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, *, 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 ϰN 2.2 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 Ͼ8*, 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.

Molecular dynamics simulations of dielectric relaxation of concentrated polymer solutions

“…1 Comparing our results with dielectric data we reached the same mapping for PI as the one proposed by Kremer and Grest. We identified crossover concentrations * that demarcate the dilute and semidilute regimes.…”

“…͑1.2͒, only the autocorrelation terms are considered, as it has been shown that cross-correlation terms are negligible for homogeneous solutions. 1,6 Using dielectric spectroscopy, Adachi and co-workers, 6-10 Imanishi and co-workers, 11,12 and Boese and Kremer 13 have investigated extensively the normal-mode relaxation of cis-polyisoprene ͑PI͒, a type-A polar polymer, in concentrated solutions and melts. There is a number of issues that were raised in these studies and it is of interest to employ molecular dynamics simulations to clarify them.…”

“…In a recent work, 1 we examined structural and dynamic characteristics of macromolecules in dilute and semidilute solutions of a good solvent using molecular dynamics simulations. The results were compared with dielectric spectroscopy experiments.…”

Relaxation of flexible chains in dilute and non-dilute systems. Dynamic Monte Carlo results for linear and star chains

“…A clear but less remarkable broadening is also shown in the simulation curves. A recent molecular dynamics study 26) also shows the same feature. In order to understand the origin of the differences between the Monte Carlo data and the Rouse theory, we have now fitted the q (t) Monte Carlo data to the expression…”

“…(12) and (13). Recent molecular dynamics simulations 26) also conclude that there is a range of chain lengths where the dielectric spectra are broadened due to chain overlapping but the normal mode relaxation times are not influenced by entanglement effects. It should be also noted that Watanabe et al 27) have explained the broad dielectric spectrum of the dielectric curves through the effect of non-sinusoidal normal modes.…”