Conformational Behavior of a Single Dipolar Chain under Stretching Force

Abstract

The stretching of a single dipolar chain by a tensile force acting on the chain end is studied using molecular dynamics simulations and theoretical considerations. In both theory and simulations, the dipoles are modeled as two oppositely charged beads; this approach makes it possible to account for the excluded volume of the charged beads and electrostatic interactions on the many-body level. In simulations, the main attention is paid to the conformational and mechanical responses of the polymer globule formed due to the charge fluctuation correlation under strong electrostatic coupling. When a force is applied, the globule undergoes a transition to an open coil in several stages. The most striking one is realized at rather strong electrostatic interactions when the stretching of the chain favors the "head-to-tail" arrangement of dipoles, with the formation of linear ionic aggregates oriented along the force vector. This state of the stretched chain appears as a quasi-plateau on the stress−strain curve. A diagram of the states of a dipolar chain under the action of a tensile force is constructed via analysis of the stress−strain dependences, as well as the microstructure of the chain, the orientational correlations of the dipoles at different magnitudes of the tensile force, and the strength of the electrostatic interactions. The developed theory allowed for the description the regimes of continuous and jump-like unfolding of the dipolar chain at f > 0.5 observed in simulations and defined by a fine interplay of conformational entropy, excluded volume, and electrostatic interactions. The results obtained provide deeper insight into the stretching behavior of ion-containing polymers in low-polar media, which is important for the design of stimuli-responsive systems.