Investigation of polyethylene by means of magic angle turning and separated-local-field experiments

The principal values of the C-13 chemical shift tensors and the H-1-C-13 se parated-local-field patterns are reported for the all-trans structures with long and short T-1 as well as the more and less mobile amorphous segments in polyethylene. It is shown that the principal values of the chemical shif t tensor, and the local H-1 field of the tensor, for the all-trans crystall ine structures with long and short T-1 are essentially the same, supporting the previous suggestion by Schmidt-Rohr and Spiess that chain diffusion be tween the amorphous and the crystalline regions in PE is the primary reason for the multiexponential C-13 T-1 relaxation observed for the all-trans pe ak observed at 33 ppm. The less mobile amorphous component adopts preferabl y a trans conformation and undergoes a fast rotation dominantly about the m olecular axis at a rate larger than the width of the CH2 group dipolar coup ling. The motion involved in the mobile amorphous structure approaches isot ropic tumbling, but a modest amount of constrained reorientation remains. C onsequently, the C-13-H-1 dipolar coupling is averaged out, but the shift t ensor is not totally averaged to its isotropic value.