Ladder‐Like Polysilsesquioxane Enable Space‐Durable Ultrathin Hard‐Yet‐Flexible Transparent Coatings

Transparent protective coatings are essential for polymer film-based space deployable systems; however, simultaneously achieving abrasion resistance, mechanical compliance, space-environment durability, and ultrathin thickness remains challenging. Here, we develop a cooperative hydrolysis-condensation strategy using organic polysilazane (OPZ) and bis[3-(trimethoxysilyl)propyl]amine (BTMSPA) to enable the controlled in situ formation of a network dominated by ladder-like polysilsesquioxane (LPSQ) from a homogeneous hybrid precursor under mild humidity. The secondary amine of BTMSPA provides a weakly alkaline environment that sustains continuous hydrolysis of Si─N and Si─H bonds in OPZ, generating reactive silanol species. Meanwhile, co-condensation between OPZ and BTMSPA regulates the condensation kinetics, suppressing the rapid formation of disordered siloxane networks in BTMSPA-only systems and promoting the gradual ladder-like growth. The resulting ultrathin (∼2 µm) coatings integrate LPSQ backbones with deformable organic linkages, delivering a rare combination of high hardness (∼0.73 GPa), strong elastic recoverability (∼80%), high optical transparency (∼94.1%), and robust adhesion to polymer substrates, while maintaining stability under space-relevant thermal, radiative, and atomic oxygen (AO) stressors. Beyond this specific material system, we establish a molecular-level design principle in which cooperative hydrolysis-condensation enables the joint control of reaction kinetics and network topology to balance hardness, deformability, and environmental durability in ultrathin coatings.