Formation and properties of composites based on microgels of a responsive polymer and TiO2 nanoparticles

Organic-inorganic composites were prepared with titanium dioxide (TiO2) nanoparticles embedded within colloidal particles of a cross-linked, thermally responsive polymer. To promote the incorporation of unaggregated nanoparticles of TiO2, temperature responsive microspherical gels (microgels) of N-isopropylacrylamide (NIPAM) with interpenetrating (IP) linear chains of poly(acrylic acid) (PAAc) were synthesized. Dynamic light scattering (DLS) measurements revealed that these microgels reversibly shrink and swell in diameter from 300-400 nm to 600-800 nm with temperature. Two types of nanoparticles of TiO2 were immobilized within the IP-microgels-fine TiO2 nanoparticles synthesized by the hydrolysis of titanium(IV) isopropoxide and commercially available Degussa P25. Characterization of the composite was conducted using transmission electron microscopy (TEM) and UV-vis absorption spectroscopy from which it was determined that the extent of loading of the TiO2 within the colloidal particles can be easily manipulated from a low value of 10% (weight) to a value as high as 75%. The TiO2 nanoparticles were in a dispersed state within the microgels and the composites showed rapid (approximately minutes) sedimentation, which is useful for gravity separations. By using turbidometry to characterize the settling behavior of the organic-inorganic composites, it was found that the settling time decreases as the content of TiO2 increases within the particles.