MICROSTRUCTURE AND MOLECULAR DYNAMICS OF ELASTOMERS AS STUDIED BY ADVANCED LOW-RESOLUTION NUCLEAR MAGNETIC RESONANCE METHODS

Abstract

ABSTRACT Nuclear magnetic resonance (NMR) certainly belongs to the most powerful spectroscopic tools in rubber science. Yet the often high level of experimental and in particular instrumental sophistication represents a barrier to its widespread use. Recent advances in low-resolution, often low-field, proton NMR characterization methods of elastomeric materials are reviewed. Chemical detail, as normally provided by chemical shifts in high-resolution NMR spectra, is often not needed when just the (average) molecular motions of the rubber components are of interest. Knowledge of the molecular-level dynamics enables the quantification and investigation of coexisting rigid and soft regions, as often found in filled elastomers, and is further the basis of a detailed analysis of the local density of cross-links and the content of nonelastic material, all of which sensitively affect the rheological behavior. In fact, specific static proton NMR spectroscopy techniques can be thought of as molecular rheology, and they open new avenues toward the investigation of inhomogeneities in elastomers, the knowledge of which is key to improving our theoretical understanding and creating new rational-design principles of novel elastomeric materials. The methodological advances related to the possibility of studying not only the cross-link density on a molecular scale but also its distribution and the option to quantitatively detect the fractions of polymer in different states of molecular mobility and estimate the size and arrangement of such regions are illustrated with different examples from the rubber field. This concerns, among others, the influence of the vulcanization system and the amount and type of filler particles on the spatial (in)homogeneity of the cross-link density, the amount of nonelastic network defects, and the relevance of glassy regions in filled elastomers.