Cellulose nanofibers reinforced nanocomposites with high strength and toughness by tunable wet-drawing and ionic cross-linking method

Although great achievements have been made for man-made cellulose nanofibers reinforced materials, the toughness is still difficult to be improved due to the stiff nature of cellulose nanofibers. Here, we provide a strategy for molecular and nanoscale engineering of stiff TEMPO oxide cellulose nanofibers (TOCNF) and soft Waterborne Polyurethanes (WPU) macromolecules (30–50 nm) that have strong interactions to form homogeneous and hierarchical compact structures by tunable wet-drawing and ionic cross-linking method. The factors such as R-value (mole ratio of NCO/OH) of WPU, amount of COOH and TOCNF were adjusted to control the inner structure of WPU-TOCNF film. The reorganized and aligned internal structure of WPU-TOCNF caused by external stretch forces increased the mechanical performance greatly. The ionic cross-linking also increased the strength and toughness further. Precisely control the structure of soft WPU macromolecules and ionic cross-linking between soft (WPU) and rigid building blocks (TOCNF) make the strength, extension at break and toughness of the obtained fiber and film reach 715 MPa (323 MPa), 41% (52%), 186 MJ m−3 (110 MJ m−3), respectively. This toughness is the highest among that all the cellulose-containing materials reported so far. Such high toughness for this nanocomposite offers promising potential for functional textile and military aerospace materials in the future.