Effects of density on the local dynamics and conformational statistics of polyethylene: A molecular dynamics study

Molecular dynamics simulations of united atom (UA) linear polyethylene were performed as a function of density, for systems of two well-separated chain lengths (N=20, N=100). The role of the exact shape of the torsional angle potential is investigated by invoking two commonly cited alkane torsional potentials [Ryckaert-Bellemans (R&B), Steele]. The increase in second-neighbor torsional angle coupling with increasing density and decreasing conformational transition rates is presented quantitatively for the first time. The simulated local orientational dynamics were also studied by means of geometric autocorrelation functions (ACFs). These ACFs were analyzed by a method that emphasizes the distribution of relaxational processes, thereby providing specific information on the relevant spectral characteristics of each process. For all densities and chain lengths studied, two peaks are observed that are well separated in time. Only for the longer chains at the highest density studied does a third intermediate peak develop. Further insight into density-induced changes to the local dynamics is obtained through the distribution of angular jumps which give rise to decay of the geometric ACFs. (C) 2000 American Institute of Physics. [S0021-9606(00)51119-2].