Grafting of alkanethiol-terminated poly(ethylene glycol) on gold

The grafting of alkanethiol-terminated poly(ethylene glycol) [HS, (CH2)(11)(OCH2CH2)(n)-OCH3; n = 34-56, MW approximate to 2224 Da] onto polycrystalline gold from dilute solutions was investigated by ellipsometry, X-ray photoelectron spectroscopy, infrared reflection-absorption spectroscopy, and in situ second harmonic generation. After immersion of a gold-coated Si wafer into a 50 muM dimethylformamide solution, the thickness of the grafted layer increases in a first rapid step up to similar to20 Angstrom. After about 10 min, the thickness rises notably again and reaches saturation after similar to2 h at similar to120 Angstrom. The kinetics of film formation clearly deviate from Langmuir kinetics, which is normally observed for the self-assembly of nonfunctionalized alkanethiols. Our observation can be explained by a conformational transition of the grafted poly(ethylene glycol) chains from amorphous coils to a brush morphology, predominantly consisting of helices with an orientation perpendicular to the surface. The second harmonic generation experiments show that the coverage at saturation of adsorption corresponds to similar to90%,that of self-assembled monolayers of alkanethiols, indicating a densely packed film.