Understanding Creep Behavior of Semicrystalline Polymer via Coarse-Grained Modeling

Understanding the deformational and failure behaviors of thermoplastic semicrystalline polymers is crucial due to the practical usages in various engineering applications. Taking isotactic polypropylene (iPP) as a semicrystalline polymer model system, atomistically informed coarse-grained (CG) molecular dynamics (MD) simulations are employed to investigate the creep behavior of iPP. The simulations reveal that there exists a threshold stress of about 20.0 MPa, above which the maximum strain of iPP within the simulation time span increases dramatically. From the strain-time analysis, it is observed that the iPP exhibits an initial elastic deformation stage and a subsequent plastic stage at lower stress levels, while a three-stage creep behavior including a third fracture stage is observed at higher stress levels. Specifically, at lower stress levels, the bonded energy increases continuously as the chains stretch steadily, while the nonbonded energy shows an initial increase followed by a steady decrease due to the interchain sliding. At higher stress levels, both bonded and nonbonded energies change dramatically at the third stage, resulting from accelerated chain stretching, unfolding, sliding, and breaking. This study provides physical insight into the creep behavior of iPP at a fundamental molecular level and highlights the important role of microstructural evolution of chains in the deformation of semicrystalline polymer materials.