The Stress—Strain Characteristics of Animal Fibers After Reduction and Alkylation

Abstract

Animal fibers were reduced to varying degrees with solutions of thiols and alkylated with methyl iodide or ethylene dibromide. The stress-strain curves of S-methylated Lincoln wool fibers showed the following characteristics. The stresses at the end of the Hookean region and yield region and the breaking stress approached zero at zero disulfide content and maintained a constant ratio. Likewise the slopes of the yield and post-yield region showed a constant ratio and extrapolated to zero at zero disulfide content. The decrease in stress or slope per disulfide bond broken was much greater at low disulfide contents than at high disulfide contents. The strains at the end of the yield region and at break increased with decreasing disulfide content and the effect of each disulfide broken was greatest at low disulfide contents. The relationship between strain at the end of the yield region and residual disulfide content was the same, within experimental error, for all fiber types. Reduction followed by reaction with ethylene dibromide did not greatly affect the stress-strain properties of the fibers. Reduction without subsequent alkylation, how ever, decreased the stiffness of the fibers much more than the rupture of an equal number of disulfide bonds by reduction and methylation. It is concluded that side-chain interactions between helical structures and matrix molecules containing many intrachain disulfide bonds are chiefly responsible for stiffen ing the fiber in the Hookean region. The yield and post-yield regions also are con sidered in terms of interactions between microfibrils and matrix, particularly with respect to the effects of disulfide bonds linking proteins in the two phases.