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.
Highlights
Many existing studies focused on Nanoscale ionic materials (NIMs) made of nanoparticles with moderate/high surface charge density and polymers with relatively long chains[1,5,10]
In neat NIMs, mobility of the polymers strongly associated with the nanoparticle can be more than ten times less mobile than bulk polymers; mobility of the polymers weakly coupled with the nanoparticle, is only moderately smaller than that of bulk polymers
How are polymers distributed across the canopy – do they form two or more layers? What is the relative population of the polymers in different layers and how does it respond to the addition of electrolyte ions? What conformation do polymers in different layers adopt? Since polymers strongly associated with nanoparticles experiences crowding, their conformation should differ from that of the bulk polymers
Summary
Many existing studies focused on NIMs made of nanoparticles with moderate/high surface charge density and polymers with relatively long chains[1,5,10]. When enough electrolyte ions are introduced, the population of the strongly associated polymers diminishes and all polymers exhibit mobility similar to that in bulk[3] These seminal works greatly improved our understanding of the polymeric canopies, but some important practical questions and conceptual issues remain to be addressed. A common feature of these simulations is that the radius of the nanoparticles is small (~1 nm), the chain length of the polymers is short and comparable to the nanoparticle’s radius (e.g., the linear polymers used in ref.[12] have ~14 monomers), and the nanoparticles carry moderate surface charge density (e.g., −2 e/nm−2) Study of these systems revealed that the charged layer of the nanoparticles approaches each other closely and the charged site of polymers can bridge neighboring nanoparticles[12]. Some important parts of NIM’s parameter space were not explored and the structure and dynamics of the polymeric canopy can exhibit new physics not studied in these studies
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