Improving Optical Models of Polarized R-SoXS for Quantitative Measurement of Molecular Orientation within Polymer Nanostructures

Polarized R-SoXS is sensitive to local molecular orientation between domains within nanostructures, but it is difficult to interpret the data due to a lack of appropriate optical models. Uniaxial optical models are typically used in both NEXAFS and ellipsometry measurements of polymer films to determine molecular orientation with respect to a substrate. The model is based on transition dipole moments (TDM’s) being aligned either parallel or perpendicular to a primary molecular axis with respect to the incident electric field. We have successfully used this model in polarized R-SoXS to go beyond global orientation to measure correlative local molecular orientation with respect to interfaces. We show how this model can break down due to TDMs in the molecule that break uniaxial symmetry and detail an optical model free of this restriction that combines R-SoXS with DFT calculations. By correlating measured and calculated features we identify resonances with molecular moieties and assign separate TDM orientations. We show how this lower symmetry model may enable further details of molecular orientation to be extracted from complex polymer nanostructures.