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Association of Poly(4-hydroxystyrene)-block-Poly(Ethylene oxide) in Aqueous Solutions: Block Copolymer Nanoparticles with Intermixed Blocks Miroslav Stepanek,* Jana Hajduova, and Karel Prochazka Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030, 12840 Prague 2, Czech Republic
Miroslav Slouf Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 16206 Prague 6, Czech Republic
Jana Nebesarova eske Budejovice, Biology Centre Institute of Parasitology, Academy of Sciences of the Czech Republic, Branisovska 31, 37005 C Czech Republic
Grigoris Mountrichas, Christos Mantzaridis, and Stergios Pispas Theoretical & Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, 11635 Athens, Greece ABSTRACT: Association behavior of diblock copolymer poly(4-hydroxystyrene)-block-poly(ethylene oxide) (PHOS-PEO) in aqueous solutions and solutions in water/tetrahydrofuran mixtures was studied by static, dynamic, and electrophoretic light scattering, 1H NMR spectroscopy, transmission electron microscopy, and cryogenic field-emission scanning electron microscopy. It was found that, in alkaline aqueous solutions, PHOS-PEO can form compact spherical nanoparticles whose size depends on the preparation protocol. Instead of a core/ shell structure with segregated blocks, the PHOS-PEO nanoparticles have intermixed PHOS and PEO blocks due to hydrogen bond interaction between OH groups of PHOS and oxygen atoms of PEO and are stabilized electrostatically by a fraction of ionized PHOS units on the surface.
’ INTRODUCTION Self-assembled nanoparticles formed by block copolymers in selective solvents have received much attention in the past three decades13 due to their interesting properties and potential technological applications, mostly in pharmacology as vessels for controlled drug release.4 Although block copolymers can selfassemble in various structures such as core/shell micelles,5,6 cylinders (worms),7,8 large compound micelles,9,10 or vesicles (polymersomes),11,12 a common feature of block copolymer nanoparticles is that they consist of at least two domains formed by segregated constituent blocks. The basic example of such compartmentalization is a diblock copolymer micelle consisting of the core formed by the solvophobic blocks and the shell formed by the solvophilic blocks. Multiblock copolymers or mixtures of block copolymers can form nanoparticles which consist of more than two compartments (“onion” micelles,13 Janus micelles,14,15 or multicompartment micelles1618). r 2011 American Chemical Society
On the contrary, in some cases, block copolymer nanoparticles contain domains formed by two types of blocks which are intermixed due to specific attractive interactions. A number of studies deal with self-assembled nanoparticles based on complexes of oppositely charged polyelectrolytes1921 or complexes stabilized by formation of hydrogen bonds between different polymer blocks.22,23 In these systems, mixing two soluble block copolymers, AB and AC (or an AB copolymer and a C homopolymer), where B and C are able to form an insoluble complex BC, triggers comicellization to core/shell particles with the core formed by the insoluble complex BC and the shell (corona) of the soluble block A. Here we report on the formation of block copolymer nanoparticles by diblock copolymer poly(4-hydroxystyrene)-blockReceived: October 8, 2011 Revised: November 22, 2011 Published: November 22, 2011 307
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poly(ethylene oxide), PHOS-PEO,24 containing intermixed PHOS and PEO chains, contrary to the anticipated coreshell structure. PHOS, sometimes referred to as poly(4-vinylphenol),25 is known to form hydrogen-bond-stabilized complexes with polymers such as PEO,26 poly(2-vinylpyridine),25 poly(ε-caprolactone),25 or poly(vinyl methyl ether).27 We demonstrate that the PHOS ability to form hydrogen bonds with PEO, together with the followed nanoparticle preparation protocol, affects dramatically the self-assembly behavior of PHOS-PEO copolymers in aqueous solutions. It was reported previously that while PHOS blocks are expected to be water-soluble above pH 9 due to ionization of phenolic groups, PHOS-PEO copolymers dissolve only in strongly alkaline aqueous solutions (pH > 11), which was attributed to hydrogen bonding between PHOS and PEO.24 In this Article, we study in detail the self-assembly of PHOSPEO in aqueous solutions as a function of the solvent composition and the physicochemical parameters of the system. We couple preparation protocol with advantages based on block copolymer chemical structure, regarding in particular hydrogen bond formation and ionization of PHOS segments in order to create functional nanoparticles. The aqueous solutions are prepared by a protocol involving dialysis from solutions in mixtures of water and tetrahydrofuran as a cosolvent. The structure of the PHOS-PEO nanoparticles is studied by static, dynamic, and electrophoretic light scattering, 1H NMR spectroscopy, transmission electron microscopy, and cryogenic field-emission scanning electron microscopy. It is shown that PHOS-PEO nanostructures with variable size, surface charge, and internal structure can be obtained.
light intensity, g(2)(t,q), related to the electric field autocorrelation function, g(1)(t,q), by the Siegert relation, g(2)(t,q) = 1 + β|g(1)(t,q)|2. The data were fitted with the aid of the constrained regularization algorithm (CONTIN) which provides the distribution of relaxation times τ, A(τ,q), as the inverse Laplace transform of g(1)(t,q) function Z ∞ t ð1Þ dτ ð2Þ g ðt, qÞ ¼ Aðτ, qÞ exp 0 τ The A(τ,q) distributions can be recalculated to the distributions of apparent hydrodynamic radii, RHapp, using the relationship, RHapp = kBTq2τ/6πη, where kB is the Boltzmann constant, T is the temperature, and η is the viscosity of the solvent. Autocorrelation functions providing monomodal relaxation time distributions by the CONTIN method were further fitted to the second order cumulant expansion ln g ð1Þ ðt, qÞ ¼ Γ1 ðqÞt þ
Γ1 ðqÞ ¼ D 1 þ CRg 2 q2 q2
ð4Þ
where C is the structural parameter reflecting the shape, polydispersity, and internal dynamics of the scattering particles. Hydrodynamic radius, RH, can be calculated from D using the StokesEinstein formula, RH = kBT/6πηD. Electrophoretic Light Scattering. ζ-Potential measurements were carried out with a Nano-ZS zetasizer (Malvern Instruments, U.K.). ζ-Potential values were calculated from electrophoretic mobilities (average of three subsequent measurements, each of which consisted of 15100 runs) using the Henry equation in the Smoluchowski approximation, μ = εζ/η, where μ is the electrophoretic mobility and ε is the dielectric constant of the solvent. Transmission Electron Microscopy (TEM). TEM micrographs were obtained with a Tecnai G2 Spirit Twin 12 microscope (FEI, Czech Republic). A small amount of the aqueous solution of PHOS-PEO nanoparticles (2 μL, c = 1 mg/mL) was sputtered onto a TEM copper grid covered with thin holey carbon film, the liquid was gently removed with filter paper, and the rest of the solution was left to evaporate at the room temperature. The dried sample was observed in the TEM microscope at 120 kV using bright field imaging. Cryogenic Field-Emission Scanning Electron Microscopy (CryoFESEM). A JSM 7401F high resolution scanning electron microscope (JEOL, Ltd., Japan) equipped with an Alto 2500 cryo-attachment (Gatan, Inc., CA) was used for the observation of the frozen aqueous solution of PHOS-PEO nanoparticles. A drop of the solution (5 μL, c = 1 mg/mL) was placed on top of the aluminum pin, and then the pin was plunged into liquid nitrogen and immediately transferred under vacuum into the chamber of the cryo-attachment. The upper part of the frozen droplet was broken off. The revealed surface was sputter-coated with platinum/palladium at 140 °C for 2 min. The thus prepared sample was observed in FESEM operated at the accelerating voltage 1 kV, the beam current of 20 μA, the working distance 8 mm, and the stage temperature 135 °C. 1 H NMR Spectroscopy. 1H NMR spectra were recorded on a Varian 300 spectrometer at 25 °C using deuterated tetrahydrofuran (THF-d8) and 10 mM NaOD in deuterium oxide as solvents. For PHOS-PEO solutions in THF-d8 and in 10 mM NaOD in D2O, solvent residual signals at 3.58 ppm (CH2O in THF) and 4.80 ppm, respectively, were used as references. Solutions in mixtures of THF-d8 with 10 mM NaOD
Materials. Copolymer Sample. Poly(4-hydroxystyrene)-block-poly(ethylene oxide) (PHOS104-PEO196) copolymer (Mw = 21.1 kg/mol, Mw/Mn = 1.13, wPEO = 0.41) was prepared by hydrolysis of poly(tertbutoxystyrene)-block-poly(ethylene oxide) copolymer prepared by anionic polymerization. Details on the synthesis and characterization of the sample are given in ref 24. Preparation of PHOS-PEO Nanoparticles in Alkaline Aqueous Solutions. Tetrahydrofuran (THF) or mixtures of THF with various amounts (up to 75 vol %) of aqueous 10 mM NaOH (THF/aq.NaOH) were used as initial solvents for PHOS-PEO. Three milliliters of THF or THF/aq.NaOH solutions containing 20 mg of the copolymer were added dropwise to 17 mL of 10 mM aqueous NaOH so that the final PHOS-PEO concentration was 1 mg/mL. The solution was then dialyzed several times against 10 mM aqueous NaOH (or 10 mM NaOD in D2O for 1H NMR measurements) to remove THF. Methods. Light Scattering (LS). The light scattering setup (ALV, Langen, Germany) consisted of a 22 mW HeNe laser, operating at the wavelength λ = 632.8 nm, an ALV CGS/8F goniometer, an ALV High QE APD detector, and an ALV 5000/EPP multibit, multitau autocorrelator. The measurements were carried out at 25 °C in for the scattering angles, θ, ranging from 30° to 150°. Static light scattering (SLS) measurements were evaluated by the Guinier method using the equation IðqÞ 1 ¼ Rg 2 q2 Ið0Þ 3
ð3Þ
where Γ1(q) and Γ2(q), respectively, are the first and the second moment of the distribution function of relaxation rates. The diffusion coefficient of the particles, D, can be evaluated by the extrapolation of Γ1(q)/q2 using the equation
’ EXPERIMENTAL SECTION
ln
Γ2 ðqÞ 2 t 2
ð1Þ
where q = (4πn0/λ) sin(θ/2) is the scattering vector magnitude (here n0 is the refractive index of the solvent), I(q) is the time-averaged scattering intensity, and Rg is the gyration radius of the scattering particles. Dynamic light scattering (DLS) measurements were evaluated by fitting the measured normalized time autocorrelation function of the scattered 308
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Figure 2. Apparent hydrodynamic radius, RHapp (curve 1), and scattering intensity per unit PHOS-PEO concentration at θ = 90°, relative to the value for the solution in pure THF, [I(90°)/c]rel (curve 2), for PHOS-PEO solutions in THF/10 mM aqueous NaOH mixtures, as functions of THF volume fraction, jTHF. The PHOS-PEO concentrations were 2 mg/mL for jTHF = 0.25 and 20 mg/mL for solutions containing more than 25 vol % THF. Figure 1. DLS distributions of apparent hydrodynamic radii (scattering angle, θ = 90°) for PHOS-PEO solutions in THF/10 mM aqueous NaOH mixtures. Numbers above the curves indicate volume fractions of THF, jTHF. The PHOS-PEO concentrations were 2 mg/mL for jTHF = 0.25 and 20 mg/mL for solutions containing more than 25 vol % THF. in D2O were referenced to sodium acetate (1.90 ppm) as an internal standard. PHOS-PEO concentrations were 10 mg/mL in solutions containing THF-d8 and 1 mg/mL for the solution in 10 mM NaOD in D2O. Potentiometric Titration. pH measurements were performed with a Radiometer PHM 93 reference pH meter equipped with a PHC 3006 combined glass microelectrode. pH of solutions was adjusted with phosphoric acid.
’ RESULTS AND DISCUSSION Figure 3. 1H NMR spectra of PHOS-PEO solutions in mixtures of THF-d8 with 10 mM NaOD in D2O. Numbers above the curves indicate volume fractions of THF in the mixtures. The spectrum for jTHF = 0 is 10 magnified. Inset: Detail of the region from 3.4 to 3.8 ppm.
PHOS-PEO Association Behavior in THF/aq.NaOH. Like
some amphiphilic block copolymers,12 the used PHOS-PEO copolymer is not directly soluble in water for kinetic reasons and a cosolvent is needed to release the copolymer chains from the solid sample into the solution. Therefore, prior to the preparation of the nanoparticles, solutions of the copolymer in THF/aq. NaOH mixtures were examined by light scattering to study the PHOS-PEO association behavior. Figure 1 shows DLS distributions (measured at θ = 90°) of apparent hydrodynamic radii of PHOS-PEO aggregates for various contents of THF in the mixtures. The mean apparent hydrodynamic radii of the most intensive relaxation mode are plotted in Figure 2 (curve 1) together with the scattering intensity at θ = 90° per the unit PHOS-PEO concentration (curve 2). In pure THF, the RHapp distribution consists of two modes corresponding to individual PHOS-PEO chains and large PHOS-PEO aggregates. Since PHOS-PEO can form hydrogen bonds both between the OH groups of PHOS units and between the PHOS OH group and the oxygen atom of the PEO unit, the formation of the large aggregates is not surprising. The aggregates are loose (as indicated by weak scattering intensity) and disappear after adding alkaline water, most probably due to electrostatic repulsion of ionized PHOS blocks. If the amount of THF is further decreased, the PHOS-PEO
association number increases as a result of prevailing strong attractive hydrophobic interactions between PHOS blocks. The formation of aggregates is accompanied by a strong increase of scattering intensity, which shows that the aggregates in water-rich mixtures have much larger molar mass than those found in THFrich solutions, which can be explained by the formation of compact PHOS domains, whose faster motion in the PHOSPEO aggregates is observed in the solution with 25 vol. % THF as an additional relaxation mode in the DLS distribution. The copolymer cannot be dissolved in THF/aq.NaOH mixtures containing less than 25 vol % THF. Figure 3 shows 1H NMR spectra of PHOS-PEO in mixtures of THF-d8 with 10 mM NaOD in D2O at various mixture compositions from pure THF-d8 to pure 10 mM NaOD in D2O. With decreasing content of THF-d8, the signals of the copolymer broaden and decrease as a result of hindered mobility of the copolymer chains due to association. It is noteworthy that unlikely to the similar copolymer PS-PEO,28 even the CH2O protons 309
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Table 1. Light Scattering Characteristics of PHOS-PEO Nanoparticles in Alkaline Aqueous Solution nanoparticles
Rg, nm
RH, nm
F
Irel(0)
PHOS-PEO-I
48
57
0.84
1
PHOS-PEO-II
165
193
0.85
16.6
Scheme 1. Structure of the PHOS-PEO Nanoparticles in Alkaline Aqueous Solution
surplus of 0.01 M aqueous NaOH or on the time scale of hours during the dialysis of the sample. The narrow range of THF/aq. NaOH mixture compositions providing stable PHOS-PEO naparticles in aqueous NaOH can be explained on the basis of PHOS-PEO association behavior in THF containing solutions: In THF-rich solutions, PHOS-PEO is dissolved as individual polymer chains or large loose aggregates which fail to form the compact hydrogen-bond stabilized particles during the sudden change of the solvent composition for kinetic reasons which results in precipitation of the copolymer. On the contrary, in solutions with lower content of THF, the formed nanoparticles are too large and tend to coagulate if the content of THF is further decreased. First, the prepared PHOS-PEO nanoparticles were characterized by light scattering measurements. In order to avoid multiple scattering and to suppress interparticle interactions, 1 mg/mL solutions of the nanoparticles were diluted 10 times with 10 mM aqueous NaOH prior to the measurements according to the turbidity of the solutions. Figure 4a shows Guinier plots for the scattering from 0.1 mg/mL solution of PHOS-PEO-I (curve 1) and PHOS-PEO-II (curve 2) nanoparticles; the scattering curve for PHOS-PEO-II exhibits a minimum in the high q-region at qmin = 24.1 μm1. Dynamic Zimm plots of the apparent diffusion coefficient, Γ1(q)/q2, for the same nanoparticles, are shown in Figure 4b, together with the corresponding CONTIN distributions of RHapp at θ = 90°. Both dynamic Zimm plots are regular; the decrease of Γ1(q)/q2 in the high q region is due to back reflected light.30 Results of the evaluation of both static and dynamic LS data are summarized in Table 1. The gyration-to-hydrodynamic radius ratio, F = Rg/RH, for both nanoparticles is close to the value for the hard sphere, Fs = 0.775. The minimum of the scattering function of the PHOS-PEO-II at qmin = 24.1 μm1 is also consistent with the spherical shape: Taking the hydrodynamic radius RH,II = 193 nm as the radius of the hard sphere, we obtain qminRH,II = 4.65 which compares well with the value for the first minimum in the hard sphere form factor, qminRs = 4.49.
Figure 4. (a) Guinier plot of the scattering curves and (b) dynamic Zimm plot of the apparent diffusion coefficients of the PHOS-PEO nanoparticles, PHOS-PEO-I (curve 1) and PHOS-PEO-II (curve 2). Inset in (b) shows corresponding DLS distributions of apparent hydrodynamic radii at θ = 90° for PHOS-PEO-I (curve 10 ) and PHOSPEO-II (curve 20 ).
of PEO blocks are visible only in pure and 75% THF-d8 with signals at 3.56 and 3.66 ppm, respectively (shift due to solvent polarity effects), despite the good solubility of PEO in water and THF/water mixtures. This result suggests that mobility of the PEO blocks is influenced by their interaction with the PHOS blocks, most probably via hydrogen bonding between the OH groups of PHOS and the oxygen atoms of PEO. On the basis of LS and 1H NMR measurements, we can assume that PHOS-PEO aggregates in THF/aq.NaOH mixtures do not have a simple core/shell structure. Instead, they contain small PHOS domains which are interconnected by PEO blocks. It is worth mentioning that the formation of a similar structure was reported previously29 for polyglycidol-block-poly(propylene oxide)-block-polyglycidol (PG-PPO-PG) copolymer in aqueous solutions, in which the connection between individual PPO hydrophobic domains was mediated by hydrogen bonding between PG chains. PHOS-PEO Nanoparticles in 10 mM Aqueous NaOH. When using PHOS-PEO solutions in THF/aq.NaOH mixtures containing 40 and 50 vol % THF, the preparation protocol described in the Experimental Section provided stable nanoparticle dispersions in 10 mM aqueous NaOH (further denoted as PHOSPEO-I and PHOS-PEO-II, respectively). In other cases, the copolymer precipitated either immediately after mixing with the 310
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The scaling of the molar mass of the nanoparticles with their hydrodynamic size can be estimated assuming that they have the same scattering contrast in PHOS-PEO-I and PHOS-PEO-II, so that the ratio of the forward scattering intensities of the solutions (of the same PHOS-PEO mass concentration), III(0)/II(0), equals the ratio of their molar masses, Mw,II/Mw,I. The fractal dimension of the nanoparticles, d, can be then estimated from the equation (RH,II/RH,I)d = III(0)/II(0). The calculation yields the value d = 2.30, which suggests that the particles are swollen to some extent and their density decreases with increasing size.31 The dependence of the size of the particles on the preparation procedure (namely, on the composition of the initial THF/aq.
NaOH mixture) indicates that the particles are kinetically frozen, nonequilibrium structures. It should be pointed out that PHOSPEO aggregates in the 50% THF solution with the hydrodynamic radius of 25 nm provide almost eight times larger nanoparticles (PHOS-PEO-II) after transfer to 10 mM aqueous NaOH, while in the case of the 40% THF solution the PHOS-PEO-I nanoparticles are on the contrary about three times smaller than the “parent“ aggregates with the hydrodynamic radius of 150 nm. It suggests that in the latter case the effect of collapse of the copolymer chains dominates over the size increase caused by the aggregation. 1 H NMR of PHOS-PEO solutions in 10 mM NaOD in D2O (Figure 3, jTHF = 0) shows no signals of the copolymer. This result proves that none of the blocks is well solvated and suggests that the blocks are not segregated into a compact PHOS core and a swollen PEO shell. Hence, we can assume that the nanoparticles consist of intermixed PHOS and PEO blocks, connected by hydrogen bonds (Scheme 1). The response of the nanoparticles (PHOS-PEO-I) to the solution pH was further studied by static, dynamic, and electrophoretic light scattering. The results are summarized in Figure 5. Small changes in the hydrodynamic radius in the alkaline region between pH 12 and pH 9 suggest that the nanoparticles are charged only on the surface so that the decreasing electrostatic repulsion between PHOS blocks does not affect the particle size. Aggregation of the particles below pH 9 manifests itself by a strong increase both in the scattering intensity (curve 1) and in their hydrodynamic radius (curve 2). The inset of Figure 5 shows the pH dependence of the ζ-potential of the nanoparticles. The negative surface charge decreases with decreasing pH as a result of protonation of ionized O groups of PHOS. In accordance with the LS measurements, the absolute value of the ζ-potential falls below the critical value of 30 mV at pH < 9, that is, in the
Figure 5. Forward scattering intensity, Irel(0) (curve 1), relatively to the value at pH 11.9, and hydrodynamic radius, RH (curve 2) of PHOS-PEO nanoparticles (PHOS-PEO-I), in aqueous solution, as functions of pH. Inset: ζ-potential of PHOS-PEO nanoparticles (PHOS-PEO-I) as a function of pH.
Figure 6. (a,c) TEM and (b,d) Cryo-FESEM micrographs of PHOS-PEO nanoparticles, PHOS-PEO-I (a,b) and PHOS-PEO-II (c,d). Insets in (a) and (c) show detailed views of the nanoparticles; the real width of both insets is 300 nm. 311
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Langmuir same region where the aggregation of the nanoparticles is observed. These results indicate that the nanoparticles are stabilized electrostatically and confirm the assumption that PEO does not form a segregated water-soluble shell which would stabilize the particles in the solution sterically. It is noteworthy that a similar type of association behavior (that is, driven by hydrogen bond interaction and controlled by electrostatic repulsion between aggregates) was reported also for homopolymers of poly(ethyl acrylic acid) (PEAA),31 which form nanoparticles after heating the PEAA solution upon its lower critical solution temperature. Similarly to PHOS-PEO, the size and aggregation number of PEAA nanoparticles depends on the preparation protocol, namely on the temperature and time of the heating/cooling cycle. In order to confirm the assumed structure, PHOS-PEO-I and PHOS-PEO-II nanoparticles were imaged by TEM in dry state and Cryo-FESEM on the fracture surface of frozen amorphous ice of the solution. The micrographs are shown in Figure 6a,c (TEM) and b,d (Cryo-FESEM). The micrographs display both individual spherical particles and their clusters. (Note that PHOS-PEO solutions used for preparation of electron microscopy samples were 10-times more concentrated that those used for LS measurements. That is why clusters were not revealed by LS measurements.) The detailed look on the micrographs reveals that they are not perfectly homogeneous but contain cavities (or channels): TEM displays the cavities as light spots (indicating a lower electron density), while in FESEM micrographs which maps the surface of the particles, they are visible as holes. The presence of cavities in the nanoparticles, however, is not in contradiction with the proposed structure because they can form as a result of local arrangement of PHOS and PEO chains (formation of small domains) in the nanoparticles. Moreover, it is in accordance with the fractal dimension of the nanoparticles (d = 2.30) estimated from LS. The average radii of the particles (∼30 nm for PHOS-PEO-I, ∼100 nm for PHOS-PEO-II) are in a reasonable agreement to the sizes obtained from LS. The lower values estimated from the electron micrographs could be attributed to the fact that the results are number-averaged, while the DLS results are based on z-averaged diffusion coefficients.12 The average size, spherical shape, and size distribution of dried particles in TEM micrographs correspond quite well to the morphology of flash-frozen nanoparticles in cryo-FESEM micrographs (compare Figure 6a and b, 6c and d), which also means that the nanoparticles, unlike micelles with highly swollen shells,6 do not collapse upon drying in vacuum conditions.
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(ii) In strongly alkaline solutions, the nanoparticles have negative ζ-potential which goes to zero with decreasing pH. The absolute value of the ζ-potential decreases below the critical value for electrostatical stabilization (|ζ| = ∼30 mV) in the same pH region (89) in which aggregation of the nanoparticles occurs as evidenced from both static and dynamic light scattering experiments. This result indicates that sterical effects from well-solvated PEO chains do not contribute to the stabilization of PHOS-PEO nanoparticles in the solution. (iii) Comparison of TEM and Cryo-FESEM images of the nanoparticles reveal no differences in their shape and size which would correspond to deswelling of a PEO shell. The obtained results demonstrate that stable water-soluble nanoparticles based on block copolymers can be formed without segregation of the solvophilic and solvophobic block of the copolymer to a core/shell structure (micelles, vesicles).
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The financial support from the Ministry of Education, Youth and Sports of the Czech Republic (Long-Term Research Project MSM0021620857), the Czech Science Foundation (Grants P208/10/0353 and P205/11/J043) and the Academy of Sciences of the Czech Republic (KAN200520704 and AVOZ40500505) is gratefully acknowledged. ’ REFERENCES (1) Hamley, I. Physics of Block Copolymers; Oxford University Press: Oxford, 1998. (2) Riess, G. Prog. Polym. Sci. 2003, 28, 1107. (3) Hadjichristidis, N.; Pispas, S.; Floudas, G. A. Block Copolymers. Synthetic Strategies, Physical Properties and Applications; J. Wiley: Hoboken, NJ, 2003. (4) Harada, A.; Kataoka, K. Prog. Polym. Sci. 2006, 31, 949. (5) Tuzar, Z.; Kratochvíl, P. Adv. Colloid Interface Sci. 1976, 6, 201. (6) F€orster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956. (7) Schmalz, H.; Schmelz, J.; Drechsler, M.; Yuan, J.; Walther, A.; Schweimer, K.; Mihut, A. M. Macromolecules 2008, 41, 3235. (8) Li, Z. B.; Hillmyer, M. A.; Lodge, T. P. Nano Lett. 2006, 6, 1245. (9) Chan, S. C.; Kuo, S. W.; Lu, C. H.; Lee, H. F.; Chang, F. C. Polymer 2007, 48, 5059. (10) Stepanek, M.; Matejícek, P.; Humpolíckova, J.; Prochazka, K. Langmuir 2005, 21, 10783. (11) Checot, F.; Lecommandoux, S.; Klok, H. A.; Gnanou, Y. Eur. Phys. J. 2003, 10, 25. (12) Shen, S.; Eisenberg, A. J. Phys. Chem. B 1999, 103, 9473. (13) Prochazka, K.; Martin, T. J.; Webber, S. E.; Munk, P. Macromolecules 1996, 29, 6526. (14) Xu, H.; Erhardt, R.; Abetz, V.; M€uller, A. H. E.; Goedel, W. A. Langmuir 2001, 17, 6787. (15) Voets, I. K.; Fokkink, R.; Hellweg, T.; King, S. M.; de Waard, O.; de Keizer, A.; Stuart, M. A. C. Soft Matter 2009, 5, 999. (16) Lutz, J. F.; Laschewsky, A. Macromol. Chem. Phys. 2005, 206, 813. (17) Zhibo, L.; Hillmyer, M. A.; Lodge, T. P. Langmuir 2006, 22, 9409. (18) Uchman, M.; Stepanek, M.; Prochazka, K.; Mountrichas, G.; Pispas, S.; Voets, I. K.; Walther, A. Macromolecules 2009, 42, 5605.
’ CONCLUSIONS In this Article, we show that well-defined compact spherical nanoparticles which are stable in alkaline aqueous solutions down to pH 9 can be prepared by dialysis from solutions of the PHOS-PEO copolymer in mixtures of water and tetrahydrofuran as a cosolvent. Hydrophilic PEO in the nanoparticles, which are stabilized electrostatically by ionized PHOS units on the surface (Scheme 1), is intermixed with collapsed PHOS due to hydrogen bond interaction between PHOS and PEO blocks, as it can be concluded from the following experimental evidence: (i) 1H NMR measurements show no signal of PEO protons in D2O solutions of PHOS-PEO, indicating that PEO chains are collapsed and immobile. 312
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