Dissociation Behavior of Surface-Grafted Poly(Acrylic Acid): Effects of Surface Density and Counterion Size

Abstract

The effects of surface density and counterion size upon the dissociation behavior of poly(acrylic acid) grafted onto a polyethylene surface were investigated. The effect of counterion size was studied in aqueous alkali metal salt solutions (LiCl, NaCl, KCl, and CsCl) by potentiometric titration methods. To study the effect of surface density, we prepared a "poly(acrylic acid) gradient surface" where the density of poly(acrylic acid) changes gradually along the sample distance. The gradient surface was produced by the treatment of a polyethylene sheet using a corona with gradually increasing power, followed by graft polymerization in 10 wt% acrylic acid solution. The prepared gradient surface was characterized by Fourier transform infrared spectroscopy in the attenuated total reflectance mode. Potentiometric titration curves for the sections of the gradient surface were obtained in different salt solutions to measure the changes of electrostatic Gibbs free energy of the poly(acrylic acid) on the gradient surface. It was observed that the changes of electrostatic Gibbs free energy increase with the increasing density of the poly(acrylic acid) grafted onto the surface and the decreasing hydrated radius of the alkali metal ions in the solution. It was difficult to remove protons from the poly(acrylic acid) on the sections with high surface density and in the solutions containing alkali metal ions with small hydrated radii.