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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]

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 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]

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]

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]

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]

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]

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]

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]

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]

Light scattering and dielectric spectroscopy measurements were performed on the room temperature ionic liquid (RTIL) [C4mim][NTf2] in a broad temperature and frequency range. Ionic conductivity was used to estimate self-diffusion of ions, while light scattering was used to study structural relaxation. We demonstrate that the ionic diffusion decouples from the structural relaxation process as the temperature of the sample decreases toward Tg. The strength of the decoupling appears to be significantly lower than that expected for a supercooled liquid of similar fragility. The structural relaxation process in the RTIL follows well the high-temperature mode coupling theory (MCT) scenario. Using the MCT analysis we estimated the dynamic crossover temperature in [C4mim][NTf2] to be Tc ∼ 225 ± 5 K. However, our analysis reveals no sign of the dynamic crossover in the ionic diffusion process. [ABSTRACT FROM AUTHOR]

In recent years it has become clear that the interfacial layer formed around nanoparticles in polymer nanocomposites (PNCs) is critical for controlling their macroscopic properties. The interfacial layer occupies a significant volume fraction of the polymer matrix in PNCs and creates strong intrinsic heterogeneity in their structure and dynamics. Here, we focus on analysis of the structure and dynamics of the interfacial region in model PNCs with well-dispersed, spherical nanoparticles with attractive interactions. First, we discuss several experimental techniques that provide structural and dynamic information on the interfacial region in PNCs. Then, we discuss the role of various microscopic parameters in controlling structure and dynamics of the interfacial layer. The analysis presented emphasizes the importance of the polymer-nanoparticle interactions for the slowing down dynamics in the interfacial region, while the thickness of the interfacial layer appears to be dependent on chain rigidity, and has been shown to increase with cooling upon approaching the glass transition. Aside from chain rigidity and polymer-nanoparticle interactions, the interfacial layer properties are also affected by the molecular weight of the polymer and the size of the nanoparticles. In the final part of this focus article, we emphasize the important challenges in the field of polymer nanocomposites and a potential analogy with the behavior observed in thin films.

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 α-relaxation time near the nanoparticle grows with cooling faster than the α-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.