Deuteron NMR resolved mesogen vs. crosslinker molecular order and reorientational exchange in liquid single crystal elastomers

Differences in the temperature behaviour of orientational ordering of structurally equivalent side-chain liquid single crystal elastomers (LSCEs) with 2H-labelled crosslinker and mesogen have been studied by deuteron quadrupole-perturbed NMR. The impact of nematic director reorientations on the deuteron NMR spectral shapes was analyzed in terms of a discrete reorientational exchange model. This provided for the determination of the degree of nematic director alignment and for the quantification of the influence of the reorientational exchange on the 2H NMR spectra in terms of two parameters, the nematic director orientational dispersion parameter σθ and the motional effectiveness parameter α. A comparative analysis of model simulations and experimental spectra reveals that mesogenic molecules in LSCEs exhibit faster reorientational dynamics as compared to crosslinker molecules and that mesogens and crosslinkers exhibit a similar and rather substantial static director orientational disorder.