Monte Carlo simulations of hydrophobic polyelectrolytes: Evidence of complex configurational transitions

Off-lattice Monte Carlo simulations, including electrostatic and Lennard-Jones potentials, are performed to investigate at the limit of counterion condensation the dilute conformations of quenched polymer chains in poor solvent conditions. The Monte Carlo (MC) search procedure is improved to achieve dense conformations in the limit of both strong attractive and repulsive interaction potentials. Configurational properties such as the radius of gyration, and single chain structure factors are calculated as a function of attractive monomer–monomer interactions, ionic concentration, and monomer number. It is observed that hydrophobic polyelectrolytes exhibit a large variety of conformations compared to flexible or semiflexible polyelectrolytes without hydrophobic groups. MC results demonstrate that there is a range of electrolyte concentration and hydrophobicity for which polymers exhibit exotic but stable conformations, namely the pearl necklace and the cigar-shape conformation. It is shown that by gradually increasing the monomer hydrophobicity of a strong polyelectrolyte, it undergoes a cascade of transitions from an extended structure to a pearl necklace, a pearl necklace to a cylinder, and a cylinder to a coil, successively. Smooth transitions are observed when competition between hydrophillic and hydrophobic interactions is important, whereas sharp transitions are obtained in the other cases. A good agreement is observed with the theoretical description of weakly charged polyelectrolytes in poor solvent conditions. To have an insight of the spatial organization of the chain monomers, scattering functions are calculated for each characteristic conformation. It is shown that large changes in the scattering curve profiles can be expected when the hydrophobicity of the backbone has a definite influence on the chain dimensions.