O.E. Philippova

Hydrophobic aggregation in aqueous solutions of hydrophobically modified polyacrylamide in the vicinity of overlap concentration

The rheological behavior and hydrophobic association of hydrophobically modified polyacrylamide with 0.5 mol% of hydrophobic n-nonylacrylamide units were studied in the vicinity of the overlap concentration. It was shown that the rheological properties of aqueous polymer solutions are determined by the formation of hydrophobic domains playing a role of physical cross-links between polymer chains. It was observed that a pronounced increase of the viscosity above the overlap concentration is accompanied by the increase of the concentration of hydrophobic domains, but the average size of one domain remains constant.

Chitosan and its hydrophobic derivatives: Preparation and aggregation in dilute aqueous solutions

This review surveys the main methods of preparing chitosan and its hydrophobic derivatives and their influence on the microstructure of polymers. The experimental data on the aggregation of these poly� mers in dilute aqueous solutions are summarized. Basic factors affecting aggregation are analyzed, and its general regularities are revealed. It is shown that, in the case of both chitosan and its hydrophobic derivatives, the formation of aggregates is governed by the competition of attraction of associating groups promoting aggregation and their repulsion arising from the presence of charged units and counterions hindering aggre� gation. Various aggregate models that were proposed for chitosan derivatives with different mainchain lengths and different contents of associating groups are discussed.

Multichain aggregates in dilute solutions of associating polyelectrolyte keeping a constant size at the increase in the chain length of individual macromolecules.

Multichain aggregates together with individual macromolecules were detected by light scattering in dilute aqueous solutions of chitosan and of its hydrophobic derivatives bearing 4 mol % of n-dodecyl side groups. It was demonstrated that the size of aggregates and their aggregation numbers increase at the introduction of hydrophobic side groups into polymer chains. The key result concerns the effect of the chain length of individual macromolecules on the aggregation behavior. It was shown that for both unmodified and hydrophobically modified (HM) chitosan, the size of aggregates is independent of the length of single chains, which may result from the electrostatic nature of the stabilization of aggregates. At the same time, the number of macromolecules in one aggregate increases significantly with decreasing length of single chains to provide a sufficient number of associating groups to stabilize the aggregate. The analysis of the light scattering data together with TEM results suggests that the aggregates of chitosan and HM chitosan represent spherical hydrogel particles with denser core and looser shell covered with dangling chains.

How a viscoelastic solution of wormlike micelles transforms into a microemulsion upon absorption of hydrocarbon: new insight

In this article, we investigate the effect of hydrocarbon addition on the rheological properties and structure of wormlike micellar solutions of potassium oleate. We show that a viscoelastic solution of entangled micellar chains is extremely responsive to hydrocarbons-the addition of only 0.5 wt % n-dodecane results in a drastic drop in viscosity by up to 5 orders of magnitude, which is due to the complete disruption of micelles and the formation of microemulsion droplets. We study the whole range of the transition of wormlike micelles into microemulsion droplets and discover that it can be divided into three regions: (i) in the first region, the solutions retain a high viscosity (∼10-350 Pa·s), the micelles are entangled but their length is reduced by the solubilization of hydrocarbons; (ii) in the second region, the system transitions to the unentangled regime and the viscosity sharply decreases as a result of further micelle shortening and the appearance of microemulsion droplets; (iii) in the third region, the viscosity is low (∼0.001 Pa·s) and only microemulsion droplets remain in the solution. The experimental studies were accompanied by theoretical considerations, which allowed us to reveal for the first time that (i) one of the leading mechanisms of micelle shortening is the preferential accumulation of the solubilized hydrocarbon in the spherical end caps of wormlike micelles, which makes the end caps thermodynamically more favorable; (ii) the onset of the sharp drop in viscosity is correlated with the crossover from the entangled to unentangled regime of the wormlike micellar solution taking place upon the shortening of micellar chains; and (iii) in the unentangled regime short cylindrical micelles coexist with microemulsion droplets.

Effects of hydrophobic substituents and salt on core-shell aggregates of hydrophobically modified chitosan: Light scattering study

In this study we examine two methods of enhancement of aggregation of hydrophobically modified chitosan in dilute aqueous solutions: by increasing the content of n-dodecyl substituents, favoring hydrophobic association, and by increasing the amount of added low molecular weight salt, screening the electrostatic repulsion between similarly charged aggregating chains. By static and dynamic light scattering it was demonstrated that at the growth of the content of hydrophobic groups in the polymer (2-4 mol %) and of the amount of salt in solution (0.025-0.1 M) the weight fraction of aggregates increases, but the aggregation number remains unchanged. This behavior was attributed to the core-shell structure of the aggregates, which provides a low surface energy and strong attraction of associating groups inside the core. At the same time, the effects of the content of hydrophobic groups in the polymer and the ionic strength of the solution on the radii of the aggregates are quite different. Increasing the content of hydrophobic groups induces growth of the gyration radii of the aggregates, but does not affect their hydrodynamic radii. These data suggest the expansion of the hydrophobic core of the aggregates and the contraction of their highly swollen shell. On the other hand, increasing the salt concentration leads to a decrease of both the gyration and hydrodynamic radii of the aggregates, which is due to partial screening of electrostatic repulsion between similarly charged units and lowering of the osmotic pressure of counterions confined inside the aggregates.

Molecular mass characterization of polymers with strongly interacting groups using gel permeation chromatography–light scattering detection

This work is aimed at studying dilute solution behavior and developing techniques for the proper Mr characterization of polymers with strongly interacting groups. In particular, we have studied ionomers based on poly(dimethylcarbosiloxane) with carboxylic groups and hydrophobically modified polyacrylamide and its charged terpolymer. Gel permeation chromatography-light scattering (GPC-LS) study of organosilicon ionomers allowed us to follow molecular mass distribution evolutions during approximately a week that evidenced the gradual dissolution of clusters formed in bulk polymers. The rate of this process depends on the polymer composition and its preliminary treatment. Observed aggregates arre found to be stable during the procedure of chromatographic analysis. For characterization of hydrophobically modified polyacrylamide and its charged derivatives, we have used aqueous NaNO3-acetonitrile mixed solvent. Fluorescence spectroscopy with pyrene as a probe did not reveal any hydrophobic association in this solvent in contrast to aqueous solutions. Reversed-phase retention as well as polyelectrolyte exclusion were suppressed at GPC analysis. GPC-LC and classical LS methods gave consistent results for uncharged and weakly charged polymers. In the case of highly charged and highly hydrophobic terpolymers LS results evidenced association, while OPC-LC gave credible Mr values. We can conclude that in the last case, the observed association was weak enough to be disrupted in the coarse of chromatography.

Polyelectrolyte networks as highly sensitive polymers

This review presents the results of theoretical and experimental studies concerning collapse of polyelectrolyte gels. Along with classical investigations, a number of new effects are described; they are associated with the formation of ion pairs and with nonuniform distribution of low-molecular-mass counterions over the volume of the polyelectrolyte gel.

Two mechanisms of gel/surfactant binding

Abstract The interaction of poly(methacrylic acid) gel with cationic surfactant, cetylpyridinium chloride (CPC), has been studied at pH of the solution ranging from 6.0 to 2.0. It has been shown that the value of pH has a dramatic effect on the swelling behavior of the gel in the surfactant solution: at pH>2.5 the gel collapses, at lower pH the gel swells. At the same time, the total amount of CPC absorbed by the gel is only weakly dependent on the value of pH. The results were explained by the interplay of two mechanisms of binding of surfactant by the gel: electrostatic and hydrophobic. The former leads to the gel collapse, while the latter one induces the gel swelling. The contribution of each mechanism has been estimated. It has been shown that at high pH the electrostatic forces play an essential role, while with decreasing pH the gel ionization is suppressed and the hydrophobic interactions between the alkyl tails of the surfactant molecules and the gel chains become dominant. Thus, it has been demonstrated that the swelling behavior of the gel upon absorption of charged surfactant can be radically altered as a result of the changeover from electrostatic to hydrophobic mechanism of the surfactant binding.

Smart polymers for oil production

Two “smart” polymer systems have been developed for the oil producing industry. One system designed for water control in a producing well finds the site of the water influx by itself and blocks it. The other system is designed for use in hydraulic fracturing and acts as a thickener in an aqueous medium, including that at elevated temperatures; at the same time, in contact with hydrocarbons, the system transforms into a low-viscosity liquid, providing for a high permeability of the medium to petroleum.

Viscoelasticity of Smart Fluids Based on Wormlike Surfactant Micelles and Oppositely Charged Magnetic Particles

Novel viscoelastic smart suspensions based on cationic wormlike micelles (WLMs) of erucylbis(hydroxyethyl)methylammonium chloride and oppositely charged submicron magnetite particles in the presence of added low molecular weight salt were prepared and investigated. The suspensions demonstrate remarkable stability against sedimentation, which can be due to the incorporation of particles into the network of entangled WLMs by linking to energetically unfavorable micellar end-caps. Added particles enhance significantly the viscosity, the plateau modulus, and the relaxation time of the system, acting as additional multifunctional physical cross-links in the micellar network. The increase of plateau modulus stops when the concentration of particles reaches ca. 1.5 wt %, indicating that all micellar end-caps available in the system are linked to the particles. Further addition of particles may lead just to the redistribution of micellar ends between the particles without creation of new elastically active chains. The increase of rheological characteristics by added particles is more pronounced in suspensions with a smaller content of low molecular weight salt KCl when the WLMs are shorter in length and therefore contain a larger amount of end-caps responsible for the interaction with the particles. Magnetite particles not only enhance the rheological characteristics but also impart magnetoresponsive properties to the suspension. Upon application of magnetic field, the liquidlike system transforms into a solidlike one, demonstrating a constant value of storage modulus in all frequency range and the appearance of yield stress, which is due to the formation of field-aligned chainlike aggregates of particles opposing the flow. A combination of responsive properties inherent to both the matrix and the particles makes these smart fluids very competitive with other magnetic soft matter materials for various applications.