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Self-Assembly Behavior of a Stimuli-Responsive Water-Soluble [60]Fullerene-Containing Polymer S. Dai,†,‡ P. Ravi,†,‡ C. H. Tan,‡ and K. C. Tam*,†,‡ Singapore-MIT Alliance and School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore Received May 11, 2004. In Final Form: July 3, 2004 A novel pH- and temperature-responsive water-soluble [60]fullerene-containing poly[2-(dimethylamino)ethyl methacrylate] (C60-b-PDMAEMA) was synthesized by atom transfer radical polymerization. The pH and temperature dependence of the physical properties of the aqueous C60-b-PDMAEMA solution was studied by potentiometric and conductometric titrations, UV-vis transmittance, and laser light scattering techniques. At low pH and at temperatures ranging from 25 to 55 °C, in addition to C60-b-PDMAEMA unimers, micelle-like aggregates are produced in the aqueous solution containing C60 hydrophobic cores and protonated PDMAEMA shells. Only unimeric C60-b-PDMAEMAs are found to exist in solution at high pH and low temperature, where PDMAEMA segments form a charge-transfer complex with C60 molecules. However, C60-b-PDMAEMA precipitates from aqueous solution at temperatures exceeing the lower critical solution temperature of PDMAEMA of ∼45 °C. The pH and temperature stimuli-responsive properties of the [60]fullerene-containing polymer in aqueous solution are completely reversible.
Introduction Since the discovery of fullerene, physical study of such a system has been an active research field due to its interesting structure, physical properties, and biological applications.1-3 Some investigations on the fullerene in nonpolar to polar solvents using UV-vis spectroscopic, light scattering, and luminescence techniques have been reported.4-8 Reversible or irreversible fullerene aggregates with different particle sizes and aggregation numbers could be produced in different solvent systems. Because of its low solubility, use of hazardous solvents, complicated preparation methods, and poor processibility, the utilization of fullerene is seriously hampered. To improve the solubility for end-use applications, several methods have been proposed and developed. Because of the electronaccepting characteristic of fullerene, it can form a chargetransfer complex (CT complex) with organic systems or polymers through the donor-acceptor interaction, which enhances its solubility in either water or organic solvents.9-16 C60 can form a CT complex with nitrogen, * To whom corresponding should be addressed. Fax: (65) 6791 1859. E-mail:
[email protected]. † Singapore-MIT Alliance, Nanyang Technological University. ‡ School of Mechanical and Production Engineering, Nanyang Technological University. (1) Geckeler, K. E.; Samal, S. Polym. Int. 1999, 48, 743. (2) Karaulova, E. N.; Bagrii, E. I. Russ. Chem. Rev. 1999, 68, 889. (3) Jensen, A.; Wilson, S. R.; Schuster, D. I. Bioorg. Med. Chem. 1996, 4, 767. (4) Bulavin, L. A.; Adamenko, I.; Prylutskyy, Y.; Durov, S.; Graja, A.; Bogucki, A.; Scharff, P. Phys. Chem. Chem. Phys. 2000, 2, 1627. (5) Bulavin, L. A.; Adamenko, I. I. Yashchuk, V. M.; Ogul’chansky, T. Y.; Prylutskyy, Y. I.; Durov, S. S.; Scharff, P. J. Mol. Liq. 2001, 93, 187. (6) Rudalevige, T.; Francis, A. H.; Zand, R. J. Phys. Chem. A 1998, 102, 9797. (7) Nath, S.; Pal, H.; Palit, D. K.; Sapre, A. V.; Mittal, J. P. J. Phys. Chem. B 1998, 102, 10158. (8) Deguchi, S.; Alargova, R. G.; Tsujii, K. Langmuir 2001, 17, 6013. (9) Tarassova, E.; Aseyev, V.; Tenhu, H.; Klenin, S. Polymer 2003, 44, 4863. (10) Ungurenasu, C.; Airinei, A. J. Med. Chem. 2000, 43, 3186. (11) Khairullin, I. I.; Chen, Y.; Hwang, L. Chem. Phys. Lett. 1997, 275, 1.
oxygen, and carbonyl functional groups through the charge-transfer or electron-transfer interaction due to the electron-acceptor property of fullerene and the electrondonating property of these organics. For example, the C60 CT complex can be easily produced with nitrogencontaining polymers or organic compounds, such as poly(vinylpyrrolidone) (PVP) and anilines using their strong donor and acceptor properties, respectively. Another way to improve the solubility is to form complexes with surfactants or cyclodextrins through solubilization and encapsulation.17-22 Fullerene can be partitioned into the micellar core of surfactants through hydrophobic interaction, while cyclodextrin can form either a 1:1 or a 2:1 inclusion complex with fullerene. Another possible way to improve the solubility of fullerene is to conjugate several organic components with a fullerene molecule. Grafting fullerene with charged groups such as acidic or basic groups could result in the production of a water-soluble fullerene system.23-31 In aqueous solution, these fullerene (12) Sushko, M. L.; Tenhu, H.; Klenin, S. I. Polymer 2002, 43, 1679. (13) Wang, Y. J. Phys. Chem. 1992, 96, 764. (14) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 4093. (15) Sibley, S.; Campbell, R. L.; Silber, H. B. J. Phys. Chem. 1995, 99, 5724. (16) Li, M.; Chen, Q. Polymer 2003, 44, 2793. (17) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903. (18) Murthy, C. N.; Geckeler, K. E. Chem. Commun. 2001, 1194. (19) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372. (20) Chen, X. L.; Jenekhe, S. A. Langmuir 1999, 15, 8007. (21) Filippone, S.; Heimann, F.; Rassat, A. Chem. Commun. 2002, 1508. (22) Samal, S.; Geckeler, K. E. Chem. Commun. 2000, 1101. (23) Cerar, J.; Skerjanc, J. J. Phys. Chem. B 2003, 107, 8255. (24) Wang, 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. 1999, 42, 4614. (25) Guldi, D. M.; Hungerbuhler, H.; Asmus, K. D. J. Phys. Chem. B 1999, 103, 1444. (26) Shiea, J.; Huang, J.; Teng, C.; Jeng, J.; Wang, L. Y.; Chiang, L. Y. Anal. Chem. 2003, 75, 3587. (27) Jeng, U.; Lin, T.; Tsao, C.; Lee, C.; Canteenwala, T.; Wang, L. Y.; Chiang, L. Y.; Han, C. C. J. Phys. Chem. B 1999, 103, 1059. (28) Felder, D.; Guillon, D.; Levy, R.; Mathis, A.; Nicoud, J.; Nierengarten, J.; Rehspringer, J.; Schell, J. J. Mater. Chem. 2000, 10, 887.
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derivatives form nanoscale aggregates through hydrophobic interaction. However, the properties of these modified fullerenes will alter depending on the substitution ratio. It has been reported that the grafting of uncharged water-soluble groups, such as cyclodextrin and poly(ethylene glycol), can also produce water-soluble fullerenes.21,22,32-34 In addition, attempts to produce fullerene anions to increase its solubility were also reported.35,36 Chu and co-workers reported the production of a double-layer vesicle from these fullerene anions in aqueous solution.35 Another method of improving the solubility and processability of fullerene is to graft different types of polymers to a fullerene molecule to produce fullerene-containing polymers. Fullerene-containing polystyrene (PS) and poly(methyl methacrylate) (PMMA) have been synthesized by several research groups.37-43 Different types of fullerene-containing poly(ethylene oxide)s (PEOs) were developed, and their applications in polymer blends have been examined.44-49 The water-soluble fullerenecontaining polymers have been synthesized using cyclodextrin as the supermolecular shielding micro-containers.50,51 In addition, various types of water-soluble fullerene systems were produced by grafting hydrophilic polymers to fullerene.52-54 However, most of these studies focused mainly on the synthesis and chemical characterization of the fullerene-containing polymers, and few studies examined their solution behaviors.37,45,46,55-58 Many of the solution properties of fullerene-containing polymer sys(29) Sano, M.; Oishi, K.; Ishi-I, T.; Shinkai, S. Langmuir 2000, 16, 3773. (30) Richardson, C. F.; Schuster, D. I.; Wilson, S. R. Org. Lett. 2000, 2, 1011. (31) Shi, Z.; Jin, J.; Li, Y.; Guo, Z.; Wang, S.; Jiang, L.; Zhu, D. New J. Chem. 2001, 25, 670. (32) Tomberli, V.; Ros, T.; Bosi, S.; Prato, M. Carbon 2000, 28, 1551. (33) Samal, S.; Geckeler, K. E. Chem. Commun. 2001, 2224. (34) Angelini, G.; Maria, P.; Fontana, A.; Pierini, M.; Maggini, M.; Gasparrini, F.; Zappia, G. Langmuir 2001, 17, 6404. (35) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944. (36) Sawamura, M.; Iikura, H.; Hirai, A.; Nakamura, E. J. Am. Chem. Soc. 1998, 120, 82856. (37) Zhou, P.; Chen, G.; Hong, H.; Du, F.; Li, Z.; Li, F. Macromolecules 2000, 33, 1948. (38) Bunker, C. E.; Lawson, G. E.; Sun, Y. Macromolecules 1995, 28, 3744. (39) Li, L.; Wang, C.; Long, Z. Fu, S. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4519. (40) Mihnard, E.; Hiorns, R.; Francois, B. Macromolecules 2002, 35, 6132. (41) Ford, W. T.; Nishioka, T.; McCleskey, S. C.; Mourey, T. H.; Kahol, P. Macromolecules 2000, 33, 2413. (42) Hawker, C. Macromolecules 1994, 27, 4836. (43) Lee, T.; Park, O.; Kim, J.; Kim, Y. C. Chem. Mater. 2002, 14, 4281. (44) Huang, X.; Goh, S. H.; Lee, S. Y. Macromol. Chem. Phys. 2000, 201, 2660. (45) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Langmuir 2003, 19, 4798. (46) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Polymer 2003, 44, 2529. (47) Song, T.; Goh, S. H.; Lee, S. Y. Macromolecules 2002, 35, 4133. (48) Jiao, H.; Goh, S. H.; Valiyaveettil, S. Macromolecules 2002, 35, 1399. (49) Goh, H. W.; Goh, S. H.; Xu, G. Q.; Lee, K. Y.; Yang, G. Y.; Lee, Y. W.; Zhang, W. D. J. Phys. Chem. B 2003, 107, 6056. (50) Nepal, D.; Samal, S.; Geckeler, K. E. Macromolecules 2003, 36, 3800. (51) Samal, S.; Choi, B. J.; Geckeler, K. E. Chem. Commun. 2000, 1373. (52) Sun, Y. P.; Lawson, G. L.; Huang, W.; Wright, A. D.; Moton, D. K. Macromolecules 1999, 32, 8747. (53) Okamura, H.; Miyazono, K.; Minoda, M.; Komatsu, K.; Fukuda, T.; Miyamoto, T. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3578 (54) Okamura, H.; Miyazono, K.; Minoda, M.; Miyamoto, T. Macromol. Rapid Commun. 1999, 20, 41. (55) Wang, X.; Goh, S. H.; Lu, Z. H.; Lee, S. Y.; Wu, C. Macromolecules 1999, 32, 2786. (56) Yang, J.; Li, L.; Wang, C. Macromolecules 2003, 36, 6060.
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tems were conducted in organic solvents, such as tetrahydrofuran (THF). For C60-containing PMMA and PS, the C60 retains its strong donor-acceptor property after modification. Core-shell micelles with different aggregation numbers were reported for C60-b-PMMA and C60-bPnBMA [poly(n-butyl methacrylate)] in THF solution. The chemical structure and composition control the micellization behaviors of C60-containing PS in THF. For example, C60-(PS)2, C60-(PS-PVP)2, and C60-(PVP)2 with similar molecular weights formed stable micelles in dilute solution with aggregation numbers of 1, 6, and 20, respectively. For C60 grafted with PEOs, large compound aggregates were produced in THF solution instead of small core-shell micelles. By varying the end-capped ratios and the molecular weight of PEO, the morphology and association mechanism were altered. For PEO with higher molecular weight and both ends capped with C60, gel-like network structure was observed in semidilute solutions, resembling that of associative thickeners.59 A watersoluble C60-containing polymer system, C60-b-poly(acrylic acid) (C60-b-PAA), was recently reported.56 C60-b-PAA produced core-shell micelles in aqueous solution, and the core-shell structure was found to enhance the photoconductivity of films. Extensive survey of the literature revealed only a few publications on the aqueous solution behavior of fullerene-containing polymers. In this study, the synthesis and characterization of a well-defined novel stimuli-responsive water-soluble fullerene-containing polymeric system is reported. The solution behaviors of the polymer were carefully studied as a function of pH and temperature. Various types of association mechanisms may result, depending on the pH and temperature. Experimental Section Materials. C60 (>99.5%) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) monomer were purchased from MTR, Ltd. (Cleveland, OH), and Sigma-Aldrich, respectively. DMAEMA was dried over CaH2 and distilled under reduced pressure. All the solvents were freshly distilled and used. Synthesis of PDMAEMA-Cl Macroinitiator. In the first step, well-defined -Cl-terminated poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) macroinitiator was synthesized using F-toluenesulfonyl chloride as the initiator and CuCl complexed with N,N,N′,N′,N′′,N′′-hexamethyltriethylenetetramine (HMTETA) as a catalyst in a 50 vol % (with respect to monomer) water/methanol (1:4) mixture at room temperature. After 90% conversion was attained, the reaction was stopped by diluting with THF and the catalyst system was removed by passing through a basic alumina column. The polymer solution was precipitated in large excess of hexane, and reprecipitation was repeated three times to completely remove unreacted monomer. The polymer was dried under vacuum at room temperature. Part of the -Cl-terminated PDMAEMA was used as the macroinitiator for the synthesis of C60-containing polymer, and the remainder was used as the homopolymer in this present study. Synthesis of C60-b-PDMAEMA. PDMAEMA-Cl macroinitiator was charged into a Schlenk flask and dissolved in a small amount of 1,2-dichlorobenzene. In a separate Schlenk flask, a C60 and CuCl/HMTETA catalyst system (1:2 molar ratio of macroinitiator) was dissolved in 20 mL of 1,2-dichlorobenzene. Three freeze-pump-thaw cycles were performed for both Schlenk flasks. Finally, under an argon atmosphere, the macroinitiator was added via a double-tipped syringe and reacted for 24 h at 90 °C. After 24 h, the reaction was diluted with THF, the catalyst was removed by passing through a basic alumina column, and the solvent was removed under vacuum. (57) Sushko, M. L.; Baranovskaya, I. A.; Bykova, E. N.; Klenin, S. I. J. Mol. Liq. 2001, 91, 59. (58) Okamura, H.; Ide, N.; Minida, M.; Komatsu, K.; Fukuda, T. Macromolecules 1998, 31, 1859. (59) Winnik, M. A.; Yekta, A. Curr. Opin. Colloid Interface Sci. 1997, 2, 424.
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Scheme 1. Synthesis Scheme of PDMAEMA and C60-b-PDMAEMA
The unreacted C60 was removed by dissolving the copolymer in THF, filtered, and passed through the basic alumina column. The filtrate was concentrated and precipitated in excess of hexane to yield a dark brown polymer. The procedure was repeated three times to ensure the complete removal of unreacted C60. The polymer was then dried under vacuum and stored in the dark. Gel Permeation Chromatography (GPC). An Agilent 1100 series GPC system equipped with a LC pump, PLgel 5 µm MIXED-C column, single RI detector, and UV/RI dual detectors was used to determine the polymer molecular weight and molecular weight distribution. The column was calibrated with narrow molecular weight PS standards. HPLC-grade THF was used as the mobile phase. The flow rate was maintained at 1.0 mL/min. For the UV detector, a wavelength of 330 nm was used to monitor the signals. Thermogravimetric Analysis (TGA). A Perkin-Elmer TGA7 thermogravimetric analyzer was used to conduct the TGA measurements. The temperature was scanned from 25 to 800 °C with a scan rate of 10 °C/min under nitrogen (N2) flow. X-ray Photoelectron Spectroscopy (XPS). A Kratos Ultra XPS system was used to perform the XPS measurements. The C60-b-PDMAEMA sample was ground into fine powder and mounted onto XPS standard sample studs using a double-sided adhesive tape. Sufficiently fine sample powder was required to ensure that the surface of the adhesive tape was fully covered. The X-ray source for the XPS is an aluminum target yielding 1086.6-eV X-rays. The electron source was operated at 15 kV and 10 mA, delivering 40 eV of energy at a rate of 0.1 eV/step for high-resolution XPS spectra analysis. The pressure in the analysis chamber was maintained at 2 × 10-8 bar or lower during the measurement at room temperature. The photoelectron takeoff angle was fixed at 45° relative to the sample surface. The XPS data were analyzed using the XPSPEAK3.1 curve-fitting software. Potentiometric and Conductivity Titration. An ABU93 tri-buret titrator equipped with the Aliquot software was used to conduct the titration experiments. The temperature was controlled with a PolyScience water bath. In addition, a Metrohm
Figure 1. GPC traces of PDMAEMA and C60-b-PDMAEMA using a single RI detector (a, b) and UV/RI dual detectors (c, d). 744 pH meter equipped with a Solitrode combined pH electrode was used to measure the pH values of the sample solutions. UV-Visible Spectroscopy. A HP8453 UV-visible spectrophotometer equipped with a HP89090A temperature control unit was used to measure the absorption and transmittance of the polymer solutions at different temperatures and pHs. For the absorption study, the wavelength was scanned from 200 to 800 nm at 25 °C, while the wavelength was set at 600 nm for the transmittance experiments. Dynamic Light Scattering (DLS). A Brookhaven BI-200SM goniometer system equipped with a 522-channel BI9000AT digital multiple τ correlator was used to perform DLS experiments. The inverse Laplace transform of REPES in the Gendist software package was used to analyze the time correlation functions with a probability of reject setting of 0.5. The deionized water used was from a Millipore Alpha-Q purification system equipped with a 0.22-µm filter, while HCl, NaOH, and NaCl were used to adjust the pH and ionic strength. A 0.2-µm filter
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Figure 2.
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NMR spectrum of C60-b-PDMAEMA in benzene-d6.
was used to remove dust prior to light scattering experiments, and the experimental temperature was controlled by a PolyScience water bath.
Results and Discussion Synthesis of C60-b-PDMAEMA. It was demonstrated recently that atom transfer radical polymerization (ATRP) is a suitable technique for synthesizing the well-defined monosubstituted C60-containing polymer.37 The synthesis details for C60-b-PDMAEMA are described in Scheme 1. First, well-defined living PDMAEMA-Cl macroinitiator was synthesized by ATRP as described in the experimental section. From GPC traces, it was found that well-defined PDMAEMA-Cl’s were obtained with Mn ∼ 18 000 and Mw/Mn ∼ 1.13 for DMA18K and Mn ∼ 7900 and Mw/Mn ∼ 1.15 for DMA8K, respectively. To obtain C60-b-PDMAEMA copolymer, ATRP was again utilized, and the details have been described in the experimental section. To avoid multiple substitution of polymer chains onto the same C60 and the formation of C60 CT complex, a higher ratio of C60/PDMAEMA-Cl (2.5:1) was used. The GPC traces from the RI detector revealed that the molecular weight distributions, Mw/Mn, of C60-b-PDMAEMAs are 1.19 (FDMA18K, Mn ∼ 18 200 Da) and 1.21 (FDMA8K, Mn ∼ 8000 Da), respectively. The comparison of the GPC traces for DMA18K and FDMA18K determined using different detectors (single RI detector and RI/UV dual detectors) are shown in Figure 1. The GPC traces of both DMA18K and FDMA18K determined using a single RI detector (a and b) showed monomodal distribution. In addition, the absence of tailing or multiple peaks confirmed the monosubstitution of PDMAEMA to C60 molecules. To further confirm the presence of a covalent bond between C60 and PDMAEMA, GPC traces of C60-b-PDMAEMAs were also carried out with UV/RI dual mode detectors as shown in Figure 1 (c and d). Because only the C60 moietycontaining polymers were detectable from both UV (λ ∼ 330 nm) and RI detectors, the results strongly confirmed that PDMAEMA chains are chemically grafted onto C60 molecules. The GPC traces of FDMA18K detected from
both UV and RI detectors (c and d) are nearly identical, indicating the chemical uniformity of the polymer. However, the observed broadening of the distribution curve (Figure 1d) with a small shoulder was observed using the UV detector, and this is possibly related to the absorption phenomena rather than size exclusion.44 As a result, a slight broadening of the subsequent RI response was observed (Figure 1d) compared to the GPC trace obtained using a single RI detector (Figure 1b). The small contribution of the C60 molecule to the hydrodynamic volume results in a small increase in the molecular weights of the fullerene-containing polymer.37 It is possible that minute fractions of unreacted PDMAEMA-Cl could be present, and this small fraction is extremely difficult to remove because of the similar characteristics of both polymeric systems. However, we believe the aggregation behavior will not be significantly altered. Both C60-containing copolymers used in this study were designated as FDMA18K and FDMA8K with degrees of polymerization of 114 and 50, respectively, while the corresponding PDMAEMA homopolymers used for the comparison are labeled as DMA18K and DMA8K, respectively. 1H NMR and 13C NMR were carried out on either PDMAEMA or C60-b-PDMAEMA to confirm the chemical composition and structure. To eliminate signal broadening of C60 in the 13C NMR spectrum due to possible association, a good solvent (benzene-d6) was used in all NMR experiments. However, there was no clear detectable peak at ∼145 ppm, corresponding to the resonance of the C60 moiety. This may be due to the low sensitivity of the C60 carbon atoms attributed by the loss of 60-fold degeneracy of the original C60 molecules after being conjugated with the polymer chains. This also reinforces the covalent linkage between C60 and PDMAEMA and not the physical trapping of C60 within the PDMAEMA matrix.60 Another possible reason may be related to the high molecular weight of the PDMAEMA homopolymer, which suppresses the C60 peak. The 1H NMR spectrum of the C60-b(60) Tang, B. Z.; Xu, H.; Lam, J. W. Y.; Lee, P. P. S.; Xu, K.; Sun, Q.; Cheuk, K. K. L. Chem. Mater. 2000, 12, 1446.
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Figure 3. UV-vis spectra of C60 in cyclohexane and C60-bPDMAEMA in toluene. The inset is the enlarged spectra in the visible region.
Figure 5. Light transmittance of PDMAEMA and C60-bPDMAEMA at various (a) temperatures and (b) pHs.
Figure 4. Potentiometric (filled symbols) and conductivity (open symbols) titration curves of PDMAEMA and C60-bPDMAEMA at different temperatures: (a) 25 and (b) 55 °C.
PDMAEMA in C6D6 is shown in Figure 2, which is identical to the spectrum for PDMAEMA homopolymer (spectra given in Supporting Information). The large ring current effect on the protons of the polymer chains is not evident from 1H NMR measurements, which may be related to the longer PDMAEMA chain. The weight percentage of C60 incorporated in the PDMAEMA or the yield of the C60-b-PDMAEMA was determined by TGA measurements. From the TGA profiles of C60, DMA18K, FDMA8K, and FDMA18K under N2 flow, it is evident that PDMAEMA is completely decomposed at 440 °C and is close to zero after 550 °C, but the C60 was still stable until 600 °C. On the basis of the TGA thermograms, the yield of FDMA8K and FDMA18K was calculated to be ∼88%. To further confirm the chemical
composition of the C60-b-PDMAEMA, XPS measurements were carried out. In the XPS spectra (figure given in Supporting Information), the binding energy at 398 eV corresponds to the nitrogen atom (N). The binding energies at 531 and 532 eV are related to CsO and CdO, respectively. The C spectrum contains two peaks, where the peak at 287.6 eV is related to the OsCdO and Os CH2, and the peak at 284.5 eV is related to the carbon from either fullerene or the rest of the carbon in the PDMAEMA chain.61 From the XPS spectra, we determined that the atomic concentrations of N, O, and C are 8.46, 17.99, and 73.54%, respectively, which agree with the theoretical values of 8.67, 17.35, and 73.98% for the FDMA18K system. This confirmed the successful grafting and the chemical composition of the C60-b-PDMAEMA system. UV-vis spectroscopy was also carried out to monitor the absorption of FDMA18K in aqueous solution as shown in Figure 3. C60 shows an obvious absorption peak at 256 and 330 nm in cyclohexane. For the block copolymers of FDMA18K in toluene, the absorption peak at 330 nm is not obvious. The C60 absorption peak at 330 nm is known to be sensitive to the chemical reaction, and its intensity weakens upon the chemical modification of C60 molecules.60 Hence, the absence of the peak at approximately 330 nm may be attributed to the conjugation of C60 to PDMAEMA chains. The inset shows some difference in absorption spectra in the visible wavelength region. The spectra is less structured, and no pronounced maxima were observed, which strongly supports the successfully conjugation of PDMAEMA chains onto C60, which perturbs the (61) Crist, B. V. Handbook of Monochromatic XPS Spectra-Polymers and Polymers Demaged by X-Rays; John Wiley & Sons: New York, 2000.
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Figure 6. Decay time distribution functions of (a) DMA18K, (b) FDMA18K, and (c) FDMA8K at different temperatures and pHs. Table 1. Physical Properties of PDMAEMA and C60-b-PDMAEMA in Aqueous Solution polymer
Mna
Mw/Mna
Mnb
Rhc (nm) pH ) 3, T ) 25 °C
Rhc (nm) pH ) 3, T ) 55 °C
Rh (nm) pH ) 10, T ) 25 °C
DMA18K FDMA18K FDMA8K
18 000 18 200 8 000
1.13 1.19 1.21
19 000 19 000 8 700
4.8 5.5 (f); 55 (s) 2.1 (f); 52 (s)
4.9 5.3 (f); 56 (s) 2.0 (f); 52 (s)
3.0 4.1 3.0
a From the GPC-RI detector. b From pH titration. c The letters f and s represent the fast and slow decay modes in the decay time distribution functions, respectively.
conjugated structure and the electronic properties of C60 molecules. This is also reinforced by the change in the color of the solution from purple (C60) to yellow (C60-bPDMAEMA). In addition, no obvious absorption at 514 nm was evident, which indicates that the grafting of PDMAEMA to C60 is a 1,2 addition, and not a 1,4 addition.37 Physical Properties of C60-b-PDMAEMA in Aqueous Solution. Because PDMAEMA is a water-soluble weak base in aqueous solution at room temperature, end group analysis can be used to determine the numberaveraged molecular weights of the polymer as well as the chain conformations in solution. The potentiometric and conductivity titration curves of DMA18K and FDMA18K at different temperatures are shown in Figure 4. On the basis of the pH titration curve, the number-averaged molecular weight was determined to be 19 000 Da, which agrees with the Mn obtained from GPC. It is also observed that the pH curves for DMA18K and FDMA18K are fairly identical, suggesting that the PDMAEMA chains are equally accessible in both cases. At all temperatures and pHs lower than 4, all the PDMAEMA chains/segments are protonated, and they are completely deprotonated at pHs greater than 9. The dependence of pKa values on the degree of neutralization is related to the polymer chain conformations. It is evident that grafting of PDMAEMA to C60 does not significantly alter the chain conformations in aqueous solution, that is, the pH and temperature sensitivity of the PDMAEMA are retained. At low pH values, PDMAEMA chains are protonated to yield a water-soluble polyelectrolyte, while at high pH values, PDMAEMA chains are deprotonated and become insoluble when the temperature exceeds the lower critical solution temperature (LCST).62 The pH and temperature sensitivity of (62) Lowe, A. B.; Billingham, N. C.; Armes, S. P. Chem. Commun. 1997, 1035.
DMA18K and FDMA18K was also examined by the UVvis spectrophotometer at a fixed wavelength of 600 nm. Figure 5a show the transmittances of 0.05 wt % DMA18K and FDMA18K over a temperature range of 20-70 °C. At a pH of 3, PDMAEMA is protonated and the solution loses its temperature-sensitive character but regains its thermal responsive property at pH ∼ 10. The LCST of DMA18K and FDMA18K was determined to be ∼45 °C. The pH dependence of the light transmittances for DMA18K and FDMA18K at different temperatures were also examined and shown in Figure 5b, where the transmittances are independent of pH at low temperatures but dependent on pH at high temperatures. At 55 °C, the DMA18K and FDMA18K become water-insoluble at pH > 7.8. It is observed that DMA18K and FDMA18K are both temperature- and pH-responsive polymers as indicated by Figure 5. Both pH titration and UV-vis transmittance studies revealed that the pH- and temperature-responsive properties of C60-b-PDMAEMA are retained, and to the best of our knowledge, such a system has not been reported before. The solution behaviors of both DMA18K and FDMA18K in aqueous solutions at different temperatures and pHs were examined by laser light scattering, and the distribution spectra of PDMAEMA are shown in Figure 6a. Only one decay mode was observed at temperatures of 25 (for pHs of 3 and 10) and 55 °C (for pH of 3). The linear relationship between the decay rates and the square of the scattering vectors (q) confirmed that the decay mode is due to the translational diffusion of the scattering object. At high pHs and high temperatures, the polymer solution becomes cloudy, and no light scattering experiment was performed in this region. On the basis of the StokesEinstein relationship, the hydrodynamic radii were calculated and summarized in Table 1. It is obvious that the Rh values do not change with temperature but decrease with pH. At a low pH, all PDMAEMA chains are
A [60]Fullerene-Containing Polymer
protonated and the polymer becomes a positively charged polyelectrolyte, which results in an apparent increase in the hydrodynamic radius. On the basis of the relationship of Rh and the molecular weight for DMA18K in a good solvent, the theoretical Rh was determined to be ∼4 nm. Hence, the decay mode in the decay time distribution function, corresponding to an experimental hydrodynamic radius of 4 nm, confirmed that the DMA18K chains are in the form of unimers. The solution properties at 25 and 55 °C of FDMA18K in a pH 3 aqueous solution are shown in Figure 6b. Two decay modes were observed in the decay time distribution, and both decay rates exhibit linear q2 dependence, where the hydrodynamic radius determined from the StokesEinstein equation are summarized in Table 1. The fast and slow modes correspond to Rh values of 5.5 and 55 nm, respectively, where the fast mode represents the unimeric FDMA18K and the slow mode corresponds to the FDMA18K aggregates. The formation of the micellar aggregate is driven by the amphiphilic properties of C60-b-PDMAEMA.55,56,58 The solution properties at 25 and 55 °C of low molecular weight C60-b-PDMAEMA (FDMA8K) in aqueous solution at pH 3 are shown in Figure 6c, where two translational decay modes are present. The unimer possesses a Rh of 2.5 nm, which is consistent with the shorter PDMAEMA chains. However, the aggregate size of 52 nm is fairly close to the FDMA18K system, which is probably related to a larger aggregation number for FDMA8K because the system is more hydrophobic, which also produces a larger proportion of micelles compared to unimers as revealed by the smaller fast peak compared with that of FDMA18K system. Both FDMA18K and FDMA8K at pH of 10 and 25 °C possess only one translational decay mode as shown in parts b and c of Figures 6, respectively. The Rh values for FDMA18K and FDMA8K were determined to be ∼4 and ∼3 nm, respectively, corresponding to the unimers of the two fullerene-containing polymeric systems. At high pH (>9) and low temperature, PDMAEMA chains are deprotonated and form a CT complex with the fullerene molecules (electron acceptor) using the lone pair of electrons on the nitrogen atom (electron donor). Such interaction precludes the formation of a micellar aggregate as the PDMAEMA/C60 CT complex, resulting in the formation of a stable isolated fullerene-containing polymer in aqueous solution. When the temperature was raised beyond the LCST of 45 °C, phase separation occurs and the solution became cloudy suggesting the formation of C60-b-PDMAEMA suspensions. On the basis of the results described above, the aggregation behavior and the microstructure of C60-bPDMAEMA in aqueous solution at varying pHs and temperatures were elucidated and summarized in the phase diagram shown in Figure 7. At low pHs and all temperatures, the PDMAEMA segments on the C60-bPDMAEMA are protonated and acquire a polyelectrolyte character. Unimers of protonated C60-b-PDMAEMA coex-
Langmuir, Vol. 20, No. 20, 2004 8575
Figure 7. Phase diagram of C60-b-PDMAEMA in aqueous solution at different temperatures and pH values.
ist with micellar aggregates. However, at pHs greater than 7.6, the PDMAEMA segments deprotonate and possess a LCST. When the temperature exceeds the LCST, C60-bPDMAEMA becomes insoluble and forms a reversible cloudy yellow suspension. When the temperature is lower than the LCST, C60-b-PDMAEMA is water-soluble, and PDMAEMA segments form a CT complex with fullerene molecules through the electron donor-acceptor interaction to produce unimeric C60-b-PDMAEMA in solution. Conclusions A novel well-defined temperature- and pH-responsive [60]fullerene-containing PDMAEMA was synthesized by ATRP. The characteristics of the C60-b-PDMAEMA polymer and its solution behaviors were examined by several characterization techniques. The aggregation behaviors of C60-b-PDMAEMA in aqueous solution can be tuned by adjusting the pH or temperature. At low pHs and all temperatures, micellar aggregates coexist with unimeric C60-b-PDMAEMA in solution. At high pHs, insoluble C60b-PDMAEMA suspensions are produced at high temperatures and they transform into unimeric C60-b-PDMAEMA particles at temperatures below the LCST of PDMAEMA via the formation of the CT complex. Acknowledgment. We wish to thank the SingaporeMIT Alliance for providing the funding to support this research. We thank Prof. Gan of National Institute of Education for the use of the NMR. Supporting Information Available: Figure S1, 13C NMR of C60-PDMAEMA in benzene-d6; Figure S2, 1H NMR of PDMAEMA in benzene-d6; Figure S3, TGA thermograms of C60, PDMAEMA, and C60-b-PDMAEMA; and Figure S4, 1s spectra of N (a), O (b), and C (c) for FDMA18K. This material is available free of charge via the Internet at http://pubs.acs.org. LA048826S
