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 (R_f) 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 R_f 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 R_f 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 R_f groups. Analysis of the band enables us to determine the end-on orientation of the R_f chain about the surface of bulk PFAS: when the R_f 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.