Mechanical properties and cross-linking structure of cross-linked natural rubber

Molecular singularity of cross-linked natural rubber was proposed to be a significant cross-linking junction of rubber, on the basis of the assignment of nuclear magnetic resonance (NMR) signals achieved in our previous work. This molecular singularity was observed to have an important role in the most outstanding mechanical properties of the rubber, through the investigation of the relationships between the structure of cross-linking junctions and the mechanical properties of various cross-linked rubbers. The various cross-linked natural rubbers were prepared by conventional vulcanization (CV), efficient vulcanization (EV) and semi-EV (SemiEV) to have nearly identical cross-link densities. These rubbers were analyzed using solid-state NMR spectroscopy. Using H-1-NMR spectroscopy, the amount of tertiary carbons linking to sulfur in the CV rubber was observed to be the same as those of the EV and SemiEV rubbers. C-13-NMR spectroscopy revealed that the number of quaternary carbons linking to sulfur in the CV rubber was the largest among the rubbers. The CV rubber exhibited the best mechanical properties; hence, a cross-linking junction of quaternary carbons linked to sulfur was assigned to the molecular singularity. Polymer Journal (2012) 44, 772-777; doi:10.1038/pj.2012.112; published online 6 June 2012