Effect of Residual Dipolar Interactions on the NMR Relaxation in Cross-Linked Elastomers

For elastomer networks above the glass transition temperature T g, a unified approach is presented to relate the residual dipolar couplings in various independent NMR experiments to the cross-link density. This is demonstrated on a series of cross-linked poly(styrene- co -butadiene) elastomers. The presence of dynamic physical and permanent chemical cross-links leads to a nonzero average of the homonuclear and heteronuclear dipolar couplings, which results in a solid-like NMR relaxation behavior. The residual dipolar couplings are expressed as a function of the effective number of statistical segments N e between the physical and between the chemical cross-link points, using a simplified network model with Gaussian statistics. These effective numbers are extracted for each sample of the series from the 13 C-edited transverse 1 H magnetization relaxation of the CH group. It is shown that the respective N e values can be used to scale the time domain of various NMR experiments such as (a) the free induction decay, (b) the 13 C-edited 1 H transverse magnetization relaxation, (c) the cross-polarization curves, and (d) the 1 H magnetization exchange between the CH and CH 2 groups. This proves the validity of the unified view on the dipolar interactions in elastomer networks and provides a way to estimate the cross-link density.