• Perm-waved human hair: A thermo-rheologically complex, shape-memory composite.

      Wortmann, Franz J; email: franz.wortmann@manchester.ac.uk; Jones, Celina; Davies, Thomas J; Wortmann, Gabriele (2021-06-29)
      A 'permanent', bent shape can be imposed on a straight human hair by a two-stage reduction/oxidation (perm-waving) process. The process relies on the molecular level on sulfhydryl (SH)/ disulfide (SS) interchange as bond exchange reaction (BER). We expected a well-documented transition temperature around 60 °C to be the trigger for the shape memory (SM) -process of perm-waved hair. We confirm the existence of the SM-process as such and investigate its time- and temperature-dependence. The results show a two-stage SM-behaviour, implying two distinct variations of the BER. The model to fit the data contains two fractional, normalised, elastic bending rigidities, which are strictly compensatory. They show Arrhenius-type temperature dependence and a common activation energy of ≈ -12 kJ/mol. The characteristic relaxation time for the first SM-process shows little, if any temperature dependence (E = -4 ± 2.7 kJ/mol). This is in contrast to the second process (E = -58 ± 5.5 kJ/mol), but in line with the expected properties of the suggested BERs. None of the parameters shows any sign of the expected trigger transition (≈ 60 °C). We hypothesize that this specific transition occurs only for large tensile deformations, when specific SS -bonds in the intermediate filaments of hair are activated. There is thus no specific 'trigger' transition for the SM of bent, perm-waved hair. [Abstract copyright: Copyright © 2021 Biophysical Society. Published by Elsevier Inc. All rights reserved.]