Effect of connectivity and charge density on the swelling and local structural and dynamic properties of colloidal PNIPAM microgels

The structural and dynamic properties of thermosensitive neutral and partially charged colloidal microgels of poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM/AAc) have been investigated by neutron and light scattering techniques. Introduction of cross-linker induces topological constraints into the polymer network, and hence reduces the swelling capability of the particles as expressed in the variation of their hydrodynamic radii. Small-angle neutron scattering experiments reveal that below the transition temperature the inhomogeneous structure of the microgel consists of local solid-like frozen regions and solution-like regions where the polymer chain behaves as in semidilute polymer solution. The temperature-induced collapse leads to a strongly segregated micro-phase separated particle. The swelling of charged microgels is determined by their charge density, controlled through the amount of acrylic acid comonomer, the pH, and the salt concentration.