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

ABSTRACT

Structural 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 φ_p) and charge density f. PAA size (R_g, R) decreases with φ_p 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 φ_p. The number of h-bonds between carboxylic acid groups and carboxylate groups remain unaffected by φ_p. 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.

KEYWORDS:

• Polyelectrolyte
• aqueous
• molecular dynamics
• semi-dilute concentration
• aggregates
• h-bonds
• self-diffusion

Acknowledgements

The authors are very grateful to the availability of the supercomputing ‘Virgo’ cluster facility at IIT Madras on which all NPT production simulations were carried out. Energy minimisation and NVT MD simulations were performed on individual workstations.

Disclosure statement

No potential conflict of interest was reported by the authors.