Deep Eutectic Solvents: A Review of Fundamentals and Applications.

Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure-property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments. This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.

The interplay of thermodynamics and kinetics: imparting hierarchical control over film formation of self-stratified blends.

Spin casting has become an attractive method to fabricate polymer thin films found in organic electronic devices such as field-effect transistors, and light emitting diodes. Many studies have shown that altering spin casting parameters can improve device performance, which has been directly correlated to the degree of polymer alignment, crystallinity, and morphology of the thin film. To provide a thorough understanding of the balance of thermodynamic and kinetic factors that influence the stratification of polymer blend thin films, we monitor stratified polymer blend thin films developed from poly(3-hexylthiophene-2,5-diyl) and poly(methyl methacrylate) blends at controlled loading ratios, relative molecular weights, and casting speed. The structures of these thin films were characterized via neutron reflectivity, and the results show that at the fastest casting speed, polymer-polymer interactions and surface energy of the polymers in the blend dictate the final film structure, and at the slowest casting speed, there is less control over the film layering due to the polymer-polymer interactions, surface energy, and entropy simultaneously driving stratification. As well, the relative solubility limits of the polymers in the pre-deposition solution play a role in the stratification process at the slowest casting speed. These results broaden the current understanding of the relationship between spin casting conditions and vertical phase separation in polymer blend thin films and provide a foundation for improved rational design of polymer thin film fabrication processes to attain targeted stratification, and thus performance.

The importance of solvent quality on the modification of conjugated polymer conformation and thermodynamics with illumination.

Device efficiency in key organic electronic devices such as organic photovoltaics, field transistors, and light emitting diodes has long been known to be closely tied to the conformation of the conjugated polymer chains which make up the active layers. Our previous results show that light exposure can have a profound effect on the structure and assembly of these optoelectronic materials in solution. In order to advance our understanding of the role which solvent quality plays in this phenomenon, we have further studied the modulation of these illumination dependent structural changes on the key benchmark conjugated polymers P3HT and MEH-PPV as a function of solvent quality over a wide range of polymer solubilities. Analysis of this data indicates that use of poorer conjugated polymer solvents ultimately results in larger absolute alterations to polymer conformation, denoting the crucial role which solution thermodynamics plays in this generic effect. This discovery opens the door to controlling final device morphology through careful manipulation of solvent composition during solution based device casting techniques, moving our efforts closer to the development of a powerful, non-destructive, and tunable method for light-driven control of polymer conformation in novel light-responsive organic materials.

Tunable synthetic control of soft polymeric nanoparticle morphology.

With a growing variety of nanoparticles available, research probing the influence of particle deformability, morphology, and topology on the behavior of all polymer nanocomposites is also increasing. In particular, the behavior of soft polymeric nanoparticles in polymer nanocomposites has displayed unique behavior, but their precise performance depends intimately on the internal structure and morphology of the nanoparticle. With the goal of providing guidelines to control the structure and morphology of soft polymeric nanoparticles, we have examined monomer starved semi-batch nano-emulsion polymerizations that form organic, soft nanoparticles, to correlate the precise structure of the nanoparticle to the rate of monomer addition and crosslinking density. The synthesis method produces 5-20 nm radii polystyrene nanoparticles with tunable morphologies. We report small angle neutron scattering (SANS) results that correlate synthetic conditions to the structural characteristics of soft polystyrene nanoparticles. These results show that the measured molecular weight of the nanoparticles is controlled by the monomer addition rate, the total nanoparticle radius is controlled by the excess surfactant concentration, and the crosslinking density has a direct effect on the topology of each nanoparticle. These studies thus provide pathways to control these 3 structural characteristics of the nanoparticle. This research, therefore provides a conduit to thoroughly investigate the effect of structural features of soft nanoparticles on their individual properties and those of their polymer nanocomposites.

Diffusion of copolymers composed of monomers with drastically different friction factors in copolymer/homopolymer blends.

Copolymers are commonly used as interface modifiers that allow for the compatibilization of polymer components in a blend. For copolymers to function as a compatibilizer, they must diffuse through the matrix of the blend to the interface between the two blend components. The diffusivity of a copolymer in a blend matrix therefore becomes important in determining good candidates for use as compatibilizers. In this work, coarse-grained Monte Carlo simulations using the bond fluctuation model modified with an overlap penalty have been developed to study the diffusive behavior of PS/PMMA random copolymers in a PMMA homopolymer blend. The simulations vary the connectivity between different monomers, the thermodynamic interactions between the monomers which manifest within a chain, and between copolymer and homopolymer matrix and define the monomer friction coefficient of each component independently, allowing for the determination of the combined effect of these parameters on copolymer chain diffusion. The results of this work indicate that PS-r-PMMA copolymer diffusion is not linearly dependent on the copolymer composition on a logarithmic scale, but its diffusion is a balance of the kinetics governed by the dominant motion of the faster styrene monomers and thermodynamics, which are governed by the concentration of styrene monomer within a given monomer's local volume.

Unraveling the Mechanism of Nanoscale Mechanical Reinforcement in Glassy Polymer Nanocomposites.

The mechanical reinforcement of polymer nanocomposites (PNCs) above the glass transition temperature, Tg, has been extensively studied. However, not much is known about the origin of this effect below Tg. In this Letter, we unravel the mechanism of PNC reinforcement within the glassy state by directly probing nanoscale mechanical properties with atomic force microscopy and macroscopic properties with Brillouin light scattering. Our results unambiguously show that the "glassy" Young's modulus in the interfacial polymer layer of PNCs is two-times higher than in the bulk polymer, which results in significant reinforcement below Tg. We ascribe this phenomenon to a high stretching of the chains within the interfacial layer. Since the interfacial chain packing is essentially temperature independent, these findings provide a new insight into the mechanical reinforcement of PNCs also above Tg.

Unexpected Molecular Weight Effect in Polymer Nanocomposites.

The properties of the interfacial layer between the polymer matrix and nanoparticles largely determine the macroscopic properties of polymer nanocomposites (PNCs). Although the static thickness of the interfacial layer was found to increase with the molecular weight (MW), the influence of MW on segmental relaxation and the glass transition in this layer remains to be explored. In this Letter, we show an unexpected MW dependence of the interfacial properties in PNC with attractive polymer-nanoparticle interactions: the thickness of the interfacial layer with hindered segmental relaxation decreases as MW increases, in sharp contrast to theoretical predictions. Further analyses reveal a reduction in mass density of the interfacial layer with increasing MW, which can elucidate these unexpected dynamic effects. Our observations call for a significant revision of the current understandings of PNCs and suggest interesting ways to tailor their properties.

3D reconstruction of carbon nanotube networks from neutron scattering experiments.

Structure reconstruction from statistical descriptors, such as scattering data obtained using x-rays or neutrons, is essential in understanding various properties of nanocomposites. Scattering based reconstruction can provide a realistic model, over various length scales, that can be used for numerical simulations. In this study, 3D reconstruction of a highly loaded carbon nanotube (CNT)-conducting polymer system based on small and ultra-small angle neutron scattering (SANS and USANS, respectively) data was performed. These light-weight and flexible materials have recently shown great promise for high-performance thermoelectric energy conversion, and their further improvement requires a thorough understanding of their structure-property relationships. The first step in achieving such understanding is to generate models that contain the hierarchy of CNT networks over nano and micron scales. The studied system is a single walled carbon nanotube (SWCNT)/poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate) ( PEDOT: PSS). SANS and USANS patterns of the different samples containing 10, 30, and 50 wt% SWCNTs were measured. These curves were then utilized to calculate statistical two-point correlation functions of the nanostructure. These functions along with the geometrical information extracted from SANS data and scanning electron microscopy images were used to reconstruct a representative volume element (RVE) nanostructure. Generated RVEs can be used for simulations of various mechanical and physical properties. This work, therefore, introduces a framework for the reconstruction of 3D RVEs of high volume faction nanocomposites containing high aspect ratio fillers from scattering experiments.

Elucidating the assembly of nanoparticle organic hybrid materials (NOHMs) near an electrode interface with varying potential using neutron reflectivity.

A critical concern regarding electrolyte formulation in an electrochemical environment is the impact of the interaction of the multiple components (i.e., supporting electrolyte or additive) with the electrode surface. Recently, liquid-like neat Nanoparticle Organic Hybrid Materials (NOHMs) have been considered as an electrolyte component to improve the transport of redox-active species to the electrode surface. However, the structure and assembly of the NOHMs near the electrode surface is unknown and could significantly impact the electrode-electrolyte interface. Hence, we have investigated the depth profile of polyetheramine (HPE) polymer and NOHM-I-HPE (nanoparticles with ionically bonded HPE polymer) in deuterated water (D2O) in the presence of two different salts (KHCO3 and ZnCl2) near two different electrode surfaces using neutron reflectometry. Moreover, the depth profile of the NOHM-I-HPE near the electrode surface in a potential has also been studied with in situ reflectivity experiments. Our results indicate that a change in the chemical structure/hydrophilicity of the electrode surface does not significantly impact the ordering of HPE polymer or NOHM-I-HPE near the surface. This study also indicates that the NOHM-I-HPE particles form a clear layer near the electrode surface immediately above an adsorbed layer of free polymer on the electrode surface. The addition of salt does not impact the layering of NOHM-I-HPE, though it does alter the conformation of the polymer grafted to the nanoparticle surface and free polymer sequestered near the surface. Finally, the application of negative potential results in an increased amount of free polymer near the electrode surface. Correlating the depth profile of free polymer and NOHM-I-HPE particles with the electrochemical performance indicates that this assembly of free polymer near the electrode surface in NOHM-I-HPE solutions contributes to the higher current density of the system. Therefore, this holistic study offers insight into the importance of the assembly of NOHM-I-HPE electrolyte and free polymer near the electrode surface in an electrochemical milieu on its performance.

When does a macromolecule transition from a polymer chain to a nanoparticle?

Frequently, the defining characteristic of a nanoparticle is simply its size, where objects that are 1-100 nm are characterized as nanoparticles. However, synthetic and biological macromolecules, in particular high molecular weight chains, can satisfy this size requirement without providing the same phenomena as one would expect from a nanoparticle. At the same time, soft polymer nanoparticles are important in a broad range of fields, including understanding protein folding, drug delivery, vitrimers, catalysis and nanomedicine. Moreover, the recent flourish of all polymer nanocomposites has led to the synthesis of soft all-polymer nanoparticles, which emerge from internal crosslinking of a macromolecule. Thus, there exists a transition of an internally crosslinked macromolecule from a polymer chain to a nanoparticle as the amount of internal crosslinks increases, where the polymer chain exhibits different behavior than the nanoparticle. Yet, this transition is not well understood. In this work, we seek to address this knowledge gap and determine the transition of a macromolecule from a polymer chain to a nanoparticle as internal crosslinking increases. In this work, small angle neutron scattering (SANS) offers insight into the structure of polystyrene and poly(ethyl hexyl methacrylate) nanostructures in dilute solutions, with crosslinking densities that vary from 0.1 to 10.7%. Analyses of the SANS data provides structural characteristics to classify a nanostructure as chain-like or particle-like and identify a crosslinking dependent transition between the two morphologies. It was found that for both types of polymeric nanostructures, a crosslinking density of 0.81% (∼ a crosslink for every 1 in 125 monomers) or higher exhibit clear particle-like behavior. Lower crosslinking density nanostructures showed amounts of collapse similar to that of a star polymer (0.1% XL) or a random walk polymer chain (0.4% XL). Thus, the transition of an internally crosslinked macromolecule from a polymer chain to a nanoparticle is not an abrupt transition but occurs via the gradual contraction of the chain with incorporated crosslinks.

Microemulsions as Emerging Electrolytes: The Correlation of Structure to Electrochemical Response.

We describe the structural studies of microemulsions (µEs) prepared from water, toluene, butanol, and polysorbate 20 (PS20) that we recently used as electrolytes. Small-angle neutron scattering was used to monitor the development of the bicontinuous system as a function of the water-to-surfactant mass ratio on a constant oil-to-surfactant dilution line, revealing how the domain size, correlation length, amphiphilicity factor, and bending moduli change with composition. Kratky and Porod analyses are also employed, providing further structural detail of the scattering domains. We demonstrate that controlling the water-to-surfactant ratio with a constant oil-to-surfactant dilution affects the bicontinuous phase, reveals a sizeable compositional region with structural similarities, and provides insight into the correlation of structure to physical properties. Voltammetric results are presented to examine how the evolution of the bicontinuous structure formed in a µE prepared from water, toluene, butanol, and PS20 contributes to the electrochemical response. These findings, therefore, provide essential information that will guide the formulation of µEs as electrolytes for energy storage.

Evolution of microscopic heterogeneity and dynamics in choline chloride-based deep eutectic solvents.

Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. Predictive understanding of the fundamental correlations between local structure and macroscopic properties is needed to exploit the large design space and tunability of DESs for specific applications. Here, we employ a range of computational and experimental techniques that span length-scales from molecular to macroscopic and timescales from picoseconds to seconds to study the evolution of structure and dynamics in model DESs, namely Glyceline and Ethaline, starting from the parent compounds. We show that systematic addition of choline chloride leads to microscopic heterogeneities that alter the primary structural relaxation in glycerol and ethylene glycol and result in new dynamic modes that are strongly correlated to the macroscopic properties of the DES formed.

Polymer loop formation on a functionalized hard surface: quantitative insight by comparison of experimental and Monte Carlo simulation results.

Polystyrene terminated with carboxylic acid end groups (telechelic polymer) was grafted from the melt onto a silicon wafer that contained a monolayer of epoxy groups. Ellipsometry and fluorimetry were employed to monitor the kinetics of the grafting and loop formation, respectively. These results are quantitatively correlated with bond fluctuation Monte Carlo (BFMC) simulations that model the grafting and loop formation process. The quantitative correlation found between experiment and simulation provides unique insight into the process of polymer loop formation. Specifically, this correlation provides a calibration of the fluorescence intensity to the amount of singly bound chains present on the surface, revealing that about 80% of the bound chains form loops on the surface at the longest reaction time studied, and provides the time evolution of singly and doubly bound chains during the reaction. Moreover, this correlation is broadly applicable and can be used to readily monitor the impact of a broad range of reaction conditions (e.g., temperature, telechelic concentration, surface density of functional groups) on the loop formation process. This correlation, therefore, provides a method to access fundamental information that is not accessible by experiment alone and yet is required to tailor surface properties through adjusting the coverage and fraction of loops in the grafted layer and to correlate surface-sensitive properties to specific grafted layer structure.

Quantitative Evaluation of the Hierarchical Porosity in Polyimide Aerogels and Corresponding Solvated Gels.

Aerogels are promising materials for many aerospace applications, including high-performance antennae and flexible insulation, because of their inherent low density and high surface areas. Polymer aerogels, especially polyimide aerogels, provide excellent mechanical properties beyond traditional silica aerogels while maintaining the required thermal stability. Polyimide aerogel surface area, porosity, and pore volume are important properties; however, these measurements are traditionally conducted on the aerogel after removal of the solvent. Because of this, the impact of synthetic control and solvent presence on the nanoscale to mesoscale structure of polyimide aerogels in functional applications is unclear. In this report, we use small-angle neutron scattering to determine the dry and solvated skeletal strut size and composition of polyimide aerogels to deduce the impact of solvation on the structure of complex aerogel struts. Our results show that the aerogel contains a hierarchical assembly of pores, with pores present both within and between the supporting struts. This translates to a material with solvent in the larger pores, as well as absorbed in the supporting polyimide skeleton. The amount of solvent uptake in the struts varies with the solvent and polyimide properties. The insight from these results provides pathways to determine the correlations between aerogel nano- and mesoscale structural characteristics, fabrication processes, and their performance in functional applications such as polymeric battery separators. These results also broaden the characterization tools of polymeric aerogels that differentiate between dry and solvated nano- and mesoscale structures that exist in common operating conditions.

Electron Transfer in Microemulsion-Based Electrolytes.

The use of flowing electrochemical reactors, for example, in redox flow batteries and in various electrosynthesis processes, is increasing. This technology has the potential to be of central significance in the increased deployment of renewable electricity for carbon-neutral processes. A key element of optimizing efficiency of electrochemical reactors is the combination of high solution conductivity and reagent solubility. Here, we show a substantial rate of charge transfer for an electrochemical reaction occurring in a microemulsion containing electroactive material is loaded inside the nonpolar (toluene) subphase of the microemulsion. The measured rate constant translates to an exchange current density comparable to that in redox flow batteries. The rate could be controlled by the surfactant, which maintains partitioning of reactants and products by forming an interfacial region with ions in the aqueous phase in close proximity. The hypothesized mechanism is evocative of membrane-bound enzymatic reactions. Achieving sufficient rates of electrochemical reaction is the product of an effort designed to establish a reaction condition that meets the requirements of electrochemical reactors using microemulsions to realize a separation of conducting and reactive elements of the solution, opening a door to the broad use of microemulsions to effect controlled electrochemical reactions as steps in more complex processes.

Liquid Structure and Transport Properties of the Deep Eutectic Solvent Ethaline.

A range of techniques including physical property measurements, neutron scattering experiments, ab initio molecular dynamics, and classical molecular dynamics simulations are used to probe the structural, thermodynamic, and transport properties of a deep eutectic solvent comprised of a 1:2 molar ratio of choline chloride and ethylene glycol. This mixture, known as Ethaline, has many desirable properties for use in a range of applications, and therefore, understanding its liquid structure and transport properties is of interest. Simulation results are able to capture experimental densities, diffusivities, viscosities, and structure factors extremely well. The solvation environment is dynamic and dominated by different hydrogen bonding interactions. Dynamic heterogeneities resulting from hydrogen bonding interactions are quantified. Rotational dynamics of molecular dipole moments of choline and ethylene glycol are computed and found to exhibit a fast and slow mode.

Design, synthesis, and characterization of lightly sulfonated multigraft acrylate-based copolymer superelastomers.

Multigraft copolymer superelastomers consisting of a poly(n-butyl acrylate) backbone and polystyrene side chains were synthesized and the viscoelastic properties of the non-sulfonated and sulfonated final materials were investigated using extensional rheology (SER3). The non-linear viscoelastic experiments revealed significantly increased true stresses (up to 10 times higher) after sulfonating only 2-3% of the copolymer while the materials maintained high elongation (<700%). The linear viscoelastic experiments showed that the storage and loss modulus are increased by sulfonation and that the copolymers can be readily tuned and further improved by increasing the number of branching points and the molecular weight of the backbone. In this way, we show that by tuning not only the molecular characteristics of the multigraft copolymers but also their architecture and chemical interaction, we can acquire thermoplastic superelastomer materials with desired viscoelastic properties.

Neutron scattering in the biological sciences: progress and prospects.

The scattering of neutrons can be used to provide information on the structure and dynamics of biological systems on multiple length and time scales. Pursuant to a National Science Foundation-funded workshop in February 2018, recent developments in this field are reviewed here, as well as future prospects that can be expected given recent advances in sources, instrumentation and computational power and methods. Crystallography, solution scattering, dynamics, membranes, labeling and imaging are examined. For the extraction of maximum information, the incorporation of judicious specific deuterium labeling, the integration of several types of experiment, and interpretation using high-performance computer simulation models are often found to be particularly powerful.

Understanding the chemistry of the development of latent fingerprints by superglue fuming.

Cyanoacrylate fuming is a widely used forensic tool for the development of latent fingerprints, however the mechanistic details of the reaction between the fingerprint residue and the cyanoacrylate vapor are not well understood. Here the polymerization of ethyl-cyanoacrylate vapor by sodium lactate or alanine solutions, two of the major components in fingerprint residue, has been examined by monitoring the time dependence of the mass uptake and resultant polymer molecular weight characteristics. This data provides insight into the molecular level actions in the efficient development of latent fingerprints by superglue fuming. The results show that the carboxylate moiety is the primary initiator of the polymerization process and that a basic environment inhibits chain termination while an acidic environment promotes it. The results also indicate that water cannot be the primary initiator in this forensic technique.

Monitoring the Effects of Illumination on the Structure of Conjugated Polymer Gels Using Neutron Scattering.

We demonstrate a protocol to effectively monitor the gelation process of a high concentration solution of conjugated polymer both in the presence and absence of white light exposure. By instituting a controlled temperature ramp, the gelation of these materials can be precisely monitored as they proceed through this structural evolution, which effectively mirrors the conditions experienced during the solution deposition phase of organic electronic device fabrication. Using small angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) along with appropriate fitting protocols we quantify the evolution of select structural parameters throughout this process. Thorough analysis indicates that continued light exposure throughout the gelation process significantly alters the structure of the ultimately formed gel. Specifically, the aggregation process of poly(3-hexylthiophene-2,5-diyl) (P3HT) nano-scale aggregates is negatively affected by the presence of illumination, ultimately resulting in the retardation of growth in conjugated polymer microstructures and the formation of smaller scale macro-aggregate clusters.