One-pot polymerization, surface grafting, and processing of waterborne polyurethane-cellulose nanocrystal nanocomposites

A series of new waterborne polyurethane (WPU)/cellulose nanocrystal (CN) composites have been successfully synthesized via in situ polymerization. The conditions were optimized to induce the grafting of part of the pre-synthesized WPU chains on the surface of cellulose nanocrystals (CNs) and the corresponding nanocomposites were processed by casting and evaporation. The morphology, structural, thermal, and mechanical properties of the resulting nanocomposite films were evaluated by scanning electron microscopy, wide-angle X-ray diffraction, differential scanning calorimetry, dynamic mechanical analysis, and tensile tests. The success of the grafting was substantiated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and differential scanning calorimetry. Thus, it was demonstrated that the grafted WPU chains formed crystalline domains on the surface of CNs which expedited the crystallization of the polycaprolactone (PCL) soft segment domains in the WPU/CN nanocomposites. This co-crystallization phenomenon induced the formation of a co-continuous phase between the matrix and filler which significantly enhanced the interfacial adhesion and consequently contributed to an improvement in the thermal stability and mechanical strength of the nanocomposites. Although the ductility of the final nanocomposites was slightly reduced, in the CN content range from 0 to 10 wt-%, the Young's modulus and strength were significantly improved as shown by the change from 1.7 to 107.4 MPa and 4.4 to 9.7 MPa, respectively.