A Universal Structural Model for Human Hair to Understand the Physical Properties 2. Mechanical and Permeation Behaviors

The mechanical properties of human hair fiber and the permeation behaviors of some dye molecules into hair have been studied, and are here discussed based on a universal structural model proposed by the authors in the previous paper. The model consists of two structural parts, both of which have two states (Two-part/two-state model). One part of the hair, which has a higher transition temperature (Tc; ca. 70 °C in water), is assigned to be macrofibril and exo-cuticle; the other part with lower Tc (ca. 0 °C in water) is inter-macrofibrillar materials, a cell-membrane complex (CMC) and endo-cuticle. The temperature dependence of the elastic modulus of a human hair in the Hookean region clearly shows two break points, indicating the above-mentioned transition temperatures. We have proposed a viscoelastic model based on the two-part/two-state structural model to understand the mechanical behaviors in the Hookean, yield and post-yield regions. The permeation rate of some dye molecules into hair fiber starts to dramatically increase at the higher transition temperature. Such permeation behaviors can also be understood from a universal structural model. The molecular size of dye is a crucial factor in permeation behaviors. Dye molecules with a size smaller than 1.0 nm migrate much more easily into hair.