Effect of Crosslinking Junctions as a Molecular Singularity Point on Mechanical Properties

Oraphin Yamamoto^{1, 2}, Yoshimasa Yamamoto³, and Seiichi Kawahara^1*

Molecular singularity of crosslinked natural rubber was proposed to be a significant crosslinking junction of the rubber, based on the assignment of NMR signals achieved in our previous work. It was found to play an important role in the most outstanding mechanical properties of the rubber, when the relationships between structure of the crosslinking junctions and mechanical properties for various crosslinked rubbers were investigated. The various crosslinked natural rubbers were prepared by conventional vulcanization (CV), efficient vulcanization (EV) and semi-efficient vulcanization (SemiEV) to be almost similar crosslink density. They were analyzed through solid-state NMR spectroscopy. The amount of tertiary carbons linking to sulfur of the CV rubber was the same as those of the EV and SemiEV rubbers, when the rubbers were analyzed by ¹H-NMR spectroscopy. ¹³C-NMR spectroscopy revealed that the amount of quaternary carbons linking to sulfur of the CV rubber was largest among the rubbers. The mechanical properties of the CV rubber were the most excellent; hence, the quaternary carbons linking to sulfur as crosslinking junction was assigned to the molecular singularity.

Key words: Natural rubber, Crosslinking junctions, NMR, Mechanical properties