Structure and dynamics of atactic Na+-poly(acrylic) acid (PAA) polyelectrolyte in aqueous solution in dilute, semi-dilute and concentrated regimes

Abstract

ABSTRACTStructural and dynamic properties of aqueous solution of atactic poly(acrylic) acid (PAA) in dilute, semi-dilute and concentrated regimes were studied by fully atomistic molecular dynamics simulations with explicit solvent description, as a function of polymer concentration c (i.e. volume fraction φp) and charge density f. PAA size (Rg, R) decreases with φp in semi-dilute and concentrated regimes, due to increase in counter-ion condensation. For all values of f, in dilute regime (c < c*) chains are expanded and in semi-dilute regime (c*< c < c**) chains are in contact with each other, while for c ≅ c** aggregates comprising of few PAA chains occur (at f = 0.2, 0.4 and 0.7). Number of PAA intrachain h-bonds is greater than PAA–PAA interchain h-bonds at all values of f and φp. The number of h-bonds between carboxylic acid groups and carboxylate groups remain unaffected by φp. The Na+ ion self-diffusion coefficient shows linear decrease with concentration for f < 1 and exponential decrease for f = 1. The PAA self-diffusion coefficient shows power law decrease with concentration for f < 1 and exponential decrease for f = 1. Aggregation of chains is favoured due to PAA–PAA interactions with increase in concentration. Our simulation results are in agreement with experiments and coarse-grained simulations in the literature.