Controlling Diffusion and Exchange in Layer-by-Layer Assemblies
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- 作者：Nicole S. Zacharia ; Dean M. DeLongchamp ; Miguel Modestino ; Paula T. Hammond
- 刊名：Macromolecules
- 出版年：2007
- 出版时间：March 6, 2007
- 年：2007
- 卷：40
- 期：5
- 页码：1598 - 1603
- 全文大小：318K
- 年卷期：v.40,no.5(March 6, 2007)
- ISSN：1520-5835
文摘
Here we present a layer-by-layer (LbL) assembled device architecture that serves as a modelheterostructure to study the atypical assembly that can result from the bottom-up combination of multiple multilayersof different compositions. Heterostructure assembly is disrupted by diffusion of linear poly(ethylene imine) (LPEI)within an LPEI/poly(acrylic acid) (PAA) polyelectrolyte multilayer and the exchange of LPEI with an aromaticpolycation (poly(hexyl viologen) or PXV) that was assembled with PAA in an underlying multilayer. We illustratethis diffusion/exchange mechanism by showing that the assembly of an LPEI/PAA multilayer on to a PXV/PAAmultilayer causes the heterostructure film to roughen and become opaque, indicating significant morphologicalchanges. FTIR analysis confirms that LPEI diffuses into the underlying polyelectrolyte multilayer and displacesPXV. Exchange experiments of the constructed PXV/PAA multilayers in the presence of LPEI solutions werecompleted. Molecular weight dependence on the rate of exchange is shown through the use of high molecularweight LPEI, which is shown to undergo displacement at rates much slower than the time frame of the typicallayer adsorption cycle. Finally, we prevent LPEI diffusion by incorporating a thin blocking layer of cross-linkedLbL film, resulting in a discrete, compartmentalized multilayer structure. We explain these phenomena by thestrength of acid-base interactions between LPEI and PAA, the ability of hydrophilic LPEI molecules to movethrough a multilayer, and the tendency of weak polyelectrolytes to redistribute their ionization in response tochanges in immediate environment. The success of assembling a heterostructured LbL film can be predicted bywhether or not the individual component layers grow superlinearly in isolation. Our system provides a modelexample of LbL assemblies in which interdiffusion destabilizes film growth, and by understanding the mechanismof this destabilization, we are able to control it.