Molecular dynamics simulation on elastic anisotropy and slip systems of crystalline phase of polypropylene

Abstract

Thermoplastic polymers are expected to be used as a matrix for fiber reinforced plastics due to their high formability and recyclability. Particularly, thermoplastic crystalline polymers have a hierarchical structure composed of crystalline phase and amorphous phase. In order to describe large deformation behavior of crystalline polymers, the polymer multiscale models considering these structures have been developed over the past few decades. However, the mechanism of plastic deformation and material parameters of crystalline phase such as anisotropic elastic coefficients, slip systems and critical resolved shear stress of each slip system have been unclear because of its difficulty of experimental measurement. In this study, we used molecular dynamics method to investigate the microscopic deformation behavior and determine the material parameters of crystalline phase of α-isotactic polypropylene (iPP). We reproduced crystalline phase of α-iPP numerically and applied various kinds of deformation. As a result, we determined thirteen anisotropic elastic coefficients. Each value was different, which means that crystalline phase of α-iPP has elastic anisotropy. Moreover observing plastic deformation behavior of molecular chain slip, we found that crystalline phase has six slip systems, i.e., longitudinal slip and transverse slip on three slip planes. We identified the critical resolved shear stress of each slip system from stress-strain responses. Finally, we performed finite element analysis on single crystal of α-iPP using the material parameters obtained by molecular dynamics simulations, which exhibited macroscopic anisotropy in elasticity and yielding.