Toughening Poly(methyl methacrylate) via Reinforcement with Cellulose Nanofibrils

The incorporation of fibers into polymers is a well-known route for realizing composite materials with improved strength, stiffness, and toughness. Cellulose nanofibrils (CNFs) are a promising reinforcement because of their high aspect ratio, high specific stiffness and specific strength, and transparency and the fact that they are derived from renewable and sustainable sources. Here, we demonstrate the application of CNFs for the reinforcement of continuous polymer fibers (filaments) to achieve a concurrent enhancement of strength, stiffness, and fracture toughness. Poly(methyl methacrylate) (PMMA)-CNF composite fibers with 0 to 3 wt % CNFs were made by solvent blending and melt-spinning/drawing techniques, resulting in fibers with diameters ranging from 200 to 300 μm. The processing route was developed to overcome challenges presented by the combination of high processing temperatures of PMMA and the low thermal stability of CNFs as well as the formation of a percolated CNF network and low volume fractions. Fourier transform infrared (FTIR) spectroscopy was used to understand the alignment of PMMA and CNFs in the fibers. The strength and Young's modulus of the fibers were characterized by tensile testing, and the fracture toughness was measured via notched fiber tests. The addition of low-weight fractions (<3%) of CNFs increased Young's modulus by 35% and yield strength by 19% compared to neat PMMA fibers. Along with this increase in stiffness and strength, the incorporation of 1 wt % CNF led to a doubling of the fracture toughness. FTIR results and imaging of the fracture surfaces provided insight into toughening mechanisms.