STRUCTURAL-CHANGES AND DYEABILITY OF SILK FIBROIN FIBER FOLLOWING SHRINKAGE IN NEUTRAL SALT SOLUTION

The fine structural changes of Bombyx mori silk fibroin fibers induced by shrinking with concentrated calcium chloride aqueous solution at e levated temperature were investigated as a function of shrinking rate. Tensile strength decreased and elongation at break increased in the s hrinkage range 13-67%, the shape of the stress-strain curve changing f rom rubber-like to brittle at high shrinkage values (70-90%). The bire fringence gradually decreased over the entire shrinking range examined , the curve becoming steep as the shrinkage exceeded about 67%. The be havior of isotropic refractive index (n(iso)) closely resembled that o f birefringence (DELTAn) in the shrinking range 13-67%. Beyond shrinka ge of 67%, the n(iso) showed a tendency to increase, especially for th e sample with 80% shrinkage. Dichroism measurements showed that the mo lecular 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-degrees remained essentially unc hanged regardless of the shrinking treatment. The results of dyeing be havior showed that the saturation value attained by shrunk silk fibers was significantly larger than that of the untreated control sample. B oth standard affinity and the heat of dyeing increased slightly for th e shrunk silk fibroin fiber, suggesting that a larger number of reacti ve sites became available for the interaction between dye molecules an d fibroin chains. A schematic model is proposed for explaining the rel ation between structural changes and enhanced dyeability of the silk f ibers following shrinkage in neutral salt solution. (C) 1994 John Wile y & Sons, Inc.