EFFECT OF RESIDUAL DIPOLAR INTERACTIONS ON THE NMR RELAXATION IN CROSS-LINKED ELASTOMERS

For elastomer networks above the glass transition temperature T-g, a u nified 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 polylstyrene-co-butadiene ) elastomers. The presence of dynamic physical and permanent chemical cross-links leads to a nonzero average of the homonuclear and heteronu clear dipolar couplings, which results in a solid-like MMR relaxation behavior. The residual dipolar couplings are expressed as a function o f the effective number of statistical segments N-e between the physica l and N-e(X) between the chemical cross-link points, using a simplifie d network model with Gaussian statistics. These effective numbers are extracted for each sample of the series from the C-13-edited transvers e H-1 magnetization relaxation of the CH group. It is shown that the r espective N-e values can be used to scale the time domain of various N MR experiments such as (a) the free induction decay, (b) the C-13-edit ed H-1 transverse magnetization relaxation, (c) the cross-polarization curves, and (d) the H-1 magnetization exchange between the CH and CH2 groups. This proves the validity of the unified view on the dipolar i nteractions in elastomer networks and provides a way to estimate the c ross-link density.