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Macromolecules 2007, 40, 8476-8482
Electrostatically Driven Coassembly of a Diblock Copolymer and an Oppositely Charged Homopolymer in Aqueous Solutions Ilja K. Voets,*,† Stefan van der Burgh,† Bela Farago,‡ Remco Fokkink,† Davor Kovacevic,† Thomas Hellweg,§ Arie de Keizer,† and Martien A. Cohen Stuart† Laboratory of Physical Chemistry and Colloid Science, Wageningen UniVersity, Dreijenplein 6, 6703 HB Wageningen, The Netherlands, Institut Max Von Laue-Paul LangeVin, F-38042 Grenoble Cedex 9, France, and Physikalische Chemie I, UniVersity of Bayreuth, 95440 Bayreuth, Germany ReceiVed June 19, 2007; ReVised Manuscript ReceiVed August 22, 2007
ABSTRACT: Electrostatically driven coassembly of poly(acrylic acid)-block-poly(acrylamide), PAA-b-PAAm, and poly(2-methylvinylpyridinium iodide), P2MVP, leads to formation of micelles in aqueous solutions. Light scattering and small angle neutron scattering experiments have been performed to study the effect of concentration and length of the corona block (NPAAm ) 97, 208, and 417) on micellar characteristics. Small angle neutron scattering curves were analyzed by generalized indirect Fourier transformation and model fitting. All scattering curves could be well described with a combination of a form factor for polydisperse spheres in combination with a hard sphere structure factor for the highest concentrations. Micellar aggregation numbers, shape, and internal structure are relatively independent of concentration for Cp < 23.12 g L-1. The Guinier radius, average micellar radius, hydrodynamic radius, and polydispersity were found to increase with increasing NPAAm. Micellar mass and aggregation number were found to decrease with increasing NPAAm.
Introduction Micelles formed by the electrostatically driven assembly of oppositely charged components are relatively novel particles in the field of “self”-assembly. The resulting particles are termed complex coacervate core micelles (C3Ms), polyion complex (PIC) micelles, block ionomer complex (BIC) micelles, or interpolyelectrolyte complexes (IPEC). Strictly speaking, the correct term would be coassembly, as self-assembly excludes structures consisting of multiple components. In this study, we focus on C3Ms consisting of a neutral-ionic block copolymer and a polyelectrolyte with an opposite charge sign. A sketch of such a system is presented in Figure 1. The novelty of C3Ms lies in the fact that the separate components are hydrophilic, i.e., no micellization occurs in solutions of single components. Yet, when mixed under appropriate conditions (pH, ionic strength, mixing ratio), C3Ms may form. A number of publications on this type of assembly can be found in the literature. The radius of the micelles is generally of the order of several tens of nanometers and electrophoretic mobility measurements indicate that the micelles carry no excess charge. An important driving force for aggregation is the entropy gain associated with the release of counterions from the polyelectrolyte double layers. Hence, C3Ms dissociate above a critical ionic strength when charges are highly screened. The micelles form in a rather small compositional window around the so-called preferred micellar composition, PMC, which corresponds to a mixing ratio of 1:1 as expressed in chargeable monomers for equal charge densities of the polyelectrolyte blocks. The following speciation as a function of mixing ratio has been proposed.1 When the composition of the system is exactly at the PMC, the system exclusively forms micelles. When the composition is chosen somewhat away from * Corresponding author. E-mail:
[email protected]. † Wageningen University. ‡ Institut Max von Laue-Paul Langevin. § University of Bayreuth.
Figure 1. Complex coacervate core micelle. The micellar core consists of the oppositely charged polyelectrolyte blocks PAA and P2MVP, whereas the corona consists of neutral PAAm blocks. Both core and corona are highly water-swollen.
the PMC, coexistence between soluble complex particles (i.e., small, soluble complexes consisting of a few polymers) and micelles occurs. Further away from the PMC, the micelles disappear altogether and soluble complex particles coexist with free polymer molecules. This paper describes light and small angle neutron scattering experiments on C3Ms consisting of poly(acrylic acid)-blockpoly(acrylamide), PAA-b-PAAm, and poly(2-methylvinylpyridinium iodide), P2MVP, intended to study the effect of concentration and polymerization degree of the corona block (NPAAm ) 97, 208, and 417) on micellar characteristics, such as shape, mass, aggregation number, radius of gyration, and internal structure. To the best of our knowledge, this is the first SANS study on micelles consisting of a neutral-ionic diblock copolymer and an oppositely charged homopolymer. Closely
10.1021/ma071356z CCC: $37.00 © 2007 American Chemical Society Published on Web 10/18/2007
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Coassembly of a Diblock Copolymer 8477
Figure 2. Chemical structure of (a) poly(acrylic acid)-block-poly(acrylamide), PAA42-b-PAAmN (NPAAm ) 97, 208, and 417) and (b) poly(2-methylvinylpyridinium iodide), P2MVP209. The numbers beside the brackets denote the degree of polymerization.
related are SANS studies on mixed micelles consisting of a neutral-ionic block copolymer and a multivalent ion2,3 or oppositely charged surfactant micelles.4,5 Berret et al. have published several articles on such systems, incorporating the same PAA-b-PAAm diblock copolymers as in this study.3-5 For mixed polymer/surfactant micelles, it was found that the aggregation number expressed as the number of diblock copolymers per micelle could be as high as 100-250 and that the surfactant micelles keep their micellar structure within the larger structure. The core thus consists of an ensemble of spherical surfactant micelles that are interconnected by the poly(acrylic acid) blocks from the diblock copolymer. The typical distance between the neighboring surfactant micelles showed up as a structure peak at high q values. C3Ms are expected to have several potential applications. In solution, the micelles can be used for encapsulation, protection, stabilization, and controlled release of virtually any charged species, which may prove to be advantageous in drug delivery, laundry, nanoparticle formation,6 and food-stuff applications. Substrates, such as silica and polyelectrolyte multilayers, may be rendered antifouling after exposure to C3Ms.7,8
and P2MVP solutions of 5 g L-1. In the LS-T experiments, PAAb-PAAm solutions were titrated with a concentrated solution of P2MVP to minimize dilution effects. The LS-T experiments were performed to determine the PMC, which is assumed to be independent of concentration. Small Angle Neutron Scattering (SANS). Small-angle neutron scattering experiments were performed at the Institut Max von Laue-Paul Langevin (ILL), Grenoble, France, on the D22 beam line. Two detector distances were chosen, such that a q-range of 0.0029-0.137 Å-1 was covered, with an incident wavelength of 0.8 nm and a wave-vector resolution ∆q/q of 10%. The spectra were treated according to standard ILL procedures, and the scattering cross sections are expressed in cm-1. The temperature was kept constant at 293 K. Micellar solutions have been prepared at the preferred micellar composition, PMC, corresponding to f+ ) 0.50 as determined from the LS-T measurements, at concentrations 1.7-38.24 g L-1 in pure D2O for contrast reasons. The q-dependence of the scattered intensity can be described according to the general equations11,12 I(q) ) npart(Fpart - Fsolv)2Vpart2P(q)S(q)
(2)
∫ p(r)
(3)
and I(q) ) 4π
q)
f+ )
[n+ ] [n+ ] + [n- ]
(1)
The stock solutions were diluted with a NaNO3 solution of equal ionic strength to obtain PAA-b-PAAm solutions of 0.5-1 g L-1
sin(qr) dr qr
with the particle number density, npart/cm-3, the particle coherent scattering length density, Fpart/cm-2, the solvent coherent scattering length density, Fsolv/cm-2, the particle volume, Vpart/cm3, the form factor, P(q), the structure factor, S(q), the pair distance distribution function, p(r)/cm-2, and the magnitude of the scattering vector, q/cm-1, defined as follows
Experimental Part Materials. The diblock copolymers poly(acrylic acid)-blockpoly(acrylamide), PAA42-b-PAAm97, PAA42-b-PAAm208, and PAA42b-PAAm417, were a kind gift from Rhodia Chimie, Aubervilliers, France. (The subscripts correspond to the degree of polymerization.) They have been synthesized according to the MADIX process, resulting in an estimated polydispersity index (PDI) e 1.3.9 The oppositely charged homopolymer poly(2-methylvinylpyridinium iodide), P2MVP209 (Mw ) 56 000 g mol-1, degree of quaternization ∼70%, PDI ) 1.09) has been purchased from Polymer Source Inc., Canada. Chemical structures are depicted in Figure 2. Stock solutions of P2MVP (38 g L-1) and PAA-b-PAAm (2538 g L-1) were prepared in Milli-Q water (light scattering-titration) or D2O (99.9% isotopic purity, Isotec Inc., Miamisburg, OH) to which NaNO3 (J. T. Baker Chemicals, Deventer, The Netherlands) was added to obtain a final concentration of 50 mM. The pH of the stock solutions was adjusted with 1 M NaOH/HNO3 (Merck, Darmstadt, Germany) or NaOD/DNO3 to obtain pH ) 7 for both solutions. All polymers and other chemicals were used as received, without further purification. Light Scattering-Titrations (LS-T). Details of the experimental setup and data analysis have been reported previously.10 Results are typically given as pH, total light scattering intensity, I90°, and hydrodynamic radius, Rh, 90°, at a scattering angle of 90° as a function of the mixing fraction, f+. The mixing fraction is defined as the ratio between the number of positively chargeable monomers (i.e., quaternized and nonquaternized monomers) and the sum of the numbers of positively and negatively chargeable monomers, i.e.,
∞
0
4π θ sin λ 2
()
(4)
with the wavelength of the incident radiation, λ, and the angle between the scattered and incident beam, θ. Hence, by indirect Fourier transformation of eq 3, one obtains the pair distance distribution function. In this study, p(r) functions were computed using generalized indirect Fourier transformation employing the GIFT software package.13-15 A hard sphere structure factor (Percus-Yevick closure,16 averaged structure factor17) has been included to describe the scattering curves of the more concentrated samples. Alternatively, I(q) may be modeled by selecting a particle form and structure factor fit to describe particle shape and interaction as present in the studied system. In this paper, we applied a hard sphere structure factor (Percus-Yevick closure, decoupling approximation using an average particle radius) and a form factor for homogeneous spheres. In the latter, size polydispersity was included via a Gaussian (NPAAm ) 97 and 208) and Schulz-Zimm (NPAAm ) 417) size distribution, f(R,〈R〉) with the polydispersity index, preal, and the average particle volume, 〈Vpart(R)〉 (see Supporting Information). The forward scattering intensity at q ) 0, I0, can be used to obtain the particle mass, Mpart/g mol-1, according to I0 )
MpartCpart(Fbb - Fsolv)2V02 NA
(5)
with the particle weight concentration, Cpart/g cm-3, the building block (see below for definition) coherent scattering length density, Fbb/cm-2, the building block specific volume, V0/cm3 g-1, and Avogadro’s number, NA/mol-1. The number of PAA42-b-PAAmN polymers, Pagg, per particle can now easily be obtained by division of the particle molar mass, Mpart, by the building block molar mass, Mbb.
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Macromolecules, Vol. 40, No. 23, 2007 V0 )
φbb,partVpartNA Mpart
(7)
For the C3Ms in this study, we define a building block as a unit consisting of one diblock copolymer and a corresponding amount of P2MVP groups, i.e., f+ ) 0.5. A building block is present Pagg times in one particle. The building block for NPAAm ) 97 equals PAA42-b-PAAm97 + P2MVP42, for NPAAm ) 208 it is PAA42-bPAAm208 + P2MVP42, and for NPAAm ) 417 a building block consists of PAA42-b-PAAm417 + P2MVP42. An overview of the coherent scattering length densities, F, specific volume, V0, and molecular weights, Mw, of the studied species are given in Table S1 (see Supporting Information).
Results and Discussion
Figure 3. Results of a light scattering titration experiment: (a) light scattering intensity, I90°, (b) hydrodynamic radius, Rh,90°, and (c) pH as a function of f+. A relatively concentrated P2MVP209 solution (∼8 g L-1) was titrated with a buret into a dilute PAA42-b-PAAm97 solution (∼0.5 g L-1) in the scattering cell. The initial pH values of the solutions were matched at pH ) 7, as was the ionic strength (50 mM NaNO3). The preferred micellar composition, PMC, was determined as the f+ corresponding to a maximum in I90° and |dpH/df+|. The PMC was assumed to be independent of concentration.
Alternatively, I0 can also be expressed in the following manner I0 )
NAnbb 2 φ (F - Fsolv)2Vpart2 Pagg bb,part bb
(6)
with the building block number density, nbb/mol cm-3, and the building block volume fraction in the particle, φbb, part. In this way, the φbb, part can be obtained by combination of eqs 5 and 6, which is equivalent to stating that
Light Scattering-Titrations (LS-T). Figure 3 shows the results of a light scattering titration where PAA42-b-PAAM97 was titrated with P2MVP209. In analogy to our previous paper,1 the PMC was found at the maximum in scattered intensity and the maximum in |dpH/df+|; i.e., the PMC is at f+ ) 0.5. Similar experiments were performed for PAA42-b-PAAm208 and PAA42-b-PAAm417 (data not shown), and the PMC was always found at f+ ) 0.5. Hydrodynamic radii, Rh,90°, at the PMC are 14.5, 20.2, and 20.4 nm for NPAAm ) 97, 208, and 417, respectively. Small Angle Neutron Scattering (SANS). The SANS scattering curves as obtained after data reduction, subtraction of incoherent scattering (both from solvent and hydrogenated polymer units), and division by concentration are presented in Figure 4. The polymer contribution to the incoherent scattering scales linearly with concentration for a given NPAAm (Table 3). Some of the samples exhibit upturns in the first 4-5 points of the scattering curves, which are caused by the presence of a small fraction of larger aggregates, which may be clusters of micelles. All scattering curves appear rather smooth, i.e., distinct features such as form factor minima are absent, which indicates a rather high polydispersity in the systems.
Figure 4. I(q)/Cp (cm-1 L g-1) versus q/Å-1 for C3Ms of P2MVP209 and (a) PAA42-b-PAAm97 (2.32 g L-1 e Cp e 38.24 g L-1), (b) PAA42-bPAAm208 (2.17 g L-1 e Cp e 35.12 g L-1), and (c) PAA42-b-PAAm417 (1.70 g L-1 e Cp e 27.53 g L-1). Scattering curves have been corrected for incoherent scattering, due to solvent and hydrogenated polymer segments, and divided by Cp.
Macromolecules, Vol. 40, No. 23, 2007
Coassembly of a Diblock Copolymer 8479 Table 2. Comparison of GIFT Analysis, Guinier Analysis, and Model Fitting for C3Ms of PAA42-b-PAAm97 and P2MVP209 Cp/g L-1
Rga/nm
Rg/Rh
Rgub/nm
Rgu/Rh
〈R〉c/nm
〈R〉/Rh
2.32 4.63 9.54 15.16 25.0 38.24
8.16 8.38 7.90 7.82 8.02 8.71
0.56 0.58 0.55 0.54 0.55 0.60
7.52 7.88
0.52 0.54
9.8 9.8 9.8 9.8 9.8 9.6
0.68 0.68 0.68 0.68 0.68 0.66
a Obtained from GIFT analysis. b Obtained from Guinier analysis. Obtained from model fitting as described below. (We estimate the uncertainties in Rg, 〈R〉, Rgu, and Rh to be in the order of 10%).
c
Figure 5. Guinier representations, ln I(q) versus q2/Å-2 for C3Ms of P2MVP209 and PAA42-b-PAAm97 (O, Cp ) 2.32 g L-1; 0, Cp ) 4.63 g L-1), P2MVP209 and PAA42-b-PAAm208 (], Cp ) 2.17 g L-1; 4, Cp ) 4.37 g L-1), and P2MVP209 and PAA42-b-PAAm97 (+, Cp ) 1.70 g L-1; ×, Cp ) 3.41 g L-1). Used q-range is 0.005 < q < 0.01 Å-1. The first four to five points were discarded as they lack statistics and/or exhibit upturns resulting from a small fraction of aggregates in the system. Table 1. Guinier Analysis for C3Ms of P2MVP209 and PAA42-b-PAAmNa NPAAm
Cp/g L-1
Rgu/nm
R/nm
I0/cm-1
Rgu/Rh
97 97 208 208 417 417
2.32 4.63 2.17 4.37 1.7 3.41
7.52 7.88 10.18 10.23 13.83 13.62
9.70 10.17 13.14 13.20 17.85 17.58
1.84 3.56 1.46 3.00 1.54 2.29
0.52 0.54 0.50 0.51 0.68 0.67
aN -1 and 4.63 g L-1. N PAAm ) 97: Cp ) 2.32 g L PAAm ) 208: Cp ) 2.17 g L-1 and 4.37 g L-1. NPAAm ) 417: Cp ) 1.70 g L-1 and 3.41 g L-1. Guinier representations are given in Figure 5. The hard sphere radius, R, has been calculated from the Guinier radius, Rgu, according to R ) x5 / 3Rgu. Rgu/Rh has been calculated using the Rh as determined from the LS-T, i.e., Rh has been assumed to be independent of concentration. We estimate the uncertainties in R, Rgu, Rh, and I0 to be in the order of 10%.
Clearly, for constant NPAAm, scattering curves of different Cp superimpose in the high q-regime (q > 0.02 Å-1) after division by Cp, indicating that concentration hardly affects micellar shape on these length scales (
