Impact-Resistant yet Fold-Durable, Transparent PFAS-Free Hard Coatings on Ultrathin Glass with UV-Crosslinked Ladder Polysilsesquioxane

Abstract PFAS-free hard coatings must simultaneously deliver optical transparency, mechanical durability, and practical water repellency, yet these functions are often limited by a trade-off between hydrophobic surface design and a mechanically load-bearing network. Here, we report a molecular-level hybrid strategy based on a highly condensed polysilsesquioxane (PSQ) framework with short-range ordering consistent with ladder-like structural motifs, synthesized from methyltrimethoxysilane (MTMS) and a bulky epoxy-functional comonomer, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (ECTMS). A stepwise sol–gel process, consisting of ECTMS pre-hydrolysis followed by controlled dropwise MTMS addition, suppresses MTMS-dominant self-condensation and premature gelation, enabling homogeneous co-condensation and soluble PSQ precursors suitable for thick-film processing. Time-resolved FT-IR and DOSY NMR track alkoxy consumption and the cooperative evolution of MTMS- and ECTMS-derived species, while GPC reveals oligomer formation, subsequent chain growth, and possible late-stage siloxane bond redistribution/equilibration; XRD and 29Si NMR support a highly condensed framework with ordering consistent with ladder-like structural motifs. The resulting ME-PSQ forms smooth, particle-free coatings on ultrathin glass at 30–35 μm thickness. After UV curing, the coatings exhibit approximately 92% visible transmittance with low yellowness, pencil hardness up to 8H, and initial water contact angles of approximately 100°. Hydrophobicity is retained after 48 h exposure to polar organic solvents and through at least 24 h exposure to 1 M acid/base media, while post-immersion thickness, AFM, ATR-FT-IR, and UV–Vis measurements after 48 h indicate limited surface erosion without catastrophic degradation of the underlying siloxane network. The coating withstands 200,000 folding cycles at 1.5R without visible cracking or delamination and increased the observed pen-drop fracture-threshold height from approximately 15 cm for bare UTG to approximately 35 cm for coated UTG under the specified comparative test conditions.