Effect of hydrophilic group on water droplet contact angles on surfaces of acid modified SiLK and Parylene polymers

The effect of surface roughness and surface chemistry of SiLK and Parylene polymers on the water droplet contact angle was investigated using Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). AFM images show that the surface morphologies of SiLK and Parylene treated with H2SO4 and H3PO4 solutions are qualitatively the same as those of the untreated SiLK and Parylene. However, the advancing (θa) and receding (θr) contact angles on SiLK dramatically decrease from θa = 70 ± 3° and θr = 63 ± 2° to θa = 37 ± 2° and θr = 34 ± 3°, respectively, after being treated with a H2SO4 solution. In contrast, the advancing and receding contact angles on SiLK treated with H3PO4 solution or Parylene treated with H2SO4 or H3PO4 solutions were changed within a few degrees. XPS results show that polar functional groups (CO and –OH) formed on the SiLK sample surface after being treated with H2SO4 solution. However, H3PO4 reacts less aggressively with the ethynyl (–CC–) bonding on the SiLK surface compared with H2SO4. Parylene lacks ethynyl chemistry and is therefore chemically more inert than SiLK. Thus, both H2SO4 and H3PO4 solutions have fewer reactions with Parylene resulting in slight changes in the advancing and receding contact angles.