Clinton G. Wiener

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

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.

Control of Mesh Size and Modulus by Kinetically Dependent Cross‐Linking in Hydrogels

Kinetically controlled cross-linking processes produce mechanically distinguishable hydrogels using identical precursor chemistry. The oxime ligation demonstrates tunable reaction kinetics with pH and buffer strength, which induce changes in the structural features of hydrogels and determine their mechanical properties. Small-angle neutron scattering and swelling studies provide an insight into how structural properties correlate with mechanical properties for this hydrogel system.

Response of Swelling Behavior of Weak Branched Poly(ethylene imine)/Poly(acrylic acid) Polyelectrolyte Multilayers to Thermal Treatment

Weak polyelectrolyte multilayers (PEMs) prepared by the layer-by-layer technique have attracted a great deal of attention as smart responsive materials for biological and other applications in aqueous medium, but their dynamic behavior as a function of exposure to a wide temperature range is still not well understood. In this work, the thermally dependent swelling behavior of PEMs consisting of branched poly(ethylenimine) and poly(acrylic acid) is studied by temperature controlled in situ spectroscopic ellipsometry. Because of diffusion and interpenetration of polyelectrolytes during film deposition, the PEMs densify with increasing bilayer number, which further affects their water uptake behavior. Upon heating to temperatures below 60 °C, the worsened solvent quality of the PEM in water causes deswelling of the PEMs. However, once heated above this critical temperature, the hydrogen bonds within the PEMs are weakened, which allows for chain rearrangement within the film upon cooling, resulting in enhanced water uptake and increased film thickness. The current work provides fundamental insight into the unique dynamic behavior of weak polyelectrolyte multilayers in water at elevated temperatures.

Tough Stretchable Physically-Cross-linked Electrospun Hydrogel Fiber Mats

Nature uses supramolecular interactions and hierarchical structures to produce water-rich materials with combinations of properties that are challenging to obtain in synthetic systems. Here, we demonstrate hierarchical supramolecular hydrogels from electrospun, self-associated copolymers with unprecedented elongation and toughness for high porosity hydrogels. Hydrophobic association of perfluoronated comonomers provides the physical cross-links for these hydrogels based on copolymers of dimethyl acrylamide and 2-(N-ethylperfluorooctane sulfonamido)ethyl methacrylate (FOSM). Intriguingly, the hydrogel fiber mats show an enhancement in toughness in comparison to compression molded bulk hydrogels. This difference is attributed to the size distribution of the hydrophobic aggregates where narrowing the distribution in the electrospun material enhances the toughness of the hydrogel. These hydrogel fiber mats exhibit extensibility more than double that of the bulk hydrogel and a comparable modulus despite the porosity of the fiber mat leading to >25 wt % increase in water content.

Supramolecular Hydrophobic Aggregates in Hydrogels Partially Inhibit Ice Formation

Prevention of ice crystallization is a challenging problem with implications in diverse applications, as well as examining the fundamental low temperature physics of water. Here, we demonstrate a simple route, inspired by water confinement in antifreeze proteins, to inhibit crystallization and provide high water mobility of highly supercooled water using supramolecular hydrogels of copolymers of dimethylacrylamide (DMA) and 2-(N-ethylperfluorooctane sulfonamido)ethyl acrylate (FOSA). These hydrogels can suppress or inhibit freezing of their water, depending on the copolymer composition. Dynamic and static neutron scattering indicate that hydrogels using the copolymer with 22 mol % FOSA partially inhibit ice formation. This behavior is attributed to confinement (<2 nm) of water between the hydrophobic FOSA nanodomains that prevents 45% of the water within the hydrogel from freezing even at 205 K. Very fast dynamics of the amorphous water are observed at 220 K with an effective local diffusivity decreased by only a factor of 2 from that observed at 295 K within the hydrogel using the copolymer with 22 mol % FOSA. The spacing between the hydrophobic nanodomains, tuned through the copolymer composition, appears to modulate the water that can crystallize. These fully hydrated hydrogels (at equilibrium with liquid water at 295 K) can enable a significant fraction of highly supercooled water to be stable down to at least 205 K.

Quantitative Rheometry of Thin Soft Materials Using the Quartz Crystal Microbalance with Dissipation

In the inertial limit, the resonance frequency of the quartz crystal microbalance (QCM) is related to the coupled mass on the quartz sensor through the Sauerbrey expression that relates the mass to the change in resonance frequency. However, when the thickness of the film is sufficiently large, the relationship becomes more complicated and both the frequency and damping of the crystal resonance must be considered. In this regime, a rheological model of the material must be used to accurately extract the adhered films thickness, shear modulus, and viscoelastic phase angle from the data. In the present work we examine the suitability of two viscoelastic models, a simple Voigt model ( Physica Scripta 1999, 59, 391-396) and a more realistic power-law model ( Langmuir 2015, 31, 4008-4017), to extract the rheological properties of a thermoresponsive hydrogel film. By changing temperature and initial dry film thickness of the gel, the operation of QCM was traversed from the Sauerbrey limit, where viscous losses do not impact the frequency, through the regime where the QCM response is sensitive to viscoelastic properties. The density-shear modulus and the viscoelastic phase angle from the two models are in good agreement when the shear wavelength ratio, d/λ n, is in the range of 0.05-0.20, where d is the film thickness and λ n is the wavelength of the mechanical shear wave at the nth harmonic. We further provide a framework for estimating the physical properties of soft materials in the megahertz regime by using the physical behavior of polyelectrolyte complexes. This provides the user with an approximate range of allowable film thicknesses for accurate viscoelastic analysis with either model, thus enabling better use of the QCM-D in soft materials research.