Coassembly of Poly(ethylene oxide)-block-poly(methacrylic acid) and

Article pubs.acs.org/Macromolecules

Coassembly of Poly(ethylene oxide)-block-poly(methacrylic acid) and N‑Dodecylpyridinium Chloride in Aqueous Solutions Leading to Ordered Micellar Assemblies within Copolymer Aggregates ̌ pánek,† Sylvain Prévost,‡,§ Borislav Angelov,∥ Jan Bednár,⊥,# Mariusz Uchman,*,† Miroslav Stě % Marie-Sousai Appavou, Michael Gradzielski,‡ and Karel Procházka† †

Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030, 128 40 Prague 2, Czech Republic ‡ Stranski Laboratorium für Physikalische und Theoretische Chemie, Technische Universität Berlin, Straβe des 17. Juni 124, 10623 Berlin, Germany § Soft Matter Department, Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany ∥ Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovský Square 2, 16206 Prague 6, Czech Republic ⊥ First Faculty of Medicine, Institute of Cellular Biology Albertov 4, Charles University in Prague, 128 01 Prague 2, Czech Republic # LIPhy UMR 5588, Univ. Grenoble 1/CNRS, Grenoble F-38041, France % Forschungszentrum Jülich GmbH, IFF-JCNS, Lichtenbergerstraße 1, D-85747 Garching, Germany S Supporting Information *

ABSTRACT: Formation of polyelectrolyte−surfactant (PE− S) complexes of poly(ethylene oxide)-block-poly(methacrylic acid) (PEO705−PMAA476) and N-dodecylpyridinium chloride (DPCl) in aqueous solution was studied by static and dynamic light scattering (SLS, DLS), small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryo-TEM). While it was found previously [Macromolecules 1997, 30, 3519] by microcalorimetric titration that in a similar system (PEO176− PMAA186) crystallization of aliphatic tails of N-dodecylpyridinium bromide did not occur, in our system it was evidenced by SAXS that upon addition of DPCl to fully ionized PEO705− PMAA476 the ordered arrangement of the surfactant occurs in a certain range of PEO705−PMAA476 concentrations and surfactant-to-polyelectrolyte charge molar ratio (Z). Our data suggest a four-step process in the behavior of the PEO705− PMAA476/DPCl system: (i) coexistence of loose aggregates of electrostatically bound surfactants to PMAA block with free and almost unperturbed copolymer coils at Z ≪ 1, (ii) formation of aggregates containing ill-defined cores formed by DPCl micelles attached to coiled PMAA chains (beads-on-a-string nanoparticles) in the range around Z = 0.5, (iii) formation of compact core− shell nanoparticles with a core formed by densely packed ordered (crystalline) DPCl micelles and PEO shell starting slightly before charge equimolarity (Z = 1), and (iv) the region of coexistence of the core−shell nanoparticles with free DPCl micelles in excess above equimolarity (Z ≫ 1). In the region around Z = 0.5, the nanoparticles with nonordered cores coexist in a mixture either with a fraction free chains and large swollen nanoparticles decorated by surfactant micelles (at lower Z) or with the core− shell nanoparticles (at higher Z). PE−S complexes were characterized in detail in terms of molar mass, size, shape, and internal structure.

■

INTRODUCTION Polyelectrolyte−surfactant (PE−S) complexes have attracted great interest of scientists and engineers not only because of a curiosity driven fundamental research of their interesting behavior but especially because of their wide industrial applications ranging from cosmetics, detergents, and paints to drug delivery and food technology.1−3 The coassembly of PE− S complexes is driven by a combination of electrostatic and hydrophobic interactions, and both components affect the phase behavior, interfacial properties, rheology, etc. To design © XXXX American Chemical Society

materials with required properties, a number of factors have to be taken into account, such as the polyelectrolyte4 and/or the surfactant concentration, molecular weight,4 charge density,5 backbone rigidity,6 and degree of branching of the polyelectrolyte7 as well as the polarity of the headgroup and the length of the aliphatic tail of the surfactant.8 The high number of parameters that can be varied allows one, on one hand, to Received: July 19, 2012

A

dx.doi.org/10.1021/ma301510j | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

complexes only, we performed a study of nanoparticles in a broad range of surfactant/polymer stoichiometric ratios, Z, which allows us to demonstrate differences in the aggregation behavior, as compared to the previously reported results.

control and tune the structure and stimuli-responsive properties of prepared nanoparticles with a high efficiency, but on the other hand, it requires complex and lengthy fundamental studies of potentially applicable systems. In the past decades, most attention has been paid to PE−S formed by homopolymer polyelectrolytes.1−3 Nevertheless, several examples of studies on PE−S of block polyelectrolytes have been published recently as well.1,6,9−15 It has been reported that PE−S formed by double-hydrophilic block copolymers containing polyelectrolyte blocks with oppositely charged ionic surfactants assemble in both vesicles9−12 and micelles.13,14 In those systems, the membrane of the vesicles or the core of the micelles is composed of the insoluble PE−S complex and the stabilizing water-soluble shell of the neutral block. It is worth-mentioning that both the micelles with disordered cores16−19 and various ordered structures19−23 containing polyelectrolyte blocks connected by densely packed surfactant micelles were observed. Interestingly, very highly ordered, e.g., macrolattice-forming materials, have been prepared by the complexation of polyelectrolyte gels and surfactants.3,24,25 Kabanov and co-workers12−14 as the first research team investigated the formation of electrostatically stabilized complexes between double hydrophilic copolymer poly(ethylene oxide)-b-poly(sodium methacrylate), of nearly symmetric (PEO176−PMAA188) and asymmetric (PEO210− PMAA35) monomeric unit composition with single-, double-, and triple-tail cationic surfactants differing in the length of the tail (C12, C16) as well as in chemical nature of the headgroup and counterions. By using electron microscopy with selective staining, the authors confirmed the formation of vesicles and micelles of various sizes in aqueous solutions. On the basis of isothermal titration calorimetry results, they concluded that surfactant micelles crystallize in cores of PEO176−PMAA188− cetylpyridinium bromide complexes; however, no ordered structures were detected in analogous nanoparticles formed with dodecylpyridinium bromide.14 Berret and co-workers19 found that the precipitated phase formed as a result of the interaction of homopolyelectrolytes with oppositely charged surfactants, poly(sodium acrylate) with dodecyltrimethylammonium bromide, PANa420/DTAB, and poly(trimethylammonium ethyl acrylate methyl sulfate) with sodium dodecyl sulfate, PTEA41/SDS, respectively, contain long-range order Pm3n cubic and hexagonal micellar structures. However, once those polyelectrolytes (regardless of their lengths) become blocks of double hydrophilic block copolymers containing a neutral poly(acrylamide) block, PAM, they do not exhibit any crystalline order in cores of PANa-b-PAM/ DTAB, or PTEA-b-PAM/SDS complexes, because of additional surfactant−polymer interactions. In this paper, we investigate microstructures of surfactant− polyelectrolyte complexes of PEO−PMAA with DPCl in detail. Analogous complexes have already been studied by other authors.12−14 However, the use of a copolymer with a significantly higher molar mass (PEO705−PMAA476) than that studied earlier enables the formation of extended PE−S complex domains in assembled structured complexes and, as it will be shown later, modifies appreciably the process of the electrostatic self-assembly and properties of formed nanoparticles. The size and structure of the nanoparticles have been studied by a combination of scattering (LS, SANS, and SAXS) and microscopy (cryo-TEM) techniques. Because most studies on this system published so far focused on stoichiometric

■

EXPERIMENTAL SECTION

Materials. Poly(ethylene oxide)-block-poly(methacrylic acid), PEO705−PMAA476, Mw/Mn = 1.5, was purchased form Polymer Source, Inc., Dorval, Quebec, Canada. N-Dodecylpyridinium chloride hydrate (98%), DPCl, was a product of Aldrich. Deuterium oxide (99.95% D, Euriso-top, France) was used instead of water in samples for SANS to increase the contrast with the hydrogenated material and to lower the background essentially due to incoherent scattering proportional to the amount of hydrogen. Sodium tetraborate (p.a. ≥99%) was purchased from Fluka and dissolved in water taken from Millipore System; the final buffer concentration was 50 mM, pH 9.3. Transmission Cryo-Electron Microscopy (Cryo-TEM). CryoTEM was used to visualize the morphology of the objects in solution. This technique provides direct imaging of the hydrated sample without perturbing the nanoparticles. The samples for cryo-TEM were prepared as described earlier.26 3 μL of the sample solution was applied to an electron microscopy grid covered with perforated carbon supporting film (C-flat 2/2-2C, Electron Microscopy Science) glowdischarged for 20 s with 10 mA current. Most of the sample was removed by blotting (Whatman no. 1 filter paper) for ∼1 s, and the grid was immediately plunged into liquid ethane held at −183 °C. The sample was then transferred without rewarming into a Tecnai Sphera G20 electron microscope using a Gatan 626 cryo-specimen holder. The images were recorded at 120 kV accelerating voltage and microscope magnification ranging from 5000× to 14 500× using a Gatan UltraScan 1000 slow scan CCD camera (giving a final pixel size from 2 to 0.7 nm) and low dose mode with the electron dose not exceeding 1500 electrons per nm2. Typical value of applied underfocus ranged between 1.5 and 2.7 μm. The applied blotting conditions resulted in the specimen thickness varying between 100 and ca. 300 nm. Light Scattering. The light scattering setup (ALV, Langen, Germany) consisted of a 22 mW He−Ne laser, an ALV CGS/8F goniometer, an ALV High QE APD detector, and an ALV 5000/EPP multibit, multitau autocorrelator at 20 °C. The copolymer concentration in all solutions was c = 1 mg/mL. Static light scattering (SLS) measurements of the corrected excess scattering intensity, I(q), of the copolymer solutions, as a function of the magnitude of the scattering vector q = (4πn0/λ) sin(θ/2) (θ being the scattering angle, n0 = 1.332 the refractive index of the solvent, and λ = 632.8 nm the wavelength of the incident light), were treated by fitting the data in the angular range from 30° to 90° to the Guinier equation

ln

I(q) 1 = − R g 2q2 I(0) 3

(1)

to obtain the forward scattering intensity, I(0), and the radius of gyration, Rg. Dynamic light scattering measurements were evaluated by fitting the measured normalized time autocorrelation function of the scattered light intensity, g(2)(t), related to the electric field autocorrelation function, g(1)(t), by the Siegert relation,27 g(2)(t) = 1 + β|g(1)(t)|2, where β is the coherence factor. A constrained regularization algorithm (CONTIN) provided the distribution of relaxation times τ, A(τ), as the inverse Laplace transform of g(1)(t) function

g(1)(t ) =

∫0

∞

⎛ t⎞ A(τ ) exp⎜ − ⎟ dτ ⎝ τ⎠

(2)

The A(τ) distributions were recalculated to the distributions of apparent hydrodynamic radii, RHapp, assuming the apparent diffusion coefficient Dapp = 1/τq2 and using the Stokes−Einstein formula, RHapp = kBT/6πηDapp, where kB is the Boltzmann constant, T the temperature, and η the solvent viscosity. B


Macromolecules

Article

Alternatively, the autocorrelation functions, collected at scattering angles from 30° to 90°, were further fitted to the second order cumulant expansion

ln g(1)(t , q) = −Γ1(q)t 2 +

Γ2(q) 2 t 2

were isotropic, they were radially averaged, and spectra from the three configurations were merged with no need for any arbitrary coefficient. Apparent Molecular Volumes and Scattering Length Densities. SANS data depend on the “contrast” between media of different scattering length densities, η. The scattering length is an atomic (nuclide)-dependent property and for a given compound depends on its chemical composition. For the data analysis, the densities or apparent volumes of the species are necessary, and the values used in this work are presented in Table 1 (further details are given in the Supporting Information).

(3)

where Γ1 and Γ2 respectively are the first and the second moment of the distribution function of relaxation rates. The diffusion coefficient of the particles, D, was obtained by the linear extrapolation to zero q values as

Γ1(q) q2

= D(1 + CR g 2q2)

Table 1. Apparent Volumes, Densities and Scattering Length Densities of the Compounds Used in This Work (4)

where C is the structure parameter dependent on the shape and degree of polydispersity of the particles. The hydrodynamic radius of the particles, RH, can be calculated from D using the Stokes−Einstein formula. Small-Angle X-ray Scattering (SAXS). Small-angle X-ray scattering measurements with PEO705-PMAA476/DPCl samples with the copolymer concentration, c = 5 mg/mL, were performed at the I22 beamline of Diamond Light Source (Didcot, U.K). In case of 1 mg/ mL PEO705−PMAA476/DPCl samples, the measurements were done at the ID02 beamline at the European Synchrotron Radiation Facility in Grenoble, France.28 Both synchrotrons are of the third generation and offer high X-ray flux (∼1012−1013 photons s−1). Samples were filled in borosilicate glass capillaries (Hilgenberg Gmbh, Germany, diameter 1.5 mm, wall thickness 10 μm), and the capillaries were mounted in a holder in air at the room temperature (21 °C) without a special vacuum chamber. The I22 station was equipped with a gas wire RAPID 2D detector,29,30 the area of which was divided into 512 × 512 pixels. The wavelength of the incident X-ray beam and the beam size at the sample areas were 0.155 nm and 350 × 275 μm2, respectively. The accessible q-range was from 0.09 to 4.17 nm−1. A metal attenuator was installed in order to prevent the samples from damage by X-radiation. The exposure time was about 1 s. The ID02 beamline was equipped by a high-sensitivity CCD FReLoN 4 M detector, the area of which was also divided into 512 × 512 pixels after 4 × 4 pixels binning. The X-ray wavelength was 0.1 nm, the beam size was 350 × 270 μm2, and the accessible q-range was from 0.07 to 3.2 nm−1. Instead of using a metal attenuator to prevent samples from X-ray radiation damage, the exposure time was decreased to 10 ms by means of a high-precision X-ray beam shutter. At both beam stations, the incident and transmitted beam intensities were recorded simultaneously. Silver behenate was used for the qrange calibration and glassy carbon for the intensity normalization. After the data acquisition, the obtained 2D images were integrated into 1D scattering curves by means of the Fit2D software.31 The backgrounds, coming from the glass capillary and the solvent, were measured and subtracted using conventional procedures. One and the same capillary was used during the measurements with and without solvent. This allowed for very precise subtraction of the background. Small-Angle Neutron Scattering (SANS). SANS Apparatus. SANS experiments were carried out on KWS-2 from the Jülich Center for Neutron Science (JCNS) at FRM-II in München, Germany. Samples were poured in quartz cuvettes (QX quality from Hellma) of 2 mm neutron pathway, thermostated at 25.0 °C. Three configurations were used with SD = 1.7 m, λ = 0.45 nm, SD = 7.7 m, λ = 0.45 nm, and SD = 7.7 m, λ = 1.2 nm, where SD is the sample-to-detector distance and λ the mean wavelength (fwhm = 20%); the final q-range obtained is 2.5 × 10−2−3.2 nm−1, corresponding to 2−250 nm in the real space (using Bragg’s law d = 2π/q), where q is the magnitude of the scattering vector, q = 4π/λ sin(θ/2), θ being the scattering angle. The beam was collimated at 8 m from the sample in all cases. The beam size at the sample position was 8 × 8 mm2. Data reduction was performed on 2D patterns by means of BerSANS,32 using the scattering by a 1.5 mm PMMA sheet to correct for pixel efficiency, taking a tabulated value for the absolute scale, and subtracting the experimental intensity for the buffer as background. Since the data

compound

ρ,b g cm−3

Vm,c nm3

η × 104,d nm−2

buffer DPCl C12H25− −C5H5NCl MA− EO PEO705−PMAA476

0.9268 + 0.0478xa 0.8009 1.3966 + 0.0375x 2.07 1.1873 1.5670

0.4451 + 0.0386x 0.3511 0.0941 + 0.0386x 0.0681 0.0616 75.8

6.31 0.23 + 0.18x −0.39 2.54−0.02x 2.86 0.67 1.61

a

Fraction of binding counterions. bDensity. cApparent molecular volume. dScattering length density. For PMAA the apparent volume depends strongly on the nature of the counterions and the ionization degree; in this work, the value was set and fixed to the value for the deprotonated polyelectrolyte. Analysis of SANS Scattering Curves. Scattering data can be expressed as a sum of two contributions, I(q) = Icoh(q) + Iinc, where Icoh is the coherent scattering depending on the shape, size, and repartition of domains in the sample and Iinc a constant background due to incoherent scattering by atoms. For a single population of monodisperse spheres, the coherent part can be separated into a normalized form factor P(q) representing the shape, size, and internal distribution of materials of the scatterers, a scaling proportional to their number density, N, their average volume ⟨V⟩, their average contrast Δη = η̅part − ηsolvent, and a structure factor S(q) to account for interactions: Icoh(q) = N ⟨V ⟩2 (Δη)2 P(q)S(q)

(5)

with NV = φ, the volume fraction. When interactions can be neglected, S(q) ≈ 1. Therefore, the forward scattering can be used to determine the volume of a particle as long as interactions are negligible:

VI(0) =

I(0) ϕ(Δη)2

(6)

If two or more populations of scatterers are present whose dimensions differ by ca. 1 order of magnitude, the intensity can be approximated as the sum of scattering contributions by each individual population (cross-terms are neglected). Because of the complexity of the system, a model-free analysis based on integral structural parameters was carried out before model fitting. After obtaining realistic parameters from the model-free analysis, data were fitted with analytical models (see Results and Discussion). In the case of the pure polymer, a model for the Gaussian coil is used Ipolym(q) = I(0)

2 [exp(− R g 2q2) − (1 − R g 2q2)] R g 4q 4

(7)

Using eq 5, I(0) can be expressed as I(0) =

cM w NAρ2

(Δη)2

(8)

where Mw is the mass-averaged molar mass of the polymer, NA the Avogadro constant, c the polymer concentration, and ρ the polymer apparent density. C


Macromolecules

■

Article

RESULTS AND DISCUSSION Light Scattering. Prior to detailed small-angle scattering studies, the formation of PEO705−PMAA476/DPCl particles was studied by light scattering to gain a global overview on the selfassembling behavior. Figure 1 shows the CONTIN distribu-

Figure 2. Gyration radius, Rg (curve 1), and forward scattering intensity of particles formed in PEO705−PMAA476/DPCl solutions normalized by that of the pure PEO705−PMAA476 solution, I(0)/I0(0) (curve 2), as functions of the stoichiometric ratio of DPCl-to-PMAA units, Z. Inset: gyration-to-hydrodynamic radii ratio, ρ, of particles formed in PEO705−PMAA476/DPCl solutions as a function of Z.

increases due to increasing association number. At Z ca. 0.6, a critical charge ratio is attained at which the transition from loose aggregates to core−shell particles occurs. The collapse of the core-forming PMAA/DPCl blocks and the decrease of the gyration radius as well as changes in the association number are clearly seen in curves obtained by SLS. The minimum Rg is observed close to the zero net charge at Z = 1. A slight increase in the size of the core−shell particles with increasing Z for Z > 1 is probably caused by an increase of the number of surfactant micelles in cores and by partial (secondary) aggregation of PEO705−PMAA476/DPCl particles as a result of their decreased solubility in solutions containing excess surfactant molecules. The dependence of the gyration radius of PEO705− PMAA476/DPCl particles on Z obtained by SLS is similar to that of RH (calculated from their diffusion coefficients at q = 0 extrapolated by means of eq 4). However, the comparison of both curves is worth mentioning because the ratio, ρ = Rg/RH, reflects the inner structure and compactness of scattering particles. The Z-dependent ratio, ρ, is shown in the inset of Figure 2. It gradually decreases from ρ = 1.46, which indicates the scattering from loose structures (for a monodisperse coil at θ-conditions, ρcoil = 1.50) to values around 0.8 which are typical for hard spheres (ρsphere = 0.778) and other compact spherical particles, e.g., block copolymer micelles. The observed trend confirms the above-mentioned increasing compactness of the aggregates with increasing Z. The dependence of the forward scattering intensity on Z (Figure 2, curve 2) is consistent with the model assuming the transition from loose aggregates to core−shell particles: A steep increase in I(0) close to Z = 0.6 reflects a significant rise of the association number as soon as the core−shell aggregates appear in the solution. However, for a correct interpretation of I(0) values, it is necessary to keep in mind that even though the forward scattering intensity is proportional to the molar mass of PEO705−PMAA476/DPCl particles, the proportionality constant depends on Z due to changes in the refractive index increment of the polymer−surfactant complex. Small-Angle Neutron Scattering (SANS). SANS curves were measured for a series of solutions prepared by mixing (i) stock solution of the polymer at 1 g L−1 (13.9 μM of the copolymer, 6.6 mM of PMAA units) in 50 mM sodium tetraborate−D2O buffer and (ii) 100 mM surfactant D2O solution. The scattering curves in the Z-range from 0.15 to 3.6

Figure 1. DLS distributions of apparent hydrodynamic radii of particles formed in PEO705−PMAA476/DPCl solutions. Values of stoichiometric ratio of DPCl to PMAA units, Z, are indicated above the corresponding curves. Scattering angle, θ = 90°.

tions of apparent hydrodynamic radii measured by DLS at θ = 90° for the pure PEO705−PMAA476 and for several PEO705− PMAA476/DPCl mixtures in 0.05 M sodium tetraborate buffer. The polymer concentration is 1 mg/mL, and the composition is given by the charge ratio of surfactant to MAA units: Z = [DPCl]/[MAA]. In the case of the block copolymer solution, the distribution is bimodal due to a small amount of strong scatterers which coexist with individual PEO705−PMAA476 chains in the solution (both observed relaxation modes correspond to diffusive motions as proven by linear dependences of the relaxation rate, Γ vs q2). This observation is not surprising because a spontaneous formation of large aggregates in polymer solutions (usually somewhat vaguely described as fairly concentrated metastable microphase-separated domains) has been reported both for neutral polymers and polyelectrolytes.33−37 Upon addition of DPCl, the surfactant interacts with the copolymer and forms micelles which condense on its chains. The solubility of the polymer decreases and new large polymer−surfactant aggregates form in the solution. At low Z, they are in equilibrium with free polymer chains, but as they (i) dominate the scattering behavior and (ii) their fraction increases with increasing Z, an apparent unimodal distribution of hydrodynamic radii is observed at Z ca. 0.4. The positions of peak maxima shift and their half-widths change with Z due to structural changes that will be discussed in subsequent paragraphs. Additional information on the structure of aggregates can be obtained by static light scattering. The Z-dependences of the gyration radius Rg of the copolymer and the aggregates and the scattering intensity at the zero scattering angle normalized by that for the pure PEO705−PMAA476 solution are shown in Figure 2. Up to Z ca. 0.6, the size of the particles steeply D


Macromolecules

Article

loose aggregates. As the relative difference between intensities at low and high q decreased as compared with the previous curve, we can conclude that the large and swollen aggregates (seen in the low q region) for Z = 0.38 are on average more compact than those at Z = 0.15. This is a reasonable conclusion as the gradual compensation of the polyelectrolyte charge by micelles attached to PMAA chains screens the electrostatic forces, lowers the solubility, and causes partial collapse of swollen aggregates and their possible reorganization.1 The curve for Z = 0.6 and intermediate q has a shape typical for small globular aggregates, which indicates that the interaction of polyelectrolyte chains with increasing content of the surfactant micelles has already reduced their solubility significantly and promoted the formation of distinct globular assemblies of arranged DPCl micelles held together by PMAA and stabilized by PEO chains. We can now even see a trace of a correlation peak at q ca. 1.6 nm−1 which will be discussed later. Nevertheless, the scattering intensity still increases steeply in the low q range without any sign of leveling-off toward q = 0, in contrast to systems with Z > 0.6. It is worth mentioning that all curves for systems with Z in the range 0.15−0.6 rise steeply with decreasing q in the low q-range without reaching a plateau, which indicates that the aggregates (at least some of them) that are present in studied systems are too large for the upper length-scale accessible by SANS measurements (which was about 250 nm in our case). To outline and summarize the changes in the conformational behavior up to Z = 0.6, for Z = 0.15 and 0.38, we assume a coexistence of free chains with loose aggregates, while for Z = 0.6, we suppose that the core−shell micelles coexist with a fraction of voluminous aggregates. With further increase in DPCl content, the curves in the low q range adopt a scattering pattern with a kink around 0.1 nm−1, which corresponds to the presence of spherical aggregates of 45 nm radius. This scattering pattern survives without significant changes up to high concentrations of DPCl. For Z > 0.6 a new correlation peak appears at high q (q ∼ 1.6 nm−1), corresponding to a 3.9 nm characteristic spacing between densely packed surfactant micelles in cores of polymer/ surfactant nanoparticles. On the basis of the general discussion of the SANS curves given above (and the more detailed analysis given below), we can deduce the following scheme of structural changes occurring in the studied system (Scheme 1). Modeling the SANS Data. To confirm the proposed qualitative scheme of the behavior in a broad range of Z and to get a more quantitative picture, we performed an extensive modeling of SANS curves based on models deduced from the standard model free data analysis which is described in more detail in the Supporting Information. The scattering intensity for the pure polymer (Z = 0) is weak and the curve is noisy due to a very low copolymer volume fraction (ca. 0.06%). The data can be reasonably fitted with a model for Gaussian coil (eq 7) with I(0) = 0.06 cm−1 and Rg = 15 nm. Using eq 8, a molar mass of 40 kg mol−1 is obtained which is appreciably less than the value provided by the producer (72 kg mol−1). Besides the already mentioned low quality of the measured curve, the discrepancy is most probably due to interchain interactions which, despite low concentration and efficient electrostatic screening, cannot be ruled out completely and may lead to the formation of a low fraction of more or less stable or just temporary high molar mass aggregates. As only a part of the curve for high q could have been successfully fitted by the model for Gaussian coil, an accurate value of molar mass have

together with those for pure solutions of the copolymer and surfactant micelles are shown in Figure 3. While the SANS

Figure 3. SANS data for the pure polymer (1 g L−1), the pure surfactant (100 mM), and mixtures of them (at constant polymer concentration of 1 g L−1), in D2O−buffer at 25 °C. The surfactant-topolymer molar ratio Z is given next to each spectrum (equimolarity at Z = 1). Intensities of the data are incrementally shifted by a factor 8 for better readability; data for the pure polymer (Z = 0) are directly at scale. Continuous lines are best fits.

curve for the pure copolymer has very low intensity in the whole q-range (and can be modeled reasonably by the curve for polymer coils; see later), the addition of a small amount of DPCl corresponding to 1 mM, i.e., to Z = 0.15 (well below cmc which is 9.8 mM in 50 mM sodium tetraborate, as determined by ITC),38 induces the intensity increase in the low q range nearly by 2 orders of magnitude. The high q regime remains less affected. This fact indicates the formation of a small fraction of very large polymer−surfactant aggregates coexisting with free coils. When discussing and interpreting the data, it is necessary to keep in mind that the excess scattering length density η of the surfactant is higher than that of the polymer which partially explains the observed intensity increase at low q. Nevertheless, it also means that the inhomogeneities monitored by SANS are mainly the fluctuations of structural patterns created by surfactant micelles interacting with (and attached to) polymer chains as will be supported by other data and their careful analysis later. An inspection of SANS data in the low Z range up to 0.6 reveals that the shapes and mainly the changes of individual curves with Z are slightly surprising. The high q part of the curve for Z = 0.38 resembles that for pure solution of surfactant micelles (above cmc), suggesting that the scattering behavior at short length scales is dominated by disordered surfactant micelles attached to coiled chains. However, in a low q range, the experimental intensity rises steeply, indicating the presence of large patterns created by the surfactant micelles in large and E


Macromolecules

Article

been used;39 for these micelles, the form factor and parameters used were taken from the fit of pure concentrated DPCl micelles, with no modification. Upon further addition of surfactant, the scattering from large spherical objects is observed and a correlation peak appears at high q. The strongly scattering core−shell nanoparticles with cores formed by densely packed surfactant micelles are modeled as follows: The form factor for densely packed micelles is the core−shell sphere model (eq 7 in Supporting Information) with Nagg = 47 (aggregation number of surfactant molecules in the DPCl micelles) as obtained for pure DPCl micelles (see below). Repulsive interactions between micelles are evaluated using the simple hard-sphere potential40 (see Supporting Information), assuming that electric charges are essentially neutralized. For the form factor for the whole nanoparticle, the core− shell model has been used (see Supporting Information). For Z ≥ 0.91, we assume that all polymer chains participate to the formation of nanoparticles. The core is composed of densely packed surfactant micelles that neutralize the charge at PMAA chains and solvent molecules, and the shell is composed of PEO moiety and the solvent. The NP core radius and the number density of micelles in the core determine the composition of the core, from which the contrast is calculated. The experimental SANS curves are relatively smooth (as compared with theoretical curves for the assumed monodisperse model species) due to slight polydispersity in sizes, which is implemented in the model as a Gaussian distribution of core radii. Finally, for Z = 3.6 the SANS data indicate the presence of free (excess) surfactant micelles in the solution. They are fitted by the same model as pure surfactant micelles in the buffer (Z = ∞), i.e., biaxial oblate ellipsoids with Nagg = 47 and core halfaxes of 1 × 2 × 2 nm3 (see Supporting Information). The shell thickness has been found to be 0.55 nm. The equivalent spherical micelle (with same aggregation number) has a core radius of 1.60 nm and a shell thickness of 0.58 nm; this simplified model is used for densely packed micelles, while the biaxial model is used for other micelles (either in clusters or as excess micelles). To summarize the analysis of the SANS curves, the data from mixtures of PEO705−PMAA476 and DPCl are modeled in the absolute scale by the sum of several contributions (maximum 5). However, individual terms are relevant (i.e., they are nonzero) only in some regions of Z, which are specified in previous parts and are outlined in Scheme 1.

Scheme 1. Representation of the Suggested Self-Assembled Structures in Solution as a Function of the Molar Ratio Za

a

(a) Pure polymer coils, (b) loose aggregates of the surfactant bound electrostatically to the PMAA block, (c) beads-on-a-string nanoparticles with a core formed by low density nonordered micelles, (d) core−shell nanoparticles with a core formed by densely packed ordered micelles, and (e) core−shell nanoparticles coexisting with excess free surfactant micelles.

not been reproduced, and we will further use the value given by the manufacturer, i.e., PMAA: 41 kg mol−1 and PEO: 31 kg mol−1. However, the modeling confirms that the copolymer dissolves fairly well under given conditions (i.e., in a sodium tetraborate buffer at pH 9.3 when the carboxylic groups are ionized and the electric charge is efficiently screened by small ions) and acquires the shape of an almost unperturbed Gaussian coil and only a small fraction of chains aggregate. With the first addition of the surfactant (Z = 0.15), the curve starts to change, but it does not show any distinct features so far. It can be modeled assuming the presence of a very low amount of large loose aggregates interconnected by surfactant micelles (formed as a result of electrostatic polymer−surfactant interaction) with the size larger than that of pure DPCl, which reflects the fact that they are wrapped by parts of PMAA chains. This yields on one side additional material to the micelle, but more importantly leads to a pronounced electrostatic shielding and thereby to a micellar growth as it would correspondingly also be observed in a solution of higher ionic strength. The next curve at Z = 0.38 supports this working hypothesis. Its shape can be described by the scattering from large aggregates stabilized by micelles with a larger size than those determined previously (aggregation number 116 as compared with 47 for pure DPCl). In agreement with other experimental data (e.g., cryo-TEM images), the curve for Z = 0.38 can be interpreted by a model that assumes small clusters of micelles attached to polymer chains (see also Scheme 1 and Supporting Information Figure S3), and hence the beads-on-a-string structure factor has

I(q) = Iinc + Icluster(q) +

∫R [INP(q) + Im,NP(q)]Φ(Rc) c

dR c + I xs(q)

(9)

where Iinc accounts for q-independent incoherent scattering (and is fixed to the value determined by a Porod analysis at high q), Icluster(q) (used up to Z = 1) is the contribution from micelles attached on polymer chains, modeled by small clusters of core−shell ellipsoids (“beads-on-a-string”), where the only parameter fitted is the fraction of surfactant (the number of micelles per string was fitted once for Z = 0.38 and then kept constant due to the lack of information resulting from the large intensity contribution from nanoparticles for larger Z), INP(q) is the contribution from the nanoparticles, modeled by core−shell spheres with a homogeneous polydisperse core composed of the PMAA moiety, solvent, and surfactant and a homogeneous F


Macromolecules

Article

shell composed of solvated PEO, Im,NP(q) is the contribution from densely packed DPCl micelles in the core of the nanoparticles modeled by core−shell spheres interacting through a simple hard-sphere potential, their size distribution is described by Schulz−Zimm distribution function, Φ(Rc), and Ixs(q) (used for Z > 1) is a contribution from free micelles in excess modeled with biaxial core−shell ellipsoids interacting through electrostatic repulsions. The individual scattering contributions can be expressed in the form (here we have used the decoupling approximation for form and structure factor (P(q) and S(q)) that is strictly valid only for the monodisperse case but is conventionally also employed for polydisperse cases of colloidal systems)42 Icluster(q) = NclusterVcluster 2Δηcluster2P ellips(q)S bead(q)

(10)

INP(q) = NNPVNP 2ΔηNP2P CS,NP(q)

(11)

Im,NP(q) = Nm,NPVm,NP 2Δηm,NP2P CS,m(q)SHS(q)

(12)

Im,xs(q) = Nm,xsVm,xs 2Δηm,xs2P ellips(q)Scharged(q)

(13)

hydrodynamic radii obtained by light scattering. Indeed, the actual shell thickness has little influence on the SANS model beyond a certain hydration. The number of polymer chains in the shell is deduced from the total concentration of polymer and from the number density of particles. The model curves are shown together with experimental data on Figure 3, and the decomposition of the model curve into individual contributions is exemplified in Figure 4, which shows that the different fitted

where N, V, and Δη are the density numbers, volumes, and average contrasts and indices “cluster”, “NP”, “m,NP”, and “m,xs” respectively correspond to the clusters, nanoparticles, micelles embedded in the nanoparticles, and excess surfactant micelles; Sbead(q) is the beads-on-a-string structure factor, SHS(q) is the hard-sphere structure factor for the densely packed micelles, Scharged(q) is the structure factor of charged spheres for the ellipsoidal micelles in excess, PCS,NP(q) and PCS,m(q) respectively are the core−shell sphere form factors for the nanoparticles and the micelles (here assumed spherical) embedded in the nanoparticles, and Pellips(q) is the form factor for the biaxial core−shell ellipsoidal micelles. The expression for all structure factors and form factors can be found in the Supporting Information. Obviously, the hard-sphere structure factor is not perfect for modeling the cores formed by densely packed micelles, which are probably neither spherical nor neutral and their ordering and packing increases with Z (see SAXS patterns later). However, SANS is a relatively low-resolution technique, in particular in this q-region with a rather large wavelength smearing, so the details are suppressed anyway and we prefer as simple and transparent model as possible. The number density of micelles in the core of the nanoparticles was therefore fixed in all fits so that the bump due to the smeared peak of the structure factor describes reasonably well the position of the experimentally observed peak. The fit is then performed in a q range up to 1 nm−1, i.e., before the inner structure of the core starts to influence experimental data. There are finally only four parameters to be fitted: the average core radius Rc and standard deviation of the nanoparticle size distribution s/Rc, the concentration of surfactant participating to the nanoparticles, and to the micelles outside the nanoparticles (corresponding to Ncluster or Nm,xs depending on Z); the difference between the known concentration of surfactant and the sum of these two concentrations from the fits is then interpreted as a cmc. We recall that for Z ≥ 0.91 all the polymer molecules are assumed to be part of the nanoparticles (this simplifying assumption reflects the charge compensation). The PEO shell thickness is fixed to a value of 25 nm so that the overall dimensions determined by SANS are in a good agreement with

Figure 4. Decomposition of the fit for Z = 1.1 into elementary scatterers: the large and polydisperse core−shell sphere for nanoparticles, core−shell spheres with a structure factor for micelles densely packed in the core of NP, and biaxial ellipsoids for micelles not included in the core of the NP. Contributions are not in absolute scale for readability.

parameters are obtained from different scattering features at different q-ranges; the output parameters are listed in Table 2. The outcome of the fits is consistent with the approximate results provided by the model-free analysis, which indicate that the fits are meaningful, in spite of the assumptions and simplifications required by the complexity of the systems. The reliability of the parameters deduced from the fits is further supported by their being physically realistic with continuous evolution as a function of Z. As the absolute scale has been used in all fits, the amount of the surfactant participating to micelles is known and we can evaluate the concentration of free surfactant by subtraction, which is increasing with Z up to 7.4 mM (for Z = 3.6). The cmc for pure surfactant in 50 mM sodium tetraborate determined by ITC and is 9.8 mM,38 which corroborates further the SANS analysis. The parameters resulting from the fits indicate a very low density of PEO chains at the surface of the nanoparticles, in the order of 60 nm2 per chain (increasing with Z from 30 up to 70 nm2). In general, we can summarize that the detailed modeling described above fully confirms the behavior of the system depicted in Scheme 1. SAXS Measurements. In order to gain complementary information and higher q resolution, additional SAXS experiments were done. Because of their high resolution, they provide deeper insight in the internal structure of PEO705−PMAA476/ DPCl nanoparticles and enable to investigate packing of surfactant micelles in the nanoparticle core. It was found that the ordering of surfactant micelles in the core depends both on the surfactant-to-polyelectrolyte molar ratio and on PEO705− PMAA476 concentration. G


Macromolecules

Article

Table 2. Outcome from the Fits of SANS Data from Core−Shell Nanoparticles Z

Zca

Cm,xs,b mM

Cm,NP,c mM

Rc,d nm

s/Rce

Nagg,polyf

Nagg,micg

aPEO,h nm2

0.91 1.13 1.44 1.81 3.63

0.63 1.13 1.37 1.54 1.92

0.77 0.24 0.08 0.06 1.67

3.9 6.9 8.3 9.1 10.2

48 46 50 57 62

0.22 0.15 0.15 0.16 0.17

1060 482 513 682 706

6799 5540 7164 10692 13842

29 57 63 62 71

a Surfactant-to-MMA molar ratio in the core of the nanoparticles. bConcentration of the surfactant outside the nanoparticle core (due to interdependencies, the error is actually close to 0.1 mM). cConcentration of the surfactant in densely packed micelles in the NP (error up to 3 mM). d Average core radius (typical error: 0.03 nm). eRelative standard deviation of Rc. fAggregation number of polymer chains in one nanoparticle (error up to 218 for Z = 1.81, in other cases much lower). gAggregation number of the micelles in one nanoparticle (error up to 16). hArea per polymer chain at the core/shell interface of the nanoparticle (error up to 20 nm2 for Z = 1.81, in other cases much lower). Evaluation of uncertainties: see Supporting Information.

Figure 5 shows SAXS curves for a series of PEO705− PMAA476/DPCl complexes in the q range 0.5−2.5 nm−1 (the

(Figure S4). This result indicates that the liquid crystalline structure of the core is kinetically frozen and remains preserved after dilution of the sample. Cryo-Transmission Electron Microscopy (Cryo-TEM). Cryo-transmision electron microscopy was performed on NP solutions in 50 mM sodium tetraborate D2O. The nanoparticles were prepared at copolymer concentration 5 g L−1 and DPClto-MAA ratios Z = 0.15, 0.38, and 1. Unfortunately, lower concentrations of the copolymer (comparable with those used for scattering experiments, ca. 1 g L−1) did not provide good quality cryo-TEM micrographs (Talmon et al. reported similar problems for aggregates formed by short chain surfactants and polyelectrolytes.)21 Figure 6 represents typical cryo-TEM images of nanoparticles obtained at Z = 0.15 (a), 0.38 (b), and 1 (c). At Z = 0.15 when the surfactant concentration is lower than cmc of DPCl, the interaction of the surfactant with PE induces the formation of micelles and their electrostatic binding to charged PMAA block.41 A low fraction of polydisperse aggregates form in which the elongated structures could be found as indicated by arrows. For a further discussion, it is important to realize that the polymer chains (both blocks) are almost invisible, and only the surfactant micelles and their assemblies are clearly seen on the images. As more surfactant is added and its concentration approaches the cmc (Z = 0.38 and cDPCl = 2.4 mM), fairly uniform beads-on-a-string structures are formed Figure 6b. Since all experiments were done in a dilute region, the elongated structures could be depicted as a few spherical DPCl micelles electrostatically interacting with PEO705−PMAA476 chains. Those elongated structures grow until Z ca. 0.6, when the electrostatic repulsion between noncompensated negative charges on PMAA chains is overbalanced by hydrophobic interactions and more compact core−shell structures start to appear in the solution. When the negative and positive charge is matched at Z = 1, a massive aggregation occurs and quite dense core−shell nanoparticles with an average diameter of 120 ± 20 nm are formed (Figure 6c). The size estimated by cryo-TEM is in a good agreement with LS and SANS results. The stabilizing PEO blocks are in good solvent condition and form the micellar shell that is invisible on the cryo-TEM images.

Figure 5. SAXS curves of PEO705−PMAA476/DPCl complexes in the q range 0.5−2.5 nm−1. Copolymer concentrations were (a) 1 and (b) 5 g L−1. Surfactant-to-polyelectrolyte units molar ratios, Ζ, are indicated above the corresponding curves.

region of the correlation peak revealed by SANS). At low PEO705−PMAA476 concentration (c = 1 g L−1), the surfactant micelles are disordered up to Z ca. 2, showing a single broad correlation peak corresponding to the characteristic distance of DPCl micelles in the core. For higher Z ratios, scattering curves contain two overlapping peaks with the maxima at 1.85 and 1.62 nm−1. The ratio of maxima positions is fairly close to 41/2:31/2, suggesting the fcc cubic packing of the DPCl micelles in the core. That a significantly higher value of Z than 1 is required for seeing such well-defined peaks could be explained such that the DPCl is carrying some of its Cl− counterions into the copolymer aggregates. Accordingly then, a larger number of it can be become incorporated than foreseen for Z = 1 and once the packing density of DPCl micelles in the copolymer aggregate interior surpasses a certain concentration this transition to a highly ordered state is observed. Increase of the surfactant concentration increases fluidity of the nanoparticles core and allows for better accommodation both surfactants and polymer chains. The measurement at high PEO705−PMAA476 concentration (c = 5 g L−1) revealed that the transition of surfactant micelles in the core to the ordered state occurs at Z ca. 1. In this case three scattering peaks appeared with the maxima at 1.81, 1.66, and 1.49 nm−1, i.e., close to 61/2:51/2:41/2, which corresponds to the Pm3n cubic packing. It is noteworthy that the well-resolved scattering peaks of the Pm3n cubic structure were still visible when the sample was diluted down to the copolymer concentration of 0.3 g L−1

■

CONCLUSIONS In this paper, we have shown by combining information from light scattering and SANS that mixing the copolymer PEO705− PMAA476 with DPCl in aqueous solution leads to the formation of different states of self-assembled structures, depending on the value of the charge ratio, Z, which can be subdivided into four different regions: H


Macromolecules

Article

Figure 6. Cryo-TEM images of nanoparticles formed at the stoichiometric ratio, Z, of DPCl to PMAA units (a) 0.15, (b) 0.38, and (c) 1 and the copolymer concentration, c = 5 g L−1; the arrow in (a) indicates the very first elongated beads-on-a-string structures. Scale bars = 100 nm.

(i) at low Z (Z ca. 0.15), a low fraction of large elongated copolymer aggregates form; the aggregates are swollen and the chains are connected by surfactant oligomers, the formation of which (below cmc) is induced by electrostatic interaction with units of PMAA blocks; the aggregates coexist with free copolymer coils; (ii) at higher Z (Z ca. 0.38), the fraction of aggregates increases, but their size slightly diminishes due to the screening and overall weakening of the electrostatic repulsion between the ionized PMAA units by the surfactant micelles of the opposite charge bound to the chains which partially neutralize electric charges; the aggregates still coexist with free copolymer chains which are partially decorated by surfactant micelles and form quite random coils; (iii) around Z = 0.6, the solubility of the copolymer−surfactant aggregates formed on PMAA blocks is low and because the solubility of PEO is unaffected, relatively compact core−shell aggregates start to form; their cores are formed by densely packed and ordered surfactant micelles attached to PMAA chains and are protected by soluble PEO shells; at first they coexist with a fraction of large aggregates, but close to Z = 1.0, all polymer chains and surfactant molecules are engaged in compact aggregates; (iv) at Z ≥ 1, the core−shell aggregates partially solubilize DPCl surfactant micelles (together with corresponding amount of Cl− counterions) and coexist with a certain excess of surfactant molecules in the solution and at concentrations above the cmc also with surfactant micelles. For the core−shell nanoparticles formed around Z ∼ 1 (regions iii and iv), with a thin PEO shell and a core containing

PMAA interacting with oppositely charged surfactant micelles, it was found by SAXS experiments that the ordering of DPCl micelles in the core depends both on surfactant-to-polyelectrolyte molar ratio and the total concentration, becoming more pronounced with increasing concentration. A fcc and Pm3n cubic packing of the DPCl micelles in the core were found in systems with Z > 2 and polymer concentration 1 g L−1 and with Z > 1 and 5 g L−1, respectively. In summary, this investigated PEO705−PMAA476/DPCl system shows a rich structural variety that can be controlled in a systematic fashion by the mixing ratio characterized by Z and by the total concentration. This renders it interesting as a rather versatile system for purposes of solubilizing and delivering active agents with them.

■

ASSOCIATED CONTENT

S Supporting Information *

Further pieces of information on apparent volumes and scattering length densities, hard-sphere structure factor, model free analysis of SANS, data fits for nanoparticles and for pure DPCl micelles in D2O−buffer, and uncertainties on fitted parameters and SAXS data for dilution experiment. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. I


Macromolecules

Article

Notes

(25) Tararyshkin, D.; Kramarenko, E.; Khokhlov, A. R. J. Chem. Phys. 2007, 126, 164905. (26) Dubochet, J.; Adrian, M.; Chang, J. J.; Homo, J. C.; Lepault, J.; McDowall, A. W.; Schultz, P. Q. Rev. Biophys. 1988, 21, 129. (27) Berne, B. J.; Pecora, R. Dynamic Light Scattering: With Applications to Chemistry, Biology and Physics, reprinted ed.; Dover Publications: Mineola, NY, 2000. (28) Panine, P.; Finet, S.; Weiss, T. M.; Narayanan, T. Adv. Colloid Interface Sci. 2006, 127, 9. (29) Lewis, R. A.; Berry, A.; Hall, C. J.; Helsby, W. I.; Parker, B. T. Nucl. Instrum. Methods Phys. Res., Sect. A 2000, 454, 165. (30) http://www.diamond.ac.uk/Home/Beamlines/I22/tech/ detectors.html/. (31) http://www.esrf.eu/computing/scientific/FIT2D/. (32) Keiderling, U. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 1455. (33) Sedlák, M. Langmuir 1999, 15, 4045. (34) Sedlák, M.; Koňak, Č .; Štěpánek, P.; Jakeš, J. Polymer 1987, 28, 873. (35) Volk, N.; Vollmer, D.; Schmidt, M.; Oppermann, W.; Huber, K. Adv. Polym. Sci. 2004, 166, 29. (36) Groehn, F.; Antonietti, M. Macromolecules 2000, 33, 5938. (37) Matějíček, P.; Štěpánek, M.; Uchman, M.; Procházka, K.; Špírková, M. Collect. Czech. Chem. Commun. 2006, 71, 723. (38) Uchman, M.; Gradzielski, M.; Angelov, B.; Tošner, Z.; Procházka, K.; Štěpánek, M., to be published. (39) Burchard, W.; Kajiwara, K. Proc. R. Soc. London, A 1970, 316, 185. (40) Ashcroft, N. W.; Lekner, J. Phys. Rev. 1966, 145, 83. (41) Hansson, P. Langmuir 2001, 17, 4167. (42) Chen, S. H. Annu. Rev. Phys. Chem. 1986, 37, 51.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the financial support from the Ministry of Education of the Czech Republic (long-term Research Project No. MSM0021620857) and the Grant Agency of the Czech Republic (Grants P208/10/0353, P208/12/P236, P205/11/J043, and P106/12/0143) and German Academic Exchange Service DAAD (Grants 2B08021 and D0804221 PPP-CZ-09-10, PKZ: 50016729). This research project has been supported by the European Commission under the 7th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract: 226507 (NMI3). For allocation of beam time we are grateful to ILL (Grenoble, France), ESFR (Grenoble, France), and Diamond (Didcot, UK). B.A. acknowledges user support from the Diamond Light Source (Didcot, Oxfordshire, UK; Proposal SM3313, Beamline I22) and ESRF (Grenoble, France, Proposal SC3113, Beamline ID02) and thanks Drs. S. Filippov, P. Štěpánek, N. Terrill, J. Gummel, and T. Narayanan for cooperation and support.

■

REFERENCES

(1) Voets, I. K.; de Keizer, A.; Stuart, M. A. C. Adv. Colloid Interface Sci. 2009, 147−148, 300. (2) Langevin, D. Adv. Colloid Interface Sci. 2009, 147−148, 170. (3) Zhou, S.; Chu, B. Adv. Mater. 2000, 12, 545. (4) Li, Y.; Xia, J.; Dubin, P. L. Macromolecules 1994, 27, 7049. (5) Hoffmann, I.; Heunemann, P.; Prévost, S.; Schweins, R.; Wagner, N. J.; Gradzielski, M. Langmuir 2011, 27, 4386. (6) Štěpánek, M.; Matějı ́ček, P.; Procházka, K.; Filipov, S. K.; Angelov, B.; S̊louf, M.; Mountrichas, G.; Pispas, S. Langmuir 2011, 27, 5275. (7) Balomenou, I.; Bokias, G. Langmuir 2005, 21, 9038. (8) Mahendra, S. B.; Surekha, D. Adv. Colloid Interface Sci. 2006, 123−126, 387. (9) Pispas, S. Soft Matter 2011, 7, 474. (10) Pispas, S. Soft Matter 2011, 7, 8697. (11) Wang, Y.; Han, P.; Xu, H.; Wang, Z.; Zhang, X.; Kabanov, A. V. Langmuir 2009, 26, 709. (12) Kabanov, A. V.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941. (13) Bronich, T. K.; Popov, A. M.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2000, 16, 481. (14) Bronich, T. K.; Kabanov, A. V.; Kabanov, V. A.; Yu, K.; Eisenberg, A. Macromolecules 1997, 30, 3519. (15) Štěpánek, M.; Škvarla, J.; Uchman, M.; Procházka, K.; Angelov, B.; Kovácǐ k, L.; Garamus, V. M.; Mantzaridis, Ch.; Pispas, S. Soft Matter 2012, DOI: 10.1039/C2SM25588J. (16) Berret, J.-F.; Hervé, P.; Aguerre-Chariol, O.; Oberdisse, J. J. Phys. Chem. B 2003, 107, 8111. (17) Berret, J.-F.; Oberdisse, J. Physica B 2004, 350, 204. (18) Courtois, J.; Berret, J.-F. Langmuir 2010, 26, 11750. (19) Berret, J.-F.; Vigolo, B.; Eng, R.; Hervé, P.; Grillo, I.; Yang, L. Macromolecules 2004, 37, 4922. (20) Nizri, G.; Magdassi, S.; Schmidt, J.; Cohen, Y.; Talmon, Y. Langmuir 2004, 20, 4380. (21) Nizri, G.; Makarsky, A.; Magdassi, S.; Talmon, Y. Langmuir 2009, 25, 1980. (22) Kogej, K. J. Phys. Chem. B 2003, 107, 8003. (23) Trabelsi, S.; Guillot, S.; Raspaud, E.; Delsanti, M.; Langevin, D. Adv. Mater. 2006, 18, 2403. (24) Yeh, F.; Solokov, E. L.; Khokhlov, A. R.; Chu, B. J. Am. Chem. Soc. 1996, 118, 6615. J


Coassembly of Poly(ethylene oxide)-block-poly(methacrylic acid) and

Recommend Documents