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

Abstract

The molecular dynamics and orientation of vulcanized natural rubber (NR) stretched at a low extension ratio (α= stretched length/ original length) were studied by carbon-13 direct polarization-magic angle spinning nuclear magnetic resonance (13C DP-MAS NMR), 13C cross-polarization (CP)-MAS NMR, 13C DP NMR without MAS, 13C CP NMR without MAS and density functional theory (DFT) calculations. Gradual peak broadening was observed in the 13C 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 13C NMR spectra of uniaxially stretched NR (α=2) changed slightly depending on the angle, θ, between the stretching direction and the applied magnetic field, although the spectra of unstretched NR did not change even if θ was changed. Thus, it is noted that NR chains oriented slightly as a time average by stretching even at a low extension ratio, α=2, although there still exists rapid rotation around the oriented NR chain. Motionally narrowed anisotropies in the 13C 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. © The Society of Polymer Science, Japan (SPSJ) All rights reserved.