Molecular Dynamics Simulations and Mechanistic Insights into Wrinkle Formation in TPU under Supercritical CO₂ Flow Field

The external force field has a profound influence on the molecular structure and properties of polymers, but the effect of the supercritical carbon dioxide (scCO2) flow field on the molecular dynamics (MD) of thermoplastic polyurethane (TPU) remains uncharted. We previously discovered the formation of wrinkled structures in TPU during scCO2 foaming in the presence of a scCO2 flow field, but the underlying mechanism is elusive. In this work, the flow-induced orientation of TPU molecular chains and the subsequent chain relaxation when the flow field is removed are systematically investigated with nonequilibrium molecular dynamics simulations. The orientation and stretching of molecular chains intensified with an increasing shear rate (Wi), which led to increases in internal stress and kinetic energy. Although TPU chains reached similar orientation conformations under various Wi, the internal stress and orientation speed are higher in the case of high Wi. In the relaxation process, the molecular chains that are stretched at a higher Wi exhibited faster stress release and conformation disordering, which caused a smaller radius of gyration (Rg) and higher stress level compared to the initial coiled state. This fast stress relaxation behavior is believed to be responsible for the formation of fine and dense wrinkles on the TPU surface. The molecular insight from our simulations reveals the wrinkle formation mechanism and is meaningful for controlling the microstructure and properties of wrinkled TPUs.