Polyelectrolytes

Polyelectrolytes are macromolecules bearing a net charge. Interest in these unique materials spans numerous and diverse areas. They have been investigated because of their essential participation in biological systems, such as, conformation transition of DNA . In ion-exchange membranes, they have been used for ion selectivity, such as recovery and separation of precious and heavy metals. Also they have been explored in polyion gels for controlled release and phase transitions, such as, drug delivery . Their molecular structure can be tailored to allow large conformational changes with pH, temperature, or added electrolytes. Molecular parameters that significantly influence behavior include: hydrophobic/hydrophilic balance, molecular weight, number, type and distribution of charge on the macromolecular backbone, distance of charged moiety from the backbone, and counterion type . Solution properties including phase behavior, hydrodynamic volume, and binding can be altered, offering utilization in flocculation, adhesion, stabilization, compatabiiization, viscosification, and suspension.

Chain conformation and solubility depend on the extent of ionization and interactions with water. A characteristic feature of polyelectrolytes is the ability to attain large hydrodynamic volumes in deionized water at low concentrations . This effect is caused by coulombic repulsion between charged groups along the polymer chain that forces the chain into a rod-like conformation. In the presence of salt, coulombic repulsions are shielded, allowing the polymer chain to assume a more random, entropically favored conformation with a subsequent decrease in hydrodynamic volume. Electrostatic repulsions not only cause an increase in hydrodynamic volume but also increase shear sensitivity or non-Newtonian behavior.

The extent of ionization of polyelectrolytes depends on the relative base or acid strength, degree of solvation, and dielectric constant of the solvent. The structure and properties of acidic (anionic) and basic (cationic) polyelectrolytes, are discussed further.

Polyelectrolytes (Anionic) are macromolecules bearing a negative charge. Depending on the strength of the acid, they can strongly interact with positive charges. Anionic polyelectrolytes that have been studied extensively have primarily carboxylic or sulfonic acid functionality, including: poly(acrylic acid), poly(vinylsulfonic acid), poly(styrenesulfonic acid), poly(2-acrylamido-2-methylpropanesulfonic acid), poly(methacrylic acid), and their salts. Applications of anionic polyelectrolytes are diverse ranging from gels for medical applications to latex stabilizers and dye receptors in synthetic fibers.

Polyelectrolytes (Cationic) are macromolecules bearing a positive charge. Cationic functional groups can strongly interact with suspended, negatively charged particles or oil droplets. Polymers containing cationic charges can be segregated into three main categories - ammonium, sulfonium, and phosphonium quaternaries. The commercially significant ammonium polymers are quaternary polyacrylamides, polyamines, and polyimines, including polyvinylammonium polymers. Cationic polyelectrolytes are of great importance in many industrial applications. They are used as flocculants in the clarification of drinking water and in the clean up of industrial wastes, sewage, and sludges, and as retention aids in the papermaking industry.