Electrochemically Controlled Swelling and Mechanical Properties of a Polymer Nanocomposite

Abstract

We present the layer-by-layer assembly of an electroactive polymer nanocomposite thin film containing cationic linear poly(ethyleneimine) (LPEI) and 68 vol % anionic Prussian Blue (PB) nanoparticles, which allow for electrochemical control over film thickness and mechanical properties. Electrochemical reduction of the PB doubles the negative charge on the particles, causing an influx of water and ions from solution to maintain electroneutrality in the film; concomitant swelling and increased elastic compliance of the film result. Reversible swelling upon reduction is on the order of 2-10%, as measured via spectroscopic ellipsometry and electrochemical atomic force microscopy. Reversible changes in the Young's elastic modulus of the hydrated composite film upon reduction are on the order of 50% (from 3.40 to 1.75 GPa) as measured with in situ nanoindentation, and a qualitative increase in viscous contributions to energy dissipation upon redox is indicated by electrochemical quartz crystal microbalance. Electrochemical stimuli maintain a mild operating environment and can be applied rapidly, reversibly, and locally. We maintain that electrochemical control over the swelling and mechanical behavior of polymer nanocomposites could have important implications for responsive coatings of nanoscale devices, including mechanically tunable surfaces to modulate behavior of adherent cells.

Charge Transport in Redox Polyelectrolyte Multilayer Films: The Dramatic Effects of Outmost Layer and Solution Ionic Strength

“…It is known that the thickness can be affected by changes in the solution salt concentration (salt-induced swelling) [30,44] and in the oxidation state of the film (redox-driven swelling). [45][46][47] The magnitude of these processes in similar systems to the osmium pyridine-bipyridine derivatized poly(allylamine) (PAH-Os)/poly(vinyl sulfonate) (PVS) films studied here is less than about 30%. [30,[44][45][46][47] Since the error that would be introduced in D app by neglecting a swelling of this magnitude [48] is much smaller than the variations observed due to the nature of the outmost layer and changes in ionic strength, we will assume a constant film thickness as determined in water: [14] 1.5 nm for Au/3-mercapto-1-propanesulfonate (MPS)/PAH-Os, 10.2 nm for Au/MPS/(PAH-Os/ PVS) 4 PAH-Os, and 11.5 nm for Au/MPS/(PAH-Os/PVS) 5 .…”

Geometrically Controlled Mechanically Responsive Polyelectrolyte Tube Arrays

“…For the force relaxation, constant indentation depth was maintained during the 1-min holding period in a similar manner. All of the experiments were performed in pH 2.0 (0.01 M HCl) and 5.5 (0.01 M NaCl) aqueous solutions via gold-coated polystyrene colloidal probe tips (end radius R = 22.5 μ m, nominal spring constant k ≈ 14 N m − 1 , Novascan), functionalized with neutral, hydrophilic, hydroxyl-terminated self-assembled monolayer by 24 h immersion in 3 m M 11-mercaptoundecanol (HS(CH 2 ) 11 OH, Sigma-Aldrich) ethanol solution, which was assumed infi nite modulus ( ≈ 4 GPa) [ 26 ] compared to the tested material. Indentation force curves were analyzed using MATLAB R2009a (MathWorks).…”

Advanced Nanomechanical Test Techniques

“…Schmidt and co-workers used a NanoTest modified to act as a electrochemical cell to study an electroactive polymer nanocomposite thin film containing cationic linear poly(ethyleneimine) and 68 vol % Prussian blue nanoparticles as a candidate stimulus-responsive polymer material [116]. Electrochemical reduction of the Prussian blue particles doubled their negative charge causing an influx of water into the film to maintain electroneutrality.…”