Sum-Frequency Generation Vibrational Spectroscopy: A Nonlinear Optical Tool to Probe the Polymer Interfaces

Abstract

The present study gives a brief introduction to sum-frequency generation (SFG) vibrational spectroscopy with an overview of the role of second-order nonlinear optical process. Here, we have emphasized on theoretical aspects of the SFG spectroscopy and the spectral analysis to extract the molecular structure and orientation of interfacial molecules. The interfacial structural information of various polymer materials plays an important role to determine properties like adhesion, friction, and wettability. Therefore, we have investigated the molecular structure of polydimethylsiloxane (PDMS) polymer films at the air/polymer interface by using SFG spectroscopy. The vibrational signatures of the PDMS polymer and the intensity of the SFG signal are recorded by varying the molecular weight of the PDMS polymer. The average orientation tilt angle and angular distribution width of methyl groups for each PDMS polymer are determined. The SFG results reveal the change in the intensity of SFG signals and the change in molecular tilt angle and angular distribution of methyl groups at air/PDMS film interface with the variation in the molecular weight of PDMS. The SFG spectral analysis reveals that the molecular tilt angle of the methyl group varies from ~49° to 75° with respect to the surface normal and the angular distribution varies from ~0° to 30° for all the PDMS polymer samples. It is interesting to find that the tilt angle of the methyl functional group of the PDMS polymer at the air/polymer interface can be controlled by varying the molecular weight of the polymer.