Preparation and characterization of silk fibroin nanocrystals

Crystalline silk fibroin nanoparticles (CSFs), a kind of natural protein nanocrystal, were prepared by treatment of silk fibroin (SF) powders with 64 wt% aqueous sulfuric acid. Various treatment periods were tested, and optimal conditions corresponded to 2 h at 45 °C. Transmission electron microscopy observation showed that the CSFs existed as short rod-like fragments and spherical particles. The content ratio of β-sheet structure to α-form/random coil and the percentage crystallinity (χc) increased, but the weight-average molecular weight of CSFs decreased with prolonged degradation time. Compared with native SF, these CSFs of nanometer size and relatively high χc showed an upward shift of the glass transition temperature and thermal degradation temperature. Furthermore, high-strength elastic nanocomposite materials were successfully prepared from the CSFs and waterborne polyurethane (WPU). One of the WPU/CSF films produced exhibited significant increases from 1070 to 2370% for elongation at break, 0.6 to 2.2 MPa for tensile strength and 0.3 to 4.0 MPa for Young's modulus. This work provides a new pathway for the preparation of natural nanocrystals and WPU-based elastomers with high strength and toughness. Copyright © 2012 Society of Chemical Industry