PHYSICAL PROPERTIES OF ALPHA-KERATIN FIBERS 393 molecules coming under this low level of strain is little affected in terms of its conductivity pathways. Conduction associated with the water present within the keratin structure is essentially a property of the non-ordered component of the structure. The dielectric properties of the keratin water system (41) are dependent on frequency of' the measurement as well as the water content. At high water contents, which correspond to the state of the fibers in a high humidity environment, the dipolaf orientational polarization of large segments of the keratin structure is plasticized that is, it is freed in its movement by water and is responsible for the major component of the total polarization. A contribution is also made by water molecules free to align with the applied field. Because dielectric and dynamic mechanical data are essentially dependent on the mobility of the same elements within the keratin structure, comparison can be made between the mechnical relaxation spectra obtained for fibers extended within the "Hookean" region and the dielectric relaxation spectrum for fibers at the same water content and temperature. THE TWO-PHASE MODEL For small distortions of a keratin fiber that exist in the "Hookean" region the fiber's behavior with changes of water content on the basis of much of the foregoing data has been expressed in terms of a two-phase model (42) consisting of a water impenetrable phase of cylindrical rods oriented parallel to the fiber direction embedded in a water penetrable matrix-phase. The matrix-phase is mechanically plasticized and weakened by the presence of water, whereas no mechanical change is expected in the cylindrical rods with water uptake by the fiber. Torsional mechanical data shows that the modulus of rigidity of a dry fiber (,• 0% relative humidity) is about 1.7 x 10 9 pascals, falling by a factor of 10-20 to a value of 1-2 x 108 pascals for a wet fiber (43,44). This major change in torsional rigidity for the fiber as the environment is changed from dry to wet occurs in parallel with a much smaller change in the longitudinal stiffness of the fiber by a factor of 3-4. In longitudinal extension in terms of the two-phase model the two phases act in parallel with both equally deformed. This means that in the wet state the unweakened cylindrical rods contribute considerably to the longitudinal stiffness of the fiber. When the structure undergoes twist about the fiber axis, however, if the matrix is weakened by the presence of water the distortion is nearly completely confined to the matrix, and the torsional stiffness depends mainly on the matrix opposition to the torsional distortion. The result would be that on the basis of the proposed model the presence of water in the structure should cause a much greater reduction in torsional rigidity than in longitudinal stiffness of the fiber going from a dry to wet environment, a result borne out by the experimental data. Ample evidence exists for the presence of a highly ordered structure of low water penetrability within the keratin fibers, containing the organized c•-helical material (11). Chemical evidence (45) based on the extraction of proteins from c•-keratin fibers after the breaking of the disulphide bonds shows that two major fractions of proteins are obtained, a high and a low sulphur component. The low sulphur component is made up'of about 50% c•-helical material, and the high sulphur protein extract has no helices. In line with the physical evidence, the microfibrils within the cortex of the c•-keratin fibers have been identified with the water impenetrable cylindrical rods of the tw$-phase model. The microfibrils contain the crystalline c•-helical material and hence are the source of the low sulphur protein extracts. However, as has been pointed out by Bendit (46), the low sulphur protein is not completely confined to the microfibrils and

394 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the non-helical "tails" of this protein probably play a vital part in the physical interactions between neighboring microfibrils and the matrix in which they are embedded. The high sulfur protein extract is confined to the matrix, and with the high glycine-tyrosine protein found in varying quantities in o•-keratin fibers forms the non-crystalline protein material of the matrix. The high sulphur and high glycine- tyrosine proteins contain fewer hydrophyllic sites, and the former protein is more crosslinked than the low sulphur protein of the microfibrils. This fact has raised the question, why does the matrix absorb more water than the microfibrils (4). Aside from the crystalline and hence less labile state of the microfibrils, the proteins of the matrix do not have to absorb water within their own structure. The matrix must be considered to consist of water plus the high sulphur and high glycine tyrosine proteins (47). The proteins themselves can form globules, and the water form an enveloping, continuous network of hydrogen bonds with the proteins acting as a filler within this network, interacting with the network, at the globule surface. The existence of protein globules has been indicated by X-ray diffraction measurements (48). The mechanical properties in water of different o•-keratins show a progressive stiffening with increase of the high sulphur plus high glycine-tyrosine content of the fiber, an increase which has been associated with the matrix (49). Further, the dry to wet change in fiber diameter is progressively reduced. This can be understood clearly if we recognize that the matrix consists of water plus these two proteins. Because of the physical limitation of intermicrofibrillar spacing (50,51) set during formation of the fiber in the follicle, an increase of these proteins results in a decrease of water content, and a resultant decrease of mechanical mobility of the matrix structure for the wet fiber. A similar effect can be obtained by introducing into the matrix of a keratin fiber large dye molecules (52). It has been shown that with increase of dye uptake the dry to wet diametral swelling of the fiber is reduced and its mechanical stiffness for small strains, particularly in torsion (51), is increased. With suitable dye molecules this procedure is quite reversible and indicates that the main physical effect of these molecules is to displace water in the matrix structure. On the basis of the two-phase model regarding the structure as consisting of microfibrils containing the organized o•-helical structure, and the matrix, broadly considered as corresponding to the rest of the structure, labile and weakened in water, estimates have been made of the mechanical contribution of each phase to the equilibrium Young's modulus of 1.4 x 109 pascals for o•-keratin fibers in water. These measurements (53,54) are based on dynamic measurements at about 102herz and at various humidities, together with conventional mechanical tests at different tempera- tures in water. All tests show the basic agreement that the contribution of the microfibrils to the equilibrium Young's modulus is close to 1.2 x 109 pascals, and the matrix contribution is of the order of 0.2 x 109 pascals. This latter contribution of the matrix is, as expected, small and is of the order expected from the elastomeric stiffness of an amorphous chains. SWELLING IN FORMIC ACID, ALCOHOLS, AND UREA SOLUTION X-ray diffraction evidence shows clearly for both formic acid (33) and the simple alcohols (55) (methanol, ethanol, n-propanol, and n-butanol) that all these solvents swell the crystalline regions of the microfibrils in o•-keratin fibers. In the case of the alcohols the range of swelling is 9-11% in the distance between the helices with no