All-Atom Molecular Dynamics Simulations of the Temperature Response of Densely Grafted Polyelectrolyte Brushes

HS Sachar and BS Chava and TH Pial and S Das, MACROMOLECULES, 54, 6342-6354 (2021).

DOI: 10.1021/acs.macromol.1c00922

Water acts a "good" solvent for most polyelectrolyte (PE) systems due to its high dielectric strength. Theoretical and experimental studies have highlighted some remarkably unconventional properties of strongly water- swollen PE brushes such as a reduction in brush height with an increase in temperature. However, an understanding of the atomistic scale response of the grafted PE chains as well as the local arrangement and organization of the brush-supported counterions and water molecules to changes in temperature is missing. Here, we conduct an all-atom molecular dynamics study to probe the influence of temperature on the "microstructure" of densely grafted, fully ionized polyacrylic acid brushes. The atomistic insights obtained from our study shed light on prior experimental observations and elucidate the key role played by changes in the hydrogen bonding network in dictating the hydrophilicity of the brushes. In addition to the temperature-mediated reduction in the height of the PE brush layer, we study the influence of temperature on several properties of the brush-supported water molecules such as the number distribution and kinetics of water-water and water-PE hydrogen bonds, mass density, orientational tetrahedral order parameter, and activation energy associated with the self-diffusion of water. The effect of temperature on the properties of the brush-trapped counterions is also quantified via changes in their solvation structure, counterion- water radial distribution function, and translational mobility. Our findings unravel several hitherto unknown phenomena such as the enhanced propensity for attaining PE brush-induced "water-in-salt"-like scenarios at elevated temperatures and an increase in the activation energy for the self-diffusion of water with an increase in the degree of PE brush- induced nanoconfinement (from "bulk water" to "sparsely grafted PE brushes" to "densely grafted PE brushes"). We anticipate that this work will be helpful in improving our predictive capabilities of the temperature response of water-swollen PE brushes, which would be instrumental for better design of several PE brush systems used in a multitude of applications.

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