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J. Phys. Chem. B 2007, 111, 778-781
Structural Evolution of Mixed Micelles Due to Interchain Complexation and Segregation Investigated by Laser Light Scattering Dinghai Xie, 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 230026, China ReceiVed: October 1, 2006; In Final Form: December 8, 2006
Polystyrene-b-poly(acrylic acid) (PS-b-PAA) diblock copolymers form micelles in toluene with PAA as the core and PS as the corona. The introduction of poly(methyl methacrylate)-b-poly(ethylene oxide) (PMMAb-PEO) solution in toluene leads to mixed micelles due to the hydrogen-bonding complexation between PAA and PEO. By using a combination of static and dynamic laser light scattering, we have investigated the evolution of the mixed micelles. Our results revealed that the complexation between PAA and PEO in the core and the segregation between PS and PMMA in the corona as a function of the molar ratio (r) of PEO to PAA manipulate the evolution. At r < ∼1.0, the mixed micelles hold a spherical structure after a longtime standing. However, at r > ∼1.0, the average radius of gyration 〈Rg〉, the average hydrodynamic radius 〈Rh〉, and the ratio 〈Rg〉/〈Rh〉 of the mixed micelles increase with time, whereas the molar mass (Mw) does not change. The facts indicate that the mixed micelle has evolved from a spherical structure to a hyperbranched structure.
Introduction Block copolymers are well known to self-assemble into nanostructures.1,2 Materials with such structures are finding applications in drug delivery, catalysis, and microelectronics. So far, a rich variety of such structures have been prepared. Among them, Janus micelles consisting of a single core and a compartment corona are particularly interesting because they exhibit unusual phase separation and interfacial properties.3-7 Actually, it is important to understand the interplay of covalent bonding, noncovalent bonding, and entropic interactions in the self-assembly.8-14 Investigation on the evolution of micelles would help us to correlate the self-assembled structures with the molecular interactions. However, such studies are still in the beginning stages.15,16 Polystyrene (PS) and poly(methyl methacrylate) (PMMA) are immiscible and readily phase separate in bulk. Because PS has a lower surface free energy than PMMA, they have a tendency to segregate on a surface.17 On the other hand, poly(acrylic acid) (PAA) and poly(ethylene oxide) (PEO) prove to form complexes in toluene due to hydrogen bonding.18 Accordingly, polystyreneb-poly(acrylic acid) (PS-b-PAA) and poly(methyl methacrylate)b-poly(ethylene oxide) (PMMA-b-PEO) are expected to form a mixed micelle with PEO/PAA complex as the core and PMMA and PS as the corona. In the present work, by using a combination of static and dynamic laser light scattering, we have investigated the evolution of the mixed micelles in toluene. Our aim is to understand the roles of hydrogen-bonding complexation and segregation played in the self-assembly of polymers. 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. E-mail: gzzhang@ ustc.edu.cn.
Figure 1. Angular dependence of the Rayleigh ratio Rvv(q) for PS-bPAA/PMMA-b-PEO mixed micelles with different standing time at 25.0 °C, where the molar ratio (r) of PEO to PAA is 2.0.
purified by azeotropic distillation in toluene. CuBr was purified by stirring in acetic acid, washing with methanol, and then drying. Styrene, methyl methacrylate (MMA), and tert-butyl acrylate (BA) were respectively passed through a column of neutral alumina to remove inhibitor. Tetrahydrofuran (THF) was distilled over potassium. Other chemicals were used as received. PMMA-b-PEO and PS-b-PEO were synthesized following a procedure in ref 19. PEO-Br macroinitiator was prepared by the reaction of the PEO with 2-bromoisobutyryl bromide. The atom transfer radical polymerization (ATRP) of styrene or MMA initiated by PEO-Br/CuBr in THF resulted in the PMMA-bPEO or PS-b-PEO diblock copolymer. The details for the synthesis of PS-b-PAA can be found elsewhere.20 First, the ATRP of BA and styrene using a alkyl bromide as the initiator and CuBr/N,N,N′,N′′,N′′-pentamethyldiethylenetriamine as the catalyst yielded PS-b-PBA copolymer. PS-b-PAA was obtained after the complete hydrolysis of PS-b-PBA by trifluoroacetic acid in dichloromethane. The number average molecular weight (Mn) and the polydispersity (Mw/Mn) of PMMA-b-PEO, PS-b-PEO, and PS-b-
10.1021/jp066438o CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007
Structural Evolution of Mixed Micelles
Figure 2. Time dependence of 〈Rg〉 and 〈Rh〉 of PS-b-PAA/PMMAb-PEO mixed micelles with different standing time at 25.0 °C, where the molar ratio (r) of PEO to PAA is 2.0. The inset shows the standing time dependence of 〈Rg〉/〈Rh〉.
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Figure 4. The molar ratio (r) dependence of 〈Rg〉/〈Rh〉 of PS-b-PAA/ PMMA-b-PEO mixed micelles with different standing time at 25.0 °C.
Figure 5. Angular dependence of the Rayleigh ratio Rvv(q) for PS-bPAA/PS-b-PEO mixed micelles with different standing time at 25 °C, where the molar ratio (r) of PEO to PAA is 2.1. The inset shows the hydrodynamic radius distributions f(Rh) of the micelles.
Figure 3. The molar ratio (r) of PEO to PAA dependence of 〈Rg〉 and 〈Rh〉 of PS-b-PAA/PMMA-b-PEO mixed micelles with different standing time at 25.0 °C. The inset shows the r dependence of the apparent molar mass (Mw).
PBA were characterized by a combination of gel permeation chromatography (GPC) and 1H NMR spectra. GPC measurements were carried out on a Waters 150C using 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 using chloroform-d (CDCl3) as the solvent and tetramethylsilane (TMS) as the internal standard. Here, we designate a block copolymer as Am-b-Bn with m and n being the numbers of the units in A and B blocks. For PMMA210-b-PEO114, PS225-bPEO114, and PS260-b-PAA164, we have Mw/Mn ) 1.14, 1.06, and 1.27. PS-b-PAA micelles were prepared by introducing 70.0 mL of toluene into 7.0 mL of PS-b-PAA solution in THF under stirring at a constant rate by an SP100i syringe pump. The THF was removed by evaporation under reduced pressure. Such a solution was further mixed with PMMA-b-PEO or PS-b-PEO solution in toluene to form mixed micelles. The final concentration of the PS-b-PAA micelles was 9.0 × 10-5 g/mL, and the concentration of PMMA-b-PEO or PS-b-PEO ranged from 1.0 × 10-5 to 2.0 × 10-4 g/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 UNIPHASE He-Ne laser (λ0 ) 632 nm) as the light source. In static LLS,21,22 we were able to obtain 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 from the
angular dependence of the absolute excess time-average scattering intensity, known as the Rayleigh ratio Rvv(q). In dynamic LLS,23 the intensity-intensity time correlation function G(2)(t,q) was measured to determine the line-width distribution G(Γ). For diffusive relaxation, Γ is related to the translational diffusion coefficient (D) of the scattering object (polymer chain or colloid particle) in dilute solution or dispersion by D ) (〈Γ〉/q2)Cf0,qf0 and further to hydrodynamic radius (Rh) from the StokesEinstein 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 a corresponding measured G(2)(t,q) using the CONTIN program. All dynamic LLS measurements were conducted at a small scattering angle (θ) of 15°. The mixed micelle solutions were kept at 25.0 °C during the measurements and the standing. The refractive index increment values of the micelles were measured by using a precise differential refractometer.24 Results and Discussion Figure 1 shows a typical angular dependence of Rayleigh ratio Rvv(q) for the mixture of PS-b-PAA micelle with PMMAb-PEO chains at different standing time, where the molar ratio (r) of PEO to PAA is 2.0. After the introduction of PMMA-bPEO chains, some of them are expected to insert into PS-bPAA micelles due to the hydrogen-bonding complexation between PAA and PEO. Because PMMA-b-PEO chains have a small refractive index increment value in toluene, they are not detected. What we observed are the PS-b-PAA/PMMA-b-PEO mixed micelles. It is known that the intercept and slope of a line in static LLS are related to the average molar mass and 〈Rg〉 of a scattering object. The time independence of the intercept of KC/Rvv(q) versus q2 indicates that the molar mass of the mixed micelles almost does not change with time. However, 〈Rg〉 increases with time, reflecting in the slope.
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Figure 6. Schematic illustration of the evolution of the mixed micelle.
Figure 2 shows that 〈Rg〉 and 〈Rh〉 of the mixed micelles increase with time in the first 2 months and tend to be constant afterward. Because of the invariant molar mass, the changes in 〈Rg〉 and 〈Rh〉 indicate the structural change of an individual mixed micelle. The inset shows the time dependence of the ratio 〈Rg〉/〈Rh〉. The ratio can reflect the structure of a polymer chain or a particle. For uniform non-draining sphere, hyperbranched cluster, and random coil, 〈Rg〉/〈Rh〉 are ∼0.774, 1.0-1.2, and 1.5-1.8, respectively.25,26 A spherical micelle generally has a 〈Rg〉/〈Rh〉 of 0.8-1.0. Obviously, the mixed micelles are spherical with a core-shell structure with a standing time of 24 h reflecting in 〈Rg〉/〈Rh〉 ≈ 0.8. Thus, PS and PMMA blocks should randomly interlace in the corona. The increase in 〈Rg〉/〈Rh〉 with time in the first 2 months is indicative that the mixed micelle has undergone structural change. Finally, 〈Rg〉/〈Rh〉 reaches ∼1.2, indicating that the mixed micelle has evolved into a more hyperbranched structure. The evolution is the result of the chain reorganization in the micelle due to the balance between the interchain complexation and segregation. We will come back to this point later. Note that the structural change of the mixed micelle has much dependence on the molar ratio (r) of PEO to PAA. Figure 3 shows the r dependence of 〈Rg〉 and 〈Rh〉 of the mixed micelles with different standing time at 25.0 °C. After the introduction of PMMA-b-PEO within 24 h, 〈Rg〉 and 〈Rh〉 increase with r at r < ∼1.0, but both of them no longer change at r > ∼1.0. The apparent molar mass (Mw) exhibits the same behavior (the inset). It is understandable because the density of the hydrogen bonds formed between PEO and PAA increases with r at r < 1.0 and has the maximum at r ) 1.0.18 As r increases, more PMMA-b-PEO chains are expected to insert in the micelles. However, the residency of PMMA-b-PEO chains also leads to an increase in the density of the chains in the corona of the micelle. At r > ∼1.0, the dense corona restricts PMMAb-PEO chains from entering the core of the micelle. This explains why 〈Rg〉, 〈Rh〉, and Mw almost do not change with r at r > ∼1.0. Note that the situation is much different after 4 months. Figure 3 shows that 〈Rg〉, 〈Rh〉, and Mw have not changed after 4 months at r < ∼1.0, suggesting stable spherical micelles. However, at r > ∼1.0, 〈Rg〉 and 〈Rh〉 increase with time while Mw does not change, clearly indicating the chain reorganization in the micelle. This can be better viewed in terms of 〈Rg〉/〈Rh〉. Figure 4 shows the r dependence of 〈Rg〉/〈Rh〉 at the standing time of 24 h and 4 months, respectively. After the addition of PMMA-b-PEO to PS-b-PAA micelles within 24 h, 〈Rg〉/〈Rh〉 is ∼0.75-0.93 in the range r ) 0-2.6, indicating that the mixed micelles are spherical with a core-shell structure. PS and PMMA blocks randomly interlace in the corona. Note that 〈Rg〉/ 〈Rh〉 slightly decreases from ∼0.93 to 0.75 with the increasing r at r < ∼1.0, indicating that a larger r leads to denser micelles
due to the stronger hydrogen-bonding complexation between PAA and PEO. The constant 〈Rg〉/〈Rh〉 at r > ∼1.0 suggests that the mixed micelles have not changed their structure within 24 h. After a 4-month-reorganization, the micelles become more extended, reflecting the increase in 〈Rg〉/〈Rh〉 in the whole range of r. On the other hand, the minimum 〈Rg〉/〈Rh〉 value at r ≈ 1 further indicates the strongest hydrogen-bonding complexation between PAA and PEO there, which leads to the densest micelle. The most important event is that the 〈Rg〉/〈Rh〉 increases from ∼0.9 to 1.3 as r increases from 1.0 to 2.6. This fact indicates that the mixed micelle with a high r value changes from a spherical structure to a more hyperbranched structure after a 4-month-reorganization. To understand the roles of complexation and segregation in the evolution of the mixed micelles, we also examined PS-bPEO/PS-b-PAA mixed micelles. Figure 5 shows a typical angular dependence of the Rayleigh ratio Rvv(q) for PS-b-PEO/ PS-b-PAA mixed micelles with the standing time of 24 h and 5 months, respectively. Note that the corona of the micelle only consists of PS blocks so that no repulsion or segregation occurs. The invariant intercept and slope of a line indicate that the Mw and 〈Rg〉 have not changed after 5 months. Meanwhile, the inset shows 〈Rh〉 also does not change with time. The facts clearly indicate that only the hydrogen-bonding complexation between PAA and PEO cannot drive the evolution of PS-b-PAA/PMMAb-PEO mixed micelles. The segregation of PS and PMMA due to the repulsion plays an important role in the structural change of the micelles. Strictly speaking, the balance between the complexation and segregation determines the micelle structure. At r < 1.0, ethylene oxide (EO) units fully form hydrogen bonds with the excessive AA units. On the other hand, the repulsion between PS and PMMA in the corona is weak because PMMA chains in corona are not enough. Consequently, the hydrogenbonding complexation between PAA and PEO dominates the segregation between PMMA and PS, and the mixed micelle holds its spherical structure. In contrast, EO units are excessive at r > 1.0, and only a part of EO units in each PEO chain form hydrogen bonds with PAA. As a result, the repulsion between PMMA and PS in the corona dominates the complexation between PAA and PEO in the core. Thus, PMMA-b-PEO chains are able to rearrange themselves so that the system has lower energy, and the mixed micelle evolves into a hyperbranched structure. The structural transformation of the micelles is schematically illustrated in Figure 6. Conclusion In conclusion, using laser light scattering, we have investigated the evolution of PS-b-PAA/PMMA-b-PEO mixed micelles
Structural Evolution of Mixed Micelles as a function of the molar ratio of PEO to PAA. Our results revealed that the complexation between PAA and PEO in the core and the segregation between PS and PMMA in the corona manipulate the evolution. When the segregation dominates the complexation, the mixed micelle develops into a hyperbranched structure after a long-time standing. Acknowledgment. Financial support from the National Natural Science Foundation (NNSF) of China (20474060 and 20474059) and The Chinese Academy of Sciences (KJCX2SW-H14) is gratefully acknowledged. References and Notes (1) Alexandridis, P.; Lindman, B. Amphiphilic Block Copolymers: SelfAssembly and Applications; Elsevier: Amsterdam, 2000. (2) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967-973. (3) Erhardt, R.; Zhang, M. F.; Bo¨ker, A.; Zettl, H.; Abetz, C.; Frederik, P.; Krausch, G.; Abetz, V.; Mu¨1ler, A. H. E. J. Am. Chem. Soc. 2003, 125, 3260-3267. (4) Li, Z. B.; Kesselman, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P. Science 2004, 306, 98-101. (5) Roan, J.-R. Phys. ReV. Lett. 2006, 96, 248301. (6) Nie, Z. H.; Li, W.; Seo, M.; Xu, S. Q.; Kumacheva, E. J. Am. Chem. Soc. 2006, 128, 9408-9412. (7) Zheng, R. H.; Liu, G. J.; Yan, X. H. J. Am. Chem. Soc. 2005, 127, 15358-15359. (8) Giebeler, E.; Stadler, R. Macromol. Chem. Phys. 1997, 198, 38153825.
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