Rheology of thermosensitive latex particles including the high-frequency limit

The flow properties of aqueous suspensions of thermosensitive latex particles are investigated as a function of volume fraction and temperature. The particles consist of a solid poly(styrene) core and a shell corn Dosed of crosslinked poly(N-isopropylacrylamide) (PNIPA) chains. The PNIPA network shrinks with increasing temperature leading to a denser layer of polymeric chains on the surface of the core pal-tides. The shear viscosity obtained from suspensions of these particles at low shear is compared to the viscosity measured in the high-frequency limit. In the limit of dilute suspensions the viscosity is modeled in terms of an effective hydrodynamic radius R-H. It is shown that R-H of highly swollen particles depends markedly on frequency. The data indicate that the swollen network on the surface of the particles is partially drained at high frequencies. For shrunken networks RH measured in the low and high frequency limit coincides again. The high frequency shear modulus G(infinity)' measured at high volume fractions demonstrates that the thermosensitive particles may he regarded as soft spheres. The repulsive interaction may be modeled in terms of a power law with an exponent of 9. (C) 2001 The Society of Rheology.