L. Z. Rogovina

Definition of the concept of polymer gel

The current ideas concerning polymer gels are outlined. The following definition of gel is proposed: a gel is a solid composed of at least two components, one of which (polymer) forms a three-dimensional network by virtue of covalent or noncovalent bonding (chemical and physical gels, respectively) in the medium of the other component (liquid), wherein the minimum amount of the liquid is sufficient for ensuring the elastic properties of the gel, although it may exceed tens to hundreds of times the amount of the polymer. It is noted that, at a high network density or high polymer-chain rigidity, the formation of fragile gels is possible. A general feature of physical gels is the existence of the yield point. Earlier results, which suggest the existence of two yield points in polymer gels, as found in studies of gelatin hydrogels over a wide concentration range, are suveyed.

Rheological properties of solutions and gels of combined systems hydrophobically modified polyacrylamides-new viscoelastic cationic surfactants

The rheological properties (viscosity and dynamic elasticity modulus) of solutions and gels of combined systems composed of hydrophobically modified polymers containing main-chain charged groups or lacking these groups and of newly synthesized viscoelastic cationic surfactants with different amounts of OH groups and long nonpolar saturated and trans monounsaturated radicals have been studied. It has been shown that the maximum viscosity of solutions of polymer-surfactant complexes corresponds to the very low concentration of the surfactant. Regions where homogeneous and heterogeneous combined solutions and gels exist are considered. An increase in the viscosity of the combined solutions with temperature is discovered and discussed. The synergistic effect of polymers and surfactants is demonstrated, and a more efficient role of surfactants in the case of uncharged polymers is revealed. The roles of the number of OH groups and the length of the nonpolar radical in a surfactant molecule and a difference in the properties of systems containing surfactants with saturated and unsaturated radicals are considered.

Molecular-Mass Characteristics and Association Behavior of Weakly Charged Hydrophobically Modified Poly(acrylamides)

The evolution of compositional heterogeneity and molecular-mass distribution in the course of micellar polymerization of weakly charged hydrophobically modified poly(acrylamides) has been studied. The relationship between the molecular characteristics, association behavior, and rheological properties of aqueous solutions and gels of these copolymers has been examined. The molecular-mass distribution broadens with conversion owing to accumulation of a low-molecular-mass fraction depleted of the hydrophobic monomer. A change in the molecular-mass distribution has a strong effect on the viscoelastic behavior of the polymers. The properties of the copolymers are dependent on the pH of the reaction mixture: the copolymers prepared in the alkaline medium show a more pronounced ability to experience hydrophobic aggregation than the copolymers prepared in the acidic medium. The aqueous solutions and the gels of these polymers are characterized by higher viscosities and dynamic moduli. The rheological characteristics of the hydrophobically modified poly(acrylamides) are improved with an increase in both their molecular mass and the length of hydrophobic sequences with a constant molecular mass.

Local mobility in network junctions of gels based on hydrophobically modified polyacrylamides

Spin-probe ESR spectroscopy is employed to study the local mobility in junctions of physical networks in gels of hydrophobically modified polyacrylamides. These junctions are formed owing to self-association of alkyl groups containing 12 carbon atoms. It is shown that, irrespective of the mode of covalent binding between the alkyl groups and the polymer backbone (via either ester or amide groups) and the degree of macromolecule blockiness, the local mobility in the gel network junctions is almost an order of magnitude lower than that in the hydrophobic core of a micelle of the low-molecular-mass surfactant sodium dodecyl sulfate.

Local mobility of micelles of new cationic surfactants and their interactions with hydrophobically modified polyacrylamides

The local dynamics and organization of micelles of new long-chain cationic surfactants with saturated hydrocarbon fragments (from C16 to C22) are investigated via the EPR spin-probe technique. The local mobility of spin probes in the hydrocarbon core of a micelle changes insignificantly, while the order parameter noticeably increases with lengthening of the hydrocarbon fragment of the surfactant molecule. The specific features of the interaction of the surfactants with network junctions of the gels formed by two types of hydrophobically modified polyacrylamides—either containing charged groups (sodium acrylate) in the backbone or lacking these groups—are studied. In both cases, the local mobility of network junctions of the gel increases after the introduction of the surfactant (C18). Moreover, for surfactant with a long alkyl group (C2), the microscopic viscosity of the gel based on the uncharged polymer decreases, although the local mobility of the network junctions increases. Possible causes of the observed specific features are discussed.

Effect of salts on local mobility and rheological properties of micelles of new long-chain surfactant

The effects of organic (sodium salicylate) and inorganic (KCl) salts on the rheological properties of micellar solutions and the local characteristics (local mobility and ordering) of micelle cores is studied for a cationic surfactant containing a long (C18) unsaturated alkyl radical. The polar head of the surfactant contains two hydroxyl groups. The local characteristics are determined employing spin probe ESR spectroscopy. It is shown that the incorporation of a salt into a micellar solution reduces the local mobility of radicals of surfactant molecules in micelle cores and increases their local Lordering and the viscosity of the solution. Sodium salicylate has a stronger influence on the solution viscosity and the local characteristics of micelle cores than KCl does. Variations in the local characteristics of micelle cores under the action of the salts are in close correlation with variations in the rheological properties of the micellar solutions.

Diversity of polymer gels and the main factors determining the properties of gels and related solid polymers

This paper is a survey of the authors’ main results on the properties and structure of polymer gels of diverse nature that are related to the ideas of S.P. Papkov and G.V. Vinogradov. The properties of physical and chemical gels of various compositions and structures and of solid polymers prepared on their basis are considered in relation to the polymer concentration and thermodynamic quality of a solvent during gel preparation. It is demonstrated that the conditions of gel synthesis affect the dependence of the elastic modulus of the gel on the volume fraction of the solvent in it. The properties of the gels based on model chemical networks for soft- and stiff-chain polymers are compared.

Local mobility and rheological characteristics of gels based on hydrophobically modified poly(acrylamides)

For hydrophobically modified poly(acryl amide), we analyze the effect of various parameters of the macromolecular structure (number and length of side hydrophobic groups, content of charged groups, type of bonding between side chains and the polymer backbone, and the degree of blocking of hydrophobic groups distributed along the chain) on the local mobility of physical network junctions and rheological characteristics of gels. We have found that the local mobility measured by the method of spin-probe EPR spectroscopy is either independent or it slightly depends on the above parameters. At the same time, these parameters exert a strong effect on the rheological characteristics of gels. This disagreement can be explained by the fact that local mobility of junctions is primarily controlled by the intermolecular interactions of hydrophobic groups and by the covalent bonding between these groups and a macromolecule. However, the rheological characteristics depend on the number of junctions, their dimensions, and other parameters.

Conductivity of nonaqueous solutions of aromatic polymers containing ionizable side groups

The concentration dependence of the equivalent conductivity has been measured for dilute DMAc solutions of K, Na, and Li salts of aromatic polymers with ionizable side groups [poly(diphenylenesulfophthalide) and poly(arylene ether ketone) with side COOH groups]. A correlation between the concentration dependences of the equivalent conductivity and reduced viscosity shows that the first parameter initially sharply decreases with an increase in concentration and then achieves a limiting value at a rather low concentration and that precisely in this concentration range the polyelectrolyte effect manifests itself in the concentration versus reduced viscosity curve. With an increasing amount of metal ions incorporated into the polymer, the equivalent conductivity grows. It was demonstrated that the type of counterion affects the reduced viscosity and equivalent conductivity of salt solutions. This effect depends on the structure of the polymer backbone: for poly(arylene ether ketone) salts, the reduced viscosity and the equivalent conductivity increase in the sequence Li → Na → K, while for poly(diphenylenesulfophthalide) salts, the reduced viscosity increases in the reverse order and the equivalent conductivity remains almost invariable. The limiting values of the equivalent conductivity and the degree of association of polyelectrolytes have been estimated in terms of the Arrhenius-Ostwald theory. It has been shown that, in the case of poly(diphenylenesulfophthalide) salts, the equivalent conductivity and the degree of association are higher than those for poly(arylene ether ketone) salts with side COOH groups. This finding is indicative of a stronger binding of counterions by poly(arylene ether ketone).

Dynamic mechanical and thermal characteristics of homopolymers and copolymers containing arylene ether ketone units with carboxylic side groups and their salts

Properties of a poly(arylene ether ketone) with carboxyl side groups, copolymers containing units of this homopolymer, and their salts with different degrees of neutralization by alkali metals were studied by the methods of dynamic mechanical analysis, thermogravimetric analysis, and differential scanning calorimetry. By varying the nature of a metal atom and the degree of neutralization of carboxylic groups of a homopolymer and their content in copolymers, one may change the properties of polymers, including their ability to form ion pairs and, hence, their glass transition temperatures. For polymer salts at the 100% neutralization of carboxylic groups, the storage modulus and the glass transition temperature are shown to increase with decreasing the radius of the metal ion. The thermal stability of poly(arylene ether ketone) with carboxylic side groups and copolymers with different contents of such groups may be controlled by varying the nature and content of the introduced metal.