Rheology of fumed silica/polydimethylsiloxane suspensions

There has been a long-running debate about reinforcement and rheological nonlinearity of suspensions with respect to filler dispersity and agglomeration depending on many factors, including molecular weight of dispersing medium, surface chemistry of nanoparticles, and aging of suspensions. Herein, rheological behaviors of fumed silica suspended in polydimethylsiloxane (PDMS) were investigated as a function of weight averaged molecular weight (Mw, 2–88 kg/mol) of PDMS, and concentration and surface characteristics (hydrophilic or hydrophobic) of fumed silica. The reinforcement at low-frequency is maximized at Mw close to entanglement molecular weight (Me) while at high-frequency it decreases with increasing Mw. A unified reinforcement mechanism related to PDMS Mw and silica nature and loading is disclosed. Hydrophobic silica (R974) suspensions exhibit linearity-softening-hardening-softening response with increasing dynamic strain at frequency 6.28 rad s−1 while as-prepared hydrophilic silica (A200) suspensions show a sustained softening beyond the linearity regime. However, aging of the A200 suspensions could induce appearance of four-stage response when the gels become critical or change into sols, which is more significant for low-Mw PDMS.