Solubility and rheological behavior of silk fibroin (Bombyx mori) in N-methyl morpholine N-oxide

As an important direct solvent for cellulose, N-methyl morpholine N-oxide (NMMO) is environmentally friendly, and potentially very economical. Silk fibroin (SF) (Bombyx mori) can also be dissolved directly in NMMO·H2O. However, it is unexpectedly difficult to obtain a silk fibroin solution with a concentration higher than 10 wt.% in this way, and extensive degradation of silk fibroin occurs if the dissolution temperature is higher than 110 °C. On the other hand, it is found that regenerated silk fibroin (RSF) film is much easier to dissolve in NMMO·H2O than ordinary SF. The RSF in NMMO·H2O can be easily concentrated to a range from 10 to 25 wt.%. The structural differences between the degummed silk fiber and the RSF film lead to this different solubility in NMMO·H2O. The rheological behavior of concentrated RSF/NMMO·H2O solutions were also investigated. Regenerated silk fiber was spun from this type of solution, and its strength can reach up to 3.07 cN/dtex.