Structural Dynamics of Weak Polyelectrolytes in Aqueous Solution

Polyelectrolytes are polymers whose repeating units bear an electrolyte group. These groups dissociate in aqueous solutions (water), making the polymers charged. Both natural and synthetic weak polyelectrolytes are used in a variety of industries, mostly related to modifying flow and stability properties of aqueous solutions and gels. For instance, they are often used in water treatment, oil recovery as well as cleaning products like soaps, shampoos and cosmetics.

In many practical applications, the performance of weak polyelectrolytes is crucially related to their structural conformations. To get incisive insight of the conformational dynamics of polyelectrolytes, the research started from polyelectrolytes with simplest architectures: linear polyelectrolytes in dilute solution at single molecular level. For weak polyelectrolytes whose charge fraction is in ionization equilibrium with the solution, the effect of pH and added salts on conformations and electrostatic states was systematically studied and discussed.

Based on the understanding with linear polyelectrolytes, the study was subsequently expanded to polyelectrolytes with complex architectures, which compose a large group of materials including dendrimers, micelles, and branched-polymers. The study was primarily focused on star-shaped polyelectrolytes in dilute aqueous solution. Polyelectrolyte stars with various architectures were synthesized and labeled with fluorophores. The effect of arm density and arm length on the conformations of polyelectrolyte stars was explored by fluorescence spectroscopy in aqueous solution. Furthermore, the inhomogeneous charge distribution inside a star-shaped polyelectrolyte was investigated.

Along with the study of single polyelectrolyte in aqueous solution, the research was furthered to a more practical form, polyelectrolyte brushes, in ionic environments. Polyelectrolyte brushes with high tenability and stimuli-responsive “smartness” have been shown to be extremely useful in cell adhesion substrates, drug delivery, biosensors, microactuators, etc. In this study, the wetting property and swelling behavior of planar polyelectrolyte brushes were carefully examined in the presence of different salts under varied solution pH and compared with previous scaling theory. The immediate implications of these experimental results are related to design of “smart” surfaces with controllable charge distribution, membrane thickness and wettability.