Polyimide (PI) films are widely used in the fields of electronics and aerospace due to their excellent thermal stability, outstanding mechanical properties, outstanding chemical and radiation resistance, and special dielectric properties. Due to its unique chemical structure, PI film has high chemical stability, which makes its surface less prone to chemical reactions with other substances, resulting in a non-polar surface and low surface energy. Therefore, special surface modification treatment is required when adhering to other materials. Therefore, in order to improve the adhesion of PI surface, it is necessary to modify its surface and increase its wettability. However, the good adhesion of PI film to other materials requires specific surface treatment. At present, the most widely used methods for modifying polymer surfaces include chemical treatment, electrochemical treatment, light irradiation, and plasma treatment. Plasma activation treatment is a fast, efficient, environmentally friendly, clean, completely dry, and solvent-free process that allows for the introduction of a wider range of functional groups onto the surface of polymer materials compared to other modification methods. Compared with other plasma technologies, atmospheric plasma can continuously process material surfaces without the need for vacuum equipment. The plasma treatment operation is carried out at the top layer of the material without affecting the overall quality.

Plasma treatment of polyimide film

Using atmospheric pressure plasma surface treatment machine to modify the surface of PI film. The atmospheric pressure plasma processing system mainly consists of a full bridge digital circuit plasma power supply host, an air compressor, and a plasma spray gun. The plasma surface treatment machine is shown in Figure 1. The plasma jet emits energy in a circular pattern. The external electrode is connected to the air compressor, with a pressure of 0.2MPa. The generator continuously provides 220V voltage with an output frequency of 25KHz. Apply plasma jets of different powers and speeds onto the surface of the PI film, fix them in a specific processing mode, and maintain a position of 4mm.

Figure 1 Schematic diagram of plasma treatment of PI film

The influence of plasma treatment power on the surface wettability of PI thin films

Surface free energy is widely used to characterize the wettability of one material to another. The surface free energy is composed of polar and dispersive components, which are obtained by measuring the contact angle between the sample and water and diiodomethane. It can be clearly seen from the results in Figure 2 (a) that the contact angles of water and diiodomethane after plasma treatment are significantly reduced. As shown in the figure, the water contact angle and diiodomethane contact angle of the original film are 61.66 ° and 75.94 °, respectively. When the plasma treatment power is 400W, the contact angle between the PI film and water decreases to 31.17 °, a decrease of 49.45%, and the contact angle of diiodomethane decreases to 53.50 °, a decrease of 29.55%; When treated with 600W, the contact angle of diiodomethane became 54.65 ° and the contact angle of water became 33.03 °; When the power was further increased to 800W, the water contact angle reached the minimum value of 28.83 °, a decrease of 53.24% compared to the original membrane, and the contact angle of diiodomethane was 46.77 °; When the power continues to increase to the maximum of 1000W, the water contact angle increases to 36.44 °, and the diiodomethane contact angle continues to decrease to 46.10 °. From the overall trend, after plasma treatment, the contact angle of water significantly decreased, indicating an improvement in the wettability of the PI film surface after treatment. The improvement in wettability proves that the number of polar groups on the PI film surface significantly increases after plasma treatment; The overall contact angle of diiodomethane also showed a decreasing trend, indicating that plasma treatment increased the roughness of the PI film surface and also led to an increase in the wettability of the film surface.

Figure 2 PI thin films treated with different plasma powers: (a) dynamic contact angle, (b) surface free energy

By measuring the contact angle and combining it with the polarity and dispersion of water and diiodomethane liquid, the dispersion and polarity components of the surface of PI thin films treated with different processing powers can be calculated. The sum of the two is the total surface free energy (Figure 2b). After atmospheric plasma treatment, the dispersion, polarity, and surface energy of the PI thin film surface undergo significant changes. The dispersion component of the original film surface is 19.62 mJ/m2, the polarity component is 21.29 mJ/m2, and the total surface energy after summation is 40.91 mJ/m2; After 400W plasma treatment, the surface polarity component increased to 32.88mJ/m2, an increase of 54.44%, and the dispersion component changed to 32.30mJ/m2, an increase of 64.63%; When the power is 600W, the surface polarity component becomes 32.31mJ/m2 and the surface dispersion component becomes 31.65mJ/m2; When the power is further increased to 800W, the dispersion component is 36.06mJ/m2 and the polarity component is 31.65mJ/m2, and the surface energy reaches its maximum value of 67.71mJ/m2; When the processing power reaches the maximum value of 1000W, the dispersion component reaches a peak of 36.42mJ/m2, the polarity component is 27.46mJ/m2, and the surface energy decreases to 63.88mJ/m2. From the trend of the overall dispersion component, it can be seen that as the plasma processing power increases, the dispersion component gradually increases. This is because the higher the power, the stronger the etching of the PI film surface by the plasma, resulting in an increase in surface roughness; The polarity component shows a trend of first increasing and then decreasing, indicating that plasma treatment of PI thin surfaces introduces oxygen-containing polar groups into the surface. Subsequently, due to the high power and energy, the introduced polar groups are destroyed, leading to a decrease in polarity component; The surface energy of the film also shows a trend of first increasing and then decreasing, reaching its maximum value at a power of 800W. Due to the lower contact angle and higher surface energy, which are beneficial for the surface wettability of PI samples and enhance the interfacial bonding of composite materials, plasma treatment improves the wettability of PI films by changing the surface roughness of the film and introducing polar groups.

The influence of plasma treatment power on the surface elemental composition and functional groups of PI thin films

Plasma treatment allows for the introduction of a wider range of functional groups onto the surface of polymer materials, thus different power plasma treatments of PI films can affect the chemical composition changes on the surface. Analyze the changes in elements and functional groups before and after film treatment using XPS. Figure 3 shows the full scan spectra of the thin film under conditions of 400W, 600W, 800W, and 1000W power without plasma treatment. The binding energy of C1s is located at 285eV, the binding energy of N1s is located at 400eV, and the binding energy of O1s is located at 532eV. Table 3 shows the percentages of C, N, and O elements and the ratios of O/C and N/C calculated from the integrated intensities of the 1s peaks of each element on the surface of the PI film before and after plasma treatment. It is not difficult to see from the XPS spectrum that the proportion of C element on the surface of the original film is higher than that of O element, while the proportion of O element and N element on the surface of the thin film with different processing powers first increases and then decreases. The true oxidation situation on the surface of the PI film can be represented by the O/C ratio. The content of C element on the surface of the original film is 77.29%, the content of O element is 15.33%, the content of N element is 7.38%, the O/C ratio is 19.83, and the N/C ratio is 9.55. After 400W plasma treatment, the O/C ratio was 33.06, the N/C ratio was 9.34, and the O/C ratio significantly increased to 33.06, an increase of 66.72%; When the processing power is 600W, the O/C and N/C ratios further increase; When the plasma processing power reaches 800W, the O/C and N/C ratios reach their maximum, which are 39.02 and 17.79, respectively. Compared with the original film, they increase by 96.77% and 86.28%, respectively. When the processing power reaches a maximum of 1000W, the O/C and N/C ratios decrease to 29.21 and 16.41, respectively.