Translational Diffusion, Relaxation Times, and Quasi-Elastic Scattering of Flexible Chains with Excluded Volume and Fluctuating Hydrodynamic Interactions. A Brownian Dynamics Study

A polymer model with excluded volume interactions represented by a relatively soft potential is employed in Brownian dynamics simulations. The model and method have been shown in previous work to reproduce equilibrium properties and diffusion coefficients calculated with preaveraged hydrodynamic interactions. Trajectories obtained with fluctuating hydrodynamic interactions are now analyzed to obtain the diffusion coefficient, relaxation times of the Rouse coordinates, and the quasi-elastic scattering form factor, together with its first cumulant, of chains with different numbers of units. The main conclusions are as follows: an abnormal increase of the diffusion coefficient with respect to its preaveraged value for the shortest chains, tentatively attributed to effects in the relatively large surface of conformations with overlapping units. These effects seem to be eliminated through extrapolations to the long-chain limit. Thus, the extrapolated ratio of the radius of gyration to the hydrodynamic radius is higher than the estimations for unperturbed Gaussian chains and is in good agreement with upper and lower bounds. The relaxation times are consistently higher than those obtained for Gaussian chains. The results for the cumulants are generally lower than those predicted by Benmouna and Akcasu, the differences being attributed to the lack of consistency between the blob model and the chain models with long-range interactions employed in simulation work. Finally, the Pecora procedure to extract diffusion coefficients and relaxation times from the quasi-elastic form factor is favorably tested for the case of excluded volume chains.