Crosslink Density Effects on the Hysteretic Behaviour of Keratin Fibers in Water Sorption Isotherms

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The fundamental physical processes behind the hysteresis phenomena in keratin water sorption isotherms are still uncertain. To examine the role of swelling in this process, hair was modified via reduction and dyeing to measure the impact of reduced inter-keratin bonding on hysteresis. Water sorption isotherms were measured using Dynamic Vapour Sorption, and fibres were characterised using Nuclear Magnetic Resonance. Hair fibres with reduced crosslinks from damage procedures showed a higher amorphous content, indications of glass transitions at lower relative humidity, and faster relaxation properties, resulting in distinctly different hysteretic properties to that of untreated hair. This work concludes that hysteresis is dependent on the swelling ability of keratin, which allows greater plasticisation of glassy regions in the fibre by water. This swelling is strongly governed by disulphide bond content.