Rheology of thermosensitive latex particles including the high-frequency limit

The flow properties of aqueous suspensions of thermosensitive latex particl es are investigated as a function of volume fraction and temperature. The p articles consist of a solid poly(styrene) core and a shell corn Dosed of cr osslinked poly(N-isopropylacrylamide) (PNIPA) chains. The PNIPA network shr inks with increasing temperature leading to a denser layer of polymeric cha ins 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 m easured 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 particle s is partially drained at high frequencies. For shrunken networks RH measur ed in the low and high frequency limit coincides again. The high frequency shear modulus G(infinity)' measured at high volume fractions demonstrates t hat the thermosensitive particles may he regarded as soft spheres. The repu lsive interaction may be modeled in terms of a power law with an exponent o f 9. (C) 2001 The Society of Rheology.