Abstract
Anisotropic dimensional changes in silk fiber immersed in aqueous LiBr solutions have been studied.
Contractility of fibers was observed at temperatures higher than ca. 40°C above in the 6M LiBr solution. A larger dimensional change of the fibers was observed by increased salt concentration and the length of contraction leveled off at the LiBr concentration 7.5M. At the concentration of 8.5M, the fibers were expanded and broken down at 75°C.
In order to examine the contraction mechanism of the fibers, infrared absorption was tested in the LiBr/D2O system using silk fibroin films containing unoriented β, α, and the random coil structure, respectively. It was found that at a salt concentration of 4.3M, only β structure was destroyed. At higher concentration of 8.6M, however, both α and β structures of silk were destroyed. Then, the transformation involved the change from crystalline to amorphous phase of the constituent polypeptide chains. The silk fibers contracted in LiBr solution showed remarkable thermoelastic properties. The fibers dialysed against deionized water gave the α structure. From these facts, it was confirmed that the silk fiber was changed by aqueous LiBr solution from the β structure to the α structure.
