COMPRESSION OF POLYETHYLENE-GLYCOL CHAINS GRAFTED ONTO SILICON-NITRIDE SURFACE AS MEASURED BY SCANNING FORCE MICROSCOPY

Monomethoxypolyethylene glycol (PEG) with a molecular weight of 2000 D a has been grafted to a silicon nitride surface that has been silanize d with 3-aminopropyltriethoxysilane (APS). A scanning force microscope (SFM) was used to measure a force-distance profile between the PEG-mo dified surface and the unmodified silicon nitride SFM tip. In 0.1 M KN O3 solution the force increased monotonically with distance, showing n o adhesion between the SFM probe and the PEG-modified surface. The mon otonically increasing force is interpreted as a steric repulsion force . Increasing solution concentration of MgSO4 caused the appearance of a weak attractive force between the SFM probe and the PEG-modified sur face. The magnitude of the attractive force increased with increasing MgSO4 solution concentration. The addition of MgSO4 generated poor sol vent conditions for grafted PEG chains and eliminated the steric repul sion force, probably through a collapse of the tethered chains. The in terpretation of the MgSO4 effect on the tethered PEG chains is hindere d by the fact that surfaces treated with APS alone caused a cantilever instability at a separation distance of approximately 25 nm. An attra ctive force at this large separation distance suggested that APS treat ment resulted in a surface coating thicker than a monolayer.