Stress-strain behaviour and volume swelling of wool fibres rendered elastomeric in LiBr solution

Abstract

Stress-strain behaviour and volume swelling of wool samples were studied, with the wool fibres in the elastomeric state produced by their absorption of 8 M LiBr solution. Tests were done with normal wool and wool samples in which the number of disulphide bonds had been decreased by reduction in thioglycollic acid or benzyl mercaptan. Experiments included fibres from all samples at the end of both first and second stages of supercontraction and also after a setting treatment. With the aid of the theoretical relationship between stress and strain the data were used to calculate relative levels of cross-linking. Results from fibres at the end of the second stage of supercontraction indicate that there are five times as many cross-links in unreduced fibres as in zero-disulphide fibres. Results for fibres at the end of the first stage of supercontraction are different; they indicate six times as many cross-links in unreduced fibres as in unreduced second-stage fibres, and fifteen times as many in unreduced fibres as in zero-disulphide fibres. It is shown that the above estimates disagree with estimates made using either length change or volume swelling data. Reasons for preferring estimates from the stress-strain equation are given. It is concluded that fibres at the end of the second stage of supercontraction (or set fibres) yield stress-strain results chiefly dependent on the number of covalent cross-links in wool, while the results from first-stage fibres are complicated by the presence of physical constraints. Application of solution theory to the volume swelling results indicates a potential level of supercontraction of 70% for wool on the point of going into solution. This high level of contraction is not observed in unbuffered LiBr solution, but has been reported for wool fibres in acid solutions of salts.