PHYSICAL PROPERTIES OF ALPHA-KERATIN FIBERS 395 apparent loss of the helical content of the fiber. In the case of the alcohols (56), Speakman obtained a progressive increase of mechanical stiffness of the fibers with increase of molecular size. However, if the tests are carried out to mechanical equilibrium so that the dynamic processes, which are slower for a fiber in the alcohols compared with water, have time to relax, then the fibers indicate a considerable reduction in their stiffness in the "Hookean" region (57). This reduction is the expected consequence of the swelling of the crystalline regions. Bendit (33) has shown that in 99% formic acid the distance between o•-helices in the crystalline region has increased by about 17% above the value in water. For an aqueous solution of formic acid up to 70% (V/V) optical birefringence measurements show no change in the crystallinity of the fibers. However, the "Hookean" Young's Modulus of the fibers up to this formic acid concentration decreases continously to a value of 108 pascals, a factor of 20 down on their stiffness in water. This indicates a progressive penetration of the microfibrils by the formic acid with the o•-helices intact but certainly weakened by the penetration. The result is in contrast to the effect of immersion of fibres in increasingly higher concentration of lithium bromide solutions, as indicated earlier in this review (33). In the latter case the rapid mechanical weakening of the fiber between 6 and 7 Molar concentrations of aqueous lithium bromide solution results from a randomization of the o•-helical structure. Up to just below 6 Molar concentra- tion the mechanical weakening of the fiber was confined to the matrix. In concentrated urea for aqueous solutions as high as 10 Molar the stiffness of the fiber suggests no weakening of the microfibrils (33), with all the change being confined to the matrix. Measurements of optical birefringence also indicate that the crystallinity of the fiber is little changed. Summarizing the effect of swelling on the physical properties of an o•-keratin fiber, we see that three distinct categories of events may occur with three different effects on the mechanical properties of the fiber: (a) The matrix is the main structure swollen, as in the case of water and urea solutions which results in only a weakening and increased mobility of the matrix with the ordered microfibrils intact. (b) The matrix and microfibrils swollen with the o•-helices intact as in the case of the alcohols and formic acid up to 70% concentration in water (V/V), results in a progressive mechanical weakening of the matrix and micro fibrils but has little effect on the optical birefringence, with the X-ray diffraction o•-pattern still present but indicating the swelling. (c) The matrix, microfibrils swollen and the o•-helices randomized as for fibers in concentrated aqueous solutions of lithium bromide at molarities greater than 6.6M. The fiber's mechanical stiffness is drastically reduced and its value corresponds to that of a purely elastomeric solid. The high angle X-ray diffraction o•-pattern and the optical birefringence have both disappeared in agreement with the structure now consisting of randomized polypeptide chains. THE YIELD REGION MECHANICAL When an cr-keratin fiber is extended at a constant rate of straining beyond 2% strain, especially in water, the value of the stress on the fiber does not increase markedly until

396 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS about 25-30% strain is reached. This region of extension with little change in stress is the Yield region. Speakman (3) originally demonstrated that for mechanical deforma- tions in this region the properties of the fiber were completely recoverable after relaxation in water in room temperature overnight, provided the deformation had not been maintained too long ( ! hour) or at a temperature greater than about 50øC. Further work in this field of recovery of mechanical properties after deformation, has shown that this property of recovery is quite sharply limited to the Yield region (58). Extension beyond the Yield region results in a very rapid increase in irrecoverability of mechanical properties accompanied by covalent bond breakdown, as demonstrated by the formation of free radical detected by election spin resonance technique (59). The behavior of fibers in creep (60) and stress-relaxation (61) for strains in the Yield region is completely non-linear visco-elastic. A complete description for this behavior has been obtained for fibers in water by the application of the Burte-Halsey model (60,62). The fiber can be considered to consist of a continuum of units which can exist in a short state A or an extended state B with an energy barrier between the two states. The description of c•-keratin fibers in water in terms of these Burte-Halsey units defines the mechanical properties of the fiber with change of temperature, force, and time (60). As the fiber is extended from the Hookean into the Yield region, the Burte Halsey units, which are in state A and tensioned when the fiber is in the "Hookean" region, begin to transform into state B units. The whole Yield region corresponds to a phase transition of state A • state B with the stress remaining constant with the temperature constant as would be expected in such a first order transition. The length of the fiber is defined by the proportion of units in the longer state B as against the proportion in state A. Extrapolation of the fixed stress values for the state A • state B situation at different temperatures for fibers in water suggests that the transformation of all the state A to state B would occur at zero stress at 160øC. This spontaneous "melt" in water assumes that the fiber is unaffected structurally by any irreversible way. Melting experiments carried out on both hair and wool keratin fibers in water have shown that the melting process is time dependent, occuring at 130øC in minutes and at 120øC in hours (63,64). These latter "melting" processes are irreversible and represent the combination of transformation of units A ---• units B and the breakdown of the disulphide crosslinks in the structure, which stabilize units A and hence result in a temperature reduction of the melt as this breakdown occurs. Recent dynamic mechanical measurements (53) using oscillatory displacement tech- niques at 118hz has shown a clear separation between two major mechanical events at all relative humidities (Figure 3). As the fiber is extended from the "Hookean" to the Yield region there is a rapid loss of dynamic stress as unfolding of units within the structure commences. This unfolding process is quite independent of the moisture content of the fiber and occurs in parallel with a moisture sensitive amorphous thixotropic structure, which during extension of the fiber goes through a gel-sol transformation. The transformation of units from state A to extended state B can be understood as corresponding to the extension of whole cooperative groups of o•-helices extending by unfolding into the extended •-configuration. The dynamic data suggests that the units unfolding as the fiber is extended into the Yield region are responsible for a loss of dynamic stress independent of the moisture content of the fiber. Equation of these A units with o•-helical units of the microfibrils is therefore