An optimized united atom model for simulations of polymethylene melts

Abstract

We present here an optimized united atom model that is able to reproduce properties of melts of n-alkane chains of varying molecular weights. This model differs from previous models in that the Lennard-Jones well depth for the terminal methyl group (0.2264 kcal/mol) differs from that of the methylene units (0.093 kcal/mol). The position of the minimum is at 4.5 Å for both units. Properties of n-C44H90 melts from this model are compared with experiments and those from an explicit atom model. Good agreement with experiment is obtained for static properties of the melt, specifically P–V–T behavior, chain conformations, and x-ray scattering profiles. The large-scale dynamics, as measured by self-diffusion, are found to agree reasonably well with experimental results, being about 30% faster with our best united atom force field. Analysis of the end-to-end vector orientation autocorrelation function in terms of the Rouse model yields a monomer friction coefficient somewhat greater than that determined from the rate of self-diffusion, reflecting the fact that the n-C44H90 chains are not sufficiently long to behave as Gaussian coils. Detailed local chain dynamics for n-C44H90 melts, as measured by the P1(t) and P2(t) orientation autocorrelation functions for C–H vectors, are found to agree reasonably well with results from simulations using an explicit atom model, and yield spin-lattice relaxation times T1 and nuclear Overhauser enhancement values in reasonable agreement with experimental 13C NMR measurements. As with large scale dynamics, local dynamics are faster in general (about 20%) than experimental results.