Structure development in silica-filled rubber composites

Abstract

Structure development in silica filled polyisoprene composites was investigated by wide line pulsed NMR and high-resolution solid-state NMR combined with transmission electron microscopy. The rubber phase in the composites was composed of so-called bound rubber and free rubber. Furthermore, the bound rubber was composed of multiple phases having different mobilities. The amount ofbound rubber and its mobility depended on the sort of silica used. NMR results revealed that chemical bonding was negligible between silanol groups and rubber molecules. The principal formation mechanism of bound rubber was physical adsorption of rubber molecules onto the silica surface. In addition, the amount of bound rubber was greatly affected by the state of aggregation of silica particles which was a function of particle size and amount of silanol groups on the particles. When the composites were annealed, the amount of bound rubber increased with increasing annealing temperature. 13 C-DD/MAS-NMR spectra of the bound rubber revealed that the chemical structure of polyisoprene molecules changed with the heat treatment. The results suggest that the increase of bound rubber with heat treatments was primarily due to the development of chemical crosslinks in both bound and free rubber phases.