Dipolar and chemical shift anisotropy relaxation in a solid polymer measured by the NMR coherence time in the rotating frame

In this paper a new approach is proposed to the study of slow dynamics in solid complex systems, such as solid polymer or dense macromolecular aggregates. The peculiar aspect of this method is its ability to separate the relaxation contribution coming from the NMR interactions, which are commonly found in such systems, namely the direct dipolar coupling and chemical shift anisotropy. In this approach, the effect of such interactions on relaxation are, to some extent, separable and observable experimentally vs. temperature and vs. the effective resonant frequency. This is realised by measuring the coherence time in the rotating frame T, which is an unusual NMR parameter whose behaviour is strictly related to the orientation of a tilted rotating frame with respect to the Zeeman field axis. Such an orientation drives the role of the secular part of spin Hamiltonian and makes it controllable experimentally. Thus 'fine' interaction effects, like the chemical shift one, may be observed to overcome the main effect of the stronger direct dipolar interaction.