Properties of poly(N-isopropylacrylamide)-grafted colloidal silica

Poly(N-isopropylacrylamide-co-3-trimethoxysilylpropyl methacrylate) was pre pared by radical polymerization and was grafted onto the surface of spheric al colloidal silica. The copolymer, which had on average 1 silyl group per chain, condensed on the silica dispersed in THF at 60 degrees C. The result ing particles had a critical coagulation concentration of calcium chloride of 500 mM I at room temperature, indicating robust colloidal stability. By contrast, heating the suspension to 50 degrees C lowered the critical coagu lation concentration by more than three orders of magnitude, giving a value of 0.1 mM. Thus, the PNIPAM shell induced temperature-sensitive colloidal stability in the silica dispersion. The coated silica particles were also s urface active. However, the surface tension of 50 mJ/m(2) at 25 degrees C i s 15% higher than the corresponding value for PNIPAM solutions. (C) 2000 Ac ademic Press.