Figure 6. Influence of chemical treatments on the denaturing peak temperature for hair () p as determined by (A) wet and (B) dry DSC. 680 JOURNAL OF COSMETIC SCIENCE

681 THE MATRIX REVISITED Yet, as before, wet and dry represent extremes, and previous DSC/DTA, experiments on wool (24) and hair (25) showed that the denaturing temperature decreases with progressively increasing water content as obtained by varying the relative humidity (see Figure 7). These experimental data appear to be well fit by the Williams–Landel–Ferry (WLF) model (26), given by the equation ln(a T Reference Reference ) ( ) C T T C T T = - + - 1 2 (8) where a T reflects primarily the temperature dependence of a segmental friction coefficient or mobility on which the rates of all configurational rearrangements depend, C 1 and C 2 are empirical constants, and T Reference is 50°C. This fit allows for the conclusion that the matrix is a viscous material (27), corresponding to the viscous component of the viscoelastic description of the mechanical behavior of hair given by Equation 1. STRUCTURE AND MORPHOLOGY OF THE MATRIX Historically, the matrix has generally been viewed as an amorphous, cross-linked polymer, but the fine details of its structure were less investigated. The relatively recent progress in the analysis of KAP structures (10) has rekindled interest in studying the matrix morphology. Some of the early proposals for the fine structure of the matrix were based on an analysis of tensile measurements on wool. Crewther suggested that a layer of matrix molecules protects the helical structure of the microfibrils (28). In a second work on the same topic, Figure 7. Evolution of the peak temperature of keratin fibers with increasing water content, as reported by Haly and Snaith for DTA measurements on wool (24) and by Cao and Leroy for DSC measurements on hair (25). The experimental points fit the WLF model (26), as shown by the dotted curve (27).