Langmuir 2006, 22, 2927-2930
2927
Morphological Transformation of [60]Fullerene-Containing Poly(Acrylic Acid) Induced by the Binding of Surfactant Chang Wang, Palaniswamy Ravi, and Kam Chiu Tam* Singapore-MIT Alliance, School of Mechanical and Aerospace Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore ReceiVed August 11, 2005. In Final Form: NoVember 18, 2005 Water-soluble pH-responsive [60]fullerene end-capped poly(acrylic acid) (PAA85-b-C60) was synthesized using atom-transfer radical polymerization (ATRP) technique. The unusual morphological transformation of the polymer induced by the binding of nonionic surfactant Triton X-100 (TX100) at different degrees of neutralization (R) was investigated using isothermal titration calorimetry (ITC), UV-vis spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM). For the 5 mM (monomer concentration) polymer solution at pH 1.7 mM in the polymer solution). The binding is significantly weakened and the complexation is disrupted with increasing pH, where the interaction completely ceased at pH >6.
Fullerene-containing amphiphilic copolymers are novel watersoluble, stimuli-responsive systems that self-assemble into micellar aggregates with well-defined size and shape.1-8 Because of their size and biocompatible property, they can potentially be used as nanoreactors, drug delivery vehicles, or nanotemplate systems. To date, the effects of hydrophobic/hydrophilic balance, pH, temperature, ionic strength, and solvent properties on the self-organization behavior of fullerene-containing polyelectrolytes have been studied, and some important features were observed.4-8 However, the interaction of surfactants with these polymers and their impact on the morphology of polymeric micelles have been less extensively studied.9-11 It is known that surfactants interact with polymers, driven by either electrostatic interaction (for oppositely charged polymer/surfactant systems),12-15 hydrophobic interactions (for nonionic polymer/surfactant systems),12,16-19 or hydrogen bonding.20,21 These interactions * To whom correspondence should be addressed. E-mail: mkctam@ ntu.edu.sg. Fax: 65-6791-1859. (1) Yang, J.; Li, L.; Wang, C. Macromolecules 2003, 36, 6060. (2) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Polymer 2003, 44, 2529. (3) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Langmuir 2003, 19, 4798. (4) Dai, S.; Ravi, P.; Tan, C. H.; Tam, K. C. Langmuir 2004, 20, 8569. (5) Tan, C. H.; Ravi, P.; Dai, S.; Tam, K. C.; Gan, L. H. Langmuir 2004, 20, 9882. (6) Tan, C. H.; Ravi, P.; Dai, S.; Tam, K. C. Langmuir 2004, 20, 9901. (7) Ravi, P.; Dai, S.; Tan, C. H.; Tam, K. C. Macromolecules 2005, 38, 933. (8) Teoh, S. K.; Ravi, P.; Dai, S.; Tam, K. C. J. Phys. Chem. B 2005, 109, 4431. (9) Burker, S. E.; Eisenberg, A. Langmuir 2001, 17, 8341. (10) Guldi, D. M. J. Phys. Chem. A 1997, 101, 3895. (11) Williams, R. M.; Crielaard, W.; Hellingwerf, K. J.; Verhoeven, J. W. J. R. Netherlands Chem. Soc. 1996, 115, 72. (12) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (13) Hayakawa, K.; Kwak, J. C. J. Phys. Chem. 1982, 86, 3866. (14) Ehtezazi, T.; Govender, T.; Stolink, S. Pharm. Res. 2000, 17, 871. (15) Fundin, J.; Hansson, P.; Brown, W.; Lidegran, I. Macromolecules 1997, 30, 1118. (16) Dai, S.; Tam, K. C.; Jenkins, R. D. J. Phys. Chem. B 2001, 105, 10189. (17) Dai, S.; Tam, K. C.; Li, L. Macromolecules 2001, 34, 7049. (18) Bloor, D. M.; Wan-Yunus, W. M. Z.; Wan-Badhi, W. A.; Li, Y.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 3395. (19) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588. (20) Yoshida, K.; Dubin, P. L. Colloids Surf., A 1999, 147, 161. (21) Wang, C.; Tam, K. C. J. Phys. Chem. B 2004, 108, 8976.
are governed by several factors such as polymer charge density, overall hydrophobicity of the polymer backbone, chain flexibility, and ionic strength.12-15,20-23 The interactions may alter the solubility and optical and physical properties of the polymer, which trigger intriguing conformational changes and induce complex phase behaviors. These phenomena, such as the solubilization of cholesterol, proteins, and enzymes by amphiphilic molecules such as bile salt, are commonly encountered in the human body. Interactions between nonionic surfactant polyoxyethylene 9 lauryl ether (C12H25EO9) with poly(acrylic acid) (PAA) and fullerene end-capped poly(acrylic acid) (PAA-b-C60) were investigated recently. It was observed that C12H25EO9 binds to compact aggregates of both polymers, driven by hydrophobic interaction. However, the binding is weak and the binding isotherms are identical for the two polymers regardless of the grafted fullerene pendant, and no structural reorganization or complexation was observed in the course of binding, suggesting that C12H25EO9 does not bind to fullerene domains. It was reported that TX100 micelles can successfully disperse fullerene molecules in aqueous medium.24,25 Recent studies also show that the functionalization of single-walled carbon nanotubes (SWNTs) using TX100 significantly reduced the nonspecific adsorption of protein onto the surface of nanotubes.26 Intrigued by the strong affinity between TX100 and fullerene/SWNTs and motivated by the potential applications of nanomaterials in drug delivery and microbiological devices, we studied the interactions of TX100 with PAA-b-C60 at different degrees of neutralization R. (R ) 0 represents the unneutralized state of the polymer, and R ) 1 represents the full neutralization of the polymer.) The investigation was corroborated by isothermal titration calorimetric (ITC), UV-vis spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM) measurements. Strong binding of TX100 to PAA-b-C60 and unusual morphological transformations of the polymer in the course of binding (22) Hansson, P. Langmuir 2001, 17, 4167. (23) Wang, C.; Tam K. C. J. Phys. Chem. B 2005, 109, 5156. (24) Bensasson, R. V.; Blenvenue, E.; Dellinger, M.; Leach, S.; Patrick, S. J. Phys. Chem. 1994, 98, 3492-3500. (25) Guldi, D. M. J. Phys. Chem. A 1997, 101, 3895-3900. (26) Shim, M.; Wong, N. S. K.; Chen, R. J.; Li, Y.; Dai, H. Nano Lett. 2002, 2, 285-288.
10.1021/la052191v CCC: $33.50 © 2006 American Chemical Society Published on Web 02/22/2006
2928 Langmuir, Vol. 22, No. 7, 2006
Letters
Scheme 1. Synthesis Scheme of PAA85-b-C60
Figure 1. Dependence of Rapp h on the TX100 concentration for 5 mM PAA85-b-C60 at R ∼0 in 0.1 M NaCl solution.
were observed. A better understanding of the complexation mechanism of PAA-b-C60/TX100 was obtained. The size, shape, and conformation of fullerene-containing amphiphilic copolymers can be tuned and controlled by the addition of nonionic aromatic surfactant, which is of scientific and industrial importance. The well-defined water-soluble, pH-responsive PAA-b-C60 copolymer was synthesized by the atom-transfer radical polymerization (ATRP) technique using group protection chemistry as shown in Scheme 1.27 First, a well-defined, stable -Brterminated poly(tert-butyl acrylate)-Br (PtBA-Br) macroinitiator was synthesized by ATRP. Subsequently, azide groups substituted for the active bromine end-group by allowing PtBA-Br to react with NaN3 in DMF. Thereafter, azide-terminated PtBA (PtBAN3) was reacted with excess C60 (1:5 PtBA/C60 to avoid multiple grafting of PtBA onto C60) in 1,2-dichlorobenzene at 130 °C for 20 h to obtain PtBMA-b-C60. The GPC traces showed a unimodal distribution with Mn ) 11 000 Da and Mw/Mn ) 1.15 for the PtBMA-b-C60. The covalent bonding between C60 and PtBA and the monosubstitution of PtBA to C60 was further confirmed using UV/RI dual-detector UV-vis spectroscopy, 13C NMR spectroscopy, and TGA measurements. Finally, the tert-butyl protecting groups were removed by hydrolysis using trifluoroacetic acid in dichloromethane at room temperature for 12 h to obtain a well-defined mono end-capped PAA-b-C60. The chemical formula of the copolymer can be expressed as PAA85-b-C60. PAA85-b-C60 is insoluble in aqueous medium at low pH (i.e., ∼3). Therefore, the polymer was dissolved in NaOH solution, where the molarity of NaOH is equivalent to that of carboxylic groups on the polymer chain. The polymer solution was continuously stirred for 12 h, until it became homogeneous. Thereafter, excess NaOH in the polymer solution was neutralized with equivalent amounts of HCl, and the polymer solution remained homogeneous and transparent. As indicated by the enthalpy profiles and particle sizes obtained from ITC and DLS, respectively, strong binding and binding-induced complexation are observed only when the surfactant contains an aromatic ring (27) Zhou, P.; Chen, G. Q.; Hong, H.; Du, F. S.; Li, Z. C.; Li. F. M. Macromolecules 2000, 33, 1948.
(TX100) and the polymer consists of fullerene (PAA85-b-C60). Among all of the polymer/surfactant systems investigated in this study (e.g., PAA/C12H25EO9, PAA85-b-C60/C12H25EO9, PAA/ TX100, and PAA85-b-C60/TX100), only TX100 micelles can partition and bind to hydrophobic fullerene domains of PAA85b-C60 and induce polymer/surfactant complexation. To further confirm the binding of TX100 on the fullerene domains of the polymer, the binding of TX100 to 5 mM PAA85-b-C60 at R ) 0 was characterized using UV-vis spectroscopy. The fullerene content of PAA85-b-C60 shows strong absorption bands at 256 and 334 nm, and they exhibit a red shift with an increase in TX100 concentration. The band at 256 nm shifts to 274 nm, and the band at 334 nm becomes barely detectable when the TX100 concentration reaches 0.5 mM and the system undergoes complexation (precipitation). The red shift may indicate the binding of TX100 to fullerene domains that enhances the π-π conjugation (between TX100 and fullerene and within fullerene molecules themselves) and reduces the spacing between fullerene molecules.28 The unique specific binding between TX100 and fullerene domains of the polymers is driven by hydrophobic interaction and enhanced by the good compatibility between the hydrophobic moieties of the polymers and surfactant because they both possess aromatic rings that trigger π-π conjugation. Dynamic light scattering and TEM studies were carried out to examine the morphological changes of the polymer/surfactant complex. The hydrodynamic radius (Rapp h ) of 5 mM (monomer concentration) PAA85-b-C60 at R ≈ 0 determined from dynamic light scattering was plotted against TX100 concentration in Figure 1. PAA85-b-C60 forms a large compound micelle (LCM) with a hydrodynamic radius of ∼120 nm in the absence of surfactant.5,6 The hydrodynamic radius remains essentially constant until the TX100 concentration reaches 0.4 mM. At 0.5 mM, Rapp h increases sharply from 130 to 570 nm, and the scattering intensity increases by several orders of magnitude as the opacity of the solution is enhanced, signaling the commencement of polymer/ surfactant complexation (PSC) induced by the specific binding of TX100 to fullerene domains of the polymeric micelles. The complexation triggers a morphological change and the precipitation of polymeric micelles. With further additions of TX100 to 1.7 mM, Rapp h increases slightly to 680 nm and then decreases rapidly, yielding a corresponding reduction in the scattering intensity resulting from the resolubilization of the precipitates. (28) Shon, Y. S.; Choo, H. Chem. Commun. 2002, 2560.
Letters
Figure 2. Micrographs demonstrating the morphology change of unneutralized PAA85-b-C60 (samples prepared from a 5 mM polymer solution) induced by the binding of TX100: (a) CTX100 ) 0 mM, (b) CTX100 ) 1 mM, (c) CTX100 ) 1.7 mM, and (d) CTX100 ) 2 mM.
In excess amounts of surfactant, the complex is dissociated by bound TX100 micelles with their hydrophilic headgroups extending outward, resulting in a decrease in particle size and scattering intensity. A series of TEM images for different regimes as marked in Figure 1 are summarized in Figure 2, showing the morphological changes of 5 mM unneutralized PAA85-b-C60 with increasing TX100 concentration. Figure 2a shows the large compound micelles (LCMs) of unneutralized PAA85-b-C60 of radius 100
Langmuir, Vol. 22, No. 7, 2006 2929
nm, which is consistent with the particle size obtained from light scattering measurements. When the TX100 concentration was increased to 1.0 mM, spherical, dense particles with smooth surfaces and much larger sizes were observed, characterizing the formation of precipitated PSC induced by surfactant binding. The hydrophobic binding is strengthened by the good compatibility between the hydrophobic segment of TX100 and fullerene, thus the surfactant micelles are able to encapsulate the fullerene clusters within the LCMs. The selective binding may expand the fullerene clusters and release some fullerene moieties. The “free” polymer chains together with bound TX100 molecules may associate to yield physical cross-linkers that enhance interparticle association between the polymeric micelles, resulting in the formation of flocculated PSC as shown in Figure 2b. The radii of PSC vary from 450 to 600 nm, which is in good agreement with that determined from dynamic light scattering. With further additions of TX100 to 1.7 mM, more surfactant micelles are bound on the fullerene clusters in the LCMs with their hydrophilic headgroups extending outward to enhance the overall hydrophilicity of the polymeric micelles. Thus, the dense precipitates of PSC disintegrate into smaller aggregates with a broad size distribution of varying shapes as shown in Figure 2c. When the concentration of TX100 exceeds 2 mM, the complex dissociates completely into smaller particles with a radius ranging from 60 to 100 nm. The polymeric micelles were coated and stabilized by a wetting layer of bound TX100 as shown in the micrograph (Figure 2d). These results demonstrated that the addition of TX100 significantly influences the self-assembly of fullerene-containing polymers and is an effective way to tune the size, shape, and solubility of the polymeric micelles. The proposed mechanism of the complexation and morphological change in the PAA85b-C60/TX100 system at R ) 0 in aqueous medium based on light scattering and TEM studies is illustrated in Figure 3.
Figure 3. Mechanism of the binding and induced morphological change in the PAA85-b-C60/TX100 system at R ) 0 in an aqueous medium. (a) CTX100 ) 0 mM, large compound micelle of PAA85-b-C60. (b) CTX100 ) 0.5 mM, selective binding of TX100 to fullerene domains that releases polymer chains. (c) 0.5 mM < CTX100 < 1.7 mM, precipitated polymer/surfactant complex. (d) CTX100 > 1.7 mM, LCMs stabilized by a wetting layer of TX100 in excess surfactant.
2930 Langmuir, Vol. 22, No. 7, 2006
Figure 4. Dependence of Rapp h on R for the mixture of 5 mM PAA85b-C60 and 0.7 mM TX100 in a 0.1 M NaCl solution.
The morphology of the PAA85-b-C60/TX100 mixture is responsive not only to surfactant concentration but also to pH. The dependence of the hydrodynamic radius on R for the mixture of 5 mM PAA85-b-C60/0.7 mM TX100 at different R (varying from 0 to 0.3) is shown in Figure 4. The polymer/surfactant mixture exists as precipitated PSC with a hydrodynamic radius of 646 nm at R ) 0. The Rapp h decreases to 372 nm when R is increased to 0.1, characterizing the gradual dissociation of PSC caused by the weakening of polymer surfactant interaction when the polymer was neutralized and became more hydrophilic. When R exceeds 0.15 (pH ∼6), Rapp h decreases to 121 nm, which is equal to the size of LCM in the absence of TX100, indicating the complete dissociation of PSC. Thereafter, Rapp h increases to 149 nm when R increases to 0.3, characterizing the expansion of the polymeric micelles driven by electrostatic repulsion between neutralized/ionized carboxylate groups. Figure 5 shows a series of micrographs depicting the morphology changes of the PAA85-b-C60(5mM)/TX100(0.7 mM) mixture at different R values. (The micrograph where the sample was taken is marked in Figure 4.) As shown in Figure 5a and b, the dense flocculated PSC is produced at R e 0.05. The radius of PSC is approximately 500 nm at R ) 0 and 400 nm at R ) 0.05, which are consistent with the particle sizes obtained from light scattering measurements. When R reaches 0.1, the PSC becomes less dense, and the size decreases significantly to approximately 280 nm as shown in Figure 5c. Neutralization of the polymer considerably reduces the polymer/surfactant affinity. Consequently, some bound TX100 molecules were expelled from the complex, leading to the disintegration of PSC and a corresponding reduction in size. When R exceeds 0.15, the PSC is completely dissociated. The abundance of spherical particles with smooth surfaces shown in Figure 5d characterizes the partially neutralized LCM of the polymer with a size distribution from 60 to 110 nm. The noncovalent nature of the binding force
Letters
Figure 5. Micrographs demonstrating the morphologies of the PAA85-b-C60/TX100 mixture (samples prepared in a solution of 5 mM PAA85-b-C60 mixed with 0.7 mM TX100) at different R values. R ) (a) 0, (b) 0.05, (c) 0.1, and (d) 0.15.
and the tunable affinity between the polymer and TX100 make the complexation reversible by simply adjusting the pH. In summary, we observed the unique specific binding of TX100 to fullerene domains within the large compound micelles of PAA85-b-C60 and opened the possibility to tune the morphology and solubility of C60 containing polymers using nonionic aromatic surfactants. The selective binding of TX100 to fullerene domains of the polymer triggers an interesting morphological change in the polymeric micelles, from large compound micelles to precipitated aggregates and finally to soluble polymeric clusters by a wetting layer of bound surfactant molecules. Moreover, the complexation is fully reversible by adjusting the pH of the system, allowing full control of the conformation of the polymeric micelles and phase behavior of the polymer/surfactant mixture. A systematic study of the binding interaction and induced morphological change of various fullerene-containing polymers and surfactants is currently in progress. We believe that the present findings unveil a possible new approach to the direct assembly and tuning of the micellar conformation of fullerene-containing polymers, which may find important applications in drug delivery, nanoscale patterning, and miniaturized biological devices. Acknowledgment. We are grateful for the financial support provided by Nanyang Technological University and the Singapore-MIT Alliance (SMA). Supporting Information Available: Experimental techniques and characterization details. Differential enthalpy curves obtained from the titration of C12H25EO9 and TX100 into 5 mM (monomer concentration) PAA and PAA85-b-C60 at R ) 0 and 1. UV-vis spectra obtained in 5 mM PAA85-b-C60 with the addition of TX100 (concentration varies from 0 to 0.5 mM). This material is available free of charge via the Internet at http://pubs.acs.org. LA052191V
