Search

We employ polymer integral equation theory to study a simplified model of semiflexible polymerized ionic liquids (PolyILs) that interact via hard core repulsions and short range screened Coulomb interactions. The multi-scale structure in real and Fourier space of PolyILs (ions chosen to mimic Li, Na, K, Br, PF6, and TFSI) are determined as a function of melt density, Coulomb interaction strength, and ion size. Comparisons with a homopolymer melt, a neutral polymer–solvent-like athermal mixture, and an atomic ionic liquid are carried out to elucidate the distinct manner that ions mediate changes of polymer packing, the role of excluded volume effects, and the influence of chain connectivity, respectively. The effect of Coulomb strength depends in a rich manner on ion size and density, reflecting the interplay of steric packing, ion adsorption, and charge layering. Ion-mediated bridging of monomers is found, which intensifies for larger ions. Intermediate range charge layering correlations are characterized by a many-body screening length that grows with PolyIL density, cooling, and Coulomb strength, in disagreement with Debye–Hückel theory, but in accord with experiments. Qualitative differences in the collective structure, including an ion-size-dependent bifurcation of the polymer structure factor peak and pair correlation function, are predicted. The monomer cage order parameter increases significantly, but its collective ion counterpart decreases, as ions become smaller. Such behaviors allow one to categorize PolyILs into two broad classes of small and large ions. Dynamical implications of the predicted structural results are qualitatively discussed. [ABSTRACT FROM AUTHOR]

We study experimentally the temperature evolution of the thickness of the interfacial layer, Lint(T), between bulk matrices and the surface of nanoparticles in nanocomposites through broadband dielectric spectroscopy. Analyses revealed a power-law dependence between the logarithm of structural relaxation time in the interfacial layer, τint(T), and the Lint(T): ln τ i n t (T) / τ 0 ∝ L i n t β (T) / T , with τ0 ∼ 10−12 s, and β index ∼0.67 at high temperatures and ∼1.7 at temperatures close to the glass transition temperature. In addition, our analysis revealed that the Lint(T) is comparable to the length scale of dynamic heterogeneity estimated from previous nonlinear dielectric measurements and the four-point NMR [ξNMR(T)], with Lint(T) ∼ ξNMR(T). These observations may suggest a direct correlation between the Lint(T) and the size of the cooperatively rearranging regions and have strong implications for understanding the dynamic heterogeneity and cooperativity in supercool liquids and their role in interfacial dynamics. [ABSTRACT FROM AUTHOR]

Understanding the microscopic mechanism of the transition of glass remains one of the most challenging topics in Condensed Matter Physics. What controls the sharp slowing down of molecular motion upon approaching the glass transition temperature Tg, whether there is an underlying thermodynamic transition at some finite temperature below Tg, what the role of cooperativity and heterogeneity are, and many other questions continue to be topics of active discussions. This review focuses on the mechanisms that control the steepness of the temperature dependence of structural relaxation (fragility) in glass-forming liquids. We present a brief overview of the basic theoretical models and their experimental tests, analyzing their predictions for fragility and emphasizing the successes and failures of the models. Special attention is focused on the connection of fast dynamics on picosecond time scales to the behavior of structural relaxation on much longer time scales. A separate section discusses the specific case of polymeric glass-forming liquids, which usually have extremely high fragility. We emphasize the apparent difference between the glass transitions in polymers and small molecules. We also discuss the possible role of quantum effects in the glass transition of light molecules and highlight the recent discovery of the unusually low fragility of water. At the end, we formulate the major challenges and questions remaining in this field. [ABSTRACT FROM AUTHOR]

Background The importance of protein dynamics for their biological activity is now well recognized. Different experimental and computational techniques have been employed to study protein dynamics, hierarchy of different processes and the coupling between protein and hydration water dynamics. Yet, understanding the atomistic details of protein dynamics and the role of hydration water remains rather limited. Scoop of review Based on overview of neutron scattering, molecular dynamic simulations, NMR and dielectric spectroscopy results we present a general picture of protein dynamics covering time scales from faster than ps to microseconds and the influence of hydration water on different relaxation processes. Major conclusions Internal protein dynamics spread over a wide time range from faster than picosecond to longer than microseconds. We suggest that the structural relaxation in hydrated proteins appears on the microsecond time scale, while faster processes present mostly motion of side groups and some domains. Hydration water plays a crucial role in protein dynamics on all time scales. It controls the coupled protein-hydration water relaxation on 10–100 ps time scale. This process defines the friction for slower protein dynamics. Analysis suggests that changes in amount of hydration water affect not only general friction, but also influence significantly the protein's energy landscape. General significance The proposed atomistic picture of protein dynamics provides deeper understanding of various relaxation processes and their hierarchy, similarity and differences between various biological macromolecules, including proteins, DNA and RNA. This article is part of a Special Issue entitled “Science for Life” Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo”. [ABSTRACT FROM AUTHOR]

Proton transport is critical for many technologies and for a variety of biochemical and biophysical processes. Proton transfer between molecules (via structural diffusion) is considered to be an efficient mechanism in highly proton conducting materials. Yet, the mechanism and what controls energy barriers for this process remain poorly understood. It was shown that mixing phosphoric acid (PA) with lidocaine leads to an increase in proton conductivity at the same liquid viscosity. However, recent simulations of mixtures of PA with various bases, including lidocaine, suggested no decrease in the proton transfer energy barrier. To elucidate this surprising result, we have performed broadband dielectric spectroscopy to verify the predictions of the simulations for mixtures of PA with several bases. Our results reveal that adding bases to PA increases the energy barriers for proton transfer, and the observed increase in proton conductivity at a similar viscosity appears to be related to the increase in the glass transition temperature (Tg) of the mixture. Moreover, the energy barrier seems to increase with Tg of the mixtures, emphasizing the importance of molecular mobility or interactions in the proton transfer mechanism. [ABSTRACT FROM AUTHOR]

We present detailed dielectric spectroscopy studies of dynamics in two hydrated proteins, lysozyme and myoglobin. We emphasize the importance of explicit account for possible Maxwell–Wagner (MW) polarization effects in protein powder samples. Combining our data with earlier literature results, we demonstrate the existence of three major relaxation processes in globular proteins. To understand the mechanisms of these relaxations we involve literature data on neutron scattering, simulations and NMR studies. The faster process is ascribed to coupled protein–hydration water motions and has relaxation time ~ 10–50 ps at room temperature. The intermediate process is ~ 10 2 –10 3 times slower than the faster process and might be strongly affected by MW polarizations. Based on the analysis of data obtained by different experimental techniques and simulations, we ascribe this process to large scale domain-like motions of proteins. The slowest observed process is ~ 10 6 –10 7 times slower than the faster process and has anomalously large dielectric amplitude Δ ε ~ 10 2 –10 4 . The microscopic nature of this process is not clear, but it seems to be related to the glass transition of hydrated proteins. The presented results suggest a general classification of the relaxation processes in hydrated proteins. [ABSTRACT FROM AUTHOR]

The detailed surface chemistry of aluminum oxide protected silver films for use specifically in surface enhanced Raman spectroscopy and tip enhanced Raman spectroscopy (TERS) was investigated. We have demonstrated that increased storage and scanning use lifetimes for silver plasmonic structures are directly connected with the elimination of chemical degradation at the plasmonic structure surface. X-ray photoelectron spectroscopy of the metal films confirmed that a 2-3 nm thick coating of aluminum oxide prevented chemical attack of the underlying silver film for three months of storage in a desiccator, significantly increasing the storage lifetime of current probes. The scanning lifetime of a TERS probe when used to image a hard patterned silicon substrate was doubled with the addition of this protective coating. These measurements were performed without laser illumination in order to separate laser-induced heating degradation from pure mechanical degradation of the metallized probe currently encountered during TERS data collection. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Reports on the nature of the glass transition which remains an unsolved problem in the physics of condensed matter. Information on the mode-code coupling theory and crossover temperatures; Details on the dynamics at the mesoscopic frequency range.

The present study demonstrates that large electric fields progressively enhance the conductivity of ionic systems up to timescales corresponding to those on which their structural rearrangements take place. Yet, in many ionic materials, some regarded as candidates for electrical energy storage applications, the structural relaxation process can be tremendously slower than (or highly decoupled from) the charge fluctuations. Consequently, nonlinear dielectric spectroscopy may be employed to access rheological information in dynamically decoupled ionic conductors, whereas the combination of large electric power density and good mechanical stability, both technologically highly desired, imposes specific experimental constraints to reliably determine the steady-state conductivity of such materials. [ABSTRACT FROM AUTHOR]

The existence of a Debye-type ultraslow process in dielectric spectra of bulk polyalcohols and similar materials has been reported repeatedly in the recent literature. Its loss peak is observed at frequencies that are decades below those of the primary structural relaxation, in a range where the loss signal is usually dominated by dc-conductivity or even electrode polarization. We show that this peak originates from an incomplete filling of the capacitor volume, e.g., as a result of gas bubbles, a situation that gives rise to a Debye process at the conductivity relaxation frequency of the material, where the values of storage and loss components of permittivity are identical. The result implies that these peaks are not endemic to the liquid and can lead to various misinterpretations of the dielectric relaxation spectra. Techniques avoiding the occurrence of such artifacts are discussed. [ABSTRACT FROM AUTHOR]

Hybrid composites combine two or more different fillers to achieve multifunctional or advanced material properties, such as lightweight and enhanced mechanical properties. The properties of the composites significantly depend on their microstructures, which can be tailored via advanced 3D printing processes. Understanding the process-structure-property relationships is critical to enable the design and engineering of novel hybrid composites for applications in aerospace, automotive, and protective coatings. Here, we develop 3D printable and lightweight hybrid composites and leverage the conventional design of experiments, a theoretical hybrid model, and an image-driven machine learning (ML) method to investigate their mechanical behaviors. The hybrid composites are formulated with elastomer matrix, microfillers, and thin-shell particles, enabling a significant degree of design freedom of microstructures with densities and mechanical properties varying up to 70% and 91%, respectively. Our statistical analysis indicates that the 3D printing path direction and the microfibers fraction are dominating process parameters with contribution percentages of 45.3% and 57.7% on the specific stiffness and strength, respectively. A hybrid mechanics model is developed based on a simple Weibull distribution function and classical single-filler models to effectively capture the variations in mechanical properties, however, it overestimates the values due to its statistical constraints and idealization of experimental uncertainty. The image-driven ML model leverages the microscale images directly without losing the structural details, shows more accurate predictions with experimental data, and has 48.6% lower root mean square error than the theoretical model. [ABSTRACT FROM AUTHOR]

We explore possible molecular mechanisms behind the coupling of protein and solvent dynamics using atomistic molecular-dynamics simulations. For this purpose, we analyze the model protein lysozyme in glycerol, a well-known protein-preserving agent. We find that the dynamics of the hydrogen bond network between the solvent molecules in the first shell and the surface residues of the protein controls the structural relaxation (dynamics) of the whole protein. Specifically, we find a power-law relationship between the relaxation time of the aforementioned hydrogen bond network and the structural relaxation time of the protein obtained from the incoherent intermediate scattering function. We demonstrate that the relationship between the dynamics of the hydrogen bonds and the dynamics of the protein appears also in the dynamic transition temperature of the protein. A study of the dynamics of glycerol as a function of the distance from the surface of the protein indicates that the viscosity seen by the protein is not the one of the bulk solvent. The presence of the protein suppresses the dynamics of the surrounding solvent. This implies that the protein sees an effective viscosity higher than the one of the bulk solvent. We also found significant differences in the dynamics of surface and core residues of the protein. The former is found to follow the dynamics of the solvent more closely than the latter. These results allowed us to propose a molecular mechanism for the coupling of the solvent-protein dynamics. [ABSTRACT FROM AUTHOR]

Trehalose-glycerol mixtures are known to be effective in the long time preservation of proteins. However, the microscopic mechanism of their effective preservation abilities remains unclear. In this article we present a molecular dynamics simulation study of the short time, less than 1 ns, dynamics of four trehalose-glycerol mixtures at temperatures below the glass transition temperature. We found that a mixture of 5% glycerol and 95% trehalose has the most suppressed short time dynamics (fast dynamics). This result agrees with the experimental analysis of the mean-square displacement of the hydrogen atoms, as measured via neutron scattering, and correlates with the experimentally observed enhancement of the stability of some enzymes at this particular concentration. Our microscopic analysis suggests that the formation of a robust intermolecular hydrogen bonding network is most effective at this concentration and is the main mechanism for the suppression of the fast dynamics. [ABSTRACT FROM AUTHOR]

Despite intensive research on sustainable elastomers, achieving elastic vitrimers with significantly improved mechanical properties and recyclability remains a scientific challenge. Herein, inspired by the classical elasticity theory, we present a design principle for ultra‐tough and highly recyclable elastic vitrimers with a defined network constructed by chemically crosslinking the pre‐synthesized disulfide‐containing polydimethylsiloxane (PDMS) chains with tetra‐arm polyethylene glycol (PEG). The defined network is achieved by the reduced dangling short chains and the relatively uniform molecular weight of network strands. Such elastic vitrimers with the defined network, i.e. PDMS‐disulfide‐D, exhibit significantly improved mechanical performance than random analogous, previously reported PDMS vitrimers, and even commercial silicone‐based thermosets. Moreover, unlike the vitrimers with random network that show obvious loss in mechanical properties after recycling, those with the defined network enable excellent thermal recyclability. The PDMS‐disulfide‐D also deliver comparable electrochemical signals if utilized as substrates for electromyography sensors after the recycling. The multiple relaxation processes are revealed via a unique physical approach. Multiple techniques are also applied to unravel the microscopic mechanism of the excellent mechanical performance and recyclability of such defined network. [ABSTRACT FROM AUTHOR]

Despite intensive research on sustainable elastomers, achieving elastic vitrimers with significantly improved mechanical properties and recyclability remains a scientific challenge. Herein, inspired by the classical elasticity theory, we present a design principle for ultra‐tough and highly recyclable elastic vitrimers with a defined network constructed by chemically crosslinking the pre‐synthesized disulfide‐containing polydimethylsiloxane (PDMS) chains with tetra‐arm polyethylene glycol (PEG). The defined network is achieved by the reduced dangling short chains and the relatively uniform molecular weight of network strands. Such elastic vitrimers with the defined network, i.e. PDMS‐disulfide‐D, exhibit significantly improved mechanical performance than random analogous, previously reported PDMS vitrimers, and even commercial silicone‐based thermosets. Moreover, unlike the vitrimers with random network that show obvious loss in mechanical properties after recycling, those with the defined network enable excellent thermal recyclability. The PDMS‐disulfide‐D also deliver comparable electrochemical signals if utilized as substrates for electromyography sensors after the recycling. The multiple relaxation processes are revealed via a unique physical approach. Multiple techniques are also applied to unravel the microscopic mechanism of the excellent mechanical performance and recyclability of such defined network. [ABSTRACT FROM AUTHOR]

Additively manufactured flexible and high‐performance piezoelectric devices are highly desirable for sensing and energy harvesting of 3D conformal structures. Herein, the study reports a significantly enhanced piezoelectricity in polyvinylidene fluoride (PVDF) achieved through the in situ dipole alignment of PVDF within PVDF‐2D molybdenum disulfide (2D MoS2) composite by 3D printing. The shear stress‐induced dipole poling of PVDF and 2D MoS2 alignment are harnessed during 3D printing to boost piezoelectricity without requiring a post‐poling process. The results show a remarkable, more than the eight‐fold increment in the piezoelectric coefficient (d33) for 3D printed PVDF‐8wt.% MoS2 composite over cast neat PVDF. The underlying mechanism of piezoelectric property enhancement is attributed to the increased volume fraction of β phase in PVDF, filler fraction, heterogeneous strain distribution around PVDF‐MoS2 interfaces, and strain transfer to the nanofillers as confirmed by microstructural analysis and finite element simulation. These results provide a promising route to design and fabricate high‐performance 3D piezoelectric devices via 3D printing for next‐generation sensors and mechanical–electronic conformal devices. [ABSTRACT FROM AUTHOR]

The transport of protons is critical in a variety of bio- and electro-chemical processes and technologies. The Grotthuss mechanism is considered to be the most efficient proton transport mechanism, generally implying a transfer of protons between 'chains' of host molecules via elementary reactions within the hydrogen bonds. Although Grotthuss proposed this concept more than 200 years ago, only indirect experimental evidence of the mechanism has been observed. Here we report the first experimental observation of proton transfer between the molecules in pure and 85% aqueous phosphoric acid. Employing dielectric spectroscopy, quasielastic neutron, and light scattering, and ab initio molecular dynamic simulations we determined that protons move by surprisingly short jumps of only ~0.5–0.7 Å, much smaller than the typical ion jump length in ionic liquids. Our analysis confirms the existence of correlations in these proton jumps. However, these correlations actually reduce the conductivity, in contrast to a desirable enhancement, as is usually assumed by a Grotthuss mechanism. Furthermore, our analysis suggests that the expected Grotthuss-like enhancement of conductivity cannot be realized in bulk liquids where ionic correlations always decrease conductivity. Although the Grotthuss mechanism of proton transfer is generally considered to be the most efficient means of proton transport, the search for experimental evidence of the mechanism has remained elusive and in certain cases confusing. Here, dielectric spectroscopy, quasieleastic neutron and light scattering, along with ab initio molecular dynamics simulations have been utilized to elucidate the mechanism of proton transfer in pure and 85% aqueous phosphoric acid. [ABSTRACT FROM AUTHOR]

Both the dispersion state of nanoparticles (NPs) within polymer nanocomposites (PNCs) and the dynamical state of the polymer altered by the presence of the NP/polymer interfaces have a strong impact on the macroscopic properties of PNCs. In particular, mechanical properties are strongly affected by percolation of hard phases, which may be NP networks, dynamically modified polymer regions, or combinations of both. In this article, the impact on dispersion and dynamics of surface modification of the NPs by short monomethoxysilanes with eight carbons in the alkyl part (C8) is studied. As a function of grafting density and particle content, polymer dynamics is followed by broadband dielectric spectroscopy and analyzed by an interfacial layer model, whereas the particle dispersion is investigated by small-angle X-ray scattering and analyzed by reverse Monte Carlo simulations. NP dispersions are found to be destabilized only at the highest grafting. The interfacial layer formalism allows the clear identification of the volume fraction of interfacial polymer, with its characteristic time. The strongest dynamical slow-down in the polymer is found for unmodified NPs, while grafting weakens this effect progressively. The combination of all three techniques enables a unique measurement of the true thickness of the interfacial layer, which is ca. 5 nm. Finally, the comparison between longer (C18) and shorter (C8) grafts provides unprecedented insight into the efficacy and tunability of surface modification. It is shown that C8-grafting allows for a more progressive tuning, which goes beyond a pure mass effect. [ABSTRACT FROM AUTHOR]

Self-healing materials open new prospects for more sustainable technologies with improved material performance and devices' longevity. We present an overview of the recent developments in the field of intrinsically self-healing polymers, the broad class of materials based mostly on polymers with dynamic covalent and noncovalent bonds. We describe the current models of self-healing mechanisms and discuss several examples of systems with different types of dynamic bonds, from various hydrogen bonds to dynamic covalent bonds. The recent advances indicate that the most intriguing results are obtained on the systems that have combined different types of dynamic bonds. These materials demonstrate high toughness along with a relatively fast self-healing rate. There is a clear trade-off relationship between the rate of self-healing and mechanical modulus of the materials, and we propose design principles of polymers toward surpassing this trade-off. We also discuss various applications of intrinsically self-healing polymers in different technologies and summarize the current challenges in the field. This review intends to provide guidance for the design of intrinsic self-healing polymers with required properties. [ABSTRACT FROM AUTHOR]

For the first time, tip enhanced Raman spectroscopy (TERS) blinking measurements are used to identify the individual isotopes of non-Raman resonant polystyrene in a miscible, binary blend. This demonstrates the sensitivity and selectivity required for nanoscale chemical imaging and broadens the types of surface components potentially identifiable with TERS. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: A self-reduction of graphene oxide (GO) at room temperature after prolonged storage on a copper substrate is evidenced by decrease of oxygen content and a dramatic, 6 orders in magnitude, increase in dc conductivity. Experiments revealed that the stored GO film contains copper hydroxide phase embedded in the reduced GO structure. [Copyright &y& Elsevier]

The aminoglycoside N3 acetyltransferase-IIIb (AAC) is responsible for conferring bacterial resistance to a variety of aminoglycoside antibiotics. Nuclear magnetic resonance spectroscopy and dynamic light scattering analyses revealed a surprising result; the dynamics of the ternary complex between AAC and its two ligands, an antibiotic and coenzyme A, are dependent upon the order in which the ligands are bound. Additionally, two structurally similar aminoglycosides, neomycin and paromomycin, induce strikingly different dynamic properties when they are in their ternary complexes. To the best of our knowledge, this is the first example of a system in which two identically productive pathways of forming a simple ternary complex yield significant differences in dynamic properties. These observations emphasize the importance of the sequence of events in achieving optimal protein-ligand interactions and demonstrate that even a minor difference in molecular structure can have a profound effect on biochemical processes. [ABSTRACT FROM AUTHOR]

We report a theoretical approach for analyzing impedance of ionic liquids (ILs) and charged polymers such as polymerized ionic liquids (PolyILs) within linear response. The approach is based on the Rayleigh dissipation function formalism, which provides a computational framework for a systematic study of various factors, including polymer dynamics, in affecting the impedance. We present an analytical expression for the impedance within linear response by constructing a one-dimensional model for ionic transport in ILs/PolyILs. This expression is used to extract mutual diffusion constants, the length scale of mutual diffusion, and thicknesses of a low-dielectric layer on the electrodes from the broadband dielectric spectroscopy measurements done for an IL and three PolyILs. Also, static dielectric permittivities of the IL and the PolyILs are determined. The extracted mutual diffusion constants are compared with the self-diffusion constants of ions measured using pulse field gradient (PFG) fluorine nuclear magnetic resonance (NMR). For the first time, excellent agreement between the diffusivities extracted from the Electrode Polarization spectra (EPS) of IL/PolyILs and those measured using the PFG-NMR are found, which allows the use of the EPS and the PFG-NMR techniques in a complimentary manner for a general understanding of the ionic transport. [ABSTRACT FROM AUTHOR]

Rheological measurement was employed to study the mutarotationof d-fructose in anhydrous state. By monitoring the evolutionof shear viscosity with time, rate constants for mutarotation wereestimated, and two different stages of this reaction were identified.One of the mutarotation stages is rapid and has a low activation energy,whereas the other is much slower and has a much higher activationenergy. Possible conversions corresponding to these two phases arediscussed. This work demonstrates that, in addition to the routinetechniques such polarimetry and gas–liquid chromatography,rheological measurement can be used as an alternative method to continuouslymonitor the mutarotation of sugars. [ABSTRACT FROM AUTHOR]

Using a combination of light scattering techniques and broadband dielectric spectroscopy, we have measured the temperature dependence of structural relaxation time and self diffusion in three imidazolium-based room temperature ionic liquids: [bmim][NTf2], [bmim||PF6], and [bmim][TFA]. A detailed analysis of the results demonstrates that self diffusion decouples from structural relaxation in these systems as the temperature is decreased toward Tg. The degree to which the dynamics are decoupled, however, is shown to be surprisingly weak when compared to other supercooled liquids of similar fragility. In addition to the weak decoupling, we demonstrate that the temperature dependence of the structural relaxation time in all three liquids can be well described by a single Vogel-Fulcher-Tamann function over 13 decades in time from 10-11 s up to 102 s. Furthermore, the stretching of the structural relaxation is shown to be temperature independent over the same range of time scales, i.e., time temperature superposition is valid for these ionic liquids from far above the melting point down to the glass transition temperature. We suggest that these phenomena are interconnected and all result from the same underlying mechanism--strong and directional intermolecular interactions. [ABSTRACT FROM AUTHOR]

The article presents a study which reported a design of tough reversible/recyclable adhesive materials enabled by incorporating dynamic covalent bonds of boronic ester into commodity triblock thermoplastic elastomers that reversibly bind with various fillers and substrates. Topics discussed include synthesis of multiphase material incorporating interfacial dynamic bonding, mechanical properties of dynamic crosslinked multiphase composites, and tough adhesive behavior of the materials.

We report measurements of the ionic conductivity, shear viscosity, and structural dynamics in a homologous series of quaternary ammonium ionic liquids (ILs) and a prototypical imidazolium-based IL over a wide range of temperatures down to the glass transition. We find that the ionic conductivity of these materials generally decreases, while the shear viscosity correspondingly increases, with increasing volume fraction of aliphatic side groups. Upon crossing an aliphatic volume fraction of ~0.40, we observe a sharp, order-of-magnitude decrease in ionic conductivity and enhancement of viscosity, which coincides with the presence of long-lived, nanometer-sized alkyl aggregates. These strong changes in dynamics are not mirrored in the ionicity of these ILs, which decreases nearly linearly with aliphatic volume fraction. Our results demonstrate that nanophase segregation in neat ILs strongly reduces ionic conductivity primarily due to an aggregation-induced suppression of dynamics. [ABSTRACT FROM AUTHOR]

The detailed nature of spatially heterogeneous dynamics of glycerol-silica nanocomposites is unraveled by combining dielectric spectroscopy with atomistic simulation and statistical mechanical theory. Analysis of the spatial mobility gradient shows no "glassy" layer, but the ɑ-relaxation time near the nanoparticle grows with cooling faster than the ɑ-relaxation time in the bulk and is ~20 times longer at low temperatures. The interfacial layer thickness increases from ~1.8 nm at higher temperatures to ~3.5 nm upon cooling to near bulk Tg. A real space microscopic description of the mobility gradient is constructed by synergistically combining high temperature atomistic simulation with theory. Our analysis suggests that the interfacial slowing down arises mainly due to an increase of the local cage scale barrier for activated hopping induced by enhanced packing and densification near the nanoparticle surface. The theory is employed to predict how local surface densification can be manipulated to control layer dynamics and shear rigidity over a wide temperature range. [ABSTRACT FROM AUTHOR]

Shape‐persistent, conductive ionogels where both mechanical strength and ionic conductivity are enhanced are developed using multiphase materials composed of cellulose nanocrystals and hyperbranched polymeric ionic liquids (PILs) as a mechanically strong supporting network matrix for ionic liquids with an interrupted ion‐conducting pathway. The integration of needlelike nanocrystals and PIL promotes the formation of multiple hydrogen bonding and electrostatic ionic interaction capacitance, resulting in the formation of interconnected networks capable of confining a high amount of ionic liquid (≈95 wt%) without losing its self‐sustained shape. The resulting nanoporous and robust ionogels possess outstanding mechanical strength with a high compressive elastic modulus (≈5.6 MPa), comparable to that of tough, rubbery materials. Surprisingly, these rigid materials preserve the high ionic conductivity of original ionic liquids (≈7.8 mS cm−1), which are distributed within and supported by the nanocrystal network‐like rigid frame. On the one hand, such stable materials possess superior ionic conductivities in comparison to traditional solid electrolytes; on the other hand, the high compression resistance and shape‐persistence allow for easy handling in comparison to traditional fluidic electrolytes. The synergistic enhancement in ion transport and solid‐like mechanical properties afforded by these ionogel materials make them intriguing candidates for sustainable electrodeless energy storage and harvesting matrices. [ABSTRACT FROM AUTHOR]

MXenes exhibit excellent capacitance at high scan rates in sulfuric acid aqueous electrolytes, but the narrow potential window of aqueous electrolytes limits the energy density. Organic electrolytes and room‐temperature ionic liquids (RTILs) can provide higher potential windows, leading to higher energy density. The large cation size of RTIL hinders its intercalation in‐between the layers of MXene limiting the specific capacitance in comparison to aqueous electrolytes. In this work, different chain lengths alkylammonium (AA) cations are intercalated into Ti3C2Tx, producing variation of MXene interlayer spacings (d‐spacing). AA‐cation‐intercalated Ti3C2Tx (AA‐Ti3C2), exhibits higher specific capacitances, and cycling stabilities than pristine Ti3C2Tx in 1 m 1‐ethly‐3‐methylimidazolium bis‐(trifluoromethylsulfonyl)‐imide (EMIMTFSI) in acetonitrile and neat EMIMTFSI RTIL electrolytes. Pre‐intercalated MXene with an interlayer spacing of ≈2.2 nm, can deliver a large specific capacitance of 257 F g−1 (1428 mF cm−2 and 492 F cm−3) in neat EMIMTFSI electrolyte leading to high energy density. Quasi elastic neutron scattering and electrochemical impedance spectroscopy are used to study the dynamics of confined RTIL in pre‐intercalated MXene. Molecular dynamics simulations suggest significant differences in the structures of RTIL ions and AA cations inside the Ti3C2Tx interlayer, providing insights into the differences in the observed electrochemical behavior. [ABSTRACT FROM AUTHOR]

Charge transport and structural dynamics in a homologous pair of ammonium and phosphonium based room temperature ionic liquids (ILs) have been characterized over a wide temperature range using broadband dielectric spectroscopy and quasi-elastic light scattering spectroscopy. We have found that the ionic conductivity of the phosphonium based IL is significantly enhanced relative to the ammonium homolog, and this increase is primarily a result of a lower glass transition temperature and higher ion mobility. Additionally, these ILs exhibit pronounced secondary relaxations which are strongly influenced by the atomic identity of the cation charge center. While the secondary relaxation in the phosphonium IL has the expected Arrhenius temperature dependence characteristic of local beta relaxations, the corresponding relaxation process in the ammonium IL was found to exhibit a mildly non-Arrhenius temperature dependence in the measured temperature range--indicative of molecular cooperativity. These differences in both local and long-range molecular dynamics are a direct reflection of the subtly different inter-ionic interactions and mesoscale structures found in these homologous ILs. [ABSTRACT FROM AUTHOR]

The slow, Debye-like relaxation in hydrogen-bonded liquids has largely remained a dielectric phenomenon and has thus far eluded observation by other experimental techniques. Here we report the first observation of a slow, Debye-like relaxation by both depolarized dynamic light scattering (DLS) and dielectric spectroscopy in a model hydrogen-bonded liquid, 2-ethyl-4-methylimidazole (2E4MIm). The relaxation times obtained by these two techniques are in good agreement and can be well explained by the Debye model of rotational diffusion. On the one hand, 2E4MIm is analogous to the widely studied monohydroxy alcohols in which transient chain-like supramolecular structure can be formed by hydrogen bonding. On the other hand, the hydrogen-bonded backbone of 2E4MIm is much more optically polarizable, making it possible to apply light scattering to study the dynamics of the supramolecular structure. These findings provide the missing evidence of the slow, Debye-like relaxation in DLS and open the venue for the application of dynamic light scattering to the study of supramolecular structures in hydrogen-bonded liquids. [ABSTRACT FROM AUTHOR]

The article presents work provides additional insights related to the effects of molecular weight in affecting structure and properties of electrodepolymer interfaces, essential for designing next-generation energy storage and harvesting devices. It mentions the electrified interfaces of an imidazolium-based polymerized ionic liquid using combinations of broadband dielectric spectroscopy.

Dynamics of water, especially in the temperature range of the 'no man's land', remain a mystery. We present detailed study of dynamics in aqueous LiCl solution that is often considered as a model for bulk water. We employ broadband dielectric and light scattering spectroscopy in a broad frequency and temperature range. Our analysis reveals no sign of the fragile-to-strong crossover (FSC) neither in structural relaxation nor in translational motions. Our experimental results combined with a large selection of literature data lead to the clear conclusion-there is no FSC in dynamics of aqueous solutions at T ∼ 200-230 K. Instead, our analysis reveals appearance of the so-called excess wing at the high frequency tail of the structural relaxation peak. We discuss the localized nature of the relaxation process that contributes to the excess wing. [ABSTRACT FROM AUTHOR]

We have carried out molecular-dynamics simulations on fully flexible all-atom models of the protein lysozyme immersed in trehalose, an effective biopreservative, with the purpose of exploring the nature and extent of the dynamical coupling between them. Our study shows a strong coupling over a wide range of temperatures. We found that the onset of anharmonic behavior was dictated by changes in the dynamics and relaxation processes in the trehalose glass. The physical origin of protein-trehalose coupling was traced to the hydrogen bonds formed at the interface between the protein and the solvent. Moreover, protein-solvent hydrogen bonding was found to control the structural relaxation of the protein. The dynamics of the protein was found to be heterogeneous; the motions of surface and core atoms had different dependencies on temperature and, in addition, the surface atoms were more sensitive to the dynamics of the solvent than the core atoms. From the solvent perspective we found that the dynamics near the protein surface showed an unexpected enhanced mobility compared to the bulk. These results shed some light on the microscopic origins of the dynamical coupling in protein-solvent systems. [ABSTRACT FROM AUTHOR]

Abstract Soiling of optical surfaces due to sand and dust accumulation is the main cause for decreased efficiency of concentrating solar power and photovoltaic installations in desert areas. Nanostructured coatings with tailored surface roughness can reduce the rate of soil accumulation and maintain the high optical performance of the solar mirrors and cover glass. Here, we investigate the correlation between the size and structure of the surface features of the coating and its anti-soiling and optical properties. To control the morphology of the surface features we developed two types of coating: (1) based on small size (30–50 nm) silica particles with nanostructured surface and (2) based on unstructured nanoparticles with bimodal size distribution (80 and 35 nm). We tailored the surface features of the coatings to achieve synergistic improvements over different length-scales and thus decrease the adhesion force between the soil particles and the surface. Adhesion force measurements were performed using atomic force microscopy. The adhesion force and energy required to separate a silica particle from the surface of coated solar glass was significantly lower than the respective values from the surface of uncoated solar glass. A falling sand abrasion test, modeled after the procedure in ASTM D968, was performed. The optical properties of coated and uncoated solar glass were measured before and after the soiling test. Coated solar mirror samples were tested in the field. The results of the field test provided evidence that the anti-soiling coating is effective at reducing soiling and improving the specular reflectances of the coated mirrors. The use of the developed anti-soiling coating can be extended to other power-grid applications where reducing the soil accumulation is valuable. Highlights • Anti-soiling coatings with tailored roughness that can retrofit solar mirrors and mitigate the soil accumulation. • Remarkable reductions in the soiling rate (up to 39%) of coated solar mirror samples that were tested in the field. • Mitigation of environmental pollutants such as dust particles is associated with the increased efficiency of solar power installations. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Star PolyIL arm dynamics are delayed relative to linear chains of similar size. • Stars have greater melt viscosity, increasing exponentially with arm length. • Nonlinear architecture has only secondary effects on segmental and ion dynamics. • Morphology and viscoelastic properties are tuned independently of ion transport. Abstract The viscoelastic and dielectric response of linear and star-shaped polymeric ionic liquids (PolyILs) based on imidazolium/bis(trifluoromethane)sulfonimide are probed over a broad range of frequencies and temperatures and are evaluated in connection to the polymer architecture and morphology. At longer timescales, the arm dynamics of star PolyILs are shown to be more sluggish and elastic relative to those of linear chains of comparable size. Arm retraction delays terminal relaxation in the melt, resulting in an enhanced zero-shear viscosity that increases exponentially with the arm length. Yet at shorter timescales, long-chain branching is shown to have only secondary effects on the segmental and ion motions. That is, star PolyIL melts are found to have a lower viscoelastic fragility, more frustrated secondary β fluctuations, and—for stars with the shortest arms—higher activation energies for ion hopping at low temperatures. Even so, all PolyILs ultimately exhibit nearly similar rates of segmental relaxation, ion disassociation, and dc conductivity over the investigated temperature range, regardless of polymer architecture including arm length. This study thus demonstrates how the disparity between chain, segmental, and ion dynamics in branched PolyILs can be exploited in the assembly of functional polymer electrolyte materials with divergent morphologies and controlled viscoelasticity properties, both can be varied independently of ion transport. [ABSTRACT FROM AUTHOR]

Abstract: Utilization of self‐healing chemistry to develop synthetic polymer materials that can heal themselves with restored mechanical performance and functionality is of great interest. Self‐healable polymer elastomers with tunable mechanical properties are especially attractive for a variety of applications. Herein, a series of urea functionalized poly(dimethyl siloxane)‐based elastomers (U‐PDMS‐Es) are reported with extremely high stretchability, self‐healing mechanical properties, and recoverable gas‐separation performance. Tailoring the molecular weights of poly(dimethyl siloxane) or weight ratio of elastic cross‐linker offers tunable mechanical properties of the obtained U‐PDMS‐Es, such as ultimate elongation (from 984% to 5600%), Young's modulus, ultimate tensile strength, toughness, and elastic recovery. The U‐PDMS‐Es can serve as excellent acoustic and vibration damping materials over a broad range of temperature (over 100 °C). The strain‐dependent elastic recovery behavior of U‐PDMS‐Es is also studied. After mechanical damage, the U‐PDMS‐Es can be healed in 120 min at ambient temperature or in 20 min at 40 °C with completely restored mechanical performance. The U‐PDMS‐Es are also demonstrated to exhibit recoverable gas‐separation functionality with retained permeability/selectivity after being damaged. [ABSTRACT FROM AUTHOR]

The full utilization of plant biomass for the production of energy and novel materials often involves high temperature treatment. Examples include melt spinning of lignin for manufacturing low-cost carbon fiber and the relocalization of lignin to increase the accessibility of cellulose for production of biofuels. These temperature-induced effects arise from poorly understood changes in lignin flexibility. Here, we combine molecular dynamics simulations with neutron scattering and dielectric spectroscopy experiments to probe the dependence of lignin dynamics on hydration and thermal history. We find a dynamical and structural hysteresis: at a given temperature, the lignin molecules are more expanded and their dynamics faster when the lignin is cooled than when heated. The structural hysteresis is more pronounced for dry lignin. The difference in dynamics, however, follows a different trend, it is found to be more significant at high temperatures and high hydration levels. The simulations also reveal syringyl units to be more dynamic than guiacyl. The results provide an atomic-detailed description of lignin dynamics, important for understanding lignin role in plant cell wall mechanics and for rationally improving lignin processing. The lignin glass transition, at which the polymer softens, is lower when lignin is cooled than when heated; therefore extending the cooling phase of processing and shortening the heating phase may offer ways to lower processing costs. [ABSTRACT FROM AUTHOR]

The self-diffusion of a single large particle in a fluid is usually described by the classic Stokes-Einstein (SE) hydrodynamic relation. However, there are many fluids where the SE prediction for nanoparticles diffusion fails. These systems include diffusion of nanoparticles in porous media, in entangled and unentangled polymer melts and solutions, and protein diffusion in biological environments. A fundamental understanding of the microscopic parameters that govern nanoparticle diffusion is relevant to a wide range of applications. In this work, we present experimental measurements of the tracer diffusion coefficient of small and large nanoparticles that experience strong attractions with unentangled and entangled polymer melt matrices. For the small nanoparticle system, a crossover from suppressed to enhanced diffusion is observed with increasing polymer molecular weight. We interpret these observations based on our theoretical and simulation insights of the preceding article (paper 1) as a result of a crossover from an effective hydrodynamic core-shell to a nonhydrodynamic vehicle mechanism of transport, with the latter strongly dependent on polymer-nanoparticle desorption time. A general zeroth-order qualitative picture for small sticky nanoparticle diffusion in polymer melts is proposed. [ABSTRACT FROM AUTHOR]

We theoretically study the diffusion of a single attractive nanoparticle (NP) in unentangled and entangled polymer melts based on combining microscopic "core-shell" and "vehicle" mechanisms in a dynamic bond percolation theory framework. A physical picture is constructed which addresses the role of chain length (N), degree of entanglement, nanoparticle size, and NP-polymer attraction strength. The nanoparticle diffusion constant is predicted to initially decrease with N due to the dominance of the core-shell mechanism, then to cross over to the vehicle diffusion regime with a weaker N dependence, and eventually plateau at large enough N. This behavior corresponds to decoupling of NP diffusivity from the macroscopic melt viscosity, which is reminiscent of repulsive NPs in entangled melts, but here it occurs for a distinct physical reason. Specifically, it reflects a crossover to a transport mechanism whereby nanoparticles adsorb on polymer chains and diffuse using them as "vehicles" over a characteristic desorption time scale. Repetition of random desorption events then leads to Fickian long time NP diffusion. Complementary simulations for a range of chain lengths and low to moderate NP-polymer attraction strengths are also performed. They allow testing of the proposed diffusion mechanisms and qualitatively support the theoretically predicted dynamic crossover behavior. When the desorption time is smaller than or comparable to the onset of entangled polymer dynamics, the NP diffusivity becomes almost chain length independent. [ABSTRACT FROM AUTHOR]

While PLA possesses modest to good strength and stiffness, broader application is hindered by its brittle nature. The aim of this study was to develop strong and tough polymeric materials from renewable biomaterials and understand the underlying interactions and mechanisms. Cellulose nanofibrils (CNFs) and epoxidized soybean oil (ESO) were compounded with poly(lactic acid) (PLA) to create a PLA-CNF-ESO tertiary nanocomposite system. Tensile and dynamic mechanical analyses were performed to see how variations in ESO and CNF content affect mechanical properties such as strength, modulus, ductility, and toughness. It was found that at low CNF levels (10 wt%) the addition of ESO can improve the ductility of the nanocomposites 5- to 10-fold with only slight losses in strength and modulus, while at higher CNF levels (20 and 30 wt%), ESO exhibited little effect on mechanical properties, possibly due to percolation of CNFs in the matrix, dominating stress transfer. Therefore, it is important to optimize CNF and ESO amounts in composites to achieve materials with both high strength and high toughness. Efforts have been made to understand the underlying mechanisms of the mechanical behavior of one class of these composites via thermal, dynamic mechanical, rheological, morphological, and Raman analyses. [ABSTRACT FROM AUTHOR]

Polymerized ionic liquids (polyILs), composed mostly of organic ions covalently bonded to the polymer backbone and free counterions, are considered as ideal electrolytes for various electrochemical devices, including fuel cells, supercapacitors, and batteries. Despite large structural diversity of these systems, all of them reveal a universal but poorly understood feature: a charge transport faster than the segmental dynamics. To address this issue, we studied three novel polymer electrolyte membranes for fuel cells as well as four single-ion conductors, including highly conductive siloxane-based polyIL. Our ambient and high pressure studies revealed fundamental differences in the conducting properties of the examined systems. We demonstrate that the proposed methodology is a powerful tool to identify the charge transport mechanism in polyILs in general and thereby contribute to unraveling the microscopic nature of the decoupling phenomenon in these materials. [ABSTRACT FROM AUTHOR]

Polymer-based membranes play a key role in several industrially important gas separation technologies, e.g., removing CO2 from natural gas, with enormous economic and environmental impact. Here, we develop a novel hybrid membrane construct comprised entirely of nanoparticles grafted with polymers. These membranes are shown to have broadly tunable separation performance through variations in graft density and chain length. Computer simulations show that the optimal NP packing forces the grafted polymer layer to distort, yielding regions of measurably lower polymer density. Multiple experimental probes confirm that these materials have the predicted increase in "polymer free volume", which explains their improved separation performance. These polymer-grafted NP materials thus represent a new template for rationally designing membranes with desirable separation abilities coupled with improved aging characteristics in the glassy state and enhanced mechanical behavior. [ABSTRACT FROM AUTHOR]

Conductivity in polymer electrolytes has been generally discussed with the assumption that the segmental motions control charge transport. However, much less attention has been paid to the mechanism of ion conductivity where the motions of ions are less dependent (decoupled) on segmental dynamics. This phenomenon is observed in ionic materials as they approach their glass transition temperature and becomes essential for design and development of highly conducting solid polymer electrolytes. In this paper, we study the effect of chain rigidity on the decoupling of ion transport from segmental motion in three polymerized ionic liquids (polyILs) containing the same cation-anion pair but differing in flexibility of the polymer backbones and side groups. Analysis of dielectric and rheology data reveals that decoupling is strong in vinyl-based rigid polymers while almost negligible in novel siloxane-based flexible polyILs. To explain this behavior, we investigated ion and chain dynamics at ambient and elevated pressure. Our results suggest that decoupling has a direct relationship to the frustration in chain packing and free volume. These conclusions are also supported by coarse-grained molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

Single-ion conducting polymer electrolytes (SCPE) are attractive candidates for high-performance energy storage devices. A star-shaped single lithium-ion conducting polymer electrolyte was synthesized by combining the “grafting to” and “grafting from” strategy utilizing a polyhedral oligomeric silsesquioxane (POSS) nanoparticle as the core. Carboxylic acid terminated polyethylene oxide (PEO) and Reversible Addition Fragmentation Chain Transfer Agent (RAFT-CTA) were attached to the POSS nanoparticle to form a POSS-cored macro RAFT CTA (PMR-CTA). The PMR-CTA was used to mediate the polymerization of potassium (styrene-4-sulfonyltrifluoromethyl-sulfonyl)imide (STF-K + ) and subsequent counter ion exchange resulted in a star-shaped single lithium-ion conducting copolymer (SSCP-Li + ). The grafting ratio of poly(STF-Li + ) and PEO per POSS nanoparticle can be tuned by modifying the feed ratio of the two polymer components. The synthesized star-shaped SCPE-Li + are characterized by 1 H NMR, FT-IR, UV-Vis, DLS, GPC and DSC, and their ionic conductivity is studied by broadband dielectric spectroscopy. The obtained results are compared to a conventional linear homopolymer poly(STF-Li + ) and block copolymer PEO- b -poly(STF-Li + ), and reveal a significant “decoupling” of ion conductivity from segmental dynamics for all of the studied SCPE-Li + . Blending extra PEOs with SCPE-Li + was also performed to improve the ionic conductivity. [ABSTRACT FROM AUTHOR]

Amidoxime-functionalized polydimethylsiloxane (AO-PDMSPNB) membranes with various amidoxime compositions were synthesized via ring-opening metathesis polymerization followed by post-polymerization modification. Compared to other previously reported PDMS-based membranes, the amidoxime-functionalized membranes show enhanced CO 2 permeability and CO 2 /N 2 selectivity. The overall gas separation performance (CO 2 permeability 6800 Barrer; CO 2 /N 2 selectivity 19) of the highest performing membrane exceeds the Robeson upper bound line, and the excellent permeability of the copolymer itself provides great potential for real world applications where huge volumes of gases are separated. This paper details how tuning the CO 2 -philicity within rubbery polymer matrices influences gas transport properties. Key parameters for tuning gas transport properties are discussed, and the experimental results show good consistency with theoretical calculations. This study provides a roadmap to enhancing gas separation performance in rubbery polymers by tuning gas solubility selectivity. [ABSTRACT FROM AUTHOR]

In order to design more effective solid polymer electrolytes, it is important to decouple ion conductivity from polymer segmental motion. To that end, novel polymers based on oxanorbornene dicarboximide monomers with varying lengths of oligomeric ethylene oxide side chains have been synthesized using ring opening metathesis polymerization. These unique polymers have a fairly rigid and bulky backbone and were used to investigate the decoupling of ion motion from polymer segmental dynamics. Ion conductivity was measured using broadband dielectric spectroscopy for varying levels of added lithium salt. The conductivity data demonstrate six to seven orders of separation in timescale of ion conductivity from polymer segmental motion for polymers with shorter ethylene oxide side chains. However, commensurate changes in the glass transition temperatures T g reduce the effect of decoupling in ion conductivity and lead to lower conductivity at ambient conditions. These results suggest that both an increase in decoupling and a reduction in T g might be required to develop solid polymer electrolytes with high ion conductivity at room temperature. [ABSTRACT FROM AUTHOR]