Preparation and Characterization of Functionalized Polyethylene Surfaces

We describe a procedure to modify the surface of polyethylene (PE) film using a combination of gas discharge and wet chemical techniques. This method generates high densities (10¹⁴-10¹⁶ cm^-2) of a specific functionality, largely unaccompanied by other groups, in a 50-100-Å surface layer. The topography of the polymer surface remains unchanged after treatment and functions as an effective starting material for subsequent derivatization by standard synthetic chemical reactions. A plasma of either oxygen, water, or hydrogen is generated under comparable experimental conditions. In all cases a 1-2-s, 5-W, 0.2-Torr treatment produces about the same degree of surface modification as does longer treatment. High-resolution X-ray photoelectron spectroscopy (XPS) shows that either an oxygen or a water plasma produces a variety of oxidation products ranging from alcohols to carboxylic acids. Chromic acid oxidizes the plasma-oxidized surface further to give high densities of carboxylic acid groups which can be readily converted to acid chlorides and derivatized. Borane/tetrahydrofuran reduces the plasma-oxidized surface to give alcohols which can be esterified readily. Contact-angle measurements show that the water-plasma-treated PE surface has a higher surface free energy (γ_s ∼ 62 dyn/cm) than the oxygen-plasma-treated surface (γ_s ∼ 50 dyn/cm). A 5-s, ambient-temperature, 0.2-Torr, 2-W hydrogen plasma generates a significant number of quenchable radical sites. XPS spectra of this treated surface, exposed to either nitric oxide or nitrosotrifluoromethane, show that both compounds bond to the surface.