Poly-NIPAM Microgels with Different Cross-Linker Densities

Thermoresponsive microgel particles made of the monomer N-isopropylacrylamide (NIPAM) and the cross-linker molecule N, N'-methylenebisacrylamide (BIS) were synthesized using three different cross-linker molar ratios. The volume phase transition behavior of these colloids was investigated by means of dynamic light scattering (DLS) and small angle neutron scattering (SANS) covering the different length scales of interest. Both methods provide the temperature of the volume phase transition in good agreement. The volume change as followed by DLS is described using the Flory-Rehner theory, leading to the determination of the spinodal temperature. Furthermore, the network correlation length xi, which is available from appropriate fits of the measured SANS profiles, was used to study the critical behavior in terms of scaling laws. The results from DLS and SANS show a strong cross-linker density dependence.