Chain dynamics in elastomers as investigated by proton multiple-quantum NMR

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 temperature-dependent 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, multi pie-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.