Cellulose Nanocrystal Reinforced Alginate Fibers-Biomimicry Meets Polymer Processing

This research takes a biomimetic approach to the design of polymer nanocomposites and demonstrates structure-property relationships that are controllable via processing conditions. Cellulose nanocrystals (CNCs) measuring 130 nm (length) x 20.4 nm (width) x 6.8 nm (height) were isolated from cotton by sulfuric acid hydrolysis and were incorporated in an alginate fiber wet spinning dope solution. Incorporating CNCs within the alginate fiber enables a nearly two-fold increase in the apparent jet stretch (J(A)), ratio of the linear draw speed to extrusion velocity. Fiber spinning at a constant J(A) resulted in an unexpected decrease in fiber modulus and increase in toughness. Alternatively, fiber spinning at the maximum J(A) resulted in modulus increases that are predicted by the Halpin-Tsai model and the Hui-Shia model. Wide-angle X-ray diffraction (WAXD) was used to elucidate the structure and orientation of cellulose nanocrystals (CNC) within the alginate nanocomposite fibers and provide correlations with mechanical property enhancements. The spread of the azimuthal intensity distribution of the CNC (2,0,0) reflection increased with higher CNC loads until the nanoparticles within the matrix spiraled around the longitudinal axis. The appearance of a spiral angle with increasing CNC load resulted in a step reduction in modulus and increase in toughness. Increased fiber stretching during spinning retarded the appearance of the spiral assembly and increased CNC alignment. This spiral orientation is also observed in native cellulose fibers as a microfibril angle and is deterministic of their mechanical properties.