Local dynamics of poly(ethylene oxide) in solution .2. Vector autocorrelation functions and motional anisotropy

The anisotropy of the local dynamics of poly(ethylene oxide) in toluene solution has been characterized using molecular dynamics simulations and NMR coupled spin relaxation experiments. Ratios of second rank correlation times calculated from the trajectories, such as that between correlation times describing the reorientation of vectors parallel (tau(2)) and perpendicular (tau(perpendicular to)) to the local chain axis, reproduce all trends seen in values obtained from NMR coupled spin relaxation experiments. The magnitude of the anisotropy is larger than that observed for PEO in aqueous solution but smaller than that of polyethylene in biphenyl solution. Correlation function shapes are markedly nonexponential, in contrast to those seen for recent simulations of polyisoprene in solution but similar to those seen for n-C44H90 melts. Comparison of correlation functions calculated from regions of the trajectory near and between conformational transitions indicates that transitions have a role in producing larger anisotropies and more nonexponential correlation functions. Decay in correlation functions in the absence of conformational transitions supports the suggestion that coupled small amplitude motions of groups of torsions (coupled librations) can be an important relaxation mechanism.