Swelling behavior of self-assembled monolayers of alkanethiol-terminated poly(ethylene glycol): A neutron reflectometry study

The swelling behavior of alkanethiol-terminated poly(ethylene glycol) with an average molecular weight of 2180 Da (i.e., similar to45 ethylene glycol, EG, units) in contact with water was investigated by neutron reflectometry as a function of the morphology of the PEG-SH layer. Amorphous films at a low grafting density show significant swelling with an increase of the film thickness from similar to25 Angstrom in the dry state to similar to70 Angstrom in contact with D2O, which corresponds to a total water uptake of similar to38 mass %. In contrast, quasicrystalline monolayers exhibit only a small amount of water penetrating into the film (similar to8 mass %) with a corresponding change of the layer thickness from similar to110 to similar to125 Angstrom. The water uptake per EG unit corresponds to the literature value of 1.5 for the amorphous layer and to only 0.25 in the case of the quasi-crystalline film.