With the development of society, polymer materials such as rubber, plastics, and synthetic fibers are ubiquitous in daily life. These materials are often combined with other materials to form composite materials for applications in various fields, including packaging, construction, automotive, textile, and electronics industries. However, in these applications, certain polymer materials require bonding to enhance their service life or prevent detachment-related losses. Therefore, the adhesive performance of polymer materials is critical for their practical use.  

Among the adhesive properties of polymer materials, interfacial adhesion is the most crucial. Interfacial adhesion refers to the bonding of two material surfaces through physical or chemical interactions, with the bond strength depending on the forces between the surfaces, such as covalent bonds, van der Waals forces, and hydrophobic interactions. When bonding two interfaces, if functional groups on the polymer surface chemically react with groups on the other material’s surface to form new covalent bonds, a strong bond is achieved. A classic example is the use of silane coupling agents to enhance interfacial adhesion in polymers.  

The generation of van der Waals forces arises from interactions between dipoles of atoms or molecules. Hydrophobic interactions typically occur between two hydrophobic surfaces, where no water-containing gaps form upon contact, ensuring that water molecules do not interfere with adhesion. For instance, polymer materials designed for underwater bonding often feature hydrophobic surfaces to avoid hydration at the interface, which could compromise adhesion.  

Additionally, factors such as surface wettability, roughness, and surface energy vary across different polymer materials, all of which influence bond strength. When two surfaces cannot bond directly or exhibit poor adhesion, surface modification is required to adjust their wettability, roughness, and surface energy to comparable levels. Consequently, identifying effective methods for modifying polymer material surfaces to improve interfacial adhesion remains a challenging yet essential task.

Plasma Treatment of Polymer Materials

Plasma is a quasi-neutral ionized gas composed of free electrons, ions, neutral particles, and photons. When sufficient energy is supplied to a gas to ionize at least one electron from its molecules, plasma is generated. Depending on the degree of ionization, plasma can be classified into "thermal" and "cold" types. Thermal plasma approaches thermodynamic equilibrium, characterized by high energy density and equal temperatures between heavy particles and electrons. In contrast, cold plasma exists in a non-equilibrium state, where electron temperatures are significantly higher than those of ions and neutral particles. In cold plasma, electrons serve as the primary energy carriers, controlling plasma chemical reactions through collisions with other matter (primarily neutral particles). During chemical reactions, high-energy electrons in cold plasma can break chemical bonds on polymer surfaces, activating the surface for adhesion. Conversely, thermal plasma, due to its high temperature, may damage polymer surfaces. Therefore, cold plasma methods are preferred for creating adhesive, coupling-ready polymer surfaces or coatings.  

Plasmas used for surface material treatment can be broadly categorized into two types:  

1. Gas-based plasmas (e.g., Ar, O₂, N₂, or air) that react with polymer surfaces. For example, O₂ plasma hydroxylates polymer surfaces.  

2. Organic molecule-based plasmas (e.g., saturated or unsaturated hydrocarbons) that enable polymer film growth on material surfaces. For enhancing interfacial adhesion, atmospheric pressure air plasma treatment is most commonly employed.  

In summary, plasma treatment is a surface modification method that leverages highly reactive plasma-generated energetic particles to interact intensely with substrate surfaces. By bombarding the substrate with plasma-state species, surface oxidation and grafting introduce active functional groups, improving the wettability and adhesion of polymeric materials.