DYNAMICS OF N-ALKANES - COMPARISON TO ROUSE MODEL

The crossover to Rouse-type behavior for the self-diffusion constant D , the viscosity eta, and the equilibrium structural statistics of n-al kanes (6 less than or equal to n less than or equal to 66) is studied numerically. For small n the chains are non-Gaussian and the mean squa red end-to-end distance [R2] is greater than 6[RG2], where [RG2] is th e mean squared radius of gyration. As n increases, [R-2]/[R-G(2)] --> 6(1 + a/n), where a depends on the interaction model. At constant dens ity, the Rouse model is used to extract the monomeric friction coeffic ient zeta and the viscosity eta independently from the diffusion const ant D and the longest relaxation time tau(R). zeta(D) extracted from D is nearly independent of chain length while zeta(tau), obtained from tau(R) is much larger than ib for small n. The viscosity measured in a nonequilibrium molecular dynamics simulation is closely approximated by the Value of eta determined from tau(R), while eta inferred from D is smaller for small n. For n greater than or equal to 60, the two est imates for both zeta and eta agree as predicted from the Rouse model. D calculated from three interaction models is studied for increasing n and compared to experimental data. (C) 1998 American Institute of Phy sics.