8034
J. Phys. Chem. B 2007, 111, 8034-8037
Polymeric Micelles Formed by Splitting of Micellar Cluster Dinghai Xie, Wei Bai, Kui Xu, Ruke Bai, and Guangzhao Zhang* Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, Department of Polymer Science and Engineering, UniVersity of Science and Technology of China, Hefei, Anhui, China ReceiVed: March 23, 2007; In Final Form: May 21, 2007
Polystyrene-b-poly(acrylic acid) (PS-b-PAA) diblock copolymer chains form aggregates with bimodal distribution in toluene. The introduction of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) chains leads to the formation of mixed micellar cluster due to the hydrogen-bonding complexation between PAA and PEO. By using laser light scattering and transmission electron microscopy, we have investigated the structural evolution of the mixed micellar cluster. As the standing time increases, the cluster split into regular complex micelles composed of PS-b-PAA and PS-b-PEO chains. Our results reveal that the hydrogen-bonding complexation between PAA and PEO in the core and the repulsion between PS chains in the corona as a function of the molar ratio (r) of PEO to PAA manipulate the evolution.
Introduction Block copolymers self-assemble into structures with various morphologies in a selective solvent, depending on the nature and the relative lengths of the blocks.1-9 They usually form spherical micelles at a low concentration. Sometimes, they also form aggregates with a bimodal distribution. Namely, the aggregates consist of two distributed populations.10-13 For example, polystyrene-b-poly(ethylene oxide) (PS-b-PEO) chains were reported to form a small and a larger aggregate in aqueous solution, which were interpreted as regular micelles and loose micellar clusters, respectively. The clustering of long PEO chains was thought to be responsible for the formation of micellar cluster.10 Poly(dimethylsiloxane)-b-poly(ethylene oxide) chains also form micellar clusters,11 which was attributed to the partial interpenetration of PEO chains between different micelles. Pispas and Hadjichristidis12,13 proposed that the micellar clusters are cylindrical micelles. In our previous work,14 we demonstrate that polystyrene-bpoly(acrylic acid) (PS-b-PAA) diblock copolymer chains form regular micelles in toluene. The introduction of poly(methyl methacrylate)-b-poly(ethylene oxide) (PMMA-b-PEO) chains leads such micelles to form mixed micelles due to the hydrogenbonding complexation between PAA and PEO.15 On the other hand, repulsion between PMMA and PS in the corona tends to split the mixed micelles. The balance between the hydrogen bonding and repulsion drives the mixed micelles to evolve into hybranched structure. In the present work, we report that PSb-PAA chains form bimodal distributed aggregates in toluene when the PS block is short enough. The aggregate can form a mixed micellar cluster with PS-b-PEO chains. As PS chain density in the corona increases,16 the mixed micellar clusters split into regular micelles. By using laser light scattering (LLS), we have investigated the structural evolution of the mixed micellar cluster. Our aim is to understand the origin of the micellar cluster and the micellization mechanism. Experimental Section Materials. Poly(ethylene oxide) (PEO) capped with a methyl at one end and a hydroxyl at the other (Mn ) 5000, Fluka) was * To whom correspondence should be addressed.
purified by azeotropic distillation in toluene. CuBr was purified by stirring in acetic acid, washing with methanol, and then drying. Styrene and tert-butyl acrylate (BA) were respectively passed through a column of neutral alumina to remove inhibitor. Tetrahydrofuran (THF) was distilled over potassium. PEO-Br macroinitiator was prepared by the reaction of the PEO with 2-bromoisobutyryl bromide. PS-b-PEO diblock copolymer was synthesized by the atom transfer radical polymerization (ATRP) of styrene initiated by CuBr/PEO-Br in THF.17 The details about the synthesis of PS-b-PAA can be found elsewhere.18 PS-bPBA diblock copolymer was prepared by the ATRP of BA and styrene, using a alkyl bromide as the initiator and CuBr/ N,N,N′,N′′,N′′-pentamethyldiethylenetriamine. The complete hydrolysis of PS-b-PBA by trifluoroacetic acid in dichloromethane yielded PS-b-PAA diblock copolymer. The number average molecular weight (Mn) and the polydispersity (Mw/Mn) of PS-b-PEO and PS-b-PBA were characterized by a combination of gel permeation chromatographs (GPC) and 1H NMR spectra. GPC measurements were carried out on a Waters 150C with a series of monodisperse polystyrenes as the calibration standard and THF as the eluent with a flow rate of 1.0 mL/min. 1H NMR spectra were measured on a Bruker DMX-500 NMR spectrometer with chloroform-d (CDCl3) as the solvent and tetramethylsilane (TMS) as the internal standard. Here, a block copolymer designated as Am-b-Bn, with m and n being the numbers of the units in A and B blocks. For PS225b-PEO114, PS173-b-PAA164, and PS260-b-PAA164, we have Mw/ Mn ) 1.06, 1.23, and 1.27, respectively (see the Supporting Information). PS-b-PAA micelles were prepared by mixing 70.0 mL of toluene with 7.0 mL of PS-b-PAA solution in THF at a constant rate with a SP100i syringe pump. THF was removed by evaporation under reduced pressure. Such a solution was further mixed with PS-b-PEO solution in toluene. The final concentration of the PS-b-PAA micelles was 9.0 × 10-5g/mL, and the concentration of PS-b-PEO ranged from 1.0 × 10-5 to 2.5 × 10-4g/mL. Laser Light Scattering. LLS measurements were conducted on an ALV/DLS/SLS-5022F spectrometer with a multi-τ digital time correlation (ALV5000) and a cylindrical 22 mW UNI-
10.1021/jp072329d CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007
Polymeric Micelles
Figure 1. Typical hydrodynamic radius distributions f(Rh) of PS260b-PAA164 and PS173-b-PAA164 aggregates.
PHASE He-Ne laser (λ0) 632 nm) as the light source. The weight-average molar mass (Mw), the root-mean-square radius of gyration 〈Rg2〉z1/2 (or written as 〈Rg〉), and the second virial coefficient A2 were obtained from the angular dependence of the absolute excess time-average scattering intensity or Rayleigh ratio Rvv(q) in static LLS.19,20 On the other hand, the intensityintensity time correlation function G(2)(t,q) was measured to determine the line-width distribution G(Γ) in dynamic LLS.21 For diffusive relaxation, Γ is related to the translational diffusion coefficient (D) of the scattering object in dilute solution or dispersion by D ) (〈Γ〉/q2)Cf0,qf0 and further to hydrodynamic radius (Rh) from the Stokes-Einstein equation: Rh ) kBT/ (6πηD), where η, kB, and T are the solvent viscosity, the Boltzmann constant, and the absolute temperature, respectively. Hydrodynamic radius distribution f(Rh) was calculated from the Laplace inversion of G(2)(t,q) with the CONTIN program. The dynamic LLS measurements were conducted at a scattering angle (θ) of 15°. All LLS experiments were performed at 25 °C. The refractive index increment values of the micelles were measured by using a precise differential refractometer.22 High-ResolutionTransmissionElectronMicroscopy(HTEM). The morphologies of the aggregates were observed on a JEOL2100 high-resolution transmission electron microscope operating at an acceleration voltage of 200 kV. After a drop of the dilute aqueous solution with a concentration of 1.0 × 10-4 g/mL was deposited onto a carbon-coated copper mesh grid, the sample was stained by RuO4 vapor at room temperature for a half hour before HTEM observation to enhance the electron density contrast.23,24 Results and Discussion Figure 1 shows PS173-b-PAA164 chains form aggregates with a bimodal distribution in toluene. Since the block copolymer has a unimodal distribution (see the Supporting Information), the bimodal distribution of the aggregates is not due to the compositional heterogeneity of the block copolymer. The contour length of the block copolymer chain is estimated to be ∼84 nm by L ) Nl, where the length per unit l ) 0.25 nm, and N is the total number of units.25 Considering that the chains cannot be fully stretched in solution, their size should be much less than 84 nm. If the block copolymer chains form regular micelles with core-shell structure, their radius also should be less than 84 nm. Our experiments demonstrate that the average hydrodynamic radius (〈Rh〉) of PS173-b-PAA164 chains in THF is ∼3 nm. Since THF is a common solvent for PS and PAA blocks,26 the size of PS173-b-PAA164 individual chains in toluene, a selective solvent, should be less than ∼3 nm. Thus, the peaks located at Rh ≈ 20 and 90 nm can be attributed to regular micelles and micellar clusters, respectively.
J. Phys. Chem. B, Vol. 111, No. 28, 2007 8035
Figure 2. Typical hydrodynamic radius distributions f(Rh) of PS260b-PAA164 micelles (a) and PS260-b-PAA164/PS225-b-PEO114 mixed micelles (b), where the molar ratio (r) of PEO to PAA is 2.1.
Figure 3. The molar ratio (r) of PEO to PAA dependence of the average hydrodynamic radius (〈Rh〉) and average radius of gyration (〈Rg〉) of PS260-b-PAA164/PS225-b-PEO114 mixed micelles. The inset shows the molar ratio (r) dependence of the excess scattering intensity (Rvv(θ)/ KC).
In contrast, the aggregates formed by PS260-b-PAA164 block copolymer chains, which have the same PAA block but a longer PS block, exhibit only one narrow distribution with a peak at Rh ≈ 40 nm. As discussed above, the contour length of the block copolymer chain is estimated to be ∼106 nm; namely, the maximum radius of the regular micelles is ∼106 nm. Thus, PS260-b-PAA164 aggregates can be attributed to regular micelles. Clearly, when the PS block is short enough, PS-b-PAA chains form a micellar cluster. Note that the formation of a micellar cluster cannot be attributed to the interpenetration or clustering of the PS blocks in different micelles because longer solvophilic chains are easier to overlap than the shorter ones. Instead, it is probably because short PS chains on the periphery of the micelle cannot shield the solvophobic PAA core completely, and some PAA segments are exposed on the periphery, which makes the clustering of the micelles possible. Figure 2 shows the hydrodynamic radius distributions f(Rh) of PS260-b-PAA164/ PS225-b-PEO114 mixed micelles, where the molar ratio (r) of PEO to PAA is 2.1. Obviously, the size of the micelles slightly increases after PS225-b-PEO114 chains are introduced to PS260-b-PAA164 micelles. This is because PS chains in the corona become more stretched as the chain density increases due to the insertion of PS225-b-PEO114 chains. Figure 3 shows 〈Rg〉 and 〈Rh〉 of the PS260-b-PAA164/ PS225b-PEO114 mixed micelles at different molar ratios (r) of PEO to PAA. 〈Rg〉 and 〈Rh〉 slightly increase with r at r < ∼1.0, but no longer change at r > ∼1.0. The inset shows Rvv(q)/KC, which is proportional to the apparent molar mass (Mw) of the mixed micelles, varies in a similar way. However, it increases more sharply at r < ∼0.5. This is because PS225-b-PEO114 copolymers have a larger dn/dC value (0.085 mL/g) than that of PS260-bPAA164 in toluene (0.067 mL/g). The introduction of the former
8036 J. Phys. Chem. B, Vol. 111, No. 28, 2007
Figure 4. Hydrodynamic radius distribution f(Rh) of PS173-b-PAA164/ PS225-b-PEO114 mixed aggregates with different standing time, where the molar ratio (r) of PEO to PAA is 2.0.
leads the dn/dC value of the mixed aggregates to increase. Therefore, Rvv(q)/KC is more sensitive to r than 〈Rg〉 and 〈Rh〉. It is known that the density of the hydrogen bonds formed between PEO and PAA increases with r at r < 1.0 and has the maximum at r ) 1.0.15 As r increases, more PS-b-PEO chains insert in the micelles. As discussed above, because PS blocks in PS260-b-PAA164 are long enough to cover the micelle surface, the insertion is limited. Moreover, the residency of PS-b-PEO chains leads the chain density of the corona to increase. At r > ∼1.0, the corona is dense enough to restrict PS-b-PEO chains from interacting with the PAA core. Therefore, 〈Rg〉, 〈Rh〉, and Mw almost do not change with r. Figures 2 and 3 further indicate that block copolymers form regular micelles when the micelle surface can be fully covered by the solvophilic blocks. Otherwise, micellar clusters might result. Unlike the case in regular micelles, the addition of PS-bPEO chains would lead to the structural evolution of the micellar cluster. Figure 4 shows the standing time (t) dependence of hydrodynamic radius distributions f(Rh) of PS173-b-PAA164/ PS225-b-PEO114 aggregates with r ) 2.0. In the initial stage, the aggregates exhibit a bimodal distribution the same as that of PS173-b-PAA164 micelles shown in Figure 1. In other words, there exist peaks at ∼20 and ∼90 nm corresponding to the regular micelles and micellar clusters, respectively. This is because PS225-b-PEO114 chains have not inserted into the micelles in such a short time. As the time increases to 3 days, the small aggregates slightly increase in size, but the larger aggregates increase much more. The facts indicate that while a few of the PS225-b-PEO114 chains pack into a regular micelle, most of them insert into the micellar cluster forming mixed micellar clusters. As discussed above, the short PS chains cannot completely cover the PAA surface in PS173-b-PAA164 micelles, and some PAA segments are exposed. A micellar cluster with a looser structure has more PAA segments exposed on the periphery, so it can accept many more PS225-b-PEO114 chains than a regular micelle. When the time reaches 15 days, the size of the micellar cluster decreases, whereas f(Rh) turns into a broad unimodal distribution. Further increasing the time to 60 days leads f(Rh) to be narrower with only one peak at ∼30 nm. Clearly, as more PS-b-PEO chains insert, the mixed micellar cluster splits up into smaller micelles. This is because the repulsion between PS chains in the corona increases as the chain density increases. When the repulsion dominates the complexation between PAA and PEO in the core, the micellar cluster has to be split. Note that such micelles have a size somewhat larger than the regular micelles formed only by PS173-b-PAA164 chains (∼20 nm), indicating
Xie et al.
Figure 5. Time dependence of the relative scattering intensity (It/I0) of PS173-b-PAA164/PS225-b-PEO114 mixed aggregates, where the molar ratio (r) of PEO to PAA is 2.0. The inset shows the time dependence of the polydispersity (µ2/Γ2).
Figure 6. Time dependence of 〈Rh〉 and 〈Rg〉 of PS173-b-PAA164/PS225b-PEO114 mixed aggregates with different standing time, where the molar ratio (r) of PEO to PAA is 2.0.
that they are complex micelles composed of PS173-b-PAA164 and PS225-b-PEO114 chains. Figure 5 shows the standing time (t) dependence of the relative scattering intensity (It/I0) of PS173-b-PAA164/PS225-bPEO114 mixed aggregates with r ) 2.0 where I0 is the scattering intensity at zero scattering angle. Since scattering intensity is proportional to the square of the apparent molar mass, i.e., I ∝ Mw ∝ nM2, the increase of It/I0 at t < 3 days further indicates that PS-b-PEO chains have inserted into the PS-b-PAA micellar cluster. Meanwhile, the polydispersity (µ2/Γ2) (the inset) decreases, suggesting that the aggregates tend to be more uniform because the weighting of the micellar clusters becomes dominant. At t > 3 days, the decrease of It/I0 reveals that the mixed micellar clusters split into smaller micelles. The polydispersity (µ2/Γ2) increases at t > 3 days and levels off at t > 20 days, suggesting that the micellar clusters split up into stable complex micelles. Figure 6 shows 〈Rg〉 and 〈Rh〉 of the aggregates greatly increase in the initial stage, indicating that PS-b-PEO chains have inserted into PS-b-PAA micellar clusters. 〈Rg〉 and 〈Rh〉 reach their maxima at t ≈ 3 days, indicating the formation of the largest mixed micellar cluster there. The decrease in 〈Rg〉 and 〈Rh〉 at t > 3 days clearly indicates the splitting of the mixed micellar clusters. The evolution of the micellar clusters has also been investigated by HTEM. Figure 7a clearly shows that PS173-b-PAA164
Polymeric Micelles
J. Phys. Chem. B, Vol. 111, No. 28, 2007 8037
Figure 7. HTEM images of the PS173-b-PAA164/PS225-b-PEO114 mixed aggregates at 25.0 °C, where the molar ratio (r) of PEO to PAA is 2.0: (a) PS173-b-PAA164 aggregates; (b) PS173-b-PAA164/PS225-b-PEO114 mixed aggregates after mixing for 3 days; and (c) PS173-b-PAA164/PS225-b-PEO114 mixed aggregates after mixing for 2 months.
aggregates consist of two species with radii of ∼25 and ∼100 nm, respectively. The sizes are close to those measured by LLS, further indicating the block copolymers form regular micelles and micellar clusters. After mixing with PS225-b-PEO114 block copolymers for 3 days, aggregates with radii of ∼35 and ∼150 nm can be observed, indicating that some PS225-b-PEO114 chains have inserted into the aggregates (Figure 7b). Two months later, only one species of aggregates with a size of ∼35 nm is observed, further indicating that the mixed micellar clusters split into complex regular micelles (Figure 7c). In our previous work,14 we show that PS260-b-PAA164 copolymers only form regular micelles in toluene. After introduction of PS225-b-PEO114, they form complex micelles. Such micelles do not split because the repulsion between PS chains in the corona is not enough to overcome the complexation between PAA and PEO chains. In the present study, because of the shorter PS block, PS173-b-PAA164 copolymers form mixed micellar clusters. In comparison with a regular micelle, a mixed micellar cluster can accept more PS-b-PEO chains because of its looser structure and larger surface area, so that PS chains in the corona would be denser. Consequently, the repulsion between PS chains can overcome the complexation between PAA and PEO chains, leading the micellar clusters to split into regular micelles.
Conclusion The present studies lead to the following conclusions. When the PS block is short enough, polystyrene-b-poly(acrylic acid) (PS-b-PAA) chains form bimodal aggregates with regular micelles and micellar clusters in toluene. The introduction of PS-b-PEO chains leads to the formation of mixed micellar clusters. The structure of such micellar clusters evolves with time. The hydrogen-bonding complexation between PAA and PEO in the core and the repulsion between PS chains in the corona manipulate the evolution. When the latter dominates the former, the mixed micellar clusters split up into regular complex micelles.
Acknowledgment. The financial support of National Natural Science Foundation (NNSF) of China (20474060) and The Chinese Academy of Sciences (KJCX2-SW-H14) is gratefully acknowledged. Supporting Information Available: Gel permeation chromatography curves of the block copolymers. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728-1731. (2) Harada, A.; Kataoka, K. Science 1999, 283, 65-67. (3) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372-375. (4) Won, Y. Y.; Davis, H. T.; Bates, F. S. Science 1999, 283, 960963. (5) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967-973. (6) Fo¨rster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956-9970. (7) Yu, K.; Zhang, L.; Eisenberg, A. Langmuir 1996, 12, 5980-5984. (8) Svensson, M.; Alexandridis, P.; Linse, P. Macromolecules 1999, 32, 5435-5443. (9) Sevensson, B.; Olsson, U.; Alexandridis, P. Langmuir 2000, 16, 6839-6846. (10) Xu, R. L.; Winnik, M. A.; Hallett, F. R.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 87-93. (11) Lombardo, D.; Micali, N.; Villari, V.; Kiselev, M. A. Phys. ReV. E 2004, 70, 021402. (12) Pispas, S.; Hadjichristidis, N. Langmuir 2003, 19, 48-54. (13) Pispas, S.; Hadjichristidis, N. Macromolecules 2003, 36, 87328737. (14) Xie, D. H.; Xu, K.; Bai, R. K.; Zhang, G. Z. J. Phys. Chem. B 2007, 111, 778-781. (15) Jiang, M.; Li, M.; Xiang, M.; Zhou, H. AdV. Polym. Sci. 1999, 146, 121-196. (16) Halperin, A.; Tirrell, M.; Lodge, T. P. AdV. Polym. Sci. 1992, 100, 31-71. (17) Zhang, H.; Sun, X.; Wang, X.; Zhou, Q. F. Macromol. Rapid Commun. 2005, 26, 407-411. (18) Davis, K. A.; Matyjaszewski, K. Macromolecules 2000, 33, 40394047. (19) Zimm, B. H. J. Chem. Phys. 1948, 16, 1099-1116. (20) Chu, B. Laser Light Scattering, 2nd ed.; Academic Press: New York, 1991. (21) Berne, B.; Pecora, R. Dynamic Light Scattering; Plenum Press: New York, 1976. (22) Wu, C.; Xia, K. Q. ReV. Sci. Instrum. 1994, 65, 587-590. (23) Trent, J. S.; Scheinbeim, J. I.; Couchman, P. R. Macromolecules 1983, 16, 589-598. (24) Hu, J. W.; Liu, G. J. Macromolecules 2005, 38, 8058-8065. (25) Teraoka, I. Polymer Solutions; John Wiley & Sons: New York, 2002. (26) Choucair, A.; Lavigueur, C.; Eisenberg, A. Langmuir 2004, 20, 3894-3900.
