NMR Imaging of Elastomers: A Review

From curiosity driven investigations about 10 years ago NMR imaging of materials has developed into a useful tool for characterization of polymers, in particular of elastomer products. A clear indicator of this development is the increasing number of imaging spectrometers in use at industrial research and quality-control laboratories. Typical applications of NMR imaging to elastomers are investigations of the homogeneity of a compound or product components like gaskets or tire profiles, studies of the aging behavior under different loading conditions, the mapping of stress, strain and temperature distributions, and the analysis of material change after application of an overload. Elastomers constitute a class of materials particularly suitable to NMR imaging, because they are rich in protons, the most sensitive, stable NMR nucleus, and the material is soft, giving rise to small dipole-dipole couplings and thus to comparatively narrow lines and favorable imaging conditions. Yet the residual dipole-dipole interaction which remains unaveraged from the thermal motion of intercrosslink chains is a most valuable source of information. It determines the relaxation times so that measurement of various relaxation parameters provides information on cross-link density, stress, strain, and temperature in rubber materials. Spectroscopic investigations on the other hand are not very informative, because they are sensitive to chemical change, which is often too small to be detectable under reasonable conditions even for chain scission or additional cross-linking during aging. The measurement of relaxation times does not require homogeneous magnetic fields, so that new NMR devices based on inexpensive, permanent magnets can be developed, which are applied to the surface of arbitrarily large elastomer products. The NMR MOUSE is such a mobile NMR scanner, which has been shown to be sensitive within limits to changes in cross-link density, strain and temperature. Thus NMR imaging is progressing from the acquisition of entire images which show distributions of material properties to the monitoring of volume-selective NMR information, which can be expected to be used not only for quality control and failure analysis but also for process control and on-line monitoring in the future.