Characterization of polyurethane and a silk fibroin-polyurethane composite fiber studied with NMR spectroscopies

Recently, Bombyx mori silk fibroin (SF) has played a crucial role in biomedical applications. However, due to the highly ordered β-sheet structure in SF, the flexibility of SF fiber is relatively poor, which limits its application in biomaterials. In this paper, a SF-polyurethane (PU) composite fiber was prepared to increase the flexibility of SF fiber. PU prepared by a reaction of mainly a copolymer of polyhexamethylene carbonate diol and polycaprolactone diol (PHC/PCL-diol) with 4,4’-diphenylmethane diisocyanate (MDI) was used. First, the 1H and 13C solution NMR peaks of PU were assigned to groups in detail. In addition, the 13C solid-state NMR spectrum of PU was analyzed. Then, regenerated SF-PU composite fiber was prepared by the wet spinning method. The elongation at break of the fiber increased by 1.3-1.8 times compared with that of regenerated SF fiber, although there was no significant increase in tensile strength. The main reason for the increased flexibility of the SF-PU composite fiber is the increase in the fraction of random coils in SF, which was clearly observed by 13C solid-state NMR.