Thermal and Mechanical Behavior of Polyimide–Polyurea Copolymers: Insights from Molecular Dynamics Simulations

Polyimide (PI) exhibits outstanding thermal stability and mechanical rigidity; however, their inherently rigid backbones lead to intrinsic brittleness, poor fracture toughness, and inferior impact resistance. Conversely, polyurea (PUA) features excellent elasticity, tunable soft–hard segment architectures, and a favorable balance of tensile strength and elongation at break. Herein, we systematically investigate the thermal and mechanical properties of 12 distinct PI, PUA, and PI-PUA copolymer systems via all-atom molecular dynamics simulations. Simulations demonstrate that rigid aromatic moieties significantly increase Tg and elastic modulus, while flexible hexamethylene diisocyanate (HDI) yields the highest elastic modulus via dense hydrogen-bond networks despite lowering Tg. Fluorine substitution effectively increases fractional free volume and moderately reduces Tg. Toughness is evaluated by K/G. System L with bulky phthalide side groups exhibits the highest K/G of 3.24, suggesting potential for improved plastic deformability as a preliminary screening indicator. In contrast, HDI-containing systems E and H show the lowest K/G ratios, as strong interchain hydrogen bonding severely restricts segmental slippage and induces brittle fracture. PI-PUA copolymerization proves to be an effective strategy to balance stiffness and toughness over a broad performance range. This work establishes structure–property correlations for PI-PUA systems, offering molecular-level insights for the rational design of advanced high-performance polymers, which require further experimental validation.