Plasma treatment is a good surface modification method widely used for surface modification of various materials. Plasma was discovered by British physicist and chemist William Crookes in 1879 while studying cathode rays. It is a gaseous technology that combines physical and chemical methods, and has the advantages of low energy consumption, high efficiency, low pollution, and good cleaning effect. It has been widely used in surface treatment. The energy of active particles in plasma gas is shown in Table 1, and the bond energies of some chemical bonds are shown in Table 2. The energy of active particles in plasma gas is slightly higher than the bond energy of common chemical bonds in organic films. Therefore, when the organic film is in plasma gas, the plasma can cause the organic film to break and generate chemical bonds. In this process, some of the generated chemical bonds will make the organic film hydrophilic.

The effect of plasma treatment time on the hydrophilicity of organic film surface

The variation of the water contact angle on the surface of the organic film after 50W plasma treatment with time is shown in Figure 1:

The results of the contact angle test on the surface of the organic film in Figure 1 (a), (b), and (c) show the plasma treatment of the organic film for 10s, 50s, and 70s

After being treated with 50W plasma for 10 seconds, 50 seconds, and 70 seconds, the surface water contact angle of the organic film in Figure 1 decreased to 36.458 °, 14.414 °, and 19.589 °, respectively;

As shown in Figure 1, the water contact angle on the surface of the organic film after plasma treatment significantly decreases. When the processing time is within 50 seconds, the surface water contact angle of the organic film changes significantly. Continuing to increase the processing time will cause the surface water contact angle of the organic film to increase within a small range. This is because in the early stage of plasma treatment, the active particles in the oxygen plasma react with the surface elements of the organic film to introduce hydrophilic groups, which can clean impurities and dirt on the surface of the organic film. Long term processing can lead to excessive oxidation of organic films, causing damage to some hydrophilic groups such as hydroxyl groups, resulting in changes in chemical structure and minor changes in hydrophilicity.

The influence of plasma treatment on the surface morphology of organic films

Figure 2 shows the AFM morphology of the organic film surface obtained after different plasma power treatments, which have been flattened and corrected. The AFM images of untreated organic film A and organic film B are shown in Figure (a). It can be seen from Figure (a) that the surface of the organic film is uniform and smooth, with almost no graininess, and the roughness is about 0.6nm. After 50W plasma treatment, the surface roughness of the two organic films increased to 0.78nm and 1.12nm, respectively, as shown in Figure (b). As the plasma processing power continues to increase, the surface roughness of the organic film also gradually increases. After being treated with 100W and 200W plasma, the surface roughness of organic film A reached 0.92nm and 1.76nm, respectively, and the surface roughness of organic film B reached 1.39nm and 2.65nm, respectively, as shown in Figure (c) and Figure (d). This is because as the excitation power of the plasma increases, the energy and velocity of particles such as electrons and ions also increase accordingly. When these particles with higher energy and velocity come into contact with the surface of the organic film, they intensify the bombardment effect on the surface of the organic film, causing more severe physical etching of the organic film. The chemical bonds on the surface of the organic film break, leading to an increase in surface roughness of the organic film. At the same time, high-power plasma triggers more intense bombardment, causing the substrate surface to become rough, thereby affecting the hydrophilicity of the organic film surface.

Figure 2 Surface morphology of organic film treated with plasma (a) Original substrate (b) 50W treatment (c) 100W treatment (d) 200W treatment

The main reasons for increasing the hydrophilicity of organic film surfaces through plasma treatment are as follows: free radicals and ions in the plasma undergo a series of physical and chemical reactions with the surface of the organic film, generating hydroxyl groups while removing surface attachments, which can increase the hydrophilicity of the organic film, enhance its chemical reactivity, and improve its surface adhesion.