Modeling dynamic swelling of polymer-based artificial muscles.

Polymer-based artificial muscles are lightweight, are flexible, can have variable stiffness, and provide actuation in applications in which heavy actuators are not feasible. Achieving device requirements, such as strain, strain rate, lifetime, achievable work, and efficiency, requires material and muscle geometry design. This study is motivated by the possibility of significant actuation from twisted and coiled polymer (TCP) fibers that rely on radial swelling to produce reversible work. Modeling the actuation of advanced polymers is essential for defining design metrics. An analytical thermodynamic expression based on Flory-Rehner Theory was combined with a numerical transport model in order to simulate transient swelling of a polymeric network driven by diffusion and migration. Radial swelling of polymer fibers was modeled, including parametric studies and comparison to experimental data. By increasing the transport distance, swelling is shown to increase the time to equilibrium, but this can be more than compensated by applying voltage to take advantage of ion migration/electroosmotic drag. This work indicates that, in addition to migration, dimensions smaller than 100 micrometers here are needed to achieve the sub-second response times of natural muscles. The impact of polymer swelling on transport in polymers is directly evaluated by locally accounting for the length increase of discrete elements due to solvent presence, which cannot be done analytically. Furthermore, strain and work done by swelling a TCP is modelled, and the benefit of anisotropic swelling and constant modulus is quantified.

Lignin-Based Solid Polymer Electrolytes: Lignin-Graft-Poly(ethylene glycol).

Lignin is an aromatic-rich biomass polymer that is cheap, abundant, and sustainable. However, its application in the solid electrolyte field is rare due to challenges in well-defined polymer synthesis. Herein, the synthesis of lignin-graft-poly(ethylene glycol) (PEG) and its conductivity test for a solid electrolyte application are demonstrated. The main steps of synthesis include functionalization of natural lignin's hydroxyl to alkene, followed by graft-copolymerization of PEG thiol to the lignin via photoredox thiol-ene reaction. Two lignin-graft-PEGs are prepared having 22 wt% lignin (lignin-graft-PEG 550) and 34 wt% lignin (lignin-graft-PEG 2000). Then, new polymer electrolytes for conductivity tests are prepared via addition of lithium bis-trifluoromethanesulfonimide. The polymer graft electrolytes exhibit ionic conductivity up to 1.4 × 10-4  S cm-1  at 35 °C. The presence of lignin moderately impacts conductivity at elevated temperature compared to homopolymer PEG. Furthermore, the ionic conductivity of lignin-graft-PEG at ambient temperature is significantly higher than homopolymer PEG precedents.

Gold Nanoparticle Monolayers with Tunable Optical and Electrical Properties.

Centimeter-scale gold nanoparticle (Au NP) monolayer films have been fabricated using a water/organic solvent self-assembly strategy. A recently developed approach, drain to deposit, is demonstrated to be most effective in transferring the Au NP films from the water/organic solvent interface to various solid substrates while maintaining their integrity. The interparticle spacing was tuned from 1.4 to 3.1 nm using alkylamine ligands of different lengths. The ordering of the films increased with increasing ligand length. The surface plasmon resonance and the in-plane electrical conductivity of the Au NP films both exhibit an exponential dependence on the interparticle spacing. These findings show great potential in scaling up the manufacturing of high-performance optical and electronic devices based on two-dimensional metallic nanoparticle superlattices.

Self-assembly of large-scale crack-free gold nanoparticle films using a 'drain-to-deposit' strategy.

Gold nanoparticles are widely studied due to the ease of controlled synthesis, facile surface modification, and interesting physical properties. However, a technique for depositing large-area, crack-free monolayers on solid substrates is lacking. Herein is presented a method for accomplishing this. Spherical gold nanoparticles were synthesized as an aqueous dispersion. Assembly into monolayers and ligand exchange occurred simultaneously at an organic/aqueous interface. Then the monolayer film was deposited onto arbitrary solid substrates by slowly pumping out the lower, aqueous phase. This allowed the monolayer film (and liquid-liquid interface) to descend without significant disturbance, eventually reaching substrates contained in the aqueous phase. The resulting macroscopic quality of the films was found to be superior to films transferred by Langmuir techniques. The surface plasmon resonance and Raman enhancement of the films were evaluated and found to be uniform across the surface of each film.

Detection of subsurface structures underneath dendrites formed on cycled lithium metal electrodes.

Failure caused by dendrite growth in high-energy-density, rechargeable batteries with lithium metal anodes has prevented their widespread use in applications ranging from consumer electronics to electric vehicles. Efforts to solve the lithium dendrite problem have focused on preventing the growth of protrusions from the anode surface. Synchrotron hard X-ray microtomography experiments on symmetric lithium-polymer-lithium cells cycled at 90 °C show that during the early stage of dendrite development, the bulk of the dendritic structure lies within the electrode, underneath the polymer/electrode interface. Furthermore, we observed crystalline impurities, present in the uncycled lithium anodes, at the base of the subsurface dendritic structures. The portion of the dendrite protruding into the electrolyte increases on cycling until it spans the electrolyte thickness, causing a short circuit. Contrary to conventional wisdom, it seems that preventing dendrite formation in polymer electrolytes depends on inhibiting the formation of subsurface structures in the lithium electrode.

Current-induced formation of gradient crystals in block copolymer electrolytes.

Conventional ordered phases such as crystals and liquid crystals have constant domain spacings. In this Letter, we report on the formation of coherently ordered morphologies wherein the domain spacing changes continuously along a specified direction. We have coined the term "gradient crystal" to refer to this structure, a signature of which is a small-angle X-ray scattering pattern that resembles a sundial. Gradient crystals composed of a gyroid morphology form spontaneously when ionic current is driven through a block copolymer electrolyte. We propose that this structure forms because it allows for a continuous change in domain spacing without requiring the introduction of defects. Previous studies have shown that applied electric fields ranging from 1000 to 40,000 V/mm can induce long-range structural order, alignment, and morphological transitions in block copolymers. Gradient crystals form under applied electric fields as low as 2.5 V/mm due to the presence of direct ionic currents that are absent in the aforementioned studies.

Thermal Gradient Infrared Spectroscopy for Diffusion in Polymers.

Time-resolved Fourier transform infrared-attenuated total reflectance spectroscopy (FTIR-ATR) was used to measure diffusion in opaque and translucent samples. FTIR-ATR was used to measure the change in the absorbance near the heated ATR crystal surface. The infrared absorbance was then related to the concentration through the Beer-Lambert law. The sample used is a polymer electrolyte composed of lithium bis-trifluoromethanesulfonylimide (LiTFSI) salt in a block copolymer polystyrene-poly(ethylene oxide) (SEO). A new approach to introduce concentration gradients is presented using a temperature gradient that creates a small salt concentration gradient due to thermally driven mass diffusion (the Soret effect). This first method was compared to a second method that we reported using two laminated polymer electrolyte films of different salt concentrations. The thermal gradient study (method 1) covered three temperature differences of 10, 15, and 20 °C, while the second study (method 2) used three average molar ratios across isothermal temperatures ranging from 80 to 120 °C. The benefits and limitations of the new approach are reported, as is the activation energy for salt diffusion in this and similar SEO electrolytes. Developing new techniques to measure diffusion coefficients effectively will aid in the development of a variety of devices, including solid-state batteries and thermogalvanic cells, that are able to convert waste heat into electricity and improve the efficiency of power-generating systems. FTIR-ATR overcomes previous limitations in experimental techniques measuring diffusion coefficients. The results prove that thermal gradient FTIR-ATR is an effective and repeatable approach for determining Fickian diffusion coefficients in viscoelastic solids.

Magnetic-Core/Gold-Shell Nanoparticles for the Detection of Hydrophobic Chemical Contaminants.

Magnetic-core/gold-shell nanoparticles (MAuNPs) are of interest for enabling rapid and portable detection of trace adulterants in complex media. Gold coating provides biocompatibility and facile functionalization, and a magnetic core affords analyte concentration and controlled deposition onto substrates for surface-enhanced Raman spectroscopy. Iron oxide cores were synthesized and coated with gold by reduction of HAuCl4 by NH2OH. MAuNPs were grafted with polyethylene glycol (PEG) and/or functionalized with 4-mercaptobenzoic acid (4-MBA) and examined using a variety of microscopic, spectroscopic, magnetometric, and scattering techniques. For MAuNPs grafted with both PEG and 4-MBA, the order in which they were grafted impacted not only the graft density of the individual ligands, but also the overall graft density. Significant Raman signal enhancement of the model analyte, 4-MBA, was observed. This enhancement demonstrates the functionality of MAuNPs in direct detection of trace contaminants. The magnetic deposition rate of MAuNPs in chloroform and water was explored. The presence of 4-MBA slowed the mass deposition rate, and it was postulated that the rate disparity originated from differing NP-substrate surface interactions. These findings emphasize the importance of ligand choice in reference to the medium, target analyte, and substrate material, as well as functionalization procedure in the design of similar sensing platforms.

Effect of Polystyrene Synthesis Method on Water Sorption and Glass Transition.

Commodity PS is synthesized via free radical polymerization, whereas PS in block copolymers (BCPs) is typically synthesized via living anionic polymerization. The purpose of this work is to investigate how the synthesis method impacts important properties such as water sorption and glass transition temperature (Tg). Water sorption is important because the performance of nanostructured polymer membranes in various applications is known to be affected by environmental conditions such as humidity. Tg is important because it dictates processing conditions, both for commodity PS as well as BCPs such as thermoplastic elastomers. Water sorption in commercial PS was found to be 0.5 mgwater/gpolymer at the highest humidities investigated (about 80%), in agreement with literature. On the other hand, syndiotactic PS synthesized anionically at low temperature absorbed more water, up to 1.5 mgwater/gpolymer, due to higher free volume. The greatest impact on water sorption was due to addition of hydrophilic hydroxyl chain ends to atactic PS, which resulted in water sorption of up to 2.3 mgwater/gpolymer. In addition to measuring water sorption and dry Tg separately, the impact of relative humidity on PS Tg was examined. Combined differential scanning calorimetry and dynamic mechanical analysis show that on going from the dry state to high humidity, the Tg of PS decreases by 5 °C. Moreover, the tensile storage modulus of PS decreases from 1.58 GPa at 0% RH to 0.53 GPa at 40% RH. In addition to the practical relevance of this study, this report fills a gap in experimental literature by using a poor solvent system, PS/water, to examine plasticization in the pure polymer limit.

Engineering Human Mesenchymal Bodies in a Novel 3D-Printed Microchannel Bioreactor for Extracellular Vesicle Biogenesis.

Human Mesenchymal Stem Cells (hMSCs) and their derived products hold potential in tissue engineering and as therapeutics in a wide range of diseases. hMSCs possess the ability to aggregate into "spheroids", which has been used as a preconditioning technique to enhance their therapeutic potential by upregulating stemness, immunomodulatory capacity, and anti-inflammatory and pro-angiogenic secretome. Few studies have investigated the impact on hMSC aggregate properties stemming from dynamic and static aggregation techniques. hMSCs' main mechanistic mode of action occur through their secretome, including extracellular vesicles (EVs)/exosomes, which contain therapeutically relevant proteins and nucleic acids. In this study, a 3D printed microchannel bioreactor was developed to dynamically form hMSC spheroids and promote hMSC condensation. In particular, the manner in which dynamic microenvironment conditions alter hMSC properties and EV biogenesis in relation to static cultures was assessed. Dynamic aggregation was found to promote autophagy activity, alter metabolism toward glycolysis, and promote exosome/EV production. This study advances our knowledge on a commonly used preconditioning technique that could be beneficial in wound healing, tissue regeneration, and autoimmune disorders.

Simultaneous electronic and ionic conduction in a block copolymer: application in lithium battery electrodes.

Charging ahead: separate values for the simultaneous electronic and ionic conductivity of a conjugated polymer containing poly(3-hexylthiophene) and poly(ethylene oxide) (P3HT-PEO) were determined by using ac impedance and dc techniques. P3HT-PEO was used as binder, and transporter of electronic charge and Li(+) ions in a LiFePO(4) cathode, which was incorporated into solid-state lithium batteries.

Lithium Salt Diffusion in Diblock Copolymer Electrolyte Using Fourier Transform Infrared Spectroscopy.

The diffusion of a lithium salt through a diblock copolymer electrolyte was studied using vibrational spectroscopy. Lithium bis-trifluoromethylsulfonimide (LiTFSI) was dissolved in a lamellar-structured, high-molecular-weight polystyrene-poly(ethylene oxide) diblock copolymer at various concentrations (0-4.51 molLiTFSI/kgPEO). The diffusion coefficient of LiTFSI was determined from time-resolved Fourier Transform infrared spectroscopy attenuated total reflectance (FTIR-ATR) as a function of the salt concentration. By the application of the Beer-Lambert law, FTIR-ATR was used to detect concentration changes. Mutual diffusion was driven by putting in contact two polymer electrolyte membranes with different salt concentrations. Thus, mutual diffusion coefficients were obtained without the influence of electric fields or electrode interfaces. The accuracy of the simple experimental approach and straightforward analysis was validated by comparison to diffusion coefficients reported from measurements in electrochemical cells. Both methods yield mutual diffusion coefficients of lithium salt that are only weakly (and nonmonotonically) dependent on salt concentration. There is some indication in the spectra that there exist two populations of salt with different dissociation states. This could explain the observed nonmonotonic concentration dependence of the mutual diffusion coefficient of the salt. This hypothesis will be examined quantitatively with complementary measurements in future work.

Diffusion of water in Nafion using time-resolved Fourier transform infrared-attenuated total reflectance spectroscopy.

Hydrogen fuel cells are attractive alternative power sources for applications such as transportation; however, fuel cell performance is a strong function of water equilibrium content and water sorption and desorption kinetics in polymer electrolyte membranes (e.g., Nafion). Although similar water sorption isotherms for Nafion have been reproduced in many laboratories, reported diffusion coefficients of water in Nafion vary by 4 orders of magnitude. In this study, sorption and desorption dynamics of water vapor in Nafion were measured as a function of water vapor activity and flow rate using time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. Both integral and differential experiments were performed, where integral experiments consisted of increasing the vapor activity from 0% RH to one of five values (22, 43, 56, 80, or 100% RH), while in differential experiments the activity was sequentially increased in smaller steps from 0 to 22 to 43 to 56 to 80 to 100% RH. For integral experiments, non-Fickian behavior was observed at both low and high vapor activities, while Fickian behavior was observed at moderate vapor activities. For differential experiments, Fickian behavior was observed at all vapor activities except at low vapor activities (0-22% RH). Sorption kinetics was found to be a function of flow rate, where mass transfer resistance at the vapor/polymer interface was significant at low flow rates but was insignificant at high flow rates. Accurate sorption and desorption diffusion coefficients were calculated in this study (measured at high flow rates with no mass transfer resistance) and were similar, on the order of 10(-7) cm(2)/s, and weak functions of water vapor activity.

Interferometric time- and energy-resolved photoemission electron microscopy for few-femtosecond nanoplasmonic dynamics.

We report a time-resolved normal-incidence photoemission electron microscope with an imaging time-of-flight detector using ∼7-fs near-infrared laser pulses and a phase-stabilized interferometer for studying ultrafast nanoplasmonic dynamics via nonlinear photoemission from metallic nanostructures. The interferometer's stability (35 ± 6 as root-mean-square from 0.2 Hz to 40 kHz) as well as on-line characterization of the driving laser field, which is a requirement for nanoplasmonic near-field reconstruction, is discussed in detail. We observed strong field enhancement and few-femtosecond localized surface plasmon lifetimes at a monolayer of self-assembled gold nanospheres with ∼40 nm diameter and ∼2 nm interparticle distance. A wide range of plasmon resonance frequencies could be simultaneously detected in the time domain at different nanospheres, which are distinguishable already within the first optical cycle or as close as about ±1 fs around time-zero. Energy-resolved imaging (microspectroscopy) additionally revealed spectral broadening due to strong-field or space charge effects. These results provide a clear path toward visualizing optically excited nanoplasmonic near-fields at ultimate spatiotemporal resolution.

Electrolyte Solvation Structure at Solid-Liquid Interface Probed by Nanogap Surface-Enhanced Raman Spectroscopy.

Understanding the fundamental factors that drive ion solvation structure and transport is key to design high-performance, stable battery electrolytes. Reversible ion solvation and desolvation are critical to the interfacial charge-transfer process across the solid-liquid interface as well as the resulting stability of the solid electrolyte interphase. Herein, we report the study of Li+ salt solvation structure in aprotic solution in the immediate vicinity (∼20 nm) of the solid electrode-liquid interface using surface-enhanced Raman spectroscopy (SERS) from a gold nanoparticle (Au NP) monolayer. The plasmonic coupling between Au NPs produces strong electromagnetic field enhancement in the gap region, leading to a 5 orders of magnitude increase in Raman intensity for electrolyte components and their mixtures namely, lithium hexafluorophosphate, fluoroethylene carbonate, ethylene carbonate, and diethyl carbonate. Further, we estimate and compare the lithium-ion solvation number derived from SERS, standard Raman spectroscopy, and Fourier transform infrared spectroscopy experiments to monitor and ascertain the changes in the solvation shell diameter in the confined nanogap region where there is maximum enhancement of the electric field. Our findings provide a multimodal spectroscopic approach to gain fundamental insights into the molecular structure of the electrolyte at the solid-liquid interface.

Diffusion and sorption of methanol and water in Nafion using time-resolved Fourier transform infrared-attenuated total reflectance spectroscopy.

Direct methanol fuel cells (DMFCs) are promising portable power sources. However, their performance diminishes significantly because of high methanol crossover (flux) in the polymer electrolyte membrane (e.g., Nafion 117) at the desired stoichiometric methanol feed concentration. In this study, the diffusion and sorption of methanol and water in Nafion 117 were measured using time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. This technique is unique because of its ability to measure multicomponent diffusion and sorption within a polymer on a molecular level in real time as function of concentration. Both the effective mutual diffusion coefficients and concentrations of methanol and water in Nafion 117 were determined with time-resolved FTIR-ATR spectroscopy as a function of methanol solution concentration. The methanol flux, calculated from FTIR-ATR, matched that determined from a conventional technique (permeation cell) and increased by almost 3 orders of magnitude over the methanol solution concentration range studied (0.1-16 M). Furthermore, the data obtained in this study reveal that the main contribution to the increase in methanol flux is due to methanol sorption in the membrane.

Self-Assembly of Large Gold Nanoparticles for Surface-Enhanced Raman Spectroscopy.

Performance of portable technologies from mobile phones to electric vehicles is currently limited by the energy density and lifetime of lithium batteries. Expanding the limits of battery technology requires in situ detection of trace components at electrode-electrolyte interphases. Surface-enhance Raman spectroscopy could satisfy this need if a robust and reproducible substrate were available. Gold nanoparticles (Au NPs) larger than 20 nm diameter are expected to greatly enhance Raman intensity if they can be assembled into ordered monolayers. A three-phase self-assembly method is presented that successfully results in ordered Au NP monolayers for particle diameters ranging from 13 to 90 nm. The monolayer structure and Raman enhancement factors (EFs) are reported for a model analyte, rhodamine, as well as the best performing polymer electrolyte salt, lithium bis(trifluoromethane)sulfonimide. Experimental EFs for the most part correlate with predictions based on monolayer geometry and with numerical simulations that identify local electromagnetic field enhancements. The EFs for the best performing Au NP monolayer are between 106 and 108 and give quantitative signal response when analyte concentration is changed.

Molecular Patterning and Directed Self-Assembly of Gold Nanoparticles on GaAs.

The ability to create micro-/nanopatterns of organic self-assembled monolayers (SAMs) on semiconductor surfaces is crucial for fundamental studies and applications in a number of emerging fields in nanoscience. Here, we demonstrate the direct patterning of thiolate SAMs on oxide-free GaAs surface by dip-pen nanolithography (DPN) and microcontact printing (µCP), facilitated by a process of surface etching and passivation of the GaAs. A quantitative analysis on the molecular diffusion on GaAs was conducted by examining the writing of nanoscale dot and line patterns by DPN, which agrees well with surface diffusion models. The functionality of the patterned thiol molecules was demonstrated by directed self-assembly of gold nanoparticles (Au NPs) onto a template of 4-aminothiophenol (ATP) SAM on GaAs. The highly selective assembly of the Au NPs was made evident with atomic force microscopy (AFM) and scanning electron microscopy (SEM). The ability to precisely control the assembly of Au NPs on oxide-free semiconductor surfaces using molecular templates may lead to an efficient bottom-up method for the fabrication of nanoplasmonic structures.

Gold Nanoparticle Monolayers from Sequential Interfacial Ligand Exchange and Migration in a Three-Phase System.

Using a three-phase system, centimeter-scale monolayer gold nanoparticle (Au NP) films have been prepared that have long-range order and hydrophobic ligands. The system contains an interface between an aqueous phase containing Au NPs and an oil phase containing one of various types of amine ligands, and a water/air interface. As the Au NPs diffuse to the water/oil interface, ligand exchange takes place which temporarily traps them at the water/oil interface. The ligand-exchanged particles then spontaneously migrate to the air/water interface, where they self-assemble, forming a monolayer under certain conditions. The spontaneous formation of the NP film at the air/water interface was due to the minimization of the system Helmholtz free energy. However, the extent of surface functionalization was dictated by kinetics. This decouples interfacial ligand exchange from interfacial self-assembly, while maintaining the simplicity of a single system. The interparticle center-to-center distance was dictated by the amine ligand length. The Au NP monolayers exhibit tunable surface plasma resonance and excellent spatial homogeneity, which is useful for surface-enhanced Raman scattering. The "air/water/oil" self-assembly method developed here not only benefits the fundamental understanding of NP ligand conformations, but is also applicable to the manufacture of plasmonic nanoparticle devices with precisely designed optical properties.

A convenient phase transfer protocol to functionalize gold nanoparticles with short alkylamine ligands.

HYPOTHESIS: Aqueous citrate-stabilized gold nanoparticles (Au NPs) cannot be directly transferred from water to an immiscible organic solution using short alkyl ligands. However, Au NPs can be transferred from water to a water-organic interface if chemical and mechanical inputs are used to modify the interfacial energy and interfacial area. Ligand exchange can then take place at this interface. After separating the particles from the liquids, they can be transferred to a different organic phase. EXPERIMENTS: Hexane, alkylamine, and acetone were added to aqueous citrate-stabilized Au NPs to form a film at the system interfaces. After removing the liquid phases, Au NPs were readily redispersed into tetrahydrofuran (THF). The size and shape of the transferred Au NPs were evaluated by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). FINDINGS: Au NPs with 13nm diameter are readily segregated from water with the aid of short alkylamine ligands. They form a thin film at the water/organic solvent interface, rendering them easy to separate from the liquid phases and possible to redisperse into another organic solvent. After the phase transfer process, Au NPs were functionalized with short amine ligands. In addition, the shape and size of Au NPs were preserved. The short amine-protected Au NPs in THF can stay stable for up to 27days or longer.