Molecular Orientations and Conformations of Air/Fluoroalkyl Acrylate Polymer Interfaces Studied by Heterodyne-Detected Vibrational Sum Frequency Generation

摘要

Air/fluoroalkyl acrylate polymer interfaces have been investigated by heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. As fluoroalkyl acrylate polymers, poly(2-perfluorohexylethlyl acrylate) (p-C6FA) with the C6F13 group in its side chain and poly(2-perfluorooctylethlyl acrylate) (p-C8FA) with the C8F17 group were examined. Although the two polymers differ only in the length of the fluoroalkyl group in their side chains, air/polymer interfaces of them gave quite different VSFG spectra. In the spectral range from 1300 to 1550 cm(-1), the VSFG spectrum of the air/p-C8FA interface was dominated by a vibrational band at 1370 cm(-1), whereas that of the air/p-C6FA did not show any discernible vibrational band. By reference to polarized HD-VSFG spectra of an air/fluorosurfactant interface combined with Raman measurements and quantum chemical calculations of the surfactant, the 1370 cm(-1) VSFG band was ascribed to the terminal C-C stretch of the fluoroalkyl group coupled with the terminal CF3 symmetric stretch. Based on the assignment, we deduced that the orientation of the perfluoroalkyl side chains of p-C8FA is almost perpendicular to the interface. In contrast, we concluded that the perfluoroalkyl side chains of p-C6FA orient randomly with gauche defects or orient parallel to the interface. This difference in molecular structures and conformations may give rise to the differences in macroscopic properties at the interfaces. This study demonstrates that the 1370 cm(-1) band observed in HD-VSFG spectroscopy can be used as a marker for the orientation and conformation of the CF3 groups in fluorinated compounds.