Capacitance of thin films containing polymerized ionic liquids

Electrode-polymer interfaces dictate many of the properties of thin films such as capacitance, the electric field experienced by polymers, and charge transport. However, structure and dynamics of charged polymers near electrodes remain poorly understood, especially in the high concentration limit representative of the melts. To develop an understanding of electric field–induced transformations of electrode-polymer interfaces, we have studied electrified interfaces of an imidazolium-based polymerized ionic liquid (PolyIL) using combinations of broadband dielectric spectroscopy, specular neutron reflectivity, and simulations based on the Rayleigh’s dissipation function formalism. Overall, we obtained the camel-shaped dependence of the capacitance on applied voltage, which originated from the responses of an adsorbed polymer layer to applied voltages. This work provides additional insights related to the effects of molecular weight in affecting structure and properties of electrode-polymer interfaces, which are essential for designing next-generation energy storage and harvesting devices

Ionically Paired Layer-by-Layer Hydrogels: Water and Polyelectrolyte Uptake Controlled by Deposition Time

Despite intense recent interest in weakly bound nonlinear (“exponential”) multilayers, the underlying structure-property relationships of these films are still poorly understood. This study explores the effect of time used for deposition of individual layers of nonlinearly growing layer-by-layer (LbL) films composed of poly(methacrylic acid) (PMAA) and quaternized poly-2-(dimethylamino)ethyl methacrylate (QPC) on film internal structure, swelling, and stability in salt solution, as well as the rate of penetration of invading polyelectrolyte chains. Thicknesses of dry and swollen films were measured by spectroscopic ellipsometry, film internal structure—by neutron reflectometry (NR), and degree of PMAA ionization—by Fourier-transform infrared spectroscopy (FTIR). The results suggest that longer deposition times resulted in thicker films with higher degrees of swelling (up to swelling ratio as high as 4 compared to dry film thickness) and stronger film intermixing. The stronger intermixed films were more swollen in water, exhibited lower stability in salt solutions, and supported a faster penetration rate of invading polyelectrolyte chains. These results can be useful in designing polyelectrolyte nanoassemblies for biomedical applications, such as drug delivery coatings for medical implants or tissue engineering matrices

Time-of-flight Bragg scattering from aligned stacks of lipid bilayers using the Liquids Reflectometer at the Spallation Neutron Source

Time-of-flight (TOF) neutron diffraction experiments on aligned stacks of lipid bilayers using the horizontal Liquids Reflectometer at the Spallation Neutron Source are reported. Specific details are given regarding the instrumental setup, data collection and reduction, phase determination of the structure factors, and reconstruction of the one-dimensional neutron scattering length density (NSLD) profile. The validity of using TOF measurements to determine the one-dimensional NSLD profile is demonstrated by reproducing the results of two well known lipid bilayer structures. The method is then applied to show how an antimicrobial peptide affects membranes with and without cholesterol.Peer reviewed: YesNRC publication: Ye

Nonlinear Layer-by-Layer Films: Effects of Chain Diffusivity on Film Structure and Swelling

We report on the role of molecular diffusivity in the formation of nonlinearly growing polyelectrolyte multilayers (<i>nl</i>PEMs). Electrostatically bound polyelectrolyte multilayers were assembled from poly­(methacrylic acid) (PMAA) as a polyanion and quaternized poly­(2-(dimethyl­amino)­ethyl methacrylate) (QPC) as a polycation. Film growth as measured by ellipsometry was strongly dependent on the time allowed for each polymer deposition step, suggesting that the diffusivities of the components are crucial in controlling the rate of film growth. Uptake of polyelectrolytes within <i>nl</i>PEMs was relatively slow and occurred on time scales ranging from minutes to hours, depending on the film thickness. Spectroscopic ellipsometry measurements with <i>nl</i>PEM films exposed to aqueous solutions exhibited high (severalfold) degrees of film swelling and different swelling values for films exposed to QPC or PMAA solutions. FTIR spectroscopy showed that the average ionization of film-assembled PMAA increased upon binding of QPC and decreased upon binding of PMAA, in agreement with the charge regulation mechanism for weak polyelectrolytes. The use of neutron reflectometry (NR) enabled quantification of chain intermixing within the film, which was drastically enhanced when longer times were allowed for polyelectrolyte deposition. Diffusion coefficients of the polycation derived from the uptake rates of deuterated chains within hydrogenated films were of the order of 10^–14 cm²/s, i.e., 5–6 orders of magnitude smaller than those found for diffusion of free polymer chains in solution. Exchange of the polymer solutions to buffer inhibited film intermixing. Taken together, these results contribute to understanding the mechanism of the growth of nonlinear polyelectrolyte multilayers and demonstrate the possibility of controlling film intermixing, which is highly desirable for potential future applications

Controlling Mechanical Properties of Poly(methacrylic acid) Multilayer Hydrogels via Hydrogel Internal Architecture

Hydrogel materials are crucial in many applications due to their versatility and ability to mimic biological tissues. While manipulating bulk hydrogel cross-link density, polymer content, chemical composition, and microporosity has been a main approach to controlling hydrogel rigidity, altering the internal organization of hydrogel materials through chain intermixing and stratification can bring finer control over hydrogel properties, including mechanical responses. We report on altering the mechanical properties of ultrathin poly(methacrylic acid) (PMAA) multilayer hydrogels by controlling the internal organization of the PMAA network. PMAA multilayer hydrogels were synthesized by cross-linking PMAA layers in poly(N-vinylpyrrolidone) (PVPON)/PMAA hydrogen-bonded multilayer templates prepared by dipped or spin-assisted (SA) layer-by-layer assembly using sacrificial PVPON interlayers with molecular weights of 40,000 or 280,000 g mol–1. The effect of PVPON molecular weight on PMAA hydrogel stratification and network swelling and hydration was assessed by in situ spectroscopic ellipsometry and neutron reflectometry (NR). In a new NR modeling of polymer intermixing, we have inferred nanoscopic structure and water distribution within the ultrathin-layered films from measured continuum neutron scattering length density (SLD) and related those to the mechanical properties of the hydrogel films. We have found that hydrogel swelling, the number of water molecules associated with the swollen hydrogel, and water density within the SA PMAA hydrogels can be controlled by choosing low- or high-Mw PVPON. While cross-link densities determined by ATR-FTIR were similar, greater swelling and hydration at pH > 5 were observed for SA PMAA hydrogels synthesized using higher-Mw PVPON. The enhanced swelling of these SA hydrogels resulted in softening with a lower Young’s modulus at pH > 5 as measured by colloidal probe atomic force microscopy (AFM). The effect of PMAA layer intermixing on hydrogel mechanical properties was also compared for dipped and SA (PMAA) multilayer hydrogels of similar thickness and cross-linking degree. Despite similar values of gigapascal-range Young’s modulus for dry PMAA multilayer hydrogel films, an almost twice greater softening of the SA (PMAA) hydrogel compared to that prepared by dipping was observed, with Young’s modulus values decreasing to tens of megapascals in solution at pH > 5. Our study demonstrates that, unlike simply changing bulk hydrogel cross-link density, programming polymer network architecture via controlling the nanostructured organization of SA PMAA hydrogels enables selective modulation of the cross-link density within hydrogel strata. Control of polymer chain intermixing through hydrogel stratification offers a framework for synthesizing materials with finely tuned hydrogel internal structures, enabling precise control of such physical properties as the internal architecture, hydrogel swelling, surface morphology, and mechanical response, which are critical for the application of these materials in sensing, drug delivery, and tissue engineering

Architecture of Hydrated Multilayer Poly(methacrylic acid) Hydrogels: The Effect of Solution pH

We report on the evolution of the internal structure of dry and hydrated poly(methacrylic acid) (PMAA) hydrogels by quantifying the extent of layer interdiffusion in hydrogen-bonded (HB) films and upon subsequent cross-linking and hydration. These hydrogels are produced by ethylenediamine (EDA)-assisted cross-linking of PMAA in spin-assisted (SA) and dipped HB PMAA/poly(N-vinylpyrrolidone) (PVPON) multilayers followed by complete release of PVPON at pH 8 due to severing of hydrogen bonds with the PMAA network. Internal hydrogel architecture was monitored by neutron reflectometry using deuterated dPMAA marker layers. We found that even in the highly stratified SA HB films, layer interdiffusion extends over three (PMAA/PVPON) bilayers. Cross-linking of this film induces marker layer interpenetration more deeply into the surrounding material, extending over five layers. The volume fraction of dPMAA at the nominal center of a marker layer decreased from 0.65 to 0.51 after cross-linking. Hydrated SA hydrogels preserve well-organized layering and exhibit a persistent differential swelling with two distinct swelling ratios corresponding to MAA cross-link-rich and cross-link-poor strata. In contrast, layer organization in dipped films decays rapidly with distance from the silicon substrate. Both types of hydrogel swelled by factors of two and four times their dry total thicknesses at pH 5 and 7, respectively, and exhibited elevated surface roughness upon hydration. To fit the neutron reflectometry data, a self-consistent model was developed wherein the amount of PMAA initially deposited was preserved through subsequent chemical modification and hydration. Our results open opportunities for the development of thin hydrogels with a regulated structure, which can be utilized for efficient sensing, protection, activation, and rapid response in an aqueous environment. The internal morphological hierarchy of these multilayer hydrogels affords a means of fine-tuning their response to pH, temperature, or light to a degree rarely possible for randomly cross-linked responsive networks or brushes