Chemical Speciation by NEXAFS Spectromicroscopy: Insights from Molecular Modelling of Polymers

Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy of polymers performed in a scanning transmission X-ray microscope (STXM) can provide chemical speciation with <0.1 gm spatial resolution in imaging mode. The core excitation spectra of molecular compounds that are structural analogues of polymers help interpret the NEXAFS spectra of polymers. The effect of nt-delocalization on polymer NEXAFS is discussed and illustrated by comparison to molecular spectra. Extended Htickel calculations are particularly useful for providing insight into the relationship between chemical structure and the molecular and polymer spectra. We report the interpretation of experimental NEXAFS spectra of polyethylene terephthalate (PET). Molecular models indicate that NEXAFS will be sensitive to structural isomerization in polyester polymers. We demonstrate the capability of NEXAFS to distinguish hard-segment and soft-segment phase segregation in polyurethanes.