The 'Internal Plasticisation' Hypothesis for Human Hair

The ‘internal plasticisation’ hypothesis for human hair: Investigations through esterification of cortical proteins Ian Jinks1, Franz J. Wortmann1, Gabriele Wortmann1, Prem Paul21School of Materials, University of Manchester, Manchester M13 9PL, UK.2Unilever Research and Development, Port Sunlight, Wirral, CH63 3JW, UK.Human hair, like other α-keratinous fibres, is a highly complex biomaterial. For the analysis of its mechanical and thermal properties it is, however, well described by a two-phase structure, which contains as morphological components the highly-ordered, crystalline intermediate filaments (IFs) and the less-ordered, amorphous matrix. A previous study [1] has shown that wool has a higher glass transition temperature Tg than human hair despite the fact that hair has a higher content of hydrophobic, high glycine-tyrosine (HGT) proteins contained within the matrix. On this basis, the ‘internal plasticisation hypothesis’ was developed, stating that the HGT-proteins in hair have a specific plasticisation effect in the matrix. To investigate this hypothesis, hydrophobic modifications of the proteins in hair have been carried out and their effects on the sorption performance and the thermal properties analysed.Results for esterification with alcohols of different alkyl chain lengths and structure show that, this modification leads to a substantial decrease of water sorption over the whole humidity range and to an increase in the Flory-Huggins interaction parameter. The glass transition of human hair, which is a property of the matrix, is systematically lowered. These observations provide consistent support for the ‘internal plasticisation’ hypothesis. However, hydrophobic interactions also play an important role for the association and the resulting stability of the IFs [2]. Though little changes are observed for the denaturation temperature TD, systematic reductions in denaturation enthalpies ΔHD are observed after esterification. This suggests that, internally plasticising the hair has also a marked effect on the hydrophobic interactions that maintain the integrity of the IFs.[1] F. J. Wortmann, M. Stapels, R. Elliot, L. Chandra, Biopolymers, 81 (2006) 371.[2] D. A. D. Parry, W. G. Crewther, R. D. B. Fraser, T. P. MacRae, J Molec Biol, 113 (1977) 449.