Controlling diffusion and exchange in layer-by-layer assemblies

Here we present a layer-by-layer (LbL) assembled device architecture that serves as a model heterostructure to study the atypical assembly that can result from the bottom-up combination of multiple multilayers of 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 aromatic polycation (poly(hexyl viologen) or PXV) that was assembled with PAA in an underlying multilayer. We illustrate this diffusion/exchange mechanism by showing that the assembly of an LPEI/PAA multilayer on to a PXV/PAA multilayer causes the heterostructure film to roughen and become opaque, indicating significant morphological changes. FTIR analysis confirms that LPEI diffuses into the underlying polyelectrolyte multilayer and displaces PXV. Exchange experiments of the constructed PXV/PAA multilayers in the presence of LPEI solutions were completed. Molecular weight dependence on the rate of exchange is shown through the use of high molecular weight LPEI, which is shown to undergo displacement at rates much slower than the time frame of the typical layer adsorption cycle. Finally, we prevent LPEI diffusion by incorporating a thin blocking layer of cross-linked LbL film, resulting in a discrete, compartmentalized multilayer structure. We explain these phenomena by the strength of acid-base interactions between LPEI and PAA, the ability of hydrophilic LPEI molecules to move through a multilayer, and the tendency of weak polyelectrolytes to redistribute their ionization in response to changes in immediate environment. The success of assembling a heterostructured LbL film can be predicted by whether or not the individual component layers grow superlinearly in isolation. Our system provides a model example of LbL assemblies in which interdiffusion destabilizes film growth, and by understanding the mechanism of this destabilization, we are able to control it.