Enhancement of mechanical properties in reactive polyurethane film via in-situ assembly of embedded cellulose nanocrystals

Comparing to the solvent-based and waterborne polyurethanes (PU), the solvent-free reactive PU (RPU) is prepared via in-situ polymerization and film-formation of isocyanate-capped prepolymers and macromolecular polyols in solvent-free system. Thus, the carbon emissions and environmental pollutions are significantly reduced. However, the rapid polymerization also challenges the well control of structure and properties, especially the ordered microstructures. In this work, an efficient approach is applied to enhance the mechanical property of RPU via chemically bonding cellulose nanocrystals (CNCs) with isocyanates in a solvent-free system. The well distributed spot-like structure in the RPU film is realized and significantly improve the mechanical property due to the synergistic effect of hydrogen bonds and urethane bonds. Only adding 1 wt% of CNCs, the tensile strength and elongation at break are profoundly increased to 25.4 ± 1.5 MPa and 748 ± 50.0%, respectively. Both values are 490% and 16% better than that without CNCs. Additionally, the thermal stability is also improved. The initial decomposition temperature is 29 °C higher than that without CNCs. This approach provides a simple method for developing bio-based RPU with well-ordered structures, showing great potential in applications such as environmentally friendly materials and high-performance polymers.