- K. Saalwächter.
1H Multiple-Quantum Nuclear Magnetic Resonance
Investigations of Molecular Order in Polymer Networks: II. Intensity Decay and
Restricted Slow Dynamics.
J. Chem. Phys. 120, 454-464 (2004).
We present an approach towards the analysis of the intensity decay in proton
multiple-quantum experiments on polymeric networks in terms of slow fluctuations
of the residual dipole-dipole coupling tensor. Solutions for individual spin
pairs as well as the three-spin system of methyl groups are derived, and the
influence of the cycle time of the multiple-quantum pulse sequence is evaluated.
The multiple-quantum strategy discussed herein features the advantage that the
magnitude of the fluctuating part of the residual dipole-dipole coupling
constant and the correlation time of the slow process can be determined
independently of the integral residual coupling constant as well as its
distribution. The theory is applied to experiments on end-linked
poly(dimethylsiloxane) model networks with mono- and bimodal chain length
distributions, where it is found that, for all samples, correlation times of the
slow processes average to about 1 ms, and that the magnitude of the fluctuating
part of the dipole-dipole coupling is significantly smaller than the average
dipole-dipole coupling constant. This observation is interpreted in terms of
considerably restricted reorientations of topological constraints.