Main Relaxation Research Articles

Thermal evolution of polar nanoregions identified by the relaxation time of electric modulus in the Bi1/2Na1/2TiO3 system

The correspondence between the temperature dependence of phase structures and the experimental physical properties was made clear for the first time in the Bi1/2Na1/2TiO3 system according to in situ XRD, in situ Raman and impedance spectroscopy. XRD profiles show pseudo-cubic symmetry independent of temperature, while one of the Raman models disappears near 620 K, indicating that the local symmetry increases. The temperature dependence of the main relaxation time of the electric modulus spectra can be divided into four regions, characterizing the thermal evolution of polar nanoregions (PNRs) with different symmetries and a phase transition between rhombohedral and tetragonal symmetry. Therefore, the relaxation times of the electric modulus provide a way to estimate the local phase transition and the thermal evolution of R3c and P4bm PNRs.

Non-polymeric asymmetric binary glass-formers. I. Main relaxations studied by dielectric, 2H NMR, and 31P NMR spectroscopy.

In Paper I of this series of two papers we study the main relaxations of a binary glass former made of the low-Tg component tripropyl phosphate (TPP, Tg = 134 K) and of a specially synthesized (deuterated) spirobichroman derivative (SBC, Tg = 356 K) as the non-polymeric high-Tg component for the full concentration range. A large Tg contrast of the neat components is put into effect. Dielectric spectroscopy and different techniques of 2H nuclear magnetic resonance (NMR) as well as of 31P NMR spectroscopy allow to selectively probe the dynamics of the components. For all concentrations, two well separated liquid-like processes are identified. The faster α2-process associated with the low-Tg component TPP shows pronounced dynamic heterogeneities reflected by quasi-logarithmic correlation functions at low TPP concentrations. The slower α1-process involves the reorientation of the high-Tg component SBC. Its correlation function is Kohlrausch-like as in neat glass formers. The corresponding time constants and consequently their glass transition temperatures Tg1 and Tg2 differ more the lower the TPP concentration is. Plasticizer and anti-plasticizer effect, respectively, is observed. At low temperatures a situation arises that the TPP molecules isotropically reorient in an arrested SBC matrix (Tg2 < T < Tg1). At T < Tg2 the liquid-like reorientation of TPP gets arrested too. We find indications that a fraction of the TPP molecule takes part in the slower α1-process of the high-Tg component. All the features known from polymer-plasticizer systems are rediscovered in this non-polymeric highly asymmetric binary mixture. In Paper II [B. Pötzschner et al., J. Chem. Phys. 146, 164504 (2017)] we study the secondary (β-) relaxations of the mixtures.

Non-polymeric asymmetric binary glass-formers. II. Secondary relaxation studied by dielectric, 2H NMR, and 31P NMR spectroscopy

We investigate the secondary (β-) relaxations of an asymmetric binary glass former consisting of a spirobichroman derivative (SBC; Tg = 356 K) as the high-Tg component and the low-Tg component tripropyl phosphate (TPP; Tg = 134 K). The main relaxations are studied in Paper I [B. Pötzschner et al., J. Chem. Phys. 146, 164503 (2017)]. A high Tg contrast of ΔTg = 222 K is put into effect in a non-polymeric system. Component-selective studies are carried out by combining results from dielectric spectroscopy (DS) for mass concentrations cTPP ≥ 60% and those from different methods of 2H and 31P NMR spectroscopy. In the case of NMR, the full concentration range (10% ≤ cTPP ≤ 100%) is covered. The neat components exhibit a β-relaxation (β1 (SBC) and β2 (TPP)). The latter is rediscovered by DS in the mixtures for all concentrations with unchanged time constants. NMR spectroscopy identifies the β-relaxations as being alike to those in neat glasses. A spatially highly restricted motion with angular displacement below ±10° encompassing all molecules is involved. In the low temperature range, where TPP shows the typical 31P NMR echo spectra of the β2-process, very similar spectral features are observed for the (deuterated) SBC component by 2H NMR, in addition to its "own" β1-process observed at high temperatures. Apparently, the small TPP molecules enslave the large SBC molecules to perform a common hindered reorientation. The temperature dependence of the spin-lattice relaxation time of both components is the same and reveals an angular displacement of the SBC molecules somewhat smaller than that of TPP, though the time constants τβ2 are the same. Furthermore, T1(T) of TPP in the temperature region of the β2-process is absolutely the same as in the mixture TPP/polystyrene investigated previously. It appears that the manifestations of the β-process introduced by one component are essentially independent of the second component. Finally, at cTPP ≤ 20% one finds indications that the β2-process starts to disintegrate. More and more TPP molecules get immobilized upon decreasing cTPP. We conclude that the β-process is a cooperative process.

Study of the molecular mobility of (±)-methocarbamol in the amorphous solid state

The experimental techniques of differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) were used to study the thermal behavior of the pharmaceutical drug (±)-methocarbamol and its slow molecular mobility (in the 10−3–10−2 Hz range) in the amorphous solid state. The possibility of polymorphism was considered based on the DSC results. The glass forming ability and the glass stability were investigated by DSC, and the general kinetic features of the main relaxation, including the fragility or steepness index, were studied by both experimental techniques. The secondary relaxations detected by TSDC revealed fast and slow (Johari-Goldstein) modes. These secondary relaxations of different nature were assigned based on physical aging studies.

Insights into the deactivation of 5-bromouracil after ultraviolet excitation.

5-Bromouracil is a nucleobase analogue that can replace thymine in DNA strands and acts as a strong radiosensitizer, with potential applications in molecular biology and cancer therapy. Here, the deactivation of 5-bromouracil after ultraviolet irradiation is investigated in the singlet and triplet manifold by accurate quantum chemistry calculations and non-adiabatic dynamics simulations. It is found that, after irradiation to the bright ππ* state, three main relaxation pathways are, in principle, possible: relaxation back to the ground state, intersystem crossing (ISC) and C-Br photodissociation. Based on accurate MS-CASPT2 optimizations, we propose that ground-state relaxation should be the predominant deactivation pathway in the gas phase. We then employ different electronic structure methods to assess their suitability to carry out excited-state dynamics simulations. MRCIS (multi-reference configuration interaction including single excitations) was used in surface hopping simulations to compute the ultrafast ISC dynamics, which mostly involves the 1nOπ* and 3ππ* states.This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.

Formation and structure of chitosan–poly(sodium methacrylate) complex nanoparticles

ABSTRACTInterpolyelectrolyte complex (IPEC) dispersions were prepared from chitosan and poly(sodium acrylate), NaPMA, by mixing their solutions, at different carboxyl-to-aminium molar ratios, rCA. Gyration radius was determined by small angle x-ray scattering (SAXS) and showed that, as rCA was increased, IPEC dimensions decreased and reached a minimum at rCA = 0.75, which was considered the ratio at which IPEC cluster dimensions were minimum, following collapse, phase segregation, nucleation, and growth of larger particles. Pair distance distributions, P(r), became narrower up to rCA = 0.75, increasing its width from this point. Relaxation-related parameters from dynamic light scattering (DLS) intensity correlation functions (ICFs) identified three main relaxation processes. The fast process, related to free polyelectrolyte molecules random motion disappeared as rCA, was increased. The other two relaxation processes also were a function of rCA and presented marked changes at rCA = 0.75. At the same value of rCA, the energy of activation for the average relaxation rate showed the occurrence of a clear change in the nature of IPEC-related interactions. As hydrodynamic diameter, determined by DLS, was much larger than the gyration radius determined by SAXS, IPEC particles could be described as being composed by a core, rich in segregated, insoluble material, enveloped by IPEC soluble clusters, possibly in the form of water-rich gels.

The Nature of Secondary Relaxations: The Case of Poly(ethylene-alt-propylene) Studied by Dielectric and Deuteron NMR Spectroscopy

Main (α-) and secondary (β-) relaxation of the linear poly(ethylene-alt-propylene) (PEP) with different molar masses are investigated by dielectric and 2H NMR spectroscopy. Regarding the α-process, time constants in the range 10–11–1 s are reported, and the correlation function is described by a Cole–Davidson function with stretching parameter β ≅ 0.34. Both methods show clear evidence of a pronounced β-process; the latter is thoroughly studied by different NMR techniques. The results are compared to toluene-d3 and toluene-d5, respectively, as reference systems. The solid-echo spectra as well as the spin–lattice relaxation display typical features of spatially highly restricted angular displacements similar to those reported in other glass-formers showing a β-process−like toluene. Above Tg all chain segments participate. As both chain and methyl group were deuterated and as the β-process is rather fast, its angular displacement growing with temperature above Tg is directly reflected in a reduction of the ...

The effect of structural properties on rheological behaviour of starches in binary dimethyl sulfoxide-water solutions.

This research study analysed the rheological properties of potato amylose and potato amylopectin in binary solutions of the following water and dimethyl sulfoxide concentrations: 90% DMSO (1), 80% DMSO (2) and 50% DMSO (3), with preparation methodology involving the dissolution at the temperature of 98°C. The studies of dynamic light scattering on the biopolymer coils and the determination of main relaxation times of the solutions were carried out. For the amylose solutions, the fast relaxation phenomena are predominant. The results of the quality tests of the hysteresis loop showed, that the amylose solutions in the solvents (1) and (2) are rheologically stable and shear-thickened. The amylose solutions in solvents (3) reveal oscillatory alterations of viscosity in the time. Amylopectin solutions are characterized by 80% share of slow relaxation phenomena, very low diffusion coefficients and hydrodynamic radii in the range of 2000 nm. The amylopectin solutions are rheologically unstable.

Predicting the Kinetics of RNA Oligonucleotides Using Markov State Models.

Nowadays different experimental techniques, such as single molecule or relaxation experiments, can provide dynamic properties of biomolecular systems, but the amount of detail obtainable with these methods is often limited in terms of time or spatial resolution. Here we use state-of-the-art computational techniques, namely, atomistic molecular dynamics and Markov state models, to provide insight into the rapid dynamics of short RNA oligonucleotides, to elucidate the kinetics of stacking interactions. Analysis of multiple microsecond-long simulations indicates that the main relaxation modes of such molecules can consist of transitions between alternative folded states, rather than between random coils and native structures. After properly removing structures that are artificially stabilized by known inaccuracies of the current RNA AMBER force field, the kinetic properties predicted are consistent with the time scales of previously reported relaxation experiments.

Mazes and meso-islands: Impact of Ag preadsorption on Ge growth on Si(111)

The preadsorption of Ag on Si(111) drastically changes the growth of Ge. In a temperature range from $400{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ to $650{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, Ag adsorption on Si leads to the formation of a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ reconstruction that exhibits a maze-like morphology on the mesoscopic scale, as observed by low-energy electron diffraction (LEED) and low-energy electron microscopy. This maze morphology can be attributed to a surface roughening on an atomic scale, induced by the re-arrangement of top layer atoms during the $7\ifmmode\times\else\texttimes\fi{}7$ to $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ transition. The subsequent deposition of Ge results in the formation of a wetting layer, the evolution of which has been found to be governed by the Ag/Si(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ template's maze structure, as the latter offers a high density of heterogeneous nucleation sites. Upon further Ge growth, three-dimensional islands with diameters in the micrometer range are formed, which exhibit a large and flat (111) top facet. X-ray photoemission electron microscopy reveals that during Ge growth, Ag is segregating to the surface very efficiently. Grazing-incidence x-ray diffraction and transmission electron microscopy have been used to study the composition, strain state and defect structure of the Ge islands in dependence of the growth temperature. The strain induced by lattice mismatch is found to be largely relaxed (80--90% relaxation) in the investigated growth temperature range from 400 to $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, which is confirmed by high-resolution LEED measurements. As a main relaxation mechanism, the formation of interfacial misfit dislocations has been identified. Interdiffusion of Si into the Ge islands becomes more and more pronounced for increasing growth temperature, whereas the formation of twinned Ge regions can drastically be suppressed at higher temperature.

Dielectric relaxation anomalies in polyacrylic acid and their relationship with “critical” points of water

ABSTRACTThe dielectric properties of polyacrylic acid were studied using dielectric spectroscopy method in the frequency and temperature ranges 0.2 Hz–2 MHz and 2–70°C (at heating), respectively. Thermogravimetric analysis and differential scanning calorimetry were also employed at 25–250°C. A linear dielectric dispersion has been observed at low frequencies. Analysis of electrical modulus showed that the main relaxation process is characterized by non-Debye relaxation time spectrum. The observed anomalies of dielectric properties (conductivity, electrical modulus, dielectric permittivity) are probably related to the presence of water in the sample.

Thermo-rheological behaviour of native silk feedstocks

The rheology of native silk protein feedstock specimens was characterised by shear and oscillatory measurements, over the temperature range from 2 to 55°C, producing no evidence of thermally-driven phase change behaviour. All specimens exhibited flow characteristics typical of a concentrated polymer solution, with visco-elastic behaviour dominated by two main relaxation modes exhibiting time constants around 0.44 and 0.055s at 25°C. The specimens showed well-behaved temperature dependence following the Arrhenius equation, consistent with the kinetics being governed by an activation energy of flow, which ranged from 30.9 to 55.4kJmol−1 based on oscillatory data. Consequently, for the first time, it was possible to compile master-curves for native silk feedstock specimens following the principles of time-temperature superposition, using oscillatory data demonstrating visco-elastic behaviour typical of a polymer solution across a wide temperature range. Our work has highlighted the processing range of natural silks and furthered our stance on the molecular mechanisms governing the flow behaviour of these interesting and important materials.

The Slow Relaxation Dynamics in the Amorphous Pharmaceutical Drugs Cimetidine, Nizatidine, and Famotidine

The slow molecular mobility in the amorphous solid state of 3 active pharmaceutical drugs (cimetidine, nizatidine, and famotidine) has been studied using differential scanning calorimetry and the 2 dielectric-related techniques of dielectric relaxation spectroscopy and thermally stimulated depolarization currents. The glass-forming ability, the glass stability, and the tendency for crystallization from the equilibrium melt were investigated by differential scanning calorimetry, which also provided the characterization of the main relaxation of the 3 glass formers. The chemical instability of famotidine at the melting temperature and above it prevented the preparation of the amorphous for dielectric studies. In contrast, for cimetidine and nizatidine, the dielectric study yielded the main kinetic features of the α relaxation and of the secondary relaxations. According to the obtained results, nizatidine displays the higher fragility index of the 3 studied glass-forming drugs. The thermally stimulated depolarization current technique has proved useful to identify the Johari–Goldstein relaxation and to measure τβJG in the amorphous solid state, that is, in a frequency range which is not easily accessible by dielectric relaxation spectroscopy.

Dynamics of Bottlebrush Networks

The deformation dynamics of bottlebrush networks in a melt state is studied using a combination of theoretical, computational, and experimental techniques. Three main molecular relaxation processes are identified in these systems: (i) relaxation of the side chains, (ii) relaxation of the bottlebrush backbones on length scales shorter than the bottlebrush Kuhn length (bK), and (iii) relaxation of the bottlebrush network strands between cross-links. The relaxation of side chains having a degree of polymerization (DP), nsc, dominates the network dynamics on the time scales τ0 τsc, bottlebrush elastomers behave as networks of filaments with a shear modulus G0BB(t) ∼ (nsc + 1)−1/4t–1/2. Finally, the response of the bottlebrush networks becomes time independent at...

Influence of crystallinity on the dielectric relaxations of poly(butylene succinate) and poly[(butylene succinate)-co-(butylene adipate)

Amorphous and semi-crystalline poly(butylene succinate) (PBS) and poly[(butylene succinate)-co-(butylene adipate)] (PBSA) are studied with Differential Scanning Calorimetry and Broadband Dielectric Spectroscopy (BDS). It was shown that 21mol% of butylene adipate group decreases drastically the degree of crystallinity and the size of the crystal lamellae of the PBSA. For the two semi-crystalline polymers, the microstructure is well described by a three-phase model including a small degree of rigid amorphous fraction. Fragility indexes of the main dielectric relaxation are identical for each sample which shows similar segmental dynamics of their amorphous phase. BDS allows detecting complex secondary relaxations in semi-crystalline PBS and PBSA. Local motions are impacted by the microstructure and a new relaxation occurs in more constrained amorphous environments. Correlations between molecular mobility and barrier properties are proposed.

Characterization and modeling of dynamic relaxation of a Zr-based bulk metallic glass

Dynamic mechanical properties of the Zr40Ti25Cu8Be27 bulk metallic glass with large supercooled liquid region were investigated in both temperature and frequency domains. In the framework of the spectrum of loss modulus G″, master curves were established by using the time-temperature superposition principle. We found that temperature dependence of the shift factor follows the Arrhenius relationship around the glass transition temperature Tg. The activation energy for the low temperature relaxation and viscous flow were 1.2 eV and 4.47 eV, respectively. In addition, the fragility index was evaluated by considering temperature effect to the shift factor. The main relaxation was investigated by using different theoretical models. Due to the non-symmetrical characteristic of the main (α) relaxation, the main relaxation can not be well described with a single relaxation time. The advantages and disadvantages of various models are discussed. The physical significance of different parameters is qualitatively and quantitatively evaluated, which could help to understand the mechanical relaxation mechanism in bulk metallic glasses.

Main and secondary relaxations in an Au-based bulk metallic glass investigated by mechanical spectroscopy

The dynamic mechanical relaxation behavior of an Au49Cu26.9Si16.3Ag5.5Pd2.3 (at.%) bulk metallic glass was investigated by mechanical spectroscopy as a function of temperature or driving frequency. The mechanical spectra show the evidence of a secondary relaxation (or β) process at low temperature or high driving frequency. The main relaxation (α) is clearly evident at higher temperature or lower frequency. Crystallization induces an increase in elastic modulus, in two steps, which correspond to the formation of two different crystalline phases, as determined by in-situ X-ray diffraction experiments. Master curves could be obtained based on the time-temperature superposition principle. From these curves the apparent activation energies for the two relaxation phenomena were determined: Eβ = 1.10 eV and Eα = 3.65 eV, respectively. These values indicate that the secondary relaxation may be attributed to local atomic movements, while the α relaxation corresponds to correlated movements.

Strengthening of perfluorosulfonic acid ionomer with sulfonated hydrocarbon polyelectrolyte for application in medium-temperature fuel cell

The mechanical properties and conductivity, as well as the water sorption and transport properties, of a new series of blends of perfluorosulfonated acid ionomer (PFSA, Aquivion®) and disulfonated poly(aryl ether ketone) (DS-PAEK) with compositions ranging between 10 and 90wt% were studied. For 10–20wt% sulfonated hydrocarbon polyelectrolyte blends, the main mechanical relaxation temperature of side chains of Aquivion® increased 10–20°C as compared to pure Aquivion®. This shift demonstrates the presence of interactions between Aquivion® and DS-PAEK while the ion exchange capacity, the capacity to absorb water, and the proton conductivity of blends remained very close to those of pure Aquivion®. In addition, these new materials show improved robustness and better performance in fuel cell operation at 105°C as compared to Aquivion®. These results indicate that PFSA materials can be strengthened by adding a sulfonated hydrocarbon ionomer.

Vasorelaxing Effect of Resveratrol on Bovine Retinal Arteries.

Resveratrol is a red wine polyphenol that causes vasorelaxation, which could be of interest in the treatment or prevention of eye diseases with an impaired blood flow. In this study, the vasorelaxant capacity of resveratrol (cis and trans) on bovine retinal arteries, its vasorelaxing mechanism, and its influence on the relaxation induced by the retinal relaxing factor (RRF) were examined. Isolated bovine retinal arteries were mounted into wire myographs for isometric tension measurements. Concentration-response curves of cis- and trans-resveratrol and concentration-response curves of resveratrol in the absence or presence of the endothelium or different inhibitors were constructed. Relaxations elicited by the RRF with and without resveratrol incubation were also compared. Both resveratrol isomers caused a similar strong concentration-dependent relaxation. Removal of the endothelium or blocking endothelium-dependent pathways did not change the relaxation. Also, K+ channel blockers did not reduce the relaxation, except the 120 mM K+ Krebs Ringer bicarbonate solution. Phorbol 12-myristate 13-acetate and phorbol 12,13-dibutyrate blocked the relaxation partially and so did the inhibition of heme oxygenase-1. Blocking adenylyl cyclase, AMP-activated protein kinase, estrogen receptors, sirtuin 1, or sarco/endoplasmic reticulum Ca2+ ATPase did not have an effect. The relaxation caused by the RRF was not altered by resveratrol incubation. Cis- and trans-resveratrol relax bovine retinal arteries similarly and concentration dependently. The main relaxation mechanism remains unclear, but K+ channels, carbon monoxide, and the myosin phosphatase pathway may be involved. Resveratrol does not have an influence on the RRF.

Spatio‐temporal dynamics in nondiffractive Bessel ultrafast laser nanoscale volume structuring

AbstractNondiffractive ultrafast optical beams with quasi‐stationary characteristics enable new regimes and scales in light‐matter interactions. We discuss the action of ultrashort Bessel laser beams in bulk fused silica, emphasizing excitation dynamics with energy localization beyond diffraction limit. We shed light on relaxation channels leading to one‐dimensional structures with nanoscale sections and morphologies ranging from densified matter to nanosized cavities. Space‐ and time‐resolved absorption and phase‐contrast microscopy reveals two main carrier relaxation paths. Fast exciton trapping in self‐induced matrix deformations results in positive index contrast driven by swift accumulation of non‐bridging oxygen hole centers and defect‐driven structural rearrangements. High excitation densities determine thermomechanical paths, with onset of phase transitions and the release of pressure waves. High‐aspect‐ratio nanosized channels are thus created via rarefaction and liquid cavitation, accompanied by molecular decomposition and generation of oxygen deficiency. The characteristic electronic relaxation identifies the nature of structural transitions up to the onset of phase transformation. Temporal pulse dispersion regulation allows driving unique carrier dynamics with precise control over energy deposition down to the 100 nm scale. Extreme high‐aspect‐ratio uniform void structures can thus be fabricated in conditions of sub‐micron transverse light confinement. image