Multiscale Modeling of Polyamide 6 Using Molecular Dynamics and Micromechanics

Abstract

Polyamide 6 (PA6) is a semicrystalline thermoplastic used in many engineering applications due to its high strength, good chemical resistance, and excellent wear/abrasion resistance. The mechanical properties of PA6 are dependent on two forms of crystallinity (α/γ). Semicrystalline thermoplastics have a hierarchical microstructure spanning length scales, necessitating the use of a multiscale model. Molecular dynamics (MD) simulation with the reactive INTERFACE force field was used to predict the elastic moduli of amorphous PA6. Semicrystalline PA6 was modeled using a multiscale modeling approach originally developed for semicrystalline polyetheretherketone (PEEK). This approach was facilitated by the NASA Glenn Research Center’s micromechanics software MAC/GMC. The inputs to the multiscale model were the elastic moduli of amorphous PA6, as predicted via MD and calculated stiffness matrices from the literature of the PA6 α and γ crystal forms. The multiscale model output was Young’s modulus, shear modulus, and Poisson’s ratio as a function of α and γ crystallinity. The predicted values of Young’s modulus and shear modulus compared well with experiment. The multiscale model predictions showed that the mechanical properties of semicrystalline PA6 with α and γ crystal forms are similar from amorphous to 40% crystalline and diverge after this limit, with the γ PA6 predictions having higher Young’s and shear moduli and lower Poisson’s ratio. Overall, the good agreement with experiment validated the use of the multiscale model for semicrystalline PA6, proving that the multiscale model may be used for semicrystalline polymers beyond PEEK.