A molecular dynamics study of macromolecules in good solvents: Comparison with dielectric spectroscopy experiments

Abstract

Molecular dynamics simulations are employed to study the static and dynamic properties of macromolecules in dilute and semidilute solutions of a good solvent. The results are compared with dielectric spectroscopy experiments. Crossover concentrations, ρ*, that demarcate the dilute and semidilute regimes are identified. The shift from self-avoiding-walk to random-walk behavior is also studied. An investigation is conducted of the normal-mode dielectric relaxation of type-A polar polymers. In dilute systems, a power law molecular weight dependence of the normal mode relaxation times τ∝N2.2 is observed, in accordance with DeGennes’s scaling analysis for self-avoiding-walk chains. This result does not agree with the experimental dependence of the normal mode relaxation time on the size of the polymers. The differences between the simulations and the experimental dynamic results in the dilute regime can be ascribed to the leading assumption of the model, the neglect of hydrodynamic interactions. For higher concentrations ρ>8ρ*, hydrodynamic and excluded volume effects are effectively screened, and the simulation results conform well with experiments. The dielectric spectrum broadens with increasing density, and for low densities the broadening can be explained in terms of chain overlapping. For densities higher than the entanglement density, entanglement effects broaden the spectrum considerably. A mapping of the model parameters to real polymer properties is also proposed.