A simple NMR method to measure crosslink density of natural rubber composite

The crosslink density of carbon black-reinforced natural rubber (NR) was correlated with 1H chemical shift by liquid-state 1H NMR spectroscopy. Instead of observing the rubber directly with the solid-state NMR technique, we used toluene molecules in the liquid state as probes to explore the inner cavities of the rubber sample. Four NR composites with different carbon black contents of 20, 40, 60, and 80 phr were tested. The toluene signals were split into two: one from the bulk solvent and the other from inside the rubber. The chemical shift difference between those two split signals was observed to be in linear relationship with the carbon black content. The samples thermally aged at 70 °C for 100 days showed larger chemical shift differences than the unaged sample. Furthermore, the chemical shift difference was directly proportional to the apparent crosslink density obtained independently by the conventional swelling method. This result offers by far the simplest method to measure the apparent crosslink density in rubber.