Advanced surface FTIR spectroscopy analysis of poly(ethylene)-block-poly (ethylene oxide) thin film adsorbed on gold substrate

This study investigates the thin film adsorption on gold substrate of PE-b-PEO block copolymer. The originality is to use Polarization Modulation - InfraRed Reflection Absorption Spectroscopy (PM-IRRAS) to probe the copolymer thin film conformational state when physisorbed. The PM-IRRAS technology allows to measure surfacespecific FTIR spectra of thin films due to differential reflectivity of p- and s-polarized light from interfaces. The anisotropy and the exaltation of the electric field created at the copolymer/metal interface are exploited to infer macromolecular adsorption and conformational changes induced by the adsorption. In addition to spectroscopic analyzes, the surface topology of the thin film of copolymer and the ability of the blocks to crystallize at the solid interface were investigated by Atomic Force Microscopy (AFM). Vibrational modes analysis of the PMIRRAS spectra unambiguously show first that the PEO sequences of the PE-b-PEO copolymer are adsorbed preferentially on the Au substrate in flatten conformations relative to the substrate surface with an orientation angle of 27 degrees between the main chain axis and the gold substrate and second that the PE sequences of PE blocks are randomly oriented compared to the gold surface and rejected as an amorphous phase to the upper surface of the copolymer film.