Study On Non-equilibrium Dynamics Of Polyelectrolyte Brushes

Firstly, we report a molecular dynamics study on non-equilibriumdynamics of polyelectrolyte brushes under external electric fields. Inthis work, the effects of chain stiffness and salt concentration on staticand dynamic responses of the brushes are addressed in detail. Oursimulations indicate that varying these parameters induce richelectro-responsive behavior of the brushes. The increase of saltconcentration results in the enhancement of an opposite electric fieldformed by non-equilibrium distribution of cations and anions, whichresists stretching or shrinkage of grafted chains. At strong positiveelectric fields, the flexible brushes are more sensitive to the change ofsalt concentration. When reversing the electric field, the stiff brushesundergo a conformational transition from collapse to completestretching. At high salt concentrations, dynamic responsive magnitudeof the brush thickness to added electric field is strongly reduced. It wasfound that the fall time for the stiff brush becomes much shorter thanthat for the flexible brush. Additionally, increasing ion concentrationleads to an excess extension or shrinkage of flexible brushes. Forstrongly stiff brushes, such phenomenon occurs in the presence orabsence of salt.Additionally, we also have performed for electroosmotic flow(EOF) confined in a polyelectrolyte-grafted nanochannel under variable grafting density and normal electric field. With decreasing the value ofnormal electric field, the brush undergoes a collapse transition and iondistribution is changed significantly. The brush thickness increasesupon increasing the grafting density at positive and weak negativeelectric fields, while a reduced brush thickness is observed at strongnegative electric field. Our results further reveal that the flow velocityis not only dependent on conformational transition of the brush, butalso related to cation and anion distribution. At low grafting density,the EOF is almost completely quenched at high electric field strengthdue to strong surface friction between ions and walls. For the case ofhighly dense grafting, the flow velocity is influenced weakly within thebrush when varying the grafting density. Additionally, a bidirectionalflow occurs at an intermediate electric field. The investigation on fluidflux indicates that the fluid flux is insensitive to the grafting densitywhen normal electric field is removed. For non-zero normal electricfields, a significant change in the fluid flux is observed at low graftingdensities.