: + 86 431 85262112; fax: + 86 431 85685653; e-mail:
[email protected] (X.C.). Tel: + 1 215 895 2650; fax: + 1 215 895 1265; e-mail:
[email protected] (Y.W.). † Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences. § Drexel University. (1) Xu, R. L.; Winnik, M. A.; Hallett, F. R.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 87. (2) Erhardt, R.; Zhang, M. F.; Böker, A.; Zettl, H.; Abetz, C.; Frederik, P.; Krausch, G.; Abetz, V.; Müler, A. H. E. J. Am. Chem. Soc. 2003, 125, 3260. (3) Mai, Y. Y.; Zhou, Y. F.; Yan, D. Y. Macromolecules 2005, 38, 8679. (4) Schilli, C. M.; Zhang, M. F.; Rizzardo, E.; Thang, S. H.; Chong, Y. K.; Edwards, K.; Karlsson, G.; Mller, A. H. E. Macromolecules 2004, 37, 7861. (5) Zhang, L. F.; Eisenberg, A. Science. 1995, 268, 1728. (6) Zhang, L. F.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168. (7) Bermudez, H.; Brannan, A. K.; Hammer, D. A.; Bates, F. S.; Discher, D. E. Macromolecules 2002, 35, 8203. (8) Loos, K.; Böker; A.; Zettl, H.; Zhang, M. F.; Krausch, G.; Müler, A. H. E. Macromolecules 2005, 38, 873. (9) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. J. Synth. Met. 1987, 18, 285. (10) Lire, L. M.; Torresi, S. I. C. Electrochem. Commun. 2005, 7, 717.
oxidation state of the polyaniline. Hua et al.11 prepared pHsensitive hyperbranched sulfonated polydiphenylamines, which have potential applications as biosensors because of the pH sensitivity of the doping states. In our previous work, a triblock copolymer, PEG1500-aniline pentamer-PEG1500 was synthesized. This copolymer only self-assembled into micelles in alkali aqueous solution and the size of the aggregates changed with oxidation states, which provides a potential application in controlled drug release.12 In a recent study, we prepared an electroactive amphiphilic ABA copolymer mPEG750-aniline pentamer-mPEG750 by simple condensation polymerization. When the block aniline pentamer was in its half-oxidized EM state, the copolymer selfassembled spontaneously into large aggregates (with diameters up to 1000 nm) when the pH was
