Characterization of Adsorbed 2-Vinylpyridine/Styrene Diblock Copolymers on Silver Surfaces Using Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) was used to determine the conformation of 2-vinylpyridine/styrene (2VP/S) block copolymers adsorbed onto silver surfaces. The SERS spectra were also obtained from polystyrene (PS) and poly(2-vinylpyridine) (P2VP) to confirm band assignments of 2VP/S diblock copolymers. The bands near 1615 and 1041 cm⁻¹ related to the styrene block were only weakly observed in the SERS spectra of 2VP/S diblock copolymers and the bands due to the 2VP block observed near 1073,1161,1336, and 1585 cm⁻¹ were relatively strong in intensity in the SERS spectra, indicating that the SERS spectra observed for 2VP/S diblock copolymers were mostly contributed by the 2VP block. Since SERS has previously been shown to be a surface-selective effect, the results obtained here confirmed that the 2VP block preferentially adsorbed to the surfaces while the styrene block was positioned away from the surfaces. The results obtained from angle-resolved X-ray photoelectron spectroscopy (XPS) were consistent with those obtained from SERS. The ratio of nitrogen to carbon (N/C) in the interface region of the adsorbed copolymer was larger than in the bulk copolymer. The N/C ratio was also found to increase with increasing “takeoff” angle in the near-interface region. Since nitrogen is a characteristic element of the 2VP block and the N/C ratio can be used to determine the composition profiles, it was again confirmed that the 2VP block preferentially adsorbed onto the silver surface and that the 2VP layers were enriched in the interface region. Differences in the relative intensities and frequencies of several bands in the normal Raman and SERS spectra of P2VP adsorbed onto the silver surfaces were related to orientation effects. It was concluded that P2VP and the 2VP block of diblock copolymers were adsorbed by -bonding through the pyridine nitrogen atoms with a vertical conformation.