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TEMPO-oxidised cellulose fibres are often explored as  nano -reinforcement for polymers. However, it is unclear whether  micrometre-sized  TEMPO-oxidised cellulose fibres  also possess similar reinforcing potential. In this work, we report the mechanical response of polypropylene (PP) composites reinforced with TEMPO-oxidised cellulose (TOC) of different fibre widths.  Micrometre-sized  TOC fibres (TOCF) containing sodium carboxylate (TOCF-Na) and free hydroxyl (TOCF-H) groups, as well as  nano-sized  TOC nanofibrils (TOCN) were produced from dissolving pulp and incorporated into PP matrix via melt-extrusion. It was found that model PP composites containing  micrometre-sized  TOCF-Na and TOCF-H possessed the highest tensile modulus of up to 2.5 GPa; 40% improvement over neat PP and 30% increase over PP/TOCN composite. No significant differences in the tensile strength of PP/TOCF-Na and PP/TOCF-H composites were observed when compared to neat PP. The incorporation of  nano-sized  TOCN into PP however, led to a 6% decrease in tensile strength. Single-edge notched beam fracture toughness test further showed that PP/TOCN composite possessed the lowest fracture toughness of 2.52 MPa m 1/2 ; a decrease of 18% over PP reinforced with  micrometre-sized  TOCF-Na and TOCF-H. Our study shows that  micrometre-sized  TOCFs serve as better reinforcement for polymers compared to  nano-sized  TOCN. This is attributed to the better dispersion of TOCF in the PP matrix. Furthermore, the presence of surface microfibrillation on TOCFs also enhanced the quality of the TOCF-PP interface through mechanical interlocking and local stiffening of the PP matrix.

From micro to nano: polypropylene composites reinforced with TEMPO-oxidised cellulose of different fibre widths