Characterisation of monodisperse poly(N-isopropylacrylamide) microgel particles

Poly(N-isopropylacrylamide) microgel particles prepared by emulsion polymerisation in the presence of anionic surfactant have been investigated in different media in the swollen and collapsed state by static and dynamic light scattering measurements. The microgel latex was found to be highly monodisperse. The particle scattering function fits well to the theoretical function of a sphere. The hydrodynamic diameter of the particles is significantly higher than the diameter calculated from the measured radius of gyration for a sphere of constant density. The discrepancy was interpreted by a core/shell structure of the microgel particles. If the particles are in the collapsed state the two kinds of diameter coincide with each other, indicating that the microgel particles are compact spheres. In the globular state the gel still has 39% (w/w) water content. The particles bear a small surface charge resulting in a long-range electrostatic interaction between the particles in an aqueous environment. It was observed for the first time that at a sufficiently low ionic strength the aqueous microgel particle system undergoes an ordered/disordered phase transition.