Effects of silanol group on the hybridization of rubber and silica

A formation mechanism of bound rubber in the silica-filled polyisoprene composites was discussed on the bases of transmission electron microscopy observation of silicas with or without rubber matrix and of NMR results of bound rubber. The averaged size of agglomerate which was formed without rubber matrix increased with increasing the silanol number per unit surface area (N) of silica particles and with decreasing the amount of water adsorbed on the silica surface. The silicas with low number of N (N<0.1 (1/nm(2))) could not form aggregates and/or agglomerates. Qualitatively similar results were obtained for silicas in the rubber matrix. The amount of bound rubber increased with increasing the number of N. In addition, no trace of bound rubber was found in the silica-filled rubber composite which was prepared from silicas with low number of N. These results suggest that the bound rubber corresponds to the entrapped rubber phase in the agglomerate. This consideration was supported by information on the chain mobility of bound rubber phase.