Synthesis and characterization of nanogels of poly(N-isopropylacrylamide) by a combination of light and small-angle X-ray scattering.

Thermo-responsive crosslinked nanogels of N-isopropylacrylamide (NIPAM) were synthesized by emulsion polymerization and the size was varied using different concentrations of surfactant (sodium dodecyl sulfate, SDS) in the polymerization process. The collapse behavior of the nanogels at the lower critical solution temperature at around 32 °C was investigated by dynamic light scattering, and by combined static light scattering (SLS) and small-angle X-ray scattering (SAXS). The combined data from SLS and SAXS were analyzed by a model for the nanogels which at intermediate temperatures included a central core and a more diffuse outer layer describing pending polymer chains with a low degree of cross linking. In the expanded state, the particles were modeled with a single component with a broad graded surface. In the collapsed state the nanogels were modeled as homogeneous and relatively compact particles. The amount of surfactant used had a profound effect on the final size of the nanogels owing to the phenomenon of colloidal stabilization of the emulsion droplets during polymerization. The combination of SLS and SAXS as applied to the nanogels is an attractive method for particle characterization as it spans a very large range of scattering vector from q = 0.0004 to 0.22 Å(-1).