Stacked polyelectrolyte build up without harsh chemicals

Easy fabrication of multilayer films having unique porous internal defect structures


Anion bridging model for the fabrication of PSS/PAH PEMLs from polyelectrolyte solutions containing sulfate and related divalent bridging anion salts. (The various shades of blue (PAH) and red (PSS) polyelectrolyte strands serve solely to illustrate the chain entanglement described by the model.)

Polyelectrolyte multilayers (PEMs) are constructed using a layer-by-layer deposition technique to create a stack of cross-linked films with possible nanometer thickness control. PEMs are used as high-performance coatings and can be doped with nanoparticles to create unique properties.

To get the required performance from PEMs, their internal structure must be arranged in a non-random manner. Processes to perform this arrangement require dissolution with cyanide or related agents. Alternate methods not relying on such dissolvable hard processes to control and fix polyelectrolyte structure within a multilayer stratum do not exist.

Navy researchers have developed an alternate method of controlling the internal structure of polyelectrolyte multilayers whereby anions capable of electrostatically bridging cationic sites on polycation chains in solution organize these chains into solution aggregates.

Aggregate size and solution concentration are tuned by the concentration and nature of the bridging anion present in the polycation deposition solution. The anion-bridged aggregates are sufficiently stable such that they adsorb relatively intact onto the polyanion terminal layer of a growing multilayer. The adsorbed aggregates control the surface morphology of the stratum that they comprise by virtue of their size and charge, which determine their surface coverage and spatial distribution at the film-solution interface.

Spaces between adjacent adsorbed aggregates constitute defect sites, which are covered by subsequent adsorption of the next polyanion layer as film growth proceeds. As a result, a multilayer film having a controllable unique porous internal defect structure is fabricated. The bridging anion salts may be readily removed from the adsorbed polycation aggregates by the rinsing process used during film fabrication. Therefore, the fabrication of the internal structure does not require the use of harsh chemical dissolution steps associated with the hard templates currently used to prepare similar films.

Given their unique internal structures, the films may be used for filtration and encapsulation applications. In addition, the use of the internal defect sites as nanoreactors for conducting chemical reactions within their ultra-small volumes inside the multilayer film is also possible. Because there is no known analogous method available for providing such a controllable unique internal defect structure within a polyelectrolyte multilayer film, the films are expected to find great use in these areas.

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