Stabilization of copper nanoparticles prepared by reduction from solutions of poly(acrylic acid) complexes with Cu2+ ions and poly(ethylene glycols)

Copper sols are prepared via the reduction of copper ions with hydrazine borane in dilute aqueous solutions of mixtures of the PAA-Cu²⁺ complex and poly(ethylene glycols) of various molecular masses at PEG: PAA = 0.25 base-mol/base-mol and PAA: Cu²⁺ = 10 base-mol/mol in the pH range 4.0–7.0. The stability of sols against oxidation (dissolution) or aggregation (enlargement) of metal nanoparticles is much higher than that of sols prepared in the absence of PEG. With an increase in the initial pH or a decrease in the molecular mass of PEG, the formed copper nanoparticles are much larger (no less than 20 nm in diameter) than copper nanoparticles occurring in the sol prepared in a solution of the PAA double complex with Cu²⁺ ions and high-molecular-mass PEG at a low initial pH (3–10 nm in diameter). Copper nanoparticles in sols prepared in solutions of complexes based on the high-molecular-mass PEG do not aggregate during exposure, thereby indicating the high stability of polymer screens on their surfaces.     

Effect of pH on complexation in mixed dilute aqueous solutions of poly(acrylic acid), poly(ethylene glycol), and Cu2+ ions

The phase states of mixed dilute solutions of PAA, PEG, and Cu²⁺ ions largely determines the mechanism governing the growth of metal nanoparticles during the subsequent reduction of copper ions. Mixtures with PAA: PEG > 1 base-mol/base-mol and PAA: Cu²⁺ ≥ 5 base-mol/mol are studied. It is shown that the simultaneous complexation of PAA with PEG and Cu²⁺ ions in these mixtures at pH values below the intrinsic pH of a solution is accompanied by phase separation related to insolubility of PAA-PEG interpolymer complexes. A decrease in the pH of the ternary mixture is caused by the release of a strong low-molecular-mass acid due to complexation with Cu²⁺ ions. The minimum pH value, above which the PAA-PEG-Cu²⁺ system becomes single-phase (a transparent solution), depends on the concentration ratio between PAA and PEG chains (the mean degree of polymerization). This value is either 6.8–7.0 (if all macromolecules are incorporated in the insoluble interpolymer complex with PEG) or 4.0 (if chains occur in excess). Methods of preparing single-phase systems in the pH range 4.0–7.0 via exchange reactions of the PAA-Cu²⁺ complex with PEG or the nonstoichiometric soluble interpolymer complex PAA-PEG are developed. Viscometry, electron microscopy, and dynamic light scattering are used to investigate the compositions and structures of soluble complexes, in which either each chain (if the chain is long) may be linked with both PEG and Cu²⁺ ions or PAA chains are redistributed between two complexes (at comparable lengths of PAA and PEG chains).     

Formation of copper nanoparticles during the reduction of Cu2+ ions in solutions and dispersions of polycation-poly(acrylic acid) interpolymer complexes in acidic media

The phase behavior of mixed acidic solutions of the high-molecular-mass cationic polyelectrolyte poly(1,2-dimethyl-5-vinylpyridinium methyl sulfate) and PAA at a PAA-to-polycation ratio of <1 base-mol/base-mol in narrow pH ranges at 6–30°C is studied. The reduction of Cu²⁺ ions in solutions and dispersions of interpolyelectrolyte complexes yields copper nanoparticles. It is shown that the mean-square diameters of nanoparticles formed in solutions of nonstoichiometric polyelectrolyte complexes are much smaller than those obtained in solutions of each component of the polyelectrolyte complex under the same conditions. These values decrease as the thermodynamic quality of the solvent with respect to the polymer of the pseudomatrix nonstoichiometric polyelectrolyte complex worsens. The experimental data may be explained in terms of the theoretical concept of pseudomatrix formation of a new phase in polymer solutions.     

Effect of urea on poly(methacrylic acid) and poly(acrylic acid) aqueous solutions

The effects of urea on aqueous solutions of both poly(methacrylic acid (PMA) and poly(acrylic acid) (PAA) have been investigated by using potentiometry, viscometry and study of the fluorescence of Auramine O, a cationic dye. The viscosity behaviour of unionized PMA obtained from direct dissolution of solid powder shows that the unneutralized macromolecules can be associated in water. The stability of such “aggregates” seems weak as indicated by their disappearance as soon as the charge density is very low. For PMA salt solution percolated through a cation (H⁺) exchange resin column, no association is observed. The pH-dependent conformational behaviour of PMA which, contrary to PAA, presents compact conformations in water at low charge density is discussed in terms of solvophobic/solvophilic interactions. It is shown that, even for urea concentration up to 8 M, the compact conformations of PMA are not completely destroyed. The formation of H⁺/urea complex is taken into account.     

Interpolymer complexes of poly(acrylic acid) with poly(2-hydroxyethyl acrylate) in aqueous solutions

Complexes formed from poly(acrylic acid) and poly(2-hydroxyethyl acrylate) were studied in aqueous solutions by viscometric, turbidimetric, FTIR spectroscopic, and thermogravimetric analysis methods. The formation of interpolymer complexes stabilized by hydrogen bonds was observed. It was found that the compositions of these interpolymer complexes are strongly dependent on the concentration of polymers, the order of mixing the solutions, and the pH. It was demonstrated that the complexation ability of poly(2-hydroxyethyl acrylate) is relatively low compared to other known nonionic water-soluble polymers. However, it can be significantly increased via hydrophobic modification of the poly(acrylic acid) using cetyl pyridinium bromide.     

Pseudotemplate synthesis of copper nanoparticles in solutions of poly(acrylic acid)-pluronic blends

It was found that the reduction of copper(II) ions in solutions of poly(acrylic acid)-pluronic blends results in a stable sol of metallic copper with a particle size below 10 nm, whereas a less stable sol with coarse aggregates of particles is formed in the presence of poly(acrylic acid) alone and an insoluble complex of this polymer with copper nanoparticles is produced in the presence of pluronic alone. The addition of poly(acrylic acid) to the complex causes the transfer of a portion of nanoparticles from the precipitate into the sol. In mixed poly(acrylic acid) and pluronic solutions, no formation of a polymeric complex with reasonable stability was detected. It was assumed that such a polycomplex is stabilized in the presence of copper nanoparticles. Owing to its amphiphilic nature, the complex forms stable protective shields on the surface of nanoparticles, and the stability of the sol is determined by free fragments of poly(acrylic acid).     

Formation of copper sols via reduction of Cu2+ ions in solutions of cationic and anionic polyelectrolytes

Zero-valence copper sols are prepared at 20°C via the chemical reduction of Cu(II) ions in aqueous solutions of high-molecular-mass cationic and anionic polyelectrolytes [(poly(1,2-dimethyl-5-vinylpyridium methyl sulfate) and poly(sodium styrenesulfonate), respectively]. In both sols, metal nanoparticles are characterized by narrow size distribution, indicating the pseudomatrix mechanism of their formation; however, the diameter of spherical copper particles formed in the polycation solution (3–14 nm) is much smaller than that of particles formed in the solution of polyanion (10–30 nm). Causes of different sizes of metal nano-particles formed in solutions of polyelectrolytes with different chain charges are discussed in terms of the pseudomatrix mechanism of new phase synthesis in polymer solutions and classical electrocapillary theory.     

Influence of chain charge and complexation on the overlap and entanglements formation in poly(acrylic acid) salt-containing aqueous solutions

Dilute-semidilute regime crossover in aqueous solutions of partly neutralized poly(acrylic acid) and of its complex with tetradecyltrimethylammonium bromide was studied by light scattering and viscometry methods. The chain charge growth causes the decrease of overlap concentration (c*) and the increase of the entanglements formation concentration (ce), hence, the semidilute unentangled regime of solution expands. Complexation of the polyelectrolyte with an oppositely charged surfactant leads to c* increase and to ce decrease. It is shown that in semidilute entangled solutions the surfactant acts as an effective structuring agent because of the binding of polyelectrolyte chains via surfactant micelles.     

The state of Cu2+ ions in concentrated aqueous ammonia solutions of copper nitrate

Stabilization of Cu²⁺ ions in concentrated aqueous ammonia solutions of copper nitrate in a wide range of ammonium ion concentrations has been studied by EPR and electronic absorption spectroscopy. Three types of Cu²⁺ associates with different types of orbital ordering have been identified. The ammonium ion concentration in a solution has a decisive effect on the type of orbital ordering of Cu²⁺ ions in associates. In all cases, Cu²⁺ ordering in associates is caused by the existence of bridging OH groups in the axial and equatorial positions of [Cu(NH₃) _n (H₂O)_{6 ? n} ]²⁺ complexes (n < 6). At a high concentration of ammonium ions, weakly bound associates of tetramminecopper with the $d_{x^2 - y^2 }$ ground state are formed. In solutions with low ammonium concentrations, bulky associates with the $d_{y^2 }$ and $d_{x^2 - z^2 }$ ground states and associates of Cu²⁺ ions with the $d_{x^2 - y^2 }$ ground state with hydroxyl groups in the equatorial plane and axial water molecules are formed.     

Interaction of positronium with Co2+ and Cu2+ ions in aqueous solutions

Interactions of positronium in aqueous solutions of Co²⁺ and Cu²⁺ ions have been investigated at room temperature (297 K) at varying concentrations using both lifetime and Doppler broadening of annihilation radiation techniques. In the case of Co²⁺, the results indicate spin conversion reaction alone. However, in the case of Cu²⁺, oxidation is predominant with a small contribution of spin conversion reaction. The corresponding rate constants have been evaluated.     

Electrochemical,spectroscopic, and thermal studies on interactions of linear poly(acrylic acid) with uranyl ions in aqueous solutions

The formation of complexes between linear poly(acrylic acid) (PAA) and uranyl ions in aqueous solutions was studied with conductometry, potentiometry, thermal analysis, Fourier transform infrared, and luminescence spectroscopy methods. The stoichiometry of the PAA/UO complex on repeating units of a PAA basis was determined to be 2:1. IR spectroscopy studies made on solid complexes showed that the carbonyl stretching absorption band of PAA was shifted to a higher wave number, and a new band at 1749 cm^?1 in the polycomplex spectrum was observed, confirming the existence of specific interactions between the carboxylate groups of PAA and metal ions. Luminescence spectroscopy studies showed an increase in the intensity of the uranyl‐ion emission spectra and new band formation at 483 nm, further confirming the interaction of UO ions with PAA in aqueous solutions. The thermal behavior of PAA/UO complexes further proved strong interactions in the complex structure. The thermal degradation of the polycomplexes included the main stages of destruction of both PAA and uranyl nitrate. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1610–1618, 2004     

Hydrogen-bonding-driven self-assembly of PEGylated organosilica nanoparticles with poly(acrylic acid) in aqueous solutions and in layer-by-layer deposition at solid surfaces

PEGylated organosilica nanoparticles have been synthesized through self-condensation of (3-mercaptopropyl)trimethoxysilane in dimethyl sulfoxide into thiolated nanoparticles with their subsequent reaction with methoxypoly(ethylene glycol) maleimide. The PEGylated nanoparticles showed excellent colloidal stability over a wide range of pH in contrast to the parent thiolated nanoparticles, which have a tendency to aggregate irreversibly under acidic conditions (pH < 3.0). Due to the presence of a poly(ethylene glycol)-based corona, the PEGylated nanoparticles are capable of forming hydrogen-bonded interpolymer complexes with poly(acrylic acid) in aqueous solutions under acidic conditions, resulting in larger aggregates. The use of hydrogen-bonding interactions allows more efficient attachment of the nanoparticles to surfaces. The alternating deposition of PEGylated nanoparticles and poly(acrylic acid) on silicon wafer surfaces in a layer-by-layer fashion leads to multilayered coatings. The self-assembly of PEGylated nanoparticles with poly(acrylic acid) in aqueous solutions and at solid surfaces was compared to the behavior of linear poly(ethylene glycol). The nanoparticle system creates thicker layers than the poly(ethylene glycol), and a thicker layer is obtained on a poly(acrylic acid) surface than on a silica surface, because of the effects of hydrogen bonding. Some implications of these hydrogen-bonding-driven interactions between PEGylated nanoparticles and poly(acrylic acid) for pharmaceutical formulations are discussed.     

Electrokinetic characterization of poly(acrylic acid) and poly(ethylene oxide) brushes in aqueous electrolyte solutions

Surfaces carrying hydrophilic polymer brushes were prepared from poly(styrene)-poly(acrylic acid) and poly(styrene)-poly(ethylene oxide) diblock copolymers, respectively, using a Langmuir-Blodgett technique and employing poly(styrene)-coated planar glass as substrates. The electrical properties of these surfaces in aqueous electrolyte were analyzed as a function of pH and KCl concentration using streaming potential/streaming current measurements. From these data, both the zeta potential and the surface conductivity could be obtained. The poly(acrylic acid) brushes are charged due to the dissociation of carboxylic acid groups and give theoretical surface potentials of -160 mV at full dissociation in 10(-)(3) M solutions. The surface conductivity of these brushes is enormous under these conditions, accounting for more than 93% of the total measured surface conductivity. However, the mobility of the ions within the brush was estimated from the density of the carboxylic acid groups and the surface conductivity data to be only about 14% of that of free ions. The poly(ethylene oxide) (PEO) brushes effectively screen the charge of the underlying substrate, giving a very low zeta potential except when the ionic strength is very low. From the data, a hydrodynamic layer thickness of the PEO brushes could be estimated which is in good agreement with independent experiments (neutron reflectivity) and theoretical estimates. The surface conductivity in this system was slightly lower than that of the polystyren substrate. This also indicates that no significant amount of preferentially, i.e., nonelectrostatically attracted, ions taken up in the brush.     

Conditions for the formation of copper nanoparticles by reduction of copper(II) ions with hydrazine hydrate solutions

Optimal conditions were found for the preparation of copper nanoparticles in aqueous solution via reduction of copper(II) ions with hydrazine hydrate. The effects of ligand environment of copper(II) in the initial solution (hydrate, ammonia, citrate, and glycine complexes), concentration, pH, surfactants, temperature, and mode of heating were examined. The obtained colloidal systems were studied by optical spectroscopy, X-ray photoelectron spectroscopy, X-ray powder diffraction, and atomic force microscopy. The examined colloids were found to contain generally spherical copper nanoparticles with a diameter of about 10 nm, which were coated with a copper(I) or copper(II) oxide and hydroxide film.     

Specific features of nonstoichiometric interpolymer complexes of poly(acrylic acid) and poly(ethylene glycol) as protectors of copper nanoparticles in aqueous sols

Advantages of interpolymer complexes for use as amphiphilic protectors of nanoparticles during the formation and stabilization of sols are considered. The effects of the ratio of poly(acrylic acid) and poly(ethylene glycol) and the molecular mass of poly(ethylene glycol) on the mean size and size distribution of copper nanoparticles in sols formed via the reduction of divalent copper ions in mixed aqueous solutions of these polymers are investigated. It is shown that sols of metal nanoparticles with small sizes and narrow size distributions are formed even when poly(ethylene glycols) with chain lengths below the “critical” chain length and a small PEG-to-PAA base-molar ratio are used. This is evidence for efficient protection of the formed copper nanoparticles by the interpolymer complex PEG-PAA under conditions of its instability and for self-organization of oligomeric PEG chains in complex macromolecular shields of nanoparticles.     

Kinetics and equilibrium of the complexation of Al3+ with poly(maleic, acrylic) acid

Kinetics and equilibrium of the complexation of Al³⁺ with a polycarboxylic acid (PCA, random copolymer of maleic and acrylic acid with a mean molecular weight of 92 kDa) are investigated by the stopped flow technique and potentiometric titration. The complexation proceeds according to the Eigen–Tamm mechanism, i.e. in first diffusion-controlled step an outer sphere complex is formed. The second rate determining step is the formation of the inner sphere complex, controlled by the exchange rate of hydration water. For this second step the rate constant is k ₁=3 s^-1. It is in the order of magnitude of the water exchange at the Al³⁺ ion as expected for the Eigen–Tamm mechanism. The activation parameters are also determined. Parallel to this direct reaction path a base catalyzed path is found, typical for complexation reactions of hydrolyzable metal ions. Stable complexes are formed for which the overall association constant K _ass=Q _o(1+K _i) is determined by two parts: a chemical (intrinsic) part, described by the inner sphere association constant K _i=3 and an electrostatically controlled part described by the outer-sphere association quotient Q _o. The evaluation of the kinetic experiments allows to determine the value of log(Q _o) as a function of pH: 3.3<log Q _o<4.6. From these data the potential is calculated in the range −67 to ∝93 mV at pH values between 2 and 4. For comparison, analogous experiments with the monomeric subunits of the polyacid, glutarate (GA), and tricarballylate (TCA), are performed. The complexation with the monomeric subunits glutaric- and tricarballylic acid can be explained within the classical view of a discrete outer sphere association constant Q _o. Received: 13 November 1997 Accepted: 24 March 1998     

Effect of copolymer composition on interpolymer complex formation of (co)poly(vinyl ether)s with poly(acrylic acid) in aqueous and organic solutions

The complex formation between vinyl ether of poly(ethylene glycol‐co‐vinyl butyl ether) with poly(acrylic acid) has been considered in aqueous and isopropanol solutions. The effect of copolymer composition on the complex formation process was clarified. It has been shown that the incorporation of hydrophobic fragments into macromolecules enhances the hydrophobic stabilization of polycomplexes in aqueous solutions. In organic media this effect disappears. The stability of polycomplexes formed both in aqueous and in organic solutions in respect to the addition of dimethylformamide has been studied.     

Controlled synthesis of oligomeric poly(acrylic acid) and its behavior in aqueous solutions

The controlled synthesis of oligomeric poly(acrylic acid) via the pseudoliving radical reversible addition-fragmentation chain-transfer polymerization of acrylic acid in bulk is developed. It is shown that, at high concentrations of reversible addition-fragmentation chain-transfer agents, the polymerization of acrylic acid in bulk occurs via the pseudoliving mechanism, as evidenced by a linear increase in the numberaverage molecular mass of oligomers with conversion and a narrow molecular-mass distribution of the reaction products. The surfactant properties and behavior of the oligomers in aqueous solutions are studied.     

Amine nitrosation via NO reduction of the polyamine copper(II) complex Cu(DAC)2+

The reaction of the fluorescent macrocyclic ligand 1,8-bis(anthracen-9-ylmethyl)-1,4,8,11-tetraazacyclotetradecane with copper(II) salts leads to formation of the Cu(DAC)2+ cation (I), which is not luminescent. However, when aqueous methanol solutions of I are allowed to react with NO, fluorescence again develops, owing to the formation of the strongly luminescent N-nitrosated ligand DAC-NO (II), which is released from the copper center. This reaction is relatively slow in neutral media, and kinetics studies show it to be first order in the concentrations of NO and base. In these contexts, it is proposed that the amine nitrosation occurs via NO attack at a coordinated amine that has been deprotonated and that this step occurs with concomitant reduction of the Cu(II) to Cu(I). DFT computations at the BP/LACVP* level support these mechanistic arguments. It is further proposed that such nitrosation of electron-rich ligands coordinated to redox-active metal centers is a mechanistic pathway that may find greater generality in the biochemical formation of nitrosothiols and nitrosoamines.     

Complexation between poly(acrylic acid) and poly(vinylpyrrolidone): Influence of the molecular weight of poly(acrylic acid) and small molecule salt on the complexation

Poly(acrylic acid) (PAA) with different molecular weight and poly(vinylpyrrolidone) (PVP) were prepared by free radical polymerization using 2,2′-azoisobutyronitrile (AIBN) as initiator in anhydrous methanol for PAA, and in distilled water for PVP. Then, the complexation between PAA and PVP in aqueous solution was studied by UV transmittance measurement and fluorescence probe technique. The result shows that (1) at low pH, the formation of complexation between PAA and PVP bases on the intermacromolecular hydrogen bond and the composition of the formed complex is around 3:2 (the unit molar ratio of PAA to PVP) at pH 2.60 over the range of pH investigated. (2) The cooperative interaction through the formation of hydrogen bond among active sites plays an important role in complex formation, and depends on the pH of solution, the required minimum chain length of poly(acrylic acid). (3) The hydrogen bond is not affected by small molecular salt, which only affects those carboxylic groups without forming hydrogen bond on the PAA chain.