A two-phase structure is proposed for keratin fibers consisting of a water-absorbing matrix M, in which non-water-absorbing cylinders of phase C are embedded. The cylinders of phase C are parallel to the fiber direction. The main assumption made is that only the mechanical moduli of phase M are affected by moisture uptake as long as the fiber is not stretched. On this basis, using the longitudinal and radial swelling data available, quantitative relationships between the dry and wet Young's Moduli and rigidity moduli were obtained which agreed with experimental results. Further, other physical observations were shown to be compatible with this model..

Alpha keratin fibers (hairs, wools, quills, and other mammalian appendages) together with fibroin fibers such as silks and spiders webs are all highly extensible fibrous proteins for which the mechanical properties are of primary importance both to the animal from which they originate and their ultimate application by man. Similarly, the collagens are highly inextensible fibrous proteins, which form the major component of mammalian skin and connecting structures such as tendons. All these fibrous proteins are biological polymers of polypeptide chains for which the mechanical and allied physical properties, such as water absorption, relate to both their macrostructure and their molecular and near-molecular structure. Because of both their commercial application and their relatively complex structure at the molecular and near-molecular level, interpretation of the physical properties of ␣-keratin fibers represents the main component of this presentation. The mechanical properties of ␣-keratin fibers are primarily related to the two components of the elongated cortical cells, the highly ordered intermediate filaments (microfibrils) which contain the ␣-helices, and the matrix in which the intermediate filaments are embedded. The matrix consists of globular proteins plus water, the content of the latter being dependent on the fibers environment. The Extended Two-Phase Model (ETPM) has been developed and results in a detailed coverage of the bulk mechanical properties of ␣-helical fibers in terms of their known molecular and near-molecular structure. The inextensible protein fibers, the collagens and fibroins, are also briefly discussed in terms of the relationship between mechanical properties and the structure of these fibers.

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Chemical and structural analysis of low and high sulphur protein extracts from α-keratin fibers has indicated that the low sulphur protein represents the protein of the microfibrils with its organized α-helical structure. Further, the high sulphur protein, together in some α-keratins with high glycine tyrosine protein, is the matrix protein. This latter protein, because of its high proportion of hydrophobic residues and covalent crosslinks of the diamino acid cystine, exists as a globular protein formed in the moist environment of the growing cortical cell. The matrix in a wet α-keratin fiber consists of water together with the globular high sulphur protein. The water acts as a continuous three-dimensional polymer network outside the globular matrix protein, interacting with the hydrophilic residues on the surface of the protein. From these observations, a simple microfibril-matrix model of the cortex of α-keratin fibers is proposed. This model quantitatively predicts the mechanical stiffening of a fiber being extended in water from the yield to post-yield regions of the load-extension curve. The prediction is obtained by two independent approaches that give similar results. The success of this model suggests that in considering the structural state of the cortex of α-keratin fibers, the biological origin, the chemical data such as amino acid residue sequences, and the physical measurements as applied for standard polymers need to be more dosely considered.The longitudinal load-extension relationship for akeratin fibers such as wools and hairs consists of three regions of mechanical behavior: the Hookean region up to about 2% strain, the yield region from 2% to 25-30% strain, and the post-yield region above 30% strain. These regions are particularly well defined for fibers of uniform cross section in water. The longitudinal properties of these fibers are primarily related to the molecular structure of their cortex. There is general agreement in the literature as to the structural factors that control the mechanical relationships in the Hookean and yield regions, but the factors controlling mechanical relationships in the post-yield region have been considered in terms of two distinctly conflicting models [ 5 ] .The first of these models considers the noncrystalline regions of the a-helical cortex as consisting of polypeptide chains crosslinked by disulphide bonds. The post-yield region's mechanical stiffening in this model relates to the limiting of extension produced by the disulphide crosslinks. In the proposed alternative model, the series zone model based in the first instance on mechanical data for fibers supercontracted in concentrated aqueous LiBr solutions, post-yield region stiffening relates to highly crosslinked zones repeated longitudinally in the fiber cortex. The first model, although the simpler of the two, is not easily compatible with the presence of known, highly crosslinked protein in the matrix structure of a-keratin fibers. In an attempt to resolve these di...

At 20°C and for all moisture contents, the mechanical behavior of wool fibers up to 1% extension in the Hookean region is linear viscoelastic. The equilibrium Young's modulus, based on the wet cross-sectional area of the wool fiber, is inde pendent of moisture content and is equal to 1.4X 1010 dynes-cm2. The dynamic or transient behavior of a fiber at any moisture content at 20°C can be replaced by a spring contributing a fixed stiffness of 1.4X 1010 dynes/cm2 to the dynamic Young's modulus together with a viscous dashpot in parallel and having moisture-dependent characteristics. The action of water, which in the original two-phase matrix-microfibril model, was proposed to weaken the matrix, must now be con sidered to increase the segmental mobility of the molecular structure of the matrix. Further, mechanical equilibrium between matrix and microfibril is taken to exist for the wet wool fiber, rather than the dry fiber.