Abstract
Moisture sorption properties of silk fibroin fibers modified with n-butyl isocyanate were investigated. The equilibrium moisture regain was decreased in the whole range of relative humidity with the increase in the degree of substitution (D. S.) of -OH groups of amino acid residue to form urethane linkages. The X-ray diffraction intensity distribution and the degree of crystallinity were not changed by the modification. The decrease in the moisture regain was due to the selective substitution of -OH groups of Ser, Thr or Try residues and these amino acids residues showed large contributions to the moisture regain. However, the moles of the sorbed water per peptide bond, [H2O]/[CONH], for treated (D. S., 68%) silk fiber were lower than that for polyglycine. The underestimation of the degree of crystallinity was one possible reason and another was that the introduction of the bulky group in the molecular chain might cause the rearrangement of the three dimensional structure in the amorphous region resulting in the formation of the hydrogen bonding between the peptide bonds or the protection of the peptide bonds from the water molecules. The heat of wetting for the dry silk fiber rapidly decreased in the low D. S. range indicating that the interaction between the water molecules and the -OH groups, especially of Ser residues, was stronger than the case of other polar amino acid residues.
