Segmental motions dynamic screening effect

The longitudinal relaxation also called spin-lattice relaxation is induced by a quasi-resonant exchange which occurs between the Zeeman energy of the spin system and the thermal energy of molecules which carry nuclear magnetic dipoles. The condition of quasi-resonant exchange of energy is used to calibrate the time scale of relaxation of molecular 

It is considered that several monomeric units are involved in segmental motions which are observed from the spin-lattice relaxation rate. More precisely, it is assumed that collective rotational isomerizations of a small number of succesive monomeric units are converted into a translational random motion of a short segment. An effective friction coefficient of one 

The property of homogeneity of the coq/Ti functions is well illustrated from polybutadiene chains, observing of nuclei [14], Polybutadiene is a flexible polymer whose viscoelastic properties have been extensively studied over a very broad temperature-frequency range [11]. The first criterion of application of this analysis is the observation of a nearly 

This equation applies to any chain microstructure provided the Ti reference temperature is defined with respect to the glass transition temperature Tg, corresponding to the observed polybutadiene chain [11]. 

Such an analysis has been also extended to several solvent-poljnner systems. A simil U approach has been applied to polyisoprene in dilute solution in toluene [15]. The maximiim value of the relaxation time is found to be independent of polymer concentrations ranging firom 0.58 to 1 it is equ il to 20 s 1. This result shows that the effective number of interacting protons does not depend on the polymer concentration interactions between protons located on different chain segments can be neglected. 

---