Synergistic Layered Coatings Enabled by Side-Chain Cation−π Interactions with Strong Adhesion, Flame Retardancy, Hydrophobicity, and Layered Recyclability

Localizing flame-retardant chemistry at material surfaces provides an effective strategy for mitigating fire hazards without compromising the bulk properties of inherently flammable substrates. However, existing flame-retardant coatings struggle to simultaneously deliver water repellency, strong adhesion, abrasion resistance, recyclability, and the retention of air and moisture permeability within a single system. Here, a side-chain functional-group synergy strategy is developed to construct a transparent bilayer coating that integrates flame retardancy and hydrophobicity. By incorporation of phosphorus-containing groups, aromatic units, and cationic moieties into polymer side chains, the coating exhibits efficient flame retardancy, achieving a limiting oxygen index (LOI) of 32.0% and rapid self-extinguishing behavior. The outward-facing aromatic layer imparts hydrophobicity, with a water contact angle of 113 ± 2°. Strong interlayer cation-π interactions within the bilayer architecture generate an adhesion strength of up to 7.96 MPa and impart appreciable resistance to mechanical abrasion. The coating maintains a LOI of 31.4% after 500 abrasion cycles, while stable triboelectric nanogenerator output voltages above 110 V after more than 1000 abrasion cycles further confirm its mechanical durability. Importantly, the bilayer design preserves the intrinsic air and moisture permeability of the substrate and enables highly efficient solvent-assisted, layer-by-layer recyclability (99.3%). This work establishes a generalizable route to durable, sustainable, and life-cycle-compatible protective coatings for advanced and next-generation fire-safety materials.