Langmuir 2007, 23, 12067-12070
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Self-Organization of Double-C60 End-Capped Poly(ethylene oxide) in Chloronaphthalene and Benzene Solvent Mixtures Hong Huo,† To Ngai,*,† and Suat Hong Goh‡ Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin N.T. Hong Kong, Hong Kong 00852, China, and Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543, Singapore ReceiVed June 14, 2007. In Final Form: September 10, 2007 Well-defined and narrowly distributed double-C60 end-functionalized poly(ethylene oxide) (C60-PEO-C60) was prepared by reacting azido-terminated PEO with C60. The self-organization behavior of such C60-modified copolymers in different mixtures of chloronaphthalene (CN) and benzene (BN) was studied by a combination of static and dynamic laser light scattering. For C60-PEO-C60 in pure CN, a selective solvent only for C60, self-organization occurred to form a large micelle-like core-shell aggregate, probably with some C60 interlocking with each other with the collapsed PEO chains as the core. The addition of BN, a second selective solvent for core-forming PEO chains, has a significant effect on the structures and compactness of the resultant aggregates because the introduction of BN increases the stretching of the PEO chains inside the core and modifies the interfacial energy of the core-corona interface. On the other hand, for C60-PEO-C60 in pure BN, a non-solvent of C60, several smaller flower-like micelles may self-organize to form a large spherical-like aggregation complex.
Introduction Fullerene (C60) and its derivatives have attracted much attention due to their unique properties and many promising applications.1-3 C60 is spherical with a diameter of about 1 nm and is known to be highly hydrophobic. Such a low solubility in water and most organic solvents has limited the processing of C60 as well as its applications. One way to overcome this problem is by connecting C60 with functional chargeable groups such as carboxylic acids4,5 or amines.6 Alternatively, one can increase the solubility by incorporating or grafting C60 along long hydrophilic polymer chains.7,8 Such C60-containing polymers not only are soluble in common organic solvents but also retain the unique properties of C60, allowing themselves to be more amenable for further research and applications. Moreover, these conjugated polymers can self-organize in some organic solvents or in aqueous solution to form various interesting morphologies, dependent on chemical structures, molar mass, and concentration of polymers, as well as solvent quality.9,10 In recent years, the self-organization phenomenon of C60containing polymers has been intensively studied. For C60containing poly(methyl methacrylate) and poly(n-butyl methacrylate) in tetrahydrofuran (THF), we found that there exists an equilibrium between individual polymer chains and micelle-like * Corresponding author. E-mail:
[email protected]; +852 2603 5057. † The Chinese University of Hong Kong. ‡ National University of Singapore.
fax:
(1) Diederich, F.; Thilgen, C. Science (Washington, DC, U.S.) 1996, 271, 371. (2) Cravino, A.; Saricifti, N. S. J. Mater. Chem. 2002, 12, 1931. (3) Kiss, I. Z.; Ma´ndi, G.; Beck, M. T. J. Phys. Chem. A 2000, 104, 8081. (4) Tokuyama, H.; Yamago, S.; Nakamura, E.; Shiraki, T.; Sugiura, Y. J. Am. Chem. Soc. 1993, 115, 7918. (5) Nakamura, E.; Tokuyams, H.; Yamago, S.; Shiraki, T.; Sujiura, Y. Bull. Chem. Soc. Jpn. 1996, 69, 2143. (6) Cassell, A. M.; Scrivens, W. A.; Tour, J. M. Angew. Chem., Int. Ed. 1998, 37, 1528. (7) Lowe, A. B.; Billingham, N. C.; Armes, S. P. Macromolecules 1998, 31, 5991. (8) Ravi, P.; Wang, C.; Dai, S.; Tam, K. C. Langmuir 2006, 22, 7167. (9) Dai, P.; Ravi, P.; Tan, C. H.; Tam, K. C. Langmuir 2004, 20, 8569. (10) Teoh, S. K.; Ravi, P.; Dai, S.; Tam, K. C. J. Phys. Chem. B 2005, 109, 4431.
core-shell aggregates with C60 as the core and the polymer chains as the shell.11 Tam et al. have synthesized a series of single- and double-substituted C60-containing polyelectrolytes and studied their aggregation behavior in aqueous solution.12 Both single- and double-C60 end-capped poly(acrylic acid), PAAb-C60 and C60-b-PAA-b-C60, were pH responsive and soluble in a high pH solution. However, at neutralized conditions, a coreshell micellar structure was obtained, with a hydrophobic C60 core and an ionized PAA shell.13 The self-aggregation behavior of single-C60 end-capped poly(t-butyl methacrylate) (PtBMAb-C60) in a mixed solvent system was also investigated. Tam et al. showed that PtBMA-b-C60 could self-assemble to produce interesting morphologies in different mixtures of chlorobenzene and ethyl acetate. The structure and compactness of the resultant aggregates depend on the volume ratios of each mixed solvent. However, the aggregate could not form, and a homogeneous solution of PtBMA-b-C60 was obtained when the content of ethyl acetate was less than 70% (v/v) because chlorobenzene is a good solvent for C60.14 Recently, we have devoted much attention to the preparation and study of single- and double-C60 end-capped poly(ethylene oxide)s, PEO-C60 and C60-PEO-C60. These hydrophobically endmodified PEO copolymers associate in polar media through interactions between hydrophobic groups to form large aggregates in aqueous and THF solutions. The aggregates have a complex structure, probably containing many small core-shell-like aggregates bundling together. The structure and size distribution of the resultant aggregates depend on the solvent polarity, the amount of C60, and the length of the PEO chains. In the present study, we examined in more detail the self-organization behavior of such C60-PEO-C60 copolymers in chloronaphthalene (CN), a selective solvent only for C60. We also extended our studies on the effect of the aggregation morphologies by inducing a second (11) Wang, X. H.; Goh, S. H.; Lu, Z. H.; Lee, S. Y.; Wu, C. Macromolecules 1999, 32, 2786. (12) Ravi, P.; Dai, S.; Wang, C.; Tam, K. C. J. Nanosci. Nanotechnol. 2007, 7, 1176. (13) Yang, J. W.; Li, L.; Wang, C. C. Macromolecules 2003, 36, 6060. (14) Tan, C. H.; Ravi, P.; Dai, S.; Tam, K. C.; Gan, L. H. Langmuir 2004, 20, 9882.
10.1021/la701762e CCC: $37.00 © 2007 American Chemical Society Published on Web 10/26/2007
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Scheme 1. Double-C60 End-Capped Poly(ethylene oxide)
selective solvent (benzene, BN), which is expected to increase the stretching of the core-forming PEO chains and at the same time modify the interfacial energy of the core-corona interface. It is helpful to note that there is a scarcity of reports on the self-assembly of C60-containing polymers; the aggregation process of them in different solvent mixtures, however, has not attracted much attention, partially due to the difficulty in finding a proper mixed system. Therefore, the studies conducted here may enable us to find some suitable solvent systems for the processing and application of C60-containing copolymers in the future. Experimental Procedures Sample Preparation. C60-PEO-C60 was prepared by a cycloaddition reaction of biazido-terminated PEO with C60, where PEO had a molar mass Mn ) 20 000 g/mol and a polydispersity of 1.08. Details on this procedure can be found elsewhere.15-18 The hydroxyl groups of PEO were converted to chlorine groups through a reaction with thionyl chloride. The chloro-terminated PEO was then reacted with sodium azide to form azido-terminated PEO, which subsequently underwent a cycloaddition reaction with C60 to form the double-C60 end-capped PEO. The molecular structure of such a C60-functionalized PEO is shown in Scheme 1. CN (Aldrich) was distilled under reduced pressure, and BN (Labscan Asia Co.) was used without further purification. Initially, the C60-PEO-C60 copolymer was dissolved directly in different mixtures of CN and BN with the desired volume ratios (CN/BN 100:0, 84:16, 75:25, 66:34, 31:69, and 0:100), but the final concentration of the copolymer was kept constant at 2.64 × 10-4 g/mL. The solutions were stirred overnight before the laser light scattering (LLS) measurements. The solutions were clarified with a 0.45 µm hydrophobic PTFE filter to remove dust particles. Alternatively, C60-PEO-C60 was first dissolved in pure CN with a concentration of 2.64 × 10-4 g/mL. Next, different amounts of BN were carefully added to the clarified solutions to vary the volume ratio of the solvent mixtures and to induce morphology changes of the aggregates. LLS. A modified commercial light scattering spectrometer equipped with an ALV-5000 multi-τ digital time correlator and a He-Ne laser (output power ) 22 mW at λ0 ) 632 nm) was used. The details of the LLS instrumentation and theory can be found elsewhere.19,20 In static LLS, the excess absolute time-averaged scattered light intensity, known as the excess Rayleigh ratio Rvv(θ), of a dilute polymer solution at concentration C (g/mL) is related to the weight-average molar mass Mw, the root-mean-square z-average radius of gyration 〈Rg2〉z1/2 (or written as 〈Rg〉), and the scattering vector q as KC 1 1 1 + 〈Rg2〉q2 + 2A2C ≈ 3 Rvv(q) Mw
(
)
(1)
where K ) 4π2n2(dn/dC)2/(NAλ04) and q ) (4πn/λ0) sin(θ/2), where NA, dn/dC, n, and λ0 are Avogadro’s number, the refractive index increment, the solvent refractive index, and the wavelength of light in a vacuum, respectively, and A2 is the second virial coefficient. (15) Song, T.; Dai, S.; Tam, K. C.; Lee, S. T.; Goh, S. H. Langmuir 2003, 19, 4798. (16) Huang, X. D.; Goh, S. H. Macromolecules 2000, 33, 8894. (17) Huang, X. D.; Goh, S. H. Macromolecules 2001, 34, 3302. (18) Song, T.; Goh, S. H.; Lee, S. Y. Polymer 2003, 44, 2563. (19) Ngai, T.; Wu, C. Macromolecules 2003, 36, 848. (20) Ngai, T.; Wu, C.; Chen, Y. J. Phys. Chem. B 2004, 108, 5532.
Figure 1. Typical normalized hydrodynamic radius (Rh) distributions of C60-PEO-C60 in pure CN solvent, where the concentration of the copolymer was 2.64 × 10-4 g/mL, θ ) 17.5°, and T ) 25 °C. The inset shows the scattering vector (q) dependence of the average characteristic line-width (Γ) of the C60-PEO-C60 copolymer selforganization aggregates in pure CN solution. The extrapolation of Rvv(θ) to θ f 0 and C f 0 leads to Mw. The plots of (KC/Rvv(θ))Cf0 versus q2 and (KC/Rvv(θ))θf0 versus C, respectively, lead to 〈Rg2〉 and A2. The respective values of dn/dC for C60-PEO-C60 in different mixtures were determined using a selfdeveloped and high-precision differential refractometer.21 It is worth pointing out that it is problematic to use static LLS to characterize the absolute Mw of a copolymer or aggregates of copolymer chains because of the preferential adsorption of solvent on different comonomers. In the current study, the situation is even worse because we have to use a mixed solvent of CN and BN for some LLS measurements. Each solvent has a potential preferential adsorption problem. The classic LLS characterization of a copolymer in at least three solvents with different refractive indexes is not feasible here. Therefore, the values of Mw and 〈Rg〉 measured in this study are apparent ones. In dynamic LLS, the intensity-intensity time correlation function G(2)(τ) in the self-beating mode was measured in the scattering angle range of 17.5-150°. The Laplace inversion of G(2)(τ) can lead to a line-width distribution G(Γ), which can be further converted to a translational diffusive coefficient distribution G(D) by Γ ) Dq2 or a hydrodynamic radius distribution f(Rh) by use of the StokesEinstein equation, Rh ) kBT/6πηD, where η, kB, and T are the solvent viscosity, the Boltzmann constant, and the absolute temperature, respectively.
Results and Discussion Figure 1 shows the typical normalized hydrodynamic radius (Rh) distribution of the C60-PEO-C60 copolymer in pure CN solution (2.64 × 10-4 g/mL) at θ ) 17.5°. The C60-PEO-C60 copolymer in CN has two distinct peaks located at 〈Rh〉 ∼ 4 and 63 nm, respectively. It is helpful to note that f(Rh) is an intensityweight distribution and that large aggregates scatter much more light than individual chains, particularly measured at a smaller scattering angle. Therefore, the position of the peak located at ∼63 nm actually represents only a very small number of large aggregates even if its area looks large. Our previous study of such a copolymer in THF solution also showed that in addition to the individual copolymer chains, C60-PEO-C60 can self-organize to form spherical micelles in THF with C60 as the core and PEO chains as the shell.15 Because CN is a selective solvent only for C60, conventional wisdom will lead us to conclude that while the position of the peak is located at ∼4 nm to the individual copolymer chains, the position of the peak located ∼63 nm might be due to interchain association of C60-PEO-C60 copolymers, resulting in a core-shell structure with collapsed PEO chains in the core stabilized by the soluble C60 shell. However, a simple (21) Wu, C.; Xia, K. Q. ReV. Sci. Instrum. 1994, 65, 587.
Self-Organization of C60-PEO-C60
Figure 2. Scattering vector (q) dependence of KC/Rvv(q) of C60PEO-C60 copolymer chains in different solvent mixtures of CN and BN.
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Figure 4. Solvent composition dependence of average chain density 〈F〉 of the resultant C60-PEO-C60 copolymer self-organization aggregates, where 〈F〉 is defined as Mw,agg/(4π〈Rh〉3NA/3). Table 1. Summary of LLS Results of C60-PEO-C60 Copolymer in Different Mixtures of CN and BN with C ) 2.64 × 10-4 g/mL VCN/VBN Mw (g/mol) 〈Rg〉 (nm) 〈Rh〉 (nm)a 〈Rg〉/〈Rh〉 100:0 84:16 75:25 66:34 31:69 0:100
8.2 × 104 1.0 × 105 1.1 × 105 1.8 × 105 2.7 × 105 5.0 × 105
70 99 109 116 139 124
63 91 103 130 158 140
1.11 1.09 1.06 0.89 0.88 0.89
F (g/cm3) 1.30 × 10-4 5.27 × 10-5 4.00 × 10-5 3.25 × 10-5 2.72 × 10-5 7.23 × 10-5
a 〈Rh〉 is related to the size of the large aggregates formed from the self-organization of the C60-PEO-C60 copolymer in the solvent mixtures.
Figure 3. Solvent composition dependence of the typical normalized intensity-intensity time correlation function (G(2)(τ)) of the C60PEO-C60 copolymer in the solvent mixtures at θ ) 17.5° and T ) 25 °C. The inset shows their corresponding weighted intensity distributions of hydrodynamic radius (f(Rh)) calculated from the Laplace inversion of each G(2)(τ).
theoretical calculation shows that the PEO chain between two C60 molecules made of 463 monomers has a Gaussian configuration of about 10 nm. In other words, the aggregates with a collapsed PEO core can only have a radius of 5 nm, which is much smaller than that measured from dynamic LLS. Therefore, the large aggregate in CN cannot be simply related to the coreshell micelle. We will come back to this point later. The inset of Figure 1 shows that the line-width relaxation mode (Γ) of the resultant aggregate is a linear function of q2 and that the extrapolation of Γ to q f 0 passes the origin, indicating that the relaxation of the resultant aggregates is purely diffusive. To study the structure of the aggregates in a solvent mixture, BN as a selective solvent for PEO was added. Figure 2 shows that the excess scattering intensity (Rvv(θ)) increases with the benzene content. Note that based on eq 1, the extrapolation of [KC/Rvv(θ)] to θ f 0 can lead to the apparent weight-average molecular weight (Mw,agg) of the large aggregates in the different solvent mixture, where apparent is used because of a small concentration effect in the dilute regime and the previously discussed preferential adsorption problem. For C60-PEO-C60 in pure CN, the apparent Mw,agg was determined to be 8.2 × 104 g/mol, which should be much smaller than the real case due to the uncorrected concentration effect in the solvent mixtures. Figure 3 shows the corresponding solvent composition dependence of the typical time-average intensity-intensity correlation function (G(2)(τ)) of C60-PEO-C60 in different mixtures of CN and BN. The intercept of [G(2)(τ)]τf0 (i.e., the apparent coherence factor (β)) increases with the second selective BN
content, and the slow relaxations become much slower. Note that their contribution to G(2)(τ) also increased, indicating the increasing number of large aggregates. The inset in Figure 3 shows the corresponding hydrodynamic radius distribution (f(Rh)) calculated from each measured G(2)(τ) value by a Laplace inversion program (CONTIN) in the correlator. Besides 〈Rh〉, a combination of static and dynamic LLS results (Figures 2 and 3) can also lead to other microscopic parameters of these aggregates at different mixtures of the CN and BN, such as Mw,agg and 〈Rg〉. Table 1 summarizes these microscopic parameters. The inset of Figure 3 shows that there always exists two relaxation modes, indicating that individual copolymer chains or small micelles coexist with large complex aggregates in the solvent mixtures. The size of the resultant aggregates (〈Rh〉) increases as the content of BN increases. This is expected because the introduction of BN increases the stretching of the collapsed PEO chains in the core and results in a large but much loosened structure. The packing of the copolymer chains in the resultant aggregates can be better viewed in terms of the size dependence of the average chain density (〈F〉) defined as Mw,agg/ (4π〈Rh〉3NA/3). Figure 4 shows that in the solvent mixture, 〈F〉 of the resultant aggregates decreases as the content of BN increases, clearly indicating that the aggregates becomes loose and highly drained. This is again due to the fact that the introduced BN is the selective solvent for PEO, which can penetrate into the core, drain the aggregates, and stretch the PEO chains, resulting in a more open structure. Note that in pure BN solvent, 〈F〉 was slightly increased, but it was still low. Such a lower chain density reveals that the C60-PEO-C60 copolymer association in benzene may follow a diffusion-limited mechanism.22,23 Figure 5 shows the solvent composition dependence of 〈Rg〉/ 〈Rh〉 of the C60-PEO-C60 aggregates in the different mixtures. It (22) Lin, M. Y.; Lindsay, H. M.; Weitz, D. A.; Klein, R.; Ball, R. C.; Meakin, P. J. Phys.: Condens. Matter 1990, 2, 3093. (23) Lin, M. Y.; Lindsay, H. M.; Weitz, D. A.; Ball, R. C.; Klein, R.; Meakin, P. Phys. ReV. A 1990, 41, 2005.
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Figure 5. Solvent composition dependence of ratio of radius of gyration to hydrodynamic radius (〈Rg〉/〈Rh〉) of resultant C60-PEOC60 self-organization aggregates in the solvent mixtures. The insets show the corresponding proposed structures of the aggregates in different solvent mixtures of CN and BN.
is well-known that 〈Rg〉/〈Rh〉 values reflect the conformation of a polymer chain or a structure of an aggregate. For a random flexible coil, hyperbranched cluster or micelle, and uniform nondraining spheres, 〈Rg〉/〈Rh〉 is 1.5 to ∼1.8, 1.0 to ∼1.2, and ∼0.774, respectively.24 For a weight volume ratio of BN of less than 0.25, self-organization of C60-PEO-C60 in the mixtures has 〈Rg〉/〈Rh〉 of 1.0 to ∼1.1, suggesting that the aggregate has a core-shell-like micellar structure. As we previously discussed, as the size determined by LLS is much larger than theoretical calculations based on the length of the PEO chain, we thereby cannot simply attribute the resultant structure to a typical coreshell micelle with PEO as the core and C60 as the shell in pure CN or a mixture containing small amounts of BN. However, considering that each PEO is double end-capped with C60, while the PEO chains undergo inter- and intrachain aggregation, it is possible that not all C60 are in the corona but that some of them are interlocked with other by the PEO chains inside the core to form a large micelle-like aggregate as shown in the inset of Figure 5. The addition of more benzene results in a more open structure because it is the selective solvent for PEO that drains the aggregates, stretches the PEO chains, and modifies the interfacial energy of the core-corona interface. Further increasing the content of BN decreases the 〈Rg〉/〈Rh〉 value from ∼1.1 to ∼0.9. It is expected that more and more collapsed PEO chains were released as the content of BN increased, probably as some C60-PEO-C60 self-assembled to form a flower-like micelle with C60 as the core and was stabilized by the PEO loops. The inset of Figure 3 has confirmed that there exist a large amount of these aggregates with 〈Rh〉 ∼10 nm; most likely, these are the flowerlike micelle. However, a large aggregate was also found that may be attributed to the bundling of several smaller flower-like micelles into a large spherical-like aggregation complex as shown in the inset of Figure 3 as well. Tam et al. recently reported a similar structure for the self-assembly of C60-containing poly(methyl methacrylate) in an ethyl acetate/decalin mixture as the solvent.25 As mentioned previously, we used two different ways to prepare C60-PEO-C60 copolymer solutions in mixed solvent solutions. One method was to directly dissolve the copolymer in the different mixtures of CN and BN but keep the concentration of the (24) Burchard, W. In Light Scattering Principles and DeVelopment; Brown, W., Ed.; Clarendon Press: Oxford, 1996; Chapter 13, p 439. (25) Ravi, P.; Dai, S.; Hong, K. M.; Tam, K. C.; Gan, L. H. Polymer 2005, 46, 4714.
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Figure 6. Comparison of solvent composition dependence of the hydrodynamic radius of the resultant C60-PEO-C60 copolymer selforganization aggregates in the solvent mixtures prepared by two different methods (0: directly dissolved C60-PEO-C60 copolymer in the desired solvent mixtures at C ) 2.64 × 10-4 g/mL; O: addition of different amounts of BN to the C60-PEO-C60 copolymer dissolved CN solution until reaching the desired volume ratios).
copolymer constant. Alternatively, C60-PEO-C60 was first dissolved in pure CN with a concentration of 2.64 × 10-4 g/mL. After that, different amounts of BN were carefully added into the copolymer solutions to produce the desired volume ratios. Figure 6 shows that there has been no significant change in the size of the resultant aggregates for solutions prepared by these two different methods. Note that for the copolymer solution prepared by the second approach, the concentration of C60-PEOC60 in the solvent mixtures was continuously diluted when BN was added. The nearly no effect on 〈Rh〉 reflects that the size of the resultant aggregates is concentration-independent.
Conclusion We used both static and dynamic LLS to study the selforganization process of a well-defined double-C60 end-capped PEO copolymer (C60-PEO-C60) in different mixtures of CN and BN. The solvent properties of the mixtures have a significant impact on the structures of the resultant aggregates. In pure CN or a solvent mixture containing small amounts of BN, C60-PEOC60 self-organized to form a loose micelle-like core-shell aggregate probably with some C60 interlocking with each other with collapsed PEO chains as the core. The addition of BN, a selective solvent of PEO, increases the stretching of the polymer chains and results in a more drained and open structure. For C60-PEO-C60 in pure BN, there exists an equilibrium between individual copolymer, small flower-like micelle, and large aggregation complex. The larger size of the interchain aggregation indicates that several smaller flower-like micelles may selforganize to result in a spherical-like large complex. This study enables us to find some suitable solvent systems for the processing and application of C60-containing polymers in the future. Acknowledgment. We thank the reviewers for valuable comments related to the proposed structures of the aggregates. Financial support of this work from the Hong Kong Special Administration Region (HKSAR) Earmarked Projects (CUHK402506, 2160291 and CUHK4025/04P, 2160424), a Strategic Investments Scheme administrated by The Chinese University of Hong Kong, and a Direct Grant for Research 2006/07 from the Chinese University of Hong Kong (CUHK 2060303) is gratefully acknowledged. LA701762E
