Structure and Dynamics of Polymeric Canopies in Nanoscale Ionic Materials: An Electrical Double Layer Perspective

Abstract

Nanoscale ionic materials (NIMs) are an emerging class of materials consisting of charged nanoparticles and polymeric canopies attaching to them dynamically by electrostatic interactions. Using molecular simulations, we examine the structure and dynamics of the polymeric canopies in model NIMs in which the canopy thickness is much smaller than the nanoparticle diameter. Without added electrolyte ions, the charged terminal groups of polymers adsorb strongly on charged walls, thereby electrostatically “grafting” polymers to the wall. These polymers are highly stretched. They rarely desorb from the wall, but maintain modest in-plane mobility. When electrolyte ion pairs are introduced, the counterions adsorb on the wall, causing some electrostatically “grafted” polymers to desorb. The desorbed polymers, however, are less than the adsorbed counter-ions, which leads to an overscreening of wall charges. The desorbed polymers’ charged terminal groups do not distribute uniformly across the canopy but are depleted in some regions; they adopt conformation similar to those in bulk and exchange with the “grafted” polymers rapidly, hence dilating the canopy and accelerating its dynamics. We understand these results by taking the canopy as an electrical double layer, and highlight the importance of the interplay of electrostatic and entropic effects in determining its structure and dynamics.

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“…The desorbed polymers adopt conformations similar to those in bulk. There is an interplay of electrostatic and entropic interactions that determine the structure and dynamics of the canopies [144]. However, these initial efforts were based on ionically grafted nanoparticles ionically bonded to oligomers and did not contain an entangled polymer matrix.…”

“…In a system without added salt, the charged terminal groups of the canopies adsorb strongly on oppositely charged walls, due to the attractive electrostatic interaction. Such ionic canopies are stretched and do not desorb from the oppositely charged surface [ 144 ]. In a system that contains electrolyte ions, the counterions adsorb on the charged walls, thus causing some ionic canopies to desorb from the charged surface.…”

“…The desorbed polymers adopt conformations similar to those in bulk. There is an interplay of electrostatic and entropic interactions that determine the structure and dynamics of the canopies [ 144 ].…”