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

Oxidation of low-density polyethylene film with chromic acidsulfur 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.