Secondary Relaxation Process Research Articles

Modification of the Dielectric and Thermal Properties of Organic Frameworks Based on Nonterminal Epoxy Liquid Crystal with Silicon Dioxide and Titanium Dioxide.

A nonterminal liquid crystal epoxy monomer is used to create an epoxy-amine network with a typical diamine 4,4'diaminodiphenylmethane. The plain matrix is compared to matrices modified with inorganic fillers: TiO2 or SiO2. Conditions of the curing reaction and glass transition temperatures in the cured products are determined through differential scanning calorimetry and broadband dielectric spectroscopy. The curing process is also followed through optical and electrical observations. The dielectric response of all investigated networks reveals a segmental α-process related to structural reorientation (connected to the glass transition). In all products, a similar process associated with molecular motions of polar groups also appears. The matrix modified with TiO2 exhibits two secondary relaxation processes (β and γ). Similar processes were observed in the pure monomer. An advantage of the network with the TiO2 filler is a shorter time or lower temperature required for optimal curing conditions. The physical properties of cured matrices depend on the presence of a nematic phase in the monomer and nonterminal functional groups in the aliphatic chains. In effect, such cured matrices can have more flexibility and internal order than classical resins. Additional modifiers used in this work shift the glass transition above room temperature and influence the fragility index in both cases.

Creep deformation in metallic glasses: A global approach with strain as an indicator within transition state theory

Within the framework of transition state theory, the isothermal creep behavior of metallic glasses is elucidated through a unique global approach, where the topological state is exclusively linked to measured strain. Our methodology allows the computation of the average activation volume and activation energy of deformation units as a function of strain. Experimental data from four representative metallic glasses (La30Ce30Ni10Al10Co20, La65Ni15Al25, La56.16Ce14.04Ni19.8Al10, and Cu46Zr46Al8) reveal two distinct characteristics. Below the glass transition temperature, the mechanical response is primarily influenced by secondary relaxation processes and excess configuration entropy, with activation volume increasing with strain. Upon reaching the glass transition temperature, the activation volume becomes notably larger and strain-independent. Additionally, the activation energy exhibits an increase with strain, and deformation units of varying sizes are progressively activated, from smaller to larger units. The decoupling and competition among relaxation events are correlated with the increase in the activation volume of deformation units. These findings provide valuable insights into the dynamic behavior of metallic glasses and their mechanical response across different states.

Physical aging of a biodegradable alicyclic polymer: poly (pentamethylene trans-1,4-cyclohexanedicarboxylate)

Physical aging of poly (pentamethylene trans-1,4-cyclohexanedicarboxylate) (PPeCE), a biodegradable alicyclic polyester, was investigated using Fast Scanning Calorimetry (FSC), a recent calorimetric technique allowing to accelerate physical aging and study the associated relaxation processes at different aging temperatures in an experimentally-reasonable time scale. Different mechanisms were highlighted by varying the aging temperature on a temperature range of more than 60 °C. At aging temperatures well below the glass transition temperature, several relaxation mechanisms were evidenced, probably related to secondary relaxation processes (β relaxations). When the aging temperature approaches the glass transition temperature, the primary relaxation process (α relaxation) becomes predominant.

Single-molecule magnet behavior in heterometallic decanuclear [Ln2Fe8] (Ln = Y, Dy, Ho, Tb, Gd) coordination clusters.

Five decanuclear lanthanide-iron clusters, formulated as [Ln2Fe8(hmp)10(μ2-OH)4(μ3-OH)2(μ4-O)4(H2O)6]·6ClO4·xH2O (x ≈ 8, Ln = Y for 1; x ≈ 6, Ln = Dy for 2; x ≈ 6, Ln = Ho for 3; x ≈ 7, Ln = Tb for 4; x ≈ 7, Ln = Gd for 5, Hhmp = 2-(hydroxymethyl)pyridine), have been synthesized and structurally characterized. Single-crystal structural analysis reveals that the cluster consists of six face-sharing defective cubane units. Dynamic magnetic investigations indicated that cluster 2 exhibits single-molecule magnet behavior under a zero dc field eliciting an effective energy barrier of Ueff = 17.76 K and a pre-exponential factor of τ0 = 7.93 × 10-8 s. Investigation of the performance of a series of FeIII-DyIII SMMs indicates that the relatively low energy barrier in 2 is associated with the weak ferromagnetic coupling between FeIII and DyIII ions, while the strength of ferromagnetic interaction in these clusters is mainly related to the bond distances between DyIII and O atoms coordinated to FeIII ions. Clusters 3 and 4 exhibit similar dual relaxation pathways under their respective optimal external applied dc field, where the direct relaxation process occurs in the low-frequency area, which impedes the extraction of the Ueff, while the secondary relaxation process appears at a higher frequency, which is probably a connection with intermolecularly driven relaxation. Our findings offer a magneto-structural correlation model for further investigating the single-molecule magnet behavior in lanthanide-iron systems.

Hydration effects on thermal transitions and molecular mobility in Xanthan gum polysaccharides.

In this work, the xanthan gum (XG) polysaccharide is studied over a wide range of temperatures and water fractions 0 ≤ hw ≤ 0.70 (on a wet basis) by employing differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS). The investigation reveals that the critical water fraction for ice formation is about 0.35. Glass transition temperature (Tg) was determined through calorimetry experiments for all the samples studied. Water acts as a strong plasticizer, i.e., decreasing Tg, for water fractions up to about 0.35. A secondary (local) relaxation process is recorded in both dry and hydrated samples, which is sensitive to the presence of water molecules. This fact indicates that this process originates due to the orientation of small polar groups of the side chain, or/and due to the local main chain dynamics. Two types of long-range charge transport processes were resolved. The first is related to the conductive paths being formed via bulk-like ice structures (at high hydration levels), whereas the second can be attributed to proton mobility via the hydrogen bond (HB) network of non-freezing water existing in XG. Interestingly, this process is exactly the same in all the hydrated samples with hw > 0.25. With respect to the sample with hw = 0.27, a Vogel-Tammann-Fulcher (VTF)-like polarization process has also been recorded which seems to be related to long-range charge mobility via interconnected water clusters. As far as we are aware, this is the first time that XG is studied in terms of glass transition and molecular mobility over a wide range of hydration levels combining DSC and BDS techniques.

Comparison of the effect of free volume fraction on the dielectric properties of polyethylene oxide bulk and thin film

Thin film polyethylene oxide has been produced by the thermal evaporation technique. The dielectric properties and molecular dynamics of bulk and thin film polyethylene oxide have been studied in the frequency range 0,1-Hz 20 MHz and temperature range 213–333 K by broadband dielectric spectroscopy to investigate the effect of structural differences. Primary (α- relaxation) and secondary (β, γ- relaxation) relaxation processes have been analyzed for bulk and thin film polyethylene oxide. The activation energies of β-relaxation for bulk and thin film were calculated as 2,52 eV and 0,78 eV, respectively. The motion of entire polymer chains, namely, the normal mode, has been determined for both bulk and thin film polyethylene oxide samples. The glass transition temperature and the relaxation behavior have been modeled by Vogel–Fulcher–Tamman and Cole–Cole equations, respectively. The free volume fraction, which is attributed to the reason for the difference between Vogel temperature and glass transition temperature, was calculated using the Dolittle equation. It is detected that the dielectric constant of polyethylene oxide thin film is 10 times greater than polyethylene oxide bulk. There is consistency among dielectric constants, activation energies, and free volume fractions for bulk and thin film samples. It is observed that polyethylene oxide thin film is more brittle than polyethylene oxide bulk sample. It has been reported that the thin-film polyethylene oxide with a high dielectric constant can be nominated as a candidate for production in biocompatible, organic coverages and supercapacitors.

Shear and longitudinal attenuations and quality factors of poly(methyl metacrylate) (PMMA) from 20 kHz to 12 MHz investigation with ultrasonic spectroscopy

PMMA is often considered as a calibration material for experimental benches dedicated to viscoelasticity investigation. Nevertheless, regarding literature, data concerning attenuation coefficients and quality factors are essentially available in the MHz frequency range and results in the low-frequency range are scarce and scattered. In this communication, thanks to the use of high-frequency ultrasonic spectroscopy between 2 and 8 MHz in the range 6 °C − 45 °C, Time-Temperature Superposition principle and Resonant Ultrasonic Spectroscopy (RUS), we show that both longitudinal and shear quality factors of PMMA decrease considerably for low frequencies (<MHz), and that the classically accepted linear laws describing attenuation as a function of frequency are valid only beyond several MHz. This variation is attributed to secondary relaxation processes such as γ relaxation considering the activation energy deduced from experimental data. Power laws are proposed to describe the evolution of quality factors and attenuation coefficients versus frequency in the 20 kHz − 12 MHz range.

A Spontaneously Electrical State of Matter.

ConspectusMolecular deposition on solid surfaces forms crystalline or amorphous/glassy thin solid films. Intermolecular interactions govern the packing and dynamics of these films. The connection between molecular structure and intermolecular interactions is based on understanding electrostatic forces, dispersion forces and hydrogen bonding. Recently, an entire class of dipolar molecular species have demonstrated counterintuitive self-organization such that the dipole moments of individual molecules are oriented in thin films. This leads to the spontaneous generation of polarized molecular films manifesting a polarization charge equivalent to tens to hundreds of volts in strength at the film-vacuum interface, relative to the film-substrate interface. These voltages, and the corresponding electric fields present in such films, result from a collective and spontaneous orientation of molecular dipoles throughout the film during film growth and represent a metastable state of polarized material. The existence of these materials should encourage reconsideration of the importance of solid-state intermolecular electrostatic interactions.This account will detail observations of the spontaneous electric fields in molecular solids, provide insights into the dynamics and structure of molecular materials that the emergence of these electric fields can facilitate, and present a dipole-alignment based mean-field model that reproduces the temperature dependence of the electric field strength. Species as diverse as carbon monoxide, nitrous oxide, freons, simple alcohols, and cis-methyl formate have been demonstrated to spontaneously generate electric fields. We have reported electric fields more than 108 V m-1, have shown how field strength varies with the film deposition temperature, and have reported temperature-dependent Stark shifts observable in both infrared and ultraviolet absorption spectra. The latter has led to the reporting of large Wannier-Mott excitons in wide band gap molecular materials, such as solid carbon monoxide and ammonia.Changes in the field strength with time, at specific temperatures, can be related to the structural dynamics of glassy molecular solids. Measurement of surface potentials is a very sensitive technique by which to observe the rotation and translation of molecular species buried in thin films. This is particularly true for polarized, supercooled molecular glasses, where surface potentials have been used to report on secondary relaxation processes that have hitherto been hidden from measurement.Characterizing spontaneously electric molecular films, and understanding their behavior, requires the inclusion of nonlocal and nonlinear effects. The mean-field model that we present describes the data by connecting the energy of interaction of an average dipole with the mean effective field in the film, where this field is itself a function of the degree of polarization. This feedback loop produces a smooth function with a nonintuitive, discontinuous differential. The condensation of thin molecular films is an important means by which molecular solids are generated in the interstellar medium and represents a key pathway for the generation of organic optically and electrically active materials. It may also be possible to manipulate chemistry with the intense, localized electric fields acting as or acting on catalysts. The repercussions of the spontaneous generation of bound surface charges and the presence of electric fields in molecular solids will be discussed in these contexts.

Dynamic relaxation behavior and its effect on mechanical properties of FePBCCu amorphous alloy

The correlation between the dynamic relaxation behavior and micro-inhomogeneous structure of amorphous alloy is of significance for exploring the mechanical deformation. In this work, we report the pronounced secondary (β) relaxation process in FeP(B)CCu amorphous alloys with different microstructures, which reveals the local heterogeneity at atomic scale. With the help of thermodynamic analysis, it was found that compared to the partially crystallized alloy, the more local structural heterogeneities were activated in the amorphous matrix due to the low activation energy of β relaxation and the high activation energy of crystallization. Physical aging far below the crystallization temperature demonstrated that atomic rearrangement supported the weakening of β relaxation amplitude as well as the increment of storage modulus, especially for FePBCCu amorphous alloy, which was consistent well with the increases of the reduced modulus and hardness by nanoindentation tests. Moreover, the fitted results from the Maxwell-Voigt model on creep curves revealed that the diffusion and rearrangement of atoms in loosely packed zones corresponding to the β relaxation were more active during aging. Our findings provide new insight into understanding the dynamic relaxation behavior in FePBCCu amorphous alloy and render it an ideal system to probe the correlation between relaxation and mechanical deformation.

Primary and secondary relaxation processes in poly(propylene glycol) monobutyl ether: a broadband dielectric spectroscopy investigation

Primary and secondary relaxation processes in poly(propylene glycol) monobutyl ether: a broadband dielectric spectroscopy investigation

Glass transition, crystallization kinetics, and inter-conformer relaxation dynamics of amorphous mitotane and related compounds

We employ differential scanning calorimetry, broadband dielectric spectroscopy and optical microscopy to investigate the glass transition, molecular relaxation dynamics, and isothermal recrystallization kinetics of amorphous mitotane, the only drug approved for the pharmacological treatment of adrenocortical carcinoma. Amorphous mitotane displays a glass transition at Tg = 243 ± 1 K, characterized by relatively low fragility index of 68 ± 2. Besides the structural and Johari-Goldstein relaxations, amorphous mitotane displays an intramolecular relaxation with activation energy of 25 ± 1 kJ mol−1. The same relaxation process, with virtually the same activation energy and relaxation times, is observed in the closely-related o,p’-dichlorobenzophenone compound, which allows identifying it as the rotation of the chlorobenzene ring with the chlorine closest to the central carbon. Such conformational relaxation is active at human body temperature, and may thus be potentially relevant for the mechanism of action of the drug. Our study shows that the comparative study of the relaxation map of related molecular species is a powerful tool to identify and classify secondary relaxation processes. The amorphous drug is found to be unstable against recrystallization at as well as slightly below room temperature, and to display-two-dimensional growth with only sporadic nucleation, characterized by an Avrami kinetic exponent of 2.05 ± 0.05. The kinetic stability of the amorphous form of mitotane, observed at room temperature in micellar formulations, is therefore limited to the nanoconfined sample and is not observed in the bulk compound.

Intermittent-contact local dielectric spectroscopy of nanostructured interfaces

Local dielectric spectroscopy (LDS) is a scanning probe method, based on dynamic-mode atomic force microscopy (AFM), to discriminate dielectric properties at surfaces with nanometer-scale lateral resolution. Until now a sub-10 nm resolution for LDS has not been documented, that would give access to the length scale of fundamental physical phenomena such as the cooperativity length related to structural arrest in glass formers (2–3 nm). In this work, LDS performed by a peculiar variant of intermittent-contact mode of AFM, named constant-excitation frequency modulation, was introduced and extensively explored in order to assess its best resolution capability. Dependence of resolution and contrast of dielectric imaging and spectroscopy on operation parameters like probe oscillation amplitude and free amplitude, the resulting frequency shift, and probe/surface distance-regulation feedback gain, were explored. By using thin films of a diblock copolymer of polystyrene (PS) and polymethylmethacrylate (PMMA), exhibiting phase separation on the nanometer scale, lateral resolution of at least 3 nm was demonstrated in both dielectric imaging and localized spectroscopy, by operating with optimized parameters. The interface within lamellar PS/PMMA was mapped, with a best width in the range between 1 and 3 nm. Changes of characteristic time of the secondary (β) relaxation process of PMMA could be tracked across the interface with PS.

Variation in the local ordering, H-bonding pattern and molecular dynamics in the pressure densified ritonavir

In this paper, high-pressure (HP) dielectric studies on H-bonded active pharmaceutical ingredient – ritonavir indicated the narrowing of α-dispersion with compression, and consequently, a breakdown of the temperature-pressure superpositioning rule. To understand such peculiar behavior, reported so far only for a few van der Waals compounds, we performed calorimetric, structural, and infrared measurements on the pressure densified glass (as well as on ordinary glass) of the examined substance. These investigations demonstrated that variations in the structural organization of molecules and H-bonding pattern are responsible for the observed changes in the shape of the structural relaxation peak. Moreover, further isobaric HP studies at pressure 400 MPa revealed the presence of an additional secondary relaxation process (not detected at p = 0.1 MPa) in the loss spectra collected below the glass transition temperature for this compound.

Температурная эволюция взаимодействия релаксационных процессов с локальной динамикой на терагерцевых частотах в полимерах с водородными связями

At terahertz frequencies, the torsional-vibrational motion is associated with dielectric relaxation in disordered solids with hydrogen bonds. The interaction between these processes is still poorly understood, especially at temperatures below the glass transition temperature, which is especially important for molecular mobility in polymers. We have studied polymers with hydrogen bonds (polyamide-6 and polyvinyl chloride) at temperatures from 90 to 400 K in the range 0.25 - 4 THz using far-infrared spectroscopy. Three general features were observed in the spectrum of dielectric losses, ”(a) at temperatures well below the glass transition (Tg), these losses are represented by the low-frequency wing of the absorption peak due to the libration of polymer monomer units. (b) At 0.7Tg &lt;T &lt;Tg, additional temperature-dependent losses are observed, which can be associated with the manifestation of secondary relaxation processes. (c) At temperatures above Tg, primary α-relaxation processes become the dominant contribution to terahertz losses. The results obtained show that the evolution of terahertz losses with temperature is caused by a change in the structure of hydrogen bonds, which, apparently, is universal for systems with similar intermolecular interactions.

Temperature evolution of the interaction of relaxation processes with local dynamics at terahertz frequencies in polimers with hydrogen bonds

At terahertz frequencies, the torsional-vibrational motion is associated with dielectric relaxation in disordered solids with hydrogen bonds. The interaction between these processes is still poorly understood, especially at temperatures below the glass transition temperature, which is especially important for molecular mobility in polymers. We have studied polymers with hydrogen bonds (polyamide-6 and polyvinyl chloride) at temperatures from 90 to 400 K in the range 0.25-4 THz using far-infrared spectroscopy. Three general features were observed in the spectrum of dielectric losses, ε''(ν) (a) at temperatures well below the glass transition (Tg), these losses are represented by the low-frequency wing of the absorption peak due to the libration of polymer monomer units. (b) At 0.7Tg&amp;lt;T&amp;lt;Tg, additional temperature-dependent losses are observed, which can be associated with the manifestation of secondary relaxation processes. (c) At temperatures above Tg, primary α-relaxation processes become the dominant contribution to terahertz losses. The results obtained show that the evolution of terahertz losses with temperature is caused by a change in the structure of hydrogen bonds, which, apparently, is universal for systems with similar intermolecular interactions. Keywords: polymers, terahertz IR-spectra, temperature dependence, α- and β-relaxations, hydrogen bonding.

Time-scale ordering in hydrogen- and van der Waals-bonded liquids.

The time scales of structural relaxation are investigated on the basis of five different response functions for 1,2, 6-hexanetriol, a hydrogen-bonded liquid with a minor secondary contribution, and 2,6,10,15,19,23-hexamethyl-tetracosane (squalane), a van der Waals-bonded liquid with a prominent secondary relaxation process. Time scales of structural relaxation are derived as inverse peak frequencies for each investigated response function. For 1,2,6-hexanetriol, the ratios of the time scales are temperature-independent, while a decoupling of time scales is observed for squalane in accordance with the literature. An alternative evaluation approach is made on the squalane data, extracting time scales from the terminal relaxation mode instead of the peak position, and in this case, temperature-independent time-scale ratios are also found for squalane, despite its strong secondary relaxation contribution. Interestingly, the very same ordering of response-function-specific time scales is observed for these two liquids, which is also consistent with the observation made for simple van der Waals-bonded liquids reported previously [Jakobsen et al., J. Chem. Phys. 136, 081102 (2012)]. This time-scale ordering is based on the following response functions, from fast to slow dynamics: shear modulus, bulk modulus, dielectric permittivity, longitudinal thermal expansivity coefficient, and longitudinal specific heat. These findings indicate a general relation between the time scales of different response functions and, as inter-molecular interactions apparently play a subordinate role, suggest a rather generic nature of the process of structural relaxation.

Characterization of Dielectric Relaxation Process by Impedance Spectroscopy for Polymers: Nitrile Butadiene Rubber and Ethylene Propylene Diene Monomer

We invented a dispersion analysis program that analyzes the relaxation processes from dielectric permittivity based on a combination of the Havriliak–Negami and conductivity contribution functions. By applying the created program to polymers such as nitrile butadiene rubber (NBR) and ethylene propylene diene monomer (EPDM), several relaxation processes were characterized: an α process due to segmental motions of the C-C bond, an α′ process attributed to fluctuations in the end-to-end dipole vector of the polymer chain, the conduction contribution by the filler observed above room temperature, and secondary relaxation processes β and γ of motion for the side group in NBR. In the EPDM specimen, the β process associated with the rotational motion of the side groups, the α process associated with the relaxation of local segmental motion, and the αβ process associated with the origin of the β process at high temperatures above 305 K were observed. The Maxwell–Wagner–Sillars effect and conduction contribution were also presented. The molecular chains responsible for the relaxation processes were assigned by building molecular models of the two polymers. The temperature dependence of the relaxation strength and the shape parameters that characterize the process were investigated. From the temperature-dependent relaxation analysis, the merged αβ process, activation energy, and glass transition temperature were determined and compared.

Hyaluronic acid — gelatin hydrogels as bioelectrets: Charge transport and dielectric polarization effects

In this work, we study the role of water molecules on the polarization processes activated on hyaluronic acid (HA), and gelatin (Gel) hydrogels at rather low hydration levels, by employing mainly broadband dielectric spectroscopy (BDS) and thermally stimulated depolarization currents (TSDC) technique. Our results suggest that the polarization capability of the hydrogels depends strongly on the water fraction, already at temperatures as low as −90 °C, and that charge (protons) long-range mobility is inevitably related with polarization processes in the biomaterials. The polarizabilty enhances strongly at temperatures above the onset temperature for structural rearrangements and conformational fluctuations in the hydrogels. At temperatures below this critical temperature, the polarizability is related to water secondary relaxation process (ν-process) and to polarization process within ice-like structures of hydration water. These findings imply that polarization effects in biomaterials are strongly affected by contributions due to hydration water (even for traces of water).

Nonmonotonous atomic motions in metallic glasses

Peculiar temporal intermittent microscopic dynamics has been recently reported during structural relaxation in metallic glasses, though its origin is poorly understood. By using x-ray photon correlation spectroscopy, we have investigated the atomic motions in a model system of metallic glass with distinct stability and in situ stress states. We find that the microscopic dynamics can be tuned from dramatically nonmonotonous to monotonous. The intermittent dynamics is always accompanied by the activation of a secondary relaxation process, as indicated by the drop of the initial nonergodic plateau of the intensity-intensity correlation function. Both phenomena depend on the material state and can be suppressed by simultaneously lowering the fictive temperature and external loading, leading the glass in a stationary metastable state which appears independent on the thermal history of the system.

Study of various relaxation processes and breakdown of Stokes-Einstein Debye relation in supercooled D-xylose

Various relaxation procedure in the supercooled state of D-xylose have been investigated using dielectric spectroscopy over a wide frequency (1 mHz – 10 MHz) and temperature range (140-350 K). The main relaxation process (α) that is accountable for the glass transition phenomena, another secondary relaxation process is also visible below glass transition temperature. The attached hydroxymethyl (-CH2OH) side group to the sugar ring might be responsible for this process. In addition, we have applied Coupling model to find the location of intermolecular relaxation below the glass transition temperature (Tg). The dependence of various dielectric parameters viz. τα, τβ, τγ and τβ cm on temperature have been critically examined. The current experimental result shows that, log10 σdc vs. log10 τα dependence is nonlinear and hence can be described with the fractional Stokes-Einstein-Debye relation.