Highly Flexible Yet Hard Transparent Omniphobic Coating Reinforced with an Electrospun Nylon Nanofiber Mat

Abstract

Photocuring GPOSS-g-FC6, 3-glycidyloxypropyl polyhedral oligomeric silsesquioxane (GPOSS) bearing a grafted (g) tridecafluoroheptanoyl group (FC6), produces a transparent omniphobic coating with the highest 9H pencil hardness. Although this coating on a poly(ethylene terephthalate) (PET) film can undergo inward bending to a curvature radius < 1 mm, it has limited bendability on the outer surface of a bending PET film, which limits the application of this coating on the touchscreen of foldable smartphones. My thesis research is aimed at solving this bendability challenge, by reinforcing a GPOSS-g-FC6 coating with a nylon 6,6 electrospun nanofiber mat. To produce a transparent nanocomposite sample, the electrospun nanofiber diameter must be sufficiently small for minimum light scattering. Thus, the commercial nylon 6,6 was fractionated to yield a higher molecular weight nylon 6,6 (108 ± 4kDa) that enables the production of smaller electrospun nanofibers. The fractionated sample molecular weight was determined via viscosity analysis. To simplify the preparation of a GPOSS-g-FC6/PET bilayer, electrospinning was done on an ITO-coated PET film. After optimization, a voltage of 20.0 kV, nozzle-to-collector distance of 18.0 cm, a 7.0 wt% nylon 6,6 in formic acid solution, and syringe pump rate of 0.254 mL/h, were used to fabricate nanofiber mats. GPOSS-g-FC6 was mixed with a photoinitiator to form infusion solutions depending on different thicknesses, and % infusions (by voids volume) of nanofiber mats. A series of nanocomposite samples with 100%, 150%, and 200% infusions at 20, 30, and 40 μm were prepared. The nanocomposite sample was investigated for its transparency, nanoindentation, pencil hardness, static contact angles, dynamic de-wettability, and wear resistance. The study shows 20 μm 150% infused sample has the best transparency at 98.1 ± 1.1 %. The sample has comparable nanoindentation properties with pure GPOSS-FC6 coating on ITO-PET film, and pencil hardness between 5H and 6H. Nanocomposites with 150% and 200% infusion have comparable de-wetting properties with pure GPOSS-FC6. Higher % infusion and thicker samples have better wear resistance. The free-standing nanocomposites are bendable on both surfaces on the smallest 2.00 mm diameter mandrel in a bend tester. In addition, the nanocomposite retains its ink contraction property after 500 bending cycles.