Modulated DSC of human hair in the dry and the wet state

Human hair belongs to the family of α-keratin materials and as such exhibits a complex morphological fine structure. However, in the context of the mechanical as well as thermal analysis the structure is suitably simplified as a two-phase, filament/matrix composite, with a 25-30% fraction of α-helical proteins in filamentous, intermediate filaments (IF). To characterize the thermal stability of these helical proteins Modulated DSC was conducted for European human hair dry as well as in excess water for heating rates of dT/dt=0.1-5oC/min. Comparing on this basis the two approaches gives detailed insight into the phase transition of helix denaturation as such, as well as into the role of water as a plasticizer for the process. Dry hair shows two processes, namely, the relatively small denaturation peak for the α-helical filaments (≈230oC), superimposed onto the dominant effect of thermal pyrolysis of cortical proteins (>220oC). Since water acts as plasticizer in the matrix phase, the denaturation peak for wet hair is dissociated from the pyrolysis process by shifting to substantially lower temperatures in the range of 150oC. The deconvolution of the MDSC-signal enables to determine the peak baseline, which shows for wet hair a clear, though small, step-wise change (ΔCp≈0.1Jg-1K-1), while the peak covers a range of about 20oC around the denaturation temperature. Such a compound thermal behavior indicates a classical, two-stage denaturation process of the helical proteins. ΔCp is furthermore found to increase substantially with heating rate, indicating that the assumed helix-random coil transition is only the limiting case. At lower heating rates less amorphous structures, such as small, random β-sheets are assumed to be formed, leading to reduced ΔCp-values. Denaturation enthalpies for dry as well as wet hair show little or no change with heating rate. The values for dry hair were consistently lower by a factor of 2-3 (≈4J/g) compared to wet hair (10-15J/g), signifying pronounced thermal damage to the helical proteins prior to denaturation. Denaturation temperatures (TD) change strongly in the dT/dt-range, yielding Arrhenius activation energies of 415 and 263kJ/mol for dry and wet hair, respectively. The values are discussed in the context of activation energies determined from the empirical modeling of DSC-curves for wet hair by 1st order kinetics. Some DSC-curves (wet) show distinct shoulders towards higher temperatures, which correspond to doublets of steps in ΔCp. These are attributed to the occurrence of groups of cells with distinct differences of helix denaturation temperatures, attributed to variations of cystine-content in the matrix. The groups are referred to as low-sulfur (LS) and high-sulfur (HS) cells, respectively, for which the fractions are estimated by DSC-peak deconvolution.