Segmental and Local Chain Mobilities in Elastomers by 13C−H Residual Heteronuclear Dipolar Couplings

Abstract

Site-selective information about local segmental motions of individual CHn fragments can be obtained by the determination of 13C−1H heteronuclear dipolar couplings. A systematical study was performed by measuring heteronuclear residual dipolar couplings of natural rubber samples with different cross-link densities using static and MAS solid-state NMR techniques such as SEDOR (spin-echo double resonance), REDOR (rotational-echo double resonance) and heteronuclear double-quantum (HeDQ) buildup curves. Even under static conditions, all five CHn fragments of the monomer unit can be separated. 13C−1H residual dipolar couplings and scaled average order parameters extracted from the different experiments show a linear correlation with the cross-link density. However, the residual dipolar couplings are also dependent on the individual functional group. In other words, the local rotational motions of the functional groups are superimposed on the segmental motions of the chain segment and have to be taken into acco...