Correlation between Macroscopic Elasticity and Chain Dynamics of Natural Rubber during Vulcanization as Determined by a Unique Rheo-NMR Combination

The cross-linking process of natural rubber (NR) is studied for the first time by a unique combination of rheology and time-domain (TD) NMR, two methods that are commonly conducted separately. The combined setup consists of a high-end rheometer and a compact 25 MHz 1 H NMR relaxometer (rheo-NMR), which allows rheological characterization and NMR relaxometry to be operated simultaneously on the same sample. During vulcanization, the elastic modulus G ′ ( t ), representing the macroscopic elasticity, was directly correlated to the molecular dynamic parameters derived from NMR, including the population-averaged transverse relaxation rate, 1/ T 2ave, and the logarithmic mean of the residual dipolar coupling constant, D̅ res . It is found that, at the same level of macroscopic elasticity, the conformational flexibility of network chains in NR cross-linked with the so-called efficient vulcanization (EV) is more restricted than that with conventional vulcanization (CV). This can be attributed to the shorter and consequently less flexible (monosulfidic) cross-links formed by EV, whereas polysulfidic links dominate in the CV vulcanizate as confirmed by high-field 13 C NMR spectroscopy.