Structural changes and dyeability of silk fibroin fiber following shrinkage in neutral salt solution

Abstract

AbstractThe fine structural changes of Bombyx mori silk fibroin fibers, induced by shrinking with concentrated calcium chloride aqueous solution at elevated temperature were investigated as a function of shrinking rate. Tensile strength decreased and elongation at break increased in the shrinkage range 13–67%, the shape of the stress–strain curve changing from rubberlike to brittle at high shrinkage values (70–90%). The birefringence gradually decreased over the entire shrinking range examined, the curve becoming steep as the shrinkage exceeded about 67%. The behavior of isotropic refractive index (niso) closely resembled that of birefringence (Δn) in the shrinking range 13–67%. Beyond shrinkage of 67%, the niso showed a tendency to increase, especially for the sample with 80% shrinkage. Dichroism measurements showed that the molecular orientation within the amorphous regions decreased sharply at the beginning of the shrinking treatment, within the range 0–13%, then attained a saturation at about 55%. The position and intensity of the major X‐ray diffraction peak at 20.5° remained essentially unchanged regardless of the shrinking treatment. The results of dyeing behavior showed that the saturation value attained by shrunk silk fibers was significantly larger than that of the untreated control sample. Both standard affinity and the heat of dyeing increased slightly for the shrunk silk fibroin fiber, suggesting that a larger number of reactive sites became available for the interaction between dye molecules and fibroin chains. A schematic model is proposed for explaining the relation between structural changes and enhanced dyeability of the silk fibers following shrinkage in neutral salt solution. © 1994 John Wiley & Sons, Inc.