Synergies of Nanocopper and Graphene as Cofillers in a Shape-Memory Polyurethane

Graphene and its related materials are commonly used as reinforcements in shape-memory polymers (SMPs) to engineer their functional properties, but at the cost of a reduction in their failure strain. Here, we demonstrate that the reduced failure strain in multilayer graphene reinforced polyurethane (PU/MGR) composites (at 2.0 wt % MGR) could be enhanced by more than two times with the introduction of copper nanoparticle (CuNP) cofillers (0.1-1.0 wt % CuNP). The CuNPs play a crucial role in disrupting the pi-pi and van der Waals interactions between the MGR sheets, which serve to reduce their agglomeration and suppress the number of stress-concentration sites. Interestingly, introduction of CuNPs was found to not only increase the failure strain but also contribute to remarkable tribological and thermal properties of the PU/MGR/CuNP composites. CuNP-decorated MGR fillers present at a sliding tribo-interface enabled the easy shearing and rolling of MGR sheets across each other, thereby significantly reducing the friction to attain a state of superlubricity exhibiting an ultralow and stable coefficient of friction of similar to 0.05-0.06. These concepts derived from using CuNPs as cofillers in graphene-reinforced PU may be extended to many other SMP systems for various functional systems.