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

• Published in: Macromolecules Vol. 33; no. 9; pp. 3359 - 3367
• Main Authors: Hu, Jian Zhi, Wang, Wei, Bai, Shi, Pugmire, R. J, Taylor, Craig M. V, Grant, D. M
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 02-05-2000
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis; Magic angle; Solid state; Polyethylene; Supramolecular structure; NMR spectrometry; Experimental study; Two dimension spectrometry
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma9916842

The principal values of the 13C chemical shift tensors and the 1H−13C 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 1H 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 13C 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 CH2 group dipolar coupling. The motion involved in the mobile amorphous structure approaches isotropic tumbling, but a modest amount of constrained reorientation remains. Consequently, the 13C−H dipolar coupling is averaged out, but the shift tensor is not totally averaged to its isotropic value.