High-flux filtration membranes constructed through scalable and sustainable methods are desirable for energy-efficient separations. Often, these criteria are difficult to be reconciled with one another. Polymeric membranes can provide high flux but frequently involve organic solvents in processing steps. Solubility of many polymeric membranes in organic media also restricts their implementation in solvent filtration. In the present work, we report a simple and high-throughput aqueous processing approach for polyelectrolyte complex (PEC) membranes with controllable porosity and stability in various aqueous and organic environments. PECs are materials composed of oppositely charged polymer chains that can form solids in aqueous environments, yet which can be dissolved in very specific salt solutions capable of breaking the interpolymer ion pairs. By exploiting the salt-induced dissolution and subsequent reformation of the complex, nano- to microporous films are rapidly synthesized which resemble membranes obtained through conventional solvent-phase inversion techniques. PECs remain stable in organic solvents because of the low dielectric constant of the environment, which enhances electrostatic interactions, making them suitable for a wide range of water and solvent filtration applications. Here, we elucidate how the polymer-phase behavior can be manipulated to exercise morphological control, test membrane performance for water and solvent filtration, and quantify the mechanical stability of PECs in relevant conditions.

Polyelectrolyte complex (PEC) films made from oppositely charged polymer chains have applications as drug-delivery vehicles, separation membranes, and biocompatible coatings. Conventional layer-by-layer techniques for polyelectrolyte coatings are low-throughput and multistep processes that are quite slow for building films on the order of micrometers. In this work, PEC films are electrochemically deposited using a rapid one-pot method, yielding thick (1 μm) films within short experimental time scales (5 min). This rapid electrodeposition is achieved by exploiting the reduction of hydrogen peroxide at mild electrode potentials that avoid water electrolysis yet trigger the pH-responsive self-assembly of a PEC film composed of poly(acrylic) acid and poly(allylamine) HCl. In situ rheology using an electrochemical quartz crystal microbalance quantified the shear modulus-density product of the deposited layer to be on the order of 10 Pa g/cm at a frequency of 15 MHz, with a viscoelastic phase angle of approximately 50°. This electrodeposition scheme furthers the development of PEC coatings for more high-throughput applications, where a fast and efficient single-step approach would be desirable for obtaining coatings.

The utility of the quartz crystal microbalance (QCM) as a high-frequency rheometer operating at 15 MHz was demonstrated. High-frequency data obtained from a series of rubbery materials were compared with results obtained from traditional dynamic mechanical analysis (DMA) at much lower frequencies.The high-frequency data enable meaningful shift factors to be obtained at temperatures much further above glass-transition temperature (T g ) than would otherwise be possible, giving a more complete picture of the temperature dependence of the viscoelastic properties. The QCM can also be used to quantify mass uptake and changes in viscoelastic properties during sample oxidation. The viscoelastic response spanning the full range of behaviors from the rubber to glassy regimes was found to fit well with a six-element model consisting of three power-law springpot elements. One of these elements is particularly sensitive to the behavior in the transition regime where the phase angle is maximized. The value of this quantity is obtained from the maximum phase angle, which can be obtained from a temperature sweep at fixed frequency, proving a means for more detailed frequency-dependent rheometric information to be obtained from a fixed-frequency measurement at a range of temperatures.

An atomic force microscope (AFM) based fast dynamic scanning indentation (DSI) nano-DMA method, which relies only on the commonly available capabilities of commercial AFMs to provide quantitatively accurate high-resolution (∼15 nm) spatial maps of local viscoelastic mechanical properties (E′, E″, and tan ϕ) in heterogeneous soft adhesive material systems, is described. The versatility of the DSI approach is demonstrated by successfully employing it on three industry-leading commercial AFMs/modules (Asylum’s Cypher ES and MFP-3D Infinity AFMs with the FastForceMapping module, and Bruker’s Dimension Icon AFM with the PeakForce QNM module). Frequency sweep thermorheological DSI experiments were performed to generate quantitatively accurate nano-DMA master curves spanning an unprecedented frequency range of 5 decades. Quantitative agreement between DSI nano-DMA and bulk DMA measurements is demonstrated for two different homogeneous elastomers (styrene butadiene rubber, SBR, and synthetic natural rubber, SNR). The capability of the DSI methodology in acquiring quantitatively accurate viscoelastic property maps of heterogeneous soft solids was validated through experiments on an SBR-SNR blend sample. Experimental factors affecting DSI data quality (e.g., shift factor and AFM tip size) are also discussed.

Polyampholyte (PA) hydrogels are a fascinating class of soft materials that can exhibit high toughness while retaining self-healing characteristics. This behavior results from the random distribution of oppositely charged monomers along the polymer chains that form transient bonds with a range of bond strengths. PAs can be dissolved in aqueous salt solutions and then re-cast via immersion precipitation, making them particularly useful as surface coatings in biomedical applications. Moreover, this immersion precipitation technique allows these PA hydrogels to be fabricated into films less than 100 nm. One critical challenge to this aqueous processing method is the recrystallization of the salt upon water evaporation. Such re-crystallization can disrupt the hydrogel morphology especially in thin films. In this study, a deep eutectic solvent (DES) formed from urea and choline chloride was used to dissolve PAs made from p-styrenesulfonic acid sodium salt and 3-(methacryloylamino)propyl trimethylammonium chloride. This DES has a freezing point of 12 • C, allowing it to remain stable and liquid-like at room temperatures. Thus, these PAs can be processed in DES solutions, without this issue of re-crystallization and with simple methods such as spin coating and dip coating. These methods allow these hydrogels to be used in thin (<100 nm) film coating applications. Finally, the complete miscibility of DES in water allows a wider range of 1 phase compositions and expands the processing window of these polyampholyte materials.