Effects of solvent power on dynamic moduli of polydimethylsiloxane-treated fumed silica suspension gels

The surface silanol groups of fumed silica powders were modified by polydimethylsiloxane (PDMS) chains with different molecular weights, and the powders were suspended in hexadecane and benzyl alcohol in a gel state. The dynamic moduli of the fumed silica suspensions were investigated as a function of the silica concentration. Hexadecane is a good solvent for PDMS, whereas benzyl alcohol is a poor solvent for PDMS. The dynamic moduli of silica suspension gels in both dispersion media as a function of strain depended on the molecular weight of PDMS at fixed silica concentrations: the storage modulus G′, loss modulus G″ in the linear region, and intersection strain of G′ and G″ in hexadecane increased with an increase in the molecular weight; the magnitudes of G′ and G″ in the linear region in benzyl alcohol decreased with an increase in the molecular weight of PDMS. An increase in the silica concentration caused an increase in G′ and G″ in the linear region as well as the critical strain γc, at which the linear viscoelastic region ended in benzyl alcohol, irrespective of the molecular weight of PDMS. These dependencies were compared to the fractal gel model, and the silica suspension gels were classified as weak-link gels. The fractal dimensions of the corresponding gels increased slightly with an increase in the molecular weight of PDMS. On the other hand, the value of G′ in the linear region in hexadecane could be fitted with the power law suggested by the fractal gel model, and an increase in molecular weight of PDMS caused a gradual decrease in the fractal dimension.