L.J. Richter, K.A. Briggman (844), J.C. Stephenson (844) and W.E. Wallace (854)

Objective: To develop measurement procedures for the study of polymer thin film interfaces by vibrationally-resonant sum-frequency generation (VR-SFG).

Problem: Polymer thin films have important roles in many industries, as varied as semiconductor devices, automobile coatings and construction materials. The structure at both the free and buried interfaces of thin films are critical to their performance as they can determine characteristics such as adhesion and wear. Molecular characterization of these interfaces is difficult; conventional probes using X-rays, neutrons, or linear vibrational spectroscopies do not have the specificity to characterize the interfaces in the presence of the bulk polymer. Div. 837, in collaboration with Div. 844, has recently improved the sensitivity of VR-SFG using a novel approach based on state-of-the-art femtosecond lasers and is employing this new approach in the Chemical Characterization of Materials Program. VR-SFG is a promising tool for the study of interfaces as it is specifically forbidden in the bulk region of centrosymmetric films but allowed at interfaces because of their reduced symmetry. However, when multiple interfaces are present as in the study of thin films, methods must be developed to distinguish the contributions to the VR-SFG signal from each interface present.

Approach:A general technique, utilizing optical interferences in thin films, has been developed to identify the interface giving rise to a specific VR-SFG feature. Additionally, the technique identifies experimental conditions that enhance signal levels, allowing for more efficient data acquisition. The technique was demonstrated in a study of the orientational distribution of the phenyl rings at the interfaces of polystyrene (PS) films spin cast onto oxidized Si substrates.

Results and Future Plans: Shown in the lower panel of the figure at the right are VR-SFG for a series of thin PS films. The features in the range 3000 - 3100 cm^-1 are due to the CH stretching vibrations of the phenyl side groups. The modulation in the spectra is due to optical interferences in the films. The upper panel compares the amplitude of the resonant component at 3067 cm^-1 (points) to theoretical calculations (lines) for phenyl groups located exclusively at the free surface, uniformly distributed through the bulk, and at the buried interface. The observed signal is clearly dominated by the free surface. Detailed analysis of optimal spectra acquired from 130 and 369 nm films indicate that the phenyl groups at the free surface point away from the polymer film, and are tilted from the surface normal in an angular range near 60 Degrees. This work was the first to establish the complete orientational distribution for a pendant side group at a polymer surface. In the future, similar optical interferences will be exploited in multilayer systems (substrate / dielectric / polymer) to characterize the dielectric / polymer interface and explore the molecular component of adhesion.