UV Curing Behavior of Five Heteroleptic POSS Bearing Methacrylate and Glycidyl Groups and Evaluation of Their Potential for Hard Yet Flexible Coatings

Octakis­(3-methacryloxypropyl) silsesquioxane (MPOSS) photocured using a free-radical initiator under N₂ has a high hardness H of 0.99 ± 0.05 GPa but has poor bendability. Octakis­(3-glycidyloxypropyl) silsesquioxanes (GPOSS) photocured in air using a cationic initiator is more flexible than the cured MPOSS coating but has a lower H value of 0.65 ± 0.02 GPa. In this study, a family of heteroleptic POSSs (G_mM_nPOSS) bearing 2, 4, or 6 glycidyl (G) groups and 6, 4, or 2 methacrylate (M) groups are photocured in N₂, in air, in air then N₂ (air/N₂), and in N₂/air. The hardest coating is obtained from a given precursor G_mM_nPOSS when it is cured in N₂/air. The G₂M₆POSS coating offers the best balance between H and bendability. At a thickness of 50 μm, this coating has an H value of 0.95 ± 0.07 GPa, a pencil hardness rating of >9H, and a critical bending radius of <1 mm for inward bending on the inner surface of a poly­(ethylene terephthalate) substrate film. At a thickness of 10 μm, the G₂M₆POSS coating has a critical outward bending radius of 1.5 mm. This study provides experimental guidelines for the proper photocuring of G_mM_nPOSS and reveals that G₂M₆POSS is the best candidate for hard yet flexible coatings, which may be used to protect the touchscreens of foldable smartphones among other applications.