Rheology of surface-modified titania nanoparticles dispersed in PDMS melts: The significance of the power law

摘要

We investigated the rheology of titanium dioxide (TiO _2) nanoparticles with various surface modifications in neat and binary blends of polydimethylsiloxane (PDMS) homopolymers of different molecular weights (4 k-77 k). The dispersions for bare, octadecyl-(C _(18)), and PDMS-grafted particles reflect different interaction forces. For bare particles, the relative viscosity decreases monotonically with increasing melt M _w or increasing fraction of long chains (f), consistent with thicker adsorbed layers. The octadecyl(C _(18))-grafted dispersions show no dependence on melt M _w or f, suggesting that the alkyl groups prevent polymer adsorption or bridging. Therefore, the van der Waals attractions are cut off at a separation on the order of twice the thickness of the C _(18) chains (≈5 nm), regardless of melt M _w or f. The PDMS-grafted suspensions show an increase in relative viscosity with increasing melt M _w or f, consistent with wetted polymer brushes for P N and dewetted layers for P < N. The power law we developed previously fits the shear-rate dependent viscosities with a structural relaxation time that scales with the magnitude of the attraction, thereby reflecting the microscale dynamics.