Effects of bead-bead interactions on the static and dynamical properties of model polymer solutions

The effects of segment-segment interactions on the static and dynamical pro perties of model polymer solutions are examined by Brownian dynamics simula tions in the free-draining limit over a wide concentration range. A bead-an d-spring model is used to describe the polymer chains at a coarse-grained l evel, in which segment-segment interactions are represented by a bead-bead pair potential with a Gaussian analytic form, beta u(ev)(r) = A exp(-r(2)/2 sigma(2)), where beta = 1/k(B)T and A and sigma are characteristic energy and distance scales, respectively. The chain dimensions, self-diffusion coe fficient, and viscosity of the systems are studied as functions of number d ensity of beads of the system, rho, at given excluded-volume potential para meters, A and sigma. Our results show that in the limit of infinite dilutio n even for short chains (N similar to 10) there is statistically significan t scaling behavior in the static and dynamical properties. For a system wit h given values of A and a the change in polymer coil size shows a realistic trend as the concentration of the system increases. In the dilute and conc entrated regions the coil size decreases as a result of increasing intercha in repulsions, while in the highly concentrated region the coil size increa ses again, showing a return to Rouse-like behavior because the intrapolymer and interpolymer segment-segment interactions become effectively indisting uishable for an arbitrary bead and to a large extent are "balanced out." In the limit of infinite dilution, the self-diffusion coefficient of the cent er of mass, D-cm, depends on N only and not on the potential parameter A, w hile in contrast the specific viscosity eta(sp) depends on both N and A. As the concentration increases D-cm decreases and eta(sp) increases consisten t with the behavior of real polymers. When the system becomes highly concen trated, however, both D-cm and eta(sp) unrealistically return to the Rouse limit. This suggests that from the concentrated region upward in concentrat ion, the entanglement or the topological constraints caused by the physical connectivity of the chains significantly influence their dynamical behavio r. The mean-field segment-segment interactions or excluded-volume effects i ncorporated in the current coarse-grained bead-spring approach cannot captu re this entanglement effect, and therefore give rise to unrealistic dynamic al behavior in the concentrated regime. [S1063-651X(99)05911-5].