Settling characteristics of composites of PNIPAM microgels and TiO2 nanoparticles

Settling behavior of organic-inorganic colloidal composites was studied optically. The colloids were composed of titanium dioxide (TiO2) nanoparticles embedded within cross-linked, thermally responsive microspherical gels (microgels) of poly(N-isopropylacrylamide) (PNIPAM). Interpenetrating (IP) linear chains of poly(acrylic acid) (PAAc) were used to incorporate nanoparticles of TiO2 in a dispersed state within the porous framework of the microgels. The extent of loading of the TiO2 within the colloidal particles can be easily manipulated from 10% (weight) to a value as high as 75%. The microgel-titania composites showed rapid sedimentation, which is useful for gravity separation of these particles in photocatalytic applications. The settling of the microgel-titania composites occurred over minutes and was much faster than solid, impermeable spheres. As the content of TiO2 increased within the particles from 10% to 75%, the increased effective particle density led to significant decrease in the settling time from approximately 2200s to approximately 100s. A simple mathematical model was used to interpret the optical measurements in terms of the distribution of settling velocities of the particles. The change in settling behavior with temperature suggests that the decrease in the permeability due to collapse of the microgels is balanced by an increase in effective density. The settling behavior of these composites provides not only a simple probe of particles characteristics but also insight into fundamental issues regarding settling of porous spheres, floes of inorganic particles within organic material, and sedimentation phenomena in marine environments. (C) 2007 Elsevier B.V. All rights reserved.