Covalent Layer-by-Layer
Assembly of Water-Permeable
and Water-Impermeable Polymer Multilayers on Highly Water-Soluble
and Water-Sensitive Substrates
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Abstract
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 <i>n</i>-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