Force driven transition of a globular polyelectrolyte.

Abstract

We have systematically studied behavior of a flexible polyelectrolyte (PE) chain with explicit counterions, subjected to a constant force at the terminal ends. Our simulations reveal that in the hydrophobic regime, a PE globule abruptly opens to a coil state beyond a critical force Fc. At the transition point, the polymer shape shows large scale fluctuations that are quantified in terms of end-to-end distance Re. These fluctuations suggest that the system coexists in globule and coil states at the transition, which is also confirmed from the bimodal distribution of Re. Moreover, the critical force associated with the globule coil transition exhibits a nonmonotonic behavior, where surprisingly, Fc decreases with Bjerrum length lB in the limit of small lB, followed by an increase in the larger lB limit. Furthermore, this behavior is also validated from a theory adopted for the PE. From the free energy analysis, we have demonstrated that predominantly, the competition between the intrachain repulsive energy, counterion's translational entropy, and adsorption energy leads to the novel feature of nonmonotonic behavior of force.