Structural and dynamic characterization of elastomeric materials by time domain NMR spectroscopy

In the last decades, many efforts have been dedicated to the improvement of the mechanical properties of elastomeric composite materials, as they are particularly attractive for several industrial applications. As a matter of fact, these properties are mainly related to the motional constraints of the polymer network, which are due to the presence of physical entanglements and chemical cross-linking between polymer chains, and may be influenced by the presence of different additives and reinforcement fillers (carbon black, nanosilica, clays) (1,2). Usually, the cross-link density is monitored by mechanical measurements (modulus, strain at stress, etc.); however, these methods provide only macroscopic observables, but are not suitable for a description of the topology and dynamics of the polymer network at the molecular scale. Indeed, this knowledge is required to have a more complete view of the factors that correlate with the mechanical properties of elastomers and, consequently, to better address the design of optimized materials. In this context, low field 1H time domain (TD) NMR can give an important contribution (3). In this work, we have investigated the effect of the formulation (polymer, additives, filler) and the vulcanization conditions on the structural and dynamic properties of different elastomeric materials, with application in the tyre industry, by a combination of TD NMR methods. 1H Multiple Quantum (MQ) experiments (4) were used to evaluate the residual 1H-1H dipolar couplings, which arise from the fast anisotropic motion of the polymer chains and are thus directly related to the amount of topological constraints within the polymer network. 1H relaxation times (T1, T2) (5) were measured to probe a wide range of motional frequencies of the polymer chains. In particular, 1H spin-lattice relaxation times (T1) were evaluated by means of Fast Field Cycling (6) experiments at different temperatures, covering Larmor frequencies from 10 kHz to 35 MHz. References: 1. Scotti, R.; D'Arienzo, M.; Di Credico, B.; Giannini, L.; Morazzoni, F. Silica-Polymer Interface and Mechanical Reinforcement in Rubber Nanocomposites. In Hybrid Organic-Inorganic Interfaces; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2017; pp. 151-198. 2. Kraus, G. Reinforcement of Elastomers; Interscience Publishers: New York, 1965. 3. Borsacchi, S.; Sudhakaran, U.; Calucci, L.; Martini, F.; Carignani, E.; Messori, M.; Geppi, M. Polymers 2018, 10, 822. 4. Saalwächter, K. Prog. Nucl. Mag. Res. Sp. 2007, 51, 1-35. 5. Maus, A.; Hertlein, C.; Saalwächter, K. Macromol. Chem. Phys. 2006, 207, 1150-1158. 6. Kimmich, R. Field-cycling NMR Relaxometry: Instrumentation, Model Theories and Applications; The Royal Society of Chemistry, 2019.