Quantitative Morphology Study of Cu-Neutralized Poly(styrene-ran-methacrylic acid) Ionomers: STEM Imaging, X-ray Scattering, and Real-Space Structural Modeling

Abstract

Our previous study showed quantitative correspondence between the size of ionic aggregates in a Cu-neutralized poly(styrene-ran-methacrylic acid) ionomer (Cu-SMAA) measured from scanning transmission electron microscopy (STEM) images and the sizes determined from X-ray scattering data interpreted using the Yarusso−Cooper model. Here we compare the average sample volume per ionic aggregate (Vp) from STEM images and X-ray scattering data from Cu-SMAA. The relationship between the 3D morphology of hard spheres as defined by the Yarusso−Cooper parameters (R1, RCA, Vp) and the projected 2D images obtained by STEM was elucidated by modeling. These simulations show that a significant number of the ionic aggregates in Cu-SMAA are obscured due to overlap effects and provide a quantitative relationship between the number of spheres in 3D and the number of features in 2D projections as a function of specimen thickness. Upon measuring the local thickness of a Cu-SMAA specimen using electron energy loss spectrometry (EELS), we find that the Vp determined from STEM images strongly agrees with that determined from X-ray scattering data. Thus, quantitative correspondence of both the size and number density of ionic aggregates has been obtained from direct imaging and X-ray scattering of Cu-SMAA. These data provide compelling evidence that the Yarusso−Cooper scattering model presents an excellent description of the nanoscale morphology of this solvent cast ionomer. Interpretation of this morphological data indicates that the ionic aggregates likely include both ionic and nonionic species. This comprehensive characterization provides a foundation on which a systematic evaluation of the factors that influence the nanoscale morphology, and consequently the macroscopic properties, can be accomplished.