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

PHYSICAL PROPERTIES OF ALPHA-KERATIN FIBERS 397 2./. 2.2 z •l.8- luJ 1.6- I- 1./.- I.Ul 1.0- u") .8 o •73.8 ._•_85 92.5 100 li) J0 3'0 ZO 5'0 6'0 STRAIN 6 Figure 3. The dynamic modulus E • of Lincoln wool fibers against longitudinal strain e for different relative humidities as indicated on each curve. The drop of E • as the fiber is extended into the Yield region corresponds to the unfolding of the ce-helical structure within the fiber (see text). particularly appropriate, as is also the moisture sensitive amorphous thixotropic structure with the matrix phase. Because in longitudinal extension matrix properties act in parallel with microfibrils but in water are relatively weak, they are best observed in the lateral or torsional behavior of the fiber where their effect is dominant. Extension at a constant rate of strain in the direction perpendicular to the fiber direction for rhinoceros horn or African porcupine quill in water (65) results in a hyperbolic relationship between stress and strain with an initial incremental modulus of about 3 x 108 pascals which rapidly drops to the order of 2 x 107 pascals (Figure 2). When extension ceases and the fiber is reextended or contracted at a constant rate the incremental modulus is again high, rapidly dropping to a low value with distortion (65). This behavior can best be described as thixotropic, corresponding to a material on distortion going from a gel to a sol state and (as indicated previously) is the behavior already associated with the matrix. X-RAY DIFFRACTION AND tr ,,• /5 TRANSFORMATION Bendit (66) has shown that for oz-keratin fibers extended in the Yield region there is a progressive loss of g-helical content as indicated by the high angle X-ray diffraction pattern. He was able to detect the presence of the extended/3-keratin configuration at a few percent strain and a progressive increase of the amount of/3-keratin present with further extension of the fiber. At the end of the Yield region about 305o of the original