Rubber-silica nanocomposites obtained by in situ sol-gel method: particle shape influence on the filler-filler and filler-rubber interactions

Silica-natural rubber composites were prepared by in situ sol-gel synthesis of silica nanoparticles functionalized with alkylthiol or alkylpolysulfide. The functionalizing groups were linked to silica particles by hydrolysis and polycondensation of a mixture of tetraethoxysilane (TEOS) with a suitable amount of (3-mercaptopropyl) trimethoxysilane (TMSPM), bis (3-triethoxysilylpropyl) disulfide (TESPD) or bis (3-triethoxysilylpropyl) tetrasulfide (TESPT). Particles from TEOS are spherical, instead those from TESPD, TESPT and TMSPM have irregular anisotropic shapes. This is due to the presence of isotropic or anisotropic interactions among the particle base units. Silica particles synthesized in the presence of TMSPM can also undergo condensation of the alkylthiol chains with the silanol groups, thus giving rise to a strong preferential direction for the anisotropic shape. Predominant filler-filler interactions and easy self-assembly were detected in particles from TEOS while those from TESPD and TESPT showed high filler-rubber interactions. Both filler-rubber and strong filler-filler interactions are present in silica particles synthesized by TMSPM. The dynamic mechanical properties of the composites, tested with stress-strain measurements, show that the storage modulus increases by increasing the filler-filler interaction, and it is maximum when also the filler-rubber interaction occurs. Strong silica-rubber interaction favors the silica dispersion in rubber, while it makes the filler network less compact and lowers the storage modulus.