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Langmuir 2004, 20, 9882-9884
Solvent-Induced Large Compound Vesicle of [60]Fullerene Containing Poly(tert-butyl methacrylate)
Scheme 1. Synthetic Scheme of the [60]Fullerene-Containing PtBMA
Chung How Tan,† Palaniswamy Ravi,†,‡ Sheng Dai,†,‡ Kam Chiu Tam,*,†,‡ and Leong Huat Gan§ School of Mechanical and Production Engineering, Singapore-MIT Alliance, and Natural Sciences, National Institute of Education, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Republic of Singapore Received January 29, 2004. In Final Form: August 6, 2004
There is a growing interest in the synthesis and physicochemical properties of self-assembly systems because of their potential applications in biological and chemical systems.1 A variety of strategies can be adopted to control the formation of different types of aggregates in solutions.2 Development of these aggregates is a physical process controlled by the kinetics and thermodynamics of self-aggregation phenomena. As a result of the special chemical and physical properties of [60]fullerene (C60), C60-containing polymer is an attractive system that exhibits self-assembly properties which are of particular interest in academic and industrial laboratories.3 Since the discovery of C60 in 1985, it has received widespread interest around the world; however, many of its potential applications are hampered by its poor solubility and processability. One strategy that was adopted in overcoming this shortcoming is by introducing charged groups or grafting polymer chains onto C60 molecules.4-6 Some progress has been made resulting in the development of several types of C60-containing polymers; however, their solution properties are still not well characterized and are poorly understood.7 For the C60-containing polymer system, the chemical structure and the molecular weight of the polymer, the polymer concentration, and the solvent play important roles in controlling the morphology of aggregates.8 Recently, C60-end-capped poly(ethylene oxide)s were synthesized and the aggregation behaviors in tetrahydrofuran (THF) were examined, where large spherical-like aggregation complexes were observed.5,9 The bola-amphiphilic C60 containing two ammonium head* 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. § Natural Sciences, Nanyang Technological University. (1) Webber, S. E.; Tuzar, Z. Solvents and Self-Organization of Polymers; Kluwer Academic Publishers: London, 1995. (2) Ding, J.; Liu, G. Macromolecules 1997, 30, 655. (3) (a) Curl, R. F.; Smalley, R. E. Science 1988, 242, 1017. (b) Kroto, H. Science 1988, 242, 1139. (c) Diederich, F.; Thilgen, C. Science 1996, 271, 317. (d) Wang, I. C.; Tai, L. A.; Lee, D. D.; Kanakamma, P. P.; Shen, C. K. F.; Luh, T. Y.; Cheng, C. H.; Hwang, K. C. J. Med. Chem. 2001, 42, 4614. (4) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Lsobe, H.; Nakamura, E. Science 2001, 291, 1944. (5) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Langmuir 2003, 19, 4798. (6) Song, T.; Goh, S. H.; Lee, S. Y. Macromolecules 2002, 35, 4133. (7) (a) Weis, C.; Friedrich, C.; Mulhaupt, R.; Frey, H.; Macromolecules 1995, 28, 403. (b) Sun, Y. P.; Ma, B.; Bunker, C. E.; Liu, B. J. Am. Chem. Soc. 1995, 117, 12705. (c) Okamura, H.; Terauchi, T.; Minoda, M.; Fukuda, T.; Komatsu, K. Macromolecules 1997, 30, 5279. (8) Riegel, I. C.; Eisenberg, A. Langmuir 2002, 18, 3358. (9) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Polymer 2003, 44, 2529.
groups was synthesized by Sano and co-workers, and the aggregation behavior in aqueous solution confirmed the formation of a vesicle-like structure.10 C60 grafted with well-defined polystyrene or poly(p-vinylphenol) was synthesized, and the micellization behavior in THF was examined.11 Stable micelles with aggregation numbers ranging from 6 to 20 were obtained by varying the amounts of OH substitution. Wang et al. studied the micellization behavior of different C60-containing polymers in THF.12 They reported the coexistence of individual polymer chains and the core-shell micellar aggregates for the C60containing poly(methyl methacrylate) and poly(n-butyl methacrylate) in THF. Yang and co-workers recently published the aggregation behavior of C60-containing poly(acrylic acid) in aqueous solution with a core-shell structure, and the microstructure dominates the photoconductive properties.13 From the literature, there is a scarcity of reports on the self-induced aggregation behavior of fullerene-containing polymers in mixed-solvent systems. In this study, a welldefined [60]fullerene-containing poly(tert-butyl methacrylate) (PtBMA) with a number-averaged molecular weight (Mn) of 14 000 Da, that is, C60-b-PtBMA98 (denoted by FBMA), was synthesized by the atom transfer radical polymerization technique (ATRP)14-16 as shown in Scheme 1. Because chlorobenzene (CB) is a good solvent for both PtBMA and C60 and ethyl acetate (EA) is a selective solvent for PtBMA,17 FBMA in the CB/EA solvent mixtures will self-assemble to produce interesting morphologies. During sample preparation, FBMA was first dissolved into CB, and then EA was added slowly to induce the formation of aggregates in the solvent mixtures. The resulting aggregate was examined by laser light scattering (LLS) and transmission electron microscopy (TEM). Direct morphological investigation determined from TEM provides (10) Sano, M.; Oishi, K.; Ishi-I, T.; Shinkai, S. Langmuir 2000, 16, 3773. (11) Okamura, H.; Ide, N.; Minoda, M.; Fukuda, T. Macromolecules 1998, 31, 1859. (12) Wang, X.; Goh, S. H.; Lu, Z. H.; Lee, S. Y.; Wu, C. Macromolecules 1999, 32, 2786. (13) Yang, J.; Li, L.; Wang, C. Macromolecules 2003, 36, 6060. (14) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921. (15) Gan, L. H.; Ravi, P.; Wei, M. B.; Tam, K. C. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 2688. (16) Zhou, P.; Chen, G. Q.; Hong, H.; Du, F. S.; Li, Z. C.; Li, F. M. Macromolecules 2000, 33, 1948. (17) Bezmel’nitsyn, V. N.; Eletskii, A. V.; Okun’, M. V. Phys.-Usp. 1998, 41, 1091.
10.1021/la049747j CCC: $27.50 © 2004 American Chemical Society Published on Web 09/30/2004
Notes
Figure 1. Hydrodynamic radius 〈Rh〉 (O) and radius of gyration 〈Rg〉 (b) of 0.7 wt % FBMA aggregates in different solvent mixtures. Inset depicts Rg/Rh (2) in different solvent mixtures.
images of the aggregates that could be used to validate the microstructure determined from light scattering study.18 Dynamic light scattering (DLS) was first performed to determine the dynamics of FBMA aggregates in the solvent mixtures. The molar composition of EA in the mixed solvent, XEA ) mEA/(mEA + mCB), plays an important role in defining the solvent-induced aggregation process. The onset composition for inducing the aggregation of FBMA in the solvent mixture corresponds to the critical precipitant composition (CPC). From light scattering, the CPC was determined to be 0.70. The relaxation time distribution function of FBMA in different solvent mixtures as determined from DLS only possesses one translational diffusional decay mode at EA molar composition XEA beyond the CPC. As a result of the higher relaxation time, the decay mode is attributed to the diffusion of FBMA aggregates instead of monomeric FBMA in the solvent mixtures. The polymer concentration dependence of the translational diffusion coefficients revealed that FBMA aggregates are produced via the closed association mechanism, where the diffusion coefficients are independent of polymer concentrations. The translational diffusion coefficients at infinitely dilute condition were determined by extrapolating to “zero” concentration, and the hydrodynamic radii (Rh) were calculated based on StokesEinstein equation. At the same time, it is also evident that the distribution of the hydrodynamic radii and the scattering intensity increase sharply after XEA ∼ 0.88. For comparison, DLS measurements of the PtBMA homopolymer in different solvent mixtures were also carried out. As a result of the fact that both EA and CB are good solvents for PtBMA, no aggregation behavior was observed. Hence, it can be concluded the driving force for the formation of C60-b-PtBMA aggregates is the solvent selectivity of C60 molecules. From static light scattering (SLS) measurements, the weight-averaged molecular weight Mw and the radii of gyration Rg of FBMA aggregates in solutions could be determined from a Zimm plot. The EA composition dependence of the Rh and Rg values is shown in Figure 1, where Rh decreases from 110 to 80 nm (18) (a) Zhang, L.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168. (b) Yu, K.; Eisenberg, A. Macromolecules 1998, 31, 3509. (c) Shen, H.; Eisenberg, A. J. Phys. Chem. B 1999, 103, 9473. (d) Riegel, I. C.; Eisenberg, A. Langmuir 2002, 18, 3358.
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Figure 2. Typical TEM micrographs: (a) 0.5 wt % FBMA in 0.75 EA molar composition, (b) 1.0 wt % FBMA in 0.92 EA molar composition, (c) close-up view of a single aggregate of 0.5 wt % FBMA in 0.75 EA molar composition, and (d) close-up view of a single aggregate of 1.0 wt % FBMA in 0.92 EA molar composition.
when the EA molar composition increases from 0.70 to 0.84. It then increases sharply to 120 nm at XEA ) 0.88 and to 130 nm at XEA ) 0.95. However, Rg decreases from 90 to 70 nm at EA molar compositions from 0.70 to 0.84 but increases to 170 nm at XEA ) 0.88, and it then increases gradually to 200 nm at XEA ) 0.95. The EA composition dependence of Rg/Rh is shown by the inset in Figure 1. The value remains essentially constant at 0.8 for EA molar composition ranging from 0.70 to 0.84, but it exhibits a sharp transition at EA molar composition between 0.84 and 0.88, where Rg/Rh increases sharply to 1.5 and remains constant thereafter. The molecular weight or the aggregation number Nagg dependence on XEA was examined. It was found that the aggregation number is 150 when XEA is less than 0.84 but increases dramatically to 1300 at XEA of 0.88 and keeps on increasing to 2200 at XEA of 0.95. LLS results revealed an obvious transition at XEA of 0.88, which signifies a step change in the conformation and compactness of FBMA aggregates. The conformational change of FBMA aggregates in different solvent mixtures was further elucidated by morphological studies using TEM. Transmission electron micrographs of FBMA in different solvent mixtures shown in Figure 2a,b revealed that both aggregates are of similar morphological shapes but different sizes. “Porous” and “fractured” spherical vesicle-like aggregates were observed. The size as determined from TEM is smaller than the hydrodynamic diameter from light scattering measurements. Figure 2c,d compares the close-up views of FBMA aggregates in different CB/EA solvent compositions. As seen in Figure 2c, the aggregate consists of many “pores” at an EA molar composition of 0.75, and this lies in the range of EA molar compositions (0.70-0.84) where Rg/Rh is ∼0.8 and Nagg is ∼150. However, the sizes of the “pores” decrease significantly and become almost negligible at an EA molar composition of 0.92 (Figure 2d), where the range of EA molar compositions lies between 0.88 and 0.96, Rg/Rh is ∼1.5, and Nagg is ∼2200. From the TEM micrographs, the shape of the aggregates has the resemblance of vesicles. Because of the large particle size and the smaller aggregation number, the
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Langmuir, Vol. 20, No. 22, 2004
Figure 3. Pictorial representation of a [60]fullerene-containing PtBMA LCV. Top section has been cut off for clarity.
aggregates are probably that of large compound vesicles (LCVs), where the shells of the vesicles contain an assembly of individual core-shell micelles as illustrated in Figure 3. Because EA is a selective solvent and CB is a good solvent for C60, the polymers would associate with each other to decrease the contact between C60 and EA. In the solvent mixtures, CB molecules would be partitioned into the core of the micelles and some may have been incorporated into the hollow cavities of the fullerene particle depending on the XEA value, while the rest of CB and all EA are located in the PtBMA shells. This kind of individual micelles makes the secondary LCV swollen and porous. During the sample drying process for TEM sample preparation, a lower boiling point EA evaporates first, followed by CB in the micellar core, which then creates porous patterns on the surface of the dried C60-b-PtBMA aggregates. When the content of EA increases from 0.70 to 0.84, the aggregation number and Rg/Rh do not change with solvent composition and the small decrease in the particle size may be attributed to the decrease of CB in the micellar core. With a further increase in XEA to 0.88, the aggregation of C60-b-PtBMA is enhanced, which gives rise to the sharp increase in the aggregation number and particle size. As a result, the change in the microstructure of the aggregate is evident from a sharp increase in the Rg/Rh values. At this condition, an insignificant amount of CB is present in the fullerene micellar core, which renders the core to be much more compact. This may explain why the porous structure is absent when Rg/Rh increases from 0.8 to 1.5 at high XEA. Figure 2d illustrates the diminishing of the size of pore on the surface of the aggregate at 0.92 mole fraction of EA. The penetration of CB molecules into the fullerene micellar core alters the core density, which shifts the radius of gyration and the translational diffusion of the aggregates. Because the density of the hollow fullerene
Notes
particle and that filled with CB are ∼1.8 and ∼2.9 g/cm3, respectively, the core becomes denser as more CB molecules are present, which yields smaller Rg and larger Rh values. When XEA is less than 0.84, CB is partitioned into the micellar cores, which produces a smaller Rg/Rh value for the vesicle-like structure. At the critical value of XEA of 0.88, CB molecules are expelled from the micellar core, and the contribution of CB molecules to the core density decreases, which results in a larger Rg, a smaller Rh, and, hence, a higher Rg/Rh value. Thus, the increases in the aggregation number within the LCV and the decrease in the core density as well as the increase in the particle size distribution (polydispersity) lead to the sharp transition observed in the light scattering data at XEA ∼ 0.88. Combined with TEM images, it is obvious that the LCV structure remains unchanged, but the aggregation number increases and the proportion of pores diminishes. Because EA and CB have different boiling points, they will vaporize at different rates when samples were prepared for the TEM imaging. For the “porous structured” LCVs, the dried PtBMA shell forms a film, and when the CB molecules diffuse to the interface, they must penetrate through the dried PtBMA film, which ruptures the shell of the LCVs giving rise to the “fractured” image. Another possible explanation for the observed morphology in the LCVs could be attributed to the surface tension effect when the solvent vaporizes from the LCVs. Shrinkage of the overall LCV structure will be experienced, and because the final dried structure is porous, it cannot withstand the large force generated from surface tension and gives rise to the fractured structure. In summary, well-defined [60]fullerene-containing PtBMA was synthesized by the ATRP technique. For the first time, the LCV structure for C60-b-PtBMA in CB/EA mixed solvents was observed when the CPC is exceeded. Light scattering data support the formation of a vesicle structure in solution. TEM micrographs revealed the “porous” and “fractured” LCVs. The size of the “pores” decreases significantly when the EA molar composition increases to greater than 0.84, where Rg/Rh increases from 0.8 to 1.5. Future study will examine the surface structure and cross section of the vesicle using techniques such as cryo-TEM and atomic force microscopy. Acknowledgment. The authors thank the financial supports provided by Nanyang Technological University and Singapore-MIT (SMA) Alliance. Supporting Information Available: Synthesis details and characterization techniques. Figure S1, SLS of C60-b-PtBMA in the mixed solvents; Figure S2, relaxation time distribution of 0.7 wt % C60-b-PtBMA in different solvent mixtures; Figure S3, TEM micrographs of LCVs for C60-b-PtBMA in 75 and 84 mol % EA; Figure S4, TEM micrographs of LCVs for C60-b-PtBMA in 88 and 92 mol % EA. This material is available free of charge via the Internet at http://pubs.acs.org. LA049747J
