Long-Time Molecular Motions and Local Chain Dynamics in n-C44H90 Melts by Molecular Dynamics Simulations

Abstract

Dynamic properties of n-C 44 H 90 melts have been investigated via molecular dynamics simulations with emphasis on long-time molecular motions and their connection to local chain dynamics. Correlation of conformational transitions is also examined. The model employs an atomistic force field which includes hydrogen atoms explicitly and reproduces the conformational energy surface ofn-butane and n-hexane, in good agreement with recent ab initio electronic structure calculations. The simulations yield self-diffusion coefficients which are in good agreement with experiment. However, the monomer friction constants calculated from the self-diffusion coefficients are considerably smaller than estimates from the Rouse model fit of the end-to-end chain vector reorientation and experimental melt viscosities, indicative of the fact that these chains are not long enough to assume Gaussian coils. The local bond vector orientation autocorrelation functions (OACF) exhibit important contributions from long relaxation time modes, resulting in long-time tails in the OACF. In these cases the OACF can be accurately described only by empirical functions and theoretical models which account for the long relaxation time modes. Relaxation times of the long-time tails for the local bond vector OACF approach those of the end-to-end vector OACF of the C 44 H 90 chains and are due to correlation of the relaxation of the local bond vectors to the relaxation of the chain backbone vectors. The torsional autocorrelation functions also show a long-time component which can be described reasonably by exponential-like decay. The relaxation times for the long-time component of the torsional autocorrelation functions are much shorter than was found for the long-time modes of bond vector OACF, indicative of local dynamic heterogeneities. For conformational transitions, directly correlated transitions are found to occur for the self, second, and fourth neighbors. Moreover, the second-neighbor correlated transitions appear to propagate more or less randomly along the chain.