Enhancing Membrane Adhesion to Polymeric Substrates via Plasma Treatment

Ensuring strong adhesion between porous polymeric membranes and supporting substrates is critical for the reliability and functionality of membrane devices. However, due to the innate low surface energy of polymers, achieving strong chemical bonding between such materials remains challenging. In addition, the small-pore size of membranes often limits effective pore intrusion (necessary for achieving effective mechanical interlocking) by polymer adhesives during high-throughput manufacturing. Plasma treatment is commonly used to modify the surface energy of polymers to improve adhesion and mechanical properties of composite systems. However, it remains unexplored whether the method is effective in improving the adhesion of surfaces containing nanoscale pores as found in membranes. Herein, we demonstrate that adhesion between poly-(ethersulfone) (PES) membranes with 20 and 200 nm pore sizes and polypropylene (PP) substrates is enhanced by low-pressure plasma treatment (power: 30 W, duration: 60 s, gas flow rate: 30 cm³/min) of the two surfaces. Thermomechanical bonding between the treated surfaces is performed, and the adhesion behavior is quantified by a T-peel test and imaging analysis. For the 200 nm PES membranes and PP substrate, the adhesion after plasma treatment (152-405 N/m), measured by the interfacial fracture toughness, exhibits an improvement by 0.12 to 2 times in comparison to untreated control samples (114-156 N/m). For the 20 nm PES membranes and PP substrate, the adhesion after plasma treatment (14-242 N/m) exhibits an improvement by 0.13 to 20 times in comparison to that of untreated control samples (12-96 N/m). Among the different types of plasma treatment tested, the oxygen-containing plasmas produce the largest enhancement in adhesion. When benchmarked against the adhesion of densified, nonporous PES film and PP substrates after plasma treatments (0-20 N/m), the adhesion is improved by 13 to 37 times for the 200 nm PES/PP specimens and by 1.5 to 17 times for the 20 nm PES/PP specimens, showcasing the importance of mechanical interlocking due to membrane pore structure for adhesion. This study shows that there is a synergistic effect of chemical bonding and mechanical interlocking on the interfacial fracture toughness between porous membranes and thermoplastic substrates, which can be useful in guiding the membrane bonding process in a variety of applications.