Phase separation in semidilute aqueous poly(N-isopropylacrylamide) solutions

The phase separation mechanism in semidilute aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions is investigated with small-angle neutron scattering (SANS). The nature of the phase transition is probed in static SANS measurements and with time-dependent SANS measurements after a temperature jump. The observed critical exponents of the phase transition describing the temperature dependence of the Ornstein-Zernike amplitude and correlation length are smaller than values from mean-field theory. Time-dependent SANS measurements show that the specific surface decreases with increasing time after a temperature jump above the phase transition. Thus, the formation of additional hydrogen bonds in the collapsed state is a kinetic effect: A certain fraction of water remains as bound water in the system. Moreover, H-D exchange reactions observed in PNIPAM have to be taken into account.     

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.     

The surface tension of poly(acrylic acid) in sodium chloride solutions

Summary Surface tension of polyacrylic acid (PAA) in sodium chloride solutions was measured at various ionic strengths. Surface activity of PAA solutions increases and shows a large time dependence with increasing salt concentration. Surface activity and time dependence of surface tension is explained by the changing flexibility of PAA chains.

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Zusammenfassung Oberflächenspannung von Polyacrylsäure in Natriumchloridlösung mit verschiedenem Ionisationsgrad wurde gemessen. Die Oberflächenaktivität und Zeitdauer der Gleichgewichtseinstellung nimmt mit der Salzkonzentration zu. Oberflächenaktivität und Zeitabhängigkeit werden durch die Änderung der Beweglichkeit der Polyacrylsäureketten erklärt.

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With 5 figures     

Electrically controlled separation of maleic acid and fumaric acid through a poly(vinyl alcohol)/poly(acrylic acid) composite membrane

The separation of maleic acid (MA) from a mixture of MA and fumaric acid (FA) was studied using an electrically activated polyelectrolyte gel membrane. The membrane was prepared through the iterative freezing-thawing of an aqueous solution containing poly(vinyl alcohol) and poly(acrylic acid). The separation of MA from an equimolar mixture (5 mM) of MA and FA using the membrane was performed under different conditions of pH (1–8) and electric fields (2–6 V). It was found that MA was separated from the mixture at pH 2 under an applied electric field of greater than 2 V because only the COOH groups of MA were dissociated at pH 2 and the MA⁻ ions were transported through the membrane toward the electrode opposite in sign to their charge.     

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.     

Osmotic deswelling of weakly charged poly(acrylic acid) solutions and gels

The osmotic pressure of weakly charged aqueous poly(acrylic acid) (PAA) solutions and the swelling pressure PAA gels were studied by osmotic deswelling at different degrees of ionization (α). In solution, the osmotic pressure was found to scale linearly with concentration, whereas the scaling power of the swelling pressure of gels was higher (1.66). The effect of the ionization degree on the osmotic coefficient in PAA solutions was in agreement with the theory of Borue and Erukhimovich [Macromolecules, 21 , 3240 (1988)]. Ionization increases the swelling capacity of the PAA gels until a plateau is reached at about 35% neutralization. The concentration at equilibrium swelling scales as C_e ～ α^?0.6. The contribution of the network to the gel swelling pressure is evaluated by subtracting the osmotic pressure of the polymer solution at the same concentration and degree of ionization. In swollen gels the extended network opposes swelling. As the gel is osmotically deswelled, a state of zero network pressure exists at a certain concentration, below which the network elasticity favors swelling. The crossover concentration shifts to lower values as the degrees of ionization increases. © 1995 John Wiley & Sons, Inc.     

Swelling of hydrophobically modified poly(acrylamide) and poly(acrylamide)-co-(acrylic acid) gels in surfactant solutions

A series of hydrophobically modified polyacrylamide and polyacrylamide-co-poly(acrylic acid) gels with systematically varying hydrophobicity were prepared by free-radical polymerization of acrylamide, n-alkylacrylamides (n = 10, 12, and 14), and acrylic acid. The swelling of these gels was examined in water and in both anionic and cationic surfactant solutions. It was found that the gels which incorporated acrylic acid showed extremely high swelling in water. Maximum swelling was observed in gels which incorporated 10 mol% acrylic acid. The swelling of these gels was much less in solutions of both anionic and cationic surfactants than in water. The gels which did not incorporate acrylic acid demonstrated little swelling in water, but showed increased swelling in both anionic and cationic surfactant solutions with increased hydrophobicity of the gel. Received: 1 February 1999 Accepted in revised form: 5 March 1999     

Synthesis and pH-sensitive micellization of doubly hydrophilic poly(acrylic acid)-b-poly(ethylene oxide)-b-poly(acrylic acid) triblock copolymer in aqueous solutions

A doubly hydrophilic triblock copolymer poly(acrylic acid)-b-poly(ethylene glycol)-b-poly(acrylic acid) (PAA-b-PEO-b-PAA) with M _w/M _n = 1.15 was synthesized by atom transfer radical polymerization of t-butyl acrylate (tBA), followed by acidolysis of the PtBA blocks. The pH-sensitive micellization of PAA-b-PEO-b-PAA in acidic solution was investigated by potentiometric titration, fluorescence spectrum, dynamic light scattering and zeta potential. The pK _a was 6.6 and 6.0 in deionized water and in 0.1 mol/L NaCl solution, respectively. The copolymer formed micelles composed of a weakly hydrophobic core of complexed PAA and PEO and a hydrophilic PEO shell in 1 mg/mL solution at pH < 5.5 due to hydrogen bonding. The critical micelle concentration was 0.168 mg/mL at pH 2.0. At pH < 4.5, steady and narrow distributed micelles were formed. Increasing pH to 5.0, unsteady and broad distributed micelles were observed. At pH > 5.5, the micelle was destroyed owing to the ionization of the PAA blocks.     

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).     

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.     

Binding of a surfactant counterion to low-charge-density poly(acrylic acid) and poly(methacrylic acid)

The binding of a cationic surfactant, dodecylpyridinium (C12Py) chloride, with a low-charge-density poly (methacrylic acid) (PMA) was investigated in buffer solutions under the condition of constant pH. The binding isotherms with PMA consisted of two and three steps at a pH lower and higher than 3.2, respectively. Bindings in the first step were independent of pH and this step was considered to correspond to the solubilization of the hydrocarbon chains of C12Py into the nonpolar region of the compact form of PMA. This is the indication of the compact form from the binding isotherm. At pH higher than 3.2, the second step was discriminated and it depended on the pH. In the third step, a sharp rise in the degree of binding (β) was observed accompanying the solubilization of the precipitates of the PMA–C12Py complex. The binding with poly(acrylic acid) (PAA) and PMA in conventional unbuffered NaCl solutions was also examined and the pH profile of the solution during the binding process was determined. In the case of unbuffered NaCl solutions, the binding with PAA took place cooperatively at the critical association concentration (cac). The binding isotherm consisted of two steps and the pH decreased with the increase in β. The binding isotherm of PMA, on the other hand, consisted of three steps: the pH decreased slightly in the first step and considerably in the second step with the increase in β but it increased with β in the third step, exhibiting a pH minimum around 3.2. The binding in the first step coincided with that obtained in the buffered solutions. Linear relationships between β and the pH were found for both polymers. In the case of PMA, no cac was observed in both buffered and unbuffered NaCl solutions. Received: 24 January 2001 Accepted: 23 May 2001     

Photophysical investigations of interpolymer interactions in solutions of a pyrene substituted poly(acrylic acid), poly(vinyl amine), poly(1-aminoacrylic acid), and poly(1-acetylaminoacrylic acid)

Photophysical properties of the pyrene chromophore covalently bound to poly(acrylic acid) were used to investigate the interactions of a pyrene substituted poly(acrylic acid) (1) with poly(vinyl amine hydrochloride) (PVAm), poly(1-aminoacrylic acid) (PDA), and poly(1-acetylaminoacrylic acid) (PADA) in aqueous solutions. A number of photophysical parameters were obtained from fluorescence emission and excitation spectra, the deconvolution of decay curves for pyrene monomer, and excited state complex fluorescence and the quenching of pyrene monomer fluorescence by nitromethane in polymer solutions. These photophysical parameters were considered to reflect the inter- and intrapolymer interactions in solutions of 1 , PVAm, PDA, and PADA. The formation of interpolymer complexes between 1 and PVAm was noticed at low (< 4) as well as high (> 8) values, whereas PDA and 1 formed interpolymer complexes at low pH only. No interpolymer complex formation was detected in solutions of 1 and PADA under low or high pH conditions. The structures of interpolymer complexes formed between 1 and PVAm under low and high pH conditions were found to be determined by the conformation of 1 . There were significant differences in the interpolymer interactions of 1 and PDA in comparison to those of 1 and PVAm; in particular, the fluorescence from the excited state complex was enhanced in solutions of 1 and PVAm but quenched in solutions of 1 and PDA. The investigations of terpolymer solutions of 1 , PVAm, and PADA indicated that the nature of interpolymer complexes formed in terpolymer solutions was determined by Coulombic interactions of the amino and carboxylic group containing polymers.     

Calorimetric investigation of poly(methacrylic acid) and poly(acrylic acid) in aqueous solution

The enthalpy of dissociation of poly(acrylic acid) and of poly(methacrylic acid) in water and in 0.5N NaCl at 25°C has been measured over a wide range of degrees of neutralization of the polyelectrolytes. In the case of poly(methacrylic acid) the calorimetric data permit the direct evaluation of the enthalpy of conformational transition of the polymer. For this transition, with the aid of standard free energy data derived from potentiometric titrations, the change in entropy was also estimated. The relative accuracy of the thermodynamic data, and the possibility of deriving therefrom information on the mechanism of transitions of the type, globular coils → expanded coils for partially hydrophobic synthetic polyelectrolytes in aqueous solution are discussed.     

Swelling kinetics of poly(aspartic acid)/poly(acrylic acid) semi‐interpenetrating polymer network hydrogels in urea solutions

Semi‐interpenetrating network (semi‐IPN) hydrogels, composed of poly(aspartic acid) (PAsp) and poly(acrylic acid) (PAAc) with various ratios of PAsp to AAc, were prepared. In this work, swelling kinetics was investigated through calculating some parameters. The swelling ratios were measured at room temperature, using urea solutions as liquids to be absorbed. Compared to in deionized water, the hydrogels showed larger swelling ratios in urea solutions, which might be attributed to the chemical composition of urea. The equilibrium swelling ratio could achieve 600 g/g, and the equilibrium urea/water contents were more than 0.99. The diffusion exponents were between 0.5 and 0.7, suggesting that the solvent transport into the hydrogel was dominated by both diffusion and relaxation controlled systems. Therefore, the PAsp/PAAc semi‐IPN hydrogels were appropriate to carry substances in a urea/water environment for pharmaceutical, agricultural, environmental, and biomedical applications. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 666–671, 2010     

Structure of poly(acrylic acid) in electrolyte solutions determined from simulations and viscosity measurements

In this work, the structure of poly(acrylic acid) (PAA) molecules in electrolyte solutions obtained from molecular dynamic simulations was compared with experimental data derived from dynamic light scattering (PCS), dynamic viscosity, and electrophoretic measurements. Simulations and measurements were carried out for polymer having a molecular weight of 12 kD for various ionic strengths of the supporting electrolyte (NaCl). The effect of the ionization degree of the polymer, regulated by the change in the pH of the solution in the range 4-9 units, was also studied systematically. It was predicted from theoretical simulations that, for low electrolyte concentration (10(-3) M) and pH = 9 (full nominal ionization of PAA), the molecule assumed the shape of a flexible rod having the effective length L(ef) = 21 nm, compared to the contour length L(ext) = 41 nm predicted for a fully extended polymer chain. For an electrolyte concentration of 0.15 M, it was predicted that L(ef) = 10.5 nm. For a lower ionization degree, a significant folding of the molecule was predicted, which assumed the shape of a sphere having the radius of 2 nm. These theoretical predictions were compared with PCS experimental measurements of the diffusion coefficient of the molecule, which allowed one to calculate its hydrodynamic radius R(H). It was found that R(H) varied between 6.6 nm for low ionic strength (pH = 9) and 5.8 nm for higher ionic strength (pH = 4). The R(H) values for pH = 9 were in a good agreement with theoretical predictions of particle shape, approximated by prolate spheroids, bent to various forms. On the other hand, a significant deviation from the theoretical shape predictions occurring at pH = 4 was interpreted in terms of the chain hydration effect neglected in simulations. To obtain additional shape information, the dynamic viscosity of polyelectrolyte solutions was measured using a capillary viscometer. It was found that, after considering the correction for hydration, the experimental results were in a good agreement with the Brenner's viscosity theory for prolate spheroid suspensions. The effective lengths derived from viscosity measurements using this theory were in good agreement with values predicted from the molecular dynamic simulations.     

Dual polyelectrolyte-ionomer behavior of poly(acrylic acid) in methanol: 2. Salt solutions

The influence of tetramethylammonium chloride on a polyelectrolyte-ionomer behavior of poly(acrylic acid) (PAA) partly neutralized with tetramethylammonium hydroxide in methanol was studied by the viscometry and conductivity methods. It was shown that the addition of a small amount of this low-molecular-weight salt decreases the size of macromolecules in the polyelectrolyte state. When the salt concentration rises, the tendency toward formation of ion pairs is enhanced and the ionomer state becomes thermodynamically more advantageous than the polyelectrolyte state. As a result, macromolecules are transformed into the ionomer state at degrees of polymer neutralization corresponding to the polyelectrolyte behavior of salt-free solutions.Translated from Kolloidnyi Zhurnal, Vol. 66, No. 6, 2004, pp. 746–749.Original Russian Text Copyright © 2004 by Volkov, Filippova, Khokhlov.     

Dual polyelectrolyte-ionomer behavior of poly(acrylic acid) in methanol: 1. Salt-free solutions

The behavior of poly(acrylic acid) (PAA) partly neutralized with tetramethylammonium hydroxide in methanol was studied by the viscometry, ¹H NMR spectroscopy, light scattering, and conductivity methods. It was shown that, when the degree of PAA neutralization increases from 0.1 to 0.2, a transition from a polyelectrolyte to ionomer state occurs. This transition is caused by the formation of ion pairs due to the binding of counterions with the charged groups of the polymer chain and is accompanied by a macromolecule collapse. Collapsed macromolecules aggregate. At degrees of neutralization above 0.4, the restored polyelectrolyte state is accompanied by polymer swelling. The data obtained are qualitatively explained.Translated from Kolloidnyi Zhurnal, Vol. 66, No. 6, 2004, pp. 739–745.Original Russian Text Copyright © 2004 by Volkov, Filippova, Khokhlov.     

The role of hydrogen-bonding interaction in poly(vinyl alcohol)/poly(acrylic acid) blending solutions and their films

<正>The aim of this work is to investigate the hydrogen-bonding interaction in poly(vinyl alcohol)(PVA)/poly(acrylic acid)(PAA) blending system and its influence on rheological properties in solution and the physical properties in solid state. Introducing PAA into PVA solutions resulted in a thickening behavior of blend solutions.The viscosity of the solutions increased with PAA content increasing,and a maximum viscosity could be obtained when the ratio of PVA/PAA was 70/30. The intermolecular hydrogen-bonding and miscibility between PVA and PAA in solid state were investigated by differential scanning calorimetry(DSC),Fourier transform infrared spectroscopy(FTIR) and mechanical measurements.The results displayed the great influence of introducing PAA on the properties of blending films.The tensile strength increased from 89.31 MPa to 119.8 MPa and Young's modulus improved by over 300%with increasing PAA concentration compared with those of pure PVA films.By systematically studying the rheological behaviors of solutions and the physical properties of films,the influence of hydrogen-bonding in solutions and solid states were discussed.     

Polyelectrolyte effects on the quenching of pyrene fluorescence in solutions of a pyrene substituted poly(acrylic acid)

The quenching of pyrene fluorescence by nitromethane, Tl⁺, Cu²⁺, I^?, and 4-dimethylaminopyridine (DMAP) in aqueous solutions of a pyrene substituted poly(acrylic acid) ( 1 ) was influenced by the “polyelectrolyte effect” of 1 . The efficiency of quenching in solutions of 1 was measured in terms of the Stern–Volmer constants for dynamic and static quenching which were obtained from comparison of the intensity and lifetime of pyrene fluorescence in solutions of 1 and a monomer model compound. The efficiency of quenching in solutions of 1 was always greater at high pH ( 9 ) in comparison to that at low pH ( 4 ). The ionization of carboxylic groups in 1 caused an expansion of the polymer mainchain and concomitant exposure of the pyrene molecules to the aqueous phase and quencher. The polyanion domain of 1 favored the condensation of cationic quenchers and could account for very efficient quenching in case of Cu²⁺ and Tl⁺. A very efficient quenching of pyrene fluorescence in solutions of 1 by DMAP at high pH was attributed to the hydrophobic interactions of DMAP and pyrene moiety. The iodide ions were less efficient quenchers of pyrene fluorescence due to electrostatic repulsion from the polyanion. The efficiency of quenching by nitromethane was not significantly affected by ionization of the carboxylic groups in 1 .