Characterization of cross-linked rubber materials via proton rotating-frame relaxation measurements

Proton relaxation times in the rotating frame, as obtained from the decay o f nuclear magnetization locked along a radio frequency field B-1, have been measured in a series of cross-linked natural rubber materials. These decay s are monoexponential and the relevant time constants, T-1p, can be monitor ed as a function of the radio frequency field strength B-1. At any B-1 valu e, they appear to be correlated in a very sensitive way to the cross-link d ensity. A novel interpretation of the T-1p dependence upon B-1 is presented . It is based on a theory describing the evolution of a two spin 1/2 system coupled by dipolar interaction in the presence of a spin-locking field. Th e measured rotating-frame relaxation rate is shown to be equal to the sum o f the motionally related 1/T-1p and a term arising from the radio frequency field inhomogeneity. Both terms are proportional to the square of the dipo lar interaction (the second moment in the present case where a distribution of dipolar interactions exists). Concerning changes of cross-link density, Tip measurements provide therefore a more direct information than transver se relaxation measurements (also performed in this study), the composite na ture of the latter decay curves making their interpretation less straightfo rward. As a complement to the present relaxation study, we show the correla tion of the self-diffusion coefficient of a solvent imbedded in the materia l with rubber T-1p values.