679 THE MATRIX REVISITED breaking of some disulfide bonds the subsequent formation of cysteic acid and a resulting increase in hydrogen bonding that stiffens the matrix, especially at low RH. Equations 2 to 7 establish the physical background for evaluating the contribution of the matrix to the mechanical behavior of hair fibers (virgin and bleached) at different RH values and reflect the experimental results well (3). THERMAL PROPERTIES DSC experiments also provide information on the nature of the matrix. Whereas the enthalpy of thermal denaturation (i.e., the area under the peak) is associated with the amount of α-helical protein contained within the fibers, the peak temperature, T p , has been associated with the state of the matrix (see Figure 5) (21). Two types of DSC experiments are commonly performed on keratin fibers. In the first, samples are placed in perforated capsules, and the hair is examined in the dry state (dry DSC). Alternatively, the samples can be sealed within high-pressure pans that also contain water (wet DSC) (22). Results recorded in these two types of DSC experiments similarly exhibit differences in the detected responses to chemical treatments. Figure 6A shows results from wet-state DSC experiments where, as is commonly reported, the hair can be denatured at lower temperatures as a result of chemical bleaching. In contrast, Figure 6B shows that the dry- state denaturing temperature rises for these same samples. This peak temperature is related to the state of the matrix (see Figure 5). In the wet state, this structure contributes a lesser stabilizing effect, and the KIFs denature at lower temperatures. However, the opposite situation arises in the dry state, which implies that the matrix becomes more rigid and contributes additional stabilization to the KIFs. These findings are consistent with the previously described observations for the mechanical properties (23). Figure 5. Relationship between the components of hair and DSC/differential thermal analysis (DTA) endothermal parameters (21).

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