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Aggregation Behavior of C60-End-Capped Poly(ethylene oxide)s T. Song,† S. Dai,‡ K. C. Tam,*,‡ S. Y. Lee,† and S. H. Goh*,† Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, and School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 Received December 12, 2002. In Final Form: March 17, 2003 Single- and double-C60-end-capped poly(ethylene oxide)s (PEOs) were prepared by reacting azidoterminated PEO with C60. The aggregation behavior of these polymers in THF and water was studied by gel permeation chromatography, static and dynamic laser light scattering, and transmission electron microscopy. The solvent polarity, the amount of C60, and the length of PEO segments significantly affect the conformation and the size distribution of aggregates or clusters. The aggregation number of singleC60-end-capped PEO exceeds 104, far larger than nonionic surfactants consisting of PEOs end-capped with paraffinic chains due to the stronger hydrophobic character of C60. Single-C60-end-capped PEO forms much larger aggregates than those of double-end-capped PEOs, possibly due to the relatively higher mobility of the former polymer. The PEO chain length of the double-C60-end-capped PEO controls the aggregate conformation and particle size distribution. It is believed that large aggregation complex comprises several smaller identical aggregates in THF solutions.
Introduction Associative polymers are macromolecules containing attractive groups that are either attached at the chain ends (telechelic system) or randomly grafted along the backbone (combed system). The properties of aggregates formed by associative polymers in aqueous solutions have recently attracted widespread interest due to their wide applications, both in the industry and in basic research.1 Hydrophobically modified poly(ethylene oxide)s (PEOs), consisting of PEO backbone with alkyl chains at both ends, have been used in applications such as thickeners, adhesives, adsorbants, coatings, flocculants, surfactants, and DNA separating sequencing media.2-4 Many studies have been conducted on aqueous solutions of these hydrophobically end-capped PEOs using techniques such as small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), fluorescence spectrometry, laser light scattering (LLS), pulsed-gradient NMR, and electronspin resonance (ESR).5-11 The polymers associate in aqueous media through the hydrophobic end groups and form micelles. However, the size of hydrophobic groups and the length of PEO spacers control the association †
National University of Singapore. Nanyang Technological University. * Corresponding authors:
[email protected] (S. H. Goh);
[email protected] (K. C. Tam). ‡
(1) Winnik, M. A.; Yekta, A. Curr. Opin. Colloid Interface Sci. 1997, 2, 424. (2) Rubinstein, M.; Dobrynin, A. V. Trends Polym. Sci. 1997, 5, 181. (3) Menchen, S.; Johnson, B.; Winnik, M. A.; Xu, B. Chem. Mater. 1996, 8, 2205. (4) Tam, K. C.; Jenkins, R. D.; Winnik, M. A.; Bassett, D. R. Macromolecules 1998, 31, 4149. (5) Vorobyva, O.; Yekta, A.; Winnik, M. A.; Lau, W. Macromolecules 1998, 31, 8998. (6) Vorobyova, O.; Lau, W.; Winnik, M. A. Langmuir 2001, 17, 1357. (7) Franc¸ ois, J.; Maitre, S.; Rawiso, M.; Sarazin, D.; Beinert, G.; Isel, F. Colloids Surf. A. 1996, 112, 251. (8) Yekta, A.; Duhamel, J.; Brochard, P.; Adiwidjaja, H.; Winnik, M. A. Macromolecules 1993, 26, 1829. (9) Alami, E.; Almgren, M.; Brown, W.; Franc¸ ois, J. Macromolecules 1996, 29, 2229. (10) Dai, S.; Tam, K. C.; Jenkins, R. D.; Bassett, D. R. Macromolecules 2000, 33, 7021. (11) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley and Sons: New York, 1989.
mechanisms and the microstructure of the aggregates. For alkyl chains greater than 12 carbons, flowerlike micelles are produced in solution via the closed association mechanism. [60]Fullerene (C60) has attracted tremendous interest not only for its many promising applications but also for its unusual molecular structure and perfect spherical shape.12-15 C60 is spherical with a diameter of about 1 nm and is highly hydrophobic. Zhou et al.16 suggested that C60-based “surfactants” may be synthesized through versatile organic chemistry designs to form a vesiclemembrane system as an alternative to lipid membranes and liposome vesicles in biology and medical applications. Instead of flexible hydrophobic tails, C60-based surfactant has a rigid hydrophobic sphere with a dominant intrinsic geometric constraint. Because of its large three-dimensional structure and the high hydrophobicity, the aggregation behavior of hydrophobic C60-end-capped watersoluble polymers is of paramount interest to the scientific community. We have previously studied the aggregation behavior of C60-containing polymethacrylates.17 However, these polymers have high polydispersity indexes, and C60 is randomly attached to the pendant groups of the polymethacrylates. It is desirable to study the aggregation behavior of C60-containing polymers with well-defined structures and low polydispersity indexes. Single-C60-end-capped PEO18 and double-C60-end-capped PEO19 can be synthesized by cycloaddition reaction of azido-terminated PEO with C60. These polymers have welldefined structures and narrow polydispersity indexes, making them ideal polymers for the study of the aggrega(12) Taylor, R. The Chemistry of Fullerenes; World Scientific Publishing Co. Pte. Ltd.: Singapore, 1995. (13) Hirsch, A. The Chemistry of the Fullerenes; Georg Thieme Verlag: Stuttgart, 1994. (14) Dai, L.; Mau A. W. H. Adv. Mater. 2001, 13, 899. (15) Geckeler, K. E.; Samal, S. Polym. Int. 1999, 48, 743. (16) Zhou, S. Q.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackeler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944. (17) Wang, X. H.; Goh, S. H.; Lu, Z. H.; Lee, S. Y.; Wu, C. Macromolecules 1999, 32, 2786. (18) Huang, X. D.; Goh, S. H.; Lee, S. Y. Macromol. Chem. Phys. 2000, 201, 2660. (19) Huang, X. D.; Goh, S. H. Macromolecules 2000, 33, 8894.
10.1021/la026992z CCC: $25.00 © 2003 American Chemical Society Published on Web 04/16/2003
C60-End-Capped PEOs
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tion behavior. In the present study, the aggregation behavior of two kinds of C60-end-capped PEOs in aqueous and THF solutions was studied by gel permeation chromatography (GPC), laser light scattering (LLS), and transmission electron microscopy (TEM). Experimental Section Materials. C60 (99.9% pure) was obtained form Peking University, China. Poly(ethylene glycol) monomethyl ether (MPEO2) was purchased from Aldrich, which possesses a numberaverage molecular weight (Mn) of 2200 and a polydispersity of 1.07 as determined by GPC. Two poly(ethylene oxide)-diols were also obtained from Aldrich, and their Mn/polydispersity values are 5000/1.06 (PEO5) and 20 000/1.08 (PEO20), respectively. The standard poly(ethylene oxide) (Mn ) 685 000) was purchased from Polymer Laboratories. Single-C60-end-capped PEO (FPEO2) was prepared by cycloaddition reaction of monoazido-terminated PEO (starting from MPEO2) with C60.18 Double-C60-end-capped PEOs (FPEO5F and FPEO20F) were prepared following the same procedure as FPEO2, using PEO5 and PEO20 as the starting materials, respectively. Basically, the hydroxyl groups of PEO were converted to chlorine groups through reaction with thionyl chloride. The chloro-terminated PEO was then reacted with sodium azide to form azido-terminated PEO which subsequently underwent cycloaddition reaction with C60 to afford C60-endcapped PEO. The molecular structures of FPEO2, FPEO5F, and FPEO20F are shown in Scheme 1. X-ray Photoelectron Spectroscopy (XPS) Measurements. XPS measurements were made on a VG ESCALAB MkII spectrometer with a Mg KR X-ray source (1253.6 eV photons) and a hemispherical energy analyzer. Various samples were ground to fine powders and then mounted on standard sample studs by means of a double-sided adhesive tape. The X-ray source was run at 12 kV and 10 mA. A pass energy of 20 eV and a rate of 0.05 eV/step were used for all the high-resolution XPS spectra acquisition with a binding energy width of 12 eV. The pressure in the analysis chamber was maintained at 10-8 mbar or lower during the measurements. All spectra were obtained at a takeoff angle of 75°, and they were curve-fitted with XPSPEAK3.1. Gel Permeation Chromatography. GPC measurements were performed on a Waters system with three Polysep-GFC-P linear (300 × 7.8 mm) columns calibrated with PEO standards, a Waters 600E system equipped with a Waters 410 differential refractometer (RI) detector, and a Waters 486 ultraviolet-visible (UV-vis) detector set at 320 nm with deionized H2O as the eluent. The concentrations of aqueous FPEO2, FPEO5F, and FPEO20F solutions are about 0.5 wt %. LLS Measurements. A Brookhaven BI200 goniometer equipped with a 532 channel BI9000AT digital correlator was used to perform the static and dynamic light scattering measurements. The goniometer was carefully aligned at scattering angles ranging from 15° to 155° to ensure that the deviation of the normalized scattering intensities of toluene is less than (1%. The light source is a power adjustable argon ion laser with a wavelength of 488 nm. At this wavelength, strong Rayleigh scattering was observed. However, we detected a very small fluorescence peak at 517 nm in the fluorescence emission spectrum for the C60-end-capped poly(ethylene oxide), where the peak remained at 517 nm at different excitation wavelengths. Since the scattered intensity was measured at 488 nm, the observed fluorescence should not contribute to the autocorrelation
Figure 1. C 1s spectrum of FPEO20F. function or the averaged total scattered intensity for the dynamic and static light scattering measurements, respectively. The measured temperature was controlled at 25 ( 0.1 °C using a Science/Electronic water bath. For static light scattering (SLS), the refractive index increment was measured by a BI-DNDC differential refractometer. The dn/dC values were found to be 0.109, 0.089, and 0.105 mL/g for FPEO2, FPEO5F, and FPEO20F, respectively. For dynamic light scattering (DLS), the inverse Laplace transformation routine of REPES supplied in the GENDIST software package was used to analyze the time correlation function, and the ratio of reject was set to 0.5.20 The sample concentrations were varied from 0.309 to 1.032 mg/mL for FPEO2, 0.612 to 1.021 mg/mL for FPEO5F, and 0.621 to 1.035 mg/mL for FPEO20F. Transmission Electron Microscopy. TEM micrographs were obtained with a JEOL CX100 operating at an accelerating voltage of 100 kV. For the observation of size and distribution of aggregates, a drop of dilute solution was placed onto a 200 mesh copper grid coated with carbon. The samples were dried in a vacuum before measurements.
Results and Discussion Figure 1 shows the XPS spectrum of FPEO20F. The C 1s spectrum shows the existence of two different states of carbon. The low-binding-energy (BE) peak is centered at 284.6 eV, corresponding to the carbon of C60. The highBE peak is centered at about 286.2 eV, corresponding to the carbon in PEO chain. The electron-withdrawing effect of oxygen makes the PEO carbon electron-deficient, leading to a higher BE. From peak area measurement, the low-BE peak represents 11% of all carbon, which agrees well with the theoretical value of 12% for FPEO20F, assuming quantitative reaction and monoaddition. We have reported earlier that the FPEO2 and FPEO5F samples predominantly consist of a monoadduct based on XPS measurements.18,21 Therefore, the majority of the C60 in FPEO2, FPEO5F, and FPEO20F are attached to a single PEO chain. GPC Measurements. Figure 2 shows the GPC traces of aqueous solutions of FPEO2, MPEO2, and a standard PEO. The two GPC traces of FPEO2, detected by UV-vis (at λ ) 320 nm, where the PEO is not detected but C60 or its derivatives absorb) and an RI detector, are nearly identical, indicating that C60 is present in the aggregates of FPEO2. The apparent molecular weight of FPEO2 is of the order 106-107, larger than that of the standard PEO (Mn ) 685 000). It is clear that FPEO2 forms aggregates in water. The aggregation is different from the micellization of C60-containing poly(n-butyl methacrylate) (PBMA), which involves an equilibrium between unimers and micelles.17 The GPC traces of FPEO5F and (20) Jakes, J. Czech. J. Phys. B 1988, 38, 1305. (21) Song, T.; Goh, S. H.; Lee, S. Y. Macromolecules 2002, 35, 4133.
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Figure 2. GPC traces of FPEO2: (a) RI detector; (b) UV detector. GPC traces of the starting MPEO2: (c) RI detector; the standard PEO (d) RI detector.
FPEO20F are similar to that of FPEO2. The GPC traces of FPEO2 and FPEOFs in THF are too complicated to afford meaningful conclusion. The difficulty may be due to the relatively weak intermolecular interactions for C60end-capped PEOs in THF so that the aggregation was destroyed to some extent during elution in the GPC column. LLS Measurements. Laser light scattering (LLS) is a sensitive tool for studying the static and dynamic properties of self-assembly systems. The solution behavior of C60-containing polymers or C60 derivatives has been studied by LLS.16,17,22-27 For example, a combination of static and dynamic LLS studies revealed that micellelike core-shell aggregates with C60 as the core and polymer chains as the shell exist in C60-containing poly(methyl methacrylate) (PMMA), C60-containing poly(n-butyl methacrylate) (PBMA), and hammerlike polystyrene (PS) in THF solutions.17,24 A static LLS study showed that fullerene grafted with two well-defined PS or poly(pvinylphenol) (PVPh) arms could form micelles in THF.22 The association behavior of a fullerene derivative, the potassium salt of pentaphenyl fullerene, in water was studied by static and dynamic LLS.16 The average aggregation number of associated particles in the large spherical vesicles is about 1.2 × 104. Static Light Scattering (SLS). Static light scattering provides information on the time-averaged properties of the system, the weight-averaged molecular weight (Mw), the second virial coefficient (A2), and the z-averaged radius of gyration (Rg) could be obtained based on the relationship28
(
)
q2Rg2 KC 1 ) 1+ + 2A2C Rθ Mw 3
Figure 3. Zimm plot of FPEO20F in THF, where T ) 25 °C, and the polymer concentration ranged from 0.621 to 1.035 mg/ mL. Table 1. Summary of Results of Static Light Scattering (SLS) for FPEO2, FPEO5F, and FPEO20F in the Dilute THF Solutions C60-end-capped PEOs
Mw (g/mol)
FPEO2 FPEO5F FPEO20F
4.03 × 8.51 × 106 1.00 × 106 107
A2 (cm3 mol/g2) 10-6
8.47 × -3.50 × 10-4 -3.25 × 10-4
Rg (nm)
Nagg
254 156 230
13800 1320 50
a vacuum, respectively, C is the polymer concentration in g/mL, and Rθ the excess Rayleigh ratio at scattering angle θ. The scattering vector, q () 4πn sin(θ/2)/λ), is defined as the wave vector difference of the scattered and the incident beams. The refractive index increment of the polymer solutions, dn/dC, was measured using a differential refractometer. In this study, SLS measurements were performed at smaller measurement angle such that qRg < 1-2. A Zimm plot was used to analyze the SLS data. Figure 3 shows the typical Zimm plot of FPEO20F in dilute THF solution. From the plot, the apparent Mw, A2, and Rg are calculated and summarized in Table 1. It was obvious that the measured Mw values of FPEO2, FPEO5F, and FPEO20F are much larger than their molecular weights of individual chains, indicating the existence of large aggregates in the solutions. The significant large radii of gyration of FPEO2, FPEO5F, and FPEO20F also suggest the presence of large aggregates or polymer clusters in solutions. The average aggregation number of C60-end-capped PEO in the solutions was calculated from eq 2:
Nagg )
Mw(aggregate) Mw(unimer)
(2)
(1)
where K () 4π2n2(dn/dC)2/NAλ4) is an optical constant with NA, n, and λ being the Avogadro’s number, the solvent refractive index, and the wavelength of incident light in (22) Okamura, H.; Ide, N.; Minoda, M.; Komatsu, K.; Fukuda, T. Macromolecules 1998, 31, 1859. (23) Weber, V.; Duval, M.; Ederle´, Y.; Mathis, C. Carbon 1998, 36, 839. (24) Li, C. Z.; Zhang, W. C.; Zhou, P.; Du, F. S.; Li, Z. C.; Li, F. M. Acta Polym. Sin. 2001, 4, 557. (25) Samal, S.; Geckeler, K. E. Chem. Commun. 2001, 2224. (26) Angelini, G.; de Maria, P.; Fontana, A.; Pierini, M.; Maggini, M.; Gasparrini, F.; Zappia, G. Langmuir 2001, 17, 6404. (27) Murthy, C. N.; Geckeler, K. E. Fullerene Sci. Technol. 2001, 9, 477. (28) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: London, 1953.
The average aggregation numbers of the FPEO2, FPEO5F, and FPEO20F in the THF solutions are summarized in Table 1. The Nagg value of FPEO2 is 1.38 × 104. For nonionic surfactant consisting of EOx chain and single hydrophobic paraffinic tail, the aggregation number increases with increasing length of the hydrophobic group and decreases with increasing number of EO units. Compared to the aggregation number of 20 for C9H19C6H4O(C2H4O)50H,11 the Nagg of FPEO2 is extremely large. Therefore, the high hydrophobicity of C60 plays a very important role in the aggregation process. However, for such a high aggregation number, the unimers would not be able to pack into a typical spherical micelle, and hence the topography of the aggregate needs further examination. In addition, the aggregate size and the average aggregation number of FPEO2 are also much larger than those of FPEO5F and
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FPEO20F. Since the hydrophobic/hydrophilic balance of FPEO2 is similar to that of FPEO5F, the more extensive aggregation of FPEO2 is due to the higher mobility of FPEO2 chain in THF than FPEO5F whose mobility is restricted by the two bulky C60 at both ends. Similarly, nonionic surfactants with single hydrophobic paraffinic tail form micelles with significant larger aggregation numbers than the flower micelle formed from the telechelic PEO with double paraffinic end groups.5,6,29,30 For example, the single- and double-n-hexadeca end-capped PEOs, with the same hydrophobilic/hydrophobic balance and with average molecular weights, Mw ) 16 000 and 32 000, respectively, were studied by static and dynamic light scattering. The aggregation number of the single-nhexadeca end-capped PEO is ∼34 and 2 times bigger than the double-end-capped PEO.29,30 By comparing the Mw and Rg values of FPEO5F to those of FPEO20F, it is noted that the larger aggregates of FPEO20F are made of about 50 polymer chains, while the relatively smaller aggregates of FPEO5F possess a large aggregation number. Vorobyova et al. reviewed the aggregation behavior of telechelic PEOs with paraffinic end groups.6 For the micelle-like structures formed by a C12H25 end-capped telechelic PEO of Mn ) 20 000 in aqueous solutions, the aggregation numbers range from 16 to 31.9 Although C60 is more hydrophobic than the paraffinic groups, C60 has a strong steric effect, and thus the aggregation number for FPEO20F in THF solution cannot be compared with the telechelic polymers with alkyl chains. However, FPEO20F has a much longer PEO spacer chain than FPEO5F, therefore, the aggregation number of FPEO20F should be smaller, suggesting that the aggregates are less compact than those of FPEO5F. On the other hand, increasing the length of PEO spacer gives rise to an increase in the accessibility of the C60 to form hydrophobic domains. The longer the PEO spacer, the easier it is to form intramolecular association. Further details on the smaller aggregation number for FPEO20F will be discussed in the DLS section. In addition to the molecular weight and the radius of gyration, the second virial coefficients could provide useful information on polymer-solvent interactions. The negative A2 values of FPEO5F and FPEO20F and the smaller positive A2 value of FPEO2 indicate that THF is not a good solvent for these polymers. With increasing amount of C60, the solvent quality decreases. Dynamic Light Scattering (DLS). Dynamic light laser scattering measures the temporal fluctuations of the scattered light produced by Brownian movement of the scattering particles. This temporal variation of the scattered radiation yields the Doppler shift, and the broadening of the central Rayleigh line could be used to determine the dynamic properties of the system. The intensity of the scattered light can be analyzed by photon correlation spectroscopy (PCS).31-33 The normalized field autocorrelation function is described by the expression
g1(t) )
∫w(Γ) exp(-Γt) dΓ
(3)
where w(Γ) is a continuous distribution function of decay rate Γ, which is the inverse of the decay time τ. If the (29) Gourier, C.; Beaudoin, E.; Duval, M.; Sarazin, D.; Maıˆtre, S.; Franc¸ ois, J. J. Colloid Interface Sci. 2000, 230, 41. (30) Mortensen, K. J. Phys.: Condens. Matter 1996, 8, 103. (31) Brown, W. Dynamic Light Scattering-the Method and Some Applications; Clarendon Press: Boston, 1993. (32) Chu, B. Laser Light Scattering-Basic Principles and Practice, 2nd ed.; Academic Press: Boston, 1991. (33) Brown, W. Light Scattering-Principles and Development; University Press: Oxford, 1996.
inverse Laplace transform (ILT) is used to analyze the autocorrelation function, the decay time distribution function w(Γ) can be obtained. For the translational diffusion mode, the translational diffusion coefficient D is related to the decay rate by expression 4 when the measurement angle θ is close to 0:
D)
Γ q2
(4)
In this study, DLS measurements were carried out for the dilute solutions at different measurement angles ranging from 15° to 30°. The angular dependence of the decay time distribution of FPEO2 in THF (0.722 mg/mL) is evident that only a single peak is present in the distribution function, where the decay time shifts to the left with increasing angle (Figure S1 in Supporting Information). The decay rates and the square of the scattering vector q exhibit a linear relationship with the fitting line passing the (0,0) point, indicating that the decay is due to the translational diffusion of the FPEO2 aggregates in solution. For the translational diffusion mode of large aggregates, the hydrodynamic radius can be determined from the Stokes-Einstein equation
Rh )
kT 6πη0D0
(5)
where η0 is the viscosity of the solvent, T the absolute temperature, D0 the translational diffusion coefficient at infinite dilution, and k the Boltzmann constant. If the diffusion coefficient in a dilute solution D is used instead of D0, the apparent hydrodynamic radius is obtained. The apparent hydrodynamic radius (Rh) for FPEO2 in THF decreases linearly with increasing concentration (Figure S2 in Supporting Information), indicating that the translational diffusion coefficients increase with polymer concentrations exhibiting a positive kD.
D ) D0(1 + kDC + ...)
(6)
A similar negative slope was also observed in the DLS measurements of aggregates of C60-containing PMMA and PBMA in THF.17 By extrapolating to zero concentration, the hydrodynamic radius of FPEO2 in infinitely dilute solution is 128 nm. The ratio of Rg/Rh equals 1.98, suggesting that the aggregates are not a simple coreshell structure with all the C60 on the hydrophobic core and PEO as the shell. However, there is a high possibility that FPEO2 chains form small core-shell structure, and these small core-shell structures further associate into a large aggregation complex. The DLS data indicate that the sizes of these aggregation complexes are identical. The decay time distribution of FPEO5F in THF (0.612 mg/mL) at different measurement shows only a single peak, indicating the presence of FPEO5F aggregates in the solution (Figure S3 in Supporting Information). The Rh values of FPEO5F remain fairly constant, suggesting that the solution is in the very dilute solution regime (see Figure S4 of the Supporting Information). Chu and coworkers also observed that the aggregation of other associative polymer systems was not significantly affected by interparticle interactions in very dilute solutions.34-36 The Rh of FPEO5F was found to be 84 nm, and the ratio of Rg/Rh is 1.86. Similar to the FPEO2, the aggregates are (34) Zhou, Z.; Peiffer, D. G.; Chu, B. Macromolecules 1994, 27, 1428. (35) Zhou, Z.; Chu, B.; Nace, V. M. Langmuir 1996, 12, 5016. (36) Zhou, Z.; Yang, Y.; Booth, C.; Chu, B. Macromolecules 1996, 29, 8357.
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Figure 4. Decay time distribution function of THF solution of FPEO20F (0.621 mg/mL), at 25 °C and different angles.
Figure 5. Hydrodynamic radius Rh distributions of FPEO20F in THF at different concentrations: 0.621 (O), 0.723 (0), 0.828 (2), 0.932 (b), and 1.035 mg/mL (4) at the measurement angle of 30° and at 25 °C.
not simple core-shell structures, but a large aggregate containing many smaller FPEO5F aggregates. As for the small FPEO5F aggregates, the flowerlike structure is difficult to form due to the short PEO segment. There is a high possibility of an open association with C60 as the core. In this condition, the density of the aggregates is smaller compared to FPEO2, which gives rise to the smaller aggregation number. Figure 4 shows the decay time distribution of FPEO20F in THF (0.621 mg/mL) at different measurement angles. Two peaks are observed in the relaxation time distribution, notably at large angles. The peaks are assigned as the fast and the slow decay mode, respectively, and they can be represented by two characteristic relaxation times, τf and τs (where τf < τs). The ratio of the two peaks, Af/As, decreases to zero as the measurement angle is decreased. Af/As represents the relative contribution to the intensity from these two components and is angle-dependent because the smaller measurement angle is only sensitive to large particles. For both the fast and slow modes, the decay rates and the square of the scattering vector exhibit a linear relationship, indicating that both peaks are attributed to translational diffusion. On the basis of the diffusion coefficients, the apparent hydrodynamic radii of the fast and slow modes were also determined from the Stokes-Einstein equation. Figure 5 shows the apparent hydrodynamic radius distributions of FPEO20F at different concentrations in THF. The size distribution consists of a large peak with an average Rh of about 155 nm, accompanied by a small narrow peak with Rh ∼ 12 nm. For the C60-containing PBMA and the hammerlike C60-containing PS, two decay modes in the relaxation time distribution were also
Song et al.
Figure 6. Relationship of concentration and the hydrodynamic radius Rh for FPEO20F in THF, at 25 °C.
observed, and they are attributed to interchain associative aggregates and intrachain associative unimers formed by individual polymer chains.17,24 Increasing the PEO spacer length facilitates the accessibility of the C60 to selfaggregate. When the PEO spacer is long enough as in FPEO20F, small micelle-like aggregates can be formed through inter- and intramolecular association. Hence, for the current system, the fast mode may correspond to the translational diffusion of small flowerlike aggregates of several FPEO20F chains, while the slow mode is attributed to the translational diffusion of large aggregate complex containing several small flowerlike aggregates.10,37 Figure 6 shows the relationship of the apparent hydrodynamic radii of FPEO20F and concentrations. The size and size distribution of the fast mode and the slow mode are essentially independent of the concentration in dilute solutions. Note that f(Rh) is an intensity-weight distribution, and the scattering intensity is proportional to Mw2; i.e., a large aggregate scatters more light than an individual chain. Thus, the peak of the fast mode represents a fairly large number of small aggregates even though its area seems small. Since the molecular weight determined from SLS is a weighted average value of both components, it is easy to comprehend as to why the molecular weight and the aggregation number are so low. In addition, the aggregate peak is detectable even down to a concentration of 0.6 mg/mL, which indicates that the critical aggregation concentration (cac) for C60-end-capped PEOs is extremely low. Since the Rg values of FPEO2, FPEO5F, and FPEO20F in THF are rather large (>150 nm), the size of the aggregate in a more polar solvent such as water would be even bigger. To accurately determine the large aggregate in water, the measurements must be performed at very small measurement angles. This is usually very difficult for systems in aqueous environment due to the anomalous scattering from minute quantities of dust at very low angles. The absolute value of Rh in water cannot be determined with confidence. TEM. Transmission electron microscopy is a reliable technique for determining the topography of micellar structure in solution38 and to demonstrate the aggregation of C60 or C60 derivatives.39-42 The aggregation of C60-endcapped PEOs in THF and water was examined by TEM using dried samples obtained from dilute solutions. Figure 7a-c show the TEM micrographs of FPEO2, FPEO5F, and FPEO20F aggregates in THF. Isolated spherical particles were observed with an average diameter of 200, 70, and 130 nm for FPEO2, FPEO5F, and FPEO20F, (37) Raspaud, E.; Lairez, D.; Adam, M.; Carton, J. P. Macromolecules 1994, 27, 2956. (38) Ba´n˜ez, M. V.; Robinson, K. L.; Vamvakaki, M.; Lascelles, S. F.; Armes, S. P. Polymer 2000, 41, 8501.
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mobility is restricted by the two bulky C60 at both ends. The other reason may be due to the lower molecular weight of PEO, which gives rise to the lower steric hindrance to the formation of aggregates. In addition, the distributions of FPEO2 and FPEO5F are identical compared to that of FPEO20F, which is higher due to the long PEO chain length. However, the apparent hydrodynamic diameters are larger than the TEM diameters because dynamic light scattering gives the size of solvent-swollen aggregates and TEM gives the size of dry particles. Furthermore, TEM gives the size of the C60 core only, and dynamic light scattering measures the size of the whole aggregates. Figure 7d-f shows the TEM micrographs of FPEO2, FPEO5F, and FPEO20F aggregates in water. The sample of FPEO2 consists of spherical structure with an average diameter of 250 nm, which are larger than FPEO2 in THF. It indicates that solvent could affect the aggregation behavior. The poorer the solvent, the stronger the aggregation and the larger the aggregates. For FPEO20F, networklike aggregates are observed in solution. The formation of network structures for telechelic linear alkyl chain modified PEO in aqueous solution has been reported in the literature.43,44 The hydrophobic groups associate and form network clusters through intermolecular interactions, leading to a large increase in the solution viscosity. Accordingly, intermolecular association of FPEO20F may be favored over intramolecular to form the networklike aggregation with increasing concentration. However, no network is produced, but polydisperse spherical clusters were observed for FPEO5F in aqueous solution. The PEO chain acts as the bridge in the network, and thus the chain length is critical for the formation of the network structure. Therefore, the longer PEO spacer (for example in FPEO20F) facilitates the formation of a network structure.
Figure 7. Typical transmission electron micrographs of aggregates prepared from (a) FPEO2, (b) FPEO5F, and (c) FPEO20F in THF and from (d) FPEO2, (e) FPEO5F, and (f) FPEO20F in water.
respectively. Since the PEO segments for these polymers could not be detected by TEM, what we observed here is the C60 component of these aggregates. When the samples were dried, the PEO segments are desolvated as the solvent is evaporated. Hence, the structure of the aggregate will shrink due to surface tension effects, and a smaller spherical structure will be observed by TEM. The size of FPEO2 aggregates is larger than those of FPEO5F and FPEO20F. One reason is that FPEO2 with a single C60 end is more mobile than FPEO5F or FPEO20F, whose (39) Cassell, A. M.; Asplund, C. L.; Tour, J. M. Angew. Chem., Int. Ed. 1999, 38, 2403. (40) Mendoza, D.; Gonzalez, G.; Escudero, R. Adv. Mater. 1999, 11, 31. (41) Ouyang, J. Y.; Goh, S. H.; Li, Y. Chem. Phys. Lett. 2001, 347, 344. (42) Yang, C. Y.; Heeger, A. J. Synth. Met. 1996, 83, 85.
Conclusions The aggregation behavior of single- and double-C60-endcapped PEOs with different chain lengths in aqueous and THF solutions were studied by GPC, SLS, DLS, and TEM techniques. All the techniques revealed that aggregates were formed in solutions. The aggregate has a large complex structure containing many small core-shell like aggregates (C60 in the core and PEO in the shell). The solvent, the amount of C60, and the molecular weight of the PEO control the size and distribution of the aggregates. For the single-C60-end-capped FPEO2, the particle size together with the aggregation in water is larger than that in THF. The aggregates of single-C60-end-capped FPEO2 are larger than those of double-C60-end-capped FPEOFs either in THF or in water. Compared with the aggregates of FPEO5F and FPEO20F in THF, the size of the aggregates for the former is smaller than the latter, but the small aggregates of the latter could coexist with large aggregates in solutions. For the small aggregate of FPEO20F, the flowerlike core-shell structure may be formed in solution due to the sufficiently long PEO chains. Supporting Information Available: Dynamic light scattering (DLS) data for aggregation behavior of the C60-endcapped poly(ethylene oxide)s (FPEO2 and FPEO5F) in THF. This material is available free of charge via the Internet at http://pubs.acs.org. LA026992Z (43) Yekta, A.; Xu, B.; Duhamel, J.; Adiwidjaja, H.; Winnik, M. A. Macromolecules 1995, 28, 956. (44) Zhang, K.; Xu, B.; Winnik, M. A.; MacDonald, P. M. J. Phys. Chem. 1996, 100, 9834.
