Molecular Mechanisms of Microphase Separation Structures on the Fracture Toughness and Impact Resistance of Nanoscale Polyurea Ultrathin Films

Recent nanoscale ballistic experiments have highlighted the significant potential of nanoscale polymer films to enhance ballistic protection. Polyurea (PU), defined by its distinct microphase-separated structure, shows excellent mechanical properties, making it a strong candidate for impact-resistant nanofilms and nanocomposites. However, the relationship between the microphase-separated structure and mechanical properties of PU ultrathin films at the nanoscale remains unclear. This study investigates the uniaxial tensile behavior and ballistic impact properties of PU ultrathin films using coarse-grained molecular dynamics (CGMD) simulations. The relationship between the microphase-separated structure and fracture toughness is examined, and the effects of impact velocity, hard segment content, and film thickness on impact resistance and energy dissipation are systematically analyzed. The results show that the hard segment content (w_H) affects the microphase separation morphology of the PU ultrathin films, which, in turn, influences the interface properties between the hard and soft domains. When w_H = 0.167-0.471, an increase in the hard segment content improves the fracture toughness of the films. However, at w_H = 0.667, the higher hard segment content significantly reduces the ductility, leading to a decrease in fracture toughness. Furthermore, increasing the hard segment content raises the penetration energy (E_p) of the film, primarily due to the dissociation of hard domains and increased interfacial energy dissipation. Reducing the film thickness increases the specific penetration energy ((Formula presented)), which is attributed to the decreased bending stiffness of the PU film, resulting in larger deformations and enhanced sliding of the polymer chains. These findings provide valuable insights into the design and development of high-performance PU nanofilms, highlighting their significant potential for impact protection applications.