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Polymer-Induced Fractal Patterns of [60]Fullerene Containing Poly(methacrylic acid) in Salt Solutions Chung How Tan,† Palaniswamy Ravi,†,‡ Sheng Dai,†,‡ and Kam Chiu Tam*,†,‡ School of Mechanical and Production Engineering and Singapore-MIT Alliance, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Republic of Singapore Received July 28, 2004. In Final Form: September 28, 2004 Well-defined water-soluble pH-responsive [60]fullerene (C60) containing poly(methacrylic acid) (PMAAb-C60) was synthesized using the atom transfer radical polymerization technique. By varying pH and salt concentration, different types of fractal patterns at nano- to microscopic dimensions were observed for negatively charged PMAA-b-C60, while such structure was not observed for positively charged poly(2dimethylaminoethyl methacrylate)-b-C60. We demonstrated that negatively charged fullerene containing polymeric systems can serve as excellent nanotemplates for the controlled growth of inorganic crystals at the nano- to micrometer length scale, and the possible mechanism was proposed.
Micro- to macroscopic branched fractal structures constitute the spontaneous pattern formation in nature.1 Most of the fractal modes of growth are caused by nonequilibrium phenomena due to interfacial instabilities dominated by surface tension.2,3 Colloidal aggregation,4,5 dielectric breakdown,6 electrodeposition,7,8 dendritic crystal growth,9,10 and viscous fingering11 are some of the commonly encountered micro- and macroscopic structural forms. Fullerene has received widespread interest around the world since its discovery in 1985; however, many of its potential applications are hampered by its poor solubility and processability.12,13 One strategy adopted in overcoming this shortcoming is the introduction of charged groups or grafting polymer chains to fullerene molecules.14-16 Water-soluble and pH-responsive poly(methacrylic acid)-block-[60]fullerene (PMAA-b-C60) has been synthesized by the atom transfer radical polymerization (ATRP) technique using group-protecting chemistry, as shown in Scheme 1.17 First, well-defined poly(tert-butyl methacrylate)-block-[60]fullerene18 (PtBMA-b-C60) was synthesized by ATRP, and it possesses a number-averaged * Corresponding author. Fax: (65) 6791-1859. E-mail: mkctam@ ntu.edu.sg. † School of Mechanical and Production Engineering, Nanyang Technological University. ‡ Singapore-MIT Alliance, Nanyang Technological University. (1) Ball, P. The Self-Made Tapestry; Oxford University Press, Inc.: New York, 1999. (2) Vicsek, T. Fractal Growth Phenomenon, 2nd ed.; World Scientific: Singapore, 1992. (3) Meakin, P. In Phase Transitions and Critical Phenomenon; Domb, C., Leibowitz, J. L., Eds.; Academic Press: New York, 1998; Vol. 12. (4) Puertas, A. M.; Fernandez-Barbero, A.; de las Nieves, F. J. J. Chem. Phys. 2001, 115, 5662. (5) Wang, S. Z.; Xin, H. W. J. Phys. Chem. B 2000, 104, 5681. (6) Sheu, C. R.; Cheng, C. Y.; Pan, R. P. Phys. Rev. E 1999, 59, 1540. (7) Nanda, K. K.; Sahu, S. N. Europhys. Lett. 2002, 60, 397. (8) Brady, R. M.; Ball, R. C. Nature 1984, 309, 225. (9) Langer, J. S. Rev. Mod. Phys. 1980, 52, 1. (10) Spengler, J. F.; Coakley, W. T. Langmuir 2003, 19, 3635. (11) Daccord, G.; Nittmann, J.; Stanley, H. E. Phys. Rev. Lett. 1986, 56, 336. (12) Curl, R. F.; Smalley, R. E. Science 1988, 242, 1017. (13) Kroto, H. Science 1988, 242, 1139. (14) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Lsobe, H.; Nakamura, E. Science 2001, 291, 1944. (15) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Langmuir 2003, 19, 4798. (16) Song, T.; Goh, S. H.; Lee, S. Y. Macromolecules 2002, 35, 4133. (17) Zhou, P.; Chen, G. Q.; Hong, H.; Du, F. S.; Li, Z. C.; Li, F. M. Macromolecules 2000, 33, 1948.
Scheme 1. Synthesis Scheme of PMAA-b-C60
molecular weight (Mn) of 16 100 Da with Mw/Mn ∼ 1.18, as shown by the gel permeation chromatography (GPC) analysis. The covalent attachment of C60 to PtBMA was confirmed by GPC using UV/RI dual detectors, UV absorption spectrum, and thermogravimetric analysis. Subsequently, PMAA-b-C60 was obtained through the hydrolysis of PtBMA-b-C60 in acidic conditions resulting in a polymer with a chemical composition of PMAA112b-C60.19 We have observed that negatively charged C60 containing polymeric systems can serve as excellent nanotemplates for the controlled growth of inorganic crystals at the nano- to micrometer length scale. In this letter, we demonstrated the use of negatively charged aggregates of PMAA-b-C60 to induce and control the formation of fractal patterns at nano- to microscopic dimensions in different monovalent salt solutions. In the PMAA-b-C60 system, the solubility of the polymer in aqueous solution depends on the degree of neutralization of PMAA segments. At a low pH value, the polymer is water-insoluble. Because the PMAA segments are transformed into a polyelectrolyte at high pH values, the strong electrostatic repulsion between these negatively charged groups enhances the solubility of PMAA-b-C60 in water. To monitor the pH-responsive behavior of PMAA(18) Gan, L. H.; Ravi, P.; Mao, B. W.; Tam, K. C. J. Polym. Sci., Part A: Polym Chem. 2003, 41, 2688. (19) Dai, S.; Ravi, P.; Tam, K. C.; Mao, B. W.; Gan, L. H. Langmuir 2003, 19, 5175.
10.1021/la0480843 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/15/2004
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Figure 1. pKa versus R curves during the forward (NaOH titrant) and the backward titrations (HCl titrant) of PMAAb-C60 in an aqueous solution. The inset depicts the possible aggregate microstructures of PMAA-b-C60 upon forward and backward titrations. PMAA-b-C60 exists as compact particles at point a before titration, well-organized aggregates at point b after forward titration, and compact aggregates at point c after backward titration.
b-C60, potentiometric titrations of the polymer in aqueous solutions were performed in the forward (addition of NaOH) and reverse directions (addition of HCl). It was found that the PMAA-b-C60 solution after backward titration remained clear, suggesting that compact aggregates are produced instead of insoluble precipitates as originally found when PMAA-b-C60 powders was dispersed into water. From the pH titration curves, the dependence of the dissociation constant pKa on the degree of neutralization (R) was obtained and is shown in Figure 1.20-22 The pKa curve (open diamond) exhibits a negative slope in the range of R from 0.1 to 0.3, which is attributed to the discontinuous conformational transition of polymer from insoluble particles (a) to aggregates (b) during the neutralization process (as depicted by the pictorial representation in Figure 1). However, the negative slope disappears (closed diamond) during the backward titration process, which indicates that the hydrophilic segments of the swollen PMAA-b-C60 aggregates shrink continuously to produce compact aggregates in solution (c). This is further confirmed by laser light scattering (LLS) measurements. LLS data revealed the coexistence of small micelles and large aggregates in solution. The hydrodynamic radius Rh of the aggregates decreases from 153 to 90 nm with decreasing degree of neutralization. In addition, the increase in salt concentration gives rise to the lower pKa values for the backward titration compared with those for the forward titration. The dependence of pKa on the chemical structure, polymer conformation, and salt concentration has been reported elsewhere.21,23 It is clear that well-defined pH-responsive water-soluble PMAA containing C60 has been successfully synthesized. This pH-responsive water-soluble C60 system not only exhibits interesting self-assembly properties in aqueous solution but they can also be used as templates for controlling the fractal structures of inorganic materials. In the presence of various salt solutions, well-defined (20) Ravi, P.; Wang, C.; Tam, K. C.; Gan, L. H. Macromolecules 2003, 36, 173. (21) Wang, C.; Ravi, P.; Tam, K. C.; Gan, L. H. J. Phys. Chem. B 2004, 108, 1621. (22) Yao, J.; Ravi, P.; Tam, K. C.; Gan, L. H. Langmuir 2004, 20, 2157. (23) Wang, C.; Tam, K. C.; Jenkins, R. D. J. Phys. Chem. B 2002, 106, 1195.
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fractal patterns at the submicrometer size range can be induced when solutions containing water-soluble [60]fullerene containing polymers are dried under room temperature. However, the fractal pattern cannot be observed for PMAA homopolymer, which indicates that the ability of C60 containing polymer to aggregate may contribute to the formation of the fractal patterns. The varieties of fractal patterns produced via the use of selfassembled nanostructures were analyzed using the transmission electron microscope. Figure 2 provides a summary of six different types of nanostructures produced by the PMAA-b-C60 aqueous system at different degrees of neutralization R and salt concentrations. An abundance of spherical aggregates were obtained at fully neutralized (R ) 1) and unneutralized (R ) 0) conditions without salt (Figure 2a,d). The radii of the aggregates in the fully neutralized (R ) 1) condition vary from 125 to 165 nm, while they vary from 85 to 120 nm in the unneutralized (R ) 0) condition. On the basis of the size of the aggregates, we believe that large compound micelles (LCMs) are formed in solution instead of the common small coreshell micelles for many types of amphiphlic block copolymers. From transmission electron microscopy (TEM), it is also evident that the size of the LCM increases with increasing degree of neutralization (Figure 2a,d). As a result of the polyelectrolyte backbone of PMAAb-C60, the presence of salt such as NaCl shifts the aggregation behaviors in solution and results in the formation of interesting morphologies at R ) 1. Fractal patterns were observed when the sample was dried at room temperature on a copper grid (Figure 2b). Under isothermal condition, crystallization of NaCl is controlled by the concentration of diffusing ionic species. The development of such a fractal morphology is governed by a competition between reaction-limited or diffusion-limited transport processes and an interfacial phenomenon radiating from the heterogeneous nucleation sites.2 In our present system, the LCMs act as the nucleation sites, resulting in the controlled growth of a nanostructured morphology such as the diverse fractal patterns observed in Figure 2b. In the presence of larger amounts of NaCl, the growth of fractal pattern becomes denser and more closely packed (Figure 2c). Interestingly, such a fractal pattern could not be observed for the unneutralized PMAAb-C60 system at R ) 0. Instead, NaCl crystallizes onto the LCMs, as depicted in Figure 2e. The LCMs serve as nucleating sites for the crystallization of NaCl, and the growth of NaCl crystals becomes denser with increasing NaCl concentration, as shown in Figure 2f. Because no fractal pattern was observed for the unneutralized PMAAb-C60 system, we deduced that the negative charges on the surface of the LCMs are critical for the fractal growth process. On the basis of the results from Figure 2a-f and the knowledge of self-assembled nanostructures, a probable mechanism for the development of nanoscale fractal patterns was proposed. Figure 3a illustrates the progressive development of fractal patterns during the roomtemperature incubation of PMAA-b-C60 in NaCl aqueous solutions. For the fully neutralized PMAA-b-C60 system (R ) 1), carboxylate groups are ionized, producing negative charges on the surface of LCMs. When NaCl was introduced, sodium ions were attracted and condensed on the external surface of LCMs (blue circles) together with the chloride ions (red circles), driven by electrostatic forces. Each LCM becomes a nucleating site that controls the crystallization of sodium chloride. The progressive diffusion-controlled growth of sodium chloride crystals during the drying process produces the nanoscale fractal patterns
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Figure 2. TEM images of PMAA-b-C60 at different degrees of neutralization and different concentrations of NaCl. (a) Charged LCM aggregates at R ) 1 and (b) fractal pattern formed with the addition of NaCl, (c) fractal pattern becomes denser with further addition of NaCl. (d) LCM aggregates at R ) 0, (e) salt crystallizes on the LCM, and (f) the crystallization of salt becomes denser. (The scale bar corresponds to 1 µm in part c and 0.5 µm in parts a, b, and d-f.)
Figure 3. Proposed mechanisms for the morphology development. (a) Formation of the fractal pattern induced by the negatively charged surface of LCM upon drying at R ) 1; (b) “neutral” LCMs act as nuclei for the crystallization of NaCl at R ) 0.
observed in Figure 2a-c. The morphology observed for the unneutralized PMAA-b-C60 system in the NaCl aqueous solution is illustrated in Figure 3b. The aggregates exist as a “neutral” LCM, and upon the addition of NaCl, sodium and chloride ions are distributed homogeneously in the solution. When the water is progressively removed by evaporation, crystallization of sodium chloride is induced at the LCM, which acts as nucleating sites for the growth of NaCl crystals, as is evident from Figure 2d,e. To further confirm that the negative charge at the surface of the LCM is the controlling parameter for producing such fractal patterns, we performed TEM on the PDMAEMA-b-C60 in NaCl solution at pH 3, where the poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) segments possess positive charges.24 No fractal pattern was observed; instead, NaCl crystallizes on the surface of the aggregates, as shown in Figure 4a, and this may be related to the large steric effect of amino groups on (24) Dai, S.; Ravi, P.; Tan, C. H.; Tam, K. C. Langmuir 2004, 20, 8569.
Figure 4. Transmission electron micrographs of (a) PDMAEMA-b-C60 in 0.5 M NaCl aqueous solution at pH 3, where NaCl crystallizes onto the surface on the aggregates, and (b) PMAA-b-C60 in 0.5 M LiBr aqueous solution after full neutralization, where a fractal pattern similar to that of NaCl is observed.
PDMAEMA segments. In addition, the charges and counterions on PDMAEMA segments are further away from the backbone. The structural difference provides the different capabilities for inducing the formation of fractal patterns. The utility of such a templating and patterning methodology was further demonstrated for other systems
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such as lithium bromide (LiBr). Figure 4b illustrates similar nanofractal patterns produced using fullly neutralized PMAA-b-C60 in the 0.5 M LiBr solution. Although such fractal patterns have been observed in nature at the macroscopic or microscopic level, we believe this is the first time the formation of fractal pattern at nanoscale level using C60 containing polymer is reported. The potential of using nanotechnology in controlling the pattern formation using this technique is demonstrated and can be potentially exploited for many different applications. Further studies on the detailed self-assembly
Letters
properties of the copolymer and the effects of multivalent salts on the fractal formation are in progress. Acknowledgment. The authors thank the financial supports provided by Nanyang Technological University and Singapore-MIT Alliance (SMA). Supporting Information Available: PMAA-b-C60 synthesis and characterization details. This material is available free of charge via the Internet at http://pubs.acs.org. LA0480843
