Wet spinning of Bombyx mori silk fibroin dissolved in N-methyl morpholine N-oxide and properties of regenerated fibres

Abstract

Silk fibroin (SF) was dissolved in N-methyl morpholine N-oxide (NMMO) at a polymer concentration of 13% (w/w); thermal and rheological solution properties were characterized. The melting/crystallization behaviour of NMMO was influenced by SF presence. Melting of NMMO hydrate decreased to 71 °C and a cold crystallization peak appeared at 35 °C on heating. None crystallization occurred on cooling. Quenching at a temperature of 50 °C or higher did not induce any crystallization on heating. Viscosity of SF–NMMO solutions decreased as a function of temperature. At 75 °C, viscosity remained constant for 360 min. SF–NMMO dope was spun by using a lab-scale wet spinning line. The extruded filament was coagulated in an ethanol bath. Regenerated SF fibres were collected at different draw ratios and their morphological, physical, and mechanical properties were characterized. Fibre diameters ranged from 133 to 19 μm, cross-section was regularly circular, and surface was generally smooth, with a very fine granular aspect. Birefringence increased with increasing the draw ratio, especially when take up and post-spinning draw were coupled. FT-IR spectra and DSC thermograms confirmed that SF fibres crystallized into Silk II structure. The IR crystallinity index did not change as a function of drawing. Regenerated SF fibres undrawn or drawn only during the coagulation step showed the mechanical behaviour typical of a brittle material. However, when both take up and post-spinning draw were applied, fibres displayed a ductile-stable behaviour. Typical values of the mechanical parameters of regenerated SF fibres were: E = 8.7 GPa, σ b = 120 MPa and ɛ b = 35%.