Perm-waved human hair: a thermorheologically complex shape memory composite

Area of Science:

• Materials Science
• Biochemistry
• Physical Chemistry

Background:

• Human hair can be permanently reshaped using a two-stage reduction/oxidation process, commonly known as perm-waving.
• This reshaping process involves sulfhydryl/disulfide bond exchange reactions (BER) at a molecular level.
• A transition temperature around 60°C was previously hypothesized to trigger the shape memory (SM) effect in perm-waved hair.

Purpose of the Study:

• To investigate the time and temperature dependence of the shape memory process in perm-waved human hair.
• To determine if a specific transition temperature acts as a trigger for the hair shape memory effect.
• To elucidate the molecular mechanisms underlying the shape memory behavior of perm-waved hair.

Main Methods:

• Experimental investigation of the time and temperature dependence of the shape memory process.
• Application of a mathematical model incorporating two fractional, compensatory elastic bending rigidities.
• Analysis of Arrhenius-type temperature dependence and activation energies for the molecular processes involved.

Main Results:

• The study confirmed a two-stage shape memory behavior in perm-waved hair, indicating two distinct BER variations.
• The molecular processes exhibit Arrhenius-type temperature dependence with a common activation energy of approximately -12 kJ/mol.
• No specific trigger transition temperature around 60°C was observed; instead, the shape memory effect appears continuous with temperature.

Conclusions:

• The shape memory behavior in perm-waved hair does not rely on a specific trigger transition temperature.
• The observed two-stage SM behavior is attributed to two distinct variations of bond exchange reactions (BER).
• It is hypothesized that the specific transition temperature might only manifest under large tensile deformations, activating specific disulfide bonds within hair's intermediate filaments.