Structural changes in PNIPAM microgel particles as seen by SANS, DLS, and EM techniques

Thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels with different cross-linker density have been investigated by complementary scattering methods and high-resolution imaging techniques. The swelling behavior of the colloidal particles as probed by dynamic light scattering (DLS), is discussed together with information on local structure and dynamics of the particles as obtained from small angle neutron scattering (SANS). Below the volume phase transition the static structure factor reveals liquid-like contributions. This is similar to macroscopic gels, but for low q the scattering behavior is different because of the influence of the particle form factor P(q). Beyond the phase transition temperature, Tc, the system becomes completely phase separated and interfacial scattering dominates the measured SANS profiles (I(q)∝q−4). Slight deviations from the ideal Porod behavior can be interpreted in terms of surface roughness of the collapsed particles. This corresponds to structures visible on AFM micrographs.