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Magnetic anisotropy effects in broadline NMR spectra of polytetrafluoroethylene

1962; Wiley; Volume: 61; Issue: 172 Linguagem: Inglês

10.1002/pol.1962.1206117211

1542-6238

C. W. Wilson,

Advanced MRI Techniques and Applications

Abstract The effects of molecular magnetic anisotropy upon the broadline NMR spectrum of a bulk polymer have been recognized and measured for the first time. The F 19 NMR absorption spectra in polytetrafluoroethylen (PTFE) show a line‐shape asymmetry over a broad range of temperatures which can be explained by the magnetic anisotropy of C‐F bonds. Nuclear magnetic resonance studies of PTFE samples whose crystalline regions are aligned by cold drawing have made possible an estimate of the anisotropy of C‐F bonds in this material: Δσ, the magnetic anisotropy of the screening constant, is found to be 171 ± 15 ppm; the screening of each F 19 nucleus is paramagnetic, being maximum when the direction of its C‐F bond is perpendicular to the large static magnetic field and minimum (probably zero) when parallel. Static magnetic susceptibility measurements of PTFE show the predicted diamagnetism, and only a small (<5%) static magnetic anisotropy of the molecular crystals is found.