Shape Instabilities of Polyelectrolytes
School of
Polyelectrolytes are charged polymers where some fraction f, of the chains monomers are charged. This imparts to the polelectrolyte many desirable properties, which make these polymers useful in many industrial applications. In a poor solvent, an (uncharged) polymer will tend to collapse to a spherical globule, so as to minimize polymer-solvent contact. Polyelectrolytes in poor solvents also collapse, but due to charge repulsion along the chain it was originally proposed the collapse would be to a cylindrical globule. Subsequently, it was shown this cylindrical globule is unstable to capillary wave fluctuations and the pearl necklace, consisting of spherical beads joined by long strings of polymer, is the optimal shape. Although this was predicted over a decade ago, direct evidence (i.e, from imaging such as using Atomic Force Microscopy where the polyelectrolyte absorbs on the solid substrate) of the pearl-necklace structure has not been definitive. Thus we theoretically analyze how a substrate can interact with a polyelectrolyte in a poor solvent. We show that the substrate can infact inhibit the shape instability and a new shape, an elliptical globule, becomes stable. The results are validated with three independent theoretical techniques.