An Investigation of the Adsorption of Polystyrene/Poly(2-vinylpyridine) Diblock Copolymers onto Silver Substrates Using Surface-Enhanced Raman Scattering

Adsorption of polystyrene/poly(2-vinylpyridine) (PS/P2VP) diblock copolymers having molecular weights of 152K and 15K in the PS and P2VP blocks, respectively, onto silver island films from toluene solutions was investigated using surface-enhanced Raman scattering (SERS). The length of the P2VP blocks was short compared to that of the PS blocks so that only bands characteristic of the PS blocks were observed in the normal Raman spectra of the diblock copolymers. When adsorption was carried out from solutions having concentrations above the critical micelle concentration (cmc), the SERS spectra were a strong function of the adsorption time. For an adsorption time of 2 min, the SERS spectra were similar to the normal Raman spectra of the diblock copolymer and only bands characteristic of the PS blocks were observed. However, for adsorption times of 1 and 5 days, bands related to the PS blocks decreased in intensity and bands related to the P2VP blocks increased. These results indicated that micelles were initially adsorbed from solutions above the cmc. However, the micelles unfolded on the silver surface as a function of time as P2VP was gradually chemisorbed. When the adsorption was carried out at solution concentrations below the cmc, the SERS spectra were similar to the normal Raman spectra of the copolymer and only bands characteristic of the PS blocks were observed regardless of the adsorption time. It was concluded that, in this case, the P2VP blocks were chemisorbed onto the silver surface and that the PS blocks extended into the solution. However, due to repulsive interactions between the PS blocks, the coverage of the surface by the P2VP blocks was very low and many surface sites remained unoccupied. During drying, the PS blocks collapsed onto the surface and occupied those sites.