COMPARISON OF EQUILIBRIUM AND DYNAMIC PROPERTIES OF POLYMETHYLENE MELTS OF N-C44H90 CHAINS FROM SIMULATION AND EXPERIMENTS

Results of molecular dynamics simulations of n-C44H90 melts, based on an explicit atom model that reproduced experimental equilibrium and dynamic properties of n-C13H28 melts, were found to reproduce experimentally observed P-V-T behavior and X-ray scattering profiles of the higher molecular weight system. Moreover, the simulated local chain dynamics yielded C-13 NMR spin-lattice relaxation times (T1) and the nuclear Overhauser enhancement (NOE) in quite good agreement with experimental values measured for different carbon positions and temperatures. The C-H vector P2 autocorrelation functions for the interior carbons from the simulations exhibited long-time tails that yielded NOE values less than 3.0. The experimental T1 values for the internal carbons of n-C44H90 in the melt differed only by a factor of 2 from those found for a high molecular weight polyethylene chain in the melt, indicating that a reasonably good understanding of local polymer dynamics is possible from our simulations of n-C44H90. Correlation times of the torsional autocorrelation function and the C-H vector reorientation for interior carbons were found to correspond to the average conformational transition times. Moreover, preliminary examination of the conformational transitions shows strong correlation with the neighboring bonds, especially with the second and fourth neighbors.