Formation of Intermicellar-Chained and Cylindrical Micellar Networks

Jun 9, 2009 - Telephone: +82 62 970 2306. ... PHIC-b-P2VP network nanostructures were used as a template for the in situ synthesis of Au nanoparticles...
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Formation of Intermicellar-Chained and Cylindrical Micellar Networks From an Amphiphilic Rod-Coil Block Copolymer: Poly(n-hexyl isocyanate)-block-poly(2-vinylpyridine) Haeng-Deog Koh, Mohammad Changez, M. Shahinur Rahman, and Jae-Suk Lee* Department of Materials Science and Engineering and Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro (Oryong-dong), Buk-gu, Gwangju 500-712, Korea Received February 22, 2009. Revised Manuscript Received May 16, 2009 Morphologies of the poly(n-hexyl isocyanate)-block-poly(2-vinylpyridine) (PHIC-b-P2VP, fP2VP = 0.3) amphiphilic rod-coil block copolymer were studied in rod-selective chloroform (CHCl3), both-block-soluble tetrahydrofuran (THF), and CHCl3/THF mixed solvent systems. Spherical, solid micelles with a P2VP core and PHIC shell were formed in CHCl3, whereas a microphase-separated liquid crystalline morphology was prominent in the presence of THF. In the CHCl3/THF mixed solvent system, a unique long-range intermicellar-chained network (v/v = 7/3) and a more evolved cylindrical micellar network (v/v = 3/7) were remarkably formed, respectively. PHIC-b-P2VP network nanostructures were used as a template for the in situ synthesis of Au nanoparticles (8 nm) selectively within the functional P2VP core domains.

Introduction Well-defined supramolecular architectures that are selfassembled from block copolymers have attracted interest in the field of materials science. Generally, block copolymers self-assemble into micro- to nanometer sized spherical, cylindrical, and wormlike micelles, as well as lamellar and vesicular structures. These depend on molecular weight, hydrophilic/ hydrophobic block ratio, block arrangement, and relative affinity of two blocks to the dispersed solvent.1-3 These well-defined self-assemblies of block copolymers have been used for various applications, including catalytic supports,4 drug delivery carriers,5 lithographic templates,6 and nanoreactors7,8 for the synthesis of metallic/semiconductor nanoparticles (NPs) or nanorods. Aggregation of an amphiphilic block copolymer comprising two distinct immiscible blocks, in a solvent that is selective for one block, leads to the formation of spherical micelles with an insoluble-block as the core and a soluble-block as the shell. In addition, a structural transition in spherical micelles can be induced by changing the solvent composition. We recently reported core-shell inversion of spherical micelles for polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) in PS-selective solvent (toluene) by adding an excess amount of a P2VP-selective solvent (methanol).9 *Corresponding author. Telephone: +82 62 970 2306. Fax: +82 62 970 2304. E-mail: [email protected]. (1) Hadjichristidis, N.; Prispas, S.; Floudas, G. A. Block Copolymers; WileyVCH: New York, 2003. (2) F€orster, S.; Plantenberg, T. Angew. Chem., Int. Ed. 2002, 41, 688. (3) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735. (4) Hong, S. C.; Rief, U.; Kristen, M. O. Macromol. Rapid Commun. 2001, 22, 1447. (5) Kim, S. Y.; Cho, S. H.; Lee, Y. M.; Chu, L. Y. Macromol. Res. 2007, 15, 646. (6) Kim, S. W.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; de Pablo, J. J.; Nealey, P. F. Nature 2003, 424, 411. (7) Spatz, J. P.; M€ossmer, S.; Hartmann, C.; M€oller, M. Langmuir 2000, 16, 407. (8) Lazzari, M.; Scalarone, D.; Hoppe, C. E.; Vazquez-Vazquez, C.; LopezQuintela, M. A. Chem. Mater. 2007, 19, 5818. (9) Koh, H.-D.; Kang, N.-G.; Lee, J.-S. Langmuir 2007, 23, 11425.

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Recently, together with the intramicellar transitions mentioned, intermicellar morphological transitions of spherical micelles into rod- or cylinder-like nanostructures have been an area of interest for material scientists. Gohy et al. elucidated the transformation of core cross-linked spherical micelles of a poly(2-ethyl-2-oxazoline-block-2-“soyalkyl”-2-oxazoline) (PEtOxb-PSoyOx) block copolymer into the rod through intermicellar fusion by adding acetone.10 A more evolved morphological transition of spherical micelles into intermicellar-chained structures was studied by Eisenberg group.11 Addition of desirable metal salt ions to an aqueous solution of polystyrene-block-poly (acrylic acid) (PS-b-PAA) spherical micelles successfully led to the formation of a pearl necklace morphology via intermicellar fusions. However, easily processed materials with uniform shapes and sizes, as well as large-scale production, have not yet been obtained. Thus, this is still an active area of interest for highefficiency applications. We have been gradually studied the interesting microphase separation behaviors, for example, a long-range anisotropic liquid crystalline thin film pattern due to rod-rod block interactions using a poly(n-hexyl isocyanate)-block-poly(2-vinylpyridine) (PHIC-b-P2VP) amphiphilic rod-coil block copolymer.12,13 The PHIC block consists of a helical main chain conformation in both the solid state as well as in solution.14 Due to unique characteristic and structural features, it has been extensively studied as a material for chiral recognition, optical switches, liquid crystals, and degradable materials.15-18 The combination (10) Huang, H.; Hoogenboom, R.; Leenen, M. A. M.; Guillet, P.; Jonas, A. M.; Schubert, U. S.; Gohy, J.-F. J. Am. Chem. Soc. 2006, 128, 3784. (11) Zhang, L.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777. (12) Kim, J.-H.; Rahman, M. S.; Lee, J.-S.; Park, J.-W. Macromolecules 2008, 41, 3181. (13) Kim, J.-H.; Rahman, M. S.; Lee, J.-S.; Park, J.-W. J. Am. Chem. Soc. 2007, 129, 7756. (14) Bur, A. J.; Fetters, L. J. Chem. Rev. 1976, 76, 727. (15) Vogl, O.; Jaycox, G. D. Polymer 1987, 28, 2179. (16) Wulff, G. Angew. Chem., Int. Ed. 1989, 28, 21. (17) Mayer, S.; Zentel, R. Prog. Polym. Sci. 2001, 26, 1973. (18) Okamoto, Y.; Nakano, T. Chem. Rev. 1994, 94, 349.

Published on Web 06/09/2009

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of a PHIC block with a pyridine-containing block, such as poly(2-vinylpyridine) (P2VP), attracted considerable attraction within the field of nanoscience. This is because of the presence of an unshared pair of electrons on the nitrogen heteroatom, which is widely used for coordination with foreign molecules. In this work, the effect of solvent composition on intermicellar fusion and networking was explored for amphiphilic PHICb-P2VP solid micelles. Furthermore, network-based nanostructures were used as templates for the in situ formation of Au nanoparticles (NPs) in the P2VP domains of the nanostructures.

Figure 1. AFM height images of the PHIC-b-P2VP block copolymer aggregates obtained by dip-coating the polymer solutions of (a) pure CHCl3 and (b) THF.

Letter

Experimental Section Preparation of Rod-Coil Amphiphilic Block Copolymer Solutions. A PHIC-b-P2VP (Mw=120 kg mol-1, MWD=1.07, fP2VP = 0.3) rod-coil block copolymer was synthesized via living anionic polymerization.19,20 The P2VP homopolymer was first synthesized in THF at -78 °C by living anionic polymerization, using a potassium diphenyl methane (DPM-K) initiator to prevent side reactions. The copolymerization of P2VP and the PHIC block was carried out using sequential polymerization in the presence of sodium tetraphenyl borate (NaBPh4). For the micellar aggregation studied, a PHIC-b-P2VP copolymer was solvated in pure CHCl3, pure THF, and a mixed solution of CHCl3/THF (v/v) with various volume ratios, at constant polymer concentration (3 mg mL-1). The resultant solutions were stirred for 48 h at room temperature. For morphological observation, aggregate polymer solutions were transferred onto glass and carbon-coated copper grid substrates using the spin-coating and drop-coating process, respectively. Synthesis of Au Nanoparticles. An amount of 0.5 equiv of HAuCl4 was coordinated with P2VP pyridine units for PHICb-P2VP solutions in CHCl3/THF. This was followed by 24 h of stirring for the complexation of PHIC-b-P2VP-H+/AuCl4-. Au NPs were finally synthesized by dispersing 5 equiv of sodium borohydride (NaBH4) reductant into the HAuCl4-dispersing block copolymer solutions. The solution color changed from yellow to deep purple. Resultant solutions were stirred for an additional 5 h to complete the reduction and then centrifuged for 10 min at 1000 rpm to remove salts and precipitates.

Figure 2. SEM (45° tilted angle) images of dried PHIC-b-P2VP block copolymer aggregates obtained by dip-coating the polymer solutions in the mixed CHCl3/THF (v/v) solvent compositions of (a) 8/2, (b) 7/3, (c) 6/4, (d) 5/5, (e) 4/6, and (f) 3/7.

Figure 3. AFM height (right) and phase (left) images of the PHIC-b-P2VP block copolymer aggregates obtained by dip-coating the polymer solutions from CHCl3/THF at (a, b) v/v = 7/3 and (c, d) v/v = 3/7. Langmuir 2009, 25(13), 7188–7192

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Characterization. Polymer aggregate morphologies for substrates from different solvent compositions were studied with atomic force microscopy (AFM), field-emission scanning electron microscopy (FE-SEM, Hitach, S-4700), and energy-filtering transmission electron microscopy (EF-TEM; EM 912 OMEGA, ZEISS, S-4700). Tapping mode AFM measurements were conducted using a Nanoscope IIIa Multimode atomic force microscope (Digital Instruments). SEM micrographs were recorded with a 45° tilt angle. For TEM analyses, the P2VP domain was selectively stained with I2 vapor. Absorbance for polymer aggregates dispersed in solution was checked using a UV-vis spectrophotometer (Varian, Win CARY 1E).

Results and Discussion The strategy used here for the design and development of nanonetwork structures using the PHIC-b-P2VP block copolymer relied on solvent manipulation. This included the adjustment of pure and mixed solvent systems of a PHIC-rod-selective solvent (CHCl3) as well as the common solvents for both blocks (THF).21 Figure 1 shows AFM images (height contrast) of the PHIC-bP2VP aggregates obtained by casting the polymer solutions made using pure CHCl3 and pure THF (3 mg mL-1). In CHCl3, PHICb-P2VP formed solid micelles with insoluble P2VP-cores and soluble PHIC-shells (Figure 1a). The average size of the spherical solid micelles was estimated at 45 nm, with a narrow size distribution. On the other hand, a liquid-crystalline-like selfassembled nanostructure was obtained by casting the THF solution of PHIC-b-P2VP onto substrates (Figure 1b). This unique liquid crystalline self-assembly could be attributed to the tendency of the rod block to form an anisotropic liquid crystallization nanostructure, due to the lateral PHIC rod-rod chain packing.22,23 Recently, it was reported that the structure of core cross-linked solid micelles obtained from coil-coil block copolymers transformed to cylindrical micelles.24,25 This was due to thermodynamic stability considerations when the relative volume of core coiled block chains was steadily increased under the influence of external stimuli. Consequently, an adequate expansion of the micellar core volume triggered intermicellar fusion. The main reason to conduct the core cross-linking was that the structure of coil-coil block copolymer micelles was transformed to others by external stimuli. In our present case, the spherical micellar structure with the P2VP core and the PHIC shell (formed in CHCl3) is more stable due to the PHIC rod-rod hydrophobic interaction as reported. Thus, the intermicellar fusion of PHICb-P2VP spherical micelles without cross-linking process of the P2VP core domain was performed by simply adding the THF, a common solvent for both blocks. In Figure 2, SEM images (45° tilt angle) show block copolymer aggregates, which were obtained by dip-coating polymer solutions with various CHCl3/THF volume ratios (v/v from 8/2 to 3/7). For the mixed solvent system with a low THF volume (v/v= 8/2, Figure 2a), weak intermicellar fusion and network formation were observed. A slight increase in THF volume led to the formation of well-defined intermicellar-chained networks (v/v = 7/3, Figure 2b). When the THF volume was increased substantially (v/v = 4/6 and 3/7, Figure 2e, f), more evolved cylindrical (19) Shin, Y.-D.; Han, S.-H.; Samal, S.; Lee, J.-S. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 607. (20) Ahn, J.-H.; Shin, Y.-D.; Nath, G. Y.; Park, S.-Y.; Rahman, M. S.; Samal, S.; Lee, J.-S. J. Am. Chem. Soc. 2005, 127, 4132. (21) Rahman, M. S.; Samal, S.; Lee, J.-S. Macromolecules 2006, 39, 5009. (22) Park, J.-W.; Cho, Y.-H. Langmuir 2006, 22, 10898. (23) Park, J.-W.; Thomas, E. L. Adv. Mater. 2003, 15, 585. (24) Zhang, L.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168. (25) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728.

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Figure 4. Schemes and corresponding SEM images of intermicellar fused chains and cylindrical micelles transformed from PHICb-P2VP spherical solid micelles obtained by spin-coating the polymer solutions in (a, b) CHCl3/THF (v/v=7/3) and (c, d) CHCl3/ THF (v/v = 3/7). A linearly intermicellar fused chain (a) and a branched chain (b) were together observed for CHCl3/THF (v/v= 7/3) solvent composition. A linearly cylinderical micelle (c) and a branched one (d) were also together observed for CHCl3/THF (v/v=3/7) solvent composition.

micellar networks appeared. These results clearly showed that spherical micelles (in pure CHCl3) were transformed into a cylindrical micellar network via an intermicellar-chained network as the amount of THF increased. Dried aggregates (Figure 2c, d) from CHCl3/THF solvent compositions of 6/4 and 5/5 (v/v) were regarded as intermediates between intermicellar-chained and cylindrical micellar networks. These resulted from the branching of intermicellar fused chains and cylindrical micelles with the aid of slow solvent evaporation during the dip-coating process. In addition, long-range monolayer nanoporous structures, regardless of their irregular shapes and sizes, were observed in networkbased nanostructures. Morphologies of two different nanostructures observed in the optimized solvent compositions were further verified by AFM height (left) and phase (right) images as shown in Figure 3. These included the intermicellar-chained network, obtained via dip-coating the polymer solution of CHCl3/THF (v/v = 7/3; Figure 3a, b), and the cylindrical micellar network, from CHCl3/THF (v/v=3/7; Figure 3c, d). Intermicellar fused aggregation behavior and cylindrical micellar structures are clearly verified by the magnified AFM images in Figure 3b and d, respectively. In Figure 4, SEM images of the intermicellar fused aggregates are shown. Polymer solutions having different CHCl3/THF compositions (v/v) were spin-coated at 4000 rpm onto glass substrates. In this case, the localized formations of intermicellar fusions and cylinder nanostructures could be induced by this fast spin-casting process while the results obtained from dip-coated process dramatically showed the long-range nanostructures. It indicated that the dip-coating-based slow solvent evaporation helps the aggregation behavior of intermicellar chains and cylindrical micelles. Specifically, an intermicellar fused pearl necklace nanostructure (Figure 4a) was observed upon casting the polymer Langmuir 2009, 25(13), 7188–7192

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Figure 5. TEM images of (a) Au-(intermicellar-chained network) from CHCl3/THF (v/v = 7/3) and (b) Au-(cylindrical micellar network) form CHCl3/THF (v/v = 3/7). (c) UV-vis absorption spectra of Au NPs embedded in network nanostructures. The solution absorption (i) was consistent with Au-(intermicellar-chained network) and (ii) with Au-(cylindrical micellar network).

solution of CHCl3/THF (v/v = 7/3). Surprisingly, branched micellar chains (Figure 4b) were also observed for this solvent composition. As the THF content was increased to 3/7 v/v in CHCl3/THF, cylindrical micelles with both linear (Figure 4c) and branched (Figure 4d) structures were observed together, which is quite different from dip-coated samples (Figure 2b and f). This indicates that the intermicellar fusion tended to form more evolved cylindrical micelles as the THF amount was increased. The dynamic light scattering (DLS) data shows broadening of participle size distribution, increase in diffusion coefficient, and decrease in relaxation time with increasing THF content in the mixed solvent of CHCl3/THF (see the Supporting Information, Figures S1-S3). These DLS data provided indirect support for network formation by PHIC-b-P2VP in the solution state which was visually observed during AFM/SEM/TEM analysis of the samples in the dried state. The intermicellar fused chain represented the intermediate nanostructure between isolated spherical and cylindrical micelles. As expected, the adjustment of THF into the CHCl3-based polymer solution played remarkably as a main factor to set off morphological reorganization of spherical, solid micelles toward intermicellar fused nanostructures. THF is more favorable for the side arm hexyl chain of the PHIC than the isocyanate backbone.26 Probably with the addition of THF in CHCl3, hexyl chains are getting solvated and facilitating hexylhexyl hydrophobic interaction among inter/intramicellar PHIC units along with the fusion of P2VP micellar cores, which lead to morphological reorganization toward intermicellar fused nanostructures, in order to offset micellar instability. The morphologically controlled intermicellar chain and cylindrical micellar network nanostructures observed here could be used (26) Liu, J.; Shi, Y.; Yang, Y. Adv. Funct. Mater. 2001, 11, 420. (27) Wu, L.; An, D.; Dong, J.; Zhang, Z.; Li, B.-G.; Zhu, S. Macromol. Rapid Commun. 2006, 27, 1949. (28) Bayley, H.; Cremer, P. S. Nature 2001, 413, 226. (29) Sch€uth, F.; Sing, K. S. W.; Weitkamp, J. Handbook of Porous solids; WileyVCH: Weinheim, 2003.

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as potential candidates for a variety of applications, for example, gas separation,27 biosensors,28 or catalysis supports,29 due to their long-range nanoporous structure. Furthermore, the unique functionality of two distinguishable, network-based nanostructures could be improved by in situ decorating of organic/inorganic NPs into the P2VP core domains around nanopores. The P2VP functional core domains could be coordinated with several metallic/semiconductor precursors. For example, we successfully synthesized Au NPs by coordinating the HAuCl4 precursor with the P2VP cores (PHIC-b-P2VP-H+/AuCl4-) of intermicellarchained and cylindrical micellar networks, with subsequent reduction using NaBH4. Figure 5 shows TEM images and corresponding UV-vis absorption spectra of intermicellar-chained and cylindrical micellar networks that have been decorated by Au NPs. Dark gray areas in both nanostructures were regarded as I2-stained P2VP domains. Au NPs were distinctly visible in this gray part. This indicated that Au NPs were dispersed in the P2VP block domains of two network-based nanostructures. However, the TEM micrographs clearly demonstrated that several Au NPs were selectively positioned at the confined P2VP core of the intermicellar-chained network (Figure 5a), while some of those were located randomly at the linearly connected core part of the cylindrical micellar network (Figure 5b). The average Au size was 8 nm for both of the network nanostructures. However, the Au size distribution dispersed in the cylindrical micellar network was found to be broader than that in the intermicellar-chained network. This was consistent with UV-vis absorption spectrum results (Figure 5c). Both absorbance spectra showed a similar maximum absorption wavelength at around 530 nm, verifying the intrinsic Au surface plasmon resonance.30 However, the absorbance of the Aucylindrical micellar network was broader (spectrum (ii) in Figure 5c) than the Au-intermicellar-chained network (spectrum (i) in Figure 5c), due to the broad size distribution of Au NPs. (30) Koh, H.-D.; Kang, N.-G.; Lee, J.-S. Langmuir 2007, 23, 12817.

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This might have resulted because of easier Au aggregation in the linearly connected P2VP core of the cylindrical micellar network during Au NP synthesis.

Conclusions The effect of solvent composition on the morphological transformation of PHIC-b-P2VP amphiphilic rod-coil block copolymer micelles to a monolayer network structure was evaluated using SEM, TEM, and AFM. In PHIC-selective CHCl3, PHIC-bP2VP self-assembled into spherically solid micelles. However, the addition of both-block-soluble THF into the CHCl3-based polymer solution resulted in intermicellar fusions, due to the thermodynamic instability of spherical micelles. With a gradual increase in the THF volume fraction of the mixed solvent composition, long-range intermicellar-chained and cylindrical micellar networks were formed. Furthermore, Au NPs were selectively incorporated in situ into the P2VP core domains of two distinguishable, network-based nanostructures. It is anticipated that

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these monolayer network structures with nanometer-scale pores could be utilized for the incorporation of various organic or inorganic guest molecules. Specific applications could be addressed using this strategy, such as in biomedical areas, catalysis, photonics, and electronics. Acknowledgment. This work was supported by the Program for Integrated Molecular Systems (PIMS) and the World Class University (WCU) Program through a grant (Project No. R3120008-000-10026-0) by the Ministry of Education, Science, and Technology (MEST). We thank the Korea Basic Science Institute (KBSI) for EF-TEM analysis. Supporting Information Available: DLS results for PHICb-P2VP aggregates in various solution conditions of CHCl3/ THF and AFM/TEM images of Au-(block copolymer nanostructures). This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2009, 25(13), 7188–7192