Chemical Surface‐Modification of a Polyimide: Characterization and a Kinetic Study on the Diffusion of Ions in a Modified Polymer

Abstract

A polyimide, based on an amic acid condensation polymer of 1,2,4,5‐benzenetetracarboxylic acid dianhydride and 4,4′‐oxydianiline (PMDA‐ODA), can be surface modified. Treatment with aqueous alkali metal hydroxide solutions followed by ion exchange with protons leaves the modified region of the polyimide rich in carboxylic acid groups. The carboxylic acid groups are capable of complexing metal salts. For example, a Pd2+ salt forms a 1:1 complex with the amic acid groups. The modified polyimide has been characterized by x‐ray photoelectron spectroscopy and capacity measurements. The modified polyimide is a polyelectrolyte and, when in contact with an electrolyte solution, is subject to ion exchange. The kinetics of ion exchange in this modified polyimide, initially in the form of the palladium salt of amic acid, in contact with a sodium ion‐containing solution have been investigated. It was found that the rate of ion exchange was diffusion‐controlled. The kinetic results were found to obey the Fickian equations for diffusion after a modification to account for time‐dependent diffusion coefficients. The measured interdiffusion coefficients were in the range 10−10 to 10−15 cm2 s−1.