A glass-like plastic resists creasing after 500,000 tight folds, offering a tougher transparent cover material for future foldable displays.
(Nanowerk Spotlight) A foldable display has to survive a demand ordinary screens never face: bending again and again, then returning to a smooth, untouched surface. Much of that burden falls on the transparent cover, the clear outer layer that protects the display beneath it. Preventing cracks is only part of the challenge. The harder problem is keeping repeated bending from leaving a permanent crease.
To a materials scientist, that crease is a visible sign of unrecovered strain: the cover has not fully returned to its original shape after repeated bending. Each cycle can leave a small mechanical trace behind. Over many cycles, those traces become visible as a line across the bending zone, turning a working display into one that looks permanently worn.
The difficulty comes from a basic materials conflict. Hard surfaces resist scratches because they resist local deformation. Flexible surfaces bend because their internal structure can move. A screen cover needs both behaviors at once. Glass gives hardness and optical clarity but can fracture. Plastic bends readily but can scratch, soften, and settle into a crease after repeated folding.
Foldable devices have managed this compromise with plastic films, ultrathin glass, or stacks that combine both. Nanowerk has previously covered the promise of foldable glass, but the trade-off remains. A protective polymer layer can make glass less fragile, yet that layer may fatigue. A hard coating can improve plastic, yet the plastic underneath can still relax.
A paper in Advanced Materials (“Super–Foldable Glass–Like Plastic”) reports a different approach: a glass-like plastic built as a nanoscale interpenetrating network rather than a hard coating on a soft film.
Instead of separating hardness and flexibility into different layers, the researchers place them inside one transparent material. One phase resists surface damage and slow relaxation. The other carries strain and helps prevent cracks from spreading.
Design and characteristics of glass–like plastic (GLP). (A) Scheme of the GLP preparation process. (B) Representative molecular structures of the GOS, including cage–like, incomplete cage–like, and ladder–like structures. (C) Schematic cross–sectional illustration of GLP–3. (D) Transmittance spectra of UTG, GLP–3, and PE–3 across a wavelength range of 200–800 nm. (E, F) Photographs of PE–3 (E) and GLP–3 (F) (100 cm2). (Image: Adapted with permission from Wiley-VCH Verlag) (click on image to enlarge)
The design needed two elements that usually conflict: a tough internal scaffold and a hard transparent matrix. The scaffold came from ultrahigh molecular weight polyethylene, a polymer whose long chains can form highly oriented nanofibers. The matrix used silsesquioxane and nanosilica in a liquid mixture that could flow into the scaffold, then cure under ultraviolet light into a stiff glass-like network around the fibers.
The liquid phase relies on silsesquioxane, a silicon-oxygen-rich building block that can process like a resin before curing into a hard hybrid network. Nanosilica adds rigid inorganic content, while polyethylene supplies the tough fiber scaffold. The critical feature is not simply that the material contains all three components. It is that the hard and soft phases interpenetrate through the film rather than sitting as separate sheets.
That shared structure addresses the weakness of many laminated cover films. In a coated plastic, the hard layer and soft substrate deform differently during folding. The surface may resist scratches, but the underlying plastic can still relax. In the new film, the silica-rich network threads through the polyethylene nanofibers, tying hardness, toughness, and strain distribution to the same internal geometry.
The same nanoscale mixing that gives the film strength could also have made it cloudy. Filled plastics often turn hazy because embedded particles and fibers scatter incoming light. Here, the components bend light by nearly the same amount, and the silica particles and polymer fibrils remain smaller than visible wavelengths. The resulting film transmits 92% of light at 550 nm with haze below 1%.
Neither component alone offers the needed balance. The cured silica-rich material provides hardness but fails at low strain. The polyethylene scaffold adds toughness and deformability but cannot provide a glass-like surface by itself. Together, the interlocked phases give the film a hardness of 1.1 GPa while allowing it to deform and recover instead of cracking like a brittle sheet.
Repeated folding produced the sharpest contrast with conventional plastic. At a bending radius of 0.5 mm, a commercial colorless polyimide film developed a visible crease after 100 cycles. A 5 µm glass-like plastic film stayed smooth and intact after 500,000 cycles at the same radius. A thicker 30 µm version also reached 500,000 cycles without visible creasing or cracking.
The material also resisted setting into a crease while held folded. The researchers kept the 5 µm film bent at a 0.5 mm radius while exposing it to alternating -20 °C and 80 °C conditions for 144 h. It showed no visible crease or crack. The result supports the proposed mechanism: the rigid hybrid network limits irreversible relaxation, while the nanofibers keep strain from concentrating into damage.
A sudden impact creates a different failure path. Ultrathin glass can resist scratches, but concentrated impact can make it fracture. In ball drop tests, the glass-like plastic reached a damage-free drop height 8.2 times higher than ultrathin glass. In a laboratory hammer demonstration, it avoided the catastrophic fracture seen in ultrathin glass, consistent with impact energy spreading through the entangled polyethylene scaffold.
A cover film also needs a surface that can withstand abrasion. Colorless polyimide showed scratches after 100 steel wool wear cycles, while the hybrid film showed no clear scratches after 2500 cycles under the reported conditions. That result places scratch resistance within the same structural film that provides foldability, rather than treating it only as a coating problem.
Device covers also face moisture, heat, light, and chemical exposure. Barrier measurements showed protection against water vapor and gases, while aging tests under ultraviolet light, humidity, solvents, and heat checked whether the film would yellow, roughen, weaken, or degrade. Under the reported conditions, it largely retained its transparency, surface condition, and mechanical behavior. Cell assays found no detectable cytotoxic effects.
Those results strengthen the case for the material, but they do not amount to full device qualification. The paper demonstrates material-level performance, not a finished commercial cover window. Real foldable displays include adhesives, touch layers, polarizers, coatings, cut edges, and interfaces that can shift stress during bending. Oils, cleaning agents, manufacturing defects, and daily handling can also change performance.
The work’s significance lies in treating the glass-plastic compromise as an internal architecture problem. A foldable cover does not have to choose between a hard brittle sheet and a soft creasing film. By interlocking a silica-rich network with polyethylene nanofibers, this glass-like plastic shows how clarity, hardness, impact resistance, and tight-radius foldability can coexist in one thin protective material.
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