摘要：

THERMAL AND MECHANICAL TRANSITIONS IN KERATINSAND THEIR RELEVANCE FOR HAIR COSMETICSF.-J.WortmannUniversity of Manchester, School of Materials, PO Box 88, Manchester M60 1QD, UKe-mail address: franz.wortmann@manchester.ac.ukKeratin fibres such as wool or hair exhibit a complex biological composite structure, which is common to all mammalian ??-keratins. In view of their physical properties, the dominant morphological components are the partly ??-helical intermediate filaments (IF) which are embedded, with largely axial orientation, in an amorphous matrix, consisting of the so-called intermediate filament associated proteins (IFAP). Principles of composite materials are applied to the hair structure in order to enable the derivation of the component properties on the basis of mechanical and thermal analysis. Keratin fibres have a stress strain curve in water which is quite similar to that of other, synthetic, semi-crystalline polymer fibres. Despite this apparently unspecific appearance, it is shown that the pertinent parameters of the stress/strain-curve such as yield-, post-yield and breaking strain are directly linked to the structural ??????? transition of specific helical segments of the IFs. This approach is discussed against the background of seemingly conflicting theories. Besides this special, strain-induced transition in the crystalline phase, keratins show those viscoelastic transitions in the amorphous matrix, which are characteristic for semi-crystalline polymers, such as a low temperature ??- and a higher temperature ??- (glass) transition. The strong humidity dependence of these transitions and differences between keratins will be discussed in relation to their protein composition. Under practical conditions keratin fibres are generally at temperatures and humidities below the glass transition, where the phenomenon of physical ageing occurs. Together with the mechanical properties of the morphological components this effect has very important consequences in practice, which are illustrated for water waving of human hair. At high temperatures (240??C dry, 150??C wet) the denaturation of the helical filaments takes place. The size and the location of the denaturation peak differ between keratins, depending on the amount of ??-helical material and on the composition of the surrounding matrix. Against this fundamental background, DSC was found to be especially suitable to assess various aspects of the changes that filaments and matrix undergo through chemical, cosmetic processes, such as bleaching and perm-waving. The dependencies between denaturation temperature and enthalpy for the two types of processes show that IFs and IFAPs are similarly affected by bleaching, while reductive damage, in comparison, is more pronounced in the IFs. For both types of treatments, changes in enthalpy follow apparent first-order kinetics with respect to the number of treatments and treatment times. For the oxidatively treated hair the course of the denaturation was further investigated by a kinetic analysis of the DSC-curves, assuming a first-order denaturation process and using the Friedman-method. The results show that the kinetic hindrance of the unfolding of the ??-helix by the matrix is in fact the primary controlling mechanism of the onset of the denaturation process.

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