Acid-Base Behavior of Carboxylic Acid Groups Covalently Attached at the Surface of Polyethylene,

Oxidation of low-density polyethylene film with chromic acid/sulfur acid at 72 C generates a material (PE-CO2H) having a high density of carboxylic acid and ketone functionalities in a thin surface layer on the polymer. Similar oxidation of low density polyethylene powder (powder PE-CO2H) and ultrahigh molecular weight polyethylene (UHMW-CO2H) generate materials having only slightly different properties. This paper determines extent of the ionization of the surface and near-surface carboxylic acis groups of these materials in contact with water as a function of pH using three experimental techniques: measurement of attenuated total reflectance infrared (ATR-IR) spectra, measurement of contact angles, and direct potentiometric titration. The first and third techniques measure the concentrations of carboxylic acid and carboxylate groups throughout the polymer surface and near-surface region. The second technique relies on differences in the hydrophilicity of carboxylic acid and carboxylate groups to produce differences in the polymer-water interfacial free energy. This technique is sensitive primarily to surface functionality. On the basis of correlations between results obtained using these three techniques, we propose an equation (eq 6) relating the contact angel of an aqueous solution having a given value of pH on PE-CO2H and UHMW-CO2H and the extent of ionization of those carboxylic acid groups which are directly exposed to the solution.