Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
Proc Natl Acad Sci U S A. 2013 Apr 23;110(17):6651-6. doi: 10.1073/pnas.1222325110. Epub 2013 Apr 8.
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.
功能有机薄膜通常需要对纳米级结构进行精确控制。材料的层间扩散可能会破坏这种精确的结构；因此，需要更好地了解层间扩散何时发生以及如何控制它。X 射线光电子能谱与 C60(+)团簇离子溅射相结合，能够对有机薄膜的原子组成和化学状态进行高分辨率的深度分析。使用这种技术，我们探索了聚电解质多层领域中常见的问题，例如多层中氢键和静电相互作用之间的竞争、聚合物层间扩散的阻断、与周围溶液的薄膜成分交换，以及层间扩散的程度和动力学。研究了溶胀氢键聚（环氧乙烷）/聚（丙烯酸）多层薄膜中壳聚糖（M = ∼100 kDa）的扩散系数，并确定其为 1.4*10(-12) cm(2)/s。利用高分辨率数据，我们表明壳聚糖扩散到氢键区域时，聚（环氧乙烷）会从薄膜中被取代。在所测试的条件下，单层聚（盐酸烯丙胺）完全阻止壳聚糖扩散。我们希望我们的结果能够增强对如何控制聚电解质多层结构、扩散过程中发生了哪些化学组成变化以及薄膜中的聚合物在什么条件下与溶液交换的理解。
