(363f) Conformation of a Single Polyelectrolyte Chain in Poor Solvents: Globule, Pearl-Necklace and Vesicle

Understanding the conformation of a polyelectrolyte (PE) is not only a fundamental challenge in polymer science but also critical for understanding the folding and aggregation of proteins. Here, we develop a theory by systematically including the electrostatic interactions into the self-consistent field theory for polymers to study the conformational behaviors of a single PE in poor solvents. For the case of low salt concentration, as the backbone charge fraction of PE increases, our theory predicts that the spherical globule (Sph) can either be elongated to a series of pearl-necklace (PN) structures or be flattened to two novel structures that have not been reported before: biconcave red cell and toroid. While the PN structures are stable conformations, the two fattened structures are metastable. We find that the cylindrical globule, of which the stability is under debate, is an unstable structure. In addition, our theory reveals a different characteristic of the globule to pearl-necklace transition: the transition from the Sph to the PN with double pearls is discontinuous, whereas those from adjacent PN structures are continuous at finite salt concentrations. Furthermore, the effect of salt concentration on the chain conformation has also been investigated. We find that a stable vesicle structure can exist at intermediate concentrations, which is a result of the competition between the surface energy, electrostatic energy and the translational entropy of salt ions. The structure of the vesicle such as the volume of the interior hole and the thickness of the shell can be fine-tuned by controlling the external salt concentration. We also find that a metastable “double vesicle” structure exists intermediate PN and vesicle, revealing an interesting kinetics of conformational transition. Our work greatly improves the understanding of the conformational behaviors of polyelectrolytes and broadens the applications of PE as smart soft nanoparticles which can be used in sensors, drug-delivery vehicles and nanoporous materials.