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., ~45 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 ~25 Å in the dry state to ~70 Å in contact with D<INF>2</INF>O, which corresponds to a total water uptake of ~38 mass %. In contrast, quasi-crystalline monolayers exhibit only a small amount of water penetrating into the film (~8 mass %) with a corresponding change of the layer thickness from ~110 to ~125 Å. 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.