Understanding the Reinforcement Effect of Fumed Silica on Silicone Rubber: Bound Rubber and Its Entanglement Network

In this work, fumed silica was compounded with silicone rubber and processed under two different extraction conditions to quantitatively analyze bound rubber for studying filler–rubber interaction. When nanocomposites were treated with toluene at 90 °C under ultrasonic (US) irradiation, physisorbed polymer chains were more substantially removed when compared to the traditional room temperature (RT) extraction method. The bound rubber fraction of silicone rubber filled with 40 phr of silica was 32.85% by RT treatment, while the US-treated one was reduced to 6.20%. A thin layer of tightly bound rubber adsorbed on the silica surface could be obtained by US treatment as observed by scanning electron microscopy and transmission electron microscopy. Low-field ¹H NMR results confirmed that the obtained tightly bound rubber chains were strongly constrained. The attenuated total reflectance-infrared spectra showed that the formation of tightly bound rubber caused redshift of the Si–O–Si characteristic peaks of silica, and the obtained tightly bound rubber has lower total surface energy than the rubber matrix, indicating the formation of a more stable structure. The reagglomeration potential energy of silica during the curing process was significantly reduced due to the presence of the bound rubber layer. In addition, the amount of tightly bound rubber is mainly related to the surface physical and chemical properties of silica and is less affected by the concentration. Fumed silica tends to form nanofiller networks in rubber that are woven of multiple fillers entangled with bound rubber chains to reinforce the rubber. Therefore, fillers with higher bound rubber content and a better-dispersed aggregate structure in the rubber could more significantly enhance the physical properties of nanocomposites.