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Depth-profiling X-ray photoelectron spectroscopy (XPS) analysis of interlayer diffusion in polyelectrolyte multilayers.

Proceedings of the National Academy of Sciences of the United States of America (2013-04-10)
Jonathan B Gilbert, Michael F Rubner, Robert E Cohen
ABSTRACT

Functional organic thin films often demand precise control over the nanometer-level structure. Interlayer diffusion of materials may destroy this precise structure; therefore, a better understanding of when interlayer diffusion occurs and how to control it is needed. X-ray photoelectron spectroscopy paired with C60(+) cluster ion sputtering enables high-resolution analysis of the atomic composition and chemical state of organic thin films with depth. Using this technique, we explore issues common to the polyelectrolyte multilayer field, such as the competition between hydrogen bonding and electrostatic interactions in multilayers, blocking interlayer diffusion of polymers, the exchange of film components with a surrounding solution, and the extent and kinetics of interlayer diffusion. The diffusion coefficient of chitosan (M = ∼100 kDa) in swollen hydrogen-bonded poly(ethylene oxide)/poly(acrylic acid) multilayer films was examined and determined to be 1.4*10(-12) cm(2)/s. Using the high-resolution data, we show that upon chitosan diffusion into the hydrogen-bonded region, poly(ethylene oxide) is displaced from the film. Under the conditions tested, a single layer of poly(allylamine hydrochloride) completely stops chitosan diffusion. We expect our results to enhance the understanding of how to control polyelectrolyte multilayer structure, what chemical compositional changes occur with diffusion, and under what conditions polymers in the film exchange with the solution.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Fullerene-C60, sublimed, 99.9%
Sigma-Aldrich
Fullerene-C60, 99.5%
Sigma-Aldrich
Poly(allylamine hydrochloride), average Mw 50,000
Sigma-Aldrich
Poly(allylamine) solution, average Mw ~15,000, 20 wt. % in H2O
Sigma-Aldrich
Poly(allylamine) solution, average Mw ~65,000, 10 wt. % in H2O
Sigma-Aldrich
Fullerene-C60, 98%
Sigma-Aldrich
Poly(allylamine hydrochloride), average Mw ~17,500 (GPC vs. PEG std.)