Swelling and dissolution of silk fibroin (Bombyx mori) in N-methyl morpholine N-oxide

Abstract

Bombyx mori silk fibers were dissolved in N-methyl morpholine N-oxide (MMNO), an organic cyclic amine oxide used for the solvent spinning of regenerated cellulosic fibers. The commercial MMNO monohydrate used in this study as a solvent for silk is a hygroscopic compound crystalline at room temperature, which becomes an active solvent after melting at 76°C. The degree of hydration of MMNO was checked by DSC measurements. The solvation power of MMNO towards silk fibroin drastically decreased at a water content ≥20–21% w/w. Dissolution of silk required both thermal and mechanical energy. The optimum temperature was 100°C. At lower temperatures dissolution proceeded very slowly. At higher temperatures, rapid depolymerization of silk fibroin occurred. The value of the Flory–Huggins interaction parameter χ for the MMNO–H2O–silk fibroin system was −8.5, suggesting that dissolution is a thermodynamically favored process. The extent of degradation of silk fibroin was assessed by measuring the intrinsic viscosity and determining the amino acid composition of silk after regeneration with an aqueous methanol solution, which was effective in removing the solvent and coagulating silk. Regenerated silk fibroin membranes were characterized by infrared spectroscopy, differential scanning calorimetry and scanning electron microscopy. The prevailing molecular conformation of silk fibroin chains was the β-sheet structure, as shown by the intense amide I-III bands at 1704, 1627, 1515, 1260, and 1230 cm−1. The value of the I1260/I1230 intensity ratio (crystallinity index) was 0.68, comparable to that of the fibers. The DSC thermogram was characteristic of a silk fibroin material with unoriented β-sheet crystalline structure, with an intense decomposition endotherm at 294°C. The SEM examination of fractured surfaces showed the presence of a dense microstructure with a very fine texture formed by densely packed roundish particles of about 100–200 nm diameter.