Surface wettability modification of polycarbonate by femtosecond laser irradiation

• Main Author: Aye Myat Thwe
• Other Authors: Lam Yee Cheong
• Format: Final Year Project
• Language: English
• Published: 2011
• Subjects: DRNTU::Science::Chemistry::Physical chemistry::Surface chemistry; DRNTU::Engineering::Manufacturing
• Online Access: http://hdl.handle.net/10356/46070
• Institution: Nanyang Technological University

Surface wettability plays an important role in lab-on-chips applications to control the flow of fluid for microfluidic devices. In this project, investigation on surface wettability of polycarbonate (PC) was carried out using femtosecond laser irradiation. The changes in water contact angle (WCA), surface morphology, surface roughness and stability of the modified wettability under various laser irradiation process conditions were studied. By varying the laser scanning speeds, a wide range of WCA, i.e., highly hydrophobic to low hydrophilic surfaces were achieved. The highly hydrophobic surfaces (WCA>146) could be obtained using the lowest laser scanning speed of 0.5 mm/s to 1 mm/s. Beyond 1mm/s, WCA decreased drastically to 44.8o (scanning speed: 2 mm/s). The WCA was less than 5o between 25 mm/s to 55 mm/s. However, beyond 55 mm/s, the WCA increased and it was 65o at the fastest scanning speed of 2060 mm/s within investigation range. By defocusing the laser beam on PC surface, highly hydrophilic, low hydrophilic, hydrophobic and highly hydrophobic surfaces, could be achieved. The laser fluence was inversely proportional to the focal distance. It was found that less defocus (nearly or at focus), i.e., high fluences, produced highly hydrophilic surfaces. With the defocus distance increasing, the surface hydrophilicity decreased and hydrophobic surfaces were obtained. However, by further increasing the defocus distance, the laser fluence was reduced and it was not enough to induce the changes on the PC surface. The surface wettability approached that of raw PC when the defocus distance (laser fluence) exceeded a certain threshold value. Post treatment of laser modified surface using ultrasonication in deionized (DI) water and ethanol, showed that hydrophobicity for the modified hydrophobic surfaces decreased and this was more pronounced for highly hydrophobic samples. Interestingly, it was also found that hydrophilicity for laser modified samples also decreased as well. The characterization on the surface morphology and roughness for modified samples shows that the morphology was changed and different morphology patterns were obtained under varied scanning speeds and focal distances. Surface roughness was increased after laser modification and the roughness values Ra ranged from 0.07 µm to 5.46 µm, Sa ranged from 0.187 µm to 6.47 µm. However, both surface morphology and roughness did not exhibit significant changes after post treatment. The experimental results indicated that changes in the morphology and the roughness of the surfaces may have contributed to the modification of the surface wettability, however, it was postulated that roughness was not a critical factor in determining the wettability modification. It was believed that the change in wetting behavior of laser irradiated PC surfaces was due to the chemical modification on the sample surface rather than the physical modification. Similarly, the causes of the changes in WCA of the laser modified surface after post treatment might be due to the secondary chemical modification of the surface by post treatment and removal of the possible debris deposited from the laser irradiation. The determination on the modification of the surface chemical composition is in progress. X-ray photoelectron spectroscopy (XPS) analysis is currently being carried out to identify the chemical bond changes in the modified PC surfaces.