Effects of various filler surfaces on tuning the hierarchical structures and reinforcement of silicone rubbers

The effects of various fillers on the hierarchical structures and reinforcement of silica filled silicone rubbers were investigated using contrast matching small-angle neutron scattering (SANS) and low-field nuclear magnetic resonance (NMR). Raw (R), modified (M), and grafted (G) surface were designed on colloidal silica particles with sizes of 20 and 80 nm. The reinforcement effects of colloidal silica on silicone rubber were compared with those of industrial (fumed and precipitated) silica. The power-law decay df,agg of fumed and precipitated filler aggregate were 2.6 and 2.4, respectively, suggesting that the fractal aggregates determined the formation of a connective filler network. The bound rubber (BR) thickness measured via CM-SANS were 8.2, 7.9, and 7.9 nm for 20 nm colloidal silica with G, M, and R surfaces, respectively. NMR results indicated that grafting could promote BR formation, resulting in the highest modulus (3.4 MPa) among those of analyzed samples. All samples had similar Dres/2π values (close to 210 Hz) but different σ/Dres values, implying that the aggregate influenced the crosslinking distribution rather than the crosslinking density. Combined with the hierarchical structures and mechanical properties, the synergistic effects of the aggregate and BR networks on the reinforcing performance of silica-filled silicone rubbers have been discussed.