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We investigated the effect of deformation of natural rubbers (NR) resulting from the high centrifugal pressure of fast magic-angle spinning (MAS) on 13C NMR spectra. The solid state 13C MAS NMR spectrum with high-power 1H dipolar decoupling (DD) of NR presents liquid-like, very narrow signals (full width at half height, FWHH, is less than 10 Hz). However, the static state 13C DD NMR spectrum of the NR after MAS presented broad and apparently anisotropic peaks for the restricted elongated (doughnut-shaped) NR (FWHH is approximately 280 Hz), although the static 13C NMR spectrum before MAS exhibited an isotropic and relatively narrow peak (FWHH is 150 Hz). Furthermore, the 13C DD NMR peaks of the maximum elongated NR in a rotor apparently became a doublet. The static 13C DD NMR spectrum of a strip cut from the maximum elongated NR showed an angle dependence to the static magnetic field, and the dependence was explained from the molecular orientation. The angle-dependent 13C NMR spectra of a strip cut from the doughnut-shaped NR differed from those of the maximum elongated NR. The magnetization measured using a SQUID (superconducting quantum interference device) indicated that the magnetic susceptibility of the strip cut from the doughnut-shaped NR was different from the maximum elongated (rolled) NR. The difference in the angle dependence of the 13C DD NMR spectra was therefore attributed to not only the molecular orientation but also the difference in magnetic susceptibility caused by the π electron interaction with the static magnetic field.

Strain-induced 13C chemical shift change of natural rubber