Synthesis and Aggregation Behavior of Amphiphilic Block Copolymers

Singapore-MIT Alliance, School of Mechanical and Production Engineering, and. National Institute of ... Singapore 639798, Republic of Singapore. Recei...
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Langmuir 2004, 20, 1597-1604

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Synthesis and Aggregation Behavior of Amphiphilic Block Copolymers in Aqueous Solution: Di- and Triblock Copolymers of Poly(ethylene oxide) and Poly(ethyl acrylate) S. Dai,†,‡ P. Ravi,†,‡ C. Y. Leong,‡ K. C. Tam,*,†,‡ and L. H. Gan§ Singapore-MIT Alliance, School of Mechanical and Production Engineering, and National Institute of Education, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore Received October 1, 2003. In Final Form: December 12, 2003 Di- and triblock copolymers of ethylene oxide (EO) and ethyl acrylate (EA) were synthesized by the atom transfer radical polymerization technique. The chemical compositions of the di- and triblock copolymers as determined by NMR and gel permeation chromatography are represented by [EO114EA10] and [EA10EO445EA10], respectively. The aggregation behaviors of these polymers in aqueous solutions were studied using a combination of static light scattering (SLS), dynamic light scattering, steady fluorescence, time-resolved fluorescence quenching (TRFQ), and surface tension techniques. The hydrodynamic radii (Rh) of PEO-b-PEA and PEA-b-PEO-b-PEA, where PEO is poly(ethylene oxide) and PEA, poly(ethylene acrylate), in an aqueous solution possess values of 15.3 and 25.5 nm, respectively, and they are independent of the polymer concentrations, which suggest that these micelles are formed via the closed association model. The critical micelle concentration (cmc) for the di- and triblock copolymers as determined from the surface tension are 0.002 and 0.007 wt %, while the aggregation numbers (Nagg) determined from the SLS and TRFQ are 18 and 26, respectively. When the di- and triblock copolymers are mixed in various proportions, unique intermediate cmc values are obtained. The Rh of the mixtures cannot be described by the simple or inverse mixing rule, which confirms that mixed micelles are produced.

Introduction Poly(ethylene oxide) (PEO) is a water-soluble and biocompatible polymer, which has found numerous applications in personal home care, pharmaceutical, and industrial products.1,2 Because of its widespread applications, research on the solution properties of PEO and the development of PEO derivatives are actively pursued by the scientific communities. Because of its water-soluble character, many PEO derivatives have been produced by grafting one or two hydrophobic blocks to the PEO chains, which impart amphiphilic characteristics to the polymer. These amphiphilic polymers can self-assemble into ordered structures, such as micelles that dissolve in polar solvents at concentrations exceeding the critical micelle concentration (cmc), where the hydrophobic inner core is shielded from water by the surrounding hydrophilic corona.3 In the past 2 decades, hydrophobically modified alkyl telechilic PEOs were developed and were found to exhibit interesting solution behavior.4-7 Such polymers generally * Author to whom correspondence should be addressed. Fax: (65) 6791 1859. E-mail: [email protected]. † Singapore-MIT Alliance. ‡ School of Mechanical and Production Engineering. § National Institute of Education. (1) Bailey, F. E., Jr.; Koleske, J. V. Alkylene Oxides and Their Polymers; Marcel Dekker: New York, 1991. (2) Harris, J. M.; Zalipsky, S. Poly(ethylene glycol) Chemistry and Biological Applications; ACS Symposium Series 680; American Chemical Society: Washington, DC, 1997. (3) Evans, D. F.; Wennerstrom, H. The Colloial Domain Where Physics, Chemistry, Biology and Technology Meet, 2nd ed.; Wiley-VCH: New York, 1999. (4) Winnik, M. A.; Yekta, A. Curr. Opin. Colloid Interface Sci. 1997, 2, 424. (5) Ma, S. X.; Cooper, S. L. Macromolecules 2002, 35, 2024. (6) Alami, S. A.; Stilbs, P. J. Phys. Chem. 1994, 98, 6539. (7) Persson, K.; Abrahamsen, S.; Stilbs, P.; Hansen, F. K.; Walderhaug, H. Colloid Polym. Sci. 1992, 270, 465.

form flowerlike micelles in a dilute solution consisting of a hydrophobic micellar core and looping PEO corona chains. At a higher polymer concentration, a polymer network is produced when the flowerlike micelles form intermolecular associations via bridging chains, and this produces a sharp increase in the solution viscosity. Enhanced hydrophobicity of the end-cap groups can be achieved by grafting of fluoroalkyl or fullerene to the ends of the PEO chains.8-11 Besides the hydrophobically modified PEOs, amphiphilic PEO block copolymers have received increased interest because of their potential applications in the biomedical and specialty chemical sectors. Systems such as the Pluronic triblock copolymers of PEO and PPO [poly(propylene oxide)] are widely used as polymeric surfactants.12,13 At concentrations greater than the cmc, micelles consisting of a hydrophobic PPO core and hydrophilic PEO corona are formed in solution. At even higher polymer concentrations, a thermoresponsive hydrogel is produced. By variation of the compositions of ethylene oxide (EO) or propylene oxide (PO) blocks, the solution and phase behaviors can be manipulated. Other similar systems, such as the di- and triblock copolymers of PEO and PBO [poly(butylene oxide)], were also synthesized and characterized by the research groups of Booth and Chu.14-18 (8) Cathebras, N.; Collet, A.; Viguier, M.; Berret, J. F. Macromolecules 1998, 31, 1305. (9) Tae, G.; Kornfield, J. A.; Hubbell, J. A.; Lal, J. Macromolecules 2002, 35, 4448. (10) Xu, B.; Li, L.; Yekta, A.; Masoumi, Z.; Kanagalingam, S.; Winnik, M. A.; Zhang, K.; Macdonald, P. M.; Menchen, S. Langmuir 1997, 13, 2447. (11) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Langmuir 2003, 19, 4798. (12) Nace, V. M. Nonionic Surfactants: Polyoxyalkylene Block Copolymers; Marcel Dekker: New York, 1996. (13) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1.

10.1021/la035837x CCC: $27.50 © 2004 American Chemical Society Published on Web 01/27/2004

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Because of the more hydrophobic PBO segments, flowerlike micelles are produced by the triblock copolymers in solution. It was found that 1 oxybutylene unit is equivalent to about 4.4 oxypropylene units. A good review on the structure and dynamics of pluronictype block copolymers was presented by Chu in a recent monograph.19 The gelation behavior of another thermoresponsive block copolymer (PEO-b-PNIPAM) was reported recently.20,21 In addition, poly(ethylene oxide)-bpolycaprolatone (PEO-b-PCL) has found potential applications as a drug delivery vehicle.22-28 These block copolymers form core-shell micellar structures in an aqueous solution, which could be loaded with hydrophobic drugs for enhanced delivery applications. Among the aggregation behavior of PEO block copolymers in an aqueous solution, PEO-b-PS, where PS is polystyrene, is one of the most widely studied systems.29-32 In a dilute aqueous solution, regular core-shell-like micelles coexist with polymeric clusters, where the core is dense and glassy, while the shell is highly swollen. The cmc is extremely low (∼4 × 10-7 M) because of the extremely hydrophobic PS segments.33 By variation of the compositions of the blocks and preparation methods, different types of morphologies were observed by Eisenberg and co-workers using the transmission electron microscope. Because polystyrene has a high glass-transition temperature (Tg) of ∼100 °C, the hydrophobic micellar core is glassy and frozen, where the lifetime of the hydrophobic segments in the micellar core is usually very long. For the drug delivery system, hydrophobic cores with lower Tg’s are preferred because they are more permeable than the glassy polymers.34 In this paper, we chose poly(ethyl acrylate) (PEA) as the hydrophobic segment for grafting onto the PEO homopolymer. Besides the lower Tg value (Tg ∼ -24 °C) and moderate hydrophobic character, PEA has found widespread application in drug delivery systems.35,36 The block copolymer of PEO-b-PEA should form a more dynamic and permeable hydrophobic (14) Zhou, Z.; Yang, Y. W.; Booth, C.; Chu, B. Macromolecules 1996, 29, 8357. (15) Liu, T.; Zhou, Z.; Wu, C.; Chu, B.; Schneider, D. K.; Nace, V. M. J. Phys. Chem. B 1997, 101, 8808. (16) Liu, T.; Zhou, Z.; Wu, C.; Nace, V.; Chu, B. J. Phys. Chem. B 1998, 102, 2875. (17) Liu, T.; Nace, V. M.; Chu, B. J. Phys. Chem. B 1997, 101, 8074. (18) Mingvanish, W.; Mai, S. M.; Heatley, F.; Booth, C.; Attwood, D. J. Phys. Chem. B 1999, 103, 11269. (19) Chu, B. Langmuir 1995, 11, 414. (20) Zhu, P. W.; Napper, D. H. Macromolecules 1999, 32, 2068. (21) Hu, T.; Wu, C. Macromolecules 2001, 34, 6802. (22) Gan, Z.; Jim, T. F.; Li, M.; Zhao, Y.; Wang, S.; Wu, C. Macromolecules 1999, 32, 590. (23) Zhao, Y.; Liang, H.; Wang, S.; Wu, C. J. Phys. Chem. B 2001, 105, 848. (24) Kim, S. Y.; Ha, J. C.; Lee, Y. M. J. Controlled Release 2000, 65, 345. (25) Ha, J. C.; Kim, S. Y.; Lee, Y. M. J. Controlled Release 1999, 62, 381. (26) Bae, Y. H.; Huh, K. M.; Kim, Y.; Park, K. H. J. Controlled Release 2000, 64, 3. (27) Allen, C.; Han, J.; Yu, Y.; Maysinger, D.; Eisenberg, A. J. Controlled Release 2000, 63, 275. (28) Choi, Y. K.; Bae, Y. H.; Kim, S. W. Macromolecules 1998, 31, 8766. (29) Chen, X.; Gao, B.; Kops, J.; Batsberg, W. Polymer 1998, 39, 911. (30) Xu, R.; Winnik, M. A.; Hallett, F. R.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 87. (31) Yu, K.; Eisenberg, A. Macromolecules 1996, 29, 6359. (32) Yu, K.; Eisenberg, A. Macromolecules 1998, 31, 3509. (33) Zhao, C.; Winnik, M. A.; Riess, G.; Croucher, M. Langmuir 1990, 6, 514. (34) Wise, D. L.; Trantolo, D. J.; Altobelli, D. E.; Yaszemski, M. J.; Gresser, J. D.; Schwartz, E. R. Encyclopedic Handbook of Biomaterials and Bioengineering, Part A: Materials; Marcek Dekker: New York, 1995; Vol. 2. (35) Ivan, B.; Almdal, K.; Mortensen, K.; Hohannsen, I.; Kops, J. Macromolecules 2001, 34, 1579.

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core in an aqueous solution compared to the block copolymer of PEO-b-PS. The dynamic nature of the micellar core of PEO-b-PEA makes it a better model for studying the physicochemical properties of micelles and their mixtures. In addition, there is no reported study on the solution behavior of the di- and triblock copolymers of ethyl acrylate (EA) and EO. In this paper, well-defined di- and triblock copolymers of EO and EA, i.e., PEO-bPEA and PEA-b-PEO-b-PEA, were synthesized and characterized, and the aggregation behavior of these polymers in aqueous solutions was examined using a combination of several physical techniques. Experimental Section Materials. The monomethyl-end-capped PEOs [degree of polymerization (DP) ) 114, Mw/Mn ) 1.05] and dihydroxy-capped PEOs (DP ) 455, Mw/Mn ) 1.08) were kindly donated by Dow Chemicals. EA (99%) purchased from Aldrich was passed through a basic alumina column that was stirred over CaH2 and distilled under reduced pressure. CuCl (99.995%), N,N,N′,N′,N′′,N′′hexamethyltriethylenetetramine, 2-bromoisobutryl bromide, anisole, and pyrene were purchased from Aldrich and used without further purification. Synthesis of PEO Macroinitiators. For a typical experiment, in a three-neck round-bottom flask, required amount of PEO was dissolved in 200 mL of dry toluene. The trace amount of water in PEO was removed from toluene using azeotropic distillation. After removal of 40-50 mL of toluene from the reaction mixture by azeotropic distillation, the reaction mixture was cooled to 0 °C. To the reaction mixture were added triethylamine (1.5 molar equiv to PEO) under continuous stirring and 2-bromoisobutryl bromide (1.5 molar equiv to PEO) dropwise over a 1 h period using a pressure-equalizer funnel. Then, the reaction mixture was stirred for 24 h at room temperature. After the reaction was completed, it was filtered to remove the triethylamine hydrobromide. The solvent was concentrated and precipitated in n-hexane, filtered, and dried under vacuum. The procedure was repeated two times to ensure the complete coupling of the end groups. Finally, the crude polymer was dissolved in water at pH 7-8 and extracted from methylene chloride. After drying over magnesium chloride for 12 h, the solvent was removed under vacuum and dried to obtain the purified macroinitiator. Synthesis of PEO-b-PEA and PEA-b-PEO-b-PEA. All of the synthetic steps were carried out under an argon atmosphere. In a typical experiment, required amounts of the PEO-Br (or Br-PEO-Br) macroinitiator, CuCl (molar equivalent to the macroinitiator), and magnetic bar were charged into a predried Schlenk flask and tightly sealed with a rubber septum. Deoxygenated anisole (minimum amount to dissolve the macroinitiator), followed by the monomer, was introduced into the flask via an Ar-washed syringe and stirred until the system became homogeneous. Three “freeze-pump-thaw” cycles were performed to remove oxygen from the polymerization solution. Finally, degassed ligand (HMTETA, molar equivalent to the macroinitiator) was introduced using an Ar-purged syringe and placed in a thermostated oil bath at 60 °C. As soon as the ligand was added, the system turned dark green, indicating the progress of the polymerization. After the polymerization was completed, the polymer was isolated by dissolving it in THF and passing it through an alumina column to remove the catalyst. Finally, the polymer was recovered by precipitating it into an excess amount of n-hexane, filtered, and dried under vacuum to a constant weight (Scheme 1). Gel Permeation Chromatography (GPC). Polymer molecular weights and molecular weight distributions were determined using GPC. An Agilent 1100 series GPC system equipped with a liquid chromatography pump, photoluminescencegel 5 µm MIXED-C column, and refractive index (RI) detector was used. The column was calibrated with the narrow molecular weight PS standards. HPLC-grade THF stabilized with butylated (36) Dittgen, M.; Durrani, M.; Lehmann, K. S.T.P. Pharma Sci. 1997, 7, 403.

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Scheme 1. Synthesis for (a) PEO-b-PEA and (b) PEA-b-PEO-b-PEA

Figure 1. GPC traces of the PEO-b-PEA and PEA-b-PEO-bPEA copolymers. hydroxytoluene was used as a mobile phase. The flow rate was maintained at 1.0 mL/min. GPC traces of both copolymers are shown in Figure 1, indicating that the molecular weights of the di- and triblock copolymers are higher than those determined from NMR. This is due to the difference in hydrodynamic volume of PEO with respect to the calibration GPC standard (PS). However, the molecular weight distributions are narrow (Mw/Mn ) ∼1.12) for both of the di- and triblock copolymers, with a slight increase of 1.05 for PEO used in this paper. NMR Spectroscopy. 1H NMR spectrum for the precursor block copolymer was measured using a Bru¨ker DRX400 instrument in CDCl3. The 1H NMR spectrum of the block copolymer as shown in Figure 2 allows the molar composition to be determined from the relative intensity at 4.12 ppm (-OCH2) of the EA block and 3.36 ppm (-OCH3) of the PEO block or 3.453.74 (-OCH2) of the PEO block. On the basis of the NMR and GPC trace results, DPs were found to be 10 (EA) and 114 (EO) for the diblock copolymer [EO114EA10] and 10 (EA for each end) and 445 (EO) for the triblock copolymer [EA10EO445EA10], respectively. Surface Tension (ST). The Dataphysics DCAT 21 tensiometer equipped with a standard Du Nou¨y ring was used to determine the cmc of the copolymers in an aqueous solution.

Figure 2. 1H NMR spectra of (a) PEO-b-PEA and (b) PEAb-PEO-b-PEA in CDCl3. Droplets of the concentrated polymer solution were gradually titrated to a glass vessel containing 50 mL of distilled water. The

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Figure 3. Decay time distribution functions of 0.1 wt % of PEO-b-PEA in aqueous solutions at 25 °C and different measurement angles.

Figure 4. Concentration dependence of the diffusion coefficients for PEO-b-PEA (b) and PEA-b-PEO-b-PEA (O) in aqueous solutions.

STs at different polymer mixtures and concentrations were recorded and analyzed using the SCAT program. Laser Light Scattering (LLS). A Brookhaven BI-200SM goniometer system equipped with a 522 channel BI9000AT digital multiple τ correlator was used to perform static and dynamic light scattering (SLS and DLS) experiments. The poweradjustable argon-ion laser with a wavelength of 488 nm was used as the light source. A 0.2 µm filter was used to remove dust prior to the light scattering experiments. For SLS, a Debye plot was used to analyze the experimental data, and the RI increments, dn/dC at 25 °C as determined by the BI-DNDC differential refractometer, are 0.131 and 0.148 mL/g for the di- and triblock copolymers, respectively. For DLS, the inverse Laplace transform of REPES in the Gendist software package was used to analyze the time-correlation functions with the probability of reject setting at 0.5. Steady-State Fluorescence Spectroscopy. Pyrene was used as the dye for all of the steady-state fluorescence experiments. The preparation method for pyrene in the polymer solutions is similar to that described by Winnik and coworkers.37,38 Perkin-Elmer Luminescence LS 50B equipped with FL Winlab software was used to record and analyze the steadystate fluorescence emission and excitation spectra. UV-Visible Spectroscopy. A Cary 50 Bio UV-visible spectroscope was used to measure the UV absorption of pyrene in the polymer solutions. On the basis of Beer’s law, the pyrene concentrations in the polymer solutions could be determined. The molar extinction coefficient of pyrene at a wavelength of 335 nm is 3.58 × 104 L/mol‚cm.37 Time-Resolved Fluorescence Quenching (TRFQ). The Edinburgh F900 single-photon-counting instrument was used to perform the TRFQ experiments. The nanosecond flash lamp was used as the light source; the wavelength of excitation was set at 338 nm, and the wavelength of emission was set at 380 nm.

diffusion of the scattering objects. The magnitude of the light scattering vector q, defined as the wave vector difference between the incident and the scattered light at a particular scattering angle θ, i.e., q ) ks - ki, is given by the equation39

Results and Discussion PEO-b-PEA. The aggregation behavior of PEO-b-PEA in aqueous solutions was determined by conducting DLS measurements for different polymer concentrations (0.08, 0.10, 0.12, 0.15, and 0.18 wt %) at scattering angles ranging from 60° to 120° at intervals of 15°. Figure 3 shows the decay time distribution functions of the 0.1 wt % PEOb-PEA solution measured at different scattering angles. Only one decay mode was observed in the decay time distribution functions, and the decay time decreases with increasing scattering angles. The decay rates Γ (the inverse of the decay time τ) exhibit a q2 dependence, which suggests that the decay mode is related to the translational (37) Yekta, A.; Duhamel, J.; Brochard, P.; Adiwidjaja, H.; Winnik, M. A. Macromolecules 1993, 26, 1829. (38) Vorobyova, O.; Lau, W.; Winnik, M. A. Langmuir 2001, 17, 1357.

q)

θ 4πn sin λ 2

()

(1)

where n is the refractive index (RI) of the solvent, λ, the wavelength of the incident beam, and θ, the scattering angle. The translational diffusion coefficient D is related to the decay rate Γ by eq 2,39 and the magnitude is given by

D)

Γ q2

(2)

the slope of the straight line in the plot of Γ versus q2. The diffusion coefficient D for 0.1 wt % PEO-b-PEA was determined to be 1.68 × 10-11 m2/s. On the basis of the Stokes-Einstein expression, the hydrodynamic radius Rh could be determined using39

Rh )

kT 6πη0D0

(3)

where η0 is the solvent viscosity, T, the absolute temperature, D0, the translational diffusion coefficient at infinite dilution, and k, the Boltzmann constant. For very dilute solutions, D was used instead of D0, and the apparent hydrodynamic radius Rhapp of 0.1 wt % PEOb-PEA was calculated to be 14.6 nm. Because the molecular weight of PEO-b-PEA is 5800 g/mol, the Rh value indicates the presence of aggregates instead of unimers. The decay time distribution functions of different concentrations of the diblock copolymer solutions revealed that the decay times are independent of polymer concentrations. To determine D0, the diffusion coefficients were plotted against the polymer concentrations as shown in Figure 4. The diffusion coefficients are independent of polymer concentrations, confirming that the aggregates are produced via the closed-association mechanism. By extrapolation to a zero concentration, D0 was obtained, which yielded a Rh of 15.3 nm for the micelle. For self-assembly systems, the cmc and aggregation number (Nagg) are two important physical parameters (39) Brown, W. Dynamic Light Scatteringsthe Method and Some Applications; Clarendon Press: Boston, 1993.

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Figure 5. Concentration dependence of the ST for PEO-bPEA (b), PEA-b-PEO-b-PEA (O), and their different mixtures in aqueous solutions at 25 °C.

Figure 6. Concentration dependence of the I1/I3 for PEO-bPEA (b) and PEA-b-PEO-b-PEA (O) in aqueous solutions at 25 °C.

commonly used to describe the characteristics of the micelle. The cmc could be determined from either ST or steady-state fluorescence spectroscopy using a pyrene probe. Figure 5 shows the dependence of the ST on the polymer concentration of di- and triblock copolymer mixtures. At very low polymer concentrations, the ST approaches that of water. With increasing PEO-b-PEA copolymer concentrations, unimers are partitioned to the air-water interface, which decreases the ST. The decrease in the ST, γ, continues until the unimer concentration reaches the cmc, where free micelles appear in solution, resulting in a change in slope. The intercept of the two tangential lines was extrapolated from low and high polymer concentrations to yield a cmc of ∼0.002 wt %. Steady-state fluorescence spectra of the copolymer solutions with a pyrene probe were also used to study the cmc of diblock copolymer solutions. Pyrene is hydrophobic and exhibits low solubility in water (6 × 10-7 M). When the pyrene environment changes from polar to nonpolar, its emission and excitation spectra are altered. In the pyrene excitation spectra, the peak at 335 nm in water shifts to 338 nm as the polymer concentration increases.38-41 This shift is due to the transfer of pyrene from water to the hydrophobic core of the polymer micelles. In the pyrene emission spectrum, the intensity ratio of the first vibrational band (I1) [S10 f S00] to the third band (I3) [S10 f S01] was used to monitor the micellization behavior of the surfactant solutions.42-44 The ratio of I1/I3 changes from ∼1.8 in water to ∼1.0 in the presence of anionic surfactant micelles.22 The fluorescence emission spectra of pyrene in the diblock copolymer solutions were measured. It is obvious that the fluorescence intensity increases with polymer concentrations, which may be attributed to the higher quantum yield. In addition, the I1 and I3 peaks in the emission spectra were observed at 372 and 383 nm, respectively, while the ratio of I1/I3 decreases with increasing polymer concentrations. Figure 6 shows that I1/I3 decreases from 1.7 to 1.4 when the polymer concentration was increased from 0.0001 to 0.30 wt %. The upper limit is close to the ratio of pyrene in

water (I1/I3 ) 1.75), while the lower limit seems to be higher than the expected value of pyrene in a hydrophobic environment. However, the value of 1.4 for pyrene in the block copolymer micelles has been previously reported by Wu and co-workers, and they attributed the apparent higher I1/I3 ratio to the lower polymer concentration and polar characteristics of the micelles.22 From the figure, the cmc of the diblock copolymer in aqueous solutions was determined to be ∼0.004 wt %, which is higher than the value determined from ST. The discrepancies in the cmc determined from the steady-state fluorescence and surface tension techniques may be due to the detection principles of these two methods.45 The I1/I3 from steady-state fluorescence detects the upper limit of the cmc, while the ST gives the lower limit of the cmc. Another possible reason is that because the cmc is extremely low, the values of I1/I3 are dependent on either the polymer or pyrene concentration. When the magnitude of the cmc approaches that of the pyrene concentration in solution, the pyrene in the micellar core and the free pyrene in water complicate the determination of the cmc.33 Under such conditions, I1/I3 does not show a decrease at the cmc when the micelle is formed because the emission spectra are dominated by the large excess of free pyrene in the water phase. I1/I3 will display an obvious decrease when all of the pyrene molecules are solubilized by the micellar core, which gives rise to a higher apparent cmc value as determined from the steady-state fluorescence experiments. SLS and TRFQ are commonly used to measure the Nagg of micelles.3 For SLS, the Debye plot can be used to find the weight-averaged molecular weight Mw when the particle size is small. However, the Zimm plot is a more generalized method for determining the weight-averaged molecular weight Mw, where the intramolecular interactions are taken into consideration. Because Rh for the PEO-b-PEA micelles is reasonably small, the Debye plot was adopted in this paper46

(40) Wilhelm, M.; Zhao, C. L.; Wang, Y.; Xu, R.; Winnik, M. A. Macromolecules 1991, 24, 1033. (41) Vorobyova, O.; Yekta, A.; Winnik, M. A. Macromolecules 1998, 31, 8998. (42) Feitosa, E.; Brown, W.; Vasilescu, M.; Vethamuthu, M. S. Macromolecules 1996, 29, 6837. (43) Winnik, F. M.; Regismond, S. T. A. Colloids Surf., A 1996, 118, 1. (44) Alami, E.; Almgren, M.; Brown, W. Macromolecules 1996, 29, 5026.

where K ()4π2n2 (dn/dC)2/NAλ4) is an optical constant, with NA, n, and λ being the Avogadro’s number, the solvent RI, and the wavelength of the light in vacuum,

KC 1 ) + 2A2C Rθ Mw

(4)

(45) Van Os, N. M.; Haak, J. R.; Rupert, L. A. M. Physicochemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: New York, 1993. (46) Chu, B. Laser Light ScatteringsBasic Principles and Practice, 2nd ed.; Academic Press: Boston, 1991.

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Figure 7. Debye plot of dilute PEO-b-PEA (b) and PEA-bPEO-b-PEA (O) in aqueous solutions.

Figure 8. Pyrene fluorescence decay profiles for 0.3 wt % of PEO-b-PEA and PEA-b-PEO-b-PEA in aqueous solutions.

respectively. The RI increment of the polymer solution (dn/dC) was measured using a differential refractometer. C is the polymer concentration in grams per milliliter, and Rθ is the excess Rayleigh ratio at the scattering angle θ. A2 is the second virial coefficient. Figure 7 shows the Debye plot for the dilute PEO-bPEA aqueous solutions determined for polymer concentrations ranging from 0.08 to 0.2 wt %. From the plot, the weight-averaged molecular weight Mw was found to be 9.99 × 104 g/mol; hence, Nagg for PEO-b-PEA (the micellar molecular weight divided by the molecular weight of the individual polymer chain) was found to be ∼17. TRFQ with pyrene self-quenching was also used to study the micellization behavior of the diblock polymer solutions, where the ground-state pyrene was used as the quencher of the excited-state pyrene probe.37,38,41 When the pyrene decay profile was fitted to the Infelta-Tachiya equation, the Nagg could be determined47

1.5. The calculated Nagg is ∼18, which agrees with the result obtained from SLS. PEA-b-PEO-b-PEA. Similar to the diblock copolymer in aqueous solutions, DLS was first used to measure the dynamic properties of the triblock copolymer solutions. The DLS experiments were carried out at different scattering angles and polymer concentrations. The scattering angles varied from 45° to 105° at intervals of 15°, while the polymer concentrations are 0.05, 0.08, 0.10, 0.12, 0.15, and 0.20 wt %. From the decay time distribution functions, it is clear that only one distribution peak was observed. The decay rates are q2 dependent, confirming the translational diffusion behavior of the scattering objects. The translational diffusion coefficients are independent of the polymer concentrations as shown in Figure 4. On the basis of the Stokes-Einstein relationship, Rh of the micelles was determined to be 25.5 nm, which corresponds to the Rh of the aggregates instead of unimers, and the micelles were produced via the closed-association mechanism.14 Because there are two hydrophobic end groups and the PEO molecular weight is 20 000 g/mol, the micellar structure is expected to be a flower or rosette shape, comprised of an EA hydrophobic core and looping PEO corona. SLS was used to determine the value of the averaged radius of gyration Rg for the triblock copolymer in a dilute solution by considering the intramolecular interactions. For the Rayleigh-Debye scattering, eq 8 can be used.46

t I(t) ) I(0) exp - - n(1 - exp(-kqt)) τ

[

]

(5)

where I(t) is the fluorescence intensity of pyrene at time t, I(0), the initial intensity, τ, the pyrene lifetime without the quencher, n, the mean number of quenchers per micelle, and kq, the pseudo-first-order rate constant for quenching in the micelle. The mean number of quenchers per micelle n is related to the Nagg through the expression

n)

[Q] [Q] ) N [micelle] C - cmc agg

(6)

where [Q] is the bulk molar quencher concentration, and C, the total concentration of polymer. From the UV absorption measurements, Beer-Lambert’s law was used to obtain [Q] as described by eq 7,

A ) [Q]d

(7)

where A is the absorbance, [Q], the concentration of the pyrene, d, the path length distance of the sample, and , the molar extinction coefficient with the value of 3.58 × 104 L/mol‚cm at a wavelength of 335 nm for pyrene. The decay profiles of pyrene for different polymer concentrations were carried out, and a typical decay profile is shown in Figure 8. The above quenching model showed a good fit to the experimental data, where χ2 is less than (47) Feitosa, E.; Brown, W.; Vethamuthu, M. S. Langmuir 1996, 12, 5985.

KC 1 1 ) 1 + q2Rg2 + 2A2C Rθ Mw 3

(

)

(8)

If the polymer concentration is sufficiently low, then the Rg values could be determined from the plot of 1/Iex versus q2, where Iex is the excess scattering intensity. From the relationship of 1/Iex and q2 for the triblock polymer in aqueous solutions, the Rg determined from the SLS was 21.8 nm. When this result is combined with the Rh obtained from DLS, the ratio of Rg/Rh was determined to be 0.855, which suggests that the micelle possesses a coreshell structure. The cmc values of the triblock copolymer in aqueous solutions were obtained from ST and fluorescence spectroscopy. The polymer concentration dependence of the ST is also shown in Figure 5, where the cmc was found to be 0.007 wt %, which indicates that the cmc for the triblock copolymers is larger than that for the diblock copolymers. One reason for the higher cmc observed for the triblock copolymers is the entropic penalty for bending the PEO backbone to produce the flower micelles. The

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Langmuir, Vol. 20, No. 5, 2004 1603

other reason is due to the longer PEO backbone, which gives rise to lower hydrophilic-lipophilic balance (HLB) values. The polymer concentration dependence of the ratio for I1 to I3 in the pyrene emission spectra was determined and plotted in Figure 6. I1/I3 for pyrene in the presence of PEA-b-PEO-b-PEA decreases from 1.7 to 1.43, and the cmc was determined to be ∼0.021 wt %. The observed difference between the values measured using ST and steady-state fluorescence spectroscopy was discussed previously. The Debye plot was used to determine the Nagg of the triblock copolymer micelles in an aqueous solution (Figure 7), where the molecular weight of the micelles was determined to be 2.93 × 105 g/mol; thus, the number of polymer chains per micelle is 13, corresponding to an Nagg of 26. The TRFQ measurements were performed on different polymer concentrations, and the decay profiles are shown in Figure 8. When the pyrene decay profile was fitted to the Poisson quenching model (eq 5), the mean number of quenchers per micelle (n), the lifetime (τ), and the pseudofirst-order rate constant (kq) were determined, where χ2 of the fitting is less than 1.5. Because there are two hydrophobic segments per polymer chain, eq 6 was modified for the triblock amphiphilic copolymer, where

n)

[Q] Nagg [Q] ) [micelle] C - cmc 2

Figure 9. Decay time distribution functions for the mixture of PEO-b-PEA and PEA-b-PEO-b-PEA at different compositions in aqueous solutions at 25 °C.

(9)

The calculated Nagg was found to be 24, which agrees with the value determined from SLS. Mixtures of PEO-b-PEA and PEA-b-PEO-b-PEA. For mixtures of the two amphiphilic polymers in an aqueous solution, two separate types of micelles may be produced if the hydrophobic blocks are not compatible. However, in some cases, mixed micelles are favored, such as in mixtures of mono- and difunctionalized PEOs reported by Lafle`che et al.48,49 In the current paper, the aggregation behavior of the mixtures of PEO-b-PEA and PEA-b-PEO-b-PEA in aqueous solutions was examined by DLS. From the paper on the dynamic properties of the mixture, the association mechanism of the copolymer mixtures could be evaluated whether the mixture of the core-shell micelle (for the diblock copolymer) and rosette micelle (for the triblock copolymer) or mixed micelles containing di- and triblock polymer chains are produced. The solution mixtures were studied at a fixed total polymer concentration of 0.2 wt %, and the ratio of the diand triblock copolymers were varied. The weight ratios of PEO-b-PEA to PEA-b-PEO-b-PEA were varied from 15 to 85, 30 to 70, 50 to 50, 70 to 30, and 85 to 15 wt %. The decay time distribution functions revealed one translational diffusional decay mode, suggesting that the mixed micelles containing a mixture of di- and triblock copolymer chains are produced. Figure 9 shows the relaxation time distribution functions for the mixture at different compositions of PEO-b-PEA and PEA-b-PEO-b-PEA. The peak shifts from left to right as the composition of the triblock copolymer is increased. The Rh was plotted against the mole fraction of the diblock copolymer in the solution mixture as shown in Figure 10. If the observed Rh corresponds to the mixture of the diblock core-shell and triblock rosette micelles, (48) Lafle`che, F.; Durand, D.; Nicolai, T. Macromolecules 2003, 36, 1331. (49) Lafle`che, F.; Nicolai, T.; Durand, D.; Gnanou, Y.; Taton, D. Macromolecules 2003, 36, 1341.

Figure 10. Dependence of the Rh on the mole fraction of the diblock copolymer in the mixture of PEO-b-PEA and PEA-bPEO-b-PEA in aqueous solutions. The filled circles are the experimental data. The dotted and dashed lines are the fitted results based on the simple mixing rule and the inverse mixing rule, respectively. The possible schematic aggregate conformations are marked in the figure.

they should obey either the simple mixing rule or the inverse mixing rule as given by eqs 10 and 11, respectively, tri Rmix ) xdiRdi h h + (1 - xdi)Rh

(10)

xdi 1 - xdi 1 ) di + mix Rh Rh Rtri h

(11)

where xdi is the mole fraction of the diblock copolymer. However, the experimental data exhibit a positive deviation from the mixing rules, which suggests that mixed micelles containing the di- and triblock chains are produced as illustrated in Figure 10. At both extremes, rosette and core-shell micelles are present for 100 wt % of the tri- and diblock copolymers, respectively. As the diblock is mixed with the triblock polymer, the diblock chains are incorporated into the rosette micelles, which displace some of the triblock polymer chains, yielding only a small decrease in the micellar size. The Rh of the mixed micelle is dominated by the higher molecular weight triblock copolymer, which concurs with the positive deviation observed in Figure 10. Our results are in agreement with the recent findings reported by Lafle`che et al. on the mixed micelles consisting of the mixture of the one- and two-end-substituted PEO system.48

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To verify the above-proposed mechanism, ST experiments on the mixtures of different ratios of the di- and triblock copolymers were also performed, and the results are shown in Figure 5. The STs of the polymer mixtures lie between those of the di- and triblock copolymers, and the transition point corresponds to the cmc. Only one transition point is evident in all of the ST curves, which reinforces the formation of the mixed micelle instead of the individual micelles because two cmc transition points should be observed for the latter. The cmc of the mixed micelle lies between those of the di- and triblock copolymers. When the weight percent of the diblock copolymer was varied from 15 to 85%, the cmc values decreased from 0.005 to 0.0024 wt %. Conclusions Well-defined amphiphilic di- and triblock copolymers of PEO-b-PEA and PEA-b-PEO-b-PEO were synthesized by the atom transfer radical polymerization technique.

Dai et al.

The aggregation behaviors of these polymer solutions were studied by LLS, ST, and fluorescence techniques. The cmc’s for PEO-b-PEA and PEA-b-PEO-b-PEO determined from the ST studies are 0.002 and 0.007 wt %, respectively. PEA-b-PEO forms core-shell micelles, while PEA-b-PEOb-PEO forms rosette micelles. The micelles for the di- and triblock copolymers possess average Nagg values of 17 and 25 and Rh values of 15.3 and 25.5 nm, respectively. For mixtures of the di- and triblock copolymers at concentrations exceeding the cmc, mixed micelles consisting of looping and dangling chains are produced instead of a mixture of individual core-shell and rosette micelles. Acknowledgment. The authors acknowledge the financial support from Singapore-MIT Alliance (SMA). In addition, we thank Prof. L. S. Chen at the Institute of Chemistry China Academy of Sciences for several useful discussions on fluorescence spectroscopy. LA035837X