Linear and nonlinear relations between DSC parameters and elastic moduli for chemically and thermally treated human hair.

Against the practical context of thermal straightening, hair samples were obtained with a chemical (bleaching) as well as a cumulative thermal history (0 - 800 s, 200 oC). On these samples tensile testing as well as DSC-analysis, both in the wet state, were conducted to obtain the elastic moduli Ew as well as denaturation temperatures TD and enthalpies ΔHD. 3D-plots show overall linearity for the relationships between the parameters for natural hair. For bleached hair pronounced non-linearities develop beyond 300 s of thermal treatment. At this stage TD as well as Ew approach limiting values, consistent with the state of a highly cross-linked, thermoset polymer. 2D-projections are used to investigate the correlations between pairs of parameters. The results show that bleaching imparts a specific sensitivity for thermal damage, namely, to the matrix proteins, which more readily than the intermediate filaments (IF) turn into a thermoset. Overall, correlations between parameters hold well prior to the thermoset range. It is thus suggested that tensile testing to determine the elastic modulus and DSC come to consistent and equivalent results, at least, for the current experimental context. However, while Ew combines contributions of IFs and matrix, DSC differentiates the specific property changes of these components.