Molecular dynamics and orientation of stretched rubber by solid-state super(13)C NMR

The molecular dynamics and orientation of vulcanized natural rubber (NR) stretched at a low extension ratio ( alpha =stretched length/original length) were studied by carbon-13 direct polarization-magic angle spinning nuclear magnetic resonance ( super(13)C DP-MAS NMR), super(13)C cross-polarization (CP)-MAS NMR, super(13)C DP NMR without MAS, super(13)C CP NMR without MAS and density functional theory (DFT) calculations. Gradual peak broadening was observed in the super(13)C DP-MAS NMR spectra of stretched NR with an increasing extension ratio, indicating that the molecular mobility of NR chains is restricted by stretching. The static super(13)C NMR spectra of uniaxially stretched NR ( alpha =2) changed slightly depending on the angle, [thetas], between the stretching direction and the applied magnetic field, although the spectra of unstretched NR did not change even if [thetas] was changed. Thus, it is noted that NR chains oriented slightly as a time average by stretching even at a low extension ratio, alpha =2, although there still exists rapid rotation around the oriented NR chain. Motionally narrowed anisotropies in the super(13)C spectra of stretched NR and the directions of chemical shift anisotropy principal axes determined by DFT calculations suggest that isoprene units of oriented rubber chains in stretched NR rotate rapidly around the axis that almost aligned with the C=C bond direction of polyisoprene.