Line broadening in the ¹³C NMR spectra of bulk polymers above T_g

Abstract

Abstract The effects of magic‐angle sample spinning (MAS), high‐power decoupling, and resonance frequency on the 13 C NMR linewidths of bulk polyisobutylene and bulk trans ‐polybutadiene are examined. The 13 C linewidths increase with resonance frequency, are unaffected by high‐power decoupling, and are reduced to different extents by MAS. The dominant contribution of the natural linewidth of the polyisobutylene lines is confirmed. The two carbons of trans ‐polybutadiene have approximately equal linewidths under all conditions, a result that eliminates residual chemical shift anisotropy as a major contributor to the linewidths. The large reduction of the trans ‐polybutadiene linewidths with MAS, coupled with the above result, suggests that microscopic variation of magnetic susceptibility is the major factor for this semicrystalline polymer. Cross‐polarized 13 C spectra of trans ‐polybutadiene were obtained with and without MAS. With MAS, resonances due to the crystalline and amorphous components were resolved. The principal components of the chemical shift tensor of the vinylene carbons were obtained from the spectrum without MAS.