Molecular dynamics simulations of dielectric relaxation of concentrated polymer solutions

Molecular dynamics simulations are conducted for concentrated solutions of flexible polymers. The results are contrasted with literature dielectric sp ectroscopy data, in an attempt to elucidate the observed phenomena from a m olecular level perspective. A bead-spring model is used and systems with ch ain sizes up to N=150 beads at reduced densities 0.5 less than or equal to rho less than or equal to 0.8 are studied. The dimensions of the chains fol low a universal behavior with rho/rho*, where rho* is the crossover density demarcating the onset of chain overlapping. All the chains are found to fo llow random-walk behavior. The global motion of the chains is investigated in terms of the dielectric loss E ". As in dielectric spectroscopy experime nts, the motion of the chains induces prominent dielectric relaxation at lo w frequencies. The shape of E " broadens with increasing density, and a nor mal-mode analysis indicates that overlapping of the chains with increasing density progressively renders the distribution of relaxation times more het erogeneous. For denser systems a second, smaller peak appears at the high f requency end of the spectrum. This secondary peak is not identified with se gmental motion, since the simulated chains lack components of the segmental dipoles perpendicular to the chain contour. Entanglement effects are inves tigated calculating the mean squared displacement g(1)(t), and the results suggest that the topological constraints of entanglements render at least t wo different relaxation mechanisms with disparate time scales important. An attempt to explain the shape of the spectra in terms of a phenomenological separation of the motion of chains into a rotational and a stretching mode showed that stretching plays no important role in the relaxation function and the shape of E-'. (C) 1999 American Institute of Physics. [S0021-9606(9 9)50427-3].