Investigation of Polyethylene by Means of Magic Angle Turning and Separated-Local-Field Experiments

The principal values of the 13 C chemical shift tensors and the 1 H− 13 C separated-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 shift tensor, and the local 1 H field of the tensor, for the all-trans crystalline structures with long and short T 1 are essentially the same, supporting the previous suggestion by Schmidt-Rohr and Spiess that chain diffusion between the amorphous and the crystalline regions in PE is the primary reason for the multiexponential 13 C T 1 relaxation observed for the all-trans peak observed at 33 ppm. The less mobile amorphous component adopts preferably a trans conformation and undergoes a fast rotation dominantly about the molecular axis at a rate larger than the width of the CH 2 group dipolar coupling. The motion involved in the mobile amorphous structure approaches isotropic tumbling, but a modest amount of constrained reorientation remains. Consequently, the 13 C− 1 H dipolar coupling is averaged out, but the shift tensor is not totally averaged to its isotropic value.