Main Relaxation Research Articles

Estimating glass transition temperature and related dynamics of molecular glass formers combining artificial neural networks and disordered systems theory

Glass transition temperature and related dynamics play an essential role in amorphous materials research since many of their properties and functionalities depend on molecular mobility. However, the temperature dependence of the structural relaxation time for a given glass former is only experimentally accessible after synthesizing it, implying a time-consuming and costly process. In this work, we propose combining artificial neural networks and disordered systems theory to estimate the glass transition temperature and the temperature dependence of the main relaxation time based on the knowledge of the molecule's chemical structure. This approach provides a way to assess the dynamics of molecular glass formers, with reasonable accuracy, even before synthesizing them. We expect this methodology to boost industrial development, save time and resources, and accelerate the scientific understanding of structure-properties relationships. • New joint theoretical and numerical approach to estimate the molecular dynamics • Machine learning for understanding structure-property relationships • Artificial neural networks for the design and development of new materials • Combining artificial neural networks and disordered systems theory • Estimating glass transition temperature and related dynamics of molecular glass formers

Spin dynamics of positively charged excitons in Cr+ -doped quantum dots probed by resonant photoluminescence

We study the dynamics of the spin system that consists of a positively charged II-VI semiconductor quantum dot doped with a single ${\mathrm{Cr}}^{+}$ ion. The resonant photoluminescence (PL) of the positively charged exciton coupled with the ${\mathrm{Cr}}^{+}$ spin is used to analyze the main spin relaxation channels. The intensity of the resonant PL is reduced by an optical pumping of the spin of the resident hole-${\mathrm{Cr}}^{+}$ complex that can be seen as a nanomagnet. The spin memory can be partially erased by a nonresonant optical excitation. This leads to an increase of the resonant PL signal. The resonant PL is cocircularly polarized and corresponds to relaxation channels that conserve the ${\mathrm{Cr}}^{+}$ spin $|{S}_{z}|$. The observation in the resonant-PL excitation spectra of optical transitions with a change of the ${\mathrm{Cr}}^{+}$ spin permits to determine the magnetic anisotropy of the magnetic atom. Optical pumping, autocorrelation measurements, and the power dependence of the PL intensity distribution show that the effective temperature of the hole-${\mathrm{Cr}}^{+}$ spin system is affected by the optical excitation through the local generation of phonons.

Non‐destructive thermal ageing evaluation of P (VDF‐HFP) film based on broadband dielectric response

Capacitors and sensors based on polymer dielectric materials are key components in electrical systems and electronic devices. Considering the temperature rise during operation, an effective evaluation method for the thermal ageing of polymer matrix is urgently needed. P(VDF-HFP) film with a thickness of 20 μm is manufactured by solution casting and a thermal ageing experiment up to 1000 h is conducted. Dielectric responses of wide ranges of temperatures and frequencies are measured for samples with different ageing durations. The universal relaxation law and Dissado–Hill dielectric response model are applied to further investigate the results. The characteristics of individual processes are obtained, which demonstrate that the main chain relaxation following the Vogel−Fulcher−Tammann equation and side chain relaxation following the Arrhenius equation both remain unchanged after thermal ageing. However, the activation energies of those two processes change significantly during the process and show different variation trends. The activation energy of the side chain relaxation gets larger monotonically due to the influence of annealing and the breaking of crystalline region. As a result, a non-destructive evaluation method for the thermal ageing of P(VDF-HFP) is proposed accordingly.

Simulations of nonradiative processes in semiconductor nanocrystals

The description of carrier dynamics in spatially confined semiconductor nanocrystals (NCs), which have enhanced electron-hole and exciton-phonon interactions, is a great challenge for modern computational science. These NCs typically contain thousands of atoms and tens of thousands of valence electrons with discrete spectra at low excitation energies, similar to atoms and molecules, that converge to the continuum bulk limit at higher energies. Computational methods developed for molecules are limited to very small nanoclusters, and methods for bulk systems with periodic boundary conditions are not suitable due to the lack of translational symmetry in NCs. This perspective focuses on our recent efforts in developing a unified atomistic model based on the semiempirical pseudopotential approach, which is parameterized by first-principle calculations and validated against experimental measurements, to describe two of the main nonradiative relaxation processes of quantum confined excitons: exciton cooling and Auger recombination. We focus on the description of both electron-hole and exciton-phonon interactions in our approach and discuss the role of size, shape, and interfacing on the electronic properties and dynamics for II-VI and III-V semiconductor NCs.

Molecular dynamics of alkyl benzoate liquid crystals in the bulk state and in the surface layer of their composites with oxide nanopowders

This paper presents the results concerning monotropic nematic liquid crystals 4-pentylphenyl 4′-alkyl benzoate (5PnB) (n = 3 or 5 carbon atoms in the alkyl chain). Their mesophase properties were supported by images of the polarized optical microscopy. Molecular dynamics in the bulk samples or in the composites prepared with aerosil A380 was investigated by broadband dielectric spectroscopy in a large temperature range, appropriately chosen. Thermo gravimetric and infrared investigations were additionally performed. The data were compared with those of structurally related nematics like cyanophenyl pentyl benzoates, which have a cyan group instead of the pentyl chain. The dielectric spectra of the bulk 3P5B and 5P5B demonstrate a dielectric behavior with several relaxation processes as expected for nematic liquid crystals. The temperature dependence of the relaxation rates (and of the dielectric strength) seems to have two distinguished regimes. Thus, in the isotropic state, at higher temperatures the data obey the Vogel–Fulcher–Tammann law, whereas an Arrhenius law is fitted at lower temperature, in a close similarity to the behavior of a constrained dynamic glass transition. Samples with a high density of silica (larger than 7 g aerosil/1 g of 5PnB) were prepared to observe a thin layer adsorbed on the particle surface; it was estimated that almost each guest 5PnB molecule interacts with the aerosil surface. For the composites only one main relaxation process is observed at frequencies much lower than those for the corresponding bulk, which was assigned to the dynamics of the molecules in the surface layer. Infrared spectroscopy shows that these molecules interact with the surface by the ester carbonyl group leading to the monolayer self-assembly at liquid–solid interface. We note once more the importance of the functional unit(s) for the interaction with the hydroxyl groups on the aerosil surface.

Approaching Polymer Dynamics Combining Artificial Neural Networks and Elastically Collective Nonlinear Langevin Equation.

The analysis of structural relaxation dynamics of polymers gives an insight into their mechanical properties, whose characterization is used to qualify a given material for its practical scope. The dynamics are usually expressed in terms of the temperature dependence of the relaxation time, which is only available through time-consuming experimental processes following polymer synthesis. However, it would be advantageous to estimate their dynamics before synthesizing them when designing new materials. In this work, we propose a combined approach of artificial neural networks and the elastically collective nonlinear Langevin equation (ECNLE) to estimate the temperature dependence of the main structural relaxation time of polymers based only on the knowledge of the chemical structure of the corresponding monomer.

Dielectric Properties of Water in Charged Nanopores.

In this study, we examine the spectral dielectric properties of liquid water in charged nanopores over a wide range of frequencies (0.3 GHz to 30 THz) and pore widths (0.3 to 5 nm). This has been achieved using classical molecular dynamics simulations of hydrated Na-smectite, the prototypical swelling clay mineral. We observe a drastic (20-fold) and anisotropic decrease in the static relative permittivity of the system as the pore width decreases. This large decrement in static permittivity reflects a strong attenuation of the main Debye relaxation mode of liquid water. Remarkably, this strong attenuation entails very little change in the time scale of the collective relaxation. Our results indicate that water confined in charged nanopores is a distinct solvent with a much weaker collective nature than bulk liquid water, in agreement with recent observations of water in uncharged nanopores. Finally, we observe remarkable agreement between the dielectric properties of the simulated clay system against a compiled set of soil samples at various volumetric water contents. This implies that saturation may not be the sole property dictating the dielectric properties of soil samples, rather that the pore-size distribution of fully saturated nanopores may also play a critically important role.

Deformations, relaxation, and broken symmetries in liquids, solids, and glasses: A unified topological field theory.

We combine hydrodynamic and field theoretic methods to develop a general theory of phonons as Goldstone bosons in crystals, glasses, and liquids based on nonaffine displacements and the consequent Goldstone phase relaxation. We relate the conservation, or lack thereof, of specific higher-form currents with properties of the underlying deformation field-nonaffinity-which dictates how molecules move under an applied stress or deformation. In particular, the single-valuedness of the deformation field is associated with conservation of higher-form charges that count the number of topological defects. Our formalism predicts, from first principles, the presence of propagating shear waves above a critical wave vector in liquids, thus giving a formal derivation of the phenomenon in terms of fundamental symmetries. The same picture provides also a theoretical explanation of the corresponding "positive sound dispersion" phenomenon for longitudinal sound. Importantly, accordingly to our theory, the main collective relaxation timescale of a liquid or a glass (known as the α relaxation for the latter) is given by the phase relaxation time, which is not necessarily related to the Maxwell time. Finally, we build a nonequilibrium effective action using the in-in formalism defined on the Schwinger-Keldysh contour, that further supports the emerging picture. In summary, our work suggests that the fundamental difference between solids, fluids, and glasses has to be identified with the associated generalized higher-form global symmetries and their topological structure, and that the Burgers vector for the displacement fields serves as a suitable topological order parameter distinguishing the solid (ordered) phase and the amorphous ones (fluids, glasses).

Mechanism behind Time Dependent Elasticity of Crumb Rubber-Nano-Asphalt Hybrids Using Discrete Relaxation Spectrum

Crumb rubber powder is a successfully used renewable material obtained from waste tire rubber, which has been incorporated into paving asphalt since the 1930s due to its good resistance to deformation and fatigue as well as its eco-friendly performance. In this study, carbon nanotubes and nano silica were incorporated into the terminal blend crumb rubber modified asphalt technology to remedy the issues of excessive desulfurization and degradation of ground tyre rubber with this technology. The mechanism behind the high temperature delayed elastic properties of the crumb rubber-nano-asphalt hybrids was experimentally investigated based on discrete relaxation spectrum. Development of the discrete relaxation spectra was accomplished by fitting on the 60°C storage modulus data tested by the dynamic shear rheometer using the generalized Maxwell model. Subsequently, the feasibility of characterizing delayed asphalt elasticity using main relaxation time was verified by test results from the 60°C creep and recovery test. Results indicated that the crumb rubber-nano-asphalt hybrids exhibited arrheodictic behavior and the asphalt elasticity was strengthened by two nano agents. Moreover, the elasticity reinforcement with carbon nanotubes was greater than with nano silica. Additionally, a good correlation was observed between the 60°C zero shear viscosity and main relaxation time, and greater 60°C zero shear viscosity was correlated to longer main relaxation times. Furthermore, longer main relaxation time of the asphalt was related to greater average recovery rate in the creep and recovery test. This research is expected to shed some light on the mechanism behind time-dependent elasticity of crumb rubber modified asphalt from the perspective of polymer physics.

Interfacial dynamics analysis in starch nanocrystal/ poly (butyl methacrylate) nanocomposites: Impact of the reinforcement’s functionalization

Nanocomposites based on waterborne polymer dispersion and biobased nanoparticles have gained significant interest because they offer a wide range of dimensionality and shape, and they are abundantly available from renewable biobased sources. Although polymer-filler interaction is key to the resulting performance of the nanocomposites, these interfacial properties are challenging to measure and analyze. In the present work, dielectric spectroscopy was used to investigate the interfacial characteristics of nanocomposites based on Poly(Butyl-methacrylate) (PBMA) and starch nanocrystals (SNCs) prepared by in-situ emulsion polymerization, where SNCs was the sole stabilizer. Unmodified and vinyltriethoxysilane-functionalized SNCs (VTES-SNCs) were used as both reinforcements and colloidal stabilizers to control the degree of binding of SNCs on PBMA. Two main relaxation processes were detected and identified as Maxwell-Wagner-Sillars (MWS) polarization and dipolar relaxation. The origin of these relaxations and their evolution based on the SNCs content and functionalization were discussed in terms of the SNCs-polymer matrix interaction mechanism. Most notably, an apparent effect of SNCs loading on the electrical conductivity of nanocomposites was revealed, pointing to evidence of percolation of the SNCs despite their non-conductive character.

Relaxing and working from home: associations between heating, ventilation and cooling system typologies and indoor soundscape evaluation

Data from an online survey conducted in January 2021 by 464 participants living in London and working from home (WFH) after the COVID-19 outbreak were analysed, focusing on: (1) types of building services at home, (2) perceived sound dominance of building services, and (3) the perception of the indoor acoustic environment (i.e. the indoor soundscape) in relation to two main activities, i.e. WFH and relaxation. Results show that most of participants’ houses had radiators for heating and relied on window opening for ventilation and cooling. Air systems (e.g., HVAC systems) resulted in higher perceived dominance compared to other systems, but only when evaluated for WFH. Sound dominance from building services was in turn related to soundscape evaluation. Spaces with less dominant sounds from building services were more appropriate for both WFH and relaxation, and spaces with fewer dominant sounds were assessed better, but just for WFH. Participants’ evaluations generally did not differ according to building service typology. The presence of air-cooling systems was associated with better perceived sound environments, most likely due to better acoustics conditions in newly built or retrofitted dwellings, more probably equipped with air cooling systems. Preliminary findings point out the importance of carefully considering the dominance of sounds by building services, especially for air systems, in relation to traditional and new uses of residential buildings.

Energy relaxation dynamics of hydrogen-bonded OH vibration conjugated with free OH bond at an air/water interface.

Vibrational energy relaxation dynamics of the excited hydrogen-bonded (H-bonded) OH conjugated with free OH (OD) at an air/water (for both pure water and isotopically diluted water) interface are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The calculated results are compared with those of the excited H-bonded OH in bulk liquid water reported previously. In the case of pure water, the relaxation timescale (vibrational lifetime) of the excited H-bonded OH at the interface is T1 = 0.13 ps, which is slightly larger than that in the bulk (T1 = 0.11 ps). Conversely, in the case of isotopically diluted water, the relaxation timescale of T1 = 0.74 ps in the bulk decreases to T1 = 0.26 ps at the interface, suggesting that the relaxation dynamics of the H-bonded OH are strongly dependent on the surrounding H-bond environments particularly for the isotopically diluted conditions. The relaxation paths and their rates are estimated by introducing certain constraints on the vibrational modes except for the target path in the NE-AIMD simulation to decompose the total energy relaxation rate into contributions to possible relaxation pathways. It is found that the main relaxation pathway in the case of pure water is due to intermolecular OH⋯OH vibrational coupling, which is similar to the relaxation in the bulk. In the case of isotopically diluted water, the main pathway is due to intramolecular stretch and bend couplings, which show more efficient relaxation than in the bulk because of strong H-bonding interactions specific to the air/water interface.

Study of the Effect of Physical Aging on the Polylactic Acid by Thermal and Optical Techniques

In this work, we investigated the influence of physical aging on polylactic acid (PLA) films using thermal and optical techniques; Differential Scanning Calorimetry (DSC), Thermally Stimulated Depolarization Current (TSDC), and Attenuated Total Reflection Spectroscopy (ATR). The PLA films were aged for different periods: 60, 90, and 120 minutes at a temperature Ta= 43 °C. The result obtained by DSC showed that the effect of physical aging appeared as an endothermic peak, which increased with increasing aging time and evolved towards higher temperatures. TSDC results showed a thermal current peak located between 30 and 80 ° C, which represented the main relaxation mode (α relaxation) of the dielectric manifestation of the glass transition. The intensity of this peak decreased and was shifted to higher temperatures when aging time increased, this result can be explained by a decrease in the molecular mobility of macromolecular chains due to the decrease in the free volume. The effect of physical aging on the PLA by the ATR technique showed a gradual decrease in all absorption bands during the aging period. In particular, the wide absorption band between 3000 and 3700 cm-1attributed to the hydroxyl group (OH), which disappeared after two hours of aging

Co-ferrite nanostructures prepared by solvothermal route; new ultra-low k dielectrics

Co spinel ferrite nanoparticles were successfully prepared using a solvothermal route. Their structure, morphology and cationic distribution were studied by X-ray diffraction, Transmission Electron Microscopy and 57Fe Mössbauer Spectrometry. Though, their electric conductivity and dielectric relaxation were studied on nanostructured pellets resulting from the compaction of nanoparticles, by complex impedance spectroscopy in the frequency range 1–106 Hz from 25 up to 250 °C. The dielectric properties are investigated in detail. The presence of two thermally activated relaxation peaks in the modulus loss spectra confirms the contribution of grains and grain boundaries to the electrical behavior of the material. The main dielectric relaxation comportment and the conduction process are governed by the grain boundaries. Ac-conductivity, which deviates from the usual Universal Dielectric Response, can be described by a double power-law evidenced by a Nearly Constant Loss trend followed by a Super-Linear Power Law that decreases with increasing temperature. The change in the cationic distribution in CoFe2O4, deduced from the 57Fe Mössbauer spectra, may explain the observed trend in conductivity. The classical dielectric relaxation behavior is observed and very low values of permittivity are estimated. This compound can therefore be classified as an ultra-low-k dielectric.

Electronic characteristics of PbS quantum dots passivated by halides on different surfaces

The electron properties of PbS quantum dots (QDs) with different halogens passivation on different surfaces were studied by first principle calculation. The relaxation analysis showed that the vertical direction of the surface is the main relaxation direction for the PbS QDs surface. Energy analysis shows that the greater the surface energy induces higher adsorption energy. The positive adsorption energy implies that all adsorption models are exothermic and thermodynamically stable. Compared with the I atom with larger atomic number, the F atom with smaller atomic number releases more heat after adsorption on the surface, which makes the reaction easier. The energy level shift results suggest that the vacuum energy level has shifted, leading to the PbS band edge shift. The investigations dealing with the partial density of states (PDOS) show that trap state energy levels could be observed in the bandgap of PbS (110). Furthermore, with the smaller atomic number and more serious the orbital localization, more energy levels are introduced into the bandgap. It can be seen from the differential charge distribution that the S-I, I-I and Pb-I covalent bonds are formed on PbS(100), PbS(110) and PbS(111) respectively.

State-to-state modeling of oxygen relaxation taking into account electron kinetics

Coupled vibrational, chemical and electronic kinetics in oxygen is studied using the detailed state-to-state model. Simulation of zero-dimensional oxygen relaxation in the presence of free electrons in the temperature range 300 – 1000 K is carried out; the master equations for the vibrational state populations and densities of excited species are solved together with the Boltzmann equation for the electron density distribution. Different models for heavy-particle interactions are assessed. It is shown that the forced harmonic oscillator model is suitable in the entire temperature range. The effect of the initial conditions and the presence of free electrons is discussed; it is shown that including electrons modifies the main relaxation mechanisms. Recommendations are given for extending the kinetic scheme to high-temperature regimes.

Dynamics in the plastic crystalline phase of cyanocyclohexane and isocyanocyclohexane probed by 1H field cycling NMR relaxometry.

Proton Field-Cycling (FC) nuclear magnetic resonance (NMR) relaxometry is applied over a wide frequency and temperature range to get insight into the dynamic processes occurring in the plastically crystalline phase of the two isomers cyanocyclohexane (CNCH) and isocyanocyclohexane. The spin-lattice relaxation rate, R1(ω), is measured in the 0.01-30MHz frequency range and transformed into the susceptibility representation χNMR ″ω=ωR1ω. Three relaxation processes are identified, namely, a main (α-) relaxation, a fast secondary (β-) relaxation, and a slow relaxation; they are very similar for the two isomers. Exploiting frequency-temperature superposition, master curves of χNMR ″ωτ are constructed and analyzed for different processes. The α-relaxation displays a pronounced non-Lorentzian susceptibility with a temperature independent width parameter, and the correlation times display a non-Arrhenius temperature dependence-features indicating cooperative dynamics of the overall reorientation of the molecules. The β-relaxation shows high similarity with secondary relaxations in structural glasses. The extracted correlation times well agree with those reported by other techniques. A direct comparison of FC NMR and dielectric master curves for CNCH yields pronounced difference regarding the non-Lorentzian spectral shape as well as the relative relaxation strength of α- and β-relaxation. The correlation times of the slow relaxation follow an Arrhenius temperature dependence with a comparatively high activation energy. As the α-process involves liquid-like isotropic molecular reorientation, the slow process has to be attributed to vacancy diffusion, which modulates intermolecular dipole-dipole interactions, possibly accompanied by chair-chair interconversion of the cyclohexane ring. However, the low frequency relaxation features characteristic of vacancy diffusion cannot be detected due to experimental limitations.

X-Ray diffraction analysis and modeling of the depth profile of lattice strains in AlGaN stacks

One of the requirements for the functionality of the AlGaN/GaN high-electron mobility transistors is the presence of a specific residual lattice strain in their active region. This lattice strain can be adjusted by depositing a multilayer stack that consists of individual AlxGa1−xN layers with harmonized [Al]/[Ga] ratio, which produces a controlled depth gradient of the lattice misfit and consequently a controlled gradient of the residual lattice strain. However, the lattice misfit at the interfaces between adjacent layers is partially compensated by misfit dislocations and other microstructure features, e.g., V-shaped interfacial pyramids. In this contribution, a non-destructive method for simultaneous quantitative analysis of remaining lattice strains and density of microstructure defects compensating the lattice misfit is presented and illustrated on the example of AlGaN stacks with different composition profiles, which were deposited using metalorganic chemical vapor phase epitaxy on (111)-oriented Si wafers. This technique is based on the mapping of selected reciprocal lattice points using X-ray diffraction. The lattice strains obtained from the reciprocal space mapping were verified by confocal micro-Raman spectroscopy. The presence of the microstructure defects and the depth gradient of their density were confirmed by transmission electron microscopy. The interplay of the composition profile and the thickness of individual layers on the lattice strain is described by a micromechanical model. From the comparison of the experimental results with the lattice strains, which were predicted for a defect-free multilayer stack, the main strain relaxation mechanisms are concluded, and their impact on the strain relaxation is quantified.

Effect of ultrasound-assisted thawing on gelling and 3D printing properties of silver carp surimi

The effects of ultrasound-assisted thawing (at ultrasonic frequencies of 45, 80 and 100 kHz) and water immersion thawing on gelling and 3D printing properties of silver carp surimi were examined. Ultrasound-assisted thawing (UT) can save 13.5%~40.4% time, and high ultrasonic frequency (80 kHz and 100 kHz) did not cause high thawing loss. Thawing at higher ultrasonic frequency could reduce the damage of the secondary and tertiary structure of myofibrillar proteins. No significant differences were observed for the main relaxation component (T22) and its peak area proportion (P22), and rheological properties, resulting in similar printing performance. After steam heating, cuboid samples in UT-100 kHz group kept the best geometrical shapes and had the highest hardness, springiness, and chewiness. Thus, ultrasound-assisted thawing provides a promising thawing method in food materials of 3D printing.

New contributions from thermal and dielectric techniques to the understanding of the dynamic properties of medium to long chain poly (propylene glycols)

AbstractThe molecular mobility of poly (propylene glycols) with molecular weights between 2000 and 18,200 g mol−1 was studied in the amorphous solid state, in the region of the glass transition and in the rubbery state; the experimental techniques used were differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). It was found that the glass transition temperature increases slightly with MW and tends to a limit value: TgDSC = −67°C, TgTSDC = −73°C.The different types of molecular mobility were analyzed: below Tg the secondary relaxations, and above this temperature the normal mode, were characterized in detail using TSDC; in the glassy transformation region, around Tg, the main relaxation was analyzed by both DSC and TSDC. It was found the dynamic fragility, m, behaves as independent of the molecular weight in the studied range: mDSC = 74 and mTSDC = 92. The TSDC results also show that, without influencing the kinetics of the segmental relaxation, the increase in molecular weight slows down the chain relaxation.