Structural analyses of the bound rubber in silica-filled silicone rubber nanocomposites reveal mechanisms of filler-rubber interaction

Although the reinforcing effect of fumed silica on silicone rubber is always better than that of precipitated silica, their enhancement mechanism is still controversial due to the cognitive defects of the interface effect and filler network effect of polymer nanocomposites with the limited experimental study. Herein, significant entanglement network structure differences were revealed in the fumed and precipitated silica-filled phenyl silicone rubber by removing the physisorbed rubber gradually via 90 °C-ultrasonic solvent extraction method, which eventually obtained tightly bound rubber to explain filler-rubber interaction. The silica wrapped by tightly bound rubber was observed by SEM and TEM, where fumed silica is entangled with tightly bound rubber to robust dendritic structure, while precipitated silica is entangled with tightly bound rubber to the irregular agglomerate structure. Low-field 1H NMR confirmed that the mobility of the tightly bound rubber was significantly lower than loosely bound and free rubber. Furthermore, during the US treatment, the Si–O–Si infrared peaks of fumed silica-filled rubber exhibited "blueshift-redshift-blueshift" while the precipitated silica one only "blueshift" appears, which directly confirmed that different nanoporous structures of nanofiller induced diversity of filler-rubber entanglement network. The tightly bound rubber of precipitated silica (16.71%) was much more than that of fumed silica (3.72%) (40 phr). Although fumed silica adsorbs less tightly bound rubber than precipitated silica, the prominent nanofiller networks formed by fumed silica-rubber greatly enhanced the Payne effect and damping properties. This work can help to deeper understand the formation of the filler-rubber entanglement network and its contribution to the physical properties.