Covalently Sewing Dual-Sided Janus Coating via Photoiniferter Surface Chain Extension

Abstract

Janus coatings can combine strong substrate adhesion with surface hydrophobicity, but often suffer from weak interlayer interfaces.Here we use visible-light photoiniferter reversible additionfragmentation chain transfer (RAFT) surface chain extension to "sew" an all-covalent Janus bilayer, delivering 9.0 MPa interlayer adhesion and ~100-fold improved corrosion protection on AZ31B, with similar performance on Q235 and H62.Coatings are typically bifunctional, with an outer surface combating environmental corrosions and an inner surface binding to the matrix. [1][2][3][4] However, conventional homogeneous coatings rarely maximize adhesion and barrier performance within one formulation simultaneously. 5 The concept of Janus coating is thus put forward, with two functionally distinct layers on opposite sides to combine distinct functions. [6][7][8] Up to now, Janus designs have attracted attentions in various applications, including oil-/water-proof films, 9, 10 advanced separation, 11,12 wound/tendon dressings, 13,14 and cardiac patches etc. 15,16 However, in these Janus coatings, general challenge is that network mismatch shifts incompatibility to the interface, weakening bonding under load. 17 To solve this, reactive covalent linkers (e.g., silane coupling agents) are used to combine the repulsive layers, 18, 19 but require complex design and elevated costs, and not all resins can react with them, limiting further application. 20 Photo-controlled reversible addition-fragmentation chain transfer (photo-RAFT) offers a new pathway with temporal