Nanocellulose has characteristics of good mechanical properties, chemical modification, and biocompatibility, which can be extracted from trees, plants, and other various biomass resources (Wu et al. 2014; Mazeau et al. 2008). It has motivated a lot of experimental and theoretical investigations to exploit its properties as well as in combination with other materials. For example, the uniform deformation and nano-indentation method are used to explore the stress-strain response and inter crystal sliding friction of nanocellulose and calculated the Poisson's ratio, elastic modulus, stiffness, and other mechanical parameters of cellulose (Dri et al. 2015; Wu et al. 2020). While experimental measurements meet significant difficulties for the uncontrollable operating conditions in these tests, for the crystal structure, defects, percent crystallinity of cellulose keep changing during the tests (Tanaka et al. 2006; Pakzad et al. 2012). More recently, molecular dynamics (MD) simulation as a valuable tool has been used to research the mechanical properties of nanocellulose at molecular-scale, which could provide the deformation and the microstructure evolution of the materials and predict the stress-strain response under tensile deformation by physical statistical method (Eichhorn et al. 2006; Wu et al. 2013; Liu et al. 2020). In addition, some researches have shown that different arrangements of nanocellulose in polymer matrix may enhance the polymer (Peng et al. 2020; Khakalo et al. 2020).
The bioinspired layered materials have drawn significant interest recently due to their the extraordinary properties. Particularly, the excellent mechanical properties of the bioinspired layer are attributed to the synergistic strengthening effect of the interaction between the building modules and the interface (Cheng et al. 2015; Papageorgiou et al. 2017; Zhang et al. 2016). Graphene is one of the best candidates for preparing bioinspired layered materials due to its excellent physical properties. Recently, many papers have reported that the mechanical properties of graphene-based layered nanocomposites are dramatically improved. It has been found that adding carbon series materials to the polymer system can significantly improve the mechanical properties of composites by using MD simulations (Gao et al. 2020; Islam et al. 2020; Li et al. 2017). Hu et al. studied the mechanical performances and processing approaches of polymer-graphene layered nanocomposites and revealed that processing conditions and interfacial interactions control these materials’mechanical and other physical properties (Hu et al. 2014); Kamaraj et al. investigated the role of graphene as matrix reinforcement in fiber-reinforced polymer composites and found that the flammability and water absorption of flax/epoxy composites decreased with the increase of graphene content, and the tensile and bending strength of the composites increased significantly (Kamaraj et al. 2020). The unique mechanical properties of graphene-based layered materials are attributed to the micro/nanoscale interface interactions. Typical interface interactions of graphene-based artificial materials can be divided into non-covalent bonds and covalent bonds. Compared with covalent bonds formed by chemical reactions, the interaction of non-covalent bonds is relatively weak (Liu et al. 2021; Dai et al. 2017; Song-Moo et al. 2018; Mohan et al. 2018).
In the current study, a nanocellulose-graphene layered structure with covalent linkages is designed in order to improve the mechanical property of nanocellulose. Molecular dynamics (MD) simulations are applied to research the stress-strain relationship and nanostructure deformation by comparing pristine nanocellulose and nanocellulose-graphene layered composites. The simulation results could provide some important connections between mechanical behavior and covalent bonding at graphene layered nanocomposites, and it could provide some basic theories and insights for optimizing the mechanical properties of polymers.