Viscoelastic Properties of Polyethylene Glycol (PEG) Boundary Layers near a Solid Substrate

The viscoelastic property of a polyethylene glycol (PEG) solution was studied using a quartz crystal resonator technique. The technique probes the solution in the region adjacent to the surface of the quartz crystal. The results reveal the properties of the boundary layer with a thickness of about the viscous penetration depth at the crystal-solution interface. Two different types of electrode surfaces on quartz crystal were used in the study; one is bare gold, and the other is gold plated with thiol-group (SH)-attached polyethylene glycol (SH-PEG). The results show that different surfaces have little effect on the measured viscoelastic properties of the fluid layer over a wide concentration range. Near the semidilute concentration of the solution, the viscosity of the boundary layer increases rapidly and follows a power law with an exponent of 1.5 in this concentration dependence. The dynamic shear modulus also displays a rapid increase above that concentration. The implications of these results are discussed.