Modulating the microstructure of waterborne polyurethanes for preparation of environmentally friendly nanocomposites by incorporating cellulose nanocrystals

Increasing environmental awareness has promoted development of ecofriendly materials incorporating renewable raw materials and using green synthesis routes such as waterborne dispersion, avoiding employment of organic solvents and thus reducing generation of volatile organic compounds. In this regard, waterborne polyurethanes (WBPU) present an opportunity to tailor material properties while meeting application requirements and avoiding use of organic solvents. In addition, WBPU dispersions offer the possibility to incorporate hydrophilic water-dispersible reinforcement materials, such as cellulose nanocrystals (CNC), which represent a suitable candidate for preparation of nanocomposites due to their renewability, availability, and unique properties resulting from their nanoscale dimension. Therefore, in this work, different WBPU having small particle size with narrow distribution were synthesized at various isocyanate/hydroxyl (NCO/OH) group ratios, and CNC were isolated for preparation of nanocomposites with 1, 3, or 5 wt% CNC by solvent casting. It was observed that, just by varying the NCO/OH ratio, the polyurethane microstructure was altered, resulting in different ordered structures. At low NCO/OH ratio, soft ordered domains were observed, whereas at higher NCO/OH ratio, hard ordered domains were obtained. These different microstructures of the matrix induced different behaviors of the CNC reinforcement, acting either as crystal growth inhibitor or nucleating agent, thereby modulating the properties of the final material in different ways.