  Light weight and mechanically robust cellular polymer nanofilms provide
materials solutions to many cutting-edge technologies, such as high-flux
membrane filtration, ultrathin flexible energy storage, and skin-conformable
devices. However, it remains challenging to fabricate hand manipulatable
cellular polymer nanofilms for use as self-standing structural materials.
Herein, we used a sequential planar extension approach to transform
low-entanglement ultrahigh molecular weight polyethylene (UHMWPE) gel films
dispersed in porogenic polyethylene oligomers into cellular nanofilms
consisting of stretch-dominated triangular cells of molecularly anisotropic
cell edges. The microstructure afforded the cellular nanofilm, which had a
thickness down to 20 nm, with a unique combination of ultratransparency
(>98.5%), ultrahigh in-plane tensile strength (1071 MPa.cm^3.g^(-1)), and
ultrahigh flexibility: a 43 nm thick film can deflect reversibly up to 8.0 mm
in depth (185,000 times) under a spherical indentation load. As an application,
we fabricated the nanofilm into a freestanding ultratransparent respiratory
face covering. The new polyethylene cellular nanofilms are expected to
represent a new class of platform membranes for advancing fundamental and
technological development.
