Stabilized Mixed Micelles with a Temperature-Responsive Core and a

Apr 30, 2009 - Corresponding author: E-mail: [email protected]. ... Finally, SPMMS were successfully exploited as templates for the preparation o...
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J. Phys. Chem. B 2009, 113, 7527–7533

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Stabilized Mixed Micelles with a Temperature-Responsive Core and a Functional Shell Petar Petrov,*,† Christo B. Tsvetanov,† and Robert Je´roˆme‡ Institute of Polymers, Bulgarian Academy of Sciences, Akad. G. BoncheV Street 103A, 1113 Sofia, Bulgaria, and Center for Education and Research on Macromolecules (CERM), UniVersity of Liege, Sart-Tilman, B6, 4000 Liege, Belgium ReceiVed: February 12, 2009; ReVised Manuscript ReceiVed: April 9, 2009

Formation and stabilization of multiresponsive micelles with a mixed poly(ethylene oxide)/polyelectrolyte shell and a temperature-responsive poly(propylene oxide) core were studied. Various poly(ethylene oxide)block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers were mixed with poly(acrylic acid)-block-poly(propylene oxide)-block-poly(acrylic acid) (PAA-PPO-PAA) or poly(dimethylaminoethyl methacrylate)-block-poly(propylene oxide)-block-poly(dimethylaminoethyl methacrylate) (PDMAEMA-PPO-PDMAEMA) triblock copolymers. The micelles formed by binary mixtures of well-defined compositions at a specific pH were additionally stabilized by loading with pentaerythritol tetraacrylate (PETA), that was polymerized and cross-linked “in situ” with UV assistance. Depending on both the composition of the copolymers and the experimental conditions, either spherical or wormlike “stabilized polymeric micelles with a mixed shell” (SPMMS) were observed by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The SPMMS that contained PAA blocks in the shell were pH-sensitive, such that a reversible transition from well-dispersed SPMMS to precipitate could be promoted. In contrast, the SPMMS with a PEO/PDMAEMA mixed shell remained well-dispersed in the 2-11 pH range. Finally, SPMMS were successfully exploited as templates for the preparation of Ag nanoparticles (Ag NPs). Introduction Micellization of amphiphilic block copolymers in water is one of the most intriguing properties of this class of polymers, that was intensively investigated because of great potential in medicine, biotechnology, catalysis, nanotechnology, etc.1-3 The predominant studies in this field deal with formation and characterization of core-shell micelles formed by AB- and ABA-type amphiphilic block copolymers above a critical micellar concentration (cmc) according to the so-called closed association mechanism. Micellization of commercially available PEO-PPO-PEO (Pluronics, Synperonics, Poloxamers) triblock copolymers has been more extensively studied because of the combination of several attractive properties, including resistance of the PEO shell to protein adsorption and cellular adhesion, ability of the temperature-responsive PPO core to solubilize water-insoluble compounds, availability of hydroxyl groups to which receptor-specific ligands can be attached, etc.4-8 Recently, micelles formed by ABC triblock terpolymers and by mixtures of AB and BC diblock copolymers received increasing attention because of the opportunity of increasing the functionalities of these nanosystems. Indeed, the specific physical and chemical properties of a third component C broaden the scope of potential activities and related applications of binary AB systems. Fustin et al.9 classified the micelles formed by ABC triblock terpolymers and by AB + BC and AB + CD mixtures of diblock copolymers in two main groups, i.e., micelles with a compartmentalized core and micelles with a compartmentalized corona, respectively. The micelles with a compartmentalized core are known as “onion”, “three-layer”, or “core-shell-corona” structures, * Corresponding author: E-mail: [email protected]. Telephone: +359(2)9792281. Fax: +359(2)8700309. † Bulgarian Academy of Sciences. ‡ University of Liege.

where the first insoluble block forms the micellar core, the second insoluble block forms a middle layer (shell), and the third soluble block extends into the solution and forms the micellar corona. The micelles of the second group consist of ternary ABC combinations with only one insoluble block. Importantly, whatever the micellar promoters, the micelles are formed only in a limited range of experimental conditions.1 Indeed, changing, e.g., solvent, concentration, temperature, and pH can shift the micellization equilibrium toward unimers or to the copolymer precipitation. The chemical cross-linking of either the core or the shell of core-shell micelles is an effective strategy to stabilize them against destructuration. Prochazka and co-workers10,11 were the first to report on the chemical stabilization of micellar structures by cross-linking a polybutadiene core. More recently, Liu and co-workers systematically studied the photocross-linking of block copolymer micelles with a poly(cinnamoylethyl methacrylate) (PCEMA) core.12,13 They demonstrated that the photo-cross-linking of PCEMA locked the original structure and, thus, basically preserved the average aggregation number and size distribution. Cross-linking of micellar shells was pioneered by Wooley and co-workers,14,15 in the specific case of a poly(acrylic acid) corona. A similar strategy was used by Armes and co-workers, who synthesized shell cross-linked onion-like micelles.16 In previous publications,17-19 some of us reported an efficient strategy for the stabilization of core-shell micelles by UV-induced formation of an interpenetrating network of poly(pentaerythritol tetraacrylate), poly(PETA), in which the polyether chains were physically entrapped. The accordingly stabilized polymeric micelles (SPMs) resisted changes in concentration and solvent and maintained their structure and size even when ultrasound irradiated at 20 kHz. This paper aims at reporting on the preparation of SPMs with a temperature-responsive PPO core

10.1021/jp901307d CCC: $40.75  2009 American Chemical Society Published on Web 04/30/2009

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and a mixed shell comprising PEO and either poly(acrylic acid) or poly(dimethylaminoethyl methacrylate) chains. Experimental Methods Materials. Poly(propylene glycols), PPG34 and PPG69 (Fluka), were dried by azeotropic distillation of toluene. 2-Bromoisobutyryl bromide, triethylamine, CuBr, CuCl, CuCl2, bipyridine (BiPy), N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA), trifluoroacetic acid, pentaerythritol tetraacrylate, (4-benzoylbenzyl)trimethylammonium chloride, and AgNO3 were purchased from Aldrich and used as received. tert-Butyl acrylate (tBA, BASF) and dimethylaminoethyl methacrylate (DMAEMA, Aldrich) were stirred over calcium hydride (Merck) overnight and vacuum distilled just before use. All the solvents were purified by standard procedures. Silica-60 gel (Merck) was used as supplied for removing residues of ATRP catalysts. Pluronics P123, P85, F38, and P65 were kindly donated by BASF. Synthesis. PPO Macroinitiators. The ATRP macroinitiators were synthesized by reacting PPG34 or PPG69 with 2.5 mol equiv of 2-bromoisobutyryl bromide in dry toluene in the presence of triethylamine (2.5 mol equiv) at 20 °C for 24 h, as reported elsewhere.20-22 The reaction mixture was then filtered for removing the insoluble hydrobromide salt, added with activated carbon under stirring for 12 h, filtered again, and dried. The reaction product was added to water (pH ) 9), and the turbid solution was extracted several times with dichloromethane. The organic solution was dried over magnesium sulfate and filtered, and the solvent was finally removed under reduced pressure. The esterification yield (ca. 98%) was calculated from the 1H NMR spectrum. PtBA-PPO-PtBA Triblock Copolymers. In a typical procedure the Br-PPO69-Br macroinitiator (1.5 g, 0.349 mmol) was dissolved in acetone (0.3 mL). This solution was degassed by bubbling nitrogen under stirring for 45 min and then added with the PMDETA ligand (0.15 mL; 0.7 mmol), the CuBr catalyst (0.1 g; 0.7 mmol), and the freshly distilled and degassed tBA monomer (2 mL, 0.014 mol). Polymerization was carried out at 50 °C for 6 h. The copolymer was precipitated in hot water (60 °C) and filtered out. It was dissolved in THF, and the solution was eluted through a silica gel column in order to remove the Cu(II) catalyst. Finally, THF was removed under reduced pressure and the copolymer was dried. PAA-PPO-PAA Triblock Copolymers. PtBA-PPO-PtBA copolymers were dissolved in freshly dried and distilled CH2Cl2, followed by addition of a 5-fold molar excess of CF3COOH (with respect to the amount of the tert-butyl groups). The reaction mixture was stirred at room temperature for 24 h, before being dialyzed against CHCl3 for 3 days. The solvent was removed under reduced pressure and the copolymer was dried. PDMAEMA-PPO-PDMAEMA Triblock Copolymer. The atom transfer radical polymerization of DMAEMA was initiated by Br-PPO69-Br in methanol, with the BiPy/CuCl/CuCl2 catalytic system. As a typical example, Br-PPO69-Br (0.99 g, 0.23 mmol), BiPy (0.1432 g, 0.92 mmol), CuCl (0.0454 g, 0.46 mmol), and CuCl2 (0.0062 g, 0.046 mmol) were degassed by three times repeated vacuum/argon cycles, dissolved in methanol (1.93 mL), and purged with dry argon under stirring for 60 min. Then, freshly distilled and degassed DMAEMA (1.93 mL, 0.0115 mol) was added, followed by polymerization at 60 °C for 6 h. The reaction mixture was poured into hot water (60 °C). After separation, the copolymer was dissolved in methanol, and the solution was eluted through a silica gel column in order to remove the Cu(II) catalyst. Finally, methanol was distilled off,

Petrov et al. TABLE 1: Molecular Characteristics of the Triblock Copolymers Synthesized by ATRP copolymer composition (1H NMR) PPO69-b-(PtBA17)2 PPO69-b-(PAA17)2 PPO34-b-(PtBA18)2 PPO34-b-(PAA18)2 PPO69-b-(PDMAEMA13)2

DP of the outer blocks Mn Mn Mw/Mn (1H NMR) (1H NMR) (SEC) (SEC) 17 17 18 18 13

8400 6500 6600 4600 8400

10800

1.18

7800

1.13

4500

1.26

and the copolymer was dried in vacuo at 50 °C. The copolymers were characterized by 1H NMR and SEC (Table 1). Micellization and Stabilization. Polymeric micelles with a mixed shell were prepared as follows: 0.5 g of a binary mixture of diblocks of a well-defined composition was dissolved in THF under stirring. THF was evaporated under reduced pressure, and 100 mL of bidistilled water (pH ) 9 or 3) was added. A known amount of PETA was dissolved in 1 mL of acetone and added to the micellar solution under stirring at a given temperature. Argon was bubbled through the solution for 45 min, followed by irradiation with a full spectrum UV light (Dymax 5000-EC UV curing equipment with a 400 W metal halide flood lamp) for 30 min. The stabilized polymeric micelles were purified by dialysis against water using a cellulose membrane (Sigma, cutoff 12 000 g mol-1) for 14 days. Synthesis of Ag Nanoparticles. AgNO3 was added to an aqueous dispersion of SPMMS (c ) 0.5 g L-1; AA/Ag mol ratio 1:1), that was stirred overnight and dialyzed against water for 2 days. Then, (4-benzoylbenzyl)trimethylammonium chloride (10 wt % with respect to SPMMS) was added, and the system was irradiated with full spectrum UV light for 30 min. Characterization. Nuclear Magnetic Resonance Spectrometry. The 1H NMR spectra were recorded in CDCl3 and acetoned6, with a 250 MHz Bruker AC-spectrometer. Size Exclusion Chromatography. PtBA-PPO-PtBA copolymers were analyzed by SEC at room temperature with PSS SDV-gel columns (5 µm, 60 cm, 1 × linear (102 to 105 Å), 1 × 100 Å), with THF as an eluent (flow rate ) 1.0 mL/min) and a refractometer for the detection. Molecular weights and polydispersity index (PDI) were determined with a polystyrene calibration. PDMAEMA-PPO-PDMAEMA and Pluronics copolymers were analyzed by SEC at 70 °C with a Waters chromatograph equipped with a refractive index detector and a Waters Styragel column eluted at 70 °C with 0.5 wt % LiBr containing dimethylformamide (DMF) at a flow rate of 1 mL min-1. PEO standards were used for calibration. Dynamic Light Scattering. DLS was performed at 25 °C with an ALV/CGS-3 compact goniometer system equipped with a 22 mW He-Ne laser (λ ) 632.8 nm) and an ALV-5000/EPP correlator at scattering angles from 30 to 150°. The normalized intensity autocorrelation function g2(t) was measured experimentally. The CONTIN method was used for analysis of g2(t) data. The diffusion coefficient, D, was calculated from the second moment of each peak as D ) Γ/q2, where q is the magnitude of the scattering vector (q ) 4πn sin(θ/2)/λ) and Γ ) 1/τ is the relaxation rate. The intensity weighted hydrodynamic radii, Rh, were calculated from the corresponding decay times (τ) and the Stokes-Einstein equation: Rh ) kBT/(6ηπD), where, η, kB, and T are the solvent viscosity, the Boltzmann constant, and the absolute temperature, respectively. All the samples were filtered through 1 µm nylon filters before measurement. Transmission Electron Microscopy. A drop of micellar solution was deposited on a TEM copper grid (3.05 mm, 200

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mesh) coated with a Formvar film, and the solvent was allowed to evaporate. A Philips CM 100 apparatus, equipped with a Gatan 675 CCD camera for digital imaging, was used at an accelerating voltage of 100 kV. Results and Discussion Dynamic PEO-PPO-PEO core-shell micelles have been effectively stabilized by loading with hydrophobic pentaerythritol tetraacrylate, followed by the UV-induced free radical polymerization of this multifunctional monomer. Stabilized spherical,17 wormlike micelles19 and vesicles23 can be prepared accordingly by changing the copolymer composition and/or the PETA content. This strategy is also applicable to core-shell micelles with a mixed shell, as exemplified by binary micelles consisting of PEO-PPO-PEO and poly(2-hydroxyethyl methacrylate)-block-poly(propylene oxide)-block-poly(2-hydroxyethyl methacrylate) (PHEMA-PPO-PHEMA) triblock copolymers.18 This work aims at extending this approach to the preparation of stabilized micelles comprising a temperatureresponsive PPO core and at least one pH sensitive polyelectrolyte block in the shell. This type of multifunctional micelles is indeed thought to broaden the range of potential applications. Two families of amphiphilic ABA triblock copolymers with a common central temperature-responsive PPO block and either poly(acrylic acid) or poly poly(dimethylaminoethyl methacrylate) outer blocks were synthesized by atom transfer radical polymerization according to the macroinitiator technique. Thus, Br-PPO-Br macroinitiators were first synthesized by reaction of PPG (MW: 2000 and 4000) with 2-bromoisobutyryl bromide as described elsewhere.20-22 Polymerization of tert-butyl acrylate was initiated by Br-PPO34-Br or Br-PPO69-Br, in the presence of the CuBr/PMDETA catalytic system in acetone. Part of the accordingly formed PtBA-PPO-PtBA copolymers was derivatized into the parent PAA-PPO-PAA triblocks by hydrolysis of the outer PtBA blocks with 5 equiv of trifluoroacetic acid with respect to the tBA units according to a known recipe.24 One PDMAEMA-PPO-PDMAEMA copolymer was synthesized by ATRP of DMAEMA initiated by the Br-PPO69Br macroinitiator in the presence of the CuCl/CuCl2/BiPy catalytic system in methanol. Monomer conversion was monitored by 1H NMR. For each monomer, the polymerization was terminated at ca. 80% conversion. All the triblock copolymers were purified by precipitation and elution through a silica gel column in order to remove traces of Cu. Then, they were analyzed by 1H NMR and size exclusion chromatography. Table 1 lists the molecular characteristics of these copolymers. The number-average degree of polymerization (DP) of PtBA and PDMAEMA was calculated by 1H NMR analysis and the copolymers composition, as well. These determinations were based on the integrals of peaks assigned (i) to the PPO protons at δ ) 3.56 ppm (2H, -O-CH2-CH-) and δ ) 3.4 ppm (1H, -O-CH2-CH-), (ii) to the PtBA protons at δ ) 1.53 (2H, -CH2C(CdO)H-) and δ ) 1.44 (9H, O-C(CH3)3), and (iii) to PDMAEMA protons (2H, -CH2-CH2-N-) at δ ) 4.1 ppm. The hydrolysis of PtBA with trifluoroacetic acid under mild conditions allowed for a quasi-quantitative conversion into PAA, as supported by the characteristic peak of the tert-butyl group that disappeared at δ ) 1.44 ppm (Figure 1). This reaction did not alter the PPO blocks. SEC traces of the triblock copolymers (Figure 2) show a monomodal distribution and a rather low polydispersity that indicate the high efficiency of the macroinitiators. According to the NMR and SEC data, one may thus conclude that the polymerization of the second block is well-controlled in all the cases.

Figure 1. Comparison of the 1H NMR spectra of (A) PPO69-b(PtBA17)2 in CDCl3 and (B) PPO69-b-(PAA17)2 in acetone-d6.

Figure 2. SEC eluograms of the macroinitiator, BrPPO34Br, and the PPO34-b-(PtBA18)2 triblock copolymer.

TABLE 2: Molecular Characteristics of the “PLURONICS” Triblock Copolymers Used in This Study code

copolymer compositiona

avg mol. massa

Mn (SEC)

Mw/Mn (SEC)

P123 P85 F38 P65

PEO20PPO70PEO20 PEO26PPO40PEO26 PEO43PPO15PEO43 PEO19PPO29PEO19

5750 4600 4700 3400

4000 2600 3400 2100

1.15 1.13 1.08 1.10

a

According to BASF.

Polymeric micelles with a mixed shell (PMMS) and a temperature-responsive PPO core were prepared by mixing welldefined amounts of a PEO-PPO-PEO triblock copolymer (Table 2) and a PAA-PPO-PAA (or PDMAEMA-PPO-PDMAEMA) triblock (Table 3). It must be noted that all the PEO-PPO-PEO triblock copolymers used in this work have a monomodal and narrow molecular weight distribution. Moreover, the pH was selected for one of the two blocks in the shell of the micelles to be ionized; thus, pH ) 3 for PDMAEMA containing micelles and pH ) 9 when PAA was a constitutive block of the shell (Table 3). These blocks were then highly hydrophilic and contributed to the stabilization of the micelles. All the PMMS formed were stabilized by loading the micelles with the hydrophobic acrylate, PETA. The UV-assisted free radical polymerization of this tetrafunctional monomer led to a tridi-

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TABLE 3: Formation and Stabilization of Polymeric Micelles with a Mixed Shell code

copolymers

SPMMS-1 SPMMS-2 SPMMS-3 SPMMS-4 SPMMS-5 SPMMS-6

P69A17×2:P123 P69A17×2:P85 P35A18×2:P65 P35A18×2:F38 P69D13×2:P123 P69D13×2:P85

mol ratio pH temp (°C) PETA (wt %) 1:1.2 1:1.6 1:1.2 1:1.6 1:1.6 1:1.6

9 9 9 9 3 3

40 70 40 60 40 70

15 20 15 20 20 20

mensional network of poly(PETA) chains, in which the polyether blocks were entrapped.17-19 All the copolymers were mixed at concentrations largely exceeding the critical micellar concentrations at the given temperature. The composition of the block copolymer mixtures and the mixing temperature were selected for either spherical (Table 3; -SPMMS-1, SPMMS-3, and SPMMS-4) or wormlike micelles. After cross-linking, SPMMS were dialyzed against water in order to remove both the nonstabilized copolymer and acetone. The cross-linking efficiency (yield) was gravimetrically determined as 78 ( 4%. The composition of the stabilized micelles was determined by 1H NMR and found to be very close to the composition of the original copolymer mixture, which indicates that both the copolymers equally participate in micellization. The shape, size, and size distribution of the prepared SPMMS were determined by DLS and TEM. DLS was carried out in the 30-150° range of scattering angles with 5 g L-1 aqueous solutions. Systematically, the autocorrelation functions were a single-exponential and the distributions were monomodal. The q2 dependence of the relaxation rate was anytime linear consistent with a diffusive behavior of the SPMMS. The CONTIN plots, illustrated in Figure 3 at only one scattering angle (Θ ) 90°), show a rather broad size distribution. Furthermore, three of the samples exhibit a pronounced angular dependence of the reduced translational diffusion coefficient (D ) Γ/q2) (Figure 4), which is the signature of anisotropic structures, such as wormlike micelles.19,25,26 Figure 5 is a typical TEM observation of this type of structure herein reported for SPMMS-2. The length of these wormlike micelles is in the 50-80 nm range, which is somewhat smaller than the DLS data. This apparent discrepancy results from the shrinkage of the stabilized micelles when dried before TEM analysis. Wormlike micelles and spherical micelles coexist in the SPMMS-5 sample (Figure 6), which contributes to the very broad particle size distribution observed by DLS (Figure 3c). The PAA and PEO shell-forming blocks are supposed to be randomly dispersed within the shell (on the basis of the known miscibility of PAA and PEO).27 Moreover, no evidence was found for the formation of “Janus” micelles28 (two distinct PEO and PAA coronal hemispheres) under the experimental conditions used in this work. It must also be pointed out that the two constitutive PEO and PAA blocks of the shell have either comparable or very different degrees of polymerization. When the PEO and PAA blocks have a comparable DP, formation of typical core-shell micelles is suggested (Scheme 1A). In contrast, when the DP of PEO exceeds that of PAA (cf. SPMMS-4 in Table 3), a three-layer micellar structure may be anticipated, thus the PPO core, a PEO/PAA central layer, and a PEO outer layer (Scheme 1B). The pH sensitivity of one of the two blocks in the micellar shell provides the herein prepared SPMMS with additional functionality and potential. Nevertheless, this pH-dependence

Figure 3. Distribution of the hydrodynamic radius for the SPMMS listed in Table 3 (Θ ) 90°; c ) 5 g L-1; T ) 25 °C).

also influences the stability of SPMMS dispersions in water. Indeed, SPMMS with a PEO/PAA mixed shell undergo a reversible transition from well dispersed particles at pH > 5 to precipitate at pH e 5 (Figure 7). Hydrogen bonding between the constitutive PEO and PAA blocks at low pH29 substantially decreases the hydrophilicity of the shell and the efficiency of the steric barrier, not only whenever these blocks are of a comparable length but also when PEO is longer than PAA, as is the case in SPMM-4 (Table 3). Substitution of PDMAEMA for PAA has a very beneficial effect on the stability of the aqueous dispersions, which is now observed in the 2-9 pH range. Being rid of any intermolecular complexation, the PEO blocks are now able to build up an effective steric barrier against coalescence, even at high pH when the hydrophobicity of the PDMAEMA blocks has increased.

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J. Phys. Chem. B, Vol. 113, No. 21, 2009 7531 SCHEME 1: Formation of Stabilized Polymeric Micelles with a Mixed PEO/PAA Shell

Figure 4. Reduced diffusion coefficient vs q2 of the SPMMS listed in Table 3 (c ) 5 g L-1; T ) 25 °C).

macroradicals could be formed in the micellar shell, that would reduce Ag+ ions bound to near-neighbor PAA chains with formation of Ag NPs.33 This formation was confirmed by color changes with the irradiation time, from colorless to pale yellow and finally to reddish brown, which is the consequence of the surface plasmon resonance of Ag nanoparticles in the UV-vis range at λ ∼ 400 nm.31,33 All the prepared dispersions of Ag NPs were stable for days, which is indirect evidence that they were embedded within the SPMMS shell and, thus, sterically stabilized. Figures 8 and 9 show TEM images of the SPMMS-1/Ag NP and Figure 5. TEM picture of the P69A17×2:P85 SPMMS (see Table 3).

Figure 6. TEM picture of the P69D13×2:P123 SPMMS (see Table 3).

Polyelectrolyte containing nanoobjects are often used as templates for the production of metal nanoparticles, as a result of the selective binding of the metal ionic precursors by the charged polymeric chains.30 Actually, the SPMMS prepared in this work are potential templates for the formation of metal nanopaticles. In order to illustrate this opportunity, templateassisted formation of Ag NPs was considered, which might confirm the random distribution of the PEO and PAA (PDMAEMA) blocks in the micellar shell. First, AgNO3 was added to the aqueous dispersion of SPMMS comprising PAA blocks followed by dialysis against water for 2 days in order to wash out the free Ag ions. Then, a water-soluble derivative of benzophenone was added, and the samples were irradiated with UV light. Benzophenone is a well-known photosensitizer able to abstract hydrogen atoms31 and to generate (macro)radicals.32 For instance, PEO

Figure 7. Turbidity of aqueous dispersion of different SPMMS over three cycles from pH 2 to pH 9.

Figure 8. TEM picture of the SPMMS-1/Ag NPs hybrids.

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Figure 9. TEM picture of the SPMMS-2/Ag NPs hybrids.

Petrov et al. and a temperature-responsive PPO core were indeed formed by mixing proper amounts of a “Pluronic” PEO-PPO-PEO triblock with either a PAA-PPO-PAA or a PDMAEMA-PPOPDMAEMA triblock copolymer at proper pH. The micelles were effectively stabilized by loading them with pentaerythritol tetraacrylate that was photopolymerized and cross-linked in situ. pH triggered a reversible transition from welldispersed SPMMS to precipitate in the case of micelles with a PEO/PAA shell. In contrast, SPMMS with a PEO/ PDMAEMA shell remained well-dispersed in the 2-9 pH range. SPMMS were exploited for the template-assisted preparation of Ag nanoparticles. The Ag NPs were better defined when formed in PAA containing SPMMS than in SPMMS with PDMAEMA in the shell. Acknowledgment. A scientific cooperation agreement between the Bulgarian Academy of Sciences and the CGRI and FNRS in Belgium made this work possible. The authors are much indebted to these three organizations. The financial support of the National Science Fund of Bulgaria (Ch-1511) is gratefully acknowledged. P.P. warmly thanks Dr. Christof Mehler and Prof. Volker Warzelhan (BASF) for “Pluronics” donation and Prof. J.-F. Gohy (Universite´ Catholique de Louvain, Belgium) for DLS measurements. References and Notes

Figure 10. TEM picture of SPMMS-5/Ag NPs hybrids.

SPMMS-2/Ag NP hybrids. Because of a much lower contrast than that for the inorganic nanoparticles, the polymeric templates cannot be observed by TEM. Spherical Ag NPs with dimensions in the 5-10 nm range seem to be localized within spherical (Figure 8) or elongated (Figure 9) domains that would foreshadow the templating micelles, consistent with the suggestion that the PAA chains to which the Ag ions are bound are not phase separated into hemispheres. So, one may conclude that SPMMS with PAA chains in the micellar shell are suitable templates for the formation and stabilization of Ag NPs. Whenever PDMAEMA blocks are substituted for PAA blocks in SPMMS, the addition of AgNO3 to the micellar dispersion results in a spontaneous color change from colorless to reddish brown within a few minutes. TEM observations (Figure 10) confirmed that Ag NPs with an average diameter of ca. 20 nm were formed, although no external reducing agent was added. A reasonable explanation is that Ag+ was reduced into Ag0 by the dimethylamino groups of PDMAEMA. Indeed, this reduction occurred when AgNO3 was added to an hyperbranched poly(amido amine).34 Similarly, Au3+ ions were reduced into Au0 upon the addition of NaAuCl4 to a poly(dimethylaminoethyl methacrylate)-block-poly(N-isopropylacrylamide) copolymer.35 However, the Ag NPs formed in PDMAEMA containing SPMMS are less well-defined than in PAA containing templates, more likely because of faster nucleation and growth. Conclusion An effective strategy for the preparation and stabilization of multiresponsive polymeric micelles with a mixed binary shell was reported in this work. Spherical or wormlike polymeric micelles with a mixed PEO/polyelectrolyte shell

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