Chemomechanical Mechanisms Of Interfacial Crosslinking In Graphene Oxide-based Layered Materials

Natural biomaterials with excellent mechanical properties resulting from their complex and hierarchically ordered microstructures have become important topics in the field of solid mechanics and material science research,and constantly inspire researchers to pursue high-performance biomimetic materials.Among these explorations,concerns upon nacre-like structural materials mainly focus on two aspects:the matrix design and the interface design.Graphene oxide(GO)is regarded as a potentially ideal matrix material because of its chemically flexible self-assembly capability while retaining the mechanical properties of graphene.The abundant oxygen-containing functional groups on its surface provide flexible design space for interfacial crosslinking and promote the wide application of GO in the preparation of nacre-like materials.In the studies of GO-based interface designs,although the mechanical properties of GO-based layered materials can be reinforced by introducing multiple interactions and crosslinkings,the conflict between strength and toughness still remains.Besides,in the application of GO,the aqueous environment will have a non-negligible effect on the chemomechanical behavior of GO system.At present,there still remains a lack of understanding on the nanoscale chemomechanical mechanism of these problems and multi-scale structure-property relations,resulting in the difficulty of a synchronous improvement of strength and toughness to the nacre-like layered materials.This paper systematically investigated the interfacial chemomechanical behaviors and their corresponding mechanisms for the GO-based layered materials from a nanoscale view,which provides a theoretical basis for exploring the design and preparation of GO-based layered materials.The strategy of controlling chemomechanical properties of the GO interface by noncovalent interactions was elucidated.The noncovalent interaction index(NCI index)was introduced to quantify and characterize the noncovalent interaction forms within the GO interface system.The atomic-scale origin of the strong interfacial interaction between GO and small organic molecules(melamine)was revealed.A chemical cooperativity between the superstrong hydrogen bonds(formed by the triazine)and other weak interactions(NH2-?)was analyzed.The relationship between the microscopic interactions of the system and the macroscopic mechanical properties was given based on the low-density and linear assumptions.The chemomechanical response of GO-based system in the acid-base aqueous environment was investigated.A binding system was established between the small organic molecules(melamine)and the functional groups on the surface and edge of GO.An enhanced trend of interfacial binding of GO interfaces brought by the aqueous-phase environment was proposed.The state transition of the GO interfacial system in response to pH variations(i.e.protonation and deprotonation)was investigated by introducing pKa parameters.The acid-base effect upon the strength of GO's interfacial binding well explains the alkaline enhancement in the experiment.Results indicate that the binding system at GO edge plays an important role in tuning the mechanical properties of GO-based layered materials.The aqueous-phase stability of oxygen-containing groups upon GO interface/edge and its effect on the crosslinking strategy between GO and the main-group metal cations are analyzed.By considering the binding affinities of GO to its functional groups in solution,the stability of functional groups in the aqueous phase was re-examined.Results in the aforementioned acid-base environment were extended to the behavioral characteristics under a global cationic environment.From the point of view of free energy,the relationship between the stability of the surface groups of GO and their relative distributions was analyzed.It was found that high-valence metal cations tend to capture hydroxyl group with weak bindings on the GO surface,showing strong reducibility.Based on the stability analysis,a potentially new crosslinking strategy between the main-group metal cations and the GO interlayer was proposed.The cross-link behavior of high-valence cations within the GO interlayer is actually a constantly competitive process of reduction/hydrolysis-neutralization-crosslinking.In conclusion,this paper deeply analyzes the unique chemomechanical behaviors of GO-based layered materials at their interfaces from the perspective of atomic-scale mechanism.Its atomic-scale chemomechanical coupling behavior and the important mechanisms behind it are expected to provide new ideas and understandings for the design and application of high-performance GO-based composites.