Choline-chloride based deep eutectic solvents (DES) have been used for several different applications (e.g., solubility, electrochemistry, and purifications) due to their relative inexpensive and readily available nature. In this work, three choline chloride-based DESs are simulated using molecular dynamics to study the hydrogen bonding interactions of the system. Three hydrogen bond donors (HBD) are studied in order to determine the changes in the hydrogen bonding interactions when the HBD is different in the DES. One dicarboxylic acid and two polyols (with different number of OH groups) were chosen as the HBDs of interest. First, the simulations are validated by comparing simulated and experimental thermodynamic and transport properties, when possible. Then, for maline (choline chloride/malonic acid), the more anomalous system studied here, molecular simulations complement results obtained from an FTIR spectroscopic study in order to further understand this unique system. Good agreement with experimenta...

Experimental and Computational Studies of Choline Chloride-Based Deep Eutectic Solvents

Deep eutectic solvents, considered ionic liquid (IL) analogues, show promise for many material science and engineering applications over typical ILs because they are readily available and relatively inexpensive. Atomistic molecular dynamics simulations have been performed over a range of temperatures on one eutectic mixture, 1:2 choline chloride/urea, using different force field modifications. Good agreement was achieved between simulated density, volume expansion coefficient, heat capacity, and diffusion coefficients and experimental values in order to validate the best performing force field. Atom–atom and center-of-mass radial distribution functions are discussed in order to understand the atomistic interactions involved in this eutectic mixture. Experimental infrared (IR) spectra are also reported for choline chloride–urea mixtures, and band assignments are discussed. The distribution of hydrogen-bond interactions from molecular simulations is correlated to curve-resolved bands from the IR spectra. Th...

Molecular dynamic simulations and vibrational analysis of an ionic liquid analogue.

The increasing use of copper oxide (CuO) nanoparticles (NPs) in medicine and industry demands an understanding of their potential toxicities. In this study, we compared the in vitro cytotoxicity of CuO NPs of two distinct sizes (4 and 24 nm) using the A549 human lung cell line. Despite possessing similar surface and core oxide compositions, 24 nm CuO NPs were significantly more cytotoxic than 4 nm CuO NPs. The difference in size may have affected the rate of entry of NPs into the cell, potentially influencing the amount of intracellular dissolution of Cu2+ and causing a differential impact on cytotoxicity.

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Size-dependent cytotoxicity of copper oxide nanoparticles in lung epithelial cells

Exploring the role of lignin structure in molecular dynamics of lignin/bio-derived thermoplastic elastomer polyurethane blends.

The dynamics of supramolecular networks made up of partially hydrolyzed poly(n-butyl acrylate) [PnBA] is investigated. These linear entangled random copolymers [PnBA-AA] self-assemble via hydrogen bonding interactions between carboxylic acid groups. Two types of supramolecular assemblies are revealed, i.e., binary assembly of carboxylic acid dimers and collective assembly of dimers into distinct poly(acrylic acid) [PAA] domains. The latter is proved by emergence of new relaxation processes in broadband dielectric spectroscopy while the former is evident by an increase of the glass transition temperature as well as retardation of segmental mobility observed by rheology. Therefore, a “sea–island” morphology containing geometrically confined PAA nanodomains embedded in a PnBA-rich matrix is suggested for the supramolecular network. Thermodynamic theories are employed to rationalize the existence of an interlayer with restricted mobility between the two phases. A fraction of PnBA-AA segments that are trapped ...

How Supramolecular Assemblies Control Dynamics of Associative Polymers: Toward a General Picture

The dynamics of intermolecularly hydrogen-bonded polymer blends of poly(p-(hexafluoro-2-hydroxyl-2-propyl)styrene) with poly(vinyl acetate), poly(ethylene[30]-co-vinyl acetate[70]) and poly(ethylene[55]-co-vinyl acetate[45]) are investigated by broadband dielectric relaxation spectroscopy and Fourier transform infrared spectroscopy. Each blend component exhibits a glassy state (β) relaxation, and these relaxations are affected by the formation of intermolecular associations. The glassy state behavior of the blends can be modeled using the Painter−Coleman association model. All blends exhibit a single Tg and a single dielectric segmental (α) relaxation, indicative of strong segmental-level coupling. The fragility of the glass-formers depends on the volume fraction of intermolecularly associated segments, and the association model predicts which compositions have the highest fragilities. A relaxation related to the breaking and reforming of hydrogen bonds is observed at temperatures above the α process, and...

Local Relaxation Behavior and Dynamic Fragility in Hydrogen Bonded Polymer Blends

Infrared bands and Raman lines recorded in the frequency domain have a counterpart in the time domain in the form of time-correlation functions, which are sensitive to molecular dynamics on the picosecond time scale. Time correlation functions and their variation with temperature have been calculated for the modes observed near 1601 and 1583 cm−1 in the infrared spectrum of atactic polystyrene. The correlation functions can be modeled by assuming that there is a fast relaxation process characterized by a single relaxation time that is inhomogeneously broadened by a slower process, also characterized by a single relaxation time. The fast modulation is of the order of 0.014 ps for both modes. Librational motions of the phenyl ring occur on the 0.01 ps time scale, suggesting that this mode of relaxation involves a rapid dephasing process. One cannot draw firm conclusions, however, because dephasing and vibrational energy relaxation mechanisms cannot be separated by studies of simple band shapes alone. A slow...

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Vibrational Relaxation and Dynamical Transitions in Atactic Polystyrene

Infrared spectroscopic study of thermal transitions in poly(methyl methacrylate)

Abstract Topological solitons commonly appear as energy-minimizing field configurations, but examples of stable, spatially localized objects with coexisting solitonic structures and singular defects are rare. Here we use a nonpolar chiral liquid crystal system to show how twist domain walls can co-self-assemble with vortices to form spatially localized topological objects with spontaneous folding. These soliton–vortex assemblies, which we call ‘möbiusons’, have a topology of the molecular alignment field resembling that of the Möbius strip’s surface and package localized field excitations into folded structures within a confinement-frustrated uniform far-field background. Upon supplying energy in the form of electric pulses, möbiusons with different overall symmetries of structure exhibit folding-dependent rotational and translational motions, as well as topological cargo-carrying abilities that can be controlled by tuning the amplitude and frequency of the applied fields. We demonstrate on-demand transformations between various möbiusons and show examples of encoding information by manipulating folds in such structures. A model based on the energetics of solitons and vortices provides insights into the origins of the folding instability, whereas minimization of the Landau–de Gennes free energy closely reproduces details of their internal structure. Our findings may provide a route towards topology-enabled light-steering designs. 

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Hanqing Zhao

Papers

Local Relaxation Behavior and Dynamic Fragility in Hydrogen Bonded Polymer Blends

Vibrational Relaxation and Dynamical Transitions in Atactic Polystyrene

Infrared spectroscopic study of thermal transitions in poly(methyl methacrylate)

Liquid crystal defect structures with Möbius strip topology

Vibrational Relaxation and Dynamical Transitions in Atactic Polystyrene