Characterization of Cross-Linked Rubber Materials via Proton Rotating-Frame Relaxation Measurements

Proton relaxation times in the rotating frame, as obtained from the decay of nuclear magnetization locked along a radio frequency field B1, have been measured in a series of cross-linked natural rubber materials. These decays are monoexponential and the relevant time constants, T1ρ, can be monitored as a function of the radio frequency field strength B1. At any B1 value, they appear to be correlated in a very sensitive way to the cross-link density. A novel interpretation of the T1ρ dependence upon B1 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. The measured rotating-frame relaxation rate is shown to be equal to the sum of the motionally related 1/T1ρ and a term arising from the radio frequency field inhomogeneity. Both terms are proportional to the square of the dipolar interaction (the second moment in the present case where a distribution of dipolar interactions exists). Concerning changes...