M. Sakai et al., A universal structural model for human hair to understand the physical properties 2. Mechanical and permeation behaviors, B CHEM S J, 73(9), 2000, pp. 2169-2177
The mechanical properties of human hair fiber and the permeation behaviors
of some dye molecules into hair have been studied, and are here discussed b
ased on a universal structural model proposed by the authors in the previou
s 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 highe
r transition temperature (T-c; ca 70 degrees C in water), is assigned to be
macrofibril and exo-cuticle; the other part with lower T-c (ca. 0 degrees
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 viscoelast
ic model based on the two-part/two-state structural model to understand the
mechanical behaviors in the Hookean, yield and post-yield regions. The per
meation rate of some dye molecules into hair fiber starts to dramatically i
ncrease at the higher transition temperature. Such permeation behaviors can
also be understood from a universal structural model. The molecular size o
f dye is a crucial factor in permeation behaviors. Dye molecules with a siz
e smaller than 1.0 nm migrate much more easily into hair.