Effects of Aqueous Exposure on the Mechanical Properties of Wool Fibers—Analysis by Atomic Force Microscopy

Abstract

Treated and untreated wool fibers are analyzed by methodologies based on atomic force microscopy. Untreated fibers are known to have an intact native surface lipid layer, while KOH treatment has the effect of removing this layer as well as disrupting the molecular substructure. The effective bulk Young's modulus of the untreated fibers is measured in air and in situ during exposure to water. There is a nearly instantaneous drop by a factor of two from approximately 1.2 GPa. after onset of aqueous exposure. The surface stiffness of both untreated and KOH treated fibers is measured in situ during aqueous exposure for durations up to 12 X 103 seconds. A decrease by a factor of two is observed for the untreated surface, while there is a more rapid decrease by a factor of ten for the KOH treated surface. Measurements of an equivalent Young's modulus for the near-surface layers obtained by transverse compression of less than 200 nm by the tip result in values of less than 1 MPa. Because wool is an inhomogeneous and anisotropic medium, the outcomes of the two different measurements of the modulus are complementary but not comparable. Tip-to-surface adhesion is also monitored during aqueous exposure, with the KOH treated surface exhibiting the lower value by a factor of approximately ten. There are also trends showing lower adhesion with duration of exposure. Rapid ingress of water into the bulk appears to be associated with decreased bulk stiffness, but has little immediate effect on surface and near-surface properties. The decrements in surface stiffness and observed effects on adhesion from aqueous exposure are then due to a much slower radial ingress of water, which is additionally rate-limited by the native lipid layer.