Molecular orientation analysis of perfluoroalkyl groups in the solid surface by backscattering Raman spectrometry.

Recently, unique surface properties of poly- and perfluoroalkyl substances (PFAS) have been revealed to be derived from a two-dimensional (2D) packing of perfluoroalkyl (Rf) groups with perpendicularly standing orientation, i.e., the end-on orientation, which is revealed by the stratified dipole arrays (SDA) model. This indicates that analysis of the 2D aggregation of Rf groups is crucial for understanding the PFAS-specific properties, and analysis of the primary structure alone is insufficient. In the present study, we employ an analytical method of backscattering Raman spectrometry to explore the 2D aggregation of Rf groups in terms of molecular orientation. Measurements of solid particles of perfluoro-n-alkane with theoretical consideration of the Raman activity on backscattering optical arrangement make us find useful marker bands derived from the Rf groups. Analysis of the band enables us to determine the end-on orientation of the Rf chain about the surface of bulk PFAS: when the Rf chains are end-on oriented, the bands assigned to a specific symmetry species selectively disappear, whereas the other Raman-active bands remain. In short, the disappearance of the marker bands makes us determine the end-on orientation very easily. In addition, we demonstrate that the marker band can be used to visualize in-plane distribution of the end-on orientation by utilizing mapping measurements.