Correlation between high power proton T2 NMR relaxation and macroscopic viscoelastic properties

The purpose of this work is to analyse the effect of the cross-link density on the (1)H NMR relaxation time, T(2), and to correlate this microscopic parameter with macroscopic viscoelastic properties in a series of vulcanized industrial Natural Rubber elastomers, NR, with different crosslink densities. The network structure of unfilled NR vulcanisates was analyzed by (1H) spin-spin NMR relaxation, dynamic mechanical experiments and analysis of stress-strain curves. The transversal nuclear magnetization decay is described by a relaxation function characterized by a time constant, T(2) characteristic of each sample. The effect of the cross-links density was expressed by the adjustable parameter 1/T(2). Then, relaxation curves were found to obey a superposition property by applying that scaling factor. Traditional techniques for the characterization of the network structure, such as mechanical measurements, are used and discrepancies are encountered in the values of the mean molar mass of the network chains, Mc. However the NMR T(2) relaxation method shows good correlation with mechanical tests.