The glass transition and interfacial layer in styrene-butadiene rubber containing silica nanofiller

The mechanical behaviour of a styrene-butadiene rubber, containing 23.5% styrene and filled with up to 55% by weight with silica nanoparticles, has been examined. The tan δ spectra of these composites show a second relaxation of relatively low intensity located about 40 °C above the main α relaxation, the glass transition Tg, which occurs at −34.5 °C in the unfilled polymer. This is attributed to an interfacial layer of polymer molecules whose chain relaxation dynamics have been altered by interaction with the filler surface. This is accompanied by a shift of the Tg itself to lower temperature when the filler surface is organophilic, but not when the filler surface is hydrophilic, and possible mechanisms for this are discussed. The amount of interfacial polymer can be quantified by resolving the glass transition and the second relaxation using a curve-fitting approach. A first-order treatment is presented which estimates the layer thickness to be of the order of a few nanometers.