Acid-Base Chemistry at Polymer-Solution Interfaces
Surface functionalized polyethylene films have been prepared by entrapping ethylene oligomers terminated with N,N-dimethyl para-aminobenzeneazobenzoic acid (p-methyl red). These surface functionalized films were exposed to sulfonic acids in various solvents to evaluate the accessibility, reactivity and mobility of functional groups at the polymer surface. Utilizing methane sulfonic acid in different solvents, the appearance of the protonated form of the dye in the film was monitored by UV-visible spectroscopy. Relative percentages of protonation were then calculated. The accessibility of functional groups at the polymer surface toward methane sulfonic acid in solution was shown to be a function of the dielectric constant of the solvent used. Solvents with low dielectric constants (e.g. CHCI3) which are more compatible with polyethylene allow the sulfonic acid greater access to the dye groups. Solvents with higher dielectric constants (e.g. acetone) protonate smaller amounts of the entrapped dye. The reactivity of the entrapped dye was also influenced by being at a polyethylene-solution interface. While a low molecular weight analog of the entrapped dye, the octyl ester of p-methyl red, was stable as its protonated from in solvents leveling effect was observed. The entrapped protonated dye was more acidic than protonated solvent and thus it transferred its proton to the solvent. This was observed to be a rapid process. This proton transfer was verified by using soluble dyes as proton acceptors. Finally the mobility of these azo dye molecules could be studied. The rate of cis-trans isomerization of these bound azo dyes was shown to be only slightly dependent on the solvent in which the polyethylene film was suspended.