Nuclear magnetic relaxation in crosslinked elastomers subjected to uniaxial tensile drawing

For samples of crosslinked polyisoprene rubber subjected to stretching at different tensile strains λ, transverse magnetization decay is measured by NMR spectroscopy at a proton frequency of 25 MHz and second moment M ₂ of the residual dipole-dipole interaction is estimated. Theoretical dependence M ₂(λ) is constructed on the basis of the classical theory of rubber elasticity, which predicts that the distribution of distances between the junctions of chemical crosslinks is Gaussian and that deformation is affine. This theoretical dependence is substantially different from the experimental curve. Agreement between the theoretical and experimental data can be attained if the nonaffine character of deformation that is the consequence of the density distribution of chemical crosslinks within the volume of the sample is taken into account and when the Gaussian chain model is replaced with the Langevin model.