Conformations, inter-molecular structure and hydrogen bond dynamics of neutral and cationic poly(vinyl amine) in aqueous solution

Abstract

ABSTRACT Intermolecular structure, hydration and conformations of Poly(vinylamine) (PVAm) in neutral as well as protonated (cationic) condition as polyelectrolyte in dilute aqueous solution, were studied via single-chain atomistic molecular dynamics simulations in explicit solvent. A parametric study of atomic partial charges was carried out at different values of degree-of-ionisation (f) of the amine group. A modification of generic GROMOS force-field is developed. The unusual non-monotonic behaviour of radius-of-gyration with an increase in f is captured, in agreement with experimental results of intrinsic viscosity. Cooperative solvation of PVAm chain occurs with an increase in f as assisted by counter-ion condensation. Three different regimes of the variation of radius-of-gyration of PVAm with change in f are identified. Changes in polymer-water hydrogen bonding is not the dominant effect for the behaviour of chain-size variation. The non-monotonous expansion of PVAm is attributed to the complex interplay between hydrogen bonding and intermolecular structure of polymer chain, water and counter-ions, all of which provide a cooperative solvation of the chain. With an increase in f at the higher range, the formation of ion pairs produces salt-bridges and bends in the chain, as well as ion-pair–ion-pair repulsion extending the chain at complete ionisation.