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J. Phys. Chem. B 2008, 112, 6358–6362
Thermoresponsive Complex Amphiphilic Block Copolymer Micelles Investigated by Laser Light Scattering Fang Zhao,† Dinghai Xie,† Guangzhao Zhang,† and Stergios Pispas*,‡ 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, China, and Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou AVenue, 11635 Athens, Greece ReceiVed: January 4, 2008; ReVised Manuscript ReceiVed: March 11, 2008
Poly(isoprene)-block-poly(ethylene oxide) (PI-b-PEO) diblock copolymers form micelles in water. The introduction of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPOb-PEO) triblock copolymer leads to the formation of mixed micelles through hydrophobic interaction. The dimension of the mixed micelles varies with the weight ratio (r) of PEO-b-PPO-b-PEO to PI-b-PEO. By use of laser light scattering, we have investigated the temperature dependence of the structural evolution of the micelles at different r. At r < 10, the size of the mixed micelles decreases with temperature. At r > 10, due to the excessive PEO-b-PPO-b-PEO chains in solution, as temperature increases, the mixed micelles aggregate into larger micelle clusters. Introduction Polymer-surfactant mixtures have received much attention in the past years since they have found a number of applications in flocculation of clays,1 mineral flotation,2 dissolution of biopolymers,3 and the recovery of crude oil.4 Generally, the polymer and surfactant form mixed micelles due to hydrophobic or electrostatic interactions between the individual components.5–9 The surfactants can strongly influence the shapes and sizes of the micelles.9 Sometimes, they may suppress the micellization of copolymers completely.5 However, most of the reported mixed systems are concerned with low molecular weight surfactants.5–12 Little is known about mixed solution with block copolymer surfactants.13–15 The possibilities for creating a large range of mixed polymeric surfactant systems with controlled structure and properties are enormous due to the plethora of available block copolymers. New knowledge toward this direction will definitely benefit the development of functional colloidal systems for practical applications. PEO-b-PPO-b-PEO copolymers or Pluronics are well-known nonionic polymeric surfactants that have been used in detergency, dispersion stabilization, emulsification, and lubrication.16,17 Their aqueous solutions have been extensively studied.18–30 At temperatures below the so-called critical micelle temperature (CMT), PEO-b-PPO-b-PEO copolymers dissolve in water as unimers in a dilute solution. At temperatures above CMT, they aggregate into micelles,18–26 i.e., their micellization is sensitive to temperature. PI-b-PEO forms micelles in water with the PI block as the core and the PEO block as the corona.31 Such micelles do not exhibit LCST. However, due to the low glass transition temperature (Tg ≈ -67 °C) of the PI chains,32 the core block is mobile and can readily interact with other hydrophobic chains. When PEO-b-PPO-b-PEO chains are introduced, e.g. mixed at temperatures below their CMT, the PPO blocks should be able * To whom correspondence should be addressed. † University of Science and Technology of China. ‡ National Hellenic Research Foundation.
to root in the PI core so that mixed micelles are formed with PI and PPO as the core and PEO as the corona. In the present work, we have investigated the structural evolution of such mixed micelles by using laser light scattering (LLS). Our aim is to understand the effect of hydrophobic interactions on the structure and properties of the self-assembled supramolecular complex aggregates in aqueous solutions. Experimental Section Sample Synthesis and Solution Preparation. The PI-b-PEO diblock copolymer was synthesized via anionic polymerization.33 Molecular weight (Mw) and polydispersity (Mw/Mn) are 63000 g/mol and 1.03, respectively. The PEO content is 90 wt %, as determined by NMR. PEO-b-PPO-b-PEO triblock copolymer was purchased from BASF. It was purified by extraction with hexane to remove impurities.18 The details can be found elsewhere.26 Gel permeation chromatography (GPC), NMR, and LLS analysis indicate the formula is EO19PO60EO19. PI-b-PEO stock solution was prepared by dissolving the sample in deionized water and stirring for two days at room temperature. The mixed micelles were prepared by adding aqueous solutions of PEO-b-PPO-b-PEO into PI-b-PEO solutions with a certain concentration under stirring. Laser Light Scattering. All LLS measurements were conducted on a ALV/DLS/SLS-5022F spectrometer equipped with a multidigital time correlation (ALV5000) and a cylindrical 22 mW He-Ne laser (λ0 ) 632 nm, Uniphase) as the light source. The weight-average molar mass (Mw) and the z-average root-mean square radius of gyration 〈Rg〉 of the micelles were obtained from the angular and concentration dependence of the excess absolute scattering intensity (Rayleigh ratio Rvv(q)) in static LLS (SLS).34
KC 1 1 1 + 〈 Rg2 〉 q2 + 2A2C ≈ Rvv(q) Mw 3
(
)
(1)
where K ) 4πn2(dn/dC)2/(NAλ04) and q ) (4πn/λ0) sin(θ/2), with NA, dn/dC, n, and λ0 being the Avogadro number, the
10.1021/jp800056k CCC: $40.75 2008 American Chemical Society Published on Web 04/26/2008
Thermoresponsive Amphiphilic Block Copolymer Micelles
Figure 1. Temperature dependence of hydrodynamic radius (〈Rh〉) and average radius of gyration (〈Rg〉) of PI-b-PEO micelles in water, where the polymer concentration is 1 × 10-4 g/mL. The inset shows the temperature dependence of Rvv(θ)/KC.
specific refractive index increment, the solvent refractive index, and the wavelength of the light in vacuum, respectively. A2 is the second virial coefficient. The specific refractive index increments (dn/dC) of the block copolymers were determined by a novel differential refractometer described before.35 The dn/dC values for the mixed micelle solutions were estimated by using an addition method from those of the block copolymers.36 The intensity–intensity time correlation function G(2)(t,q) was measured to determine the line-width distribution G(Γ) in dynamic LLS (DLS).37 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), using the CONTIN program. All DLS measurements were conducted at a scattering angle (θ) of 15°. The solution was filtered through 0.45 µm Millipore filters before the LLS measurement. Results and Discussion For comparison purpose, the behavior of the pure micelles formed by PI-b-PEO and PEO-b-PPO-b-PEO copolymers in water was investigated first. Asymmetric PI-b-PEO copolymers form spherical micelles in water.31 The critical micelle concentration (CMC) was determined to be 3.5 × 10-6 g/mL by fluorescence spectroscopy. Figure 1 shows a typical temperature dependence of the average hydrodynamic radius (〈Rh〉) and average radius of gyration (〈Rg〉) of PI-b-PEO micelles in water. Either 〈Rh〉 or 〈Rg〉 slightly change with temperature. The inset shows the temperature dependence of Rvv(θ)/KC, which is practically constant during heating, indicating the stability of PI-b-PEO micelles over the temperature range investigated. Clearly, the temperature dependence of the mixed micelles discussed below should not come from PI-b-PEO micelles. In addition, it is known that the ratio 〈Rg〉/〈Rh〉 can reflect the structure of a polymer chain or a particle. For the uniform nondraining sphere, hyperbranched cluster, and random coil, 〈Rg〉/〈Rh〉 are ∼0.774, 1.0–1.2, and 1.5–1.8, respectively.38,39 Here, 〈Rg〉/〈Rh〉 ≈ 0.7 at the temperature investigated indicating that the micelles formed by PI-b-PEO chains are spherical.
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Figure 2. Temperature dependence of the relative scattering intensities of PEO-b-PPO-b-PEO aqueous solution (CMT for two solutions is 36.4 and 28 °C). The inset shows the typical temperature dependence of 〈Rh〉 of PEO-b-PPO-b-PEO micelles with a concentration of 2.0 × 10-3 g/mL.
Figure 2 shows the temperature dependence of the relative scattering intensities (IT/I0) for PEO-b-PPO-b-PEO aqueous solution at different concentrations, where I0 represents the scattering intensities at 20 °C and a scattering angle of 15°, and IT represents the scattering intensities at T and the same angle. PEO-b-PPO-b-PEO exhibits a complex aggregation behavior.18–30 At a certain concentration, when the temperature is below the critical micelle temperature (CMT), only PEO-bPPO-b-PEO unimers exist in solution, thus low and nearly constant scattered intensity is detected. At temperatures above CMT, the PPO block becomes hydrophobic, and the triblock copolymers form micelles, which results in an increase of the scattered intensity. Note that the CMT of PEO-b-PPO-b-PEO has concentration dependence.18–26 Figure 2 shows that when the concentration increases from 5.0 × 10-5 to 2.0 × 10-3 g/mL, CMT decreases from 36.4 to 28 °C. This is understandable because a higher concentration favors aggregation. The inset shows the temperature dependence of 〈Rh〉 of PEO-b-PPO-bPEO micelles with a concentration of 2.0 × 10-3 g/mL. Micelles with 〈Rh〉 ≈ 8 nm were formed after CMT, whereas their dimensions increase with temperature. Besides, 〈Rh〉 sharply increases at a temperature above 35 °C because of the aggregation of micelles.18–21 Since PEO-b-PPO-b-PEO chains form micelles themselves at a temperature above CMT, when they are mixed with PI-bPEO micelles, the system would involve several species of micelles. Such a complex system is very difficult to characterize. Therefore, we mixed PEO-b-PPO-b-PEO copolymers at 20 °C, where PEO-b-PPO-b-PEO chains are unimers in water. Figure 3 shows the typical hydrodynamic radius distribution f(Rh) of the mixed micelles at 20 °C. In this range of r only one population of aggregates is observed. As the weight ratio (r) of PEO-b-PPO-b-PEO to PI-b-PEO increases, the size of the mixed micelles (Rh) decreases. Figure 4 shows the r dependence of the average hydrodynamic radius (〈Rh〉) and the average radius of gyration (〈Rg〉). After the introduction of PEO-b-PPO-b-PEO, both 〈Rh〉 and 〈Rg〉 decrease considerably. When r > ∼10, 〈Rh〉 and 〈Rg〉 no longer change because, presumably, the mixed micelles already attain equilibrium. Still one population of aggregates is present in solution. Similar phenomena have been observed in low molecular weight surfactant systems.8,9 The inset shows the r dependence of Rvv(θ)/KC. Since Rvv(θ)/KC is proportional to the apparent weight-average molar mass (Mw), the decrease of Rvv(θ)/KC indicates that the mass of the mixed micelles decreases. All experimental data indicate that PI-b-
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Figure 3. Typical hydrodynamic radius distributions (f(Rh)) for the mixed micelles of different r at 20 °C, where the concentration of the copolymer PI-b-PEO is fixed at 1 × 10-4 g/mL.
Zhao et al.
Figure 5. Typical hydrodynamic radius distributions (f(Rh)) of the mixed micelles during heating at r ) 0.5.
Figure 6. Temperature dependence of 〈Rh〉 and 〈Rg〉 of the mixed micelles at r ) 0.5. The inset shows the temperature dependence of Rvv(θ)/KC. Figure 4. The r dependence of 〈Rh〉 and 〈Rg〉 of the mixed micelle at 20 °C, where PI-b-PEO concentration is fixed at 1 × 10-4 g/mL. The inset shows the r dependence of Rvv(θ)/KC.
PEO micelles form new mixed micelles with the introduced PEO-b-PPO-b-PEO chains. Note that PI-b-PEO has 90 wt % of hydrophilic PEO and 10 wt % of PI with a low Tg.32 Such diblock copolymers form micelles with a soft PI core, which allows PPO blocks to root in, due to hydrophobic interactions. Namely, they form mixed micelles with the hydrophilic PEO block as the corona and the hydrophobic PI and PPO blocks as the core. Note that PPO blocks are less hydrophobic than PI blocks. That means that the mixed micelle core is less hydrophobic than the core of PI-b-PEO micelle. Therefore, fewer PEO blocks are required for stabilizing the core. That is why the size and the weight-average molar mass of the mixed micelles decrease after the introduction of PEO-b-PPO-b-PEO.40 Such a phenomenon has also been observed in other amphiphilic systems.41 We also investigated the aggregation of the mixture of the two copolymers by use of LLS when the concentration of PIb-PEO was below the CMC. In the range r ) 1-100, the scattering intensity changes only slightly, indicating no micelle formation. This is understandable because the less hydrophobic PPO chains cannot promote the association of the hydrophobic PI chains and induce the micellization. Figure 5 shows the temperature dependence of f(Rh) of the mixed micelles at r ) 0.5. Only one unimodal distribution can be observed even at temperatures above the CMT of PEO-bPPO-b-PEO. Moreover, the size of the mixed micelles decreases as the temperature increases. The data suggest that all PEO-b-
Figure 7. Typical hydrodynamic radius distributions (f(Rh)) of the mixed micelles during heating at r ) 20. The inset is f(Rh) of PEOb-PPO-b-PEO micelles in water (2.0 × 10-3 g/mL) at 31 °C.
PPO-b-PEO chains are incorporated in the mixed micelles with the PI-b-PEO chains. The interaction between PEO-b-PPO-bPEO and PI-b-PEO chains seems so strong that changes in temperature cannot lead to the formation of separate micelles from each individual component. In other words, complex micelles are formed, whose size depends on temperature. When the temperature reaches the nominal CMT of the triblock, PPO blocks become more hydrophobic, and it seems that the remaining PEO-b-PPO-b-PEO unimers in solution prefer to insert in the mixed micelles, leading to a further decrease of their size, rather than to form separate micelles containing only PEO-b-PPO-b-PEO chains. The small quantity of PEO-b-PPO-b-PEO unimers present in these solutions may also affect their behavior.
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Figure 8. Schematic of the evolution of the mixed micelles.
Figure 6 shows the typical temperature dependence of 〈Rh〉 and 〈Rg〉 at r ) 0.5. At T < ∼28 °C, both 〈Rh〉 and 〈Rg〉 change slightly. However, they decrease with temperature at T > ∼28 °C. From the 〈Rg〉/〈Rh〉 ratio at each temperature it can be concluded that the mixed micelles remain spherical at all temperatures investigated. Rvv(θ)/KC exhibits a similar behavior (see inset), indicating that the mass of the micelles also decreases. Note that PEO-b-PPO-b-PEO exhibits a CMT at about 36 °C. Obviously, the presence of PI-b-PEO leads the CMT to decrease to ∼28 °C. This further indicates that the PPO blocks strongly interact with PI blocks, which leads to an enhancement of the association of PPO blocks. Similar phenomena were observed in the mixed system of C12EO6 and PEOb-PPO-b-PEO.30 Figure 7 shows the temperature dependence of f(Rh) of the mixed micelles at r ) 20. At T < 30 °C, the micelles exhibit Rh ≈ 30 nm. As temperature increases in the range 30 °C < T < 42 °C, a bimodal distribution can be observed. The peak at ∼30 nm could be attributed to the complex micelles. However, a question concerning the origin of the peak at ∼8 nm remains. In comparison with the results shown in Figure 2, we know it should be attributed to the micelles formed by PEO-b-PPO-bPEO chains. Actually, PEO-b-PPO-b-PEO chains should be excessive at r ) 20, i.e., besides those complexing with PI-bPEO, there must be some free PEO-b-PPO-b-PEO chains in solution. At T < CMT, the triblock chains exist as unimers and they are too small to be detected by DLS. At T g CMT, the free PEO-b-PPO-b-PEO chains form micelles themselves, whereas the rest form complex micelles with PI-b-PEO. This should be the reason for observing a bimodal distribution. Further increasing the temperature (T g 42 °C) leads to only one peak. The 〈Rg〉/〈Rh〉 ratio at 42 °C reaches ∼1.2, indicating that the mixed micelle changes from a spherical structure to a more hyperbranched structure. A possible scenario in this temperature region may include fusion of the PEO-b-PPO-bPEO micelles with the mixed micelles toward the formation of larger aggregates. The structural evolution of the mixed micelles is illustrated in Figure 8. Conclusions The behavior of mixed polymeric surfactants, namely PPOPEO-PPO triblock and PI-b-PEO diblock copolymers, in water was investigated by Laser Light Scattering. PEO-b-PPO-b-PEO/ PI-b-PEO mixed micelles were formed after the introduction of PEO-b-PPO-b-PEO copolymers to PI-b-PEO micelle solutions, due to hydrophobic interactions. Their dimensions change
as a function of the weight ratio (r). The temperature dependence of the mixed micelles also depends on r. At a low r, the mixed micelles further dissociate as temperature increases because of the continuing insertion of unimolecularly dissolved PEO-bPPO-b-PEO chains. No PEO-b-PPO-b-PEO micelles are observed during heating, indicating that the existence of PI-b-PEO micelles suppresses the micellization of PEO-b-PPO-b-PEO. However, at a high r, when the temperature increases, pure PEOb-PPO-b-PEO micelles form first from excess triblock chains, and then fuse with the original mixed micelles to form larger aggregates. The two different phenomena indicate that the quantity of PEO-b-PPO-b-PEO unimers in solution plays an important role in the micellization process of the mixed system. The data presented in this work indicate the possibility of tuning micelle structure and temperature responsiveness by mixing appropriately chosen block copolymers. The resulting responsive colloidal systems can find applications in various fields, including drug delivery systems, due to their temperature-sensitive properties, as well as to the ability to tune micelle core composition and chemical nature. Acknowledgment. The financial support of the National Natural Science Foundation of China (20725414) and the Ministry of Science and Technology of China (2007CB936401) is acknowledged. S.P. acknowledges financial support from the General Secretariat for Research and Technology/Ministry of Development, Greece through the program “Excellence in the Research Institutes” (Phase I and II, projects 64769 and 2005ΣE01330081). References and Notes (1) Hanna, H. S.; Somasundaran, P. J. Colloid Interface Sci. 1979, 70, 181. (2) Przhegorlinskaja, R. W.; Zubkova, Y.; Khim, N. Tuer. Topl. 1978, 12, 125. (3) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (4) Taber, J. J. Pure Appl. Chem. 1980, 52, 1323. (5) Hecht, E.; Hoffmann, H. Langmuir 1994, 10, 86–91. (6) Fundin, J.; Brown, W. Macromolecules 1994, 27, 5024. (7) Wu, C.; Ma, R.; Zhou, B. Macromolecules 1996, 29, 228–232. (8) Kunieda, H.; Kaneko, M.; Tsukahara, M. Langmuir 2004, 20, 2164– 2171. (9) Zheng, Y.; Davis, H. T. Langmuir 2000, 16, 6453–6459. (10) Hecht, E.; Mortensen, K.; Gradzielski, M. J. Phys. Chem. 1995, 99, 4866–4874. (11) Ganguly, R.; Aswal, V. K.; Hassan, P. A. J. Phys. Chem. B 2006, 110, 9843–9849. (12) Jansson, J.; Schille, K.; Nilsson, M. J. Phys. Chem. B 2005, 109, 7073–7083.
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