Ultrathin films are an emerging type of PE product, characterized by ultra-low thickness (less than 200 nm), excellent specific mechanical strength, aspect ratio, and surface area-to-volume ratio. They hold great promise for applications in membrane filtration and flexible wearable devices. However, the latter two often require hydrophilic or highly adhesive surfaces to achieve better antifouling and adhesion properties. For ultrathin PE films, the trade-off between modification depth and mechanical integrity is more pronounced, necessitating novel modification designs. These modifications can be categorized into three main types: chemical, physical, and polymer processing. Physical modifications can be surface-based and are susceptible to environmental degradation, leading to diminishing modification effects. Chemical methods often require chemical reagents or catalysts to oxidize, sulfonate, ammonify, graft, or crosslink the PE surface to introduce various functional groups to enhance hydrophilicity. Polymer processing methods primarily involve modifying the PE surface through polymer blending, co-extrusion, and compounding. These methods can achieve a variety of surface modification goals and offer scalable production capabilities, but may also be affected by increased costs or operational complexity. Polyvinyl alcohol (PVA) is a typical hydrophilic linear polymer, a hydrolysis product of polyvinyl acetate, possessing excellent thermal and mechanical properties. It is soluble in a variety of solvents, making it an ideal hydrophilic modifier for polymer products.
Slightly different from conventional approaches, this work does not require covalent bonding via crosslinking or hydrogen bonding to immobilize PVA on the target matrix. Instead, it utilizes van der Waals interactions for modification. Due to the high surface energy and aspect ratio of ultra-high molecular weight polyethylene (UHMWPE) ultrafilms, the physical bond between PVA and PE is stable and easily established. This interaction is caused by an entropy-driven hydrophobic effect at the liquid-solid interface, reducing surface energy. By encapsulating PVA on the fibrils, the hydrophilicity of PE can be reversed (from 134.6° to 48.4°), and the microstructure remains almost intact even at low modification loadings (0.001% concentration). The prepared hydrophilic ultrathin PVA-PE membrane exhibits high transparency, high mechanical strength (380 MPa), and high porosity with a thickness of only 200 nm in a self-supported state. This work provides a simple method to switch the hydrophilicity of nonpolar polymer ultrafilms by establishing vdW physical bonds.
The related results, titled "Facile physical modification of ultrathin polyethylene by trace poly (vinyl alcohol)", were published in the journal *Polymer*. He Zhang, a master's student at the College of Polymer Science and Engineering, Sichuan University, is the first author, and Runlai Li, a specially appointed associate researcher at the College of Polymer Science and Engineering, Sichuan University, is the corresponding author. Sincere thanks are extended to the National Natural Science Foundation of China (52103042 and 52233002) and the State Key Laboratory of Polymer Materials Engineering (sklpme2022-3-09) for their support.
