Toward reconciling STEM and SAXS data from lonomers by investigating gold nanoparticles

To date, the sizes of the nanoscale ionic aggregates present in ionomers as determined by scanning transmission electron microscopy (STEM) experiments are inconsistent with small-angle X-ray scattering (SAXS) data interpreted by the Yarusso-Cooper model. To address this discrepancy, we have investigated a pair of model nanoparticles (11- and 55-atom An clusters) with both STEM and SAXS. To mimic ionic aggregates, these nanoparticles were supported by amorphous PS films of predetermined thickness. While the size of the STEM probe was inconsequential to the resolution and measurement of the particles, the optimal specimen thickness was determined to be less than 50 nm. SAXS was performed on dilute solutions of the nanoparticles and fit using a monodisperse, noninteracting hard-sphere form factor model. For Au 11, STEM finds a diameter of 1.3 +/- 0.14 nm and SAXS finds a diameter of 1.4 nm. Similarly, both STEM and SAXS determine a diameter of 1.7 nm for Au 55. For a combined system of An 11 and Au 55, STEM data suggests a bimodal distribution of particle sizes consistent with those of the individual particles while SAXS data indicate hard spheres with diameter of 1.6 nm. Analysis of this model system has allowed us to optimize several experimental conditions of potential importance in reconciling STEM and SAXS data from ionomers.