Yield stress behaviour of concentrated silica suspensions with temperature-responsive polymers

Rheological investigations have determined the influence of temperature and solution pH on particle interactions in concentrated silica suspensions dosed with poly(N-isopropyl acrylamide) (PNIPAM) or polyacrylamide (PAM). At 20 °C, suspensions with PNIPAM displayed DLVO behaviour. Heating suspensions to 50 °C, which is above the lower critical solution temperature (LCST) of PNIPAM, caused the molecular segmental repulsion to switch to attraction with higher adsorbed amounts, promoting attraction between the particles and higher shear yield stresses that deviate from DLVO behaviour. This weak flocculation was partially reversible and lower shear yield stresses resulted after cooling below the LCST back down to 20 °C. PAM caused strong flocculation through bridging-adsorption and electrical double layer screening and much higher shear yield stresses resulted that did not vary much with temperature.

At 50 °C adsorbed PNIPAM molecules collapse and make the surface of silica particles mutually attractive causing weak, partially reversible flocculation and high shear yield stresses. PAM strongly flocculates silica suspensions through a bridging adsorption mechanism.Figure optionsDownload as PowerPoint slideHighlights
► Shear yield stresses of silica suspensions flocculated with poly(N-isopropyl acrylamide).
► Thermal aggregation of PNIPAM produces strong sediments and high yield stresses.
► PNIPAM deposition at 50 °C causes the formation of hydrophobic silica surfaces.
► Polyacrylamide (PAM) flocculates sediments strongly and does not respond to temperature.
► PNIPAM is not as strong as PAM.