The chemical composition and properties of polymers mainly include the composition of polymer elements, distribution of functional groups, etc. Due to the fact that plasma discharge modifies polymers differently from conventional modification methods, the resulting products are more complex. Plasma treatment under different conditions has a significant impact on the chemical structure and composition of polyurethane (PU). The chemical properties of polyurethane surface directly affect various properties of the material, such as mechanical properties, surface energy, etc. Therefore, in-depth analysis of the surface chemical composition of polyurethane under plasma treatment is crucial for understanding the principle of improving the hydrophilicity of polyurethane.

Principle of Improving Hydrophilicity through Polyurethane Air Plasma Treatment

In this article, XPS testing technology is used to analyze the surface changes of polyurethane after air plasma treatment, and to explore the influence of air plasma treatment on the chemical structure and composition of polyurethane. By analyzing the XPS spectra, we can gain a deeper understanding of the details of surface modification caused by air plasma treatment, providing a theoretical basis for understanding the improvement of polyurethane hydrophilicity.

XPS analysis of polyurethane under different output powers of air plasma

Under the condition of maintaining a plasma treatment time of 40 seconds, the polyurethane was modified by changing the output power, and the modified results were compared with the untreated polyurethane. Firstly, perform XPS scanning on the untreated polyurethane, as shown in Figure 1.1.

Figure 1.1 XPS Spectrum of Untreated Polyurethane foam

From the scanning image, it can be clearly distinguished that there are three elements, C, N, and O, in the untreated polyurethane sample. At the same time, XPS scanning analysis was carried out on the polyurethane foam treated with different output power, and the percentage content of the three elements was calculated. The chemical surface modification of polymers after plasma treatment is revealed by the change in atomic percentage of chemical elements in each polymer.

The calculated proportions of the three elements are shown in Table 1.1. After plasma discharge treatment, the peak heights of C1s, N1s, and O1s all changed, with the C1s and O1s peaks becoming the most prominent, while the concentration of N1s was almost unaffected. This indicates that plasma treatment causes changes in the surface elements of polyurethane. The ratio of C element to O element on the surface of untreated polyurethane foam is as high as 65.98%, while the ratio of O element is only 29.08%. After plasma discharge treatment at different power, it can be seen that the content of O element has been greatly improved. When the output power is 33.03W, the proportion of C element decreases to 55.50%, while the proportion of oxygen element reaches 38.42%. As the output power of air plasma increases, the oxygen content on the surface of polyurethane is also constantly changing. According to the XPS quantitative analysis results, the change of O element content may be due to a series of reactions between the oxygen related active particles in the air plasma atmosphere and the polyurethane surface, through which a large number of oxygen containing groups are introduced on the foam surface. Therefore, the relative content of O element on the polyurethane surface increases significantly, leading to the change of the hydrophilicity of polyurethane. But further peak fitting analysis of the fine spectrum of polyurethane is needed to determine which specific functional groups have been added.

Table 1.1 XPS analysis data of surface element composition of polyurethane before and after air plasma variable output power treatment

(1) C1s Spectral Analysis

The distribution of functional groups obtained after peak fitting is shown in Figure 1.2. In the untreated polyurethane C1s spectrum, the C element exists in three forms. Around 284.8eV is a typical C-C, 286eV is C-O-C, and 288.5eV is O-C=O. C-C is a typical carbon carbon bond, indicating the presence of carbon chain structures in polyurethane. C-O-C is related to the presence of ether bonds in polyurethane. O-C=O represents ester bond, which is consistent with the actual situation of polyurethane. After plasma discharge treatment with different input voltages, the existence of surface functional groups changed to two forms: C-C and C-O-C. The increase in input voltage improved the activity of the plasma. It is evident from the figure that as the power increases, the proportion of C-O-C functional groups first increases and then decreases, which is closely related to the effect of air plasma treatment on improving the hydrophilicity of polyurethane

Figure 1.2 C1s spectra before and after air plasma treatment at different output powers (a) untreated (b) output power 33.03W (c) output power 48.06W (d) output power 57.16W (e) output power 66.54W

Peak fitting was performed on the fine spectrum of O element, and the resulting functional group structure is shown in Figure 1.3. The untreated polyurethane foam has two functional groups: 531.5eV C-O and 533eV C=O, and the structure of polyurethane group after plasma treatment has changed significantly.

Figure 1.3 O1s spectra before and after air plasma treatment at different output powers (a) untreated (b) output power 33.03W (c) output power 48.06W (d) output power 57.16W (e) output power 66.54W

Due to the potential energy provided by plasma, oxygen atoms undergo oxidation reactions under plasma conditions to form more stable C=O functional groups, promoting the enhancement of hydrophilicity. But as the output power increases, the rate of polyurethane surface reaction occurs faster and the time to reach the peak decreases, while longer processing time reduces the number of groups that can be oxidized on the polyurethane surface. With the increase of processing time, the temperature becomes higher and higher. High temperature may cause further molecular structure changes, which destroy the oxygen-containing functional groups already formed in the early stage of plasma treatment of polyurethane, resulting in a decrease in hydrophilicity and a weakening of the modification effect. It was observed that when the output power was further increased to 66.54W, the C-O functional group decreased while the C=O functional group reappeared. This indicates that excessive discharge temperature and high-energy particles can cause etching on the surface of polyurethane. The etching process will expose more functional groups or change the state of functional groups, thereby affecting the interaction between the material and water molecules. Surface etching may also alter the surface energy of materials, which has a significant impact on the interaction between materials and water molecules. This change may make the surface of the material more hydrophilic, leading to an increase in the improvement effect of hydrophilicity.