Understanding the Scattering by Polystyrene Microspheres

Using scattering methods (Static Light Scattering (SLS), Dynamic Light Scattering (DLS), and Small Angle X-ray Scattering (SAXS)) we studied the structure and dynamics of polystyrene microspheres of various sizes in suspensions of different concentrations as a model system for polymeric microgels studied/to be studied with the same methods. SLS utilizes average light scattering off samples at varying angles to determine the sample's molecular weight (Mw) and radius of gyration (Rg) using Zimm analysis. The samples form factor (P(q)), the ratio of intensity to zero angle intensity, was also determined and plotted against known form factors of simple geometrical shapes using Kratky plots. SAXS is similar to SLS but should provide a greater resolution for particle structure over a wider size range. DLS utilizes fluctuations of scattered light to determine particle diffusion and hydrodynamic radius (Rh). Mw and Rh can then be utilized to determine the apparent density (ρ) of the spheres. In this project, we were able to confirm by SLS and DLS the spherical shape and accepted sizes of microspheres. However, the methods became less accurate in determining Rg and Mw for larger and extremely low concentrations of microspheres, which is expected. Issues with low concentration emphasize the need for sample filtration while scattering by larger probes illustrated the resolution limit of light scattering setup and the strong effect of sample polydispersity. SAXS is being performed to obtain Rg, Mw, and ρ with an improved resolution for comparison with light scattering.