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

Abstract

ABSTRACT 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.