Graphene Oxide Functionalized with 2-Ureido-4[1H]-pyrimidinone for Production of Nacre-Like Films

The facile and efficient reaction of graphene oxide (GO) and 2-ureido-4[1H]-pyrimidinone (UPy), an isocyanate terminated 4-site hydrogen-bonding moiety, produces a functionalized GO (f-GO) that readily self-assembles into a freestanding nacre-like film using a vacuum filtration process. The reaction of UPy with GO occurs predominately via the epoxide and hydroxyl groups on the GO which was confirmed from a combination of Fourier-transform infrared (FTIR), Raman and X-ray photoelectron spectroscopy (XPS), and 13C solid state nuclear magnetic resonance (SSNMR) measurements. The nacre-like films obtained were typically 50–100 μm thick, from cross-section scanning-electron microscopy (SEM) imaging. The GO d-spacing (X-ray diffraction, XRD) increased with increasing UPy content from 0.934 to 1.45 nm, resulting in porous films with reduced tortuosity to oxygen, carbon dioxide, and water. However, at higher UPy content, reduced tortuosity is balanced with the ability of UPy dimers to readily dissociate and exchange with water. The tensile strength and tensile toughness of the GO nacre-like film increased by up to 470% and 1100% and the maximum strain by a factor of ∼2, for the film with the highest UPy content. These improvements are achieved through a mechanism of extension and unfolding of the linear chain of six carbon atoms in UPy, enhancing strain under tensile loading permitting the platelets to slide more before failure. This work highlights the impact of enhanced interlayer interactions via hydrogen bonding in producing polymer-free nacre-like films.