Strain-induced 13C chemical shift change of natural rubber

We investigated the effect of deformation of natural rubbers (NR) resulting from the high centrifugal pressure of fast magic-angle spinning (MAS) on C-13 NMR spectra. The solid state C-13 MAS NMR spectrum with high-power H-1 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 C-13 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 C-13 NMR spectrum before MAS exhibited an isotropic and relatively narrow peak (FWHH is 150 Hz). Furthermore, the C-13 DD NMR peaks of the maximum elongated NR in a rotor apparently became a doublet. The static C-13 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 C-13 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 C-13 DD NMR spectra was therefore attributed to not only the molecular orientation but also the difference in magnetic susceptibility caused by the p electron interaction with the static magnetic field. Polymer Journal (2012) 44, 778-785; doi:10.1038/pj.2012.120; published online 20 June 2012