Effect of Polymer Chemistry on the Rheological Trends of Polyelectrolyte Complexes

Polyelectrolyte complexation is a phase separation phenomenon resulting from the electrostatic interactions between two oppositely charged polyelectrolytes. Polyelectrolyte complexes (PECs) are widely used in products like shampoo and produced by some organisms as bioadhesives. PECs benefit from aqueous processing and avoid high temperatures or organic solvents. Additionally, their properties can be tuned by modulating the length, charge density, and hydrophobicity of the polyelectrolyte. Furthermore, it is well understood that salt concentration strongly affects the mechanics of the PEC. Salt ions disrupt the electrostatic interactions between polyelectrolytes, which typically causes the complex to soften and swell. Thus, while electrostatics are known to be important, the effect of the identity of the ionizable group of the polyelectrolytes on the mechanics of the material is not well-understood. To bridge this knowledge gap, we are studying the effect of changing the identity of the ionizable group and its relationship with salt concentration on the linear viscoelasticity and phase behavior of these materials. Specifically, we examine the effect of anionic carboxylate and sulfonate groups, along with cationic amines with different degrees of methylation. These rheological studies give us insight into properties like viscosity and solid-to-liquid transitions. We apply time-salt superposition to combine data from individual experiments and a produce master curve for each of the systems to allow for a broader examination of how the mechanical properties can be tuned with salt concentration – from glassy complexes at low salt to viscoelastic liquids at higher salt concentration. Our preliminary data for PECs created from cationic poly(vinylamine) paired with either anionic poly(vinylsulfonic acid) or poly(acrylic acid) exhibit similar rubbery behavior from 0.05 M to 1.75 M potassium bromide. We have also determined that the different ionizable groups affect the interaction strength between the polyelectrolytes, with our preliminary data showing that cationic groups have a greater effect. By understanding how salt concentration and polymer chemistry affect PEC mechanics, we can design and aqueously process PECs for specific applications, presenting a more environmentally friendly option compared to current plastics.