Molecularly Engineer Protic Ionic Liquids for Simultaneous Dissolution of Silk Fibroin/Cellulose and Wet-Spinning Composite Fibers Preparation through Hydrogen-Bonding-Acceptor-Strengthening Strategy

With over 20 years of continued development of ionic liquids (ILs) for natural polymer processing and conversion, the design and facile preparation of ILs is still a hot research topic. In this study, a hydrogen bonding acceptor strengthening strategy was applied to design ether functionalized protic ILs, which were synthesized by a solvent-free neutralization reaction of methoxyacetic acid (Mea) and 1,5-diazabicyclo [4.3.0]-5-nonene (DBN). The ether-functionalized protic ILs ([DBNH][Mea]) were identified as a satisfactory solvent for the simultaneous dissolution of silk fibroin (SF) and cellulose at mild conditions, thus delivering a new dissolution processing platform toward value-added SF-based regenerated fiber materials. The strengthened simultaneous dissolution mechanism of SF and cellulose in [DBNH][Mea] by the introduction of an ether structure moiety in the anion was experimentally and computationally verified, and the results indicated that the ether structure moiety endowed it with more hydrogen-bonding acceptor sites, thus presenting stronger hydrogen-bonding disruption ability and outstanding solubility to SF and cellulose under mild conditions. Rheological study of the SF/cellulose/[DBNH][Mea]/DMSO solutions was conducted systematically, and the correlations between apparent viscosity (η), activation energy (Εη), overlap concentration (c*), structural viscosity index (Δη), storage modulus (G′) and loss modulus (G′′) of the solutions were highly correlated to the mass ratio of SF to cellulose. A series of SF/cellulose composite fibers were prepared by wet spinning using ethanol as the coagulation bath, and the obtained composite fibers were analyzed by Fourier transform infrared (FTIR), X-ray diffraction (XRD), elemental analysis, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) and mechanical tests. It was found that the composite fibers had high compatibility and the postdrafting technique is beneficial for enhancing the mechanical robustness.