Surface Orientation of Isocyanate Groups Determined by Combined Sum Frequency Generation Vibrational Spectroscopic Study and Ab Initio Calculation

Sum frequency generation (SFG) vibrational spectroscopy is a powerful nonlinear optical technique that provides molecular-level information about surfaces and interfaces with submonolayer sensitivity. In this study, we combined SFG experiments with ab initio calculations to investigate the surface orientations of isocyanate groups on a commercial primer containing isocyanate functionalities, polymeric diphenylmethane diisocyanate (pMDI), and 4,4′-methylene diphenyl diisocyanate (MDI). We examined the effects of surface exposure to humidity and the duration of such exposure on the orientation of surface isocyanate groups. Instead of using the widely adopted orientation determination method based on the fitted signal strength ratios of SFG spectra collected with different polarizations, we employed a spectral comparison method. We calculated orientation-dependent SFG spectra using IR transition dipole moments and Raman polarizability tensors of relevant vibrational modes obtained from ab initio calculations, and then compared these spectra with the reconstructed experimental data (after removing the nonresonant signal contribution) to deduce the most likely isocyanate orientation. We found that the isocyanate stretching normal mode involves vibrations of many atoms in the molecule, and the spectral comparison method is advantageous when dealing with multiple peaks arising from isocyanate stretching. It was found that although the SFG spectra intensity decreased after sample surfaces were exposed to humid air due to chemical reactions between surface isocyanate groups and moisture, the remaining unreacted isocyanate groups retained similar orientations, regardless of humidity exposure or duration.