Freestanding Measurement of the Polar/Nonpolar Adsorption Interface for Complete Hydrophobicity Switching in Polyethylene Nanofibrous Membranes by Trace Poly(Acrylic Acid)

This study explores the underlying mechanisms of polar/nonpolar adsorption dynamics utilizing ultrathin polyethylene (PE) membranes, presenting an innovative method to directly characterize interfacial phenomena. By modifying the surface of these membranes with trace amounts of poly(acrylic acid) (PAA), we probe the entropy-driven hydrophobic and depletion interactions that dominate the adsorption process. This encapsulation significantly alters the hydrophilicity of PE—from a contact angle of 132.2° to 37.0°—while maintaining the micro-structural integrity at remarkably low PAA loadings. For the first time, this approach allows for the isolation and direct characterization of the interface through methods such as weighing, thermal cycling, and tensile testing. Key findings demonstrate that the adsorbed PAA layer is exceptionally loosely packed, exhibiting a porosity exceeding 90%. Moreover, the presence of PAA differentially impacts the crystallization behavior of FCC and ECC within PE, hindering FCC crystallization significantly while mildly affecting ECC. Post-adsorption, the modified PE membrane exhibits smart separation capabilities: it behaves amphiphilically in air, allowing simultaneous permeation of water and oil, and selectively separates water/oil mixtures in liquid environments based on the pre-wetting phase. The resulting hydrophilic ultrathin PAA-PE membranes maintain high transparency (over 90%), robust mechanical strength (309 MPa in tensile maximum stress), and substantial porosity, all within a freestanding, 340 nm-thick form. This work not only elucidates several first-time observations of the intriguing polar/nonpolar adsorption interface but also provides new approaches for thorough and stable physical modification to polymer membranes.