Scalable production of critically thin polyethylene films via multistep stretching

Plastic films are among the most used materials. In many applications, both high strength and low thickness are required. The thickness of free-standing plastic films has recently been reduced to micrometres, 200 nm and even 60 nm. Pushing this boundary further faces considerable challenges, as processability conflicts with stability at the ‘ultrathin’ scale (below ~100–200 nm). Here, to overcome this trade-off, we modulated the entanglement density of plastic chains to identify a maximum stretching processing window. Then, relaxation was introduced during stretching to kinetically stabilize the ultrathin film. Combined, polyethylene film thicknesses were reduced to ~12 nm, near its critical thickness. This critically thin polyethylene reveals physical properties different from its bulk counterparts, such as high mechanical strength (113.9 GPa (g cm^–3)^–1), abnormal interfacial properties and a high aspect ratio near 10⁸. Potential applications of these films include nuclear fusion ignition support and thin breathable epidermal sensors. Our work reveals advanced processing strategies, distinctive properties and broader applications of plastic films near the processing limit.