Swelling and Stability of Polyelectrolyte Multilayers in Ionic Liquid Solutions

Polyelectrolyte Bilayer-Based Transparent and Flexible Memristor for Emulating Synapses

Memristors will be critical components in the next generation of digital technology and artificial synapses. Researchers are investigating innovative mechanistic understanding of the memristor devices based on low-cost, solution-processable, and organic materials as promising candidates. Here, we demonstrate a novel polyelectrolyte-based memristor device, which is simply prepared by spin-coating poly(acrylic acid) (PAA) and polyethylenimine (PEI) on an indium tin oxide (ITO) substrate followed by a magnetron sputtering of the ITO as the top electrode. The device has a potential to achieve excellent resistive switching (RS) performance and synapse functionality as well as greater flexibility and transmittance when compared to the oxide-based memories. An on/off resistance ratio of 50 can be maintained without degradation for up to 20 000 cycles (flat state) and over 4000 cycles (bending to a 2 mm radius 10 000 times) in the DC sweep mode. Moreover, the device performs various synaptic functions, including spike-timing-dependent plasticity, pulse pair plasticity, and short-term and long-term plasticity in the potentiation and depression processes. The counterions and two oppositely charged polyelectrolyte chains can move in and out of each other depending on the applied electrical potential (pulse), resulting in a change in the potential drop at the interface of the polyelectrolyte bilayer and its electrodes, which can be attributed to the RS mechanism and various synaptic functions. This insight may accelerate the technological deployment of the organic resistive memories.

Effect of Increased Ionic Liquid Uptake via Thermal Annealing on Mechanical Properties of Polyimide-Poly(ethylene glycol) Segmented Block Copolymer Membranes

Proton exchange membranes (PEMs) suffer performance degradation under certain conditions-temperatures greater than 80 °C, relative humidity less than 50%, and water retention less than 22%. Novel materials are needed that have improved water retention, stability at higher temperatures, flexibility, conductivity, and the ability to function at low humidity. This work focuses on polyimide-poly(ethylene glycol) (PI-PEG) segmented block copolymer (SBC) membranes with high conductivity and mechanical strength. Membranes were prepared with one of two ionic liquids (ILs), either ethylammonium nitrate (EAN) or propylammonium nitrate (PAN), incorporated within the membrane structure to enhance the proton exchange capability. Ionic liquid uptake capacities were compared for two different temperatures, 25 and 60 °C. Then, conductivities were measured for a series of combinations of undoped or doped unannealed and undoped or doped annealed membranes. Stress and strain tests were performed for unannealed and thermally annealed undoped membranes. Later, these experiments were repeated for doped unannealed and thermally annealed. Mechanical and conductivity data were interpreted in the context of prior small angle X-ray scattering (SAXS) studies on similar materials. We have shown that varying the compositions of polyimide-poly(ethylene glycol) (PI-PEG) SBCs allowed the morphology in the system to be tuned. Since polyimides (PI) are made from the condensation of dianhydrides and diamines, this was accomplished using components having different functional groups. Dianhydrides having either fluorinated or oxygenated functional groups and diamines having either fluorinated or oxygenated diamines were used as well as mixtures of these species. Changing the morphology by creating macrophase separation elevated the IL uptake capacities, and in turn, increased their conductivities by a factor of three or more compared to Nafion 115. The stiffness of the membranes synthesized in this work was comparable to Nafion 115 and, thus, sufficient for practical applications.

High-Fidelity Hydrogel Thin Films Processed from Deep Eutectic Solvents

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 recast 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 recrystallization 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 recrystallization 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 one-phase compositions and expands the processing window of these polyampholyte materials.

Fourier transform infrared spectroscopy investigation of water microenvironments in polyelectrolyte multilayers at varying temperatures

Polyelectrolyte multilayers (PEMs) are thin films formed by the alternating deposition of oppositely charged polyelectrolytes. Water plays an important role in influencing the physical properties of PEMs, as it can act both as a plasticizer and swelling agent. However, the way in which water molecules distribute around and hydrate ion pairs has not been fully quantified with respect to both temperature and ionic strength. Here, we examine the effects of temperature and ionic strength on the hydration microenvironments of fully immersed poly(diallyldimethylammonium)/polystyrene sulfonate (PDADMA/PSS) PEMs. This is accomplished by tracking the OD stretch peak using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy at 0.25-1.5 M NaCl and 35-70 °C. The OD stretch peak is deconvoluted into three peaks: (1) high frequency water, which represents a tightly bound microenvironment, (2) low frequency water, which represents a loosely bound microenvironment, and (3) bulk water. In general, the majority of water absorbed into the PEM exists in a bound state, with little-to-no bulk water observed. Increasing temperature slightly reduces the amount of absorbed water, while addition of salt increases the amount of absorbed water. Finally, a van't Hoff analysis is applied to estimate the enthalpy (11-22 kJ mol-1) and entropy (48-79 kJ mol-1 K-1) of water exchanging from low to high frequency states.

Viscoelastic Properties of Polyelectrolyte Multilayers Swollen with Ionic Liquid Solutions

Polyelectrolyte multilayers (PEM) obtained by layer-by-layer assembly can be doped with ionic liquid (IL) via the swelling of the films with IL solutions. In order to examine the mechanical properties of IL-containing PEM, we implement a Kelvin-Voigt model to obtain thickness, viscosity and elastic modulus from the frequency and dissipation shifts determined by a dissipative quartz crystal microbalance (QCM-D). We analyze the changes in the modeled thickness and viscoelasticity of PEI(PSS/PADMAC)4PSS and PEI(PSS/PAH)4PSS multilayers upon swelling by increasing the concentration of either 1-Ethyl-3-methylimidazolium chloride or 1-Hexyl-3-methylimidazolium chloride, which are water soluble ILs. The results show that the thickness of the multilayers changes monotonically up to a certain IL concentration, whereas the viscosity and elasticity change in a non-monotonic fashion with an increasing IL concentration. The changes in the modeled parameters can be divided into three concentration regimes of IL, a behavior specific to ILs (organic salts), which does not occur with swelling by simple inorganic salts such as NaCl. The existence of the regimes is attributed to a competition of the hydrophobic interactions of large hydrophobic ions, which enhance the layer stability at a low salt content, with the electrostatic screening, which dominates at a higher salt content and causes a film softening.

Solution Conditions Tune and Optimize Loading of Therapeutic Polyelectrolytes into Layer-by-Layer Functionalized Liposomes

Layer-by-layer (LbL) nanoparticles offer great potential to the field of drug delivery, where these nanocomposites have been studied for their ability to deliver chemotherapeutic agents, small molecule inhibitors, and nucleic acids. Most exciting is their ability to encapsulate multiple functional elements, which allow nanocarriers to deliver complex combination therapies with staged release. However, relative to planar LbL constructs, colloidal LbL systems have not undergone extensive systematic studies that outline critical synthetic solution conditions needed for robust and efficient assembly. The multistaged process of adsorbing a series of materials onto a nanoscopic template is inherently complex, and facilitating the self-assembly of these materials depends on identifying proper solution conditions for each synthetic step and adsorbed material. Here, we focus on addressing some of the fundamental questions that must be answered in order to obtain a reliable and robust synthesis of nucleic acid-containing LbL liposomes. This includes a study of solution conditions, such as pH, ionic strength, salt composition, and valency, and their impact on the preparation of LbL nanoparticles. Our results provide insight into the selection of solution conditions to control the degree of ionization and the electrostatic screening length to suit the adsorption of nucleic acids and synthetic polypeptides. The optimization of these parameters led to a roughly 8-fold improvement in nucleic acid loading in LbL liposomes, indicating the importance of optimizing solution conditions in the preparation of therapeutic LbL nanoparticles. These results highlight the benefits of defining principles for constructing highly effective nanoparticle systems.

Quantifying and controlling the cation uptake upon hydrated ionic liquid-induced swelling of polyelectrolyte multilayers

Controlling the uptake of specific ions in polyelectrolyte multilayers is of interest for various fields of application. Here, we quantify the amount of cation of an ionic liquid, namely 1,3-bis(cyanomethyl)imidazolium chloride, incorporated into polyelectrolyte multilayers upon contact with an ionic liquid solution. The ion partition equilibrium is determined depending on concentration in solution, employing attenuated total reflection infrared spectroscopy. Generating an excess charge in multilayers by post-preparative manipulation of their charge balance, one can control the incorporated amount. Three multilayer systems are assembled for this purpose, i.e., PSS/PDADMAC, PSS/PAH and PAA/PDADMAC, employing poly(styrene sulfonate) (PSS), poly(diallyldimethylammonium chloride) (PDADMAC), poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). The charge balance of the latter two films is manipulated by an external pH stimulus generating an excess positive or negative internal charge, respectively. The concentration of cations in PEM amounts to 30% to 100% of the bulk concentration and scales as PAA/PDADMAC > PSS/PDADMAC > PSS/PAH. Thus, post-preparative pH treatment may be a future tool to create ion-conductive polymer gel films with a desired concentration of small cations.

Bombardment induced ion transport - part IV: ionic conductivity of ultra-thin polyelectrolyte multilayer films

The dependence of the ionic conductance of ultra-thin polyelectrolyte multilayer (PEM) films on the temperature and the number of bilayers has been investigated by the recently developed low energy bombardment induced ion transport (BIIT) method. To this end multilayers of alternating poly(sodium 4-styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) layers were deposited on a metal electrode and subsequently bombarded by a low energy potassium ion beam. Ions are transported through the film according to the laws of electro-diffusion towards a grounded backside electrode. They are neutralized at the interface between the polymer film and the metal electrode. The detected neutralization current scales linearly with the acceleration potential of the ion beam indicating Ohmic behavior for the (PAH/PSS)x multilayer, where x denotes the number of bilayers. The conductance exhibits a non-monotonic dependence on the number of bilayers, x. For 2 ≤ x ≤ 8 the conductance increases non-linearly with the number of bilayers. For x ≥ 8 the conductance decreases with increasing number of bilayers. The variation of the conductance is rationalized by a model accounting for the structure dependence of the conductivity. The thinnest sample for which the conductance has been measured is the single bilayer reflecting properties dominated by the interface. The activation energy for the ion transport is 0.49 eV.

Highly scalable, closed-loop synthesis of drug-loaded, layer-by-layer nanoparticles

Layer-by-layer (LbL) self-assembly is a versatile technique from which multicomponent and stimuli-responsive nanoscale drug carriers can be constructed. Despite the benefits of LbL assembly, the conventional synthetic approach for fabricating LbL nanoparticles requires numerous purification steps that limit scale, yield, efficiency, and potential for clinical translation. In this report, we describe a generalizable method for increasing throughput with LbL assembly by using highly scalable, closed-loop diafiltration to manage intermediate purification steps. This method facilitates highly controlled fabrication of diverse nanoscale LbL formulations smaller than 150 nm composed from solid-polymer, mesoporous silica, and liposomal vesicles. The technique allows for the deposition of a broad range of polyelectrolytes that included native polysaccharides, linear polypeptides, and synthetic polymers. We also explore the cytotoxicity, shelf life and long-term storage of LbL nanoparticles produced using this approach. We find that LbL coated systems can be reliably and rapidly produced: specifically, LbL-modified liposomes could be lyophilized, stored at room temperature, and reconstituted without compromising drug encapsulation or particle stability, thereby facilitating large scale applications. Overall, this report describes an accessible approach that significantly improves the throughput of nanoscale LbL drug-carriers that show low toxicity and are amenable to clinically relevant storage conditions.

Ionic liquids as plasticizers for polyelectrolyte complexes

Uptake of salts by insoluble polyelectrolyte complexes (PECs) leads to plasticization, and here it is shown that ionic liquids (ILs) are more effective plasticizers than simple organic salts such as NaCl. The PEC uptake of IL cation was monitored by solution (1)H NMR, and the mechanical impacts of plasticization were tracked by dynamic mechanical analysis (DMA). PECs prepared with polystyrene sulfonate (PSS) and poly(diallyldimethylammonium chloride) (PDDA) under charge stoichiometric conditions were immersed in aqueous solutions of 1-butyl-3-methylimidazolium chloride [BMIM][Cl] to cause IL uptake, which could be controlled by the solution's IL concentration: higher concentration leads to higher uptake which leads to greater plasticization. The effectiveness of plasticization was assessed through the position and height of a DMA tan(δ) peak ascribed to a glassy-to-rubbery PEC transition. Consistent with greater PEC uptake, isothermal titration calorimetry demonstrated that solution binding by PSS of [BMIM](+) was much stronger than binding of Na(+).

Adsorption and viscoelastic analysis of polyelectrolyte-surfactant complexes on charged hydrophilic surfaces

The aggregation of surfactants around oppositely charged polyelectrolytes brings about a peculiar bulk phase behavior of the complex, known as coacervation, and can control the extent of adsorption of the polyelectrolyte at an aqueous-solid interface. Adsorption kinetics from turbid premixed polyelectrolyte-surfactant mixtures have been difficult to measure using optical techniques such as ellipsometry and reflectometry, thus limiting the correlation between bulk phases and interfacial adsorption. Here, we investigated the adsorption from premixed solutions of a cationic polysaccharide (PQ10) and the anionic surfactant sodium dodecyl sulfate (SDS) on an amphoteric alumina surface using quartz crystal microbalance with dissipation (QCMD). The surface charge on the alumina was tuned by changing the pH of the premixed solutions, allowing us to assess the role of electrostatic interactions by studying the adsorption on both negatively and positively charged surfaces. We observed a maximum extent of adsorption on both negatively and positively charged surfaces from a solution corresponding to the maximum turbidity. Enhanced adsorption upon diluting the redissolved complexes at a high SDS concentration was seen only on the negatively charged surface, and not on the positively charged one, confirming the importance of electrostatic interactions in controlling the adsorption on a hydrophilic charged surface. Using the Voight based viscoelastic model, QCMD also provided information on the effective viscosity, effective shear modulus, and thickness of the adsorbed polymeric complex. The findings of viscoelastic analysis, corroborated by atomic force microscopy measurements, suggest that PQ10 by itself forms a flat, uniform layer, rigidly attached to the surface. The PQ10-SDS complex shows a heterogeneous surface structure, where the underlayer is relatively compact and tightly attached and the top is a loosely bound diffused overlayer, accounting for most of the adsorbate, which gets washed away upon rinsing. Understanding of the surface structure will have important implications toward understanding lubrication.