Covalent Layer-by-Layer Assembly of Water-Permeable and Water-Impermeable Polymer Multilayers on Highly Water-Soluble and Water-Sensitive Substrates

Aqueous methods for the layer-by-layer (LbL) assembly of polyelectrolyte multilayers (PEMs) are not, by their nature, well suited for the fabrication of thin films on substrates that are highly water-soluble or composed of water-sensitive materials. Here, we demonstrate that organic solvent-based processes for covalent (or “reactive”) LbL assembly can be used to fabricate multilayers directly on the surfaces of model water-soluble, water-reactive, and water-sensitive substrates that are either difficult or impossible to coat effectively using aqueous-based LbL methods. Our approach is based on methods for the reactive assembly of multilayers using poly(ethyleneimine) (PEI) and an amine-reactive polymer containing azlactone functionality (PVMDA) in polar-aprotic solvents. The amine-reactive nature of the resulting PEI/PVDMA films facilitated subsequent modification of film-coated surfaces by reaction with primary amine-based nucleophiles. Functionalization of films with the hydrophobic small-molecule amine n-decylamine resulted in the prolonged dissolution and release of underlying water-soluble substrates, and could be used to tune interfacial properties (e.g., to render these water-permeable films more hydrophobic). Fabrication of thicker PEI/PVDMA films resulted in coatings with superhydrophobic properties (e.g., water contact angles of ∼160°) that resisted the penetration of water and enhanced considerably the stabilities of water-sensitive substrates. This approach can, therefore, also be used, in various ways, and to varying extents, to design barriers that protect, enhance the stabilities of, or control/pattern the responses of water-soluble/reactive substrates when they are exposed to aqueous environments. Finally, our results demonstrate that the solubility of PEI and PVDMA in a range of different polar-aprotic solvents can provide flexibility with respect to coating water-soluble/sensitive materials that may also be soluble in some other polar organic solvents. The versatility of this approach could prove useful for modifying the interfacial properties of water-soluble and water-sensitive materials of interest in biotechnology, catalysis, and many other fundamental and applied contexts.