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.
Ionic liquids (ILs) exhibit unique features such as low melting points, low vapor pressures, wide liquidus temperature ranges, high thermal stability, high ionic conductivity, and wide ...electrochemical windows. As a result, they show promise for use in variety of applications: as reaction media, in batteries and supercapacitors, in solar and fuel cells, for electrochemical deposition of metals and semiconductors, for protein extraction and crystallization, and many others. Because of the ease with which they can be supercooled, ionic liquids offer new opportunities to investigate long-standing questions regarding the nature of the dynamic glass transition and its possible link to charge transport. Despite the significant steps achieved from experimental and theoretical studies, no generally accepted quantitative theory of dynamic glass transition to date has been capable of reproducing all the experimentally observed features. In this Account, we discuss recent studies of the interplay between charge transport and glassy dynamics in ionic liquids as investigated by a combination of several experimental techniques including broadband dielectric spectroscopy, pulsed field gradient nuclear magnetic resonance, dynamic mechanical spectroscopy, and differential scanning calorimetry. Based on Einstein–Smoluchowski relations, we use dielectric spectra of ionic liquids to determine diffusion coefficients in quantitative agreement with independent pulsed field gradient nuclear magnetic resonance measurements, but spanning a broader range of more than 10 orders of magnitude. This approach provides a novel opportunity to determine the electrical mobility and effective number density of charge carriers as well as their types of thermal activation from the measured dc conductivity separately. We also unravel the origin of the remarkable universality of charge transport in different classes of glass-forming ionic liquids.
The static and dynamic properties of poly(2-vinylpyridine)/silica nanocomposites are investigated by temperature modulated differential scanning calorimetry, broadband dielectric spectroscopy (BDS), ...small-angle X-ray scattering (SAXS), and transmission electron microscopy. Both BDS and SAXS detect the existence of an interfacial polymer layer on the surface of nanoparticles. The results show that whereas the calorimetric glass transition temperature varies only weakly with nanoparticle loading, the segmental mobility of the polymer interfacial layer is slower than the bulk polymer by 2 orders of magnitude. Detailed analysis of BDS and SAXS data reveal that the interfacial layer has a thickness of 4–6 nm irrespective of the nanoparticle concentration. These results demonstrate that in contrast to some recent articles on polymer nanocomposites, the interfacial polymer layer is by no means a “dead layer”. However, its existence might provide some explanation for controversies surrounding the dynamics of polymer nanocomposites.
Recent developments in additive manufacturing have demonstrated the potential for thermoset polymer feedstock materials to achieve high strength, stiffness, and functionality through incorporation of ...structural and functional filler materials. In this work, graphene was investigated as a potential filler material to provide rheological properties necessary for direct-write three-dimensional (3D) printing and electrostatic discharge properties to the printed component. The rheological properties of epoxy/graphene mixtures were characterized, and printable epoxy/graphene inks formulated. Sheet resistance values for printed epoxy/graphene composites ranged from 0.67 × 10
2
Ω/sq to 8.2 × 10
3
Ω/sq. The flexural strength of printed epoxy/graphene composites was comparable to that of cast neat epoxy (~ 80 MPa), suggesting great potential for these new materials in multifunctional 3D-printed devices.
The influence of different oxide nanoparticles on segmental and chain dynamics of poly(2-vinylpyridine) is investigated by broadband dielectric spectroscopy (BDS), dynamic mechanical measurements, ...and calorimetry. While significant changes in rheological properties are observed with increasing nanoparticle loading, only weak effects are found in the glass transition temperature, the segmental mobility, and fragility. These results are discussed in the context of recent controversies in studies of polymer nanocomposites and polymer thin films.
Charge transport and structural dynamics in the 1:2 mol ratio mixture of lidocaine and decanoic acid (LID–DA), a model deep eutectic mixture (DEM), have been characterized over a wide temperature ...range using broad-band dielectric spectroscopy and depolarized dynamic light scattering. Additionally, Fourier transform infrared spectroscopy measurements were performed to assess the degree of proton transfer between the neutral parent molecules. From our detailed analysis of the dielectric spectra, we have determined that this carboxylic-acid-based DEM is approximately 25% ionic at room temperature. Furthermore, we have found that the characteristic diffusion rate of mobile charge carriers is practically identical to the rate of structural relaxation at all measured temperatures, indicating that fast proton transport does not occur in LID–DA. Our results demonstrate that while LID–DA exhibits the thermal characteristics of a DEM, its charge transport properties resemble those of a protic ionic liquid.
Charge transport and dipolar relaxations in a sodium-based oligoether carboxylate ionic liquid are investigated in a wide frequency and temperature range by means of broadband dielectric spectroscopy ...(BDS). The dielectric spectra are described at lower temperatures in terms of dipolar relaxations whereas hopping conduction in a random spatially varying energy landscape is quantitatively shown to dominate the spectra at higher temperatures. Based on detailed analysis of the dielectric relaxation strength in its temperature dependence, the slower secondary relaxation process is attributed to molecular fluctuation of ion pairs (sodium and carboxylate ions) while the localized motion of the carboxylate anion gives rise to the faster process observed.
In order to understand the nature of the exceedingly low ionic conductivity of aprotic ammonium ionic liquids (ILs), we have measured the charge transport and structural dynamics of ...methyltrioctylammonium bis(trifluoromethylsulfonyl)imide m3oantf2 over a broad temperature range using broadband dielectric spectroscopy, depolarized dynamic light scattering (DDLS), rheology, and pulsed field gradient nuclear magnetic resonance. We demonstrate that the low level of ionic conductivity in this material is due to the combined effects of reduced ion mobility as well as reduced free ion concentration relative to other types of ILs. Furthermore, detailed analysis of the DDLS spectra reveals a slow process in addition to the structural α relaxation that we attribute to reorientational motion of alkyl aggregates. These findings indicate that hydrophobic aggregation strongly influences the charge transport mechanism of aprotic ammonium ionic liquids with long aliphatic side chains.
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