Molecular Weight Dependence of Viscosity and Shear Modulus of Polyethylene Glycol (PEG) Solution Boundary Layers

The viscosity and shear modulus of solutions of polyethylene glycol (PEG) with different molecular weights near a solid substrate were studied using a quartz crystal resonator technique. The technique probes the solution boundary layer with a thickness of about the viscous penetration depth adjacent to the surface of the quartz crystal. On the basis of the resonant frequency shift and the dissipation broadening of the quartz crystal resonator, the viscosity and shear modulus of the solution boundary layers as a function of the concentration and the molecular weight of PEG molecules are determined. The results show that, near the semidilute concentration of the solution, the viscosity of the boundary layer increases rapidly; the rise of the viscosity follows a power law with an exponent which depends on the molecular weight of PEG molecules. For solution boundary layers consisting of larger PEG molecules, their shear moduli display a rapid increase above the semidilute concentration also. The implications of these results are discussed.