Melts to Solutions: Structure, Dynamics, and Response of Polyelectrolytes

Abstract

The current research elucidates the changes in the structure and motion of polyelectrolytes as they are perturbed by solvents and shear flows. Polyelectrolytes are macromolecules whose backbone consists of long, predominantly hydrocarbon chains, substituted by ionizable groups, which tend to dissociate into ions in polar solvents. Their properties result from a delicate balance of elastic energy that arises from the polymeric backbone and electrostatic forces, which stem from the ionizable groups. Though polyelectrolytes have an immense potential to propel numerous technologies, from clean energy to drug delivery systems, understanding the delicate balance between the properties of the polymer backbone and the effects of the electrostatic interactions remains an open fundamental science challenge with an immense impact. The current study uses computational methods, particularly large-scale atomistic molecular dynamics simulations, to understand the balance between chain characteristics and electrostatic forces on a model polyelectrolyte, polystyrene sulfonate-sodium salt in the presence of solvents and under shear. As polymers are processed from molecules to viable materials under shear and in the presence of solvents, these studies resolve fundamental principles that directly contribute to the response of polyelectrolytes. The effects of two classes of solvents were studies, tetrahydrofuran (THF) that represents organic solvents and water which is an environmentally friendly, prevalent solvent. The structure and motion of these polymers were studied in their quiescent state, without any disruption, and under shear forces, because most polymeric molecules are transformed into actual materials under shear. We find that sulfonated polystyrene polyelectrolytes form distinctive branched ionic aggregates and form ionic networks as the sulfonation fraction increases. The addition of THF breaks some of the larger clusters, enhancing the polymer dynamics while having minimal effect on the ionic network. When perturbing these systems at high shear rates, the ionic clusters break, and almost all the chains elongate, while at the low shear rates, only some of the chains are affected. By introducing shear forces in the presence of a solvent, one can further disrupt and reshape these complex systems, enabling them to reassemble in new ways. As the water content increases, the polymer chains become more extended conformation but are not fully extended, even at very dilute polymer concentrations. When perturbing these solutions under shear, the chains undergo rapid coil-stretched-recoil cycles with a characteristic time that depends on the shear rate and polymer concentration. Understanding the behavior of polymer solutions under shear enables advancing the material processing to gain valuable properties from them