Biopolymer Fibers of High Strength and Enhanced Orientation by the Synergy of High/Low Molecular Weight Chitosans in Hybrid Biomaterials Processed by Gel Spinning

High-performance spun bionanocomposite fibers composed of high-molecular-weight chitosan (CHI), low-molecular-weight chitosan "oligomers", and cellulose nanofibers (CNFs) were successfully fabricated via gel spinning of viscous aqueous CHI formulations into a NaOH coagulation bath. X-ray diffraction (XRD) analysis revealed that the CHI polymer chains crystallized into the anhydrous chitosan allomorph. The spinning process which comprised of sequential acidic, basic, neutralization, stretching, and drying steps, produced CHI/CNF composite fibers with high crystallinity, further enhanced by the incorporation of chitosan oligomers. These oligomers appear to promote CHI crystallization by acting as nucleation sites for crystal growth. Two-dimensional XRD patterns demonstrated preferential alignment of both CNFs and CHI crystals along the fiber axis. Increasing the proportion of short-chain chitosan reduced the dope viscosity, potentially facilitating the spinning of solutions with higher polymer concentrations. XRD results indicated that the addition of low-molecular-weight chitosan with an intermediate molecular weight of approximately 4.4 × 10⁴ g/mol produced the greatest increase in crystallinity index (CrI) and molecular orientation, leading to optimal mechanical performance.