By changing the plasma treatment parameters (discharge power, treatment distance, and treatment speed), the influence of different treatment parameters on the wettability of PMMA surface was studied. The influence of different discharge powers on the contact angle of PMMA is shown in Figures 1 and 2. The parameters for each condition are processing speed of 60mm/s and processing distance of 6mm. The data is characterized at least four different points and averaged to obtain the average value.

Figure 1: The Effect of Plasma Discharge Power on PMMA Contact Angle

Figure 2: Effect of discharge power on contact angle: (a) Untreated; (b)300 W; (c)600 W; (d)900 W

From the graph, it can be seen that the change in power has a significant impact on the contact angle. The contact angle of the untreated PMMA surface is 86.67 °, approaching the critical values of hydrophilicity and hydrophobicity (90 °). After 300W plasma treatment, the contact angle of PMMA surface reached 99.34 °, indicating hydrophobicity. When the discharge power is increased to 600W, the contact angle on the PMMA surface decreases to 35 °. When the discharge power was further increased to 900W, the average contact angle on the PMMA surface decreased again, reaching 32.3 °.

It can be seen that when the discharge power is small to a certain extent, the contact angle of PMMA after treatment can be greater than that of PMMA before treatment, and plasma treatment has the ability to reduce wettability. As the discharge power increases, the contact angle gradually decreases, and the overall trend of change also tends to stabilize. At this point, plasma treatment enhances the wetting ability.

Under the parameters of discharge power of 900W and speed of 60mm/s, experiments were conducted on PMMA sheets using three treatment distances of 4mm, 7mm, and 11mm. Figures 3 and 4 show the effects of different treatment distances on the contact angle of PMMA. As the processing distance increases, the contact angle first decreases and then increases, reaching a minimum of 32.3 °.

Figure 3: The Effect of Plasma Treatment Distance on PMMA Contact Angle

Figure 4: The effect of plasma treatment distance on PMMA contact angle: (a) 4 mm;  (b)7 mm;  (c)11 mm

In addition, the influence of processing speed on wettability was studied. Figures 5 and 6 show the effect of processing speed on the contact angle of PMMA. As the processing speed increases from 30mm/s to 90mm/s, the change in contact angle follows a pattern of first decreasing and then increasing, with the minimum contact angle occurring at 60mm/s.

Figure 5: The Effect of Plasma Processing Speed on PMMA Contact Angle

Figure 6: Effect of plasma treatment speed on PMMA contact angle: (a) 30 mm/s; (b)60 mm/s;  (c)90 mm/s

Taking into account the influence of three factors on the contact angle, it can be seen that the impact of discharge power on the contact angle is much greater than the proportion of the impact of processing distance and processing speed on the contact angle. Changing the discharge power reduces the contact angle by 67.04 °, which is much greater than the decrease in contact angle when changing the processing distance and processing speed (18.39 ° and 20.63 °, respectively). Therefore, surface modification of PMMA through plasma treatment can adjust its surface wettability, and changing the discharge power is a convenient and effective way.

The Effect of Plasma Treatment on the Surface Chemical Composition of PMMA

Analyze the effect of plasma treatment on the surface chemical composition of PMMA by detecting the changes in surface elements before and after plasma treatment through XPS. The XPS detection results are shown in Figure 7. Figures 7 (a-c) show the surface energy spectra, C1s, and O1s spectra of PMMA before plasma treatment. The elements in PMMA before plasma treatment are shown in Figure 7a, with only C and O peaks in the spectrum, indicating that PMMA does not contain other elemental impurities. Figure 7d shows the surface energy spectrum of PMMA after plasma treatment. Figure 7e shows the C1s spectrum of PMMA surface after plasma treatment, which contains three components located at 284.8 eV (C-C), 286.30 eV (C-O), and 288.73 eV (O-C=O), respectively. Compared with before plasma treatment (as shown in Figure 7 (b)), the intensity of C-C peak decreased, while the intensity of C-O and O-C=O peaks increased. Figure 7f shows the O1s spectrum of PMMA surface after plasma treatment. Compared with the spectrum without plasma treatment, in addition to the C-O peak at 532.02eV and the C=O peak at 533.36eV, a new peak, namely the O-H peak at 533.44eV, appeared in the spectrum.

Figure 7 XPS and elemental spectra of PMMA surface before and after plasma treatment: (a) XPS before plasma treatment; (b) C1s spectra before plasma treatment; (c) O1 s spectra before plasma treatment; (d) XPS after plasma treatment; (e) C1 s spectra after plasma treatment; (f) O1 s spectra after plasma treatment

Tables 3.1, 3.2, and 3.3 show the intensities of peaks before and after plasma treatment in different spectra. It can be seen that the content of C element decreases and the content of O element increases after plasma treatment. This is due to the collision between plasma and material during the processing, which modifies the surface of the material, causing the long carbon chains on the PMMA surface to break, resulting in more O elements being connected to C elements, thereby increasing the content of C-O and O-C=O bonds. This explains well the phenomenon of decreased C-C strength, increased C-O and O-C=O strength in Table 3.2, as well as the decrease in the proportion of carbon containing chemical bonds in Table 3.3. An increase in the proportion of oxygen-containing functional groups can enhance the polarity of PMMA surface and also improve its wettability, which is consistent with the phenomenon of plasma treatment reducing the contact angle mentioned earlier.