DYNAMICS OF LINEAR AND BRANCHED ALKANE MELTS - MOLECULAR-DYNAMICS TEST OF THEORY FOR LONG-TIME DYNAMICS

Molecular dynamics (MD) simulations of united atom models for alkane m elts are compared with a recently developed theory for calculating the memory functions of flexible polymers. The theory is based upon an ap proximate solution of the diffusion equation without hydrodynamic inte ractions. The polymer dynamics are described by using time correlation functions which are expressed in terms of a set of equilibrium averag es and the approximate eigenvalues and eigenfunctions of the diffusion operator. For flexible enough chains with sufficiently high molecular weight, the hydrodynamic interactions are screened, and the simplifie d solvent model used by the theory is expected to be adequate. The onl y parameter not defined by the MD simulations is the bead friction coe fficient zeta. In the limit of weak hydrodynamic interactions (Rouse d ynamics), zeta can be determined from the molecular diffusion coeffici ent by applying the Rouse relation D = kT/N zeta(R). Given this choice of zeta(R), the time correlation functions computed from the theory a re compared with those obtained directly from the MD simulations. Exce llent agreement with the simulations is found for all correlation func tions and all times for the decane dynamics, provided the theory emplo ys one scale factor to increase zeta(R) and, hence, to compensate for the inadequacy of the Rouse relation. The same picture holds for hexad ecane and triacontane (C30H62) but With Smaller scale factors. Scaling becomes unnecessary for C44H90 which is long enough for the crossover to Rouse dynamics for D to be almost complete. Very good agreement (a fter appropriate scaling of zeta(R)) also emerges between theory and s imulations for several branched alkanes with carbon numbers C-25-C-30. Computations for hexadecane at different temperatures show that the s cale factors may be weakly temperature dependent. (C) 1998 American In stitute of Physics. [S0021-9606(98)50921-X].