Surface structure of latex particles covered with temperature-sensitive hydrogel layers

The electrophoretic mobility and the size of two types of model latex particles with a core-shell structure were measured and compared with those for latex particles with no shell structure. These latex particles have a negatively charged core, covered with negatively charged or uncharged temperature-sensitive poly(N-isopropylacrylamide) hydrogel layers. The charge density and the `softness' of the latex surface layer were obtained from the electrophoretic mobility data of the latex particles, showing a sudden change around the phase transition temperature (33 ^°C) of poly(N-isopropylacrylamide). Also, these changes are correlated with a change in the size of the latex particles around 33 ^°C. The temperature dependence of the hydrogel layer thickness on the particle accounts for the above observations. The negatively charged poly(N-isopropylacrylamide) surface gel layer shrinks as temperature increases, leading to an accumulation of the poly(N-isopropylacrylamide) segments on the particle core. This makes the particle harder. The uncharged poly(N-isopropylacrylamide) surface gel layer, on the other hand, does not change its thickness appreciably, but a change in configuration of the polymer segments occurs as the temperature increases, so that the surface layer becomes softer, probably because void spaces are created in the surface layer by the accumulation of polymer segments.