K. Saalwächter, A. Heuer.
Chain Dynamics in Elastomers As Investigated by Proton
Macromolecules 39, 3291-3303 (2006).
The influence of segmental and cooperative dynamics in elastomers on the time
evolution in NMR experiments is considered using static proton multiple-quantum
experiments. A novel experimental strategy as well as Monte Carlo simulations of
a simple rotational diffusion model is used to investigate the applicability of
the Andersen-Weiss approximation, upon which our analytical solutions are based.
On the basis of temperaturedependent experiments, the validity of one of the
most popular models used so far for the interpretation of NMR experiments is
disproved. This model, which is based on slow rotational diffusion of a
preaveraged residual coupling tensor, is refined by explicit consideration of
the time scale of fast segmental processes which average the intrasegmental
dipolar coupling toward a plateau value that is related to the chain order
parameter associated with topological restrictions by chemical and physical
cross-links. Models based on exponential or power-law loss of correlation are
shown to provide physically reasonable representations of the data. As opposed
to various earlier approaches, multiple-quantum NMR is unique in that it can be
used to verify or falsify the different models. Data measured on permanently
cross-linked systems are only weakly influenced by slow, cooperative processes,
while reptation dynamics in corresponding linear-chain melts effectively
prevents the observation of a well-defined order parameter.