In-situ and ex-situ added chitin nanocrystals as reinforcement of bio-based waterborne polyurethane-urea

Waterborne polyurethane-urea dispersions (WBPUU) are an environmentally friendly alternative for use in coatings and adhesives, as their synthesis follows green processes. To further advance sustainable material development, in this study WBPUU nanocomposites were prepared using a fully bio-based polyol in the polyurethane synthesis and reinforced with chitin nanocrystals (ChNC), known for their high length-to-diameter aspect ratio and exceptional mechanical properties. Two incorporation routes were evaluated for nanocomposite preparation, ex-situ, where ChNC (0.5–7 wt%) were added by sonication after polymer synthesis, and in-situ, where the ChNC (0.5–1.5 wt%) were added dispersed in water during the phase inversion step. The influence of the ChNC incorporation route, content and distribution on the final properties of the nanocomposites were thoroughly analyzed. Special emphasis was given to the effect of the incorporation route on the morphology and internal structure of the films, supported by detailed AFM, TEM, and SEM analysis of the surfaces and cross-sections. The addition of ChNC significantly improved thermomechanical stability and mechanical properties of the nanocomposites. However, the incorporation route influenced the interactions between the WBPUU and ChNC. The in-situ route was found to be more effective in promoting the formation of chemical bonds between the two components. These differences in ChNC distribution and matrix interactions resulting from the incorporation route led to variations in mechanical performance and hydrophilic behaviour.