RESUMO
In this work, the formation of free radicals in human hair and the evolution of the radical concentration under strain, using a combined electron paramagnetic resonance (EPR) - strain technique, has been investigated. The radicals formed in the hair as a result of homolytic bond cleavage in cystine residues of polypeptide chains were identified. Stability of the radicals formed in dry hair and in the presence of water were studied. The spin-strain curves for the grey human hair in dry state and in water are presented and compared with conventional hair stress vs. strain curves. The evolution of sulfur radical species has been found to only occur at strains of above ca. 25%, corresponding with the Post-Yield region of the stress vs. strain behaviour for hair; this indicates that the matrix of the hair in this region behaves like a highly crosslinked gel and helps to explain the reversibility in hair mechanical behaviour below the Post-Yield region.
