Insight into the Fracture Properties and Molecular Mechanism of Polyurethane

Polyurethane (PU) has an excellent mechanical property due to its unique microphase separated structure, but the molecular mechanism is still unclear. In this work, a multiscale model and the corresponding potential functions of PU are first developed via an inverse Boltzmann iterative method. Following it, the fracture toughness of PU is explored via a triaxial deformation, which exhibits a rise with the increase in the content of hard segments (HS). By analyzing the stress decomposition, morphology, and conformation of two phases, the HS phase can act as physical cross-links due to the strong phase strength, which mainly improves the tensile stress at the low strain. The soft segment (SS) phase contributes to the stress enhancement at high strain due to the large elongation. This microstructural evolution can be visualized by characterizing the von Mises strain and the local bead stress, which denote the microscopic strain and stress, respectively. Finally, the volume fraction, number, and surface area of voids are quantified to record their nucleation positions and growth process, which is based on the Voronoi volume of beads. Most of the voids are nucleated in the SS phase, while others are at the phase interface, which is consistent with the distribution snapshots of local elastic modulus. Moreover, the strong HS phase can inhibit the premature nucleation, growth, and mergence of voids, which enhances the fracture toughness. However, it is contrary with increasing the temperature. In summary, our work provides a novel and insightful understanding of the molecular mechanism of fracture toughness of PU.