Probing Molecular Structures of Polymer/Metal Interfaces by Sum Frequency Generation Vibrational Spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy has been applied to investigate the molecular structure of the buried poly(methyl methacrylate) (PMMA)/silver (Ag) interface. To elucidate such a structure, PMMA films with different thicknesses deposited on Ag substrates have been studied in the experiments. SFG signals collected from such PMMA films are interference results of the signals generated from the PMMA/air and PMMA/Ag interfaces. Such signals also include some nonresonant contributions from the samples. When the PMMA film thickness is changed, such an interference effect also varies. SFG signals from the buried PMMA/Ag interface have been successfully deconvoluted from such detected interference results, from which molecular structure of the PMMA/Ag interface can be inferred. The signals from the ester methyl groups dominate the SFG signals deduced for the PMMA/Ag interface, indicating the dominating presence of the ester methyl groups at this interface. The ester methyl groups point away from the Ag surface with a large tilt angle. They lie down more toward the interface compared to those on the PMMA surface in air. Methylene and alpha-methyl groups are also detected at the PMMA/Ag interface. This research demonstrates that SFG is a viable technique to elucidate molecular structures of buried polymer/metal interfaces.