Abstract

Understanding the behaviour of materials at interfaces is critical for improving manufacturing processes. Interfaces can be the decisive factor in the application of materials. Several properties can be obtained from studying the interfaces such as friction, roughness, heat conductivity, reactivity, corrosion resistance, surface energy and surface tension. A very powerful technique that allows the study of interfaces is Sum Frequency Generation (SFG).
SFG is a second order, nonlinear, optical technique which is specific to interfaces. Its principle is the combination of two incident photons with different frequencies (infrared and visible) into one SFG photon with frequency equal to the sum of the incident frequencies. When the frequency of the IR photon is equal to that of the molecule’s bond vibration, SFG is strongly enhanced, and its intensity increases significantly. The intensities of the collected SFG photons are graphed into a spectrum, and every peak in the spectrum is characterized by three parameters: amplitude, width and frequency center.
In this work, we intend to determine the effect of molecular weight variation of polystyrene (PS) on its SFG spectra. The peak intensities in normalized SFG spectra with respect to the product of intensities of incident IR and visible photons increase with the molecular weight. The parameters generated from fitting the experimental data into the theoretical relationship did not seem to follow the same trend like the intensities due to the complexity of this relationship. Similarly, upon calculating the orientation angle of the phenyl groups in the PS molecules, it did not seem to have any consistent pattern. However, we do not infer that there is
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no relationship between the variation of parameters and orientation angle, and molecular weight. We suggest that the relationship is complicated and beyond the scope of this work.