Robust, Functionalizable, Nanometer-Thick Poly(acrylic acid) Films Spontaneously Assembled on Oxidized Aluminum Substrates: Structures and Chemical Properties

초록

Immersion of oxidized aluminum substrates in ethanol solutions of poly(acrylic acid) (PAA), followed by extensive solvent immersion, results in tenaciously chemisorbed, nanometer scale, controllable thickness films for a wide range of solution concentrations and molecular weights. Atomic force microscope images reveal isolated polymer globules from adsorption in low-concentration solutions with crossover to conformal, highly uniform, nanometer-thickness films at higher concentrations, an indication that the chemisorbing chains start to overlap and trap underlying segments to form planar chemisorbed films only two or three chains in thickness. Quantitative IR reflection spectroscopy in combination with chemical derivitization on a standard set of 1.0(+/- 0.2) nm thick films reveals a film structure with 5.5(+/- 1) chemisorbed -CO2- groups/nm(2) and 6.3 unattached -CO2H groups/nm(2), with up to similar to 3.6/nm(2) available for chemical derivitization, a comparable number to typical self-assembled monolayer coverages of similar to 4-5 molecules/nm(2). Thermal treatment of the similar to 1 run chemisorbed films, at even extreme temperatures similar to 150 degrees C results in almost no anhydride formation via adjacent -CO2H condensation, in strong contrast to bulk PAA, a clear indication that the films have a frozen glass structure with effectively no segment and side group mobility. Overall, these results demonstrate that these limiting thickness nanometer films provide a model surface for understanding the behavior of strongly bound polymer chains at substrates and show potential as a path to creating highly stable, chemically functionalized inorganic substrates with highly variable surface properties.

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