Absolute Molecular Orientational Distribution of the Polystyrene Surface

Vibrationally resonant sum frequency generation (VR-SFG) has been used to study the absolute molecular orientational distribution of the pendant phenyl groups at the free surface of polystyrene (PS) thin films on oxidized Si substrates. Characterization of the dependence of the VR-SFG on film thickness allows unique identification of the origin of the signal, e.g., free surface, bulk, or buried interface. For films <400 nm thick, the dominant source of vibrationally resonant signal is the PS/air interface, while the dominant source of the nonresonant background is the Si/SiO2 interface. VR-SFG of a self-assembled phenylsiloxane layer is used to calibrate the relative phase between the vibrationally resonant phenyl ring hyperpolarizability and the Si/SiO2 interface nonresonant hyperpolarizability. It is found that the phenyl groups are ordered at the PS/air interface and are oriented away from the polymer film. Quantitative analyses of the orientational distribution for both the PS free interface and the phenylsiloxane monolayer are reported. The phenyl groups at the PS free surface are tilted away from the surface normal in an angular range near 57°.