Colorless and Transparent Self‐Healing Polyurethane Urea With Enhanced Mechanical and Scratch/Solvent Resistance via Micro‐Crosslinking

ABSTRACT Self‐repairable polymers with dynamic covalent bonds (e.g., disulfide bonds) are of interest because of their potential to extend tenure. Polyurethane, known for its tunable mechanical properties and versatility, has been explored for self‐healing applications. However, self‐healing polyurethane often faces compromise between mechanical strength, scratch/solvent resistance, and self‐healing efficiency, especially where colorless transparency is required. This study addresses these limitations by synthesizing a micro‐crosslinked polyurethane urea (mc‐PUU) using polycarbonate diol as soft segments, isophorone diisocyanate as hard segments, cystamine dihydrochloride as chain extender, and a small amount of 1,3,5‐tris(2‐hydroxyethyl)cyanuric acid as crosslinker. The structure and properties of mc‐PUU are investigated. The results show that compared with linear PUU, the mc‐PUU maintains colorless transparency (light transmittance), good tensile strength (58.5 ± 1.6 MPa), and elongation at break (470.7 ± 17.4%), toughness (145.1 ± 7.2 MJ/m 3 ), and self‐healing efficiency (70.7% tensile strength recovery, 80.4% elongation recovery), while significantly improving Young's modulus (179.9 MPa), Shore D hardness (52 HD), scratch resistance up to pencil hardness, and solvent resistance (more than 97.3% weight retention after 30 days immersion of acid, alkali, salt, water, and ethanol). The micro‐crosslinked structure uniquely balances mechanical robustness and self‐healing ability, making mc‐PUU a promising candidate for protective coatings in optical and industrial applications.