Molecular dynamics simulation of stretching-induced ductility for polystyrene

Abstract

Some glassy polymers have a mechanical behavior that stretching can induce the brittle-to-ductile transition (BDT). For polystyrene (PS) film, after pre-stretching above glass transition temperature (Tg), the property at room temperature transforms to ductile, showing higher elongation at break and higher yield strength. In this work, we have investigated this phenomenon of stretching induced ductility by molecular dynamics (MD) analysis referring to our experimental findings. PS specimens in united-atom model with different chain lengths were loaded by deformation at two-step temperature setting, which was 393 K at first and then 300 K, to simulate tensile behaviors with various per-stretching conditions. The result has shown that as deformations beginning at 393 K and proceeding at 300 K, root mean square radius of gyration (Rg) and the degree of orientation (P2,x) reach the maximum value higher than solely stretching at 393 K or 300 K. Therefore, the stretching-induced BDT is attributed to changing of both chain conformation and coil configuration in polymer chains in stepwise stretching, which can change their coil configuration at the pre-stretching, adjusting their chain network in a more stretchable position, leading the whole polymer amorphous from brittle to ductile. The simulation result is accordant with our experiment and previous BDT theory to glassy polymers.