Organic−Inorganic Nanoassembly Based on Complexation of Cationic

Feb 5, 2004 - Organic−Inorganic Nanoassembly Based on Complexation of Cationic Silica Nanoparticles and Weak Anionic Polyelectrolytes in Aqueous and...
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Organic-Inorganic Nanoassembly Based on Complexation of Cationic Silica Nanoparticles and Weak Anionic Polyelectrolytes in Aqueous and Alcohol Media Hideharu Mori,* Michael G. Lanzendo¨rfer,† and Axel H. E. Mu¨ller Lehrstuhl fu¨ r Makromolekulare Chemie II and Bayreuther Zentrum fu¨ r Kolloide und Grenzfla¨ chen, Universita¨ t Bayreuth, D-95440 Bayreuth, Germany

Joachim E. Klee Dentsply DeTrey GmbH, De-Trey-Straβe 1, D-78467 Konstanz, Germany Received September 24, 2003. In Final Form: December 15, 2003 Complexation of tertiary amine containing silica nanoparticles (diameter ≈ 3.0 nm) and a weak acidic polyelectrolyte, poly(acrylic acid), which has a characteristic pH-responsive property, was investigated under various environments. The salt- and temperature-induced association/transformation/dissociation behavior was studied in aqueous medium by following the turbidity as a function of time. Complex formation was observed in methanol, whereas no complexation was obtained in DMF. The size and density of the aggregate structures formed in methanol could be controlled by the temperature, as confirmed by TEM measurements. Characterization of the isolated complexes was conducted by elemental analysis, 1H NMR, and FT-IR measurements, suggesting that the silica composition in the complex isolated from water is the same as that obtained from methanol. On the contrary, the kind of solvent has a significant influence on the interaction between the components and the resulting aggregated structures. MALDI-TOF MS was used for the determination of the molecular weights and molecular weight distribution of the silica nanoparticles, which give useful information on the stoichiometry of the complex systems. Morphological and ionization changes of the complexes in response to the environmental changes are discussed in view of the interactions between the two constituents, stoichiometry, and sizes of the cationic particle and the anionic polyelectrolyte.

Introduction Nanostructured organic-inorganic hybrid materials with tunable properties and well-defined multidimensional architectures have progressively become important, because of their possible applications in optics, electronics, engineering, and biosciences. Nanometer-size inorganic nanoparticles have unique properties deriving from quantum confinement effects and from their large surface areas relative to their volumes. The study of hybrid structures with nanometer dimensions and their use in fabrication of nanoscale devices are of great interest, because the properties of a small object derive from size effects, structure variations, and interactions with a substrate, molecules, or other small objects on different length scales. Further advances of such organic-inorganic nanocomposite materials require fine-tuning of the sizes, topologies, and spatial assembly of individual constitutes and their interfaces.1 The self-assembly of nanoparticles into useful shapes and sizes is a center of developments in nanotechnology as it offers the promise of creating materials from well-characterized, nanometer-scale constituents with interesting properties. Specific intermolecular interactions, such as hydrogen bonding, acid-base interactions, and oppositely charged ionic interactions, to prepare self-organized materials have attracted much interest in academic researches as well as in important technologies, such as microelectronics and * To whom correspondence should be addressed. E-mail: [email protected]. † Dedicated to the memory of Dr. Michael G. Lanzendo ¨ rfer. (1) Sanchez, C.; de Soler-Illia, G. J.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061-3083.

biotechnology. A variety of self-assembled systems have been reported, including polymer-polymer,2-8 colloidcolloid,9-12 and polymer-colloid.13-16 For example, Boal et al. have recently demonstrated the self-assembly of colloidal gold nanoparticles (∼ 2 nm) by a “bricks and mortar” approach, in which complementarity between colloid and polymer was achieved using the diaminotriazine-thymine three-point hydrogen-bonding interaction.13 The interaction of charged polymers with oppositely charged colloids is found in many areas of science involving (2) Mun, G. A.; Nurkeeva, Z. S.; Khutorvanskiy, V. V.; Sergaziyev, A. D. Colloid Polym. Sci. 2002, 280, 282-289. (3) Dan, Y.; Chen, S.; Zhang, Y.; Xiang, F. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 1069-1077. (4) Kaczmarek, H.; Szalla, A.; Kaminska, A. Polymer 2001, 42, 60576069. (5) Beyer, P.; Nordmeier, E. Eur. Polym. J. 1999, 35, 1351-1365. (6) Nurkeeva, Z. S.; Mun, G. A.; Khutoryanskiy, V. V.; Zotov, A. A.; Mangazbaeva, R. A. Polymer 2000, 41, 7647-7651. (7) Zhumadilova, G. T.; Gazizov, A. D.; Bimendina, L. A.; Kudaibergenov, S. E. Polymer 2001, 42, 2985-2989. (8) Gazizov, A. D.; Zhumadilova, G. T.; Bimendina, L. A.; Kudaibergenov, S. E. Polymer 2000, 41, 5793-5797. (9) Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 7957-7958. (10) Galow, T. H.; Boal, A. K.; Rotello, V. M. Adv. Mater. (Weinheim, Ger.) 2000, 12, 576-579. (11) Pham, T.; Jackson, J. B.; Halas, N. J.; Lee, T. R. Langmuir 2002, 18, 4915-4920. (12) Cao, Y.; Jin, R.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 7961-7962. (13) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature (London) 2000, 404, 746-748. (14) Frankamp, B. L.; Uzun, O.; Ilhan, F.; Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2002, 124, 892-893. (15) Hara, M.; Lean, J. T.; Mallouk, T. E. Chem. Mater. 2001, 13, 4668-4675. (16) Tamaki, R.; Chujo, Y. Chem. Mater. 1999, 11, 1719-1726.

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Figure 1. (a) Synthetic route for the cationic silica nanoparticles. (b) TEM and (c) SFM height images of the silica nanoparticles. (d) Postulated mechanism of the reversible pH-induced association and dissociation behavior of the silica nanoparticle with PAA through ionic complexation and hydrogen-bonded complexation.22

biochemistry, biology, and environmental chemistry, and represents important systems in paper and pulp industry. Polyelectrolytes plus colloids are found in our bodies and the issue of complexation of DNA with positively charged liposomes or proteins is expected to find important applications in gene therapy and genetic regulation. In the field of organic-inorganic nanocomposite materials, a general classification has been proposed;17 “class I materials”, which correspond to all systems where the inorganic and organic components interact only weakly through hydrogen bonding, van der Waals contacts, or electrostatic forces, and “class II materials”, in which the constituents are more strongly linked through ionic/ (17) Sanchez, C.; Ribot, F. New J. Chem. 1994, 18, 1007-1047.

covalent bond formation. As to silica-based hybrids,18 the method most commonly used for the preparation is via hydrogen bonding between the polar functional group of the polymer and the residual silanol group of silica. Only a few reports have been published with regard to the utilization of ionic interaction to synthesize silica-based hybrids. For example, Tamaki and Chujo16 synthesized nanoscale homogeneous silica/polymer hybrids using sulfonated polystyrene as the organic polymer and silica modified with (3-aminopropyl)trimethoxysilane that forms the countercation. Since the strength of ionic interaction is much higher than that of hydrogen bonding, it should (18) Gao, Y.; Choudhury, N. R.; Dutta, N.; Matisons, J.; Reading, M.; Delmotte, L. Chem. Mater. 2001, 13, 3644-3652.

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provide a better degree of homogeneity and order in the final hybrids. So-called “intelligent” or “smart” materials that can sense signals and produce a definite dynamic response in the form of a change in shape, size, or structure are another central to developments in various scientific fields, such as actuators, shape-memory, and drug-delivery systems.19-21 Recently, we developed novel intelligent colloidal polymer/silica nanocomposites, in which the complexation of cationic silica nanoparticles and a weak anionic polyelectrolyte can be manipulated simply by pH change in aqueous medium through hydrogen-bonded interaction and ionic complexation caused by hydrogentransfer interactions between the constituents (Figure 1).22 To provide an effective route for the controlled self-ordering of nanoparticles with polymers and for the achievement of characteristic stimuli-responsive properties in aqueous medium, we developed special silica nanoparticles (diameter ≈ 3.0 nm) which have two independent protonaccepting sites, oxygen or nitrogen atoms. Because of the tiny size and high functionality, the silica particles can be uniformly dispersed in water and behave as single dissolved molecules to form a transparent colloidal solution. Poly(acrylic acid), PAA, was selected as a weak polyelectrolyte because the degree of ionization of carboxylic acids can be easily controlled by the pH value. In this system, both PAA and the silica nanoparticles formed visually transparent solutions in water, while a white turbid dispersion was obtained just after mixing the two solutions at room temperature. The complex formation in water was strongly affected by the pH value, and the pHinduced association-dissociation behavior was a reversible and rapid process. The reversible pH-induced colloid formation due to the complexation of the inorganicorganic nanomaterials can provide a viable route to the production of tailored materials with unique properties for various applications. To manipulate the pH-induced complexation and build up structured hybrid nanomaterials, knowledge of the influence of a large number of parameters is required, including molecular weight, concentration, molecular structure, charge density of polyelectrolyte; particle size, surface chemistry of the silica particles; solvent, pH, temperature, composition of two components, size and nature of additives (e.g., salt). Depending on these parameters, in general polymers can act as spacers to keep surfaces apart from one another (steric stabilization) or as a glue to hold the particles together (via ionic interaction or hydrogen bonding). The understanding of the relationship between the characteristic stimuliresponsive properties, specific interactions, and resulting assembled structures under various environments will allow us to develop new intelligent organic-inorganic hybrid nanomaterials as well as three-dimensional structured systems caused by the complexation. In this study, we investigated the complexation of the basic silica nanoparticles and PAA forming a weak anionic polyelectrolyte under various environments. The main experimental variables were salt concentration, temperature, nature of solvent, and composition of two components. (19) Petka, W. A.; Hardin, J. L.; McGrath, K. P.; Wirtz, D.; Tirrell, D. A. Science (Washington, D. C.) 1998, 281, 389-392. (20) Osada, Y.; Gong, J.-P. Adv. Mater. (Weinheim, Ger.) 1998, 10, 827-837. (21) Hiller, J. A.; Rubner, M. F. Macromolecules 2003, 36, 40784083. (22) Mori, H.; Mu¨ller, A. H. E.; Klee, J. E. J. Am. Chem. Soc. 2003, 125, 3712-3713.

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Experimental Section Materials. Preparation of special silica nanoparticles was conducted by the addition reaction of (aminopropyl)triethoxysilane and glycidol, followed by acidic condensation of the addition product, as shown in Figure 1a. A detailed description of the synthesis procedure can be found in an earlier communication.22 After the solvents (methanol and ethanol) were evaporated in a vacuum, the product was obtained as a glassy solid at room temperature. It was changed easily into a highly viscous transparent material by heating, for example, at 60 °C. Nanometer size (diameter ≈ 3.0 nm) could be achieved by a careful choice of the organic structure and condensation condition. In addition to tertiary amino groups, the resulting silica nanoparticle should have hydroxy groups on the outermost surface, which lead to water-soluble property. The resulting silica nanoparticle is soluble directly in water, methanol, DMF, and DMSO, while insoluble in most organic solvents, such as dichloromethane, acetone, and dioxane and so forth. Particle size was measured using scanning force microscopy (SFM) and transmission electron microscopy (TEM), as shown in Figure 1b and c. Linear poly(acrylic acid) (PAA) was obtained by atom transfer radical polymerization of tert-butyl acrylate with CuBr/N,N,N′,N′′,N′′-pentamethyldiethylenetriamine, followed by hydrolysis with an excess of trifluoroacetic acid.23 Mn ) 7700 (DP ) 107); Mw/Mn ) 1.15 (calculated from the molecular weights of poly(tert-butyl acrylate) precursor). Poly(tert-butyl acrylate) (Mn ) 13700, Mw/Mn ) 1.15, as determined by GPC using linear poly(tert-butyl acrylate) standards) before hydrolysis was also used for a control experiment. Complex Formation. The complex formation in water was conducted by mixing of the tertiary amine containing silica nanoparticles and PAA solutions under the same concentration ([silica] ) [PAA] ) 0.2 mg/mL) at room temperature. The mixed solution became turbid immediately, whereas both PAA and the silica solutions in water were visually transparent. As described in an earlier communication,22 the PAA/silica complexes in water indicated three different transition points at pH ) 2.3-2.5 (from transparent to white turbid), pH ) 5.4-5.7 (to semi-transparent), and pH ) 8.2-8.5 (to transparent again). As can be seen in Figure 1d, at pH ) 2.5-5.3, the complex is governed mainly by ionic complexation caused by proton-transfer interactions between the constituents. In the very acidic region (pH < 2.3), the nitrogen atoms are protonated by HCl instead of the carboxylic acid in PAA, leading to the dissociation of the ionic complex. In the intermediate pH region (pH ) 5.8-8.0), PAA is partially ionized and hydrogen bonds between the carboxylic acid and the hydroxy group are predominant. The third transition point (pH ) 8.28.5) corresponds to the complete ionization point of PAA, resulting in the breaking of the hydrogen bonding. Such transformations were observed reversibly and repeatedly, suggesting that one can cycle back and forth between the three different regimes by changing the pH value. The reversible pH-induced associationdissociation behavior was confirmed by turbidity and potentiometric titration measurements.22 In the present study, all experiments in aqueous medium were conducted at pH ≈ 3.5, at where the complex is governed mainly by ionic complexation caused by proton-transfer interactions between the constituents. The pH value of the dispersion was adjusted by adding the appropriate amount of an aqueous solution of HCl. The white turbid dispersion of the silica/PAA complex in water was allowed to stand for a predetermined time at different temperatures. The kinetic experiments were done by following the turbidity as a function of time at different conditions. The turbidity measurements were taken at room temperature. The salt concentration was also adjusted by adding NaCl to the white turbid dispersion. For the investigation of the influence of the salt concentration, each sample was prepared independently to avoid the effect of the time-dependent change in the turbidity. After the turbid dispersion of the complex with different amounts of the salt was allowed to stand for 5 min at room temperature with moderate shaking, the turbidity measurement was conducted. In all cases, the procedure was done within 30 min. (23) Mori, H.; Chan Seng, D.; Lechner, H.; Zhang, M.; Mu¨ller, A. H. E. Macromolecules 2002, 35, 9270-9281.

Organic-Inorganic Nanoassembly The complex formation in methanol was also conducted by mixing of the cationic silica nanoparticles and PAA solutions at room temperature. For the preparation of the isolated sample, the complex formation was conducted under higher concentration ([silica] ) [PAA] ) 7.5 mg/mL).A representative example is as follows: To 4 mL of methanol solution of the silica nanoparticles (60 mg), 4 mL of methanol solution of PAA (60 mg) was added at room temperature. The mixed solution became turbid immediately, and the complexes were precipitated from the methanol solution. The product was isolated via decantation and then washed and dried in vacuo at room temperature overnight to give a white solid (59.2 mg, yield ) 49%). The product was characterized by FT-IR, 1H NMR in DMSO-d6, and elemental analysis. In the preparation of the isolated sample from water, the white turbid dispersion was obtained without precipitation at pH ≈ 3.5 under the same concentration ([silica] ) [PAA] ) 7.5 mg/ mL). The product was isolated via centrifugation (4000 rpm, 20 min, 20 °C) and dried in vacuo at room temperature overnight (yield ) 68%). Instrumentation. 1H NMR spectra were recorded with a Bruker AC-250 spectrometer. FT-IR spectra were recorded on a Bruker Equinox 55 spectrometer. Spectra of the isolated complexes were recorded from KBr pellets, prepared by mixing the complexes with KBr. The sample of the silica nanoparticles was obtained by casting a methanol solution on NaCl plates. The elemental analyses were performed by Ilse Beetz Mikroanalytisches Laboratorium (Kulmbach). The pH value was measured using a Schott CG840 pH meter equipped with a glass electrode. The reference electrode was calibrated with buffer solutions of pH 4, 7, and 10 prior to pH measurements. The turbidity measurement was conducted using a PerkinElmer Lambda 15 UV/VIS spectrophotometer at 450 nm. Bright field transmission electron microscopy (TEM) was performed using a Zeiss electron microscope (CEM 902) operated at 80 kV. The samples for TEM observation were prepared by applying a drop of a diluted MeOH solution (10 mg/L) on carbon-coated Cu grids and allowed to dry in air. MALDI-TOF mass spectroscopic analysis was performed on a Bruker Reflex III equipped with a 337-nm N2 laser in the reflector mode and 20 kV acceleration voltage. To find a suitable condition, various matrixes, such as 5-chlorosalicylic acid (Aldrich 98%), 2,5-dihydroxybenzoic acid (Aldrich 99%), R-cyano-4-hydroxy cinnamic acid (Sigma), 2-(4hydroxyphenylazo)benzoic acid (Fluka > 99.5%), trans-3-(3indolyl)acrylic acid (Aldrich 99%), hydroxypicolinic acid (98%), picolinic acid (Aldrich 99%), and sinapic acid (Fluka > 99%) were screened in the presence and absence of a salt. Sodium trifluoroacetate (NaTFA, Fluka, 99.5%) was used as a salt for ion formation. However, the samples without salt showed better signals in all cases. Samples were prepared from methanol solution by mixing matrix (20 mg/mL) and the silica (10 mg/mL) in a ratio of 10:1. The number-average molecular weights, Mn, of the sample was determined in the linear mode.

Results and Discussion Effects of Temperature and Salt on Complexation in Water. The temperature- and salt-induced association/ transformation/dissociation behavior of the complexes was investigated in aqueous medium. In our system, simple mixing of the silica nanoparticles and PAA in water ([silica] ) [PAA] ) 0.2 mg/mL) led to a milky white dispersion at pH ≈ 3.5, because of the complexation, with no evidence of any microscopic precipitation, as can be seen in Figure 2a. Measuring the solution turbidity as a function of time is often used for kinetic studies on various systems, such as formation/transformation/dissociation of micelles24-26 and ATP hydrolysis.27 The time-dependent changes in the turbidity of the colloidal dispersion at different temperatures are plotted in Figure 3. The turbidity of the white (24) Burke, S. E.; Eisenberg, A. Langmuir 2001, 17, 6705-6714. (25) Burke, S. E.; Eisenberg, A. Polymer 2001, 42, 9111-9120. (26) Robinson, B. H.; Bucak, S.; Fontana, A. Langmuir 2000, 16, 8231-8237. (27) Jiang, W.; Hackney, D. D. J. Biol. Chem. 1997, 272, 5616-5621.

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Figure 2. Photographic demonstration of the silica/PAA complexes ([silica] ) [PAA] ) 0.2 mg/mL) in water at pH ≈ 3.5 (a, b) and in methanol (c, d) at different temperatures; (a, c): 23 °C, (b, d): 55 °C.

Figure 3. Time dependence of turbidity of the silica/PAA complexes in water at pH ≈ 3.5 ([silica] ) [PAA] ) 0.2 mg/mL) at different temperatures: 23 °C (O), 40 °C (0), and 55 °C (4).

dispersion decreased gradually at 23 °C, and the aqueous dispersion (pH ≈ 3.5) changed finally into a transparent, clear solution (pH ≈ 4.5) after 3-4 days. This is an indication that the complex system in water includes an equilibrium between associated complexes (white turbid dispersion, which may be due to network structures and can be called insoluble complexes) and dissociated states (or soluble complexes). The addition of HCl to the resulting transparent solution led to no significant difference in the appearance even at pH ) 3.0-3.5, where the complex was formed predominantly via ionic complexation caused by proton-transfer interactions. These results suggest that the equilibrium shifts slowly and irreversibility to a transparent solution, but the breaking of the ionic complexation is not responsible for the time-dependent change in the turbidity. In other words, a simple association-dissociation behavior based on the ionization change is inapplicable to understand the phenomenon, and it is preferable to consider another interpretation, such as a morphological rearrangement from a network structure to a nonnetwork one, which will be discussed later. Because self-assembly processes are generally governed by a balance of entropic and enthalpic effects, the complex formation should be affected by the temperature. In our system, the temperature has a significant influence on the equilibrium in water. As can be seen in Figure 2b, the white turbid dispersion obtained at 23 °C remained constant without significant change at higher temperature (55 °C) within ca. 30 min. Then, the turbidity decreased drastically and the value of the transmittance reached to more than 90% after 6 h (Figure 3). The increase of the temperature apparently led to the rapid change in the turbidity. Kinetic equilibrium is followed by monitoring the change in the turbidity, ln(∆T), as a function of time, which should be related to the dissociation or transforma-

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Figure 5. Schematic representation of the morphological change of the complex obtained from the cationic silica nanoparticles (diameter ≈ 3.0 nm) with long flexible PAAs (DP ) 107); (N) network state: a extended polymer chain is interacted with two or more nanoparticles, (I) intermediate state: a PAA chain is bound mainly by a silica nanoparticle, but not completely, and (F) final state: a particle is wrapped by a single chain. Monomer units in PAA are in red and the silica nanoparticles are in blue.

Figure 4. Plots of logarithm of change in turbidity, ln(∆T), over time for the silica/PAA complexes in water at different temperatures. Data are derived from Figure 3.

tion mechanisms of the complexes. As shown in Figure 4, nonlinear relations are seen in all cases, and the characteristic curvatures are essentially temperature independent. Although much is known about the self-assembly systems of polymer-polymer, colloid-colloid, and polymer-colloid complexes, only a few studies have reported the kinetics and mechanism of the association-dissociation behavior. In the cases of interpolymer complex of synthetic polymers, frequently insoluble complexes are formed, but also soluble complexes are observed. Beyer and Nordmeier investigated the complexation of polyions, PAA and poly(methacrylic acid), with the polycation ionene, poly((dimethylimino)ethylene(dimethylimino)methylene-1,4-phenylenemethylene dichloride).5 They demonstrated that the complexes precipitate directly after mixing the aqueous solutions because of the formation of an insoluble network, but the network rearranges and the solution becomes clear after a certain time. The complex stability depends on many factors, such as the kind of the interaction force, dissociation state of the complex, stoichiometry, conformation, and environmental conditions. To explain the time-dependent change in the turbidity, we propose the following model involving three different states; network state (N), intermediate state (I), and final state (F), as can be seen in Figure 5. The first state is a polyelectrolyte-silica network (N), in which one PAA chain

is bound with two or more silica nanoparticles. Since PAA acts as a cross-linking agent in this state, the solution should be turbid. In the intermediate state (I), the PAA chain is bound mainly by one silica nanoparticle, but not completely. Some PAA segments remain unbound, which can build a loop, whereas some PAA chains still act as a cross-linking agent by interacting with a few silica nanoparticles. In the final state (F), the PAA chain is bound completely to one silica nanoparticle, and the solution becomes clear because of the absence of a network structure. In the final state, there are no more free PAA segments, leading to a slight increase of the pH value. Possible driving force includes electrostatic interaction, as discussed later, in addition to acid-base interaction and surface energy of the silica nanoparticles. According to this model, there are two possibilities: (1) the mechanism is a first-order one-step reaction from N to F or (2) the reaction is a series of two steps, following the sequence N f I f F. The nonlinear relation of the logarithm of the change in the turbidity, ln(∆T), over time suggests that the dissociation or transformation of the complexes occur predominantly in two or more steps. These considerations suggest that the irreversible shift from a turbid dispersion to an optically transparent solution with time is mainly due to the morphological rearrangement of the PAA/silica complexes from the network structure to the non-network structure. Addition of a salt is a simple way to modify the ionic strength of a colloidal solution.The main effect of the salt is to screen the electrostatic charges. The repulsion of charges on the polymer chain is reduced by screening, and hence the polyelectrolyte in solution changes from a stretched to coiled conformation. In addition, the repulsion between polyelectrolytes of the same species is suppressed. The effect of monovalent salts (e.g., sodium chloride) on the various interpolymer complex systems involving PAA has been investigated, and both positive and negative effects have been reported on the complexation.28,29 In our complex system without salt, a white turbid, milky dispersion was obtained in water at pH ≈ 3.5 (Figure 2a). The salt concentration was adjusted by adding NaCl to the white dispersion and the turbidity measurement was conducted at room temperature. As can be seen in Figure 6, an increase of the salt concentration led to the decrease of the turbidity and the clear transparent solution was obtained at NaCl concentration > 0.2 mol/L. The variation of the turbidity on adding salt may contribute to the dissociation of the complexes because of the screening of PAA by inorganic ions. Figure 7 describes the effect of salt on the time-dependent change in the turbidity of the (28) Sivadasan, K.; Somasundaran, P.; Turro, N. J. Colloid Polym. Sci. 1991, 269, 131-137. (29) Nurkeeva, Z. S.; Mun, G. A.; Khutoryanskiy, V. V.; Sergaziev, A. D. Eur. Polym. J. 2001, 37, 1233-1237.

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Figure 6. Variation of turbidity of the silica/PAA complexes in water at pH ≈ 3.5 ([silica] ) [PAA] ) 0.2 mg/mL, at 23 °C) as a function of added salt concentration.

Figure 8. Postulated mechanisms of the association/transformation/dissociation behavior of the silica/PAA complexes through (a) morphological change and (b) ionization change.

Figure 7. (a) Time dependence of turbidity of the silica/PAA complexes in water at pH ≈ 3.5 ([silica] ) [PAA] ) 0.2 mg/mL, at 23 °C) in the presence (9: 0.01, 4: 0.02, b: 0.075 mol/L) and absence (0) of NaCl. (b) Plots of logarithm of change in turbidity, ln(∆T), over time.

complexes. In the sample with NaCl (0.01 mol/L), the white turbid dispersion became transparent after 30 h (the transmittance > 95%). This indicates that a tiny amount of salt is enough to increase the dissociation rate. On the contrary, the curvature of the logarithm of the change in the turbidity, ln(∆T), over time is little affected by the addition of the salt, as can be seen in Figure 7b. Further increase of the salt concentration (0.02 and 0.075 mol/L) has a slight influence on the dissociation rate and the curvature. These results suggest that the addition of the salt induces the dissociation of the complexes because of the change of ionization but has no significant influence on the time-dependent morphological change, sequence N f I f F. On the basis of the results obtained under various conditions, we propose two independent mechanisms of the association/transformation/dissociation behaviors of PAA/silica complexes in water: (1) morphological change and (2) ionization change, as shown in Figure 8. The ionization change involves the reversible pH-induced complexation and salt-induced dissociation, which generally show rapid transformation in response to changes in pH and salt concentration of environmental fluids. The pH-induced association-dissociation is a reversible process, whereas the salt-induced dissociation is an irreversible process because of the difficulty to remove the salt

from the charged polyelectrolyte surrounded by small, oppositely charged counterions. The time-dependent change in the turbidity belongs to the morphological change, which is irreversible and response is slow. The temperature has a significant influence on the timedependent morphological rearrangement from the network structure into the non-network structure. The selfassembly and disassembly processes induced by environmental changes, such as pH, salt concentration, time, and temperature, can be reasonably explained by the ionization and morphological changes. Complexation in Various Polar Solvents. In addition to water, the complex formation was investigated in other polar solvents, methanol and DMF. The complexation in methanol was easily detected visually as the transparent solutions become heterogeneous as a result of white precipitation just after mixing the solutions at room temperature (Figure 2c). Both the silica nanoparticles and PAA were visually transparent in methanol as well as in water. The white turbid milky dispersion was obtained and no microscopic precipitation could be detected in water (Figure 2a) under the same concentration. As discussed in our previous paper,22 the silica nanoparticle contains oxygen and nitrogen atoms, which may act as proton-accepting sites for hydrogen-bonded complexation and ionic complexation caused by hydrogen-transfer interactions between the constituents, respectively. To obtain more information on the complexation in methanol, a simple control experiment was conducted; the silica nanoparticle was mixed with poly(tert-butyl acrylate), which is a precursor of PAA and has only ester groups. As expected, no precipitation was observed upon the addition of the protected precursor solution to the methanol solution of the silica nanoparticles. The lack of

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aggregation observed in the control system demonstrates that the white precipitation was the result of specific interactions between carboxylic acids and the silica nanoparticles having hydroxy and tertiary amino groups on the surface. For a complexation in another type of polar solvent, the mixing of the silica nanoparticles and PAA was conducted in DMF, which is a strong proton-accepting solvent and is known to act as a complex-breaking solvent. A clear transparent solution was obtained from the silica nanoparticles and PAA in DMF, respectively. Mixing the DMF colloidal solutions of the two constituents gave an optically transparent solution at room temperature, indicating no complexation. Hence, water and methanol are regarded as complexing solvents, whereas DMF (and DMSO used for NMR analysis, see next section) is recognized as a noncomplexing solvent. The difference in the complex formation between methanol (and water) and DMF is related to the competition between polymer-polymer interactions and polymer-solvent interactions, which is a decisive factor governing the complexation in solutions. For example, it was shown that DMF acts as a complexbreaking solvent for systems of PAA-poly(vinyl ether) with ethylene glycol side chains,6,30 because the polymersolvent interactions in DMF are always stronger than the polymer-polymer interactions. The dielectric constants of DMF ( ) 38.25) and DMSO ( ) 47.24) are intermediate between water ( ) 80.1) and methanol ( ) 33.0).31 Hence, the dielectric constant is not responsible for the difference in the complexations in various polar solvents. In our system, the interactions between the silica nanoparticles and PAA are strong enough to keep the complexes in water and in methanol, but the silica-PAA interaction is weaker than the DMF-PAA interaction. Effect of Temperature on the Complexation in Methanol. The association/transformation/dissociation behavior of the complexes was also investigated in methanol at different temperatures. An increase of the temperature led to a change of the appearance as the white precipitation obtained at room temperature (Figure 2c) was changed within 15 min to relatively homogeneous turbid dispersion (Figure 2d). The rapid change in the appearance in methanol is different from the tendency observed in water, where the white turbid dispersion kept constant without significant change at higher temperature (55 °C) within at least 30 min. The turbidity measured in methanol changed from about 60% to 40% in transmittance by the heating to 55 °C, which is due to the transformation from heterogeneous precipitation to a homogeneous, turbid dispersion. Although the turbidity is a simple indicator for the complex formation, it is not useful for the quantitative evaluation of the association/transformation/ dissociation behavior in methanol, because the complexation provides a microscopic precipitation just after mixing two components. Further, the precipitated complexes in methanol have a tendency to stick to the walls of a glass cuvette during the measurements and the phenomenon becomes significant with time. Hence, the accurate monitoring of the turbidity change in methanol at different temperatures was impossible. Nevertheless, the relatively rapid response in the appearance induced by the increase of the temperature was observed in methanol. Similar to the dissociation or transformation behavior in water, the white turbid precipitation formed in methanol disappeared slowly and an optically transparent solution was obtained (30) Mun, G. A.; Nurkeeva, Z.; Khutoryanskiy, V. V. Macromol. Chem. Phys.1999, 200, 2136-2138. (31) Permittivity (Dielectric Constant) of Liquids. In Handbook of Chemistry and Physics, 77th ed.; Lide, D. R., Ed.; 1996.

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Figure 9. Representative ΤΕΜ images of the silica/PAA complexes formed in methanol at 23 °C (a) and 40 °C (b). Insets show higher magnification images taken from the area inside the box of each image (the scale bar is 50 nm in both cases).

after several days, when a relatively low concentration ([silica] ) [PAA] ) 0.2 mg/mL) was employed for the complexation. Transmission electron microscopy (TEM) measurements were used to characterize the size and shape of the silica nanoparticle/PAA complexes formed in methanol at different temperatures. The samples were prepared from dilute solution ([silica] ) [PAA] ) 10 mg/L), where the mixtures of the silica nanoparticles and PAA showed optically transparent solutions, regardless of the temperature. The sample obtained at ambient temperature (23 °C) revealed the presence of large and discrete spherical aggregates of about 30-100 nm in diameter (Figure 9a), whereas the image of the silica nanoparticles prepared independently under the same condition showed lack of aggregation. As can be seen in Figure 9b, aggregates with lower densities were observed in the complex obtained at 40 °C. At 55 °C, aggregates were still observed occasionally, but the number and size of the aggregates were less than those observed at lower temperature. These results suggest the feasibility of the thermal control of the size and the density of the aggregates in methanol. Further manipulation of the shape, size, and distribution of anionic polyelectrolyte-cationic silica assembles will allow the fabrication of novel hybrids with nanoscale constructs. Characterization of Isolated Complexes. To obtain structural information on the complexes in methanol and water, the white turbid products formed just after mixing the corresponding solutions were isolated by decantation or centrifugation. After drying in vacuo, the isolated complexes were characterized by elemental analysis, FTIR, and 1H NMR. The results are summarized in Table 1. Elemental analysis of the isolated products indicated that the complexes contain 5.5-5.6% of Si and 2.3-2.9% of N, suggesting that the silica contents in the complexes are about 50 wt-% in both cases, and are in good agreement with the composition in the feed. There is no significant difference in the composition between the samples isolated from water and methanol dispersions.

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Langmuir, Vol. 20, No. 5, 2004 1941 Table 1. Characterization of Isolated Complexes elemental analysis

a

solvent

methoda

yield (%)

C

Si

N

silica contentb (wt %)

MeOH H2O (pH ) 3.5) feed

decantation centrifugation

49 68

45.2 44.5 46.9

5.6 5.5 5.5

2.9 2.3 2.4

51 50 50

Complexes were isolated from turbid dispersion ([silica] ) [PAA] ) 7.5 mg/mL). b Determined by Si content.

Figure 10. FT-IR spectra of the silica nanoparticles (a), the silica/PAA complexes isolated from water at pH ≈ 3.5 (b) and methanol (c), and PAA (d).

The FT-IR spectra of the isolated complexes revealed a carbonyl stretch vibration (1729 cm-1) corresponding to the free carbonyl bond of PAA and a typical broad band around 1100 cm-1 resulting from Si-O stretching on silica, as can be seen in Figure 10. Furthermore, the acid functionality is visible as the broad absorption from 2400 to 3800 cm-1, in addition to a sharp peak between 2840 and 2940 cm-1 which is due to the C-H stretching vibration in the alkyl chain of the silica nanoparticles. The clear difference between the samples isolated from water and methanol is seen in the carbonyl band (Figure 11). In both cases, a clear band is visible at lower wavenumbers (1560 cm-1), which is attributed to the absorption of carboxylate anions. The appearance of the absorption band of the carboxylate anions in the complexes, which may be as a result of proton transfer from carboxylic groups in PAA to amine moieties in the silica, suggests that these complexes are stabilized by ionic COO--N+ bonds. In the complex isolated from methanol, additionally a shoulder on the carbonyl stretch vibration wavenumbers (1640 cm-1) is observed clearly, which is due to hydrogen-carbonyl-bonded carbony groups. These results suggest that the ionic complexation through COO--N+ bond is predominant in the complex formed in water at pH ≈ 3.5, whereas the complex formed in methanol includes ionic complexation and hydrogenbonded complexation. The isolated products were insoluble in methanol and water, whereas they dissolved in DMSO. Figure 12 compares the 1H NMR spectra of the complexes formed in methanol and water, and the simple mixture of the PAA and the silica nanoparticles (1/1 wt %) in DMSO-d6. The simple mixture and the complex formed in water revealed resonances corresponding to the silica nanoparticles and PAA, suggesting that both components could be detected reasonably by NMR in DMSO-d6. There was

Figure 11. FT-IR spectra in the region of carbonyl band; the silica/PAA complexes isolated from water at pH ≈ 3.5 (a) and methanol (b), and PAA (c).

Figure 12. 1H NMR spectra in DMSO-d6: (a) PAA/silica simple mixture (1/1 wt %), the silica/PAA complexes isolated from water at pH ) 3.5 (b) and methanol (c), and the silica nanoparticles (d).

no significant difference in the composition between the simple mixture and the complex isolated from water, which is consistent with the result of elemental analysis. The complex should break up in DMSO because of the strong DMSO-PAA interaction. However, the complex isolated from methanol revealed strong peaks at 3.2-3.5 ppm ascribed to the silica nanoparticles and very weak peaks at 2.2 and 1.7 ppm, which are attributed to the backbone of PAA. It means that the PAA component in the complex formed in methanol is hard to detect by 1H NMR. It can be speculated that the outermost surface of the complex consists of the silica nanoparticles and the PAA chains are located inside, where they are shielded by the outermost surface. Another possibility is that the PAA

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chains are constrained to form a tight stack in the complex, leading to undetectable PAA. On the other hand, the complex isolated from water has an assembled structure in which both components are distributed homogeneously in the complex or the interactions between the constituents lead to the aggregation with a less packing form. This may be related to the difference in the stability of the complexes in water and methanol. The influence of the solvent on the structure of the complexes may be more complicated because of further possibilities of interactions between PAA and the cationic silica nanoparticles, such as water-intermediated interactions.4,32 The results obtained from the characterization of the isolated complexes indicate that the silica composition in the complex isolated from water is the same as that obtained from methanol. On the contrary, the kind of solvent has a significant influence on the interaction between the components, existing states of the components in the complexes, and the resulting aggregated structures. The results also suggest the feasibility to develop different types of nanostructured organic-inorganic hybrid materials from the same constituents by simply changing the solvent employed during the mixing process. Complex Stoichiometry. The stoichiometry is another important factor to determine the size and shapes of assemblies as well as the association/transformation/ dissociation behavior. Different from the polymerpolymer systems, however, the evaluation of the precise stoichiometry of the polymer-colloid systems is complicated because of the difficulty in the determination of the number of functional groups on the surface of colloidal particles. Recently, MALDI-TOF MS has become a very powerful tool for the investigation of siloxane,33 silsesquioxane,34,35 and organic molecules on silicon36 and silica37 surfaces. In this study, MALDI-TOF MS was used for the determination of the molecular weights and molecular weight distribution of the silica nanoparticles. Several matrixes, 2-(4-hydroxyphenylazo)benzoic acid, R-cyano4-hydroxy cinnamic acid, trans-3-(3-indolyl)acrylic acid, and sinapic acid could be used without salt for the analysis and all samples showed a unimodal symmetrical peak. In the sample prepared by mixing with 2-(4-hydroxyphenylazo)benzoic acid, Mn ) 3760 and Mw/Mn ) 1.08 could be obtained, as shown in Figure 13. The kind of the matrix had no significant influence on the MALDI-TOF MS results. On the basis of the preparation method (acidic condensation of the addition product, R-Si(OEt)3), the general structure of the silica nanoparticles should be (R-SiO1.5)n, where each silicon atom is bound to an average of one and a half oxygens and to one alkyl chain, R ) CH2CH2CH2N(CH2CH(OH)CH2OH)2. The structure of the constitutional unit is supported by the result of elemental analysis; the observed atomic composition of the silica nanoparticles (C, 43,75; H, 7.86; Si, 10.98; N, 4.76) is in fairly good agreement with the value calculated from the structure, R-SiO1.5 (C, 41.86; H, 7.75; Si, 10.85; N, 5.43). (32) Ostrowska, J.; Narebska, A. Colloid Polym. Sci. 1979, 257, 128135. (33) Jaumann, M.; Rebrov, E. A.; Kazakova, V. V.; Muzafarov, A. M.; Goedel, W. A.; Moller, M. Macromol. Chem. Phys. 2003, 204, 10141026. (34) Wallace, W. E.; Guttman, C. M.; Antonucci, J. M. Polymer, 2000, 41, 2219-2226. (35) Falkenhagen, J.; Jancke, H.; Kruger, R.-P.; Rikowski, E.; Schulz, G. Rapid Commun. Mass Spectrom. 2003, 17, 285-290. (36) Wei, J.; Buriak, J. M.; Siuzdak, G. Nature (London) 1999, 399, 243-246. (37) Bauer, F.; Sauerland, V.; Glasel, H.-J.; Ernst, H.; Findeisen, M.; Hartmann, E.; Langguth, H.; Marquardt, B.; Mehnert, R. Macromol. Mater. Eng. 2002, 287, 546-552.

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Figure 13. MALDI-TOF mass spectrum (linear mode) of the silica nanoparticles.

The structure of the constitutional unit is basically the same to that of silsesquioxanes, (R-SiO1.5)n. The hydrolysis and polycondensation of substituted alkoxysilanes R-Si(OR′)3 containing a nonhydrolyzable Si-C bond give a variety of silsesquioxanes with various substituent groups and cage structures. Completely condensed silsesquioxanes, (R-SiO1.5)n with n ) 4, 6, 8, 10, and 12, have been synthesized and characterized.38 Completely condensed structures with n > 12 are not so common, but some examples have been reported.39-42 In addition to the fully condensed structures, (RSiO1.5)n, which are denoted as Tn (n ) even number), incompletely condensed structures containing Si-OH groups are known, which have the general formula Tn(OH)x(OR′)y where T ) RSiO1.5-(x+y)/2n.38,43,44 As judged from these considerations in combination with the results of MALDI-TOF MS analysis, the main structures of the silica nanoparticles developed in this study are postulated to be (RSiO1.5)n with n ) 12-18. The molecular weights are 3096, 3612, 4128, and 4644 for n ) 12, 14, 16, and 18, respectively. Recently, several efforts have been directed at the preparation of novel colloids and nanoparticles on the basis of the hydrolytic condensation of monosilanes, and the resulting products are often called as polysilsesquioxanes colloids or polyorganosiloxane nanoparticles. For example, Brostein et al.45 reported the synthesis of a new family of polysilsesquioxanes colloids on the basis of hydrolytic condensation of N-(6-aminohexyl)aminopropyltrimethoxysilane, H2N(CH2)6NHCH2CH2CH2Si(OCH3)3. Further detailed characterization of the structure and properties of the tertiary amine containing silica nanoparticles will be reported separately. (38) Pescarmona, P. P.; Maschmeyer, T. Aust. J. Chem. 2001, 54, 583-596. (39) Voronkov, M. G.; Lavrent′yev, V. I. Top. Curr. Chem. 1982, 102, 199-236. (40) Agaskar, P. A.; Day, V. W.; Klemperer, W. G. J. Am. Chem. Soc. 1987, 109, 5554-5556. (41) Frye, C. L.; Collins, W. T. J. Am. Chem. Soc. 1970, 92, 55865588. (42) Agaskar, P. A.; Klemperer, W. G. Inorg. Chim. Acta 1995, 229, 355-364. (43) Williams, R. J. J.; Erra-Balsells, R.; Ishikawa, Y.; Nonami, H.; Mauri, A. N.; Riccardi, C. C. Macromol. Chem. Phys. 2001, 202, 24252433. (44) Eisenberg, P.; Erra-Balsells, R.; Ishikawa, Y.; Lucas, J. C.; Mauri, A. N.; Nonami, H.; Riccardi, C. C.; Williams, R. J. J. Macromolecules 2000, 33, 1940-1947. (45) Bronstein, L. M.; Linton, C. N.; Karlinsey, R.; Ashcraft, E.; Stein, B. D.; Svergun, D. I.; Kozin, M.; Khotina, I. A.; Spontak, R. J.; WernerZwanziger, U.; Zwanziger, J. W. Langmuir 2003. 19, 7071-7083.

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Table 2. Effect of Feed Composition on the Complexes Isolated from Methanola feed composition ratiob

results ratioc

weight ratio (silica/PAA)

mole (silica/PAA)

mole (N: COOH)

yieldd (%)

Si contente (%)

silica contentf (wt %)

mole ratiob,f (silica/PAA)

mole ratioc,f (N: COOH)

0.2 0.5 1 2 5

0.42 1.0 2.1 4.2 10.5

1:18 1:7.2 1:3.6 1:1.8 1:0.72

16 46 49 73 39

5.7 6.0 5.6 6.7 7.9

52 55 51 61 72

2.3 2.6 2.2 3.3 5.4

1:3.3 1:2.9 1:3.4 1:2.3 1:1.4

a Complexes were isolated from methanol ([silica] or [PAA] ) 7.5 mg/mL) by decantation. b Estimated from the number-average molecular weights of the silica nanoparticles (Mn ) 3760 g/mol) and PAA (Mn ) 7700 g/mol). c Calculated from the constitutional units of the silica nanoparticles (R-SiO1.5 ) 258 g/mol) and acrylic acid (72 g/mol). d Yield ) the amount of the isolated complex/the total amount of the silica nanoparticles and PAA in the feed. e Determined by elemental analysis. f Determined by Si content.

Here, we investigated the influence of the feed composition on the structure of the isolated complexes. The complexation was conducted in methanol at different weight ratios, silica/PAA ) 5/1, 2/1, 1/1, 1/0.5, 1/0.2, respectively. The conditions and the results are given in Table 2. The weight fraction of the silica nanoparticles in the complexes determined by elemental analysis can be translated into a mole-to-mole comparison. Assuming that the silica nanoparticles are distributed homogeneously in the complex without any aggregation, the number of the silica nanoparticles that interacted with one PAA chain can be estimated from the number-average molecular weights of the silica nanoparticles (Mn ) 3760 g/mol) and PAA (Mn ) 7700 g/mol). For example, when the complex formation was carried out at a weight ratio of the silica/ PAA ) 1/1, the silica content in the complexes determined by elemental analysis was 51 wt %, which translates into a molar ratio of the silica/PAA ) 2.2/1. The decrease of the silica composition in the feed has no significant influence on the mole ratio of the isolated complexes. It means that on average one PAA chain is interacting with two to three silica nanoparticles in the lower silica compositions in the feed (silica/PAA weight ratio e1). On the other hand, higher mole ratios (silica/PAA ) 3.3 and 5.4) were observed in the complexes obtained at higher silica compositions in the feed. The complexation at the silica/PAA weight ratio ) 2 (mole ratio ) 4.2) gave a high yield (72%). The stoichiometry of the alkyl chains attached to the silica nanoparticles and the carboxylic acid in PAA can be estimated from the silica content determined by elemental analysis and the constitutional units of the silica nanoparticles (R-SiO1.5 ) 258 g/mol) and acrylic acid (72 g/mol). The structure of the silica nanoparticles is (R-SiO1.5)n and 〈n〉 ) 14.6, and a PAA chain possesses, in average, 107 carboxylic acid groups. The calculation indicates that about 1.4-3.4 carboxylic acid units are interacting with one alkyl chain containing one tertiary amino group and four hydroxyl groups on the silica, suggesting that not only the nitrogen atom but also oxygen atoms participate in the complexation. The stoichiometry is applicable for the complex isolated from methanol, which includes ionic complexation and hydrogen-bonded complexation. However, this is not the case for the complex isolated from water, in which the complex is governed predominantly by ionic interaction. As to the complex structures in actual solutions, there are two possibilities. The first case is that all repeat units in the PAA chain interact with the silica nanopartcies, where the ratios of the interacted carboxylic acids to one alkyl chain are 1.4-3.4. On the other hand, the stoichiometric ratio should be less than the values, if some PAA chains remain unbound by forming a loop or by acting as a cross-linking agent with free interparticle PAA chains. The isolated complex obtained from water may correspond to the situation, where about 27% of acrylic acid units are interacting with the tertiary amino group

to form the ionic complexation caused by proton-transfer interactions between the constituents, and the remaining acrylic acid units are free without interacting with the silica nanoparticles. Recently, it was shown that binding in complex of tertiary amine-modified small molecules (mesogens) and PAA does not occur in a 1:1 charge ratio but rather in an ionic 2:1 acid:amine composition.46,47 In this case, the probability of the free acrylic acid units should be less than that in the case of 1:1 acid:amine ratio. These considerations on the stoichiometry should be ascribed to the network state (Figure 5N), because the isolated samples were collected from the turbid aqueous and methanol dispersions. The final state, in which the PAA chain can wrap around the silica nanoparticle (Figure 5F), may have a different stoichiometry. Further special attention is required to understand our PAA/silica hybrid systems because of the tiny size of the cationic silica nanoparticles (diameter ≈ 3.0 nm). The reported hydrodynamic radius of the single PAA polyelectrolyte molecules in aqueous NaCl-containing solution (pH ) 6-8) is Rh ) 3.7 nm, as determined by DLS for Mw ) 18100 (degree of polymerization, DP ) 193), and about 2.0 nm as calculated by computer simulation for DP ) ca. 100, respectively.48 In this study, a linear PAA (DP ) 107) having low polydispersity was employed for the complexation. It means that the radius of the silica nanoparticles is close to that of gyration of PAA used as a weak polyelectrolyte. If the backbone of the PAA chain is in an extended conformation, the chain length should amount to the contour length DPn × 0.25 nm ≈ 27 nm. The contour length is much larger than the diameter of the silica nanoparticles, indicating that in principle a PAA chain can interact with several silica nanoparticles or can wrap around the nanoparticle several times. In the latter case, we expected that the size of the complex is less than 10 nm, and the complexes are soluble in water and methanol, which correspond to the final state (F) shown in Figure 5. On the contrary, the former case corresponds to the network state (N). Several theoretical approaches have been reported on investigations of polymer-particle complexation.49-54 The formation of complex between a polyelectrolyte and an oppositely charged particle was demonstrated and the conformations of the polyelectrolyte existing on the surface (46) Tork, A.; Bazuin, C. G. Macromolecules 2001, 34, 7699-7706. (47) Bazuin, C. G.; Boivin, J.; Tork, A.; Tremblay, H.; Bravo-Grimaldo, E. Macromolecules 2002, 35, 6893-6899. (48) Reith, D.; Mu¨ller, B.; Mu¨ller-Plathe, F.; Wiegand, S. J. Chem. Phys. 2002, 116, 9100-9106. (49) Netz, R. R.; Joanny, J.-F. Macromolecules 1999, 32, 9026-9040. (50) Chodanowski, P.; Stoll, S. J. Chem. Phys. 2001, 115, 49514960. (51) Stoll, S.; Chodanowski, P. Macromolecules 2002, 35, 9556-9562. (52) Brynda, M.; Chodanowski, P.; Stoll, S. Colloid Polym. Sci. 2002, 280, 789-797. (53) Feng, J.; Ruckenstein, E. Polymer 2003, 44, 3141-3150. (54) Akinchina, A.; Linse, P. Macromolecules 2002, 35, 5183-5193.

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resulted from two competing effects:50 the electrostatic repulsions between the chains, which force the polyelectrolyte to adopt extended conformations and limit the number of monomer units, which can be attached to the particle surface, especially in the cases of small particles. Another effect is the electrostatic attractive interaction between the particle and the chains forcing the charged polymer to undergo a structural transition and to collapse at the particle surface. In our system, the electrostatic repulsion between the carboxylate anions causes the PAA chain to adopt an extended chain conformation at basic pH. At low pH, on the other hand, the intrachain association of the hardly dissociated PAA chains causes each chain to collapse into a smaller conformation. Actually, the effects of added salt concentration and temperature on the complexation were studied in aqueous medium at pH ≈ 3.5. The apparent pKa value (taken as the pH at 50% ionization) of PAA is 5.8,22 and the degree of ionization of PAA is