Interface and interphase of nanocomposites tailored by covalent grafting of carbon nanotube: Hierarchical multiscale modeling

Covalent grafting between carbon nanotubes (CNTs) and a polymer matrix is the most efficient way to tailor the intrinsically weak interface in nanocomposites. To understand the grafted structure-to-improved property relationship of nanocomposites, however, the degradation of grafted CNT and the properties of surrounding interphase zone should be accounted for in constitutive model. In this study, the reciprocity of the interface, interphase, and elasticity of CNTs depending on the covalent grafting between the CNT and a polyethylene terephthalate (PET) matrix were studied through molecular dynamics simulations and a mean-field micromechanical interface/interphase model. The replacement stiffness method was used to determine the effect of the tailored interface on the overall elasticity of a nanocomposite in micromechanics. The elasticity of the CNTs and the nanocomposites was determined from molecular mechanics and molecular dynamics simulations, respectively. Despite the degraded elasticity of the nanotubes, clear improvements in the transverse modulus and shear moduli were observed in the covalently grafted nanocomposites. The elasticity of the interphase was determined in terms of the number of covalent grafting and the interfacial compliance using a two-step inverse micromechanical analysis. Regardless of the number of covalent grafting addressed, the elastic moduli of the interphase were always larger than those of a neat PET matrix. Furthermore, the accuracy of the proposed multiscale micromechanical interface/interphase model at various volume fractions of CNTs was validated.