Molecular Evidences in Rubber Science and Technology by Means of Time-domain NMR Experiments

In the last years, time-domain NMR experiments performed on low-field spectrometers have shown their potential applicability on rubber science and technology¹. This feasible, versatile and novel experimental approach allows us to obtain molecular-level information from the always difficult to analyze vulcanized rubber samples. In this sense, this contribution would like to point out the recent developments on different fields in rubber science and technology, e.g. vulcanization^2-3, ageing and reinforcement⁴, by the application of proton low-field NMR spectroscopy. It is aimed at a better and more complete understanding of structure-property relationships for a rational design and development of rubber materials with advanced properties.

Application of multiple-quantum (MQ) NMR experiments allows the quantitative and complete characterization of the most important factors that determine rubber network structures (including rubber lattices³): non-elastic network defects, number of cross-links and their spatial distribution^1-3. As a consequence, new insights in the vulcanization were pointed out by studying the formation and evolution of rubber network structure along this complex process.

Following the same procedure, it is also possible to characterize the effect of ageing in rubber compounds by correlating the changes in the network structure with the most important mechanical properties of such compounds. These studies would allow a better understanding about the competing processes, e.g. chain scission and break down/formation of cross-links, which take place during rubber degradation.

In addition, time-domain NMR experiments are also sensitive to the filler-rubber interface providing molecular-level details about the reinforcing mechanism that govern the macroscopic properties of (nano)filled rubber compounds. In this sense, MQ-NMR spectroscopy provides quantitative information about the actual cross-link density in filled rubber compounds and it offers an exceptional opportunity to explore the filler-elastomer interface by combining such NMR data with other complementary experimental approaches (e.g. equilibrium swelling experiments)⁴. The filler-rubber interactions measured by MQ-NMR experiments have been related to the effective surface of interaction (fractal nature of filler particle aggregates) and the average surface free energy of filler particles. After a proper analysis of these factors, it is possible to evaluate the interaction density between filler particles and rubber polymer and the nature of such interactions. 

¹K. Saalwächter. Rubber Chem. Technol. 85, 350 (2012)

²J.L. Valentín, P. Posadas, A. Fernández-Torres, M.A. Malmierca, L. González, W. Chassé, and K. Saalwächter. Macromolecules 43, 4210 (2010).

³J. Che, S. Toki, J.L. Valentín, J. Brasero, A. Nimpaiboon, L. Rong, B.S. Hsiao. Macromolecules. 45, 6491 (2012).

⁴J. L. Valentín, I. Mora-Barrantes, J. Carretero-González, M. A. López-Manchado, P. Sotta, D. R. Long, and K. Saalwächter. Macromolecules 43, 334 (2010).