Relaxation Dynamics in Polyelectrolyte Aqueous Solutions in Non-linear Shear Flows: A Molecular Dynamics Simulation Study

The interactions of polyelectrolytes with water are multifaceted, where this highly polar solvent would preferentially reside in the vicinity of the ionic domains, and concurrently collapses the hydrophobic polymer backbone, forming inhomogeneous dynamic systems. Their response to shear is at the core of their function and is of particular significance to novel recycling and upcycling strategies for waterborne plastic waste. A molecular-level insight into distinct dynamics of ionic assemblies and hydrophobic domains of polyelectrolytes in water is attained by fully atomistic molecular dynamics simulations using a model ionizable polymer, sulfonated polystyrene (molecular weight ~ 11 kg/mol), 148 chains at sulfonation fractions f= 0.55, 0.75 and 1.00, neutralized with Na⁺ counter ions in Tip4p water model. As a function of increasing shear rates, the system displays an initial elastic response followed by shear rate-dependent and ionic fraction-dependent stress overshoot and, eventually, a steady state. At high shear rates, ionic assemblies break and rearrange, and coiled polymer chains fully open up. The polymer relaxation dynamics after shear will be discussed.