SiO₂ Particles prepared from PDMS Foam/Sponge with Potential use as a Viscosity Enhancer

In this work we report a simple technique to produce SiO₂ particles with completely different morphologies, starting from the commercially available PDMS (Sylgard 184, Dow Chemicals). The process consists of first preparing a PDMS sponge by taking a solution of PDMS and cross-linkers in THF with suitable concentration and subjecting it to heating. The cross-linking reaction simultaneously occurring with the bubble formation due to boiling of the THF results in the formation of sponge-like PDMS. Subsequently, the sponge pieces are kept under two different thermal environment conditions which yield SiO₂ with entirely different morphologies in the resulting material. In the first case when the sponge is subject to slow heating to 550 ^oC and slow cooling, a very soft powder of SiO₂ nanoparticles is obtained. In the second case, fast heating and fast cooling of the same sponge wrapped inside an aluminum foil yields a brittle solid. The formation of amorphous SiO₂ in both the cases is confirmed from XRD and FT-IR. The SEM imaging of the soft powder shows particles of nearly spherical shape with size ranging from 100 nm to 500 nm whereas the brittle solid when crushed and ground, shows much larger particles with planar faces. The BET surface area was found to be 4.36 m²/g for the soft powders which increased considerably to 192 m²/g for the brittle solid.<br/>We further studied the rheological alterations due to soft powder samples. A mixture of PDMS-THF solution with 1.25 wt% soft and brittle SiO₂ particles enhances the viscosity nearly 1.5 and 2 times respectively at low shear rate (1 s^-1). This mixture was found to homogeneously incorporate water up to a certain extent (nearly 30 wt%) beyond which the mixture was found to phase separate. The viscosity of the PDMS-THF-particle mixture with 30 wt% DI water was found to further increase by a factor of nearly 10 for soft and 20 for brittle SiO₂ particles as compared to the mixture without any water. Our formulation which is completely based on a commercially available product, will be explored towards enhancing the rheological properties of the mixture PDMS-THF-particle solutions.<br/><b>Keywords:</b> SiO₂ nanoparticles, polydimethylsiloxane (PDMS), THF, zero shear viscosity.