(690d) Depth-Profiling X-Ray Photoelectron Spectroscopy (XPS) Analysis of Interlayer Diffusion in Nanostructured Polyelectrolyte Multilayers

Functional organic thin films often require stratified structures with a distinct purpose for each layer. Applications of stratified films, from the sequential release of therapeutics from a biomedical implant(1) to a polymeric interfacial layer in a dye-sensitized solar cell(2), all demand nanometer level control over the structure. However, due to dynamic diffusion of the species between layers the composition and function of these layers may not be as desired. Analysis of the diffusion of species between layers has proven difficult as other methods require fluorescent labeling of the materials of interest (FRET), require dueterated materials (neutron reflectivity) or have poor z-resolution (confocal microscopy, FTIR). In this talk we will describe high-resolution depth profiling XPS of polymeric nanostructured films using polymer-friendly cluster-ion C₆₀ sputtering that was recently published in the Proceedings of the National Academy of Sciences(3). This technique has never before been applied to functional nanostructured films and allows for the direct determination of the composition of polymeric films with ~15nm level resolution.

Applying this technique to a model nanostructured film fabricated using layer-by-layer technology we explore common issues in the field such as the dynamic competition between hydrogen bonding and electrostatic interactions during fabrication, blocking interlayer diffusion, the exchange of film components with a surrounding solution and the kinetics of interlayer diffusion. Using high resolution  depth profiling XPS we show that chitosan, a commonly used biopolymer, diffuses readily into a layer comprised of hydrogen bonded polymers with a diffusion coefficient of 1.4*10^-12 cm²/sec. In addition we demonstrate that the interlayer diffusion of chitosan displaces the hydrogen bonding component poly(ethylene oxide) from the film, thus changing the composition. Finally, we find that a <1nm layer of poly(allylamine hydrochloride) can stop the diffusion of chitosan and thus enables fabrication of highly structured films, each with the desired function. We propose that our results address some common issues of interlayer diffusion and could be applied to multiple fields utilizing organic thin films. Further, the ability to identify and analyze the composition of each layer of a polymeric nanostructured film displays the precision and power of cluster-ion C₆₀ depth profiling XPS that could be applied to many other fields.

References:

1.         Wood KC, Chuang HF, Batten RD, Lynn DM, Hammond PT (2006) Controlling interlayer diffusion to achieve sustained, multiagent delivery from layer-by-layer thin films. Proc Natl Acad Sci U S A 103(27):10207-10212.

2.         Kim KH, et al. (2012) Interface control with layer-by-layer assembled ionic polymers for efficient low-temperature dye-sensitized solar cells. Journal of Materials Chemistry 22(22):11179-11184.

3.         Gilbert JB, Rubner MF, Cohen RE (2013) Depth-profiling x-ray photoelectron spectroscopy (xps) analysis of interlayer diffusion in polyelectrolyte multilayers. Proc Natl Acad Sci U S A 110(17):6651-6656.