Acid-base behavior of carboxylic acid groups covalently attached at the surface of polyethylene: The usefulness of contact angle in following the ionization of surface functionality

Abstract

Oxidation of polyethylene with chromic acid/sulfuric acid generates a material (PE-CO2H) having a high density of carboxylic acid and ketone functionalities in a thin surface layer on the polymer. This paper determines the extent of ionization of the surface and near-surface carboxylic acid 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. On the basis of correlations between results obtained using these three techniques, we propose an equation relating the contact angles an aqueous solution having a given value of pH and the extent of ionization (a subscript i) of those carboxylic acid groups that are directly exposed to the solution. These carboxylic acid groups have broad titration curves, and have CO2H groups that are less acidic than soluble carboxylic acids. The initial ionization of these carboxylic acid groups occurs when the solution is approximately pH = 6. The detailed structures of these oxidized polymer surface layers and the nature of the interactions between the carboxylic acid and carboxylate ions in them are still not completely defined. Salt effects on the extent of ionization a subscript i at a particular value of pH are unexpectedly small, and suggest that charge-charge interactions between carboxylate ions may not dominate the titration curves. This work demonstrates the usefulness of contact angle in following chemical changes occurring in organic functional groups of surfaces.