Cycloolefin copolymer is an amorphous polymer material that has gradually attracted high attention in recent years. It has many advantages, such as high transmittance covering the entire spectrum, low birefringence, extremely low water absorption, low density, non fragility, high Abbe number, stable chemical properties, acid and alkali resistance, and excellent mechanical properties. Compared with traditional transparent plastics such as polymethyl methacrylate and polycarbonate, COC not only has optical properties comparable to PMMA, but also has higher temperature resistance. These characteristics are exactly the excellent properties required for the preparation of optical components. Therefore, COC is considered an ideal substitute material for PC, PMMA, polystyrene, polyvinyl chloride, and some engineering plastics, with good development prospects in the field of optics. However, COC is similar to traditional polymer materials in that due to its high coefficient of thermal expansion, film cracking or detachment may occur during or after film formation due to temperature changes. Therefore, improving the adhesion and environmental adaptability of COC surface optical films is currently a research hotspot.

The monomer structure of COC substrate is shown in Figure 1, with no active functional groups, so its surface activity is low. Therefore, during the coating process, strong chemical bonds cannot be formed between the atoms on both sides of the interface when combined with the thin film material, and only simple mechanical and physical bonding is required, which can result in very low adhesion of the thin film. Therefore, by oxidizing the monomers on the substrate surface to form - OH and increasing its oxygen content, the activity can be improved, thereby forming a chemical bond between the film material and the substrate to enhance the adhesion of the film.

Figure 1 COC monomer structure molecular formula

Plasma treatment utilizes high-energy particles such as electrons and ions in plasma to generate oxidation and etching on the substrate surface, increasing the polarity and roughness of the substrate surface. Its modification is only in the range of a few to several hundred nanometers on the material surface, and does not cause thermal decomposition or ablation of the material. Plasma treatment can excite, ionize, or break surface molecules of materials, thereby enhancing the chemical bonding and physical compatibility between the substrate and the film. Compared to other methods, it is more convenient to introduce certain functional groups or large molecular chains on the surface of the substrate, and the modification process can also be limited by process technology conditions. In concept, plasma processing technology can be used to modify the surface of polymer materials of any shape and characteristic, and only modify the surface properties of the material without affecting its own properties.

The influence of plasma treatment time on the surface chemical composition of COC

This section conducts X-ray photoelectron spectroscopy (XPS) testing on the surface of COC substrate to understand the variation of its surface chemical composition with plasma treatment time. Among them, the full spectrum scanning images of the substrate surface after untreated and plasma treatment for different times are shown in Figure 2. In the figure, two peaks appear at binding energies of 285eV and 532eV, corresponding to the information of C1s and O1s, respectively. It can be seen that the untreated substrate surface contains oxygen elements, but the content is extremely low. This may be due to the introduction of some oxygen elements into the substrate surface during injection molding or exposure to air, which is generally consistent with the elemental composition of the substrate surface. In the figure, it can also be seen that after plasma treatment for different times, the content of two elements on the surface of the substrate has undergone certain changes. This indicates that plasma treatment can significantly change the elemental content on the substrate surface, and different changes will occur with different treatment times.

Figure 2 XPS full spectrum scanning of substrate surface after plasma treatment for different times

Based on the above full spectrum scanning image, the relative element content of the substrate surface can be calculated and listed in Table 3.1. From the table, it can be seen that the C element content on the surface of the substrate without plasma treatment is 92.95%, the O element content is 7.05%, and the element content ratio to O/C is 0.076; After 5 seconds of plasma treatment, the C element content on the substrate surface decreased to 91.14%, the O element content increased slightly to 8.86%, and the element content increased to 0.097 compared to O/C; When the plasma treatment time was increased to 10 seconds, the C element content on the substrate surface decreased significantly to 84.41%, while the O element content increased significantly to 15.59%, and the element content continued to increase compared to O/C to 0.185; But when the plasma treatment time continued to increase to 15 seconds, the C content on the substrate surface actually increased to 89.49%, the O element content decreased to 10.51%, and the element content ratio to O/C decreased to 0.117.

The results indicate that there is not a simple positive correlation between the treatment time and the gain effect on the surface O element content of the substrate when the discharge power density remains constant during plasma treatment of the substrate. Extending the plasma treatment time within a certain range has a positive effect on increasing the O element content on the substrate surface. However, as the treatment time continues to increase, the O element content on the substrate surface will actually decrease, indicating that plasma treatment has the optimal treatment time for improving the chemical polarity of the substrate surface. After analysis, when the power density of plasma treatment remains constant, the energy and density of various active particles such as ions, molecules, and electrons in the plasma atmosphere remain unchanged. At a certain power density, due to the short plasma treatment time, the number of active particles acting on the substrate surface in the reaction is relatively small, resulting in only some functional groups on the substrate surface being able to react, and thus the change in O element content bound to the substrate surface is small; After extending the plasma treatment time, the active particles involved in the reaction gradually increase, so the content of O element on the substrate surface also increases accordingly; But because the total amount of functional groups on the substrate surface is constant, the reaction on the substrate surface will reach saturation after the treatment time exceeds the critical point. If the treatment time continues to increase, it will not only fail to increase the element content but also destroy the polar functional groups that have already been generated. Therefore, the content of O elements bound to the substrate surface will decrease.

In order to gain a more detailed understanding of the changes in the structure and content of functional groups on the substrate surface caused by plasma treatment time, the C1s spectral peak in the full spectrum scan of the substrate surface was processed using peak splitting modeling. The fitting results are shown in Figure 3. By calculating the total area enclosed by the fitted peaks of each functional group and its corresponding model in the figure and the baseline, the total content of each functional group on the substrate surface can be statistically obtained. From the figure, it can be seen that two characteristic peaks appear when the substrate is not subjected to plasma treatment, with binding energy positions of 286.9 eV and 284.8 eV, corresponding to functional groups - C-O - and - C-C -, with contents of 7.58% and 92.42%, respectively. The - C-O - group, as mentioned earlier, may be formed due to the introduction of oxygen elements from the air into the substrate surface during injection molding and exposure to air. After plasma treatment of the substrate for 5 seconds, the peak areas occupied by each functional group in the spectrum also changed accordingly. At this time, the content of - C-C - groups decreased to 90.28%, and the content of - C-O - groups increased to 9.72%; As the processing time was extended to 10 seconds, the functional group structure on the substrate surface changed, forming new functional groups with a binding energy position of 287.9Ev, corresponding to the functional group - C=O -. At this time, the content of - C - group decreased significantly to 81.53%, the content of - C-O - group increased to 16.16%, and the content of - C=O - group was 2.31%; When the processing time is extended to 15 seconds, the content of - C-C - groups on the substrate surface increases significantly by 88.26%, the content of - C-O - groups decreases to 3.92%, the content of - C=O - groups increases to 7.82%, and the content of oxygen-containing functional groups decreases significantly. This indicates that changes in processing time can cause changes in the structure and content of functional groups on the substrate surface, and appropriate processing time can greatly increase the content of polar functional groups, thereby improving the surface activity of the substrate and forming chemical bonds when the substrate is combined with the film material, greatly improving the adhesion of the film.

Figure 3 C1s peak spectra of substrate surface XPS after plasma treatment for different times a) untreated; b) C) d) Processing times are 5s, 10s, and 5s respectively

In summary, after oxygen plasma treatment, the content of C element on the surface of COC cyclic olefin copolymer substrate decreases and the content of O element increases, introducing oxygen-containing groups and increasing surface polarity. This helps to form chemical bonds when the film material is combined with the substrate, thereby greatly improving the adhesion of the film.