386 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In considering the physical properties of ce-keratin fibers it is necessary to recognize the fundamental role of water in the structure of the fiber. The properties of the fiber change markedly with the amount of water interacting with the structure of the fiber (3) to such a degree that the material may be considered as a keratin-water polymer. The penetration and the interaction of the water molecules depend not only on the existence of hydrophyllic sites within the keratin structure but also on the state of order within the different components of that structure. A high degree of order (crystallinity) as well as the presence of cross links may limit the ability of water to interact at what are chemically hydrophyllic sites. It is the lack of recognition for the role of order, on the sorption of water within the ce-keratin structure, that has led to some of the apparently anomalous conclusions, as to the placement of water based purely on the chemistry of the structure (4). In commercial application, human hair and to a lesser extent finger nails in the cosmetic industry, and wool fibers in the textile industry, are the most important of the ce-keratins. The prime physical properties in the application to these fibers are their mechanical properties. Regardless of the treatment that is applied to the fibers, in nearly all instances the result obtained and assessment made are directly related to the mechanical state of the fibers. For this reason the author has considered the mechanical properties of the ce-keratin fibers as the central physical properties of these fibers. The other properties such as X-ray diffraction, Infra-red absorption, electrical conduction, dielectric response, birefringence, and other measurements lead to our understanding of the relationship between the molecular and near molecular structure of ce-keratin fibers and their mechanical properties. It should be emphasised that the mechanical properties of a fiber are the summation of the properties of the whole fiber involving the molecular units both in the ordered (crystalline) regions of the fiber and the less ordered (amorphous) regions. Many of the physical properties dealt with in this review emphasize mainly one aspect of the fiber structure. High angle X-ray diffraction studies primarily lead to information about highly ordered regions of the fiber structure, whereas calorimetric measurements, being related to the freedom of movement of the molecular components within the fiber, respond more to the less ordered, more mobile regions. This review aims to broadly identify the molecular components and assemblies responsible for the variation of the physical properties of ce-keratin fibers with change of moisture content, temperature, and physical and chemical modification. The presence of swelling agents such as concentrated aqueous lithium bromide solutions and formic acid will be examined for their effect on the behavior of the fibers. Two broad structural features may be identified which control the physical behavior of any polymeric structure. The equilibrium structural organization of the fiber, which essentially relates to the equilibrium position of all the components of the fiber, is measured primarily by physical data which do not involve molecular movement. The structural dynamics of the system, that is the time dependent component of the physical behavior of the fiber, are measured by physical data which are sensitive to the freedom of movement of molecular groups within the fiber structure. Because molecular movement is mainly confined to the less ordered components of structure, and since the less ordered components are the main regions of absorption of plasticizers such as water and various alcohols, these plasticizers have a major control over the structural dynamics of the fiber. Figure 1 diagrammatically summarizes the relationship between the physical data obtained relevant to the equilibrium structural organization and structural dynamics of a fiber. Known structural modification by chemical and physical

PHYSICAL PROPERTIES OF ALPHA-KERATIN FIBERS 387 I STRUCTURI]CHEMICAL]& MODIFICATION (PHYSICAL , (ELECTRON, X-RAY)J , i CALORI ( ST.UCTUA OR• - _PROPERTIES DY• E LEC TRON M EAS URE MENTS MICROSCOPY DIELECTR IC• CONDUCT I0 N ) PLASTICISERS [WATER• ALCOHOLS• ETC.) RY Figure 1. A diagrammatic representation of the typical physical measurements employed in obtaining the data relating the structure of a fiber to its mechanical properties. techniques, such as reduction of disulphide content of a fiber or swelling in concentrated aqueous lithium bromide solution, can also be assessed in terms of the resultant change in the dynamic and equilibrium properties of the fiber. Thus, the reviewer proposes to examine the mechanical properties of c•-keratin fibers interpreted in conjunction with data obtained with a range of physical techniques. A general introduction has been given to the fibrous protein nature of c•-keratins without any indication to the morphology of such fibers, the distribution, size, and role of various components forming such fibers. Obviously the latter factors play their role in the mechanical properties of fibers and a brief discussion follows on this morphology as it relates to hairs, fur and wools. MORPHOLOGY Alpha keratin fibers such as hairs, furs, and wools have in common the structural components of cuticle and cortex with a central medulla often present in the coarser fibers. The cuticle covers the whole fiber and consists of layers of scales each about half a micron thick, overlapping to give a ratchet-like profile. The number of scales present in a cross-section of a fiber is dependent on the type of fiber. In human hair these may be as many as ten overlapping scales in one cross-section in a newly formed fiber, whereas in a wool fiber only sufficient scales for barely overlapping the fiber exist. The