| This thesis examines the surfaces of polymer gels using a novel technique that combines the contact mechanics approach of Johnson, Kendall, and Roberts (JKR) with the quartz crystal microbalance (QCM). Polymer gels are well-suited for a variety of applications, but their surface compositions often depend on their environmental surroundings due to the activity of the gel solvent. The JKR technique is sensitive to bulk mechanical properties and surface adhesive forces, while the QCM is sensitive to surface mechanical properties. In this thesis, the combined JKR-QCM technique is used to study the surfaces of polymer gels.; In a typical JKR-QCM experiment, a hemispherical gel is brought into and out of contact with the QCM surface at a controlled velocity, and the resulting load, displacement, contact area, and complex resonant frequency of the QCM are measured. It is shown that the changes in complex resonant frequency of the QCM in such an experiment are related to the material's surface mechanical properties, which include its high frequency viscosity.; Existing QCM theory is not intended to account for a changing interfacial contact area, which is a common feature in JKR experiments. Equations are presented to account for variable coverage of the QCM by considering the radial sensitivity profile across the quartz crystal. QCM theory is also modified for experiments involving a growing contact area between a viscoelastic material and the crystal surface when submerged in a liquid.; JKR-QCM studies of a model polymer gel, composed of a physically crosslinked triblock copolymer swollen by mineral oil, reveal a concentrated oil layer at the gel's surface that is transferred to the quartz crystal after loading and subsequent detachment of the gel. The same features are noted for the model gel submerged in water. A different model gel, composed of a chemically crosslinked polymer swollen by water, reveals evidence of a surface water layer when brought into contact with a QCM surface that is submerged in mineral oil. |
