Dielectric Relaxation Spectroscopy Research Articles

Dynamic disorder in the solid state: insights from dielectric relaxation spectroscopies

Judith Flippen-Anderson Memorial Structural Dynamics Poster Prize » American Institute of Physics American Crystallographic Association

Thermal and electrical behavior of hybrid thermosets based on epoxy and maleimide resins cured with p‐aminobenzoic acid

AbstractTwo thermoset systems based on maleimides and diglycidyl ether of bisphenol A (DGEBA) cured with p‐aminobenzoic acid were characterized in terms of thermal and electrical behavior. Thermal characterization has been undertaken by means of thermogravimetric analysis in nitrogen atmosphere up to 600°C using simultaneous thermogravimetric/Fourier transform infrared/mass spectrometry (TG/FT‐IR/MS) analysis. In the first stage of thermal degradation, the global kinetic parameters [activation energy (Ea) and preexponential factor (log A1 (s−1))] were calculated using the isoconversional method of Friedman. The energies variation as well as the shape of the differential thermal analysis curves suggests that the thermal decomposition process occurred in multiple stages. The evolved gases analysis was conducted by simultaneous TG/FT‐IR/MS coupled techniques. Dielectric relaxation spectroscopy characterization was also made.

Investigation of the electrical properties of conductive polymers by the DRS and RPE techniques

In this work, a polystyrene/polyaniline powder of core-shell structure was prepared by chemical polymerization of aniline, using tartaric acid as a dopant and ammonium persulfate as an oxidizer in presence of polystyrene. The matrix effect on the electrical properties of pure polyaniline was studied by means of dielectric relaxation spectroscopy (DRS) and electron paramagnetic resonance (EPR) technique at room temperature. AC conductivity and dielectric permittivity of the polystyrene/polyaniline were investigated in the frequency range of 10−1 - 106 Hz. The doped polyaniline showed a DC conductivity of 10−5 S/cm. Its incorporation in a polystyrene matrix lowered the conductivity of the mixture up to 10−7 S/cm. This value remains acceptable for semi conductive applications. EPR characterization confirmed this behavior, it gave g-factor values equal to 2.0046 and 2.0093 for polyaniline and polystyrene/polyaniline respectively, close to the free electron g-factor value (2.0023), suggesting that the resonance comes from the delocalized electrons in the main chain of polyaniline. High dielectric constants were obtained for both polyaniline and polystyrene/polyaniline allowing their use as high k-dielectrics. The composite material showed two different relaxation processes. The first one may originate from the electrode polarization while the second is associated to the bulk relaxation of charge carriers.

Dynamics Gradient of Polymer Chains near a Solid Interface.

The relaxation dynamics of polyisoprene (PI) and nitrile butadiene rubber (NBR) chains at the SiO2 interface were directly probed as a function of distance from the SiO2 surface using time-resolved evanescent wave-induced fluorescence anisotropy, dielectric relaxation spectroscopy, and sum-frequency generation spectroscopy. We found the presence of the dynamics gradient of chains in the interfacial region with the SiO2 surface and tried to assign it to the two kinds of adsorbed chains, namely, loosely and strongly adsorbed, at the interface. The segmental relaxation of chains in the strongly adsorbed layer at the interface could be slower than that of bulk chains by more than 10 orders.

Effect of ITO Nanoparticles on Dielectric Relaxation Processes and an Analysis of The Electric Impedance Characteristics of ITO/Epoxy Nanocomposites for Embedded Capacitor Devices

Epoxy/ITO nanocomposites containing various filler ITO concentrations by weight were successfully fabricated. Using Differential Scanning Calorimetry (DSC), a thermo-analytical technique used to study the glass transition temperatures, we expected to find a restriction in the molecular mobility of the polymer chains that might confirm the influence of incorporating ITO contents into the epoxy material. Several complementary techniques were investigated, such as dielectric relaxation spectroscopy (DRS) at the range of frequency (10−1–106) Hz and over the temperature range from − 80°C to 240°C, and time-domain spectroscopy (TDS) with frequency domain (10−5–10−1) Hz. The experimental data were investigated and interpreted in terms of various dielectric formalisms. For TDS results, the neat epoxy is affected by the added ITO nanoparticles. In the low-frequency domain of TDS measurements, the depolarization current curves of the nanocomposites are precisely fitted in a parallel circuit (RiCi) association model. The obtained dielectric data of TDS analysis shows two interfacial relaxation processes. The first wide peak was mainly ascribed to the charge accumulations ITO-polymer interfaces and the additional peak can be related to the charge accumulated at the interface material-electrodes. Impedance evaluated data were analyzed using a consistent electrical circuit formalism. From − 80°C to 60°C, the nanocomposites exhibited an ohmic behaviour within the added ITO nanofiller. Between 60°C and 120°C, the nanocomposites exhibit a capacitive contribution behavior which makes the material suitable for capacitors devices. Above 120°C, the Nyquist design illustrations [(− Z″) versus (Z′)] are well theoretical fitted to an equivalent circuit model achieved by impedance with designed parameters: resistance (Rb) and constant phase element (CPE) combinations.

Functional group configuration influences salt transport in desalination membrane materials

Five methacrylate-based co-polymers, which have different ratios of 2-hydroxyethyl methacrylate (HEMA) and glycerol methacrylate (GMAOH) co-monomers, were prepared to investigate the influence of functional group configuration on salt transport properties. The HEMA and GMAOH co-monomers were selected because of the differences in position and number of hydroxyl groups in the two molecules. Co-polymer composition was varied systematically from a vicinal diol-rich (GMAOH-rich) configuration to a distributed hydroxyl group (HEMA-rich) configuration. To decouple the impact of functional group configuration and of changing water content on transport properties, all of the materials were prepared to have statistically equivalent water content. Salt sorption was lower and salt diffusion was slower in the HEMA-rich configuration compared to the GMAOH-rich configuration. Microwave dielectric relaxation spectroscopy revealed lower relative permittivity (i.e., dielectric constant) for the HEMA-rich co-polymers, which is consistent with suppressed salt sorption in those materials compared to the GMAOH-rich co-polymers. This observation was complemented by state of water analysis that suggested freezable (i.e., bulk-like) water content decreased as HEMA content increased. Both results suggest that stronger interactions between water molecules and the polymer are favored when hydrophilic functional groups are distributed more evenly throughout the material. These results suggest that functional group position in hydrated polymers influences salt transport properties, and engineering polymers that have a distributed functional group configuration may suppress salt transport properties, which could result in favorable materials for desalination membranes.

Dielectric and calorimetric signatures of chain orientation in strong and tough ultrafine electrospun polyacrylonitrile

Ultrafine diameter fibers of polyacrylonitrile (PAN), obtained from electrospinning, have huge potential for structural applications since they exhibit an unusual combination of strength and toughness. However, the difficulty to characterize their supramolecular architecture limits their production at the industrial scale. In this work, the glass transition of electrospun nanofiber mats of PAN was investigated by means of thermal analysis techniques. Modulated temperature differential scanning calorimetry (MT–DSC) and dielectric relaxation spectroscopy (DRS) were used, and relaxation parameters characteristic of the glass transition were obtained. Reduction in average fiber diameter resulted in broadening of the glass transition and a shift of its midpoint to higher temperatures as observed by MT–DSC, revealing additional level of constraints in the amorphous phase. The DRS curves, obtained above the calorimetric signature of the glass transition, superimpose independently on the fiber diameter. This result, which contrasts with MT–DSC observations, shows that the constraint of mobility evidenced at the glass transition, is suppressed when driving the fiber mat to higher temperatures. The dielectric strength increases with temperature, revealing an increase in the density of dipoles participating to the relaxation dynamics. This result, commonly attributed to the progressive mobilization of initially constrained amorphous phase, supports the hypothesis that electrospinning process induces higher level of polymer chain orientation at small fiber diameters, which fades away when crossing the glass transition. The orientation impacts the temperature dependence of the relaxation time close to the glass transition, as it shows higher deviation from Arrhenius behavior with the decrease of the fiber diameter. This leads to an increase of the fragility index which comes in opposition to the decrease in the cooperativity length, estimated from the temperature fluctuation approach of the cooperative rearranging region (CRR) concept. To explain this result, both volume and thermal contributions of the fragility index have been calculated, and a strong increase in the thermal contribution has been observed for the most oriented material. This result is interpreted as a signature of an increase in the polymer chain rigidity.

New Insight Into the Molecular Dynamics of Epoxy/Aluminum Hydroxide Nanocomposites by Terahertz Time-Domain Spectroscopy

Nanodielectrics have shown many superior properties than conventional dielectrics. Nanocomposites based on epoxy resin and nanoaluminum hydroxide were fabricated to obtain material systems. In this paper, we investigated molecular dynamics of the nanocomposites by the combination of broadband dielectric relaxation spectroscopy, terahertz time-domain spectroscopy (THz-TDS), and thermally stimulated depolarization current (TSDC). We decompose the molecular dynamics into four relaxation processes according to their typical frequency at room temperature. Relaxation process in THz range is regarded as γ-process, which is well represented by Debye equation. At low-frequency range 10−1–106 Hz, α-process and Johari–Goldstein β-process play dominant role. Additionally, a fast β-process is expected to appear at intermediate frequencies. TSDCs results confirm the existence and change of all relaxation process. Each relaxation process changes differently with increasing filler content. Our results suggest that it is molecular dynamics at different time and space scales rather than free volume result in the change of dielectric property. This paper indicates that THz-TDS is extremely beneficial to understand the underlying mechanism of interface region in nanocomposites. We anticipate that THz-TDS is a prospective tool in future development and analysis of nanocomposites.

Mutifunctional Epoxy-Based Solid Polymer Electrolytes for Energy Storage Systems

For the development of next-generation safe energy storage (batteries or supercapacitors), energy conversion (solar cells), and electromechanical transduction devices (ionic actuators), solid polymer electrolytes (SPEs) having good electrochemical stability are widely considered as promising materials to substitute for conventional organic liquid electrolytes that have flammability and explosion issues. The SPEs could also offer a perfect solution to the enhancement of energy or power density, due to the possibility to use lithium metal anode that has a high theoretical specific capacity, and would even enable their use in multifunctional wearable or structural energy storage systems for portable electronics or transport applications. However, the key challenge facing the development of the SPEs for energy storage applications is to achieve high mechanical performance without sacrificing the requisite ionic conductivity: increasing conductivity typically leads to a reduction of the modulus. We prepare cross-linkable epoxy-based networked SPEs including Li salts (LiTFSI) with either plastic crystals (PCs), tetraglymes (G4s) or ionic liquids (ILs). The epoxy resins are particularly attractive as polymer matrices for solid polymer electrolytes due to their high mechanical performance combined with good adhesive properties and corrosion resistance. The selected electrolyte components (PC, G4 or IL) are allowed to boost ionic conductivity owing to solvating the lithium cation and plasticizing the epoxy matrix. As a result, the thermal curing of a homogeneous mixture of epoxy and electrolyte components can generate a two-phase system in which the epoxy phase is selected to provide mechanical strength and the electrolyte phase is selected to maximize ionic conductivity, via polymerization-induced phase separation, allowing for a bicontinuous microphase separation morphology. To further improve the physical properties of the SPEs, inorganic nanoparticles such TiO2, SiO2, or Al2O3 are incorporated in the nanocomposite SPEs, allowing for the combination of the advantages of both inorganic materials and organic polymers. This can be the strategy to achieve the simultaneous improvement in both mechanical strength and ion conduction of nanocomposite SPEs. In the current investigation, we conduct an investigation of the effect of electrolyte and inorganic types and their concentration on the ion conduction and dielectric and viscoelastic response of epoxy-based networked SPEs to investigate their ion and polymer dynamics. We measure storage and loss moduli, viscosity, ionic conductivity, dielectric constant, Young’s modulus of a series of epoxy-based SPEs, where the concentration of epoxy resin, electrolyte, and inorganic filler is varied, using oscillatory and steady shear, dielectric relaxation spectroscopy, and dynamic mechanical analyzer, respectively. These results are complemented by morphological studies from FESEM, microstructural and physical interaction studies from Fourier transform infrared (FTIR) spectroscopy, intermolecular interaction from Density functional theory (DFT) calculations, and thermal properties from DSC. Our study leads to insight regarding optimal design of multifunctional solid polymer electrolytes for energy storage devices.

Possible Proton Conduction Mechanism in Pseudo-Protic Ionic Liquids: A Concept of Specific Proton Conduction.

In a previous work, we have found that the pseudo-protic ionic liquid N-methylimidazolium acetate, [C1HIm][OAc] or [Hmim][OAc], mainly consists of the electrically neutral molecular species N-methylimidazole, C1Im, and acetic acid, AcOH, even though the mixture has significant ionic conductivity. This system was revisited by employing isotopic substitution Raman spectroscopy (ISRS) and pulsed field gradient (PFG) NMR self-diffusion measurements. The ISRS and PFG-NMR results obtained fully confirm our earlier findings. In particular, the self-diffusion coefficient of the hydroxyl hydrogen atom in AcOH is identical to that of the methyl hydrogen atoms within the experimental uncertainty, consistent with very little ionization. Therefore, a proton conduction mechanism similar to the Grotthuss mechanism for aqueous acid solutions is postulated to be responsible for the observed electrical conductivity. Laity resistance coefficients (rij) are calculated from the transport properties, and the negative values obtained for the like-ion interactions are consistent with the pseudo-ionic liquid description, that is, the mixture is indeed a very weak electrolyte. The structure and rotational dynamics of the mixture were also investigated using high-energy X-ray total scattering experiments, molecular dynamics simulations, and dielectric relaxation spectroscopy. Based on a comparison of activation energies and the well-known linear free energy relationship between the kinetics and thermodynamics of autoprotolysis, we propose for [C1HIm][OAc] a Grotthus-type proton conduction mechanism involving fast AcOH/AcO- rotation as a decisive step.

Engineering of keratin functionality for the realization of bendable all-biopolymeric micro-electrode array as humidity sensor

The technological quest for flexible devices to be interfaced with the biological world has driven the recent reinvention of bioderived polymers as multifunctional active and passive constituent elements for electronic and photonic devices to use in the biomedical field. Keratin is one of the most important structural proteins in nature to be used as biomaterial platform in view of the recently reported advances in the extraction and processing from hair and wool fibers. In this article we report for the first time the simultaneous use of naturally extracted keratin as both active ionic electrolyte for water ions sensing and as bendable and insoluble substrate into the same multielectrode array-based device. We implemented the multifunctional system exclusively made by keratin as a bendable sensor for monitoring the humidity flow.The enhancement of the functional and structural properties of keratin such as bendability and insolubility were obtained by unprecedented selective chemical doping. The mechanisms at the basis of the sensing of humidity in the device were investigated by cyclic voltammetry and rationalized by reversible binding and extraction of water ions from the volume of the keratin active layer, while the figures of merit of the biopolymer such as the ionic conductivity and relaxation time were determined by means of electrical impedance and dielectric relaxation spectroscopy. A reliable linear correlation between the controlled-humidity level and the amperometric output signal together with the assessment on measure variance are demonstrated. Collectively, the fine-tuned ionic-electrical characterization and the validation in controlled conditions of the free-standing insoluble all-keratin made microelectrode array ionic sensor pave the way for the effective use of keratin biopolymer in wearable or edible electronics where conformability, reliability and biocompatibility are key-enabling features.

Effect of Chitin Whiskers on the Molecular Dynamics of Carrageenan-Based Nanocomposites

Films of carrageenan (KC) and glycerol (g) with different contents of chitin nanowhiskers (CHW) were prepared by a solution casting process. The molecular dynamics of pure carrageenan (KC), carrageenan/glycerol (KCg) and KCg with different quantities of CHWs as a filler was studied using dielectric relaxation spectroscopy. The analysis of the CHW effect on the molecular mobility at the glass transition, Tg, indicates that non-attractive intermolecular interactions between KCg and CHW occur. The fragility index increased upon CHW incorporation, due to a reduction in the polymer chains mobility produced by the CHW confinement of the KCg network. The apparent activation energy associated with the relaxation dynamics of the chains at Tg slightly increased with the CHW content. The filler nature effect, CHW or montmorillonite (MMT), on the dynamic mobility of the composites was analyzed by comparing the dynamic behavior of both carrageenan-based composites (KCg/xCHW, KCg/xMMT).

Dielectric spectroscopic and molecular dynamic study of aqueous solutions of paracetamol

Dielectric relaxation spectroscopy was used to study the structural properties of Paracetamol (PCM) and its aqueous solutions in varying amount of concentrations. The dielectric and electrical properties of PCM pellet (at a constant temperature of 300.15 K) and of aqueous PCM solutions (over a temperature range of 293.15 K–323.15 K in steps of 10 K) were determined in the frequency range from 20 Hz to 2 MHz. Agilent precision LCR meter E4980A, with four terminal liquid test fixture Agilent 16452A (for aqueous solutions of PCM) and solid test fixture 16451B (for PCM pellet) were used for the measurements. From the measured dielectric data, other electrical parameters like complex modulus (M⁎), complex impedance (Z⁎) and complex electrical conductivity (σ⁎) were calculated. Obtained data were analyzed to gain information about structure of PCM as well as the nature of PCM- water interactions. Results can be effectively used in the field of pharmaceutical sciences particularly in designing, development and synthesis of drugs. Molecular dynamics computer simulations have been performed for selected concentrations of PCM in water in order to understand the actual mechanism of H-bonding interactions in the solutions and to confirm the inferences derived from the experimental results.

Relaxation dynamics of l-alanine in water medium investigated by dielectric relaxation spectroscopy.

The complex dielectric permittivity of L-alanine in aqueous medium at different concentrations and different temperatures were measured in the microwave (0.02 < ν/GHz < 20) frequency region by using open-ended coaxial probe technique. From the reflection coefficient and impedance data, the real and imaginary part of the dielectric permittivity values is determined. It is observed that there is a decrease in the real part of the dielectric permittivity up to certain frequency and an increase in the imaginary part of the dielectric permittivity with increase in the molar concentration of L-alanine in water medium. Based on the experimental data the average relaxation time values are calculated and its behavior is analyzed in terms of bound water and free water molecules. The theoretical dipole moment of L-alanine is calculated at gaseous state as well as in aqueous medium by using PCM and IEFPCM model at HF, DFT/B3LYP and MP2 calculations using 6-311G* basis set. Analysis between experimentally determined parameters and computed dipole moments were discussed. The mean molecular polarizability is calculated from the Lippincott δ function potential model and compared with the Le Fèvre method of polarizability values.

Morphology and ionic conductivity relationship in silk/cellulose biocomposites

AbstractThe relationship between morphology and the ionic conductivity of polysaccharide–protein bio‐electrolyte membranes is explored in this study. Structural proteins and polysaccharides form hydrophobic and electrostatic interactions, and the resulting matrices can exhibit novel and useful properties. However, transforming these natural biomacromolecules from their native state to a more usable form is challenging. The structural, morphological, thermal, mechanical and electrical properties of biomaterials composed of microcrystalline cellulose and Bombyx mori silk when regenerated together using ionic liquids and various coagulation agents were investigated using a diverse set of techniques including Fourier transform infrared spectroscopy, SEM, TGA, DSC, X‐ray scattering, AFM‐based nanoindentation and dielectric relaxation spectroscopy. The surface topography of the films reveals morphological changes with varying coagulation agents and ionic liquids. It was found that the thermal and mechanical properties were dependent on intermolecular interactions dictated by the type of ionic liquid used during the coagulation process. X‐ray scattering provided information on how the cellulose crystallinity varied with coagulation agent. Specifically, samples coagulated with hydrogen peroxide showed an increase in cellulose crystallinity impacting properties such as elasticity, hardness and ionic conductivity of the biocomposites. In addition, the results revealed a strong correlation between β‐sheet content and ionic conductivity and cellulose crystallinity. The results provided evidence that the ionic conductivity is dependent on protein β‐sheet content and cellulose crystallinity. © 2019 Society of Chemical Industry

Polymer Compositional Ratio-Dependent Morphology, Crystallinity, Dielectric Dispersion, Structural Dynamics, and Electrical Conductivity of PVDF/PEO Blend Films

The polymer blend (PB) films consisted of poly(vinylidene fluoride) (PVDF) and poly(ethylene oxide) (PEO) with different compositional ratios (i.e., PVDF/PEO =100/0, 75/25, 50/50, 25/75, and 0/100 wt%) have been prepared by solution casting method. These PB films were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and dielectric relaxation spectroscopy (DRS). The pristine films of PVDF and PEO have spherulite morphologies, which change enormously with the variation of their compositions in blend films. The EDX spectra confirm the linear variation of the amount of respective polymer elements, with the change of its compositional ratio in the PB films. The XRD and FTIR results confirm that the semicrystalline PVDF film has predominantly α- and β-phase crystals. The degree of crystallinity of these PB films exhibits non-linear increase, with increasing amount of PEO in the films. The relative fraction of the β-phase crystal of the PVDF in these complex PB films has been determined from the fractional relations based on the areas and intensities of crystalline peaks, observed in their XRD patterns which is found the maximum (~50%) for the 75 PVDF/25 PEO blend film. The dielectric dispersion of these PB films in the frequency window of 20 Hz-1MHz at 27 °C reveals that the real part of the complex permittivity is governed predominantly by the interfacial polarization effect at lower audio frequencies, whereas it mainly depends on the polymer compositional ratio at higher radio frequencies. The segmental relaxation process peak of the PEO chain observed in the loss part of the electric modulus spectra, shifts toward the lower frequency side with a significant suppression of intensity as the amount of PVDF enhances in the PB films. This result confirms that the PEO dynamics face considerable hindrance by the PVDF structures. The dc electrical conductivity of these PB films increases non-linearly with increasing amount of PEO in the films, and varies by more than an order of magnitude with the variation of the compositional ratio over the entire range. The temperature-dependent study of 50 PVDF/50 PEO blend film confirms its thermally activated dielectric properties and the structural dynamics with the relaxation activation energy of 0.23 eV. The compositional ratio-dependent dielectric properties of PVDF/PEO blend films offer a promising potential for their use as dielectric permittivity- and electrical conductivity-tunable insulating materials, with engineered functionality for flexible electronics and electrical devices.

Microstructure and relaxation behavior of flexible dielectric PVDF and HNBR blends: An assessment through small‐angle x‐ray scattering and dielectric relaxation spectroscopy

ABSTRACTThe work demonstrated the microstructure and the relaxation behavior of flexible electroactive blends of poly(vinylidene fluoride) (PVDF)/hydrogenated nitrile rubber (HNBR) by small‐angle X‐ray scattering and dielectric relaxation spectroscopy. Very few studies have been done so far on this topic for crystalline/rubbery blends. Lamellar morphology was observed for both the PVDF and its blends. HNBR suppressed the mobility of PVDF above its melting temperature, as evident from lowering of crystallization temperature, due to physical interaction. The interaction was increased with HNBR content. However, after complete crystallization, HNBR segments were expelled out from the lamella, and crystal long period remained intact in all the blends. Interestingly, some HNBR segments remained in the amorphous part of PVDF which reduced the electron density contrast of its crystalline and amorphous region. When HNBR was crosslinked, the interaction was reduced, and consequently, the crystallization became faster and electron density contrast increased. From the microscopic study, polydispersed particles were observed within the crystalline lamella. Interfacial polarization (IP) relaxation of PVDF was absent in the blends due to physical interaction, whereas IP relaxation of HNBR shifted to a higher frequency. The shift was higher at higher HNBR content and decreased when HNBR was crosslinked. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 851–866

Relaxation dynamics of Ni/epoxy composites studied by dielectric relaxation spectroscopy

Dynamic dielectric properties of nickel (Ni)/epoxy composites are investigated by broadband dielectric spectroscopy over a wide frequency range (approximately 1–107 Hz) and temperature range (approximately −20 to 200°C). Multiple dielectric relaxation processes in the Ni/epoxy composites are identified, corresponding to conductivity, interfacial polarization (IP), and α-relaxation. With increasing temperature, both dielectric permittivity and loss of composites increase, arising from enhanced dipolar mobility; approaching the glass transition temperature ( T g), a clear α-relaxation appears, which can be attributed to the onset of cooperative chain movement. Further increasing temperature activates the long-range movement of space charges, as manifested by an IP superimposed on the ionic conductivity. A transition from insulator (below T g) to semiconductor (above T g) is also observed for the studied composite system.

Computational spectroscopy of trehalose, sucrose, maltose, and glucose: A comprehensive study of TDSS, NQR, NOE, and DRS.

The bioprotective nature of monosaccharides and disaccharides is often attributed to their ability to slow down the dynamics of adjacent water molecules. Indeed, solvation dynamics close to sugars is indisputably retarded compared to bulk water. However, further research is needed on the qualitative and quantitative differences between the water dynamics around different saccharides. Current studies on this topic disagree on whether the disaccharide trehalose retards water to a larger extent than other isomers. Based on molecular dynamics simulation of the time-dependent Stokes shift of a chromophore close to the saccharides trehalose, sucrose, maltose, and glucose, this study reports a slightly stronger retardation of trehalose compared to other sugars at room temperature and below. Calculation and analysis of the intermolecular nuclear Overhauser effect, nuclear quadrupole relaxation, dielectric relaxation spectroscopy, and first shell residence times at room temperature yield further insights into the hydration dynamics of different sugars and confirm that trehalose slows down water dynamics to a slightly larger extent than other sugars. Since the calculated observables span a wide range of timescales relevant to intermolecular nuclear motion, and correspond to different kinds of motions, this study allows for a comprehensive view on sugar hydration dynamics.

Orientational glass, orientationally disordered crystal and crystalline polymorphism: A further study on the thermal behavior and molecular mobility in levoglucosan

Levoglucosan has very particular features in its phase diagram. On the one hand it has two crystalline phases, one orientationally ordered (ORC), stable at room temperature, which is transformed by heating in an orientationally disordered crystal (ODIC), a plastic crystal. On the other hand, it is difficult to form the structural glass by cooling down from the isotropic liquid, but it is easy to form the orientational glass (O-G) of levoglucosan by cooling from the ODIC phase. The stability of the O-G and ODIC phases allowed the study of the orientational glass transition, O-G → ODIC, by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) with the determination of the respective dynamic fragility. In this context a critical analysis of the procedures using the effect of the DSC scanning rate on the temperature location of the DSC glass transition signal was performed.Two polymorphs of levoglucosan, not yet described in the literature, were discovered. These crystalline ORC forms are converted into an ODIC phase by heating, as happens with the most stable polymorph (as received). The three ORCs show ORC → ODIC transitions that differ greatly in temperature, enthalpy, and entropy of transition. We must therefore consider four levoglucosan polymorphs, three ORCs and one ODIC.TSDC was also used to study the secondary relaxations in the levoglucosan orientational glass, and it was possible to identify a β-type mobility (Johari-Goldstein) in addition to fast relaxations. Finally, the two dielectric techniques, dielectric relaxation spectroscopy (DRS) and TSDC, were compared in their ability to provide relevant information on the secondary mobility.