Structure of polyelectrolytes in poor solvent

Abstract

PACS. 61.25.Hq -Macromolecular and polymer solutions. PACS. 36.20.Ey -Polymer molecules -conformation. PACS. 87.15Aa -Theory and modeling; computer simulation.Abstract. -We present molecular dynamics simulations on strongly charged polyelectrolytes in poor solvent. The resulting pearl-necklace conformations are analyzed in detail. Fluctuations in the number of pearls and their sizes lead only to small signatures in the form factor and the force-extension relation, which is a severe obstacle for experimental observations. We find that the position of the first peak in the structure factor varies with the monomer density as ≈ ρ Polyelectrolytes (PEs) are polymers that carry ionizable groups that dissociate ions in aqueous solution. Technical applications range from viscosity modifiers, precipitation agents, superabsorbers to leak protectors [1]. In biochemistry and molecular biology they are of great importance because virtually all proteins, as well as DNA, are PEs.Many PEs contain a non-polar hydrocarbon backbone, for which water is a poor solvent. Therefore, in aqueous solution, there is a competition between the tendency to precipitate, the Coulomb interaction and the entropic degrees of freedom. This can lead to elongated strings of locally collapsed structures (pearls). Such necklace conformations have been predicted on the basis of scaling arguments in ref.[2] for a weakly charged single chain PE, and have been confirmed by simulations using the Debye-Hückel approximation [2,3] and with explicit counterions [4]. The formation of the necklace structure is due to the Rayleigh instability of a charged droplet, which leads to a split once a critical charge is reached. The size of the pearls is determined by the balance between electrostatic repulsion and surface tension. The distance between two pearls is governed by the balance of the electrostatic pearl-pearl repulsion and the surface tension. However, there is up to now no clear experimental proof for the existence of necklace chains [5,6]. Our previous papers [4] dealt with smaller system sizes and showed that necklaces exist also in the presence of counterions and exhibit a variety of conformational transitions as a function of density. The focus of the present paper is to analyze by extensive computer simulations in detail three possible experimental observables, namely the form factor, the structure factor and the force-extension relation, which can be probed by scattering and AFM techniques. Analyzing the fluctuations of necklace structures we find an extended coexistence regime between different necklace structures and broad distributions for the pearl sizes and the pearl-pearl distances that smear out the necklace signatures. In addition we find that the peak in the structure factor of the solution scales proportional to c EDP Sciences

Theory and simulations of charged polymers: From solution properties to polymeric nanomaterials

“…Molecular dynamics simulations of partially charged polyelectrolytes with explicit counterions in poor solvent conditions were performed by the Mainz group [39][40][41][42] and by Liao et al [43]. These simulations have confirmed that polyelectrolyte chains at low polymer concentrations form necklaces of beads connected by strings (see Fig.…”

Hydrogels in Poor Solvents: A Molecular Dynamics Study

“…For the most intriguing one, a pearl‐necklace ‐like shape of the chains, theoretical models for weakly41–43 and strongly44–47 charged chains have been developed. Computer simulation studies47–53 were successful in proving the existence of such conformations within single PE chain systems, providing valuable insight into their basic properties. Although the theory was developed for single chains, similar conformations of strands within the gel can be expected for a PE gel under poor‐solvent conditions24 where the scaling predictions change due to the network topology.…”

Lattice Monte Carlo simulations of three-dimensional charged polymer chains

“…Such issues as approaching the thermodynamic limit ͑if it exists at all͒, finite size effects, properties of more concentrated polyelectrolyte solutions, and the effect of solvent quality on charged chains, 36,37 are still open and attractive for future research. Such issues as approaching the thermodynamic limit ͑if it exists at all͒, finite size effects, properties of more concentrated polyelectrolyte solutions, and the effect of solvent quality on charged chains, 36,37 are still open and attractive for future research.…”