Restricted segmental mobility can facilitate medium-range chain diffusion: A NMR study of morphological influence on chain dynamics of polyethylene

The influence of the morphology on the local motion in the noncrystalline regions and chain diffusion between crystalline and noncrystalline regions is studied in ultrahigh molecular weight linear polyethylene: The behaviors of samples of the same material crystallized from the melt and from solution are compared. The geometrical restrictions of the conformational transitions are probed via anisotropic NMR interactions, i.e., 13C-1H dipole-dipole couplings and 13C chemical shift anisotropy. As they are averaged out under MAS, recoupling techniques are applied to yield sideband patterns or quasi-static NMR spectra, from which residual anisotropics are determined. Chain diffusion is probed by 13C exchange NMR. As expected, the local conformational transitions are more restricted in the solution crystallized sample compared with the melt crystallized one. Moreover, the motional narrowing observed by both techniques indicates that the local mobility in the noncrystalline regions of the solution crystallized sample consists mainly of effectively axial motion of extended trans-conformers around their local chain axes. This facilitates the chain diffusion between the crystalline and the noncrystalline regions, being significantly faster in solution crystallized compared to melt crystallized samples, where the local mobility is much more isotropic. The implications of these results for the understanding of crystal thickening and cold drawing are discussed. © 2008 American Chemical Society.