pubs.acs.org/Langmuir © 2010 American Chemical Society
Synthesis of Electroactive Tetraaniline-PEO-Tetraaniline Triblock Copolymer and Its Self-Assembled Vesicle with Acidity Response Zhifang Yang,† Xiaotao Wang,† Yingkui Yang,‡ Yonggui Liao,† Yen Wei,*,†,§ and Xiaolin Xie*,† †
Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China, ‡ Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, China, and § Department of Chemistry, Drexel University, Philadelphia, Pennsylvania 19104 Received January 26, 2010. Revised Manuscript Received April 11, 2010 A novel ABA triblock copolymer containing electroactive tetraaniline [(ANI)4] and poly(ethylene oxide) (PEO600, Mn = 600), (ANI)4-b-PEO600-b-(ANI)4, was synthesized by coupling tetraaniline and PEO600 with tolylene 2,4diisocyanate. FTIR, NMR, and UV-vis spectroscopy were combined to characterize the chemical structure of (ANI)4b-PEO600-b-(ANI)4. The electrochemical properties, self-assembly, and acidity response of copolymer in aqueous solution were investigated by cyclic voltammetry, electron microscopy, and dynamic light scattering. Different from pure tetraaniline and polyaniline, the triblock copolymer in 1.0 M sulfuric acid solution only exhibits one oxidation peak in cyclic voltammetry. In a neutral aqueous solution, the triblock copolymer self-assembled into vesicles with diameter of about 258 nm. Upon acidification with HCl, the size of the vesicles increases to 471 nm and 1.19 μm when the concentration of HCl changes to 10-3 and 10-1 M, respectively. With addition of aqueous 1.0 M HCl to the triblock copolymer solution in THF, hollow spheres and bowl-like aggregates were obtained. A bilayer model was proposed for the vesicle formation, and the mechanism of acidity response was discussed.
Introduction Polyaniline (PANI), one of the most important conducting polymers, has attracted considerable attention owing to its facile synthesis, stability in ambient conditions, controllable electrical conductivity, and interesting redox properties.1 Recently, many efforts have been made on the preparation of PANI nanostructures because of their novel physical properties and potential applications, such as neuronal devices,2 molecular wires,3 chemical sensors,4 gas-separation membranes,5 supercapacitor,6 and artificial muscles,7 etc. Up to now, PANI nanostructures have been successfully fabricated by various methodologies including interfacial and templated polymerizations as well as electrospinning processing, which have been thoroughly reviewed recently.8 Block copolymers, due to their designable immiscibility between different blocks, often microphase-separate into nanostructures in bulk or self-assemble into various micelles in selective solvents such as spheric, cylindrical, tubular, and vesicular micelles. Such a characteristic of block copolymer prompted many researchers *Corresponding authors: Tel þ86-27-8754-0053, Fax þ86-27-8754-3632, e-mail:
[email protected] (X.X.); Tel þ1-215-895-2650, Fax þ1-215895-1265, e-mail:
[email protected] (Y.W.). (1) (a) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581–2590. (b) Heeger, A. J. Angew. Chem., Int. Ed. 2001, 40, 2591–2611. (2) Smerieri, A.; Berzina, T.; Erokhin, V.; Fontana, M. P. Mater. Sci. Eng., C 2008, 28, 18–22. (3) Long, Y.; Chen, Z.; Zheng, P.; Wang, N.; Zhang, Z.; Wan, M. J. Appl. Phys. 2003, 93, 2962–2965. (4) (a) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314–315. (b) Virji, S.; Huang, J.; Kaner, R. B.; Weiller, B. H. Nano Lett. 2004, 4, 491–496. (c) Forzani, E. S.; Li, X.; Tao, N. Anal. Chem. 2007, 79, 5217–5224. (d) Airoudj, A.; Debarnot, D.; B^eche, B.; Poncin-Epaillard, F. Anal. Chem. 2008, 80, 9188–9194. (5) (a) Li, D.; Huang, J.; Kaner, R. B. Acc. Chem. Res. 2009, 42, 135–145. (b) Wang, Z.; Liu, S.; Wu, P.; Cai, C. Anal. Chem. 2009, 81, 1638–1645. (6) Wang, Y. G.; Li, H. Q.; Xia, Y. Y. Adv. Mater. 2006, 18, 2619–2623. (7) Gu, B. K.; Ismail, Y. A.; Spinks, G. M.; Kim, S. I.; So, I.; Kim, S. J. Chem. Mater. 2009, 21, 511–515. (8) Wan, M. Conducting Polymers with Micro or Nanometer Structure; Springer: Berlin, 2008.
9386 DOI: 10.1021/la100382s
including us to fabricate PANI nanostructures via two methods. One was using self-assembled structures of block copolymers as templates to in situ synthesize PANI nanostructures. The other method was direct self-assembly of block copolymer containing electroactive PANI or aniline oligomers. For the former method, a block copolymer micelle is often used as a template to prepare PANI nanostructures.9-12 PANIs with nanofiber- and nanotubebased leaflike morphologies were synthesized using amphiphilic poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-b-PPO-b-PEO) as templates.9 Hollow PANI spheres were prepared via polymerization of aniline in the presence of poly(ε-caprolactone)-b-poly(ethylene oxide)-b-poly(ε-caprolactone) (PCL-b-PEG-b-PCL) micelles.10 Near-monodispersed PANI particles were prepared using poly(methyl acrylate-co-2-acrylamido2-methyl-1-propanesulfonic acid) block copolymer micelles as templates, where the moieties of sulfonic acid were dopants for PANI.11 The size of the particles was controlled by tuning the hydrophilic block length of the block copolymer. Li et al.12 reported the fabrication of highly porous PANI nanofiber films using polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) as templates. In addition, well-ordered lamellar structure of block copolymer was also employed as a template to prepare the nanocomposites with ordered structure. For example, Kim et al.13 prepared electrically conductive nanocomposites with anisotropy using the lamellar structure of polystyrene-b-poly(n-butyl methacrylate) (PS-b-PnBMA) as templates. Matyjaszewski and coworkers14 utilized a block copolymer with a hydrophobic soft (9) Han, J.; Song, G.; Guo, R. Adv. Mater. 2007, 19, 2993–2999. (10) Cheng, D.; Ng, S. C.; Chan, H. S. O. Thin Solid Films 2005, 477, 19–23. (11) Bucholz, T.; Sun, Y.; Loo, Y. L. J. Mater. Chem. 2008, 18, 5835–5842. (12) Li, X.; Tian, S.; Ping, Y.; Kim, D. H.; Knoll, W. Langmuir 2005, 21, 9393– 9397. (13) Lee, D. H.; Chang, J. A.; Kim, J. K. J. Mater. Chem. 2006, 16, 4575–4580. (14) (a) McCullough, L. A.; Dufour, B.; Tang, C.; Zhang, R.; Kowalewski, T.; Matyjaszewski, K. Macromolecules 2007, 40, 7745–7747. (b) McCullough, L. A.; Dufour, B.; Matyjaszewski, K. Macromolecules 2009, 42, 8129–8137.
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poly(methyl acrylate) block and a strongly acidic dopant poly(2-acrylamido-2-methyl-1-propanesulfonic acid) block as template to prepare PANI conducting film with high mechanical flexibility and durability via supramolecular recognition. The block copolymers containing PANI blocks have also been explored quite extensively.15-17 They can often self-assemble into various nanostructures. We have synthesized a rod-coil-rod (ANI)98-(PEO)136-(ANI)98 (the subscript represents the number of repeat units) triblock copolymer by polymerization of aniline in the presence of amino-terminated PEO. It is found that (ANI)98(PEO)136-(ANI)98 can assemble into spheric, rodlike, fiber, and petaline micelles depending on the solvent used and the concentration of copolymer.16 Yan et al.17 prepared PANI-PEOPANI triblock copolymer by a similar method, and helical superstructures were obtained. However, it is very difficult to obtain block copolymer with well-defined structure due to the poor solubility of polyaniline, lack of suitable purification method, and inability of controlling the number of ANI repeating units in the copolymers. Aniline oligomers, on the other hand, are soluble in many common solvents, exhibit similar electroactivity as PANI, and have well-defined structure, designed end group, and monodispersed molecular weight.18 Therefore, a number of block copolymers containing oligoanilines have been synthesized in order to produce electroactive nanostructures. Wei and co-workers19 prepared an electroactive and biodegradable polylactide-b-(ANI)5-bpolyactide (PLA-b-(ANI)5-b-PLA) film that could serve as scaffold for neuronal or cardiovascular tissue engineering. Later, they synthesized water-soluble and biocompatible triblock copolymers of PEO1500-b-(ANI)5-b-PEO150020 and PEO750-b-(ANI)5b-PEO750.21 It is found that these copolymers form the large aggegates, and their sizes decrease with increasing pH values or changing the redox state of aniline blocks from emeraldine base (EB) to leucoemeraldine base (LEB). Wang et al.22 synthesized a diblock oligomer of poly(L-lactide)-b-(ANI)4 ((PLLA)24-b-(ANI)4) by ring-opening polymerization of L-lactide. By changing the aniline block from LEB to EB state, the morphologies of the selfassemblies vary from spherical micelles to ringlike aggregates composed of much smaller spherical micelles. Later, they synthesized oligostyrene-b-(ANI)3-b-oligostyrene with tert-butylcarbamate-protected trianiline.23 The block oligomer self-assembled into spheric micelles and vesicles in THF before and after removing the protecting group, respectively.23 In this article, we present the synthesis of electroactive (ANI)4b-PEO600-b-(ANI)4 triblock copolymer with well-defined new structure of two hydrophobic oligoaniline end blocks sandwiching a hydrophilic PEO block. Its electrochemical properties were characterized by ultraviolet-visible (UV-vis) spectroscopy and (15) (a) Kinlen, P. J.; Frushour, B. G.; Ding, Y.; Menon, V. Synth. Met. 1999, 101, 758–761. (b) Li, S.; Dong, H.; Cao, Y. Synth. Met. 1989, 29, E329–E336. (16) Yang, Z.; Wu, J.; Yang, Y.; Zhou, X.; Xie, X. Front. Chem. Eng. China 2008, 2, 85–88. (17) Yan, L.; Tao, W. J. Polym. Sci., Polym. Chem. 2008, 46, 12–20. (18) (a) Wei, Y.; Yang, C.; Ding, T. Tetrahedron Lett. 1996, 37, 731–734. (b) Wei, Y.; Yang, C.; Wei, G.; Ding, T.; Feng, G. Synth. Met. 1997, 84, 289–291. (c) Wei, Y.; Li, S.; Jia, X.; Chen, M. H.; Mathai, M. W.; Yeh, J. M.; Li, W.; Wang, Z. Y.; Yang, C.; Gao, J. P.; Jansen, S. A.; Narkis, M.; Siegmann, A.; Hsieh, B. R. In Semiconducting Polymers: Applications, Properties and Synthesis; Hsieh, B. R., Wei, Y., Eds.; ACS Symposium Series 735; American Chemical Society: Washington, DC, 1999; pp 384-398. (d) Wei, Y.; Yu, Y.; Zhang, W.; Wang, C.; Jia, X.; Jansen, S. A. Chin. J. Polym. Sci. 2002, 20, 105–118. (19) Huang, L.; Hu, J.; Lang, L.; Wang, X.; Zhang, P.; Jing, X.; Wang, X.; Chen, X.; Lelkes, P. I.; MacDiarmid, A. G.; Wei, Y. Biomaterials 2007, 28, 1741–1751. (20) Huang, L.; Hu, J.; Lang, L.; Chen, X.; Wei, Y.; Jing, X. Macromol. Rapid Commun. 2007, 28, 1559–1566. (21) Hu, J.; Zhuang, X.; Huang, L.; Lang, L.; Chen, X.; Wei, Y.; Jing, X. Langmuir 2008, 24, 13376–13382. (22) Wang, H.; Guo, P.; Han, Y. Macromol. Rapid Commun. 2006, 27, 63–68. (23) Wang, H.; Han, Y. Macromol. Rapid Commun. 2009, 30, 521–527.
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cyclic voltammetry (CV). Different from the above-mentioned PEO1500-b-(ANI)5-b-PEO1500 and PEO750-b-(ANI)5-b-PEO750 triblock copolymers, the (ANI)4-b-PEO600-b-(ANI)4 triblock copolymer self-assembles into thin-shell vesicles in acidic media, and the self-assembled nanostructures and the size of vesicles in aqueous solution can be tuned conveniently by adjusting acidity of the media.
Experimental Section Materials. Poly(ethylene oxide) (PEO600, Mn = 600, from Aldrich Chemical Co.) was dried by azeotropic distillation with dry toluene before use. N-Phenyl-1,4-phenylenediamine (aniline dimer, from Acros Co.) was used as received. N,N0 -Dimethylformamide (DMF) was distilled and stored in 4 A˚ molecular sieves to eliminate any traces of water. Tolylene-2,4-diisocyanate (TDI), ammonium peroxydisulfate, hydrochloric acid (37%), ammonium hydroxide, 1-methyl-2-pyrrolidone (NMP), and other chemicals were purchased from Shanghai Reagents Co., China, and used as received. Water employed in all the experiments was doubly distilled. Synthesis of (ANI)4-b-PEO600-b-(ANI)4 Triblock Copolymer. Synthesis of Tetraaniline. Tetraaniline was synthe-
sized following similar procedure in the literature.24 As a typical preparation, aniline dimer (18.525 g, 0.10 mol) was dispersed in 500 mL of 0.10 M HCl aqueous solution with vigorous mechanical stirring for 5 h under a N2 atmosphere. Ferric chloride (16.201 g, 0.10 mol) was dissolved in 100 mL of 0.10 M HCl, and the solution was added to the above aniline dimer suspension at room temperature under strong stirring. After stirring at room temperature for 3 h, the suspension was filtered under vacuum, and the precipitate collected was washed using 0.10 M HCl three times to remove any residual ferric chloride. The tetraaniline base (15.5 g, 84%) was obtained by dedoping with ammonium hydroxide followed by drying in vacuum at 60 °C for 48 h. By the oxidative coupling reaction of aniline dimer, tetraaniline was obtained because its emeraldine base state (EB) cannot be further oxidized to longer oligomers in the presence of excess oxidizing agent.25 The molecular weight of tetraaniline was confirmed by mass spectroscopy. Synthesis of (ANI)4-b-PEO600-b-(ANI)4. A solution of 3.0 g of dried PEO600 (5.0 mmol) and 10 mL of dried DMF was added dropwise into the solution of 1.827 g of TDI (10.0 mmol) and 20 mL of dried DMF in ice bath under a N2 atmosphere. After stirring for 6 h at 0 °C, the reaction was kept in oil bath at 30 °C for another 6 h. Then the mixture was added dropwise to another flask charged with 4.026 g of tetraaniline (11.0 mmol) and 20 mL of dried DMF at room temperature. Before the tetraaniline was used, it was further purified by extracting with ethanol. After stirring for 12 h, the solution was precipitated twice in abundant double-distilled water and subsequently in anhydrous ether. The product collected as powder (4.8 g, 58%) was dried in vacuum at 35 °C. Preparation of Micellar Solutions. In order to study the selfassembly behaviors of (ANI)4-b-PEO600-b-(ANI)4 in different solutions, 0.2 mg/mL stock solution was prepared by dissolving 10 mg of (ANI)4-b-PEO600-b-(ANI)4 in 50 mL of THF under stirring overnight at room temperature. The solution was then filtered through a Nylon filter with a nominal pore size of 0.45 μm. Then 10 mL of double-distilled water was added into 10 mL stock solution slowly under vigorous stirring. The solution was dialyzed against double-distilled water to remove THF for 3 days. The double-distilled water was changed twice a day. Finally, the (ANI)4b-PEO600-b-(ANI)4 aqueous solution with the concentration of 0.10 mg/mL was obtained. The fresh solution was used for the (24) Zhang, W. J.; Feng, J.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1997, 84, 119–120. (25) Wei, Y.; Tang, X.; Sun, Y. J. Polym. Sci., Polym. Chem. 1989, 27, 2385– 2396.
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Yang et al. Scheme 1. Synthetic Route of (ANI)4-b-PEO600-b-(ANI)4
TEM and DLS measurements. To investigate the effect of acidity, 10-3, 10-1, and 1.0 M HCl solutions were used instead of water. Characterization. FT-IR spectra were recorded on an Equinox 55 spectrometer (Bruker) using the KBr pellet technique. UV-vis spectral measurements were carried out on a Shimadzu UV-2550 diode array spectrophotometer. 1H NMR spectra were obtained on a Bruker AV400 spectrometer using tetramethylsilane (TMS) as internal standard and deuterated dimethyl sulfoxide (d-DMSO) as solvent. Thermogravimetric analysis (TGA) was conducted on Perkin-Elmer TGA-7 instrument at a heating rate of 10 °C/min in argon flow (10 mL/min). Cyclic voltammetry (CV) was performed using CS150 electrochemical workstation (CorrTest Co., China) in a three-electrode electrochemical cell using Ag/Agþ as the reference electrode and platinum wires as the counter electrode. The working electrode was prepared by casting THF solution of the triblock copolymer onto the surface of the platinum plate electrode. The CV experiment was carried out in 1.0 M sulfuric acid aqueous solution in the range from -0.1 to 1.0 V at a scan rate of 100 mV/s. Transmission electron microscopy (TEM) was performed on a Tecnai G220 electron microscope (PEI Co., Netherlands) at an accelerating voltage of 200 kV. The specimens for TEM examination were prepared by depositing a drop of the micellar solution on carbon-coated copper grids. Scanning electron microscopy (SEM) was carried out on a Sirion 200 field emission scanning electron microscope (PEI Co., Netherlands) at an accelerating voltage of 10 kV. The samples were prepared by depositing a drop of the solution on a glass slide. Dynamic light scattering (DLS) was performed on an LB-550 Nano-Analyzer (Horiba, Japan) with a light source of 650 nm.
Figure 1. FT-IR spectra of PEO600 (a), tetraaniline (b), and (ANI)4-b-PEO600-b-(ANI)4 (c).
Synthesis of (ANI)4-b-PEO600-b-(ANI)4. As shown in Scheme 1, one PEO600 and two tetraaniline molecules are chemically connected with TDI. Because of the asymmetrical structure of TDI, the inherent reactive activity of the -NCO group in position 4 is much higher than that of the -NCO group in position 2 when reacting with a hydroxyl group.26 When PEO600 was added dropwise into a TDI solution, the 4-NCO in TDI is preferentially reacted with the hydroxyl group in PEO600 by nucleophilic addition to form the 2-NCO group-terminated PEO600 (designated as TDI-PEO600-TDI). Because the primary amine is more reactive than the secondary amine toward the NCO group, the amino group at the end of tetraaniline preferentially reacted with TDI-PEO600-TDI to form (ANI)4-b-PEO600-b-(ANI)4 with a defined structure. The resulting copolymer was purified by washing with water and diethyl ether to remove the unreacted PEO600 and tetraaniline, respectively. The chemical structure of (ANI)4-b-PEO600-b-(ANI)4 has been characterized by FT-IR and NMR spectroscopy. In Figure 1b,
the characteristic peaks for tetraaniline are located at 1596, 1504, 1303, 1171, 829, 748, and 696 cm-1, which are attributed to CdC stretching vibrations of the quinonoid ring and benzenoid ring, bending vibration of C-H (in-plane) in quinonoid ring, stretching vibration of C-N bond linked with benzenoid ring, and bending vibrations of C-H (out-of-plane) in para-disubstituted and monosubstituted benzene rings, respectively.27 Compared with tetraaniline, (ANI)4-b-PEO600-b-(ANI)4 (Figure 1c) exhibits several new peaks at 1720 and 2867 cm-1 bands which are assigned to the stretching vibrations of carbonyl group and C-H in saturated alkane, respectively. The peak at 1066 cm-1 is attributed to stretching vibrations of C-O-C in PEO blocks as that of PEO600 (Figure 1a). The peak at 875 cm-1 is ascribed to the out-of-plane bending vibration of 1,2,4-trisubstituted benzene ring in TDI. From 1H NMR spectra in Figure 2, compared with tetraaniline, several new peaks appear in the triblock copolymer besides the proton signals of the benzene ring at 6.5-7.5 ppm. The peaks at 4.2, 3.6, and 3.5 ppm are assigned to the protons (i.e., b, c and d) in PEO, and the peak at 2.1 ppm is related to the proton (e) of methyl group in TDI. Additionally, the peaks (a) above 7.5 ppm are attributed to the active hydrogen of N-H in the amide and tetraaniline. These results support that (ANI)4b-PEO600-b-(ANI)4 triblock copolymer has been synthesized successfully. Figure 3 gives TGA curves of tetraaniline, (ANI)4-b-PEO600b-(ANI)4, and PEO600. Tetraaniline exhibits a good thermal stability as evidenced by the fact that the decomposition process appears in the high-temperature range of 403-562 °C, which is attributed to the thermal degradation of tetraaniline backbone, whereas the residues at 700 °C could be highly cross-linked structures.28
(26) (a) Krol, P. J. Appl. Polym. Sci. 1998, 69, 169–181. (b) Bartelink, C. F.; Pooter, M. D.; Gr€unbauer, H. J. M.; Beginn, U.; M€oller, M. J. Polym. Sci., Polym. Chem. 2000, 38, 2555–2565.
c-Marjanovic, G.; Trchova, M.; Konyushenko, E. N.; Holler, P.; (27) Ciri Stejskal, J. J. Phys. Chem. B 2008, 112, 6976–6987. (28) Lu, X.; Tan, C. Y.; Xu, J.; He, C. Synth. Met. 2003, 138, 429–440.
Results and Discussion
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Figure 4. UV-vis spectra of tetraaniline (a) and (ANI)4-b-PEO600b-(ANI)4 (b) in NMP (1) and their doping behaviors by adding HCl (2).
Figure 2. 1H NMR spectra of PEO600, tetraaniline, and (ANI)4b-PEO600-b-(ANI)4.
Figure 3. TGA curves of tetraaniline (a), (ANI)4-b-PEO600b-(ANI)4 (b), and PEO600 (c).
PEO600 starts to degrade at 250 °C and fully decomposes at 358 °C. As expected, the decomposition of (ANI)4-b-PEO600-b-(ANI)4 takes place in two stages, corresponding to the thermal degradation of PEO at 260-328 °C and decomposition of tetraaniline block backbone at 328-517 °C. It is notable that the decomposition of PEO block in (ANI)4-b-PEO600-b-(ANI)4 shifts to higher temperature as compared with pure PEO, probably due to the restriction effect of tetraaniline block on PEO chains. On the basis of the residues at 650 °C, 55% and 21%, associated with highly cross-linked tetraaniline and the tetraaniline block in the copolymer, respectively, the mass fraction of tetraaniline blocks in the copolymer is calculated to be about 40%, which is close to the theoretical value, 44%, of tetraaniline block in the triblock copolymer. Electrochemical Properties of (ANI)4-b-PEO600-b-(ANI)4. Figure 4 shows UV-vis spectra of tetraaniline and (ANI)4b-PEO600-b-(ANI)4 in NMP and the proton doping upon addition of HCl solution. In NMP, both of them show two bands at around 310 and 580 nm (Figures 4a-1 and 4b-1), which are assigned Langmuir 2010, 26(12), 9386–9392
Figure 5. Cyclic voltammetry of tetraaniline (a) and (ANI)4-bPEO600-b-(ANI)4 (b) in 1 M H2SO4.
to the π-π* transition of the benzene ring and the benzenoid to quiniod (πB-πQ) excitonic transition.29 Noticeably, the πB-πQ band of the triblock copolymer is blue-shifted by Δλ = 8 nm compared to that of tetraaniline, which is attributed to its connection to electron-withdrawing acyl groups. Upon doping with HCl solution, two new bands for tetraaniline at 430 and 800 nm emerge (Figure 4a-2), which are attributed to the delocalized polaron bands. However, there is no significant change for the triblock copolymer (Figure 4b-2), which suggests that tetraaniline blocks in the copolymer are not protonated under the experimental conditions. The similar phenomenon is observed in (ANI)4b-PEO2000-b-(ANI)4 triblock copolymer and the tolyl isocyanate-capped tetraaniline, (ANI)4-TDI-(ANI)4 (see Supporting Information). One possible reason could be that NMP is a basic solvent in a large quantity, which would compete favorably against aniline units for protons.30 In addition, the connection to (29) Chen, L.; Yu, Y.; Mao, H.; Lu, X.; Zhang, W.; Wei, Y. Mater. Lett. 2005, 59, 2446–2450. (30) Chen, S. A.; Lee, H. T. Macromolecules 1993, 26, 3254–3261.
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Figure 6. TEM (a), SEM images (b), and particle size distributions (c) of (ANI)4-b-PEO600-b-(ANI)4 in H2O (I), 10-3 M HCl (II), and 10-1 M HCl (III).
electron-withdrawing acyl groups would reduce the basicity of nitrogen atoms in the tetraaniline blocks of the copolymer, which in turn may hinder their protonation. The mechanism is still under research. Cyclic voltammograms (CV) shown in Figure 5 were obtained in 1.0 M sulfuric acid solution with a scan rate of 100 mV/s in a three-electrode electrochemical cell. In Figure 5a, there are two oxidation peaks at 0.330 and 0.493 V vs Ag/AgCl for tetraaniline, which are corresponding to the transitions from leucoemeraldine base (LEB) to emeraldine base (EB) oxidation state and EB to pernigraniline (PA) oxidation state, respectively.29 However, only one oxidation peak at 0.559 V was observed for (ANI)4b-PEO600-b-(ANI)4 (Figure 5b), which could be attributed to the transition from LEB to EB state of tetraaniline block. Wang et al.22 also observed that the tetraaniline blocks in (PLLA)24b-(ANI)4 diblock copolymer only show one oxidation process from LEB to EB state based on UV-vis spectroscopy. These results are consistent with the electrochemical behavior of poly(methacrylamide) containing tetraaniline side chains as well.31 Moreover, the oxidation potential of tetraaniline blocks in the copolymer (0.559 V) (31) Chen, R.; Benicewicz, B. C. Macromolecules 2003, 36, 6333–6339.
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Figure 7. UV-vis spectra of (ANI)4-b-PEO600-b-(ANI)4 in THF (a) and in water without HCl (b) in 10-3 M HCl (c) and 10-1 M HCl (d). The inset is the magnified spectra.
is higher than that of tetraaniline (0.330 V), which could be caused, again, by the effect of electron-withdrawing acyl groups in the copolymer that should increase the oxidation energy barrier. Langmuir 2010, 26(12), 9386–9392
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Figure 8. Schematic illustration of self-assembling mechanism of the (ANI)4-b-PEO600-b-(ANI)4 vesicles and its acidity-responsive behaviors in aqueous solution with or without addition of HCl.
Self-Assembling Behavior of (ANI)4-b-PEO600-b-(ANI)4. Generally, the micellar solution of an amphiphilic block copolymer can be prepared by two approaches. One is direct dissolution of the copolymer in a solvent such as water. In the other method, water is added into the solution of block copolymer in a watermiscible organic solvent, and the organic solvent is then removed by dialysis or evaporation. The former is suitable for the preparation of starlike micelles, and the latter is preferred for the crewcut micelles. Unlike PEO1500-b-(ANI)5-b-PEO150020 and PEO750b-(ANI)5-b-PEO750,21 (ANI)4-b-PEO600-b-(ANI)4 is hardly dissolved in water directly due to short PEO block. Therefore, the micellar solutions were prepared by adding water to the THF solution of the copolymer following by dialyzing against water to remove THF. Figure 6 shows the aggregate morphologies and particle size distribution of (ANI)4-b-PEO600-b-(ANI)4 in aqueous solution at different HCl concentrations. In neutral aqueous solution (Figure 6I), the triblock copolymer self-assembles into vesicles and the average diameter of the vesicles is about 291 ( 89 nm as estimated from measuring from 68 particles in TEM images. This value is in reasonable accordance with 258 nm from DLS (Figure 6Ic). When water was replaced by 10-3 M HCl aqueous solution, larger vesicles were obtained (Figures 6IIa and 6IIb), and most of them were in a broken or collapsed form. DLS results in Figure 6IIc show that the mean diameter of the vesicles is about 471 nm. When the HCl concentration was increased to 10-1 M, the average diameter of vesicles is estimated to be about 1.19 ( 0.42 μm according to the TEM images (Figures 6IIIa and 6IIIb). This size is considerably smaller than that from DLS (2.87 μm), which might be due to secondary aggregation of the vesicles during DLS mesurement. Nevertheless, the overall trend is that the size of the vesicles increases with increasing the solution acidity. As reported, PEO1500-b-(ANI)5-b-PEO1500 and PEO750b-(ANI)5-b-PEO750 can form large spheres by secondary aggregation of the small starlike micelles with ANI block as core and PEO as corona in water.20,21 In this work, (ANI)4-b-PEO600b-(ANI)4 self-assembles into vesicles. The different morphologic structures are related to their difference in chemical structures, weight fraction of hydrophilic PEO block, and the rigidity of hydrophobic ANI block. On the one hand, micellar morphologies tend to change from spheres to cylinders to vesicles, as the weight fraction of hydrophilic block in the amphiphilic block copolymer is decreased.32 The weight fraction of PEO blocks is 36 wt % in (32) (a) Zupancich, J. A.; Bates, F. S.; Hillmyer, M. A. Macromolecules 2006, 39, 4286–4288. (b) Jain, S.; Bates, F. S. Science 2003, 300, 460–464.
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(ANI)4-b-PEO600-b-(ANI)4, which is much lower than 82 wt % in PEO1500-b-(ANI)5-b-PEO150020 and 69 wt % in PEO750b-(ANI)5-b-PEO750.21 On the other hand, the self-assembled structure of block copolymer also depends on the hydrophobic block rigidity and interaction.33 As shown in Figure 7, the π-π* and πB-πQ adsorption bands of (ANI)4-b-PEO600-b-(ANI)4 in THF are located at 310 and 569 nm. After THF is replaced by water, the adsorptions decrease due to the formation of aggregates, and the πB-πQ band are red-shifted from 569 to 598 nm, which indicates the strong π-π interaction of ANI blocks in the aggregates. Though the coil-coil block copolymers containing 36 wt % of PEO blocks preferentially form micelles,32 the rigid conformation of ANI blocks and strong π-π interactions between (ANI)4b-PEO600-b-(ANI)4 triblock copolymers lead to the formation of vesicles. Such π-π interaction and strong hydrogen bonding between amine group and imine nitrogen atoms should induce the side-by-side ordering of ANI blocks. In comparison, (ANI)4b-PEO2000-b-(ANI)4 triblock copolymers with longer hydrophilic PEO segments only form the spherical micelles in the same conditions (see Supporting Information). In addition, the wall thickness of vesicles is between 11 and 22 nm as estimated from TEM images in Figure 6, and the computation using the molecular mechanical models shows the length of a single stretched (ANI)4-b-PEO600-b-(ANI)4 molecular chain is about 10.9 nm (see Supporting Information). Therefore, we propose that the vesicle membrane consists of a bilayer where the ANI blocks are located in the center and the PEO600 blocks direct toward water as illustrated in Figure 8. The wall thickness difference between TEM results and theoretical value is associated with the packing defects. Generally, amphiphilic block copolymers form the vesicles via a two-step self-assembly process.34 First, the copolymers form a bilayer disk, and then the disk bends into a vesicle. In the second step, the closure of the bilayer disk takes place only under the prerequisite that the line energy of the disk (Edisk) is higher than the bending energy (Ebending).34 For a given disk area, the disk radius is twice as large as the vesicle radius, so that the balance of the line energy and bending energy defines the minimal vesicle size. The radius of the minimal vesicle (RV) can be expressed as RV =2κ/γ, wherein κ is the bending elasticity and γ is the line tension of the disk rim. Additionally, in a sharp contrast to the solutions in NMP as described earlier, the protonation (i.e., doping) of tetraaniline blocks has been achieved in acidic aqueous solutions (33) (a) Mao, M.; Turner, S. R. J. Am. Chem. Soc. 2007, 129, 3832–3833. (b) Bellomo, E. G.; Wyrsta, M. D.; Pakstis, L.; Pochan, D. J.; Deming, T. J. Nat. Mater. 2004, 3, 244–248. (34) Antonietti, M.; F€orster, S. Adv. Mater. 2003, 15, 1323–1333.
DOI: 10.1021/la100382s
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upon adding water to their solutions in organic solvents36 and in polymer latex particles upon freezing and solvent evaporation.37
Conclusions
Figure 9. TEM (a) and SEM (b) images of the aggregates of (ANI)4-b-PEO600-b-(ANI)4 obtained by adding 1.0 M HCl(aq) to the copolymer solution in THF.
as evidenced by the emergence of delocalized polaron band in the near-IR region (i.e., broad band at around and beyond 700 nm) as shown in the inset in Figure 7. Moverover, the higher the HCl concentration, the more pronounced the polaron band. The protonation (i.e., doping) convert ANI blocks from EB to emeraldine salt (ES) with increasing the chain conjugation, leading to higher rigidity of ANI blocks and, consequently, the higher bending elasticity (κ) that favors the formation of larger vesicles. Furthermore, the morphology of aggregates is usually related to a dimensionless “packing parameter” (p) expressed as p = ν/al, where v is the volume of the hydrophobic block chain, a is the optimal interfacial area per molecule, and l is the hydrophobic length, i.e., p e 1/3 for spheres, 1/3 e p e 1/2 for cylinders, and 1/2 e p e 1 for bilayers.35 For (ANI)4-b-PEO600-b-(ANI)4, the insertion of HCl to the tetraaniline blocks has relatively little effect on the interfacial area and the hydrophobic chain length but increases the volume of the hydrophobic chains and the packing parameter. Consequently, the vesicles are enlarged due to the formation of flatter bilayers. In THF, the (ANI)4-b-PEO600-b-(ANI)4 triblock copolymer forms large aggregates including hollow spheres and bowl-like structure upon addition of 1.0 M HCl aqueous solution (see Figure 9). The wall of the hollow spheres is much thicker than that of the vesicles in Figure 6, and many of the spheres have a hole on the surface (highlighted by an arrow in Figure 9a,b). The mechanism for forming these structures has not yet established. It is interesting to note that similar hollow spheres and bowl-like aggregates were also observed in amphiphilic random or block copolymers (35) Du, J.; O’Reilly, R. K. Soft Matter 2009, 5, 3544–3561. (36) (a) Riegel, I. C.; Eisenberg, A.; Petzhold, C. L.; Samios, D. Langmuir 2002, 18, 3358–3363. (b) Liu, X.; Kim, J. S.; Wu, J.; Eisenberg, A. Macromolecules 2005, 38, 6749–6751. (c) Bryaskova, R.; Willet, N.; Debuigne, A.; Jer^ome, R.; Detrembleur, C. J. Polym. Sci., Polym. Chem. 2007, 45, 81–89.
9392 DOI: 10.1021/la100382s
A novel electroactive triblock copolymer, (ANI)4-b-PEO600b-(ANI)4, was successfully synthesized by a condensation reaction of poly(ethylene oxide) (PEO, Mn = 600) and tolylene 2,4diisocyanate to afford the NCO-group-terminated PEO followed by further reacting with tetraanilines. Spectroscopic methods in conjunction with microscopic techniques clearly reveal its chemical structures and self-assembly behaviors in solutions. Cyclic voltammetry was also employed to investigate its electrochemical properties. It is found that differing from those of tetraaniline and polyaniline, (ANI)4-b-PEO600-b-(ANI)4 gives one oxidation peak in 1.0 M sulfuric acid solution, which is attributed to the transitions of LEB state to EB state. The self-assembly of (ANI)4b-PEO600-b-(ANI)4 in its aqueous solution forms vesicles with a mean diameter of 258 nm. And, the diameter of the vesicles increases to 471 nm and 1.19 μm when the concentration of HCl in water changes to 10-3 and 10-1 M, respectively. Once the concentration of HCl is raised to 1 M, hollow spheres and bowl-like aggregates are obtained, indicating a characteristic morphology dependence on acidity. These electroactive and acidity-responsive vesicles might have potential applications in drug delivery and control release, nanoreactors, and electromagnetic shielding microcapsules. Acknowledgment. We acknowledge the financial support by the Outstanding Youth Fund of the National Natural Science Foundation of China (50825301) and by the Fund from the State Key Laboratory of Materials Processing and Molding Technology at Huazhong University of Science and Technology (HUST). We thank the HUST Analytical and Testing Center for allowing us to use its facilities. Y. Wei is grateful to HUST for a guest professor appointment and to the Li Ka-shing Foundation and the Chinese Ministry of Education for an endowed Cheung-Kong Lecture-Chair Professorship that has enabled him to travel and collaborate with colleagues in China. Supporting Information Available: Experiments and results of (ANI)4-b-PEO2000-b-(ANI)4, the calculated length of a fully stretched (ANI)4-b-PEO600-b-(ANI)4 molecular, the size distribution, and wall thickness of the vesicles estimated from TEM images. This material is available free of charge via the Internet at http://pubs.acs.org. (37) (a) Saito, N.; Kagari, Y.; Okubo, M. Langmuir 2006, 22, 9397–9402. (b) Jeong, U.; Im, S. H.; Camargo, P. H. C.; Kim, J. H.; Xia, Y. Langmuir 2007, 23, 10968–10975.
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