Contact Studies of Weakly Compressed PEG Brushes with a Quartz Crystal Resonator

A quartz crystal resonator was used to characterize the contact of an elastomeric polymer membrane with a grafted poly(ethylene glycol) (PEG) brush in an aqueous environment. A two-layer model of the acoustic impedance of the system was used to measure the brush thickness before and after contact with the membrane. This model was further extended to include multiple layers, allowing characterization of other monomeric density profiles along the brush thickness. The polymer brush maintains a hydrated layer between the membrane and the quartz crystal surface, the thickness of which could be determined to within 1 nm. We show that the technique is very well suited for studying the properties of highly hydrated layers with thicknesses between 0 and 100 nm at low contact pressures corresponding to a very weak compression of the PEG brush.