Plasma assisted surface coating of porous solids

Surface modification of high-porosity solids using RF plasma has been investigated. Hydrophobic coatings were deposited on a stack of five porous filter papers using four types of plasma from different fluorine-containing monomers (TDFO, PDFOA, PFDD and PFMCH). Surface chemistry and water contact angles of each layer were analyzed using X-ray photoelectron spectroscopy (XPS) and video contact angle (VCA) measurements. It is seen that CF2 and CF3 groups are responsible for hydrophobic behavior and contact angle values correlates very well with total surface fluorine content. As the average surface fluorine concentration increases to about 15%, the surface becomes abruptly hydrophobic. This study indicates that a very thin coating (average thickness smaller than 1-2 nm) is needed for hydrophobic behavior. Additional deposition increases coating thickness, but has no effect on contact angle. All the monomers are equally effective in coating the "external" surfaces of the stack, but the extent of permeation of the plasma into the inner layers varies with monomer structure. PDFOA produces more penetrating plasma compared to other molecules. The molar ratio of the two hydrophobic functional groups (CF2/CF3) in all coatings is lower than that in the original monomer due to molecular fragmentation in the plasma. In the straight chain monomers studied, CF2/CF3 ratio of the coating follows the same trend as in the starting molecule. However, in the cyclic monomer studied, this ratio is significantly lower indicating greater fragmentation of such molecules. These results indicate that plasma-induced coatings can modify surface properties of high-porosity materials and that the extent of permeation can be controlled. (C) 2002 Elsevier Science B.V. All rights reserved.