Molecular dynamics of strongly coupled multichain Coulomb polymers in pure and salt-added Langevin fluids

The multichain effect and also the effect of added salt on randomly copolymerized charged polymers (polyampholytes) in a Langevin fluid are studied with the use of molecular dynamics simulations. The monomers of opposite signs tend to form loose complexes, which makes the Coulomb force attractive on average. With multichain polyampholytes, the typical state at high temperature is a container-bound one-phase state of separated chains with a substantial void among them. The association and dissociation processes occur repeatedly, with the former process a few times faster than the latter. A glass transition occurs when temperature is lowered. A compact and glassy globule in a segregated phase, which resembles that of a single-chain polyampholyte, is a typical state at low temperature due to the Coulomb force. The probability of losing this state is as low as P_dis∼exp(-N^3/2), with N the number of monomers. The critical temperature defined by overlapping of the chains increases with molecular weight and stiffness of the chains, and is less sensitive to the number of the chains. An alternate charge sequence makes a difference only when its block size is quite small. The addition of salt suppresses the formation of a dense globule by shielding the electric field; however, this is not effective when the salt ions are not allowed to penetrate well into the globule.