Synthesis and characterization of surface-grafted, hyperbranched polymer films containing fluorescent, hydrophobic, ion-binding, biocompatible, and electroactive groups
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We report the derivatization of hyperbranched poly(acrylic acid) (PAA) films with a wide range of aminoor alcohol-terminated molecules. These molecules can include moieties such as pyrene, ferrocene, poly(ethylene glycol), 15-crown-5, and a dye. To derivatize PAA films, we activate their carboxylic acid groups with isobutyl chloroformate and allow them to react with amine- or alcohol-containing molecules. Infrared spectroscopy demonstrates the formation of amide and ester bonds upon coupling as well as the presence of the derivative functional groups. Excimer fluorescence from pyrene-containing films implies a high density of pyrene groups. However, we can control the amount of pyrene in the film (and obtain monomer fluorescence) by varying the concentration of Py(CH2)3CONH(CH2)2NH2 in the derivatization solution. Cyclic voltammetry of ferrocene-containing films shows an electrochemically addressable ferrocenyl surface coverage of (6 3) 10-9 mol/cm2 in three-layer PAA films. PAA films and their derivatives are stable under sonication, Soxhlet extraction, and acidic and basic conditions. PAA films also respond to external stimuli. The ellipsometric thickness of PAA films increases by 45% upon deprotonation of the film's carboxylic acid groups and returns to its original thickness after acidification. Using surface acoustic wave mass sensors, we observed that pure PAA films adsorb or absorb volatile organic compounds (VOCs), although the amount is in the monolayer range. Fluorination of PAA films increases the amount of polar VOCs absorbed by an order of magnitude.
