Dynamics of conformations, hydrogen bonds and translational diffusion of poly(methacrylic acid) in aqueous solution and the concentration transition in MD simulations

The dynamic behaviour of chain conformations, hydrogen bonds and translational diffusion of aqueous poly(methacrylic acid) (PMA) solution as a function of polymer volume fraction ℙp across dilute to concentrated regimes inclusive of the pure polymer amorphous state was studied by molecular dynamics simulations. The behaviour of the relaxation time (τ) of the backbone dihedral angle auto-correlation function (ACF) reveals slower relaxation at higher level of polymer concentration and the existence of a concentration-driven relaxation transition for the aqueous polymer solution which occurs in the polymer volume fraction range, specifically 54%<ℙp < 82% for this system. The relaxation constant τ for backbone dihedral angle exhibits a linear variation with ℙp, indicating a first-order kinetic transition. The intermittent ACF for decay of the H-bond correlation shows that H-bonds among water molecules relax faster than those of the PMA-PMA and PMA-water type. The relaxation rate of PMA-water H-bonds shows a decrease up to ℙp = 72% and becomes faster at ℙp = 82% due to the confining influence of neighbouring PMA chains. PMA-water and water-water H-bond dynamics show transitions around ℙp = 72% PMA. With increase in ℙp PMA diffusion coefficient decreases exponentially and water diffusion coefficient decreases linearly, in agreement with experimental observations using fluorescence and nuclear magnetic resonance (NMR) spectroscopic studies.