Equilibrium and dynamic properties of polymethylene melts from molecular dynamics simulations. I. n-Tridecane

Abstract

Employing an explicit atom (EA) model of polymethylene, we have carried out molecular dynamics simulations of n-tridecane (C13H28) melts at experimental densities to compute both equilibrium and dynamic properties. The calibrated EA model reproduces quite well the experimental results of pressure, x-ray diffraction patterns, and self-diffusion constants at different temperatures. A united atom (UA) model that reproduces the experimental pressures also yields good agreement with experimental x-ray diffraction patterns and self-diffusion data, and the calculated degree of intermolecular orientational correlation is in good agreement with predictions of the EA model. However, the UA model yields significantly more extended chain dimensions than a previously investigated model, and, most importantly, significantly enhanced local chain dynamics compared to the EA model, as monitored by the chain vector reorientation and local torsional dynamics. The EA model simulations yield C–H vector orientational correlation times associated with each carbon of n-tridecane, in excellent agreement with experimental values deduced from C13-NMR T1 spin–lattice relaxation times. The C–H vector reorientation was found to be closely related to conformational jumps. These jumps, whose rates closely follow torsional correlation times, appear to occur mostly as unconcerted individual transitions for these short chains.