Journal of Chemical Physics, Vol.111, No.3, 1325-1334, 1999
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 spectroscopy data, in an attempt to elucidate the observed phenomena from a molecular level perspective. A bead-spring model is used and systems with chain 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 follow a universal behavior with rho/rho*, where rho* is the crossover density demarcating the onset of chain overlapping. All the chains are found to follow random-walk behavior. The global motion of the chains is investigated in terms of the dielectric loss E ". As in dielectric spectroscopy experiments, the motion of the chains induces prominent dielectric relaxation at low frequencies. The shape of E " broadens with increasing density, and a normal-mode analysis indicates that overlapping of the chains with increasing density progressively renders the distribution of relaxation times more heterogeneous. For denser systems a second, smaller peak appears at the high frequency end of the spectrum. This secondary peak is not identified with segmental motion, since the simulated chains lack components of the segmental dipoles perpendicular to the chain contour. Entanglement effects are investigated calculating the mean squared displacement g(1)(t), and the results suggest that the topological constraints of entanglements render at least two 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-'.