OBSERVATIONS IN SOLID POLYETHYLENES BY C-13 NUCLEAR MAGNETIC-RESONANCE WITH MAGIC ANGLE SAMPLE SPINNING

It is shown that the 13C NMR resonance lines of the crystalline component (CC) and the noncrystalline component (NCC) in polyethylene (PE) are chemically shifted from one another. This usually produces an upfield shoulder on the CC resonance line in a “cross-polarization with magic angle sample spinning” (CPMASS) experiment. Two peaks were resolved in a low crystallinity, high molecular weight sample, the NCC peak being 2.36 ± 0.1 ppm upfield from the CC peak. The chemical shift of the CC peak was measured against external Me4Si to be 34.06 ± 0.26 ppm. Measurements of the chemical shifts of liquid n-C17H36 and cyclohexane relative to external Me4Si were also made, and the chemical shift trends are discussed in terms of trans and gauche conformational effects on the chemical shift. Transverse relaxation times, T2C, were measured for various samples; the natural T2C broadened full width at half-height for the CC peak ranged from 1.00 to 2.6 Hz with a resonant proton decoupling field, v1H, of 64 kHz. The corresponding natural line width for the NCC peak was measured to be 14 ± 4 Hz, and it is presumed that this width is determined by molecular motion. The NCC peak has a substantial line width contribution from isotropic chemical shift dispersion. The capability of the Cpmass technique for resolving closely spaced, weak resonances was tested using a branched and a radiation cross-linked linear PE. Only methyl groups are completely resolved with a second resonance partially resolved in the cross-linked sample. It is noted that the Cpmass technique does not yield true relative intensities for the NCC and CC resonances. © 1979, American Chemical Society. All rights reserved.