Large-Scale Solvent Driven Actuation of Polyelectrolyte Multilayers Based on Modulation of Dynamic Secondary Interactions

Abstract

Polyelectrolyte multilayers (PEMs), assembled from weak polyelectrolytes, have often been proposed for use as smart or responsive materials. However, such response to chemical stimuli has been limited to aqueous environments with variations in ionic strength or pH. In this work, a large in magnitude and reversible transition in both the swelling/shrinking and the viscoelastic behavior of branched polyethylenimine/poly(acrylic acid) multilayers was realized in response to exposure with various polar organic solvents (e.g., ethanol, dimethyl sulfoxide, and tetrahydrofuran). The swelling of the PEM decreases with an addition of organic content in the organic solvent/water mixture, and the film contracts without dissolution in pure organic solvent. This large response is due to both the change in dielectric constant of the medium surrounding the film as well as an increase in hydrophobic interactions within the film. The deswelling and shrinking behavior in organic solvent significantly enhances its elasticity, resulting in a stepwise transition from an initially liquid-like film swollen in pure water to a rigid solid in pure organic solvents. This unique and recoverable transition in the swelling/shrinking behaviors and the rheological performances of weak polyelectrolyte multilayer film in organic solvents is much larger than changes due to ionic strength or pH, and it enables large scale actuation of a freestanding PEM. The current study opens a critical pathway toward the development of smart artificial materials.

Self‐Healing Actuating Adhesive Based on Polyelectrolyte Multilayers

“…While these composites require an external electric field and just available for strain change, flowable polyelectrolytes in layer‐by‐layer assembled polyelectrolyte multilayers (PEMs) can achieve intrinsic self‐healing of surface defects by simply being wet with water, a large simplification over other approaches such as the inclusion of embedded healing agents . Weak polyelectrolyte complexes become viscous upon contact with water, and even show strong adhesive property if sufficiently concentrated . Such facile self‐healing behavior can be used to extend the lifetime of actuating materials.…”

“…One of the most attractive features of electrostatically bound PEMs prepared by the LbL process is the controllable charge density of the PE chains, which can be adjusted by either controlling ionic strength of the surrounding aqueous solution for strong PEs or changing the ionic strength as well as solution pH for weak PEs with labile functional groups (e.g., carboxylic acid) . This ability to modulate polyelectrolyte charge density will in turn tune the properties of the PEMs, creating changes in thickness, swelling/shrinking states, elastic properties, porosity, wettability, and even generates determined mechanical motions . This makes PEMs, especially those made from weak polyelectrolytes, ideal for responsive materials, such as pH, electric field, and electrochemically induced swelling in water, thermoresponsive film expansion, and humidity controlled curling behavior as well as “walking.” However, for these diverse reversible actuators, thin PEMs are preferable to achieve significant dynamic motion, which limits the adhesive force as well as PE migration rate needed for effective self‐healing performance when the PEs are ionically bound to the underlying substrate …”

“…The PEM exhibits a significant contraction upon exposure to ethanol meaning that the asymmetric structure as a whole will curl in response to this stimulus and then return to flat with the evaporation of the ethanol and rehydration of the PEM. The film curl degree, as defined by degrees from horizontal reached by the actuating material, is readily controlled by the percentage of ethanol in the solution used to generate the actuation and the mechanical properties of the nonresponsive portion of the bilayer structure. By facile treatment with deionized (DI) water, the film is able to restore itself from physical damage and to regain its mechanical properties, extending the number of actuation cycles possible.…”

Fast Self‐Healing of Polyelectrolyte Multilayer Nanocoating and Restoration of Super Oxygen Barrier

“…The film shrinks to its original thickness (≈700 nm for 8‐BL) after drying at 70 °C for 30 min. It has been reported that this reversible swelling‐drying cycle can be repeated at least ten times without damaging the multilayer nanocoating . This 8‐BL PEI/PAA nanocoating reduces the OTR of 179 µm PET to the undetectable limit of commercial instrumentation (<0.005 cc (m −2 d −1 )) .…”

“…It has been reported that this reversible swelling-drying cycle can be repeated at least ten times without damaging the multilayer nanocoating. [52] This 8-BL PEI/PAA nanocoating reduces the OTR of 179 µm PET to the undetectable limit of commercial instrumentation (<0.005 cc (m −2 d −1 )). [45] This low OTR originates from the strong ionic interaction between the polyelectrolyte chains, which results in small free volume in the nanocoating.…”