Relaxation Modes Research Articles

Glass transition phenomena and dielectric relaxations in supercooled d-lyxose aqueous solutions

Differential scanning calorimeter and broadband dielectric spectroscopy in a broad range of temperatures (150–300 K) were employed to study the d-lyxose aqueous mixture at different hydration levels. Two relaxation processes were observed in all investigated d-lyxose aqueous mixtures. A relaxation process (process-I) usually known as the primary relaxation mode which is accountable for the collective motion of d-lyxose aqueous solution, was observed above the glass transition temperature (Tg). Below Tg, another process designated as process-II was found which is mainly related to the water molecule relaxation inside the d-lyxose matrix. The average relaxation times as a function of temperature and dielectric strengths of both observed relaxation processes (I & II) were analyzed for all hydration levels in d-lyxose. It was identified that the relaxation amplitude of process-II in the d-lyxose aqueous mixture was increased drastically and their activation energies were found to be approximately independent of the content of water above critical concentration, xc = 0.28. This suggests that the dynamical process observed above xc was dominated by the presence of water clusters. In the current aqueous mixture, the critical content of water (xc) is slightly higher as compared to previously reported aqueous mixtures, indicating a more cooperative nature of water molecules with a d-lyxose matrix. Additionally, the Tg of pure water was estimated at 128 ± 5.8 K from the extrapolation of DSC Tg data of the d-lyxose aqueous solution by using the well-known Gordon-Taylor equation. Our current result gives further support to the well-accepted glass transition (Tg) of pure water.

Gold Nanorod-Induced Effects in a Mesogenic Compound 4-(trans-4-n-Hexylcyclohexyl) isothiocyanatobenzene.

Gold nanorods (GNRs) have a capsule-like structure with different optical properties than spherical gold nanoparticles due to surface plasmon resonance. Liquid crystals (LCs) are mesogenic compounds having crystal-like orientation and liquid-like fluidity. They are important materials from a technological point of view. Both GNRs and LC compounds are anisotropic in shape and properties. Different nano entities show interesting results when dispersed in different liquid crystalline materials which are instrumental from the application point of view. In the present work, GNRs have been dispersed in nematic liquid crystalline materials, namely 4-(trans-4-n-hexylcyclohexyl) isothiocyanatobenzene (6CHBT). Calorimetric, texture, spectroscopic, and dielectric studies were carried out for a pure 6CHBT and its composites with GNRs. Different calorimetric and dielectric parameters such as transition temperature, enthalpy, heat flow, permittivity, dielectric strength, dielectric anisotropy, and relaxation frequency have been determined, and the effect of GNRs has been explored. This article gives an insight into the influence of GNRs on the morphology and anisotropic physical properties of the nematic liquid crystalline material.

Stress relaxation of 3D printed PLA of various infill orientations under tensile and bending loadings

AbstractIn recent years, the widespread use of 3D printing technology in various industries has highlighted the crucial issue of how 3D printed polymers behave mechanically when subjected to stress relaxation. The time‐independent mechanical properties of these polymers, as well as their stress relaxation behavior, are both affected by the 3D printing parameters used. Therefore, this study examines the stress‐relaxation behavior of PLA under tensile and bending loading modes, specifically investigating how different raster orientations affect this behavior. The findings indicate that the ±45° infill orientation had the least amount of relaxation in both tensile and bending modes compared to the other orientations. Additionally, there was higher stress relaxation in the bending loading mode across all infill orientations. The findings indicate that the experimental duration should exceed 750 s to achieve an accurate model, as both Maxwell‐Weichert elements contribute to stress relaxation during this period. On the other hand, extending the test beyond 20,000 s is not necessary since neither of the elements contributes significantly to stress relaxation.

Rubber-like elasticity in laser-driven free surface flow of a Newtonian fluid

The energy needed to deform an elastic solid may be recovered, while in Newtonian fluids, like water and glycerol, deformation energy dissipates on timescales of the intermolecular relaxation time [Formula: see text] . For times considerably longer than [Formula: see text] the existence of shear elasticity requires long-range correlations, which challenge our understanding of the liquid state. We investigated laser-driven free surface bubbles in liquid glycerol by analyzing their expansion and bursting dynamics, in which we found a flow-dominating, rubber-like elasticity unrelated to surface tension forces. In extension to findings of a measurable liquid elasticity at even very low deformation frequencies [L. Noirez, P. Baroni, J. Mol. Struct. 972, 16-21 (2010), A. Zaccone, K. Trachenko, Proc. Natl. Acad. Sci. U.S.A. 117, 19653-19655 (2020)], that is difficult to access under increased strain, we find a robust, strain rate driven elasticity. The recovery of deformation energy allows the bursting bubble to reach Taylor-Culick velocities 20-fold higher than expected. The elasticity is persistent for microseconds, hence four orders of magnitude longer than [Formula: see text] . The dynamic shows that this persistence cannot originate from the far tail of a distribution of relaxation times around [Formula: see text] but must appear by frustrating the short molecular dissipation. The longer time should be interpreted as a relaxation of collective modes of metastable groups of molecules. With strain rates of 106 s-1, we observe a metastable glycerol shell exhibiting a rubber-like solid behavior with similar elasticity values and characteristic tolerance toward large strains, although the molecular interaction is fundamentally different.

Machine learning aided dimensionality reduction toward a resource efficient projective quantum eigensolver: Formal development and pilot applications.

In recent times, a variety of hybrid quantum-classical algorithms have been developed that aim to calculate the ground state energies of molecular systems on Noisy Intermediate-Scale Quantum (NISQ) devices. Albeit the utilization of shallow depth circuits in these algorithms, the optimization of ansatz parameters necessitates a substantial number of quantum measurements, leading to prolonged runtimes on the scantly available quantum hardware. Through our work, we lay the general foundation for an interdisciplinary approach that can be used to drastically reduce the dependency of these algorithms on quantum infrastructure. We showcase these pertinent concepts within the framework of the recently developed Projective Quantum Eigensolver (PQE) that involves iterative optimization of the nonlinearly coupled parameters through repeated residue measurements on quantum hardware. We demonstrate that one may perceive such a nonlinear optimization problem as a collective dynamic interplay of fast and slow relaxing modes. As such, the synergy among the parameters is exploited using an on-the-fly supervised machine learning protocol that dynamically casts the PQE optimization into a smaller subspace by reducing the effective degrees of freedom. We demonstrate analytically and numerically that our proposed methodology ensures a drastic reduction in the number of quantum measurements necessary for the parameter updates without compromising the characteristic accuracy. Furthermore, the machine learning model may be tuned to capture the noisy data of NISQ devices, and thus the predicted energy is shown to be resilient under a given noise model.

Origin of reentrant relaxor formation in ferroelectric solid solutions

Nanoscale compositional heterogeneity created by doping in ferroelectric systems leads to the formation of conventional relaxors in most cases, but reentrant relaxors in some unusual cases. It has remained a long-standing puzzle why reentrant relaxors rather than conventional relaxors form in these unusual cases. In this study, we use a binary ferroelectric system having a solid solution of a ferroelectric with cubic (C) to tetragonal (T) transition at one end and a ferroelectric with C to rhombohedral (R) transition at the other end, with nanoscale compositional heterogeneities, to reveal the origin of the reentrant relaxor transition. Our phase field simulations based on Landau theory demonstrate that the reentrant relaxor transitions in such a system are manifested by the formation of R nanodomains in the T microdomains upon cooling at compositions near the T/R phase boundary, which is accompanied by frequency-dependent permittivity peaks below ${T}_{\mathrm{C}}$. We found that the difference in phase transition sequence at different local compositions near the T/R phase boundary created by point defect doping is essential for the formation of reentrant relaxors. This work unravels the general conditions for the formation of reentrant relaxors and may shed light on the origin of other reentrant ferroic glasses.

Resolving the Emission Transition Dipole Moments of Single Doubly Excited Seeded Nanorods via Heralded Defocused Imaging.

Semiconductor nanocrystal emission polarization is a crucial probe of nanocrystal physics and an essential factor for nanocrystal-based technologies. While the transition dipole moment for the lowest excited state to ground state transition is well characterized, the dipole moment of higher multiexcitonic transitions is inaccessible via most spectroscopy techniques. Here, we realize direct characterization of the doubly excited-state relaxation transition dipole by heralded defocused imaging. Defocused imaging maps the dipole emission pattern onto a fast single-photon avalanche diode detector array, allowing the postselection of photon pairs emitted from the biexciton-exciton emission cascade and resolving the differences in transition dipole moments. Type-I1/2 seeded nanorods exhibit higher anisotropy of the biexciton-to-exciton transition compared to the exciton-to-ground state transition. In contrast, type-II seeded nanorods display a reduction of biexciton emission anisotropy. These findings are rationalized in terms of an interplay between the transient dynamics of the refractive index and the excitonic fine structure.

Effect of doping ferroelectric BaTiO3 nanoparticles on dielectric, electro-optical, and ionic properties of antiferroelectric bent-core liquid crystal

Nano-doping in polar bent-core liquid crystals is the topic of current interest for designing multifunctional smart materials. Here, we study the effect of doping of two different-sized (5–15 nm and 50–70 nm) ferroelectric BaTiO3 nanoparticles on the dielectric, electro-optical and ionic properties of antiferroelectric smectic A bent-core liquid crystal. Doping with both the sizes of nanoparticles results in lowering of the transition temperature of the liquid crystal mesophases. Both, the pure as well as doped samples show a soft mode in their antiferroelectric smectic A phase. In comparison to pure liquid crystal, the dielectric strength of the soft mode in doped samples is decreased, while their relaxation frequencies show an increasing behavior. Additionally, a decrement has been observed in the spontaneous polarization of the doped samples. A drop in polarization as well as soft mode dielectric strength values in doped samples, simultaneously point to an antiparallel dipole–dipole correlation between the ferroelectric nanoparticles and antiferroelectric liquid crystal molecules. Higher values of molecular mode relaxation frequencies obtained in nano-doped samples indicate a decrement in viscosity and dilution of pure liquid crystal. An important electro-optical parameter, the response time shows a remarkable decrement for the higher size (50–70 nm) nanoparticles doped sample. An enhancement in direct current conductivity and ion diffusion coefficient is observed for both the nano-doped samples. The smaller size (5–15 nm) ferroelectric nanoparticles reduce the mobile ion concentration and act as ion-capturing agents for the liquid crystal. A systematic investigation of the nano-doped bent-core liquid crystal can catalyze the use of the combined media for faster switching electro-optic applications.

Effect of surface treatment on the dynamics of relaxation processes in confinement: ferroelectric liquid crystal in porous Anopore membrane

ABSTRACT Dielectric properties of a ferroelectric liquid crystal 2-methylbutyl 4-decyloxybenzylidene-4’-n-aminocinnamate (DOBAMBC) confined in untreated and treated porous Anopore membrane were investigated depending on the frequency and temperature. The confinement of a ferroelectric liquid crystal in an untreated Anopore membrane leads to the disappearance of the Goldstone mode. Moreover, despite the modification of the pore surface with various surfactants, no the Goldstone mode or other additional relaxation processes were observed. The relaxation rate of the molecular δ-process is slightly shifted for untreated and treated Anopore membrane in comparison to the bulk. Generally, the relaxation rate temperature dependence shows the Arrhenius-type course, while for lecithin-treated Anopore, it is similar to the Vogel–Fulcher–Tammann-type dependence. Dielectric spectra are dominated at low frequencies by conductivity and electrode polarization effects and are modified in geometric constraints.

Deep blue thermally activated delayed fluorescent emitters based on methyl-modified acridin-9(10H)-one acceptor

Developing deep blue thermally activated delayed fluorescence (TADF) materials with high efficiencies and good color purities is still a formidable challenge. Here, two novel deep blue TADF emitters, namely MePhCz-AD and MePhCz-AD-Py, were synthesized by incorporating two 3,6-diphenyl-9H-carbazole donors onto the methyl-modified acridin-9(10H)-one (AD) acceptor, respectively. The high rigidity of AD acceptor effectively suppressed non-radiative transition and vibration relaxation of excited states, which is favorable for a good emitting quantum yield and narrow spectral width. The presence of methyl groups at 2,7-sites of AD ring slightly reduced the acceptor strength and contributed to the deep blue emission color. Incorporation of phenyls at 3,6-sites of carbazole ring delocalized the distribution of the highest occupied molecular orbital (HOMO), promoting the radiative decay process. As a result, these two emitters showed deep blue TADF with fast radiative decay rates kR > 107 s−1. In addition, with the introduction of pyridyl (Py) group at 10-site of AD ring instead of phenyl, both the spin-orbital coupling (SOC) constant between high-lying triplet states and the lowest singlet excited state (S1) and thus the reverse intersystem crossing rate constant (kRISC) were dramatically promoted for MePhCz-AD-Py. The organic light-emitting diode with MePhCz-AD-Py as doped emitter exhibited a maximum external quantum efficiency (EQEmax) of 15.4% with a relatively narrow FWHM of 60 nm and the deep-blue color coordinates of (0.15, 0.15).

Photoluminescence in Cerium-Doped Fluoride Borate Crystals

Antizeolite fluoride borates LiBa12(BO3)7F4 (LBBF) doped by Ce3+ ions demonstrate photoluminescence (PL) and hold promise for use as phosphors in white light-emitting diodes. In this study, we present the results of modeling the electronic and optical properties of Ce-containing LBBF structures where doping atoms are located at different sites of the host lattice. We also consider the presence of F-centers, which exhibit trap states located within the band gap. We further investigate the nonadiabatic excited state electronic dynamics to elucidate electron-relaxation pathways. The nonadiabatic couplings (NACs) calculations provide transition probabilities facilitated by the nuclear movement. The relaxation rates of electrons and holes are calculated using Redfield’s theory of the formalism of the reduced density matrix (RDM). The PL spectra are calculated using molecular dynamics (MD) sampling and time-integrated methods along the excited state trajectory based on NACs. Mechanisms of PL in LBBF:Ce3+ crystals are interpreted using both computational and experimental observations. This work illustrates the dependence of transition energies, intensities, and relaxation rates of Ce-containing LBBF crystals on a selection of doping sites. In addition to analysis of emission band contributed by cerium, this work allows to identify and reproduce spectral lines hypothetically corresponding to the interband and intraband transitions in F-centers of borate crystals, available in the absence of a metal center.

Geometric theory of mechanical screening in two-dimensional solids.

Holes in mechanical metamaterials, quasilocalized plastic events in amorphous solids, and bound dislocations in a hexatic matter are different mechanisms of generic stress relaxation in solids. Regardless of the specific mechanism, these and other local stress relaxation modes are quadrupolar in nature, forming the foundation for stress screening in solids, similar to polarization fields in electrostatic media. We propose a geometric theory for stress screening in generalized solids based on this observation. The theory includes a hierarchy of screening modes, each characterized by internal length scales, and is partially analogous to theories of electrostatic screening such as dielectrics and Debye-Hückel theory. Additionally, our formalism suggests that the hexatic phase, traditionally defined by structural properties, can also be defined by mechanical properties and may exist in amorphous materials.

Detecting the exponential relaxation spectrum in glasses by high-precision nanocalorimetry

Nonexponential relaxations are universal characteristics for glassy materials. There is a well-known hypothesis that nonexponential relaxation peaks are composed of a series of exponential events, which have not been verified. In this Letter, we discover the exponential relaxation events during the recovery process using a high-precision nanocalorimetry, which are universal for metallic glasses and organic glasses. The relaxation peaks can be well fitted by the exponential Debye function with a single activation energy. The activation energy covers a broad range from α relaxation to β relaxation and even the fast γ/β' relaxation. We obtain the complete spectrum of the exponential relaxation peaks over a wide temperature range from 0.63Tg to 1.03Tg, which provides solid evidence that nonexponential relaxation peaks can be decomposed into exponential relaxation units. Furthermore, the contribution of different relaxation modes in the nonequilibrium enthalpy space is measured. These results open a door for developing the thermodynamics of nonequilibrium physics and for precisely modulating the properties of glasses by controlling the relaxation modes.

Composite Films Based on Methyl Cellulose Mixtures with Poly(urethane-imide) Obtained from Their Solutions in Dimethylacetamide

Composite films of methyl cellulose and poly(urethane-imide) are obtained. At the poly(urethane-imide) content in the films above 50% phase separation of the polymers is observed. Using the methods of dynamic mechanical analysis and X-ray diffraction the structural organization of the films is studied, the temperatures of relaxation transitions are determined, and the mechanical characteristics of the composite films are investigated.

Ambient Environment Adaptive Elastomer Constructed by Microphase Separation and Segment Complexation of Triblock Copolymers.

Elastomers with environmental adaption have attracted considerable attention for advanced applications in various areas. Here, we fabricate an ambient environment adaptive elastomer by assembling triblock copolymers polystyrene-b-poly(acrylic acid)-b-polystyrene (SAS) and polystyrene-b-poly(ethylene oxide)-b-polystyrene (SES). Owing to the microphase separation of triblock polymers and hydrogen-bonding complexation of their middle segments, the SAS/SES complex presents dichotomy of vitrified hard PS domains and soft PAA/PEO domains, which presents major relaxation transition in the temperature zone 10-30 °C and relative humidity (RH) 40-60%. The SAS/SES elastomer presents quick adaption to the ambient environment change with temperature and humidity coupling. Moreover, after a loading-unloading cycle training, the SAS/SES elastomer exhibits domain orientation, low energy dissipation, high recovery ratio, and distinct strain stiffening compared with the pristine complex. The SAS/SES elastomer has potential to be used as a sensing and adaption component for complicated intelligent systems.

THE CHARACTER PORTRAYAL OF MURIEL SPARK AND THEIR UNIQUE PROFICIENCY

Muriel Spark is considered as one of the most engaging, most tantalizing writers of her generation. What is extraordinary about Sparks achievements is that she is having a large international readership. She manages to engage with many of the most serious intellectual issues of her time. It is typical of her work that it both gestures to words and acknowledges many of the debates and concerns of the age without, ever, being wholly reliant on them. She says: I possessed one very rare book … rare books didnt interest me, for their rarity but for their content … we have all started to write our memoirs, the truth, the whole truth and nothing but the truth. And we are lodging them for seventy years in a safe place until all the living people mentioned therein shall be living no longer. 1 These works… will be both valuable to the historian of the future … you should easily be able to rectify any lack or lapse in form, syntax, style, characterization, invention, local color, description, dialogue, construction and other trivialities. 2 Her literary style, therefore, has self-consciously mixed the exactness and intensity of poetic language with the more expansive and relaxed mode of the contemporary novel. Spark, to a large extent, derived her stylistic principles from her neoclassical poetry and she has, in this regard applied the neoclassical tenets of lucidity, precision and formal elegance to her prose. The paper discusses some universal experiences and worldly phenomena which Mrs. spark very beautifully reveals through her characters.

Dynamics of supercooled liquids from static averaged quantities using machine learning

We introduce a machine-learning approach to predict the complex non-Markovian dynamics of supercooled liquids from static averaged quantities. Compared to techniques based on particle propensity, our method is built upon a theoretical framework that uses as input and output system-averaged quantities, thus being easier to apply in an experimental context where particle resolved information is not available. In this work, we train a deep neural network to predict the self intermediate scattering function of binary mixtures using their static structure factor as input. While its performance is excellent for the temperature range of the training data, the model also retains some transferability in making decent predictions at temperatures lower than the ones it was trained for, or when we use it for similar systems. We also develop an evolutionary strategy that is able to construct a realistic memory function underlying the observed non-Markovian dynamics. This method lets us conclude that the memory function of supercooled liquids can be effectively parameterized as the sum of two stretched exponentials, which physically corresponds to two dominant relaxation modes.

Ordered and amorphous phases of polyacrylonitrile: Effect of tensile deformation of structure on relaxation and glass transition

Calorimetry and mechanical tests on polyacrylonitrile PAN show two glass transitions (Tg). The lower Tg (370 K) has been attributed to its ordered phase with the amorphous regions showing a glass transition at 420 K. An understanding of these processes at the molecular level is lacking as is the counterintuitive lower Tg for the ordered phase as compared to the amorphous one. Molecular dynamics (MD) simulations of the amorphous and ordered PAN phases result in Tg in the range 445–455 K and 450–465 K, respectively. The amorphous value is in good agreement with the experimental result once rate effects are considered. However, MD predicts a slightly higher Tg for the ordered phase; this is as expected but in disagreement with the experimental observation. To explain this discrepancy, we built a large-scale amorphous PAN sample and mechanically strained it above its Tg to achieve a structure consistent with experiments, including the coexistence of amorphous and ordered regions. The resulting ordered phase shows the hexagonal pattern seen experimentally and, interestingly, these domains are surrounded by nanovoids caused by the large local deformations and molecular alignment. We characterized the backbone torsional mobility of the two phases in the deformed structures as a function of temperature and found that the amorphous region exhibits similar relaxation dynamics as compared with homogeneous samples. However, the ordered phase has significantly faster relaxation dynamics which can explain the lower experimental Tg.

Lubricating performance of polytetrafluoroethylene hybrid fabric composites with removed thermosetting resin at cryogenic temperatures

AbstractAt cryogenic temperatures, the higher friction coefficient of conventional fabric composites severely inhibits the application of hybrid polytetrafluoroethylene (PTFE)/amide fabric composites in pivotal tribological pairs such as plain bearings operated at low temperatures. In order to improve the lubricating properties of fabric composites at cryogenic and wide temperature ranges, a novel hybrid PTFE/Nomex fabric composite is prepared by peeling off the facial phenolic formaldehyde resin through the Hot‐Press Mechanical Stripping method, which is named Phenolic Formaldehyde Mechanical Stripping (PFMS). The friction coefficient of PFMS is 0.06 at 296 K, and can be as low as 0.1 at 123 K, reduced by up to 58% compared to conventional fabric composite without resin removal. Furthermore, the friction coefficient of PFMS first increases and then decreases as the temperature decreases, the turning point falls in the range around the temperature of 181 K, corresponding to the γ relaxation transition of the PTFE. The wear of the fabric composites, the microstructure, and the chemical composition of the transfer film are comprehensively investigated. The results demonstrate that thermal activation and increased mechanical properties contribute to friction coefficient variation above and below the temperature of 181 K, respectively.

Unraveling the Vibrational Spectral Signatures of a Dislocated H Atom in Model Proton-Coupled Electron Transfer Dyad Systems.

Phenol-benzimidazole and phenol-pyridine proton-coupled electron transfer (PCET) dyad systems are computationally investigated to resolve the origins of the asymmetrically broadened H-bonded OH stretch transitions that have been previously reported using cryogenic ion vibrational spectroscopy in the ground electronic state. Two-dimensional (2D) potentials describing the strongly shared H atom are predicted to be very shallow along the H atom transfer coordinate, enabling dislocation of the H atom between the donor and acceptor groups upon excitation of the OH vibrational modes. These soft H atom potentials result in strong coupling between the OH modes, which exhibit significant bend-stretch mixing, and a large number of normal mode coordinates. Vibrational spectra are calculated using a Hamiltonian that linearly and quadratically couples the H atom potentials to over two dozen of the most strongly coupled normal modes treated at the harmonic level. The calculated vibrational spectra qualitatively reproduce the asymmetric shape and breadth of the experimentally observed bands in the 2300-3000 cm-1 range. Interestingly, these transitions fall well above the predicted OH stretch fundamentals, which are computed to be surprisingly red-shifted (<2000 cm-1). Time-dependent calculations predict rapid (<100 fs) relaxation of the excited OH modes and instant response from the lower-frequency normal modes, corroborating the strong coupling predicted by the model Hamiltonian. The results highlight a unique broadening mechanism and complicated anharmonic effects present within these biologically relevant PCET model systems.